[Adsorption Capacity and Mechanism of Biochar Derived from Typical Agricultural Wastes for Cadmium in Aqueous...

Twelve biochar types were derived from animal manure (cow manure, chicken manure, and pig manure) and crop straw (wheat straw, rice straw, and corn straw) at different temperatures (300℃/700℃ and 300℃/500℃) for Cd²⁺ in a solution system in the present study. A scanning electron microscope, Fourier infrared spectrometer, X-ray diffraction, and CHN analyzer were applied to analyze the physical and chemical properties, surface structure, and elemental composition of the biochar. The adsorption capacity and related mechanism of biochar for Cd²⁺ in an aqueous solution was studied. The results showed that the maximum adsorption capacity of cow manure, chicken manure, and pig manure biochar for Cd²⁺ increased from 83.40, 19.65, and 96.74 mg·g^-1 to 106.54, 268.89, and 164.53 mg·g^-1, respectively, with the increase in pyrolysis temperature. With the increase in pyrolysis temperature, the pore structure of biochar became more abundant, oxygen-containing functional groups increased, and the aromatic structure appeared. Quantitative analyses revealed that ion exchange accounted for 12%-52%, chemical precipitation accounted for 27%-79%, complexation accounted for 1%-8%, and cation-π accounted for 1%-28% of the total adsorption capacity of Cd²⁺. With the increase in pyrolysis temperature, the proportion of cation-π interaction increased from 1%-13% to 8%-30%, but the proportion of chemical precipitation and ion exchange were still very high (70%-93%). Therefore, ion exchange and chemical precipitation could be the main mechanisms of agricultural waste biochar adsorption for Cd²⁺.