Phosphate removal and recovery using immobilized phosphate binding proteins

Progress towards a more circular phosphorus economy necessitates development of innovative water treatment systems which can reversibly remove inorganic phosphate (P_i) to ultra-low levels (<100 μg L^-1), and subsequently recover the P_i for reuse. In this study, a novel approach using the high-affinity E. coli phosphate binding protein (PBP) as a reusable P_i bio-adsorbent was investigated. PBP was expressed, extracted, purified and immobilized on NHS-activated Sepharose beads. The resultant PBP beads were saturated with P_i and exposed to varying pH (pH 4.7 to 12.5) and temperatures (25-45 °C) to induce P_i release. Increase in temperature from 25 to 45 °C and pH conditions between 4.7 and 8.5 released less than 20% of adsorbed P_i. However, 62% and 86% of the adsorbed P_i was released at pH 11.4 and 12.5, respectively. Kinetic experiments showed that P_i desorption occurred nearly instantaneously (<5 min), regardless of pH conditions, which is advantageous for P_i recovery. Additionally, no loss in P_i adsorption or desorption capacity was observed when the PBP beads were exposed to 10 repeated cycles of adsorption/desorption using neutral and high pH (≥12.5) washes, respectively. The highest average P_i adsorption using the PBP beads was 83 ± 5%, with 89 ± 4.1% average desorption using pH 12.5 washes over 10 wash cycles at room temperature. Thermal shift assay of the PBP showed that the protein was structurally stable after 10 cycles, with statistically similar melting temperatures between pH 4 and 12.5. These results indicate that immobilized high-affinity PBP has the potential to be an effective and reversible bio-adsorbent suitable for P_i recovery from water/wastewater.