Surface characteristics of crop-residue-derived black carbon and lead(II) adsorption

Previous studies demonstrated that black carbon (BC) in soils arising from the burning of crop residues is a highly effective adsorbent for organic contaminants. This work evaluated the adsorptive ability of BC for heavy metals in relation to the BC surface characteristics. Two BC samples, rice carbon (RC) and wheat carbon (WC) isolated from the burning residues of rice straw and wheat straw, respectively, were characterized for their surface properties with reference to a commercial activated carbon (AC). While RC and WC had lower surface areas than AC, the two BC samples possessed higher surface acidities and thus lower pH of the isoelectric points (pH IEP) than AC as indicated by titration, FTIR, and zeta potential measurements. The Pb(II) adsorption by RC and WC was higher than that by AC and increased significantly with increasing pH, suggesting the electrostatic interactions between positive Pb(II) species and negatively charged functional groups on RC and WC as the primary adsorptive forces. A reduction in the total positive charge of Pb(II) species with increasing pH as computed by MINTEQA2 suggested that the deprotonation of surface functional groups of RC and WC was the controlling factor of the adsorption. The Pb(II) adsorption decreased with increasing ionic strength, due to the screening role of Na+ in neutralizing the negative charge of RC and WC. This study demonstrated that BC may be an important adsorbent of heavy metals in soil and that the adsorption may be significantly influenced by environmental conditions.