NaHCO₃ activated sludge-derived biochar by KMnO₄ modification for Cd(II) removal from aqueous solutions

The surface flat pristine biochar provides limited adsorption sites for Cd(II) adsorption. To address this issue, a novel sludge-derived biochar (MNBC) was prepared by NaHCO₃ activation and KMnO₄ modification. The batch adsorption experiments illustrated that the maximum adsorption capacity of MNBC was twice that of pristine biochar and reached equilibrium more quickly. The pseudo-second order and Langmuir model were more suitable for analyzing the Cd(II) adsorption process on MNBC. Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^- and NO^-₃ had no effect on the Cd(II) removal. Cu²⁺ and Pb²⁺ inhibited the Cd(II) removal, while PO^3-₄ and humic acid (HA) promoted it. After 5 repeated experiments, the Cd(II) removal efficiency on MNBC was 90.24%. The Cd(II) removal efficiency of MNBC in different actual water bodies was over 98%. Furthermore, MNBC owned excellent Cd(II) adsorption performance in fixed bed experiments, and the effective treatment capacity was 450 BV. The co-precipitation, complexation, ion exchange and Cd(II)-π interaction were involved in Cd(II) removal mechanism. XPS analysis showed that NaHCO₃ activation and KMnO₄ modification enhanced the complexation ability of MNBC to Cd(II). The results suggested that MNBC can be used as an effective adsorbent for treating of Cd-contaminated wastewater.