NOM-Induced Dissolution of Cr_xFe_1–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 (Cr x Fe1–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 Cr x Fe1–x (OH)3 under oxic conditions, much less is known regarding the dissolution of Cr x Fe1–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 Cr x Fe1–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 Cr x Fe1–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 Cr x Fe1–x (OH)3 in redox-dynamics and organic-rich environments, which is critical for evaluating the long-term stability of Cr remediation sites.

A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives

“…Upon reaction of Cr(VI) with Fe(II)-HA colloids under anoxic conditions, Cr was completely reduced and formed highly stable Cr(III)-HA-Fe colloids that persisted for at least 20 days, without substantial changes in particle size. 254,255 Mn is considered to be predominantly present as soluble Mn(II) in anoxic environments and in particulate form (Mn(III/IV)) in oxic environments, but several studies have also observed Mn(III) colloids in anoxic environments. 173,256−258 Due to the strong affinity of Mn(III) with NOM, up to 90% of the Mn(III) in anoxic systems can be in colloidal form, and this fraction can increase with increasing molar C/Mn ratios.…”

“…Pan et al observed the formation of stable colloids (1200 nm) composed of Cr­(III), NOM, and Fe­(III) during Cr­(VI) removal via electrocoagulation at pH 8.0, but below pH 6.0 they aggregated and settled out of the aqueous phase. Upon reaction of Cr­(VI) with Fe­(II)-HA colloids under anoxic conditions, Cr was completely reduced and formed highly stable Cr­(III)-HA-Fe colloids that persisted for at least 20 days, without substantial changes in particle size. , …”

“…Upon reaction of Cr(VI) with Fe(II)-HA colloids under anoxic conditions, Cr was completely reduced and formed highly stable Cr(III)-HA-Fe colloids that persisted for at least 20 days, without substantial changes in particle size. 254 , 255 …”

Co-transport of ferrihydrite–organic matter colloids with Sb(v) in saturated porous media: implications for antimony mobility

“…37,38 Furthermore, the redox reactions between OM and Fe at redox interfaces can substantially influence the transport of a colloid and its ability to carry pollutants. [39][40][41][42][43] Previous research has confirmed that Fe-OM colloids can act as vehicles for As(V), Cr(VI), and U(VI) transport. 42,44 However, there is no comprehensive evaluation on the co-transport of FH-OM colloid-loaded Sb.…”