Mechanisms of Pb and/or Zn adsorption by different biochars: Biochar characteristics, stability, and binding energies.

Biochar has been widely studied as an amendment for use in remediation of water and soil contaminated with heavy metals such as Pb and Zn, but the effects of biochar characteristics, including stability, on the competitive adsorption of Pb and Zn by biochars from various sources are incompletely understood. In this work, biochars from three different feedstocks, including rice straw (RS), chicken manure (CM), and sewage sludge (SS), were prepared at two pyrolysis temperatures, 550 and 350 °C, and tested to investigate the influence of their stabilities and other characteristics on their adsorption of Pb and Zn in both single- and binary-metal systems. RS biochar had the highest carbon and hydrogen contents, greatest number of functional groups (e.g., OH and C=C/C=O), highest pH, most negative surface charge, and highest physical stability, and thus the highest adsorption capacity for Pb and Zn. Pyrolysis at the higher temperature resulted in a slight decrease in aromatic functional groups on biochar surfaces but higher adsorption capacities for Pb and Zn due to the decreased biochar particle size and increased specific surface area. FTIR, XRD, and XPS analyses indicated that Pb and Zn were absorbed on the biochars primarily via chemical complexation with aromatic functional groups. Quantum chemistry calculations confirmed that these functional groups (e.g., -OH and-COOH) tended to bind more strongly with Pb than with Zn due to the former's lower binding energies, which also accounted for the notable decrease in adsorption of Zn in the presence of Pb. In addition, compared to carboxyl groups, hydroxyl groups had smaller binding energies and stronger metal complexation. These findings provide a theoretical basis for improved understanding of potential applications of biochars in environmental remediation.