Phosphate-binding protein-loaded iron oxide particles: adsorption performance for phosphorus removal and recovery from water†

Adsorbents featuring high-affinity phosphate-binding proteins (PBPs) have demonstrated highly selective and rapid phosphorus removal and recovery. While immobilized PBP is promising for inorganic phosphate (orthophosphate, P_i) removal and recovery, increased adsorption capacity of PBP-based materials is essential to enhance the feasibility of PBP for scaled implementation. Here, magnetic n-hydroxy succinimide (NHS)-activated iron oxide particles (IOPs) were used to immobilize PBP (PBP–IOPs). The PBP–IOPs provided rapid P_i removal, with more than 95% adsorption within 5 min. Slightly acidic pH, room temperature (20 °C), and low ionic strength (0.01 M KCl) demonstrated the best removal efficiency. The P_i adsorption capacity of PBP–IOPs was not affected by anions such as chloride, sulfate, nitrate, bicarbonate, and borate. PBP–IOPs released 99% of total adsorbed P_i using pH adjustment. Conjugation of PBP to higher surface area per mass IOPs increased P_i attachment capacity (0.044 mg g⁻¹) relative to previous studies of PBP immobilized on Sepharose resin (0.0062 mg g⁻¹). Accordingly, PBP–IOPs have the potential to rapidly, spontaneously, selectively, and reversibly capture P_i. Theoretical capacity calculations indicated that parallel improvements in surface area to mass ratio of the base immobilization material together with reducing the size of the P_i-binding amino acid sequence (while retaining P_i specificity) are needed to further advance design and implementation of PBP-based adsorbents.