3.1 Characteristics of the bentonites
Table 3 shows the chemical details of both SBt and OBt. The important thing was that CTAB reduced the amount of Ca2+ and Mg2+, showing that the modifiers were swapping ions. Furthermore, when examining this adsorbent, an increase in calcination losses was observed. This occurred due to the depletion of the organic cation (CTAB) bonded to the SBt coatings. Analyzing CTAB showed that it contains 12.5% C, 2.4% H, and 0.7% N. These percentages match the elements present in the CTAB formula, which are C19H42BrN. Sodium bentonite had more carbon (26.7%), hydrogen (4.5%), and nitrogen (1.2%) than CTAB. This shows that it likely had a larger organic cation and possibly more of it.
Table 3
Chemical composition of SBt and modified with cationic surfactant (CTAB).
|
SiO2
(%)
|
Al2O3
(%)
|
Fe2O3
(%)
|
TiO2
(%)
|
CaO
(%)
|
MgO
(%)
|
Na2O
(%)
|
K2O
(%)
|
MnO
(%)
|
P2O5
(%)
|
SO3
(%)
|
SBt
|
52.8
|
14.5
|
5.3
|
0.8
|
20
|
2.5
|
1.5
|
1.6
|
0.1
|
0.1
|
0.1
|
modified SBt
|
60.2
|
16
|
6.1
|
0.9
|
5.5
|
2.1
|
1.7
|
1.7
|
0
|
0.1
|
n.d*
|
*n.d.: Not detected.
Table 4 shows the specific surface area (SBET) values for various types of clays. The BET-specific surface areas of the four OBts were less than the SBt. 2.0 CEC had the lowest SBET values, measuring 7.24 m2/g. Changing the clay's arrangement or covering it with an organic material likely changes how it sticks together (Gámiz et al., 2015). When there was more of it, the space between layers got a little smaller. Several past investigations have found that the spacing between layers in OBts is influenced by factors such as the clay mineral's CEC, the amount of organic cations, the number and length of alkyl chains, and how the organic cations are arranged within the clay layers (Osman et al., 2004; He et al., 2006; Onal and Sarikaya, 2007).
Table.4.
Comparison of BET Specific Surface Area (SBET), Pore Volume (Vp), and Pore Diameter for SBt and OBts
Adsorbent
|
BET specific surface area
(m2/g)
|
Basal spacing
(Å)
|
Vm (cm3(STP) g-1)
|
Mean pore diameter
(nm)
|
BET specific surface area
(m2/g)
|
Basal spacing
(Å)
|
Vm (cm3(STP) g-1)
|
Mean pore diameter
(nm)
|
|
before adsorption
|
After adsorption
|
SBt
|
25.57
|
12.5
|
5.8
|
3.46
|
-
|
-
|
-
|
|
|
0.5CEC
|
18.068
|
14.2
|
4.15
|
5.20
|
15.78
|
13.1
|
3.62
|
4.7
|
|
1.0 CEC
|
15.784
|
15.1
|
3.62
|
4.70
|
12.44
|
14
|
2.85
|
4.42
|
|
1.5CEC
|
11.48
|
19
|
2.63
|
4.47
|
10.83
|
17.2
|
2.48
|
4.67
|
|
2.0 CEC
|
7.86
|
18.5
|
1.8
|
4.59
|
7.24
|
16.5
|
1.66
|
5.01
|
|
X times the CEC of the bentonite is the equivalent amount used to prepare the organoclay in X CEC.
Figures 3 and 4 show how nitrogen is absorbed and released at 77.4 K. This suggests the materials have mostly big or medium-sized pores, with very few tiny ones (Alothman, 2012). Type II isotherms (relatively strong) show that the adsorbents don't have a clear pore size or shape. They might be disordered or have a complicated pore structure with blockages (Alothman, 2012; Bohli & Quederni, 2016). The figures indicate that there are small surface areas and pore volumes.
Table 4 shows that as the CEC increases, the BET surface area of OBts decreases in this order: SBt > 0.5CEC > 1.0 CEC > 1.5CEC > 2.0 CEC.
The results showed that when clay pores absorb more, the specific surface area decreases. Also, when SBt is modified by CTAB, the specific surface area decreases even more.
3.2 X-ray diffractometry
The X-ray diffraction (XRD) patterns of OBts featuring distinct cation exchange capacities (CECs) are presented in Figure 5. Upon intercalation, the basal spacings expand as anticipated, contingent on the concentrations of the surfactant. The discernible peaks at 4.5°, 6.4°, 7°, and 9.9° in 2θ can be seen. The values of 1.5 CEC and 1.0 CEC are at 4.5° and 4.6°, respectively, while 2.0 CEC and 0.5 CEC show the main peaks at (4.5°, 9.8°) and (6.3°, 9.9°). The results are similar to previous studies (He et al., 2005; Durán et al., 2019).
The presence of varying intensity peaks signifies kaolinite coexistence, while the characteristic peaks serve as definitive indicators of montmorillonite. The XRD analysis offers a comprehensive elucidation of the mineralogical composition of the OBts, providing critical insights into their structural attributes. Identifying smectite, quartz, kaolinite, and montmorillonite in the samples is pivotal in understanding the physicochemical properties of these OBts. According to the results, the X-ray diffraction (XRD) analysis of the OBts revealed the presence of a notable and extensive peak, along with a less prominent broad hump that peaked at 4.5◦. Conversely, it is noteworthy that all specimens exhibited analogous peaks. As mentioned earlier, the values indicate the bilayer configuration of the cations within the interlayer, consistent with prior research conducted by other scholars, and the citation originated from Roberts et al.'s (2006) work.
Due to its hydrophilic nature, SBt exhibits limited efficacy in adsorbing non-polar organic molecules. Nevertheless, substituting inorganic cations with organic counterparts, notably surfactant molecules, within SBt interlayers represents a promising strategy for enhancing its capacity to adsorb organic compounds. In this context, introducing organic cations, particularly cetyltrimethylammonium bromide (CTAB) cations, is hypothesized to replace inorganic cations such as Na+ and K+ within SBt interlayer spaces. The extended alkyl groups inherent in these organic cations are crucial in facilitating petroleum hydrocarbon entrapment. This modification of SBt through the incorporation of organic cations not only expands its applicability to adsorbing non-polar organic molecules (Ghavami et al., 2017).
3.3 Fourier Transform Infrared Spectroscopy
The Fourier transform infrared (FTIR) spectra have the potential to offer valuable insights into the surface functional groups present on OBts and the corresponding interactions. The division of the FTIR absorption spectra into distinct segments has been undertaken to discern the principal bend and stretch vibration modes intrinsic to the composite of SBt, OBt, and surfactant. These spectral ranges encompass the following key bands: the Si-O bend and stretch modes spanning the wavenumbers of 800 to 1000 cm-1, the HOH bend spanning 1600 to 1700 cm-1, the CH stretch spanning 2800 to 3000 cm-1, and the OH stretch spanning 3000 to 3700 cm-1 (Ghavami in 2017). Comprehensive FTIR examinations were executed upon the natural clay and organoclay, varying in their CECs compounds, spanning the expansive wavenumber range of 800 to 4000 cm-1. As illustrated in Figure 6, comparative scrutiny unveils the findings stemming from the SBt as juxtaposed with CTAB and naphthalene. SBt shows a pronounced band nestled within the 1,800 to 1,000 cm-1 interval, inherently attributed to the Si-O-Si bend and Si-O stretch movements transpiring upon the clay siloxane surfaces. Integrating CTAB surfactant into the mineral matrix engenders an augmented interaction magnitude with the siloxane-situated surfaces, exceeding that exerted by the hydrated cations and aqueous entities in the unaltered clay matrix.
In practice, the FTIR spectral bands inherent to the OBt evince discernible CH2 bends, and CH stretches manifesting between 1500 to 1400 cm-1 and 3000 to 2800 cm-1, affirming the successful modification of the clay substrate with CTAB, as documented by Ikhtiyarova et al. in 2012. (Fig.7) Evident is the conspicuous absence of akin bands within the OBt's spectrum, barring a solitary presence at 1,600 cm-1, which signifies the deformation of OH attributed to the water molecules accommodated within the clay's layers.
The intensity of the band above directly correlates with the abundance of interlayer water molecules, a quantity undergoing reduction consequent to the assimilation of the surfactant. This effect, in turn, imparts hydrophobic attributes to the OBt, negating the prevailing hydrophilicity endemic to the unmodified clay.
The peak magnitude within this span is notably more pronounced in standard clay, contrasting with the organoclay. Further conspicuous within the organoclay FTIR spectra are the discernible peaks populating the 3,000 to 2,800 cm-1 interval, attributed to the symmetric and asymmetric stretching vibration modes intrinsic to the CH bonds. This occurrence fundamentally validates the occupancy of a considerable fraction of the clay's interlayer voids by the CTAB surfactant. Noteworthy are the spectral peaks encompassing the range of 3,300 to 3,700 cm-1, representing the intrinsic inner OH moieties in the structure of clay alongside adsorbed water entities. However, the strength of the bands exhibits diminution concomitant with surfactant presence. The assertions arising from Figure 7 find substantiation in the FTIR spectrum of the OBts over varying CECs. Herein, the prominence of minor peaks attributed to the oscillatory vibrations of CH2 groups grows conspicuous in tandem with heightened CEC levels. The dynamic changes in the micellar configuration subsequently induce a leftward shift in said vibrational modes as the CTAB-clay proportionality is elevated. Consequently, greater quantities of surfactant yield more pronounced resultant peaks, as corroborated by Gürses et al. in 2012.
The FTIR spectra visualization of OBt post-naphthalene adsorption indicates an accentuation of the Si-O stretch band, a manifestation directly influenced by the sorption of these contaminants. Subsequent to the absorption of said pollutants, the asymmetrical CH stretch band's apex shifts towards the left, emblematic of the constrained packaging density inherent to species intercalated within the layers, a situation necessitated by the adoption of fewer gauche conformational alignments proximal to the clay surfaces. This observed phenomenon signifies the pollutant molecules' proclivity for interaction with the OBt substrate, thereby reducing the surfactant's surface domain available for gauche conformation adoption. An interesting observation is the connection between the increased basal spacing and the decrease in gauche conformation. These findings align with existing research. This shift toward higher packing density is a result of fewer gauche conformations occurring in a geometric configuration characterized by curved spatial alignments of the chain. This concept was previously discussed by Ghavami in 2017 and Lazorenko et al in 2020. In simpler terms, as explained by Chuang et al. in 2010 and Park et al. in 2011, the increase in layers and the simultaneous strengthening of water-repelling interactions happen because organic carbon between layers expands across the surface. The stretching of the OH bond and the bending of the HOH molecule indicate how water and methanol stick to the OBt material.
3.3 Hydrophilic and hydrophobicity of OBts
Similar results were derived from the assessment of water contact angles, as depicted in Figure 8. 2.0 CEC, 1.5 CEC, 1.0 CEC, and 0.5 CEC showed contact angles of 155.23, 146.15, 138.77, and 134.25, respectively. The hydrophilicity is so high in clay and OBt samples (1 and 2) that the contact angle cannot be measured. In addition to the discernible impact of the length of molecular chain on contact angles, a substantial elevation in contact angles plays a pivotal role in the case of OBts featuring CTAB substitutions. Therefore, outcomes from contact angle and liquid adsorption agree with the superhydrophobic essence of blended adsorbents.
3.3 Scanning electron microscopy (SEM analysis)
Figure 9 demonstrates the SEM micrographs of OBts. The analysis was recorded at 20 kV. The SBt surface exhibited a rough appearance. Following modification with a cationic surfactant, the OBt surface undergoes a modification, losing its layered structure and evolving into a smoother and more porous surface (Kaya et al., 2013).
The morphology of OBts is characterized by a textured surface exhibiting fractured edges arranged in a parallel-layered configuration, aligning with the X-ray diffraction (XRD) reflection planes observed for organoclays. Nevertheless, the surface texture is notably soft and spongy, marked by the agglomeration of nanoparticles resulting from their reduced particle size and the consequential amplification of surface area-to-volume effects. Upon closer examination of the images, it becomes evident that post-synthesis, the absorbent surface undergoes increased heterogeneity, accompanied by a noteworthy augmentation in the specific surface area of the absorbent; this heightened specific surface area is pivotal in enhancing adsorbent efficiency (He et al., 2006).
The outcomes indicate that the clay contains big aggregate particles with bent sides and big fragments in different areas. However, the surfactant-loaded organic particles have a less sheet-like structure with jagged edges and many fine flakes. Images of the OBt show a typically flat, aggregated morphology. The most plausible justification is the high surfactant packing density in the clay interlayer spacing, causing the bent plates to become flat coatings (Mallakpour & Dinari, 2011).
3.4 Adsorption of naphthalene
Figure 10 illustrates the outcomes of naphthalene elimination using natural clay (unmodified) and OBt with varying CECs. The findings indicate that the clay alone possesses a minimal adsorption rate. This rate is insignificant in comparison to OBt. This is while OBt exhibits a high capacity for adsorbing pollutants. The concentration of pollutants denotes the immediate occurrence of a significant portion of adsorption within a brief period. When time progresses, the sorption of pollutants decreases due to the gradual reduction in OBt capacity and the filling of interlayer spaces by the sorbed pollutant. The quantity of pollutant sorption is closely linked to the huge percentage of CEC (surfactant quantity), which results in faster adsorption. Comparatively, organoclay with 2.0 CEC adsorbed 1.13 times more than that with 0.5 CEC within the first 15 minutes. However, over longer durations, the addition of surfactant has minimal impact on adsorption but increases treatment costs, which is unfavorable. A significant portion of the contaminant is removed rapidly, even within five minutes. This behavior aligns with the findings of Changchaivong and Khaodhiar (2009), who observed that most adsorption occurs in the initial minutes. The OBt adsorption capacity rises with surfactant level up to a certain threshold. Beyond this threshold, additional surfactant does not impact the adsorption volume as the soil desorbs excess surfactant during testing. Consequently, the surfactant forms foam in the contaminated liquid (Farias et al., 2022). The maximum adsorption capacity of CTAB-bentonite and SBt for naphthalene was 14.08 and 5.22 mg/g, respectively. As a result, this study suggests that organoclay is a promising adsorbent candidate and a suitable choice for removing non-aqueous phase liquid contaminants from aqueous solutions due to its high adsorption capacity and rapid adsorption rate.
Figure 11 is a comparative assessment of naphthalene removal efficacy. The figure juxtaposes unmodified natural clay against organophilic clay with varying CEC levels. The results distinctly reveal that the inherent clay substrate exhibits a modest affinity for adsorption, a tendency significantly overshadowed by the robust adsorption capabilities demonstrated by the organophilic clay. Evidently, the organophilic clay displays a substantially heightened proficiency in capturing pollutants.
A more meticulous view of the temporal evolution of changes in pollutant concentration reveals that considerable adsorption occurs rapidly, transpiring within a brief time frame. However, with extended temporal durations, pollutant sorption efficiency gradually declines, which can be attributed to the incremental reduction in the absorptive capacity of the OBt matrix and the progressive occupancy of interlayer spacings resulting from the continuous entrapment of pollutants. The quantity of sorbed pollutants correlates notably with increased CEC levels (coupled with elevated surfactant concentrations), facilitating accelerated sorption within compressed time scales.
From an alternative perspective, it is observed that OBt adsorption, supported by a 2.0 CEC, achieves a 1.13-fold improvement compared to its counterpart associated with 0.5 CEC, particularly evident during the initial 15-minute interval. However, the extension of temporal trajectories tends to diminish the impact of surfactant augmentation on adsorption, concurrently leading to heightened operational costs necessitating prudent consideration. Importantly, a substantial amount of contaminant is promptly alleviated, demonstrating notable efficacy within a brief 5-minute period. This finding aligns with those of Changchaivong and Khaodhiar (2009) and validates the prevalence of early-stage adsorption.
The extent of adsorption exhibits an increasing gradient with the concurrent escalation in surfactant infusion up to a noticeable threshold. Beyond this critical threshold, the heightened surfactant infusion ceases to exert any incremental influence on the adsorptive capacity of the OBt. This phenomenon is attributed to desorption within the soil matrix, wherein excess surfactant is released during the experimental course, forming a frothy emulsion in the contaminated aqueous environment. Therefore, this study's findings indicate that thanks to high adsorption capacity and rapid adsorption rate, OBt can be a promising adsorbent candidate and a perfect choice for removing non-aqueous phase liquid contaminants from aqueous solution.
The adsorption of naphthalene reveals that the adsorption capacity of naphthalene on OBts with varying CECs surpasses that of SBt. Introducing a relatively modest quantity of cetyltrimethylammonium bromide (CTAB) enhances naphthalene uptake. This heightened affinity between naphthalene and OBt is attributed to the advantageous interaction facilitated by π–π interactions, as previously discussed by Huang et al. (2005). Consequently, the surface modification of clay with CTAB establishes favorable adsorption sites for Polycyclic Aromatic Hydrocarbon (PAH) molecules.
The results obtained from the experiments exhibited a better fit with the Freundlich isotherm model, with a correlation coefficient of determination (R2>0.97). All naphthalene adsorption isotherms are represented as qe=KdCe, where qe signifies the quantity of adsorbate per unit mass of adsorbent, Ce denotes the balance adsorbate level, and Kd denotes the distribution coefficient derived from the adsorption isotherm slope (Figure 12). This outcome aligns with the conceptual framework of the separation procedure between the organic phase and aqueous solution, resulting from aggregating alkyl chains in the interlayer space.
As the charge on organic cations increases, the strengthened van der Waals interactions among alkyl chains in the middle layer alleviate water interference and expand the basal spacing, facilitating the solute's partitioning into the organic phase. With higher organic cation loadings, the interstices in the OBt layers become predominantly occupied by adsorbed surfactants, significantly reducing the available free space for organic adsorbents. Consequently, the dense interlayer becomes less penetrable for organic adsorbents, restricting their absorption. Moreover, excess organic cations and their counterions with polar or charged groups can reduce the hydrophobic qualities of the interlayer space, further constraining the absorption of the organic adsorbent. Despite these limitations, smaller naphthalene molecules can still infiltrate the dense interlayer space, resulting in an increased partition coefficient (Kd) under such circumstances (Zhu et al., 2007).