Improving cathode cleaning and current efficiency by regulating loose scale deposition in scale inhibitor-containing water

Abstract

For electrochemical water softening in practical water containing scale inhibitor, unsatisfactory cathodic cleaning and low hardness removal effect greatly limit its wide large-scale application. An effective improvement is not only dependent on the performance of external mechanical cleaning, but also closely related to the scale structure itself. In this work, the deposition of loose scale layer in the presence of scale inhibitor and its promotion on current efficiency and cathode cleaning efficiency were investigated from interfacial electrochemical perspective. Results reveal that such scale formation is attributed to the relatively high enrichment degree of scaling ions at cathode-water interface. At a high current density or low flow velocity, a relatively high interfacial pH is obtained and the thickness of this alkaline interface is larger than that at lower current density or higher flow velocity. This leads to the increase of CO32− generated by the reaction of HCO3− with more OH−. Besides, due to the weak chelation ability of the scale inhibitor at the alkaline interface, more Ca2+ are released and interact with cathodically formed CO32–. Corresponding scaling driving force is higher than that at lower current density or higher flow velocity. This leads to the deposition of loose scale layer with less stable morphology and a higher specific surface area. This as-deposited scale layer not only acting as seeds accelerates the crystallization and enhances the current efficiency, but also acting as an fragile pre-deposition layer facilitates the cleaning of cathode and produces the highest cleaning efficiency over 95%. This is beneficial for the stable operation of hardness removal device.