El Farouk

Omar Merouani

Bahman Khabiri

Milad Ferdowsi

El Hadi Benyoussef

Luc Malhautier

Gerardo Buelna

J Peter Jones

Michèle Heitz
email: michele.heitz@usherbrooke.ca

Biofiltration of methane in presence of ethylbenzene or xylene

Keywords:

Landfill gases (LFGs) are gaseous emissions containing several harmful compounds such as volatile organic compounds (VOCs), volatile inorganic compounds (VICs) along with methane (CH 4 ) which is a powerful greenhouse gas. In the case of small or older landfills, disposal of LFG can be economically challenging, and a possible solution is biofiltration. Biofiltration of CH 4 in the presence of two VOCs was studied, ethylbenzene (EB) for biofilter 1 (BF1) and xylene (X) for biofilter 2 (BF2). Both biofilters were packed with inorganic packing materials and operated with an empty bed residence time (EBRT) of 4.5 min. Methane concentrations varied from 2000 to 10,000 ppmv along with 2 concentrations for both VOCs i.e., 200 and 500 ppmv. In the case of individual VOC's removal, the acclimation period of the microorganisms to EB and X were close to 30 days, indicating a similar adaptation period for X and EB. The addition of CH 4 under low inlet concentration of 2000 ppmv had minor effect on VOCs biodegradation such that the average VOC removal efficiencies (REs) remained above 85% for VOC inlet concentrations of 200 ppmv while the average CH 4 -RE were around 56% in both biofilters. When CH 4 inlet concentration increased up to 10,000 ppmv, inhibition became an issue for all VOC concentrations and dropped VOC and CH 4 -REs down to 80% of their original values. The type and concentration of the VOC played a key role in the intensity of the inhibition. In general, mass transfer was controlling the elimination of the three substrates.

Introduction

Methane (CH 4 ) with a global warming potential (GWP) approximatively 21-25 times higher than carbon dioxide (CO 2 ), is the most important greenhouse gas (GHG) after CO 2 [START_REF] Padrón | Methane emission to the atmosphere from landfills in the Canary Islands[END_REF]. Estimations in 2020 indicate that an annual global emission around 570 million metric tonnes of CH 4 has been emitted worldwide from which 60% are from anthropogenic sources such as landfills, the oil and gas industries, sewers and wastewater treatment plants, coal mines, biogas production and the agricultural and livestock industries (IEA, 2020; [START_REF] Khabiri | Bioelimination of low methane concentrations emitted from wastewater treatment plants: a review[END_REF][START_REF] Mackay | Fugitive and vented methane emissions surveying on the Weyburn CO2-EOR field in southeastern Saskatchewan, Canada[END_REF]. Methane concentration in the atmosphere is increasing by 0.6% (10 ppbv) each year [START_REF] Padrón | Methane emission to the atmosphere from landfills in the Canary Islands[END_REF]. Atmospheric CH 4 concentration was estimated around 1875 ppbv in 2019 [START_REF] Schiermeier | Global methane levels soar to record high[END_REF]. The current CH 4 emission estimation indicates the possibility of an increase in global temperature above 3 • C by the end of this century. A reduction in emissions is therefore required in order to meet the temperature increase limit of 1.5-2 • C indicated in Paris Agreement (COP21) [START_REF] Nisbet | Very strong atmospheric methane growth in the 4 years 2014-2017: implications for the Paris agreement[END_REF].

Landfills are important sources of CH 4 emission due to anaerobic degradation of organic matters producing a biogas called landfill gas (LFG) with CH 4 concentrations up to 60% v/v [START_REF] Padrón | Methane emission to the atmosphere from landfills in the Canary Islands[END_REF][START_REF] Pecorini | Mitigation of methane, NMVOCs and odor emissions in active and passive biofiltration systems at municipal solid waste landfills[END_REF]. It is estimated that 68 million metric tons of CH 4 were produced globally from waste management systems including landfills in 2019 (IEA, 2020). Along with CH 4 , other compounds can be found in LFG such as volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) [START_REF] Nikiema | Elimination of methane generated from landfills by biofiltration: a review[END_REF]. Concentration of total VOCs in LFG can vary in the range of 40-1000 mg.m -3 [START_REF] Gong | Effect of volatile organic compounds on carbon dioxide adsorption performance via pressure swing adsorption for landfill gas upgrading[END_REF]. Among VOCs, aromatic compounds such as xylene (X) and ethylbenzene (EB) are commonly present in LFGs, which can cause cardiorespiratory diseases [START_REF] Borhani | Cancer risk assessment benzene, toluene, ethylbenzene and xylene (BTEX) in the production of insulation bituminous[END_REF][START_REF] Ran | Short-term effects of ambient benzene and TEX (toluene, ethylbenzene, and xylene combined) on cardiorespiratory mortality in Hong Kong[END_REF]. For example, [START_REF] Sevimoglu | Effect of persistent trace compounds in landfill gas on engine performance during energy recovery: a case study[END_REF] reported concentrations varying in the range of 10-80 mg.m -3 and 20-50 mg.m -3 for X and CH 4 and VOCs can be either antagonistic, synergistic or neutral [START_REF] Yang | Simultaneous removal of multicomponent VOCs in biofilters[END_REF]. In antagonistic interactions, the biodegradation of CH 4 is altered by the presence of VOCs that has a toxic or inhibitory effect. For example, styrene (Khabiri et al., 2020b), toluene and chlorobenzene [START_REF] Ménard | Biofiltration of methane and trace gases from landfills: a review[END_REF] were found to have an antagonistic effect on CH 4 biodegradation. On the contrary, by considering a mixture of CH 4 and VOCs, the enhancement of CH 4 biodegradation can be explained by co-metabolism phenomena, growth-benefit effect or solubility improvement of the substrate by the presence of a hydrophilic compound [START_REF] Pineda | Enhanced removal of hydrophobic volatile organic compounds in biofilters and biotrickling filters: a review on the use of surfactants and the addition of hydrophilic compounds[END_REF]. Methane biodegradation was found to be strengthened in presence of dimethyl sulphide (DMS) [START_REF] Kim | Effects of nonmethane volatile organic compounds on microbial community of methanotrophic biofilter[END_REF]. In neutral interactions, no significant change in biodegradation occurs despite the presence of several substrates. For example, the presence of benzene and toluene had no effect on CH 4 elimination [START_REF] Kim | Effects of nonmethane volatile organic compounds on microbial community of methanotrophic biofilter[END_REF].

It is hypothesized that EB or X tend to have an inhibitory effect on CH 4 elimination due to their toxicity and possible competitive inhibition for methanotrophic bacteria (the main microorganisms responsible for CH 4 biodegradation) [START_REF] Lee | Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea[END_REF]. Methane might also have a negative effect on VOC elimination.

The aim of this study was to investigate the simultaneous biofiltration of CH 4 with a VOC as a binary mixture e.g., CH 4 /X or CH 4 /EB, in two identical BFs with inorganic packing materials at different concentrations of CH 4 and VOCs. The use of an inorganic packing material would allow a BF to have stable long-term performance but presents challenges like the absence of nutrients and indigenous microorganisms; the addition of inoculum and NS is therefore required [START_REF] La | Biofiltration of methane using hybrid mixtures of biochar, lava rock and compost[END_REF]. To the authors best knowledge, the elimination of mixtures of CH 4 /X or CH 4 /EB in BFs has never been studied. The first specific objective was to study the biofiltration of two individual VOCs (EB or X) under two levels of concentrations. The second specific objective was to study the biofiltration of both VOCs with CH 4 addition at different ILs varying between 19 and 95 g.m -3 .h -1 and to investigate the interactions between EB or X in presence of CH 4 . The possible inhibition of the VOCs on CH 4 elimination as well as the effect of high CH 4 concentrations on VOCs elimination were studied.

Materials and methods

Biofilters experimental set-up

Fig. 1 shows the experiments' set-up. Two (2) upflow BFs (BF1 and BF2) made of plexiglass with a volume of 0.018 m 3 (1 m height and 0.15 m diameter) were used for a period of 281 days for BF1 and 278 days for BF2. The BFs were composed of three beds (0.3 m height). The bottom bed was packed with an inorganic material with the following properties: an average diameter of 7.5 mm, a void fraction of 0.55, a density of 750 kg.m -3 and a specific surface area of 470 m 2. m -3 . The properties of the middle and top beds packed with inorganic materials were: an average diameter of 5.0 mm, a void fraction of 0.43, a density of 1200 kg.m -3 and a specific surface area of 1250 m 2. m -3 . The material with the highest void fraction was chosen to be in the bottom bed as it is more likely to have biomass accumulation [START_REF] Ferdowsi | Moving from alcohol to[END_REF]. The composition of the materials cannot be indicated for confidentiality reasons.

Methane (Praxair Inc., Canada) flow rate was controlled by a mass flow controller (MFC) (Brooks, USA) and was injected at the bottom of the bioreactor along with humidified air and one VOC (EB for BF1 and X for BF2). Air was humidified by passing through a bubbler humidifier providing a relative humidity of 90%. Dry air was injected through a VOC bubbler filled with either EB (>99%, Sigma-Aldrich, USA) for BF1 or X (>99%, Fisher chemicals, USA) for BF2. Both humidified and dry air flows were controlled by rotameters (Brooks, USA). EB, respectively, in LFG.

For CH 4 concentration in LFG exceeding 60% v/v [START_REF] Nikiema | Elimination of methane generated from landfills by biofiltration: a review[END_REF], LFG can be valorized for electricity production or heating [START_REF] Ciuła | Energy from landfill gas as an example of circular economy[END_REF][START_REF] Themelis | Methane generation in landfills[END_REF]. When CH 4 concentration is in the range of 35-60% v/v, valorization may not be economically sustainable, and LFG physico-chemical elimination techniques are preferred. Several LFG elimination techniques exist such as thermal and catalytic oxidation, but the most commonly used mitigation technique is flaring [START_REF] Ménard | Biofiltration of methane and trace gases from landfills: a review[END_REF]. In case of concentrations below 3% v/v (the case of small-scale landfills or landfills older than 30 years), biotechnologies present several advantages over abovementioned conventional techniques for LFG treatment in term of feasibility, environmental aspect and economical sustainability [START_REF] Malakar | Comparative study of biofiltration process for treatment of VOCs emission from petroleum refinery wastewater-a review[END_REF][START_REF] Pecorini | Mitigation of methane, NMVOCs and odor emissions in active and passive biofiltration systems at municipal solid waste landfills[END_REF].

The most used biotechnology for CH 4 and VOC mitigation is biofiltration. This bioprocess relies on microorganisms to act as biocatalysts and transforming pollutants to CO 2 , water (H 2 O), biomass, etc. [START_REF] Delhoménie | methane biofilters: an experimental study on biofilter performance and carbon distribution[END_REF]. The microorganisms are supported in a humidified packed bed (organic and/or inorganic or synthetic) forming a biofilm and are periodically supplied with a nutrient solution (NS) containing essential nutrients (Khabiri et al., 2020a).

Methane and VOC biofiltration have been intensively studied over the last two decades. [START_REF] Fjelsted | Biofiltration of diluted landfill gas in an active loaded open-bed compost filter[END_REF] eliminated 60% of CH 4 from LFG with an inlet load (IL) of 9-66 g.m -3 .h -1 and an empty bed residence time (EBRT) in the range of 24-167 min in a compost packed biofilter (BF) in a landfill in Denmark. [START_REF] Lebrero | Exploring the potential of fungi for methane abatement: performance evaluation of a fungalbacterial biofilter[END_REF] obtained a removal efficiency (RE) close to 90% at a CH 4 -IL of 40 g.m -3 .h -1 for a BF packed with compost and inoculated with a pure strain of Graphium sp. [START_REF] Cho | Ethylbenzene, o-xylene, and BTEX removal by Sphingomonas sp. D3K1 in rock wool-compost biofilters[END_REF] obtained an EB-RE of 92% in a rock-compost BF for ILs varying between 10 and 40 g.m -3 .h -1 and an EBRT of 62 s. Similar X-REs were obtained by [START_REF] Natarajan | Biodegradation of xylene in a biofilter-effect of process variables, shock loads and kinetic modeling[END_REF] in date palm tree bark packed BF for X concentrations between 0.1 and 3 g.m -3 and an EBRT around 2 min.

The biofiltration of several components in a mixture from different chemical types and structures can be challenging due to the increased complexity of genetic and enzymatic regulation mechanisms in the codegradation. For example, CH 4 was found to be inhibited by VOCs such as EB or X [START_REF] Lee | Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea[END_REF][START_REF] Dobslaw | Biological waste air and waste gas treatment: overview, challenges, operational efficiency, and current trends[END_REF]. [START_REF] Liao | Removal of benzene, toluene, xylene and styrene by biotrickling filters and identification of their interactions[END_REF] indicated that interaction effects increased as much as the complexity of gas mixture elevated along with the microbial community changes according to the components of the gaseous mixture. [START_REF] Albanna | Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers[END_REF] found that VOCs such as trichloroethane, trichloroethylene and dichloromethane act as uncompetitive inhibitors on CH 4 degradation by bonding to the enzyme-substrate complex and deactivating the enzymes.

Even though several studies of VOC elimination by biofiltration were reported in the literature, only a few of them treated CH 4 and VOCs simultaneously. Khabiri et al. (2020b) used an inorganic BF to treat a mixture of CH 4 (IL of 6-70 g.m -3 .h -1 ) and styrene (IL of 9-32 g.m -3 . h -1 ) at an air flow rate of 0.018 m 3. h -1 . A complete styrene elimination was observed (RE = 100%) while CH 4 -RE was between 43 and 80%. [START_REF] Zdeb | Microbial removal of selected volatile organic compounds from the model landfill gas[END_REF] used a compost/expanded clay BF to treat a model LFG composed of CH 4 , benzene, toluene, EB and X (BTEX). Due to a long EBRT (1680 min), all pollutants were removed with REs above 90% at a total VOCs-IL of 0.005 g.m -3 .h -1 and CH 4 -IL of 12 g.m -3 .h -1 . Although the authors in the abovementioned studies usually indicated a negative effect of VOCs presence on CH 4 elimination, further studies on simultaneous biofiltration of CH 4 and VOCs are required to understand the interactions between the pollutants. Since LFG contains CH 4 along with multiple VOCs and VICs, the studies on LFG biofiltration should consider the interactions between CH 4 and VOCs as well as the possible inhibition that can be caused by one component on another.

Although a single pollutant biodegradation is usually affected by its biodegradability and mass transfer/kinetic limitations, the simultaneous biodegradation of several pollutants presents additional challenges in form of interaction effects [START_REF] Cheng | Challenges and solutions for biofiltration of hydrophobic volatile organic compounds[END_REF]. The interaction among

Analytical tools

Gas samples were taken from the inlet and outlet of the BFs. A total hydrocarbon flame ionization analyzer equipped with a single continuous flame ionization detector (FIA 510, HORIBA, Japan) was used to analyze the concentrations of CH 4 and VOCs. A CO 2 gas analyzer (ULTRAMAT 22P, SIEMENS, Germany) was used to measure CO 2 concentrations in the BFs inlet and outlet and to monitor CO 2 production. Gas samples from the BFs were extracted by vacuum pumps and directly supplied to the hydrocarbon and CO 2 analyzers.

Performance parameters

The performance parameters used to evaluate the biofiltration process are the following:

-Inlet load: IL= C in Q/V [g.m -3 .h -1 ]
(1)

-Elimination capacity: EC= (C in -C out ) Q/V [g.m -3 .h -1 ] (2) -Removal efficiency: RE = 100*(C in -C out )/C in [%] (3)
-CO 2 production rate:

PCO 2 = (C(CO 2 ) out -C(CO 2 ) in ) Q/V [g.m -3 .h -1 ] (4)
Where Q is the gas flow rate (m 3 .h -1 ), V is the packed bed volume (m 3 ) and C in and C out are the BF's inlet and outlet pollutant concentrations, respectively (g.m -3 ). C(CO 2 ) in and C(CO 2 ) out are the CO 2 concentrations in the BFs inlet and outlet, respectively (g.m -3 ).

Experimental methods and operating conditions

Five (5) liters of leachate from an active BF treating CH 4 (IL = 60 g. m -3 .h -1 , RE = 76%) were used to inoculate the packing materials that had been thoroughly washed with tap water prior to inoculation. The leachate was recirculated 3 times on the same day from the top to the bottom of the BF in order to ensure homogeneous distribution of microorganisms through the filter bed [START_REF] Ferdowsi | Performance evaluation of a methane biofilter under steady state, transient state and starvation conditions[END_REF]. The addition of NS was started a week later. The components and composition of the NS were the same as the one used by Khabiri et al. (2020a). The NS was then recycled in a 20*10 -3 m 3 tank and fed to the BF from the top at a rate of 60*10 -3 m 3. h -1 for 2 min every 2 days. The experiments were designed based on 8 consecutive phases and the NS tank was refreshed at the end of each phase. Gas flow rate was kept constant during the whole experiments at 0.24 m 3. h -1 corresponding to an empty bed residence time (EBRT) of 4.5 min for both BFs.

At the beginning of the experiment, VOCs (EB for BF1 and X for BF2) were injected to the BFs along with humidified air at an IL of 13 g.m -3 . h -1 (Phase 1) corresponding to inlet concentration of 200 ppmv. After reaching a pseudo-steady state (constant BF performance), VOC-IL was increased to 32 g.m -3 .h -1 corresponding to inlet concentration of 500 ppmv (Phase 2). Once a pseudo-steady state was reached, CH 4 was added to the inlet gas at ILs of 19 (Phase 3), 57 (Phase 4), 95 (Phase 5) g. m -3 .h -1 corresponding to concentrations of 2000, 6000, 10,000 ppmv, respectively. The transition from one CH 4 -IL to another was only carried out after a pseudo-steady performance was obtained. The IL of VOCs was then returned to 13 g.m -3 .h -1 and CH 4 -ILs were changed again to 19 (Phase 6), 57 (Phase 7), 95 (Phase 8) g.m -3 .h -1 . Carbon dioxide concentrations were measured every 3 days. Table 1 presents the different phases of the experiments.

Results and discussion

Methane and ethylbenzene removal efficiencies for different inlet loads (BF1)

Fig. 2 presents CH 4 -REs and EB-REs for BF1 in different phases of experiments. In Phases 1 and 2, only EB was injected to BF1. Ethylbenzene-REs of 83±5 and 89±3% were obtained for ILs of 12.0± 0.2 g.m -3 .h -1 (EB concentration of 200 ppmv) and 32.0±0.4 g.m -3 .h -1 (EB concentration of 500 ppmv), respectively. Ethylbenzene-RE increase can be attributed to the development and growth of microorganisms In Phase 4, CH 4 -RE and EB-RE were 59±1 and 88± 2%, respectively when CH 4 -IL was increased to 54.0±2.0 g.m -3 .h -1 (CH 4 concentration of 6000 ppmv) while EB-IL remained unchanged at 32.0± 0.4 g.m -3 . h -1 . Methane-RE and EB-RE did not significantly differ from Phase 3 possibly due to the microorganisms' adaptation to higher CH 4 -ILs.

In Phase 5, BF1 tolerated the high CH 4 and EB concentrations (10,000 ppmv for CH 4 and 500 ppmv for EB) while being fed simultaneously. According to Fig. 2, CH 4 -RE and EB-RE stayed over 57% and 86% respectively for corresponding CH 4 -IL and EB-IL of 96± 2.0 and 32±0.4 g.m -3 .h -1 , respectively. Methane-RE (57±3%) was higher compared to REs reported in some previous studies despite the short EBRT of 4.5 min and the presence of EB in this study. [START_REF] Kim | Functional rigidity of a methane biofilter during the temporal microbial succession[END_REF] obtained CH 4 -RE of 51% for an IL of 99.5 g.m -3 .h -1 with an EBRT of 20 min (more than 4 times higher than the EBRT used in the present study) in a perlite packed BF. [START_REF] Nikiema | The influence of the gas flow rate during methane biofiltration on an inorganic packing material[END_REF] reported that for the same CH 4 concentration (10,000 ppmv), CH 4 -RE was around 50% for an EBRT around 4.5 min.

For the following Phases 6-8, the EB inlet concentration returned to the lowest level of 200 ppmv while CH 4 inlet concentration varied from 2000 to 10,000 ppmv. In other words, groups of Phases (3 and 6), (4 and 7), and (5 and 8) were identical in terms of CH 4 inlet concentration, but different in terms of EB inlet concentration.

In Phase 6, CH 4 -IL and EB-IL decreased to 20.0±0.5 and 12.0±0.5 g. m -3 .h -1 , respectively, which corresponds to the lowest values of CH 4 and EB inlet concentrations tested in this study. A comparison between Phases 3 and 6 shows higher CH 4 -RE of 64±2% in Phase 6 compared to 58±4% in Phase 3 for a similar CH 4 -IL and a lower EB-IL (32.0±0.4 g. m -3 .h -1 ) in Phase 3 compared to Phase 6 (12.0± 0.5 g.m -3 .h -1 ). It is worth mentioning that CH 4 was introduced to BF1 in Phase 3 for the first time. Therefore, the CH 4 degrading community was probably more developed in Phase 6 compared to Phase 3. Also, the reduction of EB-IL in Phase 6 compared to Phase 3 possibly moderated the inhibitory effect of EB on CH 4 degradation [START_REF] Albanna | Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers[END_REF]. Ethylbenzene-RE in Phase 6 (87±2% for an IL of 12.0± 0.5 g.m -3 .h -1 ) was mildly lower than in Phase 3 (90±2% for an IL of 32.0±0.4 g.m -3 .h -1 ). This might be due to high CH 4 -IL during Phase 5 (96.0±2.0 g.m -3 .h -1 ) which could have led CH 4 degraders to be dominant as a microbial culture at the end of Phase 5 and beginning of Phase 6. In contrast, due to the presence of EB as a single substrate, the EB degraders were likely dominant at the end of Phase 2 just before the beginning of Phase 3.

In Phase 7, CH 4 -IL was increased to 57.0±0.6 g.m -3 .h -1 while EB-IL was kept at 12.0±0.5 g.m -3 .h -1 ; the lowest REs of this study were obtained (51±2% for CH 4 and 75±4% for EB). Methane-RE and EB-RE both dropped since CH 4 -IL was 3 times higher than in Phase 6 which might cause inhibition for CH 4 and EB degradation. For a similar CH 4 -IL (54±2 g.m -3 .h -1 ) and a higher EB-IL (32.0±0.4 g.m -3 .h -1 ), higher CH 4 -RE and EB-RE had been obtained (59±1 and 88±2%, respectively) in Phase 4 compared to Phase 7. The EB-RE drop in Phase 7 compared to Phase 4 could be attributed to the dominance of CH 4 degraders over EB degraders due to significantly higher CH 4 -IL compared to EB-IL. The synergistic effect of high level of EB concentration (500 ppmv) on CH 4 and EB biodegradations in Phase 4 might be the reason for higher CH 4 and EB-REs in Phase 4.

In Phase 8, a slight improvement in the performance of BF1 was observed for an increased CH 4 -IL of 99.0±4.2 g.m -3 .h -1 and at a constant EB-IL of 12.0±0.5 g.m -3 .h -1 . Methane-RE and EB-RE were obtained as 54±2 and 80±4%, respectively. Methane and EB-REs in Phase 8 were lower than REs in Phase 5 (similar CH 4 -IL to Phase 5 but lower EB-IL of 12.0±0.5 g.m -3 .h -1 ). A decrease from 57±3 to 54±2% for CH 4 -RE and from 86±3 to 80±4% for EB-RE were observed in Phase 8 m -3 .h -1 and an EBRT of 3 min in a BF packed with black slag.

According to Figs. 2 and3, both BFs displayed an increasing VOC-RE trend over Phases 1 and 2. The percentage of VOC-RE improvement from Phase 1 to Phase 2 were 7 and 18% for BF1 (EB) and BF2 (X), respectively.

In Phase 3, CH 4 was added at an IL of 20±1 g.m -3 .h -1 under X-IL of 32.0±0.5 g.m -3 .h -1 . A CH 4 -RE of 55±9% was obtained which was similar to CH 4 -RE in Phase 3 in BF1. The addition of CH 4 to BF2 in Phase 3 had no underline effect on the VOC removal and X-RE remained above 87%.

During Phase 4 in BF2, CH 4 -IL was increased by almost 3 times (54 ± 2 g.m -3 .h -1 ) compared to Phase 3 under constant X-IL of 32.0± 0.5 g. m -3 .h -1 . Both CH 4 -RE and X-RE slightly increased to 57± 2 and 90± 2%, respectively. Methane-IL increase caused no inhibition since appropriate microorganisms for both X and CH 4 removal were probably developed and adapted to remove larger amounts of pollutants.

In Phase 5, CH 4 -IL was increased to 90±4 g.m -3 .h -1 while X-IL remained unchanged (32.0±0.5 g.m -3 .h -1 ). Unlike BF1, the CH 4 -IL increase had an inhibitory effect on both CH 4 and X biodegradation as REs decreased from 57±2 (Phase 4) to 49±2% (Phase 5) and from 90± 2 (Phase 4) to 77±1% (Phase 5) for CH 4 and X, respectively. [START_REF] Lee | Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea[END_REF] reported that X has more toxicity or inhibition effect on methanotrophs than EB during tests on methanotrophs isolated from a landfill soil which could explain the CH 4 -RE decrease.

The addition of CH 4 at increasing ILs did not significantly affect EB conversion as mean EB-RE was 87±4% for Phases 3, 4 and 5. Unlike relatively constant EB-REs in BF1, X-RE slightly decreased from 87 (Phase 3) to 77% in Phase 5 (BF2) possibly due to CH 4 inhibitory effect on X since EB has been reported to be more biodegradable than X [START_REF] Deeb | Temperature effects and substrate interactions during the aerobic biotransformation of BTEX mixtures by toluene-enriched consortia and Rhodococcus rhodochrous[END_REF].

For identical CH 4 concentrations, the lower level of VOC concentration (200 ppmv in Phases 6, 7 and 8) resulted in lower VOC-RE (Figs. 2 and3). For instance, EB-RE and X-RE decreased from 88 to 75% and from 90 to 75% respectively, when VOC inlet concentration dropped from 500 to 200 ppmv in Phase 7 compared to Phase 4 under constant CH 4 inlet concentration of 6000 ppmv.

Regarding CH 4 removal, the concentration of accompanying VOC played opposite roles in BF1 and BF2. According to Fig. 3, a 500 ppmv concentration of X (Phases 3, 4 and 5) resulted in lower CH 4 -REs (6% lower) compared to a X concentration of 200 ppmv while the EB high concentration (Fig. 2) had almost a mild synergistic effect on CH 4 -RE for CH 4 concentration ranging from 6000 to 10,000 ppmv.

Pollutants elimination capacities as a function of their inlet loads

Fig. 4 (a) presents CH 4 and EB-ECs for BF1 as a function of their ILs. Maximum CH 4 -EC was 54.0±2.9 g.m -3 .h -1 for a CH 4 -IL of 99.0±4.2 g. m -3 .h -1 . Elimination capacities were higher than those reported for the same CH 4 -IL range in inorganic packed BFs. [START_REF] Nikiema | The use of inorganic packing materials during methane biofiltration[END_REF] obtained CH 4 -EC of 49.5 g.m -3 .h -1 for CH 4 -IL of 90 g.m -3 .h -1 (EBRT = 4.1 min). Vergara-Fernandez et al. ( 2020) reported a maximum CH 4 -EC of 42.2 g.m -3 .h -1 for CH 4 -ILs in the range of 300-400 g.m -3 .h -1 . Ethylbenzene-EC increased from 8.0±0.6 to 28±2 g.m -3 .h -1 for ILs of 12.0±0.2 and 32.0±0.4 g.m -3 .h -1 , respectively. Results were slightly higher than those of [START_REF] Gallastegui | Comparative response of two organic biofilters treating ethylbenzene and toluene after prolonged exposure[END_REF] who obtained EB-EC in the range of 8-20 g.m -3 .h -1 for ILs of 10-30 g.m -3 .h -1 in a black slag BF with EBRTs ranging from 100 to 200 s.

Methane and X-ECs as a function of ILs in BF2 are presented in Fig. 4 (b). The highest CH 4 -EC obtained (54±4 g.m -3 .h -1 ) for an IL of 101±3 g.m -3 .h -1 was similar to the highest CH 4 -EC in BF1. Xylene-EC increased from 8.0±0.4 to 29.0±2.3 g.m -3 .h -1 for X-ILs of 9.0±0.6 to 32.0±0.5 g.m -3 .h -1 . The values were close to [START_REF] Natarajan | Biodegradation of xylene in a biofilter-effect of process variables, shock loads and kinetic modeling[END_REF] results who obtained X-EC of 9-29 g.m -3 .h -1 in a tree-bark BF with ILs varying between 3 and 40 g.m -3 .h -1 .

In Fig. 4, the deviation of CH 4 -EC versus CH 4 -IL from the 100% Fig. 3. Pollutants removal efficiencies for each phase in BF2 ( for X-RE and for CH 4 -RE).

compared to Phase 5. These results indicated that for high level of CH 4 -ILs in the range of 54-99 g.m -3 .h -1 , the presence of high EB-IL of 32.0± 0.4 g.m -3 .h -1 (Phases 4 and 5) led to higher CH 4 -REs compared to the presence of low EB-IL of 12.0±0.5 g.m -3 .h -1 (Phases 7 and 8). In other words, high level of EB concentration (500 ppmv) under high CH 4 inlet concentration (6000-10,000 ppmv) could support synergistic interactions. In this regard, high EB concentrations (500 ppmv) might have favored the development of bacterial communities who contributed to CH 4 biodegradation in addition to the VOC removal.

Simultaneous CH 4 and xylene biodegradation (BF2 in comparison with BF1

Methane-RE and X-RE for BF2 in 8 Phases are presented in Fig. 3. In Phases 1 and 2 (Fig. 3), X-RE increased from 76±5 to 90± 6% when X-ILs increased from 12.0±0.2 g.m -3 .h -1 (X inlet concentration of 200 ppmv) to 32.0±0.5 g.m -3 .h -1 (X inlet concentration of 500 ppmv). [START_REF] Li | Comparison and application of biofilter and suspended bioreactor in removing gaseous o-xylene[END_REF] obtained a similar X-RE of 90% for ILs of 20-30 g.m -3 .h -1 and an EBRT of 1.5 min compared to 4.5 min used in the current study. [START_REF] Gallastegui | Black slag fixed bed for toluene, ethylbenzene and p-xylene (TEX) biodegradation and meiofauna development[END_REF] also reported a X-RE of 86% for an IL of 16.5 g. removal line was an indicator of incomplete conversion of CH 4 in both BF1 and BF2 (RE<100%). However, a high proportion of EB and X were successfully eliminated and the VOC-EC versus VOC-IL curves showed a least deviation from the 100% removal line due to higher solubility of VOCs in water compared to CH 4 (0.022 g.kg -1 for CH 4 and 0.150 and 0.106 g.kg -1 for EB and X, respectively at 20 • C, 1 atm) and more biodegradability of VOCs compared to CH 4 [START_REF] Coquelet | Solubility of ethylbenzene and xylene in pure water and aqueous alkanolamine solutions[END_REF]ETB, 2021).

Elimination capacity as a function of IL for CH 4 (in BF1 and BF2) increased linearly. Therefore, the highest EC (EC max ) for both BFs could not be reached and critical ILs could not be determined. The linear trend of CH 4 -EC as a function of CH 4 -IL in BF1 and BF2 indicates that CH 4 biodegradation was under mass transfer limitations [START_REF] Ferdowsi | Elimination of mass transfer and kinetic limited organic pollutants in biofilters: a review[END_REF].

Total carbon dioxide production as a function of total elimination capacity

Carbon dioxide production rate (PCO 2 ) is an indicator of biodegradation activity. The values of EC and PCO 2 can be used to determine the nature of biodegradation reactions (biomass production, etc.) occurring within the BF. The mass ratio PCO 2 /EC in case of theoretical oxidation (no oxidation by-products or biomass production) is 2.75 for CH 4 and 3.32 for EB or X as single pollutants. The theoretical PCO 2 /EC mass ratio in a BF treating CH 4 and VOC (EB or X) simultaneously, would therefore range between 2.75 and 3.32 depending on CH 4 and VOC concentrations in the mixture (Khabiri et al., 2020b).

Fig. 5 presents the total PCO 2 as a function of total EC (the summation of CH 4 -EC and EB or X-EC) for BF1 and BF2. Carbon dioxide production increased from 17±1 to 102±8 g.m -3 .h -1 for total EC in the range of 8.0±0.6-82±6 g.m -3 .h -1 in BF1 and from 14±2 to 79±8 g. m -3 .h -1 for total EC ranging from 9.0±0.6-69±4 g.m -3 .h -1 in BF2. A higher PCO 2 for BF1 (22% higher than in BF2) can be attributed to higher EC obtained in BF1 as the mixture EB + CH 4 was easier to biodegrade than X + CH 4 .

A linear relation between PCO 2 and EC variation was obtained with slopes of 1.02 and 0.94 for BF1 and BF2, respectively. The slopes show the PCO 2 /EC ratios (mass ratio), and the deviation from the theoretical mass ratios (lower than 2.75-3.32) indicates that microorganisms produced biomass and other by-products such as extracellular polymeric [START_REF] Cruz-García | Biofiltration of high concentrations of methanol vapors: removal performance, carbon balance and microbial and fly populations[END_REF]Khabiri et al., 2020b;[START_REF] Shahi | Analysis of metabolites and carbon balance in the biofilteration of cumene using loofa sponge as biofilter media[END_REF]. Since the slope for BF1 was 7% higher than for BF2, a higher portion of the consumed carbon was converted to CO 2 compared to biomass. h According to Fig. 6 (a) when CH 4 was added at an IL of 19.0± 0.7 g. m -3 .h -1 (Phase 3) at day 82, EB-RE dropped from 90 to 63% and then gradually increased to 93% after 20 days. Ethylbenzene-RE decrease after CH 4 addition was related to the required adaptation period to the increased substrates ILs and the presence of a new substrate (CH 4 ) in biofilm.

Fig. 6 (b) presents X-RE during the first 3 phases in BF2. In Phase 1, when X was injected at an IL of 12.0±0.2 g.m -3 .h -1 , X-RE was 33% at the first day of the operation and gradually increased from day 1 to day 30 to reach a pseudo-steady state (X-RE of approximately 76%). Therefore, the acclimation period for BF1 and BF2 were similar. [START_REF] Singh | Biofiltration of xylene using wood charcoal as the biofilter media under transient and high loading conditions[END_REF] reported X-RE of 40% (IL = 12 g.m -3 .h -1 ) from first day and reached a pseudo-steady state condition after 20 days with a X-RE of 99% possibly due to a high EBRT of 59 min in a BF packed with wood charcoal.

In Phase 2, X-IL was increased to 32.0±0.5 g.m -3 .h -1 at day 40 and X-RE gradually decreased to reach 57% at day 49 (27% RE decrease). Then X-RE increased from 57 to 92% after 15 days. According to Fig. 6 (a) and (b), X-RE decrease after the X-IL increase between Phase 1 and Phase 2 was smooth and less sudden compared to EB-RE. The microorganisms responsible for X removal were likely more resistant to a sudden increase of substrate IL compared to EB's microorganisms. However, a higher RE was obtained for EB possibly due to higher biodegradability of EB compared to X [START_REF] Natarajan | Biodegradation of ethyl benzene and xylene contaminated air in an up flow mixed culture biofilter[END_REF].

At day 76, CH 4 was injected at an IL of 20±1 g.m -3 .h -1 in Phase 3. The X-RE sharply dropped to 65% and then gradually increased to 89% after approximately 30 days. BF2 behaved similarly in terms of RE fluctuation following CH 4 addition as BF1 during Phase 3.

Conclusion

Two identical inorganic-based bed biofilters were employed to eliminate either EB or X as VOC solely and in the presence of CH 4 under the same operating conditions and range of concentration variations. According to the chemical structure similarities between the target VOCs such as number of carbon and aromaticity, the behavior of the biofilters were similar in many aspects. Both biofilters successfully adapted to the elimination of the single VOCs and VOC-REs higher than 90% for VOC-ILs up to 32 g.m -3 .h -1 were obtained. Also, the addition of CH 4 could be tolerated in both biofilters owing to the initial CH 4degrading rich inoculum in the original leachate used for inoculation. However, certain substrate characteristics differences such as biodegradability, inhibitory effects and toxicity for microbial cultures resulted in some different behaviors in terms of performance in both biofilters. For CH 4 concentration of 2000 ppmv, both biofilters reached CH 4 -REs around 56% under VOC concentration of 200 ppmv. The CO 2 production rate increase when EB or X were involved in the presence of CH 4 confirmed the microbial development and activity during the operation in both biofilters.

. VOC biofiltration dynamic response to concentration increase and CH 4 addition

Fig. 6 (a) presents the variations of EB-RE as a function of time during Phases 1, 2 and 3. These first three Phases were selected to investigate VOC elimination as a single pollutant and in a mixture with CH 4 introduced during Phase 3. The BF response to VOC concentration increase as well as CH 4 addition was monitored over time. Ethylbenzene-RE was 23% one day after inoculation when EB was injected at an IL of 12.0± 0.2 g.m -3 .h -1 . Ethylbenzene-RE gradually increased from day 1 to day 30 (Phase 1) when it reached a pseudo-steady state (RE around 88%). The results indicated that the adaptation period was longer than usual for VOCs treatment in BFs. García-Pe ña et al. (2008) obtained EB-RE close to 90% after about 5 days of inoculation with a fungi consortium for EB concentration around 40 g.m -3 and an EBRT of 1.7 min in a BF packed with vermiculite. [START_REF] Son | Ethylbenzene removal in a multiple-stage biofilter[END_REF] also reported an EB-RE of 99% at day 1 in a compost-based bed BF for EB-IL of 6 g.m -3 .h -1 and an EBRT of 2 min. The difference in the adaptation period might be due to the absence of indigenous microorganism community in the inorganic packing or lack of EB degraders in the initial inoculum as the inoculum used was a leachate from a BF treating only CH 4 .

Ethylbenzene-RE dropped sharply to 40% (decreased by 55%) after EB-IL was increased to 32.0±0.4 g.m -3 .h -1 (Phase 2). This RE reduction demonstrated that the increased EB-ILs were beyond the degradation capacity of microorganisms. [START_REF] Volckaert | Ethylbenzene removal under mesophilic conditions in a biofilter with Macadamia ternifolia nutshells as a carrier material[END_REF] reported an EB-RE decrease from 95 to 60% when EB-IL was increased from 12 to 50 g. m -3 . -1 in BF packed with macadamia nutshells and with an EBRT of 90 s. Microorganisms in BF1 gradually adapted to EB-IL after 30 days and EB-RE returned to 91% similarly to the end of Phase 1.

Fig. 1 .

 1 Fig. 1. Experimental set-up for biofiltration of CH 4 and VOCs.

  responsible for EB removal and/or the increase of EB bioavailability in the biofilm phase.[START_REF] Gallastegui | Black slag fixed bed for toluene, ethylbenzene and p-xylene (TEX) biodegradation and meiofauna development[END_REF] obtained a similar EB-RE of 90% during the acclimation period for an IL varying in the range of 4.5-16.1 g.m -3 .h -1 in a black slag packed BF at an EBRT of 3 min.In Phase 3, CH 4 was added to BF1 at an IL of 19.0± 0.7 g.m -3 .h -1 (CH 4 concentration of 2000 ppmv). According to Fig.2, CH 4 -RE reached 58±4% while EB-RE remained almost constant at 90± 2% (EB-IL was kept constant at 32.0±0.4 g.m -3 .h -1 ). Regarding CH 4 elimination as a sole pollutant,[START_REF] Hernández | EPS and SMP as stability indicators during the biofiltration of diffuse methane emissions[END_REF] obtained a higher CH 4 -RE of 62% for an IL of 18 g.m -3 .h -1 and an EBRT of 4.4 min in a tree bark BF. Gómez-Borraz et al. (2017) reported a CH 4 -RE of 62% for an EBRT of 19 min and a CH 4 -IL of 21 g.m -3 .h -1 in a compost BF. About the removal of CH 4 and a VOC in a mixture,Khabiri et al. (2020b) obtained a CH 4 -RE of 64% for CH 4 -IL of 20 g.m -3 .h -1 in the presence of styrene (styrene-IL = 32 g.m -3 .h -1 ) in a BF packed with inorganic materials with an EBRT of 6 min. Methane-RE (58±4%) in Phase 3 was 8% lower than REs obtained for CH 4 -ILs close to 19 g.m -3 .h -1 in the abovementioned studies due to a possible inhibition for CH 4 biodegradation by EB presence or the shorter EBRT of 4.5 min.[START_REF] Lee | Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea[END_REF] reported a slight inhibitory effect of EB on methanotrophic bacteria in a batch bioreactor as CH 4 oxidation rate dropped from 100 to 80% when EB was added to an enriched CH 4 -oxidizing microbial culture from landfill cover soil. Methane was injected at a concentration of 5000 ppmv along with 100-200 ppmv of EB. Incubation was carried out at 30 • C and at a shaking rate of 180 rpm.

Fig. 2 .

 2 Fig. 2. Pollutants removal efficiencies for each phase in BF1 ( for EB-RE and for CH 4 -RE).

Fig. 4 .

 4 Fig. 4. Pollutants elimination capacity as a function of inlet load for (a) BF1 ( for CH 4 -EC and for EB-EC) and (b) BF2 ( for CH 4 -EC and for X-EC).

Fig. 5 .

 5 Fig. 5. Carbon dioxide production as a function of elimination capacity for BF1 and BF2 ( for BF1 and for BF2).

Fig. 6 .

 6 Fig. 6. Dynamic response to concentration increase and CH 4 addition (a) BF1 ( for EB-RE and for EB-IL) and (b) BF2 ( for X-RE and for X-IL).

Table 1

 1 Operating conditions during the experiments.

	Phase	VOC	CH 4	Duration for	Duration for
		concentration	concentration	BF1 (days)	BF2 (days)
		(ppmv)	(ppmv)
	1	200	0	37	37
	2	500	0	45	39
	3	500	2000	45	42
	4	500	6000	15	27
	5	500	10,000	54	57
	6	200	2000	34	34
	7	200	6000	27	18
	8	200	10,000	24

Acknowledgments

M. Heitz and G. Buelna would like to thank the Fonds de recherche du Québec -Nature et technologies (FRQNT) for financially supporting this project. M. Heitz and J.-P. Jones would also like to thank the Natural Sciences and Engineering Research Council (NSERC)/Discovery grants program for the financial support.

Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.

Credit author statement

El Farouk Omar Merouani: Conceptualization, Methodology, Validation, Formal analysis,

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.


Roy Chaaya

Stéphane Gaboreau

Hugues Raimbourg

Joachim Tremosa

Impact of temperature on mechanical and geochemical behaviour of swelling clay minerals

Impact of temperature on mechanical and geochemical behaviour of swelling clay minerals.

Roy Chaaya 1,2 *, Stéphane Gaboreau 1 , Hugues Raimbourg 2 , Joachim Tremosa 1 1 BRGM, 45060 Orléans, France. 2 ISTO, UMR 7327 -Université d'Orléans, CNRS, BRGM, 45071 Orléans, France. * r.chaaya@brgm.fr

The ubiquity of swelling clay minerals in surface and sub-surface geoscience systems, in sedimentary deposits, in fault zones or in their potential application in the context of nuclear waste disposal make the effect of temperature (≥ 60°C) on their hydrological, mineralogical and mechanical behaviour a major scientific issue. Therefore, we aim to assess the swelling behaviour of clays when hydrated at temperature (≥ 60°C) under confined conditions. The swelling results from water-mineral interactions at different scales, from interlayer space within crystallite to pore space between particles, depending on the charge compensating cations. We are tackling these physical and chemical issues using a combined approach of experiments and geochemical modelling in order to apprehend the osmotic and crystalline contributions. Swelling experiments were performed with homo-ionic clays with different solutes (cations and ionic strength) and at different dry densities from 1.4 to 1.6 g.cm -3 . Preliminary results acquired at temperatures ranging from 25 to 80°C show that swelling pressure of compacted Kunipia-Na form at a density of 1.4 g. cm -3 decreases by 33% at 80°C (figure 1). This temperature effect seems to become less significant with increasing density (i.e. 24% for d=1.6 g.cm -3 ). On the contrary, the swelling pressure of Kunipia-Ca remains almost constant showing a very negligible effect of temperature on Ca-form smectites. On the other hand, X-ray diffraction analysis performed on sodium and calcium smectite powders under controlled relative humidity at temperatures of 25°C and 80°C show that the temperature has a very negligible effect on the interlayer water retention capacity and thus on the crystalline component of the swelling pressure (figure 2). Complementary data of water adsorption gravimetry isotherms, in confined conditions, were obtained in the same conditions. This set of data will thus allow to assess the temperature effect on both crystalline and osmotic water retention capacity.

Preferred format: Oral Session 19 : High temperature impact on bentonite barrier systems in the context of radioactive waste disposal.

Figure 1 :

 1 Figure 1: The swelling test results of a Kunipia-Na of density 1.4 g.cm -3 hydrated by a NaCl solution of concentration 10 -1 M at temperatures of 25; 60 and 80°C.

Figure 2 :

 2 Figure 2: Recorded basal distances of Kunipia-Na at temperatures of 25°C and 80°C for relative humidities of 30% and 80% (XRD).