Metal selectivity and effects of co-existing ions on the removal of Cd, Cu, Ni, and Cr by ZIF-8-EGCG nanoparticles.

Recently, the developments of MOFs and their applications on heavy metal removal have attracted much attention. In this paper, we focused on the influence of co-existing ions on the adsorbability of epigallocatechin gallate (EGCG) modified ZIF-8 in a multi-metal system (Cd(II), Cu(II), Ni(II), and Cr(VI)). It was found that the adsorption of Cd(II), Cu(II), Ni(II), and Cr(VI) on the ZIF-8-EGCG fitted well with the pseudo second-order kinetic model and Langmuir isotherm. The adsorption selectivity of ZIF-8-EGCG in the mixed metal solution followed the order of Cu(II) ≫ Cr(VI) > Cd(II) > Ni(II). Besides, Cu(II) was the most attractive substance with a maximum capacity of 232.97 mg·g-1. Copper also disturbed the adsorption of Cd(II) and Ni(II) and contributed the most to the Cr(VI) removal. The natural anions (e.g., Cl-, NO3-, AsO2-, SO42-) significantly impeded Cr(VI) adsorption, while low concentrations of natural cations (e.g., K+, Na+, Ca2+, and Mg2+) facilitated the adsorption of Cd(II), Cu(II) and Ni(II).