Highly efficient removal of polycyclic aromatic hydrocarbons from water using organophilic clays obtained from simultaneous ion exchange and grafting

doi:10.21203/rs.3.rs-2583942/v1

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Highly efficient removal of polycyclic aromatic hydrocarbons from water using organophilic clays obtained from simultaneous ion exchange and grafting

https://doi.org/10.21203/rs.3.rs-2583942/v1

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The present work analyses the polyaromatic hydrocarbons (PAHs) removal from water through batch adsorption processes, using two low-cost surfactant-grafted bentonites (Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5).These materials were prepared via a modification of montmorillonite (Mt) and acid-activated bentonite (H5-Mt) by intercalation and grafting of (3-trimethoxysilylpropyl) octadecyldimethylammonium chloride (TPODAC). Anthracene (ANT) was used as PAH model to assess the sorption capacity of each material. It was found that the selective modification of clay mineral has higher adsorption capacity for ANT compared with raw clay. Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 removed 91 % and 85 % of ANT from water, respectively, while the raw bentonite (Mt) removed only 22 % under the same conditions. The adsorption isotherms were S-shaped and were fitted by the Frumkin-Fowler-Guggenheim (FFG) model, exhibiting an adsorption cooperative behaviour. The results indicate that hydrophobic and cation-π interactions ruled the ANT sorption on the modified clays. Simultaneous adsorption experiments with a mixture of 16 PAHs showed a high removal capacity of both modified clays. The promising results obtained in this work highlights the use of the TPODAC-grafted bentonite as potential sorbents to removal PAHs in a simple, economical, and environmentally friendly way.

adsorption

polyaromatic hydrocarbons

bentonite

surfactants

grafting

Polyaromatic hydrocarbons (PAHs) are organic compounds found naturally in coal and crude oil and can also be found in the environment mostly as a result of incomplete combustion of fossil fuels and anthropogenic sources of pollution (Akinpelu et al., 2019; Behera et al., 2018; Hedayati and Li, 2020; Satouh et al., 2021). Once in the environment, based on their physicochemical properties, PAHs are distributed among different matrices such as gases, particles, soils, and sediments. From these environmental compartments, PAHs are easily carried by rainwater and runoff to nearby water bodies (Balati et al., 2015; Hedayati and Li, 2020). Within the wide variety of PAHs present in the environment, the US EPA has defined sixteen priority PAHs for testing in environmental matrices due to their high toxicity, mutagenicity, and carcinogenicity to various organisms (Akinpelu et al., 2019; Hedayati and Li, 2020).

These concerns have driven the development of sustainable technologies to remove PAHs from water, which include a wide range of processes such as physical, chemical, thermal and biological, among others (Akinpelu et al., 2019). Considering its operational simplicity and low cost for maintenance the adsorption method is one of the best alternatives (Akinpelu et al., 2019; Lamichhane et al., 2016). Activated carbon is often used as a sorbent to remove PAHs from water (Hedayati and Li, 2020; Lamichhane et al., 2016), however, its application is limited by its high cost (Hedayati and Li, 2020) and its adsorption capacity can be drastically decreased in presence of natural organic matter (Lemić et al., 2007). Other adsorbents such as biochar is also used as adsorbent of PAHs (Lamichhane et al., 2016), but it may undergo weathering as a result of ambient climatic conditions, which can cause an oxidative chemical transformation of fresh biochar. This leads to biochar aging and reduces the availability of interior hydrophobic sites (Kim et al., 2021). Others materials such as minerals (clays and zeolites) have been proposed to successfully remove PAHs (Bruna et al., 2012; Hedayati and Li, 2020; Lamichhane et al., 2016). Clays are widely used as adsorbent, due to their extraordinary surface properties, common availability, low-cost and to be non-toxics (Ghavami et al., 2017; Guégan, 2018; Lawal et al., 2019). Its natural inorganic composition gives them a hydrophilic nature (Bergaya et al., 2006), and as a result, they have a low affinity for non-ionic organic pollutants. However, clay minerals can be modified for targeting a particular contaminant. Several clay minerals have been modified to adsorb PAHs from aqueous solutions (Chang et al., 2015; Hedayati and Li, 2020; Lamichhane et al., 2016). In particular, the organo-functionalization of their surface allows a successful sorption of hydrophobic contaminants. The main way to prepare the organoclays is via ionic exchange between a cationic surfactant and natural exchangeable cation of the clay mineral or via grafting of organic functionalities (organosilane) to the hydroxyl groups mainly presence at the edge of the clay mineral layers (Jaber and Miehé-Brendlé, 2009; He et al., 2010; Parolo et al., 2014; Chang et al., 2015; Slaný et al., 2019; Mekhzoum et al., 2020;). Nevertheless, an important drawback of organoclays prepared by intercalation is the progressive released of the surfactant that can occur into the environment resulting in secondary pollution, and for organoclays prepared by grafting is the limited amounts of grafted organic groups (Jaber and Miehé-Brendlé, 2009). Also, acid-activated is another textural modification of clays that have been utilised in a variety of industrial applications due to their vast array of outstanding advantages. These include large surface areas, high reactivity, high surface acidity, non-toxicity, easy availability, low cost, environmental friendliness, and high chemical and mechanical stability (Komadel, 2016). However, acid-activated clays have not been extensively studied regarding their potential to adsorbed contaminants to advantage this high specific surface area. To take advantage of the best features of adsorption process, overcome the mentioned limitations and explore new modified clay minerals, we previously proposed the incorporation of a cationic surfactant, (3-trimethoxysilylpropyl) octadecyldimethylammonium chloride (TPODAC), to raw bentonite surface and activated bentonite via a simple procedure, in which a relatively high amount of TPODAC was bonded to bentonite via grafting, cationic exchange, and cross-linking bonding (Funes et al., 2020). On the other hand, delocalized π electrons of PAH molecules may interact with electron-deficient or positively charged species via electron-donor–acceptor (EDA) interactions or electrostatic attractions (cation-π bonding), respectively. Previous studies suggested that, in addition to hydrophobic effects, specific π-π EDA interactions also take place between π-donor and π-acceptor compounds (Qu et al., 2008). Cation-π interactions between tetra-alkyl ammonium cations and the sorbed aromatic compounds on clay surfaces has also been proposed (Hedayati and Li, 2020; Hou et al., 2015; Qu et al., 2008). The TPODAC molecule contains hydrophobic hydrocarbon tails and a positively charged amino group that could interact with PAHs via hydrophobic and cation-π interactions. The TPODAC modified clay minerals possess high hydrophobicity and have high stability in aqueous media, highlighting their potential application as adsorbent for hydrocarbons removal (Funes et al., 2020).

The aim of this work was to evaluate the performance of two novel organophilic clays (Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5), with different textural and structural properties, for the removal of PAHs from water. To this end, anthracene (ANT) was used as a model compound of PAHs. The influence of contact time, adsorbent dosage, ANT concentration and competitive adsorption of a PAHs mix has been investigated. The ANT adsorption and is interaction mechanism was further assessed by several isotherm models and kinetic models. To the best of our knowledge, no study in the literature has reported on the binding of organic agent via grafting and cation exchange to harness the benefits of both interaction mechanisms in the pollutant´s removal processes.

2.1. Materials

Natural sodium bentonite from the Allen formation (Neuquén-Argentina) with mineral composition of ~ 90% smectite (montmorillonite), quartz (2%), gypsum (1.5% − 2%), feldspar (0.5% − 1%), calcite (0.4%) and zeolites (3% − 4%). We called this material Mt. The structural formula is (Ca_0.38Na_0.009Mg_0.027)((Si_7.706Al_0.294)(Al_2.852Fe³⁺_0.528Mg_0.564Ti_0.049)O₂₀(OH)₄) and its cation exchange capacity (CEC) 104.6 cmol kg^− 1(Musso et al., 2010). Mt has SSA_BET value of 51 m² g^− 1 and VTP of 0.08 cm³ g^− 1(Funes et al., 2020). The organic surfactant, dissolved 42% in methanol, is (3-trimethoxysilylpropyl) octadecyldimethylammonium chloride (TPODAC) and was purchased from Sigma-Aldrich. HCl, methylene chlorine, ethanol and methanol were purchased from Ciccarelli. Solid anthracene was purchased from Mallinckrodt (purity 95%) and PAHs mix solution (Naphthalene (NAP), Acenaphthylene (ACNL), 2-bromonaphthalene (2BrNAP), Acenaphthene (ACN), Fluorene (FLU), Phenanthrene (PHE), Anthracene (ANT), Fluoranthene (FLT), Pyrene (PYR), Benzo[a]anthracene (B[a]ANT), Chrysene (CHR), Benzo[b]fluoranthene (B[b]FLT), Benzo[a]pyrene (B[a]PYR), Indene[1,2,3-c,d]pyrene (IND[1,2,3-cd]PYR), Dibenzo[a,h]anthracene (DB[ah]ANT) and Benzo[g,h,i] perylene (B[ghi]P)) whose chemical properties are list in table S1 was purchased from Sigma-Aldrich (purity > 99%). The preparation of ANT aqueous solution and PAHs mix aqueous solution are described in the Supporting Information (S.1 and S.2).

2.2 Synthesis of Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5

Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 were prepared and characterized following the procedure that was previously reported (Funes et al., 2020). Briefly, 1 g Mt and 2.1 mL of TPODAC solution were dispersed in 100 mL of ethanol/water solution and heat by refluxed for 5 h at 80°C. The solid was separated, washed, and dried in oven at 30°C for 24 h. A similar procedure was applied for H5-Mt (activated clay with HCl 5M).

2.3 Adsorption experiments

All adsorption studies were performed at 21 ± 1 ◦C in triplicate. Adsorption kinetics was initially done to evaluate the time needed to achieve equilibrium, and then adsorption studies under equilibrium conditions were carried out to investigate the effect of adsorbent dose and PAHs concentration on the PAH removal. The amount of PAH adsorbed was determined using the mass balance equation:

$${q}_{e}=\frac{V.({C}_{i}-{C}_{eq})}{m} \left(1\right)$$

where q_e is the amount of PAH adsorbed per unit mass of adsorbent in µg g^− 1, C_eq and C_i are the equilibrium PAH concentration and the initial PAH concentration in aqueous solution in µg L^− 1, V is the volume of solution in L and m is the adsorbent mass in g.

The percentage removal of PAH from solution was calculated from the equation:

$$\%R=\left(1-\frac{{C}_{eq}}{{C}_{i}}\right).100 \left(2\right)$$

where %R is the percentage removal.

In each assay, a reference test without adsorbent was included to account for losses of PAHs due to processes other than adsorption to the solid (i.e. volatilization, adsorption to flask walls or filter).

2.3.1 Anthracene adsorption Kinetics

50 mg L^− 1 dosages of Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 and 1600 mg L^− 1 dosage for Mt were used for adsorption experiments. The ANT concentration was 60 µg L^− 1. Each system was first dispersed by sonication for 5 min and then shaken in orbital shaker (100 rpm). After different contact time (0.25-10 h), 3 mL aliquots were removed and filtered with Sartorius AP40 EM glass microfiber membranes and the supernatant were analysed by fluorescence spectroscopy. The assays were carried out in 250 mL glass flask at pH 6 and 21 ± 1°C.

2.3.2 Anthracene adsorption experiments

Dosages between 10–400 mg L^− 1 for Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 and between 100–6400 mg L^− 1 for Mt were used for equilibrium adsorption experiments. The ANT concentration range was 20–70 µg L^− 1. Each system was dispersed by sonication for 5 min and shaken at pH 6 and 21 ± 1°C in orbital shaker (100 rpm) for 5 h (determined by kinetic experiments). Then, 10 mL aliquots of dispersion were filtered and analysed by fluorescence spectroscopy in a Hitachi F-7000 spectrofluorometer (250 nm excitation wavelength, slit: 2.5 nm, 300 nm to 450 nm emission wavelength). Calibration curve and, quantification and detection limits, are shown in Fig. S1, in the Supporting Information). The assays were carried out at pH 6. Previous studies have shown no pH effect on adsorption, as ANT molecule does not have dissociable protons (Angove et al., 2002).

2.3.3 PAHs adsorption experiments

Simultaneous adsorption experiments with a mixture of 16 PAHs were performed. Experimental conditions were: 200 mg L^− 1 and 400 mg L^− 1 adsorbent dosages, 24 h of contact time, pH 6 and 21 ± 1°C in shaker (100 rpm). The initial concentrations of each PAH in water ranging between 2 and 35 µg L^− 1 (total concentration of PAHs in the solution was 276 µg L^− 1). The initial and equilibrium PAH concentration were determined by gas chromatography (GC calibration data is shown in Table S1, in the supporting information). 100 mL of aqueous solution was extracted, after filtration with Waltman S&S N°589 filter, with three dichloromethane volume of 5 mL each. The organic fraction was concentrated to 1 mL under a nitrogen stream (at 25°C and 3 psi pressure) and was analysed by GC in an Agilent 6890 series GC system, with a flame ionization detector (FID), equipped with a Zebron brand ZB-5 30 m x 0.25 mm ID x 0.25 µm column and automatic injector. Column temperature was 35°C for 1 min; it increased up to 300°C with ramping at 6°C min^− 1 and was held 5 min at 300°C.

3.1 Physicochemical characterization

Mt/TPODAC-1.5, H5-Mt/TPODAC-1.5 and Mt were characterized in a previous study (Funes et al., 2020). Mt/TPODAC-1.5 conserve the crystalline structure typical of montmorillonite (Mt), with an increase of interlayer space by TPODAC intercalation and cross-linking. XRD diffraction pattern of Mt/TPODAC-1.5 shown a primary reflection at 3.57° (2θ), giving an interlayer spacing (d(001)) of 2.47 nm. Additionally, TPODAC interact with montmorillonite via covalent bonds (grafting) with Mt. In particular, the mass percentage of TPODAC bound to Mt is 30%, where the percentage of TPODAC bounding by grafting and intercalation were close to 5% and 25%, respectively. In contrast, H5-Mt/TPODAC-1.5 is an amorphous material (no diffraction peaks and crystal structures were observed from the XRD diffraction patterns and TEM micrographs, respectively). H5-Mt/TPODAC-1.5 possess 12% surfactant percentage, where close of 7% and 5% of TPODAC were bounding via grafting and intercalation, respectively.

Mt/TPODAC-1.5 possesses lower BET specific surface area (SSA_BET) than H5-Mt/TPODAC-1.5 (16 m² g^− 1 vs 73 m² g^− 1). Mt/TPODAC-1.5 showed a volume total of pore (VTP) 0.06 cm³ g^− 1 (mainly mesopores), which can be ascribed to pores that were generated in the interparticle space, while that H5-Mt/PODAC-1.5 showed a VTP of 0.34 cm³ g^− 1 (mainly mesopores) which are likely due to the presence of amorphous silica. Both hybrid materials are chemically stable in water, in a wide pH range (3–10), in water with high salinity (1.4M NaCl) and hexane.

3.2 Anthracene adsorption experiments

3.2.1 Anthracene adsorption kinetics

Figure 1 shows the ANT adsorption kinetics toward Mt/TPODAC-1.5, H5-Mt/TPODAC-1.5 and Mt. It can be seen that after approximately 5 h, the equilibrium was reached for the three materials. Mt/TPODAC-1.5 showed the highest q_e value (close to 555 µg g^− 1), which is much higher than the raw bentonite (more than one order of magnitude). This highlights the significant role of TPODAC in the adsorption of ANT. Also, the difference in the adsorption capacity between Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 is consistent with a higher surfactant content in Mt/TPODAC-1.5 sample (see section 3.1).

Three adsorption kinetic models (pseudo-first order, pseudo-second order and intraparticle diffusion) were selected to estimate the adsorption mechanism and quantify the extent of ANT uptake. The three equations employed to model the sorption are expressed in lineal form as follows (Peralta et al., 2019):

pseudo–first order equation:${log} \left({q}_{e}-{q}_{t}\right)=log{ q}_{e}-{k}_{1}t \left(3\right)$

pseudo–second order equation: $\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}.{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$\left(4\right)$

intraparticle diffusion equation:${q}_{t}={k}_{id}.{t}^{1/2}+C \left(5\right)$

Where q_t and q_e are the ANT adsorption capacity (µg g^− 1) of adsorbent at time t (min) and equilibrium condition, respectively; k₁ (min^− 1) and k₂ (g µg^− 1 min^− 1) are the equilibrium rate constants for pseudo–first order sorption and pseudo–second order sorption, respectively and k_id is the intraparticle diffusion rate constant (µg g^− 1 min^− 1/2) for the diffusion process. The C (µg g^− 1) value in the intraparticle diffusion equation, relates to the contribution of the surface sorption and deviation from the origin is due to the difference in the rate of mass transfer in the initial and final stages of adsorption process(Dawodu and Akpomie, 2014). For pseudo–first order kinetic, the plot of log(q_e− q_t) vs t was used to determine the slope (k₁) and q_e. For pseudo–second order kinetic, the linear plot of t/q_t against t is used for calculated the values of q_e and k₂. From this model, the initial adsorption rate, h_i (µg g^− 1 min^− 1 can be estimated as (Costa et al., 2017):

${h}_{i}={k}_{2}.{q}_{e}^{2} \left(6\right)$

Table 1 show the experimental data. The best fit of the experimental data was obtained with the pseudo-second order model (Fig.S.3). The adsorption kinetics order suggest that the adsorption rate depend on adsorbent and adsorbate concentration. The experimental adsorption capacity (q_e(exp.)) were close to those calculated from the pseudo–second order model (q_e(cal.)), providing further confirmation of the fit of the experimental data to this model (Table 1). On the other hand, the initial adsorption rate values were higher for TPODAC-grafted bentonites than Mt (Table 1), which indicates that the presence of hydrocarbon tails in both Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 could increase the initial adsorption rates in addition to increasing the adsorption capacity.

The pseudo-second order model assumes that the rate-controlling step could be associated with chemisorption, including valency interactions via electron sharing or exchange between the adsorbent and sorbate (Qu et al., 2008; Hedayati and Li, 2020). In our experimental conditions, PAHs can share electrons with ammonium groups, where aromatic structures in PAHs can act as electron donors. Theoretical investigations explicitly demonstrate that cation − π interactions between tetramethylammonium (TMA⁺ ) cations and aromatic rings play the key role in adsorption of organics over tetramethylammonium cation modified montmorillonite (MMT − TMA) (Hou et al., 2015). The experimental study made by Qu et al. (2008) indicated that the PAHs show more than 10-fold higher sorption than the chlorobenzenes having close hydrophobicity but fewer delocalized π electrons. Taking this into account, the high ANT adsorption capacity onto organophilic clays, can be accounted for the cation − π interactions between positively-charged quaternary ammonium of TPODAC and the aromatic rings of PAHs in addition with hydrophobic interactions (Fig. 2).

In the case of Mt, the ANT adsorption capacity is very low despite having an appreciable SSA_BET value. The interaction between the clay mineral and the aromatic hydrocarbons can be explained by the presence of the interlayer cations that interact with π electrons of the aromatic ring (Ugochukwu, 2019) but the nature of the interlayer cation determines the interaction degree, thereby K⁺ interact better than divalent cations due to its least hydration energy (Ugochukwu, 2019). In addition, montmorillonite surface has a hydrophilic character due to the isomorphous substitution and the presence of hydroxyl groups, especially in edges of clay mineral particles (Bergaya et al., 2006), thus it is expected to have a low affinity for PAHs.

In order to investigate the adsorption rate-limiting step and diffusion mechanism, the intra-particle diffusion model was used. Fig. S.4 shows good fits of the intraparticle diffusion model with a single linear region and non-zero intercepts (C) for the three materials. This evidence that intraparticle diffusion is not the only mechanism controlling the adsorption process of ANT. In turn, organophilic clays showed higher values of C than Mt, indicating that there is a higher contribution of surface adsorption on the adsorption rate.

Table 1

Kinetic results for ANT adsorption onto Mt, H5-Mt/TPODAC-1.5 and Mt/TPODAC-1.5.
			Adsorbents
Model	constant	Mt	H5-Mt/TPODAC-1.5	Mt/TPODAC-1.5
	q_{e (exp)} (µg g^− 1)	15	256	555
pseudo-first order	q_{e (calc)} (µg g^− 1)	12	128	440
	K₁ (min^− 1)	1.9E-03	3.0E-03	5.9E-03
	R²	0.787	0.691	0.930
pseudo-second order	h_i (µg g^− 1 min^− 1)	0.3	13.4	13.2
	q_{e (calc)} (µg g^− 1)	16	261	611
	k₂ (g µg^− 1 min^− 1)	1.2E-03	2.0E-04	3.5E-05
	R²	0.997	0.994	0.992
	k_id (µg g^− 1 min^− 0.5)	0.4	4.4	17.1
Intraparticle diffusion	C (µg g^− 1)	7	161	185
	R²	0.938	0.766	0.975

3.2.2 Adsorbent dosage and initial ANT-concentration effects on removal efficiency

The effect of adsorbent dosage on ANT removal efficiency is shown in Fig. 3. For a given adsorbent dosages value, Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5 showed removal efficiencies much higher than Mt, which is attributed mainly to the presence of the surfactant. For instance, a dose of 200 mg L^− 1 of Mt/TPODAC-1.5 is required to remove 80% of ANT, which is 32-fold less than using raw bentonite (Mt).

Figure S6 shows the adsorption capacity at different initial concentrations of ANT and different doses of adsorbent. It is observed that the adsorption capacity increases with the increase of the ANT initial concentration. This is due to the increasing ANT concentration gradient between the solution and the adsorbent surface, which provides a significant driving force to overcome the resistance to the mass transfer. An inverse trend was obtained between the adsorption capacity of the materials and the adsorbent dosage for all ANT concentration values. The adsorption capacity of all materials decreases when the adsorbent dosage increases probably because of a decrease in the adsorption area available for ANT molecules due to overlapping or adsorption sites aggregation. Although the total adsorption area increases with increasing dosage, the available adsorption area per unit mass may be less due to agglomeration effects of the material particles, thus the number of available adsorption sites per unit mass and the amount of ANT per unit mass of adsorbent also decreases. To better visualize the effect of adsorbent dose and ANT concentration, Fig.S5 in the supporting information, shown the remaining ANT concentration in the supernatant using different dosage of adsorbent and initial ANT concentrations.

To determine the maximum adsorption capacity of adsorbents and to understand the adsorption mechanisms, Langmuir, Freundlich, Temkin, Dubinin–Radushkevich (D–R) and Frumkin-Fowler-Guggenheim (FFG) isotherms (see models’ explanation and equations in supporting information) were used to model the experimental data. The obtained isotherm parameters are shown in Table 2. For Langmuir, Temkin and D-R models the correlation coefficients (R²) are low, indicating a poor fit of the isotherm to the adsorption of ANT. In contrast, the FFG model describes an appropriate fit to the experimental data. Figure 4 shows the adsorption isotherms of ANT at pH 6 and 21 ± 1°C and the FFG model fit. The inset shows the plot in logarithmic scale to better observe the behaviour at low equilibrium concentrations.

These isotherms can be classified as S-type (sigmoid), subgroup type I, according to the classification found in the literature (Giles and Smith, 1974). The adsorption of hydrophobic compounds on clays commonly give S-type isotherms, indicating the presence of a cooperative adsorption, which occurs when the adsorbate-adsorbate interaction has an effect on the adsorption of new adsorbate molecules. (El-Nahhal and Safi, 2004; Limousin et al., 2007). ANT was poorly adsorbed on raw bentonite, whereas its adsorption on organophilic clays resulted in an increase in the adsorbed amounts at the same equilibrium concentration and the maximum adsorption capacities (q_max) obtained by the FFG model (Table 2) were significantly higher for organophilic clays. It is likely that the high adsorption capacity of Mt/TPODAC-1.5 is due to high surfactant percent (%S), which promotes hydrophobic and cation − π interactions and partitioning phenomena. In addition, H5-Mt/TPODAC-1.5 adsorbs more ANT than expected based on the %S value. This could be because it has a higher organic-grafted fraction and provides a high number of cation-π interactions since the ammonium group does not interact electrostatically with the negatively charged mineral surface because it is grafted through the alkoxy groups and is more available to interact with the electronic density of ANT rings. On the other hand, the significant curvature of ANT-Mt isotherm indicates that ANT sorption onto Mt is not caused by partitioning, but via physical attractive forces. Similar results were found for phenanthrene adsorbed toward sodium-montmorillonite and modified montmorillonite with different organic cations (El-Nahhal and Safi, 2004). The smoothest curvature of the organophilic clays isotherms is probably accounted for the relatively large amounts of surfactant bonded to the clay that allow a partitioning effect. The K values obtained from the FFG model indicate that the adsorbent-adsorbate affinity in the organophilic clays is higher for Mt/TPODAC-1.5 than for H5-Mt/TPODAC-1.5 (see Table 2), while the b values (negative in all cases) indicate that there is attraction between ANT molecules when they are adsorbed (Koopal et al., 2020). This implies a side-by-side association between ANT adsorbed molecules suggesting a vertical orientation of ANT molecules at the surfaces (El-Nahhal and Safi, 2004).

Table 2

Langmuir, Freundlich, Temkin, Dubinin–Radushkevich and Frumkin-Fowler-Guggenheim isotherm constants, for the adsorption of ANT toward Mt, H5-Mt/TPODAC-1.5 and Mt/TPODAC-1.5.
			Adsorbents
Model	Constant	Mt	H5-Mt/TPODAC-1.5	Mt/TPODAC-1.5
	q_max (µg g^− 1)	28	1845	1712
Langmuir	K_L (L µg^− 1)	0.034	0.008	0.018
	R²	0.781	0.543	0.792
	n	1.02	0.85	0.97
Freundlich	K_F (µg g^− 1)(L µg^− 1)^1/n	0.7	11.5	26.0
	R²	0.850	0.957	0.932
	A (L µg^− 1)	0.170	0.201	0.286
Temkin	B (µg g^− 1)	14	381	415
	R²	0.629	0.701	0.622
	q_max (µg g^− 1)	15	341	448
Dubinin-Radushkevich	β (mol² J^− 2)	5.0E-06	4.0E-07	2.0E-06
	R²	0.383	0.474	0.510
Frumkin-Fowler-Guggenheim	q_max (µg g^− 1)	65	2700	3300
	K (L µg^− 1)	4.9E-03	5.5E-03	6.5E-0.3
	b	-3.30	-2.60	-2.60
	R²	0.890	0.954	0.940

Figure 5 shows the PAHs removal efficiency obtained for the organophilic clays with two different dosages (200 and 400 mg L^− 1). When ANT is adsorbed from PAHs mixture with Mt/TPODAC-1.5 at 200 and 400 mg L^− 1 dosages, ANT removal efficiencies (90 ± 5% and 90 ± 3% respectively) are comparable with that obtained when ANT is only used as contaminant (82 ± 3% and 91 ± 5% respectively, see the section 3.2.3), which indicates that the adsorption sites of Mt/TPODAC-1.5 have not yet been saturated. Similar results are observed for H5-Mt/TPODAC-1.5.

3.3 PAHs mixture adsorption experiments

In order to analyse the performance of the organophilic clays in competitive experiments, the simultaneous adsorption of 16 PAHs was investigated. Figure 5 shows the removal efficiency of 16 PAHs after 24 h time contact. PAHs were ordered according to log of n-octanol-water partition coefficients (log K_ow) that is a measure of the hydrophobicity (see Table S2). The adsorption was higher for the compounds with larger log K_ow values. In particular, Mt/TPODAC-1.5 showed higher removal efficiency for the more soluble PAHs than H5-Mt/TPODAC-1.5 (low molecular weight PAHs -LMW PAHs- with 2 and 3 rings in their structure), while there were no significant differences for less soluble PAHs (high molecular weight PAHs -HMW PAHs- of 4 or more rings) between both sorbents. With a dosage of 400 mg L^− 1 for both hybrid materials, a complete removal of most of the HMW PAHs were obtained. These results indicate that the PAHs adsorption depend on K_ow values, which is consistent with a hydrophobic adsorption mechanism. However, the cation-π interactions must be considered since PAHs with higher K_ow values possesses a more extended electronic π system that can interact with the tetra alkyl ammonium cation of surfactant molecules (Hedayati and Li, 2020).

In this study, organophilic clays with a high chemical stability in water, prepared from a simple and economical method were used with an excellent performance in the removal of PAHs. The presence of TPODAC significantly improves the adsorption capacity of raw bentonite. This modification allows to significantly reduce the dosage of adsorbent with respect to the raw material to achieve the same performance. The smoothed S-type shape isotherm suggest that ANT undergoes a partitioning process between the TPODAC and water, in addition to π-cation interactions. The FFG model was the best adsorption model that fit the isothermal experimental data and suggest that there are attractive interactions between the ANT molecules when they are adsorbed on the surface of the materials. Although the acid treatment of bentonite increases the specific area of the H5-Mt/TPODAC-1.5, the lower amount of TPODAC rule the low PAH removal performance.

This study demonstrated that with a low dose of adsorbent, up to 100% PAHs removal efficiency can be achieved. The good performance of Mt/TPODAC-1.5 makes it suitable for testing their efficiency in adsorption column systems for continuous water treatments, limiting the risk of secondary pollution. This research contributes to the water remediation proposal in a simple, economical, efficient, and environmentally friendly way.

Ethical Approval: No ethical approval was necessary for this study

Consent to Participate: All authors are aware of, and accept responsibility for the manuscript.

Consent to Publish: All authors are accepting the publish of the manuscript

Author contributions: Israel G. A. Funes: Conceptualization, Investigation, Writing - Original Draft, Methodology. Luciano Carlos: Conceptualization, Supervision, Writing - Review & Editing. Maria E. Parolo: Conceptualization, Supervision, Writing - Review & Editing.

Funding: This work was supported by Universidad Nacional del Comahue, Argentina, Project 04/I217 and ANPCyT (PICT 2017-1847). This study was conducted within the Strategic Program of Training of Human Resources for Research and Development (PERHID) of the CIN.

Competing Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Availability of data and materials: All data generated or analyzed during this study are included in this published article

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Highly efficient removal of polycyclic aromatic hydrocarbons from water using organophilic clays obtained from simultaneous ion exchange and grafting

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Abstract

Figures

1. Introduction

2. Experimental Section

2.1. Materials

2.2 Synthesis of Mt/TPODAC-1.5 and H5-Mt/TPODAC-1.5

2.3 Adsorption experiments

2.3.1 Anthracene adsorption Kinetics

2.3.2 Anthracene adsorption experiments

2.3.3 PAHs adsorption experiments

3. Results and discussion

3.1 Physicochemical characterization

3.2 Anthracene adsorption experiments

3.2.1 Anthracene adsorption kinetics

3.2.2 Adsorbent dosage and initial ANT-concentration effects on removal efficiency

3.3 PAHs mixture adsorption experiments

4. Conclusions

Declarations

References

Supplementary Files

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