Muhammad Usman
email: muhammad.usman@squ.edu.om

Salah Jellali

Ioannis Anastopoulos

Yassine Charabi

Bassim H Hameed

Khalil Hanna

Fenton oxidation for soil remediation: A critical review of observations in historically contaminated soils

Keywords: Soil contamination, Soil remediation, Persistent organic pollutants, Advanced oxidation processes, Fenton oxidation

1. Introduction 2. Literature search methodology 3. Fenton oxidation-based strategies tested in historically contaminated soils 3.1. Traditional Fenton oxidation 3.2. Modified Fenton oxidation and role of chelating agents 3.3. Heterogeneous Fenton oxidation and role of iron minerals 3.3.1. Use of endogenous Fe as a heterogeneous Fenton catalyst 3.3.2. Use of exogeneous Fe minerals as heterogeneous Fenton catalysts 3.4. Photo-Fenton and Electro-Fenton oxidation 4. Integration of Fenton oxidation with other remediation strategies 4.1. Coupling of Fenton oxidation with bioremediation 4.2. Coupling of Fenton oxidation with chemical reduction J o u r n a l P r e -p r o o f 4.3. Coupling of Fenton oxidation with soil washing/soil flushing 23 5. Experimental setups tested for Fenton oxidation of historically contaminated soils 24 6. Major implications of Fenton oxidation in historically contaminated soils 25 6.1.

Poor pollutant availability and soil matrix effect 25 6.2. Impacts of Fenton oxidation on soil quality and productivity 28 6.2.1. Impacts of Fenton oxidation on soil physico-chemical properties 28 6.2.2. Effects of Fenton oxidation on soil organic matter 31 6.2.3. Impacts on the release of nutrients and co-existing pollutants 33 6.2.4. Effects of Fenton oxidation on soil microorganisms 34 6.2.5. Effects of Fenton oxidation on soil revegetation capacity 38 7. Conclusions and outlooks 38

Introduction

Soil remediation has gained particular attention in recent decades due to its implications for food security and environmental health. Scientific community is continuously exploring suitable upgrades in conventional remediation strategies or their innovative alternatives.

Advanced oxidation processes (AOPs) are emerging as viable remediation strategies for soils contaminated with organic pollutants. These AOPs rely on different oxidants which can completely mineralize the organic pollutants or improve their biodegradability for subsequent bioremediation in coupled remediation techniques.

Fenton oxidation is amongst the most popular AOPs owing to its high treatment efficiency, quick process, and wide application range. Fenton oxidation is based on the pioneering work of J.H. [START_REF] Fenton | Oxidation of Tartaric Acid in Presence of Iron[END_REF] which suggested the application of a solution of H 2 O 2 and Fe(II) salt (later termed as Fenton's reagent) for successful oxidation of tartaric acid. Hydroxyl radicals (HO • ) are predominantly formed at acidic pH through this catalytic process. A large amount of research has been devoted to understanding its reaction mechanism but, the simplest and widely recognized representation of this reaction is the following: Fe(II) + H 2 O 2 → Fe(III) + HO -+ HO • (1)

Owing to its strong oxidation potential (E° = 2.8 V), the HO • radical is amongst the most reactive chemical species which is able to oxidize organic pollutants instantaneously in a non-selective way. There are many Fenton-based processes that are distinguished by J o u r n a l P r e -p r o o f underlying reaction chemistry, treatment efficiency, and environmental impacts. A detailed description of different Fenton-based processes in the context of real soils is provided in the next section with an emphasis on their efficiency, advantages and challenges (Section 3, Table 1).

Fenton oxidation has been extensively used to remediate different organic pollutants in contaminated soils as evident from the immense amount of research in this field. It should, however, be noted that experiments denoting soil application have been performed in different contaminated matrices including artificially contaminated soils (pollutants spiked either on sand, synthetic soil composed of sand and minerals, or non-polluted real field soil)

and historically contaminated real soils. All these matrices have been focused in reviews (mostly pollutant-specific) related to the Fenton oxidation without any particular distinction between different soil matrices while reviewing the existing literature (Cheng et al., 2016a;[START_REF] Sutton | Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies[END_REF]Usman et al., 2016b;[START_REF] Venny | Current status and prospects of Fenton oxidation for the decontamination of persistent organic pollutants (POPs) in soils[END_REF][START_REF] Yap | Fenton based remediation of polycyclic aromatic hydrocarbons-contaminated soils[END_REF]. To our knowledge, this is the first review which is based exclusively on the observations regarding the applications of Fenton oxidation in real historically contaminated soils.

"Laboratory-spiked" contaminants are not sorbed in uncontaminated soils to the same extent as historically contaminated soils sorb their "native" pollutants [START_REF] Arp | Native Oxy-PAHs, N-PACs, and PAHs in Historically Contaminated Soils from Sweden, Belgium, and France: Their Soil-Porewater Partitioning Behavior, Bioaccumulation in Enchytraeus crypticus, and Bioavailability[END_REF]. This is partly due to the absence of sufficient "aging" in laboratory studies [START_REF] Arp | Native Oxy-PAHs, N-PACs, and PAHs in Historically Contaminated Soils from Sweden, Belgium, and France: Their Soil-Porewater Partitioning Behavior, Bioaccumulation in Enchytraeus crypticus, and Bioavailability[END_REF][START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Once pollutants are discharged into the soil, they become sequestered in the soil, particularly in soil organic matter, and sorption strength increases over time [START_REF] Jonsson | Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton's reagent: A multivariate evaluation of the importance of soil characteristics and PAH properties[END_REF]. Moreover, these pollutants slowly start to diffuse into the organic soil fraction and/or penetrate into cavities (Figure 1). Soil organic matter is highly heterogeneous with varying proportions of organic phases having different affinities for contaminants. The presence of non-aqueous phase liquids (NAPLs) in soils presents specific challenges to any remediation technique [START_REF] Pac | In situ chemical oxidation: Lessons learned at multiple sites[END_REF]. Moreover, the entrapment of pollutants in soil voids and pores may also affect their mobility and availability (Figure 1) [START_REF] Jonsson | Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton's reagent: A multivariate evaluation of the importance of soil characteristics and PAH properties[END_REF].

Similarly, soil mineral fractions and pollution characteristics have also shown strong impact on oxidation efficiency [START_REF] Jonsson | Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton's reagent: A multivariate evaluation of the importance of soil characteristics and PAH properties[END_REF][START_REF] Pac | In situ chemical oxidation: Lessons learned at multiple sites[END_REF]. Moreover, historically J o u r n a l P r e -p r o o f contaminated soils tend to be near industrial sites (coking industry, gasworks, oil fields, etc.),

where the soil can be enriched with strong-sorbing carbonaceous materials, which can decrease contaminant bioavailability [START_REF] Arp | Native Oxy-PAHs, N-PACs, and PAHs in Historically Contaminated Soils from Sweden, Belgium, and France: Their Soil-Porewater Partitioning Behavior, Bioaccumulation in Enchytraeus crypticus, and Bioavailability[END_REF]. The contaminant contact time (i.e.

aging effects) is also highly important in predicting the pollutant availability which is much lower in the historically contaminated soils compared with recently contaminated and spiked soils [START_REF] Esmaeili | A Comparison of In-vitro PAH Bioaccessibility in Historically Contaminated Soils: Implications for Risk Management[END_REF]. Owing to the long-term pollutant aging and soil heterogeneity, historically contaminated soils face particular pollution composition patterns, soil constituents, and complex soil-pollutant interactions. These major pitfalls pose particular challenges to soil decontamination as the treatment efficiency in historically contaminated soils can be significantly lower than that in spiked soil [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Artificially contaminated soils can facilitate the preliminary investigations but are not the true representative of historically polluted soils under field settings. Owing to the recent advancements in this field and the amount of data in historically contaminated soils, it becomes essential to review the relevant literature to have better insights into the application of Fenton-based strategies for field application.

For this purpose, research data has been compiled from numerous publications dealing with the application of Fenton oxidation in historically contaminated soils. In the first part of this review, different Fenton-based processes are critically evaluated to illustrate the promising techniques and their appropriate applications with a brief description of related challenges and their key solutions. The coupling of this process with other treatment strategies like bioremediation, reduction, soil washing, is also discussed. Multiple experimental setups (batch, column, pot and field) have been used in soil remediation studies which are also discussed. The next sections describe the limiting factors associated with these processes in soil-plant system with a description of strategies to overcome these limitations. This review is intended to provide a unique reference to facilitate the upscaling of Fenton oxidation for real field application.

J o u r n a l P r e -p r o o f has been reproduced with permission from Ref. [START_REF] Jonsson | Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton's reagent: A multivariate evaluation of the importance of soil characteristics and PAH properties[END_REF] whereas the rest of the figure has been prepared using Biorender app.

Literature search methodology

For this purpose, Web of Science database was searched (updated on March 16, 2021) by using the following keywords in manuscript titles: (("Fenton" or "chemical oxidation" or "H 2 O 2 ") and ("soil" or "soils")). This search technique is often used in bibliometric analyses where the use of quotation marks ("") ensures finding the exact phrases by deactivating the synonym feature of Web of Science [START_REF] Usman | A bibliometric study of the Fenton oxidation for soil and water remediation[END_REF][START_REF] Vanzetto | Bibliometric study of the toxicology of nanoescale zero valent iron used in soil remediation[END_REF].

Term "or" was used to find different expressions which were connected by using "and" to ensure that these words appeared in the title of each article. This search yielded 309 articles and each article was read to identify if it focuses on the remediation of historically contaminated soils by using Fenton-based treatments. Articles presenting the application of Fenton oxidation in artificially contaminated soils (by spiking) were excluded. Studies J o u r n a l P r e -p r o o f involving real contaminated soils were reviewed and are presented in the next sections. A separate search has also been performed by using TITLE ("Fenton" or "chemical oxidation"

or "H 2 O 2 ") and ("site" or "sites"). As, many irrelevant articles were found in this search (about reaction sites at catalysts), articles were first shortlisted by reading the article title on the basis of their applications in soils. Then, the shortlisted articles were read to identify the relevant articles. Table 2 provides a brief summary of all these studies.

Fenton oxidation-based strategies tested in historically contaminated soils

Different Fenton-based oxidation processes have been tested for the treatment of historically contaminated soils. These processes are categorized according to the associated number of publications in Figure 2. Among them, prominent strategies tested in real soils include

Traditional Fenton oxidation

Traditional Fenton oxidation relies on soluble Fe(II) to catalyze H 2 O 2 and requires an acidic pH (pH < 4) for optimum efficiency. This process has shown substantial potential to remove pesticides [START_REF] Villa | Oxidation of p,p′-DDT and p,p′-DDE in highly and longterm contaminated soil using Fenton reaction in a slurry system[END_REF][START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF][START_REF] Zhu | Comparison of Persulfate Activation and Fenton Reaction in Remediating an Organophosphorus Pesticides-Polluted Soil[END_REF], polycyclic aromatic hydrocarbons (PAHs) [START_REF] Jonsson | Comparison of Fenton's reagent and ozone oxidation of polycyclic aromatic hydrocarbons in aged contaminated soils[END_REF]2007;[START_REF] Lundstedt | Transformation of PAHs during ethanol-Fenton treatment of an aged gasworks' soil[END_REF], total petroleum hydrocarbons (TPHs) [START_REF] Liu | Tandem modified Fenton oxidation and bioremediation to degrade lubricant-contaminated soil[END_REF]Sutton et al., 2014b;2014a) in industrially contaminated soils (Table 2). For example, [START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF] reported 75% removal of a pesticide (DDT) when traditional Fenton oxidation (6 mM FeSO 4 , 38.1 g H 2 O 2 in increments) was applied in a soil obtained from a former pesticides warehouse in Brazil. [START_REF] Jonsson | Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton's reagent: A multivariate evaluation of the importance of soil characteristics and PAH properties[END_REF] evaluated its use in soils obtained from five Swedish industrial sites including three former wood impregnation sites, a former gas works, and a working coke plant. An overall degradation of 9-43% was observed in these soils though degradation efficiency varied according to the nature of PAHs i.e. 89%, 59%, and varying between 0-38% for PAHs with two, three, and more rings, respectively.

As soluble Fe(II) precipitates at higher pH values and loses its catalytic ability, most of the studies involving traditional Fenton oxidation were performed at acidic pH values (Table 2).

However, maintaining such an acidic pH in soils could increase the treatment cost and can also cause adverse impacts on soil biota and quality [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF][START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF].

A separate section (No. 6.2) has been devoted to illustrating the impact and environmental implications of different Fenton-based processes in real contaminated soils. It is worth mentioning that few studies also assessed the efficiency of soluble Fe(II) at circumneutral J o u r n a l P r e -p r o o f pH. However, under such conditions, either its efficiency was significantly affected [START_REF] Usman | Application of chemical oxidation to remediate HCH-contaminated soil under batch and flow through conditions[END_REF], or it was completely unable to catalyze the chemical oxidation due to the precipitation of soluble Fe(II) [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Performing this process at circumneutral pH exhibited negligible impacts on soil's indigenous microbes but its efficiency was declined [START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF]. To summarize, optimum efficiency of soluble Fe(II) in contaminated soils requires acidic conditions which can have detrimental impacts on soil quality that should be considered. It is interesting to report that initial acidification can also improve the oxidation efficiency by the acid-induced dissolution of soil carbonate (complete or partial) which is otherwise considered as an oxidant scavenger [START_REF] Usman | Chemical oxidation of hexachlorocyclohexanes (HCHs) in contaminated soils[END_REF].

Modified Fenton oxidation and role of chelating agents

To preclude the requirement of initial acidification, modified Fenton oxidation proposes the use of chelating agents (CAs) along with soluble Fe(II) to carry out the process at circumneutral pH [START_REF] Zhang | A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values[END_REF]. The CAs have a strong chelation ability for metal ions to keep them in solution even at higher pH values which extends the optimum range of reaction pH. As illustrated in Table 2, chelating agents have been used in Fenton oxidation of various pollutants such PAHs [START_REF] Laurent | Oxidation of a PAH polluted soil using modified Fenton reaction in unsaturated condition affects biological and physicochemical properties[END_REF][START_REF] Lemaire | Chemical oxidation efficiency for aged, PAH-contaminated sites: An investigation of limiting factors[END_REF][START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF][START_REF] Zhao | Removal of Bound PAH Residues in Contaminated Soils by Fenton Oxidation[END_REF], TPHs [START_REF] Chunyang | Fenton Pre-Oxidation Followed by Microbial Degradation for Removing Crude Oil from Contaminated Soil[END_REF][START_REF] Liu | Tandem modified Fenton oxidation and bioremediation to degrade lubricant-contaminated soil[END_REF][START_REF] Martínez-Pascual | Coupling chemical oxidation and biostimulation: Effects on the natural attenuation capacity and resilience of the native microbial community in alkylbenzene-polluted soil[END_REF]Sutton et al., 2014b;2014a;[START_REF] Wang | Hydrogen peroxide (H2O2) requirement for the oxidation of crude oil in contaminated soils by a modified Fenton's reagent[END_REF]Xu et al., 2011a;2011b;2016b;2017a;2018) or polychlorinated biphenyls (PCBs) [START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF]. The use of CAs with soluble Fe(II) exhibited significant enhancement in oxidation efficiency. Two types of CAs have been employed in real contaminated soils. Their first type corresponds to organic CAs such as citric acid, citrate, ethylenediaminetetraacetic acid (EDTA), ethylenediamine-N,N′disuccinic acid (EDDS), ethylenediamine-N,N′-bis(o-hydroxyphenyl) acetic acid (EDDHA), oxalic acid, sodium citrate, sodium dodecyl sulfate, sodium N,N′-bis(carboxymethyl) glutamic acid (GLDA). Inorganic CAs are the other type of CAs where sodium pyrophosphate (SP) represent the sole example tested in the context of real soils. The EDTA has been used traditionally as a stable CA with high efficiency [START_REF] Gryzenia | Production and accumulation of surfactants during the chemical oxidation of PAH in soil[END_REF]Lu et al., 2010a; J o u r n a l P r e -p r o o f Ndjou'ou and Cassidy, 2006) but its application can be limited by its non-biodegradable nature [START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF]. Therefore, [START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF] suggested EDDS, a structural isomer of EDTA, as an alternative owing to its bridgeable nature. The addition of low-molecular weight organic acids such as citrate could also remarkably improve the oxidation efficiency (Sutton et al., 2014b;2014a). Citric acid emerged as the most widely investigated CA in real contaminated soils (Table 2) probably because of its high efficiency, quick availability, and less environmental impacts. For example, in soil obtained from an oil tank storage site, Amsterdam, Xu et al. (2011b) reported that TPH degradation by modified Fenton oxidation was higher (51% removal) in the presence of citric acid than oxalic acid (9%). A similar trend was reported during the remediation of sorbed crude oil in contaminated soil [START_REF] Xu | Enhancing the Removal of Sorbed Crude Oil from Soil Through Multiple Oxidation Steps in Stepwise Fenton Processes[END_REF]. However, to remove bound PAHs by Fenton oxidation in a soil from an oil well in Chia, citric acid exhibited slightly lower efficiency than oxalic acid [START_REF] Zhao | Removal of Bound PAH Residues in Contaminated Soils by Fenton Oxidation[END_REF].

Recently, [START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF] evaluated the removal of PCBs by modified Fenton oxidation activated by Fe(II) or Fe(III) chelated with five different CAs including organic (EDDS, EDDHA, GLDA, and sodium citrate) and inorganic CAs (like SP). They reported that PCB removal was higher in CA/Fe(II)/H 2 O 2 system as compared to that by CA/Fe(III)/H 2 O 2 due to the direct implications and greater reactivity of Fe(II). Efficiency of CAs varied in the following order: SP > EDDHA > sodium citrate > EDDS > GLDA (Figure 3). Organic CAs act as radical scavengers due to their competition with the target organic pollutants and thus decrease the oxidation efficiency [START_REF] Usman | Comment on "A comprehensive guide of remediation technologies for oil contaminated soil -Present works and future directions[END_REF]. On the other hand, SP being an inorganic CA does not consume the oxidant. Findings of [START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF] also revealed that the use of SP/Fe(II)/H 2 O 2 slightly reduced the final pH (from 7.6 to 6.6) than EDDHA/Fe(II)/H 2 O 2 that caused a remarkable decrease (from 7.6 to 2.7) which can be hazardous for soil quality and could contribute towards improving the metal solubility. Therefore, the use of SP and other inorganic CAs could be advantageous as compared to the organic CAs. But till now, only two studies have explored the efficiency of inorganic CAs in real contaminated soils (Jorfi et al., 2017;[START_REF] Ma | Enhanced Fenton degradation of polychlorinated biphenyls in capacitor-oil-contaminated soil by chelating agents[END_REF]. This should be further evaluated in the context of future studies.

J o u r n a l P r e -p r o o f In addition to chelating soluble Fe(II) or Fe(III), CAs can also be advantageous in improving the catalytic ability of iron-based solids (hematite, zero-valent Fe etc.) or soil native Fe minerals [START_REF] Polli | Impact of the Fenton-like treatment on the microbial community of a dieselcontaminated soil[END_REF][START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF][START_REF] Sherwood | Modified Fenton oxidation of diesel fuel in arctic soils rich in organic matter and iron[END_REF]. These CAs facilitate the desorption of soil entrapped Fe allowing the formation of CA/soluble Fe complexes which can then participate in Fenton oxidation. It has been found that the use of EDTA resulted in significant improvement in TPH degradation (from <40% to >70%) when relied on native Fe or added ZVI [START_REF] Ouriache | Treatment of petroleum hydrocarbons contaminated soil by Fenton like oxidation[END_REF]. Soil organic matter (SOM) contains many natural CAs such as fluvic and humic acids which may also contribute in chelation. For example, in three arctic soils characterized by high SOM (10-15 wt.%) and Fe oxides, Sherwood and Cassidy [START_REF] Sherwood | Modified Fenton oxidation of diesel fuel in arctic soils rich in organic matter and iron[END_REF] reported that relying on the SOM for chelation of the native Fe improved the stability and oxidation efficiency of H 2 O 2 for diesel fuel degradation than by using EDTA as a chelate. However, it is usually difficult to achieve the optimum ratio of natural CAs (considering the small amount of SOM) to efficiently chelate the native Fe. Therefore, the addition of an appropriate amount of CA is usually required for an effective chelation of native Fe (Table 2). It should be noted that the use of these organic acids can also improve the availability of pollutants for higher removal efficiency by complexing with these pollutants [START_REF] Zhao | Removal of Bound PAH Residues in Contaminated Soils by Fenton Oxidation[END_REF]. Owing to this useful feature, the use of cyclodextrin has been suggested as a pre-treatment before applying Fenton oxidation [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF] that increased the oxidation efficiency of PAHs by increasing the pollutant availability. Though CAs enable the process to proceed at neutral pH, this process can be limited by higher treatment costs and elevated toxicity of CAs.

Moreover, organic CAs can also increase the oxidant demand as they, being organic in nature, can consume the radicals by competing with the pollutants [START_REF] Usman | Comment on "A comprehensive guide of remediation technologies for oil contaminated soil -Present works and future directions[END_REF]. This nonproductive consumption would increase the treatment cost while decreasing the treatment efficiency. The use of inorganic CAs can be advantageous but limited amount of the associated literature calls for more studies to evaluate their efficiency and associated implications in soil system.

Heterogeneous Fenton oxidation and the role of iron minerals

Heterogeneous Fenton oxidation, also termed Fenton-like oxidation, is another important strategy where iron solids/minerals are used to catalyze the reaction at circumneutral pH [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]2016b). Immobilization of Fe species within the structure of iron minerals prevents iron hydroxide precipitation. Thus, the catalytic ability of these iron minerals can be maintained at neutral pH. In this context, catalytic ability of following iron solids has been evaluated in the literature including iron oxides namely hematite [START_REF] Jorfi | Enhancement of the Bioremediation of Pyrene-Contaminated Soils Using a Hematite Nanoparticle-based Modified Fenton Oxidation in a Sequenced Approach[END_REF], magnetite [START_REF] Barzegar | Enhanced Sono-Fenton-Like Oxidation of PAH-Contaminated Soil Using Nano-Sized Magnetite as Catalyst: Optimization with Response Surface Methodology[END_REF][START_REF] Biache | Effect of pre-heating on the chemical oxidation efficiency:Implications for the PAH availability measurement in contaminated soils[END_REF][START_REF] Boulangé | Fenton-like and potassium permanganate oxidations of PAH-contaminated soils: Impact of oxidant J o u r n a l P r e -p r o o f doses on PAH and polar PAC (polycyclic aromatic compound) behavior[END_REF][START_REF] Mirzaee | Optimization of total petroleum hydrocarbons removal from Mahshahr contaminated soil using magnetite nanoparticle catalyzed Fenton-like oxidation[END_REF][START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]2017;[START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]2014;2016a;2021), zerovalent Fe (ZVI) [START_REF] Cao | Remediation of DDTs contaminated soil in a novel Fenton-like system with zero-valent iron[END_REF][START_REF] Ouriache | Treatment of petroleum hydrocarbons contaminated soil by Fenton like oxidation[END_REF][START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]2017), tourmaline [START_REF] Jian | Effects of tourmaline catalyzed Fenton-like combined with bioremediation on the migration of PBDEs in soilplant systems: Soil properties and physiological response of lettuce and selective uptake of PBDEs[END_REF][START_REF] Li | A novel technology for remediation of PBDEs contaminated soils using tourmaline-catalyzed Fenton-like oxidation combined with P. chrysosporium[END_REF], steel slag (Cheng et al., 2016b) and endogenous iron (Table 2). It should be noted that the use of soluble Fe(III) has also been proposed as a Fenton-like catalyst in other environmental matrices but its use has never been evaluated in real soils. There exists only one study where Fe(III) bearing iron citrate has been used as a catalyst for heterogeneous Fenton oxidation of oil hydrocarbons in an oil field contaminated soil (Lu et al., 2010b). At the end of this treatment, total extractable organics were reduced from 32,400 to 21,800 mg kg -1 soil. Provided below is a brief J o u r n a l P r e -p r o o f description of the catalytic activity of different iron solids including native Fe species and exogenous iron minerals.

Use of endogenous Fe as a heterogeneous Fenton catalyst

Natural soils also contain a significant amount of iron (2-50 g kg -1 ) in the form of different iron oxides which are capable of catalyzing the Fenton's process. Therefore, exploring the catalytic potential of endogenous Fe minerals is also gaining attention [START_REF] Villa | Oxidation of p,p′-DDT and p,p′-DDE in highly and longterm contaminated soil using Fenton reaction in a slurry system[END_REF]. The role of endogenous iron minerals was highlighted in PAH contaminated soils bearing 16.4 mg.g -1 of native Fe (Palmroth et al., 2006a). The addition of H 2 O 2 alone (0.4 g.g - 1 soil) removed >50% of PAHs under saturated conditions (Palmroth et al., 2006a).

However, native Fe minerals may not be able to catalyze the Fenton oxidation despite their presence in sufficient amounts potentially due to their poor availability or existence as inactive forms [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Therefore, most of the studies relying on the catalytic ability of native Fe suggested the use of chelating agents (Table 2). For example, in a highly contaminated soil obtained from pesticides warehouse, oxidation efficiency in the presence of native iron (hematite mainly) was 24% and 4% of DDT [1,1-bis(4-chlorophenyl)-2,2,2trichloroethane] and DDE [2,2-bis(4-chlorophenyl)-1,1-dichloroethylene], respectively [START_REF] Villa | Oxidation of p,p′-DDT and p,p′-DDE in highly and longterm contaminated soil using Fenton reaction in a slurry system[END_REF]. Degradation extent, however, increased to 53% (DDT) and 46%

(DDE) by adding soluble Fe (3.0 mmol L -1 ). Moreover, the use of chelating agents (Table 2) can also improve the availability and efficiency of native Fe species [START_REF] Ouriache | Treatment of petroleum hydrocarbons contaminated soil by Fenton like oxidation[END_REF][START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF][START_REF] Sherwood | Modified Fenton oxidation of diesel fuel in arctic soils rich in organic matter and iron[END_REF]. These agents enhance the desorption of the iron entrapped in soil and form soluble Fe-based complexes that remain available to catalyze Fenton's oxidation [START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF]. Endogenous iron exhibited higher catalytic ability to activate H 2 O 2 in presence of EDTA [START_REF] Ouriache | Treatment of petroleum hydrocarbons contaminated soil by Fenton like oxidation[END_REF]. It is worth mentioning that owing to the catalytic ability of endogenous Fe, natural soils have been used

as cost-free Fenton-like catalysts to degrade pollutants in water medium like pharmaceuticals and dyes [START_REF] Aktas | The adsorption and Fenton behavior of iron rich Terra Rosa soil for removal of aqueous anthraquinone dye solutions: kinetic and thermodynamic studies[END_REF][START_REF] Changotra | Natural soil mediated photo Fenton-like processes in treatment of pharmaceuticals: Batch and continuous approach[END_REF][START_REF] Khataee | Iron rich laterite soil with mesoporous structure for heterogeneous Fenton-like degradation of an azo dye under visible light[END_REF][START_REF] Koottatep | Potential of Laterite Soil Coupling Fenton Reaction in Acetaminophen (ACT) Removal in Constructed Wetlands[END_REF]. Relying on endogenous Fe, if in sufficient quantities and with good J o u r n a l P r e -p r o o f efficiency, can be economically feasible. However, the existence of endogenous Fe in sufficient amount is not the sole deciding parameter to rely on its role. Its type and the extent of its availability also dictate the efficiency of native Fe minerals which should also be assessed before implementing a treatment. In this context, it would be rewarding to make better efforts to improve the availability and efficiency of native Fe species in contaminated soils.

Use of exogenous Fe minerals as heterogeneous Fenton catalysts

To overcome the limitations associated with endogenous Fe, the use of exogeneous Fe minerals has emerged as an efficient alternative. Among iron oxides, magnetite stands out owing to its higher catalytic ability as compared to other iron oxides, which, however, has only been compared in water medium [START_REF] Matta | Fenton-Like Oxidation of 2,4,6-Trinitrotoluene Using Different Iron Minerals[END_REF]. No such reactivity comparison has been made in real contaminated soils. The higher efficiency of magnetite has been linked to its structural Fe(II) which renders it more efficient than ferric (bearing Fe(III))

minerals (Usman et al., 2018a). Our previous studies reported that magnetite-catalyzed (10% w/w) Fenton oxidation was highly efficient to treat PAHs contaminated soils obtained from coking plant sites [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. It removed 70-90% of PAHs while soluble Fe(II)

was unable to act as a catalyst at neutral pH (<10% of PAH removal). Rybnikova et al.

(2017), however, reported the inefficiency of magnetite or nZVI (5 mg g -1 of soil with 0.2-4 mM H 2 O 2 ) at neutral pH to catalyze Fenton oxidation in PCP contaminated soil (timber mill) compared to the 36% removal in the presence of soluble Fe(II). Its use, however, decreased the pH of soil suspension was decreased to 5. The removal of PCP reached 43% with higher amounts of reagents (5 or 25 g of magnetite kg -1 soil w/w with 200-500 mM H 2 O 2 ) [START_REF] Usman | Enhancement of Pentachlorophenol Removal in a Historically Contaminated Soil by Adding Ascorbic Acid to H2O2/Magnetite System[END_REF]. This discrepancy in the remediation efficiency in the same soil has been correlated to different amounts of reagents tested in these studies [START_REF] Rybnikova | Remediation of an aged PCP-contaminated soil by chemical oxidation under flow-through conditions[END_REF][START_REF] Usman | Enhancement of Pentachlorophenol Removal in a Historically Contaminated Soil by Adding Ascorbic Acid to H2O2/Magnetite System[END_REF].

The use of ZVI-catalyzed Fenton oxidation has shown success in removing organochlorine pesticides [START_REF] Cao | Remediation of DDTs contaminated soil in a novel Fenton-like system with zero-valent iron[END_REF] and petroleum hydrocarbons [START_REF] Ouriache | Treatment of petroleum hydrocarbons contaminated soil by Fenton like oxidation[END_REF] in aged contaminated soils (Table 2). Both of these studies relied on the Fenton oxidation catalyzed J o u r n a l P r e -p r o o f by ZVI applied along with EDTA, to prevent Fe precipitation at circumneutral soil pH. [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF] reported that the catalytic ability of ZVI was slightly higher (18% pollutant removal) than magnetite (7% removal) in a historically contaminated PCB soil. It is worth mentioning that in spiked sand, degradation extent was up to 90% and 77% with ZVI and magnetite respectively. Though ZVI was more efficient, magnetite exhibited higher catalytic stability [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]. Indeed, the quick oxidation and surface passivation of nZVI are considered as the most significant challenges with its successful application for contaminant removal [START_REF] Bae | Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review[END_REF]. However, the reduction potential of ZVI also enabled the coupling of reduction and Fenton oxidation in a PCP contaminated soil from a timber facility [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]. A detailed description of the coupling of Fenton oxidation with other strategies is provided in Section 4.

The use of steel slag-catalyzed Fenton-like process resulted in 93% removal of atrazine removal in soil obtained from contaminated load site of pesticide manufacturing plant (Cheng et al., 2016b). Its treatment efficiency was comparable to other catalysts like FeSO 4 and Fe 2 O 3 . Contrary to these catalysts that decreased the soil pH from 5.4 to 3.4 (FeSO 4 ) and 4.6 (Fe 2 O 3 ), the use of steel slag increased the soil pH to 6.2 which was beneficial for revegetation purposes. Therefore, the application of alkaline steel slag can be advantageous to avoid the limitations of the traditional Fenton process i.e. an acidic pH. A detailed description of the implications of Fenton-based processes on soil properties is provided in Section 6.2. It should be noted that heterogeneous catalysts reported in the context of real contaminated soils are very limited as compared to those tested in artificially contaminated soil or aqueous solutions which calls for further experiments in real soils. The catalytic ability of these minerals to catalyze Fenton oxidation strongly depends on their surface and structural characteristics and the nature of Fe species (Fe(II) or Fe(III). The oxidation efficiency of iron minerals decreases by Fe leaching and surface oxidation/passivation which, if addressed, can improve the treatment efficiency. For example, in comparison to the 43% PCP removal by magnetite/H 2 O 2 system, the introduction of ascorbic acid improved the treatment efficiency to 95% (magnetite/ascorbic acid/H 2 O 2 system) [START_REF] Usman | Enhancement of Pentachlorophenol Removal in a Historically Contaminated Soil by Adding Ascorbic Acid to H2O2/Magnetite System[END_REF].

J o u r n a l P r e -p r o o f

The presence of ascorbic acid, a reducing agent, allowed magnetite to retain its structural Fe(II) contents and catalytic ability. However, this is the only study that used ascorbic acid in a historically contaminated soil [START_REF] Usman | Enhancement of Pentachlorophenol Removal in a Historically Contaminated Soil by Adding Ascorbic Acid to H2O2/Magnetite System[END_REF] to enhance the catalytic ability of iron minerals in Fenton oxidation though its application has shown good efficiency in aqueous solution and spiked soils [START_REF] Hwang | Removal of organochlorine insecticide endosulfan in water and soil by Fenton reaction with ascorbic acid and various iron resources[END_REF]. In this context, it is highly desirable to establish iron catalysts having higher stability and lower iron leaching but without compromising the treatment efficiency.

Photo-Fenton and Electro-Fenton oxidation

The application of Fenton's reagent with the UV-vis radiation (photo-Fenton oxidation) can

trigger additional reactions to generate extra HO• radicals [START_REF] Zhu | Heterogeneous photo-Fenton degradation of bisphenol A over Ag/AgCl/ferrihydrite catalysts under visible light[END_REF]. Electro-Fenton oxidation is another major Fenton-based process that involves the in-situ generation of H 2 O 2 at the cathode with O 2 or air feeding in the presence of a suitable Fe catalyst [START_REF] Zhou | Electro-Fenton Process: New Trends and Scale-Up[END_REF]. On the other hand, the electro-assisted Fenton process comprises of the regeneration of catalyst by Fe(III) electrochemical reduction or the production of iron catalysts on the anode to form peroxi-coagulation process (Figure 2).

Despite the large number of studies using photo-Fenton or electro-Fenton oxidation to treat organics in aqueous solution or spiked soils, no study has reported their direct application in real contaminated soils. Both of these techniques were employed to treat the organic pollutants in soil extract in integration with soil washing of real contaminated soils [START_REF] Huguenot | Combination of surfactant enhanced soil washing and electro-Fenton process for the treatment of soils contaminated by petroleum hydrocarbons[END_REF][START_REF] Safa | Investigating the photo-Fenton process for treating soil washing wastewater[END_REF][START_REF] Villa | Soil remediation using a coupled process: soil washing with surfactant followed by photo-Fenton oxidation[END_REF]. The readers are directed to Section 4.3 for further readings of this coupling.

Integration of Fenton oxidation with other remediation strategies

In coupled or integrated processes, two or more treatments, which are complementary to each other, are applied sequentially or simultaneously. Coupled treatment is intended to combine the advantages of individual techniques while overcoming their challenges. Fentonbased treatments can be integrated with other strategies for an improved treatment efficiency and to minimize the impacts of chemical oxidation on soil quality (Figure 4). A

J o u r n a l P r e -p r o o f major focus of such efforts was the coupling of Fenton oxidation with bioremediation (Table 2). Its coupling with other techniques like reduction and soil washing has also been investigated but there exist very few studies on this topic. Provided below is a brief description of different coupled processes tested in historically contaminated soils.

Coupling of Fenton oxidation with bioremediation

Compared to the chemical oxidation, bioremediation is environment-friendly and more suitable for the bioavailable fraction of a contaminated site. For example, the majority of products in crude oil are biodegradable which can be removed by bioremediation alone.

However, the existence of non-biodegradable residual contaminants after bioremediation prevents it from meeting the stringent environmental cleanup standards. Once the biodegradable fraction is eliminated, contamination level reaches a plateau as observed in oilfield contaminated soil after two years of bioremediation by Lu et al. (2010a). Therefore, J o u r n a l P r e -p r o o f these researchers suggested the use of Fenton-like oxidation to eliminate the nonbiodegradable residual contaminants after bioremediation (Lu et al., 2010a). Fenton oxidation is known for its rapid and extensive removal of pollutants. It has been suggested conventionally that Fenton oxidation is not compatible with bioremediation due to its negative impacts on microbes caused by acidic pH, oxidative stress, and changes in redox conditions. However, recent studies indicate that, although Fenton oxidation can temporarily decrease microbial activity, soil microbes can re-attain their populations and contaminant degradation ability [START_REF] Sutton | Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies[END_REF]. Indeed, it has been reported in many studies (Table 2) that integration of both processes improves the treatment efficiency and soil quality issues traditionally associated with Fenton oxidation. For example, the application of a bioremediation pretreatment may improve the remediation and reduce the amount of oxidant required in subsequent oxidation as all biodegradable compounds have already been eliminated (Lu et al., 2010a;[START_REF] Usman | Treatment of hydrocarbon contamination under flow through conditions by using magnetite catalyzed chemical oxidation[END_REF]2018b). After 20 weeks of bioremediation, the combined treatment removed 88.9% of TPHs whereas biological treatment alone removed 55.1%. Bioremediation may also lead to the formation of toxic byproducts which can also be toxic to the microbes. Therefore, the application of Fenton oxidation to bioremediation has been suggested to reduce soil toxicity and to improve treatment efficiency [START_REF] Gong | Remediation of weathered petroleum oil-contaminated soil using a combination of biostimulation and modified Fenton oxidation[END_REF]. For example, the removal of petroleum hydrocarbons increased from 55% (by 20 weeks of biostimulation alone) to 88% with the combined system where Fenton oxidation was applied to the biostimulation setup after 8 weeks [START_REF] Gong | Remediation of weathered petroleum oil-contaminated soil using a combination of biostimulation and modified Fenton oxidation[END_REF].

Contaminated soil

Decontaminated soil

Similarly, the application of Fenton oxidation as a pretreatment may improve overall remediation by (i) reducing the contamination to quantities less toxic for soil microbes, (ii) increasing the availability of pollutants for subsequent bioremediation, (iii) generating the biodegradable pollutant by-products, and (iv) providing oxygen and nutrients (by release from soil constituents) to improve the aerobic transformation of pollutants (as demonstrated in Figure 5 evaluating the interactions of oxidant species with soil constituents) [START_REF] Sutton | Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies[END_REF]. Considering these advantages, the combination of Fenton oxidation with subsequent bioremediation (Table 2) has been tested in many studies by using Fenton oxidation as a J o u r n a l P r e -p r o o f pre-treatment and bioremediation as a final polishing step. The use of an integrated system exhibited better treatment efficiency than either Fenton oxidation or bioremediation alone.

For example, PAH removal by Fenton oxidation combined with bioremediation was higher (43-59%) than by bioremediation alone (22-30%) in a creosote oil-contaminated soil (Palmroth et al., 2006b). In another study, the application of Fenton oxidation as a pretreatment in an integrated system with bioremediation removed pyrene to a higher extent (82%) than by bioremediation alone (37%) [START_REF] Jorfi | Enhancement of the Bioremediation of Pyrene-Contaminated Soils Using a Hematite Nanoparticle-based Modified Fenton Oxidation in a Sequenced Approach[END_REF]. Similarly, PAH removal of 75% by the integrated system was noted in another creosote oil-contaminated soil with a 30% increase in removal contributed by the bioremediation step [START_REF] Valderrama | Oxidation by Fenton's reagent combined with biological treatment applied to a creosotecomtaminated soil[END_REF]. In a petroleum-contaminated soil, the application of Fenton oxidation decreased the extractable organics from 32,400 to 21,800 mg kg -1 soil which were further reduced to 5360 mg kg -1 soil during 10 weeks of microbial treatment (Lu et al., 2010b). The major focus of the current research is to improve the microbial recovery and the efficiency of subsequent bioremediation. For this, there exist different strategies which are briefly described below:

(i) Reducing the oxidant dose during Fenton pre-oxidation: It has been found that lower amounts of oxidant (Fe/H 2 O 2 ratio of 1:20) were sufficient to remove 43% of PAHs without inhibiting subsequent bioremediation, which caused to a total pollutant removal of 75%.

However, aggressive pre-oxidation did not favor the subsequent bioremediation [START_REF] Valderrama | Oxidation by Fenton's reagent combined with biological treatment applied to a creosotecomtaminated soil[END_REF]. A higher residual population of indigenous bacteria can be obtained by weak Fenton pre-oxidation (low H 2 O 2 concentration) (Xu et al., 2016a;2017a). As a consequence, the recovery period of microbes was shortened by 20-40 days and the efficiency of subsequent bioremediation was improved (Xu et al., 2016a). Therefore, the use of mild or weak Fenton peroxidation has been suggested for better efficiency of microbes in the subsequent bioremediation step [START_REF] Valderrama | Oxidation by Fenton's reagent combined with biological treatment applied to a creosotecomtaminated soil[END_REF]Xu et al., 2017a).

(ii) Shortening the contact time of the oxidant with microbes: Minimizing the amount of direct contact that soil microbes have with the concentrated oxidant can minimize the negative impacts of oxidation and can aid microbes to exhibit better bioremediation. This has been obtained by the incremental addition of H 2 O 2 (Xu et al., 2011b) or by instant Fenton pre-J o u r n a l P r e -p r o o f oxidation [START_REF] Xu | Efficient sustainable bioremediation of long-chain crude oil in soils using instant Fenton pre-oxidation[END_REF]. The biodegradation of long-chain alkanes after instant Fenton pre-oxidation was two times higher than that after non-instant oxidation [START_REF] Xu | Efficient sustainable bioremediation of long-chain crude oil in soils using instant Fenton pre-oxidation[END_REF]. In another effort to improve subsequent bioremediation, stepwise addition of H 2 O 2 instead of adding all amount in the beginning has been suggested (Sutton et al., 2014a;Xu et al., 2011b). Incremental or sequential addition of chemical oxidant can provide mild oxidation conditions which prevent sterilization and a sudden increase in soil temperature (Xu et al., 2011b). In addition to its advantages in the coupled treatment process, the incremental addition of H 2 O 2 also leads to higher oxidation efficiency [START_REF] Lemaire | Chemical oxidation efficiency for aged, PAH-contaminated sites: An investigation of limiting factors[END_REF][START_REF] Xu | Enhancing the Removal of Sorbed Crude Oil from Soil Through Multiple Oxidation Steps in Stepwise Fenton Processes[END_REF]. This has been attributed to the absence of scavenging reaction between H 2 O 2 and HO • (that happens at high concentration) as H 2 O 2 concentration remains low (Sutton et al., 2014a;[START_REF] Xu | Enhancing the Removal of Sorbed Crude Oil from Soil Through Multiple Oxidation Steps in Stepwise Fenton Processes[END_REF].

(iii) Biological stimulation after Fenton pre-oxidation: Biological stimulation is intended to provide nutrients to enhance the growth and activity of the existing microbes (Figure 4). A rebound in the bacterial communities and their activity has been noted when nutrients and air mixing were provided after preliminary Fenton pre-oxidation of an alkylbenzene-polluted soil [START_REF] Martínez-Pascual | Coupling chemical oxidation and biostimulation: Effects on the natural attenuation capacity and resilience of the native microbial community in alkylbenzene-polluted soil[END_REF]. This rebound in microbial population allowed an efficient coupling of both techniques. Similarly, the use of nutrients and peanut hull in an oilcontaminated soil also allowed the microbes to recover their count and activities two weeks after Fenton oxidation [START_REF] Gong | Remediation of weathered petroleum oil-contaminated soil using a combination of biostimulation and modified Fenton oxidation[END_REF]. In addition to the use of a nutrient mixture, [START_REF] Valderrama | Oxidation by Fenton's reagent combined with biological treatment applied to a creosotecomtaminated soil[END_REF] added a phosphate buffer in Fenton oxidation to control the reaction pH. The use of this buffer also provides phosphate to microbes and thus enhances their remediation potential [START_REF] Sutton | Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies[END_REF]. Sutton et al. (2014a) noted that though microbial activity increased by the nutrients released during Fenton oxidation, a nutrient amendment was necessary to obtain diesel degradation by subsequent bioremediation. It should be noted that the addition of nutrients may not always be necessary as Fenton oxidation may release the nutrients by oxidizing the soil organic matter [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF]. For example, phosphate concentration increased in soils after Fenton oxidation when the impact of J o u r n a l P r e -p r o o f chemical oxidation was studied on plant growth [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF]. A detailed description of the impact of Fenton oxidation on soil quality is provided in section 6.2.

(iv) Bioaugmentation by adding bacteria after Fenton pre-oxidation (Figure 4): Lu et al.

(2010b) reported that Fenton-like oxidation reduced the microbial population from 3.45 × 10 6 CFU g -1 to 4.21 × 10 3 CFU g -1 soil. However, after inoculation with a mixed culture separated from the same contaminated soil, the microbial count increased gradually and attained a value of 2.19 × 10 8 CFU g -1 after 4 weeks. The application of this treatment decreased the extractable organics by 84% (with 50% contributed by bioremediation).

Similarly, the addition of indigenous microbes after Fenton pre-oxidation has also been tested in another pilot-scale study resulting in additional 19% removal of petroleum hydrocarbons [START_REF] Kim | Pilot scale feasibility study for in-situ chemical oxidation using H2O2 solution conjugated with biodegradation to remediate a diesel contaminated site[END_REF].

Coupling of Fenton oxidation with chemical reduction

The coupling of Fenton oxidation with reduction has been mostly studied in aqueous solutions or spiked soils. However, the integration of both processes to remediate historically contaminated soil has been rarely investigated. There exists only one study that explored the removal of polychlorinated biphenyls (PCBs) in a historically contaminated soil by this coupled treatment [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]. This study relied on the use of four synthetic nanoparticles (Fe 0 , Fe/Ni, Fe 3 O 4 , Fe 3 - x Ni x O 4 ) which were firstly tested for the reductive transformation of PCBs followed by their use as catalysts in Fenton oxidation. In historically contaminated soil, only Fe 0 and Fe/Ni particles exhibited better efficiency for reduction (13-18 %) and chemical oxidation (18-19%). It is worth mentioning that in PCBs spiked sand, the efficiency of Fe/Ni reached 97% (PCB28). This difference in treatment efficiency between spiked sand and historically contaminated soil has been linked to the soil matrix [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]. A detailed description of the impact of soil matrix is provided in Section 6.1

where various limiting factors are highlighted. The addition of oxidant after a preliminary reduction step could lead to the mineralization of the reduction by-products as the reactive oxidant species would react faster with low-chlorinated compounds or biphenyl, than with high-chlorinated compounds having greater stability [START_REF] Rybnikova | Removal of PCBs in Contaminated Soils by Means of Chemical Reduction and Advanced Oxidation Processes[END_REF]. oxidation has been successfully used as a post-treatment to decontaminate these contaminated effluents [START_REF] Huguenot | Combination of surfactant enhanced soil washing and electro-Fenton process for the treatment of soils contaminated by petroleum hydrocarbons[END_REF][START_REF] Ma | Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4′-trichlorodiphenyl from soil contaminated with capacitor oil[END_REF][START_REF] Safa | Investigating the photo-Fenton process for treating soil washing wastewater[END_REF][START_REF] Seo | Treatment of Artificial and Real Cocontaminated Soil by an Enhanced Electrokinetic-Fenton Process with a Soil Flushing Method[END_REF][START_REF] Villa | Soil remediation using a coupled process: soil washing with surfactant followed by photo-Fenton oxidation[END_REF]. For washing in historically contaminated soils, different extracting agents have been used like non-ionic surfactants [START_REF] Dominguez | Selective removal of chlorinated organic compounds from lindane wastes by combination of nonionic surfactant soil flushing and Fenton oxidation[END_REF][START_REF] Huguenot | Combination of surfactant enhanced soil washing and electro-Fenton process for the treatment of soils contaminated by petroleum hydrocarbons[END_REF][START_REF] Ma | Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4′-trichlorodiphenyl from soil contaminated with capacitor oil[END_REF][START_REF] Seo | Treatment of Artificial and Real Cocontaminated Soil by an Enhanced Electrokinetic-Fenton Process with a Soil Flushing Method[END_REF][START_REF] Villa | Soil remediation using a coupled process: soil washing with surfactant followed by photo-Fenton oxidation[END_REF], biosurfactant [START_REF] Fanaei | Enhanced treatment of the oilcontaminated soil using biosurfactant-assisted washing operation combined with H2O2stimulated biotreatment of the effluent[END_REF], cyclodextrins [START_REF] Ma | Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4′-trichlorodiphenyl from soil contaminated with capacitor oil[END_REF], and organic solvents [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. It should be noted that coupling of soil washing/Fenton oxidation has also been studied with electrokinetic treatment (Table 2) [START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF][START_REF] Seo | Treatment of Artificial and Real Cocontaminated Soil by an Enhanced Electrokinetic-Fenton Process with a Soil Flushing Method[END_REF]. The electrokinetic treatment relies on the low-density current generated by two electrodes introduced into contaminated soil. This current stimulates the transport of soil contaminants towards the electrodes where they are finally collected and treated [START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF]. The use of soil flushing and electrokinetic Fenton treatment removed about 58% of TPHs after 27 days of treatment in soil obtained from an area of high industrial activity in Spain [START_REF] Popescu | Soil flushing and simultaneous degradation of organic pollutants in soils by electrokinetic-Fenton treatment[END_REF][START_REF] Seo | Treatment of Artificial and Real Cocontaminated Soil by an Enhanced Electrokinetic-Fenton Process with a Soil Flushing Method[END_REF].

Experimental setups tested for Fenton oxidation of historically contaminated soils

Researchers have used different experimental setups to explore treatment efficiency in real contaminated soils such as batch, flow-through conditions (saturated, unsaturated, and closed-loop columns), and field application. Batch experiments are used to optimize the experimental conditions where oxidant solution is stirred with the contaminated soil ensuring an optimum contact between oxidant and contaminant for improved efficiency. Column experiments are considered as a better representative of field conditions and are usually performed on the basis of parameters derived from batch conditions (Usman et al., 2020;2021). Oxidant solution is injected in soil columns under saturated (upward oxidant injection with a peristaltic pump) or unsaturated conditions (downward addition of oxidant where oxidant solution flows only due to gravity gradient). Preferential oxidant movement could leave the presence of preferential pathways and/or gas phase in the unsaturated columns because flow of water is mainly controlled by the gravity gradient [START_REF] Rybnikova | Remediation of an aged PCP-contaminated soil by chemical oxidation under flow-through conditions[END_REF].

Both these saturated and unsaturated systems represent open-loop setup where oxidant solution is flushed out of the system and thus any loss of unreacted oxidant would decrease the oxidation efficiency. Therefore, a closed-loop system was developed which is based on recirculation of injected oxidant solution which improves the oxidation efficiency. Generally, J o u r n a l P r e -p r o o f treatment efficiency depends upon the oxidation system and follows this order: batch > closed-loop saturated column > saturated column > unsaturated column [START_REF] Rybnikova | Remediation of an aged PCP-contaminated soil by chemical oxidation under flow-through conditions[END_REF].

Most of the remediation studies (Table 2) relied on batch experiments due to the simplicity of setup, better control over reaction conditions, and preliminary nature of investigations.

Column systems are relatively complicated and were the subject of very few studies though they offer a better representation of the field conditions (Table 2). Closed-loop system has been tested by [START_REF] Rybnikova | Remediation of an aged PCP-contaminated soil by chemical oxidation under flow-through conditions[END_REF] to remediate a PCB-contaminated soil. A similar system has been applied by [START_REF] Talvenmäki | In situ bioremediation of Fenton's reaction-treated oil spill site, with a soil inoculum, slow release additives, and methyl-β-cyclodextrin[END_REF] to treat a residential lot impacted by spills from a leaking light heating oil tank.

Major challenges and implications of Fenton oxidation in historically contaminated soils

Contrary to the aqueous solution or spiked soils, oxidation efficiency in historically contaminated soils could have different implications. Fenton oxidation can alter the physicochemical properties and microbial population of contaminated soils. Moreover, the efficiency of Fenton oxidation can be limited by poor pollutant availability in aged soils.

Provided below is a brief description of these impacts and their potential solutions.

Poor pollutant availability and soil matrix effect

Poor pollutant availability is the major limitation in historically contaminated soils due to strong hydrophobicity and partitioning of pollutants into soil constituents especially organic matter (Figure 1) [START_REF] Biache | Effect of pre-heating on the chemical oxidation efficiency:Implications for the PAH availability measurement in contaminated soils[END_REF][START_REF] Usman | Treatment of hydrocarbon contamination under flow through conditions by using magnetite catalyzed chemical oxidation[END_REF]2016a). These contaminants are deeply diffused into the soil organic matter that renders them unavailable for the hydroxyl radicals [START_REF] Bogan | Inclusion of vegetable oils in Fenton's chemistry for remediation of PAH-contaminated soils[END_REF]. Therefore, pollutant availability and Fenton oxidation efficiency have been strongly linked in PAH-contaminated soils [START_REF] Biache | Effect of pre-heating on the chemical oxidation efficiency:Implications for the PAH availability measurement in contaminated soils[END_REF][START_REF] Lemaire | Chemical oxidation efficiency for aged, PAH-contaminated sites: An investigation of limiting factors[END_REF][START_REF] Usman | Treatment of hydrocarbon contamination under flow through conditions by using magnetite catalyzed chemical oxidation[END_REF]2016a). Indeed, magnetite-catalyzed heterogeneous Fenton oxidation was not able to degrade PAHs unless their availability was increased in two coking plant soils [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Due to this strong correlation, the efficiency of Fenton J o u r n a l P r e -p r o o f oxidation efficiency has been proposed as a valid indicator to assess pollutant availability [START_REF] Biache | Effect of pre-heating on the chemical oxidation efficiency:Implications for the PAH availability measurement in contaminated soils[END_REF]. This limitation in historically contaminated soils can be addressed by using different pretreatments including chemical, extraction, and thermal. Chemical pretreatments are intended to enhance pollutant availability by using various chemical reagents like co-solvent, cyclodextrin, surfactant, and vegetable oil (Usman et al., 2016b). For example, treating ethanol pre-treated soil by Fenton oxidation increased the PAH removal by 11-28% [START_REF] Lundstedt | Transformation of PAHs during ethanol-Fenton treatment of an aged gasworks' soil[END_REF] and 20% in another study [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. However, ethanol can also act as a radical scavenger [START_REF] Lundstedt | Transformation of PAHs during ethanol-Fenton treatment of an aged gasworks' soil[END_REF]. This scavenging effect was avoided by its evaporation in the soil before introducing the oxidant [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. In the same coking plant soil, the use of cyclodextrin as a pretreatment Similarly, pretreating the PAH contaminated soil with vegetable oil also improved the efficiency of subsequent Fenton oxidation up to 45% [START_REF] Bogan | Inclusion of vegetable oils in Fenton's chemistry for remediation of PAH-contaminated soils[END_REF]. It should also be noted that vegetable oil pretreatment rendered both heavy and light PAHs available for Fenton oxidation. On the other hand, only light PAHs were degraded by Fenton oxidation in the absence of this pretreatment.

It is well established that freshly-contaminated soils (artificially spiked) are easier to oxidize than historically contaminated soils as native pollutants in the latter case are strongly sequestered onto soil constituents for years [START_REF] Jorfi | Pyrene removal from contaminated soils by modified Fenton oxidation using iron nano particles[END_REF][START_REF] Lemaire | PAH oxidation in aged and spiked soils investigated by column experiments[END_REF][START_REF] Nam | Enhanced degradation of polycyclic aromatic hydrocarbons by biodegradation combined with a modified Fenton reaction[END_REF][START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Against 84% and 96% removal of pyrene and Benzo(a)pyrene, respectively, in a spiked soil, only 40% and 20% of these pollutants were degraded when the same Fenton treatment was applied in a soil from former manufactured gas plant [START_REF] Nam | Enhanced degradation of polycyclic aromatic hydrocarbons by biodegradation combined with a modified Fenton reaction[END_REF]. Considering this, our group separated the extractable organic matter (EOM) by solvent extraction from two ancient PAH-contaminated coking plant soils and spiked it on sand as fresh PAH contamination [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Application of Fenton-like oxidation in spiked sands removed more than 90% of PAHs whereas similar treatment was almost unable to degrade any PAHs when applied directly in soil. In the next step, instead of separating the EOM from the soil, it was allowed to evaporate in the same soil. This was done to improve the availability of PAHs as the fresh contamination in the same soils. This strategy removed up to 60% of PAHs in re-contaminated soils. This decrease in pollutant removal efficiency between spiked sand (90%) and real soil (60%) was attributed to non-productive oxidant consumption by insoluble organic matter and/or soil carbonates [START_REF] Lemaire | Chemical oxidation efficiency for aged, PAH-contaminated sites: An investigation of limiting factors[END_REF][START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. These interactions of reactive oxygen species (ROS) with different soil constituents are shown in Figure 5. In general, soil organic matter can affect the Fenton oxidation in different ways: (i) as a surfactant and redox agent (Xu et al., 2017b), (ii) as a metal chelator [START_REF] Sherwood | Modified Fenton oxidation of diesel fuel in arctic soils rich in organic matter and iron[END_REF], (iii) its competition with target pollutants for the oxidant [START_REF] Lemaire | Chemical oxidation efficiency for aged, PAH-contaminated sites: An investigation of limiting factors[END_REF][START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Therefore, J o u r n a l P r e -p r o o f higher natural oxidant demand should be expected in real contaminated soils as compared to spiked sand or aqueous solutions.

Similarly, the pollutant availability and the efficiency of subsequent Fenton oxidation can be enhanced by pre-heating the contaminated soils. Pre-heating two ancient coking plant soils at different temperatures improved the subsequent Fenton oxidation of PAHs by 19, 29, and 43 % in A soil and 31, 36, and 47 % in B soil pre-treated at 60, 100, and 150 °C, respectively (Usman et al., 2016a). In another study [START_REF] Biache | Low temperature oxidation of a coking plant soil organic matter and its major constituents: An experimental approach to simulate a long term evolution[END_REF], thermal pretreatment (100 °C for 1 week under inert atmosphere) was mainly found effective in soils from former coking and gas plants due to the long-term aging rendering PAHs unavailable. While the application of a similar pretreatment did not significantly enhance the Fenton oxidation of PAHs in soils sampled from an active wood treating facility [START_REF] Biache | Low temperature oxidation of a coking plant soil organic matter and its major constituents: An experimental approach to simulate a long term evolution[END_REF]. Mechanisms underlying an increased pollutant availability by thermal pretreatment have not been yet investigated due to limited research on this theme. Moreover, a better understanding of this process is required for its upscaling to the field with due consideration to the nature of the target soil. In any case, it is essential to thoroughly characterize the soil before applying Fenton oxidation as soil nature and pollutant interactions may necessitate extended contact for treatment or additional treatments.

Impacts of Fenton oxidation on soil quality and productivity

Fenton oxidation is highly efficient to decrease pollutant load in contaminated soils.

However, it could also have negative effects on soil quality, microbial community and its productivity (Figure 6). This section describes the effects of Fenton oxidation reported in historically contaminated soils.

Impacts of Fenton oxidation on soil physico-chemical properties

Generally, the application of Fenton-based treatments in contaminated soils is accompanied by a decrease in soil's pH and a temporary increase in its temperature. The intensity of this variation is usually dependent on the dose of the reagents, the nature of the contaminated soil, and the treatment time (Cheng et al., 2016b;[START_REF] Ferguson | A field trial of in situ chemical oxidation to remediate long-term diesel contaminated Antarctic soil[END_REF][START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF] J o u r n a l P r e -p r o o f [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF][START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF]. For example, the decrease in the soil pH after oxidation varied between about 1.3 [START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF] to more than 5.5 units [START_REF] Ferguson | A field trial of in situ chemical oxidation to remediate long-term diesel contaminated Antarctic soil[END_REF]. [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF] studied, the efficiency of Fenton oxidation for the treatment of an alkaline PAH contaminated soil sampled from a coking plant site. A slight decrease in pH from 9.7 to 8.3 was reported after Fenton oxidation (H 2 O 2 and Fe(II) at concentrations of 1.12 and 0.1 mol L -1 and UV irradiation). [START_REF] Laurent | Oxidation of a PAH polluted soil using modified Fenton reaction in unsaturated condition affects biological and physicochemical properties[END_REF] also noted a prominent decrease in pH (from 7.3 to 4.8) and cation exchange capacity (CEC) (2 units) during the treatment of an industrial PAH-contaminated soil particularly at higher oxidant dose (65 g kg -1 ). Similarly, for higher reagent concentrations (10 mol L -1 of H 2 O 2 and 10 mol L -1 of FeSO 4 ), [START_REF] Ferguson | A field trial of in situ chemical oxidation to remediate long-term diesel contaminated Antarctic soil[END_REF] reported a temporarily pH decrease of 5.5 units (from 9.3 to 3.8) at the soil surface (<5 cm soil depth) in a field contaminated with petroleum products near the old Casey station, East Antarctica. This pH variation was found to be much lower (0.4 units) at a higher sampling depth (10 to 15 cm). During this field assay, a temporary but significant increase in soil surface temperature from 8 °C to 35-60 °C was reported due to the exothermic nature of Fenton oxidation. Similar increase in temperature was also observed by [START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF] during the treatment of pesticide contaminated soil at laboratory scale by traditional Fenton process (20 mM H 2 O 2 , 4 M soluble Fe(II), pH 2.8).

Temperature increased from 25 °C to about 55 °C during the first 30 min of the reaction and started gradually decreasing after 70 min of reaction. This temporary increase in temperature is mainly dependent on the concentration of H 2 O 2 (Cheng et al., 2016b).

Indeed, the average soil temperatures of 40.3, 48.4, 59.5, 66. FeSO 4 solution and 0.4 mM of citric acid per g of soil) which decreased the soil pH by about 1.3 and 5.7 units, respectively. However, the drop in pH was very small (from 7.20 to 7.10 in one soil and from 8.35 to 8.10 in soil) when Fenton-like oxidation (using magnetite) was applied in two coking plant soils [START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. Interestingly, the use of steel slagcatalyzed Fenton-like oxidation slightly increased the soil pH from 5.4 to 6.2 due to the basic nature of steel slag (pH 12.3). It should be noted that in case of modified or Fenton-like oxidation, the decrease in pH is not as harsh as it could be in traditional Fenton oxidation where initial acidification (pH of 3) is required. Still, this small decrease in pH along with higher oxidant amount could significantly decrease the soil microbial count which, however, can be regenerated (as demonstrated in the section 6.2.4). J o u r n a l P r e -p r o o f

Effects of Fenton oxidation on soil organic matter

Fenton oxidation, being a non-selective process, generally degrades both the target organic pollutants as well as the non-target SOM (Sutton et al., 2014a). The SOM is an extremely important soil constituent due to its role in maintaining soil quality and health. In historically contaminated soils, the organic matter bearing persistent pollutants can be more condensed than natural organic matter and thus may be recalcitrant to degradation. Moreover, Fenton oxidation can decrease the SOM contents to different extents depending upon the soil type, the reagents concentrations, and the treatment duration (Cheng et al., 2016b;[START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF]Sutton et al., 2014a;2014b;Xu et al., 2017a). For example, a slight decrease in SOM contents from 4.7 to 4.3% was noted during Fenton-like oxidation of pesticide-contaminated soil (Cheng et al., 2016b) (Figure 7). [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF], reported a slight decrease of the soil's total organic content (TOC from 9.9% to 8.7%) during the remediation of a coking plant soil (H 2 O 2 1.12 M and Fe(II) 0.1 M). Similarly, [START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF] reported that Fenton's reagent (H 2 O 2 + soluble Fe(II) applied at neutral pH) had a negligible impact on TOC in two different soils from a coking plant and coal gas plant. However, the use of modified Fenton oxidation with citrate as an organic chelating agent significantly increased the TOC in both soils (from about 2.5 to 5% and from 0.3 to 1.5% in soils from coking and coal gas plants, respectively).

The degradation of SOM is also accompanied by an increase in dissolved organic carbon (DOC) due to the release of natural soil organics (Cheng et al., 2016b) (Figure 7). Soils intrinsically rich in organic matter exhibited an important SOM degradation as indicated by an increase in DOC from 69 mg kg -1 to 578 mg kg -1 (peat soil) and from 337 mg kg -1 to 3746 mg kg -1 (fill soil) by traditional Fenton oxidation (150 mg L -1 FeSO 4 , 5 % H 2 O 2, pH < 3) (Sutton et al., 2014b). This increase in DOC contents was less prominent (2871 mg kg -1 in peat soil) with modified Fenton oxidation (150 mg L -1 FeSO 4 , 5 % H 2 O 2 , 1.2 g L -1 citrate, pH 6.8). Similar findings were observed by Sutton et al. (2014a) during oxidation of a contaminated soil collected from a railroad station in Poland. They reported that DOC concentrations were about 8 and 16 times higher after modified Fenton (pH 6.5-7) and the J o u r n a l P r e -p r o o f traditional Fenton process (pH 3). An increase of 3.6 times in DOC was reported by Cheng et al. (2016b) using steel slag catalyzed Fenton-like process for the remediation of pesticides-polluted soil at natural conditions. The DOC concentrations were not significantly affected with sequential oxidant addition (three times over three days) (Sutton et al., 2014a).

In another study, DOC contents increased by 15.1 times in oil-well contaminated soil (Yan'an, China) after Fenton pre-oxidation (900 mM H 2 O 2 , 2.9 mM Fe(II)) (Xu et al., 2017a).

However, DOC concentration decreased from 7750 to 5940 mg kg -1 when the H 2 O 2 concentration was doubled (from 900 mM to 1800 mM) and to about 2600 mg kg -1 when the pre-oxidation phase was performed without Fe(II). It is worth mentioning that most of these findings concern the coupling studies which are intended to provide mild treatment conditions. However, the loss of SOM could be more prominent when harsh oxidation conditions are used. J o u r n a l P r e -p r o o f

Impacts on the release of nutrients and co-existing pollutants

There exists limited data on the effect of Fenton oxidation on the release of nutrients and coexisting pollutants in the target soils (Figure 6). For example, [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF] reported that the Fenton oxidation of a PAH-contaminated coking plant soil caused an important decrease in contents of total N (from 2.20 to 1.51 g kg -1 ), N-NO 3 (from 17.0 to 1.8 mg kg -1 ), the available P (from 56 to 32 mg kg -1 ). Moreover, this treatment also changed the aggregates structure due to SOM oxidation, and increased the surface area (from 16.8 to 21.5 m 2 g -1 ) and the water retention capacity (from 9.1% to 22.8%). Furthermore, the exchangeable Ca and Mg were increased from 720 to 1280 mmol kg -1 and from 9 to 14 mmol kg -1 , respectively. The N-NH 4 content was increased from 6.7 to 20.1 mg kg -1 . A similar trend was observed by Sutton et al. (2014a) after the treatment of a diesel-polluted soil by Fenton (H 2 O 2 /Fe(II) at pH 3) and modified Fenton oxidation (H 2 O 2 /Fe(II)/citrate at pH 6.5-7) oxidation. The N-NH 4 concentration in the polluted soil (0.5 mg kg -1 ) increased by factors of 172 and 14 after Fenton and modified Fenton treatment, respectively. Moreover, the initial contents of total nitrogen (3.25 mg kg -1 ) increased by about 62 and 13 times, respectively. Similarly, N-NH 4 content after Fenton pre-oxidation was enhanced from 15 mg kg -1 to 763 mg kg -1 with an increase in the H 2 O 2 concentration from 0 mM to 1800 mM highlighting the role of oxidant dose in nutrient release (Xu et al., 2017a).

On the other hand, Fenton treatment was also found to increase the release of metals in cocontaminated soils [START_REF] Villa | Environmental implications of soil remediation using the Fenton process[END_REF]. For example, Fenton oxidation of a pesticide contaminated soil increased the dissolution potential of 36%, 12%, 8.9%, 3.1%, and 0.9% for Zn, Cu, Ni, Mn, and Cr, respectively after 84 h of reaction. This high metal release was imputed to the SOM oxidation (leading to the release of metals, Figure 5) and acidic conditions that improve the solubility of immobilized metals. This increase in mobilization of pollutants has also been linked to the presence of H 2 O 2 and other reactive species including oxidants and reductants formed during oxidation [START_REF] Bennedsen | Mobilization of metals during treatment of contaminated soils by modified Fenton's reagent using different chelating agents[END_REF]. However, chelating agents like citrate, EDTA, and pyrophosphate did not seem to enhance metal mobilization [START_REF] Bennedsen | Mobilization of metals during treatment of contaminated soils by modified Fenton's reagent using different chelating agents[END_REF].

J o u r n a l P r e -p r o o f

Further studies are, however, required to better understand the fate of different pollutants in co-contaminated soils and the mechanisms underlying metal mobilization. Along with that, field application also calls for the investigations into the evolution of SOM, DOC, nutrients, and co-existing pollutants following Fenton oxidation. This would be highly useful to better predict the efficiency of subsequent bioremediation.

Effects of Fenton oxidation on soil microorganisms

The application of oxidant affects the microbial community due to the combined effect of i) direct and harsh interactions of oxidant species with microbes, ii) generated heat, and iii) the acidic conditions during the Fenton oxidation. Several studies have studied the effect of Fenton-based processes on microbial communities' growth, diversity, and regeneration [START_REF] Ferguson | A field trial of in situ chemical oxidation to remediate long-term diesel contaminated Antarctic soil[END_REF][START_REF] Polli | Impact of the Fenton-like treatment on the microbial community of a dieselcontaminated soil[END_REF]Sutton et al., 2014a). Commonly, a dramatic decrease of the microbial community is observed after the oxidation, followed by a regeneration period of varying length [START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF]Sutton et al., 2014a). For example, the application of Fenton oxidation (H 2 O 2 + Fe II without pH adjustment) decreased the bacterial, fungal, and PAH degrading bacteria population in a PAH-contaminated soil that rebounded after five weeks of incubation [START_REF] Laurent | Oxidation of a PAH polluted soil using modified Fenton reaction in unsaturated condition affects biological and physicochemical properties[END_REF].

The response and regeneration potential of microbes is usually dependent on the nature of tested soil, oxidation conditions, incubation duration, and the provision of favorable conditions for microbes like nutrients, air, etc. Depending on the experimental conditions (especially H 2 O 2 concentration and treatment time), the total recovery of the microbial community could take longer time (Palmroth et al., 2006a;Sutton et al., 2014a). Indeed, even after 16 weeks of incubation, the soil previously treated for 10 days with a relatively high H 2 O 2 amount (0.4 g per g of soil) had lower intact microbial cells (68%) than the untreated soil (95%) (Palmroth et al., 2006a). Likewise, [START_REF] Polli | Impact of the Fenton-like treatment on the microbial community of a dieselcontaminated soil[END_REF] investigated the impact of Fenton-like process at two H 2 O 2 doses (2% and 6%) and two application modes (single and sequential addition) for the treatment of a diesel-contaminated soil collected from a gasoline station. The laboratory microcosms showed the devastating impact of all treatments on bacterial density and diversity (6S copy number decreased from 6.54*10 8 g -1

J o u r n a l P r e -p r o o f

to about 1*10 5 ). But microbial community was partially recovered (16S copy number of 10 7 to 10 8 ) after 30 days of incubation of soil treated at lower oxidant dose (2%), while reaching a complete recovery in 03 months. Whereas for H 2 O 2 dose of 6%, there was no recovery after 30 days and bacterial abundance was still two orders of magnitude lower than untreated soil even after 3 months of incubation.

Similarly, the nature of treatment and soil also affect the response of microbes. For example, [START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF] reported that the microbial community began to grow after 7 and 10 days for the Fenton and modified Fenton treatments, respectively. Similarly, Sutton et al. (2014b) investigated the effect of both the soil nature (peat and fill) and oxidation method (Fenton and modified Fenton) on the bacterial activity for a period of 8 weeks. Both processes reduced the total bacterial 16S rRNA gene abundance by approximately one order of magnitude in both soils, confirming the moderate effect of the chemical oxidation on the indigenous soil microorganisms. Bacterial regeneration started after 1-4 weeks for the peat soil and 1-2 weeks for the fill soil. At the end of the incubation period (8 weeks), a quasi-full recovery of microbial biomass was reported for both soils. In contrast to the 16S rRNA gene measurements, for the two soils, no reduction of the monooxygenase gene (alkB) abundance was reported after chemical oxidation. Moreover, a rapid increase of the alkB gene contents was observed after 1 week of incubation, especially in the fill soil. The composition of microbial community was also found to be different depending upon the nature of treatment.

Results at the laboratory scale are generally different from the last field investigation. For example, [START_REF] Ferguson | A field trial of in situ chemical oxidation to remediate long-term diesel contaminated Antarctic soil[END_REF] in a field study (Casey station, East Antarctica), showed that the application of Fenton oxidation devastatingly reduced the contents of the total heterotrophs from 138*10 3 to below detectable levels within only 3 days. New microbial growth was detected after about 3 years. The authors explained this long-time recovery by the high sensitivity of the Antarctic ecosystems.

It is important to mention that the response of microbial community was much better after The impact of the Fenton-based technologies application on plants grown in real contaminated soils has been exclusively assessed at laboratory scale through petri dishes or pot assays (Figure 6). For example, [START_REF] Sirguey | Impact of Chemical Oxidation on Soil Quality[END_REF] reported that Fenton oxidation had a positive impact on the germination and growth of ryegrass (Lolium multiflorum, L.) in pot experiments. Indeed, the kinetic of seed emergence was higher in treated soil (former coking plant soil contaminated with PAHs) as maximum germination was achieved after 11 as compared to 18 days in non-treated soil respectively. Moreover, after a growth period of three weeks, compared to the blank assay, the Fenton treatment increased the average germination rate, the fresh and dry shoot biomass from 73%, 13.6 mg, and 3.0 mg to 90%, 119.0 mg, and 17.8 mg, respectively. Moreover, fresh root biomass and length have significantly increased from 1.1 mg and 0.8 mm to 17.7 mg and 4.2 mm, respectively. They imputed this positive effect to the decrease in the soil hydrophobicity and increase of the water availability. However, [START_REF] Laurent | Oxidation of a PAH polluted soil using modified Fenton reaction in unsaturated condition affects biological and physicochemical properties[END_REF] noted that plant growth in a treated coking plant soil (contaminated with PAHs) was negatively affected at a higher oxidant dose (by modified Fenton oxidation with H 2 O 2 at 65 g kg -1 ). Moreover, treated soil also displayed a significant delay in seed germination and a decrease in root and shoot biomass. The decreased values of soil pH and higher oxidant concentration were found as the major limiting factors to affect plant growth [START_REF] Laurent | Oxidation of a PAH polluted soil using modified Fenton reaction in unsaturated condition affects biological and physicochemical properties[END_REF]. Considering the contrasting results (performed at different oxidant doses) and very limited studies (only 02) on this topic, further studies are required to better evaluate the phytotoxicity of the reagents involved in Fenton oxidation.

Conclusions and outlooks

Fenton oxidation has been applied to decontaminate soils historically contaminated with various pollutants. However, the efficiency, reaction chemistry, and environmental impacts of Fenton oxidation is significantly altered depending on the nature of the Fenton process and catalyst type in particular. In comparison to the traditional Fenton oxidation, heterogeneous J o u r n a l P r e -p r o o f

processes have displayed better decontamination potential and less hazardous impacts on soil quality and native biota. The development of efficient and environment-friendly heterogeneous catalysts is a promising field. More efforts should be focused to improve catalyst stability and longevity in heterogeneous soil medium. Moreover, it is highly demanding to explore the fate of these catalysts in contaminated soils. Nanomaterials are gaining significant attention as heterogeneous catalysts recently but their injection into soil requires many investigations prior to their field application. Moreover, the toxicity of nanomaterials is itself a widely debated topic. Therefore, a comprehensive understanding of their interactions with soil biota is required. Owing to the substantial role of endogenous iron minerals, efforts are required to further improve their role. A thorough characterization of the soil before applying Fenton oxidation is essential due to the role of soil constituents and pollutant interaction as they may necessitate alterations in Fenton oxidation conditions (in oxidant doses, contact time or method of injection, etc.) or additional pretreatments (like availability enhancement of pollutants or iron minerals).

Similarly, the application of chemical oxidants leads to a dramatic decrease in soil microbial community which, however, can be regenerated if favorable conditions are provided after Fenton oxidation. However, extremely harsh oxidation conditions may completely destroy the soil structure making it very difficult to recover. Therefore, various strategies have been suggested to reduce the environmental impacts of oxidants like by reducing the oxidant dose and or contact time with the soil. In addition to this, biological stimulation by providing nutrients after oxidation, and bioaugmentation by adding bacteria can also facilitate soil recovery. These strategies have enabled the integration of Fenton oxidation with bioremediation that emerged as an environmental-friendly strategy. Maintaining a balance between the oxidation conditions, treatment cost, and environmental impacts would be crucial to design an environment-friendly and efficient treatment. Further studies are also required to assess the cost of this technology which is dictated by the site-specific geochemical and contaminant circumstances particularly for real field applications. To assess the cost of this treatment, the amount of pollution and the polluted soil volume must J o u r n a l P r e -p r o o f be taken into account. While developing the cost information, it should be acknowledged that costs reported under a set of conditions at one site are very difficult to extrapolate to the other sites. Field scale applications are severely limited for Fenton oxidation in contaminated soils. Therefore, a thorough understanding of Fenton oxidation is required under field conditions to identify the true worth of the applicability and worth of this remediation strategy.

Figure 1 :

 1 Figure 1: The conceptual model of a soil particle demonstrating different soil components that affect the remediation extent in historically contaminated soils. The image of soil colloid

modified

  Fenton oxidation followed by heterogeneous Fenton oxidation and traditional Fenton oxidation. However, photo-Fenton and electro-Fenton oxidation were mainly used to treat the soil washing extracts. These techniques which have been tested under different experimental conditions are described in the following sections.

Figure 2 :

 2 Figure 2: Fenton-based oxidation strategies tested for the remediation of historically contaminated soils. The size of each colored slice in pie chart is intended to represent the

Figure 3 :

 3 Figure 3: The removal of PCBs affected by the type and contents of different chelating agents (GLDA, EDDHA, EDDS, SC, SP) in the presence of (a) Fe(II) and (b) Fe(III). [H 2 O 2 ] o = 500 mM, [Fe] o = 50 mM, [PCB] o = 63.9 mg kg -1 , and reaction time = 24 h. (Figure reproduced with permission from Ref. (Ma et al., 2018))

  r n a l P r e -p r o o f

Figure 4 :

 4 Figure 4: Integrated remediation of historically contaminated soils by coupling of Fenton

J

  o u r n a l P r e -p r o o f 4.3. Coupling of Fenton oxidation with soil washing/soil flushing Soil washing is an ex-situ process that involves the removal of contaminants from soil predominantly by washing with different liquids (extracting agents). In this process, different additives like surfactants and chelating agents are introduced into the washing solution to solubilize the soil pollutants which have low solubility and higher soil adherence. Similarly, soil flushing is an in-situ process where soils are flushed with extracting agents while the mobilized pollutants are collected in extraction wells. However, in both these processes, soil contaminants are only separated in soil solution. Therefore, contaminated washing effluents should undergo a suitable treatment before their discharge into the environment. Fenton

  [START_REF] Ma | Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4′-trichlorodiphenyl from soil contaminated with capacitor oil[END_REF] compared the efficiency of non-ionic surfactants (Tween 80, Brij 58, and Brij 30) and cyclodextrin to enhance the washing efficiency of PCB (PCB28) in a soil contaminated with capacitor oil. The highest extraction efficiency (up to 61%) was observed with Brij 58 surfactant. However, the use of the surfactants and CD may contribute towards oxidant scavenging and thus can reduce the treatment efficiency of post-Fenton oxidation[START_REF] Ma | Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4′-trichlorodiphenyl from soil contaminated with capacitor oil[END_REF]. Instead of separately treating the soil solution,[START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF] spiked it on sand or the same soil sampled from a former coking plant site (as a pollutant availability enhancement strategy). It has been found that contrary to cyclodextrin, the scavenging effects can be avoided by using organic solvents (like chloroform or ethanol) as they can be evaporated after the extraction treatment[START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF]. However, these solvents can have adverse impacts on the soil quality, and therefore, the use of vegetable oil has been suggested as an environment-friendly alternative for the desorption of pollutants(Bogan et J o u r n a l P r e -p r o o fal., 2003;[START_REF] Gong | Removal of polycyclic aromatic hydrocarbons from manufactured gas plant-contaminated soils using sunflower oil: Laboratory column experiments[END_REF]. However, further studies are required to elaborate their role in historically contaminated soils.

Figure 5 :

 5 Figure 5: Interactions of reactive oxygen species (ROS) including with various soil constituents in historically contaminated soils. ROS predominantly include HO • along with other species like 1 O 2 , H 2 O 2 , O •- 2 . After applications it interacts with soil pollutants (productive consumption) and non-productive consumption by soil organic matter and/or soil minerals. Among soil minerals, Fe minerals also contribute in the generation of ROS by catalyzing the added oxidant. (Figure prepared using Biorender app)

  r n a l P r e -p r o o f of Fenton oxidation resulted in slightly lower PAH removal (15%) as compared to ethanol pretreatment due to its potential contribution to oxidant scavenging[START_REF] Usman | Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation[END_REF].

  2 and 71.1 °C were reported against H 2 O 2 concentrations of 5, 10, 15, 20 and 30%, respectively in soil from a pesticide manufacturing plant treated by Fenton-like oxidation catalyzed by steel slag. Though compared to the traditional Fenton oxidation, a modified Fenton treatment allows the oxidation to take place at neutral pH, the use of acidic chelating agents can lead to a prominent decrease in pH. For example, Liao et al. (2019) studied the remediation of a coking plant soil for three days using both Fenton oxidation (1 mM of H 2 O 2 and 0.2 mM of FeSO 4 solution per g of soil) and modified Fenton treatment (2 mM of H 2 O 2 , 0.4 mM of J o u r n a l P r e -p r o o f

Figure 6 :

 6 Figure 6: Impacts of Fenton oxidation on soil quality and plant growth. The parameters marked with an upward arrow indicate an increase while that with a downward arrow represent a decrease after Fenton oxidation. Contrasting findings have been reported for the starred parameters (☆). Microbial population decreases rapidly after Fenton oxidation but rebounds once the favorable conditions are restored. Temperature, however, represents a temporary increase that may be very harsh at strong oxidant doses. (Figure prepared using Biorender app)

Figure 7 :

 7 Figure 7: Effects of the Fenton-like treatment on the pH (a), soil organic matter -SOM (b), dissolved organic carbon -DOC (c) of a pesticide contaminated soil having 617.5 mg kg -1as the initial concentration of atrazine). converter slag dosage = 80 g kg -1 ; H 2 O 2 contents = 10%. This figure is reproduced with permission from Ref.(Cheng et al., 2016b).

FentonFigure 8 :

 8 Figure 8: The structure of microbial community in a coal gas plant soil (G) and coking plant soil (C) after treatment by persulfate oxidation (PS), permanganate oxidation (KMnO 4 ), modified Fenton oxidation, (M-Fenton), Fenton oxidation(Fenton) and control. This image is reproduced with permission from Ref.[START_REF] Liao | Effect of various chemical oxidation reagents on soil indigenous microbial diversity in remediation of soil contaminated by PAHs[END_REF].

Table 1 :

 1 A brief overview of the advantages and disadvantages associated with different Fenton-based oxidation strategies

	Treatment	Activation agent Advantages		Disadvantages/Challenges
	Traditional	Soluble Fe(II)	 Comparatively simple	 Requires acidic pH for
	Fenton		operation.		optimum efficiency.
	oxidation		 Easy	availability	of	 Maintaining this pH is costly
			reagents.		and problematic in soil.
	 Iron catalysts cannot be separated from the reaction medium or reused.  Inclusion of CAs increases the treatment cost.  CAs, particularly organics, compete with pollutants and scavenge the oxidants.  Iron catalysts cannot be separated from the reaction medium or reused.  Slow reaction rate, and low formation of reactive species  Stability and efficiency of iron minerals can decrease with the reaction time due to surface passivation  Injection of iron particles in contaminated soils is challenging and calls for further investigations. J o u r n a l P r e -p r o o f Modified Fenton oxidation Soluble Fe(II) with chelating agents (CAs)  Wide working pH range.  Easy availability of reagents Heterogeneous Fenton oxidation Solid iron minerals  Wide working pH range  Low iron leaching  Iron minerals can be separated from the reaction medium or reused.  Endogenous iron minerals can also catalyze the oxidation and thus reduce the treatment cost.  Dual role of iron
			minerals can facilitate
			the coupling of Fenton
			oxidation		with
			adsorption or chemical
			reduction.
	Photo-Fenton	UV-vis radiation	 Efficient cycling of Fe(II)	 High treatment cost due to
	process		and Fe(III)		the use of light
			 Low input of initial Fe(II)	 Photoreactor	must	be
			 High radical production	designed prior
			due	to	additional	 Has been applied only to
			reactions		treat the soil washing
						extract	(no	direct
						applications so far in the

Fenton oxidation strategy Catalyst system Pollutant type Soil origin Experim ental setup pH Purpose of study Ref.

	Traditiona oxidation washing oxidation	Soluble EDTA organic mical	PAHs extractable	China productio Coking to oil wells located in TPH to fuel	Batch manually	7.4 -	impact on oxidation availability To the compare evaluate the	(Liao et 2010a) ot et al., Frasca
	Thermal l and followed	Fe 3 O 4 Fe(II) matter) reactor set	PAHs organics	n, gas 02 soils: plant field, in Dagang Pond contamina storage	Batch slurry mixed	Natur 7.5	soil To evaluate remediatio the the leachabilit	(Biache al., 2015) et al.,
	Traditiona pretreatm modified by	Soluble alone or at 1000	Chloropyrifo	Insecticid work, Former Shijiazhua Oilfield, inlet, ted site tank of a	Batch slurry and	3 al	To evaluate the n chelating remediatio y of TPH-	(Zhu et et al., 2019) 2018)
	Pesticides l Fenton ent Fenton electro-	Fe(II) with citric mA of	s and -	e factory, Sweden coking ng and a Tianjing, Inuvik and thermoele	slurry allowed		compare the role of remediatio efficiency ability of n contamina	al., 2015)
	Traditiona oxidation Traditiona followed oxidation Fenton	Soluble Soluble acid power	DDT and bromo-2-PAHs	Former Jiangsu, Soils from plant, coal gas China Kugluktuk ctric	Batch Batch to stay	2.5-3 pH 3,	Oxidation the To study a pre-n potential under native soil efficiency ted soil	(Villa 2017) (Jonsso
	l Fenton l Fenton by oxidation	Fe(II) and Fe(II) supply	DDE chloropheno	pesticides China industrial former plant field, , power	slurry slurry	25 °C	of treatment the role of heating and different organic	and n et al.,
	oxidation Fenton-Modified	native Fe Soluble	l TPHs	warehous sites for gas plant, Nanjing, Canadian Two oil station,	Batch	7.5	pesticides efficiency soil treatment impact on conditions matter and To	Nogueir 2007) (Wang
	like Modified Fenton	Soluble Fe(II) with	Dichloromet	e, Mato wood active China Soils Arctic wells, near	Batch slurry	6.5	and of Fenton properties (100 °C) soil quality To added optimize	a, 2006) (Lu et et al.,
	oxidation Modified Fenton oxidation	Soluble Fe(III) citric acid	PAHs hane-	Grosso, preservati wood-02 soils: adjacent Yan'An, Mexico	Batch slurry	7.4	evaluation oxidation and PAH on To evaluate EDTA the	(Lemair al., 2015)
	Fenton oxidation Traditiona	Fe(II) Soluble	extractable TPH	Brazil on, coke treating Former to oil wells Diesel China City	slurry Batch	3 or	of and characteri subseque evaluate the To oxidant	e et al., 2010b) (Sutton
	oxidation followed l or Fenton-	Fe(II) Relied on	organics Diesel	productio facility, coking in Dagang contamina Gasoline	slurry Saturated	6.8 7.6	oxidation persulfate stics in nt the limiting efficiency evaluate dose To explore	2019) et al., (Polli et
	by modified Modified like	alone or Soluble native Fe	TPHs organics	n, gas France plant Oilfield, ted Two oil station	Batch slurry	7.5	parameter oxidation multiple chemical factors in of coupled the To study the effect	2014b) (Xu et al.,
	Modified bioremedi Fenton Fenton treatment	Soluble with citrate Fe(II) with with added	HCH	Former work, sites, Tianjing, railway wells, site	Batch slurry	8.05	s To soils oxidation chemical remediatio recovery the effect of of	(Usman al., 2018)
	Fenton Thermal ation oxidation oxidation (KH 2 PO 4	Fe(II) Fe 3 O 4 citric acid EDTA	PAHs	gravel pit Sweden 02 soils: Neuves-China site, Yan'An, contamina	and Batch	7.2 -	particularly compare To oxidation n strategy of of chemical	et al., (Usman 2016b)
	oxidation KH 2 PO 4 pretreatm Modified followed stabilized	Soluble Soluble	PAHs TPHs	of lindane Creosote Former Maisons Oil tank Węgliniec, China ted with	column Batch slurry Batch	3 8.3 7.5	native Fe the To test the evaluate To study microbial oxidation oxidation	2017) (Valderr et al., (Xu et
	Traditiona l Fenton Soil washing followed by photo-Fenton oxidation Fenton-like system Traditiona l Fenton J o u r n a l P r e -p r o o f Soluble Fe(II) DDT Former pesticides warehous e, Mato Grosso, Brazil Batch slurry 2.5-3 To evaluate treatment efficiency and impacts on metals and organic matter in soil soluble Fe(II) with solar irradiation DDT and DDE Former pesticides warehous e, Mato Grosso, Brazil Batch slurry 2.8 To test the feasibility of a coupled process nZVI DDT and DDE Former organochl orine pesticide manufact uring plant, Jiangsu, China Batch slurry 6.6 To test the efficiency of nZVI catalyzed chemical oxidation to degrade pesticides Soluble Fe(II) or HCH Former gravel pit Batch slurry 2 and 8 To evaluate manufact uring industry, Sierentz, France efficiency of different oxidation systems (Fenton, persulfate, permanga nate) Polycyclic Aromatic Hydrocarbons (PAHs) Fenton-like oxidation Relied only on soil indigenous Fe (16.4 g kg -1 ) PAHs Creosote oil contamina ted soil from a wood impregnat ion site Downflow soil columns 5 -6 To evaluate treatment efficiency and effect on microbial activity Fenton-like oxidation + biodegrad ation Relied only on soil indigenous Fe (16.4 g kg -1 ) PAHs Creosote oil contamina ted soil from a wood impregnat ion site Downflow soil columns 5 -6 To evaluate the coupling of Fenton oxidation with bioremedi ation Ethanol pretreatm Soluble Fe(II) PAHs Former gas work Batch slurry 2-3 To test the efficiency J o u r n a l P r e -p r o o f stabilized H 2 O 2 followed by bioremedi ation Fe(II) oil contamina ted soil samples from a wood preservin g site, Spain slurry coupling of Fenton oxidation with bioremedi ation Modified Fenton oxidation Soluble Fe(III) + EDTA PAHs Former manufact ured gas plant Batch slurry 7.9 To test the formation of surfactant s during chemical oxidation Modified Fenton oxidation Soluble Fe(II) PAHs Former coking plant site, Neuves-Maisons, France Unsaturat ed columns 7.3 To explore the effect of Fenton oxidation on soil properties Fenton-like oxidation Fe 3 O 4 or soluble Fe(II) PAHs 02 soils: Former coking plant sites, Neuves-Batch slurry 7.2 -8.3 To evaluate the catalytic ability of Fe 3 O 4 to J o u r n a l P r e -p r o o f ent followed by Fenton-like oxidation coking plant sites, Neuves-Maisons and Homécour t, France different temperatur es (60, 100 & 150 °C) of thermal pretreatme nt on subseque nt chemical oxidation Sono-Fenton-like oxidation Fe 3 O 4 + ultrasonics PAHs Oilfield, Ahvaz city, Iran Batch slurry 3.4 To evaluate the efficiency of sono-Fenton-like oxidation based on central composite design optimizatio n approach. Modified Hematite + Pyrene Oil Batch 7 To J o u r n a l P r e -p r o o f and Homécour t, France Fenton-like Fe 3 O 4 PAHs and PACs 02 soils: Former coking plant of Neuves-Maisons and former gas plant, Rennes, France Batch slurry 7.4 To evaluate the behavior of PAHs and PACs Oil hydrocarbons/Total petroleum hydrocarbons (TPHs) Modified Fenton oxidation Soluble Fe(II) TPHs, VOCs, SVOCs North Creek Railroad Station' soil contamina ted with diesel, New York, USA In-situ applicatio n in saturated and unsaturat ed condition s 6-7 To remediate an historic site Modified Fenton oxidation Soluble Fe(II) Diesel polluted soil, Old small-scale field trial 9.3 To evaluate the field J o u r n a l P r e -p r o o f Fenton oxidation followed by bioremedi ation Fe(II) with citric acid or oxalic acid or SOLV-X storage site, Amsterda m, Netherlan ds slurry the role of various chelating agents and coupling with bioremedi ation. Modified Fenton oxidation Soluble Fe(II) with citric acid TPHs Cable site, Arnhem, Netherlan ds Batch slurry 7.5 Comparin g the efficiency of Fenton oxidation with CaO and KMnO 4 . Modified Fenton oxidation followed by bioremedi ation Relied on native Fe TPHs Two oil contamina ted soils, Finland Batch slurry 6.2-7.7 To evaluate chemical oxidation in composte d soil with bioremedi ation Modified Fenton Fe(III) with nitrilotriace TPHs Field site containing Batch slurry 7.5 To evaluate J o u r n a l P r e -p r o o f by bioremedi ation Poland population during this integrated treatment Traditiona l or modified Fenton oxidation followed by bioremedi ation Soluble Fe(II) alone or with citrate TPH Diesel contamina ted railway site, Węgliniec, Poland Batch slurry 3 or 6.8 To evaluate the effect of organic carbon and nutrients mobilized during Fenton oxidation on following bioremedi ation Modified Fenton oxidation followed by biostimula tion Soluble Fe(II) with citric acid alkylbenzen e Historicall y polluted soil from the leakage of an undergrou nd storage Batch slurry Spiking the naturally contamina ted soil To evaluate the natural attenuatio conditions on the removal of TPHs and soil organic matter Fenton-like oxidation Fe 3 O 4 TPHs Petroche mical site, Mahshahr , Iran Batch slurry 3 To optimize the treatment efficiency Electro-kinetic Fenton oxidation Citric acid as complexin g agent of native Fe TPHs TPH polluted industrial site, Spain -5.7 To optimize the treatment efficiency and to study the impact on soil toxicity Modified Fenton oxidation Soluble Fe(II) with citric acid TPHs Oil well, Yan'an, China Batch 7.9 To investigate the effect of Fenton treatment on nutrient mobilizatio H 2 O 2 ) followed by natural attenuatio n diesel on soil native biota Fenton-like oxidation Native iron and added ZVI with EDTA TPHs Petroleum hydrocarb on contamina ted site, Algiers Batch 7.2 To assess the remediatio n potential of ZVI and native Fe-catalyzed process Modified Fenton oxidation followed by bioremedi ation Fe(II) with citric acid TPHs Xinjiang Oilfield, China Batch 7.6 To evaluate the efficiency of coupled treatment Traditiona l and Fenton oxidation followed by bioremedi Soluble Fe(II) with citric acid Lubricant Lubricant contamina Southern Taiwan Batch pH 3 and 7 To evaluate efficiency of coupled treatment J o u r n a l P r e -p r o o f modified alone or ted site in the	(Villa et al., 2008) (Villa et al., 2010) (Cao et al., 2013) (Usman et al., (Palmrot h et al., 2006a) (Palmrot h et al., 2006b) (Lundst edt et ama et al., 2009) (Gryzeni a et al., 2009) (Laurent et al., 2012) (Usman et al., 2012) 2016a) (Barzeg ar et al., 2017) (Jorfi et (Boulan gé et al., 2019) (Vitolins et al., 2003) (Fergus on et al., 2004) al., 2011b) (Xu et al., 2011a) (Cajal-Mariños a et al., 2012) (Gong, 2012) (Sutton et al., 2014a) (Martíne z-Pascual et al., 2015) (Mirzae e et al., 2017) (Popesc u et al., 2017) (Xu et al., 2017a) (Ouriach e et al., 2019) (Chunya ng et al., 2019) (Liu et al., 2019)
	and Fenton-ent Fenton oxidation ation followed oxidation after Fenton	sodium tic acid Fe 3 O 4 pyrophosp Soluble	Long chain	of lindane site, Maisons industrial Casey weathere tank manufact Sweden and zone, Iran Station, d Oil well in	slurry Batch	pH	the use of of ethanol promote evaluate efficiency the post-n capacity n and chemical pretreatme Fenton-the and application and subseque To	2014) al., al., 2006) 2017) (Xu et
	like by followed bioremedi pre-	hate Fe(II) with	crude oil	uring Homécour East petroleum Yan'an,	and field	7.2	oxidation nt on like efficiency impact on of Fenton microbial nt evaluate	al.,
	oxidation traditional by ation oxidation	citric acid		industry, t, France Antarctica oil-northwest	experime		to subseque oxidation of coupled soil quality oxidation. resilience remediatio the	2020)
	Fenton Modified bioremedi Modified Electrokin followed	Soluble Soluble Citric acid (Fe bound	PAHs TPHs TPHs and	Sierentz, 02 soils: Sydney contamina Railroad ern China	Unsaturat Batch -nt	7.4-8 8.6	remediate nt To system To To n potential efficiency	(Lemair (Ndjou'o (Seo et
	oxidation Fenton ation Fenton etic-Modified by	Fe(II) Fe(III) + or sodium Soluble to SOM)	PAHs TPHs	France Former Tar Ponds ted soil, adjacent Three soil	ed slurry Batch	8.4 7.3-	HCH treatment evaluate (bioremedi evaluate optimize To of coupled	e et al., u and al., (Xu et
	oxidation oxidation Fenton Fenton bioremedi	EDTA dodecyl Fe(II) with		coking site, Nova Tianjing, polluted samples	columns	7.9	contamina by Fenton PAH ation and the the evaluate treatment	2013) Cassidy, 2015) al.,
	process oxidation ation	sulfate in citric acid		plant Scotia, China site, obtained			ted soils oxidation oxidation Fenton surfactant treatment stepwise and its	2006) 2018)
	Fenton-Traditiona Modified with soil	Steel Soluble Native Fe anode or oxalic	Atrazine PAHs Diesel fuel	Site of Soils from sites, Canada Three Seoul, from Oil	Batch Batch Batch	5.4 pH 3, 3.9-	To assess Compariso under oxidation) production To efficiency Fenton impact on	(Cheng (Jonsso (Sherwo
	like l Fenton Modified Fenton washing	converter Fe(II) Soluble with added chambers acid or	PAHs	pesticide industrial Neuves-Petroche aged Republic well,	slurry slurry Batch slurry	70 3 4.2	the n of the dynamic To remove during evaluate and to oxidation microbial	et al., n et al., (Zhao et od and
	oxidation oxidation Fenton oxidation	slag Fe(II) + EDTA or SOLV-X		manufact sites for Maisons mical diesel fuel of Korea Yan'an,	slurry	°C, (natur	remediatio treatment conditions bound chemical the study the in multiple biodiversit	2016b) 2006) al., Cassidy,
	oxidation	citric acid without it		uring wood and plant, contamina China		al)	n efficiency and to see PAH oxidation oxidation impact on steps y	2019) 2014)
	Modified Modified Explosives	or oxalic Soluble (relying on H 2 O 2 alone TPHs Dichloromet	plant, preservati Homécour Nanjing, Soils ted arctic Site	Batch Batch	7 8.2	efficiency of Fenton the role of residues in To efficiency soil toxicity To	(Lu et (Romer
	acid Fe(III) + soil Electroche TPH hane-Traditiona Soluble Fenton Soil Fenton hexahydro-	Xiangtan, on, coke t, France China adjacent soils Urban adjacent Drainage	slurry -slurry, Batch	8.4 45	and and ozone PAH soil evaluate and to To decrease To test	al., (Huguen o-(Bier et

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Acknowledgments

This publication was jointly supported by Qatar University and Sultan Qaboos University, Oman, under the international research collaboration grant (IRCC-2021-014). The findings achieved herein are solely the responsibility of the authors.

Declaration of Competing Interest

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Graphical abstract

Highlights

 1 st review evaluating the Fenton oxidation exclusively in real contaminated soils.

 These soils with complex pollution pose particular challenges to oxidation.

Fenton Oxidation for Soil Remediation  Their implications and potential solutions in soil-plant system are evaluated.

 Future research needs are highlighted to facilitate upscaling at field scale.

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