Visualization of a novel fluorescent-tagged bisphosphonate behavior during reverse osmosis desalination of water with high sulfate content

Abstract

Abstract Gypsum scaling during reverse osmosis desalination process is studied in presence of a novel fluorescent-tagged bisphphosphonate antiscalant: 1-hydroxy-7-(6-methoxy-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyldi(phosphonic acid), HEDP-F, by fluorescent microscopy, scanning electron microscopy (SEM) and a particle counter technique. A case study of gypsum scale formation revealed a nonconventional mechanism of antiscalant efficacy. It is found that scaling takes place in the bulk retentate phase via heterogeneous nucleation step. Herewith, the background solid particles (“nanodust”) play a key role as gypsum nucleation centers. It is demonstrated that contrary to popular belief, an antiscalant interacts not with gypsum nuclei, but with “nanodust” particles, isolating them from calcium and sulfate ions sorption. Therefore, the number of gypsum nucleation centers gets reduced, and in turn an overall scaling rate becomes diminished. It is demonstrated that the fluorescent-tagged antiscalants may become a very powerful tool in membranes scaling inhibition studies. It is also shown, that the scale formation scenario changes from the bulk medium in the beginning to the sediment crystals growth on the membrane surface at the end of desalination process.