Assessment of micellar solutions as draw solutions for forward osmosis

In this paper, the viability of using micellar draw solutions (DS) for Forward Osmosis (FO) is presented for the first time. Above the critical micelle concentration (CMC) and the Krafft temperature (Tk), the monomers in a surfactant solution aggregate to form micelles, resulting in a relatively constant osmotic pressure above the CMC. These properties can be useful for a FO system in order to maintain a constant water flux and to enable draw solution regeneration at low energy. Three cationic and two anionic surfactants were studied at different concentrations as potential draw solutions for FO applications. Both flat sheet and hollow fiber FO membranes were used in the study. The study revealed that the micellar solutions generated more stable flux compared to other inorganic DS; the hypothesis of a constant flux above the CMC was valid only for concentrations slightly above the CMC, and micellar solutions behaved similar to inorganic solutes at concentrations significantly higher than the CMC. It was shown that external concentration polarization (ECP) does not have a significant effect in micellar solutions just above the CMC. Furthermore, all surfactants demonstrated between 3 and 300 times less reverse transport compared to NaCl at similar concentrations. Finally, the surfactant solute could be regenerated with recoveries as high as between 95% and 99%. It was thus verified that micellar solutions are an attractive DS for application to Forward Osmosis and potentially in the forward osmosis membrane bioreactor (FO-MBR). © 2014 Elsevier B.V. All rights reserved.

G.G. acknowledges the CAPES Foundation, Ministry of Education of Brazil, for sponsorship under contract grant BEX-0487/10-4. M.S.N. acknowledges the financial support for this research from the INSPIRE program, which is jointly funded by the Higher Education Commission (HEC) of Pakistan and the British Council (BC) (Project No. SP-192). The authors also acknowledge Hydration Technology Innovations (HTI; Albany, USA), for providing cartridge FO membranes for this study. Micellar solutions are promising as novel draw solutions for the FOMBR and for brackish water desalination. Above the CMC, they offer a relatively constant osmotic pressure and water flux, which allows for control of flux. They also exhibit very low back diffusion. The latter avoids toxic contamination of the feed in an FO-MBR, as confirmed in our previous work [13]. Ease of regeneration/separation from the diluted draw solution by low energy techniques is an added advantage of using micellar solutions. The choice of an appropriate surfactant as draw solution may necessitate a compromise. For example, choosing a larger chain length confers a lower CMC (and hence osmotic pressure and flux), but reduces solubility and increases Krafft temperature (so that recovery is possible closer to ambient temperature, and requires less refrigeration energy) and also reduces reverse transport. Other issues, such as microbial toxicity in the FOMBR, or viscosity, may also play a role. For potable water applications, the monomer separation still needs to be investigated further, via nanofiltration. For this latter reason, it is suggested that micellar solutions would currently be most useful in FO applications such as wastewater treatment via the FOMBR, or other non-potable niche applications such as food processing.