"Partitioning of Water Contaminants into the Active Layer of Thin-film Composite Nanofiltration Membranes"

Tasuma Suzuki

Advisor: Benito J. Mariñas

Thursday, April 3, 2008

Nanofiltration (NF) membrane processes are becoming more widely used in drinking water treatment because they provide an effective barrier for most chemical and microbial contaminants.  However, state-of-the-art NF membranes provide inadequate rejection for low molecular weight, neutral compounds such as arsenious acid.  A better understanding on solute transport mechanisms through the active layer of NF membranes is required to develop a new generation of membrane materials providing better rejection capability for these contaminants.  Solute permeability is affected by both partitioning at the aqueous/active layer interface and diffusivity within the active layer.  Low solute rejection could thus be the result of high partitioning, high diffusivity, or both of these effects.  It is therefore necessary to differentiate between these two transport steps.  The primary objective for the  initial phase of this study was to develop a methodology, based on Rutherford backscattering spectrometry (RBS), for measuring the partition coefficient of solutes into the active layer of NF membranes. using.  The target inorganic solute was arsenious acid (MW = 126 Da).  In addition, iohexol (MW = 821 Da) was used to represent larger organic molecules.  Experiments were performed with the ESNA1-LF membrane, as a representative of commercial polyamide (PA) NF membranes.  Experimental results showed that the partition coefficient of arsenious acid and iohexol were 2.2 and 0.13, respectively, both expressed in terms of active layer to aqueous phase concentration ratio.  Solute permeation experiments also performed as part of this study revealed that the  permeability coefficient of arsenious acid was ~1,400 times higher than that of iohexol.  Consequently, the lower rejection of arsenious acid (<63%) compared to that of iohexol (>99.8%) is the result of arsenious acid having a partition coefficient ~17 times higher and an intra-active layer diffusivity ~80 times greater than the corresponding coefficients for iohexol.  The relatively low rejection of arsenious acid is thus the combined result of its affinity to partition into PA and a high diffusivity within this polymeric material.

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