**2.2 Fourier transform infrared (FTIR) spectroscopy of organoclays**

**Figures 4**–**7** represent the FTIR spectra of the raw clays, the organoclays loaded at 0.5 CEC, 1.0 CEC, 1.5 CEC and 2.0 CEC loading level of C12, C14, C16 and 2C12 surfactants, and pure surfactants.

The bending bands of the H-O appear at around 1600 cm<sup>1</sup> on the FTIR spectra of the raw and modified clays. The stretching vibrations of the structural hydroxyl (OH) is observed at 3622 cm<sup>1</sup> for the raw clays, and the band around 3420 cm<sup>1</sup> is attributed to the hydroxyl of water molecules contained in the clay layers. According

**Figure 5.** *FTIR spectra of AH and DI raw clays, pure C14 surfactant and C14-AH and C14-DI organoclays.*

to Madejová et al., the bands seen at around 3694 and 3620 cm<sup>1</sup> are associated to the stretching vibrations of the structural hydroxyl in AlMgO-H and Al2O-H of montmorillonite or kaolinite, respectively [31, 32]. In addition, the band observed at 3620 cm<sup>1</sup> on the spectra of raw clays and oganoclays are characteristic of smectites containing high amount of Al in the octahedra sheet [31]. The Si-O stretching band is observed at 1100 cm<sup>1</sup> and the Si-O in-plane bending appears at 470 cm<sup>1</sup> in all FTIR spectra [32]. The Al2OH bending vibration and the Al-O-Si deformations bands are observed at 913 and 536 cm<sup>1</sup> , respectively [32]. The Fe-O or Al-O out of plane bending vibrations appear at around 694 cm<sup>1</sup> [33].

*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*

**Figure 6.** *FTIR spectra of AH and DI raw clays, pure C16 surfactant and C16-AH and C16-DI organoclays.*

The asymmetric stretching bands of the CH2 group shift slightly from 2927 to 2920 cm<sup>1</sup> depending on AH or DI raw clays, and surfactant loading, 0.5 to 2.0 CEC level due to the insertion of the C12, C14, C16, and 2C12 surfactants. Likewise, the wavenumbers of the CH2 symmetric stretching vibrations shift from 2855 cm<sup>1</sup> to 2850 cm<sup>1</sup> with increasing surfactant loading levels (**Figures 4**–**7**). The wavenumbers of asymmetric and symmetric CH2 stretching bands of surfactants in the modified clays increase with the surfactant loading level's increase and approach the wavenumbers of the pure surfactant. This further sustains that the long alkyl chains are packed identically in clay layers as in the pure quaternary alkylammonium

**Figure 7.** *FTIR spectra of AH and DI raw clays, pure 2C12 surfactant and 2C12-AH and 2C12-DI organoclays.*

bromide salts when the loading level tends to saturation. In fact, Kamitori et al. [29], Silva et al. [34] and Campanelli and Scaramuzza [35] reported that the surfactants (C12, C14, and C16) are arranged in parallel plans inside a sheet and antiparallel as layers which alternate in the pure surfactant compound. The IR spectra of the organoclays support that the long alkyl chains orientation in the montmorillonite clay interlayers evolves progressively from a parallel orientation with clay sheets at lower loading level of surfactant to a standing position at higher loading level as reported in the literature [28, 36, 37].


#### **Table 1.**

*Change in CH2 symmetric and asymmetric stretching vibration frequencies depending on the surfactant loading level and/or AH and DI clays.*

The CH2 asymmetric and symmetric stretching vibration bands frequencies in the pure surfactant molecules and the modified clays are summarized in **Table 1**. Contrary to the CH2 asymmetric and symmetric stretching frequencies, the CH2 scissoring and rocking vibrations frequencies observed at 1473 and 731 cm<sup>1</sup> , respectively [33], seem to be independent of the length of alkyl chain and level of loading.

**Figures 4**–**7** show that the intensities of the band attributed to the hydroxyl of absorbed water seen at 3420 and 1635 cm<sup>1</sup> turn down progressively when the surfactant loading level increases and this behavior is certainly related to the change of the raw clay properties from hydrophilic to hydrophobic ones [33, 38]. The FTIR spectra support the model of insertion of the quaternary alkylammonium cations into mineral clay interlayers as described below, **Figure 3**.

### **3. Influence of various factors in batch studies of bisphenol A removal**

In the present work, various factors such as the adsorbents dose (clays and organoclays), the agitation time, the initial concentration, the pH, the surfactants loading level, and the temperature have been studied in order to elucidate the BPA sorption process. The methodologies used for all adsorption experiences are described elsewhere [39].

#### **3.1 Effect of the agitation time on the BPA adsorption and kinetics**

#### *3.1.1 Effect of the agitation time on the BPA adsorption*

**Figure 8a**–**d** represent the amount of BPA removed as a function of the agitation time for 200 mg as raw clays and organoclays dose. From **Figure 8a**–**d**, the BPA

**Figure 8.**

*Effect of agitation time of BPA adsorption on the raw clays and organoclays: AH, DI, C12-AH, C12-DI (a), C14- AH, C14-DI (b), C16-AH, C16-DI (c) and 2C12-AH, 2C12-DI (d).*

sorption depends on the duration of the agitation. In fact, it is seen that the amount of BPA removed from the aqueous solution by the raw and modified clays increases during the first 10 minutes and attains the equilibrium concentration within 20 minutes (**Figure 8a**–**d**).

Moreover, after 20 minutes as duration of agitation, no significant difference is observed according to the types of surfactants, the surfactant loading levels and the clays types (**Figure 8a**–**d**). This finding is unlike to those reported by Zheng et al. [10], where the quantity of BPA removed per unit of mass increases with the surfactant chain longer (C12 to C16). The agitation time required to reach the equilibrium is low and this suggests that the physisorption process is involved in the BPA removal [40]. It is also seen that the number of the surfactant long alkyl chain (C12 and 2C12) does not enhance the BPA adsorption and this results is unlike the results obtained by Park et al., where the quantity of BPA adsorbed increases with the increase of the number of long alkyl chain (C12 and 2C12) [8]. The ranking of the qe values are from 18.7 to 19.8 mg of BPA/g of adsorbent for 200 mg as adsorbent dosage and 100 mg/L as BPA initial concentration for all the organoclays after 120 min as agitation time.

#### *3.1.2 Kinetics*

The constants of the pseudo-first and the pseudo-second kinetics have been evaluated by using the result of the effect of agitation time on BPA sorption and the obtained parameters are summarized into **Table 2** [39].

According to the high determination coefficients obtained (R<sup>2</sup> > 0.98) in the case of the pseudo-second order kinetic model when compared to those of the pseudo-


*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*

#### **Table 2.**

*Kinetic constants of BPA removal on raw and modified clays.*

first-order kinetic model ones (R2 < 0.85) (**Table 2**), it can be concluded that the removal of BPA is better described by the pseudo-second-order kinetic model than the pseudo-first order kinetic model (**Table 2**). According to the literature, the pseudosecond-order kinetic model suggests that the chemisorption is the limiting step during the adsorption process [8, 16]. This result is like those reported previously on BPA removal using different adsorbents, such as graphene [9], C16-palygorskite [19], surfactant-modified zeolite [26], organo-montmorillonites [3, 7, 8, 10, 16]. Generally, BPA removal from water using organoclays is better described by the pseudo-seconorder kinetic than the pseudo-first order kinetic [7, 8, 10, 11, 16, 21].

#### **3.2 Effect of pH on BPA adsorption**

**Figure 9** represents the amount of BPA removed when the pH increases from 2 to 12. It is seen that from acidic pH up to pH 8, the amount of BPA removed on all organoclays stays relatively constant (**Figure 9a**-**d**). For pH values ranging from 2 to 8, the qe values ranged from 19.4 to 19.8 mg/g for all modified clays. This is followed by a decrease in the efficiency of organoclays in BPA sorption from aqueous solution having pH ≥ 8. In fact, the qe values decrease slightly from 19.8–19.4 mg/g to 19.6 – 16.6 mg/g when the pH value increases from 8 to 10. The qe values turn down remarkably when the pH becomes higher than 10 for all the organoclays with higher

**Figure 9.**

*Effect of pH on the adsorption of BPA by organoclays: C12-AH and C12-DI (a), C14-AH and C14-DI (b), C16-AH and C16-DI (c), 2C12-AH and 2C12-DI (d).*

decrease for those loaded at 1.0 CEC. In fact, it is seen that the qe values for all modified clays are ranged between 18 to 9.0 mg /g when the pH varies from 10 to 12. For pH = 12, the organoclays loaded at 2.0 CEC loading level give higher qe values (9.2 to 18.5 mg/g) compared to those loaded at 1.0 CEC (9.0 to 13.9 mg/g). In addition, higher qe values are obtained for the organoclays based on AH clay loaded at 2.0 CEC than the ones based on DI clay loaded at 2.0 CEC and only for C14, C16 and 2C12 surfactants at a pH = 12. An opposite effect is observed for C12 surfactant because the C12-DI organoclays yielded high qe values than the C12-AH ones at 2.0 CEC loading level. This result which is obtained in the case of high pH value (pH = 12) seems to be linked to the high ability of AH clay to insert more quaternary alkylammonium cations than the DI clay one, due to the fact that AH clay has a high CEC value than the DI clay. The BPA molecules ionization degree depends on the pH of the aqueous solution and this seems to be another reason for the decrease in the qe values when the pH reached 8 [2, 8, 9]. In fact, BPA molecules remain predominantly in their neutral form when pH ≤ 8, in their mono-ionized form when 8 ≤ pH ≤ 10 (pKa1 = 9.6) and in the di-ionized form when pH ≥ 10 (pKa2 = 11.3). Thus, the adsorption capacity's decrease of the modified clays seen in the case of alkaline pH range can be explained by the electrostatic repulsions existing between the negative charges of the clay's surface and those bisphenolate anions [9]. In the present work, the efficiency of organoclays to sorb BPA in alkaline solution (pH ≥8) is less influenced for organoclays loaded at 2.0 CEC than those ones loaded at 1.0 CEC. This result sustains that the modified clays loaded at 2.0 CEC maintain high affinity toward the mono-ionized and the di-ionized forms of BPA at high pH values. Rathnayake et al. reported a possible retention of surfactant molecules onto the clays surface, which provides sites having hydrophobic character for BPA removal [16**]**. **Figure 9** shows that in the case of a solution with

*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*

pH ≤ 8, 1.0 CEC as loading level is sufficient to get organoclays having high removal capacities for BPA, whereas in the case of a solution with pH value ≥8, 2.0 CEC as loading level is required to get organoclays having high removal capacities toward BPA. Particularly, when the pH reaches 12, we recommend the employment of either 2.0 CEC-C14-AH, 2.0 CEC-C16-AH, 2.0 CEC-2C12-AH organoclays, or 2.0 CEC-C12-DI organoclay.

#### **3.3 Effect of BPA initial concentration on BPA adsorption and isotherms**

#### *3.3.1 Effect of BPA initial concentration on BPA adsorption*

**Figure 10a**-**d** represent the amount of BPA removed as a function of the initial concentration for 12 hours as agitation duration.

It is seen that the quantity of BPA removed from the aqueous solution increases rapidly up to 200 mg/L as initial concentration. **Figure 10a**-**d** show that the equilibrium adsorption capacity of BPA on modified clays increases when the initial concentration increases and this until to the saturation of available reactive sites [11**]**. As shown in **Figure 10a**-**d**, all curves on the effect of initial concentration on BPA adsorption onto the organoclays are almost relatively linear up to 200 mg/L and become parabolic after that value. As a result, the removal of BPA onto the organoclays is affected by its initial concentration. This can be explained by the fact that before 200 mg/L is BPA initial concentration, there are more available active sites onto the organoclays which causes a better elimination rate of BPA. However, the number of active sites decrease gradually after 200 mg/L and this induces less efficient adsorption. As reported in previous studies, the present work shows that the

#### **Figure 10.**

*Initial concentration effect on BPA adsorption by organoclays: C12-AH and C12-DI (a), C14-AH and C14-DI (b), C16-AH and C16-DI (c), 2C12-AH and 2C12-DI (d).*

increase of BPA initial concentration causes an increase in the qe value. For example, for 200 mg as adsorbent dose, when we increase the initial concentration from 25 to 1000 mg/L, the qe values increase from 4.8–4.9 mg/g to 88–127 mg of BPA/g of adsorbent for all the organoclays.

### *3.3.2 Isotherms of BPA adsorption*

The effect of the initial concentration has been used to evaluate the Langmuir and Freundlich isotherms parameters [39] and the obtained values are included in **Table 3**. **Table 3** shows that the determination coefficient (R2 ) values are better in the case of the Langmuir isotherm (0.89 ≤ R2 ≤ 0.99) than the Freundlich isotherm (0.89 ≤ R<sup>2</sup> ≤ 0.95) for all the organoclays and this corresponds to a monolayer adsorption of BPA. The above results resemble to those obtained on BPA adsorption using organically modified montmorillonite [7, 10] and amphoteric modified vermiculite [17].

#### **3.4 Effect of surfactant loading level on BPA adsorption**

The **Figure 11a**-**d** represent the qe values of adsorbed BPA as a function of the surfactant loading level (aqueous solution pH around 5–6). **Figure 11a**–**d** show high elimination rate of BPA onto AH and DI modified clays compared to the untreated ones. This result states that the intercalation of the quaternary alkylammonium into the clays' interlayers induces a strong removal level of BPA from aqueous solution.


#### **Table 3.**

*Isotherm parameters of BPA adsorption on the organoclays.*

*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*

**Figure 11.** *The C12, C14, C16, and 2C12 surfactants loading level effect on the BPA adsorption.*

As a result of the surfactants intercalation, the amount of BPA sorbed per mass of adsorbent (qe) increases remarkably from 1.4 and 2.5 mg/g for DI and AH raw clays, respectively, to qe values higher than 19.5 mg/g in the case of organoclays loaded at 2.0 CEC loading level. We notice that the efficiency of the modified clays to remove BPA from aqueous solution is not significantly affected when the surfactant loading level increases from 1.0 to 2.0 CEC for pH ≤ 8. The below result is unlike those reported by Park et al. who found that the amount of BPA adsorbed increases when the surfactant loading level increases [8]. According to **Figure 11**, a better elimination rate is obtained in acidic solution when the AH and DI raw clays are loaded at least at 1.0 CEC loading level of the C12, C14, C16, and 2C12 surfactants, which result is identical to the those obtained by studying the effect of the pH on the BPA sorption.

#### **3.5 Effect of temperature on BPA adsorption and thermodynamics**

#### *3.5.1 Effect of temperature on BPA adsorption*

The temperature effect on BPA adsorption onto the organoclays was evaluated at different temperatures (303, 313 and 323 K) and the obtained results are presented in **Figure 12a**–**d**.

**Figure 12** shows that the increase of the temperature from 30 to 50°C causes a slight drop of the amount of BPA removed on the organoclays. This result shows that BPA removal is more propitious for lower temperatures. In fact, the ranking of the qe values obtained are from 19.6 and 18.5 mg of BPA/g of organoclays and this occurs when the temperature increases from 30 to 50°C. This is due to the exothermic nature of the adsorption reaction [41, 42]. This means that the temperature affects weakly

**Figure 12.**

*Effect of the temperature on BPA adsorption by the organoclays: C12-AH and C12-DI (a), C14-AH and C14-DI (b), C16-AH and C16-DI (c), 2C12-AH and 2C12-DI (d).*

the uptake of BPA onto the organoclays. The organoclays maintain high removal level toward BPA molecules at a temperature up to 50°C (qe ≥ 18.5 mg/g).

#### *3.5.2 Thermodynamics*

**Figure 12a**-**d** represent the quantities of BPA removed on the organoclays at different temperatures (303, 313, and 323 K). These figures show that the quantity of BPA removed on the organoclays drop weakly with the increase of the temperature from 30 to 50°C and this results means that the adsorption process is favored at lower temperature.

The results obtained by studying the effect of the temperature on BPA removal have been used to calculate the thermodynamic parameters such as Gibbs free energy (ΔG), enthalpies change (ΔH), and entropy change (ΔS) and the calculated values are included in **Table 4**. All enthalpies change values (**Table 4**) are less than zero (ΔH ≤ 0) and this suggests that the uptake of BPA from water using organoclays is exothermic, which is also sustained by the drop of the amount of BPA removed with the increase of the temperature. From the literature, it can be noted that when the values of ΔH are ranged from 2.1 to 20.9, a physisorption is involved in the adsorption process, but chemisorption intervenes for a value ranging from 80 to 200 kJ/mol [43]. In this work, the ΔH values evaluated from the experimental data of BPA sorption onto the organoclays are ranged from 76.5 to 13 kJ/mol. According to the ΔH values, different forces and interactions are supposed to be involved in the adsorption process [19]: between 4 and 10 kJ/mol for Van der Waals interactions, about 5 kJ/mol for hydrophobic interactions, between 2 and 40 kJ/mol for hydrogen bond


*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*


#### **Table 4.**

*Thermodynamic constants of BPA adsorption on the modified clays.*

interactions, around 40 kJ/mol for coordination exchange interactions, between 2 and 29 kJ/mol for dipole bond, and up to 60 kJ/mol for chemical bond. All ΔH values obtained in the present work are ranged from 76.5 to 13 kJ/mol and this suggests that the following interactions can intervene during the adsorption process: dipolebonding, coordination exchange, hydrogen bonding and chemical bond. From the lower adsorption efficiency of the raw clays toward BPA molecules, we suggest that the hydrogen bonding contributes weakly to the BPA sorption in the present work [11]. From the obtained results, it can be postulated that both physisorption (Van der Waals forces, hydrogen bonding) and chemisorption (hydrophobic interactions, BPA phenolic rings and alkylammonium group interactions) intervene during BPA sorption onto the organically modified clays. The results obtained in the present work resemble to those reported previously on the adsorption of Rhodamine-B onto activated carbon [44]; adsorption of Acid Red G dye onto C18-montmorillonite [45], adsorption of carbofuran onto biochars [46]. According to previous reported results, adsorption can be due to the intervention of both physisorption and chemisorption and this coexistence is described as the following: (i) a first rapid chemisorption followed by a slow physisorption [47]; (ii) contributions of both processes [48]; (iii) simultaneous occurring of both physisorption and chemisorption with predominant chemisorption [46] or without any predominant process [49].

**Table 4** indicates that ΔG ≤ 0 and this suggests that the uptake of BPA molecules by the organoclays is spontaneous. The drop of the ΔG values with the increase of the temperature shows that the capacity of the organoclays to sorb BPA molecules is inversely dependent on the temperature [9, 19, 50], which also indicates a spontaneous process at low temperatures. In addition, ΔG decreases with the increase of the temperature (**Table 4**) and this shows that BPA sorption is thermodynamically favored at room temperature than at higher temperatures [51]. Gibbs free energy ranging from 20 to 0 kJ/mol corresponds to physisorption mechanism, while values

*Solid-State Synthesis of Organoclays: Physicochemical Properties and Application… DOI: http://dx.doi.org/10.5772/intechopen.107503*

ranging from 80 to 400 kJ/mol suggest chemisorption mechanism [52]. In the present work, the Gibbs free-energy values are ranged from 11 to 6 kJ/mol (**Table 4**) and this result means that the physisorption intervenes in BPA removal from water. According to previous literature results, when ΔS value is higher than zero (ΔS > 0) the interactions at the solid–liquid interface are aleatory, while value less than zero (ΔS < 0) suggests inverse interactions [50, 53]. In the present work, the ΔS values are all negative (30 ≤ ΔS ≤ -205 J/mol) and this suggests a decrease of the disorder at the solid–liquid interface, while ΔG and ΔH negative values indicate spontaneous and exothermic processes of BPA sorption onto the organoclays [8].

Our study highlights the fact that the adsorption is spontaneous (ΔG < 0), exothermic (ΔH < 0) with negative entropy values (ΔS < 0) and these results are in agreement with those previously reported on the adsorption of BPA using organoclays [7, 8, 10], C16-modified palygorskite [19], and surfactant-modified zeolite [22, 54].

### **4. Conclusion**

The solid-state reaction methodology was successfully used to intercalate three cationic alkyltrimethylammoniums (n-C12H25(CH3)3 N+ , n-C14H29(CH3)3N+ and n-C16H33(CH3)3N<sup>+</sup> ) and one di-alkyldimethylammonium ((n-C12H25)2(CH3)2N+ ) into two natural clays containing Ca-montmorillonite as main clay type. XRPD shows that the d001 basal spacing increases after the intercalation of the cationic surfactants and this proved the swelling of the clays layers. It is observed that at 2.0 CEC as surfactant loading level, the basal spacing (d001) of the prepared organoclays are 25 Å and 32 Å for those treated with C12 and C14 surfactants respectively and up to 38 Å for those treated with C16 and 2C12 surfactants. The appearance of the CH2 symmetric and asymmetric stretching bands on the IR spectra at around 2850 and 2920 cm<sup>1</sup> , respectively, proves successful insertion of the surfactants into clays interlayers. The values of the frequencies related to the CH2 symmetric and asymmetric stretching bands of the surfactants in organoclays diminish and tend to those of the pure surfactants when the surfactant loading level increases. The hydrophilic and organophobic properties of the raw clays change to hydrophobic and organophilic ones upon the insertion of the surfactants into the clays interlayers. The present work shows that the organoclays synthetized by using AH and DI local clays lead to a high removal efficiency toward BPA molecules in aqueous solutions. In fact, higher values of qm (from 88 to 127 mg/g mg/g) were achieved for BPA adsorption onto organoclays based on AH and DI clays and n-C12H25(CH3)3N<sup>+</sup> , n-C14H29(CH3)3N+ , n-C16H33(CH3)3N+ and (n-C12H25)2(CH3)2N+ surfactants. The BPA removal was influenced by the adsorbent dose, the initial concentration, and the agitation time, the surfactant loading level, the pH and the temperature. The organoclays efficiencies to sorb BPA molecules from water decrease significantly when the pH value reached 10. It is also noted that when the level of surfactants is 2.0 CEC, the efficiency of the organoclays to sorb BPA is less influenced in alkaline solution than the one of those loaded at 1.0 CEC. The present work indicates that organoclays loaded at 1.0 CEC level are efficient for BPA removal from acidic solution, and those loaded at 2.0 CEC level are useful to achieve high elimination rate of BPA in alkaline solution. The results obtained from the thermodynamic and kinetic investigations show that both physisorption and chemisorption intervene in the retention of BPA species. The present work strongly sustains that organoclays prepared via solid-state intercalation are effective adsorbents as those obtained via solid-liquid reactions for BPA removal from water.
