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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...

An attempt to reveal the mechanisms of scale inhibition with the use of two different fluorescent-tagged antiscalants at once is undertaken. To reach the goal, a novel 1,8-naphthalimide-tagged polyacrylate (PAA-F2) is synthesized and tested separately and jointly with 1,8-naphthalimide-tagged bisphosphonate (HEDP-F) as a gypsum scale inhibitor within the frames of NACE Standard TM0374-2007. Here, it is found that at a dosage of 10 mg·dm−3 it provides a much higher inhibition efficiency (96%) than HEDP-F (32%). A PAA-F2 and HEDP-F blend (1:1 mass) has an intermediate efficacy (66%) and exhibits no synergism relative to its individual components. The visualization of PAA-F2 revealed a paradoxical effect: an antiscalant causes modification of the CaSO4·2H2O crystals habit, but does not interact with them, forming particles of its own solid complex [Ca-PAA-F2]. This paradox is interpreted in terms of the “nano/microdust” concept, prioritizing the bulk heterogeneous nucleation step, while an ability of the scale inhibitor to block the nucleus growth at the next steps is proven to be of secondary importance. At the same time, HEDP-F does not change the gypsum crystals morphology, although this antiscalant is completely located on the surface of the scale phase. The PAA-F2 and HEDP-F blend revealed an accumulation of both antiscalants in their own [Ca-PAA-F2/Ca-HEDP-F] phase with some traces of HEDP-F and PAA-F2 on the CaSO4·2H2O crystals surface. Thus, the visualization of two different antiscalants separately and jointly applied to gypsum deposition demonstrates differences in phosphonic and polymeric inhibitors location, and a lack of causal relationship between antiscalant efficiency and scale particle habit modification. Finally, it is shown that the confocal microscopy of several fluorescent antiscalant blends is capable of providing unique information on their interrelationships during scale deposition.

Описан новый подход к созданию технологических схем утилизации концентрата установок обратного осмоса с минимальным расходом воды на собственные нужды. Схема включает обработку воды в две ступени. Проведены экспериментальные исследования по определению технологических характеристик мембранных установок (выход фильтрата, интенсивность образования осадков на мембранах). На основании результатов экспериментов получены значения приведенных затрат на очистку подземных вод различного химического состава. Исследования проводились на лабораторных стендах с использованием нанофильтрационных мембран, имеющих разные значения селективности. Расход сервисных реагентов и эксплуатационные затраты на оборудование рассчитывались по программе, ранее разработанной авторами для определения технологических характеристик мембранных установок. При разработке установок следует отдавать предпочтение мембранам с низкими значениями селективности, обеспечивать снижение затрат на реагенты и энергопотребления. Получены зависимости скорости роста осадка карбоната кальция от типа мембран и кратности объемного концентрирования исходной воды. Сравнение затрат показывает, что применение мембран даже для случаев обезжелезивания воды оказывается экономичнее известных классических технологий.Experimental investigations have been conducted to determine the main process parameters of membrane units (filtrate yield, the rate of scaling on membrane surface). Basing on the results of the experimental studies the total costs of purification of underground water of various chemical composition were obtained. The studies were conducted on laboratory benches with the use of nanofiltration membranes with various selectivity rates. The consumption of service chemicals and operational costs for the equipment were calculated by the software designed earlier by the authors for determining the process parameters of membrane units. While designing membrane units, nanofiltration membranes with low values of selectivity, power consumption and expenditures for chemicals are preferred. The dependencies of the calcium carbonate scaling rates on membrane types and the multiplicity of volumetric concentration of source water were obtained. A comparison of costs shows that the use of membranes even for cases of water deferrization is more economical than the known traditional technologies.

Introduction. Understanding of crystal growth mechanism enables to develop efficient tools to control scaling and improve the process of treatment using membranes and increasing the amount of filtrate output. This investigation is aimed at studying an antiscalant behaviour in reverse osmosis (RO) process when treating ground water. Experimental dependences of calcium carbonate scaling efficiency on antiscalant dosage were found. Rates of adsorption on crystal surface of scaling deposit and on membrane surfaces were compared. Dependences of rates of inhibitor adsorption on crystal surface versus scaling rates were determined. Inhibitor adsorption on RO membrane surfaces was studied. New approaches to studying crystal growth mechanism in the presence of polymeric inhibitors are presented. Materials and methods. In the course of experiments conducted with using inhibitor dissolved in distilled water, inhibitor sorption on membrane surface was observed in the absence of calcium ions. As to experiments with dosing the inhibitor in tap water, the inhibitor sorption on the membrane did not occur: the inhibitor was adsorbed on the surface of the scaling crystals. Results. Experimental relationships are obtained that show dependencies of calcium carbonate deposit growth rates versus RO facility filtrate output values in the presence of different antiscalants with their dose values of 3, 5 and 7 mg/l. The article shows that antiscalant dose value does not provide substantial influence on antiscalant efficiency when natural water with low hardness is treated in the RO facility. This permits substantial reduction of operational costs. It was also proved that inhibitor is not adsorbed on membrane surface during natural water treatment that also confirms efficiency of low antiscalant dosing. Conclusions. Low hardness values of natural water (3–5 mill equivalents per liter) demonstrate that antiscalant efficiencies do not depend on its dose. Rate of inhibitor adsorption on crystal surface during calcium carbonate deposition also increases with scaling rate increase. Rates of antiscalant consumption increase with antiscalant dose values. In natural water the dissolved antiscalant molecules are bonded with calcium ions therefore antiscalant does not react with membranes and is not adsorbed on membrane surface.

Knowledge of the scaling mechanism makes it possible to develop effective means to control scaling and improve the membrane performance, increasing the recovery. This paper presents new approaches to the study of the mechanism of scaling in the presence of polymeric inhibitors (antiscalants); the adsorption of antiscalant molecules on the crystal and membrane surfaces has been investigated. The relations of the antiscalant adsorption rates to the antiscalant dose and the calcium carbonate scaling rate have been revealed. For the first time, the inhibition process was "visualized" by using a fluorescent antiscalant containing a fluorescent moiety-a copolymer of N-allyl-4-methoxy-1,8-naphthalamide and acrylic acid (PAA-F1). The examination of the surfaces of crystals and membranes by scanning electron and fluorescence microscopy showed new unexpected results: the antiscalant is adsorbed on the membrane surface and on the surface of calcite crystals formed. Fluorescence turned out to be more intense and noticeable on the surface of crystal faces than inside the crystal. During the nucleation phase, the "dark" part of the crystal lattice is formed and then begins to be covered with a "luminous" layer of the fluorescent antiscalant, which blocks further crystal growth. During experiments with antiscalant solutions in distilled water, the antiscalant was found to be adsorbed on the membrane surface in the absence of calcium ions. In experiments in which the antiscalant was added into the original tap water, it was adsorbed on the surface of the resulting crystals, not on the membrane. The visualization of the crystal growth inhibition process opens up new possibilities for studying the mechanism of scaling and developing of new technologies to control scaling.

Scaling of sparingly soluble salts could be recognized as a main factor that limits wide application of reverse osmosis (RO) membrane facilities in drinking water production and industrial water recycling. The report demonstrates a new approach to evaluate scaling rates and antiscalant behavior in commercial membrane spiral wound modules through the use of the fluorescence-tagged antiscalants and laser scanning confocal microscope (LSM) observations. Throughout the conducted study the “visualization” of scale inhibitors behavior appeared to be a very promising and universal tool for their activity understanding. Examination of membrane surface and calcite crystals in autopsied membrane modules demonstrated new unexpected results: antiscalant adsorbed either on membrane surface or on crystal surface. In the presence of calcium ions during ground water treatment antiscalant was adsorbed only on crystal surface and sorption on membrane was not detected. Fluorescence was more intensive on the surface and on the outer edges of crystal surface than inside crystal. To investigate antiscalant adsorption to membrane surface, experiments with distilled water containing antiscalant were performed. Intensive sorption of fluorescent inhibitor molecules to membrane was observed.

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