Arsenic-rich acid mine water with extreme arsenic concentration: mineralogy, geochemistry, microbiology, and environmental implications

Efficient reductive recovery of arsenic from acidic wastewater by a UV/dithionite process

The removal of arsenic (As(III)) from acidic wastewater using neutralization or sulfide precipitation generates substantial arsenic-containing hazardous solid waste, posing significant environmental challenges. This study proposed an advanced ultraviolet (UV)/dithionite reduction method to recover As(III) in the form of valuable elemental arsenic (As(0)) from acidic wastewater, thereby avoiding hazardous waste production. The results showed that more than 99.9 % of As(III) was reduced to As(0) with the residual concentration of arsenic below 25.0 μg L-1 within several minutes when the dithionite/As(III) molar ratio exceeded 1.5:1 and the pH was below 4.0. The content of As(0) in precipitate reached 99.70 wt%, achieving the purity requirements for commercial As(0) products. Mechanistic investigations revealed that SO2·‒ and H· radicals generated by dithionite photolysis under UV irradiation are responsible for reducing As(III) to As(0). Dissolved O2, Fe(III), Fe(II), Mn(II), dissolved organic matter (DOM), and turbidity slightly inhibited As(III) reduction via free radicals scavenging or light blocking effect, whereas other coexisting ions, such as Mg(II), Zn(II), Cd(II), Ni(II), F(-I), and Cl(-I), had limited influence on As(III) reduction. Moreover, the cost of treating real arsenic-containing (250.3 mg L-1) acidic wastewater was estimated to be as low as $0.668 m-3, demonstrating the practical applicability of this method. This work provides a novel method for the reductive recovery of As(III) from acidic wastewater.

Enhanced extraction of heavy metals from gypsum-based hazardous waste by nanoscale sulfuric acid film at ambient conditions

Low-cost, low-energy extraction of heavy metal(loid)s (HMs) from hazardous gypsum cake is the goal of the metallurgical industry to mitigate environmental risks and carbon emissions. However, current extracting routes of hydrometallurgy often suffer from great energy inputs and substantial chemical inputs. Here, we report a novel solid-like approach with low energy consumption and chemical input to extract HMs by thin films under ambient conditions. Through constructing a nanoscale sulfuric acid film (NSF) of ∼50 nm thickness on the surface of arsenic-bearing gypsum (ABG), 99.6% of arsenic can be removed, surpassing the 50.3% removal in bulk solution. In-situ X-ray diffraction, infrared spectral, and ab initio molecular dynamics (AIMD) simulations demonstrate that NSF plays a dual role in promoting the phase transformation from gypsum to anhydrite and in changing the ionic species to prevent re-doping in anhydrite, which is not occurred in bulk solutions. The potential of the NSF is further validated in extracting other heavy metal(loid)s (e.g., Cu, Zn, and Cr) from synthetic and actual gypsum cake. With energy consumption and costs at 1/200 and 1/10 of traditional hydrometallurgy separately, this method offers an efficient and economical pathway for extracting HMs from heavy metal-bearing waste and recycling industrial solid waste.

In-situ immobilization of arsenic and antimony containing acid mine drainage through chemically forming layered double hydroxides

Acid mine drainage (AMD) rich in arsenic (As) and antimony (Sb) is considered as a significant environmental challenge internationally. However, simultaneous removal of As and Sb from AMD is still inadequately studied. In this study, a highly effective and simple approach was proposed for mitigating As and Sb-rich AMD, which involves in-situ formation of layered double hydroxides (LDHs). Following the treatment, the residual concentrations of iron (Fe), magnesium (Mg), sulfate, As and Sb in field AMD were decreased from their initial concentrations of 1690, 1524, 2055, 7.8 and 10.6 mg L-1, respectively, to 1.3, 12.4, 623, 0.006 and 0.004 mg L-1, respectively. Chemical formula of the resulting As and Sb-loaded LDHs can be identified as Mg4.226Fe2.024OH2SO4AsSb0.006∙mH2O. The dissolution rates of metal(loid)s in As and Sb-loaded LDH were lower than 1% under strongly acidic and alkaline environments. In presence of the mixed adsorbates, the As immobilization capacity by LDHs was significantly decreased, with an apparent intervention from Sb. However, As did not have a significant effect on the immobilization of Sb by LDH. As was immobilized by LDHs through anion exchange and complexation with -OH groups, while Sb was captured by anion exchange and complexation with [Formula: see text] . Density functional theory (DFT) calculations further proved the above conclusions. This novel approach is effective and can be applied for in-situ AMD treatment from abandoned mines.

Acid mine drainage (AMD) treatment using galvanic electrochemical system Al-Cu

Acid mine drainage was evaluated using a galvanic (GV) electrochemical system, Al-Cu (anode/cathode), based on a 32 factorial design. The factors analyzed were anodic area/volume ratios (A/V) of 0.037, 0.072, and 0.112 cm2/cm3, and treatment time from 0.25-8 h, and analyses were performed in duplicate with 11 degrees of freedom. The response variables were the total dissolved solids and concentrations of As, Cu, Co, Cr, Pb, Fe, Ni, and S O 4 2 - . The pH, electrical conductivity, and temperature were monitored during the process. Significant differences between treatments were determined by analysis of variance with Tukey's test (p < 0.05) using Statgraphics Centurion XVI.I software. The results showed that a greater electrode surface, A/V ratio, and treatment time improved pollutant removal. The spontaneous reactions generated by the galvanic cell, through the current that flows owing to the potential difference between the Al and Cu electrodes, allows the removal of heavy metals, arsenic, and S O 4 2 - by coagulation and precipitation mechanisms. The removal efficiencies achieved were Cu (99.1%), As (76.6%), Ni (80.2%), Pb (83.6%), Cr (100%), Fe (93.71%), and 92.9% for sulfates. The X-ray diffraction and Raman analyses of the solid fraction indicated that cuprite was formed with a purity of 96%, and the recovery of Cu by the GV system may be a viable option for mining companies.

Kinetics of Na- and K- uranyl arsenate dissolution

We integrated aqueous chemistry analyses with geochemical modeling to determine the kinetics of the dissolution of Na and K uranyl arsenate solids (UAs(s)) at acidic pH. Improving our understanding of how UAs(s) dissolve is essential to predict transport of U and As, such as in acid mine drainage. At pH 2, Na0.48H0.52(UO2)(AsO4)(H2O)2.5(s) (NaUAs(s)) and K0.9H0.1(UO2)(AsO4)(H2O)2.5(s) (KUAs(s)) both dissolve with a rate constant of 3.2 × 10-7 mol m-2 s-1, which is faster than analogous uranyl phosphate solids. At pH 3, NaUAs(s) (6.3 × 10-8 mol m-2 s-1) and KUAs(s) (2.0 × 10-8 mol m-2 s-1) have smaller rate constants. Steady-state aqueous concentrations of U and As are similarly reached within the first several hours of reaction progress. This study provides dissolution rate constants for UAs(s), which may be integrated into reactive transport models for risk assessment and remediation of U and As contaminated waters.

Molecular structures of dissolved and colloidal AsV-FeIII complexes and their roles in the mobilization of AsV under strongly acidic conditions

The effect of high concentration of iron (Fe^III) on the speciation and mobility of arsenic (As) under strongly acidic conditions remains unclear. This work studied the redistribution and speciation of As^V and Fe^III at Fe/As molar ratio of 1-14 and pH 1.5-2.0 in the dissolved, colloidal, and solid phases. Results showed that the elevated Fe^III induced the decomposition of the precipitated poorly crystalline ferric arsenate by forming dissolved (< 3 kDa) and colloidal (3 kDa-0.1 µm) As-Fe complexes. The fraction of particulate As (> 0.1 µm) decreased from 70-90% to less than 20% when the Fe/As molar ratio increased from 1 to 14. The particle size of the bulk samples decreased significantly with the increase of Fe^III concentration. The FTIR results suggested that As^V in dissolved/colloidal As-Fe complexes dominantly occurred as HAsO₄^2- species. The EXAFS results indicated that each HAsO₄^2- coordinated with approximately two Fe atoms in dissolved/colloidal As-Fe complexes at Fe/As ≥ 2. The findings suggest that high aqueous Fe^III concentration can promote the mobility of As by forming dissolved/colloidal Fe-As complexes in acidic waters, potentially accelerating As transport from source to downstream in acid mine drainage systems.

Arsenic(V) Removal from Water by Resin Impregnated with Cyclodextrin Ligand

More rigorous environmental rules on hazardous metals release require the implementation of diverse methods to remove them from contaminated streams. The adsorption process is the most attractive method for removal of toxic metal ions from different aqueous solutions. In the present study, batch and fixed bed column experiments were carried out to investigate the performance of permethylated β-cyclodextrin impregnated resin for the removal of As(V) ions from aqueous solutions. Sorption of As(V) ions was performed in groups under a variety of pH and metal concentrations. It was noted that removal efficiency was the highest (98%) at pH 6.0, and at an initial As(V) concentration equal to 0.1 mg/L. Sorption model use for experimental data indicates that the removal mechanism of As(V) by the permethylated β-cyclodextrin impregnated resin was the physical sorption. The adsorption capacity of the cyclodextrin ligand immobilized onto Amberlite XAD-4 resin was 19 mg/g. In addition, the impregnated resin was regenerable, thus was able to be used over and over.

Photo-oxidation of arsenite in acidic waters containing Suwannee River fulvic acid: roles of 3SRFA* and hydroxyl radical

The photo-oxidation of arsenite (As(III)) in solution containing Suwannee River fulvic acid (SRFA) under the ultraviolet A (UVA) irradiation (λ_max = 365 nm) was studied. In a solution containing 100.0 μg·L^-1 As(III) and 10.0 mg·L^-1 SRFA at pH 3.0, SRFA induced As(III) photo-oxidation by producing the triplet excited state of SRFA (³SRFA*) and hydroxyl radical(HO˙). Approximately 82% of As(III) oxidation was attributed to HO˙ which depended strongly on HO₂˙/O₂˙^-. The remaining 18% of As(III) oxidation was attributed to the direct reaction between As(III) and ³SRFA*. The photo-oxidation of As(III) was significantly affected by solution pH. Excess SRFA inhibited As(III) photo-oxidation. The addition of a low concentration of ferric ions retarded the photo-oxidation of As(III) due to the poor photo-activity of Fe(III)-SRFA complexes. In contrast, the addition of ferric ions at high concentration greatly accelerated As(III) photo-oxidation because of the high photo-activity of Fe(III)-OH complexes. The fractions of SRFA with different molecular weight showed different oxidizing capacities under UV irradiation which was possibly related to the different contents of phenolic OH groups. The findings have important environmental implications for the photo-transformation behavior of As(III) in natural surface waters containing dissolved organic matter, especially acidic waters.

Effect of Time Interval on Arsenic Toxicity to Paddy Field Cyanobacteria as Evident by Nitrogen Metabolism, Biochemical Constituent, and Exopolysaccharide Content

Arsenic poisoning in aquatic ecosystem is a global concern that obstructs the productivity of agricultural lands (paddy fields) by targeting the growth of cyanobacteria. The cyanobacteria also tolerate and accumulate elevated concentration of arsenic (As) inside the cell and excrete out from cells in less toxic forms after the successive time interval. Thus to validate this, the study was carried out at two different time intervals, i.e., 48 h and 96 h. Two redox forms of As arsenate (As^V) and arsenite (As^III) at different concentrations (50, 100, and 150 mM As^V; 50, 100, and 150 μM As^III) caused substantial reduction in growth, pigments (Chl a/Car and phycobiliproteins: phycocyanin, allophycocyanin, and phycoerythrin), inorganic nitrogen ( nitrate (NO₃^-) and nitrite (NO₂^-)) uptake, activity of enzymes (NR, NiR, GS, and GOGAT) of nitrogen metabolism, biochemical constituents (protein, carbohydrate, and exopolysaccharide (EPS) contents of Nostoc muscorum, and Anabaena sp. PCC7120. The tested doses of As^V and As^III after 48 h of exposure exhibited adverse impact on these parameters, but after 96 h with lower doses of As^V (50 mM and 100 mM) and As^III (50 μM and 100 μM), significant recovery was recorded. Contrary to this, at higher dose of As^V (150 mM) and As^III (150 μM), the adverse impact was further aggravated with increasing time exposure. Contrary to the activity of NR, NiR, GS, and GOGAT, GDH activity (alternative NH₃⁺ assimilating enzyme) was found to increase, and after 96 h, the activity showed declining trend but still higher than the control. The biochemical constituent EPS (first protective barrier) under scanning electron microscope showed more accumulation of dry adsorbent in the case of As^III stress hence displayed more toxic nature of As^III than As^V. The study concludes that with increasing time exposure, the recovery in growth and related parameters mainly at lower doses of As^V and As^III points toward adaptability of cyanobacteria which was more pronounced in Nostoc muscorum.

Active Treatment of Contaminants of Emerging Concern in Cold Mine Water Using Advanced Oxidation and Membrane-Related Processes: A Review

Responsible use and effective treatment of mine water are prerequisites of sustainable mining. The behavior of contaminants in mine water evolves in relation to the metastable characteristics of some species, changes related to the mine life cycle, and mixing processes at various scales. In cold climates, water treatment requires adaptation to site-specific conditions, including high flow rates, salinity, low temperatures, remoteness, and sensitivity of receiving waterbodies. Contaminants of emerging concern (CECs) represent a newer issue in mine water treatment. This paper reviews recent research on the challenges and opportunities related to CECs in mine water treatment, with a focus on advanced oxidation and membrane-based processes on mine sites operating in cold climates. Finally, the paper identifies research needs in mine water treatment.

Root microbiome assembly of As-hyperaccumulator Pteris vittata and its efficacy in arsenic requisition

The assemblage of root-associated microorganisms plays important roles in improving their capability to adapt to environmental stress. Metal(loid) hyperaccumulators exhibit disparate adaptive capability compared to that of non-hyperaccumulators when faced with elevated contents of metal(loid)s. However, knowledge of the assemblage of root microbes of hyperaccumulators and their ecological roles in plant growth is still scarce. The present study used Pteris vittata as a model plant to study the microbial assemblage and its beneficial role in plant growth. We demonstrated that the assemblage of microbes from the associated bulk soil to the root compartment was based on their lifestyles. We used metagenomic analysis and identified that the assembled microbes were primarily involved in root-microbe interactions in P. vittata root. Notably, we identified that the assembled root microbiome played an important role in As requisition, which promoted the fitness and growth of P. vittata. This study provides new insights into the root microbiome and potential valuable knowledge to understand how the root microbiome contributes to the fitness of its host.

The role of efflorescent salts associated with sulfide-rich mine wastes in the short-term cycling of arsenic: Insights from XRD, XAS, and µ-XRF studies

The evaporation of As-rich leachates generated by the weathering of sulfide-rich mine wastes accumulated in abandoned tailing dams of the La Concordia mine, triggers the widespread precipitation of saline crusts and efflorescences. Because these salts are highly soluble, they may release high concentrations of arsenic after rainfall events. Thus, the goal of this work is to assess the solid speciation of As in these efflorescences, which may help to understand the short-term cycling of As in the site. The results reveal that As is present only as As(V), while its capacity to be retained in the salts highly depends on their mineralogical composition. Hydrous sulfates, such as gypsum and epsomite show a very low capacity to scavenge As, while copiapite retains the highest concentrations of this element. The spectroscopic evidences suggest that in this mineral, As(V) is included within the lattice, substituting sulfate in the tetrahedral sites. Because copiapite is highly soluble, it may be considered as one of the most important transient reservoirs of As in the site that can release high concentrations of this hazardous pollutant during the occasional rainfall events produced during the wet season.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min^-1. NaOH or Ca(OH)₂ as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O₂ for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min^-1 were optimal for As removal.

Hydrogen disorder in kaatialaite Fe[AsO2(OH)2]5H2O from Jáchymov, Czech Republic: determination from low-temperature 3D electron diffraction

Kaatialaite mineral Fe[AsO2(OH)2]5H2O from Jáchymov, Czech Republic forms white aggregates of needle-shaped crystals with micrometric size. Its structure at ambient temperature has already been reported but hydrogen atoms could not be identified from single-crystal X-ray diffraction. An analysis using 3D electron diffraction at low temperature brings to light the hydrogen positions and the existence of hydrogen disorder. At 100 K, kaatialaite is described in a monoclinic unit cell of a = 15.46, b = 19.996, c = 4.808 Å, β = 91.64° and V = 1485.64 Å3 with space group P21/n. The hydrogen sites were revealed after refinements both considering the dynamical effects and ignoring them. The possibility to access most of the hydrogen positions, including partially occupied ones among heavy atoms, from the kinematical refinement is due to the recent developments in the analysis of 3D electron data. The hydrogen bonding observed in kaatialaite provides examples of H2O configurations that have not been observed before in the structures of oxysalts with the presence of unusual inverse transformer H2O groups.

Selective Removal of As(V) Ions from Acid Mine Drainage Using Polymer Inclusion Membranes

Acid mine drainage (AMD) is globally recognized as one of the environmental pollutants of the priority concern due to high concentrations of toxic metals and sulfates. More rigorous environmental legislation requires exploitation of effective technologies to remove toxic metals from contaminated streams. In view of high selectivity, effectiveness, durability, and low energy demands, the separation of toxic metal ions using immobilized membranes with admixed extractants could ameliorate water quality. Cellulose triacetate based polymer inclusion membranes (PIMs), with extractant and plasticizer, were studied for their ability to transport of As(V) ions from synthetic aqueous leachates. The effects of the type and concentration of extractant, plasticizer content, and sulfuric acid concentration in source phase on the arsenic removal efficiency have been assessed. Under the best of applied conditions, PIM with Cyanex 921 as extractant and o-nitrophenyl octyl ether (o-NPOE) as plasticizer showed high repeatability and excellent transport activity for selective removal of As(V) from AMD.

Metagenomic insights into microbial arsenic metabolism in shallow groundwater of Datong basin, China

Elevated arsenic (As) in groundwater is an urgent environmental problem that has caused serious endemic diseases in Datong basin, China. The fate and toxicity of As are generally regulated by microbial As metabolic processes. However, little is known about the microbial community and As metabolism in Datong basin. Herein, the microbial community structure and As metabolism genes in four wells with different levels of As concentration in Shanyin county were investigated using metagenomics approach. The results showed that the presence of As influenced the microbial communities, and Rhodococcus genus was significantly enriched in elevated As wells. As resistance genes were dominant from low to high As containing wells, and As efflux genes such as arsB and acr3 were positively correlated with As concentrations, suggesting that microbes tend to pump As out of the cell as a strategy for As detoxification. Other environmental factors including oxidation-reduction potential (ORP), total organic carbon (TOC), sulfate, and temperature also played a role in shaping the microbial community structure and As metabolic processes.

Treatment of As-rich mine effluents and produced residues stability: Current knowledge and research priorities for gold mining

Refractory ores, in which gold is often embedded within As-bearing and acid-generating sulfide minerals, are becoming the main gold source worldwide. These ores require an oxidizing pre-treatment, prior to cyanidation, to efficiently breakdown the sulfides and enhance gold liberation. As a result, large volumes of As-rich effluents (> 500 mg/L) are produced through the pre-oxidation of refractory gold ores and/or the exposure of As-bearing tailings upon exposure to air and water. Limited information is available on performant treatment of these effluents, especially of pre-oxidation effluents characterized by a complex chemistry, extremely acidic or alkaline pH and high concentrations of arsenic. The treatment of As-rich effluents is mainly based on precipitation (using Al or Fe salts and/or Ca-based compounds) and (electro)-chemical or biological oxidation processes. A performant treatment process must maximize As removal from contaminated mine water and allow for the production of residues that are geochemically stable over the long term. An extensive literature review showed that Fe(III)-As(V) precipitates, especially bioscorodite and (nano)scorodite, appear to be the most appropriate forms to immobilize As due to their low solubility and high stability, especially when encapsulated within an inert material such as hydroxyl gels. Research is still required to assess the long-term stability of these As-bearing residues under mine-site conditions for the sustainable exploitation of refractory gold deposits.

Two-generational effects and recovery of arsenic and arsenate on Daphnia magna in the presence of nano-TiO2

The toxicity of arsenic (As) can be influenced by many environmental factors. Among them, nanomaterials can adsorb arsenic and alter its bioavailability in organisms. However, the studies on long-term effects of arsenic in the presence of nanoparticles are limited. Thus, the 21-d effect of titanium dioxide nanoparticles (nano-TiO₂) on chronic toxicity of arsenic (arsenate and arsenite) was investigated in two generations of Daphnia magna. The exposed concentration of nano-TiO₂ was 1 mg/L and the concentration of As(Ⅲ) or As(Ⅴ) was 0.2 mg/L which was lower than the 48 h-NOEC (no observed effect concentration). The survival, body length, average number of offspring and time of first brood were determined. Our results indicated that the exposure to nano-TiO₂ and As during the parental generation can affect the health of offspring. Nano-TiO₂ was found to significantly alleviate the mortality and reproduction inhibition of As on D. magna, and the alleviation of As(Ⅴ) was more prominent than that of As(Ⅲ). It is likely that nano-TiO₂ alters the metabolism and adsorption condition of arsenic in the gastrointestinal tract of D. magna. Overall, these results indicate that the increase of arsenic adsorption onto nano-TiO₂ in the gut of D. magna could alleviate the toxicity of arsenic. Nonetheless, further research should be conducted to study the influence of arsenic on the multi-generations of aquatic organisms, especially when it is coexisted with other substances.

Toxicity assessment of arsenate and arsenite on growth, chlorophyll a fluorescence and antioxidant machinery in Nostoc muscorum

The present study deals with impact of varied doses of arsenite (As^III; 50, 100 and 150 µM) and arsenate (As^V; 50, 100 and 150 mM) on growth, photosynthetic pigments, photochemistry of photosystem II, oxidative biomarkers, (O₂^•¯, H₂O₂ and MDA equivalents contents) and activity of antioxidant enzymes in diazotrophic cyanobacterium Nostoc muscorum after 48 and 96 h of the treatments. The reduction in growth, pigment contents (Chl a, Phy and Car) and PS II photochemistry was found to increase with enhanced accumulation of test metal in cells, and the damaging effect on photosynthetic pigments showed the order (Phy > chl a> Car). The negative effect on PS II photochemistry was due to significant decrease in the value of JIP kinetics ϕP₀, F_V/F₀, ϕE₀,Ψ₀ and PI_ABS except F₀/F_V and significant rise in values of energy flux parameters such as ABS/RC, TR₀/RC, ET₀/RC and DI₀/RC. Both the species of arsenic caused significant rise in oxidative biomarkers as evident by in vitro and in vivo analysis of (O₂^•¯, H₂O₂ and MDA equivalents contents) despite of appreciable rise in the activity antioxidative enzymes such as SOD, POD, CAT and GST. The study concludes that in among both forms of arsenic, arsenite effect was more dominant on growth, photosynthetic pigments; oxidative stress biomarkers as evident by weak induction of anti-oxidative defense system to overcome the stress as compared to arsenate.

The performance of microbial anodes in municipal wastewater: Pre-grown multispecies biofilm vs. natural inocula

In this study, different inoculation strategies for continuously operated microbial anodes are analyzed and compared. After 20daysof operation with municipal wastewater anodes pre-incubated with a biofilm of the exoelectrogenic species Geobacter and Shewanella showed current densities of (65±8) μA/cm². This is comparable to the current densities of non-inoculated anodes and anodes inoculated with sewage sludge. Analysis of the barcoded pre-grown multispecies biofilms reveal that 99% of the original biofilm was detached after 20daysof operation with municipal wastewater. This is in contrast to previous experiments where a pre-grown biofilm of exoelectrogens was operated in batch mode. To implement pre-grown biofilms in continuous systems it will thus be necessary to reveal a window of process parameters in which typical exoelectrogenic microorganisms including model organisms can be kept and/or enriched on anodes.

Probing Synechocystis-Arsenic Interactions through Extracellular Nanowires

Microbial nanowires (MNWs) can play an important role in the transformation and mobility of toxic metals/metalloids in environment. The potential role of MNWs in cell-arsenic (As) interactions has not been reported in microorganisms and thus we explored this interaction using Synechocystis PCC 6803 as a model system. The effect of half maximal inhibitory concentration (IC50) [~300 mM As (V) and ~4 mM As (III)] and non-inhibitory [4X lower than IC50, i.e., 75 mM As (V) and 1 mM As (III)] of As was studied on Synechocystis cells in relation to its effect on Chlorophyll (Chl) a, type IV pili (TFP)-As interaction and intracellular/extracellular presence of As. In silico analysis showed that subunit PilA1 of electrically conductive TFP, i.e., microbial nanowires of Synechocystis have putative binding sites for As. In agreement with in silico analysis, transmission electron microscopy analysis showed that As was deposited on Synechocystis nanowires at all tested concentrations. The potential of Synechocystis nanowires to immobilize As can be further enhanced and evaluated on a large scale and thus can be applied for bioremediation studies.

Arsenic mineralogy and mobility in the arsenic-rich historical mine waste dump

A more than 250 year-old mine dump was studied to document the products of long-term arsenopyrite oxidation under natural conditions in a coarse-grained mine waste dump and to evaluate the environmental hazards associated with this material. Using complementary mineralogical and chemical approaches (SEM/EDS/WDS, XRD, micro-Raman spectroscopy, pore water analysis, chemical extraction techniques and thermodynamic PHREEQC-2 modeling), we documented the mineralogical/geochemical characteristics of the dumped arsenopyrite-rich material and environmental stability of the newly formed secondary minerals. A distinct mineralogical zonation was found (listed based on the distance from the decomposed arsenopyrite): scorodite (locally associated with native sulfur pseudomorphs) plus amorphous ferric arsenate (AFA/pitticite), kaňkite, As-bearing ferric (hydr)oxides and jarosite. Ferric arsenates and ferric (hydr)oxides were found to dissolve and again precipitate from downward migrating As-rich solutions cementing rock fragments. Acidic pore water (pH3.8) has elevated concentrations of As with an average value of about 2.9 mg L(-1). Aqueous As is highly correlated with pH (R2=0.97, p<0.001) indicating that incongruent dissolution of ferric arsenates controls dissolved As well as the pH of the percolating waste solution. Arsenic released from the dissolution of ferric arsenates into the pore water is, however, trapped by latter and lower-down precipitating jarosite and especially ferric (hydr)oxides. The efficiency of As sequestration by ferric (hydr)oxides in the waste dump and underlying soil has been found to be very effective, suggesting limited environmental impact of the mine waste dump on the surrounding soil ecosystems.