[Structural Characteristics of Micro-nano Particle Size Biochar and Its Adsorption Mechanism for Cd²]

The aim of this study was to investigate the structural characteristics of biochar with different micro/nano particle sizes and its effect on the adsorption performance of Cd²⁺. Corn stalk biochar with different particle sizes (180-250 μm, 50-75 μm, and ≤ 20 μm, denoted as BC-1, BC-2, and BC-3, respectively) were prepared using the sieving and ball milling method. The structural properties of different particle sizes of biochar were analyzed via elemental analysis, laser particle size analysis, SEM, BET, FTIR, and XPS. Additionally, the adsorption mechanisms of Cd²⁺ by three particle sizes of biochar under initial Cd²⁺ concentrations, adsorption times, and pH conditions were comparatively studied using static adsorption experiments. The results showed that with the decrease in particle size, the pH and zeta potential of biochar were reduced; the aromaticity and polarity decreased; the specific surface area and pore volume increased; and the intensity of the characteristic peaks containing OH, C[FY=,1]C/C[FY=,1]O, and C-O groups increased. The adsorption kinetics of Cd²⁺ with different particle diameters of biochar were in accordance with the pseudo-secondary kinetic model, with chemisorption dominating. The equilibrium times were in the decreasing order of BC-1 (540 min)>BC-2 (360 min)>BC-3 (80 min). The Langmuir model could better fit the adsorption isotherm process of Cd²⁺ on biochar of different particle sizes (R²>0.97), and the maximum adsorption capacity of Cd²⁺ increased with the decrease in particle size, which was expressed as BC-3 (74.43 mg·g^-1)>BC-2 (45.71 mg·g^-1)>BC-1 (44.59 mg·g^-1). The main mechanisms of Cd²⁺adsorption by biochar were electrostatic attraction, surface complexation, and cation-π interaction.