**2.2. Preparation of CD/EPI polymers**

The α-CD/EPI, β-CD/EPI, and γ-CD/EPI polymers were prepared dissolving opportune amounts of the respective CDs in water, in presence of sodium borohydride. The mixtures were vigorously stirred at 50°C until the reactants were dissolved. Then, NaOH (40% w/w) solution was added and an excess of epichlorohydrin was slowly added dropwise. The mixtures were vigorously stirred and heated gently at 50°C. About after 5 hours, the solutions started to be viscous, and gelatinous solids were obtained. Then acetone was added, and the systems were maintained under stirring and heating for 10 min. After cooling, the insoluble polymers obtained were poured into water, filtered and the resulting solid was purified by several Soxhlet extractions. Next, the CD/EPI polymers were dried in oven, at 50°C for 12 h, crushed and utilised as adsorbent materials to remove DB78 from aqueous solution. **Figure 2** shows the scheme of CD/EPI polymer synthesis.

**2.5. Adsorption equilibrium isotherms**

ues of the linear regression correlation coefficient R2

\_\_1

dimensionless constant separation factor, RL

The linear form of Freundlich equation is:

where q<sup>e</sup>

where C0

where q<sup>e</sup>

The values of K<sup>F</sup>

slope of the linear plot ln q<sup>e</sup>

The linearised form of Langmuir is represented by Eq. (3):

ing sites (L/mg). From the intercept and slope of the plot 1/q<sup>e</sup>

*RL* <sup>=</sup> \_\_\_\_\_\_ <sup>1</sup>

Langmuir values, qm, b, and RL are presented in **Table 2**.

ln *qe* <sup>=</sup> ln *KF* <sup>+</sup> \_\_1

tion of the dye in solution at equilibrium, K<sup>F</sup>

is a favourable adsorption condition [31, 33].

*qe* = \_\_\_1 *qm* + \_\_\_\_\_\_\_\_\_ <sup>1</sup> *b qm* \_\_1 *Ce*

maximum monolayer amount of DB78 adsorbed per unit mass of adsorbent, C<sup>e</sup>

(mg/g) is the amount of DB78 adsorbed at equilibrium, C<sup>e</sup>

versus ln C<sup>e</sup>

(mg/g) is the amount of the dye adsorbed on polymer at equilibrium, qm (mg/g) is the

is the initial concentration of adsorbate (mg/L) and b (L/mg) is Langmuir constant.

that is defined by the following equation:

centration of dye in solution at equilibrium and b is the constant related to the affinity of the bind-

values of qm and b, respectively. Moreover, the Langmuir isotherm can be expressed in terms of a

The value of RL indicates the trend of the adsorption process, indeed, the isotherm can be either favourable (0 < RL < 1), unfavourable (RL > 1), linear (RL = 1) or irreversible (RL = 0). The

maximum adsorption capacity of adsorbent and n (dimensionless) is the heterogeneity factor.

ability of adsorption process: when n = 1, the adsorption is linear, when n > 1, the adsorption

and n, reported in **Table 2**, were calculated respectively by the intercept and

Adsorption isotherms, by means of accurate mathematical models, allow to evaluate the adsorption behaviour and to describe how the adsorbate interacts with the adsorbent [31]. Among all isotherm models developed, the more common models, Langmuir and Freundlich models were used in this study. The Langmuir adsorption isotherm model presumes that the adsorption occurs on homogeneous sites of adsorbent surface forming a saturated monolayer of adsorbate on the outer surface of adsorbent and that the adsorption of each molecule onto the surface has equal adsorption activation energy [32, 33]. The Freundlich adsorption isotherm is an empirical equation which describes heterogeneous systems having unequal available sites on adsorbent surface with different adsorption energies [31, 32]. The adsorption isotherms were evaluated adding different amounts of CD/EPI polymers to dye solutions and maintaining the systems at constant temperature of 25°C under continuous stirring until the equilibrium was achieved. Values of dye concentration were measured before and after the adsorption processes and the obtained experimental data were fitted with Langmuir and Freundlich models. The val-

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

give information about the best-fit model.

http://dx.doi.org/10.5772/intechopen.72502

versus 1/Ce, it is possible to obtain the

(1 <sup>+</sup> *<sup>b</sup> <sup>C</sup>*0) (4)

*<sup>n</sup>* ln *Ce* (5)

(L/g) is the Freundlich constant related to the

. The magnitude of n gives an indication of the favour-

(mg/L) is the concentra-

(3)

309

(mg/L) is the con-

**Figure 2.** Scheme of CD/EPI polymer synthesis.

#### **2.3. Instruments**

Electrochemical measurements were performed in a standard three-electrode cell using hanging mercury drop electrode (HDME) as working electrode. An Ag/AgCl, KCl sat electrode and a Pt rod were used as reference and counter electrodes, respectively. A LiClO<sup>4</sup> 0.1 M solution was used as supporting electrolyte. Voltammograms were recorded by means of the AUTOLAB PGSTAT10 potentiostat interfaced with a personal computer. Absorption spectra were recorded from 200 to 600 nm using a Shimadzu UV-1601 spectrophotometer. Calorimetric measurements were performed using an LKB 2277 Thermal Activity Monitor Isothermal Microcalorimeter equipped with an LKB 2277–204 flow mixing cell. The photographs of samples were collected by using a field emission scanning electron microscope (Merlin Compact/VP, Carl Zeiss Microscopy, Germany) with a secondary electron detector using an acceleration voltage of 2 KV and an aperture size of 30 μm.

#### **2.4. Batch adsorption experiments**

Batch mode experiments were carried out to study the dye adsorption processes by CD/EPI polymers. The required amounts of adsorbent were added to fixed volume of dye solution, at opportune concentration, under constant condition of agitation rate (170 rpm), pH and temperature. At predetermined time intervals, the dye concentration in solution was evaluated by UV–Vis absorption measurements. Different variables, such as contact time, adsorbent dosage, initial dye concentration, pH and temperature, were analysed to recognise the optimum adsorption states. These experiments were performed by varying the parameter under evaluation and maintaining the other parameters constant. Values of dye removal (%) and amount of dye adsorbed onto adsorbent q<sup>t</sup> (mg/g) at time t were respectively calculated using the following expressions:

$$\text{\%} = \frac{\text{(C}\_i - \text{C}\_i)}{\text{C}\_i} \text{ 100} \tag{1}$$

$$q\_t = \frac{\left(C\_i - C\_i\right)V}{m} \tag{2}$$

where *Ci* and *C*<sup>t</sup> (mg/L) are the dye concentration in solution at initial and at t adsorption time, respectively. V (L) is the initial volume of dye solution and m (g) is the mass of adsorbent. All tests were achieved in triplicate and the mean values were reported.

#### **2.5. Adsorption equilibrium isotherms**

**2.3. Instruments**

308 Cyclodextrin - A Versatile Ingredient

**2.4. Batch adsorption experiments**

**Figure 2.** Scheme of CD/EPI polymer synthesis.

amount of dye adsorbed onto adsorbent q<sup>t</sup>

% <sup>=</sup> (*Ci* <sup>−</sup> *Ct*

*qt* <sup>=</sup> (*Ci* <sup>−</sup> *Ct*

tests were achieved in triplicate and the mean values were reported.

the following expressions:

and *C*<sup>t</sup>

where *Ci*

Electrochemical measurements were performed in a standard three-electrode cell using hanging mercury drop electrode (HDME) as working electrode. An Ag/AgCl, KCl sat electrode

solution was used as supporting electrolyte. Voltammograms were recorded by means of the AUTOLAB PGSTAT10 potentiostat interfaced with a personal computer. Absorption spectra were recorded from 200 to 600 nm using a Shimadzu UV-1601 spectrophotometer. Calorimetric measurements were performed using an LKB 2277 Thermal Activity Monitor Isothermal Microcalorimeter equipped with an LKB 2277–204 flow mixing cell. The photographs of samples were collected by using a field emission scanning electron microscope (Merlin Compact/VP, Carl Zeiss Microscopy, Germany) with a secondary electron detector

Batch mode experiments were carried out to study the dye adsorption processes by CD/EPI polymers. The required amounts of adsorbent were added to fixed volume of dye solution, at opportune concentration, under constant condition of agitation rate (170 rpm), pH and temperature. At predetermined time intervals, the dye concentration in solution was evaluated by UV–Vis absorption measurements. Different variables, such as contact time, adsorbent dosage, initial dye concentration, pH and temperature, were analysed to recognise the optimum adsorption states. These experiments were performed by varying the parameter under evaluation and maintaining the other parameters constant. Values of dye removal (%) and

> ) \_\_\_\_\_\_ *Ci*

respectively. V (L) is the initial volume of dye solution and m (g) is the mass of adsorbent. All

(mg/L) are the dye concentration in solution at initial and at t adsorption time,

(mg/g) at time t were respectively calculated using

100 (1)

) *<sup>V</sup>* \_\_\_\_\_\_\_\_\_\_\_\_ *<sup>m</sup>* (2)

0.1 M

and a Pt rod were used as reference and counter electrodes, respectively. A LiClO<sup>4</sup>

using an acceleration voltage of 2 KV and an aperture size of 30 μm.

Adsorption isotherms, by means of accurate mathematical models, allow to evaluate the adsorption behaviour and to describe how the adsorbate interacts with the adsorbent [31]. Among all isotherm models developed, the more common models, Langmuir and Freundlich models were used in this study. The Langmuir adsorption isotherm model presumes that the adsorption occurs on homogeneous sites of adsorbent surface forming a saturated monolayer of adsorbate on the outer surface of adsorbent and that the adsorption of each molecule onto the surface has equal adsorption activation energy [32, 33]. The Freundlich adsorption isotherm is an empirical equation which describes heterogeneous systems having unequal available sites on adsorbent surface with different adsorption energies [31, 32]. The adsorption isotherms were evaluated adding different amounts of CD/EPI polymers to dye solutions and maintaining the systems at constant temperature of 25°C under continuous stirring until the equilibrium was achieved. Values of dye concentration were measured before and after the adsorption processes and the obtained experimental data were fitted with Langmuir and Freundlich models. The values of the linear regression correlation coefficient R2 give information about the best-fit model.

The linearised form of Langmuir is represented by Eq. (3):

$$\frac{1}{q\_{\epsilon}} = \frac{1}{q\_{m}} + \frac{1}{b} \frac{1}{q\_{m}} \frac{1}{C\_{\epsilon}} \tag{3}$$

where q<sup>e</sup> (mg/g) is the amount of the dye adsorbed on polymer at equilibrium, qm (mg/g) is the maximum monolayer amount of DB78 adsorbed per unit mass of adsorbent, C<sup>e</sup> (mg/L) is the concentration of dye in solution at equilibrium and b is the constant related to the affinity of the binding sites (L/mg). From the intercept and slope of the plot 1/q<sup>e</sup> versus 1/Ce, it is possible to obtain the values of qm and b, respectively. Moreover, the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor, RL that is defined by the following equation:

$$R\_{\mathbb{L}} = \frac{1}{\left(1 + b \, \mathrm{C}\_{0}\right)} \tag{4}$$

where C0 is the initial concentration of adsorbate (mg/L) and b (L/mg) is Langmuir constant. The value of RL indicates the trend of the adsorption process, indeed, the isotherm can be either favourable (0 < RL < 1), unfavourable (RL > 1), linear (RL = 1) or irreversible (RL = 0). The Langmuir values, qm, b, and RL are presented in **Table 2**.

The linear form of Freundlich equation is:

$$
\ln q\_{\epsilon} = \ln K\_{\text{p}} + \frac{1}{\text{H}} \ln \text{C}\_{\epsilon} \tag{5}
$$

where q<sup>e</sup> (mg/g) is the amount of DB78 adsorbed at equilibrium, C<sup>e</sup> (mg/L) is the concentration of the dye in solution at equilibrium, K<sup>F</sup> (L/g) is the Freundlich constant related to the maximum adsorption capacity of adsorbent and n (dimensionless) is the heterogeneity factor. The values of K<sup>F</sup> and n, reported in **Table 2**, were calculated respectively by the intercept and slope of the linear plot ln q<sup>e</sup> versus ln C<sup>e</sup> . The magnitude of n gives an indication of the favourability of adsorption process: when n = 1, the adsorption is linear, when n > 1, the adsorption is a favourable adsorption condition [31, 33].


**Table 2.** Adsorption isotherm values.

#### **2.6. Thermodynamic analysis**

Thermodynamic parameters, such as Gibb's free energy change (ΔG°) (J mol−1), enthalpy change (ΔH°) (J mol−1) and entropy change (ΔS°) (J mol−1 K−1), allow to comprehend the nature of adsorption process and the effect of temperature on adsorption. These parameters can be calculated using the following relations [34]:

$$
\Delta \, G^{\circ} = -RT \ln \mathcal{K}\_{\circ} \tag{6}
$$

N═N double bond occurs at the hydrazo stage (HN˗NH), via the consumption of 2e−

on the chemical structure of the investigated azo compound, the nature of adjacent substituents and the pH of the medium [35]. Furthermore, the electrochemical reduction of azo compounds is an irreversible process complicated by preceding and following chemical reactions leading to the cleavage of the azo bond and resulting in various degradation products [36]. In **Figure 3**, the cyclic voltammetry measurements of DB78 are reported. It presented three cathodic peaks, located in the range from −0.2 to −1.0 V. The first two weak waves (I and II) were positioned at −0.15 and −0.70 V respectively, while the more intense wave (III), were located at about −0.80 V. Ep, I and Ep, II are both attributable to the azo moieties electroreduction [37, 38]. The different potential for the azo group reduction is due to the different substituents present in ortho position respect to it. The first peak can be attributed to the electroreduction of the azo group with the ortho -OH group that facilitate the electroreduction

The electrochemical behaviour of DB78 in presence of increasing CDs concentration was then analysed. Although the addition of α-CD did not greatly influence the cyclic voltammograms of DB78 (data not showed), it is not possible to affirm that there is no interaction between dye and α-CD, but that this technique did not allow to obtain detailed information. On the contrary, the addition of β-CD and γ-CD, at increasing molar ratio, showed regular changes in the cyclic voltammograms of DB78. Indeed, in **Figure 4a** and **b**, it is possible to observe a strong increment of current intensity values at the increasing of the CD amount, particularly in the case of γ-CD, while no shifts of the potential peaks were detected. These regular variations indicate that the dye was reduced with more difficulty because its involvement in the inclusion complex. The inclusion of the azo groups of dye inside the cavity of the CDs prevents the interaction with the electrode and reduces the diffusion coefficient of the molecule determining the reduction of the peak current intensity. Consequently, the electrochemical measurements confirmed the formation of inclusion complexes between DB78 and β-CD and between DB78 and γ-CD.

**Figure 3.** Cyclic voltammetry at HMDE of aqueous solution containing Direct Blue 78.

/4H<sup>+</sup>

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

), via the consumption of 4e−

at the amine stage (─NH2

due to its electron-donating effect.

/2H<sup>+</sup> , or 311

, in one or two steps depending

http://dx.doi.org/10.5772/intechopen.72502

where R is the universal gas constant (8.314 J mol−1 K−1), T is the solution temperature (K) and Kc is defined as:

$$K\_{\rm c} = \frac{\mathcal{C}\_{\rm v}}{\mathcal{C}\_{\rm v}} \tag{7}$$

$$
\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \tag{8}
$$

Therefore, Eqs. (6) and (8) can be rewritten as:

$$
\ln K\_{\text{C}^\*} = \frac{\Delta S^\circ}{R} - \frac{\Delta H^\circ}{RT} \tag{9}
$$

ΔH° and ΔS° were obtained by plot of Eq. (9), while the ΔG° values were determined from Eq. (8).

#### **3. Results and discussion**

#### **3.1. Electrochemical measurements**

Before testing the ability of azo dye removal by CD/EPI polymers, the interactions between DB78 and α-, β- and γ-CDs were investigated in solution by electrochemical measurements. Generally, electrochemical studies of different azo dyes show that the electroreduction of the N═N double bond occurs at the hydrazo stage (HN˗NH), via the consumption of 2e− /2H<sup>+</sup> , or at the amine stage (─NH2 ), via the consumption of 4e− /4H<sup>+</sup> , in one or two steps depending on the chemical structure of the investigated azo compound, the nature of adjacent substituents and the pH of the medium [35]. Furthermore, the electrochemical reduction of azo compounds is an irreversible process complicated by preceding and following chemical reactions leading to the cleavage of the azo bond and resulting in various degradation products [36]. In **Figure 3**, the cyclic voltammetry measurements of DB78 are reported. It presented three cathodic peaks, located in the range from −0.2 to −1.0 V. The first two weak waves (I and II) were positioned at −0.15 and −0.70 V respectively, while the more intense wave (III), were located at about −0.80 V. Ep, I and Ep, II are both attributable to the azo moieties electroreduction [37, 38]. The different potential for the azo group reduction is due to the different substituents present in ortho position respect to it. The first peak can be attributed to the electroreduction of the azo group with the ortho -OH group that facilitate the electroreduction due to its electron-donating effect.

The electrochemical behaviour of DB78 in presence of increasing CDs concentration was then analysed. Although the addition of α-CD did not greatly influence the cyclic voltammograms of DB78 (data not showed), it is not possible to affirm that there is no interaction between dye and α-CD, but that this technique did not allow to obtain detailed information. On the contrary, the addition of β-CD and γ-CD, at increasing molar ratio, showed regular changes in the cyclic voltammograms of DB78. Indeed, in **Figure 4a** and **b**, it is possible to observe a strong increment of current intensity values at the increasing of the CD amount, particularly in the case of γ-CD, while no shifts of the potential peaks were detected. These regular variations indicate that the dye was reduced with more difficulty because its involvement in the inclusion complex. The inclusion of the azo groups of dye inside the cavity of the CDs prevents the interaction with the electrode and reduces the diffusion coefficient of the molecule determining the reduction of the peak current intensity. Consequently, the electrochemical measurements confirmed the formation of inclusion complexes between DB78 and β-CD and between DB78 and γ-CD.

**2.6. Thermodynamic analysis**

**Table 2.** Adsorption isotherm values.

310 Cyclodextrin - A Versatile Ingredient

Kc

Eq. (8).

is defined as:

calculated using the following relations [34]:

*KC* <sup>=</sup> *<sup>C</sup>*\_\_*<sup>i</sup>*

Therefore, Eqs. (6) and (8) can be rewritten as:

**3. Results and discussion**

**3.1. Electrochemical measurements**

ln*KC* <sup>=</sup> <sup>∆</sup>*S*° \_\_\_\_

Thermodynamic parameters, such as Gibb's free energy change (ΔG°) (J mol−1), enthalpy change (ΔH°) (J mol−1) and entropy change (ΔS°) (J mol−1 K−1), allow to comprehend the nature of adsorption process and the effect of temperature on adsorption. These parameters can be

**Polymers T (K) Langmuir Freundlich**

**b (L/mg) qm (mg/g) RL R2 KF**

β-CD/EPI 298 0.425 4.988 0.028 0.988 1.649 3.520 0.760

γ-CD/EPI 298 0.026 14.156 0.377 0.999 0.501 1.463 0.985

323 0.205 11.775 0.057 0.991 2.264 2.237 0.828 353 0.237 12.183 0.050 0.999 2.354 2.273 0.908

323 0.075 15.954 0.143 0.981 1.326 1.728 0.931 353 0.062 23.207 0.168 0.993 1.566 1.586 0.965

∆ *G*° = −*RT* ln *KC* (6)

where R is the universal gas constant (8.314 J mol−1 K−1), T is the solution temperature (K) and

∆*G*° = ∆*H*° − *T*∆*S*° (8)

ΔH° and ΔS° were obtained by plot of Eq. (9), while the ΔG° values were determined from

Before testing the ability of azo dye removal by CD/EPI polymers, the interactions between DB78 and α-, β- and γ-CDs were investigated in solution by electrochemical measurements. Generally, electrochemical studies of different azo dyes show that the electroreduction of the

*Ce*

*<sup>R</sup>* <sup>−</sup> <sup>∆</sup>*H*° \_\_\_\_

*RT* (9)

 **(L/g) n R2**

(7)

**Figure 3.** Cyclic voltammetry at HMDE of aqueous solution containing Direct Blue 78.

**3.3. Effect of contact time**

active sites of polymers was reached.

**3.4. Effect of adsorbent dosage**

further measurements.

γ-CD/EPI polymer.

To determine the effect of contact time on adsorption processes, 10 mL of DB 78 (11.00 mg/L) was maintained for 24 h under continuous stirring with 1.00 g of β- and γ-CD/EPI polymers at pH 6 and 25°C. The concentrations of dye in solution were measured at several times. **Figure 5b** shows that both polymers presented the maximum dye removal after 2 h of adsorption process and no further changes were observed after 24 h. Therefore, it is possible to affirm that the time required to achieve the equilibrium was about 2 h. During this time, the complete saturation of

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

http://dx.doi.org/10.5772/intechopen.72502

313

The amount of the adsorbent used in these experiments is another important parameter that affects the uptake of dye. Indeed, a quantitative removal cannot be achieved when the polymer is less than the optimum amount. To optimise the smallest quantity of polymer able to adsorb the greater amount of DB78, increasing dosage of adsorbents, from 0.05 to 1.25 g, was added into 10 mL of dye solution (11.00 mg/L). The systems, maintained at pH 6 and 25°C, were stirred, until equilibrium achievement, and the remaining amount of dye in solutions were measured. In **Figure 6a** and **b** are presented the effect of adsorbent dosage on β-CD/EPI and γ-CD/EPI polymers, respectively. It is possible to observe that both polymers have the same behaviour: the percentage of dye removal increased with the increase in dosage of polymers, due to the major availability of adsorbent surface sites [18]. In presence of β-CD/EPI polymer (**Figure 6a**), the removal of dye from the initial solutions increased from 41.20 to 98.90% as the adsorbent dosage increased from 0.05 to 1.00 g. When γ-CD/EPI polymer (**Figure 6b**) was used as an adsorbent, the removal of DB78 increased from 52.01 to 97.25% as the adsorbent dosage increased from 0.05 to 1.00 g. A further increase in dosage of polymers (1.25 g) did not improve the removal of both dye since the systems were achieved the maximum adsorption efficiency. Therefore, 1.00 g of polymers was used for

**Figure 6.** Effect of adsorbent dosage using 10 mL of DB78 (11.00 mg/L) and increasing dosage of polymers, from 0.05 to 1.25 g, at pH 6 and 25°C. (a) Adsorption measurements with β-CD/EPI polymer and (b) adsorption measurements with

**Figure 4.** Cyclic voltammetry at HMDE of aqueous solution containing DB78 in presence of increasing amounts of CDs. (a) DB78/β-CD and (b) DB78/γ-CD at different molar ratios.

#### **3.2. Dye adsorption efficiency by different CD/EPI polymers**

To evaluate the more appropriate material able to adsorb DB78, three different types of adsorbents, α-CD/EPI, β-CD/EPI, and γ-CD/EPI polymers, were used. Ten milliliters of dye (11.00 mg/L) at pH 6 and 25°C were analysed using 1.00 g of polymers as adsorbent. **Figure 5a** shows that β-CD/EPI polymer presented a better ability to remove DB78 from solution than the other polymers. The dye removal efficiency was 98.90% with β-CD/EPI polymer, in contrast to 97.25% and only 92.70% when γ-CD/EPI and α-CD/EPI polymers were respectively used. Consequently, all adsorption experiments were carried out on β-CD/EPI and γ-CD/EPI polymers. It is possible to suppose that the adsorption is based not only on physical adsorption process in the polymers networks but also on inclusion complex formation [39]. Therefore, β- and γ-CD, which are characterised by a wider cavity, can form more host-guest supramolecular interaction with dye than α-CD. However, β-CD/EPI, despite the intermediate size of β-CD between α- and γ-CD, showed the better efficiency in the removal of dye. This behaviour is due to the highest complexing ability and stability with cross-linking agents of β-CD [40].

**Figure 5.** Adsorption measurements using 10 mL of DB 78 (11.00 mg/L) and 1.00 g of polymers at pH 6 and 25°C. (a) Adsorption comparison between α-CD/EPI, β-CD/EPI and γ-CD/EPI polymers and (b) effect of contact time.

#### **3.3. Effect of contact time**

To determine the effect of contact time on adsorption processes, 10 mL of DB 78 (11.00 mg/L) was maintained for 24 h under continuous stirring with 1.00 g of β- and γ-CD/EPI polymers at pH 6 and 25°C. The concentrations of dye in solution were measured at several times. **Figure 5b** shows that both polymers presented the maximum dye removal after 2 h of adsorption process and no further changes were observed after 24 h. Therefore, it is possible to affirm that the time required to achieve the equilibrium was about 2 h. During this time, the complete saturation of active sites of polymers was reached.

#### **3.4. Effect of adsorbent dosage**

**Figure 5.** Adsorption measurements using 10 mL of DB 78 (11.00 mg/L) and 1.00 g of polymers at pH 6 and 25°C. (a) Adsorption comparison between α-CD/EPI, β-CD/EPI and γ-CD/EPI polymers and (b) effect of contact time.

To evaluate the more appropriate material able to adsorb DB78, three different types of adsorbents, α-CD/EPI, β-CD/EPI, and γ-CD/EPI polymers, were used. Ten milliliters of dye (11.00 mg/L) at pH 6 and 25°C were analysed using 1.00 g of polymers as adsorbent. **Figure 5a** shows that β-CD/EPI polymer presented a better ability to remove DB78 from solution than the other polymers. The dye removal efficiency was 98.90% with β-CD/EPI polymer, in contrast to 97.25% and only 92.70% when γ-CD/EPI and α-CD/EPI polymers were respectively used. Consequently, all adsorption experiments were carried out on β-CD/EPI and γ-CD/EPI polymers. It is possible to suppose that the adsorption is based not only on physical adsorption process in the polymers networks but also on inclusion complex formation [39]. Therefore, β- and γ-CD, which are characterised by a wider cavity, can form more host-guest supramolecular interaction with dye than α-CD. However, β-CD/EPI, despite the intermediate size of β-CD between α- and γ-CD, showed the better efficiency in the removal of dye. This behaviour is due to the highest complexing ability and stability with cross-linking agents of β-CD [40].

**Figure 4.** Cyclic voltammetry at HMDE of aqueous solution containing DB78 in presence of increasing amounts of CDs.

**3.2. Dye adsorption efficiency by different CD/EPI polymers**

(a) DB78/β-CD and (b) DB78/γ-CD at different molar ratios.

312 Cyclodextrin - A Versatile Ingredient

The amount of the adsorbent used in these experiments is another important parameter that affects the uptake of dye. Indeed, a quantitative removal cannot be achieved when the polymer is less than the optimum amount. To optimise the smallest quantity of polymer able to adsorb the greater amount of DB78, increasing dosage of adsorbents, from 0.05 to 1.25 g, was added into 10 mL of dye solution (11.00 mg/L). The systems, maintained at pH 6 and 25°C, were stirred, until equilibrium achievement, and the remaining amount of dye in solutions were measured. In **Figure 6a** and **b** are presented the effect of adsorbent dosage on β-CD/EPI and γ-CD/EPI polymers, respectively. It is possible to observe that both polymers have the same behaviour: the percentage of dye removal increased with the increase in dosage of polymers, due to the major availability of adsorbent surface sites [18]. In presence of β-CD/EPI polymer (**Figure 6a**), the removal of dye from the initial solutions increased from 41.20 to 98.90% as the adsorbent dosage increased from 0.05 to 1.00 g. When γ-CD/EPI polymer (**Figure 6b**) was used as an adsorbent, the removal of DB78 increased from 52.01 to 97.25% as the adsorbent dosage increased from 0.05 to 1.00 g. A further increase in dosage of polymers (1.25 g) did not improve the removal of both dye since the systems were achieved the maximum adsorption efficiency. Therefore, 1.00 g of polymers was used for further measurements.

**Figure 6.** Effect of adsorbent dosage using 10 mL of DB78 (11.00 mg/L) and increasing dosage of polymers, from 0.05 to 1.25 g, at pH 6 and 25°C. (a) Adsorption measurements with β-CD/EPI polymer and (b) adsorption measurements with γ-CD/EPI polymer.

#### **3.5. Effect of initial dye concentration**

To study the effect of initial dye concentration on adsorption mechanism onto CD-based polymers, increasing the concentrations of DB78 solutions were used. The experiments were performed at pH 6 and 25°C, using a constant volume of dye solution (10 mL) and a constant dosage (1.00 g) of β-CD/EPI polymer and of γ-CD/EPI polymer. Experimental results show that the amount of dye adsorbed onto adsorbent, q<sup>t</sup> (mg/g), increased with the increase in initial concentration of dye. This behaviour was more evident in the case of β-CD/EPI polymer (**Figure 7a**), where the amount of dye adsorbed onto polymer at equilibrium, q<sup>e</sup> , improved from 0.32 to 1.99 mg/g as the initial concentration of DB78 increased from 11.00 to 70.00 mg/L. In the case of γ-CD/EPI polymer (**Figure 7b**), q<sup>e</sup> increased from 0.24 to 1.28 mg/g when the initial concentration of dye was incremented from 11.00 to 70.00 mg/L. This occurs because the increase in the initial concentrations of dye induces the optimisation of favourable interaction raising the driving force, able to overcome the resistance to the mass transfer of dye between the aqueous and the solid phase [41]. Furthermore, these measurements demonstrate again the better adsorption ability of β-CD/EPI polymer than γ-CD/EPI polymer.

#### **3.6. Effect of initial pH**

To study the influence of pH on the adsorption of azo dye onto the two polymers, experiments were carried out at pH 2, 6 and 11 with a contact time of 2 h. In **Figure 8a** and **b** are reported the results respectively obtained with β-CD/EPI and γ-CD/EPI polymers. Generally, the initial pH of solution plays a significant role in the chemistry of adsorbent and dye, however, in this case, no significant changes in the adsorption process were observed at different pH conditions. Indeed, for both polymers, when acid and basic conditions were used, no important variations in the adsorption efficiency were observed. However, the highest percentage of dye removal was obtained at pH 6. It is possible to suppose that at alkaline pH, the presence of excess ─OH ions compete with the anionic dye for the adsorption sites. Indeed, as the pH of the system increases, the number of negatively charged sites increases as well, and the number of positively charged sites decreases. A negatively charged surface site on the adsorbent does not support the adsorption of anionic dye due to electrostatic repulsion [42]. On the other hand, at low values of pH, the sulphonate groups of dye are protonated and the number of positively charged sites increases, inducing again electrostatic repulsion. Therefore, all experi-

**Figure 8.** Effect of initial pH on the adsorption of DB78 onto polymers (1.00 g). Ten milliliters of dye solution (55.00 mg/L), at pH 2, 6, 11 and temperature 25°C, were used. (a) Adsorption measurements with β-CD/EPI polymers and (b)

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

http://dx.doi.org/10.5772/intechopen.72502

315

The adsorption isotherms of DB78 onto CD/EPI polymers were determined at pH 6 maintaining the systems at constant temperature of 25, 50 and 80°C. Various quantities of adsorbent, from 0.05 to 1.00 g, were added to 10 mL of dye (80.00 mg/L) and the adsorption process was maintained until the reaching of equilibrium state. The Langmuir and Freundlich values are listed in **Table 2**,

was better represented by Langmuir isotherm model than the Freundlich equation. The applicability of Langmuir isotherm describes a monolayer and homogeneous adsorption of the dye onto the surface of polymers, where the adsorption of each molecule onto the surface has equal adsorption activation energy [31]. These results agree with a study, reported in the literature [42], where some azo dyes have been removed by β-cyclodextrin-based polymers. Furthermore, these measurements show that increasing the temperature from 25 to 80°C induced a higher

that β-CD/EPI and γ-CD/EPI polymers are good and favourable adsorbent for DB78 removal.

The thermodynamic parameters for the adsorption of DB78 dye wastewater on β-CD/EPI and γ-CD/EPI polymers are summarised in **Table 3**. The negative values of ΔG° indicated that the dye adsorption by these polymers is a spontaneous and a favourable process. Since the

, the results show that the adsorption process with both polymers

values were between 0 and 1, it possible to underline

is used to determine

ments were performed at pH 6 that is the natural pH of DB78 aqueous solution.

respectively and the value of the linear regression correlation coefficient R2

**3.7. Adsorption equilibrium isotherms**

adsorption measurements with γ-CD/EPI polymers.

maximum adsorption capacity. Since the RL

the best-fit model. Based on R2

**3.8. Thermodynamic analysis**

**Figure 7.** Effect of initial dye concentration on the adsorption of DB78 onto polymers (1.00 g). Ten milliliters of dye solution at increasing concentrations, from 11.00 to 70.00 mg/L, at pH 6 and temperature 25°C, were used. (a) Adsorption measurements with β-CD/EPI polymer and (b) adsorption measurements with γ-CD/EPI polymer.

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers http://dx.doi.org/10.5772/intechopen.72502 315

**Figure 8.** Effect of initial pH on the adsorption of DB78 onto polymers (1.00 g). Ten milliliters of dye solution (55.00 mg/L), at pH 2, 6, 11 and temperature 25°C, were used. (a) Adsorption measurements with β-CD/EPI polymers and (b) adsorption measurements with γ-CD/EPI polymers.

the system increases, the number of negatively charged sites increases as well, and the number of positively charged sites decreases. A negatively charged surface site on the adsorbent does not support the adsorption of anionic dye due to electrostatic repulsion [42]. On the other hand, at low values of pH, the sulphonate groups of dye are protonated and the number of positively charged sites increases, inducing again electrostatic repulsion. Therefore, all experiments were performed at pH 6 that is the natural pH of DB78 aqueous solution.

#### **3.7. Adsorption equilibrium isotherms**

The adsorption isotherms of DB78 onto CD/EPI polymers were determined at pH 6 maintaining the systems at constant temperature of 25, 50 and 80°C. Various quantities of adsorbent, from 0.05 to 1.00 g, were added to 10 mL of dye (80.00 mg/L) and the adsorption process was maintained until the reaching of equilibrium state. The Langmuir and Freundlich values are listed in **Table 2**, respectively and the value of the linear regression correlation coefficient R2 is used to determine the best-fit model. Based on R2 , the results show that the adsorption process with both polymers was better represented by Langmuir isotherm model than the Freundlich equation. The applicability of Langmuir isotherm describes a monolayer and homogeneous adsorption of the dye onto the surface of polymers, where the adsorption of each molecule onto the surface has equal adsorption activation energy [31]. These results agree with a study, reported in the literature [42], where some azo dyes have been removed by β-cyclodextrin-based polymers. Furthermore, these measurements show that increasing the temperature from 25 to 80°C induced a higher maximum adsorption capacity. Since the RL values were between 0 and 1, it possible to underline that β-CD/EPI and γ-CD/EPI polymers are good and favourable adsorbent for DB78 removal.

#### **3.8. Thermodynamic analysis**

**Figure 7.** Effect of initial dye concentration on the adsorption of DB78 onto polymers (1.00 g). Ten milliliters of dye solution at increasing concentrations, from 11.00 to 70.00 mg/L, at pH 6 and temperature 25°C, were used. (a) Adsorption

To study the effect of initial dye concentration on adsorption mechanism onto CD-based polymers, increasing the concentrations of DB78 solutions were used. The experiments were performed at pH 6 and 25°C, using a constant volume of dye solution (10 mL) and a constant dosage (1.00 g) of β-CD/EPI polymer and of γ-CD/EPI polymer. Experimental results

in initial concentration of dye. This behaviour was more evident in the case of β-CD/EPI polymer (**Figure 7a**), where the amount of dye adsorbed onto polymer at equilibrium, q<sup>e</sup>

improved from 0.32 to 1.99 mg/g as the initial concentration of DB78 increased from 11.00 to

when the initial concentration of dye was incremented from 11.00 to 70.00 mg/L. This occurs because the increase in the initial concentrations of dye induces the optimisation of favourable interaction raising the driving force, able to overcome the resistance to the mass transfer of dye between the aqueous and the solid phase [41]. Furthermore, these measurements demonstrate again the better adsorption ability of β-CD/EPI polymer than γ-CD/EPI polymer.

To study the influence of pH on the adsorption of azo dye onto the two polymers, experiments were carried out at pH 2, 6 and 11 with a contact time of 2 h. In **Figure 8a** and **b** are reported the results respectively obtained with β-CD/EPI and γ-CD/EPI polymers. Generally, the initial pH of solution plays a significant role in the chemistry of adsorbent and dye, however, in this case, no significant changes in the adsorption process were observed at different pH conditions. Indeed, for both polymers, when acid and basic conditions were used, no important variations in the adsorption efficiency were observed. However, the highest percentage of dye removal was obtained at pH 6. It is possible to suppose that at alkaline pH, the presence of excess ─OH ions compete with the anionic dye for the adsorption sites. Indeed, as the pH of

(mg/g), increased with the increase

increased from 0.24 to 1.28 mg/g

,

measurements with β-CD/EPI polymer and (b) adsorption measurements with γ-CD/EPI polymer.

**3.5. Effect of initial dye concentration**

314 Cyclodextrin - A Versatile Ingredient

**3.6. Effect of initial pH**

show that the amount of dye adsorbed onto adsorbent, q<sup>t</sup>

70.00 mg/L. In the case of γ-CD/EPI polymer (**Figure 7b**), q<sup>e</sup>

The thermodynamic parameters for the adsorption of DB78 dye wastewater on β-CD/EPI and γ-CD/EPI polymers are summarised in **Table 3**. The negative values of ΔG° indicated that the dye adsorption by these polymers is a spontaneous and a favourable process. Since the


**Table 3.** Thermodynamic parameters.

obtained values of free energy change were in the range of −9.85 to −12.01 kJ mol−1, for the β-CD/EPI polymer, and in range of −11.26 to −14.25 kJ mol−1, for the γ-CD/EPI polymer, it is possible to affirm that the adsorption was principally physical. Indeed, some studies reported that the adsorption is classified as physical adsorption when the ΔG° values range between −20 and 0 kJ mol−1, and as chemical adsorption when ΔG° values range from −80 to −400 kJ mol−1 [34]. The positive values of ΔS°, for both polymers, showed that the disorder of the systems increased at the solid solution interface during the adsorption of DB78 on polymers. Also, the ΔH° values for β-CD/EPI and γ-CD/EPI polymers were 4.94 and 1.86 kJ mol−1, respectively. These positive values indicate that the adsorption followed an endothermic process as in agreement with results derived from the isotherm measurements.

**Figure 9.** Thermograms obtained by DSC analysis. (a) Thermograms of DB78, β-CD/EPI polymer and DB78 loaded

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

http://dx.doi.org/10.5772/intechopen.72502

317

**Figure 10.** Scanning electron microscopy images of polymers. (a) Unloaded β-CD/EPI polymer, (b) DB78 loaded β-CD/

EPI polymer, (c) unloaded γ-CD/EPI polymer and (d) DB78 loaded γ-CD/EPI polymer.

β-CD/EPI polymer and (b) thermograms of DB78, γ-CD/EPI polymer and DB78 loaded γ-CD/EPI polymer.

#### **3.9. Thermal analysis**

The thermal analysis of β-CD/EPI polymer, γ-CD/EPI polymer, and their respective polymers loaded with DB78 was performed with differential scanning calorimetry (DSC) under N2 atmosphere with heating rate of 20°C/min. As shown in **Figure 9a**, the DSC thermograms of β-CD/EPI polymer exhibited an endothermic peak at about 280°C [43]. After the interaction of this polymer with DB78, the thermogram presented a double endothermic peak at about 250 and 280°C. Since the first signal corresponds to the decomposition temperature of the only dye, it is possible to affirm that DB78 exhibits a thermal instability even after adsorption. This result allows to hypothesise that the interaction between DB78 and the polymers did not occur only in the internal cavities of cyclodextrins but also in the pores present on the external surface of polymer. In **Figure 9b**, the DSC thermograms of γ-CD/EPI polymer loaded with DB78 were no longer exhibit the typical thermal decomposition phenomena, shown in the thermograms of DB78 and γ-CD/EPI polymer. It confirms the interaction between DB78 and γ-CD/EPI: the inclusion of the dye into the γ-CD by weak forces stabilises both the adsorbent and the adsorbate.

#### **3.10. Morphologic study**

CD-based polymers were observed by field emission scanning electron microscope (FESEM) to examine their morphology. In **Figure 10a** and **c**, FESEM images of unloaded β-CD/EPI and Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers http://dx.doi.org/10.5772/intechopen.72502 317

obtained values of free energy change were in the range of −9.85 to −12.01 kJ mol−1, for the β-CD/EPI polymer, and in range of −11.26 to −14.25 kJ mol−1, for the γ-CD/EPI polymer, it is possible to affirm that the adsorption was principally physical. Indeed, some studies reported that the adsorption is classified as physical adsorption when the ΔG° values range between −20 and 0 kJ mol−1, and as chemical adsorption when ΔG° values range from −80 to −400 kJ mol−1 [34]. The positive values of ΔS°, for both polymers, showed that the disorder of the systems increased at the solid solution interface during the adsorption of DB78 on polymers. Also, the ΔH° values for β-CD/EPI and γ-CD/EPI polymers were 4.94 and 1.86 kJ mol−1, respectively. These positive values indicate that the adsorption followed an endothermic process as

**Polymers T (K) ΔG° (kJ mol−1) ΔS° (J mol−1 K−1) ΔH° (kJ mol−1)**

β-CD/EPI 298 −11.263 54.352 4.942

γ-CD/EPI 298 −9.847 39.266 1.860

323 −12.622 353 −14.252

323 −10.907 353 −12.006

The thermal analysis of β-CD/EPI polymer, γ-CD/EPI polymer, and their respective polymers loaded with DB78 was performed with differential scanning calorimetry (DSC) under

 atmosphere with heating rate of 20°C/min. As shown in **Figure 9a**, the DSC thermograms of β-CD/EPI polymer exhibited an endothermic peak at about 280°C [43]. After the interaction of this polymer with DB78, the thermogram presented a double endothermic peak at about 250 and 280°C. Since the first signal corresponds to the decomposition temperature of the only dye, it is possible to affirm that DB78 exhibits a thermal instability even after adsorption. This result allows to hypothesise that the interaction between DB78 and the polymers did not occur only in the internal cavities of cyclodextrins but also in the pores present on the external surface of polymer. In **Figure 9b**, the DSC thermograms of γ-CD/EPI polymer loaded with DB78 were no longer exhibit the typical thermal decomposition phenomena, shown in the thermograms of DB78 and γ-CD/EPI polymer. It confirms the interaction between DB78 and γ-CD/EPI: the inclusion of the dye into the γ-CD by weak forces stabilises both the adsorbent

CD-based polymers were observed by field emission scanning electron microscope (FESEM) to examine their morphology. In **Figure 10a** and **c**, FESEM images of unloaded β-CD/EPI and

in agreement with results derived from the isotherm measurements.

**3.9. Thermal analysis**

**Table 3.** Thermodynamic parameters.

316 Cyclodextrin - A Versatile Ingredient

and the adsorbate.

**3.10. Morphologic study**

N2

**Figure 9.** Thermograms obtained by DSC analysis. (a) Thermograms of DB78, β-CD/EPI polymer and DB78 loaded β-CD/EPI polymer and (b) thermograms of DB78, γ-CD/EPI polymer and DB78 loaded γ-CD/EPI polymer.

**Figure 10.** Scanning electron microscopy images of polymers. (a) Unloaded β-CD/EPI polymer, (b) DB78 loaded β-CD/ EPI polymer, (c) unloaded γ-CD/EPI polymer and (d) DB78 loaded γ-CD/EPI polymer.

γ-CD/EPI polymers are respectively showed. It is possible to observe that these materials presented a very porous, rough and irregular structure which cavities are able to adsorb the DB78 molecules. Moreover, the presence of loaded dye molecules on polymers did not affect significantly the morphology of the samples, as reported in **Figure 10b** and **d**, confirming the weak and physical interaction of adsorption process.

**Author details**

Paola Semeraro<sup>1</sup>

Murcia, Murcia, Spain

2014.04.002

**References**

Vito Rizzi<sup>1</sup>

, José Antonio Gabaldón2

\*Address all correspondence to: pinalysa.cosma@uniba.it

1 Department of Chemistry, University of Bari Aldo Moro, Bari, Italy

and Pinalysa Cosma1,2\*

DOI: 10.1016/j.jenvman.2016.06.026

169. DOI: 10.1016/S1385-8947(03)00100-1

jphotobiol.2016.07.040

2009.03.0102009

, Paola Fini<sup>3</sup>

2 Department of Food Technology and Nutrition, Catholic University San Antonio of

3 Department of Chemistry, National Research Council CNR-IPCF, UOS Bari, Bari, Italy

[1] Yagub MT, Sen TK, Afroze HM. Dye and its removal from aqueous solution by adsorption: A review. Advances in Colloid and Interface Science. 2014;**209**:172-184. DOI: 10.1016/j.cis.

[2] Ejder-Korucu M, Gürses A, Doğar Ç, Sharma SK, Açıkyıldız M. Removal of organic dyes from industrial effluents: An overview of physical and biotechnological applications. In: Sharma SK, editor. Green Chemistry for Dyes Removal from Wastewater: Research Trends and Applications. 1st ed. Hoboken, New Jersey: John Wiley & Sons, Inc and Salem, Massachusetts: Scrivener Publishing LLC; 2015. pp. 1-22. DOI: 10.1002/9781118721001.ch1

[3] Gupta VK, Suhas. Application of low-cost adsorbents for dye removal—A review. Journal of Environmental Management. 2009;**90**:2313-2342. DOI: 10.1016/j.jenvman.2008.11.017

[4] Peláez-Cid AA, Herrera-González AM, Salazar-Villanueva M, Bautista-Hernández A. Elimination of textile dyes using activated carbons prepared from vegetable residues and their characterization. Journal of Environmental Management. 2016;**181**:269-278.

[5] Edison TNJI, Atchudan R, Sethuraman MG, Lee YR. Reductive-degradation of carcinogenic azo dyes using *Anacardium occidentale* testa derived silver nanoparticles. Journal of Photochemistry & Photobiology, B: Biology. 2016;**162**:604-610. DOI: 10.1016/j.

[6] Tripathi P, Srivastava VC, Kuma A. Optimization of an azo dye batch adsorption parameters using Box–Behnken design. Desalination. 2009;**249**:1273-1279. DOI: 10.1016/j.desal.

[7] Ghoreishi S, Haghighi R. Chemical catalytic reaction and biological oxidation for treatment of non-biodegradable textile effluent. Chemical Engineering Journal. 2003;**95**:163-

, Estrella Núňez2

Removal of an Azo Textile Dye from Wastewater by Cyclodextrin-Epichlorohydrin Polymers

, José Antonio Pellicer2

http://dx.doi.org/10.5772/intechopen.72502

,

319
