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

Abstract

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.

Formation, aggregation, and transport of NOM–Cr(iii) colloids in aquatic environments

“…A pH of 7 was selected because it is an environmentally relevant value ubiquitous in aquatic environments. 7,13,30,36 Aliquots of NOM stock suspension were dispensed into the reactors to create a series of NOM working suspensions at concentrations of 0-50 mg C per L for AHA and 0-100 mg C per L for SRHA, respectively. The concentrations of AHA and SRHA were selected to be representative of those found in aquatic environments.…”

“…The concentrations of AHA and SRHA were selected to be representative of those found in aquatic environments. 7,29,30,[37][38][39][40] Then, 10 mg L −1 (192.3 μM) of Cr(III) from the stock solution was added to these vigorously stirred suspensions, yielding C/Cr molar ratios of 0-43.3. A relatively high Cr(III) concentration was chosen to improve the analytical sensitivity and accuracy for studying the colloidal stability of organically complexed Cr(III).…”

“…6 Remediation effectiveness and efficiency for Cr are thus determined by not only the rate of Cr(VI) reduction but also the subsequent mobility of reduced Cr(III) products. 4,7 Although Cr(VI) concentration declined to a very low concentration as a result of reductive immobilization, we still need to understand the mobility of Cr(III) since the reduced Cr(III) species could be reoxidized to Cr(VI) when the redox conditions shift to oxidizing conditions. [8][9][10] Understanding the transport propensity of Cr(III) species is thus fundamentally important for a thorough estimation of the ultimate fate of Cr and the long-term efficiency of the remediation effort.…”

“…17 However, there is increasing evidence that organically complexed metals are colloids with sizes ranging between 1 and 450 nm. 7,13,21 For example, we have recently observed the formation of NOM-Cr(III) colloids (1-220 nm) upon Cr(VI) reduction by reduced NOM under anoxic conditions. 22 Studies have shown that mobile colloids can be considered as a dominant player in facilitating transport of low solubility metals and radionuclides in subsurface environments.…”

Effects and bioaccumulation of Cr(III), Cr(VI) and their mixture in the freshwater mussel Corbicula fluminea

“…Effects of Cr(III) have also been reported on the structure and permeability of biological membranes (Filice et al, 2019;Suwalsky et al, 2008). Finally, Cr(III) is particle reactive and tends to adsorb onto suspended particulate matter and colloids eventually accumulating in bed sediments (Dominik et al, 2007;Gustafsson et al, 2014;Liao et al, 2020).…”