Preparation and characterization of polymeric ligand exchanger based on chitosan hydrogel for selective removal of phosphate

Polymeric ligand exchangers (PLEs) are typically prepared using commercial chelating resins which are often costly and less "green". In this work, we prepared a new PLE by immobilizing Cu(II) on a low-cost, natural biopolymer chitosan. It was confirmed that the Cu²⁺ ions were bonded to chitosan by complexing with the nitrogen and hydroxyl groups in the chitosan polymer chain, leading to a reduction in the size of the hydrogel and intensified density of the biopolymer. The chelating interaction between nitrogen and Cu²⁺ acts as a crosslinker that improves the physical and chemical stability of the PLE. The pH sorption tests confirmed a pK_a of ∼7.0 for the biopolymer. The PLE reverses the affinity sequence of standard anionic resins, and displayed much greater affinity toward strong ligands such as phosphate than sulfate due to concurrent electrostatic and Lewis acid-base interactions between immobilized Cu²⁺ ions and phosphate regardless of solution pH. The maximum phosphate uptake was estimated to be 70 mg g^-1 and 35 mg g^-1 in single and binary-component systems, respectively. Fixed-bed column tests revealed that the PLE may be used for selective removal of phosphate of strong ligand characteristics over sulfate.