Surface Modification of RO Membranes for Fouling Reduction

Reverse osmosis is a well established technique to produce drinking water from sea water or brackish water. The membrane consists of three destinctive layers, namely a non-woven support, a porous middle layer made from poly(sulfone) or poly(ether sulfone), which can be used as an ultrafiltration membrane (polysulfone, polyethersulfone) and a dense polyaramide top layer prepared by interfacial polycondensation. While the first two layer act as support for the top layer, the polyaramide toplayer, which is only 100 nm thick, determines the permeate flux and salt rejection. The most challenging issue in RO application today is the low chlorine stability of the polyaramide toplayer membrane. However, cleaning cycles with chlorine are necessary to remove biofouling from the membrane surface. Several methods like variation of amine component [1], variation of acid component [2] or use of sulfonated poly(ether sulfone)s [3] have been reported in the past to produce chlorine stable separation layers. However, from a performance point of view, thin film composite membranes with a polyaramide top-layer are still the benchmark.

This work is not preliminarily aimed at changing the chemical composition of the polyaramide top-layer but to make the surface less susceptible for biofouling. For this purpose, carbonyl chloride groups that remain after preparation of the polyaramide amide dense layer are used for a subsequent surface modification step. These groups are reacted for example with amine or hydroxyl group containing hydrophilic polymers like hyperbranched poly(amine)s or poly(ethylene glycol)s. The membrane surfaces were more hydrophilic after the modification step as revealed by contact angle measurements. Modification with the hyperbranched poly(amine) resulted in a membrane with a positively charged surface over a wide pH range (isoelectric point at ca. pH 9), while the poly(ethylene glycol) modified membrane had an IEP at approx. pH 5.

The effect of surface modification on membrane properties is further discussed in terms of permeate flux, salt rejection and protein adhesion.

Acknowledgement:

The financial support of this work by the German Egyptian Research Fund is gratefully acknowledged (EGY 10/029).

Key Words: reverse osmosis, surface modification, fouling

References

[1] J. E. Cadotte, Reverse Osmosis Membrane, US Patent 4259183 (1981)

[2] B. J. Trushinski, J. M. Dickson, T. Smyth, R. F. Childs, B. E. McCarry, J. Membr. Sci. 143, 181-188 (1998)

[3] H. B. Park, B. D. Freeman, Z.-B. Zhang, M. Sankir, J. E. McGrath, Angew. Chem. Intern. Ed. 47, 6019 - 6024 (2007)