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1、Analytica Chimica Acta 515 (2004) 343348A continuous approachfor the determination of Cr(VI) in sedimentandsoil basedon the coupling of microwave-assistedwaterextraction,preconcentration,derivatization andphotometric detectionS. Morales-Mu?oz, J.L. Luque-Garc ? a?, M.D. Luque deCastroDepartment of A
2、nalytical Chemistry, University of Crdoba, Marie Curie Building,Campus of Rabanales, E-14071 Crdoba,SpainReceived 19 January 2004; received in revised form 19 March 2004; accepted 19 March 2004Available online 25 May 2004AbstractA dynamic system for the continuous leaching of Cr(VI) from sedimentand
3、soil basedon both microwave assistanceand iterative changeof the ?ow direction of the extractant through the sample cell hasbeendeveloped. The microwave-assisted extractor hasbeencoupled to aphotometric detector through a ?ow injection interface in order to develop a fully automatedmethod. The Cr(VI
4、) extracted was monitoredafter derivatization with 1,5-diphenylcarbazide. Two approachesareproposedwhich differ in the inclusion of apreconcentration minicolumnpackedwith a strong anion exchangeresin. A 0.04 M ammonium buffer solution was usedasextractant and0.2 g of sample riversedimentspiked with
5、50 and 5?g g- 1for the method without preconcentration (method A) and with preconcentration (method B)wassubjected to814 min of 300 W microwave-assisted extraction. The within-laboratory reproducibility and repeatability were 2.6 and 1.9 for method A,and 4.0 and 2.6 for method B. The proposedmethods
6、have beencompared with the referenceEPA method 3060/7196.? 2004 Elsevier B.V. All rights reserved.Keywords: Microwave-assisted extraction; Hexavalent chromium; Sediment; Photometric detection1. IntroductionThe predominant use of chromium in industry (in pro-cesses such as plating, tanning,and paint
7、and pigmentproduction)unfortunatelycauses environmentalconcern.The toxicityof a metal depends on its oxidation state. Inthe case of chromium,Cr(III)is considered an essentialmicroelement 1,2 while Cr(VI)is thought to be toxic 3and carcinogenic. Thus, the identi?cation of Cr(VI) speciesin environment
8、al samples is a problem of great concern be-cause of their toxicity to aquatic and terrestrial organisms,including humans. It is not suf?cient to give a total concen-tration of metal; instead, what is required to understand thepotential toxicityof a sample is the concentration of Cr(VI)in the sample
9、.An ideal extraction method would extract the metal ef-?ciently without converting metal ions from one oxidationstate to another. Although much research hasbeen focused?Corresponding author: Tel.: + 34-957-218615;fax: + 34-957-218615.E-mailaddress: 952lugajuco.es (J.L. Luque-Garc ?a).on the extracti
10、on and detection of chromium speciesin liq-uid samples such as natural and waste waters 4 6 , the ex-traction of Cr(VI)from soil samples requires an additionaleffort 7 . The dif?cultyin determining Cr(VI)species insolid samples arises from the possible changes taking placein the chromium oxidation s
11、tate.Theenvironmentalprotectionagency(EPA)recog-nises four methods for sample preparation of hexavalentchromium:7195, coprecipitation;7196, colorimetrywith1,5-diphenylcarbazide(DPC);7197, chelation/extraction;and 7198,differentialpulsepolarography.Anof?cialmethod, namely EPA method 3060, using alkal
12、ine diges-tion of Cr(VI)is also suggested for sample preparation ofCr(VI).A study was conducted by Gurknecht in 1983 toevaluate the above four methods 8 . The study concludedthat 7195 and 7197 methods were vulnerable to effects ofmatrix composition. The 7196 colorimetricmethod basedon the coloured c
13、omplex formed between DPC and Cr(VI)9 is one of the most sensitive and selective for Cr(VI)determination.Several methods based on batch extractionhave beenproposed for the speci?c extraction of Cr(VI)from solid0003-2670/$see front matter ? 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.200
14、4.03.092名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 6 页 - - - - - - - - - 344S. Morales-Mu?ozet al. / Analytica Chimica Acta 515 (2004) 343348samples 3,10 12. There is only one case in the literaturein which a dynamic extraction system has been used 13 .Almo
15、st all thesemethods are based on the useof ultrasoundenergy to accelerate the extraction step 3,10,11,13 . Thisenergy has proved to be of great help in the extraction ofCr(VI)from solid samples withoutdisturbingthe speciesdistribution.Microwave-assisted extraction is also an expeditious, in-expensiv
16、e and ef?cient extraction technique. Over the pastfew years, this technique has been used in a discontinu-ous mode for accelerating the sample preparation step andavoiding some potential problems namely,analyte lossesand contamination 14,15 related to conventional meth-ods. Dynamic microwave-assiste
17、d sample treatment hasalsobeen used 16 19, allowing the automation of the prelimi-nary step of the analytical process.In the present research,a dynamic microwave-assisted ex-traction method has been developed and compared with therecommended EPA method 3060 21 . A commercialfo-cusedmicrowave device
18、(Soxwave-100) hasbeen employed.The use of a dynamic extraction system 20 facilitates theautomation of the whole analytical process. Thus, a fullyautomated approach in which a ?ow injection (FI) manifoldis usedasinterface for the coupling of a microwave-assistedextractor with a photometric detector i
19、s proposed. The inclu-sion of a minicolumnpacked with a strong anion-exchange(SAE) resin in the FI systemallowed preconcentration of theCr(VI) extracted before detection, thus allowing the analysisof samples with low levels of the target analyte.2. Experimental2.1. Instruments and apparatusMicrowave
20、extraction was performed witha Soxwave-100 focused microwave digestor (Prolabo, Fontenay-sous-Bois, France) with a maximum irradiation power of 300 W.A TX 32 device (Prolabo) was used for the control of the mi-crowave unit. A Gilson Minipuls-3low-pressure peristalticpump (Gilson,Worthington,OH, USA)
21、programmed forchanging the rotation direction a presetintervals, threeRheo-dyne low-pressure selection valves (Rheodyne, Cotati, CA,USA), a laboratory-made chamber of Te?on (7 cm 7.5mmi.d.) and Te?on tubing of 0.8 mm i.d. were usedto build theleaching system.Two Gilson Minipuls-3low-pressure perista
22、ltic pumps,a Rheodyne Model 5041 low-pressure injectionvalve, alaboratory-made minicolumn (3 cm2 mm i.d.) packed withSAE resin, a laboratory-made debbubler and Te?on tubingof 0.8 mm i.d. were usedto construct the two dynamic man-ifolds. One of the channel of PP2 was usedas PP1.A Waters model 490 liq
23、uid chromatographic spectropho-tometer equipped with a low-volume?ow-cell and Knauerx t recorder was employed for the photometric monitoringof the Cr DPC coloured complex at 540nm.2.2. Reagents and standardsCr(VI)and Cr(III)workingstandard solutionswerepreparedfrom1000 ?g ml- 1stockstandardsolutions
24、preparedbyK2CrO4andCr2O3(Merck,Darmasdt,Germany).The Cr(VI)complexingreagent solutionof1,5-diphenylcarbazide(DPC) (Panreac, Barcelona, Spain)was prepared by dissolving 0.2g of DPC in 40ml of ethanol,diluting to 100 ml with water andstoring in alight-excludingbottle. Ammonia solution and ammonium sul
25、fate (Panreac)were used for preparation of 0.04 and 0.5M ammoniumbuffer (pH 8) solution usedas extractant and eluent, respec-tively. Ultrapure water from a Milli-Qsystem (Millipore,Canada, USA) was used throughout. All reagents were ofanalytical reagent grade. An SAE resin (Dowex 1-X8-400;Sigma Aldr
26、ich,Steinheim, Germany) was used for precon-centration of Cr(VI). The resin was supplied in the chlorideform and was cleaned prior to use by slurring it with 3 MHCl, allowing it to stand for 10min and then decanting offthe acid. This procedure was repeated three times. Afterpouring off the last port
27、ion of the cleaning acid, the resinwas slurried with 1M HCl and dried prior to use.2.3. SamplesTwo hundred and ?fty grams of river sediment as ma-trix, spiked with Cr(VI)to obtain a ?nal concentration of50? gg- 1, was used to carry out the optimisationstudy. A250 g portion of river sediment was spik
28、ed with 25 ?g g- 1of Cr(VI) and other 250 g portion with 5 ? gg- 1of Cr(VI).These spiked levels were selected in order to obtain sedi-ments with environmentallyrepresentative concentrations.Six aliquots of 50g were spiked with Cr(VI)to obtaina ?nal concentrationin the sediment of 50 ?g g- 1in allthe
29、m and with Cr(III)to a ?nal concentration of: 5, 10,15, 20, 25 and 30? gg- 1from the ?rst to the last aliquot.The sediment thus prepared was aged for three months inorder to simulate the matrix analyteinteractionin naturalsamples. Two types of natural contaminated soil (namely,clayeyand slimy)were s
30、elected forvalidatethe pro-posed method by comparison with the EPA method 306021 .2.4. Procedures2.4.1. EPA method 3060/71962.4.1.1. Leaching step.A 50 ml volume of extractant (asolution consisting of a mixture of 0.5 M NaOH and 0.28 MNa2CO3at pH 11.5) and 2.5g of sediment sample werepoured into a b
31、eaker and heated to 90 95?C on a hot-plate during 1 h. The cooled extract (pH 12) was ?lteredthrough a 0.45 ? m membrane ?lter, and then, the ?ltrate wasneutralised to pH 7.5 with concentrated HNO3and storeduntil the followingstep.名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - -
32、 - - - - 第 2 页，共 6 页 - - - - - - - - - S. Morales-Mu?ozet al. / Analytica Chimica Acta 515 (2004) 343348345LEACHING STEPDETERMINATION STEPWMEECRMECLCSV1PPPA) WITHOUT PRECONCENTRATIONPP1SV2PP2PP2MCRCDDSAECWEIVDPCACIDStEXMCRCDBWB) WITH PRECONCENTRATIONSV3EXPP3ACIDWDBDPCFig. 1. Experimentalset-up for d
33、evelopment of the methods for Cr(VI)without (A) and with preconcentration step (B). LC, leacher-carrier; St, standard;PP, peristaltic pump; PPP, programmableperistalticpump; SV, selection valve; W, waste; ME, microwave extractor; EC, extraction cell; R, refrigerant;MEC, microwave extraction controll
34、er;EX, extract; MC, mixing coil; RC, reaction coil; DB, debubbler; D, detector; E, eluent; IV, injection valve; SAEC,strong anion-exchange column.2.4.1.2. Determinationstep.A proper amount of the neu-tralised ?ltrate and 1ml of DPC solution (2 mg ml- 1) werepoured into a 50 ml calibrated ?ask and 1%
35、 HCl was usedto adjust the solution to pH 2, necessary for the formationof the red violet complex, which was monitored photomet-rically at 540 nm.2.4.2. Proposed procedureTwo different methods were developed using the mani-folds in Fig. 1, which differ in the inclusion of apreconcen-tration unit in
36、Fig. 1B in order to achieve lower determina-tion levels.2.4.2.1. Leachingstep.Anamountof0.2 gsedimentwas placed into the sample chamber. The closed system(with a total volume of 2ml) was ?lled with the extractant(0.04 M ammonium buffer solution) impelled by peristalticpump PP1 by maintainingselectin
37、gthe leacher-carrier(LC) through valve SV1 with the valve SV2 in the closed-circuitposition.The sample chamber was placed in themicrowave vessel, which contained= 50 ml of water, andirradiated at 300W for a preset time depending on the sam-ple matrix. Duringmicrowave irradiation, the direction ofthe
38、 extractant (at 1.2 ml min- 1) was changed each 30s inan iterative manner, thus minimisingboth dilutionof theextract and increased compactness of the sample in the ex-traction chamber, and avoiding overpressure in the systemas a result. In addition, a closer sample extractant contactis achieved, thu
39、s accelerating the removal of the target an-alyte. Afterextraction, selection valve SV3 was switchedand the extract was driven either to the preconcentrationsystem (at 0.2 ml min- 1) or directly to the detection system(at 0.25ml min- 1).For introductionof the standards in the system, valveSV1 select
40、ed the standard (St) channel with valve SV2 inthe closed-circuitposition. In this way, the extraction sys-tem was ?lled with a given standard. An amount of 0.2g ofsediment without detectable level of the target analyte wasplaced into the extraction chamber in order to obtain a stan-dard volume equal
41、 to those of the extracts (when extractionis performed). Once the system was ?lled, selection valvesSV1 and SV2 were switched. In this way, the leacher-carrierdrives to the waste the standard volume between both valves(SV1 and SV2). Then, selection valves SV2 and SV3 wereswitched to the closed- and
42、open-circuit positions, respec-tively, in order to drive the standard either to the preconcen-tration or detection system.2.4.2.2. Preconcentrationstep.Theextractfromtheclosed system was driven (at 0.2ml min- 1) to a minicolumnpacked with SAE resin where the analyte was retained. Theminicolumnwas lo
43、cated in the loop of an injection valve,thus allowing elution in the direction opposite to retention.Elution was carried out by passing through the minicolumna 0.5M ammonium buffer stream at 0.25ml min- 1. The elu-ate was driven to the spectrophotometer for determinationafter derivatization with DPC
44、.2.4.2.3. Determinationstep.The extract or the eluatefrom the preconcentration step was merged with an 1% HCl名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 6 页 - - - - - - - - - 346S. Morales-Mu?ozet al. / Analytica Chimica Acta 515 (2004) 343348stream and was
45、driven to a mixing coil (1 m 0.8 mm i.d.).Then, the resulting stream was merged with 2 mgml- 1DPCsolution and driven to a reaction coil (1.5 m 0.8mm i.d.)in order to form the coloured complex. Both the HCl andDPC streams were pumped at 0.25 ml min- 1. Photometricdetection was performed at 540 nm. A
46、debubbler was con-nected in-lineto remove occluded air in the solid, whichcould give rise to parasitic signals at the detector.3. Results and discussionThe order used for optimisingthe steps involved in theoverall method was as follows:?rst, the formation of thecoloured complex was optimised for che
47、cking the other pre-vious steps; then, the variables affecting the leaching stepand, ?nally, the preconcentration step, which is an optionalstep that provides a more sensitive alternative for the anal-ysis of soils with low levels of Cr(VI) (below 1.2 ? gg- 1).3.1. Optimisationof the coloured comple
48、x formationA multifactorialdesign methodology was used for the op-timisation of the main factors affecting the formation of thered violet complex (namely, the ?ow rates and the concen-tration of the HCl and DPC solutions and the ?ow rate ofthe sample). The optimisation procedure and the results ob-t
49、ained were similar to thoseof Ref. 13. The rangesassessedand optimum values found are shown in Table 1.3.2. Optimisationof the continuous microwave-assistedextraction procedureThe variables optimised in the leaching step were theirradiationpower, the irradiationtime, the extractant ?owrate in the cl
50、osed circuit during microwave irradiation andthe time interval between successive changes of the ?owdirectionofthe extractant. A 0.04 Mammoniumbuffersolution was selected as extractant as it had proved to beef?cient.The volumeof extractant (2 ml)correspondedTable 1Ranges and optimumvalues for the va