Formation and Transport of Cr(III)-NOM-Fe Colloids upon Reaction of Cr(VI) with NOM-Fe(II) Colloids at Anoxic-Oxic Interfaces.
2020
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.
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