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Unravelling pH Changes in Electrochemical Desalination with Capacitive Deionization
Antony C Arulrajan1,2, Jouke E Dykstra1, Albert van der Wal1
1Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands.
Environmental Science & Technology
|September 29, 2021
Summary
Membrane capacitive deionization (MCDI) electrode aging alters pH changes during water desalination. Aged electrodes show non-Faradaic processes dominate pH shifts, impacting long-term stability, especially with tap water.
Area of Science:
- Water treatment technologies
- Electrochemical processes
- Materials science
Background:
- Membrane capacitive deionization (MCDI) uses charged electrodes for water desalination.
- MCDI operation can lead to pH shifts, potentially causing ion precipitation and affecting system longevity.
- pH changes are attributed to Faradaic and non-Faradaic electrochemical phenomena.
Purpose of the Study:
- To investigate how electrode aging affects pH changes during MCDI operation.
- To differentiate between Faradaic and non-Faradaic contributions to pH shifts in aged electrodes.
- To analyze the impact of different feed water compositions (NaCl vs. tap water) on pH behavior.
Main Methods:
- Long-term operation experiments with MCDI systems using NaCl and tap water.
- Monitoring of pH changes in feed and effluent water over time.
- Comparison of experimental results with theoretical models to identify dominant mechanisms.
Main Results:
- Electrode aging significantly alters the magnitude and direction of pH changes during MCDI cycles.
- NaCl solutions showed a pH decrease during regeneration, while tap water showed a pH increase.
- Non-Faradaic processes were identified as the primary cause of pH changes in aged electrodes.
- Adsorption/desorption of bicarbonate and carbonate ions influenced pH changes during tap water desalination.
Conclusions:
- Electrode aging in MCDI leads to a shift in pH change mechanisms, with non-Faradaic processes becoming dominant.
- Tap water desalination with aged electrodes poses a risk of hardness ion precipitation due to pH increase.
- Understanding these pH dynamics is crucial for improving the long-term stability and efficiency of MCDI systems.
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