Coating layer effect on performance of thin film nanofiltration membrane in removal of organic solutes
Introduction
High water permeability and rejection of solutes are features which make nanofiltration (NF) membranes economically viable in water softening applications, attractive for water purification and removal of harmful organic solutes from water [1], [2], [3]. However, theirs in general hydrophobic selective layer facilitates adsorption and the transport of organic solutes [4], [5], [6], [7], which often results with inadequate removal of, for example pesticides, pharmaceuticals and endocrine disrupting compounds, and limits their practical use [5], [8], [9]. Rejections of non-hydrophobic or ionized organics are generally higher than those of non-ionized hydrophobic solutes due to much lesser solute affinity toward membrane, or charge repulsion through NF membranes. The latter was confirmed in a recent study of Dražević et al. [10] on the RO (SWC1) membrane, where it was shown that non-ionized phenolic solutes enter the selective layer, and freely partition inside by an order of ten, even if solute radius is close to the radius of the pore. Moreover, it was shown that sorption of organics in the selective layer is unfavourable because it changes membrane characteristics in terms of both salts and water permeability.
Nabe et al. [11] correlated membrane surface hydrophilicity with membrane fouling and it can be assumed that rejection of at least some of organics could be improved by rendering the membrane surface more hydrophilic. Furthermore, Norberg et al. [6] found that RO membranes, declared as low fouling, are resistant to fouling in terms of specific flux decrease over time. It is widely accepted today that commercial brackish water RO membranes declared as low fouling, such as BW30 and LFC, have increased oxygen content and are probably coated [12]. Our past study also found [13] that LFC membrane was more resistant to fouling in terms of irreversible pesticide adsorption and flux decrease compared to other membranes.
Many studies focused on surface modification of membranes [14], [15], [16], [17], [18], [19] while the general benefits of modifications are permanently reduced membrane fouling [20] and increased rejection of proteins or organic solutes and salts. But, according to authors’ knowledge very few were focused on a mechanism which could explain why membranes became better rejecting to some organic solutes while rejections of some solutes remain the same or even decrease. This study aimed at proposing a plausible way to study such mechanism. Commercial thin film NF membrane, NF270, was coated with a thick layer of polyvinyl alcohol (PVA) and the rejections of various organic solutes of different sizes and physicochemical properties were studied before and after the addition of PVA layer. Pesticides, bentazone and tebuconazole, were used in this study as model compounds which represent an entire class of high affinity, non-ionized and hydrophobic compounds such as bisphenol A and hormones.
Section snippets
Concentration polarization corrected rejections
Mass transfer coefficients of solutes used were estimated using a well-known Gröber relation for laminar flow regime [21],where Sh is Sherwood number, Re, Reynolds number, Sc, Schimdt number, L, length of the channel and dH is hydraulic diameter. The number Re is calculated as as Re = (udH)/ν where u is cross-flow velocity and ν is kinematic viscosity. The number Sc is calculated as Sc = ν/D. Diffusivity of solutes were estimated using Wilke–Chang relation [22].
Materials
All of the chemicals used were of analytical grade. Organic solutes used in this study (Table 1) were supplied from different manufacturers: methylisobuthylketone (98%, Riedel-De Haen Ag Seelze, Hannover, Germany); 3,3-dimethyl-2-butanone (98%, Sigma Aldrich, Steinheim, Germany); cyclohexanol (99%, Riedel-De Haen Ag Seelze, Hannover, Germany); oxepane (98%, Heraeus, Karlsruhe, Germany); 4-aminopiperidine (Sigma Aldrich, St. Louis, MO), trimethylene oxide (97%, Across Organics, New Jersey, USA);
Characterization of PVA coatings by the means of FTIR spectroscopy and contact angles
Fig. 1A compares IR spectra of virgin NF270 and PVA coated NF270 membrane. IR spectra (Fig. 1A) of virgin NF270 membrane is identical to one recorded by Tang et al. [12]. PVA can be associated with two major peaks (Fig. 1A and B), CH broad alkyl stretching band (ν = 2850–3000 cm−1) and hydrogen bonded OH band (ν = 3200–3570 cm−1) [30]. Grafted PVA coatings membrane samples reveal two new signals attributed to MA (Fig. 1C); at 1650 cm−1 weak alkenyl (CC) stretch (ν = 1620–1680 cm−1) [31], [32]. Inspection
Conclusions
This study is among very few which examined rejection of hydrophobic organic solutes of identical molecular mass and similar Stokes radii on the same membrane. Similar sizes of these solutes, i.e., almost constant membrane/solute friction forces allowed to evaluate the pure effect of physicochemical properties of solutes on their rejections by membrane. Estimated solute/membrane energy of interactions, of both pristine NF270 and PVA coated NF270, correlated well with experimental log Kow.
Acknowledgements
The authors are grateful to Mrs. Sandra Milin who did a major part of experimental work in this study. This work was supported by the Croatian Ministry of Science, Education and Sports through Project 125-1253008-3009 “Membrane and adsorption processes for removal of organic compounds in water treatment”.
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