Morgane Poser

Rodolfo Luis

E Duarte

Pascal Silva

Annabelle Peu

Éric Couvert

Dumont

Silva

Pascal Peu

Annabelle Couvert

Éric Dumont
email: eric.dumont@imt-atlantique.fr

Cellular concrete waste: An efficient new way for H2S removal

Keywords: Hydrogen sulfide, Cellular Concrete Waste, Desulfurization, Separation, Calcium oxide Highlights

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Biogas, a renewable energy source produced by anaerobic digestion of raw materials such as agricultural and municipal waste, manure, green waste or food waste…, is an alternative to fossil fuels to produce energy. It is a mixture of gaseous compounds, mainly methane (CH4) and carbon dioxide (CO2) but contains also trace amounts of other components such as nitrogen (N2), water vapor (H2O), ammonia (NH3), oxygen (O2) and hydrogen sulfide H2S [START_REF] Golmakani | Advances, challenges, and perspectives of biogas cleaning, upgrading, and utilisation[END_REF].

According to the biogas plants and the feedstocks considered, the H2S gaseous concentrations can reach several thousands of ppm [START_REF] Peu | Sulphur fate and anaerobic biodegradation potential during co-digestion of seaweed biomass (Ulva sp.) with pig slurry[END_REF]. Hydrogen sulfide is known as a major contaminant in gas since it is both hazardous and corrosive. Its burning leads to the formation of sulfur oxides (SOx) which can affect both the environment and animal health. A H2S level limited to the range 200-500 ppm is recommended even if a H2S-free biogas is advised [START_REF] Khoshnevisan | A review on prospects and challenges of biological H2S removal from ACCEPTED MANUSCRIPT / CLEAN COPY biogas with focus on biotrickling filtration and microaerobic desulfurization[END_REF]. Different technologies exist for H2S removal, such as biological treatments using microalgae or sulfide oxidizing bacteria and physicochemical treatments (membrane separation, chemical absorption, and physical adsorption techniques including adsorption on activated carbon or iron oxides, and pressure swing adsorption) [START_REF] Golmakani | Advances, challenges, and perspectives of biogas cleaning, upgrading, and utilisation[END_REF][START_REF] Abatzoglou | A review of biogas purification processes[END_REF][START_REF] Andriani | A review on biogas purification through hydrogen sulphide removal[END_REF][START_REF] Dumont | H2S removal from biogas using bioreactors: a review[END_REF][START_REF] Ghimire | Technologies for removal of hydrogen sulfide (H2S) from biogas[END_REF][START_REF] Piechota | Multi-step biogas quality improving by adsorptive packed column system as application to biomethane upgrading[END_REF][START_REF] Watanabe | Chemistry of H2S over the surface of Common solid sorbents in industrial natural gas desulfurization[END_REF]. Each of these techniques have their pros and cons in terms of complexity of operation, performance and cost effectiveness. They are suitable for large industrial gas treatment systems, however, they are not well adapted to treat small gas flowrates generated by decentralized domestic biogas digesters such as encountered in India and China for instance [START_REF] Piechota | Biomethane in Poland-Current Status, Potential, Perspective and Development[END_REF]. As unpurified biogas burning from small domestic installations used for cooking and lighting leads to significant SO x emissions, the development of cheap techniques adapted to these decentralized raw gas production systems must be encouraged. A simple ACCEPTED MANUSCRIPT / CLEAN COPY filtration using a bed filled with cellular concrete waste (CC) was recently highlighted to eliminate H2S from diluted in air under abiotic conditions, i.e. without any microbial population [START_REF] Ben Jaber | Removal of hydrogen sulfide in air using cellular concrete waste: Biotic and abiotic filtrations[END_REF][START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF]. It was suggested that the ability of CC to remove H2S was due to reactions between H2S and some components of the material, mainly with calcium carbonate leading to potential gypsum formation [START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF]. Literature data highlight that H 2 S filtration using CC had never been tested to clean raw gas with high H2S content. The objective of this study was therefore to investigate the ability of a filter bed filled with cellular concrete waste to remove H2S in a mimic biogas to reach the recommended levels. The influence of the material moisture was investigated. Different operating conditions were applied to determine the filtration performance, in terms of elimination capacity (EC) and removal efficiency (RE). Interactions between H2S and cellular concrete components, were also studied.

Materials and methods

The properties and composition of the cellular concrete waste used (Figure 1) were detailed in previous studies dedicated to air cleaning [START_REF] Ben Jaber | Removal of hydrogen sulfide in air using cellular concrete waste: Biotic and abiotic filtrations[END_REF][START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF]. This material, called "copolight", is a mineral material provided by the Florentaise company (France). It is a light (density 547 ± 5 kg m -3 ) and porous (porosity 64%) material, slightly alkaline (pH of surface 9.0 ± 0.1). Its composition in weight is: 50.5% SiO2, 24.6% CaO, 19.7% SO3, 2.2% Al2O3, 1.4% P2O5, 1.3% Fe2O3, 0.2% K2O [START_REF] Ben Jaber | Removal of hydrogen sulfide in air using cellular concrete waste: Biotic and abiotic filtrations[END_REF]. The experimental set up, depicted in Figure 2, consisted of two identical PVC cylindrical columns (internal diameter 100 mm) filled with the same volume of material (1 m height, V = 7.8 L). For safety reasons, a mimic biogas was used, CH4 being replaced by N2. A nitrogen generator BrezzaNiGen LC-MS (40-1) (purity up to 99.9%) from Gengaz Company (Wasquehal, France) was used to supply N2 continuously into filters (called "Filter (a)" and "Filter (b)"). In spite of the ability of the generator to efficiently purify N2 from air, a weak fraction of oxygen, ranged from 0.5 to 0.8%, was always measured in the nitrogen gas. The N2 flow rate entering a filter was controlled by a mass flowmeter (Model 5850S, Brooks Instruments, Hatfield, USA) and a stream of H2S (99.7% purity), controlled by another mass flow meter (Model 5850S, Brooks Instruments, Hatfield, USA), was mixed with the N2 flow rate at the bottom of the filter. As a result, the gas flowing through the bed of CC was a mixture of N2-H2S-O2 mimicking an actual raw gas. Thermocouples (K type) were installed on each filter to measure temperatures. To maintain an optimal bed humidity, CC was humidified by a water drop-by-drop system. To respect the operating configuration of an industrial process, the water used was continuously recirculated in each filter. Wet conditions were controlled by humidity sensors (Model EE08, E+E Electronik, Sevres, France) located at the top of filters. H2S concentrations were measured all along filters by an electrochemical analyzer (Biogas 5000, Geotechnical Instruments Ltd, United Kingdom). Six sampling ports located at 0, 20, 40, 60, 80 and 100 cm from the bottom were used for this purpose. The sampling ports were also used for measuring the pressure drops (pressure sensor Setra, Setra Systems, Inc, Boxborough, USA; 0-700 Pa). The parameters used to determine filters performances are reported in Table 1 and the operating conditions are summarized in Table 2. Experiments were performed for two distinct phases.

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During Phase I (from 97th to 124th day), the influence of the relative humidity on RE was evaluated using Filter (a) at a constant EBRT and H2S concentration. During Phase II (from 146th to 215th day), the influence of EBRT and inlet H2S concentration on RE were studied for a relative humidity of the gas higher than 93% using the two identical Filters (a) and (b). Before Phase I, to fix the different operation problems encountered, both filters were operated intermittently during a preliminary stage for 96 days. Filters were fed with mimic biogas at day 1 and all experimental parameters were continuously recorded from day 43. The H2S concentrations selected (up to 1500 mg m -3 ) were chosen according to the concentrations encountered in a raw biogas. Gas and material temperatures were measured, not controlled. For the running period, temperatures ranged from 17 to 27 °C according to climatic conditions (June to August 2021).

The composition of CC samples during H2S filtration was determined using an energy dispersive X-ray fluorescence (XRF) spectrometer (EDX-800HS, Shimadzu Company) at ACCEPTED MANUSCRIPT / CLEAN COPY different operating times, i.e. at day 1, day 43, day 138 and day 215. X-ray diffraction (XRD) analysis was done to assess the modification in crystalline structure taking place within H2S filtration (Bruker AXS -D8 advance diffractometer with Cu Kα radiation, λ = 1.5406 Å, step size of 0.02° in 2θ ranged from 5 to 75°, step time of 576 s). Samples of the raw material at the beginning of experiment (day 1) and on the used material at the end of experiment (day 215) were analyzed.

Loading Rate LR (g m -3 h -1 ) Q V C in C in : Inlet gas concentration (g m -3 ) Elimination Capacity EC (g m -3 h -1 ) Q V (C in -C out ) C out : Outlet gas concentration (g m -3 ) Empty Bed Residence Time EBRT (s) V Q Q: Gas flow rate (m 3 s -1 )
Removal Efficiency RE (%) 100

C in -C out C in V: Bed volume of the filter (m 3 )

Influence of the relative humidity (RH)

The influence of the gas relative humidity is displayed at Figure 3(A) (phase I). It can be seen that the higher the relative humidity, the higher the removal efficiency. This finding clearly confirms that wet conditions are required to obtained H2S removal as evidenced by Lebrun et al. [START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF] for air treatment. Results from Figure 3(A) demonstrate that relative humidity close to 100% is needed to significantly increase RE. To guarantee the removal efficiency, the humidity of the gas to be treated must be controlled. To some extent, this finding can be compared to results reported in the literature dedicated to the degradation of concrete sewer pipelines in wastewater infrastructure systems [START_REF] Joseph | Surface neutralization and H2S oxidation at early stages of sewer corrosion: Influence of temperature, relative humidity and H2S concentration[END_REF][START_REF] Wells | Modelling concrete deterioration in sewers using theory and field observations[END_REF]. From controlled chamber experiments, Joseph et al.

[13] investigated the role of relative humidity, temperature and H2S concentration in air on the concrete degradation. They showed that the influence of the relative humidity was not obvious at low temperature (16-18°C) whereas at higher temperature (25 and 30 °C), its influence was quite clear. This finding was confirmed by Wells and Melchers [START_REF] Wells | Modelling concrete deterioration in sewers using theory and field observations[END_REF] Applying this empirical relationship to the data reported in Figure 3(A) for RH>85%, the RE vs moisture level is displayed at Figure 3(B). As observed, a clear linear trend between RE and the moisture level exists, even if the regression coefficient is only 0.85. This figure confirms that cellular concrete must be largely wet to achieve a significant removal efficiency. H2S is then probably first transferred from the gas phase to the aqueous phase to react with CC components.

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H2S inlet concentration = 320 mg m -3 . (A): RE vs relative humidity of the gas (RH). (B): RE

vs moisture level of the material calculated from the empirical relationship proposed by [START_REF] Wells | Modelling concrete deterioration in sewers using theory and field observations[END_REF]: moisture level = (0.1602 RH -0.1355)/(1 -0.9770 RH) for RH ranged between 85 and 100%

Influence of EBRT and H2S inlet concentration

The influence of both EBRT and H2S concentration on RE, recorded for the two identical filters, is given in Figure 4. For a high EBRT (90 s and 180 s for phases II(a) and II(b), respectively), high RE values can be achieved for the overall range of H2S concentrations applied. RE close to 100% were even recorded for H2S concentration of 910 mg m -3 at EBRT 90 s (phase II(a)).

These very high results were obtained during a period of high outdoor temperatures (near to 27 °C), well above the values measured in the experiments (from 17 to 20 °C). The temperature of the filtering material must have a large influence on RE. This finding is supported by Wells and

Melchers [START_REF] Wells | Modelling concrete deterioration in sewers using theory and field observations[END_REF] who studied the influence of the temperature of the sewers atmosphere on the corrosion of concrete pipes due to H2S presence, as indicated above. The contact time between the gas and the material has to be sufficiently long to allow the reaction to take place. In fact, for a given H2S concentration to be treated (500 and 1500 mg m -3 for phases II(a) and II(b), respectively), the decrease of EBRT led to a decrease of performances. Since wet conditions are required for high efficiency, the mass transfer of H2S between the gas phase and the liquid phase surrounding the material is likely to be limited, especially for a short EBRT. This issue is similar to the one reported in the current literature dedicated to H2S removal by biofiltration [START_REF] Huynh Nhut | Removal of H2S in biogas using biotrickling filter: Recent development[END_REF], since H2S has to be dissolved in the aqueous phase to be degraded by microorganisms suspended in the liquid or attached at the surface of the material. To consider the influence of both EBRT and H2S concentration, all results are gathered in Figure 5 introducing the elimination capacity of filters according to the loading rate to be treated (Table 1). As observed, RE ranged from 70 to 100% for LR (loading rate) up to 30 g m -3 h -1 . This finding is in the same ACCEPTED MANUSCRIPT / CLEAN COPY order of magnitude that those reported in the recent literature dedicated to the anoxic removal of H2S using conventional biotrickling filters, as reviewed by Valdebenito-Rolak et al. [START_REF] Valdebenito-Rolack | Markers for the Comparison of the Performances of Anoxic Biotrickling Filters in Biogas Desulphurisation: A Critical Review[END_REF].

Compared to anoxic biotrickling filtration, the main advantage to proceed a simple H2S filtration using CC lies in the fact that micro-organisms and chemical addition of nitrate as electron acceptor are not needed. This technology should be surely more robust, economical and therefore more reliable over the long term than conventional biofiltration at equal performance. In the future, H2S removal using CC could probably be an interesting alternative to biofiltration depending on the industrial application. Note that the present results obtained for a mimic biogas are better than results found for air treatment since a maximum elimination capacity of 7.8 g m -3 h -1 was calculated by Lebrun et al. [START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF], without controlling the humidity of the material. For LR higher than 30 g m -3 h -1 , a decay of performance is observed (Figure 5).

Experimental results are significantly scattered, probably partly due to temperature variations mentioned above. As a first approach, the decrease in EC (elimination capacity) for high LR (Figure 5) seems similar to an inhibition curve sometimes observed in H2S biofiltration although the filter bed was not inoculated. Filters can be naturally colonized by microorganisms, but the role of this potential biomass on H2S removal should be limited.

Moreover, all attempts to model the experimental results from procedures developed for biofilter analysis failed [START_REF] Ben Jaber | Biofiltration of H2S in air-Experimental comparisons of original packing materials and modeling[END_REF][START_REF] Dumont | Validation of a rapid procedure to determine biofilter performances[END_REF]. Another modelling approach mainly based on both parameters H2S concentration and EBRT must be favored.

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Modeling removal efficiency

H2S concentration profiles along the filters length (H) are given in Figure 6 for two inlet concentrations (500 and 1500 mg m -3 , respectively) and for different EBRT. Analysis of these profiles can give information about the kinetic regime operating in filter. The decrease in the H2S concentration along the height of the filter can be well fitted using quadratic forms, but functions are concave down (the second derivatives of these functions being negative) whereas the contrary is usually observed (concave up). All results are gathered in Figure 7 as RE = (1-C/Cin) versus (H/U). H/U is the time taken for the gas to reach the height H in the filter. For each experiment, H/U is then ranged between 0 at the bottom of the filter (H = 0 m) and EBRT at the top (H = 1 m). For both ranges of H2S concentration applied, RE clearly depends on the time spent by the gas in the filter. For instance, for a H2S concentration of 500 mg m -3 , RE is around 40% when the time spent is 30 s, 70% when the time spent is 45 s, and close to 100% when the time spent is 60 s. A similar finding is achieved for a H2S concentration of 1500 mg m -3 . Whatever EBRT is applied to a given inlet concentration, all experiment results are well Loading Rate (g m From a physical point of view, the logistic function (sigmoid curve) is a suitable equation for the modelling of experimental data. Indeed, for a residence time of the biogas in the filter (H/U)  ∞, RE logically tends to the asymptote 100% (Figure 7). Moreover, the parameter  Cin  , which affects the steepness of the curve, shows that the curve approaches the asymptote RE = 1 more rapidly while the inlet H2S concentration is low. The parameter  corresponds to the number of times the value RE(t=0) must be multiplied to reach RE = 1. As observed in Figure 7, modeled logistic curves begin at RE(t=0) = 0.036 (3.6%) whereas RE should be zero. More sophisticated logistic equations can be found to begin at RE(t=0) closed to 0, however, the curves are not affected for RE > 0.036 and the simple form reported in Figure 7 is preferable. This equation will be useful to design industrial and applied filters. RE profiles according to the gas residence time in filters (calculated from data reported in Figure 6). Symbols: experimental points; solid curves: logistic equation.

Possible reactions between H2S and CC

The X-ray diffraction (XRD) analysis performed on samples of the raw material at the beginning of experiment and on the used material at the end of experiment is shown in Figure 8. Comparison between spectrograms shows the loss and formation of various components. The raw material was mainly characterized by the presence of tobermorite, chemically represented by CaO.SiO2.nH2O, and calcite (i.e. calcium carbonate CaCO3). At the end of experiment, both mineral components disappeared while new components appeared, mainly elemental sulfur S 0 (under the octasulfur form S 8 ), bassanite (CaSO 4 ,0.5H 2 O) and anhydrite (CaSO 4 ), which certainly come from gypsum dehydration (CaSO4,2H2O) during the drying of samples for analysis. The disappearance of calcium oxides (CaO.SiO2.nH2O and CaCO3) and the appearance of calcium sulfate (CaSO4,nH2O) are consistent with the degradation mechanisms of concrete sewer pipelines due to H2S formation in wastewater infrastructure systems [START_REF] Yuan | Degradation modelling of concrete submitted to sulfuric acid attack[END_REF].

The formation of calcium sulfate (gypsum) is due to chemical reactions between the main calcium components of CC with H2S and sulfuric acid H2SO4 (Figure 9). Possible reactions with H2S are: CaO + H2O → Ca(OH)2 Equation 4Ca(OH)2 + CO2 → CaCO3 + H2O Equation 5CaCO3 + H2S → CaS + H2O + CO2 Equation 6CaS + 2CO2 + 2H2O → CaSO4,2H2O + 2C Equation 7

Possible reactions with H2SO4 are:

CaCO3 + H2SO4 + H2O → CaSO4.2H2O + CO2 Equation 8
Ca(OH)2 + H2SO4 → CaSO4.2H2O The presence of sulfuric acid was confirmed by the decrease in pH (from 8.0 to 1.7) of the liquid contained in the water tank and used to humidify the filters. The formation of H2SO4 can be due to reaction between the sulfur trioxide (SO3) contained in the CC and water, and to the direct oxidation of H2S with oxygen contained in the mimic biogas (up to 0.8%). According to the following stoichiometric equations, the first product of the H2S oxidation is S 0 , which can be converted into SO4 2-in the case of oxygen excess amounts: H2S + 0.5O2 → S 0 + H2O Sulfuric acid can also be formed biologically by sulfur oxidizing bacteria (SOB), but an attempt to extract DNA from the surface of a few pieces of CC taken from the filter at the end of the experiment did not reveal the presence of bacteria.

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cellular concrete components and H2S in wet conditions

The accumulation of sulfur on CC is clearly evidenced by the increase of the sulfur proportion in the material measured using the energy dispersive X-ray Fluorescence spectrometer (XRF).

As Figure 10 shows, the sulfur percentage in the material extracted from Filter (a) increased from 15.4% at day 43 (beginning of continuous measurement records) to 36.2% at day 215, corresponding to an increase of 20.8% in weight. The transformation of the material can also be seen in Figure 1 and Figure 11. The yellow deposit on the surface of the material (Figure 1)

clearly reveals the presence of elemental sulfur S 0 whereas the origin of the black color at the core has to be identified. This black color is maybe due to the reaction between H2S and Fe2O3 leading to the formation of iron sulfide (FeS) as will be discussed later. Note that the use of "iron sponges" based on chemical reactions occurring between Fe2O3 and H2S is an old technology to remove this component from natural and coal gases. Nonetheless, as observed in Figure 1 and Figure 11, the black color develops from the core to the surface which is counterintuitive since a diffusion-reaction front progresses from the surface to the core. This preliminary observation should be investigated in the future. The reactions between the CC components and the gas also modified the mechanical structure of the material, which became easily friable. As the pieces of material extracted from the filter were highly moist, a simple finger pressure was sufficient to crush them and highlight the dark coloration inside, as observed in Figure 11. Such an operation is not possible using raw cellular concrete waste.

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In addition to the reaction of H2S with O2, the production of elemental sulfur can also be due to the presence of oxides into the material itself (Figure 9). The chemistry of H2S removal at ambient temperature by oxides such as ZnO, CuO, and Fe2O3 was recently reviewed by Watanabe [START_REF] Watanabe | Chemistry of H2S over the surface of Common solid sorbents in industrial natural gas desulfurization[END_REF]. According to the composition of cellular concrete waste, Al2O3 and Fe2O3 are the main oxides of metals. Cupper, chromium, strontium and zirconium were also detected (Figure 10). Watanabe [START_REF] Watanabe | Chemistry of H2S over the surface of Common solid sorbents in industrial natural gas desulfurization[END_REF] reported that H2S reacts with metal oxides to form metal sulfides as:

MOx(solid) + xH2S(gas) ↔ MSx(solid) + xH2O(gas) Equation 14
This reaction consists of the mechanism described by the following reactions:

MO + H2S ↔ MS + H2O

Equation 15

H2O + MO ↔ M(OH)2

Equation 16

M(OH)2 + H2S(gas) ↔ MS + 2H2O

Equation 17

Where MOx is the form of the initial metal oxide and MSx is the produced metal sulfide after the reaction. Reactivity potential theoretically follows the order Fe2O3 > CuO > ZnO [START_REF] Watanabe | Chemistry of H2S over the surface of Common solid sorbents in industrial natural gas desulfurization[END_REF]. Since the proportion of ferric oxide Fe2O3 in CC is 1.3% in weight, its presence probably led to the following reactions with H2S, as reported by [START_REF] Davydov | Mechanism of H2S Oxidation by Ferric Oxide and Hydroxide Surfaces[END_REF].

Fe2O3 + 3H2S → Fe2S3 + 3H2O Equation 18
Fe2S3 → FeS + FeS2

Equation 19

FeS2 → FeS + 1/2S2 Equation 20
Subsequently, FeS and Fe2S3 can react with oxygen leading to the regeneration of ferric oxide:

4FeS + 6H 2 O + 3O 2 → 4Fe(OH) 3 + 1/2S 8 Equation 21 2Fe2S3 + 6H2O + 3O2 → 4Fe(OH)3 + 3/4S8 Equation 22 2Fe2S3 + 3O2 → 2Fe2O3 + 3/4S8 Equation 23
The overall stoichiometry is:

2H2S + O2 → 1/4S8 + 2H2O Equation 24
Because disulfur S 2 and octasulfur S 8 are allotrope forms of elemental sulfur (octasulfur being its main component) the continuous regeneration of Fe2O3 is probably one of the mechanisms ACCEPTED MANUSCRIPT / CLEAN COPY involved in H2S removal by cellular concrete waste (Figure 9). Although the mechanisms of the reactions occurring between H 2 S and the CC components are partly revealed, the macroscopic analysis from XRD and XRF confirmed the ability of the cellular concrete to react with H2S and clean the gas.

H2S elimination capacity

The cumulative amounts of H2S removed by both filters are displayed in Figure 12 for the different experimental phases including the preliminary stage. Between day 43 and day 215 (end of experiment), Filter (a) removed 422 gH2S meaning that over the period, 60% of the H2S entering the filter were removed from biogas. For Filter (b), results are lower, 262 gH2S and 45%

of the H2S entering the filter. This result is due to the fact that the EBRT applied during Phase II (Table 2) were shorter than the calculated residence time needed for high RE in relation with the high H2S concentration (1500 mg m -3 ; Figure 7). The ability of the CC to react with H2S is by and large remarkable since 169 g of H2S were removed per kg of material for Filter (a) at the end of the experiment (105 g per kg for Filter (b); Figure 12). This result is equivalent or higher than data reported in the literature [START_REF] Golmakani | Advances, challenges, and perspectives of biogas cleaning, upgrading, and utilisation[END_REF][START_REF] Watanabe | Chemistry of H2S over the surface of Common solid sorbents in industrial natural gas desulfurization[END_REF][START_REF] Asaoka | Removal of hydrogen sulfide using carbonated steel slag[END_REF][START_REF] Galera Martínez | Valorization of calcium carbonate-based solid wastes for the treatment of hydrogen sulfide in a semi-continuous reactor[END_REF][START_REF] Lee | Enhanced adsorptive removal of hydrogen sulfide from gas stream with zinc-iron hydroxide at room temperature[END_REF][START_REF] Mulu | A review of recent developments in application of low cost natural materials in purification and upgrade of biogas[END_REF] even compared with activated carbons considered as references for H2S removal (Table 3). Galera Martínez et al. [START_REF] Galera Martínez | Valorization of calcium carbonate-based solid wastes for the treatment of hydrogen sulfide in a semi-continuous reactor[END_REF] reported, for example, that the capacity of fixed-bed or fluidized-bed reactors using carbon-based materials for H2S abatement is below 70 g of H2S per kg of material. These authors studied the ability of two solid wastes containing mainly calcium carbonate to remove H2S from air at low concentration (280 mg m -3 ) in a triphasic gas-solid-liquid reactor. They reported a removal of 75 g of H2S per kg of material. The ability of CC to remove H2S in this study is more than twice as high as those reported in the literature dealing with the reuse of solid waste.

As a first approach, assuming that all H2S removed has reacted with the CC components to form mainly gypsum and elemental sulfur, the increase of the proportion of sulfur (%S) in the CC over time can be assessed by Equation 25 (Appendix A):

% S (t) = % S (t=0) + 100 EC ρ filter M S M H2S t Equation 25
With EC the elimination capacity (expressed in kg m -3 h -1 ), filter the CC bed density (kg m -3 ), MS and MH2S, the sulfur and hydrogen sulfide molar mass (kg mol -1 ), and t the operating time (h). From data reported in Figure 12, because the average EC value calculated for Filter (a) over the period day 43 to day 215 was 10.4 g m -3 h -1 , the increase in sulfur percentage calculated from Equation 25 should be around 15.9% (with filter calculated from the mass of cellular concrete introduced in the filter bed at the beginning of the experiment). The increase in sulfur proportion determined from XRF measurement (Figure 10) is 20.8% during the same period, ACCEPTED MANUSCRIPT / CLEAN COPY i.e. slightly higher that value determined from Equation 25. Assuming that the pieces of material extracted from the filter (top and bottom) for subsequent analysis are representative of all pieces of material filling the filter, the difference between the two results is probably due to the variation of the filter over time. Because the progressive accumulation of sulfur in the material led indubitably to a modification in its mass and its structure, the density of the filter bed varied over time. Although Equation 25gives only an order of magnitude of the increase in the sulfur content in the material over time, the formula does give a valuable indication of the ability of CC to remove H2S. At day 215, the sulfur and the calcium contents in Filter (a) were in the same proportion (Figure 10), the cellular concrete was possibly close to its end-of-life.

Lifetime assessment of cellular concrete

On the basis that reactions between H2S and calcium oxides lead to gypsum formation (Equation 3 to Equation 10), that fundamentally modifies the structure of the material, the lifetime of the cellular concrete can be assessed by considering the percentage of CaO in the raw material. This lifetime (t in h) can then be determined as a first approach using the Equation 26 (Appendix B):

t = ρ filter 1 EC %CaO M H2S M CaO Equation 26
As the average EC value calculated over the period (day 43 to day 215) is 10.4 g m -3 h -1 , the operating time of a cellular concrete having 24.9% of CaO, is around 4650 h, i.e. 194 days. This time is by and large consistent with the time life of the material observed during the experiment since the measured performance declined progressively after the end of experiment. Given that t is inversely proportional to EC, the operating time will be shorter as EC increases. According

to Figure 12, the maximum amount of H2S removed was 2.4 gH2S kgCC -1 day -1 corresponding to ACCEPTED MANUSCRIPT / CLEAN COPY EC = 32 g m -3 h -1 . If these conditions could be continuously maintained, the operating time will be 3 times shorter. Once transformed into gypsum CC waste becomes a new waste, its future must be considered. This new waste could eventually be used as a soil amendment, but it will have to be characterized in terms of toxicity before use. To conclude, as the lifetime of the cellular concrete was calculated for 24.9% of CaO, the result could be improved for higher contents of calcium oxide. Recent work highlighted that synthetic tobermorite (CaO Al2O3 SiO2 H2O) can be prepared using bio-waste (snail shell) and domestic waste such as soda-lime-silica glass [START_REF] Lamidi | Preparation and characterization of synthetic tobermorite (CaO-Al2O3-SiO2-H2O) using bio and municipal solid wastes as precursors by solid state reaction[END_REF]. The synthesis of a functional material especially dedicated to H2S removal, and made from various waste, would be attractive to imagine. However, the results of the present study have shown that various components of the cellular concrete react with H2S, and synergetic effects probably occurred. The porosity of the material is also a significant parameter that have to be considered [START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF].

The simple H2S removal technology based on filtration using cellular concrete waste could be applied at different scales. For large industrial applications, long horizontal filters filled with different crates of cellular concrete would be more appropriate than a very high column. For small decentralized domestic biogas digesters, the size of filters would be small (some liters) and consequently a simple system could be use. Whatever the scale of the application, maintaining the moisture content of the material and changing the material at the end of its life are the two parameters that will dictate the technology and the design of the filters. Concerning the future of the cellular concrete waste having reacted with H2S, the new waste material obtained could be possibly recyclable as a soil amendment as it is mainly composed of gypsum.

Nonetheless, it will have to be characterized in terms of toxicity before use. As the findings reported in this study were based on a mimic biogas, future experiments should be performed in real conditions by using a real biogas. The presence of others gases in the biogas, mainly carbon dioxide but also oxygen, could have a significant influence on the empirical relationship predicting the removal efficiency (Equation 2).

Conclusions

The ability of cellular concrete waste to satisfactorily remove H2S from a biogas mimic composed of N2-O2-H2S was evidenced. Many complex physico-chemical mechanisms must occur simultaneously between H2S and the components of cellular concrete waste (mainly calcium oxides and ferric oxide), leading to a modification of the composition of the material.

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During the study, 169 g of H2S were removed per kg of material at the end of the experiment, it is significantly higher than data reported in the literature. This result is probably lower than the maximum capacity of the CC since the experiment was ended before the end-of-life of the material. The H2S elimination capacity of the CC was also measured to be up to 32 g m -3 h -1 , which is equal to or better than the performance of biofiltration systems. This simple H 2 S removal technology, certainly more robust and economical than conventional biofiltration with the same performance, could take its place in numerous industrial cases. A first empirical relationship, depending of both the H2S concentration of the biogas and the EBRT applied, was proposed to predict the removal efficiency of a filtration system (logistic equation, Equation 2).

An equation was also proposed to predict the lifetime of the material. These findings suggest that H2S removal using cellular concrete waste could be suitable to treat small gas flowrates generated by decentralized domestic biogas digesters. This pre-experiment based on a mimic biogas must be, in the future, validated in real conditions by using a real biogas.

Figure 1

 1 Figure 1 Cellular concrete waste; scale in centimeters. Left: raw material; Middle: extracted from the top of filter (a) at the end of experiment; Right: extracted from the bottom of filter (a) at the end of experiment

Figure 2

 2 Figure 2 Experimental setup (Filter (a): left; Filter (b): right)

  who studied the corrosion of concrete in 6 different sewers throughout Australia during 48 months. These authors developed a model to predict the corrosion rate. According to this model, the corrosion rate is proportional to (i) the square root of the H2S concentration in the gas phase, (ii) the temperature via an Arrhenius equation, and (iii) the moisture content of the concrete expressed as the empirical functional relationship depending on the relative humidity (RH) for RH ranged from 85 to 100%: Moisture level = 0.1602 RH -0.1355 1 -0.9770 RH Equation 1

Figure 3

 3 Figure 3 Influence of the relative humidity of the gas on H2S removal efficiency. EBRT = 90 s.

Figure 4

 4 Figure 4 Influence of EBRT and H2S concentration on RE (Phase II(a) and II(b) correspond to

Figure 5

 5 Figure 5 EC vs LR for Phases II(a) and II(b) (II(a) closed symbols; II(b) open symbols)

  fitted using a logistic function (RE=1/[1 +  exp(- (H/U) Cin  )]) involving three constants ,

Figure 6

 6 Figure 6 H2S concentration profiles according to the length of filters (Left: Cin ≈ 500 mg m -3 : phase II(a); Right: Cin ≈ 1500 mg m -3 : phase II(b))

Figure 7

 7 Figure 7 RE profiles according to the gas residence time in filters (calculated from data

Figure 8 X

 8 Figure 8 X-ray diffractograms of samples of cellular concrete. Black line: raw material

Figure 9

 9 Figure 9 Main reactions between the cellular concrete components and H2S in wet conditions

Figure 10 Figure 11

 1011 Figure 10 Modification in composition of the cellular concrete over time extracted from Filter (a)

Table 1 Parameters used Parameter Definition Nomenclature

 1

Table 2 Operating conditions Phase Inlet H2S concentration Cin (mg m -3 ) EBRT (s) Loading rate LR (g m -3 h -1 ) Relative humidity RH (%) Filter (a)

 2

		I	320	90	12.8	15-98
			350		14.0
			500		20.1
			630	90	25.2	>93
		II(a)	770 910		30.3 36.4
				60	30.2
			500	45	40.2	>93
				30	60.3
			350		7.0
			500		10.0
	Filter (b)	II(b)	900 1200 1500	180 150	18.0 24.0 29.0 36.0	>93
			1500	120	45.0	>93
				100	54.0
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3 Results and discussion

  ,  determined using Microsoft Excel ® solver (the equation of the parity plot between modeled RE and experimental RE values is y = 0.98 x; R 2 = 0.98; not shown):

		EBRT 90 s;	350-910 mg m -3
		EBRT 60 s;	500 mg m -3
		EBRT 45 s;	500 mg m -3
	RE =	EBRT 30 s; EBRT 180 s; EBRT 150 s; EBRT 120 s; 1 + 26.94 exp (-321.60 ( 500 mg m -3 350-1500 mg m -3 1500 mg m -3 1500 mg m -3 H U ) C in -1.3 ) 1	Equation 2
		EBRT 100 s;	1500 mg m -3
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-3 h -1 )



Table 3

 3 Summary of main adsorbents used for H2S removal(25 °C and atmospheric pressure;

	adapted from [1])
	Adsorbents for H2S removal	H2S elimination capacity (g kg -1 )
	Iron oxide-based (Regenerative, commercial)	12.7 to 200
	Activated carbons (non-Regenerative, commercial)	1.17 to 211.3
	Functionalized adsorbents (non-Regenerative, lab-based)	1.7 to 496.6
	Zeolites (non-Regenerative, commercial)	0.5 to 53
	Metal organic frameworks (non-Regenerative, lab-based)	13.6 to 618

Figure

[START_REF] Lebrun | H2S removal using cellular concrete waste as filtering material: Reactions identification and performance assessment[END_REF]

Amounts of H2S removed by both filters over time (the operating conditions are summarized in

Table 2)

Acknowledgements

The authors would like to thank ADEME (N° TEZ19-041) and the Region of Brittany (N°1169) for their financial support.

Acknowledgments: To the Département Systèmes Energétiques et Environnement of IMT Atlantique (Nantes, France). Also special thank to Yvan Gouriou for his continuous support in process management and Denys Grekov for the XRD analysis.

Appendix A

Assuming that H2S reacts with components of CC to mainly form gypsum and elemental sulfur, the increase in sulfur proportion of CC over time can be assessed according to the following way (S = sulfur):

Mass S (t) = Mass S (t=0) + Mass S (transferred from gas to cellular concrete)

Eq. (A.1)

With Q the gas flowrate (m 3 h -1 ), Cin and Cout (kg m -3 ) the gas H2S concentrations at the inlet and outlet of the filter, respectively, t (h) the operating time, MS and MH2S, the of sulfur and hydrogen sulfide molar mass (kg mol -1 ). Dividing by the mass of CC used (MassCC in kg), the following equation is obtained:

As the mass of cellular concrete in the filter is (MassCC = V filter) with V (m 3 ) the volume of the filter bed and filter (kg m -3 ) the CC bed density, Eq. (A.3) gives:

According to the definition of the elimination capacity (EC) given in

With parameters given in Appendix A and MCaO, the molar mass (kg mol -1 ) of calcium oxide.

As previously detailed, the mass of cellular concrete MassCC = (V filter) and:

This equation can be rewritten under the following forms, either by introducing the Elimination Capacity or the Removal Efficiency and the Empty Bed Residence Time given in Table 1 using the appropriate units (EC in kg m -3 h -1 and EBRT in h):

Eq. (B.5)

Eq. (B.6)

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Conflict of interest

The authors declare no conflict of interest.

CRediT author statement

Morgane POSER: methodology, resources, validation, formal analysis, investigation, data