Role of Coordination Environment in the Oxidation of CrIII Aminocarboxylate Complexes by Hydrous Manganese Oxide

Oxidation of CrIII to CrVI by MnIII,IV (hydr)oxides plays an important role in controlling chromium mobility and toxicity in soils and sediments. Previous studies on CrIII oxidation by hydrous manganese oxide (HMO) reveals that reaction rates are highly sensitive to CrIII speciation, with aminocarboxylate complexes exhibiting reaction-induced Mn2+ accumulation at HMO surfaces, which progressively inhibits CrVI production. In this work, we compare initial CrVI production rates for well-defined, presynthesized CrIII–aminocarboxylate complexes with the chelating agents iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), and ethylenediaminetetraacetic acid (EDTA). Initial-rate measurements minimize the influence of secondary ligand oxidation, reaction-generated MnII, and associated changes in HMO reactivity, thereby providing a common basis for comparing the effects of CrIII coordination environment. Initial CrVI production rates spanned 8 orders of magnitude and generally decrease as aminocarboxylate ligands occupy a greater fraction of the CrIII coordination sphere and fewer coordination positions remain occupied by H2O and OH–. These results demonstrate that CrIII oxidation by Mn oxides is acutely sensitive to the CrIII coordination environment, with important implications for CrVI formation in natural and engineered systems. These findings demonstrate that meaningful prediction of CrVI formation requires explicit consideration of CrIII speciation.