K2FeO4-modified vinasse biochar reduces chromium accumulation in rice grains and enhances Cr(VI) reduction in paddy soil.

K2FeO4-modified biochar (KFB) has shown potential for chromium (Cr) immobilization, but its effects on Cr transformation and translocation within the soil-rice continuum remain poorly understood. We synthesized KFB from vinasse-derived biochar and conducted a pot experiment with Cr-spiked soil (300 mg·kg-1) under four treatments: control (CK), 1% unmodified biochar (VB), 1% FeCl3-modified biochar (FB), and 1% KFB. After 120 days of rice growth, iron plaque (IP) and Cr fractions were determined to elucidate the impact of KFB on Cr dynamics in this system. Compared to CK, VB, and FB, KFB significantly reduced grain Cr concentration by 20.1, 5.3, and 4.1 times, respectively. KFB also increased dithionite-citrate-bicarbonate (DCB)-extractable Fe by 49.7%, 58.3%, and 31.5% relative to the three controls, suggesting enhanced IP formation and greater Cr sequestration within IP. The Cr(VI) reduction rate peaked at 90.6% under KFB, exceeding other treatments (83.9-87.6%). KFB exhibited the highest concentrations of Fe(II) and free Fe oxides, both positively correlated with IP formation and Cr complexation. In soil, Cr was predominantly associated with Fe-Mn oxides (FM: 56.3-58.9% of total Cr), with KFB yielding the highest FM-Cr content. Elevated organic matter in biochar-amended soils further promoted Cr immobilization. KFB effectively mitigates Cr accumulation in rice grains and enhances Cr(VI) reduction in rhizosphere soil, primarily via promoting iron plaque formation and Fe-Mn oxide-bound Cr sequestration.