Gypsum-integrated, goethite-modified biochar enhances Cd and As immobilization in contaminated paddy soil under contrasting redox conditions.

Goethite-enriched biochar has been widely studied for contaminant stabilization. Yet, the development and efficacy of a single composite material that integrates goethite-biochar with a calcium-based amendment (e.g., gypsum) for co-contaminated soils remain unexplored, particularly the interactive effects on metal immobilization. To address this gap and leverage the potential of calcium sulfate to enhance GBC performance, we developed a novel gypsum-integrated, goethite-modified biochar (Gyp-GBC) and applied it to cadmium (Cd) and arsenic (As) co-contaminated alkaline paddy soil across contrasting redox regimes, demonstrating synergistic Fe-Ca interactions that increase As/Cd immobilization beyond additive effects. Gypsum or GBC alone were significantly less effective (p < 0.05) than Gyp-GBC in decreasing bioavailable Cd and As by 54.3% and 42.9% under flooded conditions and by 50.6% and 46.0% under unsaturated conditions. Moreover, Gyp-GBC reduced Cd extractable by the toxicity characteristics leaching procedures by 56.1%, 49.1% and As by 42.7%, 50.9% under flooded and unsaturated regimes, respectively, demonstrating a marked reduction in leachability and potential toxicity. Microscopic and spectroscopic techniques, such as scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), further demonstrated that immobilization was due to Fe-mediated redox processes, precipitation, and complexation. Collectively, Gyp-GBC showed increased immobilization ability in redox-variable soil. These results improve understanding of Fe-mediated geochemical processes in alkaline paddy soils.