FeSO4+ drives the H2O-losing transformation of iron(III)-phosphate-bearing secondary minerals with significant effect on immobilization of arsenic in AMD environments

The objective of this study was to explore a bioremediation strategy based on injecting NO3- to support the anoxic oxidation of ferrous iron (Fe(II)) and arsenite (As(II)) in the subsurface as a means to immobilize As in the form of arsenate (As(V)) adsorbed onto biogenic ferric (Fe(III)) (hydr)oxides. Continuous flow sand filled columns were used to simulate a natural anaerobic groundwater and sediment system with co-occurring As(III) and Fe(II) in the presence (column SF1) or absence (column SF2) of nitrate, respectively. During operation for 250 days, the average influent arsenic concentration of 567 microg L(-1) was reduced to 10.6 (+/-9.6) microg L(-1) in the effluent of column SF1. The cumulative removal of Fe(II) and As(II) in SF1 was 6.5 to 10-fold higher than that in SF2 Extraction and measurement of the mass of iron and arsenic immobilized on the sand packing of the columns were close to the iron and arsenic removed from the aqueous phase during column operation. The dominant speciation of the immobilized iron and arsenic was Fe(III) and As(V) in SF1, compared with Fe(II) and As(III) in SF2. The speciation was confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicate that microbial oxidation of As(III) and Fe(II) linked to denitrification resulted in the enhanced immobilization of aqueous arsenic in anaerobic environments by forming Fe(III) (hydr)oxide coated sands with adsorbed As(V).

Hemimorphite as a natural sink for arsenic in zinc deposits and related mine tailings: Evidence from single-crystal EPR spectroscopy and hydrothermal synthesis

Adsorption and transformation of thioarsenite at hematite/water interface under anaerobic condition in the presence of sulfide

Arsenic immobilization and release in the environment is significantly influenced by bacterial oxidation and reduction of arsenic and arsenic-bearing minerals. In this study, we tested three iron-reducing bacteria, Shewanella oneidensis MR-1, Shewanella sp. HN-41, and Shewanella putrefaciens 200, which have diverse arsenate-reducing activities with regard to reduction of an As-bearing ferrihydrite slurry. In the cultures of S. oneidensis MR-1 and Shewanella sp. HN-41, which are not capable of respiratory reduction of As(V) to As(III), arsenic was maintained predominantly in its pentavalent form, existing in particulate poorly crystalline As-bearing ferrihydrite and formed small quantities of a stable ferrous arsenate [Fe3(AsO4)2] precipitate. However, in the culture of the As(V) reducer, S. putrefaciens 200, As(V) was reduced to As(III) and a small fraction of As-bearing ferrihydrite was transformed into ribbon-shaped siderite that subsequently re-released arsenic into the liquid phase. Our results indicated that release of arsenic and formation of diverse secondary nanoscale Fe-As minerals are specifically closely related to the arsenic-reducing abilities of different bacteria. Therefore, bacterial arsenic reduction appears to significantly influence As mobilization in soils, minerals, and other Fe-rich environments.

Arsenic immobilization in soils and sediments is primarilycontrolledby its sorption onto or incorporation into reactive soil minerals,such as iron (oxyhydr)­oxides. However, coexisting ions (e.g., dissolvedbicarbonate, phosphate, silica, and organic matter) can negativelyimpact the interaction of the toxic arsenate species with iron (oxy)­hydroxides.Of special note is inorganic phosphate, which is a strong competitorfor sorption sites due to its analogous chemical and structural natureto inorganic arsenate. Much of our understanding of this competingnature between phosphate and arsenate focuses on the impact on mineralsorption capacities and kinetics. However, we know very little abouthow coexisting phosphate will alter the stability and transformationpathways of arsenate-bearing Fe (oxyhydr)­oxides. In particular, thelong-term fate and behavior regarding arsenate immobilization areunknown under anoxic conditions. Here, we document, through mineraltransformation reactions, the immobilization of both phosphate (P)and arsenate [As­(V)] in secondary mineral products and characterizetheir changing compositions during the transformations. We did thiswhile controlling the initial P/As­(V) ratios. Our results documentthat, in the absence or at low P/As­(V) ratios, the initial ferrihydriterapidly transforms to green rust sulfate (GRSO4), which further transforms into magnetite after 180 days. Meanwhile,high P/As­(V) ratios resulted in a mixture of GRSO4 and vivianite, with magnetite as a minor fraction. Invariably,the speciation and partitioning of As­(V) were also affected by theP/As­(V) ratio. A higher P/As­(V) ratio also led to a faster partialreduction of mineral-bound As­(V) to As­(III). The most important findingis that the initial ferrihydrite-bound As­(V) became structurally incorporatedinto magnetite [low P/As­(V) ratio] or vivianite [high P/As­(V) ratio]and was thus immobilized and not labile. Overall, our results highlightthe influence of coexisting phosphate in controlling the toxicityand mobility in anoxic, Fe2+-rich subsurface settings,such as contaminated aquifers.

Enhancing immobilization of arsenic in groundwater: A model-based evaluation

On the site of a former pulp mill the arsenic content of groundwater was found to be up to 2 mg/L. High costs for decontamination and remediation were estimated, due to the fact that area is still in industrial use. A long term pollution potential in the case of adopting pump and treat measures was calculated. As the early site investigations gave evidence for immobilization of arsenic in the aquifer within close range of the hot spot, detailed laboratory and field measurements were carried out 1999/2000. The goal was to understand the principles of dissolution and precipitation of arsenic in sulfate reducing environments as a basis for a MNA-concept. The interactions between arsenic sulfur and iron under the influence of organic matter (sulfate reducing conditions) is complex. Both the soluble arsenic-sulfur species and the arsenic solid phases that are formed were not known. However, experience in the remediation of groundwater contaminations by acid mine water had shown that arsenic can be immobilized under sulfate reducing conditions by iron-bearing reaction walls (Blowes & Ptacek 1996; Köber et al. 2005; Wilopo et al. 2008).

Dispersion of natural arsenic in the Malcantone watershed, Southern Switzerland: field evidence for repeated sorption–desorption and oxidation–reduction processes

Extremely arsenic-rich acid mine waters have developed by weathering of native arsenic in a sulfide-poor environment on the 10th level of the Svornost mine in Jáchymov (Czech Republic). Arsenic rapidly oxidizes to arsenolite (As2O3), and there are droplets of liquid on the arsenolite crust with high As concentration (80,000-130,000 mg·L(-1)), pH close to 0, and density of 1.65 g·cm(-1). According to the X-ray absorption spectroscopy on the frozen droplets, most of the arsenic is As(III) and iron is fully oxidized to Fe(III). The EXAFS spectra on the As K edge can be interpreted in terms of arsenic polymerization in the aqueous solution. The secondary mineral that precipitates in the droplets is kaatialaite [Fe(3+)(H2AsO4)3·5H2O]. Other unusual minerals associated with the arsenic lens are běhounekite [U(4+)(SO4)2·4H2O], štěpite [U(4+)(AsO3OH)2·4H2O], vysokýite [U(4+)[AsO2(OH)2]4·4H2O], and an unnamed phase (H3O)(+)2(UO2)2(AsO4)2·nH2O. The extremely low cell densities and low microbial biomass have led to insufficient amounts of DNA for downstream polymerase chain reaction amplification and clone library construction. We were able to isolate microorganisms on oligotrophic media with pH ∼ 1.5 supplemented with up to 30 mM As(III). These microorganisms were adapted to highly oligotrophic conditions which disabled long-term culturing under laboratory conditions. The extreme conditions make this environment unfavorable for intensive microbial colonization, but our first results show that certain microorganisms can adapt even to these harsh conditions.

Effect of hydroquinone-induced iron reduction on the stability of scorodite and arsenic mobilization

Characterization of the dissolution of tooeleite under Acidithiobacillus ferrooxidans relevant to mineral trap for arsenic removal

The arsenic minerals in the environment constitute the primary source of the environmentally occurring arsenic. The As minerals interact with the environment and this renders either their dissolution or the formation of secondary minerals, or both. The distribution of the environmental arsenic is determined by the biogeochemical transformations with respect to the redox conditions, the pH, the availability of ions, the adsorption-desorption, dissolution and the biological activity. The arsenic in the environment is sorbed primarily by metal oxides, especially the ones of iron (Fe), aluminum (Al) and manganese (Mn). These are thought to bind As(+5) readily than As(+3). The overall adsorption depends greatly on pH. Metal oxides such as the ones of hydrous ferric, manganese and aluminum are additional important sinks of arsenic that is adsorbed or co-precipitated. Their dissolution depends also on pH. The redox potential and the microbial activity. Final result is the release of arsenic chemical species into the environment. This review presents a systematic compilation of the major geochemical processes that govern arsenic fate in the environment. The paper attempts to compile the removal capacity of constituents which could be useful for the purpose of As remediation.

Arsenic, lead, fluorine, nitrogen, and carbon are common in the near-surface environment, but their concentrations in water, solids, and biota are highly variable. The distribution of As, Pb, F, N, and C in the environment is dependent on source, mineralogy, speciation, biological interactions, and geochemical controls. The As minerals interact with environment, and this renders either their dissolution or the formation of secondary minerals, or both. The distribution of the environmental arsenic is determined by the biogeochemical transformations with respect to the redox conditions, the pH, the availability of ions, the adsorption–desorption, dissolution, and the biological activity. The biological transformation and cycling of As can lead to oxidation or reduction of species that mobilize As. Besides, a significant proportion of As can also be remobilized from the soils through the process of anion exchange. Large variations can be observed on all spatial scales influenced by a variety of natural processes including nongeological influences such as climate and vegetation. Continental weathering of bedrocks contributes natural Pb to sediments, while mining and refining of Pb-bearing ores, which are subsequently used for industrial Pb applications, supply anthropogenic Pb to the environment. Lead geochemistry of rivers and costal environments plays a significant role in the biogeochemical cycling of Pb and pollutant delivery at the land–sea interface. Fluorine is ubiquitous in the environment with most deriving from natural sources, these being normal weathering processes resulting in F release from rocks and minerals, volcanic activity, and marine aerosol emission, together with biomass burning, being in part natural. However, there are several sources of anthropogenically derived F, which in some areas represent a threat to the biosphere. Together with carbon, oxygen, and hydrogen, nitrogen is one of the four most common elements in living cells and an essential constituent of proteins and nucleic acids, the two groups of substances that can be said to support life. The important nitrogen pools are soil organic matter, rocks (in fact the largest single pool), sediments, coal deposits, organic matter in ocean water, and nitrate in ocean water. The next most common gaseous form of nitrogen in the atmosphere after molecular nitrogen is dinitrogen oxide. The geochemistry of carbon is the transformations involving the element carbon within the systems of the earth. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum, or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respires back into the atmosphere. Carbon can form a huge variety of stable compound. It is an essential component of living matter. Carbon makes up only 0.08% of the combination of the lithosphere, hydrosphere, and atmosphere.KeywordsMineral elementsArsenicFluorineLeadNitrogenCarbonBiogeochemical transformationsEnvironment