Adsorption Capability of Ionic Dyes onto Pristine and Calcined Activated Clay

In this study, pristine and calcined activated clay (AC) were characterized by scanning electron microscopy, X-ray diffraction analysis, thermogravimetric-differential thermal analysis, electron probe microanalysis, surface pH measurement, specific surface area measurement, and humidity adjustment. ACs were used as adsorbents for investigating the adsorption kinetics, isotherms, thermodynamic parameters, of Basic Red 46 (BR), Basic Blue 75 (BB), Acid Red 138 (AR), and Acid Blue 185 (AB) in aqueous solutions, as well as the effect of sodium hydroxide on these dyes. The results showed that adsorption of cationic and anionic dyes was related to the specific surface area (0.974–0.984) and humidity adjustment performance (0.939–1.000), respectively. The adsorption capacity of the cationic dyes (BB and BR) onto pristine AC increased on increasing the temperature. Equilibrium data using pristine AC fitted well to both the Langmuir and Freundlich isotherm models. Kinetic data were best described by the pseudo-second order model (correlation coefficient was 0.979–0.999 and 0.945–0.999 for BB and BR). Nearly 8 h of contact time was sufficient for the adsorption of cationic dyes to reach equilibrium. Thermodynamic parameters were also evaluated for the cationic dye-adsorbent system, which revealed that the adsorption process is endothermic in nature. These results demonstrated that pristine AC could be used as a natural adsorbent for the removal of cationic dyes (BB and BR) from aqueous solution. [DOI: 10.1380/ejssnt.2016.209]


INTRODUCTION
Dyes are synthetic aromatic compounds embodied with various functional groups.They are widely used in textile, leather, paper, plastic, and other industries.More than 10,000 chemically different dyes are manufactured.The global dyestuff and dye intermediates production is estimated to be around 7108 kg per annum [1,2].Some of these dyes may degrade to produce carcinogens and toxic products [3].In addition, the high solubility of dyes in water results in their wide dissemination into the environment, making them detrimental to crops, aquatic life, and human health [4].Since cationic dyes can easily interact with the negatively charged cell-membrane surface, which can lead to health problems, they are more toxic than anionic dyes [5][6][7].Thus, the removal of dyes-particularly cationic dyes-from effluents is important for risk reduction.
Various treatment methods have been developed for decontamination purposes, including coagulation, chemical oxidation, membrane separation, electrochemical processes, and adsorption techniques.Of the abovementioned techniques, adsorption was recognized as a promising and cost-effective process to remove colors from aqueous solutions.Additionally, many kinds of adsorbents have been developed for various applications [8][9][10][11][12].
On the other hand, clay minerals have a wide range of applications in various areas of technology, due to their natural abundance and their ability to be chemically or physically modified [13].Activated clay is an expanding 2 : 1 layer silicate mineral that mainly consists of montmorillonites.The basic structural unit of the montmorillonite is a layer consisting of two inward-pointing tetrahedral sheets with a central alumina octahedral sheet.The layers are continuous in the length and width directions, but the bonds between these layers are weak and can be cleaved easily, which allows water and other molecules to enter between the layers, causing height expansion [14].Several methods have been proposed to modify the properties of natural clay minerals, among which the acid activation method is efficient [15,16].However, calcination treatment of montmorillonite has not been reported.Thus, if dye adsorption methods utilizing activated clay could be developed, the value and applicability of these materials would drastically increase.
In this study, we prepared calcined activated clay and used it for adsorption of dyes (cationic dye: Basic Red 46 and Basic Blue 75, Anionic dye: Acid Red 138 and Acid Red 185) from aqueous solutions.The properties of the adsorbents, adsorption kinetics, adsorption isotherms, and the effects of pH and temperature were investigated.Moreover, the adsorption mechanism of the dyes was elucidated.The results provide new insights into the development of high-performance adsorbents for dye adsorption.The specific surface area of the adsorbent was measured using a NOVA4200e specific surface analyzer (Yuasa Ionic, Japan).Thermogravimetry-differential thermal analysis (TG-DTA) was carried out with TG8120 (Shimadzu, Japan).The morphologies and crystallinities of the adsorbent were studied using scanning electron microscopy (SEM, SU1510, Hitachi, Ltd., Japan) and X- ray diffraction (XRD, MiniFlex II, Rigaku, Japan).The amount of aluminum, magnesium, and silica on the adsorbent surface was measured with an accelerating voltage of 15.0 kV and a beam diameter of 5 µm.The solution pH was measured using a digital pH meter (Mettler-Toledo, Japan) after adding the adsorbent (0.2 g) to distilled water at pH 7.0 (100 mL), allowing the mixture to stand for 2 min at 25 • C, and filtering it through a 0.45 m membrane filter.Humidity adjustment performance of the adsorbent was measured by the adsorption capability of moistures in relative humidity of 57.7 and 92.5%, respectively [17].Relative humidity of 57.7 and 92.5% were prepared by potassium nitrate (94.6 g/100 mL) and sodium bromide (35.7 g/100 mL), respectively.Subsequently, the adsorbent (0.3 g) was placed in the relative humidity of 57.7 and 92.5% desiccator for 24 h at 25 • C. The humidity adjustment performance was calculated using Eq. ( 1):

A. Materials
where M is the humidity adjustment performance (mg/g), F 1 is the adsorbent weight in relative humidity of 92.5% (mg), F 2 is the adsorbent weight in relative humidity of 57.7% (mg), and W is the adsorbent weight before adsorption of moisture (g).

B. Amount of dye adsorbed onto AC, AC500, and AC1000
Adsorbent (AC, AC500, and AC1000) was added to an aqueous solution of BB, BR, AB, and AR (100 mg/L, 50 mL).The suspension was shaken at 100 rpm for 24 h at 25 • C. The sample was filtered through a 0.45 µm membrane filter and the filtrate was analyzed with a spectrophotometer (UV-1200, Shimadzu Co., Ltd., Japan).The absorption wavelengths used for BB, BR, AB, and AR were = 657, 535, 622, and 519 nm, respectively.The amount of dye adsorbed onto AC was calculated using Eq. ( 2): where q is the amount of dye adsorbed onto AC (mg/g), C 0 is the initial concentration (mg/L), C e is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the mass of the adsorbent (g).

C. adsorption isotherms of the dyes
Adsorbent AC (0.05 g) was added to an aqueous solution of BB or BR (50 mL) at different initial concentrations.The suspension was shaken at 100 rpm for 24 h at 5, 25, and 50 • C. The sample was filtered through a 0.45 µm membrane filter and the filtrate was analyzed with a spectrophotometer.The amount of dye adsorbed was calculated using Eq.(2).

D. Effect of contact time on the adsorption of the dyes
Adsorbent AC (0.05 g) was added to an aqueous solution of BB or BR (100 mg/L, 50 mL).The suspension was shaken at 100 rpm for 10 min -24 h at 25 • C. The sample was filtered through a 0.45 µm membrane filter and the filtrate was analyzed with a spectrophotometer.The amount of dye adsorbed was calculated using Eq. ( 2).

E. Effect of solution pH on the adsorption of the dyes
Adsorbent AC (0.05 g) was added to an aqueous solution of BB or BR (100 mg/L, 50 mL, pH 3-11).The solution pH was adjusted with aqueous solutions of hydrochloric acid or sodium hydroxide.The suspension was shaken at 100 rpm for 24 h at 25 • C. The sample was filtered through a 0.45 µm membrane filter and the filtrate was analyzed with a spectrophotometer.The amount of dye adsorbed was calculated using Eq.(2).

A. Properties of adsorbents
SEM images of the adsorbents and the amount of aluminum, magnesium, and silica on the surface of the adsorbents are shown in Figs. 1 and 2, respectively.The particle size of the adsorbent did not changed with increasing calcination temperatures.At the same time, we observed an uneven surface on the adsorbent.The amount of silica on the adsorbent surface increased with increasing calcination temperature indicating that volatile substances and metallic impurities were removed by calcination.
Figure 3 shows the XRD patterns for AC, AC500, and AC1000.The obtained data are consistent with powder diffraction file 2010 obtained from the International Center for Diffraction Data.Accordingly, AC consisted of montmorillonite, while AC1000 consisted of montmorillonite and silicate dioxide.Montmorillonite, a member of the smectite family, has a 2 : 1 expanding crystal lattice.The smectite group refers to a family of non-metallic clays primarily composed of hydrated sodium calcium aluminum silicate, a group of monoclinic clay-like minerals with the general formula (Ca, Na, H)(Al, Mg, Fe, Zn) 2 (Si, Al droxide (Na,Ca) Potassium, iron, and other cations are common substitutes and the exact ratio of cations varies with the source [14,19].
The results for the thermal analysis of AC, AC500, and AC1000 are shown in Fig. 4.An endothermic peak at about 100 • C on the DTA curve of AC was observed.The TG curve shows a sharp weight loss of 10 wt%.This mass loss was attributed to dehydration of the compounds.Surface pH, specific surface area, and humidity adjustment performance are shown in Table I.Surface pH of AC, AC500, and AC1000 was 3.7, 3.8, and 6.7, respectively.Moreover, specific surface area of AC was greater than that of AC500 or AC1000.Humidity adjustment performance of AC1000 was smaller than that of AC or AC500.Calcination treatment clearly affected the relevant properties of the adsorbents.

B. Amount of dyes adsorbed onto AC, AC500, and AC1000
The amount of cationic (BB or BR) and anionic (AB or AR) dyes adsorbed on the adsorbent is shown in Fig. 5.The amount of BB or BR adsorbed onto AC, AC500, and AC1000 was 80.2, 55.0, and 3.4 mg/g or 95.2, 69.4,Humidity adjustment 2.0×10 −2 2.5×10 −2 2.3×10 −3 performance (mg/g) and 4.7 mg/g, respectively.The amount of cationic dye adsorbed decreased with increasing calcination temperature.On the other hand, the amount of AB or AR adsorbed onto AC, AC500, and AC1000 was 12.2, 15.8, and 0.9 mg/g or 18.5, 33.6, and 6.1 mg/g, respectively.Calcination treatment did not affect the adsorption capability of anionic dyes.We could observe the different trends in the adsorption capabilities of the dyes (the amount of cationic dyes adsorbed was greater than the amount of anionic dyes).The results obtained in this study suggest that the charge of the montmorillonite surface is generally negative; therefore, anionic dyes are repelled from the adsorbent surface.
Furthermore, we investigated the relationship between the amount adsorbed and the properties of the adsorbent for the cationic dyes (BB and BR). Figure 6 shows the relationship between the amount of cationic dye (BB and BR) and the specific surface area.The correlation coefficients between the amount of dye adsorbed and the specific surface area were 0.974 and 0.984 for BB and BR, respectively, which indicates that the specific surface area (physical property) is related to the adsorption capabilities of the cationic dyes.In addition, silicate dioxide structure was produced by the calcination treatment in AC (Fig. 3).The decrease in cationic dyes adsorption after calcination is attributed to the changes of the physical properties (decrease of specific surface area and the ratio of montmorillonite and silicate dioxide).Therefore, pristine AC could be useful as a natural adsorbent for the removal of cationic dyes (BB and BR) from aqueous solution.The relationship between the amount of anionic dye adsorbed and the humidity adjustment performance is shown in Fig. 7.The correlation coefficients between the amount of anionic dyes (AB and AR) adsorbed and the humidity adjustment performance were 1.000 and 0.939, respectively.Humidity adjustment performance shows the adsorption capability of moisture onto the adsorbent surface.In addition, the high value in humidity adjustment performance indicates the high hydrophilicity of the adsorbent surface.Therefore, wettability of the adsorbent surface is a critical factor for adsorption of anionic dyes onto ACs.However, we could not understand the adsorption mechanism of anionic dyes in detail, therefore we need to investigate it further [20,21].ACs did not show the adsorption capability of anionic dyes (Fig. 5), therefore, cationic dyes (BB and BR) were used in the following experiment.

C. Cationic dye adsorption isotherms for AC
Adsorption isotherms for cationic dyes at different temperatures are shown in Fig. 8.The amount of BB and BR adsorbed onto AC increased with increasing temperature.As it is necessary to establish the most appropriate correlations for the equilibrium data in the design of adsorp- tion systems, two common isotherm equations have been tested in the present study: Langmuir and Freundlich models [22,23].The former is applicable under the following assumption: (i) the solid has a uniform surface, (ii) there is no interaction between adsorbed molecules, and (iii) the adsorption process takes place in a single layer.The latter is an empirical model used to explain the observed phenomena for the non-ideal heterogeneous adsorption system, where the adsorbed dye on the adsorbent will increase as long as there is an increase in the dye concentration in the solution [24].The mathematical form of the Langmuir equation is expressed as: where W s is the monolayer adsorption capacity and a is the Langmuir constant.The plot of C e /q e versus C e is employed to generate the intercept value of 1/aW s and slope of 1/W s .The Freundlich model is represented as: where log k is the relative adsorption capacity of adsorbent and 1/n is a constant related adsorption intensity.The plot of log q e versus log C e gives a straight line with a slope of 1/n and intercept of log k.The calculated parameters are summarized in  Moreover, cationic dyes easily adsorbed onto the AC surface when 1/n was in the range 0.1-0.5 but not when 1/n > 2. This finding is consistent with previous reports, which showed that cationic dye adsorption onto the AC surface occurred readily when 1/n < 2 (0.37-0.96) [25].
The W s value at 50 • C for AC was greater than those at 5 or 25 • C, indicating that the adsorption temperature affects the maximum adsorption capability of cationic dyes.

D. Thermodynamic parameters
The thermodynamic data reflect the feasibility and favorability of the adsorption.The parameters free energy change (∆G), enthalpy (∆H) and entropy change (∆S) are estimated by the change in equilibrium constants with temperature.The energy change of the adsorption reaction is given by: where ∆G is the free energy change (kJ/mol), R is the universal constant (8.314J/mol K), T is the absolute temperature (K) and K c is the equilibrium constant (q e /C e ).
According to the Van't Hoff equation, the equilibrium constant K c is obtained from the slope ln(q e /C e ) as a function of C e at different temperatures.The values of ∆H and ∆S are calculated from the Van't Hoff equation: where ln K c is plotted against 1/T , and a straight line with the slope (−∆H/RT ) and intercept (∆S/R) are determined [26,27].The calculated thermodynamic parameters are depicted in Table III.The cationic dye adsorption onto AC increased with increasing temperature from 5 to 50 • C (Fig. 8), demonstrating that the adsorption process was endothermic.The positive values of ∆H (15.7  and 25.3 kJ/mol for BB and BR) illustrate the endothermic nature of cationic dye adsorption [26].The negative values of ∆S for the two dyes indicate that the two systems have decreased randomness at the solid-solution interface [28].The increasing value of ∆G with increasing temperature suggests that lower temperatures facilitate adsorption [29].Generally, the value of ∆G for physical adsorption is less than −19.7 kJ/mol while chemisorption is in the range of −74.5 to 397.5 kJ/mol [30].As shown in Table II, the values of ∆G for BB and BR suggest that the adsorption process is a typical chemical process.

E. Effect of contact time on cationic dye adsorption
The effect of the contact time on the adsorption of cationic dyes is shown in Fig. 9. BB and BR adsorption reached equilibrium within 8 h.Adsorption kinetics describe the rate of adsorbate uptake, which in turn controls the equilibrium time.When adsorption is preceded by diffusion through a boundary, the kinetics in most systems follow the pseudo-first-order equation.The model given by Lagergren and Svenska is defined as [31]: ln(q e − q t ) = ln q e − k 1 t (7) where q e and q t (mg/g) are the amounts of adsorbate at equilibrium and at any time, t (h), respectively, and k 1 (1/h) is the adsorption rate constant.Contrary to other models, the pseudo-second-order equation predicts the be-   havior over the whole time of adsorption, with chemisorption being the rate controlling step given by [32]: where k 2 is the pseudo-second-order rate constant for the adsorption process.The experimental data of cationic dye adsorption at different time intervals were examined with pseudo-first-order and pseudo-second-order models, using the plots of ln(q e − q t ) versus t and t/q t versus t, respectively.The corresponding parameters and correlation coefficients are listed in Table IV.The value q e,exp for AC was much closer to the value of q e,cal of the pseudo-secondorder model than that of the pseudo-first-order model.In addition, the correlation coefficients are closer to unity for the pseudo-second-order model (0.999) than for the pseudo-first-order model (0.945-0.979).The success of the pseudo-second-order model in fitting the experimental data suggests that the cationic dyes are chemisorbed to the AC [33].

F. Effect of pH on cationic dye adsorption
The pH of the dye solution plays an important part in the adsorption process as it alters the surface properties of the adsorbent as well as the degree of ionization of the dye.In this study, the influence of pH on adsorption capacity is investigated for the initial pH solution in the range of 3 to 11, and the results are shown in Fig. 10.The adsorption capacity of the AC for BB decreased with increasing pH, while that for BR did not change over the entire pH range.The difference in pH dependence of BB and BR could depend on the molecular structure (molecular size, chemical composition, functional groups, and charge state etc.).However, we have no data for the cationic dyes (BB and BR) structure (charge state etc.) in water.Therefore, we are now planning to investigate the main adsorption mechanism using pristine AC for removal cationic dyes from aqueous solution.In general, the amount of cationic dye adsorbed at basic conditions is greater than that at acidic conditions.Since the low adsorption capacity of cationic dyes at acidic pHs is due to the presence of excess H + ions, which compete with the cationic groups on the dye for the adsorption sites, increasing adsorption capability at basic pHs can be attributed to the electrostatic interaction between the cationic dye molecules and the negatively charge surface of AC (adsorbent) [6,34,35].However, such trends were not observed in this study.Therefore, both physical and chemical adsorption mechanisms are relevant to the adsorbing cationic dyes in this study.The first relevant property is the specific surface area of AC (physical property) as we showed the relationship between the amount of dye adsorbed and the specific surface area (Fig. 6).Another one is the description of the AC edges.Since the edges of activated clay are identical to the surface of a mixture of oxides, they act similarly to those oxide surfaces providing affinity sites for cationic dye adsorption.The degree of adsorption would be influenced by the development of pH-dependent charge on the edges (or so called layer charge) due to acid base reactions of surface groups.Thus, the adsorption of cationic dyes onto AC may be described as follows: S-OH + BB (or BR) = S-O-BB (BR) + H + (9) or S-O − + BB (or BR) = S-O-BB (BR) (10) where S represents the active surface functional groups including the silanol (Si-O − ) and aluminol (Al-O − )sites; S-OH and S-O − refer to neutral and ionized surface hydroxyl functional groups [36].

IV. CONCLUSIONS
This study shows that pristine activated clay (AC) is an effective and widely available adsorbent for the removal of cationic dyes (Basic Blue 75, BB and Basic Red 46, BR) from aqueous solutions.Pristine AC was prepared by calcination at 500 and 1000 • C (denoted AC500 and AC1000, respectively).AC was determined to have a montmorillonite structure by XRD; on the other hand, AC1000 was shown to have both montmorillonite and silicate dioxide structures.While the amount of cationic dye adsorbed onto AC was greater than that adsorbed onto AC500 or AC1000, the amount of anionic dye adsorbed onto AC did not show similar trends.Strong correlations between the amount of cationic dye adsorbed and the specific surface area were observed, with coefficients of 0.974-0.987.In addition, the correlation coefficient between the amount of anionic dye adsorbed and humidity adjustment performance was in the range of 0.939-1.000.These results suggest that the amount of cationic dye adsorbed is related to the physical property of the AC.The adsorption equilibrium was reached within 8 h and these data were subsequently fitted to a pseudo-second-order kinetic model (correlation coefficient: 0.999).The optimal pH condition for BB and BR adsorption was approximately 3.0 and 3.0-8.0,respectively.The amounts of cationic dye adsorbed onto AC increased with increasing temperatures, and the value of ∆G obtained in this study for BB and BR suggest that the adsorption process is a typical chemical process.The adsorption isotherm data provided a good fit to both the Freundlich and Langmuir equations.Taken together, the results obtained in this study illustrate that AC has promising applications for cationic dye (BB and BR) removal from aqueous solutions.

FIG. 7 :FIG. 8 :
FIG.7: Relationship between amount of dye adsorbed and the humidity adjustment performance.

FIG. 9 :
FIG.9: Effect of contact time on the adsorption of dyes onto AC.

FIG. 10 :
FIG.10: Effect of contact time on the adsorption of dyes onto AC.

TABLE I :
The properties of adsorbents.

TABLE II :
Freundlich and Langmuir constants for adsorption of dyes onto AC.

TABLE III :
Thermodynamic parameters for the adsorption of dyes on AC.

TABLE IV :
Fitting results of kinetic data using pseudo-first-order model and pseudo-second-order model.