Chromium biogeochemical cycling in basalt-derived paddy soils from the Leizhou Peninsula, South China

Although basalt-derived paddy soils contain high Cr levels and pose serious environmental risks, the mechanisms controlling Cr transformation during anoxic-oxic alternation remain unclear. In this study, the major geochemical processes controlling the release, geochemical speciation, and Cr redistribution during anoxic-oxic alternation were investigated in three basalt-derived paddy soils containing 114, 268, and 429 mg/kg of Cr. The exchangeable and carbonate-bound Cr (available Cr fractions) increased under anoxic phase but decreased under oxic phase, indicating the reversibility of Cr availability. The relationships between the dynamic changes in Fe/ C/N/S and Cr transformations were also revealed. Fe-Mn oxide-bound Cr remained relatively stable during anoxic-oxic alternation, indicating that Cr may be released through reductive dissolution of Fe(III) (hydr)oxides while it was incorporated into the structure of Fe(III) (hydr)oxides through Fe(II)-catalyzed recrystallization under anoxic phase. The organic matter and sulfide-bound Cr increased under anoxic phase but decreased under oxic phase. During the anoxic period, organic matter can form complexes with Cr, and sulfate reduction may induce the formation of Cr sulfide. Based on elementary reactions, kinetic models were established to quantitatively describe Cr transformation during anoxic-oxic alternation in basalt-derived paddy soils. This study revealed the vital factors controlling Cr transformation and redistribution, which are useful for Cr risk assessment of basalt-derived paddy soils.