Phosphate removal performances of layered double hydroxides (LDH) embedded polyvinyl alcohol / lanthanum alginate hydrogels

Lanthanum modified compounds and Layered double hydroxides (LDH) are promising adsorbents for phosphate. However, the nano-scale LDH is challenging to separate, and conventional immobilization methods weaken the adsorption performance. With the affinity between metal ions and phosphate, polyvinyl alcohol/metal ions sodium alginate (PS-M−LDH) hydrogel beads for the improvement of phosphate removal were prepared by in-situ crosslinking with different metal ion (M) solutions. Through screening, the PS-M−LDH crosslinked by La (PS-La-LDH) has the preferable phosphate adsorption performance. The phosphate adsorption behavior of PS-La-LDH hydrogel was further investigated. Results showed that the adsorption process of phosphate by PS-La-LDH hydrogel was in accordance with the pseudo-second-order kinetic model and Freundlich model. The maximum experimental adsorption capacity of PS-La-LDH was 34.2 mg P/g, converting to an equivalent LDH of 91.2 mg P/g LDH, which was 1.6 times of pristine LDH powder (58.0 mg P/g LDH). The removal of phosphate by PS-La-LDH performed well at a wide range of pH 3 ∼ 8. PS-La-LDH showed selective adsorption to phosphate with the existence of competing anions (Cl-, NO3–, and SO42–). The phosphate adsorption by PS-La-LDH hydrogel only reduced 16.57 % when the concentration of SO42– was increased to 2882 mg/L. SEM, XRD, FTIR, and XPS results showed that the electrostatic interaction, ligand exchange, and ion exchange jointly facilitated the adsorption of phosphate.