Phosphate removal by polystyrene anion exchanger (PsAX)-supporting Fe-loaded nanocomposites: Effects of PsAX functional groups and ferric (hydr) oxide crystallinity

Fe-loaded nanocomposites (NCs) have proven to be a promising category of adsorbents for efficiently removing phosphate [P(V)] from aqueous media. In this study P(V) removal and the possible adsorption mechanisms were investigated for four Fe-loaded NCs, which were developed by loading ferric (hydr)oxide (Fe[H]O) nanoparticles (NPs) of different crystal formations inside polystyrene anion exchanger (PsAX). Fe [H]O-PsAX@25 (with amorphous Fe[H]O NPs) was synthesized by in situ deposition method under ambient condition, and it served as source material to yield Fe [H]O-PsAX@75, @150, and @200 by hydrothermal treatment for 24 h under 75, 150, and 200 degrees C, respectively. Macroscopic experiments revealed that these NCs have decent P(V) adsorption abilities at pH 5-10 except Fe [H]O-PsAX@200, the functional groups (R-N+[CH3](3)) of which severely suffered from degradation or even elimination as evidenced by FT-IR spectra and its exchange capacities. XPS results spectroscopically confirmed three different P(V) adsorption mechanisms, that is, ion-exchange interactions with R-N+[CH3](3), ligand-exchange interactions with amorphous Fe[H]O NPs, and ligand-exchange interactions with crystalline NPs (i.e., hematite). The ion-exchange interactions lack selectivity toward P(V) and would be invalidated by higher concentrations of competing sulfate anions, while ligand-exchange interactions, especially from amorphous Fe[H]O NPs, exhibited great P(V) adsorption selectivity. Acid-dissolution resistance of the loaded Fe[H]O NPs was significantly improved when being transferred into crystal formation, but their P(V) adsorption capacities gradually decreased during transformation process partially because of enlarging particle sizes and thus lessening effective surface areas.