Abstract: Biochar is a commonly used material for remediation of cadmium contaminated soil, but its efficiency is not high. To improve the adsorption capacity of biochar for Cd²⁺, rice straw biochar was modified by loading Ca²⁺ and activating modification by humic acid oxidation impregnation. The materials were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Batch adsorption experiments and pot experiments were carried out to explore the adsorption fixation properties and mechanism of the materials for Cd²⁺. The results showed that: (1) The best adsorption performance was observed when the mass ratio of biochar to humic acid was 1∶1 and the concentration of Ca²⁺ was 1.5 g·L^-1 (referred to as RHB1-1), and the maximum adsorption capacity was 74.46 mg·g^-1, which was 61% higher than that of original biochar. (2) The content of acidic oxygen-containing functional groups on the surface of calcium-humic acid modified biochar was significantly increased, and the free hydroxyl group was newly formed. Cd²⁺ was efficiently adsorbed by coordination chelation and complexation co-ion exchange. The adsorption of Cd²⁺ by calcium-humic acid modified biochar was mainly chemisorption, which was consistent with Freundlich adsorption isotherm model. (3) The materials can absorb Cd²⁺ in soil and promote the transformation of Cd²⁺ from an acid extractable state to a residual state, reducing the bioavailability of Cd²⁺ in soil.

Key words: biochar, humic acid, Ca²⁺, Cd²⁺