Comparison between Asymmetric and Symmetric Gemini Surfactant

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12927−12938

pubs.acs.org/IECR

Comparison between Asymmetric and Symmetric Gemini Surfactant-Modified Novel Organo-vermiculites for Removal of Phenols Gaili Cao, Manglai Gao,* Tao Shen, Bingbing Zhao, and Hao Zeng State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, PR China

Downloaded via BUFFALO STATE on July 28, 2019 at 17:47:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: To study the effect of the asymmetry of gemini surfactants on the adsorption characteristics of organovermiculites (OVts), [C 1 4 H 2 9 (CH 3 ) 2 N + (CH 2 ) 6 N + (CH3)2C14H29]Br2 (termed 14-6-14) and [C22H45(CH3)2N+(CH2)6N+(CH3)2C6H13]Br2 (termed 22-6-6) with the same number of carbon atoms were designed to modify vermiculite for removing p-chlorophenol (PCP), phenol (P), and pmethylphenol (PMP) from aqueous solution. The resulting OVts were investigated by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetry and differential thermogravimetry, scanning electron microscopy, and zeta potential analysis. Factors affecting the adsorption capacities of OVts have been studied. The results indicated that the adsorption mechanism of phenols on both OVts was similar. The adsorption capacities of phenols on 14-6-14-Vt were slightly higher than those on 22-6-6-Vt, and the retention of phenols onto the two OVts followed the order PCP > PMP > P. Hydrophobic, electrostatic, hydrogen bond, and special interactions (cation−π and π−π) played positive roles in the adsorption of phenols onto OVts. The spent OVts were easy to regenerate with ethanol for at least three cycles. cannot be ignored.17,20 Therefore, vermiculite is selected as a precursor in this work. Surfactants are providing revolutionary advances in the territory of cleaning off organic pollutants using clay minerals; in particular, gemini surfactants possess superior properties in comparison to conventional surfactants and are employed in modifying the inherent limitations of clays to enhance their affinity for organic contaminants.20−22 Besides a massive amount of research on symmetric gemini surfactants, the considerable focus has also been on asymmetric gemini surfactants equipped with two alkyl chains of different lengths.23 The most investigated type of such dissymmetric surfactants is still the one with the general formula [CmH2m+1(CH3)2N+(CH2)sN+(CH3)2CnH2n+1]Br2 (m and n represent chain length of the two alkyl chains, and s indicates the length of the spacer).24 Studies have shown that asymmetric gemini surfactants have many advantages over symmetric surfactants, such as lower cmc, stronger solvophobic effect and hydrophobic interactions, and higher polydispersity.25−27 This asymmetric gemini surfactant is mainly used as a structuredirecting agent in the synthesis of zeolites because of its special structure.28,29 However, there is hardly any research about the use of asymmetric bisquaternary ammonium salt in modification

1. INTRODUCTION Phenols are double-edged swords: they bring benefits to humans as a raw material for many products, but they can also give rise to protein and tissue degeneration and even be life threatening.1,2 Phenols are regarded as priority pollutants by the US Environmental Protection Agency (EPA) because they are harmful to living organisms even at lower concentrations.1,3−5 Phenol, p-chlorophenol, and p-methylphenol are toxic phenolic compounds; the presence of them poses a risk to aquatic organisms and plants.5,6 Therefore, the removal of them from aquatic systems is extremely urgent. Decontaminating phenolcontaining sewage calls for an array of methods such as oxidation,6 microbial and photocatalytic degradation,7,8 solvent extraction,9 ion exchange,10 and adsorption;10−13 of these methods, adsorption is one of the most universal and widely used approaches for the removal of phenols.13,14 In recent years, vermiculites as adsorbents have received increased attention in the field of removing organic pollutants because of the advantages of operational cost, high efficiency, eco-friendliness, nontoxicity, and abundance of reserves.15 Organo-vermiculites modified by a surfactant acquire remarkable properties, such as stronger hydrophobicity, wider interlayer spacing, and higher adsorption capacity.16−18 It is worth noting that the preparation method of organo-vermiculite is simple and that the availability of the surfactant is high.17,19 Moreover, easy regeneration is also an important advantage that © 2019 American Chemical Society

Received: June 3, 2019 Accepted: June 27, 2019 Published: June 27, 2019 12927

DOI: 10.1021/acs.iecr.9b02997 Ind. Eng. Chem. Res. 2019, 58, 12927−12938

Article

Industrial & Engineering Chemistry Research

Figure 1. Chemical structures of 22-6-6 and 14-6-14 and the arrangement of modifiers between layers of vermiculite.

Figure 2. FT-IR spectra of 22-6-6 and 14-6-14 (a) and of Vt and two OVts (b).

2. MATERIALS AND EXPERIMENTAL METHODS

of vermiculites, not to mention the comparison between organovermiculites modified by asymmetric and symmetric gemini surfactants. Bisquaternary ammonium salts [C14H29(CH3)2N+(CH2)6N+(CH3)2C14H29]Br2 (termed 14-6-14) and [C22H45(CH3)2N+(CH2)6N+(CH3)2C6H13]Br2 (termed 22-6-6), with the same number of carbon atoms, are designed to modify vermiculite for investigating the effect of the asymmetry of both on retention of p-chlorophenol (PCP), phenol (P), and p-methylphenol (PMP) from aqueous solution. The structures of organo-vermiculites (OVts) were revealed by elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis (TG), and the morphology of OVts was observed by scanning electron microscopy (SEM). Batch experiments were carried out under varying dosages of surfactant, concentration of contaminants, time, temperature, and pH. In addition, the kinetic and isotherm equations inextricably linked to the adsorption mechanism of PCP, P, and PMP were studied. Of particular significance in this study are the low-cost, renewable, and eco-friendly adsorbents for the removal of phenols from aqueous solutions.

2.1. Materials. The vermiculite (Vt) material used as the precursor was purchased from Sigma-Aldrich and needed to be further made into homoionic-Vt according to previous research.19 Analytically pure p-chlorophenol (PCP), phenol (P), and p-methylphenol (PMP) used as adsorbates were purchased from J&K Chemical Ltd. 1-Bromodocosane, N,N,N′,N′-tetramethyl-1,6-diaminohexane, 1-bromohexane, and 1-bromotetradecane were acquired from AiLan (Shanghai) Chemical Technology Co., Ltd. Toluene, acetonitrile, anhydrous ether, n-butanol, absolute ethanol, and acetone were purchased from Beijing Tongguang Fine Chemical Co., Ltd. Deionized water (18 MΩ·cm) was employed throughout the experiment. [C14H29(CH3)2N+(CH2)6N+(CH3)2C14H29]Br2 (14-6-14) and [C22H45(CH3)2N+(CH2)6N+(CH3)2C6H13]Br2 (22-6-6) were synthesized according to the research by Zana et al.30 and Choi et al.,28 respectively. Their chemical structures are shown in Figure 1. In addition, the purity and structure of the obtained modifiers were detected by 1H NMR, EA, and FT-IR. The results are demonstrated in Table S1 (Supporting Information) and Figure 2a. 12928


Article


vibration of Si−O.34 The absorption peaks at 2924 and 2860 cm−1 are assigned to antisymmetric and symmetric stretching vibrations of C−H in the surfactants; the bending vibration of C−H is responsible for the band at 1470 cm−1.31,35 They provided conclusive evidence for the successful organic modification of Vt.36 3.2.2. Elemental Analysis. The elemental analysis data of prepared OVts tabulated in Table 1 indicate the increased

2.2. Preparation of 22-6-6-Vt and 14-6-14-Vt. The preparation of 22-6-6-Vt and 14-6-14-Vt was performed according to research by Zang and colleagues.19 A certain amount of surfactants (0.2, 0.4, 0.6, 0.8, and 1.0 cationic exchange capacity (CEC) of Vt) were added in a stuffed conical flask containing 50 mL of water, and the flask was vibrated for 1 h in a 60 °C water bath shaker. To this mixture was added 1 g of Vt, and the reaction was allowed to continue for 3 h under the same conditions. The products were centrifuged, washed several times, dried at 80 °C overnight, and grounded to pass a 200 mesh sieve. The resulting products were tagged as 22-6-6-Vt and 14-6-14-Vt, respectively. 2.3. Characterization. Characterizations of modifiers and organo-vermiculites (OVts) were performed on a FT-IR spectrometer (Nicolet Magna 560 E.S.P), scanning from 4000 to 400 cm−1, and the resolution was 4.0 cm−1. An X-ray diffractometer (XRD, Shimadzu XRD-6000, Cu Kα radiation, 40 kV and 40 mA) was engaged to uncover the structural characteristics of OVts, scanning from 0.5° to 10° in 2θ. The synthesized surfactants were identified by 1H NMR (CDCl3 solution, ECA 600 MHz Spectrometer). An elemental analyzer (Vario MACRO cube), a scanning electron microscope (model SU 8010 from Japan), and a TG/DTG STARe system (30−800 °C, 10 °C min−1) from Mettler Toledo and Zetasizer Nano ZS were used to determine the composition, morphology, and zeta potentials of OVts. 2.4. Adsorption and Desorption Experiments. The adsorption experiments of PCP, P, and PMP on OVTs were executed with batch experiments with the following the steps: 50 mL of phenols solution and 0.05 g of OVts were mixed and then reacted for 3 h in a water bath oscillator with an oscillation speed of 200 r/min; the reaction mixtures were centrifuged, and the concentration of the obtained supernatant was measured by an ultraviolet−visible spectrophotometer at λmax = 280, 270, and 277 nm. The regeneration ability of OVts was investigated by soaking the spent adsorbents in ethanol at 25 °C for 1 h. The resorption processes were exactly the same as the adsorption experiment.

Table 1. Elemental Analysis of 22-6-6-Vt and 14-6-14-Vt element

Vt

22-6-6-Vt

14-6-14-Vt

C H N

0.61 1.3 0.1

13.78 3.01 0.78

15.20 3.45 0.89

organic carbon content in both OVts (from 0.61% to 13.78% for 22-6-6-Vt and from 0.61% to 15.52% for 14-6-14-Vt). Furthermore, the loading of 22-6-6 on Vt (0.21 mmol/g) is lower than that of 14-6-14 (0.24 mmol/g), which may be because 22-6-6 is more hydrophobic than 14-6-14 so that its affinity for hydrophilic Vt is lower.37 3.2.3. XRD Analysis. FT-IR does not provide detailed information on structural geometry of the surfactant between clay layers, but this can be done by XRD. XRD can also provide the structure configuration of surfactant intercalation. The XRD patterns of the original vermiculite, 22-6-6-Vt, and 14-6-14-Vt are shown in Figure 3. In Figure 3a, only one peak is observed at 1.12 nm for the untreated vermiculite, and the d001 layer spacing of 22-6-6-Vt increases from 1.12 to 3.75 nm and reaches a platform at 4.16 nm with a shoulder at 3.09 nm when the level of 22-6-6 is 0.4 CEC, indicating that the arrangement of 22-6-6 transforms from the lateral bilayer (with an angle of 30° or 22° to the silicate plane) to the coexistence of lateral bilayer and dense paraffin-type bilayer (with an angle of 34° or 25° to the silicate plane).38 However, in Figure 3b, the basal spacing of 14-6-14-Vt migrates from 1.12 to 3.84 nm and stabilizes at 4.01 nm (0.4 CEC), which is attributed to the arrangement of 14-6-14 between the layers of vermiculite converting from the loose paraffin bilayer to a dense one. In addition, the angle between the alkyl chains of 14-6-14 and the silicate plane may change from 47° to 50°. Furthermore, the small shoulders appeared at 2.86 nm because of the loose paraffin bilayer.39 The difference between the configurations of the two surfactants might be ascribed to the fact that the alkyl chain on one side of 22-6-6 is so long that it extends a larger layer spacing compared to 14-6-14. The enlarged layer spacing of OVts proves that surfactants have been successfully inserted into the vermiculite layer. 3.2.4. TG-DTG Analysis. The thermodynamic stabilities of Vt and OVts were investigated by TG-DTG. Figure 4a depicts the relationship between the mass loss of raw and modified vermiculite and temperature. Inspection of Figure 4b indicates that the decomposition of modified vermiculite is mainly divided into three steps. The mass loss below 100 °C is rooted in the adsorbed water on/in particles; mass losses around 250 and 403.67 °C are caused by physically adsorbed and intercalated surfactants, respectively.40,41 Specifically, the peak of OVts below 100 °C is significantly smaller than that of Vt, implying that there are fewer water molecules in the organo-vermiculites, which is explained by the hydrophilic silicate surface being converted into a hydrophobic one by modifiers.31 Moreover, the mass loss of OVts in the second stage is significantly more than that in other stages, implying that both surfactants mainly exist

3. RESULTS AND DISCUSSION 3.1. Characterization of Surfactants. The structural formulas of the modifiers used are shown in Figure 1. The FTIR spectra of surfactants presented in Figure 2a reveal that in all case the spectra are highly similar to each other because they are provided with analogous constitution and architecture. Two bands emerged at 2920 and 2850 cm−1 born of the antisymmetric and symmetric stretching vibration of C−H; the band of the bending vibration of C−H is detected at 1470 cm−1.31 It is worth noting that the peaks at 3440 and 1630 cm−1 belong to water molecules.32 The elemental analysis and 1H NMR results of 22-6-6 with 14-6-14 are listed in Table S1 of the Supporting Information. The elemental analysis results corroborate that the experimental values are close to the calculated values. The results of 1H NMR, EA, and FT-IR reveal that 22-6-6 and 14-6-14 were successfully prepared. 3.2. Characterization of OVts. 3.2.1. FT-IR Analysis. The FT-IR results of crude and organo-vermiculites are presented in Figure 2b. As shown in Figure 2b, the characteristic bands of Vt emerged at 3710 and 3684 cm−1 are attributed to the stretching vibration of Si−O−H, and the band at 3420 cm−1 corresponds to the stretching vibration of H−O−H.33,34 Moreover, the bending vibration of H−O−H is observed at 1650 cm−1. The sharp peak appearing at 994 cm−1 arises from the bending 12929


Article


Figure 3. X-ray diffraction patterns of two OVts.

Figure 4. TG and DTG curves of Vt and two OVts.

PCP, P, and PMP have been systematically studied. As shown in Figure 5, the adsorption capacities of 22-6-6-Vt keep increasing as the surfactant dosage changes from 0.2 to 0.4 CEC for PCP and PMP, from 0.2 to 0.6 CEC for P, then descending when surfactant feeding exceeds 0.4 CEC for PCP and PMP, 0.6 CEC for P. However, the sorption of PCP, P, and PMP onto 14-6-14Vt increases when the surfactant dosage is less than 0.8 CEC and reduces when it is greater than 0.8 CEC. From the results of XRD, the clay layers are significantly expanded as the surfactant increases, promoting the adsorption of phenols. However, surfactants beyond the saturated concentration may make them densely stacked, block smaller holes of OVts, and occupy the adsorption sites existing on OVts, inhibiting retention of phenols.39 On the other hand, from the viewpoint of the

in the interlayer or on the surface of OVts by physical adsorption. The mass loss of 22-6-6 (15.95%) is lower than that of 14-6-14 (16.30%), fitting well with the results of elemental analysis. TG-DTG analysis demonstrates that the surfactants have been located on/in the vermiculites. 3.2.5. SEM Analysis. SEM images show a typical lamellar curl morphology of phyllosilicate for Vt (Figure S1).42 The surface of the organically modified vermiculite becomes rough, broken, and loose, which provides a larger place for the retention of contaminant molecules. The phenomenon affords intuitive evidence to the successful insertion of gemini surfactants into the vermiculite layer. 3.3. Adsorption Experiments. 3.3.1. Effect of Surfactant Addition. The retention capacities of the obtained OVts for 12930


Article


Figure 5. Effects of the concentration of modifiers on PCP (a), P (b), and PMP (c) adsorption by OVts (C0 = 100 mg L−1, V = 50 mL, adsorbent mass = 0.05 g, T = 25 °C).

Figure 6. Effects of contact time on the removal of PCP (a), P (b), and PMP (c) onto two OVts (C0 = 200 mg/L, V = 50 mL, adsorbent mass = 0.05 g, T = 25 °C).

solution affects surface properties of adsorbents and the ionization/disassociation of phenols. On the one hand, the Si−OH on the surface of adsorbents is partially or completely deprotonated with the increasing pH, leading to a decrease in the positive charge and an increase in the negative charge on the surface, which is corroborated by zeta potentials (Table S2).43,44 On the other hand, phenols are popular in neutral or protonated form when pH < pKa of PCP, P, and PMP (9.37, 9.89, and 10.26, respectively) and in their anionic form at pH > pKa.5 As pH shifts from 2 to 6, the adsorption capacities increase, mainly depending on the partition process;39 furthermore, at the lower pH, the more H+ in the solution may reduce the density of electron clouds formed by the conjugate of the benzene ring and the hydroxyl group in phenols, causing cation−π interactions between the protonated Si−OH or the positively charged surfactant and phenols to weaken. In addition, the lower the pH, the stronger the electrostatic repulsion between protonated phenols and the positively charged OVts.45 As pH increases to 10, the adhesion capacities of PCP, P, and PMP decline, and the least adsorption capacities are found at pH 10 because of the forming of electrostatic repulsion between the negatively

molecular structures of the adsorbates themselves, the steric hindrance with P is smaller than PCP and PMP, which may be the reason why the adsorption capacity of P begins to decrease at 0.6 CEC (Figure 5b). From 0.4 to 0.6 CEC, the adsorption capacity of P increases by only 7.10%, and adsorption capacities of PCP, P, and PMP onto 14-6-14-Vt go up by only 5.45%, 8.99%, and 0.32% as the modifier amount changes from 0.4 to 0.6 CEC, respectively. For the sake of saving costs, 0.4 CEC was selected as the best dose of modifiers. 3.3.2. Effect of Contact Time. It is shown in Figure 6 that both adsorbents exhibit rapid adsorption characteristics within 15 min then slow until the adsorption equilibrium (about 30 min) is reached, which may be attributed to the fact that the adsorption site of the adsorbent is occupied by phenols with time on-stream. The reaction time was set to 180 min to ensure that the adsorption reached equilibrium in subsequent experiments. 3.3.3. Effect of Solution pH. As shown in Figure 7, the adsorption capacities of three phenols increase when the pH increases from 2 to 6 and decrease as pH goes beyond 6. The reason for the change in adsorption capacities is that pH of the 12931


Article


Figure 7. Effects of pH on the removal of PCP (a), P (b), and PMP (c) onto two OVts (C0 = 200 mg/L, V = 50 mL, adsorbent mass = 0.05 g, T = 25 °C).

Table 2. Kinetic Parameters for Adsorption of PCP, P, and PMP on OVtsa pseudo-first-order

pseudo-second-order

adsorbent

adsorbate

qe,exp

qe,cal

k1 (×10−2)

R2

qe,cal

k2 (×10−2)

R2

22-6-6-Vt 22-6-6-Vt 22-6-6-Vt 14-6-14-Vt 14-6-14-Vt 14-6-14-Vt

PCP P PMP PCP P PMP

84.39 25.50 58.01 87.66 30.37 62.65

14.39 18.83 19.32 8.43 13.64 15.54

0.96 0.28 0.49 1.38 0.39 0.79

0.6469 0.5655 0.7227 0.5144 0.4422 0.6750

84.67 25.56 58.21 87.80 30.49 62.85

2.17 4.12 2.08 3.97 5.87 2.18

0.9999 0.9998 0.9998 1.0000 0.9999 0.9999

C0 = 200 mg/L, V = 50 mL, adsorbent mass = 0.05 g, T = 25 °C; qe,exp (mg g−1); qe,cal (mg g−1); k1 (min−1); k2 (g mg−1 min−1).

a

charged adsorbates and adsorbents.31 Electrostatic repulsion between phenolic anions adsorbed on OVts and free phenolic anions in solution also reduces adsorption capacities of OVts; in addition, dissociation of phenols brings about their poor hydrophobicity, leading to a decrease in hydrophobic interaction between adsorbates and alkyl chains of surfactants.46 Hence, pH 6 was selected as the optimum pH for the adsorption experiment. 3.4. Adsorption Kinetics. Adsorption kinetics such as pseudo-first-order and pseudo-second-order were investigated to set forth the adsorption mechanism of phenols on adsorbents, as expressed by the following equations:46,47

pseudo-second-order model with high correlation coefficients (R2 > 0.999). 3.5. Adsorption Isotherms. Langmuir and Freundlich isotherm are summarized in Table S3 for further study of the adsorption mechanism of three phenols onto 22-6-6-Vt and 146-14-Vt. Figure 8 shows the equilibrium adsorption isotherms of PCP, P, and PMP onto OVts fitted by the Langmuir model. Equations for the two isotherms are shown below:5,47 Langmuir isotherm: Ce C 1 = + e qe KLqm qm

pseudo-first-order: ln(qe − qt ) = ln qe − k1t

RL =

(1)

pseudo-second-order: t 1 1 = + t qt qe k 2qe 2

1 1 + KLC0

(3)

(4)

Freundlich isotherm: ln qe = ln KF +

(2)

where qt and qe represent the amounts of phenols adsorbed at time t and equilibrium, respectively; k1 (min−1) and k2 (g (mg min)−1) stand for rate constants. The degree of fit between the experimental data and calculated model is portrayed by the correlation coefficient (R2). Models with higher values are more suitable for depicting phenol adsorption kinetics. Parameters tabulated in Table 2 reveal that in all cases the adsorption processes conform to the

1 ln Ce n

(5)

where Ce (mg/L) is the concentration of adsorbate at equilibrium; qe (mg/g) symbolizes the equilibrium adsorption capacity; qm (mg/g) is the largest monolayer adsorption capacity of phenols in theory; KL (L/mg) represents the Langmuir equilibrium constant which is related to affinity of binding sites and energy of adsorption; RL is defined as the separation factor. KF (mg/g) and n belong to Freundlich constants. Related parameters listed in Table S3 manifest that the adsorption of 12932


Article


Figure 8. Equilibrium adsorption isotherms of PCP, P, and PMP on OVts fitted Langmuir model (V = 50 mL, adsorbent mass = 0.05 g).

capacities of three phenols are consistent with this order: PCP > PMP > P. Electrostatic interaction mainly determined by the pH of the solution has been discussed in detail in the section Effect of Solution pH. In acidic or basic solutions, electrostatic repulsion dominates rather than hydrophobic interactions. Therefore, the adsorption capacities of PCP, P, and PMP under acidic or alkaline conditions is less than that at pH 6. In addition, the cation−π interactions50 between cationic surfactants or protonated Si−OH and the electron-rich benzene ring in phenols are beneficial to the adsorption of phenols on OVts. The electron cloud density on the benzene ring decreases in the order PMP > P > PCP, as chlorine is an electronwithdrawing group and methyl is an electron-donating group,5 also making the cation−π interactions between cationic surfactants or protonated Si−OH and PMP stronger than that of PCP and P. This does not match their adsorption capacities, indicating that the cation−π interaction is weaker than the hydrophobic interaction.

phenols on two organo-vermiculites is consistent with the Langmuir isotherm with high correlation coefficients in all experiment (R2 > 0.99), implying that the adsorption of the three phenols on the two organo-vermiculites are monolayer. It is worth noting that the value of RL is always less than unity, indicating that all adsorption processes are favorable.48 3.6. Adsorption Mechanism. The adsorption of phenols onto OVts can be achieved by hydrophobic interaction, electrostatic interaction, and special interaction.5,45 22-6-6 and 14-6-14 are cationic surfactants with long alkyl chains and can form a hydrophobic environment on surfaces or between layers of vermiculite, providing an organic phase for phenols. Moreover, the hydrophobicity of phenols should also be considered, and the stronger the hydrophobicity, the stronger its affinity for organo-vermiculite.45 The solubilities of PMP, PCP, and P are 23, 27, and 93 g L−1 at 25 °C, respectively.49 However, the large steric effect of PMP inhibits it from entering the interlayer of OVts to some extent.5 Therefore, the retention 12933


Article


Figure 9. FT-IR spectra (a and b), TG-DTG curves (c and d), and XRD patterns (e and f) of spent OVts.

The hydrogen bond between the adsorbates and between the Si−OH in OVts and the hydroxyl group in phenols also promotes the adsorption of phenols. Furthermore, in order to further understand the adsorption mechanism, the characterizations of the spent OVts were

revealed by FT-IR, XRD, and TG-DTG. In FT-IR spectra of the spent OVts (Figure 9a,b), the peaks at 1490 cm−1 (Figure 9a) and 1498 cm−1 (Figure 9b) are related to the overlapped effect of the bending vibration of C−H and C=C, which are obviously stronger than that of the OVts, indicating that π−π interaction 12934


Article


Figure 10. Plots of ln K versus 1/T for adsorption of PCP (a), P (b), and PMP (c) on OVts.

Table 3. Thermodynamic Parameters for the Adsorption of PCP, P, and PMP on OVtsa ΔG° (kJ mol−1) adsorbent

phenols

298 K

308 K

318 K

328 K

ΔS° (J mol−1 K−1)

ΔH° (kJ mol−1)

22-6-6-Vt 14-6-14-Vt 22-6-6-Vt 14-6-14-Vt 22-6-6-Vt 14-6-14-Vt

PCP PCP P P PMP PMP

8.34 9.07 15.01 15.28 12.78 13.05

9.22 9.66 15.95 16.07 13.82 13.69

9.86 10.17 16.78 17.01 14.69 14.34

10.73 10.60 17.81 17.85 15.77 14.98

−78.13 −51.22 −92.43 −86.39 −98.33 −64.29

−14.93 −6.17 −12.56 −10.50 −16.53 −6.12

a

V = 50 mL, adsorbent mass = 0.05 g.

between phenols also exists in the adsorption process.17 In Figure 9c,d, significant differences between the TG-DTG curves of the OVts and that of the spent OVts are that the peaks at 258 °C for 22-6-6-Vt and 253.67 °C for 14-6-14-Vt migrate to a higher or lower temperature, which indicates that new interactions are involved in the adsorption process, which are most likely the π−π interactions between phenols17,51 or cation−π interactions between cationic surfactants or protonated Si−OH and electron-rich benzene ring in phenols. It can be clearly seen from Figure 9e,f that the interlayer spacing of the spent OVts is significantly enlarged, indicating the presence of π−π interactions and cation−π interactions within OVts.17,51 Therefore, hydrophobic, electrostatic, cation−π, hydrogen bond, and π−π interactions play positive roles in the adsorption of phenols onto OVts. 3.7. Effect of Surfactant Structure. Under the same experimental conditions, the adsorption capacities of 14-6-14-Vt for phenols are stronger than that of 22-6-6-Vt (Figure 7), which may be due to the difference in structure and arrangements of the surfactants. The organic carbon availability of 22-6-6 (3.42, 2.58, and 1.57 for PCP, PMP, and P, respectively) is higher than that of 14-6-14 (3.19, 2.30, and 1.48 for PCP, PMP, and P, respectively), implying that the interactions between adsorbates and 22-6-6 are stronger than that between adsorbates and 14-614.17 However, the adsorption capacity of 22-6-6-Vt is slightly lower than that of 14-6-14-Vt. The regions where the longer alkyl chains are aggregated may be more hydrophobic than that of the shorter alkyl chains aggregated in the interlayer of 22-6-6Vt (Figure 1), which may cause an inhomogeneous hydrophobic

environment, resulting in its adsorption capacity being lower than that of 14-6-14-Vt. In addition, an alkyl chain of 22-6-6 is so small that it may easily embed in the tunnel of OVts, blocking it and hindering the diffusion of adsorbate molecules. Although the layer spacing of 22-6-6-Vt is larger than that of 14-6-14-Vt, the amount of 22-6-6 inserted into the vermiculite layers is inferior to that of 14-6-14 according to the results of XRD and elemental analysis, which may lead to a reduction in hydrophobic interaction between the surfactant molecules and the adsorbates. 3.8. Adsorption Thermodynamics. The energy variations of the adsorption process were calculated by thermodynamic parameters (ΔG°, ΔH°, and ΔS°), being calculated from the adsorption experiments at various temperatures using the equations below:52,53 ΔS° ΔH ° − R RT

(6)

ΔG° = ΔH ° − T ΔS°

(7)

ln KL =

where KL (L mol−1) is the Langmuir constant;52−54 R (8.314 J mol−1 K−1) is the gas constant; T (K) is the reaction temperature; the values of ΔH° and ΔS° can be computed from the slope and intercept of the linear plot of ln KL versus 1/T in Figure 10. Table 3 summarizes the thermodynamic parameters at various temperatures. It is observed from Table 3 that ΔG° values at all temperatures are positive; as the temperature increases, the ΔG° values are more positive, indicating that the adsorptions of PCP, 12935


Article


Figure 11. Reusability of organo-Vts for PCP (a), P (b), and PMP (c) adsorption.

caused by the structure of surfactants as well as the hydrophobicity of phenols. Hydrophobic, electrostatic, hydrogen bond, and special interactions (cation−π and π−π) played positive roles in the adsorption of phenols onto OVts. Furthermore, adsorption kinetics and isotherm models show that the adsorption processes of PCP, P, and PMP on both OVts conform to the pseudo-second-order and Langmuir models. The adsorption behaviors occurring on the adsorbents are nonspontaneous and exothermic, and the degree of disorder in essence decreases. The spent OVts are easy to regenerate with ethanol for at least three cycles. The results obtained provide low-cost, renewable, and environmentally friendly adsorbents for the removal of phenols from aqueous solutions.

P, and PMP on 22-6-6-Vt and 14-6-14-Vt are nonspontaneous and more occur with more difficulty at higher temperatures. The negative ΔH° and ΔS° indicate that the adsorption processes are exothermic and randomness is reduced. Additionally, the absolute value of ΔH° is less than 40 kJ mol−1, indicating the adsorptions of phenols onto OVts are physical process.39 As can be seen from Figure 10 and Table 3, the adsorption of 22-6-6-Vt for PCP, P, and PMP is more susceptible to temperature than that of 14-6-14-Vt. 3.9. Desorption Tests. Addressing environmental issues needs to be carried out at low cost and with low energy consumption. Recyclability is desirable because it affects costs to a certain extent. 22-6-6-Vt and 14-6-14-Vt can be washed with ethanol to achieve regeneration. The desorption tests certify that OVts can be regenerated at least three times. As shown in Figure 11, the adsorption capacities of 22-6-6-Vt toward PCP, P, and PMP decrease from 84.48 to 74.36 mg g−1, 27.59 to 23.77 mg g−1, and 56.69 to 47.09 mg g−1 and those of 14-6-14-Vt decrease from 88.47 to 77.72 mg g−1, 31.42 to 26.89 mg g−1, 59.00 to 50.07 mg g−1 for three cycles, respectively. The results indicate that the new binding sites are produced on the surface or interlamination of the organo-vermiculites while the original binding sites are lost partially during the regeneration process, and the OVts are stable during regeneration.17

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02997.

■

Elemental analysis, 1H NMR analysis data, SEM images, and zeta potentials (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 89733680. Fax: +86 10 69727907. E-mail: [email protected].

4. CONCLUSION Novel organo-vermiculites modified by asymmetric and symmetric bisquaternary ammonium salts with the same number of carbon atoms were compared and used to remove PCP, P, and PMP from aqueous solution for the first time. The arrangement of 14-6-14 is a coexistence of dense and loose paraffin-type bilayers while that of 22-6-6 is a coexistence of lateral bilayer and dense paraffin-type bilayer in the vermiculite layers when the dosages of surfactants are 0.4 CEC. The results of EA and TG-DTG show that the load of 14-6-14 is more than 22-6-6, which may be because 22-6-6 is more hydrophobic than 14-6-14. The results indicate that the adsorption mechanism of phenols on both OVts is similar, including hydrophobic, electrostatic, cation−π, and π−π interactions and hydrogen bonds. Adsorption equilibrium is reached within 30 min for phenols onto OVts. However, the adsorption of PCP, P, and PMP on 14-6-14-Vt is stronger than that on 22-6-6-Vt, and that of them onto two OVts follows the order PCP > PMP > P (98.41, 59.67, and 33.50 mg/g on 14-6-14-Vt and 92.34, 58.68, and 31.06 mg/g on 22-6-6-Vt for PCP, PMP, and P, respectively). The difference in adsorption capacity may be

Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Grant No. 21776306). REFERENCES

(1) Yousef, R. I.; El-Eswed, B.; Al-Muhtaseb, A. H. Adsorption characteristics of natural zeolites as solid adsorbents for phenol removal from aqueous solutions: Kinetics, mechanism, and thermodynamics studies. Chem. Eng. J. 2011, 171 (3), 1143−1149. (2) Ö zkaya, B. Adsorption and desorption of phenol on activated carbon and a comparison of isotherm models. J. Hazard. Mater. 2006, 129 (1−3), 158−163. (3) Păcurariu, C.; Mihoc, G.; Popa, A.; Muntean, S. G.; Ianoş, R. Adsorption of phenol and p-chlorophenol from aqueous solutions on poly (styrene-co-divinylbenzene) functionalized materials. Chem. Eng. J. 2013, 222, 218−227. (4) Calace, N.; Nardi, E.; Petronio, B. M.; Pietroletti, M. Adsorption of phenols by papermill sludges. Environ. Pollut. 2002, 118 (3), 315−319. 12936


Article

Industrial & Engineering Chemistry Research (5) Luo, Z. X.; Gao, M. L.; Yang, S. F.; Yang, Q. Adsorption of phenols on reduced-charge montmorillonites modified by bispyridinium dibromides: Mechanism, kinetics and thermodynamics studies. Colloids Surf., A 2015, 482, 222−230. (6) Liu, Q. S.; Zheng, T.; Wang, P.; Li, Y. J. Regeneration of 4chlorophenol exhausted GAC with a microwave assisted wet peroxide oxidation process. Sep. Sci. Technol. 2014, 49 (1), 68−73. (7) Song, J. X.; Zhao, Q.; Guo, J.; Yan, N.; Chen, H. D.; Sheng, F. F.; Lin, Y. J.; An, D. The microbial community responsible for dechlorination and benzene ring opening during anaerobic degradation of 2,4,6 trichlorophenol. Sci. Total Environ. 2019, 651, 1368−1376. (8) Dubale, A. A.; Ahmed, I. N.; Chen, X. H.; Ding, C.; Hou, G. H.; Guan, R. F.; Meng, X. M.; Yang, X. L.; Xie, M. H. A highly stable metal− organic framework derived phosphorus doped carbon/Cu2O structure for efficient photocatalytic phenol degradation and hydrogen production. J. Mater. Chem. A 2019, 7, 6062−6079. (9) Fan, J.; Fan, Y. H.; Pei, Y. C.; Wu, K.; Wang, J. J.; Fan, M. H. Solvent extraction of selected endocrine-disrupting phenols using ionic liquids. Sep. Purif. Technol. 2008, 61 (3), 324−331. (10) Caetano, M.; Valderrama, C.; Farran, A.; Cortina, J. L. Phenol removal from aqueous solution by adsorption and ion exchange mechanisms onto polymeric resins. J. Colloid Interface Sci. 2009, 338 (2), 402−409. (11) Nourmoradi, H.; Avazpour, M.; Ghasemian, N.; Heidari, M.; Moradnejadi, K.; Khodarahmi, F.; Javaheri, M.; Moghadam, F. M. Surfactant modified montmorillonite as a low cost adsorbent for 4chlorophenol: Equilibrium, kinetic and thermodynamic study. J. Taiwan Inst. Chem. Eng. 2016, 59, 244−251. (12) Xue, G. H.; Gao, M. L.; Gu, Z.; Luo, Z. X.; Hu, Z. H. The removal of p-nitrophenol from aqueous solutions by adsorption using gemini surfactants modified montmorillonites. Chem. Eng. J. 2013, 218, 223− 231. (13) Lin, K. L.; Pan, J.; Chen, Y. W.; Cheng, R.; Xu, X. C. Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders. J. Hazard. Mater. 2009, 161 (1), 231−240. (14) Soto, M. L.; Moure, A.; Domínguez, H.; Parajó, J. C. Recovery, concentration and purification of phenolic compounds by adsorption: A review. J. Food Eng. 2011, 105 (1), 1−27. (15) Abollino, O.; Giacomino, A.; Malandrino, M.; Mentasti, E. Interaction of metal ions with montmorillonite and vermiculite. Appl. Clay Sci. 2008, 38 (3−4), 227−236. (16) Ding, F.; Gao, M. L.; Shen, T.; Zeng, H.; Xiang, Y. Comparative study of organo-vermiculite, organo-montmorillonite and organo-silica nanosheets functionalized by an ether-spacer-containing Gemini surfactant: Congo red adsorption and wettability. Chem. Eng. J. 2018, 349, 388−396. (17) Shen, T.; Gao, M. L.; Ding, F.; Zeng, H.; Yu, M. M. Organovermiculites with biphenyl and dipyridyl gemini surfactants for adsorption of bisphenol A: Structure, mechanism and regeneration. Chemosphere 2018, 207, 489−496. (18) Pouya, E. S.; Abolghasemi, H.; Esmaieli, M.; Fatoorehchi, H.; Hashemi, S. J.; Salehpour, A. Batch adsorptive removal of benzoic acid from aqueous solution onto modified natural vermiculite: Kinetic, isotherm and thermodynamic studies. J. Ind. Eng. Chem. 2015, 31, 199− 215. (19) Zang, W. L.; Gao, M. L.; Shen, T.; Ding, F.; Wang, J. Facile modification of homoionic-vermiculites by a gemini surfactant: Comparative adsorption exemplified by methyl orange. Colloids Surf., A 2017, 533, 99−108. (20) Wang, J.; Gao, M. L.; Shen, T.; Yu, M. M.; Xiang, Y.; Liu, J. Insights into the efficient adsorption of rhodamine B on tunable organovermiculites. J. Hazard. Mater. 2019, 366, 501−511. (21) Menger, F. M.; Keiper, J. S. Gemini surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906−1920. (22) Luo, W. H.; Ouyang, J. P.; Antwi, P.; Wu, M.; Huang, Z. Q.; Qin, W. W. Microwave/ultrasound-assisted modification of montmorillonite by conventional and gemini alkyl quaternary ammonium salts for adsorption of chromate and phenol: Structure-function relationship. Sci. Total Environ. 2019, 655, 1104−1112.

(23) Fan, Y. R.; Li, Y. J.; Cao, M. W.; Wang, J. B.; Wang, Y. L.; Thomas, R. K. Micellization of dissymmetric cationic gemini surfactants and their interaction with dimyristoylphosphatidylcholine vesicles. Langmuir 2007, 23 (23), 11458−11464. (24) Wang, X. D.; Li, Q. T.; Chen, X.; Li, Z. H. Effects of structure dissymmetry on aggregation behaviors of quaternary ammonium gemini surfactants in a protic ionic liquid EAN. Langmuir 2012, 28 (48), 16547−16554. (25) Oda, R.; Huc, I.; Candau, S. J. Gemini surfactants, the effect of hydrophobic chain length and dissymmetry. Chem. Commun. 1997, No. 21, 2105−2106. (26) Wang, X. Y.; Wang, J. B.; Wang, Y. L.; Ye, J. P.; Yan, H. K.; Thomas, R. K. Micellization of a series of dissymmetric gemini surfactants in aqueous solution. J. Phys. Chem. B 2003, 107 (41), 11428−11432. (27) Sikiric, M.; Primozic, I.; Filipovic-Vincekovic, N. Adsorption and association in aqueous solutions of dissymmetric gemini surfactant. J. Colloid Interface Sci. 2002, 250, 221−229. (28) Choi, M.; Na, K.; Kim, J.; Sakamoto, Y.; Terasaki, O.; Ryoo, R. Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts. Nature 2009, 461 (7261), 246−249. (29) Zhu, X. C.; Wu, L. L.; Magusin, P. C. M. M.; Mezari, B.; Hensen, E. J. M. On the synthesis of highly acidic nanolayered ZSM-5. J. Catal. 2015, 327, 10−21. (30) Zana, R.; Benrraou, M.; Rueff, R. Alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants. 1. Effect of the spacer chain length on the critical micelle concentration and micelle ionization degree. Langmuir 1991, 7, 1072−1075. (31) Yang, S. F.; Gao, M. L.; Luo, Z. X. Adsorption of 2-Naphthol on the organo-montmorillonites modified by Gemini surfactants with different spacers. Chem. Eng. J. 2014, 256, 39−50. (32) Ma, J. F.; Cui, B. Y.; Dai, J.; Li, D. L. Mechanism of adsorption of anionic dye from aqueous solutions onto organobentonite. J. Hazard. Mater. 2011, 186 (2−3), 1758−1765. (33) Ding, F.; Gao, M. L.; Wang, J.; Shen, T.; Zang, W. L. Tuning wettability by controlling the layer charge and structure of organovermiculites. J. Ind. Eng. Chem. 2018, 57, 304−312. (34) Madejová, J. FTIR techniques in clay mineral studies. Vib. Spectrosc. 2003, 31, 1−10. (35) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Interlayer structure and molecular environment of alkylammonium layered silicates. Chem. Mater. 1994, 6 (7), 1017−1022. (36) Liu, B.; Wang, X. Y.; Yang, B.; Sun, R. C. Rapid modification of montmorillonite with novel cationic gemini surfactants and its adsorption for methyl orange. Mater. Chem. Phys. 2011, 130 (3), 1220−1226. (37) Bai, G. Y.; Wang, J. B.; Wang, Y. J.; Yan, H. K. Thermodynamics of hydrophobic interaction of dissymmetric gemini surfactants in aqueous solutions. J. Phys. Chem. B 2002, 106, 6614−6616. (38) Chen, B.; Zhu, L.; Zhu, J.; Xing, B. Configurations of the bentonite-sorbed myristylpyridinium cation and their influences on the uptake of organic compounds. Environ. Sci. Technol. 2005, 39 (16), 6093−6100. (39) Yu, M. M.; Gao, M. L.; Shen, T.; Wang, J. Organo-vermiculites modified by low-dosage Gemini surfactants with different spacers for adsorption toward p-nitrophenol. Colloids Surf., A 2018, 553, 601−611. (40) Xi, Y. F.; Ding, Z.; He, H.; Frost, R. L. Structure of organoclays-an X-ray diffraction and thermogravimetric analysis study. J. Colloid Interface Sci. 2004, 277 (1), 116−120. (41) Xi, Y. F.; Frost, R. L.; He, H. P.; Kloprogge, T.; Bostrom, T. Modification of Wyoming montmorillonite surfaces using a cationic surfactant. Langmuir 2005, 21 (19), 8675−8680. (42) Wang, J.; Gao, M. L.; Ding, F.; Shen, T. Organo-vermiculites modified by heating and gemini pyridinium surfactants: Preparation, characterization and sulfamethoxazole adsorption. Colloids Surf., A 2018, 546, 143−152. (43) Song, Q. Q.; Liang, J. L.; Fang, Y.; Cao, C. C.; Liu, Z. Y.; Li, L. L.; Huang, Y.; Lin, J.; Tang, C. Selective adsorption behavior/mechanism 12937


Article

Industrial & Engineering Chemistry Research of antibiotic contaminants on novel boron nitride bundles. J. Hazard. Mater. 2019, 364, 654−662. (44) Park, Y.; Ayoko, G. A.; Horváth, E.; Kurdi, R.; Kristof, J.; Frost, R. L. Structural characterisation and environmental application of organoclays for the removal of phenolic compounds. J. Colloid Interface Sci. 2013, 393, 319−334. (45) Liu, Q.-S.; Zheng, T.; Wang, P.; Jiang, J.-P.; Li, N. Adsorption isotherm, kinetic and mechanism studies of some substituted phenols on activated carbon fibers. Chem. Eng. J. 2010, 157 (2−3), 348−356. (46) Shi, K. L.; Wang, X. F.; Guo, Z. J.; Wang, S. R.; Wu, W. S. Se(IV) sorption on TiO2: Sorption kinetics and surface complexation modeling. Colloids Surf., A 2009, 349 (1−3), 90−95. (47) Ho, Y. S.; McKay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34, 451−465. (48) Gu, Z.; Gao, M. L.; Luo, Z. X.; Lu, L. F.; Ye, Y. G.; Liu, Y. N. Bispyridinium dibromides modified organo-bentonite for the removal of aniline from wastewater: A positive role of π-π polar interaction. Appl. Surf. Sci. 2014, 290, 107−115. (49) Vidic, R. D.; Suidan, M. T.; Brenner, R. C. Oxidative Coupling of Phenols on Activated Carbon: Impact on Adsorption Equilibrium. Environ. Sci. Technol. 1993, 27, 2079−2085. (50) Vasudevan, D.; Arey, T. A.; Dickstein, D. R.; Newman, M. H.; Zhang, T. Y.; Kinnear, H. M.; Bader, M. M. Nonlinearity of cationic aromatic amine sorption to aluminosilicates and soils: Role of intermolecular cation-π interactions. Environ. Sci. Technol. 2013, 47 (24), 14119−14127. (51) Guo, S. X.; Gao, M. L.; Shen, T.; Xiang, Y.; Cao, G. L. Effective adsorption of sulfamethoxazole by novel Organo-Vts and their mechanistic insights. Microporous Mesoporous Mater. 2019, 286, 36−44. (52) Liu, Y.; Xu, H. Equilibrium, thermodynamics and mechanisms of Ni2+ biosorption by aerobic granules. Biochem. Eng. J. 2007, 35 (2), 174−182. (53) Liu, Y. Is the free energy change of adsorption correctly calculated? J. Chem. Eng. Data 2009, 54, 1981−1985. (54) Liu, Y.; Liu, Y. J. Biosorption isotherms, kinetics and thermodynamics. Sep. Purif. Technol. 2008, 61 (3), 229−242.

12938


Comparison between Asymmetric and Symmetric Gemini Surfactant

Recommend Documents