A pH-responsive covalent organic network: morphology change leads to capture and removal of phosphate ions from water

Abstract

Phosphate is one of the essential nutrients for the growth of various organisms. However, it increasingly infiltrates aquatic environments due to human activities, posing environmental risks. Recovering phosphates from wastewater offers a promising solution to mitigate eutrophication and protect dwindling global phosphate reserves. To address this problem, we developed a pH-responsive two-dimensional covalent organic network (2D ag-CON) with multiple interaction sites for anions. Phosphate adsorption resulted in the transition of its surface morphology from spherical to sheet-like structures, providing a distinctive characteristic of ag-CON. The pH-dependent adsorption and desorption properties of the tris-aminoguanidine-based 2D ag-CON enable the selective capture and release of phosphate by adjusting the pH of the solution. The polymer demonstrates high selectivity for phosphate ions, even in the presence of excess competing anions, with a maximum adsorption capacity (Q_max) of 719 mg g⁻¹ at pH 6, and allows efficient release of captured phosphate ions through pH adjustment (∼11.5). Overall, this study demonstrated the development of a water-insoluble COF capable of efficiently capturing phosphate from wastewater to mitigate the adverse effects of water eutrophication.