The role of hydration in the removal of glyphosate (GLY) and aminomethylphosphonic acid (AMPA) by nanofiltration membranes

Nanofiltration can remove glyphosate (GLY) and aminomethylphosphonic acid (AMPA) from water via steric, Donnan, and dielectric exclusions, although the significance of dielectric exclusion resulting from hydration has not been elucidated. This study investigates the properties of hydration and its role in GLY/AMPA removal. Results show that charge and dielectric exclusions are dominant in membranes with molecular weight cut-off (MWCO) > 150 Da. The contribution of dielectric exclusion is evident when GLY and AMPA in neutral forms are partially removed (50-80%) with >150 Da membranes at pH 2. When GLY/AMPA are negatively charged (pH from 4 to 12), GLY/AMPA removal increased from 50-80 to 90%, indicating the growing contribution of both charge and dielectric exclusions. The hydration layer can be shredded at higher applied pressures, decreasing removal from 86 to 28% (GLY) and 27 to 7% (AMPA). Both molecular dynamics and Fourier-transform infrared spectroscopy (FTIR) agree on the strong hydration of GLY/AMPA especially at pH 4-6. Understanding the role of hydration in the removal of small and charged organic micropollutants is important for tuning NF membranes for water purification.