Facile synthesis of nano ZnO/ZnS modified biochar by directly pyrolyzing of zinc contaminated corn stover for Pb(II), Cu(II) and Cr(VI) removals

Nowadays, nano mineral modified biochars show a promising adsorption capacity for pollutants removals by combining the advantages of porous structure of biochar and unique property of nano minerals. In this work, nano ZnO/ZnS modified biochar was synthesized from slow pyrolysis of the zinc contaminated corn stover obtained from a biosorption process. The characterization results indicated that the zinc mineral modified biochar had a better porous structure (BET = 397.4 m² g⁻¹ and TPV = 0.43 cm³ g⁻¹) than the common biochar (BET = 102.9 m² g⁻¹ and TPV = 0.20 cm³ g⁻¹), and zinc minerals were evenly anchored on the biochar surface as nano ZnO/ZnS. Batch sorption experiments show that the obtained nano ZnO/ZnS modified biochar had strong sorption ability to Pb(II), Cu(II) and Cr(VI) with maximum sorption capacities of 135.8, 91.2 and 24.5 mg g⁻¹, respectively, which were significantly higher than the common biochar (63.29, 27.05 and 15.23 mg g⁻¹, respectively). The adsorption kinetics of heavy metal ions on nano ZnO/ZnS modified biochar were well described by the pseudo-second-order model, and the adsorption behavior coincided with heterogeneous adsorption materials as reflected by well fitting the Freundlich model. The thermodynamic results indicated that the adsorption process was an endothermic and spontaneous process. Based on the comprehensive characterizations and adsorption performance, the enhancement of the metals removal by the nano ZnO/ZnS modified biochar were mainly attributed to the hydroxyl groups on the surface of nano ZnO/ZnS particles and well-developed porous structure catalyzed by zinc salt during pyrolysis process. These findings suggested that generation of nano mineral modified biochar from heavy metals polluted biomass could be an attractive approach to treating and utilizing the waste biomass with a highly technical and economic feasibility.