Competitive adsorption of cadmium and aluminum onto fresh and oxidized biochars during aging processes

Fresh biochar was shown to effectively reduce concentrations of heavy metal in soil, but the influences of the aging processes of biochar and competitions from other elements are warranted to precisely evaluate the long-term effect of biochar. This work investigated the effects of water washing, oxidation, and coexistence of aluminum (Al) on cadmium (Cd) adsorption by biochars and oxidized biochars. The Cd adsorption and the competitive adsorption of Cd and Al to rice straw-derived biochars, before and after oxidation by HNO3/H2SO4, were investigated. Meanwhile, the structural characteristics and surface charges of primary and oxidized biochars, with and without Cd loading, were analyzed by scanning electron microscopy, fourier-transform infrared spectroscopy, and zeta potential. The adsorption of Cd onto fresh biochars was dominated by surface complexation of oxygen-containing functional groups via esterification reactions, which was regulated by solution pH. Oxidization (aging) introduced carboxylic functional groups to biochar surfaces, which served as additional binding sites for Cd. The Cd binding to biochars was significantly affected by the coexistence of Al via acidification and competition for adsorption sites. The biochars exhibited high sorption capacities of Cd in soil, but soil acidification led to a counteractive of biochar’s liming effect and a reduction of Cd-binding sites; thus, the long-term effect of biochar for heavy metal immobilization should be paid more attention in acidic soil.