Why does pH increase with CaCl<inf>2</inf> as draw solution during forward osmosis filtration

Fundamental understanding of pH dynamics in forward osmosis (FO) processes is integral for further development of this emerging technology in desalination and wastewater reclamation. In this study, the pH changes during FO membrane filtration were investigated. CaCl 2 ·2H 2 O was used as the draw stream and NaCl as the feed stream. During the FO process, water would flow from the feed to draw so that the feed stream would become concentrated while the draw stream being diluted. It was found that the pH of the feed stream increased with time. Assuming that the pH or the concentration of hydrogen ions to be constant in the FO system, the pH of the draw stream should decrease. Surprisingly, the pH of the draw stream also increased. To explain this unexpected phenomenon, standard curves for both CaCl 2 ·2H 2 O and NaCl were constructed. In contrast to conventional belief, the sta ndard curves showed that pH of NaCl increased with the increase in concentration while pH of CaCl 2 ·2H 2 O decreased with the increase in concentration. As such, the standard curves explain the reason for pH increase in both feed and draw solutions. Furthermore, the curves showed that the pH to be dependent on salt concentration, where charges on ions have the ability to influence the pH of the solution