Kinetics, Affinity, Thermodynamics, and Selectivity of Phosphate Removal Using Immobilized Phosphate-Binding Proteins

A phosphate (P i )-selective adsorption system featuring immobilized P i -binding proteins (PBP) has recently attracted attention for ultralow P i removal followed by recovery. This study investigated the adsorption kinetics, affinity, thermodynamics, and selectivity, as well as the effect of pH and temperature on P i adsorption using immobilized PBP (PBP resin). Immobilizing PBP did not affect its P i 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 ( K L ) was 21 ± 5 μM -1 P i (220 ± 52 L/mg-P i ), which is higher than P i adsorbents recently reported in literature. The ideal operating ranges for high-affinity P i adsorption using PBP resin were pH 4.5 to 9 and temperature 14 to 37 °C. The P i -PBP resin adsorption process was not affected by the presence of common anions (Cl - , Br - , NO 2 - , NO 3 - , SO 4 2- , and HCO 3 - ). Adsorption using the P i -PBP resin was exothermic (Δ H = -6.3 ± 1.3 kJ/mol) and spontaneous (Δ G = -39.7 ± 0.1 to -43.2 ± 0.2 kJ/mol) between 14 and 43 °C. These results indicate that PBP resin's P i adsorption rate and affinity surpass those of existing adsorbents. Future work to increase the PBP resin's adsorption capacity is important to its application as a viable P i adsorbent.