[Phosphate Removal on Zirconium Alginate/Poly(N-isopropyl acrylamide) Hydrogel Beads with a Semi-interpenetrating...

Zirconium alginate/poly(N-isopropyl acrylamide) hydrogel beads with a semi-interpenetrating network (ZA/PNIPAM) were prepared by using the ionic crosslinking and radical polymerization method and investigated for phosphate removal from aqueous solutions. The effects on the adsorption performance of hydrogel beads, including initial pH, adsorbent dose, initial phosphate concentration, and co-existing anions, were evaluated systematically. Results showed that the ZA/PNIPAM could exhibit a maximum uptake capacity of phosphate at pH 2.The uptake capacity of the adsorbent increased with a decrease in the dose or an increase in the initial phosphate concentration. The presence of SO₄^2- had a more negative effect on phosphate removal compared to Cl^- and NO₃^-. The kinetics fitted a pseudo-second-order model and intraparticle diffusion model, suggesting the adsorption rate was mainly controlled by surface adsorption and diffusion into the interior of the hydrogel beads. The isotherm data could be described by the Freundlich model, indicating that the adsorption process was heterogeneous multilayer adsorption. The studies of FTIR, XPS, and zero point of charge with relevant adsorption data revealed that the phosphate adsorption mechanisms could be electrostatic attraction (physical adsorption) and ligand exchange reactions (chemical adsorption). After four cycles of regeneration, ZA/PNIPAM exhibited a stable uptake capacity, indicating favorable reusability.