A biomass carbon prepared from agricultural discarded walnut green peel: investigations into its adsorption characteristics of heavy metal ions in wastewater treatment

The removal of heavy metal ions is significantly important in industrial wastewater treatment. This study reported a cost-effective novel biomass carbon prepared from agricultural discarded walnut green peel (known as HBC), and its adsorption characteristics for three heavy metal ions: Zn2+, Ni2+, and Co2+ were studied. It was found that HBC possessed well-developed hierarchical porous structure and rich oxygen- and nitrogen-containing functional groups, which facilitated the adsorption process. Fundamental isotherm, kinetic, and thermodynamic adsorption studies were conducted, and HBC showed great adsorption capacity for all three ions by up to 249, 129, and 113 mg·g−1 for Zn2+, Ni2+, and Co2+ respectively. Fitting results to their mathematical models illustrated that the chemical adsorption mechanism dominated the adsorption by HBC. In addition, competitive and dynamic adsorption, as well as reusability studies of HBC were performed for further understanding HBC’s characteristics toward potential industrial wastewater treatment. Competitive adsorption results showed that HBC was highly selective for Zn2+ in both binary and ternary ion systems. Better adsorption performance for Zn2+ was also found in dynamic adsorption study with longer breakthrough and exhaustion time compared with Ni2+ and Co2+. Finally, the adsorption efficiencies of HBC for all three metal ions could still maintained at a high lever between 84 and 90% even after 5 adsorption-desorption repeats, suggesting promising reusability of HBC as an adsorbent.