Sustainable water and energy supply have become two of the greatest global challenges of our time due to the fast-growing population and the depletion of fossil fuels.1,2 industrialization, water contamination, and climate change further plagues the problems of water scarcity. it is reported that over one- third of the current world population live in water-stressed areas, among which 1.2 billion people lack access to safe drinking water and 2.6 billion have inadequate sanitation.3–5 the number of people that may be affected by water issues is projected to rise to over 3.5 billion by 2025. though improved water conservation, maintenance of infrastructure, and novel design of catchment and distribution systems can alleviate these stresses, they can only improve the use of available fresh water rather than increase it. desali- nation and water reuse are two of the few methods to increase water supply beyond the available amount from the hydrogeological cycle.6 therefore, it is of great importance to explore technologies to extract fresh water from seawater or wastewater efficiently.
in response to the growing water demand, substantial research efforts have been devoted to augment the water supply.7 Because the production of fresh water is an energy-intensive process, the development of low-en- ergy separation technologies is of great necessity to reduce the overall pro- duction cost.8,9 among the recently developed methods, the membrane separation process turns to be a promising one. particularly in the field of seawater desalination, reverse osmosis (ro) as an exceedingly robust and effective process has been the most widely used technology. in a typ- ical ro process, an external hydraulic pressure is applied to seawater to drive fresh water through a semipermeable membrane while retaining salt.
however, the process requires a rather high hydraulic pressure to outweigh the inhibiting osmotic pressure originated by seawater. though the energy consumption of current state-of-the-art ro processes has been brought down to about twice the minimum theoretical energy of de-mixing water from seawater (1.06 kWh m−3 for 50% water recovery from 3.5% seawater), it is still considerably high for sustainable water production.2,6 moreover, limitations such as membrane fouling and concentration polarization are difficult to overcome for ro processes.
Forward osmosis (Fo) has emerged as a promising membrane technology for seawater desalination and wastewater treatment at low-energy cost.10–13 it has also been investigated for the use of food processing,14–16 power gen- eration,17–20 and fertilization.21,22 Fo is a naturally occurring osmosis-driven process. it consists of two key components: (i) a semipermeable membrane that allows the permeation of water while rejecting salts and other unwanted elements; and (ii) a draw solution that is able to generate sufficiently high osmotic pressure to draw water from the feed solution across the membrane (Figure 2.1). Compared with ro, Fo holds great promise of low energy con- sumption if a suitable draw solute with an economical and efficient regen- eration method is available.2,6,23 in addition, Fo possesses advantages such as low and reversible membrane fouling, robust rejection of various contam- inants, high water flux and recovery, and reduced discharge of concentrated brine.24–28 the ability to maintain the properties of the feed solution also
Figure 2.1 Schematic illustration of Fo.
makes Fo a good candidate for applications in the field of life sciences, such as protein enrichment and controlled drug release.29–34
in an Fo process, solvent molecules move readily from a low concentration solution through a semipermeable membrane to a high concentration solu- tion until their overall chemical potential reach equilibrium. the high con- centration solution and low concentration solution are referred to as “draw solution” and “feed solution”, respectively.8,9,35 the difference in osmotic pressure (Δπ) between the draw solution and feed solution acts as the driving force for the separation of feed solvent and solutes. When an asymmetric membrane is used, two modes of Fo processes are defined depending on the membrane orientation: pressure retarded osmosis (pro) where the draw solution faces the dense selective layer (active layer); and Fo mode where the draw solution flows against the porous support layer. in osmotically driven membrane processes, concentration polarization is a readily encountered problem that is caused by the diluting effect of the draw solution and the con- centrating effect of the feed solution.36–38 the presence of both internal con- centration polarization (iCp) and external concentration polarization (eCp), which occurs within the porous layer of the membrane and at the surface of the dense selective layer respectively, can substantially lower the water flux across the membrane compared with the theoretical value (Figure 2.2).39–41 eCp occurs at the outer surface of the selective membrane and its effect on permeate flux can be mitigated by altering hydrodynamic conditions such as increasing the flow rate or turbulence.42 in contrast to eCp, iCp occurs within the porous layer and it cannot be easily eliminated. it has been found that the flux decline in Fo is primarily induced by iCp, which could deteriorate the water flux by over 80%.43,44 modeling results have revealed that both the structure of the membrane support layer and the diffusion coefficient of the
Figure 2.2 Schematic illustration of internal concentration polarization and exter- nal concentration polarization.
draw solution play predominant roles on iCp. the solute resistivity (K) is a parameter to measure the diffusion of solutes through the support layer:
K = tτ/εD = S/D
where t, τ, ε, and S are the membrane thickness, tortuosity, porosity, and structural parameter, respectively, and D is the diffusion coefficient of the solute. therefore, iCp may be reduced by using a membrane with a low S value and/or a draw solution with a high diffusion coefficient.
the water flux Jw, with a unit of L m−2 h−1 (Lmh), can be determined by the following equation:
Jw = AσΔπ
where A is the water permeability coefficient of the membrane and σ is the reflection coefficient.8 σΔπ refers to the effective osmotic pressure differ- ence across the membrane after taking the concentration polarization into account.
an ideal Fo membrane should have the following features: high water permeability, high rejection of solutes, significantly reduced internal con- centration polarization (iCp), and high chemical stability and mechanical strength.9 in addition to a suitable membrane with desired characteristics for the Fo process, a satisfactory draw solution is crucial for its successful operation. an ideal draw agent has to meet the following basic criteria. First, it should be able to generate a high enough osmotic pressure to extract water from the feed solution. in the case of dilute salt solutions, the osmotic pres- sure can be estimated using the Van’t hoff equation:
π = iMRT
where π is the osmotic pressure in bars, i is the number of ions per dissoci- ated solute molecule, M is the concentration of the solute in moles L−1, and R and T are the gas constant (0.08314 L bar mol−1 K−1) and absolute tempera- ture in kelvin, respectively.45 in the literature, iM is also referred as osmolal- ity, the concentration of osmotically active particles in a solution with a unit of mosm kg−1. For typical seawater with a salinity of 3.5%, the osmolality is about 1200 mosm kg−1 and the corresponding osmotic pressure is around 29 atm at room temperature. For the case of concentrated salt solutions, they deviate from the ideal solution and a factor φ, called the osmotic pressure coefficient, needs to be added into the equation to account for the non-ideal behavior. this equation indicates that suitable draw solutes should possess a high water solubility (high M), and a high degree of dissociation (high i).
in addition, they should also cause minimal iCp by having a low diffusion coefficient, i.e. draw solutes with small molecular weight and low viscosity are preferred. Second, after the Fo process, the draw solutes need to be easily recovered and regenerated from the diluted draw solution with a low energy input and operation cost. a Fo process is usually accompanied with another
step to produce clean water as well as recover the draw solutes (Figure 2.3).
the draw solutes should be able to be completely removed to produce drink- able water, and they can retain their original performance after being regener- ated for long-term operation. third, the reverse flux of the draw solute has to be minimized. For some small draw solutes, the semipermeable membranes cannot prevent them from diffusing into the feed solution completely. the back-diffusion of these compounds will cause the loss of the draw solutes, and in turn decreased driving force. Subsequently, draw solutes have to be replenished to cover the loss. moreover, the penetrated draw solutes may contaminate the feed solution, especially for proteins and pharmaceuticals which may lose their qualities upon exposure to these compounds.9,29–31 on top of these criteria, draw solutes should also be non-toxic, have good chem- ical stability, and be compatible with the Fo membrane.
over the past few decades, a variety of compounds have been investigated as Fo draw solutes. earliest studies explored the use of a mixture of h2o and volatile solutes, such as So2, or liquids, such as aliphatic alcohols, in Fo application and these solutes were recovered by heating or distillation.46,47 these processes were energy intensive and were not studied in detail. more- over, the presence of So2 causes toxicity and acidity. Sugars, i.e. glucose or fructose, have also been used as draw agents to prepare a nutritious drink by extracting water from seawater, brackish water, or polluted water.48–50 in this approach, the draw solutes are directly consumed and the recovery of the draw solution is avoided. Similarly, fertilizer has been investigated as a draw solute for the desalination of saline water for irrigation, and at the same time the diluted draw solution can be directly used for fertigation.21,22 the study of new draw solutes for Fo application has drawn rather great interest in recent years. notably, elimelech’s group employed ammonium bicarbonate to draw
Figure 2.3 Schematic illustration of Fo process with draw solute regeneration.
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.