Introduction
The increasing use of pesticides is a great global challenge due to their detrimental effect on surface water, air, soil, and living organisms. Thus, the use of pesticides has reduced the quantity and quality of global water resources. They exhibit harmful impacts and cause severe health problems, such as cancers, mutations, immuno-toxicity, paralysis, tonic clonic convulsions, and alterations to nervous and reproductive systems (El-Said et al. Reference El-Said, El-Khouly, Ali, Rashad, Al-Bogami, Elshehy and Al-Bogami2018). Most chlorinated pesticides such as endrin, dieldrin, and heptachlor epoxide are non-degradable; hence, they accumulate in human blood up to the warning levels established by international agencies (WHO 1989; IPCS 1992; WHO 2004). Several studies have focused on the removal of pesticides from water. One traditional approach for pesticide removal is based on decomposition into smaller or insoluble molecules that precipitate at the bottom of the water reservoir or stream (Morales-Pérez et al. Reference Morales-Pérez, Arias and Ramírez-Zamora2015). Decomposition of pesticides, however, may produce toxic byproducts which could represent a greater risk to the environment and human health than the parent molecule (Hladik et al. Reference Hladik, Roberts and Bouwer2005). Diversified methods have been reported for sequestration of pesticides from the aquatic environment such as oxidation, adsorption, solvent extraction, biodegradation, ozonation, and adsorption. Among the diversified methods, adsorption is still an efficient, simple, and promising fundamental technique for water decontamination (Bonilla-Petriciolet et al. Reference Bonilla-Petriciolet, Mendoza-Castillo and Reynel-Ávila2017), and various adsorbents have been used for this purpose (De Wilde et al. Reference De Wilde, Mertens, Simunek, Sniegowksi, Ryckeboer, Jaeken, Springael and Spanoghe2009).
Nowadays, designed and functionalized porous materials play an essential role in developing active surfaces which operate as adsorbents and catalysts. In order to achieve a precise molecular selectivity in the removal of organic pollutants, much effort has been made to optimize the shape and size of pores. Pillared clays (PILCs) are among the most interesting porous materials due to their permanent and engineered porosity and large specific surface area (Zhu et al. Reference Zhu, Wen, Zhang, Wang, Ma, Xi, Zhu, Liu and He2017). Bentonite (BT) is of particular interest and is one of the most promising PILCs for applications that require high-efficiency adsorbents or catalysts. Bentonites are classified into three types: Na-bentonite, Ca-bentonite, and mixed Na/Ca-bentonite (Şans et al. Reference Şans, Güven, Esenli and Çelik2017). The Na-bentonites are well dispersed, viscous, have a high swelling index, and greater electrical conductivity than the Ca-bentonites. Such forms of BT are a competitive alternative to activated carbon and zeolite because of their wide availability, low price, and the acidic nature of their surfaces, which enrich their chemical reactivity (Vicente and Lambert Reference Vicente and Lambert2003; Aznárez et al. Reference Aznárez, Delaigle, Eloy, Gaigneaux, Korili and Gil2015; Cheira et al. Reference Cheira, Mira, Sakr and Mohamed2019). The pores and surfaces of BT can be tailored further for specific purposes by exchanging the Na or Ca with large organic or inorganic cations which impart a change in pore shape and which in turn affects the selectivity toward specific molecules (Yuan et al. Reference Yuan, Xiaolin, He, Dan, Linjiang and Jianxi2006). The interlayer spaces of these layered materials can also be stabilized against swelling and expansion by intercalating rigid pillars which further define the pore structure and nature of surfaces and make them even more suitable for selectively accommodating certain molecules (Okada et al. Reference Okada, Seki and Ogawa2014).
Earlier attempts to modify PILCs were created by intercalating organic cations, such as tetraalkylammonium cations, and releasing the inorganic cations from the clay interlayer spaces through cation exchange (Barrer Reference Barrer1986). This concept was extended further to include other layered minerals and compounds with the ability to exfoliate fully the clay minerals to form individual, atomically thin layers (Abdelkader et al. Reference Abdelkader, Patten, Li, Chen and Kinloch2015). Despite their versatility and greater basal spacing, intercalation by organic cations imposed certain limitations such as low thermal stability (Aznárez et al. Reference Aznárez, Delaigle, Eloy, Gaigneaux, Korili and Gil2015). PILCs can also be created using inorganic polyoxycations to form the pillars. With a suitable choice of polyoxycation, the pillared interlayers can be kept stable at higher temperatures (Ming-li et al. Reference Ming-li, Yong-fu, Ji-zu and Ming-he2002). The preparation of inorganic PILCs involves four steps: (1) ensuring that the clay is in the sodium form 'Na-montmorillonite'; (2) formulation of the target pillaring agent; (3) intercalation by cation exchange; and (4) calcination to obtain the final stable form of the composite (Cool and Vansant Reference Cool, Vansant, Karge and Weitkamp1998).
Many Al oligomers have been prepared through dilute hydrolytic polymeric Al solution (HPA) (Del Riego et al. Reference Del Riego, Herrero, Pesquera, Blanco, Benito and González1994; Schoonheydt et al. Reference Schoonheydt, Leeman, Scorpion, Lenotte and Grobet1994; Kumararaja et al. Reference Kumararaja, Manjaiah, Datta and Sarkar2017). The formation of target Al oligomers is achieved by controlling: (1) the molar ratio of Al to OH; (2) the initial Al3+ concentration in solution; (3) the rate of base addition; (4) the hydrolyzed solution temperature; and (5) the aging time of the hydrolyzed solution (Cool and Vansant, Reference Cool, Vansant, Karge and Weitkamp1998). The transformation of [Al13O4(OH)24(H2O)12]7+ (Al13) into [Al30O8(OH)56(H2O)24]18+ (Al30) oligomers was reported by Rowsell and Nazar (Reference Rowsell and Nazar2000). The intercalation process focused on preparing concentrated suspensions from the pillaring agents (Chen et al. Reference Chen, Luan, Fan, Zhang, Peng and Fan2007). Schoonheydt and Leeman (Reference Schoonheydt and Leeman1992) prepared Al-PILBs through the addition of the clay to the Al polycation solution. Aouad et al. (Reference Aouad, Pineau, Tchoubar and Bergaya2006) developed a method for synthesis of Al-PILBs that minimizes the amount of water used, allowing for its extension to the industrial scale. Bentonite with mixed pillars, achieved using the concentration method, offers numerous potential applications of environmental interest, due to the possibility of introducing a large polyoxycation with favorable characteristics.
The objective of the current study was to design and synthesize aluminum pillared bentonites (Al-PILBs) with large surface areas and pore volumes and controllable pore size through the concentration method, beginning with the nitrate form of Al13 and Al30 Keggin cations with various molar ratios; and to investigate the possibility of using the Al-PILBs synthesized for the removal of chlorinated pesticides from aqueous solution.
Materials and Methods
Materials
All chemicals used in this study were of analytical grade and deployed without further purification. The chemicals used were aluminum chloride hexahydrate (AlCl3.6H2O), ammonium chloride (NH4Cl), and ammonium acetate (CH3COONH4) obtained from LOBA Chemie (Mumbai, India); sodium hydroxide (NaOH) pellets from Honeywell (Neuss, Germany); and silver nitrate (AgNO3), barium nitrate (Ba (NO3)2), anhydrous sodium sulfate (Na2SO4), Ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid (C9H6INO4S), 1,10-phenanthroline (C12H8N2), and hydroxylamine hydrochloride (NH2OH.HCl) from Sigma-Aldrich (Taufkirchen, Germany). The bentonite (Na form), consisting mostly of montmorillonite (assay ˃95% with some minor impurities of quartz and Fe2O3), was obtained from Egypt Bentonite & Derivatives Company (New Borg El Arab city, Alexandria, Egypt). Standard chlorinated pesticide solutions (1000 mg/L) containing 17 types of pesticides (α-BHC, β-BHC, σ-BHC, heptachlor, aldrin, heptachlor epoxide, endosufane 1, dieldrin, DDE, endrin, endosulfan 2, DDD, endrin aldehyde, endosulfan sulfate, DDT, methoxychlor, and γ-BHC) were acquired from AccuStandard (New Haven, Connecticut, USA). Deionized water (DIW) used in all experimental steps had an electric conductivity of 0.7 μS/cm.
Synthesis of Al13 and Al30 Sulfate
Aluminum oligomers of Al13 and Al30 sulfate were prepared using the usual synthesis procedures described by Aouad et al. (Reference Aouad, Mandalia and Bergaya2005). In this method, 0.6 M NaOH was added dropwise to a 500 mL round flask containing 1 M AlCl3·6H2O with a flow rate of 4 mL/min while stirring at 80°C. The temperature of the solution was then increased to 95°C with continuous stirring for various intervals (40 min for Al13 and 12 h for Al30). The solution mixtures obtained became transparent, were left to cool gradually, and were then kept for 5 days at room temperature; the pH was adjusted to 4.22 and 4.53 for Al13 and Al30 solutions, respectively. Then, 0.5 M Na2SO4 solution was added to each of the Al13 and Al30 solutions to obtain a SO4 2–/Al3+ ratio of 0.33 at which white colloidal suspensions of Al13 and Al30 sulfate were formed after stirring for an additional 10 min. The colloidal suspensions produced were kept with the mother liquor for 72 h at 25°C. The obtained precipitate of Al13 and Al30 sulfate oligomers was collected by filtration and washed several times with DIW then dried in air for 24 h to obtain Al13 and Al30 nitrate crystals (Zhou et al. Reference Zhou, Gao, Yue, Liu and Wang2006).
Synthesis of Al13 and Al30 Nitrates
The Al13 and Al30 sulfates prepared were used as precursors for the synthesis of Al13 and Al30 nitrates, according to the method proposed by Furrer et al. (Reference Furrer, Ludwig and Schindler1992). A 3.6 g portion of Al13 sulfate or 3.26 g of Al30 sulfate was dissolved in 800 mL of 1.0×10–2 M or 1.15×10–2 M of Ba(NO3)2, respectively, under vigorous stirring for 5 h. The products obtained were filtered out using 0.1 μm filter paper, and the filtrate was then crystallized in an oven at 60°C to obtain Al13 and Al30 nitrate crystals (Aouad et al. Reference Aouad, Pineau, Tchoubar and Bergaya2006).
Synthesis of Al-PILBs
1 g of BT with a grain size of ~20 μm was ground using a porcelain mortar with 0.17 g of Al13 nitrate or 0.15 g of Al30 nitrate, or a mixture of the two in a 1:1, 2:1, or 1:2 ratio; the mixture was then transferred to a conical flask containing 7.5 mL of DIW, stirred for 60 min, and then covered and left to stand overnight. The polyoxycation/BT ratio used in the synthesis of all Al-PILBs materials was 0.806 meq Al/g BT. The products obtained were filtered off using 0.45 μm Millipore filter paper, dried at 120°C for 12 h, and then calcined at 300°C for 3 h. The synthesized materials were labeled BT, BT-Al30, BT-Al13/Al30, BT-Al13/2Al30, or BT-2Al13/Al30.
Characterization Techniques
Estimation of the raw BT sample constituents (wt.%) was performed using X-ray fluorescence (Shimadzu 2400, Kyoto, Japan). The morphologies of Al-PILBs were characterized by field-emission scanning electron microscopy (FESEM, QUANTA FEG250, Eindhoven, The Netherlands). FTIR spectra were obtained using an advanced FTIR spectrometer (Shimadzu IRTracer-100, Kyoto, Japan). X-ray diffraction (XRD) patterns were measured on a Philips X-ray diffractometer model PW/103 (Eindhoven, The Netherlands). Surface area, pore volume, and pore-size distribution were measured by the N2 adsorption/desorption technique using a QuantaChrome (Boynton Beach, Florida, USA) Nova touch LX2 surface area and pore size analyzer. The various species of Al in HPA solutions were quantified by the Ferron colorimetric method (Chen et al., Reference Chen, Luan, Fan, Zhang, Peng and Fan2007) using a UV-Vis spectrophotometer (HACH, DR-3900, Colorado, USA). Finally, pesticide concentrations were determined using gas chromatography-mass spectrometry (GC-MS, Agilent Technologies, Santa Clara, California, USA), model 7890A GC System.
Removal of Chlorinated Pesticides from Water using Al-PILBs
A mixture of the 17 types of chlorinated pesticides mentioned above (1000 mg/L diluted in a mixture of n-hexane/water (1:20)) was used in this study. The pesticide mixture comprises the most abundant pesticides in environmental samples. Initially, 1 g of each Al-PILB material (BT-Al30, BT-Al13/Al30, BT-Al13/2Al30, BT-2Al13/Al30, or BT) was dried at 120°C for 3 h to remove physisorbed water. Then, a series of batch experiments was performed to examine the applicability of Al-PILB materials to pesticide removal from a dilute solution. In a typical adsorption experiment, the Al-PILB (25 mg) was mixed with 25 mL of an aqueous solution containing pesticide in the range 200–300 μg/L and adjusted to the appropriate pH at the natural value (5.8). The mixture was shaken using an orbital shaker in a temperature-controlled water bath at 25°C for 5 h at a constant agitation speed of 400 rpm. After equilibration, the Al-PILB was filtered using a 25 mm Whatman filter paper and the filtrate was used for adsorption assessment by analyzing the pesticide concentrations using GC-MS. The removal efficiency (R E) and adsorption capacity (q e) of Al-PILBs were calculated using the following equations (Eqs 1 and 2) (El-Said et al. Reference El-Said, El-Khouly, Ali, Rashad, Al-Bogami, Elshehy and Al-Bogami2018; Hamza et al. Reference Hamza, Weie, Mira, Abdel-Rahman and Guibal2019):
where C i and C e are the initial and equilibrium concentrations of the pesticide (μg/L), respectively; V is the volume of solution (L); and W is the weight of Al-PILB material (g).
RESULTS AND DISCUSSION
Materials Characterization
The synthesized Keggin cations of Al13 and Al30 were prepared in the sulfate form then converted to the nitrate form to give uniformly intercalated and stacked layers (Aouad et al. Reference Aouad, Pineau, Tchoubar and Bergaya2006). In spite of the large size of the Al30 polyoxycations, their high charge stabilizes their accommodation inside BT layers better than Al13 cations. Such systematic changes led to two significant characteristics of the Al30 polyoxycations: (1) the particulate size development and high charge lead to appreciable changes in interlayer spacing; and (2) the hydrolysis rate and species stability might provide a driving force for changing the morphology and structure of the Keggin cation formed (Lin et al. Reference Lin, Chin, Huang, Pan and Wang2008).
Chemical Composition
In the present study, the bentonite sample contained mainly montmorillonite accompanied by some impurities of kaolinite, illite, and quartz. The bentonite was used without thermal or chemical treatment. The main chemical composition (expressed as oxides) given by the XRF analysis was SiO2 (55.9%), Al2O3 (16.87%), Fe2O3 (9.07%), MgO (0.88%), CaO (2.39%), K2O (1.47%), TiO2 (1.06%), and Na2O (3.67%).
SEM Analysis
The surface micrographs of poly-aluminum species of Al13 and Al30 Keggin were elucidated using SEM analysis. The crystals of Al13 and Al30 sulfate formed showed different morphologies depending on the aging time (Wang and Muhammed Reference Wang and Muhammed1999). The SEM images for Al13 associated with the sulfate anions at various magnifications (Fig. 1a,b) revealed tetrahedral morphology with a variable aggregate size similar to that reported by Furrer et al. (Reference Furrer, Ludwig and Schindler1992). Because of the low crystallinity of Al30 sulfate aged for 72 h (Fig. 1c,d), no definite polygonal morphology could be distinguished for these Keggin cation particles. However, the particles were flake shaped and superimposed on one another, similar to those prepared by Chen et al. (Reference Chen, Luan, Fan, Zhang, Peng and Fan2007) and Motalov et al. (Reference Motalov, Karasev, Ovchinnikov and Butman2017) who also confirmed the formation of Al30 sulfate under the synthesis protocol used in the present study. On the other hand, the SEM images of the synthesized Al13 and Al30 associated with the nitrate anions (Fig. 2) revealed that the particle morphology of Al30 nitrate was highly regular and uniformly rectangular with variable aggregate sizes similar to a rhomboid-plate shape (Fig. 2b). The aggregated particles of Al13 nitrate had a shape typical of that depicted by Aouad et al. (Reference Aouad, Pineau, Tchoubar and Bergaya2006).
Fig. 1. Representative SEM images for the synthesized Al13 a, b and Al30 sulfate c, d at various magnifications
Fig. 2. Representative SEM images for the synthesized a Al13- and b Al30-nitrate
Al-Ferron Kinetics Method
The yields for Al13 and Al30 polycations from the synthesis procedures were examined using the Ferron kinetic method (Jardine and Zelazny Reference Jardine and Zelazny1986; Parker and Bertsch Reference Parker and Bertsch1992; Zhou et al., Reference Zhou, Gao, Yue, Liu and Wang2006). The Ferron kinetics method uses UV-Vis spectroscopy at λmax = 370 nm to follow the formation and distribution of the Al species synthesized during the course of the reaction in HPA solution (Fig. 3, Table 1). The percentages of Al species reported by this method were consistent with the expected results and revealed Al13 and Al30 yields of 74.3% and 75.5%, respectively.
Fig. 3. Al-Ferron reaction kinetics (at λmax = 370 nm) for the reaction of a Al13 and b Al30 with Ferron and log [Alb] unreacted Al13 and Al30 solutions as a function of time c, d, respectively
Table 1. Percentage of aluminum species present in Al13 and Al30 solutions and the rate constant (k)
FTIR Analysis
The FTIR technique was used to study the structure of polyoxycations precursors and to confirm the successful synthesis of the target Al aggregates and their intercalation inside the BT interlayers (Fig. 4). The aluminum Keggin cations are identified by the AlO4 group, which is observed as an asymmetrical stretching vibration peak at 769 cm−1. The region between 1200 and 800 cm–1 provides information about the structure of BT (Abeysinghe et al. Reference Abeysinghe, Unruh and Forbes2013). The weak peak at 1030 cm–1 in the spectra of BT, BT-Al30, BT-Al13/Al30, BT-Al13/2Al30, and BT-2Al13/Al30 was assigned to the Si–O stretching vibrations in the aluminosilicate layers of montmorillonite. After intercalation with Al Keggin cations, this peak was replaced by a shoulder, indicating the success of the intercalation. Moreover, the intensity of this shoulder increased with BT-2Al13/Al30 and decreased sharply with BT-Al13/2Al30, suggesting the intercalation of the BT with different Keggin cation ratios. Peaks at 923 and 523 cm–1 were associated with Al–OH and Al–O–Si vibrations, respectively. The position of the peak associated with the H–O–H bending vibrations of water molecules adsorbed on BT was shifted from 1639 cm–1 to 1635 cm–1 in BT-Al30 and BT-2Al13/Al30 materials. The intensity of this peak decreased sharply in the case of BT-Al13/2Al30. In addition, a strong peak at 3623 cm–1 and a broad peak at 3437 cm–1 for the BT were assigned to O–H stretching and cation hydration, respectively (Rivera-Jimenez et al. Reference Rivera-Jimenez, Lehner, Cabrera-Lafaurie and Hernández-Maldonado2011; Abdelkader & Fray Reference Abdelkader and Fray2017). This O–H stretching peak was diminished after intercalation with Keggin cations due to the octahedrally coordinated Al atoms associated with the hydroxyl group of the Al polycations, especially for BT-Al13/2Al30 material. In conclusion, variations in positions and intensity for those peaks are probably attributable to intercalation with differing types and ratios of Keggin cations.
Fig. 4. FTIR spectra for Al30 nitrate, Al30 sulfate, and BT in addition to the pillared bentonite materials (Al-PILBs)
Energy Dispersive X-ray Analysis
Analysis by EDX showed an increase in the Al content of Al-PILB materials relative to BT showing that Al polyoxycations of Al13 and Al30 were incorporated into the interlayer nanospaces. Moreover, Na content decreased in all Al-PILB materials because of the cation exchange between Na and Al Keggin cations. In most materials, both Ca and Mg exhibited a moderate decrease, indicating the low probability of participating in the intercalation process (Table 2). Finally, the Si/Al ratios in Al-PILBs materials decreased, confirming the success of the intercalation process.
Table 2. Chemical analysis (wt.%) of the energy-dispersive X-ray (EDX) for Al Keggin cations and Al-PILBs materials
X-ray Diffraction Analysis
The results of XRD analysis (Fig. 5) showed an increase in basal spacing, d 001, for all Al-PILB materials by varying degrees relative to raw BT, d 001 = 1.53 nm (El Bouraie and Masoud Reference El Bouraie and Masoud2017). The decrease in basal spacing in BT (to 1.47 nm) was due to dehydration of the interlayer and, therefore, the collapse of layers by ~0.06 nm after calcination at 300°C for 3 h. The intense, sharp quartz peak confirmed quartz as the major impurity in BT. The difference in sharpness and intensity in Al-PILBs peaks is probably attributable to the presence of polyoxycations in various ratios within the BT interlayers, regardless of the type of Al Keggin cation (Sanabria et al. Reference Sanabria, Alvarez, Molina and Moreno2008). The broadness of the peaks may be explained by the small number of stacked pillared layers that could have occurred because of slow replacement of the exchangeable Na cations by the larger Al13 and/or Al30 aggregates.
Fig. 5. XRD patterns for BT and Al-PILBs. Inset are the data from XRD analysis and basal spacing and changes in them in comparison to the raw BT basal spacing
Texture Analysis of the Al-PILB Materials Synthesized
Intercalation processes are known to create more pores, gaps, and several other types of defects in host materials. To investigate the effect of the intercalation on the porosity of BT, N2 adsorption/desorption measurements were used to calculate specific surface area (S A), pore diameter (D p), and pore volume (Fig. 6). The isotherms obtained were of the common type II of Brunauer's classification, as observed typically in multilayer materials. The hysteresis loops of Al-PILB materials belonged to type H4 (IUPAC classification); and most of the hysteresis loops were closed at low relative pressures, indicating the presence of micropores (Farrag Reference Farrag2016). From this approach, the synthesized Al-PILB materials showed the advantages of having a large surface area and large pore size after pillaring with Al polyoxycations, compared to the pristine BT (Table 3). The large pore size observed for BT-Al30 (2.59 nm) and BT-Al13/2Al30 (2.48 nm) compared to BT-Al30/Al13 (2.13 nm) and BT-Al30/2Al13 (2.13 nm) was probably due to large proportions of the bulky Al30 being intercalated inside BT interlayers. This induced differing physical morphologies for the synthesized compounds, which led to noticeable changes in their pore sizes. This interpretation coincides with XRD results which showed that the Al-PILBs peaks shifted to lower °2θ (i.e. increased in basal spacing).
Fig. 6. Nitrogen adsorption/desorption isotherms for BT and the synthesized Al-PILBs
Table 3. Textural characteristics of the Al-PILBs from N2 adsorption/desorption measurements
*S A, ES A, PS A, and D p denoted for specific surface area, external surface area, pores surface area, and average pore size, respectively.
Pesticides Adsorption Assays
The behavior of a mixture of chlorinated pesticides on adsorption into the synthesized Al-PILBs was investigated under optimal ion-adsorption conditions (natural pH, Al-PILBs weight 25 mg, solution volume 25 mL, and 25°C) (Fig. 7). Adsorption capacities of the heptachlor epoxide, dieldrin, endrin, endosulfan 1, and endosulfan 2 into Bt-Al30 and Bt-Al13/2Al30 were the largest among the other pesticides studied (Table 4). Both Bt-Al30 and Bt-Al13/2Al30 adsorbed dieldrin to almost the same extent. The data showed a significant adsorption capability for the pillared BT containing high proportions of Al30 polycations (i.e. BT-Al30 and BT-Al13/2Al30). This may be attributed to the porous characteristic represented by the large surface area and suitable pore size (21.3–25.9 Å) of synthesized Al-PILBs containing Al30 polycations. The adsorption behavior of the target pesticides, therefore, was influenced by the size and charge of Al polycations in the interlayer spaces. The adsorption of these non-ionic organochlorine pesticides into pillared clays is explained by the combination of hydrophobic interaction and by hydrogen bonding between hydroxyl groups of polycations and oxygen in the pesticide structure. The adsorption efficiency of the Al-PILB materials toward heptachlor epoxide, dieldrin, and endrin may be ascribed to the strong binding affinity of the epoxide group (CH2)2O (Rinaldi & Kristiani, Reference Rinaldi and Kristiani2017). BT has a negatively charged surface in addition to the Lewis acidic character of the intercalated Al30 polycation. The pesticide molecules thus adsorbed firmly on the Al-PILB surface through the negatively charged oxygen of Al30 polycations and the positively charged carbon centers in the pesticide molecules BT surfaces (Rinaldi and Kristiani Reference Rinaldi and Kristiani2017). The strong binding is probably due, therefore, to the epoxide group in which the BT-Al30 catalyzed the ring-opening of epoxides using water as the reaction medium (Bonollo et al. Reference Bonollo, Lanari and Vaccaro2011) (Table 4).
Fig. 7. The detailed adsorption profiles of Al-PILBs for 17 pesticides at selected optimal conditions (e.g. time 5 h, pH 5.8 (natural pH), and 25°C). The mixture of pesticides included the following: (1) α-BHC, (2) β-BHC, (3) σ-BHC, (4) heptachlor, (5) aldrin, (6) heptachlor epoxide, (7) endosulfan 1, (8) dieldrin, (9) DDE, (10) endrin, (11) endosulfan 2, (12) DDD, (13) endrin (14) endosulfan sulfate, (15) DDT, (16) methoxychlor, and (17) γ-BHC
Table 4. Comparisons among various adsorbents used for pesticide removal from polluted water
The effect of the initial concentration of three selected pesticides (heptachlor epoxide, dieldrin, and endrin) (Table 5) on uptake by BT-Al30 material was investigated within the concentration range 2.5–200 μg/L under the optimum experimental conditions (natural pH, BT-Al30 weight 25 mg, solution volume 25 mL, and 25°C). The adsorption capacities of heptachlor epoxide, dieldrin, and endrin onto BT-Al30 at optimum pH were 59.2, 59.15, and 60 μg/g, compared with 34.68, 39.45, and 38.9 for the pristine BT, respectively (Fig. 8). The uptake of target pesticides increased sharply as the concentration increased. Moreover, the removal efficiency of heptachlor epoxide, dieldrin, and endrin was reduced from 81.2% and 81% to 70% and 60%, respectively, when the Al30/Al13 ratio was decreased in Al-PILBs. These findings showed good binding interactions of the pesticides to the surface of Al-PILBs under optimal conditions. The adsorption data were plotted using several isotherm models such as Langmuir, Freundlich, and Sips, but only the Fruendlich model fitted well with the adsorption data for heptachlor epoxide, dieldrin, and endrin onto BT-Al30, with correlation coefficients (R 2 ) >0.96 for the target pesticides (Fig. 8). Freundlich (Reference Freundlich1906) used a multi-site adsorption assumption and his isotherm model is given by Eq. (3).
Table 5. The chemical structure and physico-chemical properties of the most selective pesticides for the synthesized Al-PILB materials
 |
Fig. 8. Experimental isotherms (symbols) and Freundlich isotherms (dash) for the equilibrium adsorption of heptachlor epoxide, dieldrin, and endrin onto BT-Al30 from a single ion solution; equilibrium time 5 h, pH 5.8, and 25°C
The Freundlich adsorption coefficient (K F, L1/n mg(1–1/n)/g) describes the adsorption capacity of the target pesticides onto BT-Al30 micropores at equilibrium concentration and 1/n (dimensionless) reflects the degree to which adsorption is a function of concentration. The values of 1/n < 1 obtained from adsorption of heptachlor epoxide, dieldrin, and endrin on BT-Al30 indicate that adsorption follows the L-type isotherm (Giles et al. Reference Giles, Macewan, Nakhwa and Smith1960). This means that competition for adsorption sites became greater as the concentration increased. The extent of adsorption of the endrin and dieldrin on BT-Al30 at a solution concentration of 20 μg/L was similar (60 and 59.2 μg/g, respectively); whereas heptachlor epoxide reached that same level (59.15 μg/g) at a solution concentration of 16 μg/L, indicating that its adsorption could be greater than the other two. The octanol/water partition coefficient (K ow ) is well known as a measure of the hydrophobicity of the pesticide and may predict the relative efficiency of pesticide removal. The values for log K ow of the target pesticides followed the order heptachlor epoxide > endrin > dieldrin (Table 5). The greater the log K ow value, the more hydrophobic the compound; therefore, heptachlor epoxide is the most likely of the three to adsorb into BT-Al30 pores at a low initial concentration (Gupta et al. Reference Gupta, Gupta, Rastogi, Agarwal and Nayak2011). The removal efficiencies of heptachlor epoxide, dieldrin, and endrin using 10, 15, and 25 mg of BT-Al30 material were 40, 65, and 99%, respectively. However, the adsorption efficiency varied slightly at a dose >25 mg; giving values ˃99%. In Table 6, the adsorption behavior of various pillared clays for the various organic compounds, including the pesticides, is summarized. When pesticides were desorbed using acetone; ~67.5% of the loaded pesticides were released from the BT-Al30. These results indicated that the BT-Al30 is a potential candidate for the reproducible removal of chlorinated pesticides from water in the environment.
Table 6. Behaviors of various clay pillars on the adsorption of organic molecules from aqueous solution
Conclusions
In this study, the synthesis of Al-PILBs containing various ratios of Al13 and Al30 Keggin cations was carried out through the concentration method using the nitrate form to create Al pillars. The chemical and textural characteristics of the synthesized Al-PILBs were investigated using XRD, SEM, EDX, FTIR, UV-Vis spectroscopy, and N2 adsorption-desorption measurements. The maximum adsorption of heptachlor epoxide, dieldrin, and endrin pesticides onto BT-Al30 at natural pH was 59.2, 59.15, and 60 μg/g, respectively, at equilibrium solution concentrations of 16, 20, and 20 μg/L, repsectively. The adsorption experimental results were fitted to the Freundlich model. The significant adsorption efficiency of BT-Al30, which is related to the negatively charged surface of BT in addition to the intercalation with Al30 polycations, means that the bentonite is useful for remediating pesticide-contaminated water.
Compliance with Ethical Standards
Conflict of Interests
The authors declare no conflict of interests.
