Search

[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]

• Appropriate CaSO 4 dose for HMs immobilization is in the range of 0.01 to 0.075. • Suitable temperatures for HMs immobilization are 750 °C (Cr/Pb/ Zn) and 350 °C (Cu/Ni). • The optimal holding time for HMs immobilization are 60 min (Cr/Cu) and 15 min (Pb/Ni/Zn). • Improvement in HMs immobilization is due to more crystals, pores, and tiny particles. The effects of calcium sulfate (CaSO 4) dosage (mass ratio of CaSO 4 to sludge), pyrolysis temperature and holding time on speciation distribution of Cr, Pb, Cu, Ni and Zn in biochar derived from co-pyrolysis of sewage sludge and CaSO 4 were investigated. The appropriate CaSO 4 dosages for better immobilization of different heavy metals were 0.075 (Cr), 0.025 (Pb), 0.025 (Cu), 0.025 (Ni), and 0.01(Zn), respectively. The corresponding proportions of heavy metals in stable state (oxidizable and residue fractions) were 96.99%, 89.23%, 99.55%, 87.43%, and 54.33%. The high pyrolysis temperature (750 °C) was conducive to immobilization of Cr, Pb and Zn, while the percentages of Cu and Ni in stable state were higher at low pyrolysis temperature (350 °C). The suitable holding time was 60 min (Cr, Cu) and 15 min (Pb, Ni and Zn), respectively. The characterization of samples showed that suitable pyrolysis temperature and holding time could promote the formation of crystals and spherical or ellipsoidal particles in biochar, and pyrolysis of aliphatic to form more mesopores and macropores, resulting in immobilization of more heavy metals. During co-pyrolysis process, CaSO 4 was easily decomposed and generated CaS, CaO, CaCO 3 and Ca(OH) 2 , which were beneficial to the immobilization of heavy metals. [ABSTRACT FROM AUTHOR]