Cr(VI) Reduction by Siderophore Alone and in Combination with Reduced Clay Minerals

Siderophores and iron-containing clays are known to influence the transformation of chromium in the environment. The role of clays in hexavalent chromium [Cr(VI)] reduction has been reported extensively. However, the mechanisms of Cr(VI) reduction by siderophores and their combination with iron-bearing clays are poorly known. Herein, we report the kinetics and products of Cr(VI) reduction by a siderophore alone or in combination with reduced clays. Results showed that Cr(VI) reduction by a tri-hydroxamate siderophore-desferrioxamine B (DFOB)-at a pH of 6 was achieved by one-electron transfer via the formation of Cr(V) intermediate. The formed Cr(V) was further reduced to organically complexed Cr(III). The Cr(VI) reduction rate and extent in the presence of both DFOB and reduced clays unexpectedly decreased relative to that with reduced clays alone, despite both serving as Cr(VI) reductants. The interaction between DFOB and clays (e.g., adsorption/intercalation, dissolution, and/or oxidation) was primarily responsible for Cr(VI) reduction inhibition. The extent of inhibition increased at higher DFOB concentrations in the presence of iron-rich nontronite but decreased in the presence of iron-poor montmorillonite, which may be related to their different Cr(VI) reduction mechanisms. This study highlights the importance of siderophores in chromium transformation and its impact on the reactivity of iron-bearing clays toward heavy metal reduction in the environment.

Automated summary

Siderophores and iron-containing clays have been found to influence the transformation of chromium. In this study, the reduction of Cr(VI) by a tri-hydroxamate siderophore alone or in combination with reduced clays was investigated. The results showed that the reduction of Cr(VI) by the siderophore involved a one-electron transfer mechanism, resulting in the formation of Cr(V) intermediate and subsequent reduction to organically complexed Cr(III). Surprisingly, the presence of both the siderophore and reduced clays led to a decrease in the rate and extent of Cr(VI) reduction compared to reduced clays alone, indicating an inhibitory effect of the interaction between the siderophore and clays.