Bio-Inspired phosphate adsorption by Copper-Decorated weak base anion exchanger

Hybrid-based phosphate adsorbents have high adsorption ability but are challenged with regeneration because of strong bonding. Inspired by phosphate-binding proteins (PBPs) which reached a trade-off between high adsorption and regeneration ability by binding phosphate species with multiple hydrogen bonds, a weak-base anion (WBA) exchange resin was decorated with Cu²⁺ ions to develop an alternative advanced phosphate adsorbent. Different from the role of metal species in the promoted adsorption of existing hybrid-based adsorbents, the Cu²⁺ ions mainly drove phosphate diffusion from the liquid to the resin and then induced the phosphate species to bond with multiple –NH₂⁺/–NH groups. Such adsorption exhibited a high entropy (ΔS⁰ = 132.47 J/mol-K) and was highly exergonic (ΔG⁰ = -16.83 ∼ -20.14 kJ/mol). Moreover, it enhanced the phosphate selectivity because the driving force for the sorption process originated from the complexation. During the regeneration, the Cu²⁺-to-Cu-O^- transformation could occur under weak alkaline conditions and repelled the adsorbed phosphate species to facilitate desorption. Therefore, the Cu-WBA resin not only exhibited high phosphate adsorptions (31.94 to 61.03 mg-P/g) and high competing-anion tolerance but also was successfully recovered with a diluted NaOH solution (0.05 N). Detailed adsorption behaviors under different pHs, temperatures, and interference of competing anions were examined, and the mechanisms were elucidated.