CONTROLLING SILICA IN WATER TREATMENT …...silica scale in water treatment applications include...

The International Desalination Association World Congress on Desalination and Water Reuse 2013 / Tianjin, China REF: IDAWC/TIAN13-103

CONTROLLING SILICA IN WATER TREATMENT APPLICATIONS Authors: A. Kempter, T. Gaedt, V. Boyko, S. Nied, M. Kley, K. Huber Presenter: Dr. Andreas Kempter Research Scientist – BASF SE – Germany [email protected] Abstract The development of the first reverse osmosis (RO) membrane systems in the 1950’s by Hassler, Reid and Breton [1] marks the cornerstone for todays membrane desalination technology. Since then, desalination and especially membrane technologies started to evolve rapidly with significant market growth especially in the last 20 years. With an installed capacity of 39.7 million m3 / d in 2011, RO accounts for 60% of the overall installed desalination capacities worldwide and thus is the most important technology for water purification. [2] However, treatment of feed water containing high levels of silica still remains a challenge for plant operators and water technologists. Although less common compared to biofouling or fouling of mineral salts (like carbonates or phosphates of calcium or magnesium), deposits of silica or metal silicates significantly reduces system performance and cause unexpected shutdowns and costly chemical or mechanical cleaning operations. The solution chemistry of silica and metal silicate is complex and still not fully understood. It is determined by a complex interplay between hydrolysis and condensation which strongly depends on the pH of the solution [3]: at neutral pH of 7, silica scaling is believed to occur via monomeric silica polymerization (precipitation fouling) and colloidal silica deposition (particulate fouling), whereas at higher pH the solubility of silica is indeed increased, however the risk of forming magnesium silicate (at pH > 8.5 and high levels of Mg2+ ions) or other deposits like CaCO3 becomes predominant. Additionally, parameters like the pH value, the temperature and the solution composition, e.g. presence of hardness ions (calcium or magnesium) or polyvalent ions like iron or aluminum, influence silica polymerization kinetics and the morphology of the formed material. [4] General concepts for controlling silica scale in water treatment applications include inhibition of particle formation and/or dispersion, realized by polymeric scale inhibitors (or anti-scalants) that are added to the feed water stream in very small (parts per million) quantities. Inhibition stops the formation of scale forming precipitates, whereas dispersion prevents the attachment of scale particles on surfaces. To investigate the influence of several parameters on the silica polymerization, different test methods including static and dynamic light scattering as well as soluble silica determination experiments were employed. Novel aspects of the formation of silica in aqueous solutions are presented. The influence of supersaturation levels as well as the presence of hardness ions are discussed. Additionally, the effects of different polymeric additives are investigated and a new surface adsorption test method is introduced.

The International Desalination Association (IDA) World Congress on Desalination and Water Reuse

REF: IDAWC/TIAN13-103 -- 2 --

I. INTRODUCTION The development of the first reverse osmosis (RO) membrane systems in the 1950’s by Hassler, Reid and Breton [1] marks the cornerstone for today’s membrane desalination technology. Since then, desalination and especially membrane technologies started to evolve rapidly with significant market growth especially in the last 20 years. With an installed capacity of 44.5 million m3 / d in 2011, RO accounts for 63% of the overall installed desalination capacities worldwide and thus is the most important technology for water purification [2]. However, membrane fouling is a tremendous problem all membrane separation processes have to face. Especially treatment of feed water containing high levels of silica still remains a challenge for plant operators and water technologists. Although less common compared to biofouling or fouling of mineral salts (like carbonates or phosphates of calcium or magnesium), deposits of silica or metal silicates significantly reduces system performance and membrane flux decline and cause unexpected shutdowns and costly chemical or mechanical cleaning operations. The solution chemistry of silica and metal silicate is complex and still not fully understood. It is determined by a complex interplay between hydrolysis and condensation which strongly depends on the pH of the solution [3]: at neutral pH of 7, silica scaling is believed to occur via monomeric silica polymerization (precipitation fouling) and colloidal silica deposition (particulate fouling), whereas at higher pH the solubility of silica is indeed increased, however the risk of forming magnesium silicate (at pH > 8.5 and high levels of Mg2+ ions) or other deposits like CaCO3 becomes predominant. Additionally, parameters like the pH value, the temperature and the solution composition, e.g. presence of hardness ions (calcium or magnesium) or polyvalent ions like iron or aluminum, greatly influence silica polymerization kinetics and the morphology of the formed material. II. SILICA POLYMERIZATION WITH VARIOUS SUPERSATURATION LEVELS AND pH We recently have shown [4] that the amount of soluble silica which was determined after a certain time in aqueous solution containing calcium and magnesium ions, is depending on the initial amount of silica or the silica supersaturation in such a solution. We concluded, that under the presented test conditions (500 ppm or 2.2 fold supersaturation of silica, 4 mmol/l Ca2+ (160 ppm) and 1 mmol/l Mg2+ (24 ppm) (28° german hardness) and 40°C, there is a kinetically meta-stable region at pH 7 up to an initial silica level of ~400 ppm. We became interested in gaining more detailed insight into the silica polymerization process and thus performed time-resolved measurements of soluble silica in solutions with different initial silica contents using time-resolved static and dynamic light scattering. Figure 1 shows the particle size development of a sample containing 750 ppm of initial silica under hard water conditions (4 mmol/l Ca2+,1 mmol/l Mg2+ (24 ppm)) as well as the determination of soluble silica with the silico-molybdato test. It becomes obvious, that silica polymerization starts immediately at the beginning of the test which is reflected in a rapid decrease of soluble silica content in the solution. After 4h of reaction time it reaches 250 ppm, a value already very close to the thermodynamic solubility limit. Simultaneously, a continuous growth of the particles can be observed. These results indicate that directly at the beginning of the experiment, particle formation (i.e. nucleation) occurs at 750 ppm initial silica level. The thus formed particles then start to grow over time. This growth can be caused by addition of further monomers to already existing particles or by agglomeration of individual particles present in the solution. The particle size keeps increasing although the soluble silica content reaches its



thermodynamic solubility limit after 24h reaction time which could be due to an agglomeration mechanism. It should also be noted, that the aforementioned hydrolysis and condensation equilibrium can also be the reason for the particle growth since already formed particles can redissolve or hydrolyze and promote particle growth by delivering new soluble silica species.

Figure 1: Particle size development (red) as well as SiO2 concentration (blue) for 760 ppm initial silica under hard water conditions (4 mmol Ca2+, 1 mmol Mg2+) and 40°C. Particle size was determined with dynamic light scattering. We also investigated the polymerization of silica at different initial silica concentrations. As seen in Figure 2, the scattering intensity detected in the solutions with different amounts of initial silica varies significantly. . At low silica content (300 ppm), there is only very little intensity detectable during a period of 24h, although determination of soluble silica with the silicamolybdato test reveals a loss of ~50 ppm silica. In contrast, when the initial silica content is 350 ppm, there is no intensity increase observed in the first 17h. After 17 h intensity increases which can be attributed to particle formation or growth of particles in the solution. Similar behavior is observed for 400 ppm. The start of particle formation shifts to shorter times starting after ~8h. These results are in line with the soluble silica determination (Table 1) which exhibit a decrease of 90 ppm in soluble silica after 24h. With 500 ppm silica however, there is no significant delay in particle formation. Silica conversion is complete after 24 h and the silica level has reached the solubility limit of 185 ppm. Additionally, particle formation and growth is observed directly after the beginning of the experiment for 750 ppm initial silica until all silica is consumed (up to remaining 185 ppm). It should be noted that the intensity measured is proportional to



the concentration of the particles as well as their mass and that both effects are not easily to distinguish. We therefore assume, that with increasing initial silica concentration (300 to 500 ppm), the concentration of silica nucleii increases (earlier increase in intensity) whereas the resulting mass in the equilibrium state decreases (indicated by the height of the plateau of the intensity curves). For 750 ppm however, fast particle formation as well as strong agglomeration is observed, which is also reflected in floc formation in the beaker experiments.

Figure 2: time resolved light scattering data for various initial silica level: 300 ppm (orange), 350 ppm (green), 400 ppm (blue), 500 ppm (red), 750 ppm (black) These results are currently investigated in more detail since we expect different polymerization mechanisms at different initial silica levels. However, the depletion of the silica in solution, especially at higher initial silica levels, might be a possible explanation for the observed differences in the detected intensities via light scattering. This is subject of ongoing research and will be published in due course. An induction time for the growth process of silica can be estimated. As seen in Figure 3, the induction time decreases with increasing silica concentration, or more precisely increasing supersaturation of silica. For initial silica level of 500 ppm the induction time can be estimated to be around 3h whereas for 750 ppm, there is almost no induction time and silica polymerization immediately starts at the beginning of the experiment.



Table 1: soluble silica determined with the silica-molybdate method for various initial silica level.

Figure 3: estimated induction time of the growth process of the solutions at pH 7 as a function of initial SiO2 level. The solution composition, especially the presence of hardness ions like magnesium or calcium significantly influences the polymerization kinetics and solubility of silica in solution. [5, 6, 7] Thus, Sheikholeslami et al and Koo et al. found, that increasing the concentration of calcium and magnesium also enhanced the polymerization of silica. We also tested the effect of hardness ions on the polymerization rate as well as its influence on the particle size measured via light scattering. As seen in Figure 4, the presence of hardness ions accelerates the polymerization of silica especially for lower initial silica levels. For 750 ppm however, the polymerization rate is already very high, thus the catalytic effect of hardness ions is less pronounced.



Figure 4: time resolved soluble silica determination with the silica-molybdate test method for different initial silica level; left without hardness ions, right: with 4 mmol calcium and 1 mmol magnesium Additionally, we examined the influence of the concentration of hardness ions (calcium and magnesium, Ca:Mg ratio 4:1) on the particle size of formed silica particles from a supersaturated solution with 760 ppm initial silica level (Figure 5). We found, that with increasing calcium and magnesium content also particle size increases as measured after 24h. This might be due to an increased tendency towards agglomeration of silica particles. It is expected that the zeta potential of silica particles decreases in the presence of hardness ions and thereby reduces electrostatic repulsion.

Figure 5: particle size and scattering intensity for an initial silica level of 760 ppm measured after 24h with different total hardness ion concentrations (Ca2+ to Mg2+ ratio 4:1). Particle size was determined with dynamic light scattering.



III. POLYMERIC INHIBITORS Generally, two different concepts can be applied to control silica scaling: inhibition of silica polymerization as well as dispersing precipitated particles, as schematically shown in Scheme 1. Both concepts are realized by dedicated polymeric additives (or anti-scalants), which are commonly used in water treatment applications.

Scheme 1: different attempts to control silica scale in water treatment applications There have been several reports on polymeric additives for silica inhibition: The most prominent class of silica inhibitors are reported to be cationic-based polymers and copolymers [8]. Demadis et al focused mainly on polyaminoamide-based dendrimers as well as polyoxazoline polymers as silica inhibitors for cooling water applications. They investigated the branching of these polymers and found, that higher degree of branching is beneficial for silica inhibition. [9] Other commonly used inhibitor polymers like copolymers and terpolymers with carboxylic acid, sulfonic acid and non-ionic functional groups have been studied by Amjad et al, specifically for silica inhibition for reverse osmosis applications. They found that these polymer types are poor silica inhibitors [8]. We and others [10, 4]) recently studied the inhibitor capacity of PVP and polyethers and found that these polymers can strongly interact with silica and show significant inhibitor performance, depending on molecular weight, comonomer used as well as dosage. Silica Floc formation prevents these polymer class from being useful for membrane applications. Based on the findings of Spinde et al [11], it is most likely that PVP and polyethers are able to stabilize monomeric as well as small oligomeric silica species through short-lived hydrogen bond complexes. We therefore also investigated the behavior of polyethers with light scattering. Figure 6 shows the intensity development over time for a solution containing 400 ppm, 500 ppm and 750 ppm of initial silica without hardness ions and 15 ppm polyether in the test solution. The results for 400 ppm initial silica are similar to the case without additive (induction phase of around 12h, then particle formation and mass increase), for 500 ppm and 750 ppm initial silica however, there is still an immediate particle formation observed and intensity evolution in this case is more pronounced than without polymers. This might be explained by our recent findings that polyether on the one hand are able to stabilize monomeric and small oligomeric silica species, on the other hand however, large particle or floc formation due to aggregation was observed which might be the reason for the increase in intensity. It should be again



noted that the intensity is proportional to both particle concentration (which increases during nucleation continously) and molecular mass of the particles. Therefore an unambiguous interpretation of the higher intensity is not possible based on the light scattering data alone.

Figure 6: time resolved light scattering data for various initial silica level without hardness ions in the presence of 15 ppm poyether. A and B are repetitions of experiments. It can be concluded, that addition of polyethers accelerates particle formation or agglomeration, although the respective test for soluble silica does show, that even after 24h for 500 ppm initial silica there is still a significant amount of soluble silica left (see Table 2). A possible explanation might be, that the silico-molybdate test is able to also detect polyether-stabilized monomeric or dimeric silica and thus, the determined amount of “free” silica is somehow overrated. Table 2: residual soluble silica after 24h in the presence of polyether determined with the silica-molybdate test method

Initial silica Silica after 24h 400 378 500 425 750 180

IV. MEMBRANE FOULING All results presented so far are results obtained from beaker tests and thus describe bulk silica polymerization in solution. Since in a reverse osmosis process, a continuous increase in silica level in the feed is realized until supersaturation is reached, these tests can only give limited information about



what really happens close to a membrane surface. At the membrane surface, the oversaturation is by nature higher than in the bulk solution, so it is most likely that silica polymerization induced by supersaturation will start at the membrane surface or in very close proximity and the formed silica will then settle down on the membrane and a silica film formation via polymerization occurs [12] [13]. As a result, pore blockage and thus membrane flux decline or increased pressure (if operated at constant flux) occurs. We already have presented an easy method to determine qualitative silica adsorption on a membrane surface [4]: We have seen that after 1h of exposure to the silica solution, first few silica particles adsorbed on the membrane surface can be observed. The silica particle density increases significantly until 4h of exposure and a complete coverage of the membrane by particles is observed after 9h. Note that the average particle size remains approximately constant over that time frame. We used this method to investigate the adsorption of silica in the presence of different polymers, namely PVP and polyethers, on membrane surfaces. As seen in Figure 7, after 1h particles are observed at the membrane surface in the presence of polyethers as well as PVP. The density is significantly higher than without any polymer in the solution. After 4h the complete surface is covered with silica precipitate. These results are in good agreement to the findings from light scattering as well as beaker tests, since in the beaker tests floc formation and particle growth was observed even after several minutes.

Figure 7: Atomic force microscopy of polymeric membranes treated with silica solution after different exposure time. Right: without additive, middle: with 50 ppm polyether, left: with 50 ppm PVP homopolymer. These tests are, however, only qualitative in nature since AFM is restricted to a few micrometer of sample membrane surface. Thus, we became interested in a method to quantify silica adsorption. Recently, Lin et al introduced a quartz micro balance system (QCM) to determine the crystallization behavior of calcium on polymer-coated surfaces [14]. They coated QCM sensors with different polymers and evaluated the adsorbed mass of calcium sulfate on different polymeric surfaces to



determine the kinetics of gypsum crystallization. We used this method to evaluate the performance of antiscalants in silica precipitation. Table 1 shows the results: Table 3: mass increase measured with a polyamide-coated QCM sensor after treatment with supersaturated silica solution (510 ppm initial silica, 4 mmol Ca2+, 1 mmol Mg2+, 40°C)

In the case of 250 ppm silica concentration, very low adsorption of silica can be observed in the QCM measurements. This is reasonable, since from silica-molybdato beaker tests it is obvious that under these conditions no silica polymerization occurs and thus it cannot be expected that silica particles adsorbs to a polyamide surface. At 510 ppm initial silica level however, significant increase in mass due to adsorption of silica on the QCM sensor was observed, as already expected from our beaker-tests. Also in line with these results, the addition of polyether leads to floc formation and agglomeration of silica, thus in both cases (250 ppm and 510 ppm initial silica) an increase in mass adsorption in the QCM experiment was detected. Visual inspection of the QCM sensors after the experiment reveals, that the silica adsorbed without polymer does give a homogenous layer on the surface making it hard to detect visually (Figure 8). With polyether however, a colorless precipitate was formed, which strongly adheres to the QCM sensor. The precipitate is silica since dipping the sensor into acidic solution does not dissolve the scale and thus magnesium or calcium salts can be ruled out.

Figure 8: QCM sensors treated with aqueous solutions containing 510 ppm initial silica (left) and 510 ppm initial silica + 50 ppm polyether (right). As discussed, hardness ions like calcium or magnesium significantly influence silica polymerization kinetics. As discussed in the literature, polymeric antiscalants like polyacrylic acids can bind to calcium ions thus reducing the amount of free calcium ions in solution. [15] If so, the kinetic of silica polymerisation should be slowed down if the total concentration of hardness ions is reduced. We thus



tested this idea by using a classical complexing agent (Trilon AS from BASF SE) in QCM test. Additionally, we also examined synergistic effects of complexing agents with a modified polyether. Table 4 summarizes our results. Polyethers alone show increased adsorption on the QCM sensor compared to the test without additive, as discussed earlier. However, the addition of a complexing agent to the test solution significantly delays silica polymerization and only very low adsorption could be detected. We also investigated a modified polyether, which gave much higher adsorption on the sensor, indicating that agglomeration was even more pronounced than in the case of unmodified polyether. However, a 1:1 mixture of this modified polyether and a classical complexing agent also showed no significant adsorption on the QCM sensor. This is reasonable, since hardness ions are removed from the solution by the complexing agent and silica polymerization kinetic is slowed down. However, this is no suitable approach for commercial water treatment applications since dose rates for complexing agents (usually a stoichiometric amount relative to hardness ions is needed) are by far higher than those used in desalination industries when employing threshold inhibitors in substoechiometric ppm amounts. Table 4: effect of polyether and complexing agents on the adsorption of silica in the QCM test

V. CONCLUSIONS We have examined the polymerization kinetics of silica with varying initial silica concentrations in the presence and absence of hardness ions using standard beaker tests, light scattering methods as well as atomic force microscopy and quartz crystal microbalance adsorption tests. From light scattering results we assumed, that with increasing initial silica concentration (300 to 500 ppm), the concentration of silica nuclei increases whereas the resulting particle size in the equilibrium state decreases. We also found that silica polymerization kinetics are accelerated in the presence of hardness ions and further demonstrated, that an induction period occurs for the onset of silica polymerization. Adsorption tests on real membranes have shown that polyethers as well as PVP polymers are no suitable silica antiscalants since flocculation and thus membrane blockage (indicated by increased scale formation observed in AFM analysis) occurs. Additionally, we introduced quartz crystal microbalance measurements to quantify the adsorption of silica on polyamide-coated sensors. This method helps to further distinguish between different polymers and will be used to develop new polymers as silica antiscalants for water treatment applications. We like to thank Günther Kern and Kai Bergner for the experimental work as well as Dr. Bernhard von Vacano and Peter Boeshans for developing and performing the QCM measurements and Dr. Peter Baumann for AFM analytics.

sample additive dossage

mass increase (µg/cm²)

510ppm SiO2 (Referenz) 1,32polyether 50 ppm 3,90complexing agent (Trilon AS) 50 ppm 0,09modified polyether 50 ppm 18,60modified polyether + Trilon AS (1:1) 50 ppm 0,28



VI. EXPERIMENTAL Reagents and instruments: All chemical reagents used were analytical grade and used without further purification. Sodium meta silicate (Na2SiO3 . 9H2O, 98%) was purchased from Sigma-Aldrich, magnesium chloride hexahydrate (MgCl2 . 6H2O) and calcium chloride dihydrat (CaCl2 . 2H2O) came from Merck. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were purchased from Fluka. Polyethers of various molecular weigths were commercial available. Materials and reagents for the silica analytical test were from Sigma Aldrich (Aquanal Professional). The membrane used was a commercial Toray TMC 820 C reverse osmosis membrane. The UV spectra for the determination of soluble silica was a HP 8452A Diode Array Spectrophotometer. Particle size determination was carried out with a Malvern Zetasizer nano. Quartz crystal micro balance measurements were performed using a QCM-D system from qsense. Light scattering (static and dynamic) are performed at a 5000E Kompakt Goniometer of the ALV-Laservertriebsgesellschaft (Langen) or at a home-build instrument at the university of Paderborn. Silica polymerization inhibition test method: The silica inhibition tests were done as followed: Batch tests were carried out to determine the precipitation characteristics at different saturation levels of silica (1.4 to 4.1 fold supersaturation, i.e. 250 ppm to 750 ppm) under hard water conditions (1 mmol/l Mg2+, 4 mmol/l Ca2+, 28° german hardness). All experiments were performed in plastic containers to prevent silica leeching from glassware. The supersaturated solutions were prepared by adding known volumes of sodium silicate stock solution to distilled water. After adjustment of the pH with diluted hydrochloric acid and / or sodium hydroxide solution, known volumes of a Ca2+/Mg2+ stock solution (consisting of a mixture of Ca2+ and Mg2+:in a ratio of 4:1) were added and the pH again adjusted to the desired value. In case of an inhibitor used, a stock solution of the inhibitor polymer was added to reach the desired inhibitor concentration. The resulting solution was then tempered and kept constant at 40°C under moderate stirring for 24h. After filtration of the reaction solution through a 0,22 µm filter, the filtrate was analyzed with the silico-molybdate test (see below). Determination of soluble (reactive) silica: To determine the soluble silica content the silico-molybdato spectrophotometric method was used. This method is based on the principle that ammonium molybdate forms yellow heteropolyacids with reactive silica and any phosphate present in the solution at low pH (~1.2). To prevent color interferences caused by molybdophosphoric acid, oxalic acid is added leaving only the silicomolybdate intact. This method is able to determine soluble silica in the range of 1 to 100 ppm, thus dilution of the test solutions (with silica contents of 200-750 ppm) is necessary. It should also be mentioned, that this method is only able to measure soluble (reactive) silica, which includes not only monomer silicate but also dimers, trimers and small oligomers of which the exact number is not known. The chemicals used for this test are commercial available in prepared mixtures (reagents A, B and C) from Sigma Aldrich. The determination of soluble silica was done according to the standard method. Briefly, some ml of the test solution was filtered through a 20 nm syringe filter. 1 ml of the filtered solution was diluted with 9 ml of distilled water and the molybdate containing reagent A was added.



After complete dissolution, Reagent B (containing amidosulfonic acid) was added and kept for 12 min. Finally, reagent C (contains citric acid) was added to the solution and stored for 2 min up to a maximum of 20 min. From the measured UV/Vis extinction, the respective amount of soluble silica in the test solution was determined. Adsorption tests on membrane surfaces Adsorption tests on membrane surfaces were performed using the same silica-solution than for the inhibition tests. For the experiment, a piece of the membrane was put into the stirred solution for variable amounts of time and kept constant at 40°C. The membrane was then removed, carefully washed with distilled water and analyzed using atomic force microscopy (AFM). Quartz microcrystal balance (QCM) Measurements: QCM sensors were spin-coated with polyamide 6 (commercial available under the brand name Ultramid B24 by BASF SE). Then, the QCM sensors were placed in the measurement cell and supersaturrated silica solution (e.g. 510 ppm or 2.2 fold supersaturation of silica, 4 mmol/l Ca2+ (160 ppm) and 1 mmol/l Mg2+ (24 ppm) (28° german hardness)) was flushed over the sensor at 40°C with a flow rate of 0.25 ml/min. Usual adsorption time was 14h. The frequency changes were used to calculate the amount of adsorbed material on the sensors using standard procedures. Light scattering experiments: A relationship for evaluation of static light scattering (LS) data from dilute solutions is given by

𝐾𝑐𝐼(𝑞)

=1𝑀𝑤

�1 +13

(𝑞2⟨𝑟2⟩𝑧� + 2𝐴2𝑐

In this equation, I(q) is the normalized scattering intensity at the scattering angle, c is the concentration, and q is the magnitude of the scattering vector that is related to the scattering angle, K is a contrast constant defined via refractive index of the particles and solvent. Extrapolation of the angular and concentration dependency of the scattering intensity to zero conditions leads to molecular mass. For the large particles (qRg > 1) particle size can be calculated from the angular dependency of scattered light. In most cases studied this condition is however not fulfilled. Besides, during the experiment the concentration of particles formed is not constant. Thus we consider the time evolution of zero angle scattering intensity, only keeping in mind its dependency on both molecular mass and particle concentration. A sodium metasilicate stock solution with a concentration of 0.2 mol/l Na2SiO3・9 H 2O and a stock solution containing magnesium chloride and calcium chloride in a total concentration of 0.2 mol/l chloride and a molar ratio of Mg : Ca of 1 : 4 were prepared. Solutions for the light scattering experiments with varying concentrations of SiO2 were prepared by dilution of the sodium silicate stock solution with water to the desired concentration. The pH value of this solution is adjusted to pH 7 by adding 2 M HCl. Successively, Mg / Ca – solution is added until the desired concentration is reached. A departing pH value is adjusted by adding 1M NaOH. The pH adjustment sets time zero for the time measurement. The solution is then filtered through a syringe filter with a pore size of 0.22 μm into a dust



free scattering cell. Removal of dust is warranted by flushing the scattering cell upside down with distilled acetone for several minutes. In order to avoid the silica formation by the silicate solution at the inner cell wall, the scattering cells have to be treated with a solution of 5% chlorotrimethylsilane (98%, Janssen Chimica) in toluene. Successful hydrophobization of the glass surface is noticeable by the changed surface energy. The glass surface repels water after the treatment. In case of time-resolved measurements one measurement was collected every five to ten minutes. Each of these measurements was an average over 4000 single measurements. VI. REFERENCES 1. J. Glater, "The early history of reverse osmosis membrane development," Desalination, pp. 297-309

and references herein, 1998. 2. IDA, Desalination Yearbook, 2012-2013. 3. U. Schubert and N. Huessing, Synthesis of inorganic materials, Weinheim: Wiley VCH, 2000. 4. A. Kempter, T. Gaedt, V. Boyko, S. Nied and K. Hirsch, "New insights into silica scaling on RO-

membranes," Desalination and Water Treatment, pp. 899-907, 2013. 5. T. Koo, Y. Lee and R. Sheikholeslami, "Silica fouling and cleaning of reverse osmosis

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fouling and its pretreatment by sodiumaluminate, lima and sodaash," Desalination, pp. 85-92, 2002. 7. R. Ning, "Discussion of silica speciation, fouling, control and maximum reduction," Desalination,

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Katarachia, "Inorganic foulants in membrane systems: chemical control strategies and the contribution of "green chemistry," Desalination, pp. 281-295, 2005.

10. L. A. Belyakova, A. M. Varvarin, D. Y. Lyashenko and N. V. Roik, "Study of interaction of poly(1-vinyl-2-pyrrolidone) with a highly dispersed amorphous silica," Journal of Colloid and Interface Science, pp. 2-6, 2003.

11. K. Spinde, K. Pachis, I. Antonakaki, S. Paasch and E. Brunner, "Influence of polyamines and related macromolecules on silicic acid polycondensation: Relevance to “Soluble Silicon Pools”?," Chemistry of Materials, pp. 4647-4687, 2011.

12. C. Tang, Q. She, W. Lay, R. Wang and A. Fane, "Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration," Journal of Membrane Science, pp. 123-133, 2010.

13. S. Lee, C. Boo, M. Elimelech and S. Hong, "Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO)," Journal of Membrane Science, pp. 34-39, 2010.

14. N. H. Lin, W.-Y. Shih, E. Lyster and Y. Cohen, "Crystallization of calcium sulfate on polymeric surfaces," Journal of Colloid and Interface Science, p. 790–797, 2011.

15. F. Fantinel, J. Rieger, F. Molnar and P. Huebler, "Complexation of Polyacrylates by Ca2+ Ions," Langmuir, pp. 2539-2542, 2004.

16. A. M. Comerton, R. C. Andrewsa, D. M. Bagley and C. Haoc, "The rejection of endocrine



disrupting and pharmaceutically active compounds by NF and RO membranes as a function of compound and water matrix properties," Journal of Membrane Science, pp. 323-335, 2008.

17. W. Den and C. Wang, "Removal of silica from brackish water by electrocoagulation," Separation and Purification Technology, pp. 318-325, 2008.

18. P. Sahachaiyunta, T. Koo and R. Sheikholeslami, "Effect of several inorganic species on silica fouling in RO membranes," Desalination, pp. 373-378, 2002.

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