**1. Introduction**

Reverse osmosis (RO) is becoming recently a powerful technology for the purification of sea, brackish and waste water [1–4]. However, one of the major limitations in efficient RO application is the membrane scaling [5–7]. Inorganic scaling, occurs when the solubility limits are exceeded. The most common scales are represented by calcium carbonate, calcium sulfate and silica [6]. As a result of inorganic fouling, the operation cost of an RO plant increases due to higher consumption of energy and expenses of membrane cleaning. The most common method in mitigating scaling in RO facilities is an application of antiscalants. Among these, polycarboxylates (polyacrylates, polyaspartates, etc.) and phosphonates are found to be highly efficient [6–11].

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 [12–18]. 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-1,3 dioxo-1*H*-benzo[de]isoquinolin-2(*3H*)-yl)heptane-1,1-diyl-bis(phosphonic acid), HEDP-F (H4hedp-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 Ca2hedp-F·nH2O. However, polyacrylates are much less sensitive to calcium environment than phosphonates [20,21]. In this way, it was reasonable to study the traceability of phosphorus-free fluorescent polymeric antiscalants in RO process.

The present study is focused on the scale inhibitor visualization during RO treatment of model water sample, with high sulfate content, in the presence of a fluorescent antiscalant- 1,8-naphthalimide-tagged polyacrylate, PAA-F1, Figure 1.

**Figure 1.** 1,8-Naphthalimide-tagged polyacrylate molecular structure.

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 [10–16]; (ii) its poor dependence on pH; (iii) its easily detectable crystal shapes; and (iv) the nucleation of gypsum has been investigated extensively in the past [10,11,14,16–18,24–33]. On the other hand PAA-F1 is expected to be a better antiscalant for CaSO4·2H2O deposits relative to HEDP-F. This was demonstrated for the non-fluorescent prototypes 1-hydroxyethane-1,1-bis(phosphonic acid) (HEDP) and polyacrylate (PAA) [34].The fluorescent-tagged polyacrylates have gained increasing interest as the reagents for on line antiscalant concentration monitoring in water treatment applications recently [35]. However, they have not been applied, so far, for scale formation mechanisms studies. As far as we know, this is the first communication on polymer antiscalant visualization in a RO experiment with gypsum scaling.
