Gypsum Crystallization during Reverse Osmosis Desalination of Water with High Sulfate Content in Presence of a Novel Fluorescent-Tagged Polyacrylate

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.However, in spite of numerous relevant studies, some controversy regarding both the dominant scaling mechanism in particular situations and the mechanism of antiscalant activity still exists [12][13][14][15][16][17][18]. Recent reviews on scale formation control in RO technologies [6,19] mention two main hypothetic mechanisms of inhibition: (i) antiscalant molecules adsorb on the active growth sites at the crystal surface of sparingly soluble inorganic salt and retard nucleation and crystal growth by distorting its crystal structure; (ii) antiscalant molecules provide similar electrostatic charge, and thus, repulsion between particles prevents them from agglomeration.Nevertheless, our recent static [20,21] and RO [22] experiments operating gypsum as a model scale in presence of a novel fluorescent-tagged bisphosphonate antiscalant 1-hydroxy-7-(6-methoxy-1,3dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyl-bis(phosphonic acid), HEDP-F (H 4 hedp-F) revealed a paradoxical effect: an antiscalant does not interact with gypsum at all, but provides nevertheless retardation of corresponding deposit formation. According to the classical crystallization theory [23], this is possible only in the case, when gypsum passes bulk heterogeneous nucleation, and exactly the "nanodust" plays the role of the solid phase template. Indeed, it is demonstrated that HEDP-F molecules being immersed into the stock solution (undersaturated against gypsum) occupy a significant part of "nanodust" crystallization centers and form there their own solid phase Ca 2 hedp-F·nH 2 O. However, polyacrylates are much less sensitive to calcium environment than phosphonates [20,21]. In this way, it was reasonable to study the trace...

Synthesis and Visualization of a Novel Fluorescent-Tagged Polymeric Antiscalant during Gypsum Crystallization in Combination with Bisphosphonate Fluorophore

“…Meanwhile, the recent reports on the fluorescent-tagged antiscalants [9][10][11][12] exhibit clearly a unique ability of these reagents to put light onto the mechanisms of scale inhibition. Indeed, already the first results of fluorescent antiscalants visualization during gypsum scale formation exceeded all expectations.…”

“…Our group managed to localize the antiscalant, and demonstrated, that both in static and dynamic experiments it is found far from the expected positions. Thus, a nonconventional mechanism of scale inhibition was proposed [9,10,12]. In this relevance, it was reasonable to try the same approach operating a blend of two different fluorescent antiscalants.…”

“…The gypsum scale was taken as a model of a sparingly soluble salt due to (i) its importance for the RO and other water treatment technologies [13,14]; (ii) absence of pH dependencies; (iii) its easily detectable crystal shapes; and (iv) the nucleation of gypsum having been investigated extensively [15][16][17][18][19]. HEDP-F was deliberately chosen for blending with PAA-F2 for two reasons: (i) It represents a different class of antiscalants (phosphonates); and (ii) its visualization during gypsum crystals growth was already studied [9,10,12]. As far as we know, this is the first attempt worldwide to visualize both components of antiscalants blend within the scale formation process.…”

A Review of Green Scale Inhibitors: Process, Types, Mechanism and Properties

“…However, the mechanism of CaCO 3 and CaSO 4 inhibition by the chemical process for the long-term scale inhibition application despite everything remains a matter of further consideration [83]. Very recently, Oschepkov et al proposed a mechanism of scale inhibition on CaSO 4 to locate and understand the presence of fluorescent tag scale inhibitors using a visualization study [84,85]. To better understand the new mechanism into the system of the scale formation and inhibition, a fluorescent-labeled bisphosphonate scale inhibitor 1-hydroxy-7-(6-methoxy-1,3-dioxo-1H-benzo[de] isoquinolin-2(3H)-yl)heptane-1,1-diyl-di(phosphonic corrosive), (HEDP-F) was synthesized and observed under a fluorescent microscope for CaSO 4 inhibition in a supersaturated aqueous solution [84].…”

“…The insoluble particles do not stockpile yet remains as small strong particles dispersed in the fluid phase, and develop without noticeable sorption of bisphosphonate on the crystal edges or some other crystal development centers; subsequently, obstruct the CaSO 4 crystals dynamic development sites, and retard the formation of scale [87]. Later, the same research group studied a scale inhibition efficiency and fluorescent properties of another newly developed fluorescent antiscalant-1, 8-naphthalimide-tagged polyacrylate (PAA-F1) [85]. A similar outcome was acquired for PAA-F1 that was realized in the presence of HEDP-F for CaSO 4 RO desalination process [88], batch static tests with CaSO 4 [84] and barite [89], and supports the proposed mechanism.…”