Ultrafitration surface modification for fouling mitigation with applications to water reuse

Abstract

The second goal was to improve the MFC cathode by developing a three-layer composite membrane that includes antifouling, electron conductive, and oxygen reducing properties. To achieve these goals, we prepared a modifying agent that includes DA, multiwalled carbon nanotubes (MWCNTs), and platinum (Pt) to modify PES membranes and proton exchange membranes (PEMs). We have observed the deposition of the (PDA+MWCNTs) on the PES membrane and measured a decrease contact angle while preserving water permeability. Two PEMs, BP-ArF4 and BP-SA, were tested, but the modification caused degradation of the BP-ArF4 PEM. Deposition of the (PDA+MWCNTs) was observed on the BP-SA PEM without degradation. Therefore, the BP-SA PEM is a candidate in future studies. The project future work includes modification optimization, membrane characterization, and property evaluation.

We applied a two-mechanism combined pore blockage and cake filtration model, and a and three-mechanism combined pore construction, pore blockage and cake filtration model, to study membrane fouling mechanisms and describe measured trends in flux and membrane resistance over filtration time. A model sensitivity analysis was conducted using the two-mechanism combined pore blockage and cake filtration model to evaluate different scenarios of fouling rate parameters and potential effects of modification on membrane resistance. These results were used to identify the optimum trends in membrane properties; i.e., the most successful membranes have the lowest membrane resistance (Rm) and lowest fouling rate parameters at the same initial flux. The model sensitivity provided an approach to account for changes in membrane resistance when comparing modified and as-received membranes. The three-mechanism model increased the modeling accuracy by virtue of a third fouling mechanism (pore constriction) at the expense of an additional model parameter; however, the parameter sets still appear to be unique.

A stirred dead-end filtration system was used to evaluate the filtration performance of modified membranes by filtering municipal wastewater treated via MFC, and model foulants (BSA, SA, and Suwannee River natural organic matter, SRNOM). As above, performance assessment included foulant rejection and membrane flux recovery. In general, the modified membrane water permeability decreased with coating time, which may occur due to pore constriction, partial blockage of pore entrances, and pore coverage by a permeable coating layer. Compared to the AR membrane, the total and fouling resistances and energy consumption of the modified membranes reduced with all foulants except for BSA. The resistances and energy consumption also increased with modification times. After optimizing the coating time, several coating strategies exhibited lower fouling resistance and lower energy consumption than the AR membrane, including DA (0.5- and 1-hr); DA+PDAEMA (0.5- and 1-hr), NE (3-hr) and PDAEMA alone (24-hr). Additionally, the DA+PDAEMA mixtures exhibited antibacterial properties. The membrane flux recovery and foulant rejection were either maintained or slightly improved depending on the foulants.

This research also investigated PES membranes modified by coatings of dopamine (DA) and norepinephrine (NE) polymerized in-situ, as well as mixtures of dopamine (DA) and poly(2-dimethylamino) ethyl methacrylate methyl (PDAEMA). NE modified membranes were characterized by contact angle measurements to assess surface wettability; X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance-Fourier-transform infrared spectroscopy (ATR-FITR) to provide surface chemistry and surface functional group information; atomic force microscopy (AFM) to characterize surface topography and roughness; and scanning electron microscope (SEM) to evaluate the membrane visually.

To identify membrane surface chemistries that mitigate fouling, we employed two surface modification methods, UV-induced graft polymerization and coating by physical adsorption. Graft polymerization of 2-(Methacryloyloxy)-ethyl-dimethyl-(3-sulfopropyl ammonium hydroxide) (BET(SO3)-) and 2-(Methacryloyloxy) ethyl trimethylammonium chloride (N(CH3)3+) (N(CH¬3)3+) monomers were selected using a high throughput (96-well plates) screening technique from a group of 24 candidates. The performance of BET(SO3)- and N(CH¬3)3+ monomers were verified in bench-scale tests using a dead-end filtration system to filter municipal wastewater treated via MFC, and three model foulants, serum albumin (BSA), sodium alginate (SA), Elliot humic acid (EHA). Performance assessment included measurement of foulant rejection and membrane flux recovery. Two monomer concentrations were employed, 0.2 and 0.6 M. Membranes modified with 0.6 M N(CH¬3)3+ performed better than the as-received (AR) membrane, and the other modified membranes. The effects of mixing were evaluated; mixing significantly decreased membrane fouling and energy consumption during filtration of MFC effluent, which we attribute to enhance the solute back transport. The modified membrane exhibited better flux recovery than the AR membrane; modification did not have a significant impact on solute rejection. Downstream UF treatment of MFC effluent significantly reduced scaling minerals (calcium and magnesium), and also significantly reduced COD.

Municipal wastewater can be a sustainable alternative source for water reuse for power plant cooling and other freshwater applications. Microbial Fuel Cells (MFCs), a bio-electrochemical system, has been investigated for municipal wastewater (WW) treatment; however, in many cases the effluent does not meet water re-us standards. In this work we investigate ultrafiltration (UF) as a downstream water purification step to meet water re-use standards, including cooling water standards. A major objective of this work was to identify membrane surface chemistries that mitigate fouling – the interaction of solutes (suspended colloids, dissolved organic matter and proteins) with each other and/or with the membrane – which can impair performance, especially by reducing permeation rates. A second objective was to evaluate ways to improve the MFC cathode membrane.