Overlooked Impact of Amorphous SiO2 in Biochar Ash on Cadmium Behavior during the Aging of Ferrihydrite-Biochar-Cadmium Coprecipitates.

The formation and transformation of ferrihydrite-cadmium (Fh-Cd) coprecipitates play a critical role in controlling the mobility and bioavailability of Cd in soils. However, the impact of biochar particles (BCs), which are widely applied to agricultural soils, on the transformation of metastable Fh-Cd coprecipitates remains unclear. Herein, we investigated Cd behavior during the aging of Fh-BCs-Cd coprecipitates synthesized by using different BCs: pristine BC, graphitized BC (BCP), BC enriched with oxyl groups (BCO), and ash-removed BC (BCM). The incorporation of pristine BC, BCP, and BCO significantly reduced the transformation rates (K) of coprecipitates (K = 0.06-0.22 × 10-2 day-1). In contrast, Fh-BCM-Cd (K = 0.53 × 10-2 day-1) underwent a similar transformation to Fh-Cd (K = 0.71 × 10-2 day-1), forming hematite with the {104} facet and goethite with the dominant facet of {110} and incorporating 11.61% of Cd into these minerals. As explored by FTIR, XAFS, and DFT calculations, SiO2, the dominant component in BC ash, was a key factor in inhibiting the transformation of Fh-BCs-Cd coprecipitates and suppressing Cd redistribution. This work highlights the inhibition of amorphous SiO2 in incorporated BCs on the transformation of Fh-BCs-Cd coprecipitates, providing deeper insight into Cd behavior in BC-amended agricultural soils.