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Kinetics, affinity, thermodynamics, and selectivity of phosphate removal using immobilized phosphate-binding proteins.

A phosphate (Pi)-selective adsorption system featuring immobilized Pi-binding proteins (PBP) has recently attracted attention for ultra-low Pi removal followed by recovery. This study investigated the adsorption kinetics, affinity, thermodynamics, and selectivity, as well as the effect of pH and temperature on Pi adsorption using immobilized PBP (PBP resin). Immobilizing PBP did not affect its Pi affinity. Kinetic studies at 22°C and pH 7.1 showed that the PBP resin achieved 95% of its equilibrium capacity within 0.64±0.2 min. The estimated Langmuir affinity constant (KL) was 21±5 µM-1 Pi (220±52 L/mg-Pi), which is higher than Pi adsorbents recently reported in literature. The ideal operating ranges for high-affinity Pi adsorption using PBP resin were pH 4.5 to 9 and temperature 14°C to 37°C. The Pi-PBP resin adsorption process was not affected by the presence of common anions (Cl-, Br-, NO2-, NO3-, SO42-, and HCO3-). Adsorption using the Pi-PBP resin was exothermic (ΔH=-6.3±1.3 kJ/mol) and spontaneous (ΔG=-39.7±0.1 to -43.2±0.2 kJ/mol) between 14°C and 43°C. These results indicate that PBP resin's Pi adsorption rate and affinity surpasses existing adsorbents. Future work to increase the PBP resin's adsorption capacity is important to its application as a viable Pi adsorbent.