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

Abstract

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.

Enhanced Oxidation of Cr(III)–Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore

“…Desferrioxamine B (DFOB), a commercially available trihydroxamate siderophore, was selected as a representative siderophore due to its ubiquity in soils/groundwater and significance in dissolution and transformation of Cr­(III)–Fe­(III) hydroxides. , DFOB was purchased as a mesylate salt (C 25 H 46 N 5 O 8 NH 3 + (CH 3 SO 3 ) − ) from Sigma-Aldrich and used without any treatment. The Cr­(III)-DFOB complex was prepared by mixing a Cr­(III) solution (by dissolving CrCl 3 ·6H 2 O into MQ-H 2 O, Shanghai Macklin Biochemical Co., Ltd., analytical grade) with a DFOB solution at pH 6 at a molar ratio of 1/1 for 48 h (Section S4 in the Supporting Information) . The 3-(cyclohexylamino)-1-propanesulfonic acid buffer (CAPS, p K a = 10.4 at 25 °C, Sigma-Aldrich, AR grade) was used to maintain the experimental pH because it had little influence on Cr­(III) oxidation and a weak complexing capacity with metals such as Cr and Fe …”

“…Dissolved Cr and Fe concentrations in the filtrate were measured with ICP-MS (Section S5 in the Supporting Information). The Cr­(VI) concentration in the filtrate was measured with the 1,5-diphenylcarbazide colorimetric method (DPC) (Section S6 in the Supporting Information) . In addition, another 0.5 mL sample was collected from the reactors and mixed with 0.5 mL of 20 mM PO 4 3– at pH 10 for 10 min to extract any adsorbed Cr­(VI) on Cr­(III)–Fe­(III) hydroxides. , The Cr­(VI) recovery efficiency using this method was about 90 ± 5% .…”

“…The Cr­(VI) concentration in the filtrate was measured with the 1,5-diphenylcarbazide colorimetric method (DPC) (Section S6 in the Supporting Information) . In addition, another 0.5 mL sample was collected from the reactors and mixed with 0.5 mL of 20 mM PO 4 3– at pH 10 for 10 min to extract any adsorbed Cr­(VI) on Cr­(III)–Fe­(III) hydroxides. , The Cr­(VI) recovery efficiency using this method was about 90 ± 5% . After the centrifugation and filtration, the Cr­(VI) concentration in the filtrate was measured with the DPC method (Section S6 in the Supporting Information) and represented the amount of total Cr­(VI) in the reactors (i.e., the sum of dissolved Cr­(VI) and adsorbed Cr­(VI)) .…”

“…The Cr(III)-DFOB complex was prepared by mixing a Cr(III) solution (by dissolving CrCl 3 • 6H 2 O into MQ-H 2 O, Shanghai Macklin Biochemical Co., Ltd., analytical grade) with a DFOB solution at pH 6 at a molar ratio of 1/1 for 48 h (Section S4 in the Supporting Information). 36 The 3-(cyclohexylamino)-1-propanesulfonic acid buffer (CAPS, pK a = 10.4 at 25 °C, Sigma-Aldrich, AR grade) was used to maintain the experimental pH because it had little influence on Cr(III) oxidation and a weak complexing capacity with metals such as Cr and Fe. 17 Batch Oxidation Experiments of Cr intermittently (approximately every 12 h) to allow air exchange.…”

“…21 After the centrifugation and filtration, the Cr(VI) concentration in the filtrate was measured with the DPC method (Section S6 in the Supporting Information) and represented the amount of total Cr(VI) in the reactors (i.e., the sum of dissolved Cr(VI) and adsorbed Cr(VI)). 36 The Cr(VI) generation rate from Cr 1−x Fe x (OH) 3 oxidation was described by a zero-order kinetic model without including the zero time point for all experimental groups since both Cr(III) and oxygen concentrations in the reactors were in excess compared with the amounts of consumption. 17 To check if Fe(III) reduction occurred along with Cr(III) oxidation, the concentration of structural Fe(II) in the residual solids was measured with a wet chemical method (Section S7 in the Supporting Information).…”