Iron(II/III) Alters the Relative Roles of the Microbial Byproduct and Humic Acid during Chromium(VI) Reduction and Fixation by Soil-Dissolved Organic Matter

Though reduction of hexavalent chromium (Cr(VI)) to Cr(III)
by
dissolved organic matter (DOM) is critical for the remediation of
polluted soils, the effects of DOM chemodiversity and underlying mechanisms
are not fully elucidated yet. Here, Cr(VI) reduction and immobilization
mediated by microbial byproduct (MBP)- and humic acid (HA)-like components
in (hot) water-soluble organic matter (WSOM), (H)WSOM, from four soil
samples in tropical and subtropical regions of China were investigated.
It demonstrates that Cr(VI) reduction capacity decreases in the order
WSOM > HWSOM and MBP-enriched DOM > HA-enriched DOM due to the
higher
contents of low molecular weight saturated compounds and CHO molecules
in the former. The presence of Fe(II/III) selectively coprecipitates
with high molecular weight components (e.g., tannins, lignin, and
CHON-rich compounds) to form ferrihydrite and greatly inhibits Cr(VI)
transformation and fixation in MBP-enriched DOM but enhances that
in HA-enriched DOM. This is probably owing to the combined effects
of (1) the increase of DOM electron-donating capacity and Fe(II) generation
during the reactions of HA with Fe(II) and Fe(III), respectively;
(2) the enrichment of phenolic and carboxyl groups, aromatic compounds,
and carbon defects on ferrihydrite surfaces; and (3) the acceleration
of HA decomposition and MBP mineralization by hydroxyl radicals. These
findings enhance our understanding of the chemodiversity of soil DOM,
the complex interactions between Cr(VI), DOM, and Fe(II/III), and
can help design remediation strategies for contaminated environments.