Multistep mineral fouling growth on a cation-exchange membrane ruled by gradual sieving effects of magnesium and carbonate ions and its delay by pulsed modes of electrodialysis

The aim of this study was to reveal the mechanisms ruling a fouling growth on both sides of a CMX-SB cation-exchange membrane (CEM), run after run during three consecutive electrodialysis (ED) treatments. A model solution containing a high magnesium/calcium ratio (2/5) was demineralized under two different pulsed electric field (PEF) on-duty ratios and dc current. The results showed a series of mechanisms ruling a multilayer mineral fouling growth and its delay by PEFs. The nature of the fouling layer, during a first run, depended on the diluate pH-value evolutions and the ion migration rates through the membrane. A subsequent multilayer fouling growth during consecutive treatments was ruled by the already formed mineral layers, where gradual sieving effects inverted the migration rates and led to a multistep crystal growth. Calcium carbonate grew on the diluate side of CEM, starting from its amorphous phase to then crystallize in a coexisting presence of aragonite and calcite. Amorphous magnesium hydroxide appeared on CEM apparently through fouling dehydration ruled by the mineral layers themselves and by overlimiting current regimes. A delayed fouling growth was observed for PEF ratio 0.3. A long pause lapse during pulse modes was demonstrated as an important parameter for fouling mitigation.

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► dc current leads to severe membrane fouling by calcium and magnesium salts.
► Pulsed electric field (PEF) modes of electrodialysis delay mineral fouling growth.
► Primary fouling growth depends on the diluate pH evolutions and ion migration rates.
► Sieving effects rule subsequent fouling growth and are triggered by fouling layers.
► Inverted migration rates result in marked multistep fouling crystallizations.