In recent years, membrane technologies have been developed to address water shortage and energy crisis. Forward osmosis (FO), as an emerging membrane-based water treatment technology, employs an extremely concentrated draw solution (DS) to draw water pass through the semi-permeable membrane from a feed solution. DS as a critical material in FO process plays a key role in determining separation performance and energy cost. Most of existing DSs after FO still require a regeneration step making its return to initial state. Therefore, selecting suitable DS with low reverse solute, high flux, and easy regeneration is critical for improving FO energy efficiency. Numerous novel DSs with improved performance and lower regeneration cost have been developed. However, none reviews reported the categories of DS based on the energy used for recovery up to now, leading to the lack of enough awareness of energy consumption in DS regeneration. This review will give a comprehensive overview on the existing DSs based on the types of energy utilized for DS regeneration. DS categories based on different types of energy used for DS recovery, mainly including direct use based, chemical energy based, waste heat based, electric energy based, magnetic field energy based, and solar energy based are proposed. The respective benefits and detriments of the majority of DS are addressed respectively according to the current reported literatures. Finally, future directions of energy applied to DS recovery are also discussed.

Osmotic energy, as a sustainable energy source with little environmental impact, has drawn much attention in both academia and industry in recent years. Osmotically driven membrane processes can harvest the osmotic energy and thus have great potential to produce sustainable clean water or electric energy. The draw solution, as an osmotic component, has been more and more explored by scientists in recent years in order to achieve a high osmotic pressure and suitable molecular size. In this work, a novel draw solute—sodium tetraethylenepentamine heptaacetate (STPH)—is synthesized and identified by nuclear magnetic resonance spectroscopy (¹H-NMR) and Fourier transform infrared (FTIR). Its solution properties are optimized in terms of the solution pH and concentration, and related to the forward osmosis (FO) performance. A water flux of 28.57 LMH and a low solute flux of 0.45 gMH can be generated with 0.5 g/mL STPH draw solution and de-ionized water (DI water) as the feed solution under pressure retarded osmosis (PRO) mode, which is superior to the FO performance with many other draw solutes reported. Further FO desalination test shows a stable water flux of 9.7 LMH with 0.3 g/mL STPH draw solution and 0.6 M NaCl feed solution. In addition, the draw solution recovery is also investigated.