Iron Oxyhydroxide Minerals Research Articles

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.

Soil Covers for Controlling Acid Generation in Mine Tailings: a Laboratory Evaluation of the Physics and Geochemistry

To evaluate the effectiveness of soil covers, column experiments were conducted on tailings protected by a three-layer soil cover and tailings directly exposed in the open laboratory for a period of 760 days. Periodic rain application was performed to simulate field conditions, and at four times during the experiments the pore water was completely flushed out of each column for analysis. Profiles of oxygen, temperature, and volumetric water content were measured throughout the experiment, and the post-testing pore water quality was also characterized. A one-dimensional semi-analytic diffusion model was used to simulate oxygen profiles in the uncovered tailings. Modelling performed using the geochemical code MINTEQ showed that in the laboratory, aluminium concentrations in the tailings pore water were controlled by Al(OH)SO4, sulphate by gypsum and Al(OH)SO4and iron by lepidocrocite in the upper half and by ferrihydrite in the lower half. In the field, however, the iron oxyhydroxide minerals formed in the oxidized zone appear to be dissolving. It was found that the cover was effective in preventing significant desaturation of the clay, even over a 150-day drying period. The covered tailings did not oxidize much during the experiments. In the uncovered tailings, oxygen modelling and examination of the geochemistry show that the rate of gross oxidation and the advancement of the oxidation front decreases with time. However, pore water quality is controlled by geochemical processes other than oxidation, as reported in the field.

The distribution of rare earth and other elements and the mineralogy of the iron oxyhydroxide phase in marine ferromanganese concretions from within Slupsk Furrow in the southern Baltic

Rare earth element concentrations in ferromanganese concretions sampled from Slupsk Furrow in the Polish Exclusive Economic Zone are similar to those of concretions from the Gulf of Bothnia. The lack of positive Ce anomalies in the concretions from Slupsk Furrow indicates that they are formed under less oxidizing conditions than spheroidal concretions from the Gulf of Bothnia. Mössbauer studies indicate that poorly crystalline lepidocrosite is the principal iron oxyhydroxide mineral present in these concretions.

An EXAFS Investigation of the Mechanism of Cd Attenuation on Fe and Mn (Oxyhydr)Oxide Minerals

The structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (~-FeOOH), lepidocrocite (?-FeOOH), a k a g a n e i t e ( ~ F e O O H ) , s c h w e r t m a n n i t e (FesOs(OH)6SO4), and cryptomelane (KMn8OI6) have been determined in situ (in the presence of water) using Extended X-Ray Adsorption Fine Structure (EXAFS) spectroscopy. Sorption on these minerals may limit the mobility of Cd in soils and groundwater.

Microbially catalyzed dissolution of iron and aluminum oxyhydroxide mineral surface coatings

This experimental study investigated the processes by which microbes interact with oxyhydroxide mineral surface coatings using an approach designed to better represent the conditions of natural subsurface environments. The interactions of Shewanella putrefaciens, a facultative anaerobe capable of dissimilatory iron reduction, with coatings of Fe 3+ and Al 3+ oxyhydroxides on natural quartz and silica glass surfaces were examined. Using synthetic groundwater solutions having- compositions that simulated a typical aquifer, bacteria were seeded onto mineral surfaces (and coatings) and incubated in parallel with abiotic controls for up to 96 h under aerobic and anaerobic conditions. Microbial-mineral surface interactions were determined using the direct observational technique, Fluid Tapping Mode™ Atomic Force Microscopy (TMAFM) in combination with measurements of ferrous iron concentrations and pH of the incubating solutions. Observations of live bacteria-surface interactions exposed to aerobic conditions showed localized pitting on Fe 3+ oxyhydroxide coated quartz surfaces within 72 h of incubation. These pits corresponded directly to sites of bacterial surface adhesion and the extent of pitting was accompanied by the accumulation of ferrous iron to low but steady-state concentrations. Localized pitting was not observed on any Al 3+ oxyhydroxide coated surfaces. In contrast, iron coated surfaces exposed to bacteria under anaerobic conditions revealed progressive, nonlocalized Fe loss over 96 h. This correlated with a temporal increase in ferrous iron concentrations in the bacteria-exposed solutions compared to the abiotic controls. Aqueous chemical measurements combined with the Fluid TMAFM observations indicate biologically-catalyzed iron reduction under both aerobic and anaerobic incubation. The pitting mechanism observed under aerobic conditions is proposed to result from a redox reaction at the bacteria-iron interface followed by the reoxidation of Fe 2+ onto the surface. The evidence suggests that bacteria under anaerobic conditions maximize rates of dissimilatory reduction by remaining passively mobile on the surface. The weaker bacterial adhesion under anaerobic conditions enhances opportunities for bacteria-iron mineral surface contact. These findings may improve our understanding of relationships between the redox environment and bacterial mobility in the subsurface.

Molecular statics calculations for iron oxide and oxyhydroxide minerals: Toward a flexible model of the reactive mineral-water interface

Molecular statics calculations are used to model the major FeOOH polymorphs and hematite. The potentials were taken from a previous investigation of Fe(III) in aqueous solutions which involved the extrapolation of the gas-phase Fe(III)-OH 2 potential energy surface to the solvated hexaaqua complex. Using this model for the solid phases, lattice parameters for goethite, akaganeite, lepidocrocite, and hematite are generally within 4% of experiment. Internal energies (at 0 K) were computed for each structure; lepidocrocite is energetically the most stable polymorph, followed by akaganeite, with goethite being the least stable. While the model exhibits some variances with experiment, it performs remarkably well, despite the challenging constraint of being consistent with a dissociating molecular dynamics model for water in its gas, aqueous, and solid phases. Because of this consistency, the model allows qualitative theoretical treatment of previously unapproachable problems in mineral-water interface geochemistry. We apply the model to identify surface species on the solvated (110) surface of goethite.

Groundwater chemistry in the Schefferville, Quebec iron deposits

Summary Input precipitation quality, soil clay mineral reactions and aquifer mineral dissolution control the quality of groundwater in an area of discontinuous permafrost in the Laborador Trough iron deposits near Schefferville, Quebec. Na and Cl are derived from precipitation and K from clay minerals. Ca, Mg and HCO3 concentrations and pH are governed by the extent of mixing of saturated water from a dolomite unit with water from other, non-carbonate units. Local conditions govern the equilibrium concentrations in the dolomite-saturated water. On ridge tops carbonate materials are absent and the closed-system evolution path is followed but in one lower area, where a carbonate-containing till is present, evolution is open-system. Iron species concentrations and pE values suggest that groundwater in equilibrium with the iron oxyhydroxide minerals is anoxic. Groundwater temperatures are similar to the annual recharge temperature (4.7°C) and pumping associated with mining may cause an alteration in the permafrost distribution by inducing groundwater flow.