Removal of Cd(II) from Acidic Wastewater on Sludge Biochar with NaHCO3 Activation Preparation

The residual sludge from urban wastewater treatment plants was utilized to prepare sodium bicarbonate-activated sludge biochar, which was then employed for removing Cd(II) from wastewater. In comparison with the inactivated sludge biochar (UBC), the activated sludge biochar (ABC) displayed an enhanced pore structure and increased graphitic nature. The adsorption experiments indicated that at pH 5 and an adsorbent dosage of 25 mg, both UBC and ABC achieved optimal removal performances for Cd(II). The coexisting ions of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^– and \({\text{NO}}_{3}^{ - }\) were found to have negligible impact on removing Cd(II) by UBC and ABC, whereas Cu²⁺ and Pb²⁺ were observed to exert inhibitory effects on removing Cd(II) by both UBC and ABC. In the presence of humic acid (HA) at concentrations ranging from 0 to 25 mg/L, it was noted that HA exerted a beneficial effect on removing Cd(II) by UBC and ABC. The pseudo-second-order kinetic model and Langmuir model were found to effectively describe the process of Cd(II) removal by UBC and ABC. Meanwhile, the maximum adsorption capacities of UBC and ABC for Cd(II) were 73.30 and 196.47 mg/g, respectively. The Cd(II) removal by UBC and ABC was influenced by mechanisms such as electrostatic interactions, complexation, mineral dissolution−precipitation, and cation−π interactions. This further illustrated the efficacy of sodium bicarbonate as an activator in enhancing the Cd(II) removal from acidic wastewater by sludge-derived biochar.