Abstract
Highly reactive nano-scale biogenic magnetite (BnM), synthesized by the Fe(III)-reducing bacterium Geobacter sulfurreducens, was tested for the potential to remediate alkaline Cr(VI) contaminated waters associated with chromite ore processing residue (COPR). The performance of this biomaterial, targeting aqueous Cr(VI) removal, was compared to a synthetic alternative, nano-scale zero valent iron (nZVI). Samples of highly contaminated alkaline groundwater and COPR solid waste were obtained from a contaminated site in Glasgow, UK. During batch reactivity tests, Cr(VI) removal from groundwater was inhibited by ∼25% (BnM) and ∼50% (nZVI) when compared to the treatment of less chemically complex model pH 12 Cr(VI) solutions. In both the model Cr(VI) solutions and contaminated groundwater experiments the surface of the nanoparticles became passivated, preventing complete coupling of their available electrons to Cr(VI) reduction. To investigate this process, the surfaces of the reacted samples were analyzed by TEM-EDX, XAS and XPS, confirming Cr(VI) reduction to the less soluble Cr(III) on the nanoparticle surface. In groundwater reacted samples the presence of Ca, Si and S was also noted on the surface of the nanoparticles, and is likely responsible for earlier onset of passivation. Treatment of the solid COPR material in contact with water, by addition of increasing weight % of the nanoparticles, resulted in a decrease in aqueous Cr(VI) concentrations to below detection limits, via the addition of ⩾5% w/w BnM or ⩾1% w/w nZVI. XANES analysis of the Cr K edge, showed that the % Cr(VI) in the COPR dropped from 26% to a minimum of 4-7% by the addition of 5% w/w BnM or 2% w/w nZVI, with higher additions unable to reduce the remaining Cr(VI). The treated materials exhibited minimal re-mobilization of soluble Cr(VI) by re-equilibration with atmospheric oxygen, with the bulk of the Cr remaining in the solid fraction. Both nanoparticles exhibited a considerable capacity for the remediation of COPR related Cr(VI) contamination, with the synthetic nZVI demonstrating greater reactivity than the BnM. However, the biosynthesized BnM was also capable of significant Cr(VI) reduction and demonstrated a greater efficiency for the coupling of its electrons towards Cr(VI) reduction than the nZVI.
Highlights
Chromium is a priority pollutant found at contaminated sites associated with a variety of previous land uses; from its initial mining and processing as a chromite ore (Kimbrough et al, 1999) through to its use in metallurgy, leather tanning, pigment production and wood preservatives (Kamaludeen et al, 2003)
BET specific surface areas of 17.1 m2 gÀ1 and 14.6 m2 gÀ1 were recorded for the biogenic nano-magnetite (BnM) and nano-scale zero valent iron (nZVI) respectively
This study demonstrated that both NPs investigated were capable of appreciable quantities of reductive Cr(VI) removal from alkaline solutions
Summary
Chromium is a priority pollutant found at contaminated sites associated with a variety of previous land uses; from its initial mining and processing as a chromite ore (Kimbrough et al, 1999) through to its use in metallurgy, leather tanning, pigment production and wood preservatives (Kamaludeen et al, 2003). Sensitive, and under environmental conditions, typically occurs in the Cr(III) and Cr(VI) oxidation states. The oxidation state exerts a major control over its physicochemical and biochemical behavior (Kotasand Stasicka, 2000). Cr(VI) typically occurs as the hydrolyzed species; H2CrO4, HCrO4À, CrO42À, and Cr2O72À, which are readily soluble and weakly adsorb to mineral surfaces (Kimbrough et al, 1999), causing oxidative damage upon entering the cell (Dayan and Paine, 2001).
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