Natural Attenuation Of Arsenic Research Articles

In situ metabolic activities of uncultivated Ferrovum sp. CARN8 evidenced by metatranscriptomic analysis

Amongst iron-oxidizing bacteria playing a key role in the natural attenuation of arsenic in acid mine drainages (AMDs), members of the Ferrovum genus were identified in mine effluent or water treatment plants, and were shown to dominate biogenic precipitates in field pilot experiments. In order to address the question of the in situ activity of the uncultivated Ferrovum sp. CARN8 strain in the Carnoulès AMD, we assembled its genome using metagenomic and metatranscriptomic sequences and we determined standardized expression values for protein-encoding genes. Our results showed that this microorganism was indeed metabolically active and allowed us to sketch out its metabolic activity in its natural environment. Expression of genes related to the respiratory chain and carbon fixation suggests aerobic energy production coupled to ferrous iron oxidation and chemolithoautotrophic growth. Notwithstanding the presence of nitrogenase genes in its genome, expression data also indicated that Ferrovum sp. CARN8 relied on ammonium import rather than nitrogen fixation. The expression of flagellum and chemotaxis genes hints that at least a proportion of this strain population was motile. Finally, apart from some genes related to metal resistance showing surprisingly low expression values, genes involved in stress response were well expressed as expected in AMDs.

Role of unsaturated soil above a heavily contaminated aquifer in the natural attenuation of arsenic

Large attenuation of arsenic is observed in a shallow aquifer in central Mexico, where the concentration decreases from 91.5 to 11.3 mg/L, over 1.3 km horizontal distance. To investigate possible mechanisms of attenuation of this pollutant, we dug a pit between the surface and the saturated zone. We then described the soil profile and determined arsenic and iron concentrations in soil samples as a function of depth. Next, we determined particle size distribution, bulk density, particle density, soil moisture and porosity of the soil material. We also analyzed arsenic concentration in the groundwater. The 2.2 m deep profile intersected 4 soil horizons (A, AB, B1 and B2). We found arsenic accumulation in the B2 horizon, directly overlying the saturated zone. These accumulations coincide with the thickness of the capillary fringe, revealing that arsenic is drawn up in the soil profile by capillary rise of arsenic-contaminated groundwater. Furthermore, arsenic accumulation showed a direct relationship with iron, likely due to arsenic adsorption on iron oxides. Results from this study contribute to understanding a capillary-driven natural attenuation mechanism that removes contaminants from groundwater by sinking them in the capillary fringe.

Natural attenuation of arsenic in the environment by immobilization in nanostructured hematite

Iron (hydr)oxides are known to play a major role in arsenic fixation in the environment. The mechanisms for long-term fixation into their crystal structure, however, remain poorly understood, especially arsenic partitioning behavior during transformation from amorphous to crystalline phases under natural conditions. In this study, these mechanisms are investigated in Fe–Al-oxisols exposed over a period of 10years to a sulfide concentrate in tailings impoundments. The spatial resolution necessary to investigate the markedly heterogeneous nanoscale phases found in the oxisols was achieved by combining three different, high resolution electron microscopy techniques – Nano-Beam Electron Diffraction (NBD), Electron Energy-Loss Spectroscopy (EELS), and High Resolution Transmission Electron Microscopy (HRTEM). Arsenic (1.6±0.5wt.%) was unambiguously and precisely identified in mesocrystals of Al-hematite with an As/Fe atomic ratio of 0.026±0.006. The increase in the c-axis (c=1.379±0.009nm) compared to standard hematite (c=1.372nm) is consistent with the presence of arsenic in the Al-hematite structure. The As-bearing Al-hematite is interpreted as a secondary phase formed from oxyhydroxides, such as ferrihydrite, during the long-term exposure to the sulfide tailings. The proposed mechanism of arsenic fixation in the Al-hematite structure involves adsorption onto Al-ferrihydrite nanoparticles, followed by Al-ferrihydrite aggregation by self-assembly oriented attachment and coalescence that ultimately produces Al-hematite mesocrystals. Our results illustrate for the first time the process of formation of stable arsenic bearing Al-hematite for the long-term immobilization of arsenic in environmental samples.

Natural attenuation of geothermal arsenic from Yangbajain power plant discharge in the Zangbo River, Tibet, China

The Yangbajain geothermal field located in central Tibet is characterized by the highest measured reservoir temperature among all hydrothermal systems in China. The high-temperature geothermal fluid extracted from Yangbajain has been used for electricity generation for over 30years. The geothermal wastewater generated by the Yangbajain power plants, with arsenic (As) concentrations up to 3.18mg/L, drains directly into the Zangbo River, the major surface water at Yangbajain, which has elevated arsenic concentrations in the segments downstream of wastewater discharges. However, along the flow direction of the river, the arsenic concentration decreases sharply. Further inspection reveals that the concentrations of weakly bound arsenic, strongly adsorbed arsenic and total arsenic in riverbed sediment were affected by the drainage of geothermal wastewater, indicating that the sediment serves as a sink for geothermal arsenic. A logarithmic relationship between the integrated attenuation coefficients (IAC) for three river segments and the corresponding adsorption distribution coefficients of riverbed sediment samples also suggests that besides the dilution of geothermal arsenic in the Zangbo River, natural attenuation of arsenic may be caused by sorption to riverbed sediment, thereby reducing its health threat to local residents using the Zangbo River as a drinking water source.

Natural arsenic attenuation via metal arsenate precipitation in soils contaminated with metallurgical wastes: II. Cumulative evidence and identification of minor processes

Abstract Accurate identification of individual As species in contaminated environments is critical because the toxicology, mobility and adsorptive properties of this element may vary substantially with its chemical forms and oxidation states. The goal of this work was to relate the geochemical behavior of As in soils contaminated by a lead smelter in Mexico, with its chemical speciation, and to achieve direct identification of low-solubility poorly-crystalline metal arsenates. Arsenic was identified as the most mobile trace element in the wastes from the smelting plant. Arsenic solubility in soils was significantly lower than its solubility in wastes, showing natural attenuation of this element. Its solubility in soil was quantitatively described in selected samples through thermodynamic equilibrium modeling. The results indicated that As solubility is controlled by solid Pb and Cu arsenate formation. The behaviors of the sequential chemical extractions were consistent with the presence of the predicted arsenates. Microscopic evidence of the formation of solid metal arsenates were obtained in fine soil fractions of selected samples with high As contents, by using the following complementary techniques: X-ray diffraction, scanning electron microscopy and transmission electron microscopy, both coupled with energy dispersive X-ray spectroscopy, and the latter with a high angle annular dark field detector. All results supported the formation of low-solubility Pb arsenates as controlling As mobility in the samples studied, in which As(V) adsorption to Fe (hydr)oxides was not the dominant process of natural attenuation.

Natural arsenic attenuation via metal arsenate precipitation in soils contaminated with metallurgical wastes: III. Adsorption versus precipitation in clean As(V)/goethite/Pb(II)/carbonate systems

► Modeling describes competition between precipitation and adsorption in clean systems. ► Relatively simple lab experiments can reproduce the two main As(V) geochemical processes. ► Adsorption is only dominant at very low As/Fe molar ratios. ► Lead arsenate precipitation is dominant in most conditions. ► In soils contaminated with mine-related wastes predominance of precipitation is predicted. Soil contamination with As and potentially harmful metals is a widespread problem around the world especially from mining and metallurgical wastes, which release substantial amounts of these elements to the environment in potentially mobile species. Recently, it has been found that in various Mexican soils contaminated with these types of wastes, arsenate is not in the form of sorbed species on Fe oxides present in the soils, as generally reported in the literature, but in the form of very insoluble compounds such as Pb, Cu and Ca arsenates. Here a thermodynamic model is applied and validated with the results from wet chemical experiments to determine the fundamental geochemical conditions governing the mobility of As in the presence of Pb. For this purpose, a relatively simple but fundamental system of goethite (α-FeOOH)/As(V)/Pb(II)/carbonate was defined as a function of the As(V)/Fe(III) ratio, in a pH range of 5–10. The speciation model included the simultaneous inclusion of triple layer surface complexation and arsenate precipitation equilibria. The model predicts that from very low total As(V)/Fe(III) molar ratios (0.012 at pH 7) the precipitation mechanism significantly influences the attenuation of As(V), and rapidly becomes the dominant process over the adsorption mechanism. Model results identify the quantitative conditions of predominance for each mechanism and describe the transition conditions in which relatively large fractions of adsorbed, precipitated and dissolved As(V) species prevail. Experimental measurements at selected As(V)/Fe(III) ratios and pH confirmed the predictions and validated the coupled thermodynamic model utilized.

Natural attenuation of arsenic in soils near a highly contaminated historical mine waste dump

Arsenic-contaminated soils near historical As-rich mine waste in Jáchymov (Czech Rep.), resulting from the smelting and seepage of the mine waste pore water, were studied to examine As partitioning between solid phases and pore waters. Mineralogical and geochemical analyses showed that As is exclusively associated with unidentified amorphous Fe oxyhydroxides, poorly crystalline goethite and hematite as adsorbed and coprecipitated species (with up to 3.2wt.% As). Adsorption of As by Fe oxyhydroxides is likely to be a major control on the migration of As in the soil pore water containing only up to 15μgL−1 As(V). The slight variations in the dissolved As(V) concentrations do not follow the total contents of As in the soil or adsorbed As, but appeared to be a function of pH-dependent sorption onto Fe oxyhydroxides. The geochemical modelling using PHREEQC-2 supported the efficiency of As(V) adsorption by Fe oxyhydroxides in the soil affected by As-rich waste solution seepage. It also suggested that active Fe oxyhydroxides has a strong attenuation capacity in soil that could effectively trap the aqueous As(V) from the unremitting waste seepage for the next approx. 11600years.

Acid drainage at the inactive Santa Lucia mine, western Cuba: Natural attenuation of arsenic, barium and lead, and geochemical behavior of rare earth elements

Abstract A detailed geochemical study was conducted at the inactive Zn–Pb mine of Santa Lucia, in western Cuba. The studied mine-wastes are characterized by high total concentrations of potentially toxic elements (PTE), with average values of 17.4% Fe, 5.47% Ba, 2.27% Pb, 0.83% Zn, 1724 mg/kg As and 811 mg/kg Cu. Oxidation of sulfide minerals in mine-waste dumps and in the open pit produces acid mine effluents (pH = 2.5–2.6) enriched in dissolved SO 4 2 - (up to 6754 mg/L), Fe (up to 4620 mg/L) and Zn (up to 2090 mg/L). Low pH values (2.5–2.8) and high dissolved concentrations of the same PTE were found in surface waters, up to 1500 m downstream from the mine. Nevertheless, concentrations of As, Ba and Pb in acid mine effluents and impacted surface waters are relatively low: 0.01–0.3 mg/L As, 0.002–0.03 mg/L Ba and 0.3–4.3 mg/L Pb. Analysis by X-ray diffraction and electron microscopy revealed the occurrence of lead–bearing barite and beudantite and the more common solid phases, reported elsewhere in similar environments including Fe-oxyhydroxides, jarosite, anglesite and plumbojarosite. Because the reported solubilities for barite and beudantite are very low under acidic conditions, these minerals may serve as the most important control in the mobility of As, Ba and Pb. In contrast, Fe-oxyhydroxides are relatively soluble under acidic conditions and, therefore, they may have a less significant role in PTE on-site immobilization. Mine-wastes and stream sediments show a light REE (LREE) and middle REE (MREE) enrichment relative to heavy REE (HREE). In contrast, acid mine effluents and surface waters are enriched in HREE relative to LREE. These results suggest that the LREE released during the oxidation of sulfides are captured by secondary (weathering) minerals, while the MREE are removed from the altered rocks. The low concentrations of LREE in acid stream water suggest that these elements can be retained in the sediments more strongly than HREE and MREE. One possible explanation for the sharp decrease in dissolved LREE might be their retention by low-solubility secondary minerals such as anglesite, barite and jarosite.

Natural attenuation of arsenic in the Tinto Santa Rosa acid stream (Iberian Pyritic Belt, SW Spain): The role of iron precipitates

Acid waters and sediments of the Tinto Santa Rosa acid stream (Iberian Pyritic Belt; SW, Spain) were analysed to determine the role of sedimentary phases in the behaviour of arsenic. Aqueous arsenic and iron concentrations decreased markedly from the adit mouth to 300 m downstream indicating iron minerals precipitation as well as arsenic sorption onto these newly-formed phases. This was confirmed by the high arsenic concentrations observed in bed-stream precipitates, which play a major role in controlling arsenic mobility. To unravel the complex nature of the AMD sediments a combination of techniques including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), total solid digestions, X-Ray Fluorescence (XRF) and X-Ray Absorption Spectroscopy (XAS) were employed. Results showed that (1) arsenic was present predominantly in its pentavalent state; (2) upstream arsenic was sorbed onto the main phase, schwertmannite, whereas downstream it was chiefly associated with goethite and jarosite; and (3) changes in arsenic speciation with depth were observed in the consolidated terrace sediments, where arsenic appeared primarily associated with schwertmannite in the upper part of the terraces, but with goethite at depth. Arsenic mobilization was controlled by sorption onto newly formed precipitates (schwertmannite, goethite and jarosite), causing natural arsenic attenuation.

Natural Arsenic Attenuation via Metal Arsenate Precipitation in Soils Contaminated with Metallurgical Wastes: I. Wet Chemical and Thermodynamic Evidences

Arsenic from natural and anthropogenic sources is a worldwide contaminant of aqueous environments, such as groundwater and soils. The present investigation was performed on Mexican soils contaminated with residues from metallurgical processes that have shown a natural As attenuation. Experimental aqueous arsenic extractions in these were successfully simulated for almost half of the soil samples using a database updated for all known metal arsenate formation constants, revealing the predominance of solubility-controlled As mobility via Pb, mixed Pb–Cu, and Ca arsenate solid formation. The relatively low total Fe/As ratios (2–13 w/w) present in the soils studied, together with the high and equivalent contents of As, Pb, and Cu in these, favor the precipitation process over As(V) adsorption to Fe oxides, despite a 2% average Fe content in the soils studied. Under these conditions bicarbonate was found to be a highly unsuitable extractant due to its indirect As release from the solid arsenates, via heavy metal carbonate precipitation processes.

人工池でのヒ素の自然浄化

An arsenic removal unit (GSF) was constructed by the Miyazaki University research group in the Marua village in Bangladesh. The sludge drained from the GSF was directed to a sludge tank and allowed to settle down. Then the supernatant in the tank was released to an artificial pond. The water, sediment and sludge samples from the pond and tank were analyzed, and the microbial statuses of the pond and tank were examined. The ORP result was observed to have changed from the reduction state to the oxidation state in the tank within one year from 2005 and 2006. Therewith, the concentration of soluble arsenic and iron in the tank decreased compared with those one year ago. The amount of arsenic included in the entire pond hardly changed in 2005 and 2006. From the result of an enrichment culture using sediment from the pond, monomethylarsonic acid and dimethylarsinic acid were detected in the medium, and the amount of total arsenic compounds in the medium decreased. It is thought that this loss was contributed by biogenic activity induced by inorganic arsenic biomethylation to gasified volatile organic species, such as monomethylarsine and dimethylarsine. These results suggested that the natural attenuation of arsenic in the pond was performed by biologic gasification.

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5–6.5 when Fe(II) and hydroxylapatite (Ca 5(PO 4) 3OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO 2(g) and O 2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe III[(OH) 3(PO 4)(As VO 4)]· nH 2O or Fe III[(OH) 3( PO 4)(As VO 4)(As IIIO 3) minor]· nH 2O, where As IIIO 3 is a minor component. The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe III(OH) 3 and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe III[(OH) 3, PO 4]· nH 2O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO 4·2H 2O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption. Significantly, the chemically-mixed, ferric oxyhydroxide–phosphate–arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.

Sulfide oxidation and the natural attenuation of arsenic and trace metals in the waste rocks of the abandoned Seobo tungsten mine, Korea

Mineralogical examinations were performed to characterize the formation of secondary minerals and natural removal process of dissolved As and trace metals (Pb, Zn and Cu) from sulfide oxidation. Laboratory-based leaching tests were also conducted to determine whether the concentrations of As and trace metals in the leachates from waste-rock materials and contaminated soil could be affected by the presence acids such as acid rainwater or acid mine drainage. Waste-rock materials and contaminated soil were compared by 4-day leaching tests using HNO 3 solutions of increasing acidity (0.00001–0.1 mole/L). Mineralogical studies of the waste rocks confirmed the presence of Fe-(oxy)hydroxides (e.g. goethite), jarosite, elemental S, Fe-sulfates, amorphous Fe–As phases, anglesite and covellite as secondary minerals. These secondary minerals act as mineralogical scavengers of dissolved trace metals, SO 4 2 - and acidity released by sulfide oxidation. Arsenic was attenuated by the adsorption on Fe-(oxy)hydroxides and/or the formation of an amorphous Fe–As phase, with a Fe/As ratio = 1 (maybe scorodite: FeAsO 4 · 2H 2O). Electron probe microanalyses data showed that the Fe-(oxy)hydroxides had high concentrations of Pb (up to 21 wt%), with appreciable amounts of As (up to 7.7 wt%), Zn (up to 4.6 wt%) and Cu (up to 2.5 wt%) indicating that dissolved metals were co-precipitated and adsorbed onto Fe-(oxy)hydroxides, Fe(Mn)-hydroxides and Fe-sulfates. The results of the leaching experiments within the pH-range 3.5–5.0 indicated that acidic rainstorms may leach minor amounts of Pb (ca. 1.7–4.0% of total), Zn (ca. 0.8–2.2% of total), Cu (ca. 0.0–0.2% of total) and As (ca. 0.02–0.1% of total) from waste rocks, including the dissolution of soluble secondary minerals previously formed during prolonged dry periods, while dissolution of these elements was negligible from the contaminated soil. In the pH-range 1.0–3.0, the leaching of Pb (ca. 2.4–31% of total) and As (ca. 0.1–5.8% of total) from the waste rocks was significant, which could influence the concentration of these metals in mine runoff. Strongly acidic solutions may also appreciably dissolve Zn (0.0–48% of total) and Cu (0.0–34% of total) in contaminated soil. Leach tests showed that the formation of less soluble secondary minerals had high retention of As, Pb, Zn and Cu, unless their solubilities were increased after the addition of strongly acidic solutions (pH of below 2.0). The precipitation of secondary minerals and the adsorption of trace metals are efficient mechanisms for decreasing the mobilities of As and other trace metals in the surface environment. Geochemical modeling indicated that the leach solutions were supersaturated with respect to goethite, hematite, and magnetite at pH greater than 2.9. Ferrihydrite, Fe(OH) 3, was not in equilibrium with the solution during the entire experiment, suggesting that amorphous Fe-hydroxide can only form in small amounts. X-ray analyses of Seobo mine-waste samples indicated the presence of jarosite and goethite, which retain As, Pb, Zn and Cu during precipitation and co-precipitation.

A natural attenuation of arsenic in drainage from an abandoned arsenic mine dump

At the abandoned As mine in Nishinomaki, Japan, discharged water from the mining and waste dump area is acidic and rich in As. However, the As concentration in the drainage has been decreased to below the maximum contaminant level (0.01 mg/l for drinking water, Japan) without any artificial treatments before mixing with a tributary to populated areas. This implies that the As concentration in water from the waste dump area has been naturally attenuated. To elucidate the reaction mechanisms of the natural attenuation, analysis of water quality and characterization of the precipitates from the stream floor were performed by measuring pH, ORP and electric conductivity on-site, as well as X-ray diffraction, ICP-mass spectrometry and ion-chromatography. Selective extractions and mineral alteration experiments were also conducted to estimate the distribution of As in constituent phases of the precipitates and to understand the stability of As-bearing phases, respectively. The water contamination resulted from oxidation of sulfide minerals in the waste rocks, i.e., the oxidation of pyrite and realgar and subsequent release of Fe, SO 4, As(V) and proton. The released Fe(II) transformed to Fe(III) by bacterial oxidation; schwertmannite then formed immediately. While the As concentrations in the stream were lowered nearly to background level downstream, those in the ochreous precipitates were up to several tens of mg/g. The As(V) was effectively removed by the formed schwertmannite and had been naturally attenuated. Although schwertmannite is metastable with respect to goethite, the experiments show that the transformation of schwertmannite to goethite may be retarded by the presence of absorbed As(V) in the structure. Therefore, the attenuation of As in the drainage and the retention of As by schwertmannite are expected to be maintained for the long term.

Studies of interfacial reactions between arsenic and minerals and its significance to site characterization

Natural attenuation remediation is based on the intrinsic attenuation capacities of the subsurface. Geochemistry of the subsurface controls the fate, transport, transformation, and bioavailability of contaminants. This paper demonstrates that interfacial reactions (e.g., adsorption, desorption, oxidation, or reduction) between arsenic and minerals play an important role in the natural attenuation of arsenic, and could present important pathways for arsenic cycling in the subsurface environment. Iron oxides and kaolinites were used for this experimental study. Based on the different behavior of minerals, it is suggested that some types of minerals can act as mineral indicators for site characterization. Integration of mineralogical and geochemical indicator parameters would greatly improve the quality of site characterization. The current MCL in the USA for arsenic in drinking water may soon be lowered to 5 µg/L. It is estimated that lowering the arsenic MCL would cost billions of dollars per year. The natural attenuation could be a potential cost-effective approach for solving the problem of arsenic-contaminated groundwater.

An Analysis of Soil Arsenic Records of Decision

In 1986 the US EPA created the National Priority List (NPL) that now comprises in excess of 2,000 sites nationwide, with arsenic the second most common inorganic constituent. A survey of 69 Records of Decision (RODs) written between 1985 and 1998 for which arsenic was a major driver found that 84% of cleanup goals were risk-driven and 16% were background-driven, with a wide range of soil-arsenic cleanup standards for 10(-6) residential risk goals (2-305 mg/kg). In comparison, the range of background-based cleanup goals was much narrower (8-21 mg/kg). ROD soil arsenic concentrations exhibit no statistically significanttemporal trend, but on a geographic basis, EPA Regions 6, 8, 9, and 10 had some of the higher decisions. The risk assessment process is important in defining cleanup goals; however routine use of site-specific variables (i.e., bioavailability, realistic tenure in both residential and occupational settings, natural attenuation of arsenic in groundwater, etc.) is necessary to ensure an accurate assessment of potential site risks and to preclude over-remediation that may result from the use of default risk variables.