Enhanced removal of cadmium from aqueous environments and soil during electrokinetic remediation using Si-Mg modified sawdust-based biochar as an adsorbent and permeable reactive barrier material

Abstract

In this study, Si-MgSB, created by modifying sawdust with NaSiO₃ and MgCl₂, was used to remove Cd²⁺ from aqueous solutions and as a permeable reactive barrier (PRB) for enhancing electrokinetic remediation in soil. Batch adsorption experiments showed that Si-MgSB effectively removed Cd²⁺ at pH 3–7, achieving a maximum capacity of 1185.91 mg/g. Characterization indicated that Si-Mg co-doping increased surface area, pore volume, and oxygen-containing groups, enhancing ion exchange, mineral precipitation, and surface complexation. The main adsorption mechanisms were identified as ion exchange (44.65 %), mineral precipitation (39.99 %), and Cd²⁺-π interactions (15.00 %). Adsorption followed a pseudo-second-order kinetic model, suggesting chemisorption, while Freundlich isotherms indicated multilayer adsorption. In practical tests, Si-MgSB achieved around 99 % Cd²⁺ removal in simulated wastewater and maintained over 80 % efficiency after eight cycles. As a PRB material, it enhanced soil Cd²⁺ removal by 15.85 % and inhibited the migration of OH^– ions during electrokinetic remediation. While coexisting ions slightly reduced effectiveness, the removal efficiency remained above 87 %. The addition of humic acid improved Cd²⁺ removal efficiency and reduced energy consumption, although further increases showed limited effects. Overall, Si-MgSB is an effective adsorbent and PRB material, offering a green, cost-effective method for heavy metal removal in complex environments.