Polymer self-assembly for novel membranes: Water treatment and chemical separations

I. Sadeghi, S. Lounder, P. Bengani-Lutz, A. Asatekin
Tufts University,
United States

Keywords: membrane, zwitterion, selectivity, fouling


Separation of complex mixtures lies at the heart of the oil and gas industry. Refineries separate crude oil, a highly complex mixture, into fractions valuable as fuel and chemical feedstocks. Contaminated water associated with oil and gas extraction, several times more in volume compared with the oil itself, needs to be treated and purified. Natural gas needs to be purified and upgraded. Broader use of membranes can potentially improve the energy efficiency of each of these processes, but membranes on the market today lack the selectivity and/or robustness to successfully perform most of these separations. In this presentation, we will discuss our approach for addressing two of these processes through the development of new polymeric materials designed to self-assemble to impart improved and/or new functionality to separation membranes by controlling nano-scale morphology and surface functionality. One research direction targets the treatment of wastewater streams associated with oil extraction and refining. These streams are high in oil content, which leads to severe membrane fouling. Zwitterions, functional groups with equal numbers of positive and negative charges, strongly resist fouling, defined as performance loss due to the adsorption and adhesion of feed components onto the membrane. They also easily self-assemble due to strong intermolecular interactions. We have developed high flux, fouling resistant, size-selective membranes utilizing the self-assembly of random copolymers of zwitterionic and hydrophobic monomers. The effective membrane pore size or ~1 nm closely matches the size of self-assembled zwitterionic nanodomains. These membranes are exceptionally fouling resistant, showing little to no flux decline during the filtration of a wide range foulants including oil, and complete flux recovery with a water rinse. This makes them highly promising for treating oil well produced water and oily wastewater streams. We also aim to develop membranes that can separate mixtures of small molecules of similar size based on their chemical properties, relevant for the separation of petroleum derivatives from each other. For this purpose, we prepared membranes by depositing micelles formed by random copolymers of a highly hydrophobic fluorinated monomer with methacrylic acid on a porous support. The gaps between the micelles act as 1-5 nm nanochannels functionalized with carboxylic acid groups. These membranes show charge-based selectivity between organic molecules. Furthermore, the carboxyl groups can be functionalized to alter the selectivity of the membrane. We used this method to prepare membranes that exhibit aromaticity-based selectivity. We believe these approaches will eventually lead to novel membranes that are capable of new separations and can replace more energy intensive methods such as distillation or extraction.