Langmuir 2002, 18, 8295-8301
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Effect of Competitive Adsorption of Zn2+ on the Flocculation of Lauryl Sulfate Micelles by Al3+ P. Paton-Morales and F. I. Talens-Alesson* TALENCO Consulting, Psge Canti, 8, 2, 2, 08005 Barcelona, Spain Received January 25, 2002. In Final Form: July 16, 2002 In micellar flocculation of lauryl sulfate micelles with solutions containing Zn2+ and Al3+ cations, the charge ratios polyvalent cation/micellar surfactant may be greater than 1 above [Zn2+] ) 0.031 M. This is unlike the case of flocculation of lauryl sulfate micelles with Al3+ or Al3+/Na+ solutions, where the charge ratio Al/sodium dodecyl sulfate in the floc is lower than or nearly equal to 1. Only below [Zn2+] ) 0.031 M may the localized adsorption model be used to describe the binding of cations to micelles. The apparent charge inversion may be explained by assuming that a flocculating micelle drags the ions contained within its hydrodynamic radius. A reduction of the Stern and diffuse layer radii under increasing electrolyte concentrations may collapse these within the hydrodynamic radius of the micelle, allowing the hydrodynamic radius to reach even into the bulk solution. If all the solution included within the hydrodynamic radius is dragged into the floc, this would explain the apparent charge inversion. Such a mechanical effect would explain the observed enhancement of pollutant removal by adsorptive micellar flocculation in the presence of Zn2+. Binding of phthalic acid is shown as an example.
Introduction Binding of cations to micelles has been mathematically described using double-layer models and triple-layer models (Figure 1). These models assume the existence of regions where the isotropy of the solution is altered by the existence of an electric field generated by the electrically charged surface of the micelles. Simpler models, such as Temkin-Frumkin isotherms, have also been used successfully to prove that it is not correct to assume constancy in the charge ratio between bound cations and surfactant micelles and micellar surface potential when comparing the binding of monovalent and divalent cations1 such as Cu2+, as assumed in earlier work on modeling of the binding of Ca2+/Na+ mixtures on lauryl sulfate (DS-) micelles,2,3 where a charge ratio of 0.65 has been assumed, equal to the charge ratio reported by Rathman and Scamehorn for binding of Na+ on DS- micelles.5 Double-layer models, such as the Gouy-Chapman model, assume a diffuse layer and a bulk solution. These models overpredict concentrations at the surface of the micelles and potential gradients.1 The most comprehensive models for binding of counterions to micelles are triplelayer models, applied to the binding of Na+ to DSmicelles4,5 and to the precipitation of Ca(DS)2 in the presence of NaCl and nonionic surfactants.2,3 These models consider three regions: the Stern layer, where cations are bound to the micellar surface; the diffuse layer, intermediate between the Stern layer and the bulk solution; and the bulk solution. The earlier models2,3,5 were simplified by assuming average values for the diffuse layer concentrations and avoiding the explicit inclusion of a thickness for the diffuse layer and by assuming constant charge ratios regardless of the binding counterion. Lin and Jafvert4 propose a more rigorous formulation. It calculates the surface charge density at the divide between the Stern layer and the diffuse layer (eq 1) and the Stern layer potential (eq 2), (1) Hafiane, A.; Issid, I.; Lemordant, D. J. Colloid Interface Sci. 1991, 142, 167-178. (2) Stellner, K. L.; Scamehorn, J. F. Langmuir 1989, 5, 70-77. (3) Stellner, K. L.; Scamehorn, J. F. Langmuir 1989, 5, 77-84. (4) Lin, C.-C.; Jafvert, C. Langmuir 2000, 16, 2450-2456. (5) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984, 88, 58075816.
Figure 1. Schematic view of a double-layer model (A) and a triple-layer model (B). Surfactant molecules (circle with minus sign and tail) are present at the micellar surface and the bulk solution (at their cmc). Cations are present within the diffuse layer and the Stern layer. Other anions are present in the bulk solution.
which for the particular case of binding of Na+ onto DSin a NaDS/NaCl solution are
σS ) σS2
2000kTN
{
(
( ))
VSN F 1 + ([DS-]S + [Cl-]S - [Na+]S) SDS η )
( )
[Na+]W exp
( ( ) )
∑i Ci,w exp -eψS kT
-zieψS kT
(1)
-1 )
( )}
+ ([DS-]W + [Cl-]W) exp
eψS
kT {[Na+]W + [Cl-]W + [DS-]W} (2)
10.1021/la0200820 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/04/2002
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using the Boltzmann equation for the calculation of both Stern and diffuse layer potentials,
( )
Ciatx ) Ci,w exp
-eψx kT
(3)
and the Poisson-Boltzmann equation for the potential at any distance from the micellar surface. The result of this formulation, which uses as fitted parameters the thickness of the Stern layer and the intercept K1 of the relationship between critical micelle concentration (cmc) and Stern layer potential,
log [DS-]W ) K1 -
eψS 2.3kT
(4)
provides the following expression for the binding factor of Na+ onto DS- micelles:
βNa+ ) 0.0382[Na+] + 0.314
(5)
The result differs considerably from the experimental value of 0.65 reported in the literature.5 A possible explanation,1 considering the good agreement of the model with other experimental results, is that measures of concentrations in the bulk solution by an ion-selective electrode in stirred cells may ignore not only the cations within the Stern layer but also part of the cations present in the diffuse layer, and this would mean that the region around the micelle where ions could be considered “bound” or associated would extend beyond the Stern layer and extend to the hydrodynamic radius of the micelle. The results from the model for the system DS-mic/Na+/(Cl-, DS-) show the concentration of anions to be insignificant within the Stern and diffuse layers and the diameter of the diffuse layer to decrease with [Na+] following the equation
log λSD ) -0.505 log [Na+] + 1.61
(6)
The model predicts [Na+] at the Stern layer to be about 8 M, for [NaCl] + [NaDS] ) 0.1 M. This shows that very high cation concentrations may be found near the micellar surface. While Lin and Jafvert did not apply their model to multivalent cations and anions, the application would be straightforward, as long as information such as micelle radius, aggregation number, and surface area is available. An interesting case of multivalent cation adsorption is the binding of Al3+ to sodium dodecyl sulfate (SDS)6,7 or R-olefinsulfonate8 (AOS). It leads to the suppression of electrostatic repulsion between micelles and to their flocculation.9,10 Unlike in precipitation of insoluble salts of anionic surfactants from micellar solutions, zero ζ-potentials have been reported.11 The flocculate redissolves at sufficiently high [Al3+], and no micelles are detected by argon light scattering above [Al3+] ) 0.013 M for [SDS] ) 0.05 M.10 It is assumed that Al-DS aquacomplexes form, eventually sequestering the micellar (6) Porras, M.; Talens, F. I. Sep. Sci. Technol. 1999, 34 (13), 26792684. (7) Porras-Rodriguez, M.; Talens-Alesson, F. I. Environ. Sci. Technol. 1999, 33 (18), 3206-3209. (8) Porras, M.; Talens, F. I. Sep. Sci. Technol. 2000, 35 (12), 19731978. (9) Paton, P.; Talens, F. I. J. Surfactants Deterg. 1998, 1 (3), 399402. (10) Talens, F. I.; Paton, P.; Gaya, S. Langmuir 1998, 14, 50465050. (11) Talens-Alesson, F. I. J. Dispersion Sci. Technol. 1999, 20 (7), 1861-1871.
surfactant. As [Al3+] increases above the value for the maximum flocculation region and into the region where deflocculation takes place, the ζ-potential becomes again more negative.10,11 The same behavior has been observed in the SDS/Fe3+ system.12 All these systems show the ability to remove organic pollutants of anionic nature (e.g., the pesticide 2,4-D, phthalic acid, phenol). The flocculation of micelles and the binding of the anionic pollutant are simultaneous6,8 and the flocculate has good filterability, thus allowing the removal of small organic molecules without using membranes (e.g., MEUF). This separation technique has been called adsorptive micellar flocculation (AMF). Problems avoided by AMF include membrane fouling,13,14 retentate with substantial water content15 (1/5 to 1/10 of the initial volume of effluent are normal), or permeation of the anionic surfactant in micellar form16 at [SDS] above 0.1 M in the ultrafiltration cell. Equilibrium in AMF is reached after contact times of 2-3 min with SDS6 and up to 10 min with AOS.8 The fact that filtration is a low-cost maintenance operation and the contact time is short makes the technique promising. AMF has been described as a secondary adsorption of the pollutants to the cationic Stern layer.17 Previous research on the applicability of AMF has covered issues such as the effect upon surfactant flocculation of high salinity18 and hydrocarbons.19 Their presence has been found to reduce the ratio of surfactant flocculation. The effect of pH20 is more complex as it affects the chemistry of Al3+ in the solution. Within the pH range 4-6, pH modifiers allow full flocculation at high [Al3+], where it would be prevented at the pH naturally occurring of about 3. At very alkaline or acidic pH, no flocculation takes place. The effect of heavy metals in reducing the ratio of flocculated surfactant has also been briefly mentioned in previous work.10 It can lead to the complete prevention of flocculation. This is unlike other interfering agents such as NaCl or hydrocarbons, which seem to have an upper concentration above which they cannot further reduce the ratio of flocculated surfactant. Although divalent cations interfere, their coadsorption onto flocculating micelles may be a separation method for the removal of heavy metals. Reiller et al.21 have described the sequential combination of MEUF (to concentrate heavy metals) and complexation (to strip the retentate of the heavy metals by means of a second ultrafiltration step, separating them from the micelles). A similar approach may allow the recovery of heavy metals from micellar flocculates. The effect of this binding process on the removal of organic pollutants by AMF is also a matter to be elucidated. The system studied here is NaDS/ZnSO4/Al2(SO4)3. Work has involved determination of the final concentrations of DS-, Al3+, and Zn2+ in the solutions and the pH after flocculation and filtration. From these results, ratios (12) Talens-Alesson, F. I.; Hall, S. T.; Hankins, N. P.; Azzopardi, B. J. Colloids Surf., A 2001, 204, 85-91. (13) Dunn, R. O.; Scamehorn, J. F. Sep. Sci. Technol. 1987, 22, 763789. (14) Akay, G.; Wakeman, R. J. J. Membr. Sci. 1994, 88, 177-195. (15) Scamehorn, J. F.; Christian, S. D.; El-Sayed, D. A.; Uchiyama, H. Sep. Sci. Technol. 1994, 29, 809-830. (16) Azoug C.; Steinchen, A.; Charbit, F.; Charbit, G. J. Membr. Sci. 1998, 145, 185-197. (17) Talens-Alesson, F. I. Colloids Surf., A 2001, 180, 199-203. (18) Paton-Morales, P.; Talens-Alesson, F. I. Langmuir 2001, 17, 6059-6064. (19) Paton-Morales, P.; Talens-Alesson, F. I. Colloid Polym. Sci. 2000, 278, 697-700. (20) Paton-Morales, P.; Talens-Alesson, F. I. Colloid Polym. Sci. 2001, 196-199. (21) Reiller, P.; Lemordant, D.; Hafiane, A.; Moulin, C.; Beaucaire, C. J. Colloid Surf. Sci. 1996, 177 (2), 519-527.
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between Zn and Al in the flocs and flocculated/total surfactant ratios have been calculated. A number of experiments involving removal of phthalic acid and phenol have been included to highlight some of the findings. The experimental results have been used to benchmark a version of the localized adsorption model already used for micellar flocculation of SDS in the presence of Al3+ and Na+. The evolution from micelles to the final flocculates has not been characterized, and adequate values for aggregation numbers and their evolution during flocculation, for micellar sizes, and for micellar surfaces are unavailable. This prevents a rigorous use of the Lin and Jafvert model, which is used only for a qualitative discussion of the results. The results of the localized adsorption model show that at lower [Zn2+] it is still applicable and gives consistent results, but this ceases to happen at higher [Zn2+]. A test to confirm the possible existence of phenol-Al complexes has also been performed, as the results of this work suggest that removal of pollutants by AMF is due to the coadsorption of Al3+ and positively charged pollutant-Al complexes. As phenol has already been reported as removable by AMF, it is essential to elucidate whether under acidic conditions in aqueous solution it does form complexes with Al. The Localised Adsorption Model (LAM) This model is analogous to the adaptation of the original LAM5 used by Paton-Morales and Talens-Alesson18 for the description of the binding of Al3+ during flocculation of SDS micelles in the presence of varying concentrations of NaCl. Mass Balance of the Flocculate and the Solution. The species to be considered are Al3+, Zn2+, DS-, SO42-, and Na+. Na+ can be present free or bound to micelles or flocs. Al3+ can be present free, bound to micelles, bound to flocs, or allegedly forming aqua-complexes19 with DS-. Under the pH of the experiments, below 3.75, the predominant form is Al3+. Zn2+ can be considered free, bound to flocs, or bound to micelles, again with the same binding ratio as in flocs. SO42- can initially be considered free.
[Al]total ) [Al3+]free + [Al3+]bound + [Al3+]other
(7)
[Na+]total ) [Na+]free + [Na+]bound
(8)
[Zn2+]total ) [Zn2+]bound + [Zn2+]free
(9)
[Al]total ) [Al3+] + βAl([SDS]total - [SDS]mon) + [Al3+]other (10) [Zn2+]total ) βZn([SDS]total - [SDS]mon) + [Zn2+]free (11) Some statements valid for Al3+/SDS and Al3+/Na+/SDS cease to be acceptable: the residual surfactant concentration at the minimum surfactant concentration does not necessarily correspond to the cmc, and the Stern potential in the conditions of optimum flocculation is not necessarily zero, as flocculation is significantly less efficient. Electrostatic Binding Ratio. The binding ratios can be calculated as in previous work:
βAl ) [KAlRAl e(-zAlΨS/kT)]/ [zAl(1 + KAlRAl e(-zAlΨS/kT) + KZnRZn e(-zZnΨS/kT) + KNaRNa e(-zNaΨS/kT))] (12)
βZn ) [KZnRZn e(-zZnΨ0/kT)]/ [zZn(1 + KAlRAl e(-zAlΨS/kT) + KZnRZn e(-zZnΨS/kT) + KNaRNa e(-zNaΨS/kT))] (13) Dividing the right-hand terms by the charge of the cations is necessary because the original equation for the localized adsorption model3-5 described the fraction of neutralized charge and not the ion ratio. KNa is taken as 1 from the literature,5 and KAl as 17.67 as estimated in our previous work.16 To calculate the value of KZn from the ion ratios, the bootstrap is that the Stern potential is the same for eqs 12 and 13 in a given experiment. The binding ratios βAl and βNa are assumed to be equal to the molar ratios between cations and surfactant in the floc. The free cation concentrations in the bulk solution may be estimated assuming the binding ratios of cations onto micelles and flocs to be equal, but only for the range of [Al3+] where deflocculation is not observed. This restriction is a consequence of ignoring the fraction of surfactant forming complexes in an experiment within the deflocculation region. Only if we can consider that surfactant is either monomeric or micellar/flocculated can we estimate the bound/unbound cation concentrations. The activities are calculated from the concentrations and the activity coefficients f estimated using the Davies equation log f ) -0.5z2(I0.5/(1 + I0.5) - 0.2I). The equation is not valid for ionic strengths I > 0.5. Experimental Section Reagents. Sodium lauryl sulfate kindly provided by KAO Corp. (Barbera del Valles, Spain) was used as received. The surfactant had a content in active matter of 98% w/w and 2% of unreacted alcohol and was used as received because the behavior of a purified material is irrelevant from the point of view of technological applications. The residual concentration of SDS was determined by the Hyamine (or Shell) method. The concentrations of Al3+ and Zn2+ were determined by ICP using a Jovyn-Ivon JI-38. High-purity ZnSO4 from Panreac, AR grade Hyamine 1622 from Carlo Erba, A.R. grade Disulfine Blue V150 from Merck Schuchardt, and high-purity dimidium bromide, Al2(SO4)3, and CHCl3 from Probus were used. Water was Milli-Q grade. Procedure for Flocculation Experiments. The experimental procedure consisted of preparing stock solutions and keeping them in a thermostatic bath at 25 °C. Appropriate volumes of each solution were mixed, vigorously shaken, and kept still, settling for 1 h before further handling. Then the flocculates were vacuum-filtered through 45 µm cellulose nitrate filters. The pH was measured in the filtered solutions. Residual concentrations were determined as well on the filtered solution. The experimental series were all carried out with a 0.05 M SDS concentration. For the various series, [Zn2+]total was 0.031, 0.062, 0.075, 0.094, and 0.125 M. For the last two series, no flocculation was observed. [Al3+] ranged from 6 × 10-3 to 0.013 M. Complexation Test. The conclusions of this work raise the question of whether Al-phenol complexes may exist in the acidic conditions of AMF. Phenol has been reported not to form complexes within the usual pH ranges used in flocculationcoagulation for water treatment, 4.6-5.5.22 The existence of complexes may be elucidated by a difference in light absorbance between solutions of a given compound at a set concentration in the absence and in the presence of a suspected complex-forming counterion. The existence of a difference is attributed to the presence of complexes. Under some specific restrictions, it is also possible to calculate the stoichiometry of those complexes.22 A test has been carried out at a pH around 2.5 obtained by means of a pH 2.28 phosphoric-based buffer solution (20-100 mL of sample). The pH is slightly more acidic than the average pH in AMF.6,7,8 Two series of samples with [Al3+] ) 1.8 × 10-4 and 4.5 (22) Cathalifaud, G.; Ayele, J.; Mazet, M. Water Res. 1997, 31, 689698.
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Figure 2. Variation of absorbance of phenol caused by the presence of Al3+ cations with respect to the same concentration of phenol (moles per liter). The contributions of different complexes cannot be allocated, but the fact that they must exist is clear.
Figure 4. The fraction of flocculated Al vs the total aluminum sulfate concentration (moles per liter).
flocculation within a given series. From the aqueous equilibrium for Al23 and the pH of the various experiments (Table 1), it can be found that the occurrence of Al(OH)2+ and [Al13O4(OH)24]7+ will be small at pH 3.75 or lower. The deflocculation observed at high [Al3+] and attributed to complexes Al-DS is observed earlier. Table 1 shows pH and residual SDS concentrations for the flocculation series discussed and for other experiments with higher [Zn2+] where flocculate did not form. The pH values are comparable in both flocculating and nonflocculating series, indicating that the cation concentrations removed from the solution are part of the flocculate itself and are not due to simultaneous coprecipitation of hydroxides during flocculation. Flocculation being prevented, there is no precipitation of Al(DS)3. The main difference between the effect of Zn and NaCl or hydrocarbons as interfering agents is that while above a certain concentration of the latter two there is no significant effect on disrupting the flocculation, as [Zn2+] increases, smaller increments in its concentration reduce more markedly the flocculation ratio. This is shown in Figure 3: while between [Zn2+] ) 0.031 and 0.062 M the reduction in flocculated surfactant is 0.01 M, between [Zn2+] ) 0.062 and 0.075 M the reduction raises to about 0.015 M. Coadsorption of Cations. Figures 4 and 5 show the removal of both Al3+ and Zn2+ from the solution during micellar flocculation. Under optimum flocculation conditions, some 0.005 mol/L of Zn2+ is removed from a solution initially 0.031 M Zn2+, with a flocculated [SDS] of about 0.0475 M. Some 0.012 mol/L is removed at [Zn2+] ) 0.061 M with a flocculated [SDS] of about 0.04 M, and some 0.005 mol/L is removed at [Zn2+] ) 0.075 M with a flocculated [SDS] of about 0.025 M. Another interesting
Figure 3. The concentration of flocculated surfactant (white symbols, left Y-axis) is plotted together with the combined Al plus Zn charge ratio relative to micellar surfactant (black symbols, right Y-axis). Both are plotted against the aluminum sulfate concentration. The horizontal solid line indicates an apparent charge ratio of 1. × 10-4 M, each with phenol concentrations ranging from 1.6 × 10-3 to 4.8 × 10-3 M, were tested. Figure 2 shows that there is a significant variation in absorbance and that it changes with the ratios Al3+/phenol. The measures were taken on a Jenway 6405 spectrophotometer at a wavelength of 285 nm. Therefore, Al-phenol complexes may exist under acidic conditions. We will come later to the significance of this finding.
Results and Discussion Micellar Flocculation. Increased [Zn2+] reduces the surfactant flocculation (Figure 3), and an increase in [Al3+] from 0.03 to 0.05 M is required to achieve the maximum
Table 1. pH and Residual SDS Concentration at Five Different Zn2+ Concentrations over a Range of Concentrations of Al3+ Ions [Zn2+] (M) 0.031 [Al3+]
(M)
0.010 0.012 0.015 0.020 0.025 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100 0.110 0.120
pH
[SDS]res
3.74
0.044
3.64 3.58 3.58 3.51 3.48 3.46 3.42
0.025 0.011 0.006 0.004 0.003 0.003 0.003
3.38 3.38 3.38 3.38 3.38
0.006 0.009 0.012 0.019 0.027
0.063 pH
[SDS]res
0.075 pH
[SDS]res
3.79 3.79 3.79
0.050 0.041 0.027
3.79 3.79 3.79
0.050 0.050 0.044
3.74 3.64 3.58 3.51 3.48 3.46 3.46 3.42 3.46 3.38
0.015 0.011 0.010 0.012 0.014 0.018 0.026 0.036 0.046 0.050
3.74 3.64 3.58 3.51 3.48 3.46 3.46 3.46
0.035 0.027 0.026 0.029 0.033 0.042 0.050 0.050
0.094
0.125
pH
[SDS]res
pH
[SDS]res
3.79
0.050
3.79
0.050
3.79
0.050
3.79 3.74 3.64 3.58 3.51
0.050 0.050 0.050 0.050 0.050
3.79 3.79 3.74
0.050 0.050 0.050
3.64 3.58
0.050 0.050
3.48
0.050
3.51
0.050
3.46
0.050
3.48
0.050
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Figure 5. The fraction of flocculated Zn vs the total aluminum sulfate concentration (moles per liter).
result (Figure 3) is that the combined charges of micellebound Al3+ and Zn2+ are smaller than the SDS charges at [Zn2+] ) 0.031 M, roughly equal in the optimum flocculation for [Zn2+] ) 0.062 M, and greater for [Zn2+] ) 0.075 M. This suggests an inversion of polarity around the micelle. Lacking information on the binding of Na+, we cannot ensure whether charge inversion takes place at [Zn2+] ) 0.062 M or below. When the localized adsorption model is applied, only the results for the series [Zn2+] ) 0.031 M are reasonable. For the other series, the localized adsorption model yields unreal results such as negative values of KZn or Stern potentials above 10 000 mV. Table 2 shows the values of KZn and Stern potentials obtained for [Zn2+] ) 0.031 M. The values of KZn show some scatter but are consistent with the values for KNa+ and KAl3+. The estimates of the Stern potential are consistent with the fact that flocculation in the presence of Zn2+ is not as extensive: the Stern potentials are reasonably negative, and their values are smaller as flocculation improves. So the question is why the model does not work at higher concentrations of multivalent cations. Lin and Jafvert4 suggest that the values of binding experimentally measured by Rathman and Scamehorn5 might be too high because the chemical analysis by ionselective electrode awarded as bound part of the cations within the diffuse layer. They reason that the physical barrier defining “bound” or nondetectable to the sensor may the hydrodynamic radius of the micelle and not the Stern layer, which would be smaller than said radius. Their model may be fitted to the results assuming a compression of the Stern layer at increasing counterion concentration. In the case of multivalent cations, the Stern and diffuse layers might contract. The volume within the hydrodynamic radius of the micelle could include part of the outer diffuse layer where the anion concentration is not negligible or extend into the bulk solution where the anion and cation concentrations comply with the requirement of electroneutrality. We lack detailed data for the evolution of colloid aggregates in micellar flocculation. Available details are that the micelles seem to be 10 nm in diameter,9,10 while for the NaDS/NaCl system Lin and Jafvert use 3.7 nm, and the colloidal aggregates found under some conditions remaining in equilibrium with the
Figure 6. Schematic view of the effect of changing thickness of the Stern-diffuse layer with respect to the hydrodynamic radius.
floc are from 100 to 400 nm9,10 depending on the conditions. The hydrodynamic radius is going to be larger as the micelles begin to flocculate and migrate. At this point, it has to be stressed that the flocculate does not form with a large number of fragments scattered across the bulk of the solution but that they aggregate together in one single floc. Therefore, they sweep through the solution and this effect of hydrodynamic drag of whatever combination of the Stern layer, the diffuse layer, and the bulk phase is encompassed within the hydrodynamic radius of the particles is important. Most of this material will be trapped within the floc fragments as they grow, and the composition of the final floc will depend on the concentration profile within the hydrodynamic radius, and not within the Stern layer. Some simple tests allowing freshly filtered floc to dry for 24 h by evaporation at room temperature indicate that the floc may initially contain at least 50% w/w of water, which at high salt concentrations would provide a non-negligible contribution of both anions and cations to the final floc. The motion and aggregation of micelles during flocculation should be very similar regardless of whether charge ratio inversion occurs or not. In the experiments with Al/Na, the inversion of the charge ratio is not seen. The conclusion is that the difference between both cases is that with Al or Al/Na the Stern-diffuse layer does not collapse within the hydrodynamic radius, and it is a good approximation to assume the Stern layer to be equivalent to the hydrodynamic layer (Figure 6). Enhanced Binding of Anionic Species. Results of binding of phthalic acid24 during micellar flocculation in the presence of Zn2+, in the presence of high [NaCl], and on flocs formed only in solutions containing SDS and aluminum sulfate (Figure 7a,b) show the effect of inversion in the apparent (bound cation/micellar surfactant) charge ratio. In the presence of the same concentration of free pollutant, the binding ratio pollutant/SDS in the floc is
Table 2. [Al3+]
(M)
0.01 0.0125 0.015 0.02 0.025 0.03
[SDS]floc (M)
ionic strength I
βAl
βZn
Ψ0 (V)
KZn (m3 kmol-1)
0.039 0.044 0.046 0.047 0.047 0.047
0.246 674 7 0.282 761 2 0.318 948 0.405 671 9 0.481 557 6 0.555 313 7
0.180 512 8 0.181 192 7 0.191 984 6 0.205 474 3 0.208 996 8 0.146 526 5
0.136 366 7 0.121 979 4 0.092 679 1 0.070 223 7 0.067 809 0.051 202 8
-0.049 369 -0.045 689 -0.043 581 -0.041 439 -0.039 805 -0.028 828
5.734 244 4 5.888 031 9 4.519 885 2 3.961 742 8 4.494 842 5 3.868 615 3
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of pollutants above charge ratio inversion must be therefore a consequence of the higher concentration of polyvalent cations and their complexes with pollutants, plus the compression of the Stern and diffuse layers. The proof that phenol forms complexes is important because phenol is removed by AMF,12 and all the other pollutants susceptible to capture by AMF are known to form complexes with Al3+. The mechanism proposed here overcomes the weakness of the former hypothesis of a two-step adsorption mechanism mostly controlled by electrostatic interactions, in which indeed there is no clear reason for a successful competition by phenoxide, phthalate, or benzoate against chloride or sulfate (with higher charge ratios or higher diffusivity). Conclusions
Figure 7. Removal of phthalic acid by various flocculates: (a) ratio phthalic acid/flocculated surfactant vs concentration of phthalic acid (moles per liter); (b) ratio phthalic/flocculated surfactant vs Al concentration.
higher for [Zn2+] in the region of charge inversion than for the rest of the cases. All the other cases follow the same functional dependence of pollutant/SDS ratio versus [pollutant]free. To make Figure 7a clearer, a point has been removed which is still shown in Figure 7b. It is the leftmost point of the [Zn2+] ) 0.07 M data and is the highest binding ratio found. There is no clear explanation of why in particular the points to the left of both Zn series have higher binding ratios. The pH is the same as, for instance, the NaCl series, so a higher proportion of acid present as phthalate (a chelate) is not a valid explanation. Why the capture of pollutants occurs at all requires further explanation, given the new mechanism proposed for micellar flocculation. In the cases where charge ratio inversion does not occur, only the Stern and maybe part of the diffuse layer are likely to be within the hydrodynamic radius. The anion concentration will be very low within the hydrodynamic radius, in fact much lower than in the bulk solution, yet pollutants are captured with separation ratios6-8 of up to 95% in some cases. According to the mechanism proposed here, only after charge inversion should some pollutant bind. It is known that humic and fulvic acids form complexes with Al3+, and among these are cationic complexes.22 Due to high [Al3+] within this region, cationic complex stoichiometries [Al2(pollutant)3]3+ and [Al(pollutant)2]+ are expected to form and incorporate into the Stern and diffuse layers. The enhanced removal (23) Stumm, W.; Morgan, J. J. Aquatic Chemistry; John Wiley & Sons: New York, 1981; Chapter 5. (24) Talens-Alesson, F. I.; Anthony, S.; Hankins, N. P.; Azzopardi, B. J. Proceedings of the 32 Jorn. Com. Esp. Deterg, Barcelona, 2002, pp 433-436.
Micellar flocculation seems better described as the consequence of three phenomena: (1) the anisotropic reorganization of the ions around the charged surface of ionic micelles with the thickness of the region with a concentration gradient (Stern plus diffuse layers) dependent on the ion concentrations; (2) the electroneutralization of the micelles, leading to their flocculation; and (3) the drag by the micelles and their growing aggregates, which move to further aggregate, of the ions contained within their hydrodynamic radii. This mechanism explains how it is possible to capture a higher proportion of cations than would be expected assuming that binding affects only the cations within the real Stern layer. To model micellar flocculation, it would be required to solve the triple-layer model and to calculate the hydrodynamic radius of the micelles and their aggregates. This would allow the calculation of the ion binding by integration of the concentration profiles within the hydrodynamic radius. The capture of pollutants by AMF may be explained by the accumulation of cationic complexes within the Stern-diffuse layer. Acknowledgment. The authors are indebted to the Bureau of Clean Technologies of the Department of the Environment of the Autonomous Government of Catalonia for funding this research between 1993 and 1996. F. I. Talens-Alesson acknowledges also U.K. EPSRC Grant GR/N 24377/01 (2000-2001). Nomenclature e F f I K k N SDS T z
1.6 × C Faraday constant (96 472 C/mol) activity coefficient ionic strength, kmol m-3 dissociation constant, m3 kmol-1 Boltzmann constant, 1.38 × 10-23 J molecule-1 K-1 Avogadro’s number (6.022 × 1023/mol) surface area of DS- micelle temperature, K ionic charge 10-19
Greek Symbols Rx β γ η λ
chemical activity of species x cation/surfactant binding ratio (ion/ion) chemical activity aggregation number sum of the Stern and diffuse layer thicknesses (Å)
Flocculation of Lauryl Sulfate Micelles ψS ζ
Stern potential, V zeta-potential, V
Subindexes bound free mic
concentration which is actually bound to micelles concentration existing in solution as the form indicated within brackets present in micellar form
Langmuir, Vol. 18, No. 22, 2002 8301 mon other total W X
present as monomer concentration of the ion in other forms (complexes) total concentration of the species added to the sample in aqueous solution chemical species
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