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 Pb²⁺ and Zn²⁺, but the effects of biochar characteristics, including stability, on the competitive adsorption of Pb²⁺ and Zn²⁺ 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 Pb²⁺ and Zn²⁺ 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 Pb²⁺ and Zn²⁺. Pyrolysis at the higher temperature resulted in a slight decrease in aromatic functional groups on biochar surfaces but higher adsorption capacities for Pb²⁺ and Zn²⁺ due to the decreased biochar particle size and increased specific surface area. FTIR, XRD, and XPS analyses indicated that Pb²⁺ and Zn²⁺ 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 Pb²⁺ than with Zn²⁺ due to the former's lower binding energies, which also accounted for the notable decrease in adsorption of Zn²⁺ in the presence of Pb²⁺. 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.