Abstract
A hybrid nanofiltration-electrodialysis process was developed and tested for highly-selective separation of MgSO4 from seawater. The magnesium-rich solution was produced primarily for replenishing Mg2+ into desalinated water within the post-treatment step in seawater desalination plants. The new three-step process consists of: (1) seawater nanofiltration (NF) for generation of a magnesium-rich retentate; (2) a Dia-NF step in which the NF retentate is used as feed, which is aimed at minimizing unwanted solutes (primarily B, Na+ and Cl−); and (3) a selective electrodialysis (ED) step on the retentate from step 2, aimed at further lowering the Cl−:Mg2+ mass ratio to below 0.2, to produce a high-purity MgSO4-rich product solution. Results are presented with an emphasis on the ED step, for which the effects of flow velocity, current density and two operation modes (constant-current and constant-potential) were assessed. The results show a highly-dependent selectivity of the monovalent-selective ion-exchange membrane to the cell potential, suggesting that it can serve as a fine-tuning tool for selective ED processes. The cost of adding 20 mgMg/L to one m3 of desalinated water was estimated at ~$0.014/m3, which is competitive compared to other separation techniques, and certainly versus the alternative of direct dosage of food-grade magnesium salts.
Original language | English |
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Article number | 114357 |
Journal | Desalination |
Volume | 481 |
DOIs | |
State | Published - 1 May 2020 |
Keywords
- Desalinated water
- Electrodialysis
- Ion exchange membranes
- Mg replenishment
ASJC Scopus subject areas
- General Chemistry
- General Chemical Engineering
- General Materials Science
- Water Science and Technology
- Mechanical Engineering