VI) in environmental samples

Author’s Accepted Manuscript Ultrasound assisted-deep eutectic solvent based on emulsification liquid phase microextraction combined with microsample injection flame atomic absorption spectrometry for valence speciation of chromium(III/VI) in environmental samples Erkan Yilmaz, Mustafa Soylak www.elsevier.com/locate/talanta

PII: DOI: Reference:

S0039-9140(16)30574-4 http://dx.doi.org/10.1016/j.talanta.2016.08.001 TAL16765

To appear in: Talanta Received date: 9 April 2016 Revised date: 29 July 2016 Accepted date: 1 August 2016 Cite this article as: Erkan Yilmaz and Mustafa Soylak, Ultrasound assisted-deep eutectic solvent based on emulsification liquid phase microextraction combined with microsample injection flame atomic absorption spectrometry for valence speciation of chromium(III/VI) in environmental samples, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ultrasound assisted-deep eutectic solvent based on emulsification liquid phase microextraction combined with microsample injection flame atomic absorption spectrometry for valence speciation of chromium(III/VI) in environmental samples *

Erkan Yilmaz, Mustafa Soylak ** Erciyes University, Faculty of Sciences, Department of Chemistry, 38039 Kayseri-TURKEY

Abstract A new type of deep eutectic solvents (DESs) have been prepared and used as extraction solvents for ultrasound assisted-deep eutectic solvent based emulsification liquid phase microextraction method (UA-DES-ELPME) for the speciation of total chromium, chromium(III) and chromium(VI). The chromium concentration in DES rich phase (extraction phase) was determined by using microsample injection flame atomic absorption spectrometer (FAAS). The detection limit (LOD), the quantification limit (LOQ), preconcentration factor and relative standard deviation were found as 5.5 µg L−1, 18.2 µg L-1, 20 and 6 %, respectively. The accuracy of the developed method was evaluated by the analysis of water the certified reference materials (TMDA-53.3 Fortified environmental water and TMDA-54.4 Fortified Lake Water) and addition-recovery tests for water samples. Keywords:

Deep

eutectic

solvent, Green

chemistry, Liquid

phase

microextraction,

Emulsification, Chromium, Speciation. *This study is a part of PhD thesis of Erkan Yilmaz **corresponding author, fax number: +90 352 4374933, E-mail: [email protected]

1

1. Introduction Cr(III) is considered as an essential trace element for mammals metabolism, whereas Cr(VI) can be toxic and carcinogenic for linivg organisms because of the its oxidizing potential and easy permeating of biological membranes [1, 2]. Water-soluble Cr(VI) compounds are also more extremely irritating and toxic to human body tissue [3]. Because of the difference in toxicity of Cr(III) and Cr(V) species to aquatic biota, Today there is an increasing demand to found a simple, accurate and reliable method for the speciation of these two oxidation states in water samples [4, 5]. In the determination of chromium species in water samples, sample preparation and/or extraction techniques are necessity prior to detection steps such as flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), ultraviolet visible absorption spectrometry (UV–vis), high performance liquid phase chromatography (HPLC), gas chromatography (GC) and inductively coupled plasma optical emission spectrometry (ICP-OES) [6-10]. Of these methods, FAAS is a very common and popular measurement technique and has a variety of advantages including simple operation, low cost and good selectivity [11, 12]. However, FAAS technique can only measure total concentration of chromium and the direct determination of Cr(III) and Cr(VI) at very low concentrations is always impossible because of the insufficient sensitivity of this method and matrix effects of foreign ions in water samples. As a result, preliminary species separation and preconcentration step is required before detection by FAAS. For the speciation and preconcentration of chromium, the analytical methods reported in the literature are usually based on liquid–liquid extraction (LLE), cloud point extraction (CPE), solid phase extraction (SPE), coprecipitation, solid phase microextraction (SPME) [17] and liquid phase microextraction [1, 7, 9, 13-15]. Within these methods, an important effort has been shifted to the development of liquid phase microextraction methods, mainly

2

characterized for eliminating or minimizing the time consuming, tedious and multistage operations, consumption of potentially organic solvents, production of secondary laboratory waste, necessisty of large and complex laboratory equipments, which commonly faced in conventional speciation and preconcentration methods [16-18]. The biggest obstacle for the application of liquid phase microextraction methods is the utilization of environmentally friendly green solvents. The selection of the ‘‘right’’ solvent is the most important task not only for an effective extraction but also for the development green analytical methods. Because of the reasons mentioned above, the scientists focused on the search of alternative and novel solvents as separating agent. As a result of research, a new generation solvent system called deep eutectic solvents (DESs) has emerged [19, 20]. A DES is generally consisting of two or more than two safe and inexpensive components that are capable of associating with each other through hydrogen bonds, to form a eutectic mixture with a melting point lower than that of each individual component [21-23]. Deep eutectic solvents (DESs) are typically formed by mixing choline chloride salt (ChCl, Vitamin B4) (e.g. cheap, non-toxic and biodegradable) with safe hydrogen bond donors (HBDs) (e.g. cheap and non-toxic urea, glycerol, sugars, carboxylic acids, etc.). When compared to organic solvents and ionic liquids used in extraction methods, DESs prepared from ChCl provide many advantages such as low price, easy to prepare by simply mixing components, no need to further purification, most of them are biodegradable, biocompatible and non-toxic [24-26]. In literature, some applications of DESs in material and organic compound synthesis, catalysis, electro-chemistry and substance dissolution have been reported [19-16]. Only a few studies have focused on the use of DESs in the separation process. According to our best knowledge, there is no any use of DESs as extraction solvent for speciation of inorganic analytes.

3

The aim of this work is the utilization of DESs as extraction solvent in ultrasound assisted-deep eutectic solvent based emulsification liquid phase microextraction method (UADES-ELPME) for speciation and preconcentration of Cr(III)/Cr(VI) in environmental water samples. The chromium concentration was measured by using microsample injection flame atomic absorption spectrometer. Suitable microextraction conditions for the developed method were investigated in detail.

2. Experimental 2.1. Apparatus In order to produce nanosized and/or microsized emulsion in water phase, an ultrasonic water bath (Norwalk, CT, USA) was used. The centrifugation for phase separation was performed using an ALC PK 120 model centrifuge (Buckinghamshire, England). An Analyst 300 Model flame atomic absorption spectrometer (Norwalk, CT, USA) including air–acetylene flame and equipped with a handmade micro-injection system, which consist of a mini home-made Teflon funnel with an Eppendorf Pipette [27]. The measurements were carried with the continuous aspiration mode of FAAS. The Teflon funnel was connected to the nebulizer of FAAS with capillary tubing. A 100 μl of extraction phase was injected to the micro-injection (microsample injection) system by using Eppendorf pipette and peak heights were recorded as signals [27]. A Sartorius PT-10 model pH meter with glass-electrode was used for pH adjustments of sample solutions (Sartorius Co., Goettingen, Germany).

2.2. Chemicals and reagents All reagents used were of at least analytical grade and needed no further purification. Choline chloride (ChCl) (Alfa Aesar, Germany), tetrabutylaamonium chloride (TBACl) (Acros,

4

Germany) and methyltrioctylammonium chloride (MTOACl) (Sigma-Aldrich, USA) were used as quaternary ammonium salts, while phenol (Ph) and Decanoic acid (DA) (SAFC, USA) were used as hydrogen bond donors. KMnO4 and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultra pure water purified through reverse osmosis (18.2 MΩ cm, Millipore) was used as the working medium.

2.3. Synthesis of DESs Five different DESs were synthesized as follows: (1) Mixing ChCl with Ph at different molar ratios ((molar ratios 1:2 (DES1), 1:3(DES2) and 1:4 (DES3)), (2) DES4: Mixing 1 M of TBACl with 2 M of DA (3) DES5: Mixing MTOACl with DA. The components of DESs were mixed in a beaker and then stirred at room temperature until a clear liquid was formed about 5 min.

2.4. Ultrasound assisted-deep eutectic solvent based emulsification liquid phase microextraction a 10.00 mL standard solution of Cr(VI) (or sample solution), 0.375 mL of 0.5 M H2SO4, 0.4 mL of 0.125 % (w/v) sodium diethyldithiocarbamate (NaDDTC) and 450 µL of DES 2 (as water-miscible extraction solvent) were placed in a 50 mL conical bottom centrifuge tube; a homogeneous solution was obtained. 450 µL of THF was injected into the sample solution and the mixture was kept in an ultrasonic bath for 2 min leading to aggregation of DES molecules and consequently a turbid solution. At this stage, the aggregated DES droplets progressively broke into tiny droplets because of the ultrasonic irridation involved by transient cavitation near the interface of DES droplets. The mixture was centrifuged at 4000 rpm for 10 min to accelerate the complete separation of the water phase and DES rich phase. The DES phase was situated at the bottom of the tube. The water was taken using

5

micropipette and the volume of DES phase remain in tube was completed to 750 µL with ethanol and then an aliquot 100 μl of the solution was injected to the nebulizer of the flame atomic absorption spectrometer by micro sampling unit and absorbance value was measured. The graphical representation of the UA-DES-ELPME method was shown in Fig. 1.

2.5. Determination of chromium species (1) Cr(VI): After the UA-DES-ELPME method, the extracted analyte concentration was directly analyzed by FAAS (2) Total Cr: In order to determine total chromium, 5 drops of KMnO4 (0.02 mol L−1) solution and 0.5 mL of H2SO4 (0.5 mol L-1) solution were added into 10 mL of the sample solution include Cr(VI) and Cr(III). Then, the sample solution was heated for 25 min at 60 0C for completely oxidation. After the oxidation, the UA-DES-ELPME method was applied to resulted solution and total chromium concentration was measured by FAAS. (3) Cr(III): The concentration of Cr(III) in water samples was calculated by subtracting Cr(VI) from the total chromium.

2.6. Applications The proposed liquid phase microextraction method was applied to tap water, two different chromium plating factory waste water samples (Kayseri, Turkey) and lake water (Van, Turkey) samples. Real water samples were filtered through a 0.45-μm cellulose membrane and utilized for further derivatization and UA-DES-ELPME. The certified reference water samples (TMDA-53.3 Fortified environmental water and TMDA-54.4 Fortified Lake Water) for the determination of total chromium were diluted appropriately with double distilled water prior to UA-DES-ELPME.

6

3. Results and discussion 3.1. Effect of acid concentration Cr(VI) reacts with DDTC to give a hydrophobic complex in acidic medium, hence acid concentration has a unique role on the complex formation and subsequent extraction. The influence of acid concentration on the recovery of Cr(VI) and Cr(III) was checked in the range of 0.005–0.1 mol L−1 H2SO4. The recovery results obtained was given in Fig. 2. As could be seen that, Cr(VI) was completely extracted in the range of 0.005-0.0250 mol L−1 acid concentration and it reached the maximum level at 0.0175 mol L−1, but extraction efficiency for Cr(III) was lower than 5 % for all acid concentration. In further studies, the acidity of the model solutions was adjusted to 0.0175 mol L−1 sulfuric acid.

3.2. Selection of extraction solvent The selection of suitable DESs for extraction methods is based on hydrophobic, electrostatic and π-π interactions with target analytes. In our study, three kinds of DESs with same molar ratios (1:2) ((Chlonine chloride-phenol), (Tetrabuthylaamonium chloride-Decanoic acid) and (Trioctylammonium chloride-Decanoic acid)) as extraction solvents were investigated by adding 450.0 μL of these DESs in the sample solution, respectively. Table 1 shows the changes in the recovery of Cr(VI). The results indicated that the quantitative recovery (> 95 %) for Cr(VI) was obtained by using chlonine chloride-phenol DES, while the recoveries of Cr(VI) was not quantitative for tetrabuthylaamonium chloride-Decanoic acid DES and trioctylammonium chloride-Decanoic acid DES. Hence, chlonine chloride-phenol DES was used as extraction solvent for further studies. After selection of suitable DES as extraction solvent, three Chlonine chloride-phenol DESs with different molar ratios were checked. The experimental results were shown in Table

7

1, which suggested that the recovery is observed by increasing the content of phenol from 2 to 4 and quantitative recovery was obtained by using molar ratios of chlonine chloride-phenol (1:3) (Table 1). In order to reduce required amount of phenol, DES2 was used for further steps since its preparation.

3.3. Effect of DES volume To examine the effect of DES volume as extraction solvent, solutions containing different volumes of DES (0.25-0.5 mL) were tested in the same UA-DES-ELPME method. The results are depicted in Fig. 3. Results revealed that the recoveries of Cr(VI) were quantitative in the range of 0.4-0.5 mL, but recoveries for Cr(III) were lower than % 10 in the range of 0.25-0.30 mL. Hence 0.45 mL of DES2 as extraction solvent was used for the rest of the future work.

3.4. Effect of THF volume Self-aggregation and separation of DESs molecules in aqueous phase were occurred by adding THF as an emulsifier agent and aggregates become insoluble in the water/THF. Hence, the volume of THF in sample solution is important parameter. The effect of THF volume on the recovery of Cr(VI) was evaluated within the range of 0.25–1.0 mL (Fig. 4). In UA-DES-ELPME, during the addition of THF in sample solution, a cloudy solution was formed. The quantitative results were obtained by adding 0.25–1.0 mL of THF. But after centrifugation, the phase separation of water-DES phases was not complete and a cloudy solution was remaining for 0.25 and 0.40 mL of THF. Thus, phase separation was very difficult. 0.45 mL of THF was chosen as the optimum volume.

8

3.5. Effect of amount of NaDDTC NaDDTC, a sulphur containing complexing agent, forms very stable and hydrophobic complexes rapidly with Cr(VI) in acidic medium, which increase solubility of Cr(VI)-DDTC complex in extraction phase. Hence, the amount of NaDDTC are important factor for effective and quantitative extraction Cr(VI) from acidic sample solution medium. Based on the results shown in Fig. 5, The recovery of Cr(VI) increases significantly as the volume of 0.125 (w/v) of NaDDTC increases from 0.1 mL to 0.3 mL and quantitative recoveries were obtained between 0.3 mL and 0.6 mL of NaDDTC. Hence, 0.4 mL of 0.125 (w/v) of NaDDTC was added to sample solution for subsequent experiments.

3.6. Effect of ultrasonication time Analytical application of ultrasonic radiation has an important role in the mass transfer of the analyte from water phase to extraction phase by increasing the interactions between analyte in sample solution and extraction solution. In our study, ultrasonic water bath was used to obtain micro and nanosized aggregate and increase extraction efficiency. The effect of the ultrasonication time was checked in the range of 1–5 min. keeping constant experimental conditions. The extraction efficiency stabilized when the ultrasonication time was longer than 2 min. So, In further studies, the samples were put in ultrasonic water bath for 2 min.

3.7. Effect of salt addition Ionic strength of the working media is another critical parameter on the microextraction studies to obtain quantitative recoveries of the analyte elements. In otder to investigate the effect of ionic strength on performance of DLLME, different amount of NaCl (0–2 g) was added in sample solutions and the developed method was applied to the sample solutions by keeping constant other experimental conditions. The results were shown in Fig. 6. Salt

9

addition has important effect on the recovery of Cr(VI). Quantitative recoveries were obtained till 0.25 g of NaCl. The recovery of Cr(VI) decreases when the amount of NaCl increases and this situation cause an increase in the volume of extraction phase and a decrease for enrichment factor.

3.8. Effect of sample volume The effect of sample volume on recovery of Cr(VI) was examined from 5.0 mL to 20.0 mL by fixing the optimized conditions. The recoveries would be less than 85.0 % when the volume of sample solution was over 15.0 mL. Quantitative recoveries were obtained in the sample volume range of 5.0-15.0 mL.

3.9. The effect of coexisting ions In the instrumental detection of the elements at the trace levels, coexisting ions are problematic and a separation step are necessity [28-32]. Most common coexisting ions in real samples, such as alkali and alkaline earth elements, do not complex with NaDDTC. However, high concentrations of transition metal ions react with NaDDTC and can cause a decrase on the extraction efficiency of Cr(VI). Hence, the effects of common coexisting ions in natural water samples on the recovery of Cr(VI) were also checked in optimum conditions (Table 2). Table 2 show that, the developed microextraction method has strong selectivity and the common coexisting ions have no interference effect on the determiantion of Cr(VI).

3.10. Analytical performance of UA-DES-ELPME method The detection limit (LOD) and the quantification limit (LOQ) for the developed analytical procedure were calculated from the equation 3s/B and 10s/B, respectively. Where s is the standart deviation of the seven blank solution and B is the slope of the calibration curve. The

10

LOD and LOQ were found as 5.5 µg L−1 and 18.2 µg L−1, respectively. The relative standard deviation (RSD) resulting from the analysis of 10 replicates of 10 mL solution containing 0.1 mg L−1 of Cr(VI) was 6.0 %. The linear equation was A=8.2x10-5 + 7.4x10-4C with a correlation coefficient (R2) of 0.997 (A is the absorbance and C is the concentration of Cr (VI) in the extraction solution). For comparative purposes, the performance characteristics of the developed UA-DESELPME method and other methods in the literature are given in Table 3. Our method shows low LOD, high preconcentration factor and these characteristics are comparable or even better than these methods [36-40].

3.11. Applications of the method To check the accuracy of the developed liquid phase microextraction method, the method was applied to the determination of chromium in the TMDA-53.3 Fortified environmental water and TMDA-54.4 Fortified Lake Water certified reference materials. The results given in Tables 4 show that there are in good agreement with the certified values. The method was also employed for the determination of Cr(VI) and Cr(III) species in several water samples. The accuracy of the proposed method was also evaluated by addition-recovery tests. For this purpose, different amounts of the standard Cr(VI) and Cr(III) solutions were spiked in water samples. The analytical results were summarized in Table 5. The results for addition-recovery studies showed that recoveries of the method for Cr(VI) and Cr(III) in water samples were from 97 to 109 % with standart deviation less than 6.0. The obtained analytical results for validation of the UA-DES-ELPME demonstrated that the UA-DES-ELPME method was not affected samples matrices and applicable for speciation and determination of chromium in water samples.

11

Conclusion An

ultrasound

assisted-deep

eutectic

solvent

based

emulsification

liquid

phase

microextraction method (UA-DES-ELPME) combined with microsample injection flame atomic absorption spectrometry was developed for speciation, preconcentration and determination of chromium. DES was used as a green microextraction phase and an alternative to traditional volatile organic solvents. An ultrasonic water bath was used to accelerate micro and nanosized aggregate formation and increase extraction efficiency. The suggested new method provides some advantages, such as simplicity of experimental steps, use of low price and low toxicity extraction solvents since most of DESs can be prepared from readily accessible chemicals and especially DESs derived from ChCl and renewable chemicals, relatively high speed of sample preparation. These notable advantages of the developed microextraction method and DESs now open alternative routes for the analytical applications.

Acknowledgement The authors are grateful for the financial support of the Unit of the Scientific Research Projects of Erciyes University (FBA-2015-5783) (Kayseri, Turkey).

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Figure captions Fig. 1. Graphical representation of the UA-DES-ELPME method Fig. 2. Effect of H2SO4 concentration on the recovery of Cr(III) and Cr(VI) (Type of DES : chlonine chloride-phenol (1:3), Volume of DES : 0.45 mL, Volume of THF : 0.45 mL, Volume of 0.125 (w/v) of NaDDTC: 0.4 mL, N=3). Fig. 3. Effect of the DES volume on the recovery of Cr(VI) (N=3, Concentration of H2SO4 : 0.0175 mol L−1, Type of DES : chlonine chloride-phenol (1:3), Volume of THF : 0.45 mL, Volume of 0.125 (w/v) of NaDDTC: 0.4 mL). Fig. 4. Effect of the THF volume on the recovery of Cr(VI) (N=3, Concentration of H2SO4 : 0.0175 mol L−1, Type of DES : chlonine chloride-phenol (1:3), Volume of DES : 0.45 mL, Volume of 0.125 (w/v) of NaDDTC: 0.4 mL). Fig. 5. Effect of the volume of NaDDTC on the recovery of Cr(VI) (N=3, Concentration of H2SO4 : 0.0175 mol L−1, Type of DES : chlonine chloride-phenol (1:3), Volume of DES : 0.45 mL, Volume of THF : 0.45 mL Volume of 0.125 (w/v) of NaDDTC: 0.4 mL). Fig. 6. Effect of the salt addition on the recovery of Cr(VI) (N=3).

18

Table 1. Effect of DES type on the recovery of Cr(VI) (N=3). DES

DES component

Molar ratio

Recovery, %

DES1

ChCl:Ph

1:2

89±7

DES2

ChCl:Ph

1:3

100±5

DES3

ChCl:Ph

1:4

100±4

DES4

TBACl: DA

1:2

20±2

DES5

MTOACl: DA

1:2

67±5

19

Table 2. Effect of foreign ions on the recovery of Cr(VI) (N=3). İon

Addes as

Concentration, mg L-1

Foreign ion/Cr(VI)

Recovery, %

Mg2+

Mg(NO3)2.6H2O

500

1250

98±10

Ca2+

Ca(NO3)2.4H2O

500

1250

20±0

Fe3+

Fe(NO3)3 9H2O

5

12.5

100±0

Mn2+

Mn(NO3)2.4H2O

5

12.5

104±4

Zn2+

Zn(NO3)2.6H2O

5

12.5

80±0

SO42-

Na2SO4

1000

2500

91±0

20

Table 3. Comparison of the UA-DES-ELPME-FAAS method with other preconcentration and determination methods combined with different detection techniques.

Method

LOD, µg L-1

PF or EF

RSD, %

Sample

References

CP-FAAS

1.17

40

7.7

Water and soil

1

SPE-FAAS

45

25

-

Water

33

LLE-spectrophotometer

7.5

5

75

Water

34

CPE-HPLC

5.2

19

1.0

Water

35

SPE-FAAS

4.08

30

-

Plant

36

SPME-HPLC-UV

2

-

7

Water

38

SPE-FAAS

7.7

75

5.7

Water and Red lentil

37

UA-DES-ELPME-FAAS

5.5

20

6.0

Water

This study

21

Table 4. The analysis results of certified reference materials (N=3).

Certified Reference Material

Certified Value, µg L-1

Found, µg L-1

Recovery, %

438

413±15 a

94

344

319±20

93

TMDA-54.4 Fortified environmental water TMDA-54.4 Fortified Lake Water a

Mean ± standard deviation.

22

Table 5. Addition-recovery test for UA-DES-ELPME of Cr(III) and Cr(VI) in water samples (N=3). Tap water Added, µg

Found, µg

Calculated, µg

Cr(III)

Cr(VI)

Cr(VI)

Total Cr

0

0

BDLb

1.5

1.5

1.6±0.1 a

3.1±0.2

3.0

3.0

3.0±0.2

6.2±0.1

BDL

Cr(III)

Recovery, % Cr(III)

Cr(VI)

-

-

1.5

100

107

3.1

105

100

-

-

BDL

Lake water 0

0

BDL

BDL

BDL

1.25

1.25

1.2±0.2

2.6±0.1

1.3

104

98

1.0

1.5

1.5±0.1

2.7±0.1

1.0

100

103

Factory waste water-I 0

0

0.3±0.1

0.3±0.1

BDL

-

-

1.5

1.0

1.3±0.3

2.9±0.1

1.5

100

100

1.5

1.5

1.8±0.3

3.2±0.1

1.7

93

105

Factory waste water-II 0

0

0.4±0.1

0.4±0.1

BDL

-

-

1.0

1.0

1.5±0.2

2.9±0.1

1.1

110

107

2.0

2.0

2.5±0.6

4.4±0.2

2.0

100

104

a

Mean ± standard deviation.

b

Below of the detection limit

23

Fig. 1.

24

100

Recovery, %

80 60 Cr(VI)

40

Cr(III) 20 0

0

0.02

0.04 0.06 0.08 -1 H2SO4 concentration, mol L

Fig. 2.

25

0.1

100

Recovery, %

80 60

Cr(VI) 40 20 0 0.25

0.3

0.35

0.4

Volume of DES, mL

Fig. 3.

26

0.45

0.5

100

Recovery, %

80

60

Cr(VI) 40 20 0 0.25

0.5

0.75 Volume of THF, mL

Fig. 4.

27

1

1.25

100

Recovery, %

80 60 40

Cr(VI)

20 0 0.1

0.2

0.3

0.4

DDTC volume of DDTC, mL Fig. 5.

28

0.5

0.6

100

Recovery, %

80

60

Cr(VI) 40

20 0

0.5

1

1.5

Amount of NaCl , g

Fig. 6.

Highlights < A new type of deep eutectic solvents (DESs) have been prepared for chromium speciation. < The effects of analytical parameters were investigated and optimized. < Matrix effects were investigated.. < The method has been applied to speciation of chromium in water samples.

29

2

GRAPHICAL ABSTRACT

30