Synthesis, Characterization, and Dye Removal of ZnCl₂-Modified Biochar Derived from Pulp and Paper Sludge

In this study, pulp sludge-derived biochar synthesized through modification with ZnCl₂ under multistep pyrolysis conditions was investigated for the effective removal of methylene blue (MB) from aqueous solution. Results showed that when the Zn2PT350-700 dosage was 10 mg, the largest adsorption amount of MB was 590.20 mg/g within 24 h under pH = 8. Kinetics and thermodynamics revealed that the adsorption process of MB can be described by the Freundlich isotherm model and the pseudo-second-order kinetic model, which means that multilayer sorption occurred on the heterogeneous surface of Zn2PT350-700. The analysis of the adsorption mechanism showed that electrostatic attraction between the deprotonated functional groups and MB⁺, cation exchange, and π-electron interaction played a major role in MB adsorption, followed by physical adsorption. After six cycles of desorption-adsorption, Zn2PT350-700 still maintained good adsorption performance. All results demonstrated that Zn2PT350-700 could perform as promising adsorbents for efficient MB removal from wastewater. Using biochar from paper and pulp sludge for wastewater remediation is an ingenious method, which can reduce the environmental and health risks related to industrial waste disposal, while providing remediation of water contaminated with industrial dye effluents.