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

Shu, L, Obagbemi, I, Liyanaarachchi, S, Navaratna, D, Parthasarathy, R, Aim, R and Jegatheesan, V 2016, 'Why does pH increase with CaCl<inf>2</inf> as draw solution during forward osmosis filtration', Process Safety and Environmental Protection, vol. 104, pp. 465-471.


Document type: Journal Article
Collection: Journal Articles

Title Why does pH increase with CaCl<inf>2</inf> as draw solution during forward osmosis filtration
Author(s) Shu, L
Obagbemi, I
Liyanaarachchi, S
Navaratna, D
Parthasarathy, R
Aim, R
Jegatheesan, V
Year 2016
Journal name Process Safety and Environmental Protection
Volume number 104
Start page 465
End page 471
Total pages 7
Publisher Elsevier
Abstract 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.
Subject Applied Mathematics not elsewhere classified
Chemical Engineering not elsewhere classified
Maritime Engineering not elsewhere classified
Keyword(s) Brine
Calcium chloride
Forward osmosis
High salt concentration
Saline solution
Sodium chloride
DOI - identifier 10.1016/j.psep.2016.06.007
Copyright notice © 2016 Institution of Chemical Engineers
ISSN 0957-5820
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