Biochar for Lead Ion Removal from Aqueous Solutions: the Effect of Inorganic Carbonate

Biochar, a carbon-rich material that can be produced from forest byproducts and waste, is emerging as a valuable material for removing heavy metal ions from aqueous solutions. Biochar can be produced by many processes and equipment. Among these, mobile air curtain burners, developed to achieve lower levels of particulate matter and smoke than open pile burning, can be deployed in the field where biomass feedstocks are located. To elucidate lead metal ion (Pb²⁺) removal mechanisms, this study characterized seven biochars generated by a novel mobile curtain burner, representing the first such investigation of materials produced by this platform. The Pb²⁺ removal capacities ranged from 35 to 224 mg·g^-1 of biochar. To understand the factors influencing this removal capacity, various physicochemical properties of the biochars were characterized, including the specific surface area, electrical conductivity, zeta potential, and pH. The mineral content, particularly calcium carbonate (CaCO₃), was determined by analyzing the X-ray diffraction (XRD) patterns of the biochar samples using the Rietveld refinement method. Quantitative analysis showed that CaCO₃ was the primary driver in Pb²⁺ removal. Precipitates of cerussite and hydrocerussite crystals were observed in the biochar XRD patterns after Pb²⁺ removal. A linear correlation was observed between the Pb²⁺ removal capacities of these biochars and the CaCO₃ contents. The results indicate that the Pb²⁺ removal capacities of these biochar samples were predominantly determined by the CaCO₃ contents through the chemical precipitation mechanism.