The clay mineral attapulgite was purchased from GEOHELLAS company (Greece). The origin of the atttapulgite sample is from the region of Grevena (North Greece). It is a commercial product, specifically sold for agricultural use, and in the present study it was used without further pretreatment. The properties of attapulgite are listed in Table 1 as specified by manufacturer’s data sheet and by [18] . The cation exchange capacity (CEC) of this mineral is generally 200 - 300 meq/kg [8] .

The surfactant cetyltrimethylammonium chloride (CTMA) was purchased from Aldrich and was of technical grade. The reagents sodium carbonate, sodium hydroxide, the dye bromophenol blue and the solvent 1,2-dichloromethane were obtained from Fluka and were of analytical grade.

For the CTMA kinetic study, two grams of attapulgite were added into 25 mL of CTMA solution with initial concentration of 10⁻³ M in 50 mL centrifuge tubes. The mixtures were shaken at 150 rpm for 1, 2, 4, 6, 24 h. After, they were centrifuged at 4000 rpm for 15 min and the supernatant was filtered through a 0.45 μm filter.

For the CTMA sorption study, 5, 10, 20, 50, 200 mg of attapulgite were added into 25 mL of CTMA solution with concentration of 10⁻³ M in 50 mL centrifuge tubes. The mixtures were shaken at 150 rpm for 1 h. After, they were centrifuged at 4000 rpm for 15 min and the supernatant was filtered through a 0.45 μm filter.

CTMA has been determined by colorimetric methods [19] [20] . In this work it was determined by the colorimetric method described by [19] . An adequate volume of the aqueous solution was placed into a 25 mL volumetric flask. Then 5 ml of 10 % carbonate solution, 1 mL of bromophenol blue (freshly prepared by dissolving 40 mg of dye in 100 mL of 0.001N NaOH) and 10 mL of 1,2-dichloroethane. The mixture was shaken for 5 min and after it was transferred in a separating funnel. After settling the organic phase was transferred to a stoppered centrifuge tube and centrifuged for 5 min. The absorbance of the colored organic phase was measured at 600 nm, with the solvent as blank.

The calibration was done by standard samples within the 0.02 × 10⁻⁵ - 1 × 10⁻⁵ M range.

The absorbance (A) of standard solutions of CTMA was plotted versus their concentration, C (M) as is shown in Figure 1. In the same figure, the linear

equation is presented that was used for the calculation of the unknown concentrations of CTMA and also the correlation coefficient, R².

Figure 2 shows the results from the kinetic study. The concentration of CTMA in equilibrium, Ce (M) is plotted versus time (hours). It is clear from the figure that the equilibrium has been achieved in the first hour. The next batch experiments that have been described in the experimental part in order to calculate the isotherm were done in one hour.

The most common adsorption isotherms are the Langmuir and Freudlich ones. The Langmuir isotherm proposes that sorption occurs on homogenous active sites by monolayer sorption (surface with a finite number of identical sites) without interactions between sorbed molecules and has the following form:

C s = K L × S m × C e 1 + K L × C e (1)

where C_s is the amount of solute sorbed on solid at equilibrium (mol/g), C_e is the solute concentration in liquid phase at equilibrium (mol/L), S_m is the apparent sorption capacity or sorption maximum (mol/g) and K_L is the Langmuir coefficient (L/mol) related to the adsorption energy.

Equation (1) can be rearranged to a linear form:

C e C s = 1 K L × S m + ( 1 S m ) × C e (2)

So that K_L and S_m can be determined by the linear equation.

Freudlich equation proposes that sorption occurs on heterogenous active sites and/or there are interactions between sorbed molecules leading to multilayer sorption. This isotherm is used widely, especially at low to medium concentration range and has the following form:

C s = K F × C e n (3)

where C_s is the amount of solute sorbed on solid at equilibrium (mol/g), C_e is the solute concentration in liquid phase at equilibrium (mg/L), K_F is the Freudlich coefficient (L/mol) indicating of the sorption capacity and n is a coefficient indicating of the sorption intensity.

Equation (3) can be rearranged to a linear form:

log C s = n × log C e + log K F (4)

The results from the isotherms are shown in Figure 3 and Figure 4. Comparing the two plots it is obvious because of the higher correlation coefficient that the adsorption equilibrium data fit much better to the Langmuir model. The sorption capacity (S_m), as calculated from the slope of Equation (2), is 240 mmol/kg, a similar value with the CEC of this material (264 meq/kg), as was calculated by [8] .

Generally, the mechanisms for the physical adsorption are: Van der Waals forces, hydrophobicity, hydrogen bonds, polarity and steric interaction, dipole dipole interaction, π-π interaction. The mechanisms for the adsorption of cationic surfactants on different clay and zeolite minerals include: ion exchange and hydrophobic interactions.

There are many studies in literature about the adsorption of cetyltrimethylammonium bromide, but not of the chloride salt, on clay and zeolite minerals.

From the clay minerals, montmorillonite is reported mostly to have the greatest sorption capacity. The mechanisms for the adsorption of CTMA on montmorillonite were found to be both ion exchange and interactions such as induced and π-π [21] .

Kaolinite is another clay mineral that has also been studied. Harendra and Vipulanandan [22] have studied the adsorption of cetyltrimethylammonium bromide on kaolinite and blasted sand and found that kaolinite had the higher sorption capacity of the two. Kaolinite had a CEC of 15 - 20 meq/kg and a sorption capacity of 10 mmol/kg.

Li and Gallus [23] also have studied the adsorption of cetyltrimethylammonium bromide on well clystallized kaolinite and on poorly clystallized kaolinite and found that the hydrophobic interaction was mostly responsible for the

adsorption compared to cation exchange. Kaolinite there had a CEC of 30 - 37 meq/kg and an almost twice sorption capacity of 58 mmol/kg.

From the zeolite minerals, clinoptilolite is one of the most common and abundant ones.

Li [24] has studied the adsorption of cetyltrimethylammonium bromide on clinoptilolite and found a sorption capacity of 200 mmol/kg, a value that is smaller than the sorption capacity found in the present work. The clinoptilolite had a CEC of 90 - 110 mmol/kg.

Another research that used clinoptilolite as sorbent, with a very high CEC value of 1360 meq/kg and cetyltrimethylammonium bromide as sorbate [25] , found a sorption capacity of 284 and the adsorption mechanism that was mostly the ion exchange.

Attapulgite has been found [16] that its sorption capacity for removing Ca, Mg, K and Na from sea water is due to its ion exchange properties. The equilibrium data fitted best with the Langmuir isotherm. It showed a sorption capacity of 248 mmol/kg of Na, slightly higher than the other ions. This result is similar with the result of the present study for CTMA.

Attapulgite has been investigated for the removal of heavy metals and for the removal of cationic dyes [12] [13] [14] [15] . The majority of these papers found that equilibrium data fitted best to the Langmuir isotherm.