Effects of added calcium-based additives on swine manure derived biochar characteristics and heavy metals immobilization

Abstract

Although pyrolysis is a promising way for treating animal manure, the application is restricted with some limitations of biochar. To improve the quality of biochar derived from swine manure and enhance the immobilization of heavy metals (Cu and Zn) in it, swine manure was mixed with four types of Ca-based additives (CaO, CaCO3, Ca(OH)2, and Ca(H2PO4)2) prior to pyrolysis at 300–700 °C. The thermogravimetric characteristics of swine manure were obviously influenced The addition of CaO, CaCO3, and Ca(OH)2 during the whole decomposition process. Furthermore, with the addition of CaO and Ca(OH)2, the emission of CO2 and CO was substantially decreased at 200–500 °C, whereas the formation of CO, H2, CO2, and CH4 was drastically increased at 600–800 °C. The biochar produced with CaO addition had the highest pH, surface area and carbon content. Moreover, by addition of Ca-based additives, except for Ca(H2PO4)2, the transformation of labile Cu and Zn to the stable fraction was promoted, and the leachability and environmental risk of them were simultaneously reduced. In contrast, CaO and Ca(OH)2 were more favorable for the immobilization of Cu and Zn than CaCO3. Our study indicated that the catalytic pyrolysis using CaO was an effective and valuable method of animal manure treatment.