Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic–Oxic Interfaces

Natural organic matter-iron (NOM-Fe) colloids are ubiquitous at anoxic-oxic interfaces of subsurface environments. Fe(II) or NOM can chemically reduce Cr(VI) to Cr(III), and the formation of Cr(III)-NOM-Fe colloids can control the fate and transport of Cr. We explored the formation and transport of Cr(III)-humic acid (HA)-Fe colloids upon reaction of Cr(VI) with HA-Fe(II) colloids over a range of environmentally relevant conditions. Cr(VI) was completely reduced by HA-Fe(II) complexes under anoxic conditions, and the formation of Cr(III)-HA-Fe colloids depended on HA concentration (or molar C/Fe ratio) and redox conditions. No colloids formed at HA concentrations below 3.5 mg C/L (C/Fe ratio below 1.6), but Cr(III)-HA-Fe colloids formed at higher HA concentrations. In column experiments, Cr(III)-HA-Fe(III) colloids formed under oxic conditions were readily transported through sand-packed porous media. Colloidal stability measurements further suggest that Cr(III)-HA-Fe colloids are highly stable and persist for at least 20 days without substantial change in particle size. This stability is attributed to the enrichment of free HA adsorbed on the Cr(III)-HA-Fe colloid surfaces, intensifying the electrostatic and/or steric repulsion interactions between particles. The new insights provided here are important for evaluating the long-term fate and transport of Cr in organic-rich redox transition zones.