Daqing Jia
email: daqing.jia@uca.fr

Marcello Brigante

Changbo Zhang

Gilles Mailhot
email: gilles.mailhot@uca.fr

Impact of Mn 2+ in

Keywords: Manganese oxides, ligands, manganese ions, complexation, water treatment

come   L'archive ouverte pluridisciplinaire

Introduction

Manganese oxides are one common transition metal-based minerals, which possess various oxidation states [START_REF] Wang | Anaerobic microbial manganese oxidation and reduction: A critical review[END_REF]. The application of manganese oxides in dealing with different organic pollutants in waters has become one popular water treatment technology [START_REF] Tu | The degradation of printing and dyeing wastewater by manganese-based catalysts[END_REF]. High-valent manganese oxides such as Mn IV O 2 , Mn VII O 4 -can directly oxidize most organic contaminants through the electron exchange processes [START_REF] Chiam | Highly efficient oxidative degradation of organic dyes by manganese dioxide nanoflowers[END_REF][START_REF] Huang | Effect of MnO 2 Phase Structure on the Oxidative Reactivity toward Bisphenol A Degradation[END_REF]. However, the low-valent manganese oxides including , Mn III OOH, Mn II O exhibit weak ability to oxidize organic contaminants, although Mn III species were reported to have higher reduction potential ( = 1.54 V vs Normal Hydrogen Electrode (NHE)) than MnO 2 ( = 1.24 V vs NHE) [START_REF] Jia | Caffeine degradation using peroxydisulfate and peroxymonosulfate in the presence of Mn2O3. Efficiency, reactive species formation and application in sewage treatment plant water[END_REF][START_REF] Yamaguchi | The Redox Chemistry of Manganese(III) and -(IV) Complexes[END_REF][START_REF] Watts | Oxidative and Reductive Pathways in Manganese-atalyzed e to 's Reactio s[END_REF]. To improve the reactivities of all-valence manganese oxides, various radical precursors (e.g., hydrogen peroxide (H 2 O 2 ), persulfate, and bisulfite) have been employed to generate different oxidative radicals and high-valence manganese species (e.g., Mn(IV), Mn(V)). For instance, Huang et al. reported the efficient degradation of bisphenol A (BPA) by MnO 2 -activated peroxymonosulfate (PMS) through the generation of sulfate radicals ( ) [START_REF] Huang | Effects of MnO2 of different structures on activation of peroxymonosulfate for bisphenol A degradation under acidic conditions[END_REF]. Jia et al.

elucidated the positive role of generated Mn(IV) species in the manganite/PMS system for the degradation of estrogenic compounds [START_REF] Jia | Efficient removal of estrogenic compounds in water by MnIII-activated peroxymonosulfate: Mechanisms and application in sewage treatment plant water[END_REF].

The use of ligands in the transition metal-based advanced oxidation process (AOPs) to improve the degradation efficiency of organic pollutants has been widely reported [START_REF] Domingues | Sulfate radical based advanced oxidation processes for agro-industrial effluents treatment: A comparative review with e to 's pero idatio , ci[END_REF].

Ligands can complex transition metal ions to form stable metal ion-ligand complexes, avoiding the precipitation of metal ions and broadening the applicable pH range of the system [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]. One classical representative of the ligands-assisted AOPs technology is the chelatemodified Fenton/Fenton-like reactions. For example, Jia et al. demonstrated the improved degradation of trichloroethylene in groundwater by Fe 3 O 4 -activated H 2 O 2 with the addition of different ligands (e.g., ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA)) [START_REF] Jia | TCE degradation in groundwater by chelators-assisted Fenton-like reaction of magnetite: Sand columns demonstration[END_REF]. The application of ligands in the homogeneous and heterogeneous manganese-based AOPs (Mn-AOPs) for removing different organic pollutants has also been reported. For instance, Gao et al. reported the efficient degradation of p-nitrophenol using Mn(II) activated PMS in the presence of NTA and EDTA [START_REF] Gao | Enhanced peroxymonosulfate activation via complexed Mn(II): A novel non-radical oxidation mechanism involving manganese intermediates[END_REF]. In addition, Gao et al. found that the oxidation rate of triclosan by permanganate was accelerated with the addition of several ligands, including pyrophosphate (PP), NTA, and humic acid (HA) [START_REF] Gao | Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced Transformation of Triclosan by Mn(VII) in the Presence of Ligands?[END_REF].

The principal goal of employing ligands in the Mn-AOPs is to obtain more stable Mn(III)ligand (Mn(III)-L) complexes. This can be explained by two reasons. Firstly, ligandsstabilized Mn(III) species can directly oxidize organic pollutants, enhancing the degradation efficiency of contaminants [START_REF] Qin | Enhanced transformation of phenolic compounds by manganese(IV) oxide, manganese(II) and permanganate in the presence of ligands: The determination and role of Mn(III)[END_REF]. Secondly, ligands-stabilized Mn(III) species can provide more opportunities to form higher valent Mn such as Mn(IV) and Mn(V), improving the oxidation efficiency of pollutants and promoting the cycling of Mn species [START_REF] Gao | Enhanced peroxymonosulfate activation via complexed Mn(II): A novel non-radical oxidation mechanism involving manganese intermediates[END_REF]. However, the generation of Mn(III)-L complexes strongly depends on the types of participating complexing agents and manganese oxides. It is reported that inorganic ligands such as pyrophosphate can react with MnO 2 and manganite (MnOOH) to generate soluble Mn(III)-PP species through the ligands-assisted dissolution processes [START_REF] Liu | Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)[END_REF][START_REF] Mcardell | Reaction of EDTA and Related Aminocarboxylate Chelating Agents with CoIIIOOH (Heterogenite) and MnIIIOOH (Manganite)[END_REF]. However, the amino carboxylic ligands such as NTA and EDTA only can form Mn(II)-L species with manganite [START_REF] Mcardell | Reaction of EDTA and Related Aminocarboxylate Chelating Agents with CoIIIOOH (Heterogenite) and MnIIIOOH (Manganite)[END_REF]. Therefore, the application of appropriate ligands provides a novel and effective approach to enhance the reactivity of manganese oxides.

Dissolved divalent manganese ions (Mn 2+ ) are ubiquitous in the natural aquatic environment [START_REF] Post | Manganese oxide minerals: Crystal structures and economic and environmental significance[END_REF]. Mn 2+ , as the essential Mn species, can be oxidized to high valent manganese oxides such as Mn 2 O 3 , MnO 2 , and [MnO 4 ] -. Similarly, in the manganese oxides-participated redox reactions, Mn 2+ will be produced as the basic form of Mn species. Therefore, Mn 2+ widely exists in different waters such as the natural waters and wastewaters. The influence of Mn 2+ on the reactivity of manganese dioxides has been investigated. The impact of Mn 2+ can be described from several aspects including transforming the crystal structure of MnO that the kinetic constant of m-aminophenol degradation was decreased from 1.04 10 -3 M -1 min -1 to 0.84 10 -3 M -1 min -1 with the addition of Mn 2+ from 0 to 7 mM [START_REF] Huang | Transformation of m-ami ophe ol by bir essite (δ-MnO2) mediated oxidative processes: Reaction kinetics, pathways and toxicity assessment[END_REF]. Therefore, the treatment of organic pollutants by MnO 2 in the presence of metal ions such as Mn 2+ is a challenge. Moreover, extensive research has been conducted on the reactions between Mn 2+

and MnO 2 , ligands and MnO 2 , as well as organic compounds and MnO 2 . However, the efficiency and underlying mechanisms of organic pollutants degradation by the MnO 2 /ligands/Mn 2+ system have been scarcely reported in the literature. Sun et al. reported the rapid BPA degradation by the soluble Mn(III) species at circumneutral pH with the formation of fourteen BPA degradation products [START_REF] Sun | Mn(III)-mediated bisphenol a degradation: Mechanisms and products[END_REF]. Qin et al. demonstrated the removal of 2,4-dicholophenol (2,4-DCP) using the Mn(VII)/PP/Mn 2+ system [START_REF] Qin | Enhanced transformation of phenolic compounds by manganese(IV) oxide, manganese(II) and permanganate in the presence of ligands: The determination and role of Mn(III)[END_REF]. The results showed that the addition of Mn 2+ improved the degradation of 2,4-DCP by the PP-complexed Mn(VII). However, the previous studies primarily emphasized the significance of Mn(III) species in the removal of contaminants. However, the role of Mn 2+ in the context of the ligand-assisted MnO 2 system is rarely studied and and if it is without enough details to understand the exact roles of Mn 2+ .

In the current investigation, Bisphenol A was employed as a representative compound owing to its substantial reactivity with acid birnessite (MnO 2 ) and the existence of a welldocumented redox mechanism that underpins the reaction [START_REF] Zong | Synergistic multiple active species drive fast estro e o idatio by δ-MnO2 in the existence of methanol[END_REF]. The effect of PP and Mn 2+ concentrations on the reactivity of MnO 2 was firstly evaluated. Two suitable concentrations were selected for the following experiments. The main reactive species and the role of Mn 2+ in the MnO 2 /PP/Mn 2+ system was emphatically investigated. Furthermore, various ligands, namely NTA, EDTA, and citrate, were employed to examine the reactivity of MnO 2 in the presence of Mn 2+ . The impact of critical parameters, such as solution pH and the presence of inorganic anions, were also assessed. Subsequently, the efficiency of the proposed system in degrading BPA in actual sewage treatment plant outlet water was evaluated. The findings of this study accentuate the advantageous role of Mn 2+ in the MnO 2 /ligand system and provide a valuable reference for the treatment of organic contaminants in ligands-assisted manganese oxides system.

Synthesis and characterization of acid birnessite

Acid birnessite (MnO 2 ) was synthesized using the method proposed by McKenzie [START_REF] Mckenzie | The synthesis of birnessite, cryptomelane, and some other oxides and hydroxides of manganese[END_REF]. The synthesis process involved adding 166 mL of concentrated hydrochloric acid (HCl) dropwise to 2.5 L of KMnO 4 (0.4 M) while maintaining the temperature of the suspension at 90°C.

After 10 minutes of reaction at 90°C, the slurry was stirred vigorously at room temperature for 15 hours. The resulting precipitate was collected through centrifugation and washed with ultrapure water u til the co ductivity reached 0.6 μ cm -1 . The acid birnessite was stored in polypropylene containers in the form of a suspension at 4 °C for further use. The acid birnessite was characterized using X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET), Scanning Electron Microscope (SEM), and High-Resolution Transmission Electron Microscope (HRTEM). Further details regarding the characterization conditions and results can be found in the Supplementary Materials (SM, Fig. S8).

Kinetic experiments and analysis

The chemical reactions were performed in a borosilicate brown bottle in which 50 mL of solution was kept at room temperature (298 ± 2 K) under stirring (350 rpm) to ensure homogeneity. 25 μ BPA was added into the 50 mL reaction solution containing a certain concentration of PP and Mn 2+ , then the solution pH was adjusted to the desired value using NaOH (0.1 M) and HClO 4 (0.1 M). At last, manganese dioxide (MnO 2 , 0 μ ) was added to start the reaction. 1 mL of sample was withdrawn at the predetermined interval times and was immediately mixed with 20 µL L-ascorbic acid (0.5 M) to quench the reaction. The samples were filtered by a 0. 0 μm H-PTFE filter and then analyzed by HPLC.

BPA concentration was determined using a high-performance liquid chromatography system (HPLC) equipped with a diode array detector (DAD, waters 2998). Chromatographic separation (Waters 2695) was performed using a NUCLEODUR 100-3 C18 column (150 4.6 mm, 5 mm of particle size, MACHEREY-NAGEL). The detection wavelength of BPA was set at 225 nm (one of the absorption maxima of BPA), and the column temperature was kept at 30 . Acetonitrile (CARLO ERBA Reagents) and water (with 0.1% acetic acid) were mixed as the mobile phases with a volume ratio of 60/40, and the flow rate was set at 1.0 mL min -1 .

The pseudo-first-order rate constant of BPA ( ) was calculated based on the slope of ln(C t /C 0 ) vs reaction time as following l t 0 t where C 0 and C t are the initial and time t concentration of BPA in the solution.

Mn(III)-PP solution was prepared following the procedures of Qian et al. [START_REF] Qian | Geochemical Stability of Dissolved Mn(III) in the Presence of Pyrophosphate as a Model Ligand: Complexation and Disproportionation[END_REF]. Briefly, a certain amount of potassium pyrophosphate was first dissolved in ultrapure water to achieve the concentration of 5 mM, and the solution pH was adjusted to 6.0. A suitable amount of Mn(III) acetate dehydrate was then added to the prepared PP solution under vigorous stirring to form 1 mM of Mn(III)-PP complex. According to the need of study, the molar ratio of PP and Mn(III) always keeps at 5:1 to maintain the state of complexation. The final pH of the mixture solution was around 5.0. Before using, Mn(III)-PP solution was filtered through a 0. μm H-PTFE filter to remove any potential particles. Mn(III)-PP complexes was measured on the UV-visible spectrophotometer (Varian Cary 3) at 480 nm and quantified using the calibration curve (Fig. S1).

The complexe of Mn(III)-EDTA was monitored using a UV-visible spectrophotometer (Varian

Cary 3) at wavelengths of 488 nm to track its change over the course of the reaction [START_REF] Klewicki | Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate[END_REF].

Sewage treatment plant (STP) outlet water was collected from one wastewater treatment plant amed " rivières" (on June 2022, Clermont-Ferrand, France) and used after the filtration by 0.45 µm PTFE filter. The main physicochemical characteristics of STP outlet water are reported in Table S1.

Results and discussion

Reactivity of MnO 2 with PP in the presence of Mn 2+

The adsoprtion of BPA on MnO 2 was verified and as mentioned in a previous article it remains negligible [START_REF] Jia | Impacts of environmental levels of hydrogen peroxide and oxyanions on the redox activity of MnO2 particles[END_REF]. The degradation of BPA by PP complexed manganese dioxide (MnO 2 ) in the presence of Mn 2+ was investigated. Fig. 1 shows that ~ 66% of BPA was removed by MnO 2 alone in 180 min, which can be explained by the high oxidation potential of MnO 2 [START_REF] Yamaguchi | The Redox Chemistry of Manganese(III) and -(IV) Complexes[END_REF]. The addition of PP obviously accelerated the degradation of BPA by MnO 2 (100% BPA removal in 120 min). This might be attributed to the generation of oxidative Mn(III)-PP complexes [START_REF] Liu | Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)[END_REF]. More details about the degradation processes of BPA by the MnO 2 /PP system will be discussed in section 3.3 & 3.4. The relative experimental error lies at 5% for BPA.

Moreover, the effect of Mn 2+ on the reactivity of MnO 2 in the presence of PP was investigated. It can be seen that an improved BPA degradation by MnO 2 /PP system was achieved by adding 10 µM of Mn 2+ (100% BPA degradation in 90 min). As a comparison, in the absence of PP, the degradation of BPA by MnO 2 was slightly decreased with the addition of Mn 2+ due to the competition between Mn 2+ and BPA for the reactive sites of MnO 2 .

Moreover, the blank experiment showed that the reaction of Mn 2+ and PP achieved no BPA degradation. Therefore, the presence of PP reversed the role of Mn 2+ on the reactivity of MnO 2 , promoting the degradation of BPA instead of inhibiting the removal of BPA.

Effects of PP and Mn 2+ concentrations

The effects of PP and Mn 2+ concentrations on the degradation of BPA by MnO 2 were investigated. As shown in Fig. 2(a), the kinetic constants of BPA degradation ( ) were increased from 0.0051 min -1 to 0.0335 min -1 with the increase of PP concentrations from 0 to 600 µM. The results might be explained by the improved formation of Mn(III)-PP complexes with the increase of PP dosages. However, with the continuously increasing PP concentration to 1200 µM, the did not change much. This conclusion is consistent with the findings of Zhong et al. that increasing PP concentration from 100 to 400 µM, the kinetic constants of pentachlorophenol degradation by the -/Mn(III)-PP system kept stable since the excess PP has no effect on the reactivity [START_REF] Zhong | Mn(III)-ligand complexes as a catalyst in ligand-assisted oxidation of substituted phenols by permanganate in aqueous solution[END_REF]. The effects of Mn 2+ concentrations on the degradation of BPA were studied as well. Fig. 2(b) shows that with the addition of Mn 2+ from 2 to 20 µM, the was increased from 0.0246 to 0.0398 min -1 . The improved removal of BPA can be attributed to the formation of more Mn(III)-PP complexes with the increase of Mn 2+ concentrations. The at 0 M of Mn 2+ was excluded from this comparison due to the fact that the degradation of BPA by the MnO 2 /PP system in the absence of Mn 2+ does not obey the pseudo-first-order rate (Fig. 1). In order to minimize the environmental toxicity associated with higher Mn 2+ concentrations, a concentration of 10 µM was deemed appropriate for subsequent experiments, even though larger quantities of Mn 2+ have been shown to enhance BPA degradation. In addition, to ensure all the produced Mn(III) species can be complexed by PP, slight excess of PP concentration (1.2 mM) will be employed in the following steps.

The primary reactive species

Mn(III)-ligand complexes are considered as the main reactive species for organic pollutants degradation in the ligands-assisted manganese oxide systems [START_REF] Qin | Enhanced transformation of phenolic compounds by manganese(IV) oxide, manganese(II) and permanganate in the presence of ligands: The determination and role of Mn(III)[END_REF]. Therefore, in the present study, the formation of Mn(III)-PP complexes in different processes were followed using the UV-visible spectrophotometer. Ten times higher concentrations of all participating chemical compounds were used to measure the formation of Mn(III)-PP complexes due to the low absorption intensity of Mn(III)-PP complexes at 480 nm (ε 65 M -1 cm -1 ) [START_REF] Webb | Evidence for the presence of Mn(III) intermediates in the bacterial oxidation of Mn(II)[END_REF]. Moreover, the blank experiments were conducted to exclude the possible influence of BPA degradation products on the measurement of Mn(III)-PP complexes (Fig. S2). The relative experimental error lies at 5% for Mn(III)-PP complex.

Fig. 3 shows that no obvious formation of Mn(III)-PP complex was observed in the system of MnO 2 (2.3 mM) and PP (12 mM) in 3 h. It might be attributed to the low concentration of Mn(III)-PP complex formed in reaction and the low absorbance intensity of Mn(III)-PP complex at 480 nm. To verify whether Mn(III)-PP complexes were really generated in the reaction of MnO 2 and PP, the absorbance signals of complex at 258 nm were checked (258 and 480 nm are two absorbance peaks of Mn(III)-PP complex on UV-Vis spectrum). As shown in Fig. S3, the absorbance intensity of Mn(III)-PP complex at 258 nm slightly increased with the reaction time, indicating the formation of Mn(III)-PP complexes. However, the increased scale of Mn(III)-PP complexes absorbance is small (∼0.01 in 3 h). This can be explained by the low reactivity of MnO 2 with PP for the generation of Mn(III)-PP complexes.

Similar results were reported by Liu et al.. They found that -MnO 2 (0.01 g L -1 ) takes three days to form 15.6 M of Mn(III)-PP complexes with 5 mM PP [START_REF] Liu | Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)[END_REF]. The low performance of MnO 2 for PP complexation at low solution pH can be attributed to the strong electrostatic repulsion between them. The point of zero charges (PZC) of MnO 2 is 2.3-2.9, thus the surface of MnO 2 is filled with negative charges at solution pH 5.0 [START_REF] Li | Alteration of birnessite reactivity in dynamic anoxic/oxic environments[END_REF]. The pKa values of PP are 0.85, 1.96, 6.60, and 9.41, resulting in a negatively charged PP (H -) at pH 5.0 [START_REF] Yadav | Induced fit activity-based sensing: a mecha istic study of pyrophosphate detectio with a "fle ible" e-salen complex[END_REF].

Therefore, the contact between MnO 2 and PP at pH 5.0 is weak, leading to the slow generation of the Mn(III)-PP complexes.

In the presence of Mn 2+ (100 M), the generation of Mn(III)-PP complex by the MnO 2 /PP system was much more rapid (∼125 M Mn(III)-PP complexes were formed in 30 min). The results clearly show the positive role of Mn 2+ in catalyzing MnO 2 in the presence of PP. As discussed above, in the acid environment, the reaction between MnO 2 and PP will be suppressed due to the occurrence of electrostatic repulsion. However, with the addition of positively charged Mn 2+ , the negative charges on the surface of MnO 2 (especially the vacancies sites) will be neutralized. Therefore, the presence of Mn 2+ can significantly improve the adsorption of PP by MnO 2, leading to the accelerated generation of Mn(III)-PP complexes.

However, it is worth noting that no more Mn(III)-PP complex was generated after 30 min reaction in the system of MnO 2 /PP/Mn 2+ . This result can be attributed to the missing of reductants (such as BPA) which are necessary for the cyclic generation of Mn(III) species.

In comparison, in the presence of BPA, the amount of Mn(III)-PP complexes generated in the MnO 2 /PP system was increased (∼800 M in 3 h). The result can be explained by the fact that BPA as a reductant was oxidized by high valent Mn species (e.g., Mn(IV) and Mn(III)), leading to the generation of byproducts (BPA ox ) and Mn 2+ . The generated Mn 2+ can participate in the reaction of MnO 2 and PP accelerating the formation of Mn(III)-PP complexes. Fig. S4 shows the removal efficiency of BPA was improved with the increase of the concentrations of Mn(III)-PP complexes. However, it is worth noting that in the first 30 min, the formation rate and amount of Mn(III)-PP complexes in the MnO 2 /PP/BPA processes are very low (∼36.7 M) (Fig. 3), indicating the existence of an induction period of BPA degradation by the MnO 2 /PP system. The result is consistent with the degradation tendency of BPA in the MnO 2 /PP system (Fig. 1).

Moreover, it is easy to see that the amount of Mn(III)-PP complexes generated in the MnO 2 /PP/Mn 2+ system in the first 30 min is higher than that in the MnO 2 /PP/BPA system (Fig. 3). The results indicate that Mn 2+ is better than BPA at activating MnO 2 even though both Mn 2+ and BPA can react with MnO 2 to generate Mn(III) species. This conclusion was confirmed by the results that a higher generation rate of Mn(III)-PP complexes was achieved in the MnO 2 /PP/BPA process with the addition of Mn 2+ (∼250 M Mn(III)-PP complexes generated in 30 min) (Fig. 3). Additionally, the cycle between Mn species (e.g. Mn 2+ , Mn(III),

and Mn(IV)) is favored due to the presence of BPA in the MnO 2 /PP/Mn 2+ system. The amount of Mn(III)-PP complexes formed in the MnO 2 /PP/Mn 2+ /BPA system (∼1000 M in 3 h) is much higher than that in the MnO 2 /PP/Mn 2+ process (125 µM). In summary, Mn(III)-PP complexes are the main reactive species for the efficient degradation of BPA by the MnO 2 /PP/Mn 2+ system. Moreover, Mn 2+ plays an important role in BPA degradation by PPassisted MnO 2 .

The role of Mn 2+

In previous research, greater emphasis has been placed on the role of Mn(III) in the reactivity of MnO 2 , while the contribution of Mn 2+ has been overlooked. In the present study, the multiple roles of Mn 2+ in BPA degradation by the PP-assisted MnO 2 system were investigated in detail. Firstly, positively charged Mn 2+ acts as a role of bridge to increase the connection between MnO 2 and PP. The results can be explained by the naturally electrostatic adsorption.

This conclusion is supported by the study of Wang et al. that reported the adsorption of fulvic acid by MnO 2 was enhanced with the addition of divalent cations [START_REF] Wang | Effects of metal cations on coupled birnessite structural transformation and natural organic matter adsorption and oxidation[END_REF]. However, it is worth noting that the formation of MnO 2 -metal cations-PP bridge does not equally imply the degradation of organic pollutants can be improved. For example, Fig. 4 shows that adding Na + , Ca 2+ , and Cu 2+ inhibited the removal of BPA by PP-assisted MnO 2 . It can be explained by the fact that the presence of Na + , Ca 2+ , and Cu 2+ blocked the direct association of PP and Mn 2+ with MnO 2 leading to the reduced generation of Mn(III)-PP complexes. Therefore, constructing an effective bridge is the key step for improving the degradation of organic pollutants in the MnO 2 /PP system. Comparing to Na + , Ca 2+ , and Cu 2+ , Mn 2+ is more susceptible to be oxidized by MnO 2 to form higher valent Mn ions due to the high redox potential of MnO 2 ( = 1.24 V vs NHE) [START_REF] Yamaguchi | The Redox Chemistry of Manganese(III) and -(IV) Complexes[END_REF]. Thus, the reductive activity of Mn 2+ could be the reason that Mn 2+ are able to improve the degradation of BPA in the MnO 2 /PP system while Na + , Ca 2+ , and Cu 2+ cannot. To verify this hypothesis, ferrous ion ( e e 0 = 0.771 V vs NHE) as a substitute of Mn 2+ was added in the reaction and the removal efficiency of BPA by the MnO 2 /PP system was monitored [START_REF] Zhou | Critical review of reductant-enhanced peroxide activation processes: Trade-off between accelerated Fe3+/Fe2+ cycle and quenching reactions[END_REF]. As shown in Fig. 4, the addition of Fe 2+

was obviously improved the degradation rate of BPA in the system of MnO 2 /PP. Therefore, the successful bridge should be established on the reductive metal ions which can be oxidized by MnO 2 . In addition, Mn 2+ also exhibited the catalytic effect on the reactivity of MnO 2 in the presence of PP. This conclusion can be verified by several phenomenon. For instance, the induction period existed in the degradation of BPA by MnO 2 /PP was disappeared in the presence of Mn 2+ (Fig. 4). Moreover, the amounts and the generation rates of Mn(III)-PP complexes were obviously improved in the MnO 2 /PP system due to the addition of Mn 2+ (Fig. 3). In summary, Mn 2+ ion presented multiple roles, including bridging, reductive, and catalytic roles, which significantly enhance the degradation of BPA by MnO 2 in the presence of PP.

Effect of ligand types

In order to study the effect of different ligands on the degradation of BPA in the MnO 2 /Mn 2+ system, three other common ligands (i.e., NTA, EDTA, and Citrate) were employed to activate the MnO 2 /Mn 2+ system. The degradation efficiency of BPA by various ligands-assisted MnO 2 system is shown in Fig. S5. To better demonstrate the kinetic of BPA degradation by MnO 2 in the presence of different ligands, the variation of -l ( t ) vs time was depicted in Fig. 5. It can be seen that without the addition of ligands, the degradation of BPA by the MnO 2 /Mn 2+ system obeys the pseudo-first-order rate where the is 0.0051 min -1 . However, the addition of different types of ligands leads to various BPA degradation kinetics. For example, in the presence of PP (a non redox-type ligand), the removal of BPA by the MnO 2 /Mn 2+ system followed the pseudo-first-order rate. In comparison, with the addition of organic types ligands (e.g., NTA and EDTA), the processes of BPA degradation by the MnO 2 /ligand/Mn 2+ systems were separated into acceleration and deceleration parts. In the first 30 min, the presence of NTA and EDTA obviously accelerated the degradation of BPA by MnO 2 /Mn 2+ with the of 0.0470 min -1 and 0.0322 min -1 , respectively. However, after 30 min, the kinetics of BPA removal were decelerated. Especially for the EDTA used as organic ligand, after 30 min of reaction, the was decreased to 0.0064 min -1 which is close to the reaction rate of the blank process. The observed high values in the first 30 min can be attributed to the generation of high amounts of Mn(III)-NTA and Mn(III)-EDTA complexes due to the higher stability constants of the complexes (as shown in Table S2). The decreased after 30 min reaction can be explained by the rapid decomposition of Mn(III)-NTA and Mn(III)-EDTA complexes via different pathways. For instance, it is reported that Mn(III)-EDTA complexes can be decomposed through the inter-or intra-molecular electron transfers as shown in R [START_REF] Wang | Anaerobic microbial manganese oxidation and reduction: A critical review[END_REF] and R(2) [START_REF] Mcardell | Reaction of EDTA and Related Aminocarboxylate Chelating Agents with CoIIIOOH (Heterogenite) and MnIIIOOH (Manganite)[END_REF][START_REF] Klewicki | Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate[END_REF]. Moreover, the same authors reported that the half-life time of Mn(III)-EDTA decomposition with the molar ratio of [Mn]/[EDTA] 1:50 is less than 10 min in the pH range of 5.0-6.0 [START_REF] Klewicki | Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate[END_REF]. The variation of the absorbance intensity of Mn(III)-EDTA vs time was followed (Fig. S6). The absorbance rapidly increased and reached its maximum value of approximately 0.0095 after 30 minutes, followed by a gradual decline. These results indicate the formation and accumulation of Mn(III)-EDTA complex during the initial 30 minutes, followed by its gradual consumption. In addition, NTA and EDTA as the organic ligands can directly consume MnO 2 through the redox processes resulting in the decrease of BPA degradation [START_REF] Wang | Reaction of MnIII,IV (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds[END_REF]. The low efficiency of citrate on the reactivity of MnO 2 verified this point as well. It is clear to see that the occurrence of citrate did not improve the degradation of BPA by the MnO 2 /Mn 2+ system. Even, the presence of citrate slightly inhibited the removal of BPA by the MnO 2 /Mn 2+ system after 30 min reaction. In conclusion, the addition of PP can facilitate the degradation of organic pollutants by MnO 2 in the presence of Mn 2+ . However, the performance of organic ligands (e.g., EDTA, NTA) on the reactivity of MnO 2 depends on the stability of the complex.

-(aq) Mn 2+ (aq) + o (aq) R( 1)

-(aq) + EDTA -(aq) + o (aq) R(2)

Effects of solution pH and inorganic anions

The effects of solution pH and inorganic ions were investigated to evaluate the adaptability of the MnO 2 /PP/Mn 2+ system in a natural aquatic environment. Moreover, based on the previous report, increasing solution pH from 4.0 to 8.0 leads to the reductive potential of MnO 2 decreasing from 0.99 to 0.76 V [START_REF] Jia | Impacts of environmental levels of hydrogen peroxide and oxyanions on the redox activity of MnO2 particles[END_REF]. Therefore, both the direct oxidation of BPA by MnO 2 and the amounts of Mn(III) species generated in the reaction will be reduced at higher solution pH. Furthermore, the oxidative reactivity of Mn(III)-PP complex under different solution pH needs to be considered since the complexes are the main reactive species for BPA degradation in the MnO 2 /PP/Mn 2+ system. Fig. S7 shows that increasing solution pH from 5.0 to 8.0, the degradation efficiency of BPA by Mn(III)-PP complexes at 5 min was decreased from 65% to 35 %. Thus, considering both the reactivity of MnO 2 and Mn(III)-PP complex, the decrease of at higher solution pH is understandable.

Inorganic anions such as l -, -, and H -are commonly found in wastewater. The removal of organic pollutants by the advanced oxidation processes (AOPs) is usually disturbed by the presence of inorganic anions [START_REF] Wang | Effect of inorganic anions on the performance of advanced oxidation processes for degradation of organic contaminants[END_REF]. The effect of inorganic anions on the degradation of BPA by the MnO 2 /PP/Mn 2+ system under solution pH 8.0 was investigated.

Fig. 6 (b) shows that the addition of inorganic anions (e.g., l -, -, and H -) slightly inhibited the removal of BPA by the MnO 2 /PP/Mn 2+ system. In the presence of l -, -, and H -, the (0.0042 min -1 ) was decreased to 0.0038, 0.0032, and 0.0029 min -1 , respectively. The results might be explained by the competition between inorganic anions and negatively charged PP species for the connection to Mn 2+ adsorbed on the surface of MnO 2 .

Assessing the degradation of BPA in STP outlet water

To assess the performance of the MnO 2 /PP/Mn 2+ system in treating real wastewater, the removal of BPA by the MnO 2 /PP/Mn 2+ system in the sewage treatment plant (STP) outlet water was evaluated. The initial total organic carbon (TOC) was measured to be 5.59 mgC L -1 (where 3.86 mgC L -1 comes from adding BPA). Fig. 7 shows the degradation of BPA by the MnO 2 /PP/Mn 2+ system in the STP outlet water was inhibited ( 20% removal in 3 h) compared with the pure water results. Around 1.49 mgC L -1 of TOC was removed by the MnO 2 /PP/Mn 2+ system in the first 1 h, however in the following 2 h, only 0.31 mgC L -1 of TOC was degraded. The results can be attributed to the presence of inorganic anions, organic matter, and metal ions in the STP outlet water. Table S1 shows 62.93 mgC L -1 of inorganic carbon and various anions such as l -, -, and -were present in the STP outlet water. The negative effect of inorganic anions on the MnO 2 /PP/Mn 2+ system has been discussed in section 3.6. Moreover, it is known that organic matter can compete with organic pollutants (e.g., BPA) for the reactive species which are generated in the reaction. Thus, the presence of 1.73 mgC L -1 of organic carbon in the employed STP outlet water will inhibit the degradation of BPA by the MnO 2 /PP/Mn 2+ system. In addition, the presence of various less-reductive cations further blocked the formation of Mn(III)-PP complexes in the MnO 2 /PP/Mn 2+ system leading to the suppressed degradation of BPA as discussed in section 3.4.

Conclusion

Mn(III)-mediated organic contaminants degradation have attracted an increasing attention.

Many efforts have been taken to increase the amount of Mn(III) species during the processes of organic compound oxidation by the manganese oxides. In comparision to previous, the role of Mn 2+ in enhancing the reactivity of manganese oxides is studied in detail. In this study, the efficiency and mechanism of BPA degradation by the MnO 2 /PP/Mn 2+ system was investigated. 25 µM of BPA was completely removed by the MnO 2 /PP/Mn 2+ system in 90 min. Mn(III)-PP complexes were verified as the primary reactive species for BPA oxidation.

Moreover, Mn 2+ was demonstrated to be an important compound for the final efficiency and to act bridging, reductive, and catalytic roles in the MnO 2 /PP/Mn 2+ system. The substituted ligands such as EDTA and NTA demonstrated superior performance in activating MnO 2 within a restricted timeframe, while citrate was found to have an insignificant impact on the degradation of BPA. The results of the investigation also indicate that the MnO 2 /PP/Mn 2+ system exhibits a preference for oxidizing BPA under acidic conditions. Furthermore, close to natural pH around 8, the presence of inorganic ions such as chloride, nitrate, and bicarbonate

Fig. 1 .

 1 Fig. 1. BPA degradation by the MnO 2 /PP system in the presence of Mn 2+ . Experimental conditions: [MnO 2 ] = 230 M, [Mn 2+ ] = 10 M, [PP] = 1.2 mM, [BPA] = 25 M, pH = 5.0.

Fig. 2 .

 2 Fig. 2. Degradation of BPA ( ) in the system of MnO 2 /PP/Mn 2+ . (a) Effect of PP concentrations in the presence of 10 M Mn 2+ . (b) Effect of Mn 2+ concentrations using 1.2

Fig. 3 .

 3 Fig. 3. The formation of Mn(III)-PP complexes in different processes. Experimental conditions: [MnO 2 ] = 2.3 mM, [Mn 2+ ] = 100 M, [PP] = 12 mM, [BPA] = 250 M, pH = 5.0.

Fig. 4 .

 4 Fig. 4. BPA degradation by the MnO 2 /PP system in the presence of various metal cations. Experimental conditions: [MnO 2 ] = 230 M, [ligands] = 1.2 mM, [BPA] = 25 M, [Metal cations] = 10 M, pH = 5.0. The relative experimental error lies at 5% for BPA.

Fig. 5 .

 5 Fig. 5. Kinetic analysis of BPA degradation by MnO 2 /ligands/Mn 2+ system using pseudofirst-order model. Experimental conditions: [MnO 2 ] = 230 M, [ligands] = 1.2 mM, [BPA] = 25 M, [Mn 2+ ] = 10 M, pH = 5.0.

Fig. 6 .

 6 Fig. 6. BPA degradation by MnO 2 /PP/Mn 2+ . (a) Effect of solution pH. (b) Effect of inorganic

Fig. 7 .

 7 Fig. 7. BPA degradation efficiency and TOC removal in STP outlet water by the MnO 2 /PP/Mn 2+ system. Experimental conditions: [MnO 2 ] = 230 M, [PP] = 1.2 mM, [BPA] = 25 M, [Mn 2+ ] = 10 M, pH = 8.0. The solid lines show the fit of data using a polynomial function.

  2 ,destabilizing the aggregation of MnO 2 nanoparticles in solution, and suppressing the oxidative reactivity of MnO 2 . For example, Elzinga reported that MnO 2 could be transformed into feitk echtite (β-Mn III H) a d ma ga ite (γ-Mn III OOH) by aqueous Mn 2+ at pH 7.5 via the interfacial electron transformation[START_REF] Elzinga | Reductive Transformation of Birnessite by Aqueous Mn(II)[END_REF]. Lefkowitz et al. introduced the mineralogical transformation of hexagonal MnO 2 by Mn 2+ , where manganite was formed when the solution pH was kept at 7.0-8.0[START_REF] Lefkowitz | Influence of pH on the Reductive Transformation of Birnessite by Aqueous Mn(II)[END_REF]. Cheng et al. demonstrated that the presence of Mn 2+ in solution is highly efficient in decreasing the stability of MnO 2 nanoparticles[START_REF] Cheng | Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging[END_REF].Huang et al. indicated

2 Materials and methods 2.1 Materials

	Manganese (II) sulfate monohydrate, bisphenol A, ethylenediaminetetraacetic acid disodium
	salt hydrate (EDTA), nitrilotriacetic acid (NTA), trisodium citrate dihydrate, sodium sulfate,
	calcium chloride dehydrate, copper sulfate pentahydrate, sodium nitrate, sodium bicarbonate,
	sodium chloride, ferrous sulfate, and L-ascorbic acid were purchased from Sigma-Aldrich,
	France and were all of analytical reagents grade. Potassium pyrophosphate (96%) was bought
	from Alfa Aesar. All chemicals were used as received without further purification. All
	solutions were prepared in ultrapure water obtained from a water purification system
	(Millipore, resistivity 18.2 M cm).

Acknowledgements

he work was supported by the " édératio des Recherches e viro eme t" ( R R ) and the I-Site project CAP 20-25 of University Clermont Auvergne (UCA). This work also financed by the Foreign Experts Program of the Ministry of Science and Technology of the people's Republic of China (No. G2022051018L). The authors gratefully acknowledge financial support from China Scholarship Council provided to Daqing Jia (No, 201806920034) to study at the University Clermont Auvergne in Clermont-Ferrand, France.

ions exhibited only a slight inhibition on the degradation of BPA by the MnO 2 /PP/Mn 2+ system. Furthermore, the application of the proposed system in the sewage treatment plant outlet water was found to be inhibited due to the presence of organic matters, inorganic anions, and metal cations. The findings of this research provide comprehensive insights into the role of Mn 2+ in the ligand-assisted MnO 2 system and serve as a valuable reference for understanding the degradation of organic pollutants by manganese dioxide in the natural environment.

Credit authorship contribution statement

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.


Jian Wu

Zhao

Ribault Le

Vinkovic Ivana & Catherine
email: catherine.le-ribault@ec-lyon.fr

Serge Simoëns

ECL Lmfa

Large eddy simulation of dune evolution by wind erosion, sand transport and deposition

Keywords: large eddy simulation, particle transport, wind erosion, dune

), is performed. Numerical results match with the experimental ones. A consistent small-scale tool for predicting particle transport and wind erosion of a deforming dune is constructed and validated in this work.

Introduction

By wind erosion, large areas of cultivated land loose their top soil nutrients, resulting in higher production costs and an increasing use of fertilizers. Understanding, modeling and predicting wind erosion is crucial for reducing land degradation and improving landscape planning. Most studies focused on the understanding and modeling of dune dynamics without accounting for turbulence effects on particle transport [START_REF] Wiggs | Desert dune processes and dynamics Progress in[END_REF]. Large scale, global models need reliable parametrization of small-scale particle transport and pickup. Hence, the broad objective of the present study is to develop a consistent small-scale tool for predicting particle transport and wind erosion around a deforming dune.

Numerical Method and Transport Models

The turbulent boundary layer flow is resolved by Large-eddy simulations (LES) with the atmospheric code ARPS (Xue 1995). The complete model with particle transport has already been used by our team to study saltation within a flat turbulent boundary layer, over gaussian hills (Huang 2018) or above square obstacles with varying separation (Le [START_REF] Ribault | Large eddy simulation of particle transport and deposition over multiple 2D square obstacle s in a turbulent boundary layer[END_REF]). An immersed boundary method (IBM) is used here to account for dune deformation.

To initiate saltation, a particle entrainment model based on the forces acting on individual particles within the bed has been developed according to [START_REF] Huang | Large eddy simulation of erosion and deposition over multiple twodimensional Gaussian hills in a turbulent boundary layer[END_REF]. The advantage of this model is that it relies on instantaneous quantities provided by the LES. It therefore avoids the use of averaged and global flow characteristics such as the friction velocity. When the instantaneous impulse is greater than a threehold value, particles take-off. Another threehold crite-rion is imposed for rolling or sliding. Particles in the air or rolling on the wall are tracked using a Lagrangian method. Considered forces, exerted on particles are gravity and drag. Particle-surface interactions and particle deposition are also accounted for. Due to gravity, particles fall and may either deposit, rebound or eject new particles by splashing. Here, the rebound model proposed by (Kok and Reno 2009) and the splash model given by [START_REF] Beladjine | Collision process between an incident bead and a threedimensional granular packing[END_REF]) are applied. An avalanche model that reduces the dune slope when locally it exceeds the angle of repose is also used. The dune shape is computed in function of the instantaneous and local deposition and erosion rates.

Results and Discussion

Very few laboratory experiments of deforming dunes in air exist in literature. [START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF] measured the deformation of a small sand dune during 12 minutes in a wind tunnel. Initially, the dune height is H = 6cm and the fluid velocity 9m/s. Dune shapes are given at 2, 4, 6, 8 and 12 minutes.

Simulations of particle transport over this deformable dune are carried out to validate our model. Initially, the dune is sinusoidal. The incident turbulent boundary layer has a Reynolds number based on the average velocity and dune height Re H = 36400. The considered domain spans 53H × 6H × 8.5H, respectively in the streamwise, spanwise and vertical directions. The corresponding number of nodes is 1063 × 63 × 100. Parallel computations are performed on 128 MPI nodes. However, simulations are computationally expensive and last two months since dune evolution up to 12 minutes is achieved.

Figure 1 presents the dune, flow-laden sand particle posi-tions and streamwise flow velocity isovalues at 2 minutes. At this time, a small recirculation zone is present after the dune playing an important role in dune erosion and particle deposition. Particles with positive and negative vertical velocities are driven by the flow just after the dune crest. Figure 2 presents the dune shape at different times available by the experiment. The height of the dune decreases with time and the dune spreads in the streamwise direction. Particles deposit on the lee side. This is enhanced by the presence of the back flow at the beginning in the recirculation region mentioned above. The simulated dune shape at 6min differs slightly from the experimental one. Experiments show an upwind shift of the dune crest as the dune becomes asymmetric. This asymmetry is less pronounced in the LES. At 8 and 12min, the experiments predict high erosion after the dune crest starting from x/H = -1. Our simulations give higher erosion before x/H = 0 and higher deposition after, leading to a more flattened shape. Figure 3 presents the total dune mass evolution with time. Numerical results are compared to experiments. As expected, the dune mass decreases both in the simulations and in the experiments. This decrease comes from sand particles that take off and do not deposit on the dune but are transported by the mean flow. An overall satisfying agreement is achieved for the time evolution of the total dune mass between our LES and the experiments leading to a global validation of the presented model.

Conclusion

A consistent small-scale tool for predicting particle transport and wind erosion around a deforming surface is presented. Points -experiments by [START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF].

LES is used for the flow and a complete physical model has been developed for particle transport. The model is validated by comparison with the experimental dune evolution of [START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF]. An overall agreement between experimental and computational results is obtained. Further insight on particle transport in the vicinity of a deforming dune will be given by analyzing the results of our simulations.

Figure 1 :

 1 Figure 1: Sand particle positions, dune shape and instantaneous streamwise fluid velocity isovalues. Black (white) particles move away from (towards) the wall.

Figure 2 :

 2 Figure 2: Dune shape at different times. Lines -our LES.Points -experiments by[START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF].

Figure 3 :

 3 Figure 3: Time evolution of total dune mass. Line -our LES.Points -experiments by[START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF].

th International Conference on Multiphase Flow, ICMF 2023, Kobe, Japan, April 2-7, 2023

Acknowledgments

This work was granted access to the HPC resources of CINES.