Chemically modified polymeric substrates for aqueous molecular separations: applications to water treatment

Abstract

Even though we live in a planet which has a surface composition of 70% water, less than 1% is in the form of freshwater. Changes in society, population growth, and industrialization have significantly increased the water demand; thus, water scarcity has become more apparent and significant as the years pass. A feasible alternative to minimize this issue is the reuse of resources, i.e. wastewater reclamation. Even though wastewater streams have been treated since 1970’s, treatment plants do not remove all the pollutants to provide potable water. To attend this need and move towards a more adequate wastewater treatment, advanced technologies need to be implemented.

Membrane-based technologies are considered as the state-of-the-art for the application of wastewater reclamation due to their high selectivity. There are various types of membranes, and the main differences are the employed driving force and the pore size, the latter of which determines selectivity. Herein, we focused our attention on the forward osmosis (FO) membrane process. FO is a spontaneous process which can be considered as a cost-effective approach and can reach promising performances. The main components of this process are the selected membrane or substrate, and the draw solution (i.e. driving force). Nevertheless, this type of technology suffers from fouling, where the membrane pores get clogged and the overall efficiency decreases considerably.

In the substrate aspect, the most commonly studied ones are the thin film composite (TFC) membranes. The main reason for this are its constituents: a highly porous support and an active-selective surface layer. Both layers can be tailored to enhance the overall efficiency of the membrane in the FO separation. Typically, the addition of additives within the layers can incorporate features and functionalities to improve the water permeability while minimizing the membrane fouling.

In this study, we evaluated three different membranes with modifications in one or both layers of the TFC structure. In Chapter 3, we present the effect of modifying the support layer of the membrane by the addition of a metalized nanocellulose composite. We explored the utilization of these modified membranes in the FO separation process using different feed solutions: nanopure water, aqueous urea, and primary wastewater samples. The presented approach has proven to have a high efficiency in terms of water flux and selectivity towards water; however, the membranes suffered from fouling after treating the wastewater sample. In the attempt to mitigate and prevent the fouling phenomena, we focused our attention on the active layer of the TFC membrane.

Chapter 4 presents the anti-fouling effect of an electroconductive membrane. Moreover, we deposited polyaniline, an electroconductive polymer, in the substrate surface and explored its feasibility after intentionally fouling the membrane. After a 30- minutes cathodic treatment, the membrane proves to recover its performance up to an 84%. Also, electrochemical impedance spectroscopy proved to be a useful tool for monitoring the fouling phenomena.

Furthermore, in an effort to combine the finding from the previous chapters, Chapter 5 presents the effect of modifying both layers of the TFC membrane. The addition of porous carbon black particles to the support enhances the water permeability by 71%. However, the addition of a multilayered electroactive surface decreased the FO performance of the membrane but demonstrated promising electrochemical features.

These features can be potentially used to electrochemically reduce organic foulants present in wastewater streams and to avoid the fouling phenomena.

One of the impediments for the FO separation process is the recovery of the draw solute used to generate the osmotic potential gradient, i.e. the driving force. In Chapter 6, we present the utilization of a stimuli responsive draw solute, i.e. tertiary amine, which responds to the presence of carbon dioxide. In all the studied feed solutions, this tertiary amine outperformed the water fluxes of the typical draw solute: NaCl. Moreover, it proved to be a promising draw solute for wastewater reclamation and desalination approaches.

In the past years, FO membranes have gained interest due to their outstanding performance and sustainable properties. In this study, we demonstrated the advantages of different modifications in the support layer, as well as the antifouling properties attributed by an electroactive surface. We also reported the enhancement of the separation process by the use of a highly responsive draw solution. Thus, the viability of the FO separation process for wastewater reclamation and desalination was validated.