Polymeric membranes are crucial to many industries such as wastewater treatment, food processing, and resource extraction. However, since membrane-based separation primarily rely on either size or charge exclusion, the selective targeting of specific solutes remains a challenge. Enabling membrane based selective separation process would allow for the continuous recovery of target molecules, enabling resource recovery, nutrient extraction, and production of valuable commodity chemicals. In this study, we demonstrate a facilitated transport membrane capable of selectively extracting phosphate from a solution containing a mixture of competing anions. The membrane is synthesized by growing hydrous manganese oxide nanoparticles, in-situ, in a cation exchange membrane (CEM). Owing to the negative fixed charges native to the CEM polymer matrix, the composite membrane blocks the passage of co-ions (anions). However, phosphate is known to form relatively weak outer-sphere interactions with hydrous manganese oxide. Therefore, embedding hydrous manganese oxide nanoparticles within the CEM matrix provides a diffusion pathway for the transport of phosphate ions across the composite, a pathway which is blocked to other anions that are not capable of forming these interactions. Upon incorporating the HMO NPs, the membrane’s phosphate permeability increased to 25E-8 mol/m2/s/V compared to the 3.2E-8 mol/m2/s/V through an unmodified CEM. Moreover, the membrane showed improved separation of phosphate over other competing anions with a selectivity of about 33, 10, and 30 for sulfate, nitrate, and chloride respectively. Finally, the membrane modification did not affect the cation transport achieving a maximum flux of about 4.7E-5 mol/m2/s.

Reverse-selective ion exchange nanocomposite membrane: Selective phosphate recovery via an outer sphere complexation-diffusion pathway

ACS Spring 2022

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