Promotion of pH buffering capacity and immobilization of cadmium in three paddy soils by adding modified rice straw biochars

ABSTRACT To evaluate the effects of biochar on pH buffering capacity (pHBC) of paddy soils, rice straw biochar (RC) was modified by hydrogen peroxide (H2O2) and nitric acid/sulphuric acid (HNO3/H2SO4), respectively, and then added to the paddy soils developed from sandy Ultisol, clay Ultisol and granite Ultisol at the same proportion. After submerging and draining incubation, soil pHBC, soil pH and the available cadmium (Cd) extracted with calcium chloride were determined. The results indicated that the two modified biochars effectively promoted pHBC of three paddy soils and HNO3/H2SO4 modified biochar (HNO3/H2SO4-RC) led to greater increase in soil pHBC. Compared with control, HNO3/H2SO4-RC apparently increased pHBC of the paddy soils from sandy Ultisol, clay Ultisol and granite Ultisol by 32.7%, 37.4% and 25.3%, respectively. Correspondingly, soil pH increased by 1.59, 1.15 and 0.88 units, and the available Cd decreased by 74.1%, 67.5% and 75.0% in the paddy soils due to the incorporation of HNO3/H2SO4-RC. HNO3/H2SO4 modification significantly increased the carboxyl groups on the surface of RC. The dissociation of these functional groups produced a large number of organic anions, which could consume exogenous protons, and thus retarding the decline of soil pH and decreased soil available Cd.