China Nonferrous Metallurgy

Acidic Cd(Ⅱ)-containing wastewater is commonly treated by adsorption for the removal of Cd(Ⅱ). Biochar exhibits a strong affinity for heavy metals; however, raw biochar generally has a relatively low specific surface area and limited active sites. Activation can enhance the pore structure of biochar. In this study, municipal sludge was used as the precursor for biochar synthesis, and potassium dihydrogen phosphate (KH2PO4), a low-cost and environmentally friendly agent that causes no secondary pollution, was co-pyrolyzed with the sludge to produce phosphate-modified sludge biochar (PBC). The effect of the mixing ratio on the adsorption performance of PBC was investigated. In addition, single-factor experiments, adsorption kinetics, isotherm studies, and FTIR analyses were conducted, leading to the following main conclusions: PBC possesses a well-developed pore structure and a high degree of graphitization. When the mass ratio of KH2PO4 to municipal sludge was 1∶1, PBC exhibited the highest Cd(Ⅱ) removal efficiency, maintaining a removal rate above 98% within a solution pH range of 4-7. Under conditions of 25℃, an adsorbent dosage of 1g·L^-1, and an initial Cd(Ⅱ) concentration of 10-200mg·L^-1, the maximum adsorption capacity of PBC for Cd(Ⅱ) reached 132.77mg·g^-1. The coexisting ions Na⁺, Ca²⁺, and Mg²⁺ had negligible effects on Cd(Ⅱ) removal by PBC, whereas Pb²⁺, Zn²⁺, and Cu²⁺ exhibited inhibitory effects. The adsorption behavior of PBC toward Cd(Ⅱ) followed the pseudo-second-order kinetic model and the Langmuir isotherm model, indicating that the removal process was dominated by monolayer chemical adsorption. The mechanisms involved in Cd(Ⅱ) removal by PBC included complexation, Cd(Ⅱ)-π interactions, and co-precipitation.