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) 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^-, NO₂^-, NO₃^-, SO₄^2-, and HCO₃^-). Adsorption using the P_i-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 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.