Arsenic and cadmium are contaminants of concern in many rice-growing regions, such as Southeast Asia. Arsenic is a concern due to the geology and the increased mobility and toxicity of As under reduced/flooded soil conditions. Methylated As species (MMA, DMA, TMAO, e.g.) are formed by microbes in flooded rice paddies and can accumulate in rice grain. Arsenic can limit rice yield through straighthead disorder and is also toxic to humans. Minimizing flooding can lower As mobility, but increases Cd mobility in acidic soils and decreases yield. Strategies that minimize both As and Cd uptake while minimizing yield loss are critically needed. The benefits of Si to rice are well known, and these include yield increases and resistance to environmental stressors. Recent research suggests that increasing Si may limit uptake of As via competition of silicic acid and arsenous acid for transporters, and Cd via several mechanisms including increasing soil pH, growth dilution, and retention in cell walls. Release of Si through soil mineral dissolution is slow, and plant available Si is dominated by biogenic sources such as plant residues that are Si-rich such as rice husks. We posit that biochar of rice husk can be optimized for maximum retention of organic As forms as well as Cd²⁺.

The properties of biochar are altered by production parameters such as feedstock type and pyrolysis temperature. Therefore, we investigated the physicochemical changes of rice husk biochar created at various pyrolysis temperatures to determine the biochar for maximum sorption of Cd and organic As species. A key factor was the decrease in biochar surface functional groups seen at high pyrolysis temperatures. Pyrolysis of rice crop residues can transform many of the minerals present, including Si. Thus, we also investigated the effect of biochar production parameters on Si availability. Total Si supply and rate of Si dissolution were evaluated, showing maximum release rate at low pyrolysis temperatures. Through correlation of these results, optimal pyrolysis conditions were identified to maximize both contaminant sorption and Si release to soil by rice husk biochar. Next, we performed sorption and desorption isotherms of organic As species MMA and DMA and Cd²⁺ onto biochars created at different temperatures. We also examined the effects of different biochar additions on methylated As species production in paddy soil, to test if amendment addition provided substrates for methylation. These data provide insight into methylation dynamics in rice paddy soil, and the potential for, and nature of, organometallic sorption by biochar. These optimized biochars can have applications for rice paddies as well as in water treatment to minimize human exposure to As and Cd.