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Separation of anionic surfactant in paste form from its aqueous solutions using foam fractionation
Accepted Manuscript Title: Separation of Anionic Surfactant in Paste Form from its Aqueous Solutions using Foam Fractionation Authors: Shashank Shekhar Srinet, Ananya Basak, Pallab Ghosh PII: DOI: Reference:
S2213-3437(17)30059-3 http://dx.doi.org/doi:10.1016/j.jece.2017.02.008 JECE 1474
To appear in: Received date: Revised date: Accepted date:
1-11-2016 20-1-2017 4-2-2017
Please cite this article as: Shashank Shekhar Srinet, Ananya Basak, Pallab Ghosh, Separation of Anionic Surfactant in Paste Form from its Aqueous Solutions using Foam Fractionation, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2017.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Separation of Anionic Surfactant in Paste Form from
its
Aqueous
Solutions
using
Fractionation Shashank Shekhar Srinet, Ananya Basak, and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati Guwahati – 781039, India
and
Jaideep Chatterjee, Unilever R&D, Bangalore Laboratory 64, Main Road, Whitefield, Bangalore 560066, India
*
author to whom all correspondence should be addressed
E-mail: [email protected] Tel: +91.361.2582253 Fax: +91.361.2690762
1
Foam
Graphical abstract
Highlights
Surfactant was recovered in paste/solid form by foam fractionation
Separation efficiency and solid recovery increased with increasing air flow rate
Steady state foam volume increased with increasing air flow rate
Separation efficiency and solid recovery decreased in the presence of NaCl
High recovery of water was achieved
Abstract Foam fractionation is an effective method for the removal and recovery of a surfactant from water. In this work, removal of an anionic surfactant (i.e. sodium dodecylbenzenesulfonate) was studied, at high concentrations, in a batch foam fractionation column. The anionic surfactant could be separated in a solid/paste form near the top of the column by using this process High surfactant removal from water was achieved for surfactant concentrations varying from approximately near its critical micelle concentration to six times this concentration. The removal efficiency was reduced in the presence of salt (i.e. NaCl). The surfactant separation was enhanced by increasing the air flow rate. The recovery of surfactant was reasonably good. The recovery of water was good as well. The volume of foam and its age played important roles in the separation efficiency and the surfactant recovery. The adsorption of surfactant and the electric charge at the air–water interface were measured and analyzed to explain the separation efficacy. The bubble size increased with increasing air flow rate, and decreased with increasing surfactant and salt concentrations. 2
Keywords: Batch foam fractionation; Salt effect; Surfactant recovery; Wastewater treatment; Water recycling.
Nomenclature Hamaker constant, J
AH
A surface area of foam lamella, m2 Hamaker constant, J
AH
concentration of surfactant in the bulk solution, mol m–3
cs c
bulk concentration of ion, mol m–3
C1
constant in Eq. (10), Pa
C2
constant in Eq. (10), Pa
Ce
concentration of the effluent stream, mg dm3
C0
concentration of the original feed solution, mg dm3
d
diameter of sparger pore, m
e
electronic charge, C
g
acceleration due to gravity, m s2
E Gibbs film elasticity, N m1– h thickness of foam film, m k
Boltzmann constant, J K–1
KL
equilibrium constant in the Langmuir adsorption equation, m3 mol–1
NA
Avogadro’s number, mol–1
rb
radius of bubble, m
rp
radius of Plateau border, m
Greek Symbols R gas constant, J mol–1 K–1 T temperature, K z valence of ion surface tension of surfactant solution with/without NaCl, N m–1 3
surface tension of pure water, N m–1
0
surface excess concentration of surfactant, mol m–2
maximum adsorption capacity of the air–water interface, mol m–2
p
pressure difference, Pa
dielectric constant of the aqueous phase
0
permittivity of free space, C2 J–1 m–1
zeta potential, V
Debye–Hückel parameter, m–1
surfactant separation efficiency, %
disjoining pressure, Pa
density of water, kg m3
surfactant recovery, %
charge density at the air–water interface, C m–2
chemical potential, J mol–1
potential at air–water interface, V
Abbreviations CMC critical micelle concentration EDL electrostatic double layer SDBS sodium dodecylbenzenesulfonate SR short range UV-Vis ultraviolet-visible vdW van der Waals
1. Introduction Water treatment is vital for reducing the impact of numerous chemicals that are released to the natural water sources (e.g. ponds, lakes and rivers) from a wide variety of industrial and
4
domestic applications. Pollution of water has a huge impact on the environment. Treatment of wastewater is necessary to reuse the water, and it is an ultimate resource. Adsorption [1-6], biodegradation [7,8] and oxidation [9-11] are some of the common methods used for removing the pollutants from wastewater. Surfactants are released from laundries, chemical industries (e.g. soaps and detergents, personal care products, food, paper, and fabric), as well as from household washing and cleaning operations [12]. Most of the surfactants used in soaps and detergents are biodegradable, except a few. However, complete biodegradation may take weeks. Common oxidizing agents used in water treatment, such as chlorine and ozone, break the surfactant molecules into smaller ones, which are more biodegradable. However, in some cases, the intermediate ozonation products are more toxic than the base surfactant [13]. Recovery of the surfactant is not possible by these methods. Surfactants can have poisonous effects on aquatic life, if their concentration is high. They can destroy the external mucus layers of the fish, which protect them from bacteria and parasites. They can damage their gills also. Many fishes will die if the surfactant concentration exceeds 15 mg dm3 [14]. Surfactants also reduce the breeding of aquatic organisms. Most surfactants used in the industrial and domestic purposes reduce the surface tension of water. Fishes significantly absorb harmful organic chemicals (e.g. pesticides) when the surface tension of water is low. Another hazard created by surfactants is the foaming in the river and lake (see Fig. 1). Such foaming reduces oxygen transfer to water, and also causes inconvenience to the nearby residents and traffic. A promising method for the removal of surfactants from wastewaters is foam fractionation [15-17]. This method can also help to recover the surfactant. It is a simple and low-cost method, which isolates the surfactant molecules based on their surface activity [18,19]. This method is not limited to the surfactants, but can also be used to remove proteins and other surface active compounds. It is commonly used, albeit on a small scale, for the removal of organic wastes from aquariums. These units are known as “protein skimmers”. Apart from the enrichment of bioproducts, the foam fractionation technique is increasingly being used for the removal of surface active contaminants from wastewater streams. In this method, air is sparged to produce bubbles, which rise along the column, producing a good amount of foam. As the bubbles form and travel through the continuous aqueous phase, the surfactant molecules adsorb at the air–water interface. This leads to the formation of a surfactant-rich foam phase at the top of the column, and an aqueous phase lean in surfactant, at the bottom of the column. Thus, there is a depletion of surfactant in the bulk solution. It
5
has been reported that there is a concentration gradient in the column, which is dependent on the transfer between the liquid flowing up and down the column [20]. Foam fractionation can be performed either batch-wise or in a continuous manner. There are more sophisticated modes of operation incorporating enrichment and/or stripping [21]. The foam fractionation column can be either single- or multi-stage. Several studies have investigated the recovery of surfactant using foam fractionation, and examined the effects of various parameters on separation efficiency [22,23]. Removal of anionic surfactants (e.g. sodium dodecyl sulfate) from water has been reported [24]. The effects air flow rate, feed flow rate, time, foam height and liquid height have been studied [25,26]. The foam fractionation method has been applied to binary surfactant systems as well [27]. The surfactants can act as a collector and bind metal cations into chelates that can be easily removed by foam fractionation [22]. Foam fractionation has been extensively studied for the purpose of removing a variety of pollutants (e.g. heavy metals) from water by adding a surfactant [24,28]. Simple models for foam fractionation of surfactants have been developed without considering liquid drainage and coalescence of bubbles, and such models are valid only for dilute surfactant solutions [28]. Foam is a good medium for adsorptive separation of surfactants because it has a very high specific surface area on which the target species adsorbs. The amount of interstitial liquid is low, especially when the foam is dry. It is the surface rather than the interstitial liquid that drives the separation process because the target species tends to move from the interstitial liquid to the surface [15]. Nonionic organic pollutants can also be effectively removed by foam fractionation [29]. Despite a large number of publications, the use of foam fractionation for removal/recovery of surfactant from wastewater has seldom been commercialized, which clearly demands more research in this area to develop more insights on the subject. The aim of the present work was to study the removal of sodium dodecylbenzenesulfonate (SDBS), an anionic surfactant widely used in the detergent industries, from water by batch foam fractionation. The concentration of surfactant in water was high, near and up to six times the critical micelle concentration (CMC). Our work involved a systematic study of the effects of important parameters such as air flow rate, surfactant feed concentration, and the concentration of salt on the removal of the surfactant by foam fractionation, and its recovery in the solid paste form. The unique feature of our experimental system is that batch foam fractionation was performed under conditions wherein at steady state, the volumetric flow rate of air entering the foam phase from the underlying liquid phase matched the volumetric rate of air escaping the foam phase due to 6
foam collapse from the top surface of the open column. This effectively conveyed most of the dissolved surfactant from the underlying aqueous phase to a surfactant paste phase, which formed at the top surface of the foam column.
2. Experimental 2.1. Material The anionic surfactant, sodium dodecylbenzenesulfonate, was purchased from Sigma-Aldrich (India). It had an assay of > 99%, and was clearly soluble in water at 298 K. Sodium chloride (NaCl) was purchased from Merck (India), and had an assay > 99.5%. The water used for preparing the aqueous solutions was purified with a Millipore® water purification system. Its conductivity was 5×10−5 S m−1 and its surface tension was 72.5 mN m−1.
2.2. Foam fractionation The foam fractionation setup was designed and developed to operate in the batch mode. A schematic diagram of the setup is shown in Fig. 2. The column was made of Plexiglas. The height of the column was 1.5 m and its internal diameter was 29 cm. The thickness of the column wall was 5 mm. A disc sparger was installed at the bottom of the column to form air bubbles. Air was introduced from the bottom of the column with the help of an oil-free compressor (Tarsons, Rockyvac 320, India). A rotameter was installed in the air flow line to measure the flow rate. The flow rate was controlled by valves. Samples of the liquid phase were collected by using a syringe inserted through a port, fitted with a Teflon septum and located 3 cm above the base of the column. In this study, foam fractionation was performed with SDBS solutions having concentrations of 500, 1000, 2000 and 3000 mg dm–3. NaCl was added to investigate the effect of salt on surfactant removal. The feed solution was slowly poured into the column from the top. 3 dm3 surfactant solution was used in each run. Air was sparged with a flow rate in the range of 0.4 − 1.6 dm3 min–1. Each experiment was run for 7 h. Samples of the aqueous phase were collected at every 1 h interval. After ~1 h of air sparging, solid surfactant particles began to appear at the top of the foam column (Fig. 3), where the foam was dry. The solid particles (in the form of flakes) were recovered by skimming. The solid collected, was dried in a vacuum oven at 323 K for 24 h, and the weight was measured. The foam column reached its steady state height in about 1 2 h, depending on the operating air flow rate. At the end of 7
7 h, the liquid was drained out and the foam was left to collapse. Afterwards, the inside of the column was washed with a known quantity of water to remove the surfactant adhering to the wall. The surfactant concentration in this liquid was measured. The foam fractionation column was thoroughly cleaned with water before starting the next experiment. An overall material balance was performed to check the consistency of the experimental data obtained from the foam fractionation system. Experiments were conducted with 10, 50 and 100 mol m– 3
concentrations of NaCl at different concentrations of SDBS.
2.3. Measurement of SDBS concentration The concentration of SDBS in the aqueous phase was measured by a UV-Visible spectrophotometer (Perkin Elmer, Lamda-35, Switzerland). The absorbance versus wavelength profiles are shown in Fig. 4 at different SDBS concentrations. The spectra agree well with that reported in the literature [30]. The absorbance was measured at 261 nm wavelength, inasmuch as SDBS has a well-defined peak at this wavelength. An absorbance versus concentration calibration curve was developed to determine the concentration of SDBS. The SDBS concentration was varied in the range of 50–300 mg dm3, because the absorbance versus concentration curve was linear in this concentration range. A zerointercept equation was developed from the calibration plot. The concentration of SDBS in the aqueous phase was determined by collecting 5 cm 3 sample through the collection port. The sample was diluted with a known amount of water to ensure that the absorbance would lie in the range covered by the calibration curve. The sample was transferred to the measuring cuvette and the absorbance was obtained from the UV-Vis spectrophotometer. The concentration of SDBS was determined by using the calibration equation. By applying the dilution factor, the concentration of SDBS in the original sample was determined.
2.4. Measurement of surface tension The adsorption of SDBS at the air–water interface was studied by the surface tension measurements. The surface tensions of the aqueous surfactant solutions were measured by a digital tensiometer (Krüss, K9, Germany). The du Noüy ring method was used to measure the surface tension by following the procedure described in the ASTM Standard D1331-14. The ring used in the measurements was made of platinum and iridium. The sample vessel and the ring were methodologically cleaned before each measurement by following the procedure 8
described in the ASTM Standard D1331–14. The du Noüy ring burnt in a Bunsen burner before each experiment. It was dipped into the aqueous phase and then slowly pulled out through the interface to measure the surface tension.
2.5. Measurement of zeta potential
Zeta potential at the air–water interface was measured by using a zeta potentiometer (Beckman Coulter, Delsa Nano C, Switzerland). Micro-nanobubbles of air were generated in an ultrasonic water bath (PCI Analytics, 3.5L, India). A small amount of the dispersion (3 cm3) containing the micro-nanobubbles was quickly transferred to the flow-cell of the zeta potentiometer and the zeta potential was measured by the electrophoretic method.
2.6. Measurement of bubble size distribution The diameter of the foam bubbles was measured by a photographic technique [31]. The photographs of the bubbles were taken at the lowest portion of the foam, just above the liquid, after 30 min of operation. The photographs were taken by a digital camera (Nikon, D5100, India), and were analyzed by the Digimizer digital image analysis software (MedCalc software, version 4.1, Belgium). About 100 bubbles were analyzed to develop the bubble size distributions.
2.7. Experimental conditions All experiments were carried out in an air-conditioned room where the temperature was maintained at 2981 K. The temperature fluctuation was so small that it did not have any effect on the experimental results. The experiments were repeated three times and the average values of these readings are reported in Section 3.
3. Results and Discussion As air was continuously passed through the surfactant solution, the volume of foam increased. This increase in foam volume matched the air flow rate, as the air introduced into the system went into the foam phase in entirety. The only available means of air escaping from the system was by foam collapse near the top surface of the column. Since the air flow rates used in this work were relatively low, the rate of increase in foam height was rather 9
slow. This ensured that the foam near the top of the column got sufficient time to dry and eventually collapse when its age had increased to above 1 h. The surfactant molecules were continuously transferred from the aqueous phase to the foam phase. The concentration of SDBS in the aqueous phase decreased with time, as shown in Figs. 5–8. The formation and stability of foam is important in the transfer of surfactant from the aqueous to the foam phase. The foam is generated as a result of the incorporation of air into the aqueous phase. It involves the distribution of surfactant molecules at the air–water interface, which is regulated by their surface activity, bulk concentration, and the interactions at the interface. Surfactant concentration and air flow rate play a pivotal role in deciding the rate of foam growth and the foam volume. The surface tension variation is shown in Fig. 9. These data agree well with those reported in the literature [32]. The CMC of SDBS is ~500 mg dm3. The solutions used in this study had surfactant concentrations near and far above the CMC. Therefore, the surface tensions of these solutions were low, and the variation of surface tension with SDBS concentration was small. However, the surface tension significantly decreased with the addition of NaCl, which reflects a strong electrostatic interaction between the surfactant headgroups. Foam formation involves dynamic adsorption of surfactant molecules at the air–water interface. As the SDBS concentration was high in these experiments, the rate of adsorption of surfactant molecules was high. The concentration of the surfactant in the bulk solution and the ability of the surfactant molecules to stabilize the foam films determine the structure and stability of foams. After the formation of the foam, its stability depends on factors such as the gravity drainage, capillary suction, surface elasticity, bulk and surface viscosity, electrostatic double layer (EDL) repulsion, attraction due to van der Waals (vdW) force, and steric repulsion [33]. Foams are non-equilibrium systems and metastable in nature. Freshlyprepared foam contains bubbles of a wide range of diameters. Smaller bubbles have higher internal pressure as compared to the bubbles with larger diameter. The excess pressure inside the spherical bubbles is given by the YoungLaplace equation p
2 rb
, where
rb
is the
radius of the bubble. Higher excess pressure in the smaller bubbles causes the air inside them to diffuse into the larger bubbles. Therefore, the larger bubbles coarsen and increase in size with time, which is known as Ostwald ripening. The coarsening of foam causes the small bubbles to disappear. As the bubble area evolves, bubbles of five or less sides shrink and those of seven or more sides grow. This evolution of foam causes the thin foam films to get
10
stretched and dilated. In such foam films, surface waves are generated due to the mechanical and thermal perturbations, leading to the rupture of the thin film. The surfaces of foam cells are curved at the edges and the bordering foam films are joined at the Plateau borders. Pressure is smaller at these Plateau borders, compared to the center of the film, and thus, it induces a radial flow, which reduces its thickness with time [34]. The drainage process of a liquid film involves several intermediate steps until the entire liquid is drawn out. When two bubbles approach each other, hydrodynamic interaction sets in between them, and a thick lamella is formed. A dimple is formed due to a strong deformation of the two bubble surfaces. Gradually the dimple disappears, leaving behind an essentially plane parallel film [35]. The physical characteristics of the surfactant (e.g. its charge and structure) and its concentration influence the thinning of the foam film down to a critical thickness. When the thickness of the film becomes less than about 100 nm, the EDL, vdW and short-range repulsive forces start to influence its stability [36]. The net disjoining pressure acting on the thin foam film is given by (1)
vdW E D L SR
where
vdW
,
EDL
, and
SR
are the disjoining pressures due to the vdW, EDL, and the
short-range hydration forces. The balance between the capillary pressure and the disjoining pressure,
rp
(where
rp
is the radius of curvature of the Plateau border) may
decrease the drainage rate and prevent the coalescence. However, depending on the concentration of surfactants or electrolytes, bubbles may rupture at a critical thickness, which lies in the range of 10–50 nm [36,37]. Film thinning accelerates when the net disjoining pressure is negative, while a positive disjoining pressure opposes it. The vdW force is attractive and tries to drive the molecules from the thinner to the thicker parts of the film. On the other hand, the repulsive EDL force, originating from the negatively-charged headgroups of the surfactant molecules, tries to repel the surfaces. When two bubble surfaces approach each other, the EDL force is sufficiently large as the double layers overlap. The higher concentration of ions in the film leads to an osmotic pull for the water molecules to move from the bulk to the film, thereby stabilizing the film [38]. Short-range hydration force originates when water molecules are strongly bound to the headgroups of the surfactant molecules. The repulsive force between the surfaces is entropic in origin, and arises due to the overlap of hydrated headgroups, as the surfaces 11
approach each other [38]. These forces inhibit the surfaces from coming closer than ~5 nm [39]. The strength of these forces depends on the energy required to disrupt the hydrogenbonding network and dehydrate the surfaces when they approach each other. The drainage of the foam films can be retarded by the Marangoni effect. For foams generated at low surfactant concentrations, the Marangoni effect is not strong, because the diffusion rate of surfactant molecules at the air–water interface (i.e. at the deformed part of the foam) is low, resulting in a small surface tension gradient. Hence, the Marangoni effect does not play a major role in film drainage at low surfactant concentrations. However, at high surfactant concentrations, the density of surfactant molecules at the interface is high, causing movement of surfactant molecules from the surfactant-rich area to the surfactant-deficient area of the film. This results in a strong surface tension gradient during the deformation of the air–water interface. This can lead to a strong Marangoni effect, which can stabilize the foam film. The rate of foam collapse increases with increasing surfactant concentration. This may be due to the decrease in Gibbs elasticity of the foam film with increasing surfactant concentration. The Gibbs elasticity is (E) is defined as E 2A
d
(2)
dA
Using Langmuir and Gibbs adsorption equations, the Gibbs elasticity is given by [40] 2
E
2
4 R T K Lcs h 1 K L c s
2
(3)
2 K L
A film having high elasticity has more stability [41]. The film elasticity increases with increasing surfactant concentration, reaches a maximum, and then decreases at the high surfactant concentrations [42], which leads to a rapid collapse of the foam. The steady state foam volume increased with the increasing air flow rate as shown in Fig. 10 for 500 mg dm3 SDBS concentration. At the lower air flow rates (i.e. 0.4 and 0.8 dm3 min1), the foam volume became almost constant after about 2 h of operation. However, at the higher air flow rates (i.e. 1.2 and 1.6 dm3 min1), the foam volume rapidly reached its maximum value and became almost constant well before 2 h. Note that the foam fractionation experiments were conducted for 7 h and the concentration of SDBS reduced with time during
12
the entire duration, as depicted in Figs. 5–8. The foam volume profiles show that the rate of foam formation was high in the initial stages. These foams were essentially wet foams. However, after some time, the top portion of the foam column was quite dry, and the foam started to collapse leaving solid flakes of SDBS (see Fig. 3). As the foam lamella drained and finally ruptured, some portions of the foam were destroyed. Therefore, after some time, the rate of foam generation at the bottom of the foam column equaled the loss of foam at the top of the column, signifying steady state operation. This is confirmed by a constant steady foam volume, as depicted in Fig. 10. Similar trends in the foam volume profiles were observed for 1000, 2000 and 3000 mg dm3 SDBS concentrations [see Figs. SM1 – SM3 (Supplementary Materials)]. The volume of foam only slightly increased with the surfactant concentration from that for the 500 mg dm3 system. This is probably due to the fact that the air–water interface became saturated with surfactant adsorption at the CMC, and a complete monolayer formed. Further adsorption was less pronounced, which is evident from the surface tension profiles (Fig. 9). Separation efficiency is a good indicator of the performance of a foam fractionation column in removing surfactant from the aqueous phase. The separation efficiency in a batch process gives a measure of how much surfactant has been removed in the targeted time interval. It is defined as (4)
C0 Ce 100 C0
where
C0
and
Ce
are surfactant concentrations in the feed and in the effluent (at the end of
the run), respectively. The variation of separation efficiency with air flow rate at different SDBS concentrations is shown in Fig. 11. The separation efficiency increased with increasing air flow rate because the latter increased the rate of surfactant removal from the aqueous solution. More than 80% separation efficiency was achieved at the high air flow rates for 500 mg dm3 SDBS concentration. However, the separation efficiency was decreased at the high surfactant concentrations. This is probably because the surfactant removal rate did not vary significantly with the initial surfactant concentration. Since this rate was almost constant, the relative drop in concentration for the 500 ppm system was much more significant as compared to the same at the higher concentrations. The ultimate or natural end-point for this method would be the point at which foam generation stops. In the experiments reported in this paper, the end-point of the experiments was 7 h, which is shorter than the duration
13
required to reach the natural end-point. This caused the apparent separation efficiency and surfactant recovery to be higher for the lower initial surfactant concentrations. We have experimentally verified that high removal efficiencies at the higher surfactant concentrations can be achieved if the batch runs are conducted for a longer duration. Salts of sodium, calcium and magnesium are commonly present in surface and ground waters in varying quantities. Some of these salts precipitate out the anionic surfactant from the solution and considerably reduce the volume of foam. In this work, the effect of NaCl was studied. The effect of NaCl on the concentration decline profiles of SDBS is shown in Figs. 5–8. These results show that the presence of salt reduced the extent of surfactant removal. This has a direct relationship with the volume of foam produced. The foam volume slightly decreased by the addition of salt (Fig. 10). It has been reported [22] that the wetness of foams increases by the addition of salt. With the addition of salt, the repulsion between the charged headgroups of the surfactant molecules decreased, which resulted in more adsorption of the SDBS molecules at the air–water interface. This is manifested in the reduction of surface tension in the presence of salt (Fig. 9). Similar results have been reported in the literature [19,32]. The surface tension depends on the surface potential and the surface charge density as [43] 0 d d
The term,
d
(5)
, represents the electrostatic contribution to the energy per unit area of the
airwater interface. However, with increase in the NaCl concentration, the disjoining pressure due to EDL (i.e.
EDL
) is reduced. It depends on the surface potential and the
thickness of the diffuse part of the EDL (i.e. the Debye length). For a flat foam lamella [38] EDL 64 RTc
(6)
2 z e ta n h exp h 4kT
where R is the gas constant, T is the temperature,
c
is the concentration of electrolyte in the
bulk solution, z is the valence of the ions in the electrolyte, e is the electronic charge, k is Boltzmann’s constant, and is the Debye–Hückel parameter. The surface potential depends on the electrolyte concentration by the Grahame equation [38]. (7)
1 2 2kT 1 s in h 8 R T c 0 ze
14
Equation (7) shows that the surface potential depends on the salt concentration and the surface charge density. The zeta potential was measured in presence and in absence of NaCl. The results are shown in Fig. 12. The zeta potential represents the potential at the surface of shear, which is located at a few molecular diameters away from the actual air–water interface. Therefore, its value gives an approximate measure of the surface potential (i.e.
). The
charge of the air–water interface increased with increasing SDBS concentration, which increased the magnitude of the zeta potential (i.e.
became more negative). With the
addition of a small amount of NaCl (i.e. at 10 mol m3 concentration), the magnitude of zeta potential increased, due to the increase in adsorption of the SDBS molecules at the interface, which increased the surface charge density . However, with further increase in the salt concentration
c , the magnitude of zeta potential decreased (i.e.
became less negative).
This is expected from Eq. (7), inasmuch as the effects of the surface charge density and salt concentration are counteracting. The Debye length
1
N e2 2 A zi ci 0 kT i
1
also depends on the electrolyte concentration as [38]
1 2
(8)
Therefore, the Debye screening length decreases and the Debye–Hückel parameter, , increases with increasing salt concentration. Therefore, it is apparent from Eq. (6) that the addition of salt would reduce the repulsive disjoining pressure in the foam film, which would destabilize the foam. With the reduction in the stability of the foam, the removal of SDBS from the aqueous phase was reduced in the presence of NaCl. The disjoining pressure due to van der Waals force is given by [44] vdW
(9)
AH 6 h
3
The Hamaker constant,
AH
, is rather insensitive to the addition of salt [38]. The disjoining
pressure due to short-range hydration forces is given by [44] S R C1 e x p C 2 h
(10)
15
The constants,
C1
and
C2 ,
are not affected by the addition of salt. Therefore, the EDL force
is the only component in the total disjoining pressure that is significantly reduced by the addition of NaCl. The recovery of the surfactant for reuse may be desirable (e.g. when a single surfactant is recovered and the compound is expensive). The recovery is defined as the ratio of the amount of surfactant present in the paste recovered from the top of the foam column (i.e. the surfactant cake) to the amount present in the feed. It can be expressed as
100 A m o u n t o f s u r f a c ta n t p r e s e n t in th e f e e d s o lu tio n
A m o u n t o f s u r f a c ta n t r e c o v e r e d a s s o lid
(11)
The recovery of solid SDBS is shown in Fig. 13. These profiles showed a similar trend as that of the separation efficiency profiles shown in Fig. 11. A good amount of solid was actually recovered from the dry foam at the top of the column. The rest of the surfactant remained in the foam lamellae, which was not recovered. This material was collected as an aqueous surfactant concentrate after the completion of the run, when the bottom liquid was drained out, the foam collapsed, and the column was washed with water. The solid recovery decreased with the increasing salt concentration. While the data suggest that surfactant recovery was less at the higher surfactant concentrations, this is due to the duration of the experiments. The recovery of surfactant-lean water is another important aspect of foam fractionation. This can enable reuse of the water. The water recovery is shown in Fig. 14. The results show that only a small amount of water was lost in the process. With increasing air flow rate, the volume of foam increased, which caused more loss of water in the foam. With the addition of salt, slightly more amount of water was lost due to the formation of the wet foam. Bubble size is an important variable in the foam fractionation process. The bubble size distribution has a significant effect on foam quality. With time, the spherical bubbles (i.e. kugelschaum) in fresh foam take the polyhedral shape (i.e. polyederschaum), as they become drier. Smaller bubbles enhance the interfacial adsorption of surfactant (because more interfacial area is available for a given volume of foam for the surfactant molecules to adsorb) and give a higher recovery rate. On the other hand, liquid drainage rate from foams made of small bubbles can be slower, resulting in the formation of wet foam, which would compromise the solid recovery [22]. 16
The bubble size depends on the concentrations of SDBS and NaCl, and the air flow rate. The mean bubble radii are shown in Table 1. A sample bubble size distribution is shown in Fig. 15. As the superficial air velocity was increased, collision between the bubbles led to coalescence, and hence, gave rise to larger mean size. The residence time of the bubbles in the aqueous phase was shorter when the air velocity was increased. The shorter residence time in the bulk liquid may lead to incomplete adsorption of surfactant molecules onto the bubble surface (i.e. the adsorption was less than the equilibrium amount). When a bubble is about to be released from a sparger pore, the force balance on it gives the bubble radius rb as 3 d rb 4 g
(12)
1 3
Therefore, the bubble radius depends on surface tension. If the adsorption of surfactant at the air–water interface is incomplete, the value of would be higher, which would result in a larger bubble radius. With increasing surfactant concentration, the surface tension reduced, which reduced the bubble size. When NaCl was present, the surface tension reduced as well, and the bubbles became smaller. At higher surfactant concentration, the coalescence between the bubbles was considerably reduced [45], which contributed to the reduction in bubble size. The results presented in Table 1 corroborate those reported in the literature [46,47].
4. Conclusions Based on the experimental studies reported in this work, the following conclusions were reached. (1) A dissolved surfactant can be separated from its aqueous solution by foam fractionation in a paste or wet-solid form. (2) Increasing the air flow rate increased the foam volume at steady state. Low air flow rates led to smaller foam volume at which steady state operation could be maintained. (3) For the fixed batch duration of 7 h, the experiments with the lower initial surfactant concentration showed higher separation efficiency and surfactant recovery.
17
(4) A good recovery of water was observed. Water recovery decreased with increasing air flow rate. Surfactant concentration did not have a significant effect on the water recovery. (5) In the presence of NaCl in the feed solution, reductions in the surfactant recovery and separation efficiency were observed. Both of these parameters decreased with the increasing salt concentration. (6) The steady state foam volume increased with increasing air flow rate. It decreased in the presence of NaCl. However, the steady state foam volume only slightly increased with increasing surfactant concentration. (7) Bubble size increased with increasing air flow rate. It decreased with increasing surfactant concentration at a given air flow rate. The bubble size also decreased with increasing NaCl concentration. (8) The surface tension slightly decreased with increasing surfactant concentration. However, a significant reduction of surface tension was observed in the presence of NaCl. (9) By the addition of a small amount of NaCl (i.e. 10 mol m−3 concentration), the absolute value of zeta potential of the air–water interface increased. However, at the higher salt concentration (i.e. 50 and 100 mol m−3), the absolute value of the zeta potential decreased.
Acknowledgments The authors thank M/S Unilever Industries Private Limited (India) for financial support of the work reported in this article through the project no. MA-2015-00434 (dated: 12 May 2015). The authors also wish to thank Mr. Shajahan A, Unilever Industries Private Limited (India), for the photograph in Figure 1.
18
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22
Captions for figures
Fig. 1 Foaming in a lake in Bangalore (India) causing overflow of foam on the road.
Fig. 2 Experimental setup for batch foam fractionation: (a) air compressor, (b) air flow control valve, (c) air rotameter, (d) air sparger, (e) cylindrical Plexiglas column, and (f) sample collection port.
Fig. 3 Solid flakes of SDBS formed inside the dry foam at the top of the column.
Fig. 4 UV-Vis absorption spectra of SDBS at different concentrations.
Fig. 5 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 500 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 6 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 1000 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 7 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 2000 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 8 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 3000 mg dm3. The effect of NaCl is also shown in the figure.
23
Fig. 9 Surface tensions of the SDBS solutions with and without NaCl.
Fig. 10 Variation of foam volume with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 500 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 11 Variation of separation efficiency with air flow rate: (a) 500, (b) 1000, (c) 2000, and (d) 3000 mg dm3 SDBS concentrations. The effect of NaCl is also shown in the figure.
Fig. 12 Zeta potential at the air–water interface at different concentrations of SDBS and NaCl.
Fig. 13 Solid recovery at different air flow rates: (a) 500, (b) 1000, (c) 2000, and (d) 3000 mg dm3 SDBS concentrations. The effect of NaCl is also shown in the figure.
Fig. 14 Water recovery at different air flow rates after 7 h of operation. Initial SDBS concentration = 500 mg dm3.
Fig. 15 Bubble size distribution in 500 mg dm−3 SDBS solution in presence of 100 mol m−3 NaCl at the air flow rate of 1.2 dm3 min–1.
24
Fig. 1
Fig. 2 25
Fig. 3
Fig. 4
26
(a)
(b)
(c)
(d)
Fig. 5
27
(a)
(b)
(c)
(d)
Fig. 6
28
(a)
(b)
(c)
(d)
Fig. 7
29
(a)
(b)
(c)
(d)
Fig. 8
30
Fig. 9
31
(a)
(b)
(c)
(d)
Fig. 10
32
(a)
(b)
(c)
(d)
Fig. 11
Fig.
12
33
(a)
(b)
(c)
(d)
Fig. 13
Fig. 14
34
Fig. 15
35
Table 1 Mean bubble radius at different surfactant and salt concentrations, and different air flow rates
Air flow rate = 0.4 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (m) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
603
582
563
547
1000
584
565
546
529
2000
572
549
531
516
3000
564
538
517
502
Air flow rate = 0.8 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
611
587
570
562
1000
599
574
556
538
2000
585
561
545
524
3000
578
548
529
514
Air flow rate = 1.2 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
625
592
577
569
1000
614
579
565
554
2000
602
570
558
549
36
3000
588
559
547
535
Air flow rate = 1.6 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
642
615
598
581
1000
629
604
579
562
2000
617
588
565
555
3000
605
576
551
540
37
S2213-3437(17)30059-3 http://dx.doi.org/doi:10.1016/j.jece.2017.02.008 JECE 1474
To appear in: Received date: Revised date: Accepted date:
1-11-2016 20-1-2017 4-2-2017
Please cite this article as: Shashank Shekhar Srinet, Ananya Basak, Pallab Ghosh, Separation of Anionic Surfactant in Paste Form from its Aqueous Solutions using Foam Fractionation, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2017.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Separation of Anionic Surfactant in Paste Form from
its
Aqueous
Solutions
using
Fractionation Shashank Shekhar Srinet, Ananya Basak, and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati Guwahati – 781039, India
and
Jaideep Chatterjee, Unilever R&D, Bangalore Laboratory 64, Main Road, Whitefield, Bangalore 560066, India
*
author to whom all correspondence should be addressed
E-mail: [email protected] Tel: +91.361.2582253 Fax: +91.361.2690762
1
Foam
Graphical abstract
Highlights
Surfactant was recovered in paste/solid form by foam fractionation
Separation efficiency and solid recovery increased with increasing air flow rate
Steady state foam volume increased with increasing air flow rate
Separation efficiency and solid recovery decreased in the presence of NaCl
High recovery of water was achieved
Abstract Foam fractionation is an effective method for the removal and recovery of a surfactant from water. In this work, removal of an anionic surfactant (i.e. sodium dodecylbenzenesulfonate) was studied, at high concentrations, in a batch foam fractionation column. The anionic surfactant could be separated in a solid/paste form near the top of the column by using this process High surfactant removal from water was achieved for surfactant concentrations varying from approximately near its critical micelle concentration to six times this concentration. The removal efficiency was reduced in the presence of salt (i.e. NaCl). The surfactant separation was enhanced by increasing the air flow rate. The recovery of surfactant was reasonably good. The recovery of water was good as well. The volume of foam and its age played important roles in the separation efficiency and the surfactant recovery. The adsorption of surfactant and the electric charge at the air–water interface were measured and analyzed to explain the separation efficacy. The bubble size increased with increasing air flow rate, and decreased with increasing surfactant and salt concentrations. 2
Keywords: Batch foam fractionation; Salt effect; Surfactant recovery; Wastewater treatment; Water recycling.
Nomenclature Hamaker constant, J
AH
A surface area of foam lamella, m2 Hamaker constant, J
AH
concentration of surfactant in the bulk solution, mol m–3
cs c
bulk concentration of ion, mol m–3
C1
constant in Eq. (10), Pa
C2
constant in Eq. (10), Pa
Ce
concentration of the effluent stream, mg dm3
C0
concentration of the original feed solution, mg dm3
d
diameter of sparger pore, m
e
electronic charge, C
g
acceleration due to gravity, m s2
E Gibbs film elasticity, N m1– h thickness of foam film, m k
Boltzmann constant, J K–1
KL
equilibrium constant in the Langmuir adsorption equation, m3 mol–1
NA
Avogadro’s number, mol–1
rb
radius of bubble, m
rp
radius of Plateau border, m
Greek Symbols R gas constant, J mol–1 K–1 T temperature, K z valence of ion surface tension of surfactant solution with/without NaCl, N m–1 3
surface tension of pure water, N m–1
0
surface excess concentration of surfactant, mol m–2
maximum adsorption capacity of the air–water interface, mol m–2
p
pressure difference, Pa
dielectric constant of the aqueous phase
0
permittivity of free space, C2 J–1 m–1
zeta potential, V
Debye–Hückel parameter, m–1
surfactant separation efficiency, %
disjoining pressure, Pa
density of water, kg m3
surfactant recovery, %
charge density at the air–water interface, C m–2
chemical potential, J mol–1
potential at air–water interface, V
Abbreviations CMC critical micelle concentration EDL electrostatic double layer SDBS sodium dodecylbenzenesulfonate SR short range UV-Vis ultraviolet-visible vdW van der Waals
1. Introduction Water treatment is vital for reducing the impact of numerous chemicals that are released to the natural water sources (e.g. ponds, lakes and rivers) from a wide variety of industrial and
4
domestic applications. Pollution of water has a huge impact on the environment. Treatment of wastewater is necessary to reuse the water, and it is an ultimate resource. Adsorption [1-6], biodegradation [7,8] and oxidation [9-11] are some of the common methods used for removing the pollutants from wastewater. Surfactants are released from laundries, chemical industries (e.g. soaps and detergents, personal care products, food, paper, and fabric), as well as from household washing and cleaning operations [12]. Most of the surfactants used in soaps and detergents are biodegradable, except a few. However, complete biodegradation may take weeks. Common oxidizing agents used in water treatment, such as chlorine and ozone, break the surfactant molecules into smaller ones, which are more biodegradable. However, in some cases, the intermediate ozonation products are more toxic than the base surfactant [13]. Recovery of the surfactant is not possible by these methods. Surfactants can have poisonous effects on aquatic life, if their concentration is high. They can destroy the external mucus layers of the fish, which protect them from bacteria and parasites. They can damage their gills also. Many fishes will die if the surfactant concentration exceeds 15 mg dm3 [14]. Surfactants also reduce the breeding of aquatic organisms. Most surfactants used in the industrial and domestic purposes reduce the surface tension of water. Fishes significantly absorb harmful organic chemicals (e.g. pesticides) when the surface tension of water is low. Another hazard created by surfactants is the foaming in the river and lake (see Fig. 1). Such foaming reduces oxygen transfer to water, and also causes inconvenience to the nearby residents and traffic. A promising method for the removal of surfactants from wastewaters is foam fractionation [15-17]. This method can also help to recover the surfactant. It is a simple and low-cost method, which isolates the surfactant molecules based on their surface activity [18,19]. This method is not limited to the surfactants, but can also be used to remove proteins and other surface active compounds. It is commonly used, albeit on a small scale, for the removal of organic wastes from aquariums. These units are known as “protein skimmers”. Apart from the enrichment of bioproducts, the foam fractionation technique is increasingly being used for the removal of surface active contaminants from wastewater streams. In this method, air is sparged to produce bubbles, which rise along the column, producing a good amount of foam. As the bubbles form and travel through the continuous aqueous phase, the surfactant molecules adsorb at the air–water interface. This leads to the formation of a surfactant-rich foam phase at the top of the column, and an aqueous phase lean in surfactant, at the bottom of the column. Thus, there is a depletion of surfactant in the bulk solution. It
5
has been reported that there is a concentration gradient in the column, which is dependent on the transfer between the liquid flowing up and down the column [20]. Foam fractionation can be performed either batch-wise or in a continuous manner. There are more sophisticated modes of operation incorporating enrichment and/or stripping [21]. The foam fractionation column can be either single- or multi-stage. Several studies have investigated the recovery of surfactant using foam fractionation, and examined the effects of various parameters on separation efficiency [22,23]. Removal of anionic surfactants (e.g. sodium dodecyl sulfate) from water has been reported [24]. The effects air flow rate, feed flow rate, time, foam height and liquid height have been studied [25,26]. The foam fractionation method has been applied to binary surfactant systems as well [27]. The surfactants can act as a collector and bind metal cations into chelates that can be easily removed by foam fractionation [22]. Foam fractionation has been extensively studied for the purpose of removing a variety of pollutants (e.g. heavy metals) from water by adding a surfactant [24,28]. Simple models for foam fractionation of surfactants have been developed without considering liquid drainage and coalescence of bubbles, and such models are valid only for dilute surfactant solutions [28]. Foam is a good medium for adsorptive separation of surfactants because it has a very high specific surface area on which the target species adsorbs. The amount of interstitial liquid is low, especially when the foam is dry. It is the surface rather than the interstitial liquid that drives the separation process because the target species tends to move from the interstitial liquid to the surface [15]. Nonionic organic pollutants can also be effectively removed by foam fractionation [29]. Despite a large number of publications, the use of foam fractionation for removal/recovery of surfactant from wastewater has seldom been commercialized, which clearly demands more research in this area to develop more insights on the subject. The aim of the present work was to study the removal of sodium dodecylbenzenesulfonate (SDBS), an anionic surfactant widely used in the detergent industries, from water by batch foam fractionation. The concentration of surfactant in water was high, near and up to six times the critical micelle concentration (CMC). Our work involved a systematic study of the effects of important parameters such as air flow rate, surfactant feed concentration, and the concentration of salt on the removal of the surfactant by foam fractionation, and its recovery in the solid paste form. The unique feature of our experimental system is that batch foam fractionation was performed under conditions wherein at steady state, the volumetric flow rate of air entering the foam phase from the underlying liquid phase matched the volumetric rate of air escaping the foam phase due to 6
foam collapse from the top surface of the open column. This effectively conveyed most of the dissolved surfactant from the underlying aqueous phase to a surfactant paste phase, which formed at the top surface of the foam column.
2. Experimental 2.1. Material The anionic surfactant, sodium dodecylbenzenesulfonate, was purchased from Sigma-Aldrich (India). It had an assay of > 99%, and was clearly soluble in water at 298 K. Sodium chloride (NaCl) was purchased from Merck (India), and had an assay > 99.5%. The water used for preparing the aqueous solutions was purified with a Millipore® water purification system. Its conductivity was 5×10−5 S m−1 and its surface tension was 72.5 mN m−1.
2.2. Foam fractionation The foam fractionation setup was designed and developed to operate in the batch mode. A schematic diagram of the setup is shown in Fig. 2. The column was made of Plexiglas. The height of the column was 1.5 m and its internal diameter was 29 cm. The thickness of the column wall was 5 mm. A disc sparger was installed at the bottom of the column to form air bubbles. Air was introduced from the bottom of the column with the help of an oil-free compressor (Tarsons, Rockyvac 320, India). A rotameter was installed in the air flow line to measure the flow rate. The flow rate was controlled by valves. Samples of the liquid phase were collected by using a syringe inserted through a port, fitted with a Teflon septum and located 3 cm above the base of the column. In this study, foam fractionation was performed with SDBS solutions having concentrations of 500, 1000, 2000 and 3000 mg dm–3. NaCl was added to investigate the effect of salt on surfactant removal. The feed solution was slowly poured into the column from the top. 3 dm3 surfactant solution was used in each run. Air was sparged with a flow rate in the range of 0.4 − 1.6 dm3 min–1. Each experiment was run for 7 h. Samples of the aqueous phase were collected at every 1 h interval. After ~1 h of air sparging, solid surfactant particles began to appear at the top of the foam column (Fig. 3), where the foam was dry. The solid particles (in the form of flakes) were recovered by skimming. The solid collected, was dried in a vacuum oven at 323 K for 24 h, and the weight was measured. The foam column reached its steady state height in about 1 2 h, depending on the operating air flow rate. At the end of 7
7 h, the liquid was drained out and the foam was left to collapse. Afterwards, the inside of the column was washed with a known quantity of water to remove the surfactant adhering to the wall. The surfactant concentration in this liquid was measured. The foam fractionation column was thoroughly cleaned with water before starting the next experiment. An overall material balance was performed to check the consistency of the experimental data obtained from the foam fractionation system. Experiments were conducted with 10, 50 and 100 mol m– 3
concentrations of NaCl at different concentrations of SDBS.
2.3. Measurement of SDBS concentration The concentration of SDBS in the aqueous phase was measured by a UV-Visible spectrophotometer (Perkin Elmer, Lamda-35, Switzerland). The absorbance versus wavelength profiles are shown in Fig. 4 at different SDBS concentrations. The spectra agree well with that reported in the literature [30]. The absorbance was measured at 261 nm wavelength, inasmuch as SDBS has a well-defined peak at this wavelength. An absorbance versus concentration calibration curve was developed to determine the concentration of SDBS. The SDBS concentration was varied in the range of 50–300 mg dm3, because the absorbance versus concentration curve was linear in this concentration range. A zerointercept equation was developed from the calibration plot. The concentration of SDBS in the aqueous phase was determined by collecting 5 cm 3 sample through the collection port. The sample was diluted with a known amount of water to ensure that the absorbance would lie in the range covered by the calibration curve. The sample was transferred to the measuring cuvette and the absorbance was obtained from the UV-Vis spectrophotometer. The concentration of SDBS was determined by using the calibration equation. By applying the dilution factor, the concentration of SDBS in the original sample was determined.
2.4. Measurement of surface tension The adsorption of SDBS at the air–water interface was studied by the surface tension measurements. The surface tensions of the aqueous surfactant solutions were measured by a digital tensiometer (Krüss, K9, Germany). The du Noüy ring method was used to measure the surface tension by following the procedure described in the ASTM Standard D1331-14. The ring used in the measurements was made of platinum and iridium. The sample vessel and the ring were methodologically cleaned before each measurement by following the procedure 8
described in the ASTM Standard D1331–14. The du Noüy ring burnt in a Bunsen burner before each experiment. It was dipped into the aqueous phase and then slowly pulled out through the interface to measure the surface tension.
2.5. Measurement of zeta potential
Zeta potential at the air–water interface was measured by using a zeta potentiometer (Beckman Coulter, Delsa Nano C, Switzerland). Micro-nanobubbles of air were generated in an ultrasonic water bath (PCI Analytics, 3.5L, India). A small amount of the dispersion (3 cm3) containing the micro-nanobubbles was quickly transferred to the flow-cell of the zeta potentiometer and the zeta potential was measured by the electrophoretic method.
2.6. Measurement of bubble size distribution The diameter of the foam bubbles was measured by a photographic technique [31]. The photographs of the bubbles were taken at the lowest portion of the foam, just above the liquid, after 30 min of operation. The photographs were taken by a digital camera (Nikon, D5100, India), and were analyzed by the Digimizer digital image analysis software (MedCalc software, version 4.1, Belgium). About 100 bubbles were analyzed to develop the bubble size distributions.
2.7. Experimental conditions All experiments were carried out in an air-conditioned room where the temperature was maintained at 2981 K. The temperature fluctuation was so small that it did not have any effect on the experimental results. The experiments were repeated three times and the average values of these readings are reported in Section 3.
3. Results and Discussion As air was continuously passed through the surfactant solution, the volume of foam increased. This increase in foam volume matched the air flow rate, as the air introduced into the system went into the foam phase in entirety. The only available means of air escaping from the system was by foam collapse near the top surface of the column. Since the air flow rates used in this work were relatively low, the rate of increase in foam height was rather 9
slow. This ensured that the foam near the top of the column got sufficient time to dry and eventually collapse when its age had increased to above 1 h. The surfactant molecules were continuously transferred from the aqueous phase to the foam phase. The concentration of SDBS in the aqueous phase decreased with time, as shown in Figs. 5–8. The formation and stability of foam is important in the transfer of surfactant from the aqueous to the foam phase. The foam is generated as a result of the incorporation of air into the aqueous phase. It involves the distribution of surfactant molecules at the air–water interface, which is regulated by their surface activity, bulk concentration, and the interactions at the interface. Surfactant concentration and air flow rate play a pivotal role in deciding the rate of foam growth and the foam volume. The surface tension variation is shown in Fig. 9. These data agree well with those reported in the literature [32]. The CMC of SDBS is ~500 mg dm3. The solutions used in this study had surfactant concentrations near and far above the CMC. Therefore, the surface tensions of these solutions were low, and the variation of surface tension with SDBS concentration was small. However, the surface tension significantly decreased with the addition of NaCl, which reflects a strong electrostatic interaction between the surfactant headgroups. Foam formation involves dynamic adsorption of surfactant molecules at the air–water interface. As the SDBS concentration was high in these experiments, the rate of adsorption of surfactant molecules was high. The concentration of the surfactant in the bulk solution and the ability of the surfactant molecules to stabilize the foam films determine the structure and stability of foams. After the formation of the foam, its stability depends on factors such as the gravity drainage, capillary suction, surface elasticity, bulk and surface viscosity, electrostatic double layer (EDL) repulsion, attraction due to van der Waals (vdW) force, and steric repulsion [33]. Foams are non-equilibrium systems and metastable in nature. Freshlyprepared foam contains bubbles of a wide range of diameters. Smaller bubbles have higher internal pressure as compared to the bubbles with larger diameter. The excess pressure inside the spherical bubbles is given by the YoungLaplace equation p
2 rb
, where
rb
is the
radius of the bubble. Higher excess pressure in the smaller bubbles causes the air inside them to diffuse into the larger bubbles. Therefore, the larger bubbles coarsen and increase in size with time, which is known as Ostwald ripening. The coarsening of foam causes the small bubbles to disappear. As the bubble area evolves, bubbles of five or less sides shrink and those of seven or more sides grow. This evolution of foam causes the thin foam films to get
10
stretched and dilated. In such foam films, surface waves are generated due to the mechanical and thermal perturbations, leading to the rupture of the thin film. The surfaces of foam cells are curved at the edges and the bordering foam films are joined at the Plateau borders. Pressure is smaller at these Plateau borders, compared to the center of the film, and thus, it induces a radial flow, which reduces its thickness with time [34]. The drainage process of a liquid film involves several intermediate steps until the entire liquid is drawn out. When two bubbles approach each other, hydrodynamic interaction sets in between them, and a thick lamella is formed. A dimple is formed due to a strong deformation of the two bubble surfaces. Gradually the dimple disappears, leaving behind an essentially plane parallel film [35]. The physical characteristics of the surfactant (e.g. its charge and structure) and its concentration influence the thinning of the foam film down to a critical thickness. When the thickness of the film becomes less than about 100 nm, the EDL, vdW and short-range repulsive forces start to influence its stability [36]. The net disjoining pressure acting on the thin foam film is given by (1)
vdW E D L SR
where
vdW
,
EDL
, and
SR
are the disjoining pressures due to the vdW, EDL, and the
short-range hydration forces. The balance between the capillary pressure and the disjoining pressure,
rp
(where
rp
is the radius of curvature of the Plateau border) may
decrease the drainage rate and prevent the coalescence. However, depending on the concentration of surfactants or electrolytes, bubbles may rupture at a critical thickness, which lies in the range of 10–50 nm [36,37]. Film thinning accelerates when the net disjoining pressure is negative, while a positive disjoining pressure opposes it. The vdW force is attractive and tries to drive the molecules from the thinner to the thicker parts of the film. On the other hand, the repulsive EDL force, originating from the negatively-charged headgroups of the surfactant molecules, tries to repel the surfaces. When two bubble surfaces approach each other, the EDL force is sufficiently large as the double layers overlap. The higher concentration of ions in the film leads to an osmotic pull for the water molecules to move from the bulk to the film, thereby stabilizing the film [38]. Short-range hydration force originates when water molecules are strongly bound to the headgroups of the surfactant molecules. The repulsive force between the surfaces is entropic in origin, and arises due to the overlap of hydrated headgroups, as the surfaces 11
approach each other [38]. These forces inhibit the surfaces from coming closer than ~5 nm [39]. The strength of these forces depends on the energy required to disrupt the hydrogenbonding network and dehydrate the surfaces when they approach each other. The drainage of the foam films can be retarded by the Marangoni effect. For foams generated at low surfactant concentrations, the Marangoni effect is not strong, because the diffusion rate of surfactant molecules at the air–water interface (i.e. at the deformed part of the foam) is low, resulting in a small surface tension gradient. Hence, the Marangoni effect does not play a major role in film drainage at low surfactant concentrations. However, at high surfactant concentrations, the density of surfactant molecules at the interface is high, causing movement of surfactant molecules from the surfactant-rich area to the surfactant-deficient area of the film. This results in a strong surface tension gradient during the deformation of the air–water interface. This can lead to a strong Marangoni effect, which can stabilize the foam film. The rate of foam collapse increases with increasing surfactant concentration. This may be due to the decrease in Gibbs elasticity of the foam film with increasing surfactant concentration. The Gibbs elasticity is (E) is defined as E 2A
d
(2)
dA
Using Langmuir and Gibbs adsorption equations, the Gibbs elasticity is given by [40] 2
E
2
4 R T K Lcs h 1 K L c s
2
(3)
2 K L
A film having high elasticity has more stability [41]. The film elasticity increases with increasing surfactant concentration, reaches a maximum, and then decreases at the high surfactant concentrations [42], which leads to a rapid collapse of the foam. The steady state foam volume increased with the increasing air flow rate as shown in Fig. 10 for 500 mg dm3 SDBS concentration. At the lower air flow rates (i.e. 0.4 and 0.8 dm3 min1), the foam volume became almost constant after about 2 h of operation. However, at the higher air flow rates (i.e. 1.2 and 1.6 dm3 min1), the foam volume rapidly reached its maximum value and became almost constant well before 2 h. Note that the foam fractionation experiments were conducted for 7 h and the concentration of SDBS reduced with time during
12
the entire duration, as depicted in Figs. 5–8. The foam volume profiles show that the rate of foam formation was high in the initial stages. These foams were essentially wet foams. However, after some time, the top portion of the foam column was quite dry, and the foam started to collapse leaving solid flakes of SDBS (see Fig. 3). As the foam lamella drained and finally ruptured, some portions of the foam were destroyed. Therefore, after some time, the rate of foam generation at the bottom of the foam column equaled the loss of foam at the top of the column, signifying steady state operation. This is confirmed by a constant steady foam volume, as depicted in Fig. 10. Similar trends in the foam volume profiles were observed for 1000, 2000 and 3000 mg dm3 SDBS concentrations [see Figs. SM1 – SM3 (Supplementary Materials)]. The volume of foam only slightly increased with the surfactant concentration from that for the 500 mg dm3 system. This is probably due to the fact that the air–water interface became saturated with surfactant adsorption at the CMC, and a complete monolayer formed. Further adsorption was less pronounced, which is evident from the surface tension profiles (Fig. 9). Separation efficiency is a good indicator of the performance of a foam fractionation column in removing surfactant from the aqueous phase. The separation efficiency in a batch process gives a measure of how much surfactant has been removed in the targeted time interval. It is defined as (4)
C0 Ce 100 C0
where
C0
and
Ce
are surfactant concentrations in the feed and in the effluent (at the end of
the run), respectively. The variation of separation efficiency with air flow rate at different SDBS concentrations is shown in Fig. 11. The separation efficiency increased with increasing air flow rate because the latter increased the rate of surfactant removal from the aqueous solution. More than 80% separation efficiency was achieved at the high air flow rates for 500 mg dm3 SDBS concentration. However, the separation efficiency was decreased at the high surfactant concentrations. This is probably because the surfactant removal rate did not vary significantly with the initial surfactant concentration. Since this rate was almost constant, the relative drop in concentration for the 500 ppm system was much more significant as compared to the same at the higher concentrations. The ultimate or natural end-point for this method would be the point at which foam generation stops. In the experiments reported in this paper, the end-point of the experiments was 7 h, which is shorter than the duration
13
required to reach the natural end-point. This caused the apparent separation efficiency and surfactant recovery to be higher for the lower initial surfactant concentrations. We have experimentally verified that high removal efficiencies at the higher surfactant concentrations can be achieved if the batch runs are conducted for a longer duration. Salts of sodium, calcium and magnesium are commonly present in surface and ground waters in varying quantities. Some of these salts precipitate out the anionic surfactant from the solution and considerably reduce the volume of foam. In this work, the effect of NaCl was studied. The effect of NaCl on the concentration decline profiles of SDBS is shown in Figs. 5–8. These results show that the presence of salt reduced the extent of surfactant removal. This has a direct relationship with the volume of foam produced. The foam volume slightly decreased by the addition of salt (Fig. 10). It has been reported [22] that the wetness of foams increases by the addition of salt. With the addition of salt, the repulsion between the charged headgroups of the surfactant molecules decreased, which resulted in more adsorption of the SDBS molecules at the air–water interface. This is manifested in the reduction of surface tension in the presence of salt (Fig. 9). Similar results have been reported in the literature [19,32]. The surface tension depends on the surface potential and the surface charge density as [43] 0 d d
The term,
d
(5)
, represents the electrostatic contribution to the energy per unit area of the
airwater interface. However, with increase in the NaCl concentration, the disjoining pressure due to EDL (i.e.
EDL
) is reduced. It depends on the surface potential and the
thickness of the diffuse part of the EDL (i.e. the Debye length). For a flat foam lamella [38] EDL 64 RTc
(6)
2 z e ta n h exp h 4kT
where R is the gas constant, T is the temperature,
c
is the concentration of electrolyte in the
bulk solution, z is the valence of the ions in the electrolyte, e is the electronic charge, k is Boltzmann’s constant, and is the Debye–Hückel parameter. The surface potential depends on the electrolyte concentration by the Grahame equation [38]. (7)
1 2 2kT 1 s in h 8 R T c 0 ze
14
Equation (7) shows that the surface potential depends on the salt concentration and the surface charge density. The zeta potential was measured in presence and in absence of NaCl. The results are shown in Fig. 12. The zeta potential represents the potential at the surface of shear, which is located at a few molecular diameters away from the actual air–water interface. Therefore, its value gives an approximate measure of the surface potential (i.e.
). The
charge of the air–water interface increased with increasing SDBS concentration, which increased the magnitude of the zeta potential (i.e.
became more negative). With the
addition of a small amount of NaCl (i.e. at 10 mol m3 concentration), the magnitude of zeta potential increased, due to the increase in adsorption of the SDBS molecules at the interface, which increased the surface charge density . However, with further increase in the salt concentration
c , the magnitude of zeta potential decreased (i.e.
became less negative).
This is expected from Eq. (7), inasmuch as the effects of the surface charge density and salt concentration are counteracting. The Debye length
1
N e2 2 A zi ci 0 kT i
1
also depends on the electrolyte concentration as [38]
1 2
(8)
Therefore, the Debye screening length decreases and the Debye–Hückel parameter, , increases with increasing salt concentration. Therefore, it is apparent from Eq. (6) that the addition of salt would reduce the repulsive disjoining pressure in the foam film, which would destabilize the foam. With the reduction in the stability of the foam, the removal of SDBS from the aqueous phase was reduced in the presence of NaCl. The disjoining pressure due to van der Waals force is given by [44] vdW
(9)
AH 6 h
3
The Hamaker constant,
AH
, is rather insensitive to the addition of salt [38]. The disjoining
pressure due to short-range hydration forces is given by [44] S R C1 e x p C 2 h
(10)
15
The constants,
C1
and
C2 ,
are not affected by the addition of salt. Therefore, the EDL force
is the only component in the total disjoining pressure that is significantly reduced by the addition of NaCl. The recovery of the surfactant for reuse may be desirable (e.g. when a single surfactant is recovered and the compound is expensive). The recovery is defined as the ratio of the amount of surfactant present in the paste recovered from the top of the foam column (i.e. the surfactant cake) to the amount present in the feed. It can be expressed as
100 A m o u n t o f s u r f a c ta n t p r e s e n t in th e f e e d s o lu tio n
A m o u n t o f s u r f a c ta n t r e c o v e r e d a s s o lid
(11)
The recovery of solid SDBS is shown in Fig. 13. These profiles showed a similar trend as that of the separation efficiency profiles shown in Fig. 11. A good amount of solid was actually recovered from the dry foam at the top of the column. The rest of the surfactant remained in the foam lamellae, which was not recovered. This material was collected as an aqueous surfactant concentrate after the completion of the run, when the bottom liquid was drained out, the foam collapsed, and the column was washed with water. The solid recovery decreased with the increasing salt concentration. While the data suggest that surfactant recovery was less at the higher surfactant concentrations, this is due to the duration of the experiments. The recovery of surfactant-lean water is another important aspect of foam fractionation. This can enable reuse of the water. The water recovery is shown in Fig. 14. The results show that only a small amount of water was lost in the process. With increasing air flow rate, the volume of foam increased, which caused more loss of water in the foam. With the addition of salt, slightly more amount of water was lost due to the formation of the wet foam. Bubble size is an important variable in the foam fractionation process. The bubble size distribution has a significant effect on foam quality. With time, the spherical bubbles (i.e. kugelschaum) in fresh foam take the polyhedral shape (i.e. polyederschaum), as they become drier. Smaller bubbles enhance the interfacial adsorption of surfactant (because more interfacial area is available for a given volume of foam for the surfactant molecules to adsorb) and give a higher recovery rate. On the other hand, liquid drainage rate from foams made of small bubbles can be slower, resulting in the formation of wet foam, which would compromise the solid recovery [22]. 16
The bubble size depends on the concentrations of SDBS and NaCl, and the air flow rate. The mean bubble radii are shown in Table 1. A sample bubble size distribution is shown in Fig. 15. As the superficial air velocity was increased, collision between the bubbles led to coalescence, and hence, gave rise to larger mean size. The residence time of the bubbles in the aqueous phase was shorter when the air velocity was increased. The shorter residence time in the bulk liquid may lead to incomplete adsorption of surfactant molecules onto the bubble surface (i.e. the adsorption was less than the equilibrium amount). When a bubble is about to be released from a sparger pore, the force balance on it gives the bubble radius rb as 3 d rb 4 g
(12)
1 3
Therefore, the bubble radius depends on surface tension. If the adsorption of surfactant at the air–water interface is incomplete, the value of would be higher, which would result in a larger bubble radius. With increasing surfactant concentration, the surface tension reduced, which reduced the bubble size. When NaCl was present, the surface tension reduced as well, and the bubbles became smaller. At higher surfactant concentration, the coalescence between the bubbles was considerably reduced [45], which contributed to the reduction in bubble size. The results presented in Table 1 corroborate those reported in the literature [46,47].
4. Conclusions Based on the experimental studies reported in this work, the following conclusions were reached. (1) A dissolved surfactant can be separated from its aqueous solution by foam fractionation in a paste or wet-solid form. (2) Increasing the air flow rate increased the foam volume at steady state. Low air flow rates led to smaller foam volume at which steady state operation could be maintained. (3) For the fixed batch duration of 7 h, the experiments with the lower initial surfactant concentration showed higher separation efficiency and surfactant recovery.
17
(4) A good recovery of water was observed. Water recovery decreased with increasing air flow rate. Surfactant concentration did not have a significant effect on the water recovery. (5) In the presence of NaCl in the feed solution, reductions in the surfactant recovery and separation efficiency were observed. Both of these parameters decreased with the increasing salt concentration. (6) The steady state foam volume increased with increasing air flow rate. It decreased in the presence of NaCl. However, the steady state foam volume only slightly increased with increasing surfactant concentration. (7) Bubble size increased with increasing air flow rate. It decreased with increasing surfactant concentration at a given air flow rate. The bubble size also decreased with increasing NaCl concentration. (8) The surface tension slightly decreased with increasing surfactant concentration. However, a significant reduction of surface tension was observed in the presence of NaCl. (9) By the addition of a small amount of NaCl (i.e. 10 mol m−3 concentration), the absolute value of zeta potential of the air–water interface increased. However, at the higher salt concentration (i.e. 50 and 100 mol m−3), the absolute value of the zeta potential decreased.
Acknowledgments The authors thank M/S Unilever Industries Private Limited (India) for financial support of the work reported in this article through the project no. MA-2015-00434 (dated: 12 May 2015). The authors also wish to thank Mr. Shajahan A, Unilever Industries Private Limited (India), for the photograph in Figure 1.
18
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22
Captions for figures
Fig. 1 Foaming in a lake in Bangalore (India) causing overflow of foam on the road.
Fig. 2 Experimental setup for batch foam fractionation: (a) air compressor, (b) air flow control valve, (c) air rotameter, (d) air sparger, (e) cylindrical Plexiglas column, and (f) sample collection port.
Fig. 3 Solid flakes of SDBS formed inside the dry foam at the top of the column.
Fig. 4 UV-Vis absorption spectra of SDBS at different concentrations.
Fig. 5 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 500 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 6 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 1000 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 7 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 2000 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 8 Effect of air flow rate on the concentration profiles of SDBS with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 3000 mg dm3. The effect of NaCl is also shown in the figure.
23
Fig. 9 Surface tensions of the SDBS solutions with and without NaCl.
Fig. 10 Variation of foam volume with time: (a) 0.4, (b) 0.8, (c) 1.2, and (d) 1.6 dm3 min1. Initial SDBS concentration = 500 mg dm3. The effect of NaCl is also shown in the figure.
Fig. 11 Variation of separation efficiency with air flow rate: (a) 500, (b) 1000, (c) 2000, and (d) 3000 mg dm3 SDBS concentrations. The effect of NaCl is also shown in the figure.
Fig. 12 Zeta potential at the air–water interface at different concentrations of SDBS and NaCl.
Fig. 13 Solid recovery at different air flow rates: (a) 500, (b) 1000, (c) 2000, and (d) 3000 mg dm3 SDBS concentrations. The effect of NaCl is also shown in the figure.
Fig. 14 Water recovery at different air flow rates after 7 h of operation. Initial SDBS concentration = 500 mg dm3.
Fig. 15 Bubble size distribution in 500 mg dm−3 SDBS solution in presence of 100 mol m−3 NaCl at the air flow rate of 1.2 dm3 min–1.
24
Fig. 1
Fig. 2 25
Fig. 3
Fig. 4
26
(a)
(b)
(c)
(d)
Fig. 5
27
(a)
(b)
(c)
(d)
Fig. 6
28
(a)
(b)
(c)
(d)
Fig. 7
29
(a)
(b)
(c)
(d)
Fig. 8
30
Fig. 9
31
(a)
(b)
(c)
(d)
Fig. 10
32
(a)
(b)
(c)
(d)
Fig. 11
Fig.
12
33
(a)
(b)
(c)
(d)
Fig. 13
Fig. 14
34
Fig. 15
35
Table 1 Mean bubble radius at different surfactant and salt concentrations, and different air flow rates
Air flow rate = 0.4 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (m) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
603
582
563
547
1000
584
565
546
529
2000
572
549
531
516
3000
564
538
517
502
Air flow rate = 0.8 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
611
587
570
562
1000
599
574
556
538
2000
585
561
545
524
3000
578
548
529
514
Air flow rate = 1.2 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
625
592
577
569
1000
614
579
565
554
2000
602
570
558
549
36
3000
588
559
547
535
Air flow rate = 1.6 dm3 min–1 SDBS concentration (mg dm–3)
Mean bubble radius (µm) No NaCl
10 mol m–3 NaCl
50 mol m–3 NaCl
100 mol m–3 NaCl
500
642
615
598
581
1000
629
604
579
562
2000
617
588
565
555
3000
605
576
551
540
37