On the nanobubbles interfacial properties and future applications in flotation

Minerals Engineering 60 (2014) 33–40

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On the nanobubbles interfacial properties and future applications in flotation S. Calgaroto, K.Q. Wilberg 1, J. Rubio ⇑ Minerals Engineering Department, PPGE3M – Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 9500, Prédio 75, Sala 124, Porto Alegre, Brazil2

a r t i c l e

i n f o

Article history: Received 5 October 2013 Accepted 7 February 2014

Keywords: Nanobubbles Stability Zeta potential Size Surfactants Flotation

a b s t r a c t Nanobubbles, generations forms, basic studies and applications constitute a growing research area, included their usage in advanced mineral flotation. Yet, there are investigation needs for sustainable generation procedures, stability and understanding the nanobubbles interfacial properties and structures. Results proved that a reduction in pressure makes the super-saturated liquid suffers cavitation and nanobubbles were generated. Medium pH and solutions tested were adjusted, in the air saturation vessel, before the nanobubbles were formed, and this allowed to control (in situ) the surface charge/zeta potential-size of the forming nanobubbles. Measurements obtained with a ZetaSizer Nano equipment showed zeta potential values, in the presence of 102 mol L1 NaCl, displaying sigmoidal pH behaviour between pH 2 (+26 mV) and 8.5 (28 mV); an isoelectric point was attained at pH 4.5 and were positively charged (up to 23 mV) in acidic medium, a phenomenon which has not been previously observed. In alkaline medium, bubbles were highly negative zeta potential (59 mV) at pH 10. The double layers appear to play a role in the formation of stable nanobubbles providing a repulsive force, which prevents inter-bubble aggregation and coalescence. Accordingly, the sizes of the nanobubbles depended on their charge and increased with pH, reaching a maximum (720 nm) around the isoelectric point (±5 mV). Highly charged and small nanobubbles (approximately 150–180 nm) were obtained in the presence of surfactants (104 mol L1 of alkyl methyl ether monoamine or sodium dodecyl sulphate); the zeta potential values in these experiments followed a similar trend of other reported values, validating the technique used with the nanobubbles sizes varying with pH from 150 to 400 nm. Thus, charged and uncharged stable nanobubbles can be tailor-made with or without surfactants and it is expected that their use will broaden options in mineral flotation especially if collectors coated nanobubbles (‘‘bubble-collectors’’) were employed. A detailed and updated review on factors involving stability, longevity and coalescence of nanobubbles was made. It is believed that future trend will be on sustainable formation and application of nanobubbles at industrial scale contributing to widen applied research in mineral, materials processing and liquid effluent treatment by advanced flotation. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The electrokinetic zeta potential of air bubbles plays a significant role in bubble-mineral particle interaction, bubble-oil droplet attachment and bubble coalescence affecting several industrial Abbreviations: SDS, sodium dodecyl sulphate; IEP, isoelectric point; DAF, dissolved air flotation; Psat, saturation pressure; Qsat, flow-rate; EDA3B, alkyl methyl ether monoamine; SDS, sodium dodecyl sulphate; DAH, dodecyltrimethyl ammonium chloride; DTAC, dodecylamine hydrochloride. ⇑ Corresponding author. Tel.: +55 51 33169479; fax: +55 51 33169477. E-mail address: [email protected] (J. Rubio). 1 Visiting Scientist. 2 www.ufrgs.br/ltm. http://dx.doi.org/10.1016/j.mineng.2014.02.002 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.

processes, namely mineral flotation, oil sands separation, and water and wastewater treatment. In oil sands, the bitumen-bubble attachment depends on the magnitude of the zeta potential, which thus affects the bitumen recovery and froth quality (Schramm and Smith, 1987; Liu et al., 2000; Schramm et al., 2003; Creux et al., 2009; Hampton and Nguyen, 2010; Fan et al., 2012; Sobhy and Tao, 2013). Effects of various parameters on the zeta potential of air bubbles in aqueous solution have been studied thoroughly for years, and many authors have found that solution chemistry, medium pH, surfactant type and concentration, bubble size, the experimental technique employed and water temperature all play significant roles ( Collins et al., 1977; Usui and Sasaki, 1977; Kubota et al.,

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1983; Kubota and Jameson, 1993; Yoon and Yordan, 1986; Li and Somasundaran,1992; Saulnier et al., 1996, 1998; Yang et al., 2001; Takahashi, 2005; Najafi et al., 2007; Elmahdy et al., 2008 and Weihong et al., 2013). On the other hand, only a few studies have been conducted with stable nanobubbles in water alone, with inert electrolytes or in the presence of surfactants (Takahashi, 2005; Najafi et al., 2007; Weihong et al., 2013). The most common approach for determining a bubble’s surface charge is through electrophoretic mobility measurements from which zeta potential values are calculated (Collins et al., 1977; Kubota et al., 1983; Yoon and Yordan, 1986; Okada et al., 1990; Li and Somasundaran, 1992; Han and Dockko, 1998; Phianmongkhol and Varley, 2003; Najafi et al., 2007; Elmahdy et al., 2008; Oliveira and Rubio, 2011; Fan et al., 2012; Weihong et al., 2013). However, the main source of error in the size of bubbles employed in mineral flotation (0.8–3 mm diameter) stems from the high rising velocities of the bubbles in a gravitational field, which are sometimes much higher than the electrophoresis velocities. Therefore, researchers have attempted to devise techniques to generate smaller bubbles to reduce the rising velocity or to minimise the effects of buoyancy during electrophoretic mobility measurements, which can all lead to inconsistent results. Recently, Uddin et al. (2013) used a novel apparatus to measure electrical charge of bubble swarms (1 mm mean bubble diameter) and claimed that this approach (bubble sedimentation potential) was in good agreement with the values of the isoelectric point for purified waterreported in the literature. Another alternative approach has been to reduce bubbles size either physically or with the use of surfactants (Kubota et al., 1983; Cho and Laskowski, 2002; Grau et al., 2005; Li and Somasundaran, 1992; Kukizaki and Goto, 2006; Najafi et al., 2007; Fan and Tao, 2008; Fan et al., 2010a,b,c; Zimmerman et al., 2011; Fan et al., 2012). Many authors claim that the gas bubbles in aqueous media always develop negative charges on their surfaces, which suggests that cations (protons) are more likely hydrated and therefore have a tendency to stay in the bulk aqueous medium, whereas the smaller, less hydrated and more polarised anions tend to adsorb on the bubbles’ surfaces. However, this specific adsorption has not been fully explained, and its existence has not been universally accepted (Hampton and Nguyen, 2010; Paluch, 2000; Karraker and Radke, 2002; Takahashi, 2005). Many authors have measured the charge of bubbles in the presence of surfactants, frothers and/or organic solvents (Usui and Sasaki, 1977; Collins et al., 1977; Kubota et al., 1983; Yoon and Yordan, 1986; Saulnier et al., 1996, 1998; Yang et al., 2001; Takahashi, 2005; Najafi et al., 2007; Elmahdy et al., 2008; Weihong et al., 2013). However, all these reagents, especially the surfactants, not only affect the intensity of the zeta potential but also determine the nature of bubble surface charges and bubble sizes; these effects constitute a major disadvantage when one wishes to measure a bubble’s charge (Li and Somasundaran, 1992; Usui and Sasaki, 1977). The generation, properties and applications of nanobubbles have been discussed at length over the last two decades. The experimental evidence of their formation is incontestable, yet a sound theoretical background of their behaviour is still lacking (Hampton and Nguyen, 2010; Ohgaki et al. 2010; Takahashi, 2005). According to Hampton and Nguyen (2010), several mechanisms have been proposed for the hydrophobic force, but in many cases, the force may be due to the presence of nanobubbles at the liquid-hydrophobic solid interface, a fact of great importance in mineral flotation of fine and coarse particles. These authors, among others, review some features of nanobubble stability and formation theories.

Moreover, nanobubble interfacial properties, their kinetic growth to microscopic sizes and their applications in many areas, including ore flotation, are interesting areas to explore. It is believed by a number of authors (Hampton and Nguyen, 2010; Attard et al., 2002; Attard, 2003; Schubert, 2005) that dissolved gases accumulate preferentially at the hydrophobic solid–water interface, and this fact has been revealed by AFM – atomic force microscopy (Attard, 2003; Hampton and Nguyen, 2010). The latter reported that these bubbles may co-exist as nanobubbles, nanopancakes and nanobubble–nanopancake composites and influence the attraction between hydrophobic surfaces in water, bubble– particle attachment and hydrophobic coagulation between particles. Schubert (2005) reported that the long-range attractive interaction forces between hydrophobic surfaces in aqueous systems are caused by the capillary forces of gas bridges which form at the coalescence of nanobubbles adhering on the surfaces. More, this coalescence would be selective onto hydrophobised particles and that jointly with coarser bubbles would initiate the jump into the three-phase contact at the attachment events in flotation. Recently, a number of studies confirmed these findings in applications in mineral flotation; main claimed advantages of the presence of nanobubbles are: 1. The presence of nanobubbles increased the contact angles and subsequently enhances the probability of flotation (coal, phosphates), mainly the bubbles–particles attachment and stability (Fan et al., 2012; Sobhy, 2013; Sobhy and Tao, 2013a,b). 2. Nanobubbles enhance particles flotation recoveries of coal particles at lower collector and frother dosages and at high kinetic flotation rate (Sobhy, 2013; Sobhy and Tao, 2013a,b). The formation and separation (by splitting off from microbubbles) of a fraction of nanobubbles during depressurisation of saturated air in water (as in DAF-dissolved air flotation) have been neglected, although they seem to be critical factors for applications such as pollutants removal and mineral flotation of fine and coarse particles (Rodrigues and Rubio, 2007; Zimmerman et al., 2011). The current work presents the results of some interfacial phenomena of nanobubbles at the water/air interface in the presence and in the absence of surfactants. The reduction in pressure of super-saturated water with air caused rapid formation of microbubbles (30–100 lm), and a fraction of nanobubbles (varying from 130 to 720 nm) were separated and thoroughly studied. Contrary to other reported studies, medium pH was found to determine both zeta potential and hence the bubble size.

2. Experimental 2.1. Materials Bubbles were generated (Fig. 1) by depressurising air–saturated water solutions at a high flow velocity through a needle valve into an empty glass column (50 cm high; 2 cm inner diameter). Saturation of air in water was achieved in a steel vessel containing an internal glass container with a height of 15 cm, an inner diameter of 12 cm and a wall thickness of 1 cm. The container had a height of 14 cm, an inner diameter of 10 cm, a wall thickness of 0.5 cm and a real capacity of 0.7 L (Fig. 1). The depressurisation-cavitation stage employed had a 2 mm internal diameter needle valve (Globo 012-SantiÒ, made of steel); the sampling flux of 150 mL was bypassed from the column by clamping the rubber tubing (Fig. 1). A KrussÒ 8451 tensiometer was utilised for the water/air surface tension measurements, and a ZetaSizer Nano ZS (red badge)-

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Fig. 1. Experimental setup for nanobubble generation and measurements of bubbles zeta potentials and sizes: (1) rubber stopper; (2) glass column; (3) ZetaSizer Nano ZS instrument; (4) clamp; (5) latex tube; (6) needle valve; (7) saturation vessel.

ZEN3600 – MalvernÒ Instrument coupled to disposable folded capillary cells (gold plated Beryllium Copper electrodes) measured both the size and the zeta potential of the bubbles.

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applied to the dispersion of bubbles migrating at a velocity that is dependent on zeta potential. This velocity was measured by laser interferometry-M3-PALS (phase analysis light scattering), which enables the calculation of the electrophoretic mobility that is subsequently converted into a zeta potential (measured in millivolts), according to Smoluchowski0 s equation. To measure the dispersion size of the nanobubbles, the ZetaSizer Nano ZS uses a dynamic light scattering technique, which monitors the diffusion of the bubbles moving under Brownian motion and then converts this signal to equivalent diameters according to the Stokes–Einstein relationship. The high sensitivity of this equipment was ensured by a non-invasive backscatter technology. Measurements of zeta potential and size were performed at a scattering angle of 90°, a wavelength of 290 nm and a temperature of 296 K. Fifteen runs of 1 s each were performed with no delay between them in triplicate. Each zeta potential and size measurement obtained corresponded to the mean values calculated from 90 measurements (forty-five for two different samples). The mean and standard deviation values of the zeta potentials were calculated at the highest equipment sensitivity yielding averaged (narrow) distribution of the generated data (1–5% for the zeta potential values and 2–10% for the bubble size results).

2.2. Reagents NaCl supplied by SynthÒ was employed to maintain a constant ionic strength during the size and zeta potential measurements. Flotigam EDA 3B, from ClariantÒ, which corresponds to a 195 g/ gmol molecular weight commercial cationic alkyl methyl ether monoamine having the molecular formula: [R–(O–CH2)3–NH3] + CH3COO (where R is a hydrocarbon chain of ten carbon atoms) was utilised as a cationic surfactant. Sodium dodecyl sulphate, supplied by NuclearÒ, 288 g/gmol molecular weight was utilised as the anionic surfactant. Both the electrolyte and surfactant solutions were prepared in deionised water (Fisher Scientific) at 25 °C with a conductivity of 2.5 lS/cm, a surface tension of 72.3 ± 0.1 mN/m (L/air) and a natural equilibrium pH of 6.1. NaOH and HCl, from VetecÒ, were used for pH adjustments. 2.3. Methods 2.3.1. Generation of nanobubbles Nanobubble dispersions were produced either in 102 mol L1 NaCl alone or with 102 mol L1 NaCl and 104 mol L1 surfactants solutions (Flotigam and SDS) as a function of medium pH varying from 2 to 10, adjusted with either HCl or NaOH. The solutions (600 mL) were transferred to the steel vessel (Fig. 1), and the air was dissolved at an internal gauge pressure (Psat) of 66.1 psi for 25 min (batch-mode saturation). In line air filters were employed to purify the pressurised air dispersing in water. Then, an air–saturated solution (150 mL) was injected into the column of bubbles rising after passing through a needle valve (Fig. 1) at a flow-rate (Qsat) of 0.1 L min1. Larger microbubbles (approximately 40–100 lm) rose rapidly through the column, while a fixed volume 0.75 mL of nanobubbles (with very low rising rates), was introduced through a tube connection on the capillary cell (Fig. 1) endowed with a rubber constrictor valve. The time for the zeta potential and size measurements was approximately 1 min after the depressurisation. Nanobubbles were stable and lasted for at least two h; therefore, 1 min was considered a reasonably representative time for the readings. 2.3.2. Characterisation of the nanobubbles The ZetaSizer Nano ZS instrument employs a Laser Doppler Micro-electrophoresis technique whereby an electric field is

3. Results and discussion Differently to mineral flotation, nanobubbles have to be generated and not injected by simply pumping air in water or auto aspirate (air sucking). Thus, medium pH was adjusted (in the air saturation vessel) before the nanobubbles were formed, and this led to control (in situ) the surface charge/zeta potential of the forming bubbles (nanobubbles discovered to exist after depressurisation of saturated water in air through a flow constrictor). Results found are that the magnitude of the zeta potential determined the nanobubble size, in a certain relationship. The maximum size of the nanobubbles (around 720 nm) was obtained when the zeta potential values were less than 5 mv, positive or negative. Fig. 2 shows zeta potential values of the air nanobubbles as a function of medium pH (a standard deviation of triplicate measurements is indicated by the error bars) and Fig. 3 compares different results obtained by different authors. The addition of reagents (HCl, NaOH) at the saturation stage appears to make a substantial difference in the nanobubble

Fig. 2. Zeta potential values and mean diameters of nanobubbles as a function of pH in 102 mol L1 of NaCl.

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Table 1 Selected values reported for the zeta potential of different types of bubbles as a function of pH. Author (year)

Bubble mean sizes (approximate diameter values)

Zeta potential (IEP values)

General comments

This work

Air nanobubbles 350–620 nm

Oliveira and Rubio (2011)

Air microbubbles 30–60 lm

Zeta potential values: +26 mV to 59 mV, in 102 mol L1 of NaCl IEP = pH 4.5 Zeta potential values: 0 mV to 66 mV, in 102 mol L1 of NaCl IEP = pH 2 (extrapolated)

Najafi et al. (2007)

Nanobubbles

Yang et al. (2001)

N2 microbubbles > 80 lm

Li and Somasundaran (1992)

Air microbubbles < 5 lm

Bubbles size distributions exhibited a strong dependence on zeta potential. The latter displayed sigmoidal pH behaviour between pH 2 (+26 mV) and 8.5 (28 mV) Zeta potential values measured with an adapted microelectrophoresis technique (Rank Brothers Mark II). The study also included the charge of bubbles in the presence of different polyacrylamides Measurements with a laser electrophoresis technique (ZETA PALS). The charges of oxygen, nitrogen and air bubbles were measured in the presence of frothers Studies with a microelectrophoresis technique coupled to a video camera. The N2 bubbles generated by electrolysis in different electrolyte solutions Bubbles generated in a fritted-glass gas disperser) in NaCl and AlCl3 solutions. Effect of salt concentrations, aluminium hydroxide species formation and solution pH

Zeta potential values: +2 mV to 30 mV, in 102 mol L1 of NaCl IEP = pH 2.2–2.4 Zeta potential values: +6 mV to 51 mV, in 102 mol L1 of NaCl IEP = pH 3.5 IEP = pH 1.5 (N2). Zeta potential values: 10 to 60 mV

Fig. 3. Zeta potentials of bubbles as a function of pH: results reported by different authors. Experimental conditions in Table 1.

formation after pressure reduction; this is a topic that has received little or no attention, despite the long research in dissolved air bubble production and flotation. Figs. 2 and 3 and Table 1 showed that in alkaline medium, the adsorption of negative OH ions at the gas/water interface increased the negative zeta potential values to approximately 59 mV at pH 10, results observed by a number of authors (Uddin et al., 2013). Fig. 2 shows zeta potential values of the nanobubbles displaying sigmoidal pH behaviour between pH 2 (+26 mV) and 8.5 (28 mV); IEP at pH 4.5 and a high negative zeta potential (59 mV) at pH 10. These findings appear not to be observed by other authors with nano or microbubbles (Li and Somasundaran, 1992; Yang et al., 2001, Oliveira and Rubio, 2011, Cho et al., 2005). The results demonstrate that below the IEP, the surface is indeed highly positive, which is consistent with spectroscopic observations (Creux et al., 2009; Hampton and Nguyen, 2010) presumably a result of the depletion of hydroxide ions and the adsorption of protons. Yang et al. (2001) reported positively charged nanobubbles near the IEP (pH 3.0–3.5); however, the positive zeta potential values carried a high standard deviation in this range. Fig. 2 also shows that the nanobubbles grew (almost six times the diameter) as pH increased, reaching a maximum (720 nm) around the IEP (pH 4.5) where bubbles are practically uncharged

(±5 mV). Results showed that the higher the charge of the bubbles, the smaller the resulting nanobubbles, especially at low pH. The reasoning is that because the medium pH was adjusted before the bubbles were formed, the surface charge and hence the zeta potentials values were fixed (in situ, in nature) and as a result; their magnitude influences the bubble size. Thus, it appears that the size of the resulting nanobubbles depends on their surface charges, contrary to Usui et al. (1981), who reported that the zeta potential of gas bubbles (bigger than nanobubbles) depended strongly on bubble size. This causative relationship is indeed certainly questionable because these authors employed a surfactant (CTAB: cetyltrimethyammonium bromide) at different concentrations, which may affect the size of the bubbles (microbubbles). The results of this work are also contrary to those published by Cho et al. (2005) who did not encountered size dependence on pH. Differences on generation procedures (depressurisation of saturated air water against ultrasonification) and nanobubbles size: 130–720 (this work) and 600–110 (Cho et al., 2005) might explain these effects. It is believed that the developed double layer plays a critical role in the formation and stability of nanobubbles in aqueous solutions by providing a fairly high repulsive force, which prevents inter-bubble aggregation and coalescence of the stable bubbles (Ushikubo et al., 2010; Hampton and Nguyen, 2010). The stability and charge of the bubbles in acid medium, obtained in this work, may also be explained by two main facts: 1. High concentration of dissolved air in the water (supersaturating) at the operating saturation pressure of 65.1 psi. Thus, the depressurisation through the needle valve will cause cavitation leading to the spontaneous formation of a cloud of electrically charged tiny bubbles (Hampton and Nguyen, 2010). 2. The addition of reagents (HCl, NaOH) at the saturation stage appears to make a substantial difference on the nanobubble formation after pressure reduction; this is a topic that has received little attention. According to many authors the H+ ions are more likely hydrated and therefore would have a tendency to stay in the bulk aqueous medium while the smaller less hydrated and more polarised anions would adsorb onto the bubble surface (Uddin et al., 2013; Gray-Weale and Beattie, 2009; Yoon and Yordan, 1986; Kim et al., 2000; Takahashi, 2005). This would indicate that hydroxide ions would be specifically adsorbed compared to other anions, but there is no spectroscopic evidence for this theory and some

S. Calgaroto et al. / Minerals Engineering 60 (2014) 33–40

Fig. 4. Zeta potential values of bubbles as a function of pH in the presence of amine derivatives. This work: 104 mol L1 of Alquil methyl ether monoamine; Weihong et al.: 4  104 mol L1 of Dodecyltrimethyl ammonium chloride; Najafi et al., and Yoon and Yordan: 103 mol L1 of Dodecylamine hydrochloride. For other experimental conditions, see Table 2.

authors have offered evidences contrary to this mechanism (Creux et al., 2009; Hampton and Nguyen, 2010). Also, the literature reported similarities among the pH dependences of the zeta potentials of air bubbles, oil drops, and solid inert surfaces such as Teflon, which indicates that water behaves similarly at each of these low dielectric hydrophobic surfaces with IEP0 s at pH 2–4 (Hampton and Nguyen, 2010 and Preocanin et al., 2012). A possible explanation has recently been provided by (Takahashi, 2005; Gray-Weale and Beattie, 2009), who claim that the endothermic suppression of dipole fluctuations with the surrounding bulk water would be minimised if the hydroxide ion approaches the low dielectric interface. This force might account for the pH dependence of the zeta potential and for all of the other experimental observations of hydrophobic interfaces involving approximately neutral water at low ionic strengths. Regarding nanobubble stability and longevity; surface potential and repulsion forces appear to be only a small part of a dilemma not solved in decades (Lima et al., 2008; Craig, 2004; Weijs and Lohse, 2012; Ohgaki et al., 2010; Seddon et al., 2011; Hampton and Nguyen, 2010). The DLVO theory describes the stability of a colloid dispersion as a balance between the electric double layer repulsion and the van der Waals attraction. The range of the former may be 1–100 nm, scaling inversely with the square root of the ionic strength, and the measurable or effective range of the latter is practically 1 nm or so. The theory generally works well under the circumstances for which it was intended, low salt, inert surfaces and interactions at separations greater than a few nanometers. The long-ranged attractions, measurable up to several hundred nanometers, between hydrophobic nanobubbles do not fit in this treatment. This makes unfortunately, calculations difficult to make, and values if obtained will be probably of low quality, with high standard deviations; this due to the fact that the forces are presumably operating in this very same size range of the nanobubbles. The stability is probably due to their nanoscopic size whereby other phenomena, than surface forces take place, many authors claim that the investigation of nanobubbles is still in its infancy and many challenges remain. There is an apparent paradox between the Laplace–Young equation and the observed stability of nanobubbles. The main objection is that bubbles with a radius of curvature of 10–100 nm should, by the Laplace–Young equation, have an internal gas pressure of hun-

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dreds or tens of atmospheres and it is argued that they should dissolve within milliseconds. In marked contrast, the experiments reveal that nanobubbles are stable for periods as long as hours, violating classic thermodynamic laws. Is this because the bubbles are not in equilibrium with the atmosphere due to the rather long time it takes to reach diffusive equilibrium or the macroscopic thermodynamics, is not valid at the nanoscale level? Some authors now believe that gas molecules appear not to interact with each other as bubbles are very small. These gas– liquid systems are sometimes referred to as ‘‘nanobubble solutions’’ although, nanobubbles should be thermodynamically unstable in aqueous solution. Yet, fundamental aspects of nanobubble solutions, such as bubble size, gas content, diffusivity and internal pressure, remain unclear. Agarwal (2005) discussed the effects of line tension at the three-phase contact line, for the observed set of nanobubbles. The length-scale of a typical nanobubble is found to be comparable to the mean-free-path of gas molecules inside the bubble. Thus, the Young–Laplace equation- used in continuum theories for calculating pressure difference across an interface- might not be applicable in its unmodified form for the case of nanobubbles. Observations show a virtual disappearance of the buoyant force for these minute bubbles and the appearance of physical stability. Ohgaki et al. (2010) explained by Raman spectroscopy an apparent solubility of 50 nm bubbles, about 30 times higher than the saturated solubility under atmospheric condition at 298 K. Herein, almost no nitrogen molecules are dissolved homogeneously in water, but are present in the form of nitrogen nanobubbles. The problem is now why nanobubbles can persist over two weeks and what is responsible for the high resistance to diffusion of gas through the boundary. The same authors claim that the nanobubble surface contains hard hydrogen bonds that would reduce the diffusivity of the gases from the surface as shown by attenuated total reflectance infrared spectroscopy. Weijs and Lohse (2012) presented a theoretical model for the experimentally found regarding the exceptionally long lifetime of surface nanobubbles. The model was based on the limited gas diffusion through the water in the far field, the cooperative effect of nanobubble clusters, and the pinned contact line of the nanobubbles leading to a slow dissolution rate. Are nanobubbles coated with diffusion limiting molecules? Or does the gas indeed diffuse out, but is balanced by an equivalent influx? Seddon et al. (2011) provided a model for this remarkable nanobubbles stability to bulk dissolution. Their argument is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. The model appears to be in good agreement with experimental atomic force microscopy. Thus, nanobubbles last for so long because gas molecules inside them do not escape into the main liquid, but instead hitch a ride on a circular. Thus, because nanobubbles are so small, a gas molecule would be able to travel from one side of a bubble to the other, without colliding with any other gas molecule. The other is that a gas molecule sticking to the surface inside the bubble is most likely to leave the surface in the perpendicular direction. Regarding bubble coalescence inhibition in electrolytic-aqueous solutions was detected mostly in the dynamic-approach in contradiction to DLVO theory (Lima et al., 2008). This theory has not been able to explain the experimentally observed ion specific forces acting between air–bubbles in electrolyte solutions and bubbles coalescence is still unclear and remains unresolved (Marcelja, 2006). It appears that hydrophobic nanobubbles approaching other similar bubbles do not form stable films neither the film thickness (Debye length) described by DLVO theory (Lima et al., 2008). Even if a film were present, because of the hydrophobicity the film would ruptures rapidly and at large separations, typically >100 nm, distance higher than nanobubbles diameters. It is concluded that the pres-

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Table 2 Zeta potential values of bubbles in the presence of different surfactants: method of measurement from the literature. Author (year)

Surfactants

This work

Cationic: 25 mV to 2 mV Cationic: Flotigam EDA3B (1) Anionic: SDS (2) Anionic: 17 mV to 75 mV Cationic: DAH (3) Cationic: 70 mV to 15 mV

Najafi et al. (2007)

Anionic: SDS Weihong et al. Cationic: DTAC (2013) (4) Anionic: SDS Cationic: DAH Yoon and Yordan (1986) Anionic: SDS

Zeta potential values

Anionic: 40 mV to 56 mV Cationic: 20 mV to 2.5 mV Anionic: 28 mV to 45 mV Cationic: 50 mV to 15 mV

General comments Measurements of electrophoretic mobilities and size distribution of nanobubbles in a laserelectrophoresis technique (NanoZetaSizer)

Oxygen, nitrogen and air bubbles were nucleated in gas-saturated aqueous solutions. Measurements of the electrophoretic mobility of bubbles were made with a laser electrophoresis technique (ZETA PALS) Very small bubbles generated by the same method of Najafi et al. (2007); measurements of the zeta potential of air bubbles with different surfactants, pH0 s and water temperature Zeta potential of microbubbles generated by a microfoam method at different concentrations of anionic, cationic and nonionic surfactants in aqueous solutions over a wide range of pH

Anionic: 36 mV to 40 mV

(1) Alkyl methyl ether monoamine; (2) sodium dodecyl sulphate; (3) dodecyltrimethyl ammonium chloride; (4) dodecylamine hydrochloride.

Fig. 5. Zeta potential values of bubbles as a function of pH in the presence of sodium dodecyl sulphate-SDS (various concentrations). This work: 104 mol L1; Weihong et al.: 4  104 mol L1; Najafi et al., and Yoon and Yordan: 103 mol L1. For other conditions, see Table 2.

ence of nanobubbles can confound some scientific studies such as the measurement of forces between hydrophobic surfaces (Craig, 2004). The zeta potential of bubbles with ordinary cationic and ionic surfactants (Figs. 4 and 5 and Table 2) reported by many authors, show similarities. The charge of the nanobubbles depends on surfactant molecules type and concentration and the adsorption mechanisms depend highly on medium pH. With cationic amine derivatives, the positive charge of bubbles caused negative adsorption, whereas beyond the IEP, the negative charge was highly neutralised. Conversely, SDS drastically neutralised the positive charge in acidic medium reversing the bubble’s overall charge to a negative value. This means that surfactants were adsorbed through electrostatic forces with the polar head towards the bubble, and then the molecules rearranged inversely, presumably by hydrophobic forces, similar to micelle formation causing a change in the zeta potential of the bubbles according to their polar group charge. Similarly, Cho et al. (2005) claimed that the stabilisation of nanobubbles occurred by the adsorption of surfactants and equilibrium state between fully-ionised monomers and partially-ionised micelles. Fig. 6 shows the effect of surfactants on mean nanobubbles size as a function of medium pH. Results show that independently of the charge of the surfactant the bubbles size changed between

Fig. 6. Mean diameters of nanobubbles as a function of pH in the presence of 104 mol L1 Flotigam EDA 3 B (alkyl methyl ether monoamine) and SDS-sodium dodecyl sulphate, in 102 mol L1 of NaCl.

180 and 400 nm (almost twice), between pH 2 and 8. Herein, this variation is considered small and are in agreement with similar findings (see Table 2 and Cho et al., 2005) with small or coarse bubbles by other authors who claim that surface active agents ‘‘stabilise’’ bubbles inhibiting coalescence (Castro et al., 2013). Interestingly, Fig. 6 shows that at pH 10, whereby the amines begin to form insoluble species (Laskowski, 1989) results of the nanobubbles sizes are masked because the equipment does not differentiate the bubbles from these colloidal precipitates. 3.1. Final considerations In this study, a detailed review (last decade) of nanobubbles formation and stability was made bringing out the fascinating theme of these minute bubbles. The process of nucleating and formation of very stable nanobubbles occurred, after pressurizing aqueous ´ s and surfactants) with air and solutions water (different pH depressurising the saturated air (66.1 psi) through a needle valve. Curiously, all bubbles formed in the similar DAF process have been described as microbubbles (of the order of 30–100 lm) and the presence of nanobubbles has been strikingly neglected or not measurable in conventional equipment’s. The nanobubbles sizes exhibited a strong dependence on zeta potential values and the technique employed was validated when compared to other studies. It is believed that the scope for micro and nanobubbles to impact current industrial scale flotation (ores

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and effluents) processes and other gas–liquid transfer processes is very promising (Sobhy and Tao, 2013a,b). This work revealed a method of obtaining a mass fraction of nanobubbles which appears to have good potential in flotation. Future work will continue exploring this approach, maximising the concentration ratio of nanobubbles to microbubbles. It is believed that this might be possible after saturation of air at low surface tension aqueous solutions and depressurising the dissolved air in modern, high-efficiency flow constrictors. We can already control the conditions under which nanobubbles (charged or not) they are produced, so it appears that we will be able to manipulate nanobubbles to our advantage. The presence of a gas phase at mineral particle interfaces is crucial in getting super hydrophobicity and the manipulation of nanobubbles could be used to actively change the contact angle of a surface or the flow properties of the pulp at the froth/air interface. Yet, the sustainable generation of nanobubbles at high rate, with a high hold up is another big challenge; some alternatives (among others) are (Sobhy and Tao, 2013a): (i) power acoustic or ultrasonic cavitation; (ii) blowing gas through turbulent flow (shearing) in cavitation tubes, swirlflows, microporous (sintered and hydrophobic materials, (iii) formation of nanobubbles in fluidic oscillators, at several tens of kHz. This enhanced bubbles ratio would certainly improve fine and coarse particle recoveries by flotation (Rubio et al., 2007; Gontijo et al., 2007; Jameson, 2010; Sobhy, 2013; Sobhy and Tao (2013a,b); Fan et al., 2010a,b,c). Moreover, the use of collectors (xanthate, amine or oleate) coated nanobubbles (tailor made ‘‘bubble-collectors’’) must broaden options of difficult-to-treat minerals by flotation. For liquid effluent treatments such as algae and nano-pollutants removal, improving the nanobubble/microbubble ratio should enhance DAF processes because nanobubbles have demonstrated disinfectant characteristics, in addition to having an extremely high surface area for full aeration (Zimmerman et al., 2011). Thus, a new generation of DAF equipment can be designed to maximise nanobubble formation. 4. Conclusions Very stable nanobubbles were discovered to be formed after rapid depressurisation of saturated air in water (66.1 psi), and their sizes exhibited a strong dependence on zeta potential values. These values displayed sigmoidal pH dependence between pH 2 (+26 mV) and 8.5 (28 mV); an IEP at pH 4.5. Those bubbles grew from 130 nm (at pH 2) to 720 nm near the isoelectric point (±5 mV). Double layers appear to play only one of the many mechanisms reviewed to explain nanobubbles formation, stability, growing and longevity. Highly charged nanobubbles, having known size can be obtained either by modifying the pH or introducing ionic surfactants (collectors coated nanobubbles) and it is expected that their usage widen applied research in mineral, materials processing and liquid effluent treatment by non-conventional flotation. Acknowledgements The authors would like to thank all the Brazilian Institutes supporting this research, namely CNPq, Fapergs, and UFRGS. Special thanks to Arthur Debiasi and to all students at our laboratory and his friendly atmosphere. References Agarwal, A., 2005. An experimental study of nanobubbles on hydrophobic surfaces. MSc thesis, Citable URI (December 2013):
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