automated of andtotal in wastewater andother waters by cold … · 2019. 8. 1. · figure 2....

Journal of Automatic Chemistry, Vol. 10, No. 3 (July-September 1988), pp. 140--143

Automated continuous monitoring ofinorganic and total mercury in wastewaterand other waters by flow-injection analysisand cold-vapour atomic absorptionspectrometry

$. E. BirnieAssociated Octel Company Ltd, Research & Development Department, EllesmerePort, South Wirral L65 4HF, UK

An automated continuous monitoring system for the determinationofinorganic and total mercury by flow-injection analysis followedby cold-vapour atomic absorption spectrometry is described. Themethod uses a typicalflow-injection manifold where digestion andreduction ofthe injected sample takes place. Mercury is removed byaerationfrom theflowing stream in a specially designed air-liquidseparator and swept into a silica cellfor absorption measurement ata wavelength of 253.7 nm. A calibration curve up to 10 #g Hgm1-1 using three different path length cells is obtained with adetection limit of 0"02 #g Hg ml-. The sampling rate of aninjection every 3 min produces 20 results per hourfrom a flowingstream.

Introduction

The determination of mercury in aqueous solution at lowconcentrations is usually carried out by the Cold VapourAtomic Absorption (CVAA) method which was intro-duced by Puleukov et al. [1] and Hatch and Ott [2]. Themethod is based on the chemical reduction of mercuryions to elemental mercury, usually with SnCI2 or NaBH4.The elemental mercury is then swept out of solution witha carrier gas into an absorption cell where the atomicabsorption at 253.7 nm is measured. This procedurenormally uses a discrete sample method in which all themercury from a given volume ofsolution is swept throughthe absorption cell to produce a peak response. Manyimprovements to the method, Hawley and Ingle [3],Christmann and Ingle [4] have reduced the detectionlimit to 2 pg in a ml sample. This method is widelyaccepted as a standard for determining traces of mercuryin aqueous solutions and is well documented.

Relatively few automated procedures for the determina-tion of mercury are described in the literature. Most ofthese are based on the use of a continuous air-segmentedflow system such as those described by E1-Awady et al.[5], Agemian and Chau [6] and Agemian and DaSilva[7]. Other automated systems described are continuoussample flow by Oda and Ingle [8] and a continuousmonitoring system by Goto, Shibakaw, Arita and Ishii[9]. A flow-injection analysis (FIA) method described byDeAndrade, Pasquini, Baccan and Vantoon [10] uses aTeflon membrane phase separator, a detection limit of0"66 ng and a linear calibration up to 70 ng in a ml

140

sample are obtained with CVAA detector. The CVAAmethod was initially used in conjunction with FIA byRuzicka and Hansen [11 and 12] but more recently byAstrom [13] for the determination of bismuth using anatomic absorption/hydride generation system. Thepresent paper describes an automated monitoringmethod, which allows economic and completely continu-ous determination of inorganic and total mercuy inwa.stewater and other waters.

Experimental

ApparatusA schematic diagram of the flow-injection system isshown in figure 1. The sample, the reagents and thecarrier solutions were pumped by a Watson Marlow 202Fperistaltic pump using Technicon Tygon pump tubes tomaintain the correct flow rate in each feedline. PTFEtubing (0"7 mm i.d.) was used as transmission tubing.Mixing coils were also of PTFE, the first coil (250 cm x1"5 mm i.d.) was heated to 65 C in a water bath to ensurecomplete conversion of any organic mercury to theinorganic state. The second coil (60 cm x 0.7 mm i.d.)was used to mix the reducing solution with the samplestream. Automatic sample injection was accomplished bythe use ofa timing control switch which activated a pair ofRheodyne solenoid valves (type 7163), which in turn

SOL "O’,O F-----I PRO6ESSIVALVES I---- TIMER

’CTUATOR

B

MPLE PROBE

/.MIXING CO7

PUMP

Figure 1. Configuration ofthe flow injection analysis system. A"Sample stream, B" Carrier solution, C: Acid reagent, D"Oxidizing reagent, E: Reducing reagent.

S. Eo Birnie Automated continuous monitoring of inorganic and total mercury

TO SILICA CELL

CONDENSER WITHTURN 4mm OD

SPIRAL COIL

VENT

WASTE

EQUAL HEIGHTSAMPLE

No.SINTER

I/,mm 0 O TUBING

Figure 2. The air separator.

AIRINLET

operated a Rheodyne pneumatic activator (type 5001).This activator charged or discharged a Rheodyne rotarysample injection valve (type 5020).

The following sample stream enters a specially designedglass air/liquid separator, shown in figure 2, where, aftersample reduction, the mercury vapour was extracted byaeration. The liquid phase was drained to waste byoverflowing a U-bend whose dimensions were such thatcomplete extraction of mercury took place and all the gasphase passed through a water cooled condenser into asilica flow cell.

A Perkin-Elmer model 101 atomic absorption spectro-photometer was used to measure the mercury absorptionat 253"7 nm using a hollow-cathode mercury lamp source.The signal from this was recorded as sharp peaks by aServoscribe model RE571 recorder.

ReagentsAll reagents used were of analytical grade, unlessotherwise stated. Distilled water was used for all solutionpreparations. The reagent solution and their concentra-tions are as follows"

(4) Reducing reagent, 10% m/v tin (11) chloridedissolved in 10% v/v hydrochloric acid solution,Spectrosol low in mercury grade (BDH Ltd).

Mercury standard solutions in the range 2-10 g Hg ml-1were prepared by suitable dilution of mg Hg m1-1Spectrosol grade solution (BDH Ltd). All the dilutemercury standards were prepared in a matrix similar tothe solution to be analysed.

Procedure

A flow diagram of the manifold for continuous mercurymonitoring is given in figure 1. The procedure for thedetermination of inorganic mercury is as follows.

The sample stream and the carrier reagent are continu-ously pumped at a flow rate of 1"0 and 2"9 ml min-,respectively. After injection of the sample, the stream ismixed first with the acid reagent and then with water at aflow rate of 0"6 ml min-1 for each. The stream is passedinto the reaction coil for further mixing. The samplestream is mixed with the reducing reagent at a flow rate of0"8 ml min-1 and passes into the second reaction coil.Here the mercury (11) in the sample is reduced to theelemental state. The mixed stream is introduced into thespecially designed air-liquid separator where it is volati-lized by aeration at a flow rate of 50 ml min-. Thesurplus liquid is allowed to drain to waste and all thevapour is carried by the air through the cold water cooledcondenser which removes excess water vapour beforepassage of the vapour to the silica flow cell placed in theatomic absorption spectrophotometer operated atwavelength of 253"7 nm. The absorbance of the mercuryis continuously recorded for each sample injection todetermine the inorganic mercury present.

The procedure for the determination of total mercury is asfollows.

The sample stream and the carrier reagent are continu-ously pumped at a flow rate of 1"0 and 2"9 ml min-1respectively. After injection of the sample, the stream ismixed first with the acid reagent and then with theoxidizing reagent at a flow rate of 0’6 ml min-1 for each.The stream is passed into the reaction coil, placed in awater bath at 65 C for sample digestion. In the reactioncoil, organic compounds present in the sample aredecomposed and all the organic mercury is converted tothe inorganic state. The digested sample stream is mixedwith the reducing reagent at a flow rate of 0"8 ml min-and then passes into the second reaction coil. Theabsorbance of the mercury is measured in the mannerdescribed above for the determination of inorganicmercury.

Results and discussion

(1) Carrier reagent, 1% v/v nitric acid solution.

(2) Acid reagent, 50% v/v sulphuric acid solution.

(3) Oxidizing reagentT 4% m/v potassium persulphatesolution, laboratory reagent, low in mercury grade(BDH Ltd).

The air-liquid separator shown in figure 2 was designedso that maximum aeration of the sample stream occurs asit flows through the separator. This was achieved byallowing the sample stream to have sufficient residencetime to completely vaporize all the mercury present: Theoverflow after the aeration sinter was of equal height to

141

S. E. Birnie Automated continuous monitoring of inorganic and total mercury

CELL DIMENSION

(mm}

30 x 25 o.d.

95 x 25 o.d.

150 x 15 o.d.

MERCURY RANGE

(tg Hg ml-)

0to 10

0to3

0to

Figure 3. Effect ofvarying the dimensions of the flow cell.

0.125-

0"075-z

, O.OSO-

0"025-

B

A

:2 ..o o 0:6 8 I’.OMERCURY CONCENTRATION (,g Hgmt -I

Figure 4. Comparison ofinorganic and methyl mercury absorbancesignal response: A inorganic mercury, B methyl mercury.

the overflow from the waste, ensuring that no backpressure was produced which would cause unevengeneration of mercury vapour.

Figure 3 shows the range of the method for varying celldimensions. The flow cell must be. constructed with silicaglass end windows for operation at a wavelength of 253"7nm. Mercury concentrations from 0 to 10 zg Hg ml-] canbe determined directly without pre-dilution using threedifferent flow cells.

Figure 4 shows a comparison of absorbance signalresponses to inorganic and methyl mercury using thetotal mercury method. A similar response is obtained upto 0.4 g Hg ml-1 but between 0"6 and 1’0 zg Hg ml-1 adifference of up to 15% is observed. For a total mercurydetermination in samples such as wastewater from anindustrial process generally the major mercury species isinorganic mercury so it is satisfactory to use inorganicmercury solutions for calibration purposes.

Figure 5 shows typical signal profiles using the optimizedconditions. The linear equation for a calibration curve upto 1.0 g Hg ml- was Y (absorbance units) 0"228x +

B

A

10’05LABSORBANCE

Figure 5. Typical flow injection signal profiles, A2 g Hg m1-1, B 4 tg Hg ml-, C 6 g Hg ml-.

0.005 where x is the mercury concentration in g ml-land has a correlation coefficient of 0.998. The limit ofdetection (defined as two times the average of thebase-line signal) was 0"02 g Hg ml-]. The relativestandard deviation, estimated from 30 replicate determi-nations was 0.93% absorbance units calculated at a levelof 6 g ml-1 of mercury.

Some elements which may occur in natural and waste-waters can cause interference in the procedure describedwhich could result in a depression of the mercury FIAsignal. Sulphide may combine with the mercury as HgSand not be reduced to the elemental state. Chloride isoxidized by persulphate to liberate chlorine gas whichwill reduce the signal. Hypochlorite has the same effectbut this can be eliminated by boiling with potassiumbromide prior to injection. Selenium also depresses thesignal, probably due to the formation ofmercury selenide,and copper ions have the same effect. The noble metals(palladium, platinum, gold and silver) form amalgamswith mercury.

Conclusion

The proposed method is suitable for the continuousmonitoring of inorganic or total soluble mercury inwastewater and other waters. The determination couldbe made at a sampling frequency of 20 samples per hour.The amount ofreagents required are much less than thosepreviously reported in conventional auto-analysermethods.

142

$. E. Birnie Automated continuous monitoring of inorganic and total mercury

AcknowledgementsThe author wishes to thank his colleagues, Mr G.Rowlands and Mr P. Scarisbrick, for their co-operationduring long-term monitoring trials. Also Mr D. Young forcheck analysis carried ou during development of themethod, and Mr F. Wainwright for construction of theair-liquid separator and flow cells, and, finally, theAssociated Octel Company for permission to publish thispaper.

References

1. POLUEKOV, N. S., VITKUM, R. A. and ZELYUKOVA Yu. V.,Zh. Anal. Khim, 19 (1964), 937.

2. HATCH, R. and OTT, W. L., Analytical Chemistry, 40 (1968),2085.

3. HAWLEY, J. E. and INGLE, J. D., Analytical Chemistry, 46(1974), 719.

4. CHRISTMANN, D. R. and INGLE, J. D., Analytica Chimica Acta,86 (1976), 53.

5. EL-AWADY, A. A., MILLER, R. B. and CARTER, M. j.,Analytical Chemistry, 48 (1976), 110.

6. AOEMIAN, H. and CHAU, A. S. Y., Analytical Chemistry, 50(1978), 13.

7. AOAMIAN, H. and DASILvA, J. A., Analytical Chimica Acta,104 (1979), 285.

8. ODA, C. E. and INGLE, J. D., Analytical Chemistry, 53 (1981),2030.

9. GoTo, M., SHIBAKAWA, T., ARITA, T. and IsHn, D.,Analytica Chimica Acta, 140 (1982), 179.

10. DEANDRADE, J..C., PASQUINI, C., BACCAN, N. andVALooN,J. c., Spectrochimica Acta, 38B (10), (1983), 1329.

11. RuzxcKA, J. and HANSEN, E. H.,.Analytica Chimica Acta, 78(1975), 145.

12. RUZICKA, J. and HANSEN, E. H., Flow Injection Analysis(Wiley, New York, 1981).

13. ASTROM, O., Analytical Chemistry, 54 (1982), 190.

THIRD INTERNATIONAL SYMPOSIUM ON KINETICS IN ANALYTICAL CHEMISTRY

To be held in Dubrovnik, Yugoslavia, from 25 to 28 September 1989

The International Symposium on Kinetics in Analytical Chemistry 1989 KAC’89 will be held in Cavtat(Dubrovnik) from 25 September to 28 September 1989. The Symposium is the next in the series of triennialconferences, initiated in Cordoba in Spain in 1983, as a result of rapid development in kinetic methods Ofanalysis. The success of the first conference prompted the second meeting which was held in Preveza, Greecein 1986.

Scientific programmeThe scientific programme will be organized around plenary, invited and contributed papers and posters. Thescope of the symposium will be similar to that of the earlier ones, and will include catalytic (enzymatic ornon-enzymatic) and non-catalytic methods, differential reaction rate methods, unsegmented flow methods, andany other kinetic aspect of analytical interest.

The Scientific Committee includes:Professor H. A. Mottola (Stillwater, Oklahoma, USA)Professor G. Werner (Leipzig, GDR)Professor M. Valctrcel (C6rdoba, Spain)Professor M. I. Karayannis (loannina, Greece)Professor G. A.. Milovanovib (Belgrade, Yugoslavia)Professor F. F. Gagtl (Novi Sad, Yugoslavia)

LanguageThe offical language of the Symposium will be English.

General information and social programmeThe Symposium will be held in the Croatia Hotel in Cavtat, a small town on the Adriatic, close to Dubrovnik. Avaried social programme is being prepared to accompany KAC’89 and will include a Welcome Party, aSymposium Banquet and several tours. Post-Symposium excursions will be also arranged.

Travel AgencyThe official travel agency for KAC’89 (accomodation and post-symposium tours) will be YUGOTOURS,Congress Department Dure Dakoviba 31, 11 000 Belgrade, Yugoslavia.

Further informationfrom Professor Gordana A. Milovanovid, Department of Chemistry, University ofBelgrade, KA C, P. O.Box 550, 11001 Belgrade, Yugoslavia.

143

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