A novel cyano functionalized silica-titania oxide sol–gel based ionic liquid for the extraction of hazardous chlorophenols from aqueous environments

Abstract

In the present investigation a cyano functionalized silica-titania oxide sol–gel based ionic liquid (Si-Ti@CN/IL) has been successfully synthesised via sol–gel immobilization of cyanopropyltriethoxysilane and 1-benzyl-3-(trimethoxysilylpropyl)imidazolium chloride ionic liquid (BTMP-IM) on the silica-titania mixed oxide surface. The newly prepared Si-Ti@CN/IL was used as an adsorbent for the separation/preconcentration of chlorophenols from water samples prior to their HPLC-UV determination. Synthesis of Si-Ti@CN/IL was confirmed by Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), Brunauer–Emmett–Teller (BET), thermo gravimetric analysis (TGA), X-ray diffraction (XRD) and elemental (CHNS) analysis. During the extraction process, various kinds of interactions such as π–π interactions and hydrogen bonding between the anionic/cationic sites were monitored. The extraction of chlorophenols on Si-Ti@CN/IL is highly pH dependent and a significant percent extraction was achieved at pH = 2. The Si-Ti@CN/IL provided low limit of detection (LOD = 0.83–0.95 μg L⁻¹) with a linearity range from 10–1000 μg L⁻¹ (LOQ = 2.77–3.17 μg L⁻¹). The field studies with high recovery (73.39–105.54%) as well as acceptable precision (%RSD 0.82–4.19) also supported the effectiveness of Si-Ti@CN/IL, which could be useful for the extraction of selected chlorophenols from tap water, lake water and leachate from landfill sites. The comparative data support Si-Ti@CN/IL as an effective extractant for selected chlorophenols.

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1. Introduction

Phenols and their derivatives are well-known pollutants, additionally their widespread usage makes them one of the most noxious environmental wastes.^1–3 Chlorophenols are considerably used as intermediates for the generation of drugs, plastics and dyes.^4,5 The massive use of chlorophenols by pesticides, fertilizers, detergents, preservatives and dyeing manufacturing units is noticeable and considered as one of the major environmental issues of the modern era because these units release a huge quantity of chlorophenol contaminated effluents to natural streams.^4–8 From an environmental aspect, the exposure of chlorophenol contaminated effluents to the natural aqueous environment is of great concern.^9,10 Chlorophenols can be easily transferred to drinking water and can bring a threat to human beings due to their acute toxicity, potential carcinogenicity and moderate bioaccumulation.^11,12 As a consequence, many chlorophenols are listed by the US Environmental Protection Agency (EPA), and European Community (EC). Also, Japanese Ministry of Health, Labour and Welfare has stated the maximum contaminant level for phenols in drinking water is 5 μg L⁻¹.¹¹ However, exceeded concentration of chlorophenols i.e., 4.596 mg L⁻¹ in drinking water has been reported.¹² Hence, for safer environment the precise determination and removal of chlorophenols from aqueous environment is extremely essential.

Literature survey declared that liquid–liquid extraction (LLE) and solid-phase extraction (SPE) are mostly used for the extraction of chlorophenols from aqueous media.^13–16

Solid-phase extraction (SPE) is renowned more applicable method due to ease of operation, high recovery and low cost with the possibility of being automated and integrated into on-line mode as compared to liquid–liquid extraction (LLE).^17,18

Variety of the adsorbent has been employed in SPE for the separation/preconcentration of chlorophenols.^11,16,19 The conventional adsorbents have some drawback in term of selectivity, affinity and capacity. In order to overcome this matter, various attempts have been made to modify the traditional/conventional adsorbents and there is still need new innovative and efficient adsorbent for the preconcentration of chlorophenols.²⁰ In this respect cyanopropylsiloxanes deserve particular attention, because cyano-based materials due to the polarizable nature are considered as highly efficient adsorbents for the extraction of polar analytes from water samples. Since the cyano group have unshared electron pair of nitrile nitrogen which can easily forms intermolecular hydrogen bonds with the hydrogen of donor molecules/target analyte such as phenols, ketones, alcohols, esters and molecules bearing π-electrons.^21–23 As chlorophenols have aromatic core and it is reported that compounds having benzene ring are considered as good extractant agents for the extraction of chlorophenols by virtue of π–π interaction.²⁴ In this regard, recently a new class of ionic liquids (ILs) whether the introduction of a benzene ring into the cationic part of ionic liquid or imidazolium-based ILs adsorbents deserve particular attention as for the decontamination of chlorophenol contaminated wastewater.^25–28 But the solubility in water and air oxidation of ILs often limits their frequent uses. These drawbacks can be abridge by the immobilisation of ILs onto polymers and solid supports like silica as SPE materials.²⁰ Due to the unusual dual nature of ILs, they can act as a low polarity phase in front of non-polar analytes and high polarity in front of analytes bearing strong proton donor groups, through multiple interactions including electrostatic, hydrophobic and π–π.²⁹

Previously various ILs modified adsorbents were used for the extraction of the chlorophenols, so here in this study, we have reported the synthesis of a new SPE adsorbent (Si-Ti@CN/IL) and evaluation of its adsorption behaviour towards chlorophenols. The main consideration of this new material is the presence of aromatic core and benzyl groups (cationic part) of the ionic liquid, which is capable of imparting π–π interactions between the adsorbent and aromatic moieties in the chlorophenol compounds. In addition, the presence of unshared electron pair of cyano groups in cyanopropyltriethoxysilane, which is our site of interest to attract phenols through hydrogen bonds. The combination of these two functional groups can increase the affinity and capacity towards chlorophenols.

2. Experimental

2.1. Chemical and reagent

Titanium(IV) butoxide (Ti(OBu)₄), HPLC grade methanol, acetonitrile, ethylacetate and n-hexane were purchased from (Merck, Germany). Cyanopropyltriethoxysilane, triethoxysilane (TEOS), 1-benzylimidazole, 3-(chloropropyl)-trimethoxysilane were obtained from (Sigma-Aldrich, Germany). Selected 2-chlorophenol (2-CP), 3-chlorophenol (3-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TTCP) and pentachlorophenol (PCP) as shown in Fig. 1 were from (Sigma-Aldrich, Germany). Tetrahydrofuran (THF), hydrochloric acid (HCl) and diethyl ether (Fisher-Scientific, UK), acetic acid 96% from (Acros, USA). All chemicals and reagents were of analytical and chromatographic grades.

Fig. 1 Chemical structures of selected chlorophenols used in the study.

2.2. Instruments

IR spectra were recorded in ATR mode in the range of 400–4000 cm⁻¹ using Perkin Elmer X1FTIR, USA. The surface morphology was analysed by Auriga field emission scanning electron microscopy (FESEM) instrument at 1.00 kV (Carl-Zeiss, Germany). The surface area and pore size were measured using Brunauer–Emmett–Teller (BET) by nitrogen adsorption–desorption isotherms at 77.350 K in Micromeritics ASAP2020, USA. The elemental analysis was conducted using (Perkin Elmer, USA) series II CHNS 2400. Thermogravimetric analysis (TGA) was conducted under nitrogen atmosphere in the range of 50–900 °C at a heating rate of 10 °C min⁻¹ using (TGA7, Perkin-Elmer, USA). X-ray diffraction (XRD) was measured with a PANalytical Empyrean X-ray diffractometer (40 mA, 40 kV) using Cu Kα irradiation in the scanning range of 10 to 80°. The SPE vacuum manifold (12-port) obtained from Thermo Fischer Scientific (Waltham, MA, USA). HPLC system (Shimadzu, Japan) consisted of LC-20AT pump, SPD-M20A diode array detector, SIL-20A HT autosampler and CTO-10AS VP column oven was used for chlorophenols determination. The system was equipped with C-18 reverse column (250 × 4.6 mm) Hypersil gold, particle Sz. (μ) 5 (Thermo science, USA).

2.3. HPLC conditions

The chromatography analyses were performed using HPLC system. The column temperature was set at 40 °C. Mobile phase was at flow rate of 1 mL min⁻¹. The mobile phase consisted of purified water/methanol (20 : 80), both acidified to 1% with acetic acid. Detection was carried out using UV-Vis wavelength of 280 nm for all samples. The inject volume was constant on 20 μL.

2.4. Synthesis

2.4.1. Synthesis of methoxysilane functionalized imidazolium ionic liquid. 1-Benzyl-3-(trimethoxysilylpropyl)imidazolium chloride (BTMP-IM), was synthesized as follow; 1-benzylimidazole (316 mg, 2 mmol) and 3-(chloropropyl)-trimethoxysilane (3.67 mL, 2 mmol) were added to a two neck 50 mL round-bottomed flask and reaction mixture was stirred under nitrogen atmosphere at 80 °C for 48 h. The reaction mixture was cooled to room temperature and rinsed with 25 mL diethyl ether. Following the removal of diethyl ether by rotary evaporator the resulting viscous yellowish oil (yield% 97) was obtained and kept in the desiccator for 24 h.³⁰ IR (cm⁻¹) (Fig. 4A): 3342, 3156, 2959, 2845, 1500, 1565, 1457, 1368, 1191, 1039, 816, 663 & 464. ¹H NMR (400 MHz, CHCl₃) (Fig. 2B): 10.85–10.75 (1H, Im-H), 7.49–7.40 (3H, benzyl-H), 7.38–7.28 (1H, Im-H), 7.27–7.20 (2H, Im-H), 5.40–5.70 (2H, benzyl-H), 4.25–4.32 (2H, alkyl-CH₂), 3.41–3.57 (9H, O–CH₃), 1.12–1.20 (2H, alkyl-CH₂), 0.53–0.62 (2H, alkyl-CH₂). ¹³C NMR (400 MHz, CHCl₃) (Fig. 2A): 138.06, 133.30, 129.60, 129.21, 121.9, 53.56, 51.99, 50.91, 50.73, 24.24, 6.05.

Fig. 2 (A) ¹³C NMR spectra of BTMP-IM and (B) ¹H NMR spectra of BTMP-IM.

2.4.2. Synthesis of cyano functionalized silica-titania oxide sol–gel based ionic liquid (Si-Ti@CN/IL). Silica-titania mixed oxide functionalised with cyano–ionic liquid (Si-Ti@CN/IL) as shown in Fig. 3 was synthesized as follow; in first step for the preparation of silica-titania oxide cyano-functionalized backbone, titanium(IV) butoxide (3.27 mL, 9.6 mmol) and cyanopropyltriethoxysilane (2.31 mL, 9.6 mmol) in 5 mL of THF were stirred at room temperature for 30 min. Following the functionalization process, 1.71 g, 9.6 mmol of freshly prepared ionic liquid i.e., 1-benzyl-3-(trimethoxysilylpropyl)imidazolium chloride (BTMP-IM) were added to same reaction mixture and it was further stirred for 30 min. Then, on the drop wise addition of 0.69 mL (1.2 M HCl aqueous solution) a clear and homogeneous hybrid wet gel was obtained as result of attachment of the hydroxysilyl head of ionic liquid to the hydroxyl groups on the surface of backbone. The resultant hybrid wet gel (Si-Ti@CN/IL) was dried in vacuumed oven at 80 °C for 48 h. The dried hybrid wet gel (Si-Ti@CN/IL) was grounded into fine powder with mortar and pestle, rinsed with 30 mL acetone as well as with surplus deionised water. The whitish precipitates of newly synthesized Si-Ti@CN/IL adsorbent were dried in vacuumed oven at 100 °C for 24 h.

Fig. 3 Schematic route for the synthesis of Si-Ti@CN/IL.

For the synthesis of titania-silica oxide functionalised with cyano group (Si-Ti@CN) same procedure was as adopted as described for Si-Ti@CN/IL. But in case of Si-Ti@CN the adsorbent was not modified with 1-benzyl-3-(trimethoxysilylpropyl)imidazolium chloride (BTMP-IM). In case of Si-Ti@IL the titanium(IV) butoxide (Ti(OBu)₄) was not modified with cyanopropyltriethoxysilane, while all the steps were followed. For the synthesis of titania-silica oxide (Si-Ti) same procedure was as adopted as described for Si-Ti@CN/IL. But in case of Si-Ti the titanium(IV) butoxide (Ti(OBu)₄) was modified with (9.60 mmol, 2.13 mL) triethoxysilane (TOES), while all the steps were followed. Si-Ti, Si-Ti@CN and Si-Ti@IL were synthesized as reference materials to evaluate the performance of cyano groups and ionic liquid for the extraction of chlorophenols.

2.5. SPE procedure

100 mg of newly synthesized Si-Ti@CN/IL adsorbent were packed in a 3 mL SPE polypropylene cartridge fixing with upper and lower frits. The Si-Ti@CN/IL filled SPE cartridge was placed in the SPE vacuum manifold and 5 mL of methanol as well as 5 of water, respectively were used for the conditioning of Si-Ti@CN/IL filled SPE cartridges. For the optimization process, 10 mL of spiked sample (2 ppm mixed solution of selected chlorophenols) was loaded into the newly fabricated pre-conditioned cartridges at a flow rate of 1 mL min⁻¹. During the SPE process, the packed adsorbent in the cartridges was not allowed to dry at any time. Following sample loading, the SPE cartridges were left to be dry under vacuum for 30 min. Then the retained chlorophenols were eluted from the Si-Ti@CN/IL filled SPE cartridges by using 4 mL of methanol acidified with 1% acetic acid as modifier. Finally, the eluted samples were evaporated over vacuum air stream and dispersed with 0.5 mL of methanol acidified with 1% acetic acid. The dispersed solution was injected to the HPLC for analysis. For the comparative study, all synthesized Si-Ti@CN, Si-Ti@IL and Si-Ti adsorbents were packed in 3 mL SPE cartridges and a similar above mentioned SPE process was employed.

2.6. The real sample preparation

In line to identify the matrix effect of the optimised method, different environmental samples i.e., tap water, lake water and leachate were used. Tap water and lake water sample were collected from analytical chemistry laboratory and Varsity Lake, University of Malaya, Kuala Lumpur respectively, while leachate water samples were collected from landfill sites, Jeram Kuala Selangor. The collected tap/lake and leachate water samples were filtered off by 0.45 μm Millipore cellulose and 110 mm filter paper respectively and then stored at 4 °C in glass bottle foiled with aluminium.

3. Result and discussion

3.1. Synthesis and characterization of Si-Ti@CN/IL

The main goal of this study was to design a new hybrid cyano–IL based adsorbent, having aromatic core as well as cyano functional groups and exploration of its extraction properties towards the selected chlorophenols. To achieve the desired goal, 1-benzylimidazole was modified with 3-(chloropropyl)-trimethoxysilane. In order to acquire the hybrid nature and chemical stability (pH stable) the titanium(IV) butoxide was functionalized with cyanopropyltriethoxysilane and the resultant hybrid Si-Ti@CN was chosen as precursor. Since, it is reported that under the harsh chemical conditions (high/low pH conditions) sol–gel titania is more stable as compared to the sol–gel silica. So the presence of titanium in structure of adsorbent makes it as chemically more stable and increase the chemical strength of gel skeleton as well as improves the pH stability which is very important for effective extraction under high or low pH conditions. Ti(OBu)₄ was chosen as the source of titanium since Ti(OBu)₄ has comparatively slow reaction with water and does not require special care and handling precautions.^31,32 In second step aromaticity/aromatic core was achieved by the attachment of ionic liquid containing benzene ring (BTMP-IM) in the presence of acidified water in THF. The morphology and structure of the Si-Ti@CN/IL adsorbent was fully characterized by means of Fourier transformed infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), BET, XRD and TGA analysis.

The FTIR spectral analysis for the confirmation of different functional group in newly synthesised materials is represented in Fig. 4A–C. In the FT-IR spectrum (Fig. 4A) presence of C–H (aromatic), C=N, C=C, C–C, C–N and Si–O groups stretching at 3156, 1565, 1500, 1457, 1367 and 1039 cm⁻¹ respectively is a qualitative evidence for the successful functionalization of 1-benzylimidazole with 3-(chloropropyl)-trimethoxysilane. Since, raw 1-benzylimidazole does not have any stretching vibration at around 1039 cm⁻¹ (Si–O) but in FT-IR spectrum (Fig. 4A) the presence a diagnostic sharp peak for Si–O confirmed the synthesis of BTMP-IM. Successful functionalization of Ti(OBu)₄ with cyanopropyltriethoxysilane or synthesis of Si-Ti@CN can be predict by the presence of characteristic peak for cyano group (C≡N) stretching (Fig. 4B) at around 2253 cm⁻¹. Both synthesised materials i.e., BTMP-IM as well as Si-Ti@CN showed some diagnostic absorption IR bands at specific frequencies. Since BTMP-IM (Fig. 4A) does not have any stretching vibration at around 2253 cm⁻¹, same way Si-Ti@CN don't have any aromatic stretching band at around 1560 & 1500 cm⁻¹. But FT-IR spectral analysis (Fig. 4C) due to the presence of characteristic peaks for cyano group (C≡N) as well as aromatic stretching band confirmed the formation of new hybrid cyano–IL based adsorbent (Si-Ti@CN/IL). The C–H (aromatic), C≡N, C=N and C=C stretching in Si-Ti@CN/IL can be predicted by the characteristic peaks at 3156, 2250, 1570 and 1533 cm⁻¹ respectively.

Fig. 4 FT-IR spectra of BTMP-IM (A), Si-Ti@CN (B) & Si-Ti@CN/IL (C).

The morphological behaviour of Si-Ti@CN and Si-Ti@CN/IL was examined from FESEM results as presented in Fig. 5. The FESEM image of Si-Ti@CN (Fig. 5A) showed microsporus compact particles rough morphology. Following the attachment BTMP-IM as it was expected the microspores were filled and aggregation of BTMP-IM on the surface of Si-Ti@CN/IL (Fig. 5B) can be clearly seen. So the deposition of foreign materiel i.e., BTMP-IM on the surface of Si-Ti@CN confirmed the successful formation of new hybrid Si-Ti@CN/IL adsorbent.

Fig. 5 FESEM image of Si-Ti@CN (A) and Si-Ti@CN/IL (B).

The porous structure characterization by gas sorption analyser via the BET showed a decrease in the surface area, pore volume and pore size of Si-Ti@CN/IL in comparison with Si-Ti@CN. This might be due to pore coverage of IL in the surface of Si-Ti@CN/IL.

The pore sizes related to both materials are consistent with the IUPAC definition of mesopores type, (Table 1). The isotherm for the Si-Ti@CN/IL (Fig. 6B) exhibited a tendency to change the isotherm from type IV to I. This phenomenon in isotherm is typical in materials situated between mesopore to micropore sizes,^33,34 while the Si-Ti@CN (Fig. 6A) showed isotherm type IV, indicating the formation of the typical mesopores structures.²² The adsorption average pore diameter (D) of Si-Ti@CN and Si-Ti@CN/IL was determined by 4V/S_BET equation. In this equation, V stands for the adsorption total pore volume and S_BET stands for BET surface area of pores which obtained using the BET model.

Table 1 Physical properties of Si-Ti@CN/IL and Si-Ti@CN

Sample	SBET (m^2 g^−1)	V (cm^3 g^−1)	D (nm)	Elemental composition (wt%)
				C	H	N
Si-Ti@CN	13.54	0.04	11.7	17.03	3.08	3.09
Si-Ti@CN/IL	0.6	0.001	6.6	11.72	1.95	4.03

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Fig. 6 N₂ adsorption–desorption isotherms of Si-Ti@CN (A) and Si-Ti@CN/IL (B).

The thermal stability of adsorbents was examined at the temperature range of 50–900 °C in nitrogen atmosphere. BTMP-IM thermometric curve (Fig. 7A) showed two main degradation steps, in first step about 10% weight loss from 50–150 °C was assigned to physically adsorbed water, in second step 70% weight loss from 150–600 °C was assigned to the combustion of benzene ring in BTMP-IM. For both synthesised adsorbents i.e., Si-Ti@CN & Si-Ti@CN/IL (Fig. 7A) approximately 5% weight loss from 50–200 °C was attributed to the physically adsorbed waster. The 23% weight loss from 200–600 °C for Si-Ti@CN was assigned for the combustion of cyanopropyltriethoxysilane functionalities. While in case of Si-Ti@CN/IL same degradation pattern was observed.

Fig. 7 (A) TGA profiles of BTMP-IM, Si-Ti@CN and Si-Ti@CN/IL, (B) XRD analysis of Si-Ti@CN and Si-Ti@CN/IL.

Fig. 7B shows the XRD pattern structure of Si-Ti@CN and Si-Ti@CN/IL in the range of 0–80°. The presence of a peak in both patterns at 2 theta near 25 degrees is merely indicates that the silica supported Si-Ti@CN and Si-Ti@CN/IL are amorphous in nature.³³ However, the absence of higher angle peaks in diffraction patterns exhibits no long-range ordering and amorphous structure.

The elemental analysis i.e., CHNS results as represented in Table 1 also confirmed the successful attachment of BTMP-IM with Si-Ti@CN, it is obvious from the CHNS results that Si-Ti@CN contains 3.09% of N, while Si-Ti@CN/IL contains 4.03%. The increase in N percentage is due to the attachment of BTMP-IM.

3.2. SPE optimization study

3.2.1. Elution solvent. Optimization or selection of appropriate elution solvent is a very crucial step in the SPE process in order to desorb the retained chlorophenols. Four elution, solvents with different polarities i.e., methanol, acetonitrile, tetrahydrofuran (THF) and n-hexane were used to get the quantitative desorption of retained chlorophenols. Since the selected elution solvents can be categorize into polar as well as no-polar solvents, while selected chlorophenols are polar in nature. Consequently, as expected results (Fig. 8A) showed that polar solvents have higher affinity towards chlorophenols due to the higher solvents strength compared to the non-polar solvents. The obtained results showed that compared to the other tested eluting solvent methanol is more efficient for to desorption of targeted chlorophenols from the surface of Si-Ti@CN/IL adsorbent. The high desorption in case of methanol might be due to the protic nature, since methanol is a highly protic solvent and it can easily form a hydrogen bond with the hydroxyl groups of adsorbent.³⁵ In order to optimize the minimum volume of elution solvent, 1 to 20 mL of eluent solvent were passed through the cartridge. As the eluent volume enhanced from 1 mL to 4 mL, the efficiency of chlorophenols increased and best recovery for all chlorophenols was achieved by using 4 mL of eluent solvent. Beyond the, 4 mL there was no significant change was observed. Thus, 4 mL of methanol was chosen as the optimum eluent volume (Fig. 8B).

Fig. 8 SPE method development: (A) elution solvent, (B) eluting solvent volume (mL).

3.2.2. Effect of sample volume. In order to investigate the optimum volume of sample for quantitative extraction recovery, the volume of sample solution was varied from 1–25 mL and the SPE cartridges were loaded with different quantities (1, 5, 10, 15, 20, 25 mL) of 2 ppm mixed solution of selected chlorophenols. As shown in Fig. 9A, the peak area improved by increasing the sample volume up to 10 mL for all the selected target analytes. Thus, 10 mL volume was selected for subsequent experiments.

Fig. 9 SPE method development: (A) sample volume (mL), (B) solution pH.

3.2.3. Solution pH. The solution pH has momentous influence on the extraction performance, because by the variation of solution pH the extraction of target analytes can be significantly increased or decreased.¹¹ So herein the effect of solution pH on the extraction of selected chlorophenols was investigated by varying the pH from 2–10 (Fig. 9B). Results indicated that strong acidic medium is more suitable for the extraction of selected chlorophenols and maximum efficiency for all the selected chlorophenols was obtained at pH 2. While by increasing the pH it was observed that peak area turns to decrease. The highest peak area for 2,4,6-TCP as well as for 3-CP and 2,4-DCP at pH 2 can be explain on the basis of pK_a values of selected chlorophenols that are in the range 9.12 to 7.42.^23,24 The dramatic decrease in peak area in case of 2,4,6-TCP, 2,3,4,6-TTCP and PCP at basic pH i.e., 8–10 might be due to the abundance of chloro groups. Since, synthesised adsorbent Si-Ti@CN/IL contains variety of anionic terminal functional group so at basic pH there may be strong repulsive forces between the anionic sites of both i.e., adsorbent as well as targeted analytes. In addition, strong π–π interaction between benzyl group from imidazolium ring (ionic liquid) and phenolic compounds can increase the extraction.^11,24

3.2.4. The effect of acetic acid as solvent modifier. To investigate the possible effect of modifier on desorption of targeted chlorophenols acetic acid 1–15% (as ratio with methanol) was added to the eluting solvent. The highest recovery was observed with the addition of 1% acetic acid to methanol. As it is obvious from Fig. 10 that by increasing the % of acetic acid, the recovery of target chlorophenols significantly decreased except PCP. Thus, results confirmed that acetic acid as a modifier plays vital role during the desorption of retained chlorophenols from the surface of newly synthesised Si-Ti@CN/IL adsorbent. This is probably because of the high competition of acetic acid with retained chlorophenols to form hydrogen bond with the functional groups of adsorbent. However, higher % of acetic acid would not promote the elution step, due to the formation of hydrogen bond with methanol, there would be less OH group of solvent to desorb retained chlorophenols from Si-Ti@CN/IL adsorbent.³⁶

Fig. 10 SPE method development; % modifier (as ratio with methanol).

After each SPE run, the used SPE cartridges were rinsed thoroughly with methanol containing 1% acetic acid, followed by ultra-purified water to make sure that the analytes are removed from the adsorbent.

3.3. Method validation

On the basis of acquired results for the extraction of chlorophenols using Si-Ti@CN/IL, important parameters affecting the SPE were optimized as pH 2, 5 mL of methanol, followed by 5 mL of ultra-purified water as conditioning solvent, 10 mL sample volume and 4 mL of methanol containing 1% acetic acid as the eluting solvent finally eluted samples evaporated over air stream vacuum and dispersed with 0.5 mL solvent.

To validate the applicability of SPE methods for Si-Ti@CN/IL, the series of experiments were carried out to obtain the linearity, limit of detection (LOD) and limit of quantification (LOQ). SPE was carried out using the optimum extraction conditions. The linearity of the proposed method was investigated at different concentration levels of chlorophenols in the range of 10–100 μg L⁻¹ for Si-Ti@CN/IL. Each concentration was repeated three times and the mean value of peak area was taken for the calibration curve. The results showed a good linearity for the extraction of target chlorophenols with a coefficient of determination (r²) ranging from 0.9968–0.9999.

The LOD and the LOQ for SPE extraction of chlorophenols as given in Table 2 using Si-Ti@CN/IL adsorbent (0.83–0.95; 2.77–3.17 μg L⁻¹) were lower as compared to the compared to the (Si-Ti, Si-Ti@CN and Si-Ti@IL). The lowest value of LOD and LOQ of Si-Ti@CN/IL compared to other tested adsorbents indicates the effect of cyano group as well as ionic liquid on increasing the sensitivity of the adsorbent.

Table 2 Analytical figures of merits of Si-Ti, Si-Ti@CN, Si-Ti@IL and Si-Ti@CN/IL in SPE: repeatability (%RSDs, n = 5), LOD (μg L⁻¹), and LOQ (μg L⁻¹) of chlorophenols^a

Analyte	Si-Ti			Si-Ti@CN			Si-Ti@IL			Si-Ti@CN/IL
	RSDs	LOD	LOQ	RSDs	LOD	LOQ	RSDs	LOD	LOQ	r^2	RSDs		LOD	LOQ
											Intra (n = 5)	Inter (n = 3)
a Linearity ranges: Si-Ti, Si-Ti@CN and Si-Ti@IL (100–5000 μg L^−1), and Si-Ti@CN/IL (10–1000 μg L^−1).
2-CP	3.21	11.6	38.7	3.84	1.51	5.03	4.76	5.78	19.3	0.9990	2.31	1.61	0.85	2.82
3-CP	4.92	9.99	33.3	4.78	1.62	5.4	4.44	4.85	16.2	0.9968	3.98	3.68	0.84	2.79
2,4-DCP	1.36	10.5	34.9	4.08	1.38	4.6	3.6	6.48	21.6	0.9999	3.3	3.53	0.83	2.77
2,4,6-TCP	4.97	8.83	29.5	4.86	1.72	5.74	2.1	4.89	16.3	0.9999	4.71	4.16	0.93	3.1
2,3,4,6-TTCP	3.78	8.55	28.5	2.62	2.03	6.75	3.96	8.17	27.2	0.9999	4.17	4.23	0.95	3.17
PCP	3.65	8.71	29	2.25	1.71	5.7	2.66	3.31	11.0	0.9993	4.09	4.26	0.85	2.82

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The intra-day precision of optimum method was obtained by performing 5 variances at a concentration of 10 μg L⁻¹ with 100 mg of adsorbent. Inter-day precision was determined by 3 variances with the same concentration as inter-day batch, during 3 days. The optimum method demonstrated to be precise with RSDs of 2.31–4.71% (n = 5) for intra-day and 1.61–4.26% (n = 3) for inter-day batch (Table 2).

3.4. Comparative study

For the comparison of SPE performance similar amount (100 mg) of all the synthesized adsorbent i.e., Si-Ti, Si-Ti@CN, Si-Ti@IL and Si-Ti@CN/IL were packed in 3 mL SPE cartridges and optimum conditions were applied. The obtained results (Table 2, Fig. 11) indicated that Si-Ti@CN/IL adsorbent has highest percent (76.57–106.67%) with the relative standard deviation (%RSDs) of 2.31–4.71% as compared to Si-Ti (3.6–37.4; 1.36–4.97%), Si-Ti@CN (21.5–59.9; 2.25–4.86%) and Si-Ti@IL (26.1–105.6; 2.10–4.76%). Results justified the worth of Si-Ti@CN/IL as SPE adsorbent. Comparably 2-CP, 3-CP and 2,4-DCP have very low affinity for the Si-Ti@CN and Si-Ti@IL, while their percent recovery increased dramatically by using Si-Ti@CN/IL. The second concern is that the Si-Ti@CN has much lower affinity towards 2,4,6-TCP, 2,3,4,6-TTCP and PCP in comparison with Si-Ti@CN/IL. Additionally, Si-Ti showed very poor capacity and affinity towards the selected chlorophenols. So on the basis of acquired results it is concluded that combination of cyano group and ionic liquids on the surface of adsorbent can significantly enhance the affinity and capacity of the adsorbent for chlorophenols.

Fig. 11 SPE performance of Si-Ti@CN/IL with Si-Ti, Si-Ti@CN and Si-Ti@IL for selected chlorophenols.

The higher affinity of Si-Ti@CN/IL towards the chlorophenols can be explained by the fact that the imidazole ring and benzyl group (cationic part of ionic liquid) are able to form a strong π–π interaction as well H-bonding with the aromatic core and hydroxyl groups respectively of chlorophenols. In addition the synthesised Si-Ti@CN/IL contains different anionic sites i.e., S, O and N while selected chlorophenols contains Cl and OH groups. At acidic pH, hydrogen (H) of hydroxyl (OH) groups of selected chlorophenols shows binding abilities toward the anionic (S, O and N) sites of Si-Ti@CN/IL. While at the same time hydrogen (H) of hydroxyl (OH) group of Si-Ti@CN/IL shows binding abilities towards the Cl group of chlorophenols (Fig. 12). Consequently, in this scenario selected chlorophenols are extracted through strong hydrogen bonding.

Fig. 12 Proposed mechanism of interaction between the chlorophenols and Si-Ti@CN/IL.

3.5. Real samples

The optimum SPE method using Si-Ti@CN/IL adsorbent was applied to the real environmental water samples (tap water, lake water and leachate from landfill site). To investigate the effect of the sample matrix, the real samples were spiked with chlorophenols at a concentration of 5 μg L⁻¹ and 1000 μg L⁻¹ under the optimum. Table 3 shows the recovery and repeatability for real samples. The recovery of leachate, lake water and tap water was in the range of 77.99–95.57%, 75.17–105.54% and 73.87–101.17% respectively. The RSDs of leachate, lake water and tap water was in the range of 1.07–3.40%, 1.22–3.93% and of 0.82–4.19% respectively.

Table 3 Recovery and %RSDs (n = 5) of chlorophenols in the real water samples with a spiked concentration of 5 μg L⁻¹ and 1000 μg L⁻¹

Analyte	Leachate		Lake water		Tap water
	Recovery (RSDs)% 5 μg L^−1	Recovery (RSDs)% 1000 μg L^−1	Recovery (RSDs)% 5 μg L^−1	Recovery (RSDs)% 1000 μg L^−1	Recovery (RSDs)% 5 μg L^−1	Recovery (RSDs)% 1000 μg L^−1
2-CP	79.3 (1.91)	93.4 (2.27)	75.2 (2.28)	77.3 (1.97)	75.2 (2.27)	74.2 (1.99)
3-CP	95.6 (3.40)	90.1 (1.07)	83.9 (1.80)	93.8 (3.93)	82.4 (1.89)	97.4 (4.11)
2,4-DCP	90.1 (1.69)	92.5 (1.16)	105.5 (1.51)	89.0 (1.95)	96.6 (1.64)	88.6 (1.64)
2,4,6-TCP	89.1 (1.58)	78.6 (1.39)	79.8 (1.79)	86.5 (1.55)	77.1 (0.82)	87.0 (3.67)
2,3,4,6-TTCP	84.8 (3.34)	82.3 (2.45)	83.8 (2.40)	83.8 (1.36)	82.2 (2.30)	96.2 (4.19)
PCP	83.9 (2.05)	78.0 (1.32)	100.8 (1.22)	98.4 (1.43)	100.6 (1.62)	101.2 (3.91)

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The high percent recovery of targeted analytes in environmental samples shows their high affinity towards Si-Ti@CN/IL and applicability of the adsorbent for environmental samples, respectively. The chromatogram illustrations of unspiked lake water as well as spiked with analytes is given in Fig. 13.

Fig. 13 Chromatograms of lake water using Si-Ti@CN/IL as the SPE adsorbent; (A) lake water spiked with 5 μg L⁻¹ chlorophenols mixture and (B) unspiked lake water.

3.6. Comparison of newly Si-Ti@CN/IL with reported adsorbents

For the validation of the study, the results obtained were compared with previously reported adsorbents from literature. Comparison of %recovery, RSDs% and LOD (Table 4) reveals that Si-Ti@CN/IL is more efficient compared to the previously reported adsorbents.

Table 4 Comparison of %recovery, %RSDs and LOD of the current study with other reported adsorbents

Sample	Technique/adsorbent	%recovery	RSDs%	LOD	Ref.
River	SPE-HPLC/PS-DVB	51.06–104.07	2.4–5.59	23–4596	12
Drinking water	SPE-HPLC/PS-DVB	67.9–99.6	0.12–9.25	0.01–2.0	16
Tap-River-Sea	MSPE-HPLC/3D-G@Fe3O4	85.1–101.2	3.1–6.1	0.91–39.64	37
Tap-Stream	SPE-HPLC/MWCNTs	82.9–97.2	6.8–7.4	5–10	38
Tap-River	SPE-HPLC/XAD-2	8.01–79.9	0.16–4.69	—	39
Tap-River	SPE-HPLC/XAD-4	39.6–88.6	0.13–3.80	—	39
Tap–Lake–Leachate	SPE-HPLC/Si-Ti@CN/IL	73.39–105.54	0.82–4.19	0.83–0.95	Current study

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4. Conclusion

In summary, a new silica-titania oxide sol–gel based ionic liquid (Si-Ti@CN/IL) was successfully used as a new adsorbent in solid phase extraction (SPE) for the simple, fast and efficient extraction/preconcentration of chlorophenols from environmental water samples using HPLC-UV. Extraction/preconcentration of chlorophenols onto Si-Ti@CN/IL from aqueous solution was systematically investigated under various conditions and it is concluded that the presences of cyano group and cation of IL in new solid-phase extractor (Si-Ti@CN/IL), played a key role and enhanced the adsorption capability for the selected chlorophenols. The extraction of chlorophenols is sturdily pH dependent, and field studies showed that Si-Ti@CN/IL could be successfully used for the extraction of chlorophenols from aqueous environment. Therefore, Si-Ti@CN/IL will be a promising adsorbent for determination of chlorophenols in environmental samples.

Acknowledgements

I would like to acknowledge the valuable support from University of Malaya for the Research Grants (Project No. PG027-2013A and RP011B-14SUS) provided.

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