Defective metal organic framework encapsulated phytase membranes for continuous phosphate production from phytate in agricultural wastes

Utilization of phytase to convert phytate in agricultural wastes into phosphate provides a renewable and green approach for inorganic phosphate production. However, the industrial application of phytase still faces significant challenges of high sensitivity to environmental variation and difficulty in recovery and reuse, etc. In this work, phytase-MOF functionalized membrane was developed, wherein the phytase was encapsulated within the defective MIL-88A framework for enhancing the application stability of phytase without compromising its catalytic performance. The prepared catalytic membrane was further utilized in a membrane reactor for continuous phosphate production. The results demonstrated successful encapsulation of phytase within the channels of the defective MIL-88A framework, forming Phy@MIL-88A with core-shell structure, which substantially improved phytase stability under various conditions of temperatures up to 100 °C, pH range of 2-7.5, and organic solvents. A quite thick Phy@MIL-88A catalytic layer exceeding 15 μm was formed on the porous membrane surface with the assistances of polydopamine and polyethylenimine. The functionalized membrane exhibited optimal catalytic performance at 60 °C, as opposed to 40 °C for free phytase, reflecting the increased heat resistance imparted by MIL-88A. The introduction of structural defects via Zn doping and Fe/FA adjustment significantly enhanced the phosphate conversion rate of the membrane with the maximum value of 69.26% obtained, compared to 52.24% for intact Phy@MIL-88A membrane. Kinetic analysis revealed that defect introduction strengthened the affinity between phytase and phytate, thereby improving catalytic efficiency. The defective Phy@MIL-88A membrane exhibited high reusability over seven consecutive operational cycles, achieving the maximum productivity of 6182 mmol/(m²·d) and phosphate concentration of 6.44 mM. Consequently, this work broadened the industrial applicability of phytase and offered an effective strategy for green production of phosphate from agricultural wastes.