Substantia. An International Journal of the History of Chemistry 4(2) Suppl.: 95-107, 2020 Firenze University Press www.fupress.com/substantia ISSN 2532-3997 (online) | DOI: 10.36253/Substantia-1031 Citation: A. Wan Nafi, M. Taseidifar, R.M. Pashley, B.W. Ninham (2020) Con- trolled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibi- tion Effects of a Bubble Column Evap- orator. Substantia 4(2) Suppl.: 95-107. doi: 10.36253/Substantia-1031 Copyright: © 2020 A. Wan Nafi, M. Tasei- difar, R.M. Pashley, B.W. Ninham. This is an open access, peer-reviewed arti- cle published by Firenze University Press (http://www.fupress.com/substan- tia) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The Author(s) declare(s) no conflict of interest. Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Effects of a Bubble Column Evaporator Atikah Wan Nafi1, Mojtaba Taseidifar1, Richard M. Pashley1,*, Barry W. Ninham2 1 School of Science, UNSW Canberra, Northcott Drive, Canberra, Australia 2 Department of Applied Mathematics, Research School of Physical Sciences, The Austral- ian National University, Canberra, Australia *Corresponding author: r.pashley@adfa.edu.au Abstract. In the oil industry, strontium sulfate (SrSO4) scale deposits have long plagued oilfield and gas production operations. This remains an unsolved problem. We here show how the bubble column evaporator (BCE) can be used to control aqueous precipitation from salt solutions. Mixtures of strontium nitrate and sodium sulfate in the BCE system were used to precipitate strontium sulfate at different degrees of super- saturation. The effectiveness of the BCE system was compared to standard mechani- cal stirring. The precipitation of strontium sulfate in both processes was monitored through turbidimeter, particle counting, Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The results show that the BCE system has a significant inhibition effect and so can be used to control precipitation growth rate, even from supersaturated solutions. This remarkable effect also provides new insights into mecha- nisms of crystallisation, of bubble interactions and mineral flotation. Keywords: strontium sulfate, aqueous precipitation, nanobubbles, supersaturation, bubble column evaporator, particle growth rates, crystallisation, bubble interactions, mineral flotation. 1. INTRODUCTION Coatings of partially soluble salts pose significant problems. The growth of deposits on the surface of industrial equipment like boilers, heat exchang- ers, wastewater treatment plants and in oil and gas drilling operations is always an issue. The scales generally contain sparingly soluble carbonates and sulfates of calcium, barium and strontium. The formation of scale depos- its from mixing of two incompatible solutions, such as seawater and natu- ral brines, presents a serious problem in industry, e.g. in the operation of oil fields, desalination plants and geothermal wells. Calcium carbonate and cal- cium sulfate scales are typical. Strontium sulfate (SrSO4) scale is not so com- mon. However, SrSO4 deposits have long plagued oilfield and gas production 96 Atikah Wan Nafi, Mojtaba Taseidifar, Richard M. Pashley, Barry W. Ninham operations. Its removal by fast, spontaneous precipita- tion remains an unsolved problem in the oil industry, despite significant research efforts.1-8 Formation of sulfate scales reduces the diameters of pipes. This causes operational difficulties which may lead to additional capital cost and operating costs.9-12 Further, severe plugging of equipment causes loss of production, increases the cost of oil extraction and causes many safety issues. According to Howarth et al.13 wastewater facilities in the oil industry were simply not designed to handle the amount of strontium which can also include radioactive wastes. And indeed, the failure of equipment caused by strontium scale can result in safety issues due to its radioactivity.14,15 To try to mitigate these detrimen- tal effects, research has focused on several treatment options. These are demineralization systems, thermal evaporation, condensation, and reverse osmosis. How- ever, all these treatment processes suffer from difficulties in operation.16,17 For instance, the addition of reagents in chemical precipitation methods can result in separa- tion problems, in which the acidic conditions produce toxic gases as by products. So, these processes have to be carefully monitored. SrSO4 scales are categorised as insoluble scales since they are not easily dissolved, and they are also relatively difficult to treat. The addition of acid to treat SrSO4 scales to reduce clogging and build- up of scale poses environmental issues. It also leads to the risk of interruption the whole operation.18-20 The reverse osmosis membrane technique offers another common treatment method. However, exposure to high salt level concentrations present in feed water can read- ily block the pores of the membrane sheets and so fouls the expensive membranes. These have to be regularly cleaned or replaced.21-23 This leads to low efficiency of the process, high maintenance costs and a decrease in the quality of water produced. To protect the membrane and maintain the efficiency of the process, regular feed- water pre-treatment is essential. This again increases costs, besides complicating the process.24,25 Currently, there are no reports on efficient water treatment which affordably and simply inhibits SrSO4 particle growth. Prior to disposal, wastewater treatment typically aims to maximize the concentration of contaminant using low cost energy, for example, from industrial- waste vent gases, solar heat or wind turbines. These energy sources could be used with the BCE system, which concentrates wastewater and at the same time inhibits the growth of precipitate particles, as has recent- ly been reported24,25 in work that precedes ours. It was discovered that precipitation inhibition naturally occurs in the bubble column evaporator process for supersatu- rated solutions of calcium sulfate. In this process, a con- tinuous, high density, flow of rising bubbles apparently disrupts the growth of nano-particles, even in super- saturated solutions. Comparison with standard mixing methods indicates that the BCE process offers a cost effective and simple method to create precipitation inhi- bition.26 There, precipitated particles of CaSO4.2H2O were maintained at a steady size of <100 nm within a BCE, whilst stirred solutions, at the same supersatu- ration rate (of about 32 times the solubility product) formed particles with sizes increasing rapidly above 1 mm.26 Apart from scale deposit problems in the oil indus- try, SrSO4 is a multifunctional inorganic material used in various chemical applications, such as in electron- ics, ceramics, pigments, cosmetics, paper making and as a compound used for thermo-stimulated lumines- cence.27,28 This study is aimed at the determination of suitable treatments to prevent scale formation, and in order to do this it is important to understand in depth the precipitation reaction of SrSO4 in the first place. Studies on the production of fine particles, through precipitation, have received vast attention with a view to controlled production of fine particles. Nanoparticle materials can be obtained by several methods; such as, precipitation, hydrolysis, electrolysis etc. Compared with other methods, the precipitation reaction has potential advantages of homogeneity, high productivity and con- trollability of the process.29,30 However, the stirred tanks often used in the precipitation process can produce par- ticles with a broad size distribution due to inhomoge- neous mixing combined with rapid and spontaneous reaction of the mixed components.31,32 The formation of particles of small size is often followed by agglomeration, which hinders the ability to produce fine particles. Few studies on simple, effective, additive-free methods for controlled precipitation have been reported. This paper is one such new method. Some unresolved fundamental issues on mechanisms of crystallisation come up in the course of this research that will leave for later discussion in the Appendix. One of such experimental studies reports the effect of a magnetic field in combination of temperature on the precipitation of insoluble salts of alkaline earth metals, such as carbonates of calcium, strontium and barium, which were precipitated from supersaturated conditions. The structure of CaCO3 crystals were studied by a com- bination of X-ray diffraction, optical microscopy and fluorescence. All show that the application of a magnetic field with about 0.4 T, leads to inhibition of particle pre- cipitation.33 It was discovered that insoluble salts, espe- cially CaCO3, when the solution underwent magnetic treatment for about 15 min before mixing caused a sup- 97Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Eff ects of a Bubble Column Evaporator pression of particle nucleation and increased the crystal size, with a reduction in crystal number density. Precipi- tation of supersaturated salt solutions can be retarded through a combination of high magnetic fi elds and high temperature of 60 °C.34 In comparison with our experiments, the magnetic field generated in the magnetic stirring system used was likely to be too low to have any signifi cant eff ect. In addition, it was observed that in the stirring system using a magnetic bar, the particles precipitated readily and in a shorter time and with a higher growth rate. Th e BCE process, used in this work, exhibits an excellent ability to control the SrSO4 precipitation. Th e process employs vigorous mixing to form a uniform sol- ute concentration. From this a more controlled precipita- tion process was achieved than from a standard mixing method. Th is study shows that the BCE method can be used to successfully inhibit precipitation of SrSO4 from supersaturated solutions and reduce the rate of particle growth. Th e inhibition phenomenon is clearly of much wider application than for SrSO4. Based on these experi- mental results, the BCE process could also be used for reducing precipitation of SrSO4 and other nanoparticles scales that have potential applications especially in the oil and gas industry and in industrial water treatment plants. 2. MATERIALS AND METHODS 2.1. Materials Th e salts Sr(NO3)2 and Na2SO4 used in these experi- ments were analytical reagents with purity level ≥ 99%, purchased from Sigma- Aldrich. Double-distilled water, Milli-Q water and purified bottled drinking water, ‘Woolworths Select Mountain Spring’, were used to prepare the salt solutions and to produce a low parti- cle count comparison. At room temperature, the Milli- Q water had a conductivity of less than 3.0 µS cm-1 and pH of 7.06. All concentrations are given in molarity (M) units at room temperature. 2.2. Precipitation processes Figure 1 shows a schematic diagram of the BCE sys- tem, in which bubbles are sparged into the mixed salt solutions containing Sr(NO3)2 and Na2SO4 and also a standard stirring system to compare the precipitation process for SrSO4 at 25 °C. Th e air gas is pumped from an air pump (Hiblow HP40, Philippines) that passes through a silica gel column to dehumidify. A fl ow meter is used to control the fl ow rate of the inlet air which was placed aft er the desiccator. Normal air was used in the BCE, which was pre-fi ltered using a Whatman large High Effi ciency Particulate Air (HEPA) fi lter capsule to fi lter inlet air. Th e inlet air passes continuously through the BCE set up, which was operated within a fi ltered air, laminar fl ow cabinet. Th e HEPA capsule can retain 99% of particles below 0.3 mm. Th e air fl ow was passed through the gas heater to provide the required tempera- ture. Th e heater temperature was controlled by a digital variac power supply and maintained using a thermom- eter (Control company 4000 Traceable). Th en the con- trolled hot gas fl ow was pumped into the bubble column containing the mixed salt solution. To start the precipi- tation process, the concentration of salt solutions was set at a suitable supersaturation level. Th e bubbles produced in the columns were fairly uniform within the size range of about 2-4 mm. For comparison with the BCE experiment, a stand- ard stirring experiment using the same salt solutions at the same temperature were operated using diff erent cylindrical magnetic bars with lengths of 2 to 5.5 cm at diff erent stirring rates of 0, 120, 240 and 480 revolutions per min (rpm). Th e liquid samples from the BCE system and standard stirring systems were taken directly using syringes and fi ltered through Whatman Millipore 0.22 µm at diff erent times for further characterisation and by analysis of the removed dry particles. 2.3. Analytical methods Th e induction and precipitation growth of SrSO4 particles over time were monitored by turbidity meas- urement (HACH 2100AN Turbidimeter). Th e solution Figure 1. Schematic diagram of the bubble column evapora- tor (BCE) system and a photograph of a bubble column (a) and standard stirring process (b) for mixing 0.00152 M2 Sr (NO3) 2 and Na2SO4. 98 Atikah Wan Nafi, Mojtaba Taseidifar, Richard M. Pashley, Barry W. Ninham turbidity at 0.2 NTU was taken as the onset of precipi- tation. This is a useful indicator because the clear water and solutions without obvious precipitates normally gave turbidity values less than 0.2 NTU. A Spectrex Laser Particle Counter (model PC-2300) was used to deter- mine the purity of the solutions. The particle counter with detection size range 0.5–100 µm was able to detect the presence of contaminant particles that may affect the precipitation process. The presence of these particles in the solution prepared using distilled water, bottled water and Milli-Q water when filtered by the Whatman Milli- pore 0.22 µm was tested before each precipitation exper- iment. In addition, a Malvern Zetasizer (Model ZS) with detection size range 0.3 nm–10 µm was used to monitor the growth of the precipitated particles. 1 mL samples were collected into polystyrene cells using a syringe for dynamic light scattering measurement using the Mal- vern Zetasizer. Solution samples were filtered before the DLS analysis when precipitation became visible. Then the filtered samples were kept in the open air to dry completely before examination with a FEI Quanta QEM- SCAN Scanning Electron Microscope (SEM), to study particle morphology. 3. RESULTS AND DISCUSSION 3.1. SrSO4 precipitation The precipitation process typically starts when the concentration of a compound in solution is greater than its solubility, i.e. from a supersaturated solution. In the case of the reactive precipitation of SrSO4, as in this work, the reaction is given, by convention, as Sr2+(aq)+SO42-(aq)⇄SrSO4(S) (1) The precipitate of SrSO4 is produced when the prod- uct of the concentrations of Sr2+ and SO42- ions is greater than the solubility product. The precipitation of SrSO4 and its morphology was determined using SEM to deter- mine the formation of SrSO4 precipitated from a stirred solution and from a BCE. The supersaturation degree (denoted S) of a SrSO4 salt solution is defined as: S= (2) Where Ksp, the solubility product, equals [Sr2+]eq x [SO42-]eq which is the equilibrium product at the solubili- ty limit, assuming ideal conditions. All ion activity coef- ficients are assumed equal to 1 and hence the activities of all the ions are equal to their concentration. Because of the dilute concentrations involved, this assumption is reasonable. Besides the main factor of supersatura- tion level, other factors can affect the precipitation pro- cess: impurities, temperature, contact time, pH, agitation intensity and overall ionic strength.35-39 In this study, the main factors were the degree of supersaturation, purity of background solution and the BCE process as it affects precipitation compared with simple solution stirring. Several studies have discussed the effect of turbu- lence on morphology, scale deposition and minimal inhibitor concentration.40, 41 However, there is no con- sistent finding on the effect of turbulence on scale depo- sition. Barium sulfate precipitation was conducted under turbulent conditions and compared with lamina precipi- tates inside oilfield pipes. The results show that there is no difference in sulfate precipitation kinetics without inhibitors (polymeric based additives) in both condi- tions.42, 43 The present work using the BCE and standard stirring system was conducted inside a laminar flow cab- inet. The results obtained might be developed for appli- cation to the SrSO4 precipitation problem in the oil and gas industries. Particle counts obtained using the Spectrex coun- ter were used to determine the purity of water sam- ples. It was found that the particle count of all types of water samples inside the laminar cabinet are more sta- ble compared to samples exposed to ambient air. These results also showed that the purity of water was ranked as: Milli-Q > Bottled water > Tap water > Distilled water, as expected because the laboratory water distilla- tion system used collected and stored the initially clean distilled water in a vessel which was exposed to atmos- pheric air. Based on Spectrex Laser Particle Counter test results reported in Table 1, the normal distilled water contained the highest number of particles, which is more than 227 counts per mL of particles less than 3 µm in size. This value for the salt solutions which are prepared inside laminar flow cabinet using Milli-Q water was increased to 87 counts per mL despite the particle count initially being only 2 counts per mL. These results suggest that, it is almost impossible to achieve a blank solution with zero particles. The lowest consistent particle count was obtained for the salt solution samples prepared using Milli-Q water inside a laminar flow cabinet, and this sample was used as blank solution in this work. In pre- vious work, it was reported that with careful filtration, the particles can be reduced to less than 1000 counts per mL, however it is impossible to achieve zero parti- cles.44 In addition, it is difficult to prepare systems com- pletely free of fine particles and nucleation sites because 99Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Effects of a Bubble Column Evaporator of impurities in supersaturated solutions, which are dif- ficult to remove and act as nucleation catalysts. Also, the walls of the retaining vessel, can catalyse nucleation. The presence of some impurities can even cause inhibition of crystal growth or nucleation and so affect the rate of sul- fate precipitation.45-47 By filtering the solutions prior to making up the supersaturated solutions, in a laminar flow cabinet, it was possible to reduce particle densities below 50 per mL. This was the typical background level of particles used in this study. The supersaturation degree of each solution in the Table 1 was calculated based on the solubility product values at 25 °C obtained from the CRC handbook.48 The salt solutions of SrSO4 with different types of water source and degree of supersaturation were test- ed using a standard stirring system at 25 °C with a 3.5 cm length cylindrical stir bar (of 1 cm diameter) at a rate of 120 rpm. The induction time at which turbid- ity reached 0.2 NTU was recorded. Figure 2 shows that with increase in degree of supersaturation, the onset precipitation time drops down noticeably. The induction time is also affected by the presence of foreign particles. Solutions with the same degree of supersaturation but prepared using distilled water, precipitate in less time compared to salt solutions prepared using Milli-Q water. It was found that SrSO4 solutions in Milli-Q water with lower than 2 degrees of supersaturation had the longest induction time. By comparison, reported results show that the time of the appearance of nuclei (induction time) in water vapour is 103 years when the supersatura- tion degree is at 3.44 3.2. Effects of Stirring Rates The induction time for the salt precipitation in a standard stirring system is also affected by different lengths of magnetic stir bar used. As shown in Figure 3, SrSO4 solutions at similar solution conditions (0.00152 M2) were stirred at 120 rpm at 25 °C with different lengths of stirring rod. This change in length affected the induction time. It was found that longer magnetic stir bar length reduces the induction time for precipita- tion. However, comparison from Figure 3 showed some- thing quite unexpected: the induction time starts fluctu- ating when magnetic stirring bar lengths of 3.5 cm and 5.5 cm were used. Under the same conditions the BCE system for the precipitation of solutions provides a long- er induction time of 380 min. The solution in the BCE system has a stable induction time compared to a stand- ard stirring system or even quiescent conditions. Differ- ent types of impellers show significantly different results in the precipitation process, in which parameters like different length change, line speed and the shear speed within the solution during stirring. All affect the precip- itation process.49 Table 1. Purity of SrSO4 solution based on particle counts and supersaturation degree. Solution types Prepared using distilled water (particle counts per ml) Prepared using Milli Q water (particle counts per ml) Solution filtered by 0.22µm membrane (particle counts per ml) Supersaturation degree (S) Distilled water 227 - - - Milli Q water - 2 - - 0.0008 M Sr(NO3)2 + 0.0008 M Na2SO4 608 87 7 1.86 0.001 M Sr(NO3)2 + 0.001 M Na2SO4 629 96 19 2.91 0.0013 M Sr(NO3)2 + 0.0013 M Na2SO4 662 112 30 4.91 0.0015 M Sr(NO3)2 + 0.0015 M Na2SO4 668 126 34 6.54 0.002 M Sr(NO3)2 + 0.002 M Na2SO4 686 138 45 11.63 0.0025 M Sr(NO3)2 + 0.0025 M Na2SO4 698 156 49 18.17 0.005 M Sr(NO3)2 + 0.005 M Na2SO4 704 198 91 72.67 0 100 200 300 400 500 0 2 4 6 8 10 12 In du ct io n tim e (m in ) Supersaturation degree Distill ed water Milli Q water Figure 2. The induction time of SrSO4 precipitation in the solutions with different level of purity and supersaturation degrees by simple stirring (120 rpm) at 25 °C. 100 Atikah Wan Nafi, Mojtaba Taseidifar, Richard M. Pashley, Barry W. Ninham 3.3. Comparison of SrSO4 precipitation using the BCE and a standard stirring system Figure 4 shows the precipitation results for 0.00152 M2 SrSO4 solutions in a standard stirring system with different speed of stirring and in the BCE system. In these experiments, particle growth by the precipitation process was monitored using turbidity measurements. The turbidity in a standard stirring system of 120 rpm shows similar behaviour to a salt solution without stir- ring. Both slowly reach onset precipitation (turbidity > 0.2). On the other hand, the salt solution that was placed in the same stirring system at 480 rpm significantly increased precipitation after 60 min. By comparison, the turbidity in the BCE system remained constant below 0.2 NTU for more than 350 min, even though the salt solution used in the BCE was at the same level of satura- tion. An increase in temperature will increase the solu- tion solubility of SrSO4.50-52 Initially, it was expected that continuous water evaporation within the BCE will slowly increase the supersaturation level hence causing precipitation. However, based on the results given in Fig- ures 3 and 4, the BCE system shows a clear inhibition effect on particle growth. The rising bubbles in the BCE show complex behaviour due to coalescence and non- coalescence and bubble shape and trajectory that rise with rotational oscillation effects in the salt solution.53, 54 This behaviour appears to disturb the processes involved in particle growth, even in supersaturated solutions. Low turbidity values do not indicate definitively the absence of particles in the sample, since the turbidimeter involves a light extinction method which may not be able to detect very small particles.55, 56 Hence, the average size of SrSO4 precipitated particles in the BCE and standard stirring processes were monitored using a Malvern zeta- sizer, as shown in Figure 5. Precipitated SrSO4 particles were detected by a Malvern Zetasizer even at the initial point of mixing of the two solutions, with particles of around 0.3 µm, while the turbidity of the solution was found to be below 0.2 NTU. It was shown in a previous study that once particles become large, i.e. more than about 5 μm during the precipitation process, their pres- ence can be correlated with an increase in solution tur- bidity.57 As a possible explanation, the salt solutions of Sr(NO3)2 and Na2SO4 mixed at the beginning of the test might was not be fully homogenous, allowing local pre- cipitation of SrSO4 through a spontaneous heterogeneous process.58 Numerous studies have been concerned with spontaneous precipitation processes, but no satisfactory explanation has yet been accepted.59-63 The results given in Figures 4 and 5 show that increases in solution turbidity is correlated with an increase in particle size. The average size of SrSO4 par- ticles was persistent with the SEM analysis given in Fig- ure 7, in which the image of the particle changes from agglomerates to microrod/pod. The precipitation process in a stirring system led to broad size particle distribu- tion, probably due to rapid reaction.64 In comparison with SrSO4 precipitation in the BCE system, the turbid- ity of solution remained below 0.2 NTU until 380 min and the average size of particle remained constantly below 1 µm. It can be seen from Figure 5 (a) that the BCE system successfully inhibits particle growth, com- pared to a standard stirring system. The results suggest that the BCE system for precipitation has the potential to be used as a method of controlled particle growth for the production of fine particles. 0 100 200 300 400 500 0 1 2 3 4 5 In du ct io n tim e (m in ) Stir bar length (cm) Stirri ng at io nic pro duct of 0.0015 M BCE at i onic produc t o f 0.0 015 M BCE at i onic produc t o f 0.0 02 M Figure 3. The induction time of SrSO4 precipitation by simple stirring (120 rpm) with different stir bar length and using a BCE at 25 °C. 0 0,2 0,4 0,6 0,8 1 1,2 1,4 0 100 200 300 400 T ur bi di ty ( N T U ) Time (min) Precipitation without stirring Stirring precipitation (120 rpm) Stirring precipitation (480 rpm) BCE precipitation Figure 4. SrSO4 precipitation at ionic product of 0.00152 M2 (Super- saturation degree: 6.54) in a standard stirring process and a BCE monitored by turbidity measurements. 101Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Effects of a Bubble Column Evaporator The uniformity of SrSO4 particles over time was based on the change in the Polydispersity Index (PDI) using the Malvern Zetasizer, as given in Figure 5(b). A PDI value close to 0 indicates the formation of a narrow range of particle sizes. For values that are close to 1, it correlates with a random distribution of sizes. The pre- cipitates of SrSO4 in the standard stirring system were expected to experience rapid growth and so a broader size distribution. After 45 mins, the PDI was around 0.5 for the standard stirring system, whereas SrSO4 par- ticles precipitated by the BCE system showed a grad- ual increase in the PDI value. It is clear that the BCE offers significant improvement in uniformity of particle growth and a controlled size distribution. Figure 6 and Figure 7 show images of the particles of SrSO4 formed in the precipitation process in the BCE system and standard stirring system, respectively. The images were captured from the initial mixing point and at 60 min of the BCE precipitation and at 0, 60 and 75 min of the standard stirring system. As can be seen, SEM images of the precipitates formed in the standard stirring system (Figure 7) show mixtures of microrods and flower-like shapes, with sizes ranging from 1.2 µm to 9.7 µm. According to these results, with the same solution conditions as given in Figure 5 and Figure 6, the BCE system shows controlled and uniform particle growth compared to the standard stirring system. The pattern of the SrSO4 particles observed, consisting of microrods and flower-like shaped particles were similar to previous studies on calcium sulphate.26 The formation of SrSO4 particle growth follows a “two-step formation mecha- nism”, in which primary microcrystals are involved in the formation of monopods, followed by the continuous growth nucleation of monopods that leads to secondary microstructure growth, through the spontaneous aggre- gation of pods docking on planar structures.65 The latter are formed due to their similar crystallographic orienta- tion.58 The DLS measurements carried out for monitoring the particle size, gave further support to the inhibition effects in the BCE system, as shown in Figure 8, where 0 1000 2000 3000 4000 5000 10 110 210 310 410 A ve ra ge s iz e of d ia m et er ( nm ) Time (min) BCE precipitation Stirring precipitation (240 rpm) 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 50 100 150 200 250 300 350 400 Po ly di sp er si ty in de x Time (min) BCE precipitation Stirring precipitation (240 rpm) Figure 5. (a): SrSO4 precipitation at ionic product of 0.00152 M2 (Supersaturation degree: 6.54) in BCE and 240 rpm stirring sys- tem by average size at around 25 °C; (b): SrSO4 precipitation at ionic product of 0.00152 M2 (Supersaturation degree: 6.54) in BCE and 240 rpm stirring system studied by polydispersity index (PDI) measurements at around 25 °C. (b) (a) Figure 6. SEM image of SrSO4 precipitated particles at an ionic product of 0.00152 M2 in a BCE at 0 min (left), 60 min (right). 102 Atikah Wan Nafi , Mojtaba Taseidifar, Richard M. Pashley, Barry W. Ninham the peak size at 180 min remained close to the initial peak. It was found that through a continuous process, the BCE was clearly able to produce fi ne particles in a specifi c size range due to the inhibition eff ect apparently inherent to the BCE process. It might be thought that the eff ect of electrolytes on bubble coalescence inhibition53,54 might also be related to the inhibition of precipitated particle growth. How- ever, if there is a link it is unclear from the results pre- sented here simply because the initial mixed salts were at concentrations signifi cantly below those where bubble coalescence eff ects have previously been observed. Of course, only soluble mixed electrolytes have been studied for bubble coalescence eff ects but it would be a reason- able to assume that mixing SrCl2 with Na2SO4 should be similar to mixing CaCl2 with Li2SO4, both of which salts have been separately studied for their eff ects on bubble coalescence inhibition. For example, CaCl2 has a transition concentration (i.e. corresponding to 50% coa- lescence) at about 0.04 M and Li2SO4 has a transition concentration at about 0.025 M.54 Both these concentra- tions are well above even the highest concentrations (of 0.005 M) used in the present study and so would not be expected to signifi cantly aff ect bubble coalescence in 0 min by BCE 60 min by BCE 180 min by BCE Figure 7. SEM image of SrSO4 precipitated particles at an ionic product of 0.00152 M2 in a standard stirring system at 0 min (a), 60 min (b) and 75 min (c). 103Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Eff ects of a Bubble Column Evaporator the column, even though at higher concentration these mixed electrolytes would be expected to aff ect bubble coalescence.66 In this study the precipitation levels seen in the BCE process, even for supersaturated solutions, were very low and so no attempt was made to measure the par- ticle yield, which would be very low. It might be possi- ble, however, to continuously remove the fi ne particles using, for example, a membrane nano-fi ltration system to increase the product yield. Th is study of the comparison between the BCE pro- cess and a standard stirring system, for SrSO4 precipi- tates, was found to be similar to that reported earlier26 for CaSO4.2H2O precipitates. Both studies suggest that the BCE system, due to its inhibition of particle growth, can play an important role in precipitation control, espe- cially in particle-growth-control applications and in industrial water treatment. In addition, the technique could be used in the production of fi ne particles for industrial applications in ceramics, catalysis, cosmetics, pharmaceuticals and in food products.67 4. CONCLUSIONS A standard stirring system and a BCE system were compared for the precipitation of SrSO4 particles from supersaturated solutions of Sr(NO3)2 and Na2SO4. Th e precipitation of SrSO4 in both systems was monitored by turbidity measurements, Spectrex particle counting and a DLS Malvern Zetasizer, with particle morphologies observed using SEM. It was found that the BCE system, compared to the standard stirring system, had an inhibi- tory eff ect on the precipitation induction time and the precipitate growth rate; which allows for the production of particles over a wide size range, from nanometer to micrometer. Th e results obtained were found to be simi- lar to those observed earlier with CaSO4.2H2O precipita- tion, which leads to the proposition that this might be a general property of bubble column evaporators. Th e BCE system potentially off ers useful applications in various industrial processes, such as in the treatment of waste- water and other industries that are required to produce fi ne particles in a controlled manner. Particle size dis- tribution is a key parameter in quality control. Th is has been highlighted in various industries, especially from material-research and processing, fresco and paper res- toration. 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APPENDIX FACTORS CONTROLLING PRECIPITATION The “reaction” Eq. (1) for the formation of particles is impeccably correct. Sr2+(aq)+SO42-(aq)⇄SrSO4(S) However, the reactants have no respect for equa- tions on pieces of paper and something more inscruta- ble lies beneath it. We note first that the entire field of physical chemistry is undergoing revision due to sins of omission and commission at this time.68 The problem of Hofmeister effects with all its consequences is on the way to resolution. But the more important issue is the omission in classical theory of the effects of dissolved gas. The omission is monumental in its consequences, a fact still only dimly perceived. We showed the effects of cavitation on a propeller which is explored in detail in a paper in this Substan- tia Journal issue.69 The reactions that produce propeller corrosion is due to the collapse of nanobubbles, that are accompanied by free radical production. The same free radicals are also produced by spontaneous cavitation in the active sites of an enzyme and are due to coopera- tive harnessing of all the weak van der Waals and other molecular forces to produce a chemical energy available to do the job.70 Nanobubbles containing reactive hydrogen perox- ide are produced even by shaking, without any need for heavy sonication.70,71 In this paper both stirring at different speeds and turbulence accompanying the BCE process would there- fore be expected to produce nanobubbles of different sizes and stability. These should also produce sites for the selective adsorption of both anions and cations and enhanced reactivity. We have already discussed another related funda- mental work that studies the effects of magnetic fields and temperature on the precipitation of alkaline earth metal salts.33 The magnetic field seems too small an inf luence here. But who knows? Permanent magnets placed outside steel pipes prevent scaling. This works but no one knows why. The effects of temperature are also large and spe- cific in Ref [33] and that too can be put into the nano- bubble causality camp. Water is generally agreed to lose its hydrogen bonding between 80-90 °C. So, our elusive nanobubbles will be different at higher temperatures, as will specific ion adsorption and reactivity. Of all the different methods used to produce mono- disperse nano or micro particles, the work Ref [29] is closest. In our study we used a mixture of 0.005M Sr(NO3)2 + 0.005 M Na2SO4 and precipitated out particles of strontium sulphate. In Ref [29] the precipitated particles were Mg(OH)2. The initial solution contained soluble salts of magnesium in a background of sodium hydroxide. The counterions of the magnesium source salts affect the size and the spe- cific surface area of crystallites with a trend that fol- lows a Hofmeister series of anions: (sulfate < chloride < nitrate < perchlorate). It was also shown that depending on the concentration of background salt the precipitated particles could vary in size from a micron to zero radius. The two experiments are difficult to compare. With Mg(OH)2 the background of indifferent anions are at high concentrations 0.2, 0.4, 1, and 2 M. The solutions were vigorously stirred as for our case. The same con- trolled variability in particle size can be achieved with sugars in solution. 107Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Effects of a Bubble Column Evaporator The concentrations are well above the ‘critical con- centration’ for bubble-bubble fusion inhibition.53,54,66 So, it might be thought reasonable to assign the phenomena of size variability to ‘water structure’. By contrast, the situation for strontium sulfate here the concentrations were typically low at about 0.005 M and hence water structure cannot possibly be an issue. This throws us firmly back into the wide-open arena of nanobubbles. Our reference [72] brings the case into strong con- tention as a crucial, long-neglected hidden variable. Finally, we remark on two matters that mineraliza- tion of calcium carbonate in the presence of proteins is probably the most used chemical reaction in nature by weight, being responsible for shells of all invertebrates.73 This mineralization is of much interest too because of alarms about climate change. Ref [73] is exceptionally important as it produced amorphous calcium carbonate for the first time. Ref [74] opens up a new arena too in medicine and biology. Carbon dioxide foam of nanobubbles is the key com- ponent of a crucial new organ in the body, the Endothe- lial Surface Layer. It plays a key role in protection against viruses and cab and is being used industrially for sterili- sation of water. See Editorial and Ref [75, 76].