Chemical and physical characterization of rice husk biochar and ashes and their iron adsorption capacity

After flooding in rice crops, the Fe3+ ions from iron oxide minerals are reduced to Fe2+ in the anaerobic conditions, making it soluble. The excess of Fe2+ in soil solution can be toxic to plants, resulting in decreasing rice yield. Pyrolyzed materials from rice crop residues, such as rice husk, can be an environmentally friendly option to reduce iron availability in soil solution, provided they have appropriate chemical and physical characteristics regarding iron adsorption. In this study, rice husk biochar (RHB) and rice husk ashes (RHA1 and RHA2) were characterized regarding physical and chemical characteristics and the iron adsorption capacity. The different oxygenation conditions in obtaining the materials resulted in chemical and physical differences (e.g., biochar carbon content of 46% and ashes of 16% and 0.93%), but there were no significant differences related to iron adsorption capacity in aqueous solution. The iron adsorption capacity of the biochar was 5.53 mg Fe2+ g−1 and of the ashes was 6.74 and 7.22 mg Fe2+ g−1 for the two materials tested, which demonstrates potential of these materials to mitigate iron toxicity in flooded rice crops.