Ion-exchange membrane processes for desalination and waste valorization

• Main Author: Darshan S/O Tamilselvan
• Other Authors: She Qianhong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/176565
• Institution: Nanyang Technological University

Electrodialysis (ED) is a membrane-based process that is relatively new to the field of membrane technology. Unlike other conventional membrane-based processes, ED utilizes current to achieve the desired desalination or waste valorization efficiencies. Despite the promise shown by ED in its field, its performance and efficiency are still plagued by numerous issues, such as concentration polarization (CP), which is an unavoidable problem for most membrane-based processes. Beyond CP, ED is also specifically hindered by water transport from its dilute solution to the concentrate solution chamber, which is known reduce the concentration of salts recovered and decrease the overall pure water recovered as well. However, recent studies utilizing porous ion-exchange membranes (IEMs) have shown that the usage of porous IEMs to facilitate water transport might be able to reduce the overall CP incurred in the ED stack, which in turn will increase the desalting process. Hence, this project was devised to study the usage of water transport in an ED stack containing 10 mM NaCl as the feed dilute solution to affirm that inducing water transport would allow CP in the stack to be reduced and raise the overall amount of salts recovered in the concentrate chamber. 10 mM NaCl solution was prepared in the laboratory along with other configurations concentrate solutions, which consisted of 10 mM NaCl, 100 mM NaCl and 300 mM NaCl. In investigating the effects of natural organic matter (NOM) in the concentrate solution, 200 mM glucose and 600 mM glucose were also used in tandem with 10 mM NaCl in the concentrate solutions. Through this study, it was found that when comparing to the baseline of 10 mM NaCl in the dilute solution paired with 10 mM NaCl solution in the concentrate solution (where little water transport occurs), the other configurations of 10 mM NaCl with 100 mM NaCl and 10 mM NaCl with 300 mM NaCl yielded greater amounts of water flux from the dilute solution to the concentrate solution. This in turn allowed for increased desalting concentrations in the dilute solution, thereby confirming that the increase in water transport had facilitated salt removal from the dilute solution at lower concentrations as the water transport increased. This in turn produces a higher purity of water recovered in the dilute solution. Moreover, by analyzing the sudden voltage changes with respect to time in the dilute solution, it was found that an increase in water transport in the dilute solution coincided with a delay in the sudden spike of the voltage change with respect to time in the ED stack, which means that the limiting current density of the ED stack had increased when greater water transport was facilitated. Hence, the concentration of ions on the membrane surface in the dilute solution will not deplete as quickly, showcasing that CP was reduced as water transport increased from the dilute to the concentrate solutions, proving that water transport can be used to mitigate CP in ED. However, when the 10 mM NaCl was paired with the 2 configurations of NOM, the water transport observed was significantly less compared to the previous setup, albeit being greater than baseline, suggesting that NOM may have reduced the number of pores available for water and salt transport due to pore blockage.