Application of fluorescent-tagged antiscalants for mitigation of membrane scaling by calcium carbonate and calcium phosphate in electrodialysis stack

Wide application of electrodialysis, which is proven to be highly effective in waste waters treatment and in a broad spectrum of other technologies, is restrained by membrane fouling.The present study is dedicated to the application of fluorescent-tagged antiscalants (ASs) for mitigation of membrane scaling by calcium carbonate and calcium phosphate in the electrodialysis stack within a set of model experiments.Hydroxyethylidenebis(phosphonic acid), conjugated with naphthalimide (HEDP-F), and two fluoresceine-tagged polyacrylates (PAA-F2 and PAA-F2S) are tested.It is demonstrated that all three ASs are highly efficient in CaCO3 scaling retardation: PAA-F2S>PAA-F2>HEDP-F.The similar efficiency is found for the case of CaHPO4•2H2O: PAA-F2S>PAA-F2 >>HEDP-F.At the same time, PAA-F2S and PAA-F2 affect the crystal habit of both CaCO3 and CaHPO4•2H2O, while HEDP-F does not.However, due to the visualization of antiscalants it is found that there is no direct relevance between scale inhibition efficacy and an ability of ASs to affect the CaCO3 crystal growth kinetics and the crystal form of a deposit.The major scaling inhibition effect is detected at the nucleation step: an increase of induction time.However, no definite decrease of the crystal growth rate at the stages following nucleation is registered relative to the blank run.Meanwhile, most of AS molecules are found to stay in aqueous phase till the end of scale formation and only then do they begin to get adsorbed by the deposit.Notably, in the case of CaCO3, PAA-F2S and PAA-F2 reveal selective sorption on vaterite, a bit less intensive sorption on flake-like modification of calcite and the least sorption on rhomboherdal modification of calcite surfaces.In the same way, PAA-F2S and PAA-F2 demonstrate location on plate-like modifications of CaHPO4•2H2O, but reveal no presence on the pellet-like modifications of this mineral.It is shown that there is no direct correlation between location of ASs and crystal growth rate.The observed effects are interpreted in terms of AS interactions with natural nanoimpurities acting as crystallization centers at the nucleation step.