Insights into selective adsorption mechanism of copper and zinc ions onto biogas residue-based adsorbent: Theoretical calculation and electronegativity difference.

Modified biomass-based adsorption technique has attracted much attention in heavy metal ions removal, but selective adsorption behavior and mechanism of heavy metal ions adsorption onto biosorbent still need to be further clarified. Herein, a carboxylated biogas residue (BR-COOH) was prepared to remove the Cu and Zn from single/binary heavy metal ions solution and explore selective adsorption mechanism. The results exhibited that the adsorption capacities of BR-COOH for Cu was higher than that for Zn obviously, whether in the single or binary heavy metal ions solution. Meanwhile, the introduced carboxy groups were identified as the main sites for metal ions adsorption. Density functional theory (DFT) calculation results exhibited that the adsorption energy of Cu (-0.51 eV) onto BR-COOH was lower than that of Zn (-0.47 eV), indicating that the Cu adsorbed on BR-COOH was more stable than Zn. Moreover, the metal ions adsorption capacity of BR-COOH was positively correlated with their electronegativity, which was due to that the metal ions with stronger electronegativity was more easily interacted with the negatively charged oxygen in carboxyl groups. The same results were also verified in the control experiment conducted with two other biosorbents. Therefore, the work provided a new and in-depth insight into selective adsorption of metal ions onto carboxylated biosorbent.