Sorption Properties of Bentonite-Based Organoclays with Amphoteric and Nonionic Surfactants in Relation to Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a major scientific challenge due to their profound impact on public and environmental health. Therefore, studying ways to detoxify PAHs is important. In this research, the adsorption ability of bentonite modified with five surfactants, including amphoteric (cocoamphodiacetate disodium and sodium cocoiminodipropionate) and nonionic (lauramine oxide, cocamide diethanolamine, and alkylpolyglucoside) substances for the adsorption of high-molecular benzo(a)pyrene and low-molecular naphthalene from the PAH group was studied. The bentonite and bentonite-based organoclays were characterized using X-ray diffraction and Fourier transform infrared spectroscopy. The results showed that the maximum adsorption of benzo(a)pyrene by organoclays increased compared with the initial mineral. The adsorption of benzo(a)pyrene is higher than that of naphthalene. The adsorption process of benzo(a)pyrene by bentonite and organoclays is predominantly monolayer, as it is better described by the Langmuir model (R² 0.77–0.98), while naphthalene is predominantly multilayer, described by the Freundlich model (R² 0.86–0.96). According to the effectiveness of sorption capacities of organoclays—including the degree of sorption, Langmuir and Freundlich constants, the value of maximum adsorption, Gibbs free energy, and the index of favorability of the adsorption process—the most effective modification was found. For the adsorption of benzo(a)pyrene the best was cocoamphodiacetate disodium, and for naphthalene it was sodium cocoiminodipropionate.

1. Introduction

On a global scale, there are accumulations of pollutants by the components of the biosphere because the inflow of pollutants into natural and anthropogenic landscapes exceed their natural removal [1]. Polycyclic aromatic hydrocarbons (PAHs) are formed due to heat treatments of hydrocarbon materials and are widespread in all-natural environment [2]. PAHs are carcinogenic to living organisms [3]. The structure of PAHs is represented by two or more condensed benzene rings, and as the number of rings in the PAHs molecule increases, their molecular weight, hydrophobicity, environmental stability, and toxicity increase [4,5]. Benzo(a)pyrene is the most dangerous PAHs and a carcinogen of the first hazard class, while one of the most common PAHs is naphthalene [4,6,7]. The PAH content of water bodies, soils, and sediments in different regions of the world corresponds to a concentration range from 0.04 to 0.78 mmol kg⁻¹ [8,9,10,11].

Sorption remediation of natural mediums is the most common among all existing methods of soil detoxification and sediments and water detoxification from organic pollutants [12]. Due to the large specific surface area and developed system of meso- and micropores, natural and modified clay minerals are widely used to restore the environment [13,14].

Soil organic matter and clay minerals are the main components of natural mediums that regulate the adsorption–desorption behavior of organic pollutants [15]. Several studies have demonstrated the effectiveness of bentonite-based organoclays in wastewater treatments [16,17], remediation of soil contamination [18,19], as well as bottom sediments [20]. Previous research has shown that the efficiency of removing priority PAHs from soils and wastewater when using modified clay minerals could reach 99% [21,22]. This is due to the porous structure of minerals, primarily micropores, as well as the properties of their surface and the composition of functional domains [19]. Nanometer-scale pores are widespread in porous geological environments and could account for more than 90% of the total surface area of minerals [23]. Surfactants were chosen as modifiers due to their availability (widespread use for the manufacture of detergents), as well as the discovered adsorption efficiency of pollutants by some of them [24].

The process of PAH adsorption consists of three stages, i.e., (1) the diffusion of pollutant molecules from the solution to the outer surface of minerals, (2) intraparticle diffusion of PAHs through internal pores, and (3) adsorption of PAH molecules on active sites located on the inner surface of sorbent [25]. In the absence of water, organic molecules are adsorbed in both hydrophilic and hydrophobic micropores due to enhanced dispersion interactions in pore spaces of molecular sizes [23]. The microscopic density of hydrophilic sites, such as surface cations and surface hydroxyl groups, which actively interact with water molecules, controls the hydrophobicity of the surface and results in the degree of sorption of PAHs by minerals [21]. Nanometer pores surrounded by surfaces with different densities of cations and hydroxyl groups also exhibit hydrophilic–hydrophobic properties like the flat surfaces of mineral particles [23,26]. The surface hydroxyl group, molecular-level adsorbed water, hydrophobic surfaces of Si-O bridges, hydrated cations, and surface metal ions of minerals provide possible sites for PAH adsorption [27]. However, significant adsorption of hydrophobic organic pollutants can occur only in hydrophobic micropores, since water is a competitive adsorbate in hydrophilic micropores [23]. Modification of bentonite leads to a change in its sorption properties in relation to PAHs. At the same time, the type of modifier directly affects the degree of sorption and the strength of the bond in the pollutant–sorbent system [21,28,29].

The use of bentonite-based organoclays can be a key solution for the remediation of natural environments subject to anthropogenic loads. In addition, bentonite deposits are quite common in the world, which makes it possible to create highly effective sorbents in unlimited quantities. Large sources are located in many countries, and the largest exporters of bentonite are Russia, the United States of America, Greece, Azerbaijan, Japan, Italy, Argentina, Spain, and Turkey [30]. The profitability of the technology for the use of bentonite-based organoclays significantly depends on the proximity of the mineral deposit to industrial production and the contaminated site. For effective sorption of PAHs, it is necessary to develop new bentonite-based organoclays with a high sorption capacity and adsorption selectivity. The aim of this research was to study the structural and sorption properties of synthesized bentonite-based organoclays and a number of amphoteric and nonionic surfactants.

2. Materials and Methods

2.1. Synthesis of Organoclays

Bentonite from the Sarigyugh deposit (Armenia) was used for the synthesis of organoclays. The reserves of this deposit are more than 70 million tons. Samples of the mineral were provided by Bento Group Minerals, Moscow. To prepare the monoionic sodium form of the mineral, a 0.2 M NaCl solution was added to the purified natural phyllosilicate, previously crushed to particles less than 75 μm, twice—for 6 h, and then within 18 h—with a ratio of 1:50 (weight/volume). At the end of the procedure, the mineral was washed several times with deionized water until a negative test for chloride ions with AgNO₃. Separation of the solid phase from the liquid phase was carried out by centrifugation at 15,000 rpm (10,000× g) for 15 min. After that, the mineral was oven-dried at 60 °C to a constant weight and then crushed.

One gram of the corresponding surfactant was dissolved in 95 mL of deionized water and then added to 5 g of the mineral in a conical flask. The mixture was thoroughly stirred with a glass rod, and was shaken on an orbital shaker in a closed flask at a frequency of 180 rpm for 24 h at a temperature of 22–23 °C. After that, the organoclay and the liquid phase were separated by centrifugation at 9000 rpm (6000× g) for 15 min, followed by decanting of the supernatant.

The precipitate was washed three times in 40 mL of deionized water in centrifuge tubes by stirring thoroughly with a glass rod for 60 s. The separation of wash water was carried out by centrifugation at 9000 rpm (6000× g) for 15 min, followed by decantation of the supernatant. After combining the sediments in a petri dish, the organoclay was oven-dried at 60 °C to a constant weight. This temperature was chosen to prevent the decomposition and transformation of the sorbed organic matter. After grinding the organoclay in a laboratory mill, the resulting material was stored in sealed laboratory beaker in a desiccator with a lapped lid.

For the synthesis of organoclays, the monoionic sodium form of bentonite and amphoteric or zwitterionic and nonionic surfactants were used:

• Surfactant 1—Sodium cocoiminodipropionate;
• Surfactant 2—Lauramine oxide, C12–C14;
• Surfactant 3—Cocamide diethethanolamine;
• Surfactant 4—Disodium cocoamphodiacetate;
• Surfactant 5—Alkylpolyglucoside C8–C10.

For the synthesis of bentonite-based organoclays, commercial preparations of amphoteric surfactants provided by the UTS Group of Companies (Unified Trading System), Moscow, were used. Structural formulas and properties of the amphoteric and nonionic surfactants used are given in

Table 1. Structural formulas and properties of the amphoteric and non-ionic surfactants.

Name	Structural Formula	Formula
Amphoteric Surfactants
Disodium cocoamphodiacetate	n = 6, 8, 10, 12, 14, 16	Imidazolium, 1-[2-(carboxymethoxy)ethyl]-1-(carboxymethyl)-4,5-dihydro-2-norcoalkyl, hydroxides, sodium salts
Properties: Clear liquid with a faint odor. Cocoamphodiacetate disodium is compatible with all surfactants and tolerant to electrolytes. It is a water-soluble amphoteric surfactant with mild foaming, cleansing, and conditioning properties.
Sodium Cocoiminodipropionate		β-alanine, N-(2-carboxyethyl)-, N-cocoalkyl derivatives, disodium salts
Properties: High-foaming PAH, inhibitor of corrosion of iron and other metals. Stable in the entire pH range; is an effective hydrotropic. It has a synergistic effect with non-ionic surfactants providing high degreasing and dispersing ability. It is available in the form of a 30% solution and aqueous.
Nonionic PAHs
Lauraminoxide, C12-C14		Lauryl-Myristyl Dimethylamine Oxide; Dodecyl dimethylamine N-oxide (C12)
Properties: It is a tertiary amine oxide formed because of the formal oxidation of an amino group. It is compatible with all types of surfactants and activates an antistatic effect. It plays the role of a plant metabolite and detergent. It is an emulsifier, foaming agent, foam stabilizer, and thickener.
Cocamid dietanolamine	n = 6, 8, 10, 12, 14, 16	CH3(CH2)nC(=O)N(CH2CH2OH)2, n ~ 6–18; coconut oil diethanolamine condensate; diethanolamide
Properties: It is a mixture of diethanolamides of fatty acids that are part of coconut oil, which consists of approximately 48.2% lauric acid, 18% myristic acid, 8.5% palmitic acid, 8% caprylic acid, 7% capric acid, 6% oleic acid, 2.3% stearic acid, and 2% linoleic acid. Cocamide diethanolamine or cocamide DEA is a diethanolamide produced by reacting to a mixture of coconut oil fatty acids with diethanolamine. It is a viscous liquid that is used as a foaming agent in bath products and as an emulsifier in cosmetics.
Alkylpolyglucoside C8-C10		C6H11O5-O-(CH2)7-9-CH3; Mixture of C8-10-alkyl glucoside oligomers
Properties: Alkylpolyglucoside 70%. It is made from vegetable starch and fatty alcohols of palm oil. It shows excellent solubility, stability, and surface and interfacial activity in concentrated salt and alkaline solutions. It can be used as a binder in concentrated surfactants. It is a good hydrotrope, wetting agent, and dispersant.

.

Despite the toxicity of the surfactants themselves and the possible toxicity of synthesized organoclays, previous studies have shown that the products of the interaction of organoclays with pollutants were not toxic [14,24].

The phase composition of bentonite was established using X-ray diffractometric analysis performed at the Institute of Physical Chemistry and BPP RAS using Bruker D2 Phaser diffractometer (Bruker Optik AXS GmbH, Karlsruhe, Germany).

The IR spectra of the mineral and organoclays were obtained using the FTIR spectrometer FSM-2202 (SPECTR Experimental Design Bureau, Saint-Petersburg, Russia), using the complete diffuse reflection attachment in the range of 4000–400 cm⁻¹, mode—reflection, resolution—4 cm⁻¹, number of scans—16, Norton-Beer apodization—average. The substance was ground in a laboratory mill and then dried in the air under the rays of a 500 W IR lamp at a temperature of 95 °C for 1 h. The temperature was monitored by an IR pyrometer (RGK PL-12, Hefei survey optical instrument Co., Ltd., Hefei, China).

2.2. Adsorption of PAHs

2.2.1. Experiment Design

The adsorption of PAHs by bentonite and organoclays obtained by its modifications of surfactants were studied. The study was conducted considering benzo(a)pyrene and naphthalene as model pollutants. These compounds differ greatly in physical and chemical properties. Benzo(a)pyrene belongs to high-molecular compounds, and naphthalene belongs to low-molecular PAHs, and they both have carcinogenicity. To study the features of the adsorption of PAHs by organoclays, the method of adsorption of polyarenes by [31,32] was used. Benzo(a)pyrene (CAS 50-32-8, Sigma-Aldrich, Saint Louis, MO, USA) and naphthalene (CAS 91-20-3, Sigma-Aldrich, Saint Louis, MO, USA) were used for the preparation of working solutions. Since the water solubility of PAHs is very low, the working solution was prepared in the presence of an acetonitrile solution with a PAH concentration of 2 mg·mL⁻¹. The mass ratio of the solid phase and liquid phase was 1:100. For this purpose, 0.2 g were placed in a 50 mL centrifuge tube, and after that, 1, 3, 5, 6, 8, 9, and 10 mL of the working solution were added, brought to 12 mL with acetonitrile, and 8 mL of water was added to achieve the specified concentrations: 10, 30, 50, 60, 80, 90, and 100 μg mL⁻¹, corresponding to 0.08, 0.23, 0.39, 0.47, 0.63, 0.70, and 0.78 mmol kg⁻¹ of naphthalene and 0.04, 0.12, 0.20, 0.24, 0.32, 0.36, and 0.40 mmol kg⁻¹ of benzo(a)pyrene. Saturation of PAH by sorbents occurs in 4–24 h [21,28,29]. The tube with the contents was shaken for 24 h on a reciprocating shaker ULab US-1350L (ULAB, Jinan, Shandong Province, China) at 180 rpm, after which the mixture was centrifuged on a Biobase BKC-TH21 centrifuge (ULAB, Jinan, Shandong Province, China), at 12,000 rpm for 15 min. The experiment was carried out three times.

2.2.2. Study of the Mechanisms of Adsorption of PAHs

For the general characteristics of sorption processes, the degree of sorption of PAH (S) was calculated according to Formula (1):

$$\mathrm{S}={\displaystyle \frac{(\mathrm{C}-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{C}}}\times 100\mathrm{\%}$$

(1)

where C and C_e are the initial and equilibrium concentration of the element in the solution, mmol L⁻¹.

The study of the mechanisms of adsorption of PAHs by bentonite and organoclays based on it was carried out using the two-parameter models of Langmuir and Freundlich. Langmuir’s model assumes that adsorption occurs at adsorption sites that do not affect the adsorption capacity of the other sites. Each of the sites interact with only one adsorbate particle, resulting in a monolayer of adsorbed particles. The adsorption process is in dynamic equilibrium with the desorption process. The equation of two-parameter Langmuir sorption model (2) is as follows:

$$\mathrm{Q}={\mathrm{Q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\displaystyle \frac{{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{\left(1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}\right)}}$$

(2)

where Q is the number of absorbed PAHs, mmol L⁻¹; Q_max is the value of the maximum adsorption of the substance, mmol L⁻¹; K_L—Langmuir’s constant, L⁻¹ mmol; and C_e—concentration of the substance in the equilibrium solution, mmol L⁻¹.

The Langmuir equation can be expressed in terms of the partition coefficient (R_L) (3):

$${\mathrm{R}}_{\mathrm{L}}={\displaystyle \frac{1}{\left(1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{M}}\right)}}$$

(3)

where C_M—maximum initial concentration of adsorbate (mmol L⁻¹). The R_L value indicates the favorability of the adsorption process and the capacity of the adsorption system. R_L values between 0 and 1 indicate favorable adsorption under given operating conditions, R_L > 1 indicates an adverse reaction, and R_L = 0 indicates irreversible adsorption [26,33].

Using the Langmuir constant, the Gibbs free energy (∆G) is calculated as a measure of the adsorption strength in the sorbate–sorbent system (4):

$$∆\mathrm{G}=-\mathrm{R}\mathrm{T}\ln {\mathrm{K}}_{\mathrm{L}}$$

(4)

where R—universal gas constant (8.314 J/mol∙K); T—absolute temperature; and K_L—Langmuir’s constant.

According to Freundlich’s theory, the surface of most adsorbents is heterogeneous, there is an interaction between adsorbed particles, and adsorption is often not limited to the formation of a monomolecular layer. The two-parameter Freundlich sorption model (5) is as follows:

$$\mathrm{Q}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}}$$

(5)

where Q—number of PAHs absorbed, mmol L⁻¹; K_F—Freundlich’s constant, L⁻¹ mmol; C_e—concentration of the element in the equilibrium solution, mmol L⁻¹; and 1/n—empirical exponent of degree.

2.2.3. Chromatographic Analysis

PAH content was determined by high-performance liquid chromatography (HPLC) on Agilent Technologies (Santa Clara, CA, USA) chromatograph with fluorescence and ultraviolet detection (UV-1000 and FL-3000) [34]. A mixture of acetonitrile (75%) and bidistillated water (25%) was used as the liquid phase at a flow rate of 0.5 mL/min and a temperature of 20 °C. The volume of the injected solution (injection) was 20 μL. Identification was carried out by the relative retention times of the standard sample when simultaneously detected on two existing detectors, which makes it possible to identify PAH peaks with a high degree of reliability due to the large difference in the sensitivity of the detectors. The calculation of the results was carried out according to the method of an external standard according to the Formula (6).

$$\mathrm{X}={\displaystyle \frac{{\mathrm{S}}_{\mathrm{x}}\times {\mathrm{V}}_{\mathrm{e}}\times 100}{\mathrm{A}\times {\mathrm{C}}_{\mathrm{c}}\times \mathrm{M}}}\times 1000$$

(6)

where

• X—mass concentration of individual PAH in the extract, ng/g;
• S_x—Peak area of the determined PAHs, MV × s;
• V_e—volume of extract, cm³;
• A—Relative calibration coefficient, mV × s × cm³/μg;
• C_c—Correction factor considers sample preparation losses;
• M—weight of the sample taken for analysis, g.

Quantification limits (LOQ) and detection limits (LOD) were 0.17 ng/g and 0.09 ng/g for naphthalene, and 0.10 ng/g and 0.05 ng/g for benzo(a)pyrene. During chromatographing, the retention time of naphthalene was 5.2 min and that of benzo(a)pyrene was 26.8 min.

2.2.4. Statistical Analysis

Calculation of statistical parameters and fitting of adsorption isotherms were performed in the program Origin 2018 (Origin Lab Corporation, Northampton, MA, USA).

3. Results and Discussion

3.1. Components and Synthesized Organoclays

According to the manufacturer, the bentonite used is characterized by the oxide composition: SiO₂—58.3%, Al₂O₃—14.3%, Fe₂O₃—4.4%, MgO—3.6%, Na₂O—2.3%, K₂O—1.2%, and CaO—2.1%. The cation exchange capacity of the mineral is 105 mg eq/100 g. The mineral had the 9.6 pH of an aqueous suspension. The content of montmorillonite in the initial bentonite was more than 80%.

The results of the X-ray diffractometric analysis of the initial bentonite are shown in Figure 1. The analysis data show a high content of montmorillonite (7.10°), as well as insignificant impurities of quartz (22.04°, 26.73°, 28.02°) and feldspars (22.04°, 26.73°) (

Table 2. Mineral composition of commercial bentonite.

#	Phase Name	Formula	Space Group	Crystal System
1	Montmorillonite	Al0.86Fe0.1HLi0.08Mg0.14O10Si3.9	C 1 2/m 1 (12)	monoclinic
2	Quartz	SiO2	P 31 2 1 S (-1)	trigonal (hexagonal axes)
3	Feldspar	CaAl2Si2O8	C-1 (-1)	triclinic (anorthic)

).

The results of X-ray diffraction analysis showed that the initial bentonite is mainly monovalent cations (d001 = 12.49 Å, 2ϴ = 7.10°). When the initial bentonite is converted to the monoionic sodium form, the packet distance practically does not change (d001 = 12.45 Å).

IR spectra of the initial bentonite and organoclays obtained using amphoteric and nonionic surfactants are shown in Figure 2. In the IR spectrum of the original bentonite (Figure 2, Table 2), the bands in the range 3000–3700 correspond to stretching vibrations of the OH group, corresponding to the montmorillonite rim surface and water of hydration. In the area of 1400–400 cm⁻¹, there are strips of silicate structure. The IR spectra contain bands characteristic of the main phase of montmorillonite in the region of stretching and bending vibrations of structural groups, as well as bands of clearly characteristic impurities—kaolinite and quartz. The band at 1638 cm⁻¹ refers to the bending vibrations of adsorbed H₂O molecules. The bands at 1197, 838, and 798 cm⁻¹ correspond to bending vibrations of the Si–O–Si group. Characteristic absorption bands of Al-O in Al₂O₃ (1111 cm⁻¹) and bending vibrations of Si-O-Al (645–669 cm⁻¹) are also observed. The bending vibrations of the O-Si-O rings and the lattice oscillations of the Fe-O bond in the SiO₄ tetrahedra correspond to the bands of 426–499 cm⁻¹.

The interpretation of the absorption bands of the IR spectrum of the original bentonite is presented in

Table 3. Interpretation of the IR spectrum of the original bentonite.

Wave Number of the Band, cm−1	Structural Fragment
3634	ν * free -OH groups
2512	chelated H-bridge -OH
1796	CO32
1638	δ -OH
1428	νas O-C-O CO32−
1244	ν Si-O in layered silicates
1197	νas O-Si-O
1111	ν Al-O в Al2O3
918	δ Al-O-H
879	δasCO32−
838	νs Si-O-Si
798	νs Si-O-Si
669	ν Si-O-Al
645	ν Si-O-Al
499	δ Si-O in tetrahedra O-Si-O
448	δ Si-O-Fe
439	δ Si-O in tetrahedra O-Si-O
431	δ Si-O in tetrahedra O-Si-O
426	δ Si-O in tetrahedra O-Si-O

* ν—stretching, δ—bending, ν_as—asymmetrical stretching, ν_s—symmetrical stretching.

.

When comparing the IR spectra of samples of organoclays from amphoteric surfactants with the spectrum of unmodified bentonite, it was noted that in addition to the band’s characteristic of bentonite (see above), there are additional absorption bands due to organic compounds in the studied objects (Figure 2). Absorption bands in the regions 2853–2927 cm⁻¹ refer to stretching vibrations of CH₂ groups, while a small band in the region of 1465 cm⁻¹ is attributed to bending vibrations of CH₂. The appearance of bending vibrations in the region of 1378 cm⁻¹ is attributed to the presence of structural fragments of CH₃, which indicates the presence of organic compounds of surfactants in the modified bentonite. Absorption bands of 1583, 1542, and 1320 cm⁻¹ that appeared in the IR spectrum belong to amino groups and acid residues stretching symmetric vibrations of C-O-carboxylates. The appearance of bands corresponding to adsorbed H₂O indicates that not all of it has been replaced with surfactant molecules.

Due to the low concentration of organic substances in the inorganic matrix, absorption bands with a high extinction coefficient and the largest number of similar groups in the molecule were recorded on the IR spectra. An example the vibrations of the –CH₂– groups in the alkyl constituents of surfactant molecules were marked.

When comparing the IR spectra of samples of organoclays from non-inogenic surfactants with the spectrum of unmodified bentonite, it was found that in the studied objects, in addition to the band’s characteristic of bentonite, as well as in the case of bentonites modified with amphoteric surfactants, there are additional absorption bands due to organic compounds (Figure 2). However, there is a shift to the long-wave region of bending vibrations of OH groups in the region of 1608 cm⁻¹. A decrease in the intensity of the bands representing OH groups indicates a decrease in the H₂O content between the layers and the replacement of H₂O water with surfactant molecules [35]. In the region of 1718 cm⁻¹, symmetrical stretching vibrations were revealed, indicating a small number of carbonyl groups C=O of the surfactant molecule C-N when modifying bentonite cocamide with diethanolamine.

Due to the low concentration of organic substances in the inorganic matrix, absorption bands with a high extinction coefficient and the largest number of similar groups in the molecule were recorded on the IR spectra of modified organoclays. These are mainly oscillations of –CH₂– groups in the alkyl constituents of surfactant molecules.

3.2. Adsorption of PAHs by Synthesized Organoclays

It was found that the value of S benzo(a)pyrene by bentonite varies from 5.3% to 19.9%, by naphthalene—from 2.1% to 3.9%. The highest S value is characterized at the initial concentration of benzo(a)pyrene—30 μg g⁻¹, of naphthalene—at 60 μg g⁻¹. With a further increase in the concentration of pollutants in the initial solution to 100 μg g⁻¹, a decrease in the S value to 11.0% and 3.0% for benzo(a)pyrene and naphthalene, respectively, is observed (

Table 4. Degree of sorption of benzo(a)pyrene and naphthalene (S, %) as a function of the initial concentration of PAHs in solution.

Initial Concentration of PAHs in Solution, μg mL−1	Bentonite	Organoclay with Surfactant 1	Organoclay with Surfactant 2	Organoclay with Surfactant 3	Organoclay with Surfactant 4	Organoclay with Surfactant 5
Benzo(a)pyrene
10	5.3	96.8	88.7	91.7	89.0	47.4
30	19.9	73.9	68.5	70.3	86.5	82.8
50	17.1	74.2	66.1	67.3	71.7	75.7
60	14.8	68.3	65.0	67.2	65.7	70.2
80	12.4	61.7	59.3	65.1	51.2	56.6
90	11.8	54.0	53.4	60.0	45.0	52.8
100	11.0	53.2	45.9	55.4	40.8	50.5
Naphthalene
10	2.1	0.1	7.8	1.2	0.3	9.6
30	3.4	2.8	6.8	3.9	5.6	9.5
50	3.7	4.2	6.5	4.9	6.5	6.8
60	3.9	4.7	6.6	4.9	6.7	6.0
80	3.7	4.2	6.3	4.3	5.4	4.8
90	3.4	3.9	5.8	3.9	5.0	4.2
100	3.0	3.6	5.0	3.7	4.6	3.8

). At the initial stage of adsorption, when the diffusion transport of PAH molecules from the volume of the solution to the outer surface of the porous structures of the mineral is carried out, the degree of sorption increases with an increase in the concentration of the pollutant in the solution, since there are enough sites for the adsorption of PAHs [36]. With an increase in the initial concentration of PAHs in the solution as the mesopore is filled and the PAHs are diffused into the micropores, the S value decreases, due to the manifestation of steric effects that prevent the access of pollutants to the adsorption sites [27,37,38]. At the same time, steric effects are more pronounced during the adsorption of benzo(a)pyrene, which is due to its greater molecular weight [25,38].

Modification of bentonite surfactants of various nature led to an increase in the parameter S of benzo(a)pyrene by 3.7–18.1 times, and of naphthalene by 1.1–4.5 times (Table 4). According to several researchers, intercalation of clays with surfactants increases the fractional content of organic carbon of geosorbents, which makes them a more effective sorbent in relation to PAHs in a similar way to natural organic matter [39,40,41]. Intercalated surfactants on bentonite can be subjected to an aggregation procedure on a solid surface to form a surface micelle (admicella) or bilayers, providing places for the adsorption of PAHs [39]. Intercalation of bentonite with nonionic and amphoteric surfactants contributes to hydrophobization of the sorbent surface, which leads to more intensive adsorption of PAHs on organoclays compared with bentonite. This is most pronounced for lipophilic benzo(a)pyrene [42,43]. The highest S values (up to 96%) were observed for the adsorption of benzo(a)pyrene at its initial concentration in a solution of 10 μg g⁻¹. As the initial concentration of benzo(a)pyrene in the solution increased, the S parameter decreased to 40.8–53.2%—at 100 μg g⁻¹. In the adsorption of naphthalene on modified bentonites, the highest S values are characterized at the initial concentration of the pollutant in a solution of 60 μg g⁻¹, which is identical to the initial bentonite. This indicates that the adsorption of benzo(a)pyrene is more affected by the affinity of the pollutant with a hydrophobic surface, and the adsorption of naphthalene is influenced by the porosity characteristics of the sorbent. In general, organoclays with amphoteric PAHs (surfactant 1 and surfactant 2) adsorb polyarenes to a greater extent than organoclays with nonionic ones (surfactants 3, 4, 5). On average, the degree of sorption of benzo(a)pyrene by organoclays decreases in the series: organoclay with surfactant 1 ≈ organoclay with surfactant 3 > organoclay with surfactant 4 > organoclay with surfactant 2 > organoclay with surfactant 5 > bentonite. In terms of naphthalene adsorption by organoclays, this decreases in the series: organoclay with surfactant 2 ≈ organoclay with surfactant 5 > organoclay with surfactant 4 > organoclay with surfactant 3 > organoclay with surfactant 1 ≈ bentonite (Table 4).

The obtained isotherms of adsorption of benzo(a)pyrene and naphthalene by bentonite and its modifications of surfactants belong to the L-form according to the Giles classification [44] and to the I(b) group according to the IUPAC classification [45], which indicate the presence of micropores in the studied materials (Figure 3) [42]. The process of adsorption of benzo(a)pyrene and naphthalene by bentonite and its modifications by amphoteric and nonionic surfactants is practically monolayer, since in more cases it is best described by the Langmuir model (R² 0.766–0.984) [22,46] (Table 4). Nevertheless, the line of adsorption isotherms does not reach the plateau. This indicates that the non-sorption centers on the surface of organoclays and bentonite are not fully occupied by PAH molecules [46]. The best models for PAH adsorption were shown in Figure 3. Under the same experimental conditions, the adsorption of naphthalene is one order of magnitude lower than that of benzo(a)pyrene [47]. Studies by [48,49] noted that the sorption capacity of sorbents in relation to PAHs increased as the lipophilicity of pollutants increased. The proximity of adsorption isotherms to the ordinate axis is characteristic of benzo(a)pyrene, with the surface of organoclays at a pollutant concentration in solution not exceeding 30 μg g⁻¹ (0.12 mmol L⁻¹). With an increase in the concentration of benzo(a)pyrene to 80–100 μg g⁻¹ (0.40 mmol L⁻¹), the curve reaches a plateau, which indicates saturation of the surface of sorbents with a pollutant and a decrease in the binding strength in the benzo(a)pyrene–organoclay system [50]. The bond strength of naphthalene, based on the shape of the isotherms, is less pronounced due to the lower hydrophobicity of its molecule [51]. The shape of the isotherms of naphthalene adsorption by bentonite is close to the axis of the abscissa, since the sorbent and sorbate have a low bond strength (Figure 3).

The calculated parameters of Q_max benzo(a)pyrene and naphthalene by bentonite correspond to 0.08–0.10 mmol kg⁻¹. Modification of bentonite surfactant led to a significant increase in the predicted Q_max of benzo(a)pyrene and practically does not affect the Q_max of naphthalene. At the same time, in terms of the Q_max value of benzo(a)pyrene, organoclays form a series as follows: organoclay with surfactant 3 (0.36 mmol kg⁻¹) > organoclay with surfactant 1 (0.26 mmol kg⁻¹) > organoclay with surfactant 2 (0.25 mmol kg⁻¹) = organoclay with surfactant 5 (0.25 mmol kg⁻¹) > organoclay with surfactant 4(0.17 mmol kg⁻¹), for naphthalene: organoclay with surfactant 2 (0.10 mmol kg⁻¹) > surfactant 4 (0.09 mmol kg⁻¹) > organoclay with surfactant 1 (0.07 mmol kg⁻¹) ≈ organoclay with surfactant 3 (0.07 mmol kg⁻¹) > surfactant 5 (0.04 mmol kg⁻¹). The adsorption reaction of PAHs to bentonites at the R_L value is favorable, since the value of the indicator is <1 [33]. According to the results of the ΔG parameter, the process of adsorption of benzo(a)pyrene by the studied sorbents and naphthalene organoclay with surfactant 5 occurs spontaneously [22], with physical sorption predominating. This is consistent with the results of [52], where phenanthrene adsorption took place by capillary condensation in interlayer nanopores. The bond strength is defined by values ΔG of benzo(a)pyrene with the sorbent decreases in the following series: organoclay with surfactant 4 > organoclay with surfactant 1 ≈ organoclay with surfactant 5 > organoclay with surfactant 2 > organoclay with surfactant 3 > bentonite. Zero ΔG values during the adsorption of naphthalene on bentonite, organoclay with surfactant 1, surfactant 2, surfactant 3, and surfactant 4 indicate that the adsorption process of naphthalene can be reversible [53] (Table 4).

The K_F value indicates the amount of PAHs adsorbed at the unit steady-state concentration of the substance in the solution which correspond to the one. The K_F value in the Freundlich equation for benzo(a)pyrene and naphthalene sorbed on bentonite is 0.09 L mmol ⁻¹ and 0.03 L mmol ⁻¹, respectively. The use of organoclays led to an increase in the number of adsorbed PAHs; the value of K_F increased by 2.8–6.5 times for benzo(a)pyrene and 1.1–1.6 times for naphthalene. The highest values of K_F of benzo(a)pyrene were observed during adsorption by organoclay with surfactant 3, of naphthalene—with surfactant 2 and surfactant 4. The value of 1/n characterizes the homogeneity of the adsorbate in relation to the adsorbent [54]. With an increase in this parameter from 0 to 1, the homogeneity of the surface increases, and the limiting adsorption energy decreased with an increase in the surface concentration of pollutants [55]. The values of 1/n in the adsorption of benzo(a)pyrene are lower (0.36–0.63) than in the adsorption of naphthalene (0.41–0.99). With an increase in the homogeneity of the adsorbent surface, a decrease in the bond strength in the pollutant–organoclay system is observed, since the ΔG values for benzo(a)pyrene are higher than for naphthalene (

Table 5. Parameters of isotherms of adsorption of benzo(a)pyrene and naphthalene obtained by approximation by the Langmuir and Freundlich equations.

Sorbents	Langmuir					Freundlich
	KL, L∙mmol−1	Qmax, mmol kg−1	R2	ΔG, kJ∙L∙mol−1	Rl	KF, L∙mmol−1	1/n	R2
Benzo(a)pyrene
Bentonite	4.1	0.08	0.920	−3.50	0.38	0.09	0.63	0.869
organoclay with surfactant 1	21.3	0.26	0.924	−7.58	0.11	0.39	0.36	0.945
organoclay with surfactant 2	17.2	0.25	0.936	−7.04	0.13	0.39	0.41	0.904
organoclay with surfactant 3	10.0	0.36	0.951	−5.71	0.20	0.58	0.52	0.952
organoclay with surfactant 4	78.4	0.17	0.984	−10.81	0.03	0.25	0.24	0.812
organoclay with surfactant 5	21.1	0.25	0.766	−7.55	0.11	0.41	0.41	0.685
Naphthalene
Bentonite	1.0	0.07	0.538	0.00	0.56	0.03	0.87	0.955
organoclay with surfactant 1	1.0	0.07	0.855	0.00	0.56	0.04	0.99	0.928
organoclay with surfactant 2	1.0	0.10	0.971	0.00	0.56	0.05	0.75	0.960
organoclay with surfactant 3	1.0	0.07	0.939	0.00	0.56	0.04	0.85	0.930
organoclay with surfactant 4	1.0	0.09	0.918	0.00	0.56	0.05	0.79	0.891
organoclay with surfactant 5	5.0	0.04	0.955	−3.98	0.20	0.04	0.41	0.856

).

The results of PAH adsorption experiments on modified bentonites are comparable to the studies of Zhao et al. [41] and El-Nahhal and Safi [56]. At the same time, the obtained bentonites modified with nonionic and amphoteric surfactants more effectively adsorb PAHs in comparison with organoclays used in other studies. Thus, Zhao et al. [41] demonstrated that sepiolite modified with gemini surfactant adsorbs up to 0.0005 mmol kg⁻¹ phenanthrene. In the research of El-Nahhal and Safi [56], bentonite modified with chloride and bromide salts of benzyltriethylammonium were able to adsorb up to 0.0004 mmol kg⁻¹ phenanthrene. The adsorption of pyrene by bentonite by modified hexadecyltrimethylammonium bromide did not exceed 0.08 mmol kg⁻¹ [57]. The adsorption of acenaphthene, fluorene, and phenanthrene on montmorillonite composites with sodium alginate reached 0.0002 mmol kg⁻¹ [49].

To select the most effective sorbent, the values of the parameters S, K_L, Q_max, ΔG, R_L, and K_F are ranked according to the degree of influence of sorption efficiency and the average rank for each organoclay is calculated. Parameter ranking S, K_L, Q_max, and K_F is carried out in descending order, since the highest values of these parameters indicate a greater adsorption of PAHs by organoclays. For ΔG and R_L, the ranking is made in ascending order, since the lowest values indicate a stronger bond in the adsorbate–adsorbent system and emphasize the irreversibility of the adsorption process (

Table A1. Ranked adsorption parameters of benzo(a)pyrene and naphthalene isotherms of benzo(a)pyrene and naphthalene adsorption obtained by approximation by Langmuir and Freundlich equations, as well as the degree of sorption.

Sorbents	S, %	KL, g−1 μg	Qmax, μg g−1	ΔG, kJ∙L∙mol−1	RL	KF, µg g−1	Average Rank
Benzo(a)pyrene
Bentonite	6	6	6	6	6	6	6.0
organoclay with surfactant 1	1	2	2	2	2	3	1.8
organoclay with surfactant 2	4	4	3	4	4	4	3.8
organoclay with surfactant 3	2	5	1	5	5	1	3.6
organoclay with surfactant 4	3	1	5	1	1	5	2.2
organoclay with surfactant 5	5	3	4	3	3	2	3.6
Naphthalene
Bentonite	6	2	3	2	2	6	3.0
organoclay with surfactant 1	5	2	4	2	2	3	3.0
organoclay with surfactant 2	1	2	1	2	2	1	1.6
organoclay with surfactant 3	4	2	5	2	2	4	3.0
organoclay with surfactant 4	3	2	2	2	2	2	2.2
organoclay with surfactant 5	2	1	6	1	1	5	2.2

). The lowest values obtained when calculating the average rank for all parameters indicate a greater efficiency of the synthesized organoclays for the adsorption of PAHs, considering the adsorption capacity and bond strength of polyarenes with sorbates. Based on the results of the ranking, it was shown that the most effective in adsorption of benzo(a)pyrene and naphthalene are organoclay with surfactant 1 (amphoteric)with an average rank of 1.18 and organoclay with surfactant 2 (amphoteric)with an average rank of 1.6, respectively. At the same time, the adsorption of benzo(a)pyrene organoclay with surfactant 1 is characterized by K_L—21.3, Q_max 0.26 mmol kg⁻¹, K_F—0.39 L∙mmol⁻¹, and an average S value of 68.9%, with a favorable process of physical adsorption on a heterogeneous adsorbate surface. Adsorption of naphthalene on synthesized organoclay using surfactant 2 is characterized by the value of K_L—1.0, Q_max 0.07 mmol kg⁻¹, K_F—0.04 L∙mmol⁻¹, and an average S value of 3.3%, with a favorable physical adsorption process on a more homogeneous adsorbate surface.

4. Conclusions

The IR spectroscopy method confirmed that organoclays synthesized based on bentonite and various amphoteric (disodium cocoamphodiacetate and sodium cocoaminodipropionate) and nonionic (lauramine oxide, cocamide diethanolamine, and alkyl polyglucoside) organoclay surfactants have additional absorption bands due to additions of organic compounds, mainly represented by alkyl groups of surfactants used.

The processes of adsorption of low-molecular naphthalene and high-molecular benzo(a)pyrene by bentonite and organoclays made on its basis using amphoteric and nonionic surfactants are characterized. It was found that the maximum adsorption of benzo(a)pyrene by synthesized organoclays increased compared with the initial mineral, whereas the maximum adsorption of naphthalene did not change, whether increased or decreased (depending on the surfactant used for modification). At the same time, the adsorption of benzo(a)pyrene by bentonite and organoclays is an order of magnitude higher than their adsorption of naphthalene. The S values of benzo(a)pyrene bentonite vary from 5.3% to 19.9%, of naphthalene—from 2.1% to 3.9%, decreasing as the initial concentration of PAHs in the solution increases. The degree of sorption of benzo(a)pyrene, depending on the organoclay, decreases in the following series: organoclay with surfactant 1 ≈ organoclay with surfactant 3 > organoclay with surfactant 4 > organoclay with surfactant 2 > organoclay with surfactant 5 > bentonite. In terms of naphthalene adsorption, organoclays form a different series in descending order: organoclay with surfactant 2 ≈ organoclay with surfactant 5 > organoclay with surfactant 4 > organoclay with surfactant 3 > organoclay with surfactant 1 ≈ bentonite. The maximum adsorption value reaches 0.36 mmol kg⁻¹ during adsorption of benzo(a)pyrene by organoclay with surfactant 3 and 0.10 mmol kg⁻¹ during adsorption of naphthalene by organoclay with surfactant 2.

The experimentally obtained isotherms of adsorption of benzo(a)pyrene and naphthalene by bentonite and its modifications of surfactants belong to the L-form according to the Giles classification [40] and to the I(b) group according to the IUPAC classification [41]. The process of adsorption of benzo(a)pyrene by bentonite and organoclays is predominantly monolayer, while that of naphthalene is mainly multilayered. The adsorption of naphthalene by organoclays is an order of magnitude lower than that of benzo(a)pyrene. Modification of the organoclays leads to a significant increase in the predicted Q_max of benzo(a)pyrene and practically does not affect the Q_max of naphthalene. With an increase in the homogeneity of the adsorbent surface in relation to PAHs, a decrease in the bond strength in the pollutant–organoclay system is observed, which is more pronounced for benzo(a)pyrene.

Considering the favorability of the adsorption process and the strength of the bond in the sorbate–sorbent system, the most effective are organoclays based on bentonite and amphoteric surfactants: disodium cocoamphodiacetate for benzo(a)pyrene and sodium cocoiminodipropionate for naphthalene.