Investigating the pathways of enhanced Pb immobilization by chlorine-loaded biochar

Chlorides are usually neglected during heavy metal remediation. In this study, rice husk (RB) and sludge (SB) biochars were applied to investigate the influences of Cl on Pb remediation. Cl modification significantly increased Pb removal (RB: 336.84% and SB: 404.56%) as it increased the surface area, micropore volume and P release of the two biochars. The adsorption kinetics showed that Cl modification improved the coexistence of physical adsorption and chemical precipitation on biochar. However, the adsorption correlation of physical factors (BET, R = 0.299–0.502) to Pb is significantly lower than that of chemical factors (P and Cl, R = 0.875–0.982). Meanwhile, the contributions of specific surface area (R = 0.299–0.502) and micropore volume (R = 0.471–0.693) to Pb adsorption were significantly lower than that of Cl (R = 0.944–0.953), based on redundancy analysis. The loading degree of Cl (RB 79.1%, SB: 77.8%) on modified biochar was confirmed as the primary factor in stabilization of Pb. In addition, the pH had limited influences on Pb adsorption (Pvalue = 0.524). Abundant C–Cl functional groups appeared on the surface of the two biochars after Cl loading according to ATR-IR and XPS analysis. Then, Pb2+ diffused into the interior of biochars via mesopores and formed C–Cl–Pb–P complexes. The active Pb (water soluble and exchangeable) could be finally transformed into stable fraction (acidic and non-bioavailable), i.e., pyromorphite [Pb5(PO4)3Cl] based on both experimental and GWB modeling results. In particular, the adsorption/stabilization of Pb was more favorable by the biochar with <15 μm particles. The contrasts between RB and SB can be attributed to that low-temperature pyrolysis of RB (450 °C) has a large amount of CO32− that limits formation of pyromorphite. This study hence proposed that Cl loading technology can substantially improve the Pb remediation effect of biochar.