Cloud point-dispersive liquid-liquid microextraction for preconcentration and separation of mercury in wastewater samples by methylsulfanyl thiophenol material

Anal. Method Environ. Chem. J. 3 (1) (2020) 63-71 64 Anal. Method Environ. Chem. J. 3 (1) (2020) 63-71 as low concentration of Hg and high interferences ions in wastewaters, the preconcentration and extraction processes must be done [8,9]. Recently, different extraction or microextraction mechanisms were used for this purpose. The micro-solid phase extraction (μ-SPE) [10]. CPE [11], LLME based on ionic liquids [12], The DLLME and LLME are a strategy promotes the complexation processes between metal and ligand [13]. CP-DLLME technique can be assisted by ultrasonic accessory [14]. Solidified floating organic drop DLLME (SFO-DLLME) was developed by Kocurov et al and many other techniques introduced by liquid extraction procedure [15-22]. The cloud point extraction (CPE) caused to two phases for solution by temperature. The surfactants such as T-X100 were used for metal separation by clouding phenomena (S-CPE). The S-CPE has many advantages as compared to traditional extraction. The two components, salt and surfactant solutions separate into immiscible phases [23]. The metal mineral can analysis by different methods such as electrochemistry, ionic liquids and nano sorbents [24]. In the presence of salt, ionic liquids and surfactants self assemble in liquid phase at special temperature change to micelles [25-28]. Many metals interacted to micelles and so preconcentrated into the surfactant-rich phase. The aim of this study is to develop a new analytical method for rapid preconcentration and determination of trace mercury in wastewater samples based on the combination of CP-DLLME technique coupled to CV-AAS. Ionic liquid of [BMIM][PF6] dispersed in ethanol was used as trapping solvent for separation of MSTP from liquid phase. All factors which were affected on mercury extraction were studied and performance of the proposed method was validated. 2. Experimental 2.1. Apparatus and Reagents Mercury was determined by atomic absorption spectrometer with a cold vapor accessory (GBC 932, CV-AAS, AUS), deuterium-lamp (UV), Hg HCL, and a unit of circulating cooling. The conditions of CV-AAS were shown in Table 1. The pH values of the solutions were measured by a digital pH meter Metrohm (744, Swiss). A Hettich centrifuge (Germany) and an ultra sonic accessory (Tecno-GAZ, Germany) were used. All reagents with high purity and analytical grade were purchased from Merck (Germany). All standard solutions were prepared with deionized water (DW) from Millipore (USA). The Hg (II) standard stock solution (1000 ppm in 1% HNO3) was prepared from Sigma Aldrich, Switzerland. The experimental standard mercury were prepared daily by diluting of DW. The standards from 0.056.2 ppb were freshly prepared and stored in a fridge (4 °C). A 0.5% (w/v) sodium borohydride (NaBH4) was prepared daily by dissolving an appropriate amount of NaBH4 in 0.5% (w/v) of NaOH and used for hydration of mercury (HgH2). 1-Benzyl-3-methylimidazolium hexafluorophosphate ≥97.0% ([BMIM][PF6]; CASN: 39447), 1-Butyl-2,3-dimethylimidazolium hexafluorophosphate (CASN: 70869), 1,3-Diethoxyimidazolium hexafluorophosphate ≥ 97% (CASN: 688649), 1,3-Dimethoxyimidazolium hexafluorophosphate 98% (CASN: 690821) were purchased from Sigma–Aldrich ,Germany. The 4-methylsulfanyl thiophenol (CAS N: 1122-97-0, 96%, MSTP, C7H8S2) was purchased from Sigma Aldrich, Germany. The pH adjusted to 6.5 by using sodium phosphate (Na2HPO4/NaH2PO4,) as pH of 5.8-8.2. All the laboratory glasses were cleaned and washed by nitric acid and DW. Table 1. The CV-AAS conditions for mercury analysis Features Value Linear range, μg L-1 1-62 Wavelength, nm 253.7 Lamp current, mA 3.0


Introduction
Mercury compounds (Hg, R-Hg) as toxic pollutants enter to environment from wastewater factories and cause different disease in humans. There are three forms of Hg ( Hg 0 , Hg (II), R-Hg) which was used in different industries [1]. The high exposure of inorganic mercury damaged the human organs such as renal, liver and CNS [2]. Although the exposure to organic mercury or fish food can be created an main problem in blood brain barrier (BBB) and cortex of brain but is weaker than inorganic compounds [3]. The hazardous defect in humans organs such as, CNS, respiratory, cells, renal and liver caused to different diseases, hypertension, chromosomal aberrations, tremor and MS [4]. So, as high toxicity, mercury determination in wastewaters is very important as industries samples. The mercury concentration in water is less than 6 μg L -1 [5] and in blood is less than 1-2 μg dL -1 [6]. So, the reliable, accurate and fast analytical methods must be used for wastewater samples. Among different analytical methods cold vapor atomic absorption spectrometry (CV-AFS) has been widely used for mercury determination in water samples due to simple, lower LOD and good sensitivity [7]. But, as low concentration of Hg and high interferences ions in wastewaters, the preconcentration and extraction processes must be done [8,9]. Recently, different extraction or microextraction mechanisms were used for this purpose. The micro-solid phase extraction (μ-SPE) [10]. CPE [11], LLME based on ionic liquids [12], The DLLME and LLME are a strategy promotes the complexation processes between metal and ligand [13]. CP-DLLME technique can be assisted by ultrasonic accessory [14]. Solidified floating organic drop DLLME (SFO-DLLME) was developed by Kocurov et al and many other techniques introduced by liquid extraction procedure [15][16][17][18][19][20][21][22]. The cloud point extraction (CPE) caused to two phases for solution by temperature. The surfactants such as T-X100 were used for metal separation by clouding phenomena (S-CPE). The S-CPE has many advantages as compared to traditional extraction. The two components, salt and surfactant solutions separate into immiscible phases [23]. The metal mineral can analysis by different methods such as electrochemistry, ionic liquids and nano sorbents [24]. In the presence of salt, ionic liquids and surfactants self assemble in liquid phase at special temperature change to micelles [25][26][27][28]. Many metals interacted to micelles and so preconcentrated into the surfactant-rich phase. The aim of this study is to develop a new analytical method for rapid preconcentration and determination of trace mercury in wastewater samples based on the combination of CP-DLLME technique coupled to 6 ] dispersed in ethanol was used as trapping solvent for separation of MSTP from liquid phase. All factors which were affected on mercury extraction were studied and performance of the proposed method was validated.

Apparatus and Reagents
Mercury was determined by atomic absorption spectrometer with a cold vapor accessory (GBC 932, CV-AAS, AUS), deuterium-lamp (UV), Hg HCL, and a unit of circulating cooling. The conditions of CV-AAS were shown in Table 1.

Sampling
Samples of wastewater (paint factory, Tehran, Iran), wastewater ( industrial factories, Jajrood, Iran), oil company wastewater (Tehran, Iran) and chemical factory wastewater (Tehran, Iran) collected and filtered (0.45 µm) with polyethylene tubes and cellulose membrane (CMF), respectively before we used. The pH was tuned up to 7.0 with phosphate buffer solution. Then, the cloud point dispersive liquidliquid microextraction (CP-DLLME) procedure was used for mercury extraction and determination in wastewater samples. The standard reference materials NIST NIST-SRM 1641e (total mercury in water) from the National Institute of Standard and Technology (NIST, Gaithersburg, USA) were also analyzed in a similar manner according to the general procedure.  (Fig.1).

CPE procedure
The blank solutions proceeded the same way and are used for the preparation of the calibration solutions and for measurement of the blanks. The extraction mercury based on MSTP-CP-DILLME method was shown in Table 2.

Optimization of pH
The effect of pH on complexation of mercury ions based on MSTP was investigated in different pH from 2 to 10 for 0.05 μg L -1 , 0.5 μg L -1 , 6.2 μg L -1 Hg(II) as a LLOQ, MLOQ and ULOQ ranges.
The complexation was strongly depended on the pH sample and subsequently caused to increase the extraction efficiency of mercury in wastewater samples. Based on results, the maximum extraction efficiency for mercury was obtained at pH=7.0 and the recovery were below 5% in acidic or basic pH. Therefore, the pH=6-8 was selected as optimum pH for mercury extraction from wastewater samples by the developed MSTP-CP-DILLME method with high recovery (Fig. 2). In pH=6-10, the sulfur (-) has negative charge but mercury based on positive charge (+) can be complexed with sulfur in pH more of 6 and less than 8. The results showed the maximum extraction was achieved at pH=7 for mercury by coordinating covalent bond of sulfur (Hg 2+ ---: S 2-).

Optimization of ionic liquid
By procedure, the wastewater samples were shaked by ultrasonic bath for 10 min and cloudy solution was achieved by ionic liquid/ethanol micelles at pH=7.0. So the kind and amount of ionic liquid has critical role as generation micelles in liquid phase and extraction process by MSTP-CP-DILLME method. For this purpose, different ILs such as, 1-Benzyl-3methylimidazolium hexafluorophosphate, 1-Butyl-2,3-dimethylimidazolium hexafluorophosphate, 1,3-Diethoxyimidazolium hexafluorophosphate, 1,3-Dimethoxyimidazolium hexafluorophosphate were selected and used for mercury extraction in optimized conditions. Based on results, the extraction efficiency was remarkably affected by amount ionic liquid amount, so it was examined within the range of 20-200 mg. Quantitative extraction was achieved more than 60 mg of   6 ] was chosen as optimum leading to a final IL (Fig. 3).

Optimization of amount of MSTP
The amount of methylsulfanyl thiophenol (MSTP, C 7 H 8 S 2 ) was evaluated by CP-DLLME. By procedure, The concentration of MSTP between 1.0 × 10 −6 -50.0 × 10 −6 mol L −1 was prepared and optimized for maximum extraction mercury in wastewater samples in pH=7.0. The results showed the recovery has high extraction more than 5.7 × 10 −6 and then no effected on mercury extraction by increasing MSTP. In fact, the 5.7 × 10 −6 mol L −1 of MSTP was the minimum concentration which was necessary for high recovery for mercury extraction from wastewater samples. So, the 10×10 −6 mol L −1 of MSTP was selected as optimum concentration as interference ions in wastewater and more than the signal remained constant (Fig. 4).

Optimization of acids
The ionic liquids cannot directly use by CV-AAS, because of high viscosity and low interaction with redacting reagents such as NaBH4. By MSTP-CP-DILLME procedure, the Hg-MSTP loaded on [BMIM][PF6] was back-extraction by the mineral acidic/basic solution. By changing of pH, the covalence bond between sulfur and mercury leads to dissociation and mercury ions release to liquid phase of acid. Different concentration of mineral reagents from 0.1-3 mol L −1 (HCl, HNO3, CH3-COOH, NaOH) were used for back-extraction mercury from IL. The results showed that 1.5 mol L −1 of HNO 3 can back-extracted of Hg(II) from the IL phase. Then, different of volume of reagents between 0.1-1.0 mL was studied and optimized. The results showed, 0.5 mL, 1.5 M of HNO 3 had maximum back-extraction mercury in wastewater samples (Fig. 5).

Optimization of sample volume
The different sample volume for extraction mercury was studied. The effect of sample volume was evaluated between 5.0 to 35 mL of wastewater/ standard samples for 0.0.5 μg L −1 and 6.0 μg L −1 of Hg(II). Quantitative extraction was achieved less than 15 mL. In addition, the higher sample volumes caused to trace soluble the ionic liquid in liquid phase and lead to non-accurate results. So, a sample volume of 10 mL was selected for further work by CP-DLLME procedure (Fig. 6).

Interferences Ions
For analytical application of the CP-DLLME procedure, the effect of interference of coexisting ions for mercury extraction in wastewater samples was studied. The various amounts of the interfering ions were added to 10 mL of wastewater sample containing 6.2 μg L -1 of Hg (II). As Table 3, the most of the probable concomitant ions have no considerable effect on the recovery efficiencies of Hg (II) ions under optimized conditions.

Real sample analysis
The developed CP-DLLME procedure was used for mercury determination in wastewater samples. The results showed the three separate determinations mercury in water samples. The results was verified by spiking of samples with standard concentration of Hg mercury. Table 4 showed, high recovery (more than 95%) between the added and found of mercury amount by procedure which confirms the accuracy of the procedure. The recoveries of spiked samples for mercury were ranged from 96% to 105%, which demonstrated satisfactory of mercury results. In order to validate the method described, the certified standard reference materials, NIST-SRM 1641e (total mercury in water), was analyzed and the results were given in Table 5. The results of the SRM were satisfactorily in agreement with the certified values.

Conclusions
A simple, fast and sensitive method based on MSTP was used for preconcentration and speciation of mercury in wastewater samples by CP-DILLME procedure. After extraction , the mercury concentration was determined by CV-AAS. The [BMIM][PF6] as ionic liquid was used as trapping agent of MSTP-Hg for rapid separation in short time. Utilizing ionic liquid micelles and MSTP together introduced a CPE procedure based on environmentally friendly for mercury extraction  from wastewaters. This procedure provides low LOD values as well as good RSD with quantitative recoveries more than 95% in optimized conditions. The CP-DILLME procedure based on MSTP and [BMIM][PF6] can be considered as effective sample preparation for mercury extraction from wastewater samples. a Mean of three determinations ± confidence interval (P = 0.95, n =5). b NIST, SRM 1641e, total mercury in water (p=0.95).