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.

Automated summary

The development of phosphate-selective adsorbents is crucial for tackling eutrophication-related issues and scarcity of phosphorus resources. In this study, a new imine-functionalized adsorbent (N=C@CMPS) was synthesized, which exhibited excellent phosphate selectivity and adsorption capacity, primarily attributed to the imine group's specific hydrogen bonding interaction with phosphate species at neutral pH. Furthermore, the adsorbent showed broad applicability over a wide pH range and could be efficiently regenerated with a combination of dilute NaOH and HNO3 solutions.