Adsorption Of Lead And Cadmium On Calcium Sulfate/Sludge Based Biochar

Due to highly chemical toxicity,carcinogenicity and bioaccumulation,lead and cadmium industrial wastewater is considered as one of the most harmful industrial wastewater to the ecological environment.Most frequently used technologies such as chemical precipitation,electro-flocculation,adsorption,ion exchange,membrane separation,and biological treatment were applied to remove heavy metals from aqueous solutions.Compared with the conventional technologies,adsorption method has gained much attention owing to its advantages of simple operation and high removal efficiency.However,high coat of adsorbent material has extremely limited its wide application during the wastewater treatment.In this study,a low-cost with high-efficiency adsorption material calcium sulfate/sludge based biochar（SBC）was successfully synthesized by co-pyrolysis of excess sludge and calcium sulfate,and was further applied for the removal of lead and cadmium from wastewater.The comprehensive removal mechanism of lead and cadmium from aqueous solution by SBC was also studied,which could provide a solid theoretical basis for the treatment of lead and cadmium wastewater and open up a new way for the utilization of excess sludge.The experimental results are summarized as follows:（1）The results of batch adsorption experiments showed that the biochar dosage,p H value,initial concentration of solution,contact time and temperature all had significant effect on the removal of Pb²⁺and Cd²⁺onto SBC.The removal efficiency of SBC for Pb²⁺and Cd²⁺could reach over 99%when the initial the initial concentrations of Pb²⁺and Cd²⁺solutions were 100 mg/L and 50 mg/L with the adsorbent dosage of 0.4 g/L and 0.8 g/L,respectively.SBC exhibited superior removal performance at p H value of 4-8.Adsorption equilibrium could reach within 240 min and 120 min for Pb²⁺and Cd²⁺at same operation condition when the the initial concentrations of Pb²⁺and Cd²⁺solutions were 100 mg/L and 60 mg/L,respectively.The higher initial concentration and reaction temperature were all beneficial to adsorption of SBC towards Pb²⁺and Cd²⁺.（2）The adsorption process of Pb²⁺and Cd²⁺onto SBC could be better described by Langmuir model,and the isothermal adsorption process also followed Temkin model well,which indicated that the adsorption process was dominated by chemisorption.Kinetics study indicated that the adsorption process followed pseudo-second-order model well,and the adsorption rate was controlled by both liquid film diffusion and intraparticle diffusion,while the liquid film diffusion played main role.The thermodynamic study indicated that enthalpy changeΔH^θand entropy changeΔS^θwere all positive,and Gibbs free energyΔG^θwas negative,which further implying that the adsorption of Pb²⁺and Cd²⁺by SBC was a endothermic process,and rising temperature was favorable for the adsorption.（3）Characterization measurements including SEM-EDS,FTIR,XRD,XPS,and GC-MS have been conducted for SBC sample before and after adsorption of heavy metal ions,and the results showed that the removal mechanism was mainly attributed to the ion exchange between calcium sulfate or calcium carbonate in SBC and Pb²⁺or Cd²⁺.Besides,surface complexation of functional groups on the surface of biochar to Pb²⁺should not be ignored.Apart from that,analysis of the adsorbed solutions by GC-MS further indicated that organic matter in biochar was also involved in the adsorption process of SBC for Pb²⁺and Cd²⁺.