Effects of Concentration Polarization and Membrane Orientation on the Treatment of Naproxen by Sulfate Radical-Based Advanced Oxidation Processes within Nanofiltration Membranes with a Catalytic Support

Abstract

The structure with a selective nanofiltration (NF) layer on top of a catalytic ultrafiltration (UF) membrane provides the possibility of treating micropollutants (MPs) by both rejection and degradation. However, such a dense selective layer unavoidably induces a formation of a highly concentrated retentate and a low utilization of the oxidant due to its rejection. A different membrane orientation is expected to solve the problems mentioned above since the concentrated MPs can be degraded within the catalytic support, and the rejection of the oxidants can be avoided when the pressure is applied from the porous support side. However, the resulting complex concentration polymerization (CP) effects are not well understood, and the effects of the following concentration changes of the MPs and the oxidants around the catalyst within the porous support membrane are unclear as well. In this work, three polyelectrolyte multilayers with different selectivity were fabricated on PES@CoFe2O4 catalytic UF membranes by sequential dip-coating. Concentration polarization models are utilized to predict the concentrations of naproxen and peroxymonosulfate (PMS) within the porous catalytic support under different membrane orientations. The results of naproxen removal after adding PMS show that a higher naproxen removal can be obtained with a higher concentration ratio of PMS to naproxen (cPMS/cNPX). Moreover, it is shown that the MPs in the feed solution can be degraded in a catalysis-separation sequence, exhibiting the potential of rejecting and simultaneously degrading MPs. However, the coating of a selective polyelectrolyte multilayer on the catalytic UF membranes also causes lower accessibility of PMS and naproxen to the catalysts embedded within the polymeric membranes, resulting in the decline of degradation efficiency. By coating only one side of the membranes, this negative effect caused by the polyelectrolyte coating can be mitigated. Overall, a 97% removal of naproxen on the permeate side and a 12% degradation of naproxen on the feed side were observed with the one-side-coated membranes under a catalysis-separation sequence. This work highlights the key role that concentration polarization can play in the degradation efficiency of naproxen in catalytic NF membranes, providing valuable guidance for the design of further improved catalytic membranes.