Abstract
Gypsum scaling in reverse osmosis (RO) desalination process is studied in presence of a novel fluorescent 1,8-naphthalimide-tagged polyacrylate (PAA-F1) by fluorescent microscopy, scanning electron microscopy (SEM), dynamic light scattering (DLS) and a particle counter technique. A comparison of PAA-F1 with a previously reported fluorescent bisphosphonate HEDP-F revealed a better PAA-F1 efficacy, and a similar behavior of polyacrylate and bisphosphonate inhibitors under the same RO experimental conditions. Despite expectations, PAA-F1 does not interact with gypsum. For both reagents, it is found that scaling takes place in the bulk retentate phase via heterogeneous nucleation step. The background “nanodust” plays a key role as a gypsum nucleation center. Contrary to popular belief, an antiscalant interacts with “nanodust” particles, isolating them from calcium and sulfate ions sorption. Therefore, the number of gypsum nucleation centers is reduced, and in turn, the overall scaling rate is diminished. 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 the desalination process. It is demonstrated that the fluorescent-tagged antiscalants may become very powerful tools in membrane scaling inhibition studies.
Highlights
Reverse osmosis (RO) is becoming recently a powerful technology for the purification of sea, brackish and waste water [1,2,3,4]
All experiments were monitored in a liquid phase along the retentate saturation and were followed by a final scanning electron microscopy (SEM) analysis of solid membrane surface after each run
Location of PAA-F1 on membrane might provide an isolation of potential gypsum crystallization centers there
Summary
Reverse osmosis (RO) is becoming recently a powerful technology for the purification of sea, brackish and waste water [1,2,3,4]. Model water sample, with high sulfate content, in the presence of a fluorescent antiscalant 1,8-naphthalimide-tagged polyacrylate, PAA-F1, Figure 1. Antiscalant PAA-F1 (Figure 1) was synthesized by our group as described elsewhere [36] along with its scale inhibition efficiency against gypsum scaling and its fluorescent properties It has the mean molecular mass 4000 Da with c.a. 1% mass of 1,8-naphthalimide moiety. PAA-F1 concentration bearing in mind that the mean molecular mass is 4000 Da) in gypsum scaling experiment, and equilibrated there no less than 30 min It is well-known that the heterogeneous nucleation, in the presence of such solid impurities, as clay minerals or other foreign particles, is characterized by a lower free energy barrier than the homogeneous one [23].
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