(449br) Improving the Performance of Commercial Polyethylene Based Anion Exchange Membranes in an Electrodialysis Process Using Functionalized Iron Oxide Nanoparticles

Desalination is becoming essential in the context of an increasing water demand and a growing world population. The desalination capacity on 2015 was around 86.5 million m³·day^-1 (http://idadesal.org/). Due to economic reasons, brackish water tends to be the preferable water source when available. Electrodialysis is one of the desalination technologies used in the production of freshwater from brackish water (Fernandez-Gonzalez et al., 2015). This option has been recently pointed out as a cost-effective technology for the desalination of brackish water (McGovern et al., 2014).

Fouling is an issue that affects membrane desalination in general and electrodialysis in particular. Since most of organic foulants are negatively charged, this phenomenon is more pronounced in anion exchange membranes. At the same time, in order to avoid scaling, the anion exchange membranes must be impermeable to sulphate ions (Mulyati et al., 2013). Thus, to prevent fouling and scaling in electrodialysis, the desired characteristics of anion exchange membranes are high monovalent ion selectivity and improved anti-fouling properties.

In the present work, nanocomposite monovalent anion exchange membranes, by coating a standard commercial available anion exchange membrane with a sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (sPPO) and different loads of sulfonated iron oxide nanoparticles (0.2% g·g^-1 - 0.6% g·g^-1), have been developed, characterised and tested for anti-fouling and anti-scaling properties .

According to previous studies (Güler, 2014) this coating is expected to provide a negative charged surface to the anion exchange membrane that will also change the hydrophilicity of the surface thus the anti-fouling properties of the membrane. Additionally, due to electrostatic repulsions between divalent anion and the negative charge of the membrane surface, much more important than the repulsion to monovalent ions, this modification will confer monovalent ion selectivity to the nanocomposite anion exchange membrane. Thus the modification of the membrane will boost its performance against fouling and scaling.

Material and Methods

The commercial membranes modified in this work were the heterogeneous anion exchange membranes RALEX AM-PP by a physical coating (Yin & Deng, 2015) using sPPO and Fe₂O₃-SO₄^2-. There are no chemical bonds but physical interactions between the membrane surface and the film of sPPO and Fe₂O₃-SO₄^2-. The surface modification does not affect the bulk membrane structure.

Several analytical techniques were used in order to characterize the surface properties of the membranes. The surface roughness was determined by means of Atomic Force Microscopy (AFM). Scanning Electron Microscopy (SEM) was used to observe the morphology of the surface. Fourier Transform Infrared spectroscopy (FITR) was used to determine the chemical structure of the membrane and the composition of the negatively charged layer. The modifications in the hydrophilicity of the surface were characterised by measuring the water contact angle. The monovalent ion selectivity of membranes was investigated by measuring the ion flux of Cl^- and SO₄^2- through the anion exchange membrane in a four-compartment electrodialysis cell. Concentrations of Cl^-, SO₄^2- and Na⁺ were determined by Ion Chromatography. The antifouling potential of the membranes was evaluated by measuring the potential difference between both sides of the membranes over time in the presence of a reference foulant (sodium dodecyl sulphate).

Results and Conclusions

FTIR results confirm the successful coating of the membrane with sPPO and iron oxide nanoparticles. Both results examined by SEM and AFM showed an improvement in the surface homogeneity of the modified membrane. Water contact angle measurements of the commercial (100º) and the modified membranes (up to 59.2º) showed an increase in surface hydrophilicity. The best results obtained from the modified membrane can improve the anti-fouling performance up to 24% when the surface homogeneity and hydrophilicity increase. An improvement in monoselectivity of membranes up to 34% was also observed. The evolution of the concentration of Na⁺ in the dilute compartment confirmed no changes in the selectivity Cl^-/Na⁺ of the membranes.

In conclusion, the modification of a commercial anion exchange membranes with a coating of sPPO and iron oxide nanoparticles proved the simultaneous improvement of the monovalent ion selectivity of the membrane and its anti-fouling and anti-scaling properties without losing Cl^-/Na⁺ permselectivity.

Acknowledgements

Financial support from MICINN under project 2014-57833-R and CTQ2013-48280-C3-1-R-D is gratefully acknowledged. The author C. Fernandez-Gonzalez thanks the Ministry of Education for a FPI grant BES-2012-053461.

References

Fernandez-Gonzalez, C., Dominguez-Ramos, A., Ibañez, R. and Irabien, A. (2015), Sustainability assessment of electrodialysis powered by photovoltaic solar energy for freshwater production. Renewable and Sustainable Energy Reviews, 47, 604-615 

McGovern, R.K., Zubair, S.M.andLienhard V, J.H. (2014), The cost effectiveness of electrodialysis for diverse salinity applications. Desalination, 348, 57-65

Mulyati, S., Takagi, R., Fujii, A., Ohmukai, Y. and Matsuyama, H. (2013), Simultaneous improvement of the monovalent anion selectivity and antifouling properties of an anion exchange membrane in an electrodialysis process, using polyelectrolyte multilayer deposition. J. Membr. Sci, 431, 113-120

Yin, J. and Deng. B. (2015), Polymer-matrix nanocomposite membranes for water treatment. J. Membr. Sci., 479, 256-275

Güler, E., van Baak, W., Saakes, M. and Nijmeijer, K. (2014), Monovalent-ion-selective membranes for reverse electrodialysis. J. Membr. Sci., 455 254-270