NOM-Induced Dissolution of CrxFe1–x(OH)3 Precipitates and Formation of Cr(III)-NOM-Fe Colloids under Oxic and Anoxic Conditions

Abstract

Mixed Cr(III)–Fe(III) (hydr)oxides (CrxFe1–x(OH)3) are common reduction products of Cr(VI) that have long been considered as the sink of Cr in subsurface environments. While current field and laboratory studies have demonstrated that natural organic matter (NOMox) can dissolve CrxFe1–x(OH)3 under oxic conditions, much less is known regarding the dissolution of CrxFe1–x(OH)3 by reduced NOM (NOMred) and geochemical behaviors of released Cr(III) under anoxic conditions, which limited our ability to completely predict the cycle of Cr. This study provided new knowledge regarding the simultaneous dissolution of CrxFe1–x(OH)3 and formation of Cr(III)-NOM-Fe colloids by NOMox and NOMred under oxic and anoxic conditions. We showed that NOM dissolved CrxFe1–x(OH)3 via ligand-promoted dissolution under oxic conditions and reductive dissolution under anoxic conditions, releasing aqueous Cr(III) and Fe(III/II). Size fractionization results showed that the aqueous Cr(III) and Fe observed at high NOM concentration were Cr(III)-NOM-Fe colloids (ca. 3–450 nm). Dynamic light scattering results further revealed that the colloids had a particle size ranging from 79 to 167 nm and strongly negative surface charges (−40––17 mV). Cryogenic X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements further indicated a close association of NOM with Cr and Fe within the particle structure. This study provides insights into the fate of CrxFe1–x(OH)3 in redox-dynamics and organic-rich environments, which is critical for evaluating the long-term stability of Cr remediation sites.