Application and Modification of the Selectivity of a Nanoporous, Bicontinuous Cubic, Lyotropic Liquid Crystal Polymer Membrane for Aqueous Separations

Dischinger, Sarah Marie

Graduate Thesis Or Dissertation

Application and Modification of the Selectivity of a Nanoporous, Bicontinuous Cubic, Lyotropic Liquid Crystal Polymer Membrane for Aqueous Separations Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/t722h8974

Abstract

The recently-developed, nanoporous, bicontinuous cubic, lyotropic liquid crystal, thin-film composite polymer membrane (TFC Qᵢ membrane) has a unique selectivity for small dissolved solutes driven by the uniform, ca. 1-nm in size, highly charged pores present in the nanostructured polymer selective layer. During the treatment of hydraulic fracturing wastewater, this selectivity enables the TFC Qᵢ membrane to isolate low-molecular-weight organic nutrients in a reduced salinity permeate to a greater degree than some commercial polymeric nanofiltration (NF) and reverse osmosis (RO) membranes. This separation offers an alternative approach to the treatment of hydraulic fracturing wastewater by enabling biodegradation of organic nutrients to occur post-desalination. The industrial feasibility of the TFC Qᵢ membrane was suggested by its performance during a 66 h cross-flow filtration of pre-treated hydraulic fracturing produced water. Preliminary results from this experiment suggest that the fouling propensity of the TFC Qᵢ membrane is significantly less than that of a commercial NF membrane.In additional to its unique selectivity, the TFC Qᵢ membrane is advantageous over commercial polymer membranes for the ability to easily manipulate its rejection of small uncharged solutes via anion exchange at the membrane’s pore wall. Exchanged-in organosulfonate anions increasing simultaneously in molecular volume and hydrophobicity induced an increase in small uncharged solute rejection and a decrease in water flux. The impact of exchanging-in triflate on the membrane’s performance suggests that factors other than anion size and hydrophobicity need to be considered in describing small uncharged solute rejection by the TFC Qᵢ membrane. Furthermore, it was shown that a bidentate organosulfonate anion associated at the Qᵢ pore wall resisted exchange to a greater degree than its monodentate derivative during exposure to a saline solution. In summary, the nanostructure present in the TFC Qᵢ membrane gives rise to a unique selectivity that enables an alternative approach to molecular separations in aqueous industrial streams while also allowing anion exchange to manipulate the membrane’s selectivity for small uncharged solutes. This work presents the TFC Qᵢ membrane as a membrane of inherent value and as a platform to be manipulated for specific molecular separations.

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