A facile approach for achieving an effective dual sorption ability of Si/SH/S grafted sodium montmorillonite

Abstract

In this study, sodium montmorillonite was functionalized with SH, S and Si functional groups using four different soil modifiers, 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane [BAT], 3,3′-tetrathiobis(propyl-triethoxysilane) [TP], thiodiglycol bis(3-aminocrotonate) [TDBA] and (3-mercaptopropyl)trimethoxysilane [MPTMS], for the effective uptake of both organic and inorganic pollutants. The uptake of inorganic (Cu²⁺, Zn²⁺) and organic [benzene, toluene, ethylbenzene and p-xylene (BTEX)] pollutants was studied with individual as well as binary mixtures. Based on the experimental results, the soil modifier MPTMS best improved the sorption characteristics and, among two metal ions, Cu²⁺ showed enhanced adsorption. Sorption of BTEX was not correlated with a single parameter and hence it differed for the different cases based on the organic matter content. The obtained log K_OC and log K_OM values of BTEX in this study for modified montmorillonite are comparatively larger than those of unmodified montmorillonite or natural soil. Sorption carried out in the binary mixtures showed that there is no interaction between these pollutants and the presence of one did not retard the adsorption of another. The uptake phenomenon was influenced by various combined factors such as the nature, surface charge and surface area of the modified soils.

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1. Introduction

In general, pollutants are introduced into the environment through natural phenomena and human activities.¹ Although certain organic and inorganic materials are in small amounts necessary for the usual process of biological cycles, most of them become toxic at high concentrations which is extremely risky.¹ Thus, preserving soils from dilapidation and particularly from pollution is of more and more concern. The concentration and mobility of such pollutants in soils and clays have been extensively studied in recent decades since these soils or clays are used as barriers in landfills to avoid contamination of the subsoil and groundwater.² These linings are mainly constituted of bentonite (montmorillonite), which can take up both organic and inorganic pollutants via different mechanisms.³ In order to predict the nature of these pollutants in soil or clay, it is important to know the behavior of such soil or clay systems.⁴ A variety of surface and structural properties determine the uptake ability of clay or soil which can be desirably altered by various modification processes.⁵ Clay and other layered silicate clays are naturally hydrophilic and the natural form of soil or clay is considered to be an ineffective adsorbent for neutral organic contaminants such as polycyclic aromatic hydrocarbons. This limitation of adsorption capacity of clays and soils can be overcome by modification processes. Thus, the uptake properties of clay or soil for neutral polycyclic aromatic hydrocarbons can be significantly improved by replacing the neutral interlayer cations with large organic cations.^6,7 These modified organo-clays have been effective in the removal of various neutral organic contaminants.^8–10 Handling and modification of the adsorbents are easily achieved¹¹ and such modification processes are really rapid and particle size-controlled. Surface modified soils or clays by means of cationic surfactants have been successfully proven to be suitable adsorbents for inorganic and organic contaminants.¹² During the modification process, the structure and properties, in particular the basal spacing of the modified soil or clay, are significantly affected. However, the treatment with a cationic surfactant is not effective in the case of simultaneous pollution with both organic and heavy metal pollutants because cationic heavy metals are weakly adsorbed, due to the hydrophobic nature of the modified adsorbents. Thus, the results reported by other researchers^13,14 clearly indicated that cationic surfactant-modified adsorbents favored only the uptake of organic pollutants, but simultaneously decreased the adsorption of inorganic pollutants. In the case of thiol-functionalized montmorillonite, a problem of accessibility of the grafted sites was witnessed.¹⁵ Thus, a special surface modifier, an amphoteric modifier which has both negative and positive charges that could adsorb cationic and anionic pollutants, came into use.¹⁶ However, it is obvious that as an amphoteric modifier has both negative and positive charges, there is no significant surprise in acting as a dual adsorbent for both organic and inorganic pollutants. Also, so far a majority of soil or clay systems have focused on organic pollutants only and those on the adsorption of several pollutants are limited. The main restrictions of these above mentioned techniques are found either in their selectivity or in the extent of their effects. Thus, it is important to go for alternative approaches which are promising, effective and interesting for the removal of both organic and inorganic pollutants. We believed that the introduction of either S or thiol (SH) groups into montmorillonite would definitely lead to an improved affinity of modified montmorillonite in terms of improving the sorption capacity for certain metal ions such as Cu²⁺, Pb²⁺ and Zn²⁺, which are classified as chalcophiles. In the case of S or SH functionalized soils, the electron pair of the functional group is not shared after it is bound to the soil due to the special structures, which is the advantage of this method. Due to the functionalization, the surface energy of montmorillonite would decrease and the interlayer spacing is expected to expand. Thus, the basal spacing of the resulting modified montmorillonite depends on the chemical structure of the modifier, degree of ion exchange, and silicate layer thickness. So, we have chosen these different modifiers and studied how these different modifiers affect the above said properties of montmorillonite. Thus, these modifiers are considered to be more significant as they can maintain the physiochemical properties for the simultaneous adsorption of both organic and inorganic pollutants. Hence, enhanced sorption properties are expected in this study. The main objective of this study was to increase the organic content of montmorillonite by different soil modifiers with special structures, 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane [BAT], 3,3′-tetrathiobis(propyl-triethoxysilane) [TP], thiodiglycol bis(3-aminocrotonate) [TDBA] and (3-mercaptopropyl)trimethoxysilane [MPTMS], and also to obtain a simultaneous uptake of both organic and inorganic pollutants.

Thus, we attempted to graft either S or SH groups into the interlayer space of montmorillonite soil, in order to obtain modified soils which are accessible to both inorganic and organic pollutants, and compare their activities. Thus, these modified soils behave as a dual sorbent rather than a single or mixed adsorbent. Such modification treatments markedly modify the surface area, adsorption energy, and porosities of typical montmorillonite soil, and checking this hypothesis on soils was also the aim of this paper. A practical importance of such studies is that the above soil properties control the soil water retention, hydraulic conductivity, solute transport, and sorption of pollutants, which can modify the efficiency of remediation processes. According to our knowledge, there have been only few studies on surfactant-modified, and SH functionalized montmorillonite, but so far there have been no reports on S functionalized montmorillonite. In these cases, the formation of possible covalent bonds between the S or SH atoms of the respective modifier molecules and the montmorillonite surface may occur, leading to a high density and fixation during grafting. Simultaneously, the total Si content was also greatly enhanced in the Si grafted soil due to the molecular structures. In this study, the influence of the presence of Si, SH and S groups on the structural properties was also explained. The results indicated that the uptake of pollutants was mainly influenced by various factors. The uptake of inorganic pollutants was altered by the surface area and structure of the modifiers, and the uptake of organic pollutants was controlled by the solubility and organic matter content. The observed log K_OC and log K_OM values of BTEX in this study for the modified soils are comparatively greater than those of natural soils. However, the uptake was not dependent on a single factor and deep insight is yet to be gained. Previous reports just discussed the adsorption phenomenon,^16–18 but the detailed information was not explained properly. Thus, the present methodology is very significant and novel in its approach.

2. Experimental section

2.1. Chemicals

Montmorillonite used in this study was obtained from the Clay Minerals Society as SWy-2-Na-montmorillonite (Wyoming) and the chemical formula of the Na-montmorillonite is expressed as Na_0.73[Si_7.66Al_0.34][Al_3.07Fe_0.44Mg_0.56]O₂₀(OH)₄. The surface area is 28.02 m² g⁻¹ and the cation exchange capacity (CEC) is 78 meq./100 g. The montmorillonite was used without further purification. High purity laboratory grade copper and zinc standard solutions (1000 ppm) were purchased from J. T. Baker. All the reagents were of analytical grade and used without further purification. Deionized water was used in all experiments.

2.2. Modifiers

The modifiers used in this experiment were 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane [BAT] (Sigma-Aldrich, molecular weight = 248.51), 3,3′-tetrathiobis(propyl-triethoxysilane) [TP] (Sigma-Aldrich, molecular weight = 538.95), thiodiglycol bis(3-aminocrotonate) [TDBA] (Sigma-Aldrich, molecular weight = 288.36) and (3-mercaptopropyl)trimethoxysilane [MPTMS] (Sigma-Aldrich, molecular weight = 196.34). The molecular structures and chemical formulae of these four modifiers are shown in Fig. 1.

Fig. 1 Molecular structures and chemical formulae of the four different modifiers.

2.3. Preparation of modified Wyoming (SWy-2-Na-montmorillonite)

In this study, modified Na-montmorillonite was obtained by two different methods using different modifiers. One is by simple mixing in water and another is by refluxing in toluene.

A suspended solution containing Na-montmorillonite was prepared by dispersing 10 g of Na-montmorillonite in 1 L deionized water. The solution was stirred for 2 h and then allowed to stand for 1 h. Then, the clay fraction was separated by sedimentation of the clay suspension. The supernatant clay fraction was then removed and put aside. 20 mL of an appropriate amount of the respective modifier (usually double CEC) was added to the supernatant clay fraction (20 mL) and the reaction mixture was stirred for 2 h. Then, all the products were washed and the resulting modified Na-montmorillonites were filtered and lyophilized (freeze-dried). Finally, they were ground in an agate mortar and stored in a vacuum desiccator. The BAT, TP, TDBA and MPTMS modified montmorillonites obtained by the simple mixing in water method were denoted as BAT-M, TP-M, TDBA-M and MPTMS-M, respectively.

For the reflux method in toluene, the two modifiers TP and MPTMS were chosen. 5 g of Na-montmorillonite was added into 200 mL of anhydrous toluene and stirred for 5 min. To this an appropriate amount of the respective modifier (usually double CEC) was added and stirred for 5 min. Then, the reaction mixture was refluxed at 110 °C for 24 h. After cooling to room temperature, toluene was removed and the samples were collected, rinsed with toluene to remove any unreacted modifier and vacuum dried for 12 h. Finally, they were ground in an agate mortar and stored in a vacuum desiccator. The TP and MPTMS modified montmorillonites obtained by the reflux grafting method were denoted as TPG-M and MPTMSG-M, respectively.

2.4. Batch sorption studies

To test the efficiency of the modified montmorillonites as adsorbents, we used three methods. The batch sorption experiment included three pollutant-type treatments: (1) inorganic pollutants (Cu²⁺ and Zn²⁺) only, (2) organic pollutants (benzene: >99%, toluene: >99.9%, ethylbenzene: >99.6% and xylene: >99.5%; in brief BTEX) only, and (3) a mixture of both inorganic and organic pollutants. Stock solutions (5, 10, 20, 50, 100, 200, 300, 400 and 500 mg L⁻¹) of various pollutants were prepared using the standard solutions in deionized water.

2.4.1. Adsorption of inorganic pollutants (Cu²⁺ and Zn²⁺). The uptake experiments were performed at 25 °C in glass centrifuge tubes containing different heavy metal ion solutions (20 mL) and the modified montmorillonites (2 g), shaken at pH 2 for 24 h at 130 rpm in a Lab-Line orbit environ shaker. The reaction mixture was centrifuged at 6000 rpm for 10 min to precipitate the soil samples. Finally, the supernatant liquid was drawn and analyzed using a Varian AA280Fs atomic absorption spectrophotometer (AAS) (<2% error – no more than dilution errors) to determine the concentrations of the metal ions in solution.

2.4.2. Uptake of organic pollutants (BTEX). The experiments were performed at 25 °C in glass centrifuge tubes containing different organic pollutant solutions (20 mL) and the modified montmorillonites (2 g), shaken for 24 h at 130 rpm in a Lab-Line orbit environ shaker. The reaction mixture was centrifuged at 6000 rpm for 10 min to precipitate the soil samples. Then, the supernatant liquid was extracted with carbon disulfide (1 : 4) and drawn into a brown bottle, and the bottle was immediately covered. Thus, the phases were separated and finally the sample was analyzed using a Hewlett-Packard (GC, Model: HP-6890) gas chromatograph equipped with a flame ionization detector (GC-FID) in 5 mL aliquot samples to determine the concentrations of the organic pollutants in solution.

2.4.3. Uptake of both organic and inorganic pollutants. The experiments were performed at 25 °C in glass centrifuge tubes containing a mixture of inorganic (metal ion) (20 mL) and organic pollutants (20 mL) and the modified montmorillonites (2 g), shaken at pH 2 for 24 h at 130 rpm in a Lab-Line orbit environ shaker. The reaction mixture was centrifuged at 6000 rpm for 10 min to precipitate the soil samples. Then, the supernatant liquid was extracted with carbon disulfide (1 : 4) and drawn into a brown bottle, and the bottle was immediately covered. Thus, the phases were separated and finally both the extracted sample and the supernatant sample were analyzed to quantitatively determine the organic (GC) and inorganic pollutants (AAS), respectively.

2.5. Determination by an analytical procedure

The Langmuir equilibrium adsorption of various pollutants was calculated using the following equation;

q_e = [(C₀ − C_e)V]/M

where q_e is the amount of pollutants adsorbed per unit amount of the adsorbent (mg g⁻¹), C₀ and C_e are the concentrations of the pollutants in the initial solution (mg L⁻¹) and at equilibrium, respectively, V is the volume of the adsorption medium (L), and M is the amount of the adsorbent (g). The value of q_max was calculated from q_e using the Langmuir isotherms and the corresponding q_max values were obtained using a linear method.

2.6. Characterization techniques

The Brunauer–Emmett–Teller (BET) specific surface area and average pore diameter were measured by degassing the samples at 368 K, and measuring them at 77 K on a Quantachrome NOVA 1000 using the standard continuous adsorption/desorption procedure. Fourier transform infrared spectra (FT-IR) of the samples were obtained using a Necolit 6700 model spectrometer by mixing the sample with KBr in a ratio of 1 : 100 and pressing it into a disc. X-ray diffraction (XRD) patterns were obtained on a Siemens X-ray diffractometer D-5000 equipped with a Cu Kα radiation source operating at 40 kV and 30 mA. Elemental analysis was carried out with a CE EA-1110 elemental analyzer.

3. Results and discussion

3.1. FT-IR, BET, elemental analyses, and XRD

In this work, SWy-2-Na-montmorillonite was modified using four different modifiers by two different grafting methods either in water or toluene. The grafting mechanism may possibly be explained based on the covalent bond formation between the grafting groups (Si, S or SH) of the modifiers and the montmorillonite surface. There may be single, double or multiple covalent bond formations depending on the nature and affinity of the Si, S or SH groups present in the modifiers towards the surface of the montmorillonite soil. Accordingly, different modified montmorillonites with varying physicochemical properties were obtained and characterized using various techniques. Upon grafting of the modifiers onto the montmorillonite surface, the organosilanes of the BAT, TP and MPTMS modifiers undergo self assembly, as shown in Fig. 2. However, TDBA contains organosulphur and this organosulphur will in this case undergo self assembly. Thus, the molecular self assembly of the organosilanes or organosulphur groups of the modifiers has been proven to be a powerful technique for grafting of montmorillonite surfaces.¹⁹ In this process, the organosilanes or organosulphur groups of the modifiers are hydrolyzed in the presence of the SiO₂ group of montmorillonite to create corresponding hydroxysilane or hydroxysulphur. Then, these are hydrogen bound to the SiO₂ groups of montmorillonite. Eventually, aggregation of these hydrogen bound species results in the condensation between the hydroxysilane/sulphur and SiO₂ groups of montmorillonite.¹⁹ The modified and unmodified montmorillonites were subjected to FT-IR analysis, as shown in Fig. 3, in order to confirm the presence of the functional groups. In the case of unmodified Na-montmorillonite, the broad peak around 3400–3900 cm⁻¹ is assigned to ν(O–H) and ν(H₂O), the peak around 1050 cm⁻¹ is assigned to ν(Si–O–Si), and the peak around 1600 cm⁻¹ is assigned to interlayer water deformation vibrations. The appearance of new peaks²⁰ apart from the original peaks in the modified montmorillonites confirmed the successful modification using the different modifiers. In the modified montmorillonites, the peak around 2800–2900 cm⁻¹ is due to ν_stretching(CH₃ and CH₂), the peak around 1595 cm⁻¹ is due to ν_stretching(NH₂), the peaks around 1040 cm⁻¹ and 800 cm⁻¹ are due to ν_stretching(Si–O–Si), and ν_bending(Si–O–Si) is observed around 450 cm⁻¹. In the case of TP-M and TPG-M, the peak around 620 cm⁻¹ is assigned to ν_stretching(S–S). However in the case of MPTMS-M and MPTMSG-M the peaks due to ν(SH) and ν(OCH₃) are not clearly distinguished due to peak merging. Invariably in all cases, the positions of the Na-montmorillonite peaks almost remained unaltered also after the modification. Upon modification, the broadening of the water peak decreased and the peak around 3400–3900 cm⁻¹ became sharp due to the hydrophobic nature of the modified montmorillonites. The elemental analysis data, extent of functionalization, BET surface area and pore size of both the unmodified and modified montmorillonites, are shown in Table 1. From Table 1, we can easily understand that the extent of functionalization determined by the potassium dichromate method varied for the different modifiers and in the case of MPTMS, functionalization is comparatively greater than those of the others due to its simple and shorter chain length. The functional group content values explained the modification ability of the different modifiers. Thus, it is obvious that the functionalization with linear longer carbon chains is less effective due to steric hindrance. Thus, the molecular structures of the modifiers played a significant role in controlling the functionalization process. Thus, the degree of functionalization of the modifiers could be arranged in the order as MPTMS > TP > TDBA > BAT. From Table 1, it is seen that the weight percentage of C, H and N significantly increased for the modified soils which also confirmed the effective modification of the soil. The BET surface area of montmorillonite significantly decreased after modification, but the pore size drastically increased. However, the BET surface areas of the modified soils are do not depend on the functionalization. Based on the BET surface area, the modified soils could be arranged in the following order: TPG-M > MPTMSG-M > BAT-M > TDBA-M > MPTMS-M > TP-M, but the pore sizes of the modified soils did not show any direct correlation with the surface areas, and according to the pore size the modified soils could be arranged in the following order: TDBA-M > MPTMS-M > MPTMSG-M > BAT-M > TPG-M > TP-M. Among the modifiers, TP which contains both Si and S groups favorably increased the extent of functionalization, and the surface area. However, BAT with Si groups moderately increased the surface area. TDBA with a sulphur group and MPTMS with an SH group resulted in soils with a smaller surface area. Thus, the Si groups present in the modifier structure significantly favored the structural properties of the modified soils. Thus, from Table 1, it is obvious that the modification of soils significantly enhanced the pore size of the soils but decreased the surface area. It can also be seen that in all the cases, the pore volume of the modified soils decreased significantly due to pore blocking by the modifiers upon modification. This pore blocking depends on the nature of the modifier and has no significant correlation with the pore size of the modified soil. The basal spacing in the modified montmorillonites was expected to significantly increase due to the grafting of such modifiers into the interlayer framework.^8,21,22 This is confirmed by the obtained XRD patterns of the montmorillonites before and after modification, as shown in Fig. 4. The XRD patterns consisted of three peaks corresponding to the [001], [002] and [004] reflections along with the quartz peaks, as shown in Fig. 4. The XRD patterns also revealed a shift in peak position corresponding to the (001) reflection at around 2θ = 6.7° to lower 2θ values (2θ ≈ 6.7 to 4.1°) after the modification, indicating a relatively larger increase in the basal distance of these planes from 1.32 to 2.20 nm. This increase clearly confirmed the occurrence of intercalation for the modifiers between the montmorillonite clay layers. Thus, compared with Na-montmorillonite (corresponding to a basal plane spacing [d₀₀₁] of 1.32 nm), the modified montmorillonites have a larger basal spacing. Based on the basal spacing, the modified montmorillonites can be arranged in the order of BAT-M (2.20 nm) > TPG-M (2.10 nm) > TDBA-M (2.01 nm) > TP-M (1.75 nm) > MPTMS-M (1.52 nm) > MPTMSG-M (1.42 nm). However, the basal distance of the planes corresponding to the [002] and [004] reflections were not significantly altered, which highlights the successful intercalation of the [001] planes of montmorillonite. It is thus confirmed that the interlayer spacing of the montmorillonite is significantly increased by the modifiers. Based on the above results, it is confirmed that both the nature and structure of the modifier play a significant role in the structural characteristics of the modified soils.

Fig. 2 Schematic representation of the grafting of the modifiers onto montmorillonite.

Fig. 3 FT-IR spectra of (a) unmodified montmorillonite, (b) BAT-M, (c) TP-M, and (d) MPTMS-M.

Table 1 Surface area, pore size, elemental analysis and functional group content of the modified and unmodified Na-montmorillonites

Soils	BET surface area (m^2 g^−1)	Pore size (nm)	Elemental weight (%)				Functional group (mmol/100 g)
			C	H	N	S
Na-montmorillonite	28.02	9.45	0.41	1.41	0.12	—	—
BAT-M	6.91	15.55	6.49	2.65	1.15	—	40.89
TP-M	0.22	12.00	8.52	—	—	8.04	62.81
TDBA-M	1.81	51.77	11.82	2.72	1.34	—	47.68
MPTMS-M	1.52	38.67	8.33	—	—	3.71	115.94
TPG-M	12.99	15.19	4.92	—	—	2.89	22.54
MPTMSG-M	7.87	17.98	4.46	—	—	1.95	60.94

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Fig. 4 XRD patterns of the unmodified and modified montmorillonites.

3.2. N₂ adsorption isotherms

The specific surface area and pore size distribution are important indicators of an adsorbent’s adsorption capacity. The obtained nitrogen adsorption isotherms before and after modification, as shown in Fig. S1 (see Fig. S1†), obviously conveyed that all the samples exhibited typical type-II S-type curves. A hysteresis was obviously not seen and at a low relative pressure (P/P₀ < 0.01) the adsorption was low. The adsorbed volume increased linearly upon increasing the pressure P/P₀ in the range of 0.1 to 0.5. This region corresponds to a monolayer-multilayer adsorption on the pore walls. Beyond P/P₀ = 0.5, a sharp increase in the adsorbed volume was observed, which could be attributed to capillary condensation of the multilayer adsorption. At this stage, the changes in nitrogen adsorption can be used as a measure of a uniform pore size distribution. The greater the adsorption, the larger the uniform size distribution. At higher relative pressures (P/P₀ > 0.5–0.8), multilayer adsorption occurred on the external surface and larger holes appeared, thus resulting in a drastic increase in the adsorbed volume. At a value of P/P₀ of 1, saturation was reached indicating that all of the pores were filled with the condensed adsorbents. The isotherms of the modified soils exhibited sharp inflection characteristics of capillary condensation within a narrow pore size distribution,²³ where the value of P/P₀ of the inflection point is related to the average diameter of the pores. These N₂ adsorption data confirmed the uniformity of the pore size distribution of the soils. Thus, it is clear that the structural properties of the soils were not altered upon soil modification. The pore size distribution is shown in Fig. 5, and it is observed that in the range between 2 to 40 nm, particles with smaller pore sizes are distributed, and in the range between 70 to 150 nm, particles with larger pore sizes are distributed. A uniform and narrow pore size distribution was accumulated in the case of MPTMS-M, TP-M and TDBA-M, whereas in the case of TPG-M, BAT-M and MPTMSG-M, the pore size distribution was broadened and less accumulated. These nitrogen adsorption and pore size distribution curves clearly revealed that the different modifiers could affect the pore size and its distribution. This study vividly revealed that the modifiers with siloxane or thiol groups resulted in modified soils with a larger surface area and smaller pore size compared to those of soils modified with sulphur atoms which resulted in modified soils with a lower surface area but a relatively larger pore size.

Fig. 5 BJH pore distribution diagram of the modified soils.

3.3. Adsorption of inorganic pollutants (heavy metal ions)

In this study, the adsorption of heavy metal ions such as Cu²⁺ and Zn²⁺ by both modified and unmodified montmorillonite soils was explored. The adsorption of both Cu²⁺ and Zn²⁺ by unmodified montmorillonite soil is shown in Fig. 6. It was found that as the initial concentration of unmodified soil increased, the adsorption capacity also increased. When the initial concentration reached about 15 mg L⁻¹, an equilibrium was attained beyond which the adsorption became almost unaltered. At this saturation point, the availability of active sites of the adsorbent for further adsorption is limited and thus a further increase in initial concentration caused no more increase in the adsorption of metal ions. The adsorption of Cu²⁺ and Zn⁺ by the modified soils is given in Fig. 7. For the modified soils, the adsorption of inorganic pollutants drastically increased compared to that of the unmodified soil, which is due to the shrinkage of water in the interlayers of the modified soils. During the modification, the modifiers are assumed to penetrate into the interlamellar region of the soil/clay by the expansion of the clay sheets. Such expansion occurs due to the grafting of modifiers to the silanol groups within the interlayer where the silica framework is in contact with the clay layers. Thus, in the presence of modifiers, the water is removed and the interstitial layers of the surface morphology are easily stretched. By comparing the Langmuir and Freundlich adsorption isotherms of the modified soils, the calculated R² value of the Langmuir adsorption model is found to be 0.9, as shown in Table 2, indicating that the Langmuir isotherm model fitted well in all cases. It can also be seen that for the unmodified soil the adsorption capacity for Cu²⁺ was comparatively larger than that for Zn²⁺. This is because of the difference in electronegativity of the metal ions. Thus, it is very clear that the adsorption of heavy metals is not dependent on the surface area but it is dependent on the coordination properties (affinity) of the metal ions towards the adsorbents. For the modified soils, the adsorption of Zn²⁺ was also significantly increased which is due to the presence of S or SH functional groups in the modifier structures. These functional groups are capable of forming strong coordination bonds with Zn metal ions and thus in the case of the modified soils, both the adsorption of Cu²⁺ and Zn²⁺ were significantly enhanced. This suggested that the adsorption process involved a chemical adsorption with the adsorption capacity not only depending upon the surface area of the adsorbents but also on the nature (surface charge) and structure of the modifiers and the affinity of the metal ions. From Fig. 7 and Table 2, it is obvious that, even though MPTMS-M and TP-M possessed a lower surface area than those of BAT-M and TDBA-M, they interestingly showed enhanced adsorption for both Cu²⁺ and Zn²⁺, and the order of adsorption ability of the modified soils for the metal ions could be arranged as MPTMS-M > TP-M > TDBA-M > BAT-M. Thus, despite the larger surface area, BAT-M still showed a lower adsorption capacity for the metal ions. This is because MPTMS-M and TP-M contained more unshared electron pairs and thus formed strong coordination bonds with the metal ions causing enhanced adsorption. Thus, this discrepancy in behavior can be explained based on the structure and nature of the modifiers used in the modified soils. The basic mechanism of adsorption was based on the complexation of functional groups present in the modified soils (nitrogen, sulphur or SH groups) with metal ions or on hole-filling (adsorption by ion-exchange). In addition, the surface charges due to the modifiers could also contribute to the adsorption of pollutants. Thus, the adsorption phenomenon is dependent on various combined factors such as the surface charge, surface area and structure of adsorbents and also the electronegativity of the metal ions. Thus, based on the combined effect it was concluded that MPTMS-M showed an enhanced adsorption tendency towards metal ions in this study. Based on the experimental results, it was observed that, among the two metal ions, the adsorption of Cu²⁺ was significantly greater than that of Zn²⁺.^5,24,25 This is due to the comparatively smaller ionic radius and higher electronegativity of Cu²⁺ than those of Zn²⁺.

Fig. 6 Adsorption isotherms of Cu²⁺ and Zn²⁺ ions of the unmodified montmorillonite.

Fig. 7 Adsorption isotherms of (a) Zn²⁺ and (b) Cu²⁺ of the modified montmorillonites.

Table 2 Adsorption of inorganic pollutants by the modified soils and Langmuir isotherm parameters

Adsorbents	Cu^2+		Zn^2+
	qmax (mg kg^−1)	R^2	qmax (mg kg^−1)	R^2
Na-montmorillonite	2987	0.9677	2948	0.9129
BAT-M	2967	0.9718	5789	0.9490
TP-M	9061	0.9801	8019	0.9891
TDBA-M	4324	0.9658	4088	0.9259
MPTMS-M	24 803	0.9977	8750	0.9765
TPG-M	4103	0.9234	7753	0.9811
MPTMSG-M	14 180	0.9964	4626	0.9690

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As already discussed, two different modification methods were adapted in this study. Thus, in order to obtain a better understanding of both methodologies, a comparative study was carried out, as shown in Table 2. Under the same conditions, the adsorption of metal ions by the modified soils obtained by simple mixing in water (TP-M and MPTMS-M) was greater than that of the modified soils obtained by refluxing in toluene (TPG-M and MPTMSG-M). This can be explained based on different adsorption mechanisms. In the case of the modification by simple mixing in water, the modifiers can be attached to the soils on the upper surface of the interlayer and thus the resulting physical adsorption may be a multilayer adsorption. Whereas in the case of the modification by refluxing in toluene, the modifiers can be attached to the soils onto the lower surface of the interstitial layer and thus the resulting chemical adsorption may be a monolayer adsorption. Thus due to the structural behavior of the MPTMS and TP modifiers, the grafting of the modifiers by refluxing in toluene is less effective due to structure bending during refluxing which caused steric hindrance. Thus, expansion of the interstitial layer is limited and grafting of the modifiers is less efficient in refluxing toluene than that by simple mixing in water. Thus, it is suggested that compared to MPTMSG-M and TPG-M, MPTMS-M and TP-M were considered to be effective adsorbents for metal ions.

3.4. Uptake of organic pollutants (BTEX)

In this study, as already discussed montmorillonite soil was modified using special modifiers which contain specific functional groups that have a greater affinity for metal ions (inorganic pollutants) and long carbon chains due to which the organic carbon content of the modified soil also greatly improved. Thus, these modified soils could also function as partition media for nonionic organic pollutants such as BTEX. In order to explore their uptake capacity for organic pollutants, this section will explore the role of the distribution coefficient (partition coefficient, k_d), the role of soil–water saturated systems, the nature of the soil and organic pollutant and the equilibrium concentration (C_e) of organic pollutants. Calibration for the recovery of BTEX was done in order to avoid any errors due to the highly volatile nature of BTEX by covering the batch adsorption mixture, and about 95% of BTEX was recovered. The distribution coefficients (k_d) of the organic compounds between the solid and solution may be calculated using a linear relationship as follows:

x/m = k_dC

where x is the compound uptake on the solid (mg), m is the weight of the solid (kg) and C is the equilibrium compound concentration in the solution (mg L⁻¹).

The uptake of BTEX by the modified montmorillonites is shown in Fig. 8, and the distribution constants (k_d) are displayed in Table 3. From Table 3, it is clear that the distribution of BTEX in the soil–water systems is not similar to the metal ion adsorption, and it mainly depends on the organic matter content of the modified soils. Differences in water solubility of BTEX will definitely affect both the distribution constant (k_d) and equilibrium concentration (C_e) values. From Table 3, it was also observed that the modifiers TP and MPTMS caused an invariably more significant increase in k_d values than those of the BAT and TDBA modifiers due to the functional group structures and organic matter content. In general both BAT and TDBA contain more polar functional groups (oxygen or nitrogen), a more planar structure and a limited distribution environment and thus due to this hydrophobic nature, the distribution of organic pollutants is limited. The uptake results also indicated that the solubility of BTEX in water and the organic matter content may affect the partitioning of BTEX,²⁶ but at the same time the uptake phenomenon did not show any significant correlation with only the single parameters. Thus, the process appeared quite complex and the uptake of organic pollutants by the modified soils was different for the different systems. As it is known that the uptake mainly occurred on organic carbon matter, the organic content (OC) was determined and the total percentage of the organic content (OC%) is reported in Table 3. However, the total organic content reported in Table 3 is slightly different from the C% reported in Table 1. This difference in C% perhaps may be due to the presence of small amounts of inorganic carbonates in the modified soils. Thus, all three the organic content (OC%), water solubility (S_w) and distribution constant (k_d)^27,28 mainly influenced the uptake of BTEX by the modified soils.

Fig. 8 Adsorption isotherms for the uptake of BTEX by the modified montmorillonites.

Table 3 Organic carbon content and distribution constant values for different systems

Adsorbates	OC%	BAT-M	TP-M	TDBA-M	MPTMS-M	TPG-M	MPTMSG-M
		6.7	8.8	12.21	8.61	5.08	4.61
Benzene	kd	302.79	564.65	552.2	409.84	445.91	500.55
Toluene		317.34	648.32	414.62	1691.2	966.95	241.36
Ethylbenzene		—	1702.4	—	5194.5	5648.9	307.26
p-Xylene		441.44	2253.1		103.17	524.78	529.41

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It is very important to understand the water solubility of the adsorbate molecules in soil–water systems. In general, adsorbate molecules with a higher water solubility tend to easily form an aqueous solution, whereas the adsorbate molecules with a lower water solubility are easily distributed in the organic phase of the hydrophobic adsorbent rather than dissolving in the medium. Thus, for adsorbate molecules with a higher water solubility, the k_d values are always smaller than those of less water soluble adsorbate molecules, i.e. S_w and k_d are inversely proportional to each other. However, due to this distribution of non-ionic organic pollutants into the organic phase of the adsorbent in soil–water system, again the total organic content has to be corrected, as shown in Table 4, i.e. the total mass transfer of the contaminant which is dependent on the solubility, has to be considered in particular. Thus, both the equilibrium constants K_OC and K_OM could provide an indication of the constituent sorption onto soil or organic matter, respectively, and K_OC is always greater than K_OM. Hence, there is a gap between the K_OC and K_OM values in natural soil systems which is a limiting factor that has to be seriously considered. Thus, larger values of both constants indicate the preference of the constituent to be sorbed onto soil or organic matter. Thus, the significance of both the K_OC and K_OM values can be understood in terms of their respective log K_OC and log K_OM values. In this case, OC is the actual organic content before the addition of organic pollutants and OM is the total organic content after the addition of organic pollutants. Thus, this study provided significant insights into the investigation of the distribution constant (k_d), organic content (OC and OM), corrected constant values (K_OC and K_OM) and water solubility (S_w). Invariably in all cases, the value of K_OC is greater than that of K_OM. From Table 4, several observations could be derived and both K_OC and K_OM are mainly used to predict the value of the non-ionic organic pollutants present in the soil.^28,29 Although K_OC seems to be greater than K_OM, the difference in the log K_OC and log K_OM values is much smaller, i.e. they are almost identical to each other. Thus, this study successfully eliminated the gap between the K_OC and K_OM values and both obtained log K_OC and log K_OM values in this study are significantly greater than those of the natural soil system as well so far reported literature values,^28,30 as shown in Table 5. Thus, from Table 5, it is confirmed that the modified soils obtained in this study overall enhanced the effective uptake of organic pollutants.

Table 4 Corrected organic carbon content and constant (K_OC and K_OM) values for the different systems

Adsorbates	OM%	BAT-M	TP-M	TDBA-M	MPTMS-M	TPG-M	MPTMSG-M
		26.62	17.6	24.42	17.22	10.16	9.22
Benzene	KOC	2274.91	6416.48	4522.52	4760.05	8777.76	10 857.92
	log KOC	3.36	3.81	3.66	3.88	3.94	4.04
	KOM	1137.45	3208.24	2261.26	2380.02	4388.88	5428.96
	log KOM	3.06	3.51	3.35	3.38	3.64	3.73
Toluene	KOC	2992.29	2384.22	7367.27	3395.74	19 034.45	5235.57
	log KOC	3.38	3.87	3.53	4.29	4.28	3.72
	KOM	1496.14	1192.11	3683.64	1697.87	9517.22	2617.79
	log KOM	3.08	3.57	3.23	3.99	3.98	3.42
Ethylbenzene	KOC	—	19 345.45	—	60 331.01	111 198.82	6665.08
	log KOC	—	4.29	—	4.78	5.05	3.82
	KOM	—	9672.73	—	30 165.51	55 599.41	3332.54
	log KOM	—	3.99	—	4.48	4.75	3.52
p-Xylene	KOC	3316.60	25 603.41	—	1198.26	10 330.31	11 483.95
	log KOC	3.52	4.41	—	3.08	4.01	4.06
	KOM	1658.30	12 801.70	—	599.13	5165.16	5741.97
	log KOM	3.22	4.11	—	2.78	3.71	3.76

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Table 5 Comparison of the log K_OC and log K_OM values of BTEX obtained in this study with the natural soil system as well as values reported in literature

Values	Benzene		Toluene		Ethylbenzene		p-Xylene
	log KOC	log KOM	log KOC	log KOM	log KOC	log KOM	log KOC	log KOM
Estimated values	2.09	1.21	2.85	1.65	3.59	2.08	3.48	2.02
In ref. 28 and 30	2.24	1.30	2.06	1.19	2.32	1.35	2.52	1.46
In this study	3.36 to 4.04	3.06 to 3.73	3.38 to 4.29	3.08 to 3.99	3.82 to 5.05	3.49 to 4.48	3.08 to 4.41	2.78 to 4.11

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The water solubility (S_w) of BTEX was found to follow the order of: benzene > toluene > p-xylene > ethylbenzene (1780 > 515 > 197 > 152). However, the order of k_d varied for the different systems. The irregularities in the sorption mechanisms, as shown in Fig. 5 and 6, could perhaps be either due to surface irregularities of the adsorbents or the attraction or repulsion of different species under different reaction conditions.¹⁶ The different species of inorganic and organic pollutants and the modifiers as a function of pH are apparently important parameters to be taken into account. Again due to the differences in structural properties of the modified soils obtained by two different methods in this study, the k_d values of modified soils from the toluene reflux system are found to be relatively smaller than those of the soils from simple mixing in water, due to steric hindrance by structural bending and also the lower availability of a distribution environment. Thus, the k_d values of TP-M and MPTMS-M are always greater than those of TPG-M and MPTMSG-M, and the nature of the organic modifiers played a vital role in the adsorption of both organic and inorganic pollutants.

3.5. Uptake in binary systems (both inorganic and organic pollutants)

In order to evaluate the interaction between inorganic and organic pollutants, the adsorption in a binary system using both inorganic and organic pollutants was also carried out and discussed in this study. For this purpose, both MPTMS-M and MPTMSG-M were selected as adsorbents, as the modifier MPTMS showed an increased affinity towards metal ions than those of the others. In addition, benzene was mainly selected as the organic pollutant simply to avoid any errors, as it has a higher water solubility. In the case of the binary mixture, the total adsorption was greatly enhanced, however, no competitive adsorption phenomenon could be witnessed due to the different mechanisms involved in both inorganic and organic pollutant adsorption. The results obtained in this study are compared with those of literature reports¹⁶ and the total adsorption was greatly increased in this study. This clearly indicated that there is no interaction between the cations and BTEX. However, in most of the reported studies^16–18 the presence of organic pollutants caused crowding and thereby decreased the adsorption of metal ions due to steric hindrance.

The significant effect of the soil modifiers used in this study on the adsorption of heavy metal ions can be better explained by comparing them with the few published reports^1,5 as well as commercially available granular activated carbon (GAC)³¹ and zeolite,³² as shown in Table 6. From Table 6, it is very clear that the adsorption capacities reported in this study by MPTMS-M and the other modified montmorillonite soils are definitely greater than those of other adsorbents, and thus the importance, significance and capability of the modifiers with special Si, S or SH groups used in this study were witnessed and they are considered to be superior to those of the so far reported modifiers. In overall, our present work is considered to be more significant by obtaining different versatile modified montmorillonite soils for the enhanced adsorption of both organic and inorganic pollutants.

Table 6 Comparison of the adsorption capacities of the modified Na-montmorillonite soils obtained in this study with those from literature reports

Adsorbents	Modifiers	Ref.	Cu^2+		Zn^2+
			qmax (mg kg^−1)	R^2	qmax (mg kg^−1)	R^2
Na-montmorillonite	SDS	1	5275	—	2086	—
Na-montmorillonite	HDS	5	3333	—	2500	—
GAC	—	31	5845	0.9900	—	—
Zeolite	—	32	8968	0.9775	—	—
Na-montmorillonite	TDBA	In this study	4324	0.9658	4088	0.9259
Na-montmorillonite	BAT	In this study	2967	0.9718	5789	0.9490
Na-montmorillonite	TP	In this study	9061	0.9801	8019	0.9891
Na-montmorillonite	MPTMS	In this study	24 803	0.9977	8750	0.9765

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4. Conclusions

In this study, modified sodium montmorillonite soils were prepared using four different soil modifiers and two different methods. The modified soils have the advantage of adsorbing both organic and inorganic pollutants due to the special structures provided by the different soil modifiers. Thus, the uptake of both organic and inorganic pollutants was reported in this study from both single and binary mixtures in order to investigate the feasibility and the sorption abilities of the obtained modified soils. The role of different parameters on the uptake characteristics was mainly discussed. Upon modification, the structural morphologies were significantly altered. This study provided detailed insight into the effect of organic matter content, solubility and distribution constant of the pollutants on the uptake mechanisms. Regardless of the surface area, the nature of the surface charge and organic matter content of the soils significantly affected the uptake phenomenon. Among the different modifiers used in this study, the modified soil obtained using the MPTMS modifier was concluded to be an efficient adsorbent. In addition, the modified soils obtained by simple mixing in water (MPTMS-M and TP-M) act as better adsorbents than the corresponding adsorbents obtained by refluxing in toluene (MPTMSG-M and TPG-M). This is due to the fact that two different mechanisms operated during the grafting of the modifiers in these two methods. Enhanced adsorption for Cu²⁺ ions was shown than that for Zn²⁺ ions. The uptake of organic pollutants was significantly affected by both the solubility and k_d values. The uptake of BTEX was different for the different systems and the observed log K_OC and log K_OC values for BTEX were invariably enhanced in this study. In the case of the binary mixture, no competitive adsorption was observed due to the different adsorption mechanisms operating both in the organic and inorganic pollutant adsorption. The results of this study suggested a superior adsorption ability of the modified soils over that of the unmodified soil, and the methodology used in this study is also cost-wise more economic.

Acknowledgements

We thank the National Central University and National Science Council (NSC, Grant no.: NSC102-2221-E-008-002-MY3), Taiwan, Republic of China (ROC), for financial support.

Notes and references

1. O. Abollino, M. Aceto, M. Malandrino, C. Sarzanini and E. Mentasti, Water Res., 2003, 37, 1619.
2. S. E. Bailey, T. J. Olin, R. M. Bricka and D. D. Adrian, Water Res., 1999, 33, 2469.
3. L. Mercier and C. Detellier, Environ. Sci. Technol., 1995, 29, 1318.
4. F. Barbier, G. Duc and M. Petit-Ramel, Colloids Surf., A, 2000, 166, 153.
5. S.-H. Lin and R.-S. Juang, J. Hazard. Mater. B, 2002, 92, 315.
6. S. Y. Lee and S. J. Kim, Appl. Clay Sci., 2002, 22, 55.
7. C. F. Chang, C. Y. Chang, K. H. Chen, W. T. Tsai, J. L. Shie and Y. H. Chen, J. Colloid Interface Sci., 2004, 277, 29.
8. M. Thirumavalavan, Y.-T. Jhuang and J.-F. Lee, RSC Adv., 2015, 5, 20583.
9. J. M. Hwu, G. J. Jiang, Z. M. Gao, W. Xie and W. P. Pan, J. Appl. Polym. Sci., 2002, 83, 1702.
10. C. N. Owabor, U. M. Ono and A. Isuekevbo, Adv. Chem. Eng. Sci., 2012, 2, 330.
11. L. Seifi, A. Torabian, H. Kazemian, G. N. Bidhendi, A. A. Azimi and A. Charkhi, Water, Air, Soil Pollut., 2011, 217, 611.
12. H. Kazemian and M. H. Mallah, Iran. J. Environ. Health Sci. Eng., 2008, 5, 73.
13. V. A. Oyanedel-Craver and J. A. Smith, J. Hazard. Mater., 2006, 137, 1102.
14. V. A. Oyanedel-Craver, M. Fuller and J. A. Smith, J. Colloid Interface Sci., 2007, 309, 485.
15. R. Prost and B. Yaron, Soil Sci., 2001, 166, 880.
16. Z.-F. Meng, Y.-P. Zhang and Z.-Q. Zhang, J. Hazard. Mater., 2008, 159, 492.
17. S. Andini, R. Cioffi, F. Montagnaro, F. Pisciotta and L. Santoro, Appl. Clay Sci., 2006, 31, 126.
18. J. Y. Yoo, J. Choi, T. Lee and J. W. Park, Water, Air, Soil Pollut., 2004, 154, 225.
19. G. E. Fryxell, S. V. Mattigod, Y. Lin, H. Wu, S. Fiskum, K. Parker, F. Zheng, W. Yantasee, T. S. Zemanian, R. S. Addleman, J. Liu, K. Kemner, S. Kelly and X. Feng, J. Mater. Chem., 2007, 17, 2863.
20. R. C. Weast, M. J. Astle and W. H. Bayer, CRC Handbook of Chemistry and physics, CRC Press Inc., Boca Raton, Florida, 68th edn, 1988.
21. M. C. Pazos, M. A. Castro, M. M. Orta, E. Pavón, J. S. V. Rios and M. D. Alba, Langmuir, 2012, 28, 7325.
22. L. H. Bin and X. H. Ning, J. Inorg. Mater., 2012, 27, 780.
23. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area, and Porosity, Academic Press, London, Inc., 2nd edn, 1982.
24. T. Vengiris, R. Binkiene and A. Sveikauskaite, Appl. Clay Sci., 2001, 18, 183.
25. M. P. F. Fontes and P. C. Gomes, Appl. Geochem., 2003, 18, 795.
26. S. A. Boyd, J.-F. Lee and M. M. Mortland, Nature, 1998, 333, 345.
27. Y. Xi, R. L. Frost and H. He, J. Colloid Interface Sci., 2007, 305, 150.
28. J.-F. Lee, Y.-T. Chang, H.-P. Chao, H.-C. Huang and M.-H. Hsu, J. Hazard. Mater., 2006, 129, 282.
29. C. T. Chiou, P. E. Porter and D. W. Schmedding, Environ. Sci. Technol., 1983, 17, 227.
30. F. Xu, X. Liang, B. Lin, K. W. Schramm and A. Kettrup, J. Chromatogr. A, 2002, 968, 7.
31. A. H. Sulaymon, B. A. Abid and J. A. Al-Najar, Chem. Eng. J., 2009, 155, 647.
32. E. Erdem, N. Karapinar and R. Donat, J. Colloid Interface Sci., 2004, 280, 309.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10155g

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