Adsorption Characteristics and the Kinetics of the Cation Exchange of Rhodamine-6G with Na+-Montmorillonite

Journal of Colloid and Interface Science 251, 235–241 (2002) doi:10.1006/jcis.2002.8410

Adsorption Characteristics and the Kinetics of the Cation Exchange of Rhodamine-6G with Na+ -Montmorillonite Ali H. Gemeay1 Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt E-mail: [email protected] Received January 13, 2002; accepted April 1, 2002; published online June 18, 2002

The adsorption and the kinetics of the cation exchange of rhodamine-6G (Rh-6G) with Na+ -montmorillonite (Na+ -MMT) have been studied. The binding parameters of Rh-6G have been determined by applying Freundlich and D–R isotherms. The enthalpy and the entropy of adsorption have been determined. The isosteric heat of adsorption has also been determined and decreases with increasing the concentration of Rh-6G. Increasing the concentration of Rh-6G led to a decrease in the adsorption capacity, which attributed to the formation of Rh-6G aggregates. Kinetic measurements of the cation exchange were followed up using a stopped-flow electrical conductivity detection unit. The cationexchange process exhibited first-order kinetics with respect to the dye concentration and inversely proportional to the clay concentration. The measurements were accomplished at different temperatures and the activation parameters were determined. Increasing the Na+ -MMT concentration led to a decrease in the rate constant. The latter is also affected by changing the exchangeable cation. C 2002 Elsevier Science (USA) Key Words: rhodamine-6G; montmorillonte; adsorption; thermodynamics; stopped-flow; kinetics; cation exchange.

INTRODUCTION

Clay minerals have many desirable properties including high sorption capacity and shape specificity. Among the kinds of clay minerals, montmorillonite (MMT) has often been used in organic chemical applications, both technological and environmental (1, 2). The practical use of MMT–organic complexes in paints, inks, polishes, cosmetics, and fixation of pollutants has stimulated some research (3). The final application, however, should be preceded by basic investigations of model systems in order to optimize conditions to achieve the most desirable properties. An important property of MMT is that its layers have a negative charge due to the isomorphic replacement of some cations in the MMT structure by others of lower charge and similar 1

To whom correspondence should be addressed. 235

size. These negative charges are normally balanced by hydrated cations placed in the interlayer spaces (4–6). Cationic dyes can be attracted toward the anionic layers and are, therefore, quite suitable for studying the adsorption properties of MMT in aqueous solution (7, 8). Cationic and anionic dyes like sulfur blues have also been adsorbed on MMT (9, 10). The amount of cationic dye adsorbed is larger than the anionic one and dependent on pH. The competitive adsorption of methylene blue, thioflavin-T, and caessium on MMT has been studied (10). Most of these studies were done using spectrophotometric methods (11–14). An important characteristic of these systems is that the absorption spectrum of the dye changes significantly as time passes. The information about the orientation of the dye molecules at the clay surfaces has been pointed out. The interaction between the π -system of the dye and the lone pair of electrons on the oxygen on the clay surface and the aggregation of the dye at the internal and external clay surfaces have been investigated (8, 15, 16). The interaction between the π -system of the dye and the lone pair of oxygen electrons does not seem to be effective for rhodamine dyes, since these dyes have an O-carboxyphenyl group nearly perpendicular to the xanthene plane skeleton, which sterically prevents this interaction (17, 18). The changes of photophysical properties observed for the rhodamine-6G/Laponite-B and rhodamine-6G/MMT systems have been attributed to the aggregation of the dye and depended on the loading of the dye and the aging of the samples (19–22). Dye aggregation plays an important role in many practical applications such as photography, xerography, and light energy conversion devices (23–25). A variety of environmental factors such as hydrophobic dispersions and electrostatic forces affects dye–dye interaction (26). It was found that surfactant micelles promote the deaggregation of dyes to their monomer forms (27, 28). The aggregation of dye at the clay surface was dependent not only on the relative dye/clay concentration but also on the dispersion degree of the clay particles (20). The adsorption isotherm of rhodamine dyes on various adsorbents such as coir pit carbon (29), modified silica gel surfaces (30), bioadsorbent (31), and poly(ethyleneterephthalate) (PET) and polycarbonate (PC) membranes (32) has been investigated from an environmental viewpoint. 0021-9797/02 $35.00

 C 2002 Elsevier Science (USA)

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ALI H. GEMEAY

In the present work, our investigations are extended to study the thermodynamics and the kinetics of the cation-exchange of Rh-6G with Na+ -MMT. UV/vis spectroscopy and stopped-flow, electrical conductivity measurements were employed for this purpose. EXPERIMENTAL

All chemicals were of high-grade quality and were used as received. Montmorillonite (Southern Clay Products, Inc., Gonzales, TX) containing 6.39% Fe3+ , 0.88% Mg2+ , 15.50% Al3+ , and 77.28% Si4+ under the trade name of mineral colloid BP was used. Na+ -, H+ -, and Fe+3 -exchanged samples were prepared by immersing the clay into 1 M solution of the corresponding chloride or acid. In the case of the Fe3+ -exchanged sample the pH was kept constant at 6 by the addition of diluted HCl. The samples were suspended in deionized water and centrifuged at appropriate speed. The procedure was repeated several times and finally the Cl− -free samples were dried at room temperature. The cation exchange capacity (CEC) of the clay (Na+ form) equals 108 meq/100 g as determined by the BaCl2 method (33). The external surface area determination of MMT was based on the Brunauer–Emmett–Teller (BET) method using a Micrometry Gemini-2375 N2 -adsorption surface area analyzer at 77 K. The Na+ –MMT sample was degassed overnight at 80◦ C. The BET surface area of the clay particles is equal to 21 m2 /g and has an average hydrodynamic radius of 0.0728 µm. Laser-grade Rh-6G was supplied by Aldrich and used without additional purification. A stock solution of Rh-6G (2 × 10−4 mol/L) was always freshly prepared in water. The stock solution of clay (5 g/L) was also prepared in distilled water and stirred until a colloidal suspension was obtained (approximately 4 h). The desired experimental concentrations of the clay and dye were obtained by dilution and are shown in the figure captions.

Adsorption Measurements Adsorption isotherm measurements were carried out at room temperature unless stated otherwise. To a definite volume of clay solution containing 0.01 g of Na+ –MMT, various amounts of Rh-6G and deionized water were added to obtain a fixed volume (25 ml). The mixture was stirred for 24 h to ensure adsorption equilibrium. Thus, the suspension was centrifuged, and the aliquots of the supernatant were analyzed using a UV/vis spectrophotometer by monitoring the absorbance at λmax = 552 nm (molar extinction coefficient, ε = 106000 ± 300 L mol−1 cm−1 ). The adsorbed amount of Rh-6G was calculated from the difference between the initial and final concentrations of Rh-6G in solution. Kinetic Measurements The kinetic measurements of the cation exchange of the Rh-6G with Na+ –MMT were carried out conductimetrically. The solution of Rh-6G was transferred into one syringe of the stopped-flow instrument and the other syringe was filled with colloidal dispersed Na+ –MMT solution whose concentration was kept at least in a 10-fold excess of the Rh-6G. The reaction was followed in terms of the changes of the electrical conductivity. RESULTS AND DISCUSSION

The driving force for the adsorption of cationic Rh-6G dye onto Na+ –MMT is the cation exchange process (19). Adsorption isotherms were constructed by plotting the adsorbed amount of Rh-6G dye (Cads mol/g) vs [Rh-6G] in the supernatant (Cw mol/L). Figure 1 shows the adsorption isotherm at different temperatures. All adsorption isotherms are the average of triplicate experiments. From the initial segments of the adsorption

Instruments

80

60 o

20 C

4

Cads x 10 (mol/g)

The kinetics of the cation exchange process was conducted in a Hi-Tech stopped-flow Model CAK-501 (Salisbury, UK) with an electrical conductivity detection unit. It consists of two simple syringes containing the individual reactants and is capable of measuring the conductivity in the range 10−8 to 0.25 . The delivery tubes are constructed of Teflon with an internal diameter of 1.6 mm. Each tube is approximately 40 cm long. This length of the delivery tube ensures that about 0.8 cm3 of each reagent is thermostated prior to the stopped-flow kinetics run. An analogto-digital converter monitored the output of the conductance amplifier. The instrument was interfaced with an Apple IIe computer to collect data as changes in the conductivity signal vs time. A Shimadzu UV/Vis 2001S spectrophotometer was used to determine the absorption spectra of Rh-6G. To avoid interference due to light scattering by the clay particles a reference sample was prepared with the same concentration of the clay suspension. Centrifugation was carried out with a πe-ttich EBA-8 automatic centrifuge at 6000 rpm.

100

o

25 C

40

o

35 C o

40 C 20

0 0

5

10

15

20

25

30

35

6

Cw x 10 (mol/L) FIG. 1. Adsorption isotherm of Rh-6G onto Na+ –montorillonite at different temperatures, [Na+ –MMT] = 1.0 g/L, and at varying concentrations of Rh-6G in the range 2–100 × 10−6 mol/l.

RHODAMINE-6G WITH Na+ -MONTMORILLONITE

vs ln Cw at different temperatures, and the K and (1/n) values are computed and tabulated in Table 1. As shown in this table, the degree of heterogeneity of the MMT surface is increased with increasing temperature. The surface heterogeneity is due to the existence of crystal edges, type of cations, surface charges, and degree of crystallinity of the surface (42). The net effect of these factors is temperature dependent. The (1/n) value indicates the relative distribution of energy sites and depends on the nature and strength of the adsorption process; for example, (1/n) = 0.81 refers to the fact that 81% of the active sites have equal energy where adsorption takes place. The FI does not predict the saturation of the adsorbent surface by the adsorbate (36). The K value can be taken as a relative indicator of the adsorption capacity of Na+ –MMT for a narrow subregion having equally distributed energy sites toward Rh-6G cations.

-4

-5

ln Cads

-6

20°C 25°C 30°C 40°C

-7

-8

-9 -16

-15

-14

-13

-12

-11

-10

ln Cw

FIG. 2. Ferundlich isotherm plot of Rh-6G/Na+ –MMT system at different temperatures.

isotherms, it is inferred that the affinity of Rh-6G for MMT decreases with increasing temperature. The adsorption of the positively charged organic compounds onto MMT is essentially a cation exchange process, due to the negatively charged surface of MMT, and involves electrostatic forces, although hydrophobic interactions may also play a role (34, 35). Therefore, the decreasing of the adsorption affinity with increasing temperature is due to the weakening of the aforementioned force at higher temperature. Furthermore, this indicates that the adsorption of Rh-6G is an exothermic process. Freundlich Isotherm (FI) The FI isotherm is an empirical expression that encompasses the heterogeneity of the surface and an exponential distribution of the sites and their energies. This isotherm has been further extended by considering the influence of adsorption sites and the competition between different ions for adsorption on the available sites. Isotherms of this form have been observed for a wide range of heterogeneous surfaces including activated carbon, silica, clays, and polymers (36–41). The linear form of FI is ln Cads = ln K + (1/n)ln Cw

237

Dubinin–Radushkevich (D–R) Isotherm The D–R isotherm was applied to distinguish between the physical and chemical adsorption of Rh-6G with Na+ –MMT using the linear relationship ln Cads = ln Cm − Bε 2

where Cm is the maximum amount of Rh-6G adsorbed, B is a constant with energy dimension, and ε is   1 ε = RT ln 1 + . [3] Cw The plot of ln Cads vs ε 2 at different temperatures gives straight lines, and the values of Cm and B were determined from the intercept and slope using the linear regression method. As shown in Table 1, the B and Cm values increase as temperature increases, which suggests an increase in the selectivity of Rh-6G toward Na+ –MMT at higher temperature. This can be ascribed to the dissociation of the aggregated Rh-6G at elevated temperature. The mean sorption energy, E, which is the free energy of transfer of 1 mole of solute, Rh-6G, from infinity (bulk solution) to the surface of Na+ –MMT (36), is given by E=√

[1]

where (1/n) is the degree of heterogeneity of MMT (0 < (1/n) > 1) and K is constant. Figure 2 shows the plot of ln Cads

[2]

1 −2B

.

[4]

The E value (Table 1) decreases with increasing the temperature and is in good agreement with those reported for the intercalation

TABLE 1 Freundlich and D–R Isotherm Parameters of the Rh-6G/Na+ –MMT System Ferundlich isotherm parameters

D–R isotherm parameters

Temp. (◦ C)

K (mol/g)

(1/n)

Corr. coeff.

B (kJ2 mol−2 )

Cm (mol g−1 )

E (kJ mol−1 )

Corr. coeff.

20 25 30 35

1.22 ± 0.12 1.95 ± 0.18 4.15 ± 0.30 5.01 ± 0.30

0.52 ± 0.009 0.61 ± 0.014 0.81 ± 0.023 0.91 ± 0.023

0.999 0.999 0.997 0.998

−0.0028 −0.0036 −0.0054 −0.0055

0.056 0.120 0.372 0.550

13.36 11.62 10.02 9.52

0.998 0.993 0.992 0.996

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ALI H. GEMEAY

of some organic compounds onto MMT (43). Furthermore, the numerical value of E reflects the chemical adsorption based on ion exchange.

-11

-12

Thermodynamic Parameters

  R(T1 T2 ) K1 H = − ln (T2 − T1 ) K2   K1 S o = −R ln K2

[6]

∂ ln Ce Hads . = ∂T RT 2

[7]

On integration, the linear form becomes −Hads RT

[8]

where Ce is the equilibrium concentration of Rh-6G. From the plot of ln Ce vs 1/T , Fig. 3, the value of Hads was calculated and listed in Table 2. As shown in this table, the Hads value

TABLE 2 Enthalpy and Entropy Change for the Adsorption of Rh-6G onto Na+ –Montmorillonite H o (kJ/mol)

1 × 10−5 3 × 10−5

−68.80 −46.45

∗H ads

(kJ/mol)

−35.94 −23.20

Note. ∗ Hads = isosteric heat of adsorption.

-5

[Rh-6G] = 1x10 mol/L -5

[Rh-6G] = 3 x 10 mol/L

-15

-16

3.15

3.20

3.25

3.30

3.35

3.40

3.45

3

(1/T) x 10 (K)

Illustration of the linear form of the Clausius–Clapeyron equation.

[5]

where K 1 and K 2 are the partition ratios at T1 and T2 , respectively. As shown in Table 2, the negative value of enthalpy reflects the exothermic adsorption process. The negative change of the entropy indicates that the freedom of motion of Rh-6G is more restricted in Na+ –MMT than in solution and the rapid adsorption of Rh-6G with the active sites of Na+ –MMT (46). The isosteric heat of adsorption (Hads ), the standard enthalpy of adsorption at a fixed surface coverage, was also determined using the linear form of the Clausius–Clapeyron equation (36, 47):

Rh-6G (mol/L)

-14

FIG. 3.

o

ln Ce =

-13 ln Ce

During the adsorption process, Rh-6G partitions between the solution and the Na+ –MMT surface. The partitioning of Rh-6G can be mathematically expressed as the distribution ratio, which is the ratio of the equilibrium amount of Rh-6G in solution (Cw ) to its amount in Na+ –MMT (Cads ). This ratio could be changed with temperature because the association, aggregation, and the formation of Rh-6G dimers would be changed with temperature. The enthalpy H o and the entropy S o of adsorption can be calculated from the partition ratios at two different temperatures or over a temperature range (44, 45) with the assumption that H o and S o are independent on temperature. Therefore,

S o (J/K mol) −6.50 −4.64

decreases with increasing concentration of Rh-6G. This can be attributed to the formation of Rh-6G aggregates at higher concentrations; therefore, the possibility of adsorption via cation exchange at higher concentrations is diminished. Moreover, the adsorption is exothermic in nature and the H o value determined using the partition ratio is nearly half of that of Hads as shown in Table 2. The higher value of Hads reflects that the adsorption is predominantly chemical with higher complexing stability and is in good agreement with that found elsewhere (48, 49). Effect of [Na+ –MMT] The interaction of Rh-6G/Na+ –MMT was investigated in situ by UV-visible spectroscopy. When Rh-6G is added to the clay suspension, the initial process is the adsorption of the dye molecules on the external surface of MMT particles. This increases significantly the local concentration of Rh-6G, giving rise to the formation of dye aggregates. Figure 4 shows the absorbance changes upon the addition of various amounts of Na+ –MMT. The effect of clay concentration on the absorption maximum of Rh-6G is shown in Fig. 5. It is clear that λmax is increased with clay concentration, obtaining a maximum at approximately 0.2 g/L, then decreases again. The exhibited red shift of Rh-6G may be ascribed to the strong adsorption of cationic dye on the clay. Moreover, irrespective of λmax , the absorbance of Rh-6G decreases as the concentration of clay increases, obtaining a minimum at approximately 0.2 g/L then increases as shown in Fig. 5. These results can be discussed on the basis of the adsorption and aggregation of the dye on the external and internal surfaces of MMT tactoids. At low clay concentration the particles could be more dispersed, increasing their external surface compared with the internal surface. On the other hand, at higher concentrations of clay the absorbance and λmax of Rh-6G have a value approximately similar to those of Rh-6G in aqueous solution. The latter suggests again that Rh-6G is localized in an environment similar

RHODAMINE-6G WITH Na+ -MONTMORILLONITE

FIG. 4. Effect of [Na+ –MMT] on the absorption spectrum of Rh-6G (5 × 10−6 M) adsorbed on Na+ –MMT after stirring for 10 min: (a) 1.2, (b) 1.0, (c) 0.8, (f) 0.5, (g) 0.2, (e) 0.1, and (d) 0.05 g/L (at 25◦ C).

to an aqueous solution. The external and internal adsorption has also been observed when methylene blue adsorbed on different types of clay (50). The formation of Rh-6G dimers in water has been characterized by an equilibrium constant of K d = 6200 ± 300 at 20◦ C (51). As shown in Fig. 5, it is clear that in dilute clay suspensions (<0.1 g/L) necessary to obtain a high adsorption of Rh-6G, the platelets of the clay are separate and dispersed (21). In this case the adsorption at the external surface would be predominant, and for this reason the adsorption of Rh-6G would be higher; i.e., the probability of adsorbing more than one Rh-6G monomer unit at the same platelet increases, favoring the formation of Rh-6G dimers. However, an augmentation of the clay 0.6

575

concentration (>0.2 g/L) increases the probability of the formation of the clay tactoids in the aqueous solution, leading to adsorption in the interlayer space and reduction of the dye aggregation at the external surface. The same has been proposed by Rosta and Von Gunten (52) for laponite suspension solution in the concentration range 0.1–5.0 g/L. Therefore, for the highly concentrated clay suspensions necessary to obtain low loading, the equilibrium between single platelets and tactoids is displaced toward the latter structure, leading to both the decreases of the internal and external surfaces. The migration from the external to internal surface of adsorbed molecules on clay suspensions on the time scale of days has been reported for dye/clay systems (53). In this system two mechanisms are expected: a very rapid adsorption on the external surface of clays (in a few minutes) and a slow adsorption (on a time scale of weeks or longer) owing to the diffusion of the adsorbed molecules into the interlayer space of the clay. The fact that the adsorption of the dye favors the aggregation of clay particles is corroborated by different experimental results (19). Thus, the adsorption of Rh-6G on Na+ –MMT causes the aggregation of the clay particles, changing the rheological properties of the dye/clay suspension (54). Kinetics of Cation Exchange The adsorption of Rh-6G on Na+ –MMT has been reported to take place mostly by a cation exchange mechanism. The stoppedflow electrical conductivity unit has been used to investigate the kinetics of the cation-exchange process. The measurements were carried out under pseudo-first-order conditions with respect to [Rh-6G]. The progress of the cation-exchange process was followed by recording the changes of the electrical conductivity as a function of time. From the semilogarthmic plot of the typical process curve, the observed rate constant, ko , was determined (Fig. 6). The rate law can thus be expressed as ν = k [Rh-6G] [Na+ –MMT]m

[9]

where k, [Na+ –MMT], and m are the true rate constant, the concentration of the clay (in terms of g/L), and the order with respect to clay, respectively. Since the clay is present in large

change of wavelength change of absorbance

570 0.5 565 0.4 560

λ (nm)

Abs. (λmax = 560 nm)

239

0.3 555

0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

550 1.4

+

[Na -MMT] g/L

FIG. 5. Variation of the absorption spectra and λmax of Rh-6G (5 × 10−6 mol/L) as a function of [Na+ –MMT].

FIG. 6. First-order plot of the cation-exchange process of Rh-6G with Na+ –MMT recorded using stopped-flow electrical conductivity. [Rh-6G] = 2 × 10−4 mol/L, [Na+ –MMT] = 5 g/L at 30◦ C.

240

ALI H. GEMEAY

TABLE 4 The Rate Constants of the Cation Exchange Process of Rh-6G on MMT of Different Exchangeable Cations

3.0

2.5 -4

[Rh-6G] = 5x10 M

2.0 o

ko s

-1

t= 22 C

1.5

k (s−1 )

H+ Na+ Fe3+

6.43 0.82 0.13

Note. [Rh-6G] = 2 × 10−4 M, [MMT] = 5 g/L, and at 26◦ C.

1.0

0.5

0.0

0

1

2

3

4

5

6

+

[Na -MMT] (g/L)

FIG. 7. of ko .

Exchangeable cation

Illustration of the effect of Na+ –MMT concentration on the value

excess compared to [Rh-6G] then Eq. [9] can be reduced to ν = ko [Rh-6G]

[10]

where ko = k[Na+ –MMT]m ; m can thus be determined from the equation ln ko = ln k + m ln[Na+ –MMT].

[11]

The effect of [Na+ –MMT] on the rate of cation exchange has been investigated. The concentration of Rh-6G was kept constant at 5 × 10−4 M, while the amount of clay was varied in the range 1–5 g/L. It was found that increasing the [Na+ – MMT] led to a decrease in the rate constant (Fig. 7). This could be attributed to the formation of tactoids composed of two to four single platelets of MMT at high concentration. The formation of the tactoids could reduce the available exchangeable sites. Moreover, the aggregation of clay at higher concentrations may lead to increasing the viscosity of the medium, which leads to decreasing the rate of diffusion of Rh-6G. Furthermore, the dimerization of Rh-6G may also decrease the diffusion rate of

TABLE 3 Rate Constants and the Activation Parameters for the Cation Exchange Process of Rh-6G with Na+ –MMT Temp. (◦ C)

k (s−1 )

18 22 26 30 34

0.67 0.79 0.82 0.86 0.92

Ea (kJ/mol)

H # (kJ/mol)

G # (kJ/mol)

S # (J mol−1 K−1 )

Rh-6G toward the exchangeable sites. Therefore, the amount of Rh-6G adsorbed would be decreased with increasing the concentration of MMT, which would lead to a decrease in the rate constant. Taking into account that the tendency of Rh-6G to adsorb on Na+ –MMT increases with time, the short time (5 s) employed in stopped-flow measurements should affect the rate constant of the cation-exchange process. Therefore, in this case the adsorption of dye on the internal surface of the clay should be reduced and only the external adsorption will be predominating. By applying Eq. [11] the order with respect to [Na+ –MMT] was determined and equals 1.19, which is nearly equal to 1. The cation exchange was conducted at various temperatures and the rate constants were determined. The activation energy, E a , was obtained from an Arrhenius plot. The other activation parameters, H # , G # , and S # are also calculated and shown in Table 3. The values of the rate constant are smaller than that determined in the case of the cation-exchange process of the radical cation of metanil yellow dye and p-NH2 -diphenylamine with Na+ –MMT (43). This can be attributed to the high energy of the radical in the cation-exchange process. Also, as shown in Tables 1 and 3, the value of the sorption energy and the activation energy are nearly the same (13.4 kJ/mol), which confirms that the adsorption of the Rh-6G dye takes place by the cation-exchange mechanism. This agreement is properly due to the external and internal surface adsorptions being governed by the diffusion of Rh-6G to the clay surface. The effect of the exchangeable cation on the rate of the cationexchange process was also investigated. Replacing the Na+ by H+ or Fe3+ led to a change in the value of ko , Table 4. The largest ko value was observed for the H+ cation. This could be explained on the basis that the Rh-6G cation is much more selective to MMT than the H+ cation. When the Na+ cation is replaced by the Fe3+ cation, the value of ko decreases. This can be ascribed to the Fe3+ cation being more selective to MMT than the Rh-6G cation and also to the largest charge of the Fe3+ cation. CONCLUSIONS

13.41

10.92

73.8

Note. [Rh-6G] = 2 × 10−4 M, and [Na+ –MMT] = 5 g/L.

−220.0

The increasing interest in the intercalation of organic compounds into clay minerals has led us to investigate the adsorption, thermodynamics, and kinetics of the cation-exchange process of Rh-6G onto montmorillonite. The dye chosen in this study is cationic and interacts strongly with montmorillonite via

RHODAMINE-6G WITH Na+ -MONTMORILLONITE

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