Separation of Phenols and Aniline from Water Streams using Poly(ether-block-amide) Membranes
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Date
2021-10-14
Authors
Cao, Xiaotong
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Phenols and aniline are aromatic compounds in which a hydroxyl group and amine group are attached to the benzene ring, respectively. Both are common organic pollutants in a wide range of effluents from such industries as textile, pharmaceutical, rubber, plastic, wood processing, petrochemical, and coal-tar production. Currently, available technologies to remove these aromatic compounds from water streams includes extraction, sorption, biological treatment, and advanced oxidation. However, none of them is technically viable for all scenarios. Recently, membrane-based processes have drawn attention as a promising way for water treatment due to such advantages as small footprint, no regeneration required, no secondary pollution, and low energy consumption. Compared with the commonly used nanofiltration (NF) and reverse osmosis (RO) for organics/water separation, pervaporation and perstraction are more energy-efficient as the minor component (i.e., organics) are allowed to preferentially permeate through suitable organophilic membranes. For such applications, poly(ether-block-amide) (PEBA) is in general a popular membrane material in view of the easy processing, good mechanical/chemical stability, and high permselectivity towards polar organics. Thus, in this study, the removal of aniline and phenols from water streams using poly(ether-block-amide) (PEBA)-based membranes via pervaporation and perstraction was investigated.
Four representative phenols (including phenol (PhOH), p-cresol (MePhOH), p-chlorophenol (ClPhOH), and p-nitrophenol (O2NPhOH)) and aniline are selected in the study in view of their wide occurrence in human surroundings. First, nonporous PEBA 2533 membrane was prepared via the solution-casting technique to separate four phenols (PhOH, MePhOH, ClPhOH, and O2NPhOH) from aqueous solutions by pervaporation. The effects of feed concentration and operating temperature on the separation performance were investigated. While the permeation fluxes of phenolic compounds increased with an increase in feed concentration, the increase in the flux was less than proportional, leading to a decrease in the enrichment factor. It was also shown that both the permeation flux and the enrichment factor increased with increasing temperature. Of particular interest were the coupling effects of co-existing phenolic compounds due to permeant-permeant interactions, which were found to be significant in the permeation of multiple phenolic compounds that were relevant to practical applications. The permeation of PhOH, MePhOH, and ClPhOH was all affected adversely by the presence of other phenolic compounds in the feed solution, while the opposite was true for the permeation of slow-permeating O2NPhOH.
Given the good permselectivity of PEBA towards phenolic compounds, the nonporous PEBA membrane was applied in perstraction to remove those phenolic compounds. Based on the resistance-in-series model, the mass transfer characteristics of the liquid/membrane/liquid perstraction system were analyzed, and the individual mass transfer resistances from the various steps of the perstraction process were estimated. It was revealed that not only the membrane resistance was significant, the mass transfer resistance at the downstream side of the membrane as a result of sorbate desorption from the membrane and boundary layer effects was also not negligible. It was found that the use of an alkaline stripping agent can effectively enhance the removal of phenolic compounds from water.
To further understand the mass transfer fundamentals of phenols in the membrane, the sorption and diffusion of the four phenols in dense PEBA membranes were investigated. The sorption isotherms for four phenolic compounds fit the Freundlich model most properly. PEBA has high sorption uptake towards phenols, much higher than that of water, and the high solubility selectivity led to the high permselectivity of PEBA. The diffusivity was found to be determined by both the molecular size and the mutual interactions between permeants and membrane.
The mass transfer characteristics (i.e., permeability, solubility, and diffusivity) of aniline in PEBA membrane pertaining to aniline removal by pervaporation and sorption was investigated. The solubility and diffusivity of aniline in PEBA were determined separately by analyzing the sorption isotherm and sorption kinetics. Membrane permeability is affected significantly by the environment in which the membrane equilibrated with, and membrane permeability follows a decreasing order that a “fully wet” membrane (i.e., in liquid permeation) > a “partially wet” membrane (i.e., in pervaporation) > a “dry” membrane (i.e., in vapor permeation).
Finally, zeolite imidazole framework-71 (ZIF-71), a hydrophobic filler with relatively large pore window sizes and cavity sizes, was prepared and incorporated into PEBA membrane to form mixed matrix membranes (MMMs) to inhibit water permeation in order to further improve the membrane permselectivity. The incorporation of ZIF-71 crystals in PEBA membrane significantly inhibited water permeation, while the permeability of two aromatics was largely unaffected (for phenol) or slightly reduced (for aniline), leading to increased enrichment factor. Though the permeability coefficient of aniline was lower than phenol, the permeation flux of aniline was higher because of the higher transmembrane driving force. The ZIF-71/PEBA MMMs showed good stability, demonstrating its applicability in separation of aromatic compounds from their aqueous solutions.
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Keywords
PEBA, pervaporation, perstraction, phenols, aniline, mass transfer