Interactions of Aluminum with Biochars and Oxidized Biochars: Implications for the Biochar Aging Process

Interactions of aluminum with primary and oxidized biochars were compared to understand the changes in the adsorption properties of aged biochars. The structural characteristics of rice straw-derived biochars, before and after oxidation by HNO3/H2SO4, were analyzed by element composition, FTIR, and XPS. The adsorption of Al to primary biochars was dominated by binding to inorganic components (such as silicon particles) and surface complexation of oxygen-containing functional groups via esterification reactions. Oxidization (aging) introduced carboxylic functional groups on biochar surfaces, which served as additional binding sites for Al3+. At pH 2.5–3.5, the Al3+ binding was significantly greater on oxidized biochars than primary biochars. After loading with Al, the −COOH groups anchored to biochar surfaces were transformed into COO– groups, and the negative surface charge diminished, which indicated that Al3+ coordinated with COO–. Biochar is suggested as a potential adsorbent for removing Al from acidic soils.