Hierarchically porous biochar templated by in situ formed ZnO for rapid Pb2+ and Cd2+ adsorption in wastewater: Experiment and molecular dynamics study

3D hierarchical porous biochar (HPBC) was synthesized by a thermally removable template without post-activation. Zn(NO3)2 decomposition produced gases and ZnO in situ to activate and expand the three-dimensional micro-and mesopores. Compared with pristine biochar (BC), the specific surface area and pore volume of HPBC were increased by 223 and 75 times, respectively. The abundant pore structure of HPBC significantly enhanced the diffusion rate of heavy metals. For example, compared to BC, the time required for HPBC to adsorb Pb2+ reach adsorption equilibrium was reduced by 87.5% (40 min vs 5min). Such an adsorption performance of HPBC was also insensitive to different background ions (K+, Na+, Ca2+, and Mg2+) with a much higher concentration than that of heavy metals. When applied to treat desulfurization wastewater from power plants, HPBC yielded 100% removal of Pb2+ and Cd2+, much higher than that by using commercial activated carbon (28%). Molecular dynamics simulation revealed different locations preferred by the adsorption of Pb2+ (micropores) and Cd2+ (mesopores) in the hierarchical pore structures. The adsorption of Pb2+ and Cd2+ on HPBC was mainly achieved by diffusion, oxygen functional group complexation, and precipitation. These results provided better knowledge to understand the microscopic adsorption mechanisms of heavy metals in hierarchical pores and a facile yet robust strategy to design such structures in biochar for efficient wastewater treatment.