Ionic liquid-based dispersive liquid–liquid microextractionfor the determination of formaldehyde in wastewatersand detergents

Majid Arvand & Elahe Bozorgzadeh &

Shahab Shariati & Mohammad Ali Zanjanchi

Received: 12 August 2011 /Accepted: 2 January 2012# Springer Science+Business Media B.V. 2012

Abstract Spectrophotometry in combination withionic liquid-based dispersive liquid–liquid microex-traction (DLLME) was applied for the extraction anddetermination of formaldehyde in real samples. Themethod is based on the reaction of formaldehyde withmethyl acetoacetate in the presence of ammonia. Thevariation in the absorbance of the reaction product wasmeasured at 375 nm. An appropriate mixture of etha-nol (disperser solvent) and ionic liquid, 1-hexyl-3-methylimidazoliumhexafluoro-phosphate [C6MIM][PF6] (extraction solvent) was rapidly injected into awater sample containing formaldehyde. After extrac-tion, sedimented phase was analyzed by spectropho-tometry. Under the optimum conditions, thecalibration graph was linear in the range of 0.1–20 ng mL−1 with the detection limit of 0.02 ng mL−1

and limit of quantification of 0.08 ng mL−1 for form-aldehyde. The relative standard deviation (RSD%, n05)for the extraction and determination of 0.8 ng mL−1 offormaldehyde in the aqueous samples was 2.5%. Theresults showed that DLLME is a very simple, rapid,

sensitive, and efficient analytical method for the deter-mination of trace amounts of formaldehyde in waste-waters and detergents, and suitable results wereobtained.

Keywords Formaldehyde . Ionic liquid . Dispersiveliquid–liquid microextraction .Wastewaters .

Spectrophotometry

Introduction

The most commonly used aldehyde is formaldehydeknown as oxymethylene or formalin. It is a cheappreservative, more easily soluble in water than in oiland fat, used in watery concoctions like shampoo,conditioner, shower gel, liquid hand wash, and bubblebath (Salvador and Chisvert 2007). Formaldehyde isunfavorable for our health because at low concentra-tions, it can cause irritation of eyes, nose, throat, andskin. Furthermore, people with asthma may be moresensitive to the effects of inhaled formaldehyde (Li etal. 2007; Kiba et al. 1999). Textiles are often finishedwith formaldehyde-containing chemicals which canprovide crease resistance, flame retardance, anddimensional stability (Priha 1995).

Thus, there is a need to develop a simple, specific,and sensitive sample preparation and analytical tech-nique for the detection of trace quantities of this com-pound. In order to determine trace or ultra-traceamounts of formaldehyde, a chemical separation and

Environ Monit AssessDOI 10.1007/s10661-012-2521-4

M. Arvand (*) : E. Bozorgzadeh :M. A. ZanjanchiDepartment of Chemistry, Faculty of Science,University of Guilan,Namjoo Street, P.O. Box 1914, Rasht, Irane-mail: arvand@guilan.ac.ir

S. ShariatiDepartment of Chemistry, Science and Research Branch,Islamic Azad University,Guilan, Iran

preconcentration step is often necessary prior to anal-ysis. Some analytical procedures including gas chro-matography (Hopkins et al. 2003; Pierotti 1990), solidphase microextraction (SPME) (Rivero and Topiwala2004), fluorometry (Li et al. 2007), spectrophotometry(Toda et al. 2005; Kawamura et al. 2005; Gámiz-Gracia and Luque de Castro 1999), and biosensor(Horstkotte et al. 2006) have been reported for thedetermination of formaldehyde.

Dispersive liquid–liquid microextraction (DLLME)is a very effective separation technique, finding nu-merous applications in analytical chemistry. Simplici-ty of the operation, speed, low sample volume, lowcost, high recovery, and high enhancement factor aresome advantages of DLLME (Rezaee et al. 2006). Theperformance of DLLME was illustrated by extractionof different organic and inorganic compounds (Jiang etal. 2008; Liang et al. 2008; Shamsipur and Ramezani2008; García-López et al. 2007; Rezaee et al. 2009;Nagaraju and Huang 2007; Shariati et al. 2011) fromwater samples.

Ionic liquids (ILs) have been considered as greensolvent and have been applied in many fields. Room-temperature ionic liquids (RTILs) can be used as theextraction solvent instead of organic solvents due to theirunique physicochemical properties, such as negligiblevapor pressure, good solubility for organic and inorganiccompounds, high thermal stability, and environmentalbenignity (Pandey 2006; Zhang and Shi 2010).

The purpose of the present study is to develop aninexpensive, selective, and sensitive method for thedetermination of ultra-trace of formaldehyde in compli-cated matrices. The detection is based on the derivatiza-tion of formaldehyde using Hantzsch reaction, whichinvolves the cyclization between methyl acetoacetateand formaldehyde in the presence of ammonium acetate.

Experimental

Reagents and solutions

All reagents were of analytical reagent grade and usedwithout further purification. Triply distilled water wasused throughout the study. A stock solution(1,000 μg mL−1) of formaldehyde was prepared byappropriate dilution of a 37% (v/v) formaldehyde so-lution (Merck, Darmstadt, Germany). A 0.2 mol L−1

methyl acetoacetate stock solution was prepared by

diluting 2.15 mL of commercially available methylacetoacetate solution (Aldrich, Milwaukee, WI,USA) to 100 mL with purified water. An ammoniumacetate stock solution was prepared by dissolving7.71 g of ammonium acetate (Aldrich) in the purifiedwater and diluting it to 250 mL. 1-Hexyl-3-methyli-midazolium hexafluorophosphate [C6MIM][PF6] and1-butyl-3-methylimidazolium hexafluoro-phosphate[C4MIM][PF6] were obtained from Merck (Darmstadt,Germany). The ammonium acetate buffer was pre-pared in the range of pH 5.0–7.5: the pH was adjustedby adding acetic acid or NaOH to the ammoniumacetate solution.

Apparatus

UV–Vis absorbance digitized spectra were collectedon a Shimadzu UV-2100 ultraviolet–visible spectro-photometer, using a quartz microcuvette (45 mm high,internal width 2 mm, and path length 10 mm). Meas-urements of pH were made with a Metrohm 744 pHmeter using a combined glass electrode. A centrifuge(Superior, Germany) with 10-mL calibrated tubes wasused to accelerate the phase separation process. AHaake Model FK2 circulation water bath (Hamburg,Germany) with a good temperature control within ±1°Cwas used.

Analytical procedure

For DLLME under optimum conditions, 10 mL ofsample solution containing the formaldehyde,0.05 mol L–1 of methyl acetoacetate as derivatizingagent, and 20% (w/v) of NaCl was adjusted to pH 6.4in a glass test tube with conic bottom. The mixedsolution was left to react for 10 min at 60°C in a waterbath, and then cooled down in water for 5 min. Thenthe injection of 1.0 mL ethanol (dispersive solvent)containing 75 μL ionic liquid [C6MIM][PF6] wasperformed by using 1.0 mL syringe, rapidly. A cloudysolution (water, ethanol, and [C6MIM][PF6]) wasformed in the test tube. The produced cloudy solutionwas centrifuged for 5 min at 3,000 rpm to acceleratephase separation. After this process, 10 μL of sedi-mented phase was removed using a 100-μL syringeand diluted with 100 μL ethanol and injected into amicro-cell. Subsequently, the micro-cell was located inspectrophotometer to obtain related spectra.

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Results and discussion

The detection reaction is based on the Hantzsch reac-tion, which was first explained by Nash (1953). Thereaction is characterized by a cyclization of methylacetoacetate and formaldehyde in the presence of am-monia at 60°C to form a color product, 2,6-dimethyl-1,4-dihydropyridine-3,5-di(methylcarboxylate)(Scheme 1). Methyl acetoacetate is one of the mostsoluble reagents in water. It is most reactive, selective,and sensitive for formaldehyde, and its product has alarge molar absorptivity (7.8×103 dm3 mol−1 cm−1 at60°C) (Li, Ma et al. 2008).

In the proposed procedure, to achieve maximumextraction efficiency, various parameters affecting theDLLME of formaldehyde were studied and optimized.

Preconcentration factor (PF) and percent extractionrecovery (ER%) as analytical responses were calculatedbased on the following equations:

PF ¼ Csed=C0 ð1Þ

ER% ¼ Csed � Vsed=C0 � Vaq

� �� 100 ð2Þwhere Csed and C0 are the concentration of the analytein the sedimented phase and initial concentration ofthe analyte in the aqueous sample, respectively. Vsed

and Vaq are the volume of the sedimented phase andvolume of the aqueous sample, respectively. Csed iscalculated from a calibration curve which wasobtained by direct injection of formaldehyde with theconcentrations in the range of 0.05–6 μg mL−1.

On the other hand, the relative recovery (RR%) wasobtained from the following equation:

RR% ¼ Cfound � Creal=Cadded½ � � 100 ð3Þwhere Cfound, Creal, and Cadded are the concentrationsof analyte after addition of known amount of standardin the real sample, the concentration of analyte in realsample, and the concentration of known amount ofstandard which was spiked to the real sample,respectively.

Selection of extractant solvent

The extraction solvent has tomeet three requirements: (1)to extract analytes well, (2) to have higher density thanwater to sediment at the bottom of the extraction tube,and (3) to form a cloudy solution containing tiny dropletsin the presence of dispersive solvent when injected intoaqueous solution (Shariati et al. 2001). In this study, twohydrophobic ionic liquids, including [C4MIM][PF6] and[C6MIM][PF6], were investigated. By comparing themas extraction solvent, it was observed that formaldehydeexhibited a better affinity for [C6MIM][PF6] (Fig. 1).Therefore, [C6MIM][PF6] was selected as extractionsolvent in the subsequent experiments.

Selection of dispersive solvent

Dispersive solvent should be miscible in both extrac-tion solvent and aqueous sample. For this purpose,different solvents such as acetonitrile (ACN), metha-nol (MeOH), acetone (AC), and ethanol (EtOH) werestudied. A series of standard solutions, adjusted at pH6.4 and containing 0.8 ng mL−1 concentration of ana-lyte, were studied using 1.0 mL of each dispersivesolvent containing 100 μL of [C6MIM][PF6]. Accord-ing to the absorbance values obtained from the extrac-tion results, the extraction efficiency was increased inthe order of EtOH>MeOH>ACN>AC. Therefore,ethanol was selected as dispersive solvent. This higherrecovery can also be attributed to the better dispersionobtained in alcohols.

Effect of extractant solvent volume

In order to examine the effect of the extraction solventvolume, solutions containing 1.0 mL ethanol plusdifferent volumes of [C6MIM][PF6] (65, 70, 75, 80μL) were subjected to the same DLLME procedure.According to Fig. 2, net signal (ΔA) increased byincreasing in [C6MIM][PF6] volume up to 75 μL andthen decreased at higher volume because the sedimentedphase volume increases and preconcentration factor

Scheme 1 Cyclization of methyl acetoacetate and formaldehyde in the presence of ammonia

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decreases. In the following studies, 75 μL of [C6MIM][PF6] was selected as an optimal volume of the extractantsolvent.

Selection of dispersive solvent volume

In order to study the effect of the dispersive solventvolume on the extraction efficiency, different volumesof ethanol in the range of 0.5, 1.0, 1.5, and 2.0 mLwere examined. It was found that in order to obtain aconstant sedimented phase volume, the volumes of thedispersive solvent and the extraction solvent should bechanged simultaneously. Thereby, the volume of[C6MIM][PF6] was changed in the range 70, 75, 85,and 100 μL for each selected volume of ethanol. The

results indicated that when the volume of ethanolexceeded 1.0 mL, preconcentration factor decreased;the reason was that the solubility of analyte in waterincreases. On the other hand, the dispersive solventvolume of 500 μL was not enough for a good disper-sion of the extraction solvent (Fig. 3). Thus, in order toachieve the best extraction efficiency, as well as toobtain a stable cloudy solution, 1.0 mL ethanol wasselected as the optimum volume.

Salt effect

The effect of salt addition on the extraction efficiencywasevaluated by adding NaCl (0–30%, w/v) into the aqueoussolution containing 0.8 ng mL−1 of formaldehyde. As

c

b

a

Fig. 1 Absorption spectrafor blank (a), the product ofHantzsch reaction after ex-traction with [C4MIM][PF6](b), and [C6MIM][PF6](c). Conditions: watersample, 10 mL; dispersivesolvent (ethanol), 1.0 mL;methyl acetoacetate,0.05 mol L−1; NaCl, 20%(w/v); pH06.4;formaldehyde,7.0 ng mL−1

0.005

0.013

0.021

0.029

0.037

0.045

0.053

60 65 70 75 80 85

Volume of extraction solvent/mL

-10

20

50

80

110

140

170

200

PF

ΔA

Fig. 2 Effect of the volumeof extraction solvent([C6MIM][PF6]) on theabsorbance (filled circles)and preconcentration factor(open circles) of formalde-hyde. Conditions: watersample, 10 mL; dispersivesolvent (ethanol), 1.0 mL;methyl acetoacetate,0.05 mol L−1; NaCl, 20%(w/v); pH06.4;formaldehyde, 0.8 ngmL−1

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shown in Fig. 4, there was an increase in absorbance withan increase in salt concentration up to 20% and remainedconstant at higher concentrations. At the beginning, theabsorbance was increased due to “salting-out” wherebywater molecules form hydration spheres around the ionicsalt molecules. These hydration spheres reduce theamount of water available to dissolve the analyte mole-cules in water, thereby driving the additional analyte intothe organic droplets. Based on these observations, anoverall salt concentration of 20% (w/v) was used forfurther studies.

Effect of pH

The effect of solution pH on the extraction was inves-tigated in the pH range of 5.0–7.5 while keeping othervariables constant. The results demonstrated in Fig. 5

reveal that the absorbance was nearly constant in thepH range of 6.0–6.8. Accordingly, pH 6.4 was selectedfor further experiments.

Analytical figures of merit

Calibration curve was obtained under the optimizedconditions with linear dynamic range of 0.1–20 ng mL−1 and correlation coefficient (r2) of 0.997.The PF of the method was 158.5, at the concentrationof 0.8 ng mL−1 of formaldehyde and the sample vol-ume of 10 mL. The relative standard deviation (RSD%, n05) at the concentration of 0.8 ng mL−1 was2.5%. The limit of detection (LOD) based on signal-to-noise ratio (S/N) of 3 was 0.02 ng mL−1. The limitof quantification, defined as CQ010SB/m, where CQ islimit of quantification, was 0.08 ng mL−1.

100

110

120

130

140

150

160

0 0.5 1 1.5 2 2.5

Volume of dispersive solvent/mLP

F

Fig. 3 Effect of the volumeof dispersive solvent(ethanol) on the absorbanceof formaldehyde. Condi-tions: water sample, 10 mL;extraction solvent ([C6MIM][PF6]), 75 μL; methyl ace-toacetate, 0.05 mol L−1;NaCl, 20% (w/v); pH06.4;formaldehyde, 0.8 ngmL−1

0

0.01

0.02

0.03

0.04

0.05

0.06

0 5 10 15 2 25 30 35

NaCl/% w/v

ΔA

Fig. 4 Effect of NaCl con-centration on the extractionof formaldehyde. Condi-tions: water sample, 10 mL;dispersive solvent (ethanol),1.0 mL; extraction solvent([C6MIM][PF6]), 75 μL;methyl acetoacetate,0.05 mol L−1; pH06.4;formaldehyde, 0.8ng mL−1

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Effect of foreign ions

In order to identify the cations and anions that couldinterfere in the extraction and determination of formal-dehyde, the solutions with 0.8 ngmL−1 of formaldehydecontaining given amounts of foreign ions were treatedaccording to the proposed procedure under the optimumconditions. An ion was considered to be interferentwhen it caused a variation greater than ±5% in theabsorbance of the sample. For the determination of0.8 ng mL−1 formaldehyde by this method, the foreignions can be tolerated at the levels given in Table 1. Someions including Mn2+, Ni2+, Co2+, Pb2+, Ag+, NO3

−,NO2

−, and PO43− did not cause any significant interfer-

ence on the DLLME of formaldehyde. Cu2+ and Fe3+

interfered at 8.0 ng mL−1 and 4.0 ng mL−1 levels,respectively. Their interfering effects up to1,000 ng mL−1 were completely removed by the addi-tion of 1.0 mL of 0.01 mol L−1 ascorbic acid and0.01 mol L−1 KI solutions for Cu2+ and the addition of1.0 mL of 0.02 mol L−1 SCN− solution for Fe3+. Sulfite

interfered extensively, but it could be tolerated at mod-erate concentration (500 ng mL−1) in the presence of0.5 mL of 0.01 mol L−1 Hg2+ ion.

Analytical application

Extraction of formaldehyde from cosmetic samples

Due to the importance of analysis of formaldehyde incosmetic samples, the proposed method was applied todetermine the concentration of formaldehyde in twoshampoos. The obtained results are summarized inTable 2. For analysis of cosmetic samples, a portionof 100 mg of cosmetic sample was weighed into a100-mL volumetric flask and was made up to volumewith distilled water. Two milliliters of the mixture wasadded to a glass test tube with conical bottom and wasextracted using the proposed method to determineformaldehyde concentration. The amount of formalde-hyde was determined by reference method (Nash1953) too. The calculated t values are less than thetabulated values with 2 degrees of freedom at 95%confidence interval. This indicates the proposed methodto be as precise and accurate as the standard methods.

Extraction of the formaldehyde from different watersamples

In order to evaluate the analytical applicability of theproposed method, it was applied to the determinationof formaldehyde in different water samples (Table 2).Some tested water samples were found to be free from

0

0.01

0.02

0.03

0.04

0.05

0.06

5 5.5 6 6.5 7 7.5 8

pHΔ

A

Fig. 5 Effect of pH on theextraction of formaldehyde.Conditions: water sample,10 mL; dispersive solvent(ethanol), 1.0 mL; extractionsolvent ([C6MIM][PF6]),75 μL; methyl acetoacetate,0.05 mol L−1; NaCl,20% (w/v); formaldehyde,0.8 ng mL−1

Table 1 Tolerance ratios of diverse ions on the determination of0.8 ng mL−1 of formaldehyde

Ion Tolerance ratio

Ca2, Na+, K+, Mn2+, Zn2+, Ni2+, Cd2+, Co2+,Pb2+, Ag+, Cl−, Br−, I−, NO3

−, NO2−, PO4

3−1,000

Cu2+, Fe3+a 1,000

SO32a 500

a After removing as described in text

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Table 2 Determination offormaldehyde in different spikedsamples

atcri. value is 4.30 at P00.05bConcentration of the formalde-hyde in spiked samplescSample volume is 10 mLdND not detectedeLower than detection limit ofreference methodfA dilution of 1:10 was used

Sample Content texpa Recovery (%)

Proposed method Spectrophotometrymethod (Nash 1953)

Shampoo 1

Initial concentration (ng g−1) 75

Foundb (75 ng g−1 added)c 151.5 148.9 0.85 101

RSD% (n03) 3.5

Shampoo 2

Initial concentration (ng g−1) 50

Found (50 ng g−1 added) 98.3 99.1 0.34 98.3

RSD% (n03) 4.1

Well water 1

Initial concentration (ng mL−1) NDd

Found (5.0 ng mL−1 added) 4.8 <DLe – 96

RSD% (n03) 2.5

Well water 2

Initial concentration (ng mL−1) ND

Found (5.0 ng mL−1 added) 5.1 <DL – 102

RSD% (n03) 2.0

Dairy industrial factory wastewater

Initial concentration (ng mL−1) ND

Found (5.0 ng mL−1 added) 5.2 <DL – 104

RSD% (n03) 4.0

Textile industrial factory wastewater

Initial concentration (ng mL−1) 6.0

Found (6.0 ng mL−1 added) 11.7 <DL – 97.5

RSD% (n03) 4.5

Wood and paper industrialfactory wastewater

Initial concentration(ng mL−1)f

100

Found (100 ng mL−1 added) 198.6 202.3 1.08 99.3

RSD% (n03) 3.0

Table 3 Comparison of the proposed method with the other techniques for determination of formaldehyde

Analytical method Detection limit(ng mL−1)

Linear range(ng mL−1)

RSD (%) Reference

FIA–spectrophotometrya 0.09 1.5–120 1.7 Li et al. 2007

LPME–fluorospectrometryb 26 – 4.6 Sáenz et al. 2011

Mini-CE–EDc 9.1 0.05–50 (μg mL−1) 5.9 Zhang et al. 2011

FIA–spectrophotometryd 23 – 0.29 Li et al. 2008

IL-based DLLME/spectrophotometry 0.02 0.1–20 2.5 This work

a,d Flow injectionb Liquid-phase microextractioncMiniaturized capillary electrophoresis with electrochemical detection

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formaldehyde so it was determined after spiking thesamples. The recovery range was from 96% to 104%in steady-state analysis. Comparing real to experimen-tal formaldehyde concentrations, the recoveries wereclose to 100% and considered both positive and neg-ative errors, thus suggesting the accuracy of the pro-posed method. The results from the statisticalcalculation indicate good agreement between the meanvalues (t test) for two methods (for P00.05).

Comparison with other methods

A comparison between the proposed method withthe previously reported methods (Li et al. 2007;Li, Sritharathikhum et al. 2008; Sáenz et al. 2011;Zhang et al. 2001) for determination of formalde-hyde shows that the proposed method has a lowerdetection limit in comparison to other methods.Also, the proposed method is comparable in RSDand linear range to the previous studies (Table 3).

Conclusions

Spectrophotometry in combination with ionic liquid-based dispersive liquid–liquid microextraction was ap-plied for the extraction and determination of formalde-hyde in different wastewaters and detergents. The use ofDLLME as an alternative to other methods of separationand preconcentration offers several advantages includ-ing experimental convenience, safety, and being an in-expensive method. Utilizing an ionic liquid as extractionsolvent, consumption of toxic organic solvents wasomitted. In addition, only small quantities of solventwere required.

Acknowledgment We gratefully acknowledge the post-graduate office of Guilan University for supporting this work.

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