Single-step hydrothermal synthesis of biochar from waste industrial hemp stalk core for Pb2+ sorption: Characterization and mechanism studies

Abstract

Industrial hemp stalk core (IHSC)-driven hydrothermal biochars were synthesized by single-step hydrothermal with different H2SO4 concentrations as hydrothermal media for the first time. The characterization results indicated that the concentration of H2SO4 had a significant effect on the carbonization degree and affected the physicochemical properties of biochar, which in turn influenced the Pb2+ sorption ability. Among these biochars, the 50% H2SO4-activated hydrothermal biochar (HBS50) not only achieved a significant yield (49.2%), but also obtained the largest specific surface area (359.11 m2/g) and pore volume (0.357 cm3/g). Besides, the abundant functional groups were found to be exposed on the surface of HBS50. In particular, HBS50 showed excellent sorption capacity for Pb2+, reaching a maximum sorption capacity of 195.9 mg/g in a short period of time (2 h). Sorption performance of HBS50 conformed well to the Langmuir isotherm model and pseudo-second order model. Based on the sorption performance and characterization analysis, the sorption mechanisms were proposed: -OH and -COOH groups on surface of biochar took deprotonation firstly to form negatively groups and then attracted Pb2+ cations. Subsequently, most of these attracted Pb2+ cations were further fixed by surface -OH and -COOH through complex reaction. In addition, the contribution of π-π coordination with unsaturated groups (such as CC and COOR) and the electrostatic interaction for the further attachment of Pb2+ cations could not be excluded. However, ion exchange and precipitation reaction had little assistance to the sorption of Pb2+. The work offers an effective and economic method for removing aqueous Pb2+.