Fluorescent-tagged antiscalants as a tool of homogeneous membrane scaling mitigation and inhibition mechanism investigation in electrodialysis processing of solutions prone to calcium sulfate precipitation

The application of antiscalants (ASs) is a widely used method for scale inhibition in various water treatment technologies, being economically sound and rather simple in terms of technical implementation. However, despite its advantages, this method has not found wide application in electrodialysis. This study focuses on reducing the scaling of homogeneous cation-exchange membrane by using fluorescent antiscalants in the electrodialysis processing of solutions prone to calcium sulfate precipitation. Two novel antiscalants are tested: 1,8-naphthalimide-based bisphosphonate (HEDP-F) and fluorescein-tagged polyacrylate (PAA-F2). It is shown that the studied antiscalants themselves do not cause any significant change in the transport properties of the cation-exchange membrane CSE (Astom, Japan) under the conditions of an applied electric current. Chronopotentiometry and scanning electron microscopy results reveal significant antiscaling effectiveness for both scale inhibitors at sub-stoichiometric concentrations. PAA-F2 is found to be more efficient than HEDP-F, completely preventing any deposition on the membrane surface during the three-hour electrodialysis experiment. Fluorescence microscopy reveals that under the conditions of an applied electric field, HEDP-F forms a distinct phase and does not interact with the deposit structure, as it was detected previously in absence of electric current. Apparently, the mechanism of its activity primarily involves blocking impurities that function as crystallization centers of precipitate. It is suggested that PAA-F2 blocks solid impurities more efficiently than HEDP-F, since it is a polymeric AS.