Chitosan-Coated Sewage Sludge–Coal Gangue Biochar Spheres for Efficient Cr(VI) Removal from Water and Soil

A chitosan-coated biochar sphere (BC-CG-CT) was fabricated via facile copyrolysis of municipal sewage sludge (SS) and coal gangue (CG), followed by ionic gelation encapsulation. Under optimized conditions (650 °C pyrolysis, 100 min, SS:CG = 1:1), BC-CG-CT achieved a maximum Cr(VI) adsorption capacity of 23.85 mg·g⁻¹ and removal rates above 97% at pH 5 in aqueous systems. Kinetic and isotherm studies revealed a dominant pseudo-first-order behavior, monolayer adsorption, and energetically favorable interactions. Thermodynamic analysis indicated spontaneous, endothermic adsorption. In soil incubation tests (90 days, up to 300 g·kg⁻¹ dosage), BC-CG-CT reduced soluble Cr(VI) by up to 57.6%, shifted Cr fractionation from labile (EX/CB) to stable (OX/RS) pools, elevated soil pH and TOC, and suppressed CaCl₂-extractable Cr to below 0.1 mg·kg⁻¹. Mechanistic investigations combining SEM-EDS, XRD, FTIR, and XPS confirmed synergistic immobilization via pore-filling, electrostatic adsorption, Cr(VI) reduction by electron-donor groups, and complexation/precipitation. The robust spherical morphology enhances recoverability and reuse. This work demonstrates a promising and sustainable strategy for simultaneous remediation of Cr(VI) in water and soil, leveraging industrial and municipal waste valorization.