Study on the Adsorption Performance of Methylene Blue by FeCl₃-Modified Hydrothermal Rice Straw Biochar and Activated Clay Composite

In this study, rice straw biochar (RCHC) and activated clay (AC) were innovatively combined and modified with FeCl₃ to prepare an adsorbent (RCHC-ACFC). The adsorption performance of RCHC-ACFC toward methylene blue (MB) was investigated. The effects of varying RCHC-to-AC mass ratios, FeCl₃ solution concentrations, pH conditions, and loading amounts on the adsorbent's performance were investigated. The adsorption capacity, isotherms, kinetics, and thermodynamics of MB on RCHC-ACFC were explored. Surface morphology, functional groups, and phase composition of RCHC-ACFC were characterized using SEM, FT-IR, and XRD to elucidate the adsorption mechanism. Experimental data indicate that RCHC-ACFC exhibits superior adsorption performance for MB compared to RCHC and AC alone. Isotherm studies reveal that MB adsorption on RCHC-ACFC follows the Langmuir adsorption model. MB adsorption adheres to pseudo-second-order kinetics. The theoretical maximum adsorption capacity of RCHC-ACFC for methylene blue in aqueous solution reaches 539.35 mg/g. Thermodynamic studies confirmed that MB adsorption onto the adsorbent is endothermic and spontaneous. Mechanism pathway analysis indicated the presence of electrostatic forces, ion exchange, chemical bonding, and physical adsorption during the adsorption process. The results demonstrate that this RCHC-ACFC exhibits excellent adsorption performance for methylene blue in aqueous media.