Evaluation of N2 and CO2 pyrolyzed biochars in remediation studies: Adsorption and immobilization of potentially toxic elements in water and contaminated soil, and the impact on soil properties

Using CO2 medium (instead of inert gas) during pyrolysis has been reported to improve biochar properties, particularly textural and surface properties; however, its investigation in remediation applications has received little attention. In this study, biochar prepared from rice husk pyrolysis under CO2 atmosphere (BCC) and under N2 atmosphere (BCN) was investigated for efficacy in removing zinc (Zn2+) and lead (Pb2+) in water, immobilizing potentially toxic elements (PTE) in soil, and improving physicochemical properties of a mining soil. Results showed that for aqueous metals adsorption, both biochars followed the pseudo-second order kinetic model. BCC exhibited greater Zn2+ adsorption capacity and efficiency (26 mg/g, 52 %) compared to BCN (22 mg/g, 45 %). In contrast, both biochars had comparable Pb2+ adsorption efficiency (40 %). Furthermore, five-week biochar-soil incubation experiments revealed that, compared to control soil, both biochars significantly improved soil physicochemical properties, such as increasing total and organic carbons, pH, water holding capacity, and cation exchange capacity as the dose increased from 3 to 10 %. However, both biochars showed statistically insignificant differences at similar doses, revealing that soil improvements were strongly dose-dependent rather than biochar type. PTE immobilization experiments revealed that both biochars numerically reduced the bioavailability of all PTEs (Zn, Pb, Cr, Cu, Cd, Ni, Mn) in soil, but the difference was not statistically significant under the tested conditions.