Synergistic Effects of Reduced Nontronite and Organic Ligands on Cr(VI) Reduction

Widespread iron-bearing clay minerals are potential materials that can reduce and immobilize Cr(VI) as insoluble Cr₂O₃/Cr(OH)₃. The kinetics of this process is controlled by various environmental factors, yet the effects of such factors on Cr(VI) transformation by iron-bearing clays are poorly understood. Herein, we report the synergistic effects of reduced nontronite (rNAu-2) and environmentally prevalent organic ligands on Cr(VI) reduction under near-neutral pH conditions. The presence of ligands belonging to α-hydroxyl or carbonyl carboxylates, such as tartrate, malate, lactate, pyruvate, and mandelate, significantly promoted the rate and extent of Cr(VI) reduction by rNAu-2, likely because of the formation of Cr(V)-ligand complexes and resulting electron transfer from the ligand to Cr(V). In contrast, ligands containing carboxyl groups only, such as succinate and propionate, had a slightly inhibitory or no effect, likely because of their weak complexing ability with Cr(V) and lack of electron transfer from the ligand to Cr(V). In addition, α-hydroxyl carboxylates are probably more easily oxidized by Cr(V)/Cr(IV) than carboxylates. Soluble Cr(III)-organic complexes were the dominant products of Cr(VI) reduction in the presence of tartrate and malate. This study highlights the importance of organic ligands in the biogeochemical cycling of chromium and has significant implications for chromium remediation in contaminated environments.