Sorption mechanism of zinc on reed, lignin, and reed- and lignin-derived biochars: kinetics, equilibrium, and spectroscopic studies

Abstract

Biochar is considered as a substitute for stabilizing heavy metals, yet systematic studies regarding the stabilization mechanism of Zn on biochars from different biomass sources under different pyrolysis temperatures are still lacking. This study was endeavored to assess the sorption behavior and mechanism of Zn on pristine reed, lignin, and reed- and lignin-derived biochars at different pyrolysis temperatures (350, 450, and 550 °C). Batch experiments were performed to investigate the Zn sorption on pristine reed, lignin, and their derived biochars. Major functional groups on the surface of samples were measured using Fourier transform infrared (FTIR) spectroscopy and Boehm titration method, while the speciation and binding mechanisms were elucidated using X-ray absorption near-edge structure (XANES). The FTIR results showed an abrupt decline in the oxygen-containing functional groups with elevating pyrolysis temperature. The total amount of three acidic functional groups (carboxyl, lactone, phenol) was highest (2.39 mmol/g) in lignin, and it significantly decreased in reed- and lignin-derived biochars (0.05–0.94 mmol/g). Lignin exhibited the highest sorption capacity to Zn (qe = 3360 mg/kg) in the batch sorption experiments, while the sorption capacities for biochars were reduced with increasing pyrolysis temperature, irrespective of biomass sources. The XANES results indicated that soil organic matter (SOM), iron, and phosphate mineral components in the tested sorbents played an indispensable role in Zn immobilization. A large fraction of Zn-OM complex was observed in lignin due to its highest abundance of acidic functional group, leading to the highest sorption capacity for lignin toward Zn. The combination of XANES with FTIR, Boehm titration, batch sorption, and kinetic experiments altogether revealed that the decline in sorption capacity of Zn on reed- and lignin-derived biochars with elevating pyrolysis temperature was attributed to the loss in O-containing functional groups. The complexation of Zn with surface functional groups contributed to the Zn sorption on reed, lignin, and their derived biochars. The projected results will not only provide the detailed stabilization mechanism of Zn on lignin and reed- and lignin-derived biochars but also develop a novel and effective material (lignin) for heavy metal remediation.