Microsized particles of Aza222 polymer as a regenerable ultrahigh affinity sorbent for the removal of mercury from aqueous solutions

•Aza222 exhibits selectivity and extremely high affinity for Hg2+.•Hg2+ sorption capacity can be predicted from the stoichiometry of complexation.•DFT calculations reproduce known stability constants remarkably well.•Sorption performance of Aza222 exceeds that of the commercial resin.•Aza222 is a selective Hg2+ sorbent in the presence of interference cations (Ca2+).

This paper reports the preparation and characterization of a microparticulate sorbent based on a novel p-xylylene-peraza[2.2.2]cryptand (Aza222) polymer that exhibits selectivity and high affinity for mercury cations [1], [2] and [3]. The sorbent particles dispersed into an aqueous suspension were characterized in terms of surface morphology, size distribution, porosity, and surface charge. Based on measured adsorption isotherms, two possible mechanisms of adsorption by this novel sorbent are proposed: high affinity stoichiometric complexation of Hg2+ within the Aza222 cage, which proceeds until the stoichiometric capacity of the sorbent is saturated; then excess mercury removal by nonspecific physisorption onto the surface of the Aza222 polymer. The experimentally determined mercury loading capacity of the sorbent was in accord with stoichiometric calculations at both high (mg/L) and low (μg/L) concentrations, opening possibilities for the use of the polymer for mercury detection or other analytical applications. Desorption studies were carried out and a preliminary study demonstrated that the sorbent is regenerable. The mercury capacity and selectivity of the sorbent in the presence of calcium ions compared favorably against corresponding properties of both a commercial ion exchange resin used for mercury removal and a related but non-macrocyclic polyamine-based polymer. Density functional (B3LYP) calculations using the 6-31G* basis set for light elements (C, H, Li, N, O) and the LANL2DZ effective core potential and basis set combination were used to compute the relative stability constants of the various metal-cryptand complexes. The agreement between theoretical and experimental Ks values, where known, was generally excellent, supporting the prediction of unknowns. Of particular relevance to the experimental work herein, the calculated difference between log KS for Hg2+ vs. Ca2+ ion binding in the hexamethylated peraza[2.2.2]cryptand is 23.0, indicating an overwhelming preference for binding mercury in this all-nitrogen complexant model for the ligand covalently embedded in the Aza222 polymer sorbent.