Insight into mass transfer mechanism and equilibrium modeling of heavy metals adsorption on hierarchically porous biochar

One-step pyrolysis of biomass/zinc salts composites was investigated to clarify the effects of zinc salts on the pore structure of biochar and its performance for removing heavy metals from wastewater. The characterization results indicated that zinc salts not only promoted the pyrolysis of biomass but also decomposed to generate volatile gas and thermally removable pores templates in situ that established the hierarchical porous biochar (HPBC). More specifically, HPBC exhibited a significantly increased pore volume compared with pristine biochar (by a factor of up to 80) and large specific surface areas ranging from 769.91 to 1192.37 m2/g. To connect these improved properties with the heavy metals removal performance, the pore volume and surface diffusion model (PVSDM) was used to numerically simulate the adsorption process and investigate the diffusion and mass transfer mechanism. Notably, the fitted pore volume diffusion coefficient of HPBCs (DpHPBC) was at least three orders of magnitude larger than their surface diffusion coefficient (DSHPBC), whereas these two parameters (DpBC&DSBC) for pristine biochar (BC) were of the same order of magnitude but were much smaller in value than those obtained by HPBCs. These results suggested that the improved biochar pore structure by co-pyrolyzing with zinc salts mainly enhanced the pore volume diffusion of heavy metals during their adsorption. The adsorption process of Pb2+ on HPBCs was controlled by the intra-particle diffusion mechanism, while the adsorption process of Cd2+ was controlled by external and internal mass transfer.