Electrochemical water softening: influence of water composition on the precipitation behaviour

Abstract

Hardness in membrane concentrates implies lower water recoveries in desalination. Electrochemical water softening is considered as a possible solution to control this hardness. This technique is based on the electrochemically forced precipitation of insoluble compounds of hardness ions onto the cathode surface. In this work we studied, in practical conditions, the effect of water composition on the technique efficiency and on the properties of the electroprecipitates. Experiments were conducted with a filter-press reactor and a 3D stainless steel wool cathode and for waters of a fixed hardness but varying the following composition parameters between them: alkalinity, Mg/Ca relationship and presence of an anti-scalant compound. Precipitates were fully characterised by SEM, XRD, TG-DTA and N2 adsorption. Efficiency was measured in terms of hardness removal, demonstrating that the technique removes Ca more efficiently than Mg and that the presence of an anti-scalant does not alter the results. The properties of the electroprecipitates (morphology, texture, porosity and components) were related to water composition, evidencing its relevant effect. Geochemical modeling was realised to explain the electroprecipitation behaviour and why calcite, aragonite, vaterite and brucite are the only phases detected. This information contributes to the technique development, necessary for a future application in desalination practice.