Ionic liquid-based dispersive liquid–liquid microextraction for the determination of formaldehyde in wastewaters and detergents

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<ul><li><p>Ionic liquid-based dispersive liquidliquid microextractionfor the determination of formaldehyde in wastewatersand detergents</p><p>Majid Arvand &amp; Elahe Bozorgzadeh &amp;Shahab Shariati &amp; Mohammad Ali Zanjanchi</p><p>Received: 12 August 2011 /Accepted: 2 January 2012# Springer Science+Business Media B.V. 2012</p><p>Abstract Spectrophotometry in combination withionic liquid-based dispersive liquidliquid 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 0.120 ng mL1 with the detection limit of 0.02 ng mL1</p><p>and limit of quantification of 0.08 ng mL1 for form-aldehyde. The relative standard deviation (RSD%, n05)for the extraction and determination of 0.8 ng mL1 offormaldehyde in the aqueous samples was 2.5%. Theresults showed that DLLME is a very simple, rapid,</p><p>sensitive, and efficient analytical method for the deter-mination of trace amounts of formaldehyde in waste-waters and detergents, and suitable results wereobtained.</p><p>Keywords Formaldehyde . Ionic liquid . Dispersiveliquidliquid microextraction .Wastewaters .</p><p>Spectrophotometry</p><p>Introduction</p><p>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).</p><p>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</p><p>Environ Monit AssessDOI 10.1007/s10661-012-2521-4</p><p>M. Arvand (*) : E. Bozorgzadeh :M. A. ZanjanchiDepartment of Chemistry, Faculty of Science,University of Guilan,Namjoo Street, P.O. Box 1914, Rasht, Irane-mail:</p><p>S. ShariatiDepartment of Chemistry, Science and Research Branch,Islamic Azad University,Guilan, Iran</p></li><li><p>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; Gmiz-Gracia and Luque de Castro 1999), and biosensor(Horstkotte et al. 2006) have been reported for thedetermination of formaldehyde.</p><p>Dispersive liquidliquid 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; Garca-Lpez et al. 2007; Rezaee et al. 2009;Nagaraju and Huang 2007; Shariati et al. 2011) fromwater samples.</p><p>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).</p><p>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.</p><p>Experimental</p><p>Reagents and solutions</p><p>All reagents were of analytical reagent grade and usedwithout further purification. Triply distilled water wasused throughout the study. A stock solution(1,000 g mL1) of formaldehyde was prepared byappropriate dilution of a 37% (v/v) formaldehyde so-lution (Merck, Darmstadt, Germany). A 0.2 mol L1</p><p>methyl acetoacetate stock solution was prepared by</p><p>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 5.07.5: the pH was adjustedby adding acetic acid or NaOH to the ammoniumacetate solution.</p><p>Apparatus</p><p>UVVis absorbance digitized spectra were collectedon a Shimadzu UV-2100 ultravioletvisible 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 1Cwas used.</p><p>Analytical procedure</p><p>For DLLME under optimum conditions, 10 mL ofsample solution containing the formaldehyde,0.05 mol L1 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 60C in a waterbath, and then cooled down in water for 5 min. Thenthe injection of 1.0 mL ethanol (dispersive solvent)containing 75 L 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 L of sedi-mented phase was removed using a 100-L syringeand diluted with 100 L ethanol and injected into amicro-cell. Subsequently, the micro-cell was located inspectrophotometer to obtain related spectra.</p><p>Environ Monit Assess</p></li><li><p>Results and discussion</p><p>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 60C 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 (7.8103 dm3 mol1 cm1 at60C) (Li, Ma et al. 2008).</p><p>In the proposed procedure, to achieve maximumextraction efficiency, various parameters affecting theDLLME of formaldehyde were studied and optimized.</p><p>Preconcentration factor (PF) and percent extractionrecovery (ER%) as analytical responses were calculatedbased on the following equations:</p><p>PF Csed=C0 1</p><p>ER% Csed Vsed=C0 Vaq 100 2</p><p>where Csed and C0 are the concentration of the analytein the sedimented phase and initial concentration ofthe analyte in the aqueous sample, respectively. Vsedand 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 0.056 g mL1.</p><p>On the other hand, the relative recovery (RR%) wasobtained from the following equation:</p><p>RR% Cfound Creal=Cadded 100 3where 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.</p><p>Selection of extractant solvent</p><p>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.</p><p>Selection of dispersive solvent</p><p>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 mL1 concentration of ana-lyte, were studied using 1.0 mL of each dispersivesolvent containing 100 L of [C6MIM][PF6]. Accord-ing to the absorbance values obtained from the extrac-tion results, the extraction efficiency was increased inthe order of EtOH&gt;MeOH&gt;ACN&gt;AC. Therefore,ethanol was selected as dispersive solvent. This higherrecovery can also be attributed to the better dispersionobtained in alcohols.</p><p>Effect of extractant solvent volume</p><p>In order to examine the effect of the extraction solventvolume, solutions containing 1.0 mL ethanol plusdifferent volumes of [C6MIM][PF6] (65, 70, 75, 80L) were subjected to the same DLLME procedure.According to Fig. 2, net signal (A) increased byincreasing in [C6MIM][PF6] volume up to 75 L andthen decreased at higher volume because the sedimentedphase volume increases and preconcentration factor</p><p>Scheme 1 Cyclization of methyl acetoacetate and formaldehyde in the presence of ammonia</p><p>Environ Monit Assess</p></li><li><p>decreases. In the following studies, 75 L of [C6MIM][PF6] was selected as an optimal volume of the extractantsolvent.</p><p>Selection of dispersive solvent volume</p><p>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 L for each selected volume of ethanol. The</p><p>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 L 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.</p><p>Salt effect</p><p>The effect of salt addition on the extraction efficiencywasevaluated by adding NaCl (030%, w/v) into the aqueoussolution containing 0.8 ng mL1 of formaldehyde. As</p><p>c</p><p>ba </p><p>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 L1; NaCl, 20%(w/v); pH06.4;formaldehyde,7.0 ng mL1</p><p>0.005</p><p>0.013</p><p>0.021</p><p>0.029</p><p>0.037</p><p>0.045</p><p>0.053</p><p>60 65 70 75 80 85Volume of extraction solvent/mL</p><p>-10</p><p>20</p><p>50</p><p>80</p><p>110</p><p>140</p><p>170</p><p>200</p><p>PFA</p><p>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 L1; NaCl, 20%(w/v); pH06.4;formaldehyde, 0.8 ngmL1</p><p>Environ Monit Assess</p></li><li><p>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.</p><p>Effect of pH</p><p>The effect of solution pH on the extraction was inves-tigated in the pH range of 5.07.5 while keeping othervariables constant. The results demonstrated in Fig. 5</p><p>reveal that the absorbance was nearly constant in thepH range of 6.06.8. Accordingly, pH 6.4 was selectedfor further experiments.</p><p>Analytical figures of merit</p><p>Calibration curve was obtained under the optimizedconditions with linear dynamic range of 0.120 ng mL1 and correlation coefficient (r2) of 0.997.The PF of the method was 158.5, at the concentrationof 0.8 ng mL1 of formaldehyde and the sample vol-ume of 10 mL. The relative standard deviation (RSD%, n05) at the concentration of 0.8 ng mL1 was2.5%. The limit of detection (LOD) based on signal-to-noise ratio (S/N) of 3 was 0.02 ng mL1. The limitof quantification, defined as CQ010SB/m, where CQ islimit of quantification, was 0.08 ng mL1.</p><p>100</p><p>110</p><p>120</p><p>130</p><p>140</p><p>150</p><p>160</p><p>0 0.5 1 1.5 2 2.5Volume of dispersive solvent/mL</p><p>PF</p><p>Fig. 3 Effect of the volumeof dispersive solvent(ethanol) on the absorbanceof formaldehyde. Condi-tions: water sample, 10 mL;extraction solvent ([C6MIM][PF6]), 75 L; methyl ace-toacetate, 0.05 mol L1;NaCl, 20% (w/v); pH06.4;formaldehyde, 0.8 ngmL1</p><p>0</p><p>0.01</p><p>0.02</p><p>0.03</p><p>0.04</p><p>0.05</p><p>0.06</p><p>0 5 10 15 2 25 30 35 NaCl/% w/v</p><p>A</p><p>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 L;methyl acetoacetate,0.05 mol L1; pH06.4;formaldehyde, 0.8ng mL1</p><p>Environ Monit Assess</p></li><li><p>Effect of foreign ions</p><p>In order to identify the cations and anions that couldinterfere in the extraction and det...</p></li></ul>