Preparation and Modification of Rape Straw Biochar and Its Adsorption Characteristics for Methylene Blue in Water

To solve the double problems of methylene blue (MB) pollution in water and waste of straw resources, rape straw biochar (RSB600) was made by thermal decomposition of discarded rape residues at 600 °C, and modified RSB600(M-RSB600) was prepared after treatment with NaOH for MB adsorption. The physicochemical properties, MB adsorption properties and mechanism of RSB600 and M-RSB600 were studied. According to the experimental findings, M-RSB600 has stronger aromaticity, higher hydrophilicity, and more polarity than RSB600. Under the same adsorption conditions, M-RSB600 obtained better MB adsorption effect with less dosage. The MB equilibrium adsorption capacity of M-RSB600 reached 268.46 mg/g, which was 65.6% higher than that of RSB600. The quasi-second-order kinetic model better described the adsorption kinetics of MB on RSB600 and M-RSB600 (R2 > 0.98), and the Freundlich and Temkin models better described the adsorption isotherms (R2 > 0.95). The adsorption process was spontaneous, internal heat and entropy driven multilayer adsorption, dominated by chemical adsorption, specifically involving multiple interaction mechanisms, including electrostatic adherence, hydrogen-bond, π-π bond and ion-exchange. The results demonstrated that NaOH modification obviously improved the structure of RSB and make it had better adsorption and regeneration performance. M-RSB600 can be utilized as the ideal substance for effectively removing MB from effluent.