Dynamic Adsorption of Organic Compounds Dissolved in Synthetic

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Dynamic Adsorption of Organic Compounds Dissolved in Synthetic Produced Water at Air Bubbles: The Influence of the Ionic Composition of Aqueous Solutions Mona Eftekhardadkhah and Gisle Øye* Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway ABSTRACT: A detailed analysis of the dynamic adsorption of crude oil components dissolved in produced water onto air bubble surfaces is presented. Synthetic produced water was prepared using three crude oils and aqueous solutions with varying ionic composition. The total organic carbon content, molecular mass, and dynamic surface tension was determined for the samples. A modified version of the Ward−Tordai model was used to evaluate the experimental data. This allowed for determination of the characteristic diffusion coefficients and investigation of the adsorption mechanisms. The adsorption process was diffusion-controlled on short time scales, while adsorption barriers occurred on longer time scales. The presence of salts in the aqueous solutions had a strong influence on the time at which deviations from diffusion-controlled kinetics took place. This was explained in terms of salting-out of larger hydrocarbons. The presence of divalent cations did not have a major influence on the salting-out effect nor the time where the adsorption barrier appeared. carbons like BTEX and polycyclic aromatic hydrocarbons.14 The solubility of the polar compounds depends on the molecular weight and type of heteroatoms in the molecules but is normally orders of magnitude higher than the solubility of the pure aromatic hydrocarbons.14 The dissolved polar components can often be considered to be acidic or basic in nature and are anticipated to have affinity to air−water surfaces. The dynamic adsorption of model surfactants at air−water interfaces has been reported in some detail in the literature.17−23 Previously, we reported dynamic adsorption of components dissolved in synthetic produced water onto air/ water interfaces. 24 This paper is part of a series of investigations, where the overall aim is to improve the understanding of how interfacial mechanisms can affect flotation processes within produced water treatment. The particular focus of the current paper was to determine how the ionic composition of water influenced the partitioning of crude oil components into the aqueous phases, and subsequently the affinity of these components for air−water interfaces. The amount and molecular weight distribution of the dissolved hydrocarbons in the samples were determined by total organic carbon analysis and electrospray ionization mass spectroscopy, respectively. The dynamic surface tension was measured by maximum bubble pressure tensiometry, and a modified version of the Ward−Tordai model was applied for detailed evaluation of adsorption mechanisms.

1. INTRODUCTION Water naturally present in reservoirs is normally produced along with oil and gas. The volume of this produced water is significant and can make up as much as 90% of the fluids in single wells. The composition of the produced water is a complex mixture of dispersed oil and solids, dissolved hydrocarbons and minerals, as well as residual production chemicals.1 The physical and chemical properties will also continuously change during the lifetime of a field. The components in produced water can influence the environment, and various treatment operations are particularly targeted toward removal of dispersed oil from the water streams.2 Gas flotation is one of the most widely used processes for removal of dispersed oil from wastewater.3,4 Important parameters that influence the oil removal efficiency include size distributions of oil droplets and gas bubbles, the presence of production chemicals, and pH.5−8 Furthermore, the crux of efficient flotation processes is the attachment of oil droplets to gas bubbles,9,10 and the properties at oil/water and gas/water interfaces are important in this respect. Interfacially active compounds can contribute to reduced size distributions of dispersed oil and bubbles,11 and they can affect the spreading of oil droplets on gas bubbles.12 Clearly, a better understanding of interfacial phenomena and adsorption mechanisms of interfacially active compounds at the various interfaces would contribute to ensuring optimal operating conditions, and thereby improved removal efficiency, in flotation processes. Particularly water−gas interfaces have received little attention in this respect. Chemical characterization of hydrocarbons dissolved in produced water has been reported in several studies,13−16 and it has been suggested that the dissolved organics can be divided into polar and nonpolar compounds. The polar compounds typically contain heteroatoms (nitrogen, sulfur, and oxygen), while the nonpolar compounds are mostly aromatic hydro© 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Characterization of Crude Oils. Three crude oils (A, B, and C) were characterized with respect to density, viscosity, SARA (saturate, aromatic, resin, and asphaltene) fractions, total acid number Received: May 18, 2013 Revised: August 18, 2013 Published: August 30, 2013 5128

dx.doi.org/10.1021/ef400925e | Energy Fuels 2013, 27, 5128−5134

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(TAN), and total base number (TBN). The density and viscosity were measured at 20 °C using an Anton Paar density meter (DMA-5000) and a Physica (MCR 301) rheometer. The TAN and TBN measurements were performed according to the ASTM D66425 and ASTM D289626 methods, respectively. During SARA fractionation, the asphaltenes were initially precipitated by n-hexane (1:40) while the remaining maltene fraction was separated by amino and silica chromatography using a fully automated HPLC instrument. The carbon content of the crude oils was determined at the Oil and Gas Institute (Kraków, Poland) using gas chromatography (EA 1108, CHNS-O Element Analyzer, Fisons Instruments). More detailed descriptions of the characterization procedures can be found elsewhere.27 2.2. Preparation of Brine. Simulated brines were prepared by dissolving analytical grades of NaCl (99.5%, Merck, Germany), Na2SO4 (99%, Acros, USA), NaHCO3 (99.5%, Merck, Germany), MgCl2·6H2O (99%, Merck, Germany), and CaCl2·2H2O (99.5%, Fluka, Czech Republic) in water provided by a Millipore ultrapure water system. Two compositions, with and without the divalent cations (Ca2+ and Mg2+), were prepared. The ionic strength was kept constant by increasing the amount of NaCl when the divalent cations were absent. The ionic compositions of the brines are summarized in Table 1. In order to prevent carbonate precipitation, the solutions were stored in a refrigerator and brought to the required experimental temperature just before use.

Figure 1. The PW samples prepared with crude oils A, B, and C and brine with all ions.

(Agilent Technologies, Palo Alto, California) 6520 LC/MS Q-TOF system. The molecular masses were determined as the weighted average of the negative and positive ion mass spectra. The measurements were carried out at the Department of Biotechnology, SINTEF Materials and Chemistry. 2.6. Dynamic Surface Tension Measurements. Dynamic surface tensions were measured at a maximum bubble pressure tensiometer (Krüss BP100, Hamburg, Germany). A hydrophobized glass capillary, with 0.110 mm inner radius at the tip, was immersed into a 30 mL PW sample, where air bubbles with surface ages from 10 ms to 100 s were created. The surface tension of ultrapure water was confirmed to be 71.99 ± 0.11 mN/m prior to each PW sample measurement. All measurements were carried out at room temperature, and the dynamic surface tension curves are given as the average of three parallel measurements with standard deviation ±0.063 mN/m. 2.7. Theory. The formation of fresh gas−liquid interfaces in solutions containing surface active compounds gives rise to dynamic adsorption processes and reduction of surface tension until an equilibrium value is reached. The decay in surface tension is due to the transport of surfactants to the surface, a process that can be divided into two steps. The first step is molecular diffusion of the surfactants from the bulk to subsurface layer (an imaginary plane a few molecular diameters below the interface), while the second step is adsorption of the surfactants from the subsurface layer onto the surface. Depending on which of these steps is the fastest, the rate of adsorption can be accounted for as follows:28,29 (1) the dif f usion controlled model, which assumes instantaneous adsorption of surfactants to the surface once they have diffused from the bulk to the subsurface layer (i.e., diffusion is the rate-controlling step), and (2) the mixed kinetic-dif f usion model, which also assumes that the surfactants diffuse from the bulk to the subsurface layer, but the rate-controlling step is the transfer to the surface. In this case, there is an adsorption barrier that may prevent surfactants from reaching the surface and cause back diffusion into the bulk. The Ward−Tordai equation is commonly used to describe adsorption kinetics at gas−liquid interfaces.30−33 The equation accounts for diffusion from the bulk to a planar interface as well as back diffusion into the bulk when the interface becomes more crowded and is given as follows:34

Table 1. Ionic Composition of the Brine Solutions concentration (ppm) ion −

Cl Na+ Ca2+ Mg2+ HCO3− SO42‑

brine with all ions

brine without divalent cations

62810 35393 3253 909 218 49

67020 43483 0 0 218 45

2.3. Preparation of Synthetic Produced Water. The watersoluble components of the crude oils were partitioned into aqueous phases by mixing 50% crude oil and 50% simulated brine solution (or ultrapure water) with a shaker. The mixing speed was 250 rpm, and the mixing was maintained at room temperature for 24 h. Subsequently, the oil and water phases were separated by centrifugation. The centrifugation was continued until no oil droplets were present in the water phase by visual inspection. The resulting aqueous phases were considered to be synthetically produced water, and the samples are denoted PW-A, PW-B, and PW-C, in accordance with the oil used for preparation. The PW-B and PW-C samples were colorless, while the PW-A samples had a slightly yellowish color, as seen in Figure 1. It should be noted that the preparation procedures for the PW samples were investigated prior to setting the conditions described above. In order to determine the effect of temperature, two sets of samples of each crude oil and the brine with all ions were mixed at 25 and 50 °C. Furthermore, the possibility of oxidation of the oil components during mixing in air was investigated by carrying out the mixing procedure under nitrogen purging (both at 25 and 50 °C). Neither the temperature nor the nitrogen purging had major effects on the surface tension. 2.4. Total Organic Carbon (TOC) Analysis. The total organic carbon measurements were performed on an Apollo 9000 TOC combustion analyzer operating in the range of 680−1000 °C. The instrument was equipped with a nondispersive infrared detector which directly and specifically measured the carbon dioxide generated by oxidation of the organic carbon in the samples. 2.5. Electrospray-Ionization Mass Spectroscopy (EI-MS). Negative and positive ion mass spectra of the produced water samples prepared with ultrapure water were determined with the Agilent

Γ(t ) =

D⎡ ⎢2C0 t − π ⎣

∫0

t

⎤ ϕ(τ ) dτ ⎥ ⎦ t−τ

(1)

where Γ(t) is the dynamic surface adsorption, D is the diffusion coefficient, C0 is the bulk concentration of surface active compounds, t is the time, ϕ(t) is the concentration of surface active compounds in the subsurface layer, and τ is the integration variable. 5129


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Table 2. Physicochemical Properties of the Crude Oils

a

crude oil

densitya (g/cm3)

viscositya (cP)

saturates (wt %)

aromatics (wt %)

resins (wt %)

asphaltenes (wt %)

TANb (mg/g)

TBNb (mg/g)

C (wt %)

A B C

0.81 0.92 0.94

2.71 78.44 487.00

79.66 44.35 25.61

18.29 38.76 49.55

1.92 16.39 10.91

0.12 0.49 13.93

0.20 2.28 0.57

0.59 4.37 1.34

83.26 84.24 80.94

Measured at 20 °C. bUnits: mg KOH/g of oil.

Several analytical solutions of the Ward−Tordai equation that describe the short and long time scales of adsorption have been reported.33,35 For a spherical interface, the ratio between the surface area of a bubble and the volume surrounding the bubble decreases when the curvature of the interface increases. This will result in faster diffusion toward the interface and decrease the time scale for molecular diffusion, and it has been demonstrated that the bubble radius at which curvature effects become significant must be evaluated for individual surfactant systems.36−39 Modifications of the Ward−Tordai equation were suggested by Liu and Messow, who solved the diffusion equation for a spherical interface by Laplace transformation.40 This resulted in accurate expressions of the dynamic surface adsorption for the following cases: (1) short-time limit adsorption (no-back diffusion), (2) adsorption with back diffusion, and (3) long-time limit adsorption. The following equations give the dynamic surface adsorption onto a spherical interface within the short- and long-time approximation, respectively:40

Γ(t ) =

DC0t Dt + 2C0 r0 π

4. RESULTS AND DISCUSSION 4.1. Characterization of Crude Oils. The physicochemical properties of the crude oils, listed in Table 2, showed that crude oil A was the lightest oil with lowest density and viscosity. This oil also contained low amounts of resins and asphaltenes, as well as low TAN and TBN. The highest TAN and TBN were found in crude oil B, which had intermediate density and viscosity. Crude oil C was the heaviest oil with high contents of resins and asphaltenes, and intermediate TAN and TBN. 4.2. The Influence of Ionic Composition of Water on the Properties of Synthetic Produced Water. 4.2.1. Bulk Characterization of PW Samples. The amount of total organic carbon (TOC) in the PW samples is compared in Figure 2. It is

(2)

D⎛ t ⎞ Γ(t ) = (C0 − Cs) ⎜t + 2r0 ⎟ + Q (t ≥ t1) r0 ⎝ Dπ ⎠

(3)

where r0 is radius of capillary, t1 is a given long time at which the concentration at the subsurface has reached a constant value of Cs, and Q is only a function of t1 and Cs. Furthermore, the dynamic surface adsorption can be expressed in terms of the dynamic surface tension by using an appropriate adsorption isotherm, often the Langmuir isotherm, to describe the adsorption of surfactants from the subsurface layer onto the surface. The equations for the short- and long-time adsorption, respectively, are then given as follows:40

γ(t ) = γ0 −

RTD ⎛ C 0⎜ t + ⎝ r0

2 r0 ⎞ RTr0 ⎟ + C0 ⎠ πD π

γ(t ) = γ0 −

⎛ RTD (C0 − Cs)⎜ t + ⎝ r0

Figure 2. The TOC concentration of the PW samples prepared in the different aqueous solutions.

(4)

evident that the ionic composition of the water phase affected the partitioning of water-soluble compounds into the water phase. The concentration of dissolved hydrocarbons decreased significantly in the presence of inorganic ions; i.e., there was a salting-out effect. Salting-out of hydrocarbons from aqueous solutions is well-known41,42 and can be explained by enhanced structuring of the aqueous phase when salts are present.43 In other words, the cohesive energy in water is increased in the presence of salts, and the result is reduced solubility of hydrophobic species. The pH’s of the various produced water samples are listed in Table 3. For all crude oils, it is clear that

2 r0 ⎞ RTr0 ⎟ + (C0 − Cs) ⎠ πD π

− RTQ (t ≥ t1)

(5)

where γ(t) is the dynamic surface tension and γ0 is the equilibrium surface tension of the pure solvent. In order to calculate the diffusion coefficient, eq 4 can be rearranged in the following form:

F=

r0(γ0 − γ(t )) RTC0

+

r02 = π

Dt +

r0 π

(6)

Table 3. pH of the Aqueous Solvents and PW Samples

Equation 6 indicates that for a diffusion controlled adsorption process, there should be a linear relationship between F and the square root of time. The results presented in this paper are calculated from the equations for spherical interface presented above. Calculations using the analogous equations for planar interfaces29 gave no significant difference in the results (data not shown). Consequently, it was assumed that the effect of interfacial curvature was small enough to be neglected in our experiments. Similar assumptions have also been made by Nguyen et al.39

pH solvent/PW samples pure solvent PW-A PW-B PW-C 5130

ultrapure water 6.55 5.07 6.66 4.38

± ± ± ±

0.01 0.05 0.10 0.11

brine with all ions 6.69 7.50 7.22 7.42

± ± ± ±

0.02 0.19 0.05 0.07

brine without divalent cations 7.56 7.50 7.51 7.77

± ± ± ±

0.01 0.15 0.09 0.05


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the pH of the PW samples prepared with ultrapure water is significantly lower than that of the samples prepared with the brines. This revealed that at least part of the components that were salted-out in the presence of ions were acidic compounds. Figure 2 also revealed that the TOC for the PW-A and PW-B samples had similar values and that these were higher than for PW-C. About 30% of the hydrocarbons were salted out for the PW-A and PW-B, while about 25% were salted out for PW-C. The composition of the brines had minor influence on the salting-out effect, but the amount of hydrocarbons was slightly higher in the brines without the divalent cations for the PW-A and PW-B samples. No difference was seen for PW-C. The average molecular masses of the soluble hydrocarbons in the PW samples are shown in Table 4. The values are within a Table 4. Average Molecular Mass of Dissolved Hydrocarbons in the PW Samples Prepared with Ultrapure Water sample

Mm (g/mol)

PW-A PW-B PW-C

346 400 473

relatively narrow range for all three samples, which is in agreement with observations made by Stanford et al. on watersoluble compounds originating from different crude oils.14 Nevertheless, the highest molecular mass was seen for the PWC sample. The reason for the lower amount of dissolved hydrocarbons in this sample could therefore be that crude oil C generally contains larger molecules than the two other crude oils. This seems reasonable since it is the heaviest of the investigated oils. Furthermore, the lower salting-out effect for this sample suggests that the dissolved components are more polar than for the PW-A and PW-B samples, since increased polarity will decrease the salting out effect.43 4.2.2. Dynamic Surface Tension and Adsorption Mechanisms for the PW Samples. The surface tensions of the pure aqueous solutions were initially measured to be 71.99 ± 0.11 mN/m (ultrapure water), 75.65 ± 0.08 mN/m (brine with all ions), and 75.73 ± 0.11 mN/m (brine without divalent cations). The higher surface tension of the brine solutions is due to the negative surface excess of the ions at air−water interfaces.44 The dynamic surface tensions of the PW samples are shown in Figure 3. The initial surface tensions were in all cases similar to the values given above for the pure aqueous solutions. This showed that the air-PW interfaces were clean (without adsorbed components present) immediately after formation, i.e. when t → 0. However, the surface tension decreased rapidly within short time intervals and continued to decrease gradually with time toward equilibrium for long time limit adsorption. There were marked differences in the dynamic surface tension between the samples prepared in brines and ultrapure water, and the differences became more pronounced at longer times. The PW-B and PW-C samples showed similar behavior in the way that the samples prepared in ultrapure water had lower surface tension than the samples prepared in the brines. This corresponds well to the higher TOC values in the ultrapure water samples. At long times, however, the PW-A samples had higher surface tension in the ultrapure water than the brines, despite higher TOC value in pure water. Moreover, when considering the brine solutions, the surface tension was somewhat lower in the absence of the divalent cations for PW-

Figure 3. The dynamic surface tension of the PW samples prepared with different aqueous solutions and (a) crude oil A, (b) crude oil B, and (c) crude oil C.

A and PW-B, while no difference was seen for PW-C. These trends were similar to those observed for the TOC values. According to eq 6, adsorption at the interface is diffusion controlled within the short time limit if there is a linear relationship between F and the square root of time. Figure 4 shows the linear relationships for the PW-A samples between 10 and 100 ms. Similar linearities were also observed for the PW-B and PW-C samples within this time range (data not shown). This indicated that the adsorption mechanism of hydrocarbons onto the air surface was purely diffusion 5131


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from all three crude oils. No significant influence was observed in the results. Next, the average molecular masses (Table 4) were used to calculate the molar concentrations of hydrocarbons in the samples (i.e., convert mg/L to mol/L). As mentioned earlier, both polar and nonpolar hydrocarbons can be present in the PW samples, and the direct relationship between TOC and C0 can be questioned due to less surface activity of nonpolar hydrocarbons. The solubility of polar compounds is orders of magnitude larger than nonpolar compounds. Furthermore, PW samples were prepared using brines at high ionic strength, which reduces the water solubility of nonpolar hydrocarbons dramatically (salting-out effect).14 This suggests an abundance of polar (N−S−O containing) compounds, which have affinity for the air−water interface. Therefore, the relationship between TOC and C0 seems like a reasonable assumption to make. The calculated diffusion coefficients, listed in Table 5, are referred to as characteristic diffusion coefficients for the

Figure 4. The relation between F and √t in the short-time limit for adsorption of dissolved hydrocarbons in PW-A samples prepared with different aqueous solutions (r0 = 0.110 mm).

Table 5. Characteristic Diffusion Coefficients (Dch), Times at Which the Adsorption Starts to Deviate from the Diffusion Controlled Mechanism (td), and Surface Pressure (π) at td for the PW Samples

controlled at short time scales. According to eq 5, the adsorption process would also be diffusion controlled at longer time scales if there was a linear relationship between the surface tension and (√t + r0/(πD)1/2)2. However, Figure 5 shows

Dch (m2/s) PW-A-with all ions PW-A-without divalent cations PW-A-ultrapure water PW-B-with all ions PW-B-without divalent cations PW-B- ultrapure water PW-C-with all ions PW-C-without divalent cations PW-C- ultrapure water

8.1 8.5 9.0 1.0 1.0 2.5 6.4 4.2 3.1

× × × × × × × × ×

−11

10 10−11 10−12 10−10 10−10 10−11 10−11 10−11 10−11

td (ms)

π at td (mN/m)

391 438 1259 312 344 705 790 698 1008

3.0 3.8 2.5 2.8 4.1 2.1 2.2 2.1 1.6

samples, because they represent distributions of dissolved hydrocarbonsnot single components. All the samples prepared with ultrapure water had higher characteristic diffusion coefficients than the samples prepared with brine, even though the difference was smaller for PW-C than the other samples. This indicated that the largest molecules, which will contribute to lower characteristic diffusion coefficients, were consistently salted-out in the presence of salts. The characteristic diffusion coefficients were also used to predict the dynamic surface tension by the model of eq 4. A comparison of the experimental and predicted dynamic surface tensions is shown in Figure 6 for the PW-A samples prepared with brine and ultrapure water. It is evident that the model resulted in good predictions at short times. However, deviations from the experimental measurements occurred with time, and the surface tension was increasingly underestimated. The times (td) where the predicted and measured values started to deviate notably (deviation ≥1 mN/m), indicated by arrows in Figure 6, are listed for all the samples in Table 5. The precision of td is in the range of ±1 ms for the three parallels of the same PW samples. Up to td, the adsorption of dissolved molecules at the surface was diffusion controlled. Beyond td, the adsorption was no longer purely diffusion controlled, and the adsorption process was slowed down by the appearance of an adsorption barrier. This barrier was likely due to reduced availability of vacant adsorption sites on the surface and, in accordance with the mixed kinetic diffusion model, molecules will diffuse back into the bulk instead of adsorbing at the surface. It is evident

Figure 5. The relation between γ(t) and (√t + r0/(πD)1/2)2 in the long-time-limit for adsorption of dissolved hydrocarbons in PW-A samples prepared with different aqueous solutions (r0 = 0.110 mm).

nonlinearity for the PW-A samples when t ≥ 4000 ms, indicating the presence of an adsorption barrier for adsorption at long time scales. The same conclusions were reached for the PW-B and PW-C samples. From the slopes of the linear curves at short time scales, characteristic diffusion coefficients for the PW samples were calculated. First, the amounts of dissolved hydrocarbons in the samples were estimated by assuming that the total organic carbon values accounted for 80 to 84% of the mass of hydrocarbons depending on the sample. This assumption was based on the carbon content of the crude oils measured by elemental analysis (see Table 2). Naturally, there can be differences between the carbon content of the crude oils and their respective water-soluble compounds. Therefore, calculations were made using the value of 80% for the PW samples 5132


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5. CONCLUSIONS Synthetic produced water samples were prepared from three crude oils and aqueous solutions with different ionic composition. TOC and dynamic surface tension measurements were used together with a modified version of the Ward− Tordai model to evaluate the adsorption process of dissolved organic hydrocarbons onto the surface of air bubbles. The adsorption process was found to be purely diffusion controlled at short time scales, while an adsorption barrier occurred and increased at longer time scales. The presence of dissolved inorganic ions shortened the time range for diffusion controlled adsorption considerably. This was explained by a salting-out effect of the largest hydrocarbons and thereby faster diffusion to the surface that would result in more rapid crowding (i.e., adsorption barrier) at the surface. It was also suggested that the ions themselves could contribute to the adsorption barrier. The divalent cations had only a minor effect on the time scales for the adsorption mechanisms.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+47) 73 59 41 35. E-mail: [email protected]. no. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors are grateful to the industrial sponsors (ConocoPhillips Skandinavia, ENI Norge, Schlumberger Norge Divison M-I EPCON, Statoil Petroleum, and Total E&P Norge) of the joint industrial program “Produced Water Management: Fundamental Understanding of the fluids” for financial support. We are also grateful to Anders Brunsvik (SINTEF Materials and Chemistry) for valuable assistance with mass spectroscopy analysis.

Figure 6. Experimental (points) and predicted (lines) dynamic surface tension for PW-A samples prepared with (a) brine with all ions and (b) distilled water.

(1) Dudásǒ vá, D.; Flåten, G. R.; Sjöblom, J.; Øye, G. Stability of binary and ternary model oil-field particle suspensions: A multivariate analysis approach. J. Colloid Interface Sci. 2009, 337 (2), 464−471. (2) Discharges, spills and emissions from offshore oil and gas installations; OSPAR Commission: London, 2010. (3) Rubio, J.; Souza, M. L.; Smith, R. W. Overview of flotation as a wastewater treatment technique. Miner. Eng. 2002, 15 (3), 139−155. (4) Melo, M. V.; Sant’Anna, G. L.; Massarani, G. Flotation techniques for oily water treatment. Environ. Technol. 2003, 24 (7), 867−876. (5) Biswal, S. K.; Reddy, P. S. R.; Bhaumik, S. K. Bubble size distribution in a flotation column. Can. J. Chem. Eng. 1994, 72 (1), 148−152. (6) Rodrigues, R. T.; Rubio, J. New basis for measuring the size distribution of bubbles. Miner. Eng. 2003, 16 (8), 757−765. (7) Watcharasing, S.; Kongkowit, W.; Chavadej, S. Motor oil removal from water by continuous froth flotation using extended surfactant: Effects of air bubble parameters and surfactant concentration. Sep. Purif. Technol. 2009, 70 (2), 179−189. (8) Painmanakul, P.; Sastaravet, P.; Lersjintanakarn, S.; Khaodhiar, S. Effect of bubble hydrodynamic and chemical dosage on treatment of oily wastewater by Induced Air Flotation (IAF) process. Chem. Eng. Res. Des. 2010, 88 (5−6), 693−702. (9) Grattoni, C.; Moosai, R.; Dawe, R. A. Photographic observations showing spreading and non-spreading of oil on gas bubbles of relevance to gas flotation for oily wastewater cleanup. Colloids Surf., A 2003, 214 (1−3), 151−155.

that the adsorption kinetics is diffusion controlled for longer periods for the samples prepared in ultrapure water. The faster appearance of an adsorption barrier in the brine samples can be explained by two factors. First, larger molecules were salted-out in the brine samples. Overall, the dissolved components will then diffuse faster toward newly created surfaces (i.e., into the subsurface layers), and the instantaneous adsorption will result in more rapid reduction of vacant adsorption sites in these samples. The consistently higher surface pressures (i.e., the difference between the surface tension of the pure surface and the surface tension with dissolved compounds present) at td for the brine samples, listed in Table 5, seem to justify this description. Second, the diffusion coefficients for the ions are at least 1 order of magnitude higher (i.e., proportional to 10−9 m2/s) than the characteristic diffusion coefficients determined for the dissolved components. Consequently, the dissolved ions will approach the fresh surface faster, and repulsion might restrict the access of charged dissolved components. Notably, the influence of the ions was most pronounced for the PW-A samples, while the absence of divalent cations in the brines did not systematically influence the adsorption process to any major extent. 5133


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