Article pubs.acs.org/est
Redox Interactions Between Cr(VI) and Fe(II) in Bioreduced Biotite and Chlorite Diana R. Brookshaw,*,† Victoria S. Coker, Jonathan R. Lloyd, David J. Vaughan, and Richard A. D. Pattrick Williamson Research Centre for Molecular Environmental Science, and School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, United Kingdom S Supporting Information *
ABSTRACT: Contamination of the environment with Cr as chromate (Cr(VI)) from industrial activities is of significant concern as Cr(VI) is a known carcinogen, and is mobile in the subsurface. The capacity of Fe(II)-containing phyllosilicates including biotite and chlorite to alter the speciation, and thus the mobility, of redox-sensitive contaminants including Cr(VI) is of great interest since these minerals are common in soils and sediments. Here, the capacity of bacteria, ubiquitous in the surface and near-surface environment, to reduce Fe(III) in phyllosilicate minerals and, thus, alter their redox reactivity was investigated in two-step anaerobic batch experiments. The model Fe(III)-reducing bacterium Geobacter sulfurreducens was used to reduce Fe(III) in the minerals, leading to a significant transformation of structural Fe(III) to Fe(II) of 0.16 mmol/g (∼40%) in biotite and 0.15 mmol/g (∼20%) in chlorite. The unaltered minerals could not remove Cr(VI) from solution despite containing a larger excess of Fe(II) than would be required to reduce all the added Cr(VI), unless they were supplied in a very high concentration (a 1:10 solid to solution ratio). By contrast, even at very low concentrations, the addition of bioreduced biotite and chlorite caused removal of Cr(VI) from solution, and surface and near surface X-ray absorption spectroscopy confirmed that this immobilization was through reductive transformation to Cr(III). We provide empirical evidence that the amount of Fe(II) generated by microbial Fe(III) reduction is sufficient to reduce the Cr(VI) removed and, in the absence of reduction by the unaltered minerals, suggest that only the microbially reduced fraction of the iron in the minerals is redox-active against the Cr(VI).
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INTRODUCTION The ability of iron oxides and (oxyhydr)oxides (such as green rust,1 magnetite2) and Fe-bearing phyllosilicates (including smectite, biotite and chlorite3−5), to immobilize redox-sensitive contaminants through surface-mediated reduction is of great interest for the assessment and remediation of contaminated land. These minerals contain reduced iron (Fe(II)) which can alter the speciation and thus the mobility of redox-sensitive contaminants, such as Cr(VI) or high oxidation state radionuclides such as Np(V). The effect that microbially mediated Fe(III) reduction can have on the redox reactivity of different minerals is key to predicting how the minerals will affect contaminant dynamics, yet the importance of this indirect reduction pathway is only now being realized.5−7 The capacity of phyllosilicates for immobilization of contaminants is of particular interest because they are key sorbents of pollutants both in natural and engineered environments.8,9 The phyllosilicates biotite and chlorite are common and reactive components of soils, and have been found at sites impacted by contamination from industrial activities or radioactive waste storage and disposal (e.g., the Hanford site8). Biotite and chlorite both contain Fe(II) and Fe(III) in their octahedral © 2014 American Chemical Society
sheets, making them important in redox cycling of contaminants in environments where they are the dominant Fe-containing phases.4,8,10,11 Previous work on the interactions of biotite and chlorite with Cr(VI) show that these minerals can reduce Cr(VI) to Cr(III), although the details of the mechanisms remain uncertain.4,5,12−15 Chromate is widespread in environments impacted by industrial contamination.8 It is carcinogenic and toxic when present as (Cr(VI)),7 which is highly mobile in many near surface environments as the chromate ion, Cr(VI)O4−.16 Removal of Cr(VI) from solution in circum-neutral systems can be achieved by reduction to insoluble Cr(III).7 Reduction may be combined with sorption of the Cr(III) to mineral surfaces whereby the Cr will be removed from solution; Cr(III) can also form a precipitate. For example, Fe1−x,Crx(OH)3 is formed by reaction with any available OH and Fe in the system. The rate of abiotic reduction of Cr(VI) to Received: Revised: Accepted: Published: 11337
July 1, 2014 September 1, 2014 September 7, 2014 September 8, 2014 dx.doi.org/10.1021/es5031849 | Environ. Sci. Technol. 2014, 48, 11337−11342
Environmental Science & Technology
Article
for biotite was determined as 9.02 m2/g and for chlorite as 6.43 m2/g. Electronprobe microanalysis (EPMA) was used for iron quantification The biotite was found to contain on average 17.1 ± 0.3 wt % iron and the chlorite, 38.4 ± 0.4 wt % iron. Microbial Fe(III) Reduction. The model Fe(III)-reducing microorganism Geobacter sulf urreducens was used to mediate the reduction of Fe(III) associated with biotite and chlorite. Batches of the bacterium were grown in defined minimal medium with acetate (20 mM) supplied as the electron donor, and fumarate (40 mM) as the electron acceptor,25 at 30 °C in the dark to late log phase, and harvested by centrifugation for 20 min at 5000 g and 4 °C (Sigma centrifuge). The cells were washed twice with 30 mM NaHCO3 buffer at pH 7. Cells were suspended in a small volume of buffer and stored at 4 °C before use in experiments. The capacity of G. sulf urreducens to reduce the mineralassociated Fe(III) was quantified in batch anaerobic experiments. The bacterium was added, at an optical density at 600 nm wavelength (OD600) of ∼0.2, to suspensions of 0.25 g of biotite or chlorite in 10 mL NaHCO3 buffer solution (pH 7; 30 mM) under an atmosphere of 80:20 N2/CO2. Sodium acetate (10 mM) was supplied as the electron donor and the mineralassociated Fe(III) was the only electron acceptor in these experiments. Bottles were incubated at 30 °C in the dark. The reduction was studied in the presence and absence of the artificial electron shuttle anthraquinone-2,6-disulfonate (AQDS) (0.01 mM). To produce large quantities of the bioreduced minerals for Cr(VI) reduction experiments, biotite and chlorite were reduced in batches of 2 g of either mineral in 80 mL of the NaHCO3 buffer (pH7; 30 mM) in the presence of AQDS (0.01 mM). When Fe(II) concentration reached a maximum, determined by ferrozine assay, the contents of the bottles were centrifuged and the supernatant solution was removed. The mineral was washed twice with previously deoxygenated deionized water (18 Ω resistivity) under N2 in a glovebox and pelleted before being suspended in a known volume of deionized water. The reduced state of the Fe-containing mineral stock solution was confirmed by ferrozine assay after washing. Cr(VI) Reduction Experiments. Biotite and chlorite were used in Cr(VI) reduction experiments, either in their unaltered state or after enzymatic Fe(III) reduction mediated by cells of Geobacter sulf urreducens (referred to as bioreduced). An aliquot of mineral was added to 30 mM NaHCO3 buffer (pH 7 ± 0.5) (under an N2 atmosphere) and spiked with K2CrO4 to a final concentration of Cr(VI) of 1 mM. The mineral was supplied in a range of solid to solution ratios (1:10, 1:40 or 1:100). Control experiments were set up where no mineral was added or microbial cells only (final OD600 of ∼0.2) were added. All experiments were conducted at room temperature in the dark. Iron Characterization. The speciation of labile Fe in samples, which was acid-extractable in HCl (0.5 M), was determined using the ferrozine colorimetric assay (with detection limit of ∼1 mM).28,31,32 A slurry sample was removed from an experimental bottle under anaerobic conditions, reacted with 0.5 M HCl for 1 h and the acid-extractable Fe(II) quantified by measurement of the absorbance at 562 nm against known standards using a Camspec M501 single beam scanning UV/vis spectrophotometer. Acid-extracted Fe(III) was then reduced to Fe(II) by addition of excess of hydroxylamine hydrochloride and reaction for 1 h, and the total extractable iron quantified as above. Analysis of the iron chemistry of the bulk mineral was carried out using Mö ssbauer spectroscopy. After completion of bioreduction experiments, the supernatant solutions of the
Cr(III) and the reduction products can vary, depending on the distribution and amounts of Fe(II) in the minerals involved.17,18 Under some conditions, Cr(VI) reduction is predominantly homogeneous,19,20 mediated by aqueous Fe(II). The Fe(II) can be generated by the dissolution of iron-containing minerals, such as biotite dissolution at alkaline pH.15 Chromate reduction in the presence of biotite and chlorite can also occur heterogeneously, mediated by structural Fe(II),4,12 and this may accelerate the rate of reduction as with other phyllosilicates.21 Thus, reduction of structural Fe(III) in minerals can increase the amount and alter the speciation of Fe(II) in the system which, in turn, can affect contaminant immobilization.2,22−24 As bacteria are ubiquitous in these subsurface environments and can mediate Fe(III) reduction, they have a controlling influence on mineral-pollutant interactions. In anoxic conditions, many species can couple the oxidation of organic matter to the reduction of transition metal ions found in minerals, including manganese (Mn(IV/III)) and iron (Fe(III)).3,25,26 As phyllosilicates are common in soils and rocks, they are often intricately involved in this aspect of microbial anaerobic metabolism27 and the availability of Fe(III) in biotite and chlorite for microbial reduction by Shewanella species has been demonstrated previously.5,28 Microbially mediated Fe(III) reduction can give rise to either aqueous or adsorbed Fe(II); for example, where bioreduction of Fe(III) oxides such as ferrihydrite leads to mineral dissolution,29 or to the formation of structural Fe(II) if the reaction is predominantly solid-state (e.g., in some phyllosilicates30). Bacteria, therefore, have the potential to indirectly drive the reduction of Cr(VI) by changing the reactivity of minerals and a recent study by Bishop et al. (2014)5 showed that biogenic Fe(II) in smectites and to a limited extent chlorite, can reduce Cr(VI). However, the effect of this bioreduction on the reactivity of biotite toward Cr(VI) (as a model redox active contaminant) has not been defined and further characterization of the reactivity of the related mineral, chlorite, is warranted. This study is an investigation of biotite and chlorite reduction capacity toward chromate, as controlled by changes in Fe speciation after bioreduction. The objectives of this study were to (i) investigate microbial reduction of Fe(III) in biotite and chlorite using the model Fe(III)-reducing microorganism Geobacter sulf urreducens; (ii) determine the effect of bioreduction of biotite and chlorite on Cr(VI) removal and fate, and (iii) use the data to evaluate the relative reactivities of the natural and biogenic Fe(II) pools in the minerals studied and explore the stoichiometry of the reaction to better understand the mechanism of this microbial-mineral reduction pathway.
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MATERIALS AND METHODS Minerals. The biotite and chlorite used in these experiments were sourced from Excalibur Minerals Company and have been characterized previously (ref 28, see Supporting Information (SI) for further details). The minerals were prepared for use in the experiments as follows. Biotite flakes were powdered using an agate ball mill, and separated into different grain sizes using a shaking sieve stack (grains with diameters of 180−500 μm were used in all biotite experiments). The chlorite used in all experiments was extracted by hand and powdered using a metal file before separating the fine fraction of the powder using a 100 μm sieve. The biotite sample used in the experiments was confirmed as monomineralic by X-ray powder diffraction (XRD) and the chlorite powder was similarly identified as clinochlore. From Brunauer−Emmett−Teller (BET) analysis the surface area 11338
dx.doi.org/10.1021/es5031849 | Environ. Sci. Technol. 2014, 48, 11337−11342
Environmental Science & Technology
Article
experiments were removed and the slurry washed with deoxygenated deionized water. The supernatant was removed and the slurry allowed to dry in an anaerobic Coy cabinet (
