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International audience

Arsenic toxicity and occurrence in the environment lead to the research of easy to handle and cheap water treatment process. Zero Valent Iron (ZVI) supports meet these requirements. ZVI trapping capacity is due to iron by-products, such as green rusts, lepidocrocite... According to physico-chemical conditions applied, the nature of by-products changes and arsenic trapping capacity is highly impacted. The aim of this work concerns the influence of aeration onto arsenic trapping in ZVI/sand columns. Four up-flow reactors filled with a ZVI/sand mixture were fed with synthetic water spiked with AsV. Two columns were aerated by air diffusers placed at the bottom while the two others were not aerated. Arsenic concentrations were monitored for three months in order to characterize arsenic sorption capacity of both configurations. A nearly complete removal was observed during the first five days into aerated system, whereas arsenic removal was stable, close to 50% during the first 20 days for the non aerated units. After this period an inversion of arsenic trapping capacity was observed, the non-aerated columns showed a higher arsenic removal than the aerated ones. Finally, when all columns reach their saturation, highest As trapping capacities were obtained in the non-aerated systems. According to arsenic monitoring, these retention capacities were estimated at 220 and 140 mg(As)/g(Fe) for non-aerated and aerated columns respectively. Solid support samples were collected along the reactor and four different layers were distinguished. After acid mineralization, analysis of total arsenic highlighted different repartitions inside the reactors: a homogeneous concentration in the non-aerated systems and a decreasing concentration form inlet to outlet for the aerated pilot units. In order to characterize As sorption into ZVI/sand columns, breakthrough curves were modelized using Thomas model. The determined KTh constants were different for aerated and non-aerated columns, which confirmed that the affinity of arsenic for solid support was different in the two systems. Oxygen measurement showed that synthetic water was close to saturation for aerated systems while it increased from 0.5 to 8 within 70 days in the non-aerated units. According to iron corrosion mechanisms, aeration induced a fast ZVI iron by-products generation, and thus a higher arsenic trapping capacity at the beginning of the experiment. Arsenic sorption into non-aerated reactors was lower but lasted longer, due to the regular ZVI-by-products generation. The lower trapping capacity of aerated systems could be due to a conversion over time of the early-created byproducts to less reactive forms.

Decrease of As authorized level in drinking waters to 10 μg.l-1 requires developing treatments with low costs. In water, arsenic is mainly found as inorganic forms: As(V) and As(III), the latter being more toxic and mobile. Water treatments are therefore more effective to remove As(V) and thus, a pre-oxidation step is generally necessary. Biological treatment using As(III)-oxidising bacteria, followed by As adsorption on Fe0 coated sand bed reactor, could be a promising treatment process. Thiomonas arsenivorans, an autotrophic bacteria isolated from a gold mine in France, was used to oxidise As(III) in fixed bed bioreactors. The objectives of this work were to study the influence of filling material (sand or Fe0-coated sand) on bacteria development and to compare performances obtained for biological oxidation in a single oxidation-adsorption reactor or in an oxidation reactor followed by an adsorption one. Experiments were carried out in three thermostated reactors (12°C). C1 was filled with sand (sieved at 1-2 mm) and 1% Fe0, C2 was identical but inoculated with Thiomonas arsenivorans and C3 was filled with sand only and inoculated. Bed volume and hydrodynamic parameters of all reactors were characterised. After inoculation of C2 and C3, the three bioreactors were continuously up-flow fed with a synthetic water containing 10 mg.l-1 As(III). This high concentration was chosen in order to favour bacteria development. During experiments, water samplings were performed at several heights along the reactors for arsenic speciation analysis using HPLC-HG-AFS. At the end of the experiment, the contents of bioreactors were collected and divided into different parts according to their height in the reactor in order to study bacterial distribution. For each collected fraction, batch experiments were performed to study kinetics of As(III) bacterial oxidation. Bacteria diversity was also analysed on these fractions by PCR-DGGE.

In drinking water supply, arsenic exists mostly as As(III) and As(V) forms which are toxic to living organisms. According to WHO recommendations, the drinking water standard was reduced from 50 to 10 μg.L-1 and many regulatory agencies have recently accepted this new standard. Most of the existing treatment processes are effective only on As(V) which forms anionic complexes unlike As(III) which forms mobile neutral complexes. As a consequence, the removal of arsenic from water requires a preliminary oxidation step of As(III) form which can be done by using Bayoxide, GEH but their cost remains high for small drinking water treatment units. This could be overcome thanks to biological treatment using As(III)-oxidising bacteria associated with As(V) adsorption process. Numerous autotrophic bacteria are able to oxidise arsenic. Among them, Thiomonas arsenivorans is able to oxidise As(III) up to 100 mg As(III).L-1 and appears to be an interesting bacterium in that it is an autotrophic bacterium which uses arsenic as an energy source and carbon dioxide or carbonates as carbon source The objectives of this work were (1) to optimise the As(III) oxidation from water drinking resources in fixed bed bioreactor and (2) to study the biofilm development during the treatment in order to improve the efficiency of the treatment. Experiments were performed in plastic bioreactors equipped with intermediate water outflows, filled with pozzolana sieved at 5-7 mm and inoculated with a culture of T. arsenivorans. Bioreactors were then continuously up-flow fed with As(III) and a synthetic water at pH 7 whose composition and temperature (12°C) are closed to that of a natural groundwater. Residence time (30 and 15 minutes) and As(III) concentrations (15 and 50 μg.l-1) were studied. Arsenic speciation and analysis was performed using GF-AAS. At the end of the experiment, bioreactors were dismantled for biolfim analysis: bacterial counting, bacterial cell viability, bacterial diversity by CE-SSCP, diversity of aoxB gene (As(III)-oxidase enzyme) by PCR-DGGE were performed on the pozzolana samples.

International audience; Lake Courtille was a polymictic eutrophic lake prone to large pH variations and sediment resuspension. Short term P release (96 hr) under laboratory resuspension conditions for two sediment sample types, surface sediments which represent sediment accumulated over several years and trap sediments which represent current year sediment, was studied. The experiments were carried out in oxic conditions at different pH values. According to phosphorus fractionation, Fe-bound-P, Albound- P and Organic-bound-P comprised the largest phosphorus pool (80% of Total Phosphorus). Phosphorus release was from the Fe-P, Al-P and organic fractions. These represent 35% of the total phosphorus content at pH 10. At circumneutral pH, in oxic conditions, a lower quantity of P was released (roughly 4% of total P sediment content) than at pH 10. The organic matter mineralization occurring at the end of summer can also influence P release as illustrated by the difference in total phosphorus content between sediment samples of 2000 and of the last five years. This type of P release was observed in Lake Courtille. High soluble reactive phosphorus content was measured in the water column at the end of summer. Aluminium treatment was thus not efficient in preventing P release. Moreover, a higher alum dose was needed to reduce P content in the water column in order to decrease primary productivity and subsequent organic matter sedimentation.

L'arsenic est un métalloïde toxique dont la présence, relativement fréquente, dans les eaux et les sols est liée soit au fond géochimique, soit aux activités humaines. En ce qui concerne les eaux destinées à la consommation, la législation impose une concentration maximale en arsenic de 10 µg.L-1. Les effets nocifs de l'arsenic sur la santé humaine rendent nécessaire le développement de technologies efficaces et peu couteuse pour éliminer cet élément des eaux potables, ainsi que dans les aquifères pollués et dans les effluents miniers (Wang et Zhao, 2009). Une unité de traitement biologique d'eaux potabilisable faiblement arséniée (As< 50µg/L), couplée à une unité de piégeage de l'As en sortie du bioréacteur, a été mise en œuvre sur un site réel afin d'étudier la robustesse du bioprocédé. Un bioréacteur contenant de la pouzzolane (matériau utilisé dans les traitements d'eaux) a été préalablement ensemencé par une souche bactérienne As(III) oxydante autotrophe (Thiomonas arsenivorans) (Battaglia-Brunet et al., 2002, Michon et al., 2010 ; Wan et al., 2010) puis alimenté par l'eau issue du forage à température ambiante (15-17°C) avec un fonctionnement discontinu (asservissement de l'alimentation du bioréacteur à la pompe du forage d'alimentation en eau). Le suivi du développement du biofilm As(III) oxydant au cours du traitement biologique a été réalisé par la recherche des gènes codant pour l'ARNr 16S (diversité bactérienne totale) et ceux codant pour une arsénite oxydase (aoxB) (diversité des bactéries As(III)-oxydantes). Ce suivi a montré une colonisation rapide et stable du support minéral par des bactéries endogènes de l'eau à traiter. Le rendement d'oxydation de l'étape d'oxydation biologique est compris entre 54 et 100 % avec des temps de séjour de 30 minutes à 7 minutes qui sont comparables à des temps de séjour de techniques classiques de traitement. Les concentrations résiduelles en As en sortie du procédé complet (oxydation biologique + piégeage) sont inférieures à 1 µg/L, et qui sont donc très encourageants pour une application industrielle.

La contamination de l'arsenic des eaux destinées à la consommation humaine est un problème environnemental international majeur. De nombreuses études toxicologiques ont clairement mis en évidence la nature carcinogène de l'arsenic et la recommandation du seuil maximal admissible de l'As dans l'eau de l'OMS (10 µg/L) a été suivie par l'Union européenne en 1998. La mise au point de technologies efficaces et peu couteuse pour éliminer cet élément des eaux destinées à la consommation humaine, ainsi que dans les aquifères polluées et dans les effluents miniers est ainsi rendue nécessaire. Une unité de traitement biologique d'eaux potabilisable faiblement arséniée, couplée à une unité de piégeage de l'As en sortie du bioréacteur, a été mise en œuvre sur un site réel afin d'étudier la robustesse du bioprocédé biologique en conditions réelle. Ce bioréacteur à lit fixé a été préalablement ensemencé par une culture de bactéries As(III) oxydante autotrophe (Thiomonas arsenivorans) puis alimentée par l'eau issue du forage dans des conditions de température ambiante (15-17°C). Le suivi du développement du biofilm As(III) oxydant au cours du traitement biologique, qui a été réalisé par des techniques de CE-SSCP et PCR-DGGE -aoxB, a permis de démontrer une colonisation du support bactérien par des bactéries endogène de l'eau à traiter. Le rendement d'oxydation de l'étape d'oxydation biologique est compris entre 54 et 100 % selon le temps de séjour et des concentrations résiduelles en As en sortie du procédé complet (oxydation biologique + piégeage) inférieures à 2 µg/L, qui sont très encourageants pour une application industrielle.

Les effets nocifs de l'arsenic sur la santé humaine rendent nécessaire le développement de technologies efficaces et peu couteuse pour éliminer cet élément des eaux potables, ainsi que dans les aquifères polluées et dans les effluents miniers (Wang et Zhao, 2009). La limite maximale admissible dans les eaux destinées à la consommation humaine a été fixée à 10 µg/L par l'Organisation Mondiale de la Santé. Une unité de traitement biologique d'eaux potabilisable faiblement arséniée (As< 50µg/L), couplée à une unité de piégeage de l'As en sortie du bioréacteur, a été mise en œuvre sur un site réel afin d'étudier la robustesse du bioprocédé en conditions réelle. Ce bioréacteur à lit fixé a été préalablement ensemencé par une souche bactérienne As(III) oxydante autotrophe (Thiomonas arsenivorans) (Battaglia-Brunet et al., 2002) puis alimenté par l'eau issue du forage dans des conditions de température ambiante (15-17°C). Le suivi du développement du biofilm As(III) oxydant au cours du traitement biologique, par la recherche des gènes codant l'ARNr 16S et une arsenite oxydase (aoxB), a montré une colonisation par des bactéries endogènes de l'eau à traiter. Le rendement d'oxydation de l'étape d'oxydation biologique est compris entre 88 et 100 % avec des concentrations résiduelles en As en sortie du procédé complet (oxydation biologique + piégeage) inférieures à 1 µg/L, qui sont très encourageants pour une application industrielle.

International audience; Inorganic arsenic, a term which encompasses both As(III) and As(V) species, constitutes the highest toxicological risk associated with arsenic in water opposite to organic arsenic species. Besides, Arsenic standard in drinking water has been lowered from 50 to 10 µg.L-1. Nevertheless, the degrees of toxicity for As(III) and As(V) are so different that it is not sufficient to determine the total content of arsenic in a given sample in order to estimate its physiological or environmental risks. Therefore, speciation of arsenic in water sample shows an increasing interest. In this work, a simple and rapid procedure was developed for the direct determination of As(III) in drinking water by Hydride Generation Atomic Absorption Spectrometry (HG-AAS) without pre-reduction of As(V). The principle of this method is to volatilise arsenic from As(III) to arsine AsH3(g) according to an apparatus equipped with a gas-liquid separator and a quartz cell. Nevertheless, if determination of total arsenic by HG-AAS technique using sodium borohydride (NaBH4) and hydrochloric acid (HCl) is well known, the application of the method for As(III) estimation is more difficult. Indeed, it is important to define a pH range in the tri-junction point of sample borohydride-acid. So, in order to overcome this problem, citric acid has been used for selective hydride formation of As(III). The following parameters were evaluated: first, the needed quantity of citric acid and NaBH4 to achieve complete reduction of As(III) without reduction of As(V); next, the carrier flow rate and the time necessary to perform the best arsine formation. Finally, measurements were performed in de-ionised water and artificial water but also in natural water. The optimised protocol for As(III) determination in drinking water was the next one: the sample was mixed with 0.2 M citric acid and 0.1% (w/v) NaBH4, this last one prepared in a 0.2% (w/v) NaOH solution. The flow rates were of 6.5, 1.0 and 1.0 mL.min-1for sample, NaBH4 and citric acid respectively. The optimised pH range obtained in the tri-junction point was of 2.5 - 3.5. This method was shown to be satisfactory for determination of traces arsenic in water samples. Thus, a calibration range was obtained for As(III) concentration between 0.5 and 20 µg.L-1 with a detection limit of 0.1 µg.L-1 and a quantification limit of 0.3 µg.L-1. A relative standard deviation value, based on three replicate measurements, below 3% was attained. Recovery studies of different added concentrations of As(III) in artificial and natural water samples were between 96 and 105%. The essay of a certified reference material solution was 5.2 ± 0.3 µg.L-1 (certified value 5.0 ± 0.1 µg.L-1).