Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH.

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO3- solutions at circumneutral pH. The accumulation of U in plants (3.1-21.3 mg kg-1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30-700 µg L-1) and Ca (0-240 mg L-1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V0) ranged from 4.4 to 62 µg g-1 h-1 in experiments with no added Ca and from 0.73 to 2.07 µg g-1 h-1 in experiments with 12 mg L-1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L-1 Ca. Ternary Ca-U-CO3 complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy-energy-dispersive spectrometry detected U-P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Arsenic remediation by formation of arsenic sulfide minerals in a continuous anaerobic bioreactor.

Arsenic (As) is a highly toxic metalloid that has been identified at high concentrations in groundwater in certain locations around the world. Concurrent microbial reduction of arsenate (As(V) ) and sulfate (SO4 (2-) ) can result in the formation of poorly soluble arsenic sulfide minerals (ASM). The objective of this research was to study As biomineralization in a minimal iron environment for the bioremediation of As-contaminated groundwater using simultaneous As(V) and SO4 (2-) reduction. A continuous-flow anaerobic bioreactor was maintained at slightly acidic pH (6.25-6.50) and fed with As(V) and SO4 (2-) , utilizing ethanol as an electron donor for over 250 d. A second bioreactor running under the same conditions but lacking SO4 (2-) was operated as a control to study the fate of As (without S). The reactor fed with SO4 (2-) removed an average 91.2% of the total soluble As at volumetric rates up to 2.9 mg As/(L · h), while less than 5% removal was observed in the control bioreactor. Soluble S removal occurred with an S to As molar ratio of 1.2, suggesting the formation of a mixture of orpiment- (As2 S3 ) and realgar-like (AsS) solid phases. Solid phase characterization using K-edge X-ray absorption spectroscopy confirmed the formation of a mixture of As2 S3 and AsS. These results indicate that a bioremediation process relying on the addition of a simple, low-cost electron donor offers potential to promote the removal of As from groundwater with naturally occurring or added SO4 (2-) by precipitation of ASM.

Post Gold King Mine Spill Investigation of Metal Stability in Water and Sediments of the Animas River Watershed.

We applied spectroscopy, microscopy, diffraction, and aqueous chemistry methods to investigate the persistence of metals in water and sediments from the Animas River 13 days after the Gold King Mine spill (August 5, 2015). The Upper Animas River watershed, located in San Juan Colorado, is heavily mineralized and impacted by acid mine drainage, with low pH water and elevated metal concentrations in sediments (108.4 ± 1.8 mg kg-1 Pb, 32.4 ± 0.5 mg kg-1 Cu, 729.6 ± 5.7 mg kg-1 Zn, and 51 314.6 ± 295.4 mg kg-1 Fe). Phosphate and nitrogen species were detected in water and sediment samples from Farmington, New Mexico, an intensive agricultural area downstream from the Animas River, while metal concentrations were low compared to those observed upstream. Solid-phase analyses of sediments suggest that Pb, Cu, and Zn are associated with metal-bearing jarosite and other minerals (e.g., clays, Fe-(oxy)hydroxides). The solubility of jarosite at near-neutral pH and biogeochemical processes occurring downstream could affect the stability of metal-bearing minerals in river sediments. This study contributes relevant information about the association of metal mixtures in a heavy mineralized semiarid region, providing a foundation to better understand long-term metal release in a public and agricultural water supply.

Cerium oxide and neodymium oxide phytoextraction by ryegrass in bioenhanced hydroponic environments.

Sustainable technologies for the recovery of rare earth elements (REE) from waste need to be developed to decrease the volume of ore mining extractions and its negative environmental consequences, while simultaneously restoring previously impacted lands. This is critical due to the extensive application of REE in everyday life from electronic devices to energy and medical technologies, and the dispersed distribution of REE resources in the world. REE recovery by plants has been previously studied but the feasibility of REE phytoextraction from a poorly soluble solid phase (i.e., nanoparticles) by different plant species has been rarely investigated. In this study, the effect of biostimulation and bioaugmentation on phytorecovery of REE nanoparticles (REE-NP) was investigated by exposing ryegrass seeds to REE-NP in hydroponic environments. This was studied in two sets of experiments: bioaugmentation (using CeO2 nanoparticles and Methylobacterium extorquens AM1 pure culture), and biostimulation (using CeO2 or Nd2O3 nanoparticles and endogenous microorganisms). Addition of M. extorquens AM1 in bioaugmentation experiment including 500 mg/L CeO2 nanoparticles could not promote the nanoparticles accumulation in both natural and surface-sterilized treatments. However, it enhanced the translocation of Ce from roots to shoots in sterile samples. Moreover, another REE-utilizing bacterium, Bacillus subtilis, was enriched more than M. extorquens in control samples (no M. extorquens AM1), and associated with 52% and 14% higher Ce extraction in both natural (165 µg/gdried-plant) and surface-sterilized samples (136 µg/gdried-plant), respectively; showing the superior effect of endogenous microorganisms' enrichment over bioaugmentation in this experiment. In the biostimulation experiments, up to 705 µg/gdried-plant Ce and 19,641 µg/gdried-plant Nd could be extracted when 500 mg/L REE-NP were added. Furthermore, SEM-EDS analysis of the surface and longitudinal cross-sections of roots in Nd2O3 treatments confirmed surface and intracellular accumulation of Nd2O3-NP. These results demonstrate stimulation of endogenous microbial community can lead to an enhanced REE phytoaccumulation.

Evaluation and optimization of FTIR spectroscopy to quantify PHA production by municipal wastewater sludge.

Polyhydroxyalkanoate (PHA) is a family of naturally-occurring biopolymers synthesized by more than 300 microorganisms in the environment. These biopolymers have been investigated as a source material to substitute fossil fuel-based polymers; hence the synthesis of biopolymers and their characterization is a critical step in optimizing the process. Because of this, the biological production of PHA using PHA-producing microorganisms is currently the dominating process; however, the use of microbial mixed culture (MMC), such as wastewater sludge, is gaining attention. Different than pure cultures, MMC has higher culturing condition tolerance since the complex species composition and is easily obtained from wastewater treatment plants, which shortens the culturing time, lowers the cost, and promotes the application. The main constraint in MMC-based PHA is the extraction and quantification of PHA from the more complex matrix. In this paper, Fourier-transform infrared (FTIR) spectroscopy is evaluated to be used as a quantification method of PHA in MMC systems. Firstly, commercially available analytical standards, which consist of PHA/PHB, and two different solvents (chloroform and dichloromethane), were used and tested by this method, with KBr card and liquid cell methods, and the results are validated by gas chromatography mass spectrometry (GC/MS). The method was then tested using 12 samples from wastewater treatment plants. The PHA content in biomass varied from 3.42 w/w% to 1.22 w/w% following extraction with chloroform as solvent as determined by this method. In the four different combination standards, the best one is consisted of PHB and chloroform, and FTIR-liquid cell showed higher promise for PHA quantification in complex matrices.

Biointeraction of cerium oxide and neodymium oxide nanoparticles with pure culture Methylobacterium extorquens AM1.

Rare earth elements (REE) are valuable raw materials in our modern life. Extensive REE application from electronic devices to medical instruments and wind turbines, and non-uniform distribution of these resources around the world, make them strategically and economically important for countries. Current REE physical and chemical mining and recycling methods could have negative environmental consequences, and biologically-mediated techniques could be applied to overcome this issue. In this study, the bioextraction of cerium oxide and neodymium oxide nanoparticles (REE-NP) by a pure culture Methylobacterium extorquens AM1 (ATCC®14718™) was investigated in batch experiments. Results show that adding up to 1000 ppm CeO2 or Nd2O3 nanoparticles (REE-NP) did not seem to affect the bacterial growth over 14-days contact time. Effect of methylamine hydrochloride as an essential electron donor and carbon source for microbial oxidation and growth was also observed inasmuch as there was approximately no growth when it does not exist in the medium. Although very low concentrations of cerium and neodymium in the liquid phase were measured, concentrations of 45 µg/gcell Ce and 154 µg/gcell Nd could be extracted by M. extorquens AM1. Furthermore, SEM-EDS and STEM-EDS confirmed surface and intracellular accumulation of nanoparticles. These results confirmed the ability of M. extorquens to accumulate REE nanoparticles.

Leaching composition and associated microbial community of recycled concrete aggregate (RCA).

Recycled concrete aggregate (RCA) has been used as an alternative sustainable material in the construction industry, but RCA long-term environmental impacts are unknown. In this study, the bacterial enrichment potential to reduce the alkalinity of two different types of RCA was examined, from laboratory-produced concrete and from a stockpile of demolished concrete that had been in service in transportation applications. Washed and un-washed lab and field RCA were biostimulated by being exposed to ATCC® Medium 661 in batch experiments. pH, metal composition and microbial community changes in the leachates were monitored over time. Results show that initial pH of field RCA leachate could be decreased to less concerning values, as low as 8, but concentrations of some metals in the leachate exceeded groundwater quality standards. However, the biostimulated RCA released lower metal concentration and was more resistant to pH increases than non-biostimulated RCA during a long-term leaching experiment with DI water. The microbial community was enriched on anaerobic, halotolerant and alkaliphile microorganisms, resistant to extreme environmental conditions. The outcome of this research suggests a baseline for field RCA pretreatment before field application, using a biostimulation method that would generate a less environmentally detrimental runoff.

Flexible bacterial strains that oxidize arsenite in anoxic or aerobic conditions and utilize hydrogen or acetate as alternative electron donors.

Arsenic is a carcinogenic compound widely distributed in the groundwater around the world. The fate of arsenic in groundwater depends on the activity of microorganisms either by oxidizing arsenite (As(III)), or by reducing arsenate (As(V)). Because of the higher toxicity and mobility of As(III) compared to As(V), microbial-catalyzed oxidation of As(III) to As(V) can lower the environmental impact of arsenic. Although aerobic As(III)-oxidizing bacteria are well known, anoxic oxidation of As(III) with nitrate as electron acceptor has also been shown to occur. In this study, three As(III)-oxidizing bacterial strains, Azoarcus sp. strain EC1-pb1, Azoarcus sp. strain EC3-pb1 and Diaphorobacter sp. strain MC-pb1, have been characterized. Each strain was tested for its ability to oxidize As(III) with four different electron acceptors, nitrate, nitrite, chlorate and oxygen. Complete As(III) oxidation was achieved with both nitrate and oxygen, demonstrating the novel ability of these bacterial strains to oxidize As(III) in either anoxic or aerobic conditions. Nitrate was only reduced to nitrite. Different electron donors were used to study their suitability in supporting nitrate reduction. Hydrogen and acetate were readily utilized by all the cultures. The flexibility of these As(III)-oxidizing bacteria to use oxygen and nitrate to oxidize As(III) as well as organic and inorganic substrates as alternative electron donors explains their presence in non-arsenic-contaminated environments. The findings suggest that at least some As(III)-oxidizing bacteria are flexible with respect to electron-acceptors and electron-donors and that they are potentially widespread in low arsenic concentration environments.

Mobilization of As, Fe, and Mn from Contaminated Sediment in Aerobic and Anaerobic Conditions: Chemical or Microbiological Triggers?

We integrated aqueous chemistry, spectroscopy, and microbiology techniques to identify chemical and microbial processes affecting the release of arsenic (As), iron (Fe), and manganese (Mn) from contaminated sediments exposed to aerobic and anaerobic conditions. The sediments were collected from Cheyenne River Sioux Tribal lands in South Dakota, which has dealt with mining legacy for several decades. The range of concentrations of total As measured from contaminated sediments was 96 to 259 mg kg-1, which co-occurs with Fe (21 000-22 005 mg kg-1) and Mn (682-703 mg kg-1). The transition from aerobic to anaerobic redox conditions yielded the highest microbial diversity, and the release of the highest concentrations of As, Fe, and Mn in batch experiments reacted with an exogenous electron donor (glucose). The reduction of As was confirmed by XANES analyses when transitioning from aerobic to anaerobic conditions. In contrast, the releases of As, Fe and Mn after a reaction with phosphate was at least 1 order of magnitude lower compared with experiments amended with glucose. Our results indicate that mine waste sediments amended with an exogenous electron donor trigger microbial reductive dissolution caused by anaerobic respiration. These dissolution processes can affect metal mobilization in systems transitioning from aerobic to anaerobic conditions in redox gradients. Our results are relevant for natural systems, for surface and groundwater exchange, or other systems in which metal cycling is influenced by chemical and biological processes.

Coupled high and low-frequency ultrasound remediation of PFAS-contaminated soils.

Solids such as soils and sediments contaminated with per- and polyfluorinated alkyl substances (PFAS) from exposure to impacted media, e.g., landfill leachate or biosolids, direct contaminated discharge, and contaminant transport from atmospheric deposition, have caused significant environmental pollution. Such solids can act as secondary sources of PFAS for groundwater and surface water contamination. There are currently no proven technologies that can degrade PFAS in soil and sediments in a cost-effective, environmentally-friendly, and energy-efficient manner. This study examines the use of coupled high and low-frequency ultrasound in desorbing and degrading PFAS in soil, thereby achieving concurrent treatment and destruction of PFAS in soil. Two common PFAS, namely perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), were used to evaluate treatment performance in soils with both low and high organic matter contents. The test results showed that the ultrasound treatment could significantly reduce PFAS concentrations in artificially contaminated soil; however, no significant degradation was achieved. Ultrasound treatment did improve desorption of PFAS from solid particles, particularly from the highly absorbent organic soil; 68.8 ± 1.8% of PFOA and 45.4 ± 4.1% of PFOS were leached from the soil after ultrasound treatment compared to only 28 ± 0.2% of PFOA and 1 ± 3.1% of PFOSafter desorption in water. This work shows that sonication treatment is an effective technology for the removal of PFAS from solids, however, the presence of solids in the solid-liquid slurry can negatively impact ultrasonic cavitation, inhibiting the sonolytic degradation of desorbed PFAS.

Emerging investigator series: entrapment of uranium-phosphorus nanocrystals inside root cells of Tamarix plants from a mine waste site.

We investigated the mechanisms of uranium (U) uptake by Tamarix (salt cedars) growing along the Rio Paguate, which flows throughout the Jackpile mine near Pueblo de Laguna, New Mexico. Tamarix were selected for this study due to the detection of U in the roots and shoots of field collected plants (0.6-58.9 mg kg-1), presenting an average bioconcentration factor greater than 1. Synchrotron-based micro X-ray fluorescence analyses of plant roots collected from the field indicate that the accumulation of U occurs in the cortex of the root. The mechanisms for U accumulation in the roots of Tamarix were further investigated in controlled-laboratory experiments where living roots of field plants were macerated for 24 h or 2 weeks in a solution containing 100 µM U. The U concentration in the solution decreased 36-59% after 24 h, and 49-65% in two weeks. Microscopic and spectroscopic analyses detected U precipitation in the root cell walls near the xylems of the roots, confirming the initial results from the field samples. High-resolution TEM was used to study the U fate inside the root cells, and needle-like U-P nanocrystals, with diameter <7 nm, were found entrapped inside vacuoles in cells. EXAFS shell-by-shell fitting suggest that U is associated with carbon functional groups. The preferable binding of U to the root cell walls may explain the U retention in the roots of Tamarix, followed by U-P crystal precipitation, and pinocytotic active transport and cellular entrapment. This process resulted in a limited translocation of U to the shoots in Tamarix plants. This study contributes to better understanding of the physicochemical mechanisms affecting the U uptake and accumulation by plants growing near contaminated sites.

Effect of bicarbonate and phosphate on arsenic release from mining-impacted sediments in the Cheyenne River watershed, South Dakota, USA.

The mobilization of arsenic (As) from riverbank sediments affected by the gold mining legacy in north-central South Dakota was examined using aqueous speciation chemistry, spectroscopy, and diffraction analyses. Gold mining resulted in the discharge of approximately 109 metric tons of mine waste into Whitewood Creek (WW) near the Homestake Mine and Cheyenne River at Deal Ranch (DR), 241 km downstream. The highest concentrations of acid-extractable As measured from solid samples was 2020 mg kg-1 at WW and 385 mg kg-1 at DR. Similar sediment mineralogy between WW and DR was identified using XRD, with the predominance of alumino-silicate and iron-bearing minerals. Alkalinity measured in surface water at both sites ranged from 1000 to 2450 mg L-1 as CaCO3 (10-20 mM HCO3- at pH 7). Batch laboratory experiments were conducted under oxidizing conditions to evaluate the effects of NaHCO3 (0.2 mM and 20 mM) and NaH2PO3 (0.1 and 10 mM) on the mobilization of As. These ions are relevant for the site due to the alkaline nature of the river and nutrient mobilization from the ranch. The range of As(v) release with the NaHCO3 treatment was 17-240 µg L-1. However, the highest release (6234 µg L-1) occurred with 10 mM NaH2PO3, suggesting that As release is favored by competitive ion displacement with PO43- compared to HCO3-. Although higher total As was detected in WW solids, the As(v) present in DR solids was labile when reacted with NaHCO3 and NaH2PO3, which is a relevant finding for communities living close to the river bank. The results from this study aid in a better understanding of As mobility in surface water sites affected by the mining legacy.

Anoxia stimulates microbially catalyzed metal release from Animas River sediments.

The Gold King Mine spill in August 2015 released 11 million liters of metal-rich mine waste to the Animas River watershed, an area that has been previously exposed to historical mining activity spanning more than a century. Although adsorption onto fluvial sediments was responsible for rapid immobilization of a significant fraction of the spill-associated metals, patterns of longer-term mobility are poorly constrained. Metals associated with river sediments collected downstream of the Gold King Mine in August 2015 exhibited distinct presence and abundance patterns linked to location and mineralogy. Simulating riverbed burial and development of anoxic conditions, sediment microcosm experiments amended with Animas River dissolved organic carbon revealed the release of specific metal pools coupled to microbial Fe- and SO42--reduction. Results suggest that future sedimentation and burial of riverbed materials may drive longer-term changes in patterns of metal remobilization linked to anaerobic microbial metabolism, potentially driving decreases in downstream water quality. Such patterns emphasize the need for long-term water monitoring efforts in metal-impacted watersheds.

Uranium mobility and accumulation along the Rio Paguate, Jackpile Mine in Laguna Pueblo, NM.

The mobility and accumulation of uranium (U) along the Rio Paguate, adjacent to the Jackpile Mine, in Laguna Pueblo, New Mexico was investigated using aqueous chemistry, electron microprobe, X-ray diffraction and spectroscopy analyses. Given that it is not common to identify elevated concentrations of U in surface water sources, the Rio Paguate is a unique site that concerns the Laguna Pueblo community. This study aims to better understand the solid chemistry of abandoned mine waste sediments from the Jackpile Mine and identify key hydrogeological and geochemical processes that affect the fate of U along the Rio Paguate. Solid analyses using X-ray fluorescence determined that sediments located in the Jackpile Mine contain ranges of 320 to 9200 mg kg-1 U. The presence of coffinite, a U(iv)-bearing mineral, was identified by X-ray diffraction analyses in abandoned mine waste solids exposed to several decades of weathering and oxidation. The dissolution of these U-bearing minerals from abandoned mine wastes could contribute to U mobility during rain events. The U concentration in surface waters sampled closest to mine wastes are highest during the southwestern monsoon season. Samples collected from September 2014 to August 2016 showed higher U concentrations in surface water adjacent to the Jackpile Mine (35.3 to 772 µg L-1) compared with those at a wetland 4.5 kilometers downstream of the mine (5.77 to 110 µg L-1). Sediments co-located in the stream bed and bank along the reach between the mine and wetland had low U concentrations (range 1-5 mg kg-1) compared to concentrations in wetland sediments with higher organic matter (14-15%) and U concentrations (2-21 mg kg-1). Approximately 10% of the total U in wetland sediments was amenable to complexation with 1 mM sodium bicarbonate in batch experiments; a decrease of U concentration in solution was observed over time in these experiments likely due to re-association with sediments in the reactor. The findings from this study provide new insights about how hydrologic events may affect the reactivity of U present in mine waste solids exposed to surface oxidizing conditions, and the influence of organic-rich sediments on U accumulation in the Rio Paguate.

Sonochemical degradation of perfluorinated chemicals in aqueous film-forming foams.

Aqueous film-forming foams (AFFFs) are complex mixtures containing 1-5% w/w fluorocarbons (FCs). Here, we have investigated degradation of two commercial AFFF formulations, 3M and Ansul, using sound field at 500kHz and 1MHz, with varying initial concentrations ranging from 200 to 930× dilution. The foams were readily degraded by 1MHz, with percentage of defluorination ranging from 11.1±1.4% (200× dilution of 3M) to 47.1±5.8% (500× dilution of Ansul). Removal of total organic carbon (TOC) ranged from 16.0±1.4% (200× dilution Ansul) to 39.0±7.2% (500× dilution Ansul). Degradation of AFFF was affected by sound frequency with rates of defluorination 10-fold greater when the frequency was 1MHz than when it was 500kHz. Mineralization of TOC was 1.5- to 3.0-fold greater under 1MHz than 500kHz. Rate of fluoride release was 60% greater for the greatest initial concentration of FC in Ansul compared to the least initial concentration. While the rate of mineralization of AFFF was directly proportional to the initial concentration of Ansul, that was not the case for 3M, where the rates of mineralization were approximately the same for all three initial concentrations. Results of the study demonstrate that sonolysis is a promising technology to effectively treat AFFFs.

Effect of sound frequency and initial concentration on the sonochemical degradation of perfluorooctane sulfonate (PFOS).

Perfluoooctanesulfonic acid (PFOS) is a perfluorinated compound (PFC) highly resistant to conventional advance oxidation processes, which was widely used in industrial activities due to its surfactant nature, olephobic-hydrophobic properties, and chemical inertness. Sonochemical treatment has been suggested as an effective approach to treat aqueous solutions containing minimal levels of PFCs. This study investigates PFOS sonochemical degradation and its dependency on the initial concentration (10-460 µM), and the applied sound frequency (25 and 500 kHz, and 1 MHz). PFOS was degraded by sonochemical treatment at concentrations as high as 460 µM, as demonstrated by fluoride release and total organic content data. PFOS degradation rate was higher at megasonic frequencies (1MHz) compared to ultrasonic frequencies (25-500 kHz). PFOS degradation was controlled by saturation kinetics as indicated by an increase in PFOS degradation rate with increasing PFOS concentration until a maximum, after which the degradation rate was independent of the concentration. The saturation conditions were dependent on the sound frequency, and they were reached at a lower concentration under 1 MHz (100 µM) compared to the 500 kHz frequency (>460 µM). Overall, the results of this study demonstrate that high PFOS concentration can be effectively sonochemically treated using megasonic frequencies.

Adaptation of a methanogenic consortium to arsenite inhibition.

Arsenic (As) is a ubiquitous metalloid known for its adverse effects to human health. Microorganisms are also impacted by As toxicity, including methanogenic archaea, which can affect the performance of process in which biological activity is required (i.e. stabilization of activated sludge in wastewater treatment plants). The novel ability of a mixed methanogenic granular sludge consortium to adapt to the inhibitory effect of arsenic (As) was investigated by exposing the culture to approximately 0.92 mM of AsIII for 160 d in an arsenate (AsV) reducing bioreactor using ethanol as the electron donor. The results of shaken batch bioassays indicated that the original, unexposed sludge was severely inhibited by arsenite (AsIII) as evidenced by the low 50% inhibition concentrations (IC50) determined, i.e., 19 and 90 µM for acetoclastic- and hydrogenotrophic methanogenesis, respectively. The tolerance of the acetoclastic and hydrogenotrophic methanogens in the sludge to AsIII increased 47-fold (IC50 = 910 µM) and 12-fold (IC50= 1100 µM), respectively, upon long-term exposure to As. In conclusion, the methanogenic community in the granular sludge demonstrated a considerable ability to adapt to the severe inhibitory effects of As after a prolonged exposure period.

Biomineralization of arsenate to arsenic sulfides is greatly enhanced at mildly acidic conditions.

Arsenic (As) is an important water contaminant due to its high toxicity and widespread occurrence. Arsenic-sulfide minerals (ASM) are formed during microbial reduction of arsenate (As(V)) and sulfate (SO4(2-)). The objective of this research is to study the effect of the pH on the removal of As due to the formation of ASM in an iron-poor system. A series of batch experiments was used to study the reduction of SO4(2-) and As(V) by an anaerobic biofilm mixed culture in a range of pH conditions (6.1-7.2), using ethanol as the electron donor. Total soluble concentrations and speciation of S and As were monitored. Solid phase speciation of arsenic was characterized by x-ray adsorption spectroscopy (XAS). A marked decrease of the total aqueous concentrations of As and S was observed in the inoculated treatments amended with ethanol, but not in the non-inoculated controls, indicating that the As-removal was biologically mediated. The pH dramatically affected the extent and rate of As removal, as well as the stoichiometric composition of the precipitate. The amount of As removed was 2-fold higher and the rate of the As removal was up to 17-fold greater at pH 6.1 than at pH 7.2. Stoichiometric analysis and XAS results confirmed the precipitate was composed of a mixture of orpiment and realgar, and the proportion of orpiment in the sample increased with increasing pH. The results taken as a whole suggest that ASM formation is greatly enhanced at mildly acidic pH conditions.