Assessing the chronic toxicity of spreading organic amendments on agricultural soil: Tests on earthworms and plants.

Recycling organic wastes on agricultural soils improves the soil quality, but the environmental and health impact of these organic amendments closely depends on their origins, their bio-physicochemical characteristics and the considered organisms potentially affected. The aim of this study was to assess the potential chronic ecotoxicity of spreading organic amendments on agricultural soils. To do this, we characterized three different organic amendments: sewage sludge from an urban wastewater treatment plant, cow manure and liquid dairy manure. Their chronic ecotoxicity was studied through assays exposing earthworms of the species Eisenia fetida and two plants: Medicago sativa and Sinapis alba. Of the three amendments, the sewage sludge presented the highest concentrations of micropollutants and a considerable fraction of available and biodegradable organic matter. The cow manure and liquid dairy manure had lower chemical contamination and similar characteristics with lower biodegradable fractions of their organic matter. No chronic phytotoxicity was evidenced: on the contrary, particularly with sewage sludge, the germination rate and aerial and root biomass of the two plants increased. Considering earthworms, their biomass increased considerably during the reproduction assays in soil amended with sewage sludge, which contained the more bioavailable organic matter. Nonetheless, the earthworms presented an inhibition close to 78% of the production of juveniles when exposed to sewage sludge exceeding 20 g.kg-1 DW (that means 2 times the agronomic dose). This reprotoxic effect was also observed in the presence of liquid dairy manure, but not with cow manure. At the end of the assays, the glycogen and protein reserves in earthworms exposed to sewage sludge were inferior to that of control earthworms, respectively around 50% and 30%. For the earthworms exposed to liquid dairy manure, protein and lipid reserves increased. In the case of liquid dairy manure, this reprotoxic effect did not appear to be linked to the presence of micropollutants. In conclusion, our results confirm the need to use several ecotoxicity assays at different biological levels and with different biological models to assess the ecotoxic impacts of soil amendments. Indeed, although certain organic wastes present a strong nutritional potential for both plants and earthworms, a not inconsiderable risk was apparent for the reproduction of the latter. An integrated ecotoxicity criterion that takes into account a weighted sum of the different results would guide the utilization of organic amendments while ensuring the good health of agricultural ecosystems.

Assessing the Potential Ecotoxicological Risk of Different Organic Amendments Used in Agriculture: Approach Using Acute Toxicity Tests on Plants and Earthworms.

In Europe, spreading organic wastes to fertilize soils is an alternative commonly used instead of chemical fertilizers. Through their contributions of nutrients and organic matter, these wastes promote plant growth and thus agricultural production. However, these organic amendments can also contain mineral and organic pollutants requiring chemical and ecotoxicological analyses to guarantee their harmlessness on soil and its organisms during spreading. The purpose of this study was to assess the potential toxicity of three organic amendments from different sources (sewage sludge, dairy cow manure, dairy cow slurry) by performing chemical analyses and acute toxicity tests on three types of organism: earthworms, plants, soil microbial communities. Chemical analysis revealed a higher content of certain pharmaceuticals, polycyclic aromatic hydrocarbons and metals in sewage sludge in comparison with the two other types of organic wastes. The ecotoxicological assessment showed a dose-dependent effect on soil organisms for the three organic amendments with higher toxic effects during the exposure tests with a soil amended with dairy cow slurry. However, at realistic spreading doses (10 and 20 g kg-1 dry weight of organic amendments) on a representative exposed soil, organic amendment did not show any toxicity in the three organisms studied and had positive effects such as increased earthworm biomass, increased plant root growth and earthworm behavior showing attraction for organic amendment. On the contrary, exposure assays carried out on a limited substrate like sandy soil showed increased toxicity of organic amendments on plant germination and root growth. Overall, the ecotoxicological analysis revealed greater toxicity for soil organisms during the amendment of cow slurry, contrary to the chemical analysis which showed the potential high risk of spreading sewage sludge due to the presence of a higher quantity of pollutants. The analysis of the chemical composition and use of acute toxicity tests is the first essential step for assessing the ecotoxicological risk of spreading organic amendments on soil organisms. In addition to standard tests, the study suggests using a representative soil in acute toxicity tests to avoid overestimating the toxic effects of these organic amendments.

Co-hydrothermal carbonization of pine residual sawdust and non-dewatered sewage sludge - effect of reaction conditions on hydrochar characteristics.

Waste valorization is mandatory to develop and consolidate a circular bioeconomy. It is necessary to search for appropriate processes to add value to different wastes by utilizing them as feedstocks to provide energy, chemicals, and materials. For instance, hydrothermal carbonization (HTC) is an alternative thermochemical process that has been suggested for waste valorization aiming at hydrochar production. Thus, this study proposed the Co-HTC of pine residual sawdust (PRS) with non-dewatered sewage sludge (SS) - two wastes largely produced in sawmills and wastewater treatment plants, respectively - without adding extra water. The influence of temperature (180, 215, and 250 °C), reaction time (1, 2, and 3 h), and PRS/SS mass ratio (1/30, 1/20, and 1/10) on the yield and characteristics of the hydrochar were evaluated. The hydrochars obtained at 250 °C had the best coalification degree, showing the highest fuel ratio, high heating value (HHV), surface area, and N, P, and K retention, although presenting the lowest yields. Conversely, hydrochar functional groups were generally reduced by increasing Co-HTC temperatures. Regarding the Co-HTC effluent, it presented acidic pH (3.66-4.39) and high COD values (6.2-17.3 g·L-1). In general, this new approach could be a promising alternative to conventional HTC, in which a high amount of extra water is required. Besides, the Co-HTC process can be an option for managing lignocellulosic wastes and sewage sludges while producing hydrochar. This carbonaceous material has the potential for several applications, and its production is a step towards a circular bioeconomy.

Start-up phase optimization of two-phase anaerobic digestion of food waste: Effects of organic loading rate and hydraulic retention time.

The effects of organic loading rate (OLR) and hydraulic retention time (HRT) on the dynamics of acidogenic and methanogenic processes in two-phase anaerobic digestion (TPAD) of food waste (FW) were investigated to determine the start-up operational conditions. Seven arrangements of TPAD systems under mesophilic conditions were evaluated, each containing one acidogenic reactor and one methanogenic reactor. The work analyzed two HRTs (2 and 3 days) and four OLRs (2, 3, 4 and 5 kgVS.m-3.d-1). The 2D5KG system obtained VS and COD removal of 68% and 72%; SMP of 273 Lmethane.kg.VS-1. The 3D4KG system obtained VS and COD removal of 70 and 66%; SMP of 252 Lmethane.kg.VS-1. Valeric acid predominated in the acidogenic reactor in both HRT and OLR evaluated, followed by butyric acid. In the methanogenic reactor, the main methane production route was the butyric acid conversion into acetic acid and finally methane. Higher OLR benefits the methane production. The microbiological profile indicated the pathway of methanogenesis by acetoclastic methanogenesis. The canonical correlation analysis allowed to verify that the groups are independent and, therefore, the variables analyzed in the acidogenic reactor have an influence on the methanogenic reactor.

Methods for the Evaluation of Industrial Mechanical Pretreatments before Anaerobic Digesters.

Different methods were tested to evaluate the performance of a pretreatment before anaerobic digestion. Besides conventional biochemical parameters, such as the biochemical methane potential (BMP), the methane production rate, or the extent of solubilization of organic compounds, methods for physical characterization were also developed in the present work. Criteria, such as the particle size distribution, the water retention capacity, and the rheological properties, were thus measured. These methods were tested on samples taken in two full-scale digesters operating with cattle manure as a substrate and using hammer mills. The comparison of samples taken before and after the pretreatment unit showed no significant improvement in the methane potential. However, the methane production rate increased by 15% and 26% for the two hammer mills, respectively. A relevant improvement of the rheological properties was also observed. This feature is likely correlated with the average reduction in particle size during the pretreatment operation, but these results needs confirmation in a wider range of systems.

An improved procedure to assess the organic biodegradability and the biomethane potential of organic wastes for anaerobic digestion.

In this study a fractionation procedure was developed and applied to evaluate the potential of some organic wastes (two cattle manures and two catch crops, fresh and after ensiling) for anaerobic digestion. This procedure was based on water extraction of the raw sample, which enabled the evaluation of the contributions of water-soluble and particulate phases to the investigated properties. Biomethane potential (BMP) and chemical oxygen demand (COD) were determined and used to assess the anaerobic biodegradability of raw materials. Analysis of structural carbohydrates, total Kjeldahl nitrogen, water-soluble carbohydrates, volatile fatty acids and pH were also included to explain the main phenomena involved in methane production from the tested biomass. Results show that the origin and the preparation mode had a significant impact on BMP distribution. Based on a COD balance, the biodegradability of the various feedstocks ranged from 45% to 75%. Biodegradability of fresh materials was negatively correlated with the sum of structural carbohydrates and lignin content. Among the feedstock used, the water-soluble phase represented 8-69% of the total COD and 7-46% to the total BMP. Solubilization of organic matter during ensiling was due to the production and accumulation of organic acids from particulate carbohydrates and organic nitrogen. This procedure detects kinetic and biodegradability differences among biomass and thus it can be useful for the design of anaerobic digestion plants. Furthermore, it can be applied to evaluate the efficiency of biomass pretreatments.

Assessment of long term ecotoxicity of urban stormwaters using a multigenerational bioassay on Ceriodaphnia dubia: A preliminary study.

Standardized ecotoxicity bioassays show some limits to assess properly long-term residual toxicity of complex mixture of pollutants often present at low concentration, such as stormwaters. Among invertebrate organisms used for ecotoxicity testing, the microcrustacean Ceriodaphnia dubia (C. dubia) is considered as one of the most sensitive, especially regarding reproduction impairment as a toxicity endpoint. Consequently, this work explores the interest to perform a multigenerational assay based on the study of the reproduction of C. dubia to assess long-term ecotoxicity of complex mixture, using stormwater samples. With this in mind, a battery of standardized bioassays (Daphnia magna mobility, Pseudokirchneriella subcapitata population growth, Heterocypris incongruens growth and one generation C. dubia reproduction inhibition assays) was performed in parallel to a three generation C. dubia reproduction inhibition assay on 2 stormwater samples. Results highlighted that while all standardized bioassays failed to reveal residual toxicity in the stormwater samples, the C. dubia multigenerational assay exhibited an higher sensitivity than the previous ones. No adverse effect was observed for the first exposed generation, but an increase in mortality and a reproduction disturbance was obtained in the second and third exposed generation depending of the sample. Further experiments are now needed to optimize the exposure protocol of this multigenerational assay.

Contribution of different land use catchments on the microplastic pollution in detention basin sediments.

The assessment of microplastic (MP) pollution in urban areas is essential considering its abundance in freshwater, particularly due to urban wet weather discharge. The precise sources of MPs must be identified to better understand its characteristics. This study examines the relationship between MP pollution in detention basin sediments and land use in the investigated catchments. The study of stormwater management infrastructure, mainly in detention basins, has enabled the quantification of MP abundance in sediments conveyed by stormwater in urban areas. Sediment sampling was conducted in ten detention basins and one combined sewer overflow (CSO) structure in the Lyon metropolitan area, France. These basins correspond to stormwater outlets of representative urban catchment areas. MP extraction involves densimetric separation and organic matter degradation. MPs were then characterized using micro-Fourier infrared spectroscopy and siMPle software. This protocol identified MPs between 50 and 500 µm in the study sites. This study highlights the high abundance in the collected sediment samples, ranging from 2,525 to 1,218,82 MP kg-1 by dry weight sediment. The MPs found have a median size around 115 µm, making them very small MPs that are mainly composed of polypropylene followed by polyethylene and polystyrene or polyethylene terephthalate. The abundance of MPs in sediments is associated with the land use type. Catchments in predominantly industrial and commercial zones were more significantly polluted with MPs compared with those in predominantly agricultural or heterogeneous zones. Finally, statistical analyses revealed links between sedimentary and urban parameters and MPs concentrations. Several recommendations are given for future research, notably concerning the analyzing of stormwater sediments to understand the sources of MP pollution.

A review of plastic debris in the South American Atlantic Ocean coast - Distribution, characteristics, policies and legal aspects.

The presence of plastics in the oceans has already become a pervasive phenomenon. Marine pollution by plastics surpasses the status of an emerging threat to become a well-established environmental problem, boosting research on this topic. However, despite many studies on the main seas and oceans, it is necessary to compile information on the South American Atlantic Ocean Coast to identify the lack of research and expand knowledge on marine plastic pollution in this region. Accordingly, this paper conducted an in-depth review of monitoring methods, sampling, and identification of macroplastics and microplastics (MPs) in water, sediments, and biota, including information on legal requirements from different countries as well as non-governmental initiatives. Brazil was the country with the highest number of published papers, followed by Argentina. MPs accounted for 75 % of the papers selected, with blue microfibers being the most common morphology, whereas PE and PP were the most abundant polymers. Also, a lack of standardization in the methodologies used was identified; however, the sites with the highest concentrations of MPs were the Bahía Blanca Estuary (Argentina), Guanabara Bay (Brazil), and Todos os Santos Bay (Brazil), regardless of the method applied. Regarding legislation, Uruguay and Argentina have the most advanced policies in the region against marine plastic pollution due to their emphasis on the life cycle and the national ban on certain single-use plastics. Therefore, considering its content, this expert review can be useful to assist researchers dealing with plastic pollution along the South American Atlantic Ocean Coast.

Hydrochar from sawdust and sewage sludge - a potential media for retaining heavy metals in sustainable drainage systems (SuDS).

Waste valorization is an essential aspect of sustainable development. From this perspective, co-hydrothermal carbonization (Co-HTC) is a promising thermochemical process for converting organic waste into hydrochar. Hydrochar is a solid material whose physicochemical properties could make it suitable for adsorbing pollutants such as heavy metals. Accordingly, this work evaluated the hydrochar from Co-HTC of sawdust and non-dewatered sewage sludge as a potential adsorbent of heavy metals at low concentrations. In the context of sustainable drainage systems (SuDS), it is notable that heavy metals are present at very low but still potentially harmful concentrations, which presents a potential opportunity for the application of hydrochar. Thus, three hydrochars (H-180, H-215, and H-250), produced by Co-HTC at 180, 215, and 250 °C, were tested herein for their ability to retain lead (Pb2+). The H-180 presented better performance than other hydrochars (H-215 and H-250), suggesting that chemisorption could be the main adsorption mechanism. Interestingly, the presence of other cationic heavy metals (Cu2+, Zn2+, Cd2+, Cr6+, and Ni2+) did not hinder the Pb2+ adsorption, for which the removal efficiency remained close to 100%. In fact, in such a multi-metal system, hydrochar can be suitable for capturing both lead and cadmium. Therefore, the hydrochar from Co-HTC of sawdust and non-dewatered sewage sludge can be useful for removing heavy metals at low concentrations, such as those found in urban runoff waters. Although further studies are required, these findings suggest hydrochar as a potential material for application in SuDS.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process.

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Mechanical pre-treatment of source-collected municipal biowaste prior to energy recovery by anaerobic digestion.

Pre-treatments are usually necessary to prepare biowaste to anaerobic digestion. The major objectives are (i) to remove undesirable materials such as plastics and metals, which may contaminate the biowaste even if separated source-collection systems are implemented, and (ii) to extract the most readily biodegradable organic fractions from the waste stream. In this study, two wet mechanical pre-treatments, namely air-compressed press and worm screw press, were investigated on urban household biowaste. Two liquid to solid ratios were tested in each pre-treatment. Anaerobic digestion of pre-treated biowaste was studied by measuring their biomethane potentials and by controlled experiments in a continuously stirred-tank reactor with a feed load of 3.5 gVS.L-1.d-1. It was observed that increasing liquid to solid ratio in the pre-treatments allowed to increase the proportion of biodegradable organic matter extracted from the biowaste, up to 949 gCOD.kgTS-1 from household biowaste. The biomethane potentials of pre-treated waste were very high (up 525 LCH4.kgVS-1) and COD (949 gCOD.kg-1TS) from household biowaste. Anaerobic digestion in continuously stirred-tank reactor showed a very strong conversion of COD load (81%) and a high methane production up to 345 LCH4.kgVS-1.

Mechanical pretreatment of municipal biowaste to produce an aqueous slurry dedicated to anaerobic digestion.

The present study investigated a wet mechanical pretreatment to improve methane production by anaerobic digestion from biowaste material by separating a biodegradable aqueous slurry fraction (ASF) from a more recalcitrant particulate fraction (PF). Four source-sorted municipal biowastes were studied, namely household (HBW), supermarket (SBW), restaurant (RBW), and green biowaste (GBW). The treatment consisted in soaking the waste in water and then pressing the slurry through a grid with 3-mm openings to separate the two fractions. Methane production of ASF and PF obtained from the four biowastes were measured using the BMP protocol and compared to the potential of the respective untreated biowaste. Results were very different for GBW as compared to the other three BWs. With GBW, which was the most lignocellulosic of the BW studied, only 17% of the initial methane potential was recovered in the ASF. The extraction was much better on the other biowastes and increased in the following order: HBW (58%) ≃ RBW (57%) < SBW (67%). The ASF from these biowastes exhibited low total solid contents and high BMPs (416, 408, and 423 NLCH4.g-1vs for HBW, RBW, and SBW respectively). The experimental results obtained in this study therefore showed that wet pressing separation was an efficient pretreatment to improve and facilitate methane production by anaerobic digestion of biowaste such as HBW, RBW, and SBW.

Adsorption of heavy metals on to sugar cane bagasse: improvement of adsorption capacities due to anaerobic degradation of the biosorbent.

In this work, anaerobic degradation of sugar cane bagasse was studied with a dual objective: the production of biogas and the improvement of the material's characteristics for its implementation in adsorption processes. The biogas production was determined by means of biomethane potential tests carried out over two months of incubation at 35 degrees C. Biogas and methane cumulative productions were assumed to follow a first-order rate of decay. Theoretical cumulative methane and biogas productions were calculated using Buswell's equation. The anaerobic digestion resulted in a 92% decrease in the leachable organic fraction and a 40% mass loss of bagasse. The average productions of biogas and methane from the whole set of experiments were 293 +/- 6 and 122 +/- 4 mL g(-1) of volatile solids, respectively. The anaerobic incubation of the raw material led to an increase in adsorption capacities towards metal ions, which were multiplied by around 2.0 for Zn2+ and 2.3 for Cd2+.

Microalgal whole-cell biomarkers as sensitive tools for fast toxicity and pollution monitoring of urban wet weather discharges.

Urban wet weather discharge (UWWD) management is an important issue. UWWD often represents a significant source of pollution in all aquatic bodies. The occurrence of this pollution is difficult to predict due to the variability of storm events and the unknown contents of urban watershed leached out by rain. Previous studies have tried to demonstrate the ecotoxic impact of UWWD. However, most of them merely highlight the limitations of classic monospecific bioassays, given the high dilution of micropollutants or the presence of nutrients masking toxic effects. Overcoming this problem is therefore of great interest. In this study, we demonstrated the utility of a battery of biomarkers (e.g. membrane permeability, chlorophyll fluorescence, esterase and alkaline phosphatase activities) on the microalgae Chlorella vulgaris to detect the toxic effects of 7 UWWD samples after short exposures (2 and 24 h). These biomarkers are linked to microalgal life traits. Complementarily, monospecific bioassays were carried on Pseudokirchneriella subcapitata, Chlorella vulgaris, Daphnia magna and Heterocypris incongruens to compare their sensitivity to the UWWD samples. No toxic effect was detected in any of the bioassays. Yet, algal biomarkers indicated a disturbance in microalgae physiology, and particularly a perturbation of chlorophyll fluorescence, which was observed in all of the samples tested. While algal membrane permeability was affected by only one UWWD, these two enzymatic activities were stimulated or inhibited depending on the sample. Finally, this study demonstrates the sensitivity of algal biomarkers and the need to develop new, fruitful approaches to characterizing UWWD toxicity.

Xurography-based microfluidic algal biosensor and dedicated portable measurement station for online monitoring of urban polluted samples.

A critical need exists to develop rapid, in situ, and real-time tools to monitor the impact of pollution discharge toxicity on aquatic ecosystems. The present paper deals with the development of a novel, simple-to-use, low-cost, portable, and user-friendly algal biosensor. In this study, a complete and autonomous portable fluorimeter was developed to assess the A-chlorophyll fluorescence of microalgae, inserted by capillarity into low-cost and disposable xurography-based microfluidic chips. Three microalgae populations were used to develop the biosensor: Chlorella vulgaris, Pseudokirchneriella subcapitata, and Chlamydomonas reinhardtii. Biosensor feasibility and sensitivity parameters, such as algal concentration and light intensity, were optimized beforehand to calibrate the biosensor sensitivity with Diuron, a pesticide known to be very toxic for microalgae. Finally, the biosensor was employed in 10 aqueous urban polluted samples (7 urban wet-weather discharges and 3 wastewater) in order to prove its reliability, reproducibility, and performance in the detection of toxic discharges in the field.

Identification and assessment of ecotoxicological hazards attributable to pollutants in urban wet weather discharges.

Urbanization has led to considerable pressure on urban/suburban aquatic ecosystems. Urban Wet Weather Discharges (UWWD) during rainfall events are a major source of pollutants leached onto and into urban surfaces and sewers, which in turn affect aquatic ecosystems. We assessed the ecotoxicity of the different compounds identified in UWWD and identified the hazard represented by each of them. To this end, hazard quotient (HQ) values were calculated for each compound detected in UWWD based on their predicted no effect concentration (PNEC) values and their maximum measured effluent concentrations (MECmax) found in the dissolved part of UWWD. For the 207 compounds identified in UWWD, sufficient data existed for 165 of them to calculate their PNEC. The ecotoxicity of these compounds varied greatly. Pesticides represented a high proportion of the wide variety of hazardous compounds whose HQ values were calculated (94 HQ values), and they were among the most hazardous pollutants (HQ > 1000) transported by stormwater. The hazard of combined sewer overflows (CSO) was linked mainly to heavy metals and pharmaceutical compounds. Consequently, the monitoring of these pollutants should be a priority in the future. The hazard level of certain pollutants could have been underestimated due to their adsorption onto particles, leading to their low concentration in the dissolved phase of UWWD. Hence, an in-depth study of these pollutants will be required to clarify their effects on aquatic organisms.

Mobilisation of arsenic from a mining soil in batch slurry experiments under bio-oxidative conditions.

Laboratory investigations were performed to estimate the potential mobility of arsenic (As) from a highly contaminated gold-mining soil under bio-oxidative aerobic conditions as a potential remediation process. The selected soil was sampled from a gold-mining site in the South of France. It contained 27700 mg kg(-1) total As, with only 0.01% present under water-soluble forms. The nature of the immobilization mechanisms was identified by using complementary physical and chemical techniques. As was found to be strongly associated to iron (oxy)hydroxide solid phase by adsorption and/or co-precipitation. Determination of iron (Fe) and As mobility as a function of pH showed that the release of As was related with the dissolution of Fe (oxy)hydroxide at very low pH values. Bioleaching experiments were conducted with the objective to enhance the mobilization of As from the source material via biological oxidation of elemental sulfur (S degree) into sulfuric acid by autotrophic exogenous or indigenous bacteria naturally located in the soil (i.e. Acidithiobacillus species). Tests conducted at 30 degrees C in shaker flasks supplemented with S degree resulted in very acidic (pH < 1) and oxidative conditions (oxidation/reduction potential (ORP) around +800 mV vs. NHE) and induced the extraction of up to 35% of As over 84 days of incubation. Under the experimental conditions of the study (batch experiments), As mobilization was strongly correlated to the dissolution of Fe solid phases. As mobilization was probably limited by the saturation of the liquid phase. Chimiolithotrophic exogenous population appeared to have a minor effect on As bioleaching. Endogenous populations were shown to rapidly develop their capacity to oxidize S degree and mobilize As from the mining soil in the form of arsenate when elemental S degree was supplemented. The use of microbial population adapted to high As concentrations reduced significantly the lag period to reach optimal pH/ORP conditions, and increased As extraction rate to a maximum of 41% within 70 days of incubation. However, As reprecipitation was subsequently observed, suggesting that the solution should be periodically replaced in order to optimize the process.

Effect of indigenous bacterial activity on arsenic mobilization under anaerobic conditions.

Batch biochemical leaching tests were carried out to investigate the mobility of arsenic from a contaminated soil collected from a French gold mining site. The specific objective of this research was to examine the effect of indigenous bacterial activity on arsenic mobilization under anaerobic conditions. In a first step, physical and chemical characterizations were performed to provide data concerning the liquid-solid partitioning and mobility of arsenic and other inorganic constituents. In a second step, batch bioleaching tests were conducted in shaker flasks to determine the effect of indigenous bacterial activity under different anaerobic conditions (i.e., addition of mineral nutrients and carbon sources) on arsenic mobilization. Results indicated that arsenic release during contact with deionized water was limited by its very low solubility in the interstitial solution and by the stability of the different arsenic compounds formed with the amorphous solid phases of the soil (mainly iron (oxy)hydroxides). However, an increased mobilization potential was observed over the long term under anaerobic conditions with indigenous bacterial activity enhanced by the addition of carbon sources.

Effect of experimentally induced reducing conditions on the mobility of arsenic from a mining soil.

A method for estimating the release of contaminants from contaminated sites under reducing conditions is proposed. The ability of two chemical reducing agents, sodium ascorbate and sodium borohydride, to produce different redox environments in a gold mining soil contaminated with arsenic was investigated. Liquid-solid partitioning experiments were carried out in the presence of each of the reducing agents at different pH conditions. Both the effect of varying concentrations of the reducing agent and the effect of varying pH in the presence of a constant concentration of the reducing agent were studied. Concentrations of sodium ascorbate ranging from 0.0075 to 0.046 mol L(-1) and concentrations of sodium borohydride ranging from 0.0075 to 0.075 mol L(-1) were examined. The addition of varying concentrations of sodium borohydride provided greater reducing conditions (ranging from -500 to +140 mV versus NHE) than that obtained using sodium ascorbate (ranging from -7 to +345 mV versus NHE). The solubilization of arsenic and iron was significantly increased by the addition of sodium ascorbate for all concentrations examined and pH tested, compared to that obtained under oxidizing conditions (as much as three orders of magnitude and four orders of magnitude, respectively, for the addition of 0.046 mol L(-1) of sodium ascorbate). In contrast, the alkaline and highly reduced soil conditions obtained with sodium borohydride lead to a lower effect on arsenic solubilization (as much as one order of magnitude for pH values between ca. 7 and 10 and no effect for pH values between ca. 10 and 12) and no effect on iron solubilization for all concentrations examined and pH tested. At similar ORP-pH conditions the results of extraction for arsenic and iron were different for the two reagents used.