ReviewA review on the recovery methods of draw solutes in forward osmosis
Introduction
Since the beginning of the 2000s, forward osmosis (FO) has gained increasing popularity. It has been viewed as one of the most promising technologies in the 21st century [1], [2], [3], [4], [5], [6], as it is a green membrane technology for desalination and water treatment with advantages of lower energy cost and less membrane fouling compared with traditional membrane processes such as reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), and microfiltration (MF) [4], [7], [8], [9], [10]. In a FO process, a draw solution is used to “attract” water from feed solution by osmotic pressure difference through a semi-permeable membrane, while most solutes are rejected by the FO membrane. This is followed by a process to reconcentrate the diluted draw solution and produce clean water. Although not all FO applications require recovery of draw solutes (DSs) (e.g., for nutritious drinks during emergencies [11], irrigation [12], and desert restoration [13]), several major FO applications (e.g., desalination for drinking water, wastewater reclamation, etc.) need DSs to be separated from water or reconcentrated for reuse [14], [15]. The selection and use of a suitable draw solute can greatly influence the efficiency and sustainability of FO operations. Typically, an ideal draw solute in FO for water production should have the desired properties of high osmotic pressure, minimal reverse solute diffusion, easy separation from water, economic feasibility, reusability, nontoxicity, and compatibility with FO membranes [16], [17], [18], [19].
In recent years, a number of novel DSs have been proposed together with a variety of remarkable recovery technologies besides traditional DSs like sodium chloride (NaCl) [20] to advance FO technology for various applications (Table 1). These DSs may be generally classified into inorganic salts, and organic compounds according to their physicochemical properties. The inorganic salts including ammonia–carbon dioxide (CO2/NH3) [21], fertilizers [12], seawater [22], [23], and aluminum/copper/magnesium sulfate [24], [25], [26], have been proposed to reduce the energy cost for regeneration. However, the inorganic DSs with monovalent ions may cause high reverse solute flux. The organic compounds include 2-methylimidazole-based compounds [27], polyacrylic acid sodium salts (PSA) [28], hexavalent phosphazene salts [29], switchable polarity solvents (SPS) [30], hydroacid complexes [31], EDTA sodium salts [32], sodium lignin sulfonate (NaLS) [13], zwitterions [33], Na+-functionalized carbon quantum dots (Na-CQD) [34], and smart materials [18], which are expected to have minimal reverse solute flux and require low energy cost for recovery. Among the organic DSs, smart materials with intelligent response have attracted growing interest for FO applications recently, including functionalized magnetic nanoparticles (MNPs) [35], [36], [37], [38], [39], [40], [41], [42], thermosensitive polyelectrolytes [43], [44], [45], [46], and stimuli-responsive hydrogels responding to different stimuli such as heating [10], [47], a combination of heating and hydraulic pressure [48], sunlight [49], [50], [51], gas pressure [52], and magnetic heating [53]. These smart DSs may be recovered at a relatively low-energy cost under different stimuli, and induce minimal reverse solute flux due to their large sizes. However, a comprehensive review of the recovery methods of these DSs is not available. Thus the objectives of this paper are (1) to conduct a critical and detailed review on the existing recovery methods of DSs, and (2) to illustrate how the newly developed technical innovations on recovery of DSs have led to preference for FO technology as a membrane-based separation process.
Section snippets
Thermal separation
In as early as the 1960s, a variety of reagents were tested for use as DSs in the FO process for seawater desalination. However, previous efforts on these reagents are mostly presented for patents; thus, they have limited technical and performance details. In 1965, Batchelder used volatile solutes such as sulfur dioxide (SO2) in FO [54]. SO2 was added to seawater or freshwater to create a solution with high osmotic pressure, which could be used as the driving force to extract water from
Challenges and prospects for the future
A number of recovery methods based on the types of DSs in FO processes have been summarized and highlighted including thermal separation, membrane separation, precipitation, stimuli–response, combined processes, and direct use without recovery. All of them have demonstrated their potential in FO processes for various applications. However, they are still facing many challenges related to being energy-efficient and environmental-friendly, high water recovery rates, high water quality, easy
Acknowledgements
We would like to thank the Natural Science Foundation of China (NSFC, No. 51308239) and Huazhong University of Science and Technology for the financial support (No. 2012TS034).
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