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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]

Biochar prepared by co-pyrolysis of nano-zero-valent iron and sewage sludge (nZVISB) was used to remove As(Ⅴ) from aqueous solution. When the initial pH was 2, the initial As(Ⅴ) concentration was 20 mg L−1, the dose of nZVISB was 10 g L−1, the contact time was 24 h, and the adsorption temperature was 298K, the removal efficiency of As(Ⅴ) was greater than 99%. The isothermal removal of As(Ⅴ) followed the Freundlich model better, and the maximum adsorption capacity of As(Ⅴ) was 60.61 mg g−1. The removal process of As(Ⅴ) could be better described by pseudo-second-order kinetic model, and the rate-controlling step should be liquid film diffusion and chemical reaction. Thermodynamic analysis indicated that the removal of As(Ⅴ) was a spontaneous and endothermic process dominated by chemical adsorption. The characterizations of nZVISB before/after adsorption and the solution after adsorption suggested that the iron-containing substances (Fe0, Fe2+, FeOOH) and organics in the nZVISB had a great effect on the removal of As(Ⅴ), and the As was mainly immobilized on nZVISB by speciation of As–O–Fe. • As(Ⅴ) removal followed Freundlich model better, and the q max of biochar to As(Ⅴ) was 60.61 mg/g. • The removal of As(Ⅴ) was better described by pseudo-second-order kinetic model. • Liquid film diffusion and chemical reaction together determined the removal rate of As(Ⅴ). • The removal of As(Ⅴ) was a spontaneous and endothermic process dominated by chemical adsorption. • Fe0, Fe2+, FeOOH and organics participated in reduction and removal of As(Ⅴ). [ABSTRACT FROM AUTHOR]