Perspective Chapter: Hydrogel Draw Agent Desalination Systems – Outlook

Abstract

The chapter intends to discuss an application of hydrogel material as draw agent for a forward osmosis desalination system. This refers to systems that allow a continuous process of extraction of desalinated water with low energy costs and minimal environmental pollution. One of the most prominent properties of hydrogel materials is their ability to spontaneously absorb large quantities of water from saline solution separated by a semipermeable membrane. This process is energetically favorable due to the difference in the chemical potentials of water in the solution and hydrogel. Thermodynamic equilibrium between hydrogel and external saline solution corresponds to the strictly defined amount of water retained by the hydrogel in the given conditions. The excess pressure of water in hydrogel relative to the pressure of the pure external in this state is defined as the osmotic pressure difference. In contrast to the absorption of water molecules by hydrogel, their extraction is usually a process that requires large energy consumption and disruption of the continuity of the desalination cycle. However, known several opportunities to overcome this bottleneck and they are discussed in detail.

1. Introduction

Osmosis-based water desalination is an effective technique to produce high-quality water and the production rates are easily adjustable. Although two types of the osmosis-based desalination, namely forward osmosis (FO) and reverse osmosis (RO), are known currently, only the RO continuous process has found wide industrial implementation and its market share in water desalination is rapidly increasing. However, the RO based water desalination is generally recognized as energy-intensive (2.2–3.5 kWh/m³) process [1]. The high energy usage during RO desalination causes environmental concerns such as air pollution and heating associated with water cooling using energy production from fossil fuels. Another drawback of RO-based desalination is the production of high-salinity brine, which contains plenty of substances and chemicals that are harmful to the environment and ecosystem. Several studies have suggested solutions to reduce the high energy consumption and brine impacts. However, the industry community is developing the opinion that RO-based desalination has reached the theoretical and practical limit. The energy limitation and environmental damage must be overcome through different technical solutions [2]. One of desalination techniques considered as the possible alternative is forward osmosis (FO) process. In contrast to RO, in which work is done to push water molecules through the membrane against a pressure drop, FO is an energetically favorable process due to the difference in the chemical potentials of water in the solution and in draw agent. Water molecules accommodated by draw agents have different liquid water properties depending on the interaction within the agent material. Diffusion of water through the membrane is affected by the level of the agent hydration. Thermodynamic equilibrium between draw agent and external aqueous solution corresponds to the strictly defined amount of water retained by the draw agent in given conditions. A successful FO desalination process is critically reliant on the availability of a draw agent that offers both high osmotic pressure and a facile regeneration mechanism. It is generally accepted that FO process inevitably requires post-process for all types of draw agents to obtain the final water production, which creates significant obstacles to its use as a stand-alone low-energy desalination process and commercialization [3].

2. Peculiarities of hydrogel-based FO desalination process

Hydrogels are crosslinked three-dimensional hydrophilic polymer networks that can absorb a huge amount of water and not be dissolved in it. Some hydrogels called smart or stimuli-responsive polymer hydrogels can undergo a reversible volume change or solution-gel phase transition in response to external environmental stimuli, including temperature, light, pressure, solvent composition, and pH. These intrinsic properties led to search among plenty of hydrogels for such that have lowest regeneration energy consumption and correspond to such requirements as high osmotic pressure, be nontoxic, exhibit low reverse flux and acceptable cost [4]. Various hydrogels were studied as draw agents in the FO process over the past few years with varying degrees of success. The results of these efforts have been summarized by Wang group and presented in review [5]. However, as noted in the review, there are some issues that still need to be addressed in hydrogels application as draw agents, including low water flux, high external concentration polarization, and especially the in-continuous operation. Overcoming of the last drawback is of primary complexity. Let us briefly consider basics of functioning of the FO desalination system permanently extracting water from a hydrogel draw agent. In order to facilitate the description, but without limiting the generality, we will consider a notional one-dimensional water desalination processes that can take place in a vessel divided into three parts. Let the first third of the vessel be filled with running saline water and separated from the middle part by a semipermeable membrane for FO desalination. The middle part is crowded with granular unsaturated hydrogel material and separated by grid from the third part of the vessel, which is intended for collection of the freshwater. Saline water pressure P_f at upper vessel’s part does not exceed pressure values in ordinary drinking water supply systems that is of about 0.5 MPa. ^. Only water molecules penetrate through the membrane and cause swelling of the constrained hydrogel material. The swelling leads to an increase of hydraulic pressure and probably efflux of part of water through the wick starting from some pressure value. Hydrogels consist of two phases, the polymer network, which is constant in quantity, and the aqueous phase, which is variable. The system under consideration would not reach the state of thermodynamic equilibrium between these phases if a part of the desalinated water flux entering the hydrogel is diverted through the grid to the third part of the vessel under the influence of some factors. In what follows we will consider only two such factors, specially selected wicks and solar radiation. An implementation of the described above scheme imposes number of specific requirements for the properties of used materials. These requirements are discussed below.

2.1 Hydration of hydrogels

One of the most prominent properties of hydrogels is their ability to absorb large quantity of water and to swell as a consequence of this process. Water molecules accommodated by hydrogels have different properties depending on the position and interactions within the hydrophilic network. A model, presented in 1973 by John Andrade [6], defines three types of water in hydrogels—nonfreezing or bound water, free or bulk water, and freezing interfacial or intermediate water. Molecules of free water are not affected by the polymer and freeze/melt similarly to pure water; molecules of bound and to some degree intermediate water are immobilized by binding to the polymer chains through hydrogen bonds. Other properties of the bound and the intermediate water (relaxation time, polarization, etc.) also differ from properties of free water.

Diffusion in hydrogels is affected by the level of hydration. Experiments with tracer molecules dissolved in water elucidate that at a high levels of hydration, the process occurs primarily as the free water diffusion; however, at low hydration, it takes place as diffusion in the bound water [7].

Note that the thermodynamic equilibrium between hydrogel and external aqueous solution corresponds to the strictly defined amount of all three types of water retained by the hydrogel in the given conditions. The excess pressure of water in hydrogel relative to the pressure of the pure external in this state is defined as the osmotic pressure difference and can be expressed through the difference of the corresponding chemical potentials [8, 9] for ideal elastomeric gels. The hydrogel’s osmotic pressure, which can be called the “driving force” of the hydration process, is a complex parameter. The total osmotic pressure in a swelling hydrogel Π can be divided into three separate parts using the Flory–Rehner theory. This theory [10] states the perfect separability of the total free energy, (ΔF), into an elastic, mixing, and ionic contributions, each with an associated osmotic pressure (Π_elastic, Π_mixing, and Π_ionic).

$$\mathrm{\Pi }={\mathrm{\Pi }}_{\text{ionic}}+{\mathrm{\Pi }}_{\text{mixing}}+{\mathrm{\Pi }}_{\text{elastic}}$$E1

The mixing and ionic contributions are commonly seen as the cause of gel swelling while the elastic portion restricts the large expansion of the material.

The mixing osmotic pressure refers to the attraction of solvent molecules in the external solution to the hydrophilic polymer chains and can be expressed [11] by the Flory–Huggins

$${\mathrm{\Pi }}_{\text{mixing}}=-\left[ln\left(1-\mathrm{\phi }\right)+\mathrm{\phi }+{\mathrm{Ⱦ\phi }}^2\right]$$E2

where φ is the molar volume of the solvent, R is the universal gas constant, T is the temperature (Kelvin), V is the current solid volume fraction, and Ⱦ is Flory–Huggins parameter derived from the solid-fluid interaction. This parameter is material and environment dependent and defines deswelling properties of hydrogels.

For Ⱦ > 0.5 the hydrogel solution is unstable for small fluctuations [12] and gives off water relatively easily.

The equilibrium pressure for real hydrogels is different from the osmotic pressure and refers here as osmotic swelling pressure. Its value as well as equilibrium swelling ratio and swelling kinetics differs for free-standing and confined hydrogels.

A steady state water flow if such established along the porous medium like confined hydrogel can be described by following equation

$$Q=\frac{\mathit{SoK}\left(∆\Pi -∆P\right)}{\mathit{d\mu }}$$E3

where Q is the rate of water flow, K is the hydraulic conductivity of the hydrogel block, d is the hydrogel layer thickness, S₀ is the hydrogel cross-section, μ is the solvent viscosity, and ΔP and ΔΠ are the hydraulic and osmotic pressures difference between the input and the output edges of the hydrogel block, respectively.

Note that only movement of unbounded water contributes to flow, while liquid molecules held by absorptive forces are essentially immobile [13].

2.2 Forward osmosis membrane assembly—hydrogel interface

A flow of water through a semipermeable membrane can be described by Darcy’s law in its complete form [14]:

$$\mathit{Qm}=\frac{\mathit{\text{SDwCm}}}{\lambda }\left\{1-\mathit{exp}\left[\frac{\mathit{Vw}}{\mathit{RT}}\left(\mathit{\Delta \pi }-\mathit{\Delta Pm}\right)\right]\right\}$$E4

where Q_m is the rate of water flow, S is the effective cross-section of membrane, D_w is the average water diffusion coefficient in the membrane, C_m is the equilibrium concentration of water in the membrane, λ is the membrane thickness, V_w is the partial molar volume of water, R is universal gas constant, T is the temperature (Kelvin), ΔP is the pressure difference across the membrane assembly, that is, ΔP_m = P_p − P_f, where P_p is hydraulic pressure at the cross-section of hydrogel located at the distance of free path of water molecules from the membrane assembly and P_f is the feed pressure, and Δπ is the “the driving force” of water flow that is difference of osmotic pressures Π_p and Π_f at the above-mentioned cross section and the feed solution, respectively, while Πf directly proportional to the molality a of the solution M:

$${\mathrm{\Pi }}_{\mathrm{f}}=\mathrm{MRT}$$E5

Carr [15] defined the characteristic timescale for a diffusion process τ as the maximum value of the mean action time across the layer

$$\tau =0.5\frac{l^2}{D}$$E6

where l is the layer’s thickness and D is the coefficient of diffusion.

The characteristic timescale of water diffusion in hydrogel block exceeds by many orders of magnitude, the same parameter for the membrane assembly. As a result, a gel layer with a thickness approximately equal to the pore size in a rigid membrane’s base and in close proximity to it approaches the state of local equilibrium with the feeding solution however does not reach it [16]. In this, the water flow through the membrane decreases significantly compared to its value in the absence of a hydrogel (effect of the concentration polarization). Because osmotic flow and hydraulic flow require the same pressure drop along the membrane pore to generate equal flow [17], this effect can be expressed by system of Eq. (7)

$$\left\{\begin{array}{l}{\mathbf{\Pi }}_{\mathbf{p}}\mathbf{=}{\mathbf{\Pi }}_{\mathbf{f}}\mathbf{-}{\mathbf{\Delta \Pi }}_{\mathbf{eff}}\\ {\mathbf{P}}_{\mathbf{p}}\mathbf{=}{\mathbf{P}}_{\mathbf{f}}\end{array}\right.$$E7

A magnitude of ΔΠ_eff depends on the properties of both the membrane assembly and the hydrogel and lies in the range of 1200–2200 KPa [18].

Considering the effect of the concentration polarization relations given in Eq. 5 for steady-state water flow along the hydrogel block can be rewritten as:

$$Q=\mathit{SoK}$$E8

Since the resistance of the hydrogel block is at least four orders of magnitude greater than the analogous parameter of the membrane assembly (an asymmetric membrane normally consists of a dense layer of 0.1–1 μm thick and supported by a highly porous, 100–200 μm thick support layer [19]), the same equation can be used to describe the water flow through complete membrane-hydrogel block subsystem.

Based on Eq. (8), water flow is decreasing function of P_l and reaches zero at its ceiling value (or upper limit) P_lmax

$${\mathrm{P}}_{\text{lmax}}={\mathrm{\Pi }}_{\mathrm{l}}-{\mathrm{\Pi }}_{\mathrm{f}}+{\mathrm{\Delta \Pi }}_{\mathrm{eff}}+{\mathrm{P}}_{\mathrm{f}}$$E9

3. FO continuous desalination by wicking of pH-sensitive hydrogel agent

Freshwater recovery is a major embarrassment in direct osmosis desalination technology in general and hydrogel-based FO desalination in particular.

One of the possible ways to provide an energy-efficient process with a continuous duty cycle to overcome the above bottleneck is proposed and experimentally tested in the article [20]. The idea was to provide local stimuli impact on grains of pH-sensitive hydrogel. Figure 1 illustrates typical swelling dependency of superabsorbent hydrogel on pH value. As follows from the graph the water content in the hydrogel reaches a maximum at a certain pH value, and any change in this value may be accompanied by spontaneous emission of water.

Figure 1.

pH-dependent swelling of the superabsorbent hydrogel (according to [21]).

The local release of water from hydrogels under such factors as laser pulse and mechanical puncture was observed in works in ref. [22, 23]. However, these methods cannot provide collection of the released water.

The authors of ref. [20] proposed to use wicks with surface pH different from the pH of hydrogel medium as stimulus for local release of water from hydrogel granules into the intergranular space. This construction proved to be capable of passive extraction of water from swelling hydrogel draw agent in three-stage process:

• efflux of water from hydrogel grains into intergranular space in the interface with embedded wicks,

• soaking up intergranular water with the wicks, and

• dripping or evaporation of water from the wicks.

Wicking is a spontaneous movement of liquids into porous media under the action of the capillary suction pressure. Value of the suction forces is governed by the properties of the liquid, liquid-medium surface interactions, and geometric configurations of the pore structure in the medium.

At the conditions of steady-state, all liquid entering the wick per unit of time will leave it in the same period by the drop flow (evaporation-protected wicks).

Recently invented types of wicks can drain freshwater with corresponding suction pressure P_sw as high as hundreds of kilopascals.

The lowest hydraulic pressure P_t (“threshold pressure”) starting from which the water enters the wick is determined from the balance of promoting and hindering forces acting on an element of free water at the hydrogel-wick interface. Generally, this pressure is unattainable if you try to extract water directly from hydrogel grains since water retaining component of the osmotic pressure (“suction pressure of hydrogel”), which far exceeds P_sw value. However, intergranular water initiated by pH-difference is easily extractable with wicks as demonstrated in experimental results of Ref. [20] shortly presented below.

A verification of water extraction feasibility from swelled hydrogel preceded by complete removal of liquid from the intergranular space of the washed hydrogel by multistep procedure using equilibration solutions of different salinity separated from hydrogel by FO membrane.

The effect of the wick’s surface pH-initiated water release was investigated by simultaneous immersing of one end of each of the two test wicks into the bicker filled with potassium super absorbent polymer hydrogel while the other end of the wicks hung loosely down. The phenomenon of water extraction by wicks has been detected experimentally by measurement of liquid front propagation rate at various pH values of the wick and hydrogel media as well as the salinity of equilibration solution. The measurement of average rate of the liquid front advancement along the wicks has been performed by optical image analysis method [24] (the wicks were protected from water evaporation).

Figure 2 reflects an exponential drop in the rate of the waterfront advancement occurring with an increase in the concentration of the equilibrating solution and a corresponding decrease in the amount of water in the hydrogel characterized by pH value equal to 6.5.

Figure 2.

Waterfront rate as function NaCl content in the equilibration solution for the wicks with surface pH values 7.8 (red line) and 7.2 (blue line).

Two wicks 1 and 2 differing in the values of their surface pH (7.8 and 7.2 respectively) were used in this experiment. As can be seen from the same graph, the rate of water extraction by the first wick W1 is higher than by the second one (W1 − W2 > 0). There are two possible reasons for this phenomenon: inequality of the suction forces of two wicks and inequality of local hydrogel shrinking because of the wick's pH difference. In the first case the sign of difference (W1 − W2) is independent but in the second case must be dependent on the hydrogel pH value change in a certain range. The results of the experiment with a change in the pH value of the hydrogel are presented in Figure 3.

Figure 3.

Rate of waterfront propagation along wicks 1 and 2 inserted in hydrogels equilibrated with distilled water and having different pH values.

About the same ratio between the extraction rates is maintained for hydrogel with pH equal to 6.5 and 6.9. However, the test provided with hydrogel whose pH was 7.6 resulted in a change in the sign of the difference (W1 − W2 < 0).

The effects described above have been confirmed by exploitation of a prototype of continuously operating FO desalination system providingspontaneous flow of freshwater outflow from the container with saline water and consistently passing through the semi-permeable membrane, the hydrogel block, and system of the specified wicks. At conditions where the salinity of the source water is in the range of 0–10 g/l, the potassium polyacrylate hydrogel acts as a “water pump” and a “water bridge” simultaneously. This phenomenon can be used for desalination of underground water for needs of irrigation whereas desalination of sea water requires application of pH-sensitive hydrogels with higher osmotic pressure.

4. FO continuous desalination by solar-powered heating of pH-sensitive hydrogel agent

Desalination of seawater by solar energy is a hot topic of hydrogel draw agent concept. In the routine solar dewatering process, polymer hydrogels deswell under solar-induced heating resulting in the recovery of pure water and the recycling of composite polymer hydrogels for another FO process. The core element of the composite draw agent is the photothermal conversion materials like black carbon particles or more sophisticated like carbon nanotubes and aluminum-based plasmonic absorbers for example. The hydrophilic groups in the hydrophilic polymer network are beneficial in reducing the evaporation enthalpy of water molecules, accelerating water transport and improving the solar energy conversion efficiency [25, 26]. Wang et al. for the first time proposed the intermittent stimuli impact on hydrogel draw agent for quasi-continuous production of freshwater by using heating by solar energy-cooling cycles. Actually, they proposed to replace uniform bulk hydrogel body by a bilayer structure with no need to remove the draw agent from the membrane module [5, 27]. The feasibility of bilayer polymer hydrogels as draw agents in FO process has been investigated. The dual-functionality hydrogels consist of a water-absorptive layer to provide osmotic pressure, and a dewatering layer to allow the ready release of the water absorbed during the FO drawing process at lower critical solution temperature (LCST) (Figure 4).

Figure 4.

Schematic illustration of bifunctional polymer hydrogel layers process [27]. Dewatering flux of thermoresponsive hydrogel as draw agent in the bilayer arrangement (solar intensity = 0.5 kW/m² (W_input = 2 kW/m²), Q = 15, 0.2 g dewatering layer, 0.01 g absorptive layer).

Few years later the idea of Wang group was amplified by Chen et al. [28]. They developed laminated temperature-responsive hydrogel based on poly(N-isopropylacrylamide-co-sodium acrylate) (P(NIPAAm-co-SA)) with variable content of SA. The concentration of sodium acrylate decreased away from the FO membrane in the drawing layer, and the releasing layer was pure PNIPAm. Adding intermediate layers or employing the intermittent dewatering strategy increased the dewatering ratio for the multilayer hydrogel because water molecules could be transported from the drawing layer to the releasing layer more easily. The design illustrated in Figure 5 can effectively decrease reverse osmotic pressure, resulting in an increase in water flux.

Figure 5.

Schematic of experimental setup and characterization of material (a) forward-osmosis process, (b) Dewatering process with thermal input, (c) multilayer material with the drawing layer for FO desalination, (d) releasing layer for fast water release, and (e) multi-layer design with gradual reduction of SA concentration along the water transport pathway (after [28]).

The multi-layer hydrogel released ∼60% of the absorbed water at the fully swollen state in 60 min due to the LCST phase transition of the P-NIPAAm releasing layer while the uniform hydrogels only released ∼35% of the absorbed water purely by evaporation. The FO flux of multilayer hydrogel is still low compared to the inorganic hydrogel, which is a key obstacle for most organic draw agents. However, the multi-layer temperature-responsive hydrogel had a low energy consumption compared to other regeneration methods for nonresponsive draw agents due to the LCST phenomenon, which was rid of the latent heat penalty. The concept of multilayer design showed promising application by replacing the releasing layer with highly ionic polyelectrolyte [28].