Inhibition of FeS on Chromium(III) Oxidation by Biogenic Manganese Oxides

Reductive immobilization of Cr(VI) has been widely explored as a cost-effective approach for soil and water remediation. The long-term stability of the immobilized Cr(III), however, has to be addressed for the immobilization technology to be widely deployed. Cr(III) can be oxidized chemically by Mn-oxides produced through microbially mediated Mn(II) oxidation. Whether Cr(III) could be remobilized through the oxidation process under more specific environmental conditions, however, needs to be investigated. This study examined the inhibitive effect of FeS on Cr(III) oxidation by biogenic Mn-oxides that were produced in the culture of a known species of Mn(II) oxidizers, Pseudomonas putida. The results showed that 120 mg/L of well-aged Merck granular FeS did not affect Cr(III) oxidation in the culture of P. putida initially containing Mn(II). In contrast, freshly precipitated FeS slurry at a much lower concentration (10 mg/L) significantly delayed Mn-oxide production and Cr(III) oxidation, without affecting the microbial growth. In the presence of excessive FeS slurry, both Cr(VI) and biogenic Mn-oxides were reduced rapidly. The reduced Cr(III) was not reoxidized by biogenic Mn-oxides as long as freshly formed FeS was present in the systems. The study suggested that FeS produced during the soil and water treatment by technologies such as In Situ Gas Reduction (ISGR) could inhibit Mn(II) oxidation and thus prevent Cr(III) from reoxidation by biogenic Mn-oxides.