ferrihydrite and arsenopyrite

Mining activities release arsenopyrite into calcareous soils where it undergoes weathering generating toxic compounds. The research evaluates the environmental impacts of these processes under semi-alkaline carbonated conditions. Electrochemical (cyclic voltammetry, chronoamperometry, EIS), spectroscopic (Raman, XPS), and microscopic (SEM, AFM, TEM) techniques are combined along with chemical analyses of leachates collected from simulated arsenopyrite weathering to comprehensively examine the interfacial mechanisms. Early oxidation stages enhance mineral reactivity through the formation of surface sulfur phases (e.g., S n (2-)/S(0)) with semiconductor properties, leading to oscillatory mineral reactivity. Subsequent steps entail the generation of intermediate siderite (FeCO3)-like, followed by the formation of low-compact mass sub-micro ferric oxyhydroxides (α, γ-FeOOH) with adsorbed arsenic (mainly As(III), and lower amounts of As(V)). In addition, weathering reactions can be influenced by accessible arsenic resulting in the formation of a symplesite (Fe3(AsO4)3)-like compound which is dependent on the amount of accessible arsenic in the system. It is proposed that arsenic release occurs via diffusion across secondary α, γ-FeOOH structures during arsenopyrite weathering. We suggest weathering mechanisms of arsenopyrite in calcareous soil and environmental implications based on experimental data.

Topics: Arsenicals; Calcium Carbonate; Carbonates; Electrochemistry; Environmental Monitoring; Ferric Compounds; Iron Compounds; Microscopy, Electron, Scanning; Microscopy, Electron, Transmission; Minerals; Mining; Models, Theoretical; Oxidation-Reduction; Soil; Soil Pollutants; Sulfides; Sulfur; Surface Properties; Weather

Arsenic (As) is commonly associated with Cu ore minerals, with the resultant risk that As can be released offsite from mine tailings. We used synchrotron-based fluorescence X-ray absorption near-edge spectroscopy (XANES) imaging to provide in situ, laterally-resolved speciation of As within tailings which differed in magnetite content (5-12%) and organic matter content (0-5%). Although the total As content was lower in tailings with low magnetite (LM), the soluble (pore water) As was actually 7-times higher in LM tailings than in high magnetite (HM) tailings. Additionally, amendment with 5% sugarcane mulch residues (SMR) (for revegetation) further increased soluble As due to the dissolution and oxidation of arsenopyrite or orpiment. Indeed, in HM tailings, arsenopyrite and orpiment initially accounted for 88% of the total As, which decreased to 48% upon the addition of SMR - this being associated with an increase in As

Topics: Arsenic; Arsenicals; Copper; Ferric Compounds; Ferrosoferric Oxide; Iron; Iron Compounds; Minerals; Mining; Saccharum; Soil; Soil Pollutants; Solubility; Sulfides; Water Pollutants, Chemical

Arsenic is a toxic trace element, which commonly occurs as contaminant in riverine floodplains and associated wetlands affected by mining and ore processing. In this study, we investigated the solid-phase speciation of As in river floodplain soils characterized by circumneutral pH (5.7-7.1) and As concentrations of up to 40.3 g/kg caused by former mining of arsenopyrite-rich ores. Soil samples collected in the floodplain of Ogosta River (Bulgaria) were size-fractionated and subsequently analyzed using a combination of X-ray fluorescence (XRF) spectrometry, powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and selective chemical extraction of poorly crystalline mineral phases. Arsenic and Fe were found to be spatially correlated and both elements were strongly enriched in the fine soil particle size fractions (<2 μm and 2-50 μm). Between 14 and 82% of the total As was citrate-ascorbate extractable. Molar As/Fe ratios were as high as 0.34 in the bulk soil extracts and increased up to 0.48 in extracts of the fine particle size fractions. Arsenic K-edge XAS spectra showed the predominance of As(V) and were well fitted with a reference spectrum of As(V) adsorbed to ferrihydrite. Whereas no As(III) was detected, considerable amounts of As(-I) were present and identified as arsenopyrite originating from the mining waste. Iron K-edge XAS revealed that in addition to As(V) adsorbed to ferrihydrite, X-ray amorphous As(V)-rich hydrous ferric oxides ("As-HFO") with a reduced number of corner-sharing FeO6 octahedra relative to ferrihydrite were the dominating secondary As species in the soils. The extremely high concentrations of As in the fine particle size fractions (up to 214 g/kg) and its association with poorly crystalline Fe(III) oxyhydroxides and As-HFO phases suggest a high As mobilization potential under both oxic and anoxic conditions, as well as a high bioaccessibility of As upon ingestion, dermal contact, or inhalation by humans or animals.

Topics: Arsenic; Arsenicals; Bulgaria; Ferric Compounds; Iron; Iron Compounds; Minerals; Mining; Particle Size; Rivers; Soil; Soil Pollutants; Spatial Analysis; Spectrometry, X-Ray Emission; Sulfides; X-Ray Absorption Spectroscopy; X-Ray Diffraction

Floodplain soils are frequently contaminated with metal(loid)s due to present or historic mining, but data on the bioaccessibility (BA) of contaminants in these periodically flooded soils are scarce. Therefore, we studied the speciation of As and Fe in eight As-contaminated circumneutral floodplain soils (≤ 21600 mg As/kg) and their size fractions using X-ray absorption spectroscopy (XAS) and examined the BA of As in the solids by in-vitro gastrointestinal (IVG) extractions. Arsenopyrite and As(V)-adsorbed ferrihydrite were identified by XAS as the predominant As species. The latter was the major source for bioaccessible As, which accounted for 5-35% of the total As. The amount of bioaccessible As increased with decreasing particle size and was controlled by the slow dissolution kinetics of ferrihydrite in the gastric environment (pH 1.8). The relative BA of As (% of total) decreased with decreasing particle size only in a highly As-contaminated soil--which supported by Fe XAS--suggests the formation of As-rich hydrous ferric oxides in the gastric extracts. Multiple linear regression analyses identified Al, total As, C(org), and P as main predictors for the absolute BA of As (adjusted R(2) ≤ 0.977). Health risk assessments for residential adults showed that (i) nearly half of the bulk soils may cause adverse health effects and (ii) particles <5 μm pose the highest absolute health threat upon incidental soil ingestion. Owing to their low abundance, however, health risks were primarily associated with particles in the 5-50 and 100-200 μm size ranges. These particles are easily mobilized from riverbanks during flooding events and dispersed within the floodplain or transported downstream.

Topics: Arsenic; Arsenicals; Environmental Pollution; Ferric Compounds; Iron Compounds; Minerals; Mining; Rivers; Soil; Soil Pollutants; Sulfides; X-Ray Absorption Spectroscopy

The solid-state speciation of arsenic (As), iron (Fe), and lead (Pb) was studied in the mine waste-derived fertilizer Ironite using X-ray absorption spectroscopy, Mössbauer spectroscopy, and aging studies. Arsenic was primarily associated with ferrihydrite (60-70%), with the remainder found in arsenopyrite (30-40%). Lead was observed almost exclusively as anglesite (PbSO4), with <1% observed as galena (PbS). The identification of As in oxidized Fe oxides and Pb as PbSO4 is in disagreement with the dominant reduced phases previously reported and suggests As and Pb contained within the mine waste-derived product are more bioavailable than previously considered. Aging studies in solution result in Ironite granules separating into two distinct fractions, an orange oxide precipitate and a crystalline fraction with a metallic luster. The orange oxide fraction contained As adsorbed/precipitated with ferrihydrite that is released into solution when allowed to equilibrate with water. The fraction with a metallic luster contained pyrite and arsenopyrite. A complete breakdown of arsenopyrite was observed in Ironite aged for 1 month in buffered deionized water. The observations from this study indicate As and Pb exist as oxidized phases that likely develop from the beneficiation and processing of mine tailings for commercial sale. The potential release of As and Pb has important implications for water quality standards and human health. Of particular concern is the quantity of As released from mine waste-derived products due to the new As regulation applied in 2006, limiting As levels to 10 microg L(-1) in drinking water.

Topics: Adsorption; Arsenic; Arsenicals; Environmental Monitoring; Ferric Compounds; Fertilizers; Iron; Iron Compounds; Lead; Minerals; Oxygen; Spectrometry, X-Ray Emission; Spectroscopy, Mossbauer; Sulfides; Temperature; Water Pollutants; Water Pollutants, Chemical; Water Supply