We used three purple soil (Hechuan, Jialing, and Cangxi) samples from the Jialing River basin as the research objects and added different proportions of an acid–base modified Alternanthera philoxeroides biochar (C_m) to the purple soil to study the effect of C_m on the adsorption of chlortetracycline (CTC) in the purple soil. The results indicated the following: (1) At 30 °C and pH = 6, the soil adsorption capacity increased with an increasing initial concentration of CTC. The maximum adsorption amount of CTC for each tested sample was in the range of 2054.63–3631.21 mg/kg, and the adsorption capacity in different C_m amended soils was ranked in the order of 10% C_m > 5% C_m > 2% C_m > CK. The adsorption capacity of CTC increased with an increase in the proportion of C_m. Furthermore, under the same addition ratio of C_m, Hechuan soil was found to have a better adsorption effect for CTC than Jialing and Cangxi soil. (2) The Langmuir model was the most suitable for fitting the adsorption behavior of CTC on different purple soils, and the fitting coefficients were all greater than 0.9, indicating that the adsorption of CTC on each soil sample occurred via monolayer adsorption. The thermodynamic experiment results showed that an increase in temperature was beneficial to the process of CTC adsorption, which was a spontaneous, endothermic, and entropy-adding process. (3) At pH = 6, the ionic strength ranged from 0.01 to 0.5 mol/L and the adsorption capacity of CTC of the soil samples decreased with an increase in ionic strength. In the range of pH 2–10, the adsorption capacity of CTC in all the soil samples decreased with an increase in pH. The inhibition capacity of CTC in the soil samples under acidic conditions was notably higher than that under alkaline conditions.

Rice straw and corn straw were selected as raw materials to prepare biocharby anoxic carbonization and the biochar was loaded on the surface with FeCl₃, MnCl₂ and Fe(NO₃)₃ & KMnO₄, respectively, and then two types of straw biochar and six types of modified biochar were prepared. FT-IR, SEM, and XRD were used to characterize and analyze the physical and chemical properties of the biochar. The adsorption kinetics and adsorption isothermal tests of Cd²⁺ ions in aqueous solution were carried out. The results showed that modified biochars attached more active sites and surface group, especially iron-manganese-modified biochar (FMBC1, FMBC2). The kinetic adsorption tests showed that the adsorption process of eight kinds of biochar all conformed to the quasi-second-order kinetic equation, and chemisorption maybe dominated the adsorption process. The adsorption isothermal test showed that the adsorption process of Cd²⁺ ions by FeCl₃-modified biochar (FBC1, FBC2) and Fe(NO₃)₃ & KMnO₄-modified biochar (FMBC1, FMBC2) conforms to the Freundlich model, and the adsorption process of Cd²⁺ ions by MnCl₂-modified biochar (MBC1, MBC2) conforms to Langmuir model. Compared with other kinds of biochar, the K_F value of Fe(NO₃)₃ & KMnO₄-modified biochar of rice straw biochar (FMBC1) was the largest, reached 18.602 L·mg⁻¹, and its 1/n value was the smallest, it reached 0.474, indicating that the adsorption effect on Cd²⁺ of FMBC1 was the best.