By Dr Andrew Blok
Whilst water is one of the Earth’s most abundant resources, the majority of it exists as salt water which is unsuitable for human needs. Governments around the world are turning to desalination to help secure drinking water supplies. Drought conditions were behind the decision to develop the Adelaide Desalination Plant, and whilst the drought eventually broke, the plant will secure Adelaide’s drinking water supply during future drought situations. Desalinated water is generally produced using either thermal or membrane based methods. Membrane methods have proven to be the most popular; however these methods all face similar problems due to biofouling of the membrane surface.
Research at the Flinders Centre for NanoScale Science and Technology has focused on the development of new antifouling coatings for membranes to be used in the desalination of salt water. In particular we explored the use of poly((2-methacryloyloxyethyl)trimethylammonium chloride) (poly(MTAC)) coatings. The antimicrobial properties of MTAC have been known for quite some time and it was proposed that MTAC modified membranes would show increased resistance to biofouling compared to unmodified membranes.
In our research we have taken commercially available polyamide (PA), thin film composite (TFC) reverse osmosis (RO) desalination membranes and modified them in two steps. Initially the commercial membrane was modified to introduce an adhesive polydopamine (PDA)-initiator layer. This layer was then used to perform activators regenerated by electron transfer atom transfer radical polymerisation (ARGET ATRP) of MTAC onto the membrane surface.
Testing was undertaken on the PDA-g-PMTAC modified membranes to ensure they were still useful in the desalination process. A slight increase in water permeance (the amount of water passed through the membrane in a given time under a given pressure) was observed, along with good salt rejection properties (comparable to commercially available desalination membranes).
The amount of biofouling on the membrane surface was studied using confocal laser scan microscopy (CLSM). Samples of both modified and unmodified membrane were immersed in a nutrient solution for six days. After fixation and staining with a fluorescent dye, samples of both a modified and unmodified membrane were imaged using CLSM. Fig. 1 (a) shows CLSM images of bacteria on the unmodified PA RO membrane after six days, while Fig. 1 (b) shows bacteria on a PDA-g-PMTAC modified PA RO membrane after six days. Clearly, there is considerably less biofouling on the PDA-g-PMTAC modified RO membrane (Fig. 1 (b)). This suggests that PDA-g-PMTAC coatings would be suitable for use on desalination membranes as they reduce biofouling, whilst maintaining water permeance (slightly increased) and salt rejection properties.
Figure 1: a) CLSM images of batcteria adhered on an unmodified polyamide (PA) TFC membrane after 6 days biofouling, b) CLSM images of bacteria adhered on a PDA-g-PMTAC modified membrane after 6 days biofouling.
For more details about this research please refer to:
Blok, A.J.,Chhasatia, R., Dilag, J., Ellis, A.V., Journal of Membrane Science, Volume 468, Pp 216-223. http://www.sciencedirect.com/science/article/pii/S0376738814004542
This research was supported by the National Centre of Excellence in Desalination Australia which is funded by the Australian Government through the Water for the Future initiative.