Relating Contaminant Rejection by Thin-Film Composite Membranes to the Charge Density In Membrane Active Layers

Motivation and background:  The top ultrathin (~50-200 nm) active layer in the thin-film composite structure of reverse osmosis (RO) and nanofiltration (NF) membranes constitutes the main barrier to the permeation of water and solutes [1].  Existing models describing water permeation and contaminant rejection by RO and NF membranes have been used to fit experimental data [2, 3]; however, the models have not been used to make a priori predictions of membrane performance based on independent measurements of the physico-chemical properties of the membrane active layers.  The recent development of characterization procedures [4-7] for the quantification of properties that are key parameters in current transport models have enabled using corresponding experimental results as input in the existing transport models to make a priori predictions of membrane performance.

Project description: We used the solution-diffusion model , the theory of Donnan potential in permeable charged films [8], and charge density measurements in the active layers of four RO and NF membranes [6] to predict the rejection by each membrane of a weak acid (i.e., arsenious acid) and a strong electrolyte (i.e., potassium iodide) as a function of pH based on corresponding rejection results at one pH condition (pH=6.3).  Membrane permeation experiments were performed at room temperature (20^oC) using a cross flow, flat-sheet membrane system.  Membrane charge density was measured by probing with silver ion (Ag⁺) the ionized functional groups in the active layers of the membranes studied and subsequently measuring the silver concentration in the active layer surface using the experimental procedures developed in our previous work [6].    

Results: Model predictions satisfactorily described as a function of permeate flux (i.e., applied pressure) the ~2-6% and ~25-40% increase in potassium iodide rejection and arsenious acid rejection, respectively, measured experimentally as the pH of the feed waters was increased from ~6.0 to ~10.3.  The agreement between model predictions and results from experiment indicates that (i) the model satisfactorily describes the effect that the pH of the feed solution has on solute rejection by RO and NF membranes, and (ii) the procedures used for the quantification of charge density provide a valuable characterization of membrane active layers.  The experimental results, model developed and model predictions will be presented. 

Significance of the study: The results from this work provide valuable information towards the advancement of the understanding of the mechanisms of transport of water and contaminant transport through RO/NF membranes.

References

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3.         Urama, R. I.; Mariñas, B. J., Mechanistic interpretation of solute permeation through a fully aromatic polyamide reverse osmosis membrane. J. Mem. Sci. 1997, 123, 267-280.

4.         Bason, S.; Oren, Y.; Freger, V., Characterization of ion transport in thin films using electrochemical impedance spectroscopy II: Examination of the polyamide layer of RO membranes. J. Mem. Sci. 2007, 302, 10-19.

5.         Coronell, O.; Mariñas, B. J.; Zhang, X.; Cahill, D. G., Quantification of functional groups and modeling of their ionization behavior in the active layer of FT30 reverse osmosis membrane. Environ. Sci. Technol. 2008, 42, 5260-5266.

6.         Coronell, O.; Mariñas, B. J.; Cahill, D. G., Depth heterogeneity of fully aromatic polyamide active layers in reverse osmosis and nanofiltration membranes. Environ. Sci. Technol. 2011, 45, 4513-4520.

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8.         Ohshima, H.; Ohki, S., Donnan potential and surface potential of a charged membrane. Biophys. J. 1985, 47, 673-678.