Effect of humic acid-loaded biochar on the adsorption performance of Mn²⁺ in soil leachate from mining area

Humic acid modified biochar (CSB-HA) was prepared using corn stover-based biochar (CSB) loaded with humic acid for the adsorption of Mn²⁺ from the soil leachate in the mining area and its adsorption characteristics were analyzed. With the help of SEM, BET, XRD, XPS and FT-IR, the prepared samples were characterized and analyzed. The results show that the specific surface area of CSB-HA is 7.911 m²/g, which is 1.98 times that of CSB, and the pores are mostly mesoporous. The excellent pore structure provides a large number of adsorption sites and promotes the contact with pollutants, so that the adsorption performance is enhanced. The surface of CSB-HA is enriched with the oxygen-containing functional groups such as C=O, O=C—O and —COOH, and the oxygen atoms in the functional groups can be complexed and coordinated with heavy metal ions to form a stable ligand bond, thus removing Mn²⁺. The maximum adsorption of Mn²⁺ by CSB-HA is 21.493 mg/g, and the quasi-primary kinetic model, quasi-secondary kinetic model, Langmuir isotherm and Temkin isotherm can fit the adsorption process well, and the Temkin constant b<1 kJ/mol can prove that the adsorption process Physical adsorption exists. The quasi-secondary kinetic model and Langmuir isotherm fit the adsorption process better and closer to the actual detection value, so the adsorption process is a single molecule layer adsorption, which is subject to the joint action of physical adsorption and chemical adsorption, and the chemical adsorption is dominant. Therefore, the type and mass fraction of oxygen-containing functional groups have an important effect on the adsorption of Mn²⁺ by CSB-HA. The diffusion process was analyzed by the liquid-film diffusion model and the intra-particle diffusion model. The fitted line of the liquid-film diffusion model has a small intercept with the longitudinal axis, so the process is not a velocity-controlled phase. The intra-particle diffusion model can be fitted into a membrane diffusion stage (0−60 min) and an intra-particle diffusion stage (60−240 min), the latter adsorption rate is significantly lower (k_id1=1.159>k_id2=0.287), and the fitted lines do not pass through the origin, so that the intra-particle diffusion stage is the key stage of the adsorption process for controlling the rate. In the thermodynamic simulations, the separation coefficients R_L are in the range of 0.4−0.6, ΔH=15.41 kJ/mol>0, and ΔG is negative and decreases with increasing temperature, indicating that the adsorption process is reversible and spontaneous heat absorption, and the adsorption reaction can be facilitated by warming up. ΔS=49.90 J/(mol·K)>0, which proves that the solid-liquid interface stoichiometry increases during the adsorption process. The simulated adsorption experiments on leaching solution show that CSB-HA has a good adsorption effect on Mn²⁺ and a regulating effect on the pH of leaching solution.