polymer containing 4% Sa segment could give both satisfying Fo flux and good water recovery. the resultant average water flux was 0.347 Lmh when using pure water as the feed solution and its water recovery fraction was 65.2% at a separation temperature of 45 °C. this low flux value was attributed to the relatively low proportion of ionic groups in the polyelectrolytes. to achieve better Fo flux performance, the chemical characteristics and struc- tures of the polymers have to be further explored and refined.
in a later work by our group, poly(sodium styrene-4-sulfonate-co-n-isopro- pylacrylamide) (pSSS-pnipam) was employed as a draw solute.96 pSSS was a strong polyelectrolyte and was able to produce a high osmotic pressure. after the Fo process, the draw solute was then re-collected with membrane distil- lation (md) at a temperature above the LCSt of pnipam. the agglomeration of pnipam resulted in decreased osmotic pressure of the solution and conse- quently higher water vapor pressure. the combined Fo-md process is shown in Figure 2.13. pSSS-pnipam with different weight percentages of SSS (5%, 10%, 15%, 20%) – denoted as 5Sn, 10Sn, 15Sn, and 20Sn – were prepared. as expected, both the LCSts and osmolalities increased with the SSS content in the copolymer since polyelectrolyte pSSS pre-dominantly contributed to the hydrophilicity of the copolymer and at the same time it could partially hin- der the thermo-sensitivity of pnipam (Figure 2.13b). it is worth noting that the LCSt for 20Sn was not observed within the tested temperature range and it was not suitable as the draw solute. 15Sn solution with a concentration of 33.3 wt% was run for Fo tests and it produced a water flux as high as 4 Lmh with simulated seawater (0.6 m naCl solution) as the feed solution.
Figure 2.12 polyelectrolyte solution as draw agent in Fo process and the recovery of the water by hot ultrafiltration. reprinted from r. ou, et al., thermo- sensitive polyelectrolytes as draw solutions in forward osmosis process, Desalination, 2013, 318, 48–55, Copyright (2013) with permis- sion from elsevier.
2.4.3 CO
2Switchable Dual Responsive Polymers
hu’s group has achieved high Fo water flux with the development of a Co2
and thermally dual responsive polymer, poly[2-(dimethylamino)ethyl meth- acrylate] (pdmaema).65 the pdmaema were synthesized via atom transfer radical polymerization and three samples with number average molecular weights of 4000, 9000, and 13 000 g mol−1 were prepared. the protonation and deprotonation of this polymer can be reversibly switched by purging with Co2 or an inert gas. the overall process of using this polymer as draw Figure 2.13 (a) Schematic illustration of the combined Fo-md process with pSSS-pnipam as the draw solute; (b) transmittance at 500 nm of the copolymers with different weight percentages of SSS (5%, 10%, and 15% for 5Sn, 10Sn, and 15Sn), respectively; (c) osmolalities of pSSS-pnipam copolymers with different weight percentages of SSS at a concentration of 33.3 wt%. reprinted from d. Zhao, et al., thermo- responsive copolymer-based draw solution for seawater desalination in a combined process of forward osmosis and membrane distillation, Desalination, 2014, 348, 26–32, Copyright (2014) with permission from elsevier.
solute for water recovery is shown in Figure 2.14. initially, the unproton- ated polymer was dissolved in water and the resultant solution had a rel- atively low osmolality. When protonated via purging with Co2, the draw solute became a polyelectrolyte and possessed sufficiently high osmolality for seawater desalination and induced high water flux in the subsequent Fo process. after the Fo process, the dilute draw solution was purged with inert gas while being heated above the LCSt of the polymer (around 43 °C).
during this process, the polymer was deprotonated, in turn re-gained its thermal sensitivity and precipitated, enabling the ease of its recovery. then the precipitate and the supernatant were separated through a syringe filter.
the remaining polymer in the permeate was further removed via an ultra- filtration membrane. Since the concentration of draw solute in the perme- ate was very low, a low pressure at 1.5 bar was enough to recover >95%
water in the uF efficiently. prominently, the osmolality of the polymer can be significantly enhanced after protonation via Co2 purging. For example, after protonation, the osmolality of the draw solution with a concentra- tion of 0.4 g g−1 increased by two times from 0.6 osm kg−1. this protonated draw solution generated a water flux over 8 Lmh with di water as the feed solution.
Figure 2.14 Schematic illustration of the dual responsive draw solute for Fo desali- nation. reproduced from ref. 65 with permission from the royal Soci- ety of Chemistry.
2.4.4 Switchable Polarity Solvents
recently, a series of reversible non-polar-to-polar solvents have been devel- oped, and they have been tested to extract soybean oil from soybean flakes and to extract bio-oil from algae.97–100 these “smart” solvents not only can be used to efficiently extract organic materials and as solvents for Co2 detection, they can also be potentially applied in the “green” production of high-value chemicals such as pharmaceuticals. typical switchable-polarity solvents involve the introduction of a trigger, Co2, to transform the solvent from a low polarity form to a high polarity form and this process can be revers- ibly achieved by removing Co2 through purging inert gas or mild heating.
Both several amidines and tertiary amines have been proven as good can- didates to prepare switchable-hydrophilicity solvents. these polarity-switch- able solvents were attempted as novel draw solutes for Fo by mark and et al.67 in this work, the solvent was prepared by exposing a steady stream of carbon dioxide to a mixture of deionized water and N,N-dimethylcyclohex- ylamine [n(me)2Cy]. this solvent can reversibly change from its non-polar water immiscible form to its polar water miscible form through the following mechanism:
in the water miscible form, the solvents formed highly concentrated ionic solutions that could generate sufficiently high osmotic pressure for Fo pro- cess. analytical results have indicated that a fully concentrated [hn(me)2Cy hCo3] solution (59 wt%) was able to generate an osmotic pressure ten times that of seawater (3.5 wt%), which implies that the removal of 90% of the water from seawater is achievable. after the Fo process, n2 purging and a low grade heat of 60 °C were introduced into the dilute draw solution to induce phase separation between the purified water and the non-polar sol- vent, as shown in Figure 2.15. the draw solvent can be mechanically sepa- rated from the two phases and re-purged with Co2 to be used in the next Fo cycle. the separated water had a purity over 98% due to the presence of a trace amount of n(me)2Cy, which was limited by its water solubility. the trace amounts of draw solute can be further removed from the separated water through an ro process. the ro processes for conventional salt draw solute such as naCl, KCl, and mgCl2 have problems of concentration polarization and fouling when the feed solution volume is reduced. Subsequently, they can only produce a limited amount of water. the ro process associated with these polarity-switchable draw solutes avoids these problems, since once the solute reached saturation during the reduction of the volume of the ro feed solution, phase separation would occur. moreover, residue protonated n(me)2Cy generated a much lower osmotic pressure than conventional draw solutes and this would minimize the required energy and pressure. however, degradation of the hti cartridge membrane was observed when using the [hn(me)2Cy hCo3] solvent as draw solute since their relatively high ph of 8.8
fell off the normal ph operation range of the membrane. as a result, polyam- ide thin-film composite (tFC) membranes may be a better choice when using
“smart” draw solutes that operate at acidic or basic conditions since these membranes can withstand severe operating conditions.