Sorption and desorption of cadmium and zinc in two tropical soils amended with sugarcane-straw-derived biochar

PURPOSE: Biochars may enhance the retention capacity of metals in soils, especially in highly weathered tropical soil whose low cation exchange capacity renders heavy metals mobile, and thus be able to leach from soils. We evaluated the effect of sugarcane-straw-derived biochar on sorption and desorption of Cd(II) and Zn(II) in two tropical soils in particular to distinguish primary and secondary mechanisms of metal retention. MATERIAL AND METHODS: To test the efficiency of biochar to retain heavy metals, sugarcane-straw-derived biochar was mixed with a clayey Oxisol and an Entisol both from the state of Sao Paulo, Brazil, in batch testing to obtain sorption-desorption isotherms of Cd(II) and Zn(II) and measure the release/displacement of cations (Ca²⁺ and Mg²⁺) or precipitation with phosphate during the sorption process. RESULTS AND DISCUSSION: Biochar increased the sorption (including adsorption and precipitation) of both metals in both soils but that most sorption reactions were reversible under buffer acidic conditions, due to dissolution of precipitates in low pH values (