Nitrogen-Based conjugated microporous polymers for efficient Hg(Ⅱ) removal from Water: Performance and mechanism

Abstract

The development of efficient adsorbents for the removal of toxic Hg(Ⅱ) is crucial in environmental engineering. In this study, we have modified three nitrogen-based conjugated microporous polymers (TAA, PDA, DDA) with nitrogen functional groups and microporosity, resulting in ideal adsorbents for this purpose. Of these, PDA exhibited a record-breaking maximum adsorption capacity to Hg(Ⅱ) of 1582.6 mg·g−1, and fast adsorption kinetics. Additionally, DDA effectively reduced the Hg(Ⅱ) concentration from 11.2 ppb to less than 0.01 ppb. They demonstrated robust reusability, retaining over 90% adsorption capacity for at least 25 cycles. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) analyses revealed that tertiary nitrogen approached Hg in Hg(OH)2, forming chelate adsorption with Hg2+, while imine and amino nitrogen formed hydrogen bonds with –OH in Hg(OH)2 before forming chelate with Hg2+. The strength of binding ability of nitrogen functional groups to mercury followed the order of cyclic N in the oxidized unit (-5.01 eV) > cyclic N in the reduced unit (-5.00 eV) > =N- (-4.85 eV) > –NH- (-2.17 eV). A higher adsorption capacity and faster adsorption equilibrium time resulted from a larger specific surface area and a distribution of micropores closer in size to the mercury ion.