Hydrogen bonding-orientated selectivity of phosphate adsorption by imine-functionalized adsorbent

The development of phosphate-selective adsorbents is essential for addressing eutrophication-associated problems and issues about lack of phosphorus resources. In this study an imine-functionalized adsorbent (N=C@CMPS) was synthesized for this purpose; briefly, the commercially available precursor, chloromethylated polystyrene adsorbent (CMPS), was grafted with ethylenediamine (EDA) to yield the intermediate (EDA@CMPS), which was subsequently subjected to synchronous elimination to yield the product with imine functional group (N=C@CMPS). Spectroscopic analyses and macroscopic experiments were then employed to evaluate the as-prepared adsorbent. As compared with CMPS, EDA@CMPS, and two commercial amine-based adsorbents, N=C@CMPS exhibited much higher adsorption capacity and selectivity toward phosphate [P(V)]; such improvement can be primarily attributed to the imine group of N=C@CMPS and their difference in the acid-base property. The imine group, acting as the recognition unit to distinguish P(V) from background anions, can solely serve as H-bond acceptors and exclusively interact with the protonated P(V) species (H2PO4- and HPO42-, H-bond donors) under neutral pH but not with the deprotonated anions such as Cl-, NO3-, and SO42-. the thermodynamic parameters confirm the spontaneous and slightly exothermic nature of P(V) adsorption on N=C@CMPS, according with the domination of hydrogen bonding interactions. N=C@CMPS also exhibited very broad applicability for P(V) adsorption over a wide range of pH (3 ~ 10). Furthermore, the exhausted N=C@CMPS can be efficiently regenerated with the combination of dilute NaOH solution and dilute HNO3 solution for repeated use.