Mineral–microbe interactions affect the biogeochemical fluxes and cycling of many toxic heavy metals and radionuclides. These contaminants enter the environment from various sources. Because many of these contaminants are carcinogens, their release into the environment, fate cause intense public concern and are the subject of substantial research. There are several mechanisms for the remediation of heavy metals and radionuclides by clay minerals. One classical strategy is adsorption and absorption. Although this strategy may be effective in the initial removal of heavy metals, it may not be permanent because these contaminants can be remobilized if the environmental conditions change. Other strategies involve the co-precipitation of metals and radionuclides into clay mineral structures. This strategy may be more “permanent” as clay minerals are fairly stable under a wide range of conditions. A recent approach is redox manipulation facilitated by microbes. It is assumed that Fe(II) in smectite, possibly produced via bioreduction of structural Fe(III), can reduce oxidized forms of heavy metals to reduced form as a way of immobilization. Many have examined the interactions of clay-associated Fe(II) with heavy metals and radionuclides, including Tc, U, and Cr. Even when excess Fe(II) is available, not all Fe(II) is reactive. Fe(II) reactivity depends on the crystal chemical environment of Fe(II) in clay minerals. For Tc(VII) reduction, Fe(II) in smectite is the most reactive, and Fe(II) in illite the least. Surface area, surface charge, layer expandability, and layer charge between smectite and illite are important factors in accounting for this difference. Not only does smectite reduce and immobilize Tc(VII), but the reduced Tc(IV) resides inside clay mineral matrix, minimizing the potential for remobilization. Reduced structural Fe(II) in smectite is an effective reductant for Cr(VI) as well. Sorption of Cr(VI) onto a clay mineral surface is a prerequisite for subsequent reduction. In contrast, U(VI) reduction by clay-associated Fe(II) is kinetically slow. For some clays like nontronite, redox chemistry may instead favor the oxidation of U(IV) by structural Fe(III) in clay minerals. Because of the rapid oxidation of U(IV) by structural Fe(III) in nontronite, uranium can serve as an electron shuttle for microbial reduction of structural Fe(III) in chlorite.

Heavy metal remediation by nanoscale clay mineral

251st National Meeting (2016)

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