**4. Conclusions**

Visualization of fluorescent-tagged antiscalants provides a deeper insight of antiscaling mechanisms in reverse osmosis facility desalination. A case study of gypsum scale formation revealed a nonconventional mechanism of antiscalant efficacy. Scaling supposedly takes place in the bulk retentate phase via heterogeneous nucleation step. The "nanodust" particles 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 are reduced, and in turn, the overall scaling rate is diminished.

At the same time, significant amounts of work is still necessary, in order to understand how much the case of gypsum is a universal one. Therefore, a study of fluorescent-tagged polyacrylates and phosphonates in RO desalination of carbonate brines is underway now.

**Author Contributions:** Conceptualization, K.P.; methodology; writing—original draft, supervision, V.G.; Investigation (all RO membrane operations); Methodology; Writing—Original Draft (Figures preparation); M.O., Investigation (SEM; calcium concentration measurements, water ICP analysis); writing—review and editing; resources (PAA-F1 synthesis); S.T., Investigation (fluorescence intensity measurements), writing—reviewing and Editing; A.R. (Anatoly Redchuk), Investigation (particle counter measurements; polymer conformation modeling); A.R. (Anastasia Ryabova): Investigation (confocal fluorescent microscope measurements); A.P.: Conceptualization, Supervision, Writing—Review & Editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to thank Russian Science Foundation, Project No. 19-79-10220 for the financial support of the present study.

**Acknowledgments:** This work was carried out with the using equipment of the Center for collective use No. 74834 "Technological and diagnostic center for the production, research and certification of micro and nanostructures" in GPI RAS.

**Conflicts of Interest:** The authors declare no conflict of interest.
