Adsorption characteristics and mechanisms of Pb2+ and Cd2+ by a new agricultural waste-Caragana korshinskii biomass derived biochar.

Abstract

In order to explore the comprehensive utilisation and recycling technology of Caragana korshinskii resources, a new agricultural biomass waste, 15 kinds of Caragana korshinskii biochar (CB) were prepared by controlling the pyrolysis temperature and time at the anaerobic environment. Moreover, we pay more attention to deriving the adsorption mechanisms and exploring the difference in adsorption characteristics of Pb2+ and Cd2+. The optimal preparation conditions and the batch adsorption experiments were evaluated, and the adsorption characteristics and mechanisms were discussed using 8 theoretical adsorption models and multiple characterisation methods. The results showed that the CB prepared at 650°C for 3h presented the best performance. The Langmuir and Freundlich models can well simulate the isotherm adsorption process of CB for Pb2+ and Cd2+, respectively. The adsorption kinetics of CB for Pb2+ and Cd2+ were best fitted by the pseudo-second-order model. The adsorption equilibrium for Pb2+ and Cd2+ was reached within 3h, and their maximum adsorption capacity reached 220.94mgg-1 and 42.43mgg-1, respectively. In addition, the best addition amount was 3gL-1 and 2.2gL-1 for Pb2+ and Cd2+, respectively. The optimum pH range was 3-6 for Pb2+ and 6-7.5 for Cd2+. The adsorption mechanisms of CB for Pb2+ and Cd2+ were physicochemical composite adsorption processes, mainly including physical sorption on surface sites, intraparticle diffusion, electrostatic adsorption, ion/ligand exchange, cationic-π interactions, surface complexation and precipitation. Furthermore, the ash of CB also presented a positive effect on the adsorption of Pb2+. Compared with other cellulose- and lignin-based biomass materials, CB showed low cost and efficient performance without complicated modification conditions. Therefore, this study demonstrates that CB is a promising raw material in water pollution control to immobilise heavy metals.