Water resources demand continually increases with the global economic development and the population explosion. 97.3% of the water on the Earth is salt water and the available fresh water from surface water and groundwater sources only accounts for 0.01%. Therefore, seawater desalination is becoming one of the key options to meet such demands.

Conventional seawater desalination can generate high-quality fresh water, however, the high concentration of salt water produced simultaneously is usually discharged into the oceans, resulting in serious environmental and ecological problems. The valorization of high salt water after desalination process has encountered certain key bottlenecks, such as the separation of ions with different valences. 

Recently, a novel process may give a perfect solution to the complete utilization of the high salt water (mainly containing Na⁺, Cl^-, Mg²⁺, SO₄^2-). This process has been successfully developed by researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of Chinese Academy of Sciences. The study was published in Journal of Membrane Science.  

The first step of the process is to use a commercial membrane (namely Nanofiltration membrane) to separate the monovalent and divalent ions. Secondly, both the separated water (one aqueous solution mainly contains NaCl, and the other MgSO₄) enters different compartments, where electrodes are set on the two ends of these compartments. During the so-called electrodialysis step, Mg²⁺ migrates to the direction of cathode and meanwhile SO₄^2- to anode. Finally, both the ions move to adjacent compartments, respectively, where Mg²⁺ meets Cl^- and SO₄^2- meets Na⁺. The Cl^- and Na⁺ ions come from the first step. The value-added products Na₂SO₄ and MgCl₂ will therefore be obtained and reused, instead of MgSO₄, which may highly increase the risk of scaling. Certainly, the process can produce high-purity drinking water at the same time. 

Figure: Model of electrodialysis for zero-discharge and valorization of seawater (Image by QIBEBT)