Role of Inherent Inorganic Constituents in SO2 Sorption Ability of Biochars Derived from Three Biomass Wastes.

Biochar is rich in both organic carbon and inorganic components. Extensive work has attributed the high sorption ability of biochar to the pore structure and surface chemical property related to its organic carbon fraction. In this study, three biochars derived from dairy manure (DM-biochar), sewage sludge (SS-biochar), and rice husk (RH-biochar), respectively, were evaluated for their SO2 sorption behavior and the underlying mechanisms, especially the role of inherent inorganic constituents. The sorption capacities of SO2 by the three biochars were 8.87-15.9 mg g(-1). With the moisture content increasing from 0% to 50%, the sorption capacities increased by up to about 3 times, mainly due to the formation of alkaline water membrane on the biochar surface which could promote the sorption and transformation of acidic SO2. DM- and SS-biochar containing larger mineral constituents showed higher sorption capacity for SO2 than RH-biochar containing less mineral components. CaCO3 and Ca3(PO4)2 in DM-biochar induced sorbed SO2 transformation into K2Ca(SO4)2·H2O and CaSO4·2H2O, while the sorbed SO2 was converted to Fe2(SO4)3·H2SO4·2H2O, CaSO4·2H2O, and Ca3(SO3)2SO4·12H2O in SS-biochar. For RH-biochar, K3H(SO4)2 might exist in the exhausted samples. Overall, the chemical transformation of SO2 induced by biochar inherent mineral components occupied 44.6%-85.5% of the total SO2 sorption. The results obtained from this study demonstrated that biochar as a unique carbonaceous material could distinctly be a promising sorbent for acidic SO2 removal in which the inorganic components played an important role in the SO2 sorption and transformation.