Titanium dioxide-loaded biochar composite simultaneously reduces arsenic mobilization and methane emissions in flooded paddy soils

Abstract Arsenic (As) contamination and methane emissions from flooded paddy soils pose persistent challenges to food safety and environmental sustainability. Biochar-based amendments have attracted increasing attention for their capacity to regulate soil redox processes and associated biogeochemical cycling; however, achieving simultaneous mitigation of arsenic mobilization and methane emissions remains challenging. In this study, we developed a titanium dioxide (TiO 2 )-loaded biochar composite and evaluated its effectiveness in controlling As mobilization and methane production in flooded paddy soil. The TiO 2 -loaded biochar composite exhibited superior adsorption capacity for arsenite [As(III)] compared with arsenate [As(V)], even in the presence of competing anions. In laboratory assays, the composite effectively captured As(III) released during microbial reduction of ferrihydrite by Shewanella oneidensis MR-1, while As(V) immobilization remained largely unchanged. When applied to flooded paddy soils, the composite significantly reduced dissolved organic matter (DOM) concentrations, thereby disrupting its dual role as an electron shuttle and microbial carbon source. This reduction suppressed Fe(III) reduction and subsequent As mobilization. Under methanogenic and sulfate-reducing conditions, where pore-activated biochar showed limited effect, TiO 2 -loaded biochar composite decreased porewater As concentrations by 88.3% over 30 days and substantially inhibited methane emissions. These results demonstrate that the TiO 2 -loaded biochar composite can simultaneously mitigate As mobilization and methane production by combining high As(III) affinity with redox-mediated regulation of soil microbial processes. Graphical Abstract