NMR spectroscopy has been used to assist in the development of chlorine-resistant membranes for use in water desalination plants. The new membrane materials could avoid degradation by chlorine disinfectants and reduce operating costs and inefficiencies.

Sustainable access to fresh drinking water is increasingly a global problem facing the developed and the developing world.

The most commonly used technology for converting salt-laden seawater into potable water is membrane-assisted desalination. This is an energy-efficient and relatively environment-friendly process but for one problem. The aromatic polyamides commonly used to separate the H₂O from the NaCl degrade when exposed to chlorine disinfectants. Cellulose acetate membranes are also commercially. available but succumb to microbiological degradation.

Now, Ho Bum Park of the School of Chemical Engineering and Bioengineering at the University of Ulsan, in South Korea, Benny Freeman, of the Department of Chemical Engineering at the University of Texas at Austin, USA, James McGrath, Zhong-Bio Zhang, and Mehmet Sankir, of the Macromolecules and Interfaces Institute and Chemistry Department at the Virginia Polytechnic Institute, in Blacksburg, USA, have developed a novel membrane material that addresses the issues. They point out that unlike the current aromatic polyamide membranes their new materials are able to tolerate chlorinated water. Writing in the Wiley journal Angewandte Chemie they describe the development of their sulfonated copolymers in detail.

Chlorination is the most frequently used oxidising biocide for water treatment. It is relatively inexpensive but nevertheless very effective even at low "chlorine" concentration. Park and colleagues explain that disinfection is a crucial first step undertaken prior to water being pumped into a membrane-based desalination facility.

Without this initial disinfectant step, the growth of biofilms within the system occurs rapidly. This not only reduces efficiency, but can encourage the growth of pathogenic microbes. However, this step while necessary causes significant problems and costs.

"The sensitivity of polyamide membranes to chorine leads to significant additional processing steps and, in turn, increased water purification costs," the researchers say. Of course, prior to entering the water supply, the desalinated water must then be chlorinated again to bring it up to safe drinking water specifications.

Park and colleagues explain that membranes based on polysulfone instead of polyamide are much tougher, sulfur-containing engineering thermoplastics. These materials are not susceptible to chlorine degradation in the same way as polyamides. However, these materials are very hydrophobic and so do not allow water to pass through them at an adequate rate for desalination.

The international research team has now discovered that by attaching additional charged sulfonic acid groups to their polysulfones they can reduce the hydrophobicity. This improves the water transport rate through the membrane without affecting the materials' other chemical and physical properties.

Earlier efforts at creating chlorine-resistant membrane polymers had focused on modifying the materials after polymerization. However, success came to Park and colleagues when they realised that the simultaneous polymerization of disulfonated monomers (a building block containing two hydrophilic sulfonic acid groups) and another type of monomer could be used to make a novel copolymer. This approach precluded many of the unwanted side-reactions, avoided cross-linking between polymer chains and prevented breaks in the polymers that reduce the uniformity and quality of the final membrane material.

Moreover, this copolymerisation approach, tracked with NMR spectroscopy, allowed the team to control with precision the degree to which hydrophobicity is reduced, or hydrophilicity raised. Variations on the theme can be produced by altering the number of charged (hydrophilic) sulfonic acid groups on the polymer chain. This, the team says, allows them to fine-tune the properties of membrane for different degrees of permeability to water and salts for specific desalination methods, such as nanofiltration and reverse osmosis. These are the dominant technology for desalination of water.

"[Our] materials could expand the processing window for using chlorine in conjunction with desalination membranes, remove costly process steps, such as dechlorination and rechlorinations, and yield significant increases in membrane lifetime," the researchers say. "The removal of processing steps, leading to process intensification, and increase in membrane lifetime could lower water desalination costs," they add.

Related links:

• Angew Chem Int Edn, 2008, 46, 6019-6024
• Freeman Page

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.