The effect of coordination state on the activity of TA-Zr supramolecular networks for heavy metals removal from water

Abstract

The self-assembly behavior of the MPNs-based adsorbent is substantially influenced by changes in solution pH. In this study, a highly negatively charged tannic acid‑zirconium (TA-Zr) macromolecular network has been successfully synthesized. As pH increases from 3 to 11, the coordination structure of TA with Zr(IV) changes from mono- to tri-complexation state, resulting in enhanced electronegativity and increased structural stability of tri-complex TA-Zr(9) due to its higher charge density. The TA dissolution experiments and zeta potential measurements confirm this conclusion. The tri-complex TA-Zr(11) underwent structural changes in its macromolecular network due to partial zirconium hydroxide formation under strongly alkaline conditions, as confirmed by X-ray photoelectron spectroscopy analysis, which in turn impacted its stability. Meanwhile, the adsorption capacity of TA-Zr materials for Pb(II) ion increases with the pH of the synthesis solution. XPS analysis suggested that the underlying adsorption mechanism involved ligand exchange between Pb(II) ion and phenolic hydroxyl groups on TA-Zr(9). After batch adsorption-regeneration cycles, TA-Zr(9) could be regenerated with an acidic solution while maintaining consistent Pb(II) ion removal efficiency. Furthermore, TA-Zr(9) effectively treated 12,150 bed volumes (BV) of synthetic water at pH 5.6, which is 18 times more than commercial D001. These results confirm that TA-Zr(9) shows great potential as an effective adsorbentfor Pb(II) removal in practical wastewater treatment, making it a promising candidate for water remediation.