Ferric Arsenite Research Articles

Extraction of andradite from flotation tailings as a potential material for As(III) removal from simulated wastewater

Arsenic is a toxic and carcinogenic metalloid widely distributed in nature. It can enter water bodies through natural or man-made routes, posing a serious threat to the ecological environment and human health. In recent years, arsenic pollution in water has become a global problem. This study introduces the utilization of andradite from scheelite flotation tailings as a potential material for As(III) removal. The experimental results showed that after 30 min of milling, 5 g/L andradite achieved a removal rate of over 90 % for As(III), reducing its concentration from 1 mg/L to 0.09 mg/L. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) were carried to reveal the underlying mechanism of As(III) removal with andradite. Results indicated that the calcium atom and iron atom on the surface of andradite may react with As(III) to form calcium arsenite and ferric arsenite, and the products adhere to the surface of andradite. Based on the obtained results, andradite is a promising material for the remediation of arsenic pollution. Moreover, the source of andradite is very wide, and the cost of extraction and preparation is low. The extraction of andradite from non-ferrous metal flotation tailings as an adsorbent to remove As(III) not only improves the utilization rate of tailings resources, but also provides a new method for the prevention and control of arsenic pollution.

Effects of nitrate concentrations on As(III) immobilization via new ferric arsenite hydroxynitrate precipitates

The Fe(III)-As(III)-anion precipitation as layered double hydroxide-like compounds such as tooeleite and ferric arsenite hydroxychloride (FC) has been found in As(III)-contaminated acidic mine soils. However, the question of whether As(III) is immobilized by Fe(III) in aqueous nitrate media, a common constituent in soils and metallurgical wastewater, remains unclear. Herein, we investigated the effects and mechanism of nitrate on As(III) immobilization via Fe(III)-As(III)-nitrate precipitation. Our results indicated that new ferric arsenite hydroxynitrate (FN) with variable Fe(III)/As(III)/nitrate molar ratios crystallized in an initial nitrate content range of 11,160–74,400 mg·L−1 at pH 2.3. This FN precipitation resulted in 60.3–87.9% of As(III) removal and a fall-rise immobilization trend as a function of the initial nitrate concentration. The optimal initial nitrate concentration for As(III) fixation was 44,640 mg·L−1. Although FN shared similar morphology and crystal structure with tooeleite and FC, the substitution of NO3− for SO42− and Cl− in the FeO6-AsO3 interlayers caused a significant local structural distortion due to different ionic radii. The characteristic infrared and Raman bands of the nitrate group in FN occurred at 1,363 cm−1, 1,384 cm−1 and 152 cm−1, 722 cm−1, 1,055 cm−1, respectively. The FN induced significantly enhanced As(III) immobilization relative to its tooeleite and FC counterparts at equimolar concentrations of NO3−, SO42−, and Cl− as the media.

Effects of nitrate and Fe/As molar ratio on direct iron(III)-arsenite precipitation in high-sulfate-chloride wastewaters.

The addition of an arsenite-chloride solution into an arsenite-sulfate solution is extremely beneficial for the removal of As(III) via Fe(III) salt precipitation at pH 2.3. However, the applicability of this method to complicated high-As(III) metallurgical wastewaters still requires further verification. This work investigated the effects of nitrate and Fe/As molar ratio on As(III) immobilization using Fe(III) in three acid radical media including sulfate, chloride, and nitrate at pH 2.3. Our results indicated that 72.1‒93.5% of As(III) was precipitated, which was 5‒10% less than those obtained in thenitrate-free systems. The Fe/As molar ratio of 4 was the optimal condition with an average of 93% As(III) removal based on a broad sulfate/chloride molar ratio range (1:1‒16). However, a maximum of 96% As(III) removal was observed under the Fe/As molar ratio of 1.5 and the sulfate/chloride condition of 1:16. The negative correlation between complexation and precipitation was attributed to the enhanced initial complexation by the synergistic effect of the mononitratoiron complex and FeH2AsO32+. The variation of Fe/As molar ratios resulted in the diverse solid species, thus further affecting the As(III) removal efficiency. Despite producing tooeleite as a major As(III) host phase, ferrihydrite and poorly crystalline ferric arsenite hydroxysulfate formed simultaneously at the Fe/As molar ratio of 4 participated in As(III) immobilization compared with the solid products at Fe/As molar ratios ≤ 2.

Enhanced removal of high-As(III) from Cl(-I)-diluted SO4(-II)-rich wastewater at pH 2.3 via mixed tooeleite and (Cl(-I)-free) ferric arsenite hydroxychloride formation

An advanced cost-saving method of removal of high-As(III) from SO4(-II)-rich metallurgical wastewater has been developed by diluting the SO4(-II) content with As(III)-Cl(-I)-rich metallurgical wastewater and then by the direct precipitation of As(III) with Fe(III) at pH 2.3. As(III) removal at various SO4(-II)/Cl(-I) molar ratios and temperatures was investigated. The results showed that 65.2‒98.2% of As(III) immobilization into solids occurred at the SO4(-II)/Cl(-I) molar ratios of 1:1‒32 and 15‒60 °C in 3 days, which were far higher than those in aqueous sole SO4(-II) or Cl(-I) media at the equimolar SO4(-II) or Cl(-I) and the same temperature. SO4(-II)/Cl(-I) molar ratio of 1:4 and 25 °C were optimal conditions to reach the As removal maximum. Mixed aqueous SO4(-II) and Cl(-I) played a synergetic role in the main tooeleite formation together with (Cl(-I)-free) ferric arsenite hydroxychloride (FAHC) involving the substitution of AsO33− for Cl(-I) for enhanced As fixation. The competitive complexation among FeH2AsO32+, FeSO4+ and FeCl2+ complexes was the main mechanism for the maximum As(III) precipitation at the SO4(-II)/Cl(-I) molar ratio of 1:4. Low As(III) immobilization at high temperature with increased Fe(III) hydrolysis was due to the formation of As(III)-bearing ferrihydrite with the relatively high Fe/As molar ratio at acidic pH.

Effect of co-existent Al(III) in As-rich Acid Mine Drainage (AMD) on As removal during Fe(II) and As(III) abiotic oxidation process

The effect of co-existent Al(III) in As-rich AMD on As removal via oxygen-induced abiotic oxidation caused concern due to weak affinity between Al(III) hydroxide and As(III). The removal of As from As-rich AMD with and without Al(III) at various pH and Fe/(Al)/As molar ratios was investigated. The results showed that 92.3–99.8% of As in Al(III)-containing systems was removed by 70–100% of Fe(II) and 40.2–67.8% of As(III) oxidation in 1–4 h at pH 3–9 and Fe/Al/As molar ratios of 2–8:1:1. Arsenic removal efficiencies generally increased with increasing the pH and Fe/Al/As molar ratio. Arsenic oxidation efficiencies first increased from pH 3 to 5 and then decreased from pH 5 to 9, while an opposite trend occurred with increasing the Fe/Al/As molar ratio. As(III) oxidation was controlled by ROS contents involving the Fenton-like reaction between Fe(II) and O2. Ferric arsenite hydroxysulfate, ferric arsenate and As-bearing ferrihydrite+schwertannite were the main As sinks. Co-existent Al(III) promoted As(III) oxidation. The proposed method for aqueous As removal with the total As (As(T)) leaching solid product at pH 5 and Fe/Al/As molar ratio of 8:1:1 can bring the As level to the required limit as emphasized in regulatory frameworks.

Arsenic (III) Removal from a High-Concentration Arsenic (III) Solution by Forming Ferric Arsenite on Red Mud Surface

Arsenic (As) is considered one of the most serious inorganic pollutants, and the wastewater produced in some smelters contains a high concentration of arsenic. In this paper, we purified the high-concentration arsenic solution with red mud and Fe3+ synergistically. In this system, arsenite anions reacted with Fe(III) ions to form ferric arsenite, which attached on the surface of red mud particles. The generated red mud/Fe1−x(As)x(OH)3 showed a better sedimentation performance than the pure ferric arsenite, which is beneficial to the separation of arsenic from the solution. The red mud not only served as the carrier, but also as the alkaline agent and adsorbent for arsenic treatment. The effects of red mud dosage, dosing order, pH, and molar ratio of Fe/As on arsenic removal were investigated. The efficiency of arsenic removal increased from a pH of 2 to 6 and reached equilibrium at a pH of 7. At the Fe/As molar ratio of 3, the removal efficiency of arsenic ions with an initial concentration of 500 mg/L reached 98%. In addition, the crystal structure, chemical composition, and morphological properties of red mud and arsenic removal residues (red mud/Fe1−x(As)x(OH)3) were characterized by XRD, XPS, X-ray fluorescence (XRF), SEM-EDS, and Raman spectroscopy to study the mechanism of arsenic removal. The results indicated that most of the arsenic was removed from the solution by forming Fe1−x(As)x(OH)3 precipitates on the red mud surface, while the remaining arsenic was adsorbed by the red mud and ferric hydroxide.

Geochemical fractionation and mineralogy of metal(loid)s in abandoned mine soils: Insights into arsenic behaviour and implications to remediation

Trace element contamination from abandoned mine sites is a major threat to the environment. The distribution of trace elements in various particle size fractions of soils from abandoned mine sites plays a critical role in designing remediation approaches. This study investigated the geochemical distribution of trace element enrichment and mineralogical composition in various particle size fractions from contrasting abandoned mine sites (Webbs Consols, Halls Peak and Mole River, Australia). Results revealed that arsenic and other element concentrations increased with decreasing particle size for samples from Webbs Consols and Halls Peak. The highest arsenic (3.05%), lead (3.23%) and zinc (1110 mg/kg) were found in the finest fraction (<0.053 mm). In Mole River, the highest concentration of arsenic (10.8%), lead (209 mg/kg) and zinc (351 mg/kg) were observed in coarse fractions. Arsenic fractionation by sequential extraction showed that arsenic was strongly associated with the amorphous and crystalline iron phases. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies revealed that tooeleite (a ferric arsenite mineral, also confirmed by Transmission electron microscopy (TEM)), arsenopyrite, scorodite and arsenolite were the dominant arsenic minerals. The study showed elevated levels of arsenic bearing minerals across particle sizes which has significant implications for remediation approaches at abandoned mine sites.

Formation and stability of biogenic tooeleite during Fe(II) oxidation by Acidithiobacillus ferrooxidans.

Tooeleite is the only known ferric arsenite sulfate mineral and has environmental significance for arsenic remediation. This study investigated the formation and stability of biogenic tooeleite in Fe(II)-As(III)-SO42- environment using Acidithiobacillus ferrooxidans under the ambient conditions. The results show that bacteria facilitated the formation and crystallization of tooeleite owing to the microbial oxidation of Fe(II) to Fe(III). Due to the better growth of bacteria, the higher removal of As(III) by tooeleite formation was achieved under 8.978-10.806g/L initial Fe(II) concentration and 2.00-3.00 initial pH, and the highest efficiency was ~95%. Fe(III) and As(III) precipitated simultaneously into two types of tooeleite. The relatively stable tooeleite is featured by the developed (020) crystal face and the bulk-like structure with thick flakes. This study yields a better understanding of biogenic tooeleite, and the importance of tooeleite formation in As(III)-rich environment for arsenic remediation.

Arsenic(III) biotransformation to tooeleite associated with the oxidation of Fe(II) via Acidithiobacillus ferrooxidans

Tooeleite (Fe6(AsO3)4(SO4)(OH)4·4H2O), the only known ferric arsenite sulfate bearing mineral, has great potential for arsenic remediation due to its structure favoring incorporation of As(III). Based on the natural attenuation of removing As(III) directly by the formation of tooeleite via microorganisms, an iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans ATCC 23270 (At.ferrooxidans) was selected to facilitate the formation of tooeleite. The optimized condition for the biogenic tooeleite was obtained at pH of 2.0, 30 °C and an initial arsenic of 500 mg/L. The process of biological mineralization is accompanied by the removal of 95.4% arsenic. What’s more, biosynthetic tooeleite crystallization via a three-stage process was revealed using a combination of liquid and solid analyses (ICP-OES, XRD, XPS, FT-IR, SEM, STEM, particle distribution). The three stages included Fe2+ oxidation by At.ferrooxidans, Fe3+ hydrolysis and an initial Fe–As amorphous precursors formation, and finally transforming to tooeleite crystal. Moreover, RT-qPCR was used to reveal the relationship between functional gene expression of At.ferrooxidans and the mineral formation. The results showed the biogenic tooeleite exerts significant control on the geochemistry of arsenic contaminated systems.

Characterization of Fe5(AsO3)3Cl2(OH)4·5H2O, a new ferric arsenite hydroxychloride precipitated from FeCl3–As2O3–HCl solutions relevant to arsenic immobilization

Tooeleite (Fe6(AsO3)4SO4(OH)4·4H2O) is widely precipitated for direct As(III) removal from sulfate-rich industrial effluents. However, whether or not Fe(III)–As(III)–Cl(-I) precipitate is produced in chloridizing leaching media for As immobilization is almost unknown. This work founded the existence of ferric arsenite (hydroxy)chloride as a new mineral for As(III) removal. Its chemical composition and solid characterization were subsequently studied by using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), infrared (FT-IR), Raman spectroscopy and thermogravimetric (TG) curve. The results showed the formation of a yellow precipitate after 3-days reaction of Fe(III)/As(III) with molar ratio ≈ 1.7 in chloride solution at pH 2.3 neutralized with NaOH. Compared with tooeleite, chemical analysis and solid characterization indicated that Cl(-I) replaces SO4(-II) producing ferric arsenite hydroxychloride with formula Fe5(AsO3)3Cl2(OH)4·5H2O. This new plate shaped solid showed better crytallinity than tooeleite, although it has similar morphology and characteristic bands to tooeleite. The FT-IR bands at 628, 964 cm−1 and the Raman bands at 448, 610, 961 cm−1 were assigned to Fe–O or As(III)–O–Fe or As(III)–O bending/stretching vibration, indicating that both arsenite and chloride substituted for the position of sulfate for ferric arsenite hydroxychloride produced due to the lack of the SO42− vibrations. Cl-(I) also contributed to increase As removal efficiency in aqueous sulfate media under acidic pH conditions via the probable formation of sulfate-chloride ferric arsenite.

Enhanced stability of tooeleite by hydrothermal method for the fixation of arsenite

A novel method was proposed in this study to increase the stability of tooeleite, which is a ferric arsenite sulfate mineral, by synthesizing pure and well-crystallized tooeleite using the hydrothermal method. The effects of temperature, pH and Fe/As were systematically studied; the optimized tooeleite can be obtained under the conditions of pH1.6, Fe/As 1.5, and 120°C. Mössbauer spectrum was employed, which proved that the acquired tooeleite was pure and did not contain amorphous phases. The short-term stability of tooeleite was tested using Toxicity Characteristic Leaching Procedure, which exhibited relatively good stability compared with that synthesized by the currently existing method. A silica coating method was further utilized to help the tooeleite successfully reach the standard. Finally, the competitive formation of tooeleite and natrojarosite that occured during the hydrothermal process was researched and the preferential formation range of tooeleite was acquired. This study suggests that tooeleite has the potential to be used in the fixation of arsenite.

Formation of tooeleite and the role of direct removal of As(III) from high-arsenic acid wastewater

In this study, an earth-mimetic method was proposed for the direct removal of As(III) by the formation of tooeleite, a ferric arsenite sulfate mineral. A series of batch experiments was used to study the relationship between the formation of tooeleite and the removal of As(III). The results indicate that As(III) removal efficiency reached up to 99% under the treatment condition of pH 1.8⿿4.5, initial As(III) concentration higher than 0.75g/L, and Fe/As ranged from 0.8 to 2 at room temperature. Various characterizations confirm that the precipitate obtained by this treatment was tooeleite with relatively high stability. In addition, it is assumed that ferrihydrite exists as a precursor, which is vital to the formation of tooeleite and the removal of As(III). This study suggests that tooeleite formation may be an alternative method in the direct removal of As(III) from high-arsenic acid wastewater.

Spectroscopic characterization and solubility investigation on the effects of As(V) on mineral structure tooeleite (Fe6(AsO3)4SO4(OH)4·H2O)

Tooeleite is an unique ferric arsenite sulfate mineral, which has the potential significance of directly fixing As(III) as mineral trap. The tooeleite and various precipitates were hydrothermally synthesized under the different of initial As(III)/As(V) molar ratios and characterized by XRD, FTIR, XPS and SEM. The crystallinity of tooeleite decreases with the amount of As(V). The precipitate is free of any crystalline tooeleite at the level of that XRD could detect when the ratio of As(III)/As(V) of 7:3 and more. The characteristic bands of tooeleite are observed in 772, 340, 696 and 304cm−1, which are assigned to the ν1, ν2, ν3 and ν4 vibrations of AsO33−. These intensities of bands gradually decreases with the presence of As(V) and its increasing. An obviously wide band is observed in 830cm−1, which is the ν1 vibration of AsO4. The result of XPS reveals that the binding energies of As3d increase from 44.0eV to 45.5eV, which indicates that the amount of As(V) in the precipitates increases. The concentrations of arsenic released of these precipitates are 350–650mg/L. The stability of tooeleite decreases by comparison when the presence of coexisting As(V) ions.

The mineral tooeleite Fe6(AsO3)4SO4(OH)4⋅4H2O – An infrared and Raman spectroscopic study-environmental implications for arsenic remediation

The mineral tooeleite Fe6(AsO3)4SO4(OH)4⋅4H2O is secondary ferric arsenite sulphate mineral which has environmental significance for arsenic remediation because of its high stability in the regolith. The mineral has been studied by X-ray diffraction (XRD), infrared (IR) and Raman spectroscopy. The XRD result indicates tooeleite can form more crystalline solids in an acid environment than in an alkaline environment. Infrared spectroscopy identifies moderately intense band at 773cm−1 assigned to AsO33- symmetric stretching vibration. Raman spectroscopy identifies three bands at 803, 758 and 661cm−1 assigned to the symmetric and antisymmetric stretching vibrations of AsO33- and As-OH stretching vibration respectively. In addition, the infrared bands observed at 1116, 1040, 1090, 981 and 616cm−1, are assigned to the ν3, ν1 and ν4 modes of SO42-. The same bands are observed at 1287, 1085, 983 and 604cm−1 in the Raman spectrum. As3d band at binding energy of 44.05eV in XPS confirms arsenic valence of tooeleite is +3. These characteristic bands in the IR and Raman spectra provide useful basis for identifying the mineral tooeleite.

Removal of As(III) from acidic waters using schwertmannite: Surface speciation and effect of synthesis pathway

The removal of arsenic(III) from aqueous solution by three different schwertmannite specimens that were synthesized by i) biogenic precipitation (SHM_MS), ii) slow abiotic dialysis (SHM_DS) and iii) fast abiotic oxidation of Fe(II) (SHM_FS) was studied through sorption experiments at pH 3.0. Marked differences in dissolution rates, Fe:S ratios (4.7–6.4) and structural properties were found among the specimens. The sorption mechanisms are governed by multilayer processes as indicated by highly non-linear Freundlich adsorption isotherms. The affinities increased in the sequence SHM_FS ( K F = 0.02 L 0.57/(mol − 0.43 g)) < SHM_MS ( K F = 0.04 L 0.64/(mol − 0.36 g)) < SHM_DS ( K F = 0.10 L 0.66/(mol − 0.34 g)) possibly owing to the corresponding increase in specific surface area (5.3, 14.7, and 210 m 2/g, respectively). XANES results show that no detectable oxidation of As(III) occurred on any of the schwertmannite surfaces within 5 days equilibration time even though oxidation both by O 2 and Fe(III) was favorable thermodynamically. A relationship between As(III) uptake and sulfate release was observed at high initial As(III) concentrations only suggesting that As(III) retention through ligand exchange is of minor relevance under the experimental conditions examined in this study. Higher As(III) loading caused morphological degradation yielding angular particles with porous centers and the extent of degradation varied accordingly with As(III) partitioning. Appearance of new infrared absorption bands after As(III) sorption and shifting in diffractogram peaks with increasing As(III) concentrations suggest the formation of a ferric arsenite surface precipitate.

Confirmation that tooeleite is a ferric arsenite sulfate hydrate, and is relevant to arsenic stabilisation

The mineral tooeleite is without doubt an arsenic(III) mineral, as declared by Morin et al. [Morin, G., Rousse, G., Elkaim, E., 2007. Crystal structure of tooeleite, Fe 6(AsO 3) 4SO 4(OH) 4 · 4H 2O, a new iron arsenite oxyhydroxy-sulfate mineral relevant to acid mine drainage. American Mineralogist 92, 193–197], whereas in previously published work by Cesbron and Williams [Cesbron, F.P., Williams, S.A., 1992. Tooeleite, a new mineral from the US Mine, Tooele County, Utah. Mineralogical Magazine 56, 71–73] the arsenic was considered to contain arsenic(V). There has also been suggestion that in certain acid mine drainage there is evidence of bacterial mediation to form nano-particular and poorly ordered iron(III)–arsenic(III)/(V) hydroxy sulfates which may be efficient bioremediators, as in Duquesne et al. [Duquesne, K., Lebrun, S., Casiot, C., Bruneel, O., Personné, J.-C., Leblanc, M., Elbaz-Poulichet, F., Morin, G., Bonnefoy, V., 2003. Immobilization of arsenite and ferric iron by Acidithiobacillus ferrooxidans and its relevance to acid mine drainage. Applied Environmental Microbiology 69, 6165–6173], and Morin et al. [Morin, G., Juillot, F., Bruneel, O., Personné, J.-C., Elbaz-Poulichet, F., Leblanc, M., Ildefonse, Ph., Calas, G., 2003. Bacterial formation of tooeleite and mixed As(III)/(V)–Fe(III) gels in the Carnoulès acid mine drainage. France. A XANES XRD, and SEM study. Environmental Science and Technology 37, 1705–1712]. Tooeleite was originally described by Cesbron and Williams (1992) as a new mineral from the US Mine at Gold Hill in Tooele County, Utah, USA, where gold had previously been mined for many years. These authors suggested a formula in which the arsenic was present as arsenic(V). The mineral occurrences at that US Mine had been earlier described by Nolan [Nolan, T.B., 1935. The Gold Hill Mining District. U.S.G.S. Professional Paper 177 pp.]. The work reported here now confirms that the mineral tooeleite is in fact an arsenic(III) compound which can be easily prepared in the laboratory by precipitation from pure components in aqueous solutions at temperatures up to at least 90 °C. Other conclusions reached here are supported with the results from analysis of solutions and solids, as well as by X-ray diffraction analysis, thermogravimetric-differential thermal analysis, scanning electron microscopy, and also by X-ray photoelectron spectroscopy analysis.

Bacterial formation of tooeleite and mixed arsenic(III) or arsenic(V)-iron(III) gels in the Carnoulès acid mine drainage, France. A XANES, XRD, and SEM study.

The oxidation of Fe(II) in acid mine drainage (AMD) leads to the precipitation of Fe(III) compounds which may incorporate toxic elements, such as arsenic (As), within their structure or adsorb them at their surface, thus limiting their mobility. The present work provides evidence for spatial and seasonal variations of microbial activity that influence arsenite oxidation and As immobilization in the heavily contaminated AMD from the Carnoulès mine, Gard, France ([As III] = 80 to 280 mg x L(-1) in the acidic spring draining the waste-pile). In the first tens of meters of the AMD, the rapid oxidation of Fe(II) leads to the coprecipitation of large amounts of As with Fe(III) in bacterial mats. XRD, XANES, and SEM analyses of sediments and stromatolite samples revealed the unusual formation of As(III)-rich compounds, especially nanocrystalline tooeleite, Fe6(AsO3)4(SO4)(OH)4 x 4H2O, a rare ferric arsenite sulfate oxy-hydroxide mineral, together with XRD-amorphous mixed As(III)/As(V)-Fe(III) oxy-hydroxide compounds. In the wet season, the suspended sediments of the upstream zone essentially consist of tooeleite associated with am-As(III)-Fe(III) oxy-hydroxides, while am-As(V)-Fe(III) oxy-hydroxides, having As:Fe molar ratios as high as 0.6-0.8, dominate in the dry season. Comparing natural and bioassay samples revealed that the formation of As(III)-rich compounds in the wet season may be related to the metabolic activity of bacterial strains able to oxidize Fe(II) but not As(III). One of these strains, having an Acidithiobacillus ferrooxidans genotype, has been isolated from the Carnoulès AMD. In contrast, the formation of As(V)-rich compounds in the dry season can be related to both biotic and abiotic oxidation of As(III) to As(V). Some Thiomonas strains isolated from the Carnoulès AMD were shown to be able to catalyze the oxidation of As(III) to As(V) in solution. Therefore, they can promote the formation of mixed As(V)-Fe(III) oxy-hydroxides, provided enough Fe(II) oxidizes. These results yield a better understanding of natural processes at this site and may help in designing efficient As-removal processes.