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 seawater. After FO, SO2 was separated from water by heating or air stripping. However, the experiments only demonstrate that a desirable water flux is observed without providing detailed data on the recovery rates. Later, McGinnis described a two-stage FO process using a combination of potassium nitrate (KNO3) and SO2 for seawater desalination [55]. This process utilized the temperature-dependent solubility of KNO3 and SO2 to enhance the water recovery rate. However, it is costly as cooling and heating are required to recycle KNO3 and SO2. Despite the fact that the recovery of SO2 from water is easily available, the process involving SO2 may be very dangerous because SO2 is volatile, corrosive, and highly reactive toward both oxidants and reductants.