(399i) Mitigation of Thin Film Composite Membrane Biofouling Via Immobilizing Nano-Sized Biocidal Reservoirs in the Membrane Active Layer

Thin film composite (TFC) membranes based on polyamide materials have been used predominantly as osmotic membranes in water and energy applications [1]. Biofouling poses a unique challenge to these membranes, because the membranes cannot tolerate oxidants such as chlorine that is inexpensive and widely used [2]. Therefore, effective biofouling mitigation strategies are needed. Metal-organic frameworks (MOFs) are very attractive materials for developing high-performance FO membranes. They have great potential in improving the biofouling resistance of FO membranes that require both antibacterial and anti-adhesion features.

This work investigates the use of a silver-based MOF for mitigating biofouling in forward-osmosis TFC membranes. This is the first study of the use of MOFs for biofouling control in membranes. MOF nanocrystals are immobilized in the active layer of the membranes via dispersing them in the organic solution used for interfacial polymerization [3]. Field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscope (XPS) characterization results show the presence of the MOF nanocrystals in the active layer of the membranes. The immobilization improve the membrane active layer in terms of hydrophilicity and transport properties, without adversely affecting the selectivity. It imparts antibacterial activity to the membranes; the number of live bacteria attached to the membrane surface is over 90% less than that of control membranes. Additionally, the MOF nanocrystals provide biocidal activity that lasted for 6 months. This implies that the MOF can work as an Ag+ reservoir immobilized in the membrane active layer, providing a controlled sustained Ag+ release. These results indicate that the MOF imparts a long-lasting antibacterial activity to the membrane surface to mitigate biofouling in a prolonged period of the membrane operation. As a result, the biofouling resistance in the membranes is improved, whose flux has a decline of 8% after 24 hours of operation in biofouling experiments, while that of the control membranes had a greater decline of ~21%. Fluorescence microscopy and FE-SEM indicate simultaneous improvement in anti-adhesive and antimicrobial properties of the TFN membranes, resulting in a limited biofilm formation. Consequently, the modified membranes exhibit an enhanced FO seawater desalination performance in comparison with the control membranes. The performance stability of TFC membranes is improved by the presence of MOF in the active layer (as seen by a water flux decline of about 7% for modified membrane against about 18% for unmodified membrane when tested with real seawater). This study shows that biocidal activity imparted by MOF nanocrystals contributes to the mitigation of biofouling on TFN membranes and hence, improving their desalination performance. It demonstrates that it is possible to improve both the biocidal activity and the hydrophilicity of the membrane active layer, without any adverse effects on the membrane selectivity.

References

(1) Yip, N. Y., Tiraferri, A., Phillip, W. A., Schiffman, J. D., Hoover, L. A., Kim, Y. C., Elimelech, M., Thin-film composite pressure retarded osmosis membranes for sustainable power generation from salinity gradients. Environ. Sci. Technol. 2011,45 (10), 4360-4369.

(2) Glater, J., Hong, S.-k., Elimelech, M., The search for a chlorine-resistant reverse osmosis membrane. Desalination 1994,95 (3), 325-345.

(3) Zirehpour, A.,. Rahimpour, A., Arabi Shamsabadi, A., Soroush, M., Mitigation of TFC-Membrane Biofouling via Immobilizing Nano-Sized Biocidal Reservoirs in the Membrane Active Layer. Environmental Sci. & Tech. 2017, accepted.