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Adsorption of sodium dodecylbenzene sulfonate on organophilic bentonites
Applied Clay Science 18 Ž2001. 173–181 www.elsevier.nlrlocaterclay
Adsorption of sodium dodecylbenzene sulfonate on organophilic bentonites ) David Christian Rodrıguez-Sarmiento, Jorge Alejo Pinzon-Bello ´ ´ Chemistry Department, UniÕersidad Nacional de Colombia, P.O. Box 14490, Santa Fe´ de Bogota, ´ Colombia Received 27 September 1999; received in revised form 3 July 2000; accepted 18 July 2000
Abstract A bentonite from the Cauca Valley in Colombia was treated with tetramethylammonium bromide ŽTAB., hexadecyltrimethylammonium bromide ŽCTAB., hexadecylbenzyldimethylammonium chloride ŽCDAC., and alkylbenzyldimethylammonium chloride ŽBTC., in order to obtain organophilic compounds. The treatment was carried out at 50% and 100% of the cationic exchange capacity ŽCEC. of the natural bentonite ŽNB.. The adsorption of an aqueous solution of sodium dodecylbenzene sulfonate ŽDBS. over the natural and modified bentonites was studied. The experimental data points were fitted to several equations applicable to adsorption in solution such as: Langmuir, BET and Freundlich. The Langmuir isotherm adequately describes the adsorption process in all cases. The adsorption grade can be quantified by the distribution coefficient, for low concentrations in solution expressed in molality, K dm . The affinity of organophilic bentonites by DBS can be expressed in terms of the standard adsorption Gibbs energy. The class and structure of the hydrocarbon chains of the quaternary ammonium ions adsorbed over the bentonite determine the DBS adsorption grade. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Montmorillonite; Organophilic bentonites; Adsorption; Surfactants; Langmuir isotherm
1. Introduction Bentonite is a clay mainly composed of montmorillonite which is a 2:1 type aluminosilicate, that is, its crystalline structure presents an alumina octahedral layer between two tetrahedral layers of silica which, by isomorphous substitutions, require cations,
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denominated exchange cations, to compensate the negative charges of their laminar edges. Bentonite has the capacity to exchange these cations with the ones present in aqueous solutions of organic or inorganic salts. This property is mainly responsible for the great adsorbent power of bentonite, especially toward ions in solution. This property can be used to chemically modify the natural bentonite ŽNB., thus obtaining the denominated homoionic bentonites, that are bentonites in which the exchange cations have been substituted by a single cation from the selected inorganic salt. Therefore, homoionic bentonites of
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 0 . 0 0 0 2 2 - 3
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D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
Na, K, Ca, Al, Cr, etc., can be obtained. When NB is in contact with aqueous solutions of quaternary ammonium salts, the exchange cations of the clay are substituted by organic cations. The obtained compounds are known as ‘organophilic bentonites’ ŽHauser, 1950.. Since then, research on clay–organic interactions has increased steadily. For example, Dekany ´ ´ and Nagy Ž1991a,b., Patzko´ and Dekany ´ ´ Ž1993., and Regdon et al. Ž1994. have studied the liquid sorption and wetting on hydrophilicrhydrophobic layer silicates. They determined the adsorption displacement free energy functions, which are characteristic of the polarities of the surfaces. Nowadays, the widespread use of detergents in industry as well as at home is producing an important environmental impact, since despite almost all detergents that have branched-chain alkylbenzenesulfonates ŽABS. as surfactant ingredients, which are not 100% biodegradable, have been eliminated from the market, the second generation surfactant compounds such as linear-chain alkylbenzene sulfonates ŽLAS., which are supposed to be biodegradable depending on the conditions of the water, can behave as not biodegradable. This happens in waters with low levels of dissolved oxygen or in those in which phenols or phenol compounds are present. In the former case, the microbial degradation diminishes, and in the latter, is completely inhibited. The contribution of clay minerals to the adsorption of organic or inorganic compounds has been studied by several authors Že.g. Gonzalez ´ Pradas et al., 1994, 1996; Del Rey Bueno et al., 1983; Volzone and Travani, 1995; Gutierrez and Fuentes, 1993; ´ Klumpp et al., 1993; Capovilla et al., 1991.. It was found that the lipophilic tails from cations of long chain quaternary ammonium salts, previously retained on the bentonite, help to the adsorption of organic compounds ŽKlumpp et al., 1993; Capovilla et al., 1991.. The effect of progressive substitution of the exchange cations of calcium bentonite and of illite by quaternary ammonium compounds on the adsorption of 2-naphthol was studied. The results show that adsorption progressively increases as the hydrophobicity of the clay increases, due to a synergic effect between the quaternary ammonium compounds and 2-naphthol ŽKlumpp et al., 1993.. The objective of the present work is to study the adsorption of sodium dodecylbenzene sulfonate
ŽDBS. in aqueous solutions, over a bentonite from the Cauca Valley, in natural state and modified with quaternary ammonium salts to 50% and 100% of its cationic exchange capacity ŽCEC..
2. Methodology 2.1. Materials An NB from the Cauca Valley was used. The crystalline phases present in the sample were determined by using X-ray diffraction with a scanning between 28 and 648 on the following samples: unoriented powder, oriented aggregates both in water and ethylene glycol, calcined Žat 1108, 3008 and 5008. oriented aggregates in water as well as calcined Žat 6508. unoriented powder. The presence of montmorillonite with a small quantity of kaolinite, quartz, and feldspar was confirmed. A semiquantitative analysis by using X-rays was also performed with the following results: laminate minerals ) 90%, quartz - 5%, feldspar - 5%. CEC: 836.4 meqrkg. BET surface area Ždetermined by N2 adsorption.: 34 m2rg. The grain fraction ranges between 0.6 and 100 mm; four population groups with peaks at 2.5, 9, 20 and 50 mm, corresponding to each component of the sample, were observed ŽCorredor and Pinzon, ´ 1994; Pinzon ´ and Requena, 1996.. The organophilic bentonites were prepared to 100% and 50% of CEC of the NB by treatment with the following quaternary ammonium salts: hexadecyltrimethylammonium bromide ŽCTAB., tetramethylammonium bromide ŽTAB., alkylbenzyldimethylammonium chloride ŽBTC. ŽR s C 8 H 17 to C 18 H 37 ., and hexadecylbenzyldimethylammonium chloride ŽCDAC.. The organophilic bentonites were named as follows: the abbreviations of quaternary ammonium ion are used, followed by the Roman number I for the highest percentage: CTA-I, TA-I, BT-I, CDA-I, and by the Roman number II for the lowest percentage: CTA-II, TA-II, BT-II, CDA-II, of cationic exchange sites covered with the ammonium ion. The DBS employed in the kinetic assays and in the adsorption isotherms, is a Sigma product Tech. grade, with 80% purity.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´ Table 1 pH of the clay suspension systems under study Bentonite
pH of bentonite suspending in solution
pH of bentonite suspending in water
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
7.60 7.16 7.20 7.12 7.50 7.95 7.19 8.00 8.30
7.20 6.83 6.80 6.92 7.60 7.70 6.93 8.43 8.40
2.2. Preparation of the organophilic bentonites To 100 ml dispersion of NB in distilled water Ž100 grl., 100 ml of a solution of the quaternary ammonium salt was added; the mixture was mechanically stirred for 1 h and then it was washed and decanted until end of halides; the sample was dried at 1008C for 24 h, then it was powdered to 120 mesh ŽCorredor, 1991.. 2.3. Determination of the adsorption speed To 100 ml of DBS solution between 45 and 105 mgrl, magnetically stirred, 15 ml of a dispersion of either natural or organophilic bentonite in distilled water was added. Both samples were previously
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thermostated at 258C, in order to avoid thermal shocks. The final clay solution ratio was 1 grl in all cases. The adsorption time was counted from the addition of the bentonite dispersion until half of the 10-ml aliquot of the solution was separated with a syringe that had a cotton filter. Subsequently, the DBS concentration of the solution was colorimetrically determined. The following reagents were mixed in this order: 40 ml of distilled water, 1 ml of solution of 300 ppm of sodium hypochlorite, 2 ml of o-tolidine Ž1 grl HCl at 10%., 5 ml of DBS and it is completed at 50 ml with distilled water. After the solution had been homogenized, its transmittance was read at 525 mmm by using distilled water as blank ŽHarris, 1943.. This procedure was repeated with all the studied bentonites at different times, until the equilibrium was reached.
2.4. Determination of the adsorption isotherms To a 50-ml dispersion of natural or organophilic bentonite in distilled water, in a proportion of 2 grl, previously stirred for 1 h, 50 ml of a solution of DBS in distilled water were added, in concentrations ranging from 0 to 415 mgrl. The temperature of the bentonite dispersion and of the DBS solution was 258C. The samples were mechanically stirred for 2 h at 258C after the mixing process. Then, the dispersions were centrifuged for 5 min and the DBS concentration of the supernatant, Ce ,
Fig. 1. DBS adsorption kinetics on bentonite modified at 100% of its CEC with the CTA ion at 298 K.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
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was determined colorimetrically. The amount of DBS adsorbed, X, was calculated from the difference between the initial and the final concentration. The pH in each case was given by each clay suspension system Žsee Table 1..
3. Results and discussion 3.1. Adsorption rate The results of the adsorption kinetics for the different adsorbents show a behavior similar to that shown in Fig. 1 for the bentonite CTA-I. In all cases, the adsorption process of DBS over the studied bentonites was fast; the equilibrium was reached at approximately 20–30 min. According to Langmuir, the adsorption kinetics pattern corresponds to a reversible process in which the adsorption and desorption take place simultaneously until the equilibrium is reached ŽDaniels and Alberty, 1967., in which the DBS concentration remains constant, as shown by the flat part of the ‘Concentration vs. Time’ curve. According to these results, a time of 2 h was chosen to obtain the adsorption isotherms that were considered enough to reach the equilibrium. 3.2. Adsorption isotherms All the adsorption isotherms ‘ X Žmgrg. vs. Ce Žmgrl.’, except for TA-II and NBs, show a type L
behavior, according to the classification of Giles and MacEwan similar to the one shown in Fig. 2 for the bentonite CTA-I ŽOsipow Lloyd, 1962.. In this classification, type L assumes a monolayer formation in the active sites of the surface, and all the adsorption sites are supposed to be equivalent. TA-II and NBs show a type S behavior according to the above mentioned classification, which indicates that the forces involved in the adsorption process are small, that is, there is a low affinity for the adsorbate, at low adsorbate concentrations. This is explained because the NB has a negatively charged surface that repels the also anionic DBS. On the other hand, TA-II bentonite is only slightly covered by very short hydrocarbon chains, showing only a small difference with the NB. Fig. 3 shows the isotherms of the organophilic bentonites from I and II series, and also that of NB. The results indicate that DBS adsorption by the organophilic bentonites studied depends on the surface percentage covered by the lipophilic tails from the corresponding quaternary ammonium ion; indeed, the adsorption is greater when all the sites of cationic exchange are covered by the corresponding ion, like in series I clays, and the adsorption is smaller when there is lesser number of sites covered by the same ion, that is, series II clays. It is also observed that the adsorption follows the order CTA ) BT ) CDA ) TA when all the exchange sites are covered, like in series I. In the case of series II bentonites, the adsorption order is CTA ) CDA ) BT ) TA. This result will be quantitatively ex-
Fig. 2. DBS adsorption isotherm on bentonite modified at 100% of its CEC with the CTA ion at 298 K.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
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at 50%; whereas the adsorption of DBS on NB is almost negligible. 3.3. Adsorption models For the data treatment, the data that lied outside the 95% confidence interval, established by the method of least squares for the linear function, were discarded. The uncertainty of the parameters of the adjusted straight line corresponds to the standard deviations. The experimental data points were fitted to the Langmuir equation ŽDaniels and Alberty, 1967.: CerX s 1r Ž Xm K e . q CerXm
Ž 1.
where Ce is the concentration of adsorbate in the equilibrium solution, X is the amount of adsorbate adsorbed per gram of adsorbent, X m is the maximum amount of adsorbate that can adsorbed in a monolayer and K e is a constant related to the energy of adsorption. The plot of experimental values for CerX against Ce gives a straight line. Except for NB and for that treated to 50% of its CEC with TA, the linear correlation coefficients are very good Žgreater than 0.99. for the Langmuir isotherm Žsee Table 2.. In case of liquid adsorption, the BET isotherm is expressed in the following equation ŽDas and Chatterjee, 1993.: CrX Ž C0 y C . s 1r Ž Xm K . q Ž K y 1 . r Ž Xm K . Ž CrC0 . Fig. 3. DBS adsorption isotherms at 298 K: Ža. series I, bentonite covered at 100% of its CEC with the following ions: CTA-I, CDA-I, BT-I, TA-I; Žb. series II, bentonite covered at 50% of its CEC with the same ions. NB is included in both series.
plained by means of the isotherm parameters of Langmuir, BET, and Freundlich models. As an example, Fig. 4 shows the isotherm of the individual adsorption of DBS over CTA-I and II bentonites, as well as that of NB. This plot clearly shows the effect of the substitution of exchange cations by the quaternary ammonium ion on DBS adsorption. When the bentonite was modified at 100% of its CEC, it adsorbs approximately two times more DBS than what it adsorbs when it is modified
Ž 2.
where C0 is the initial concentration of the solution used, C, the final concentration of the solution, X,
Fig. 4. Adsorption isotherms of the CTA-I, CTA-II, and NB at 298 K. ŽAbbreviations have the same meaning as those of Fig. 3..
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Table 2 Parameters for the Langmuir model Bentonite
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
Parameter Data number
Correlation coefficient
Slope Žgrmg.
Intercept Žgrl.
8 10 8 7 7 9 9 5 13
0.998 0.996 0.996 0.999 0.996 0.999 0.993 y0.732 y0.444
0.00464 " 0.00012 0.00779 " 0.00026 0.00587 " 0.00021 0.00758 " 0.00013 0.00461 " 0.00018 0.00961 " 0.00013 0.0241 " 0.0010 y0.024 " 0.013 y0.11 " 0.07
0.038 " 0.008 0.159 " 0.026 0.090 " 0.022 0.276 " 0.014 0.150 " 0.026 0.160 " 0.015 2.30 " 0.25 14.1 " 2.5 53 " 12
the amount of adsorbate adsorbed, X m , the quantity of adsorbate required for a complete monolayer and K is a constant related to heat of adsorption. The plot of experimental values for CrX Ž C0 y C . against CrC0 gives a straight line. For the BET isotherm, the correlation coefficients were as good as those found for the Langmuir isotherm, except for NB and for TA bentonite TA-I. The Langmuir isotherm corresponds to a particular case of the BET isotherm, thus, explaining the good data adjustment to this model, despite that in most cases, the CrC0 ratio is greater than 0.35, which corresponds to the highest limit of linearity allowed for the BET isotherm in the literature ŽOsipow Lloyd, 1962.. However, the intercept of this isotherm does not allow to calculate the distribution coefficient for the adsorption process, K dm , because negative values andror very high uncertainty ranges were obtained for this parameter. The mathematical form of the Freundlich isotherm is ŽDaniels and Alberty, 1967.: X s KC n
Ž 3.
or: log X s log K q nlogC
Ž 4.
where C is the final concentration of the solution, X the amount of adsorbate adsorbed per gram of adsorbent, n and K are constants. The plot of log X against logC gives a straight line. In the case of Freundlich isotherm, only BT-I, BT-II, TA-I, and NBs show correlation coefficients
greater than 0.99; in the other cases, this coefficient lies between 0.99 and 0.90. On the other hand, the Freundlich isotherm, which is an empirical model that does not take into account the distribution coefficient, allows to determine the adsorption grade by means of its K and n parameters. The greater the K and n are, the greater the X value. The sequence of greater to lower the K value is: CTA-I, BT-I, CDA-I, TA-I, NB, and BT-II, CTAII, CDA-II, TA-II, NB; while for the n value this sequence is: NB, TA-I, CTA-I, BT-I, CDA-I, and NB, TA-II, CDA-II, CTA-II, BT-II. In no case, the sequence of these parameters coincides with that found for K dm values. Taking into account the empirical character of the isotherm, and that in most cases the correlation coefficient is lower than that calculated for the Langmuir isotherm, the use of K and n in the adsorption analysis of DBS over the organophilic clays was not considered. The above mentioned results allow to conclude that the Langmuir isotherm is the one that best describes the adsorption process of DBS over the organophilic clays. 3.4. Distribution coefficient and the standard Gibbs energy change of adsorption Several researchers Že.g. Del Rey Bueno et al., 1983; Gutierrez and Fuentes, 1993; Comans et al., ´ 1991; Klumpp et al., 1993. use the constant K d Žconstant of distribution or of apparent equilibrium.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
as an indicator of the extension of adsorption or as a relative measure of the adsorbent affinity from the adsorbate. The constant K d that represents the relation between the quantity of adsorbate present in each of the two phases, that is in the adsorbent and in the solution, is defined as follows: K d s XrCe Ž lrg . or: K d s XrCe Ž 10 3 mlrg . .
Ž 5.
This constant is calculated experimentally by the inverse of the intercept of the linear equation of the Langmuir isotherm ŽEq. Ž1.., that is: K d s X m K e ; or also by the slope of the diluted region Žthe linear part. of the isotherm X vs. Ce . If Ce is expressed in molality, the constant K d becomes adimensional and equals the thermodynamic constant of distribution Žwhich is adimensional and is calculated for concentrations approaching zero., that is the reason why it is called K dm . The distribution coefficient that was calculated from the Langmuir isotherm is shown in Table 3; it clearly shows the effect of the substitution grade of the exchange cations by quaternary ammonium ions in the adsorption of DBS, since K dm is greater for all the salts considered when this substitution is 100% than when it is 50%. Likewise, K dm for NB is the lowest registered value.
Table 3 Distribution coefficient, K dm , and standard Gibbs energy change of adsorption Žprocess: adsorbate over the adsorbent™adsorbate in the solution., DG ) , for various clays Bentonite
a K dm
DG ) Žkcalrmol. a
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
26,000"6000 6300"1000 11,100"2700 3620"180 6700"1200 6300"600 430"50 71"13 19"4
6.02"0.17 5.18"0.13 5.52"0.18 4.85"0.06 5.21"0.14 5.18"0.09 3.59"0.10 2.53"0.14 1.74"0.16
a
tion.
The uncertainties were calculated using the variance propaga-
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The K dm values for the hydrocarbon chains of quaternary ammonium ions follow the sequence 26,000 CTA-I, 11,100 CDA-I, 6700 BT-I, and 430 TA-I. When the K dm of CTA-I Ža chain of 16 carbon atoms and three methyl groups. is compared with that of CDA-I Ža chain of 16 carbon atoms, one benzyl group and two methyl groups., it is found that substitution of a methyl group by a benzyl group lowers the adsorption, as indicated by the K dm values, maybe due to steric effect. Likewise, the change of the hydrocarbon chain from 16 to 13 carbon atoms lowers the adsorption, as shown by the CDA-I and BT-I Ža chain of 13 carbon atoms on average, one benzyl group, and two methyl groups. K dm values. Finally, when the chains are short, with four methyl groups, a very small coefficient is obtained. The K dm values in series II show the following sequence: 6300 CTA-II; 6300 BT-II; 3620 CDA-II; 71 TA; this order changes for BT-II and CDA-II related to series I; this behavior seems to indicate that at low covering percentages of bentonite with organic cations, the steric effect is of low importance. These results are in good agreement with those found by Klumpp et al. Ž1993., for the adsorption of 2-naphthol over a calcic bentonite of Baviera in presence of dodecyltrimethylammonium and dodecyldimethylammonium bromides; these authors establish that the adsorption depends on the substitution grade of the exchange cations, on the number, and on the nature of the hydrocarbon chains. The behavior, according to Langmuir isotherm, allows to infer that in the experimental range, there is only monolayer formation and that all the adsorption sites are equivalent. This behavior can be explained according to the model established by Van Olphen Ž1963.: once the monolayer has been formed, the polar heads of the DBS molecules are oriented toward the aqueous phase, avoiding the accommodation of more layers because of electrostatic repulsion. Besides, as has already been mentioned, if the adsorption is due to the presence of lipophilic tails on the bentonite surface, the adsorption sites are equivalent and therefore the Langmuir model is the one that best describes the adsorption process, except for TA-II and NB, that do not have the characteristics described previously. Indeed, the adsorption
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D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
isotherms for the case of TA-II and NBs show a type S behavior, according to the classification of Giles and MacEwan ŽOsipow Lloyd, 1962., which indicates that there is a low affinity for the adsorbate, especially at low concentrations of it. This is a plausible result, because TA-II bentonite as well as NB lack of sites that are able to sufficiently retain DBS. The K dm values found indicate that in CTA, CDA, BT, and TA organophilic clays, the amount of adsorbed DBS is greater than the one that remains in solution. The standard Gibbs energy change of adsorption for the process: adsorbate over the adsorbent™ adsorbate in the solution, is calculated by the equation DG ) s RT ln K dm ŽEwert, 1983; Castellan, 1964.. These values calculated for each of the used bentonites is also shown in Table 3. These DG ) values indicate that the DBS molecules are more stable in the adsorbed phase than in the aqueous phase, and therefore, the adsorption process is spontaneous. It is also important to notice that DG ) values are always greater for series I bentonites, when the covering is 100%, than for the series II, when the covering is 50%. Similarly, the DG ) value for NB is the lowest of all, indicating the DBS low affinity for this adsorbent.
4. Conclusions ŽI. The substitution of the exchange cations by quaternary ammonium cations transform the bentonite in a material capable of adsorbing DBS. ŽII. The adsorption capacity is directly proportional to the substitution grade of the exchange cations in the bentonite by quaternary ammonium ions. ŽIII. The adsorption of DBS over organophilic bentonites is a relatively fast process. ŽIV. The Langmuir isotherm model is the one that best describes the adsorption process of DBS over the organophilic bentonites studied. Therefore, the adsorption of this compound on organophilic bentonites takes place by monolayer formation. ŽV. The adsorption grade can be appropriately quantified by means of the distribution coefficient, K dm .
ŽVI. The affinity of the organophilic bentonites by DBS can be expressed in terms of the standard adsorption Gibbs energy. ŽVII. The class and structure of the hydrocarbon chains of the cations of quaternary ammonium salts adsorbed on the bentonite determine the adsorption grade of DBS.
References Capovilla, L., Labbe, P., Reverdy, G., 1991. Formation of cationicranionic mixed surfactant bilayers on laponite clay suspensions. Langmuir 7 Ž10., 2000–2003. Castellan, G.W., 1964. Physical Chemistry. Addison-Wesley, Massachusetts, p. 287. Comans, R., Haller, M., Preter, P., 1991. Sorption of cesium on illite: non-equilibrium behaviour and reversibility. Geochim. Cosmochim. Acta 55, 433–440. Corredor, Y., 1991. Obtencion a partir ´ de una arcilla organofılica ´ de una bentonita del Valle del Cauca. Tesis de Grado, Departamento de Quımica, Facultad de Ciencias, Universidad Na´ cional de Colombia, Bogota. ´ Corredor, Y., Pinzon, ´ J.A., 1994. Obtencion ´ de arcillas organofılicas. Rev. Colomb. Quim. 23 Ž1., 33–43. ´ Daniels, F., Alberty, R.A., 1967. Physical Chemistry. 3rd edn. Wiley, New York, p. 338. Das, S.K., Chatterjee, M.K., 1993. Specific surfaces and heat of adsorption of some Indian clays by dye-adsorption technique. Bull. Mater. Sci. 16 Ž3., 205–211. Dekany, I., Nagy, L.G., 1991a. Liquid sorption and wetting on ´ ´ hydrophilicrhydrophobic layer silicates. Colloids Surf. 58 Ž3., 251–261, C.A. 115, 240818j. Dekany, ´ ´ I., Nagy, L.G., 1991b. Immersional wetting and adsorption displacement on hydrophilicrhydrophobic surfaces. J. Colloid Interface Sci. 147 Ž1., 119–128, C.A. 115, 240753j. Del Rey Bueno, F., Gonzalez ´ Pradas, E., Valenzuela Calahorro, C., Lopez Gonzalez, J.D., 1983. Retencion ´ ´ ´ de parathion en bentonitas homoionicas: II. Isotermas de retencion y magni´ tudes termodinamicas relacionadas. An. Edafol. Agrobiol. ´ XLII, 655–673. Ewert, C., 1983. Funciones termodinamicas relativas a la transfer´ encia de algunos hidrocarburos aromaticos desde el ciclohex´ ano hasta el agua. Tesis de grado, Departamento de Quımica, ´ Facultad de Ciencias, Universidad Nacional de Colombia, Bogota. ´ Gonzalez M., Canton ´ Pradas, E., Villafranca Sanchez, ´ ´ Cruz, F., Socıas Perez, M., 1994. Adsorption of ´ Viciana, M., Fernandez ´ ´ Cd and Zn from aqueous solution on natural and activated bentonite. J. Chem. Technol. Biotechnol. 59, 289–295. Gonzalez M., Fernandez Perez, ´ Pradas, E., Villafranca Sanchez, ´ ´ ´ M., Socıas ´ Viciana, M., 1996. Sorption of diurom, athrazine, MCPA and paraquat on bentonite, humic acid and peat. Fresenius Environ. Bull. 5, 178–184.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´ Gutierrez, M., Fuentes, H., 1993. Langmuir isotherm based pre´ diction of competitive sorption of Sr, Cs and Co in Ca montmorillonite. Waste Manage. 13, 327–332. Harris, J.C., 1943. Colorimetric determination of alkyl benzene sulfonates. Ind. Eng. Chem., Anal. Ed. 15 Ž4., 254–256. Hauser, E.A., 1950. Organophilic clays. In: Alexander, J. ŽEd.., Colloid Chemistry: Theoretical and Applied, vol. VII. Reinhold Publishing, New York, pp. 431–441. Klumpp, E., Heitmann, H., Schwuger, M.I., 1993. Synergistic effects between cationic surfactants and organic pollutants on clay minerals. Colloids Surf., A 78 Ž1–3., 93–98. Osipow Lloyd, I., 1962. Surface Chemistry. Reinhold Publishing, New York, pp. 23–58. Patzko, ´ A., Dekany, ´ ´ I., 1993. Ion exchange and molecular adsorp-
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Adsorption of sodium dodecylbenzene sulfonate on organophilic bentonites ) David Christian Rodrıguez-Sarmiento, Jorge Alejo Pinzon-Bello ´ ´ Chemistry Department, UniÕersidad Nacional de Colombia, P.O. Box 14490, Santa Fe´ de Bogota, ´ Colombia Received 27 September 1999; received in revised form 3 July 2000; accepted 18 July 2000
Abstract A bentonite from the Cauca Valley in Colombia was treated with tetramethylammonium bromide ŽTAB., hexadecyltrimethylammonium bromide ŽCTAB., hexadecylbenzyldimethylammonium chloride ŽCDAC., and alkylbenzyldimethylammonium chloride ŽBTC., in order to obtain organophilic compounds. The treatment was carried out at 50% and 100% of the cationic exchange capacity ŽCEC. of the natural bentonite ŽNB.. The adsorption of an aqueous solution of sodium dodecylbenzene sulfonate ŽDBS. over the natural and modified bentonites was studied. The experimental data points were fitted to several equations applicable to adsorption in solution such as: Langmuir, BET and Freundlich. The Langmuir isotherm adequately describes the adsorption process in all cases. The adsorption grade can be quantified by the distribution coefficient, for low concentrations in solution expressed in molality, K dm . The affinity of organophilic bentonites by DBS can be expressed in terms of the standard adsorption Gibbs energy. The class and structure of the hydrocarbon chains of the quaternary ammonium ions adsorbed over the bentonite determine the DBS adsorption grade. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Montmorillonite; Organophilic bentonites; Adsorption; Surfactants; Langmuir isotherm
1. Introduction Bentonite is a clay mainly composed of montmorillonite which is a 2:1 type aluminosilicate, that is, its crystalline structure presents an alumina octahedral layer between two tetrahedral layers of silica which, by isomorphous substitutions, require cations,
)
Corresponding author.
denominated exchange cations, to compensate the negative charges of their laminar edges. Bentonite has the capacity to exchange these cations with the ones present in aqueous solutions of organic or inorganic salts. This property is mainly responsible for the great adsorbent power of bentonite, especially toward ions in solution. This property can be used to chemically modify the natural bentonite ŽNB., thus obtaining the denominated homoionic bentonites, that are bentonites in which the exchange cations have been substituted by a single cation from the selected inorganic salt. Therefore, homoionic bentonites of
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 0 . 0 0 0 2 2 - 3
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D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
Na, K, Ca, Al, Cr, etc., can be obtained. When NB is in contact with aqueous solutions of quaternary ammonium salts, the exchange cations of the clay are substituted by organic cations. The obtained compounds are known as ‘organophilic bentonites’ ŽHauser, 1950.. Since then, research on clay–organic interactions has increased steadily. For example, Dekany ´ ´ and Nagy Ž1991a,b., Patzko´ and Dekany ´ ´ Ž1993., and Regdon et al. Ž1994. have studied the liquid sorption and wetting on hydrophilicrhydrophobic layer silicates. They determined the adsorption displacement free energy functions, which are characteristic of the polarities of the surfaces. Nowadays, the widespread use of detergents in industry as well as at home is producing an important environmental impact, since despite almost all detergents that have branched-chain alkylbenzenesulfonates ŽABS. as surfactant ingredients, which are not 100% biodegradable, have been eliminated from the market, the second generation surfactant compounds such as linear-chain alkylbenzene sulfonates ŽLAS., which are supposed to be biodegradable depending on the conditions of the water, can behave as not biodegradable. This happens in waters with low levels of dissolved oxygen or in those in which phenols or phenol compounds are present. In the former case, the microbial degradation diminishes, and in the latter, is completely inhibited. The contribution of clay minerals to the adsorption of organic or inorganic compounds has been studied by several authors Že.g. Gonzalez ´ Pradas et al., 1994, 1996; Del Rey Bueno et al., 1983; Volzone and Travani, 1995; Gutierrez and Fuentes, 1993; ´ Klumpp et al., 1993; Capovilla et al., 1991.. It was found that the lipophilic tails from cations of long chain quaternary ammonium salts, previously retained on the bentonite, help to the adsorption of organic compounds ŽKlumpp et al., 1993; Capovilla et al., 1991.. The effect of progressive substitution of the exchange cations of calcium bentonite and of illite by quaternary ammonium compounds on the adsorption of 2-naphthol was studied. The results show that adsorption progressively increases as the hydrophobicity of the clay increases, due to a synergic effect between the quaternary ammonium compounds and 2-naphthol ŽKlumpp et al., 1993.. The objective of the present work is to study the adsorption of sodium dodecylbenzene sulfonate
ŽDBS. in aqueous solutions, over a bentonite from the Cauca Valley, in natural state and modified with quaternary ammonium salts to 50% and 100% of its cationic exchange capacity ŽCEC..
2. Methodology 2.1. Materials An NB from the Cauca Valley was used. The crystalline phases present in the sample were determined by using X-ray diffraction with a scanning between 28 and 648 on the following samples: unoriented powder, oriented aggregates both in water and ethylene glycol, calcined Žat 1108, 3008 and 5008. oriented aggregates in water as well as calcined Žat 6508. unoriented powder. The presence of montmorillonite with a small quantity of kaolinite, quartz, and feldspar was confirmed. A semiquantitative analysis by using X-rays was also performed with the following results: laminate minerals ) 90%, quartz - 5%, feldspar - 5%. CEC: 836.4 meqrkg. BET surface area Ždetermined by N2 adsorption.: 34 m2rg. The grain fraction ranges between 0.6 and 100 mm; four population groups with peaks at 2.5, 9, 20 and 50 mm, corresponding to each component of the sample, were observed ŽCorredor and Pinzon, ´ 1994; Pinzon ´ and Requena, 1996.. The organophilic bentonites were prepared to 100% and 50% of CEC of the NB by treatment with the following quaternary ammonium salts: hexadecyltrimethylammonium bromide ŽCTAB., tetramethylammonium bromide ŽTAB., alkylbenzyldimethylammonium chloride ŽBTC. ŽR s C 8 H 17 to C 18 H 37 ., and hexadecylbenzyldimethylammonium chloride ŽCDAC.. The organophilic bentonites were named as follows: the abbreviations of quaternary ammonium ion are used, followed by the Roman number I for the highest percentage: CTA-I, TA-I, BT-I, CDA-I, and by the Roman number II for the lowest percentage: CTA-II, TA-II, BT-II, CDA-II, of cationic exchange sites covered with the ammonium ion. The DBS employed in the kinetic assays and in the adsorption isotherms, is a Sigma product Tech. grade, with 80% purity.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´ Table 1 pH of the clay suspension systems under study Bentonite
pH of bentonite suspending in solution
pH of bentonite suspending in water
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
7.60 7.16 7.20 7.12 7.50 7.95 7.19 8.00 8.30
7.20 6.83 6.80 6.92 7.60 7.70 6.93 8.43 8.40
2.2. Preparation of the organophilic bentonites To 100 ml dispersion of NB in distilled water Ž100 grl., 100 ml of a solution of the quaternary ammonium salt was added; the mixture was mechanically stirred for 1 h and then it was washed and decanted until end of halides; the sample was dried at 1008C for 24 h, then it was powdered to 120 mesh ŽCorredor, 1991.. 2.3. Determination of the adsorption speed To 100 ml of DBS solution between 45 and 105 mgrl, magnetically stirred, 15 ml of a dispersion of either natural or organophilic bentonite in distilled water was added. Both samples were previously
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thermostated at 258C, in order to avoid thermal shocks. The final clay solution ratio was 1 grl in all cases. The adsorption time was counted from the addition of the bentonite dispersion until half of the 10-ml aliquot of the solution was separated with a syringe that had a cotton filter. Subsequently, the DBS concentration of the solution was colorimetrically determined. The following reagents were mixed in this order: 40 ml of distilled water, 1 ml of solution of 300 ppm of sodium hypochlorite, 2 ml of o-tolidine Ž1 grl HCl at 10%., 5 ml of DBS and it is completed at 50 ml with distilled water. After the solution had been homogenized, its transmittance was read at 525 mmm by using distilled water as blank ŽHarris, 1943.. This procedure was repeated with all the studied bentonites at different times, until the equilibrium was reached.
2.4. Determination of the adsorption isotherms To a 50-ml dispersion of natural or organophilic bentonite in distilled water, in a proportion of 2 grl, previously stirred for 1 h, 50 ml of a solution of DBS in distilled water were added, in concentrations ranging from 0 to 415 mgrl. The temperature of the bentonite dispersion and of the DBS solution was 258C. The samples were mechanically stirred for 2 h at 258C after the mixing process. Then, the dispersions were centrifuged for 5 min and the DBS concentration of the supernatant, Ce ,
Fig. 1. DBS adsorption kinetics on bentonite modified at 100% of its CEC with the CTA ion at 298 K.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
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was determined colorimetrically. The amount of DBS adsorbed, X, was calculated from the difference between the initial and the final concentration. The pH in each case was given by each clay suspension system Žsee Table 1..
3. Results and discussion 3.1. Adsorption rate The results of the adsorption kinetics for the different adsorbents show a behavior similar to that shown in Fig. 1 for the bentonite CTA-I. In all cases, the adsorption process of DBS over the studied bentonites was fast; the equilibrium was reached at approximately 20–30 min. According to Langmuir, the adsorption kinetics pattern corresponds to a reversible process in which the adsorption and desorption take place simultaneously until the equilibrium is reached ŽDaniels and Alberty, 1967., in which the DBS concentration remains constant, as shown by the flat part of the ‘Concentration vs. Time’ curve. According to these results, a time of 2 h was chosen to obtain the adsorption isotherms that were considered enough to reach the equilibrium. 3.2. Adsorption isotherms All the adsorption isotherms ‘ X Žmgrg. vs. Ce Žmgrl.’, except for TA-II and NBs, show a type L
behavior, according to the classification of Giles and MacEwan similar to the one shown in Fig. 2 for the bentonite CTA-I ŽOsipow Lloyd, 1962.. In this classification, type L assumes a monolayer formation in the active sites of the surface, and all the adsorption sites are supposed to be equivalent. TA-II and NBs show a type S behavior according to the above mentioned classification, which indicates that the forces involved in the adsorption process are small, that is, there is a low affinity for the adsorbate, at low adsorbate concentrations. This is explained because the NB has a negatively charged surface that repels the also anionic DBS. On the other hand, TA-II bentonite is only slightly covered by very short hydrocarbon chains, showing only a small difference with the NB. Fig. 3 shows the isotherms of the organophilic bentonites from I and II series, and also that of NB. The results indicate that DBS adsorption by the organophilic bentonites studied depends on the surface percentage covered by the lipophilic tails from the corresponding quaternary ammonium ion; indeed, the adsorption is greater when all the sites of cationic exchange are covered by the corresponding ion, like in series I clays, and the adsorption is smaller when there is lesser number of sites covered by the same ion, that is, series II clays. It is also observed that the adsorption follows the order CTA ) BT ) CDA ) TA when all the exchange sites are covered, like in series I. In the case of series II bentonites, the adsorption order is CTA ) CDA ) BT ) TA. This result will be quantitatively ex-
Fig. 2. DBS adsorption isotherm on bentonite modified at 100% of its CEC with the CTA ion at 298 K.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
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at 50%; whereas the adsorption of DBS on NB is almost negligible. 3.3. Adsorption models For the data treatment, the data that lied outside the 95% confidence interval, established by the method of least squares for the linear function, were discarded. The uncertainty of the parameters of the adjusted straight line corresponds to the standard deviations. The experimental data points were fitted to the Langmuir equation ŽDaniels and Alberty, 1967.: CerX s 1r Ž Xm K e . q CerXm
Ž 1.
where Ce is the concentration of adsorbate in the equilibrium solution, X is the amount of adsorbate adsorbed per gram of adsorbent, X m is the maximum amount of adsorbate that can adsorbed in a monolayer and K e is a constant related to the energy of adsorption. The plot of experimental values for CerX against Ce gives a straight line. Except for NB and for that treated to 50% of its CEC with TA, the linear correlation coefficients are very good Žgreater than 0.99. for the Langmuir isotherm Žsee Table 2.. In case of liquid adsorption, the BET isotherm is expressed in the following equation ŽDas and Chatterjee, 1993.: CrX Ž C0 y C . s 1r Ž Xm K . q Ž K y 1 . r Ž Xm K . Ž CrC0 . Fig. 3. DBS adsorption isotherms at 298 K: Ža. series I, bentonite covered at 100% of its CEC with the following ions: CTA-I, CDA-I, BT-I, TA-I; Žb. series II, bentonite covered at 50% of its CEC with the same ions. NB is included in both series.
plained by means of the isotherm parameters of Langmuir, BET, and Freundlich models. As an example, Fig. 4 shows the isotherm of the individual adsorption of DBS over CTA-I and II bentonites, as well as that of NB. This plot clearly shows the effect of the substitution of exchange cations by the quaternary ammonium ion on DBS adsorption. When the bentonite was modified at 100% of its CEC, it adsorbs approximately two times more DBS than what it adsorbs when it is modified
Ž 2.
where C0 is the initial concentration of the solution used, C, the final concentration of the solution, X,
Fig. 4. Adsorption isotherms of the CTA-I, CTA-II, and NB at 298 K. ŽAbbreviations have the same meaning as those of Fig. 3..
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Table 2 Parameters for the Langmuir model Bentonite
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
Parameter Data number
Correlation coefficient
Slope Žgrmg.
Intercept Žgrl.
8 10 8 7 7 9 9 5 13
0.998 0.996 0.996 0.999 0.996 0.999 0.993 y0.732 y0.444
0.00464 " 0.00012 0.00779 " 0.00026 0.00587 " 0.00021 0.00758 " 0.00013 0.00461 " 0.00018 0.00961 " 0.00013 0.0241 " 0.0010 y0.024 " 0.013 y0.11 " 0.07
0.038 " 0.008 0.159 " 0.026 0.090 " 0.022 0.276 " 0.014 0.150 " 0.026 0.160 " 0.015 2.30 " 0.25 14.1 " 2.5 53 " 12
the amount of adsorbate adsorbed, X m , the quantity of adsorbate required for a complete monolayer and K is a constant related to heat of adsorption. The plot of experimental values for CrX Ž C0 y C . against CrC0 gives a straight line. For the BET isotherm, the correlation coefficients were as good as those found for the Langmuir isotherm, except for NB and for TA bentonite TA-I. The Langmuir isotherm corresponds to a particular case of the BET isotherm, thus, explaining the good data adjustment to this model, despite that in most cases, the CrC0 ratio is greater than 0.35, which corresponds to the highest limit of linearity allowed for the BET isotherm in the literature ŽOsipow Lloyd, 1962.. However, the intercept of this isotherm does not allow to calculate the distribution coefficient for the adsorption process, K dm , because negative values andror very high uncertainty ranges were obtained for this parameter. The mathematical form of the Freundlich isotherm is ŽDaniels and Alberty, 1967.: X s KC n
Ž 3.
or: log X s log K q nlogC
Ž 4.
where C is the final concentration of the solution, X the amount of adsorbate adsorbed per gram of adsorbent, n and K are constants. The plot of log X against logC gives a straight line. In the case of Freundlich isotherm, only BT-I, BT-II, TA-I, and NBs show correlation coefficients
greater than 0.99; in the other cases, this coefficient lies between 0.99 and 0.90. On the other hand, the Freundlich isotherm, which is an empirical model that does not take into account the distribution coefficient, allows to determine the adsorption grade by means of its K and n parameters. The greater the K and n are, the greater the X value. The sequence of greater to lower the K value is: CTA-I, BT-I, CDA-I, TA-I, NB, and BT-II, CTAII, CDA-II, TA-II, NB; while for the n value this sequence is: NB, TA-I, CTA-I, BT-I, CDA-I, and NB, TA-II, CDA-II, CTA-II, BT-II. In no case, the sequence of these parameters coincides with that found for K dm values. Taking into account the empirical character of the isotherm, and that in most cases the correlation coefficient is lower than that calculated for the Langmuir isotherm, the use of K and n in the adsorption analysis of DBS over the organophilic clays was not considered. The above mentioned results allow to conclude that the Langmuir isotherm is the one that best describes the adsorption process of DBS over the organophilic clays. 3.4. Distribution coefficient and the standard Gibbs energy change of adsorption Several researchers Že.g. Del Rey Bueno et al., 1983; Gutierrez and Fuentes, 1993; Comans et al., ´ 1991; Klumpp et al., 1993. use the constant K d Žconstant of distribution or of apparent equilibrium.
D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
as an indicator of the extension of adsorption or as a relative measure of the adsorbent affinity from the adsorbate. The constant K d that represents the relation between the quantity of adsorbate present in each of the two phases, that is in the adsorbent and in the solution, is defined as follows: K d s XrCe Ž lrg . or: K d s XrCe Ž 10 3 mlrg . .
Ž 5.
This constant is calculated experimentally by the inverse of the intercept of the linear equation of the Langmuir isotherm ŽEq. Ž1.., that is: K d s X m K e ; or also by the slope of the diluted region Žthe linear part. of the isotherm X vs. Ce . If Ce is expressed in molality, the constant K d becomes adimensional and equals the thermodynamic constant of distribution Žwhich is adimensional and is calculated for concentrations approaching zero., that is the reason why it is called K dm . The distribution coefficient that was calculated from the Langmuir isotherm is shown in Table 3; it clearly shows the effect of the substitution grade of the exchange cations by quaternary ammonium ions in the adsorption of DBS, since K dm is greater for all the salts considered when this substitution is 100% than when it is 50%. Likewise, K dm for NB is the lowest registered value.
Table 3 Distribution coefficient, K dm , and standard Gibbs energy change of adsorption Žprocess: adsorbate over the adsorbent™adsorbate in the solution., DG ) , for various clays Bentonite
a K dm
DG ) Žkcalrmol. a
CTA-I CTA-II CDA-I CDA-II BT-I BT-II TA-I TA-II NB
26,000"6000 6300"1000 11,100"2700 3620"180 6700"1200 6300"600 430"50 71"13 19"4
6.02"0.17 5.18"0.13 5.52"0.18 4.85"0.06 5.21"0.14 5.18"0.09 3.59"0.10 2.53"0.14 1.74"0.16
a
tion.
The uncertainties were calculated using the variance propaga-
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The K dm values for the hydrocarbon chains of quaternary ammonium ions follow the sequence 26,000 CTA-I, 11,100 CDA-I, 6700 BT-I, and 430 TA-I. When the K dm of CTA-I Ža chain of 16 carbon atoms and three methyl groups. is compared with that of CDA-I Ža chain of 16 carbon atoms, one benzyl group and two methyl groups., it is found that substitution of a methyl group by a benzyl group lowers the adsorption, as indicated by the K dm values, maybe due to steric effect. Likewise, the change of the hydrocarbon chain from 16 to 13 carbon atoms lowers the adsorption, as shown by the CDA-I and BT-I Ža chain of 13 carbon atoms on average, one benzyl group, and two methyl groups. K dm values. Finally, when the chains are short, with four methyl groups, a very small coefficient is obtained. The K dm values in series II show the following sequence: 6300 CTA-II; 6300 BT-II; 3620 CDA-II; 71 TA; this order changes for BT-II and CDA-II related to series I; this behavior seems to indicate that at low covering percentages of bentonite with organic cations, the steric effect is of low importance. These results are in good agreement with those found by Klumpp et al. Ž1993., for the adsorption of 2-naphthol over a calcic bentonite of Baviera in presence of dodecyltrimethylammonium and dodecyldimethylammonium bromides; these authors establish that the adsorption depends on the substitution grade of the exchange cations, on the number, and on the nature of the hydrocarbon chains. The behavior, according to Langmuir isotherm, allows to infer that in the experimental range, there is only monolayer formation and that all the adsorption sites are equivalent. This behavior can be explained according to the model established by Van Olphen Ž1963.: once the monolayer has been formed, the polar heads of the DBS molecules are oriented toward the aqueous phase, avoiding the accommodation of more layers because of electrostatic repulsion. Besides, as has already been mentioned, if the adsorption is due to the presence of lipophilic tails on the bentonite surface, the adsorption sites are equivalent and therefore the Langmuir model is the one that best describes the adsorption process, except for TA-II and NB, that do not have the characteristics described previously. Indeed, the adsorption
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D.C. Rodrıguez-Sarmiento, J.A. Pinzon-Bellor Applied Clay Science 18 (2001) 173–181 ´ ´
isotherms for the case of TA-II and NBs show a type S behavior, according to the classification of Giles and MacEwan ŽOsipow Lloyd, 1962., which indicates that there is a low affinity for the adsorbate, especially at low concentrations of it. This is a plausible result, because TA-II bentonite as well as NB lack of sites that are able to sufficiently retain DBS. The K dm values found indicate that in CTA, CDA, BT, and TA organophilic clays, the amount of adsorbed DBS is greater than the one that remains in solution. The standard Gibbs energy change of adsorption for the process: adsorbate over the adsorbent™ adsorbate in the solution, is calculated by the equation DG ) s RT ln K dm ŽEwert, 1983; Castellan, 1964.. These values calculated for each of the used bentonites is also shown in Table 3. These DG ) values indicate that the DBS molecules are more stable in the adsorbed phase than in the aqueous phase, and therefore, the adsorption process is spontaneous. It is also important to notice that DG ) values are always greater for series I bentonites, when the covering is 100%, than for the series II, when the covering is 50%. Similarly, the DG ) value for NB is the lowest of all, indicating the DBS low affinity for this adsorbent.
4. Conclusions ŽI. The substitution of the exchange cations by quaternary ammonium cations transform the bentonite in a material capable of adsorbing DBS. ŽII. The adsorption capacity is directly proportional to the substitution grade of the exchange cations in the bentonite by quaternary ammonium ions. ŽIII. The adsorption of DBS over organophilic bentonites is a relatively fast process. ŽIV. The Langmuir isotherm model is the one that best describes the adsorption process of DBS over the organophilic bentonites studied. Therefore, the adsorption of this compound on organophilic bentonites takes place by monolayer formation. ŽV. The adsorption grade can be appropriately quantified by means of the distribution coefficient, K dm .
ŽVI. The affinity of the organophilic bentonites by DBS can be expressed in terms of the standard adsorption Gibbs energy. ŽVII. The class and structure of the hydrocarbon chains of the cations of quaternary ammonium salts adsorbed on the bentonite determine the adsorption grade of DBS.
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