Application and Visualization of Fluorescent-Tagged Antiscalants in Electrodialysis Processing of Aqueous Solutions Prone to Gypsum Scale Deposition

Membrane scaling is a serious problem in electrodialysis. A widely used technique for controlling scale deposition in water treatment technologies is the application of antiscalants (AS). The present study reports on gypsum scale inhibition in electrodialysis cell by the two novel ASs: fluorescent-tagged bisphosphonate 1-hydroxy-7-(6-methoxy-1,3-dioxo-1Hbenzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyl-bis(phosphonic acid), HEDP-F and fluorescein-tagged polyacrylate, PAA-F2 (molecular mass 4000 Da) monitored by chronopotentiometry and fluorescent microscopy. It was found that cation-exchange membrane MK-40 scaling is sufficiently reduced by both ASs, used in 10(-6) mol center dot dm(-3) concentrations. PAA-F2 at these concentrations was found to be more efficient than HEDP-F. At the same time, PAA-F2 reveals gypsum crystals' habit modification, while HEDP-F does not noticeably affect the crystal form of the deposit. The strong auto-luminescence of MK-40 hampers visualization of both PAA-F2 and HEDP-F on the membrane surface. Nevertheless, PAA-F2 is proved to localize partly on the surface of gypsum crystals as a molecular adsorption layer, and to change their crystal habit. Crystal surface coverage by PAA-F2 appears to be nonuniform. Alternatively, HEDP-F localizes on the surface of a deposit tentatively in the form of [Ca-HEDP-F]. The proposed mechanisms of action are formulated and discussed. The application of antiscalants in electrodialysis for membrane scaling mitigation is demonstrated to be very promising.

Automated summary

This study investigates the inhibition of gypsum scale in electrodialysis cell using fluorescent-tagged antiscalants. It is found that a polyacrylate with a molecular mass of 4000 Da, used at a concentration of 10(-6) mol center dot dm(-3), is more effective than a bisphosphonate in reducing membrane scaling and also modifies the crystal habit of gypsum. The application of antiscalants in electrodialysis for membrane scaling mitigation is shown to be very promising.