water from saline water.10,11,23 the highly soluble ammonium bicarbonate was able to provide a high water flux as well as a high feed water recovery.
after the Fo process, moderate heating at about 60 °C was applied to induce the decomposition of ammonium bicarbonate into ammonia and carbon dioxide gases so that they can be recovered and further used as draw solutes.
however, ammonia could not be completely removed and the presence of residual ammonium plagued the use of the product water as safe drinking water.
many other chemicals, such as polyelectrolytes, 2-methylimidazole-based compounds, hexavalent phosphazene salts, hydroacid complexes, and sodium lignin sulfonate, have also been prepared and investigated as draw solutes in recent years.51–56 these draw solutes were mainly regenerated with applied hydraulic pressure (e.g. ro, nanofiltration, and ultrafiltration) or heat input (e.g. membrane distillation). But these processes generally involve the consumption of a high amount of energy. hence, draw agents with facile recovery methods have to be developed. Smart materials, which can respond to stimuli such as temperature, magnetic field, and light, offer opportunities as Fo draw agents to lower the energy consumption of the process at a great potential; and recent years have witnessed the growing interests in develop- ing such materials. So far, hydrophilic magnetic nanoparticles,30,57–62 stimu- li-responsive magnetic nanoparticles,63,64 smart polymers and solvents,65–67 and smart hydrogels,68–75 have been investigated as draw agents in Fo appli- cation. in the following sections, these smart draw agents are discussed.
hydrophilic magnetic nanoparticles as Fo draw solutes have been prepared by several methods: one-pot polyol reaction,30,57,60–63 multi-step reactions in water,83 and post-coating of pre-synthesized polymer onto hydrophobic mag- netic nanoparticles through ligand exchange.64 Chung’s group has prepared a series of hydrophilic magnetic nanoparticles functionalized with different surface groups using a one-pot synthesis through the reaction of an iron pre- cursor – iron(iii) acetylacetonate and the functional polymers in triethylene glycol (teg) at the refluxing temperature (Figure 2.5).
Current research results have revealed that the surfactants, nanoparticle size, concentration, and the dissociation extent all have an important impact on the Fo performance.57,60–63 it is found a higher hydrophilic surface cap- ping group can result in a much higher water flux. For example, among the four surfactants: 2-pyrrolidone (2-pyrol), triethylene glycol (treg), poly- acrylic acid (paa), and poly(N-isopropylacrylamide) (pnipam), the water flux exactly followed the order of their hydrophilicity: paa > treg > 2-pyrol >
pnipam (Figure 2.6). mnp-paa produced a flux over 10 Lmh in pro mode when using di water as the feed solution, while mnp-pnipam only gave a flux less than 2 Lmh.60,62 the water flux also increased with increasing draw solu- tion concentration, and this was attributed to the increased effective osmotic pressure. however, the water flux did not increase proportionally with the draw solution concentration due to concentration polarization. moreover, a higher solute concentration could result in greater viscosity.57,84 the size of the nanoparticles can also affect the water flux. nanoparticles with a smaller size have a larger surface area and a larger number of hydrophilic groups can anchor on the iron atoms on nanoparticle surface via chelation; hence, a larger osmotic pressure can be produced.57,62,63 in the case of mnp-paa, Figure 2.4 Schematic illustration of the Fo process using magnetic nanoparticles
as draw solute.
Figure 2.5 Synthesis of hydrophilic magnetic nanoparticles.
Figure 2.6 (a) highest water flux and (b) osmolality of mnps with different sur- factants; effect of paa-mnp size on (c) water flux and (d) osmolality.
reprinted (adapted) with permission from ref. 62. Copyright (2010) american Chemical Society.
nanoparticles with a diameter of 4 nm generate an osmotic pressure twice that of the 20 nm-sized nanoparticles (Figure 2.6).62 however, the magnetic strength of the nanoparticles decreases as the size reduces and capturing of the nanoparticles with a high gradient magnetic separator (hgmS) is more difficult. thus, a compromise has to be reached between high water flux and satisfactory recovery in terms of the nanoparticle size.
on top of that, the surface dissociation extent is found to be another factor that influences the Fo performance. For example, paa had limited dissoci- ation in aqueous solutions at its inherent ph range, and the –Cooh groups were the sole contributor that created osmotic pressure. through the addi- tion of naoh, the paa on nanoparticle surfaces could be further dissociated to generate a much higher osmotic pressure.61 Besides, increasing the load- ing of hydrophilic groups onto the magnetic core could also increase the Fo flux, but it would sacrifice the magnetic property of these nanoparticles.62,64
to recover the magnetic nanoparticles from the diluted draw solution, an external magnetic field needs to be applied and the generated magnetic forces on particles have to outweigh the opposing forces caused by Brownian motion, viscous drag, and sedimentation. in the case where the particles are too small and the field gradient is not high, the particles will not be cap- tured.78 thus, to separate relatively small nanoparticles (less than a critical size of 20 nm), a high-gradient magnetic separator is required to create large applied fields.59 the magnetic separator consists of an electromagnet in con- junction with an iron circuit with packed ferromagnetic stainless steel wool inside the column. during the recovery process, the magnetic field is first turned on to collect the nanoparticles by attracting and trapping them on the surfaces of the stainless steel wool. ideally, clean water will pass through the column and be collected. afterwards, the magnetic field is turned off and di water is flushed through the matrix to remove the magnetic nanoparticles from the wool and these nanoparticles are then recycled as the draw solute for the next round of the Fo process.
the use of hgmS to recover magnetic nanoparticles in the Fo process exhibits several challenges: retaining the nanoparticles on the stainless steel wool after the flushing of water; the failure to capture relatively small nanoparticles; and the aggregation of nanoparticles after magnetic separa- tion. Kim’s group has found that the choice and replacement of stainless steel wool played an important role on the complete recovery of nanoparti- cles.59 after the magnetic field was turned off, the stainless steel wool could not be completely de-magnetized due to its ferromagnetic feature and this residue magnetization prevented the complete removal of the nanoparticles via the flushing of water. these residue nanoparticles on the wool might neg- atively impact the performance of the recovery of nanoparticles in the fol- lowing cycles. thus, replacement of wool was required after each recycling process and this puts a limit on the practical application. on the other hand, the ferromagnetic property of the stainless wool was necessary to create a non-uniform field gradient to achieve the 100% absorption of nanoparticles.
therefore, high-performance magnetic separators that can achieve both
complete retention and reuse of the nanoparticles have to be designed for future practical applications.
as mentioned earlier, as the critical size of the nanoparticles to be cap- tured decreases, the required magnetic gradient increases substantially.
When the prepared magnetic nanoparticles have a wide size distribution, it is likely that only large nanoparticles can be captured while the smaller out- liers remain in the effluent water. this incomplete capture of small nanopar- ticles not only affects the quality of the water product, but also results in the loss of Fo performance in the following cycles.64,78 to address this problem, an additional treatment step such as ultrafiltration is required to recover the small nanoparticles as well as to provide a clean water product. another solution is to pre-treat the nanoparticles before using them for Fo applica- tion. the smaller nanoparticles can be separated through dialysis and the remaining mono-dispersed large nanoparticles can be further used as Fo draw solutes. For some magnetic nanoparticles that are designated small in order to generate a high osmotic pressure and enhanced Fo performance, they fall off the range that hgSm can capture and magnetic separation is no longer feasible to recover them, which is the case for paa-mnp with an aver- age diameter of 4 nm.62 in this regard, different separation techniques have to be employed to recover them. Chung’s group explored several methods, which included ultrafiltration, a novel dual-stage Fo system, and an inte- grated electric field and nanofiltration system. though these recovery meth- ods demonstrated effective recovery of magnetic nanoparticles, they failed to employ the unique magnetic responsive properties of these nanoparticles.
in that case, some other non-magnetic nanoparticles, such as carbon dots,85 can also serve the role as a draw solute and provide better Fo performance.
it should be noted that current magnetic nanoparticle-based draw solutes all use Fe3o4 as the magnetic core. other magnetic materials, such as CoFe and CoFe–Fe3o4, have higher magnetization and the use of them as the core may result in better magnetic separation.86–88
on top of the two challenges mentioned above, the most prominent prob- lem encountered is the aggregation of the nanoparticles under the high strength of the hgmS. this will lead to the subsequent deterioration of the Fo performance and steady water flux cannot be consistently achieved in multiple testing cycles. the earliest research on the aggregation behavior of nanoparti- cles by Ling et al. implied the gradual increase of nanoparticles after magnetic separation. For example, the sizes of 2-pyrol-mnp, treg-mnp, and paa-mnp all increased after being recycled (from initial 28, 24, and 21 nm to 68.1, 58.8, and 50.8 nm, respectively).62 Later study by ge et al. on using poly(ethylene glycol) diacid-functionalized nanoparticles also revealed the gradual increase of nanoparticle sizes and the resultant loss of 21% water flux at the end of nine runs of recycle (Figure 2.7).57 it was found that ultrasonication could break the agglomerated treg-mnp nanoparticles without changing the surface chem- istry and partially restored the lost Fo performance caused by nanoparticle aggregation. however, sufficient ultrasonication time has to be provided to effectively reduce the nanoparticle size and this requires a lot of energy input.
moreover, there was a significant drop observed on the saturation magnetiza- tion of the nanoparticles, and it might be attributed to the conversion of the Fe3o4 core to Fe2o3 during the ultrasonication process.