Contrasting effects of rice husk pyrolysis temperature on silicon dissolution and retention of cadmium (Cd) and dimethylarsinic acid (DMA).

Arsenic (As) and cadmium (Cd) are two toxins that affect rice, and their ability to do so may be lessened by soil incorporation of rice husk residues. Rice husks are typically removed from fields and used as a fuel source at rice mills but contain silicon (Si) and other nutrients. It has previously been shown that soil incorporation of rice husk or charred husk can release Si to soil solution to decrease As uptake and promote As methylation, and studies suggest char can additionally decrease Cd availability through several potential mechanisms including adsorption, precipitation, liming, and growth dilution. Charring conditions will impact husk Si dissolution rate and potential to immobilize Cd and possibly methylated As. Here, we compared uncharred husk to husk biochars pyrolyzed at 450, 600, 750, and 900 °C for differences in Si dissolution rate and adsorption of Cd and dimethylarsinic acid (DMA)-the dominant methylated As species present in paddy soils and grain. We hypothesized that Si dissolution rate and Cd adsorption would decrease, and DMA adsorption would increase with pyrolysis temperature. Si release decreased with pyrolysis temperature in the general order: uncharred husk > 450 °C > 600 °C = 750 °C = 950 °C but those differences were not due to SiO2 crystallization with increasing temperature. Additionally, short (< 5 d) lab-based extractions underestimate Si release from uncharred husk while overestimating release from biochars. Controlling for pH changes/liming effect, adsorption isotherms showed very weak DMA adsorption, while Cd adsorption was favored on higher temperature (950 °C) biochar and was not predicted well by cation exchange capacity (CEC). When applied in a soil incubation study using non-contaminated soil, the biochar had no impact on Cd porewater concentrations while low temperature (450 °C) rice husk biochar led to the highest Si:As ratio. Biochar did not strongly influence Cd and DMA solubility at 1% w/w amendment.