Multiple chemical modifications and Cd²⁺ adsorption characteristics of sludge-based activated carbon

Sludge resource utilization is commonly realized through carbonization, but the use of direct carbonization to obtain sludge-based activated carbon (SAC) is not functional yet. The multiple chemical modifications were carried out to achieve N-doping and pore-making to modify SAC. The SAC_U-PF' was synthesized by activating sludge simultaneously with uric acid and potassium ferrate. Moreover, SAC_N', SAC_U, and SAC_PF' were prepared with no additives, uric acid, and potassium ferrate, respectively. The results indicated that the different modifications affected the chemical properties and structure of SAC. The BET of SAC_U-PF' was 56.73 m² g^-1, which was higher than that of SAC_N' and SAC_PF'. SAC_U-PF' possessed abundant functional groups, such as C[double bond, length as m-dash]N and C-O. The adsorption capacity of SAC_U-PF' for Cd²⁺ was 9.69 mg g^-1, 5.5 times that of SAC_N', the adsorption process of Cd²⁺ by SAC_U-PF' fitted well for the second-order kinetic model and Langmuir isothermal adsorption model. The XPS and chemical analysis revealed that SAC_U-PF' and Cd²⁺ were bonded by functional groups, and the Cd²⁺ removal by SAC_U-PF' was through complexation, anion exchange, electrostatic attraction, and pore filling. The SAC_U-PF' was exhibited different removal capacities for different metals, Pb²⁺ and Mn²⁺ correspond to adsorption capacities of 4.9 and 8.1 mg g^-1. In addition, the adsorbed SAC_U-PF' can be regenerated by sodium hydroxide. The study highlights the importance of multiple chemical modifications performed on SAC, the double coupled chemical modifications to ensure its good performance in the treatment of heavy metals in wastewater treatment.