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[Display omitted] • CSSB was an adsorbent with better removal efficiency for Cd(Ⅱ). • The Cd(Ⅱ) removal was a multi-layer adsorption process dominated by chemisorption. • As Cd(Ⅱ) initial concentration increased, physisorption gradually became dominant. • The Cd(Ⅱ) removal was divided into 3 stages with different mass transfer mechanisms. • Cd(II) was mainly attached to CSSB by ion exchange, complexation and coprecipitation. Biochar derived from co-pyrolysis of sewage sludge and calcium sulfate was used to remove Cd(II) from aqueous solution. The results showed that the Cd(Ⅱ) adsorption better followed Freundlich model, and the maximum adsorption capacities were 109.0 mg/g (288 K), 127.9 mg/g (298 K) and 145.4 mg/g (308 K). The Cd(Ⅱ) removal was a multi-layer adsorption process dominated by chemisorption, which was also a spontaneous and endothermic process. The contribution of physisorption gradually increased as the Cd(Ⅱ) initial concentration. The Cd(Ⅱ) removal process which better followed pseudo-second-order kinetic model, was divided into three stages. The first (0–0.3 h) and second stages (0.3–2 h) were separately controlled by liquid film diffusion/intraparticle diffusion/chemical reaction and liquid film diffusion/chemical reaction, while the third stage (0.3–24 h) was the dynamic equilibrium process. The speciation distribution of Cd on biochar surface was mainly CdCO 3 /CdOOC and CdO/CdSiO 3 , indicating coprecipitation, ion exchange and complexation contributed more to the Cd(Ⅱ) removal. [ABSTRACT FROM AUTHOR]