SPECTROFLUOROMETRIC DETERMINATION OF MINERAL OIL CONTENT IN WASTE WATERS

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SPECTROFLUOROMETRIC DETERMINATION OF MINERAL OIL CONTENT IN WASTE WATERS

By

I. KASA and G. R-\.J~6czy

Department for Applied Chemistry, Technical University Budapest (Received Juli 15, 1975)

Presented by Prof Dr. E. Pungor

The continuous and reliable control of the mineral oil content in natural waters and in industrial waste waters is very important from the aspect of environmental protection. Several analytical methods are known for the quantitative determination of 'Oil contamination in water. For the determination of higher oil contents, the simple gravimetric method can also be used, however, in certain cases the reliability of the results obtained is questionable. On the other hand, the method is unsuitable for the determination of very low oil content.

In the ppm-ppb region, only modern instrumental analytical methods, such as e.g. spectrophotometry, gas chromatography, IR spectrometry and spectrofluorometry can be taken into consideration. .

As compared to th~ other methods, the spectrofluorescent ~ethod has several advantages. First of all, the high sensitivity of the method ~ust be mentioned. The sensitivity of spectrofluorescent oil content determi~ation is as high as 1 ,ugjl oil concentration [1]. A further advantage of the method, as compared to spectrophotometry is that quantitative determinations can be carried out in a broader concentration range, and moreover, it furnishes two kinds of information on the test sample:. the excitation .sp.ectrum and the emission spectrum. Owing to these advantages, the analy-tical importance of the spectrofluorescent method steadily gains ground.

In recent years, several papers discussed the extraction of the oil content of waste waters and sea water with organic solvents, and the subsequent fluo-, rescent determination of oil concentration [1-4].

According to the method, from the intensity of the fluorescent light, emitted by the luminescent compounds to be found in the oil products investigated, conclusions can be drawn on the oil content present. It can be established in general that in the case of most of the mineral oils the luminescent compounds mentioned give the strongest fluorescent light emission, when an excitation light beam of 310-365 nm 'wavelength is used. Therefore, in the analytical procedure published so far, the authors always used the excitation wavelengths mentioned.

5 Periodica Polytechnica CH. XX. 2.

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170 I. KAsA and G. BAJNOCZY

However, the intensity of emitted light changes with the different mineral oils, and particularly with the distillates. In the determination of mineral oil concentration, the fact that mineral oils and oil distillates of identical concentration but of different origin emit light of different intensity, may cause substantial errors. The selection of the standard is very important in the reduc- tion of this error. If the origin, the type of the contaminating oil or fraction is exactly known, the error of determination can be reduced to a minimum. In the contrary case, the relative error may be as large as several hundred per cent.

The object of our work was to determine on the basis of the fluorescent spectra of the various oil fractions the distribution of the luminescent com- pound present in mineral oil between the single fractions, and to establish which are those distillates, which can actually be measured by spectrofluorescent determinations.

In the course of the analysis of emission spectra, recorded at various excitation wavelengths, parameters were to be found, at which the difference between the fluorescent light intensity of various distillates of identical concentration is the smallest.

Material and apparatus

Oil samples used for the investigation were Soviet mineral oil supplied to Hungary and fractions of different boiling point, prepared from it.

The distillation temperature intervals of the distillates investigated, pressure values belonging to them, and the results of the visual observations of fluorescence are summarized in Table 1.

Table 1

The fluorescence of the 1000 ppm solutions of mineral oils in n-heptane, produced by UV light

Distillation Distillation Fluorescence of 1000

Distillates temperature pressure ppm solution in n-hep ..

('C) (mm Hg) tane under an tJV lamp i

Gasoline 60-170 760

I

,,"k bIn<

~<lumi.

Kerosene 140-240 760

Light gas oil 170-300 760

Heavy gas oil 220-370 760

nescene

Light paraffin distillate 180-280 10 week blue light lumi- Medium paraffin distillate

I ne scene

210-310 10 i strong blue light lumi- i nescene

Heavy paraffin distillate 280-350 10 i strong blue light lumi-

I nescene

I

Goudron 350 10 jstrong diffuse light

Mineral oil strong diffuse light

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},HNERAL OIL CONTENT IN WASTE WATERS 171

For the spectrofluorometric investigations, solutions of identical concentration (10 ppm) have been prepared from the different oil fractions. The solvent used was n-heptane of "alt" purity, which has been previously further purified by repeated distillation.

A spectrofluorometer Model HITACHI MPF-2A was used for the spectrofluorometric investigations. Spectra presented are uncorrected.

Results and discussion 1. Fluorescent spectra of the mineral oil fractions

First, the spectrofluorometric investigation of the solutions of the mineral oil and the mineral oil fractions in n-heptane has been carried out at the excitation and emission wavelengths recommended in the literature. Fluo- rescent emission spectra recorded at various excitation wavelength (310 nm;

330 nm; 360 nm; 400 nm/are shown in Figs 1-4.

30 5 1. Light gas oil

I

/

on

2. Heavy gas oil

3. Light paraffin distil/ale

'u 4. l1edium paroffin distlf/cie

5. Heavy paraffin distil/ate

60 6. Goudron

7. l1ineral oil J.£xc=31O nm 50

40

30

20

10 .

320 340 360 380 400 420 460 480 500 520 nm

Fig. 1. Emission spectra of mineral oil and mineral oil fractions.

Excitation wavelength: 310 nm

5*

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172

80

70

5D _I

::JU

::0

3D

20

iC 2

340 360 380

I. Kj{SA and G • . BAJNOCZY

400 420 1;30

1. Light gas oil 2 Heavy gas oil

3. Ught paraffin distillate 4. Hedium paraffin distillate 5. Heavy paraffin distillate 6. Gaudron

7. Mineral oif

450 480 500 520 51t0"17 Fig. 2. Emission spectra of mineral oil and mineral oil fractions.

It can be established on the basis of the emission spectra that the intensity of fluorescent light differs greatly for the solutions of various mineral oil fractions of identical concentration. On increasing the excitation 'wavelength,

differences in intensity further increase.

Next, fluorescent emission spectra, recorded at excitation wavelengths shorter than 310 nm, have been studied. (Figs 5 - 7)

Though the absolute value of the intensity of fluorescent light, emitted by the fractions, decreased with decreasing excitation wavelength, at certain emission vt'avelengths the intensity values of different fractions of identical concentration lie rather close to each other.

It can be established from data in Table 1 and from the fluorescent spectra that in the case of a common contamination of the fractions, only fractions with higher boiling point than light gas oil can be measured.

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!IHNERAL OIL CONTENT IN WASTE WATERS 173 70

60 3. Light paraffin distillate

4. Medium paraffin distillale 5. Heavy paraffin distillate

50 _6._G{Judf'on

7 Mineral oil

1;0 A£xc=360 nm

30

20 7

5 - -...

10

380 {'oo 420 440 1;60 1;80 500 520 540 nm

BD

I

70

4. Medium paraffin distillate 5. Heavy paraffin distil/ale 6. Goudron

60

7. !1inerol oil

50 5l.£xc=400nm

40

30

20

10

420 440 1,60 480 500 520 51,0 560 nm

Fig. 4. Emission spectra of mineral oil and mineral oil fractions.

Excitation wavelen gth: 400 nm

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174 1. KAsA and G. BAJNOCZY

90 I

I. Light gas oil

80 2 Heavy gas oil

3. Light paraffin distillate ft. Medium pal'affindisli'llate

70 5. Heavy paraffin distillate

6. Goudron 7. Mineral oil

{jO A£xc=280 nm

50

40 .

30

20

10

300 ]20 340 360 380 ~OO 420 '-140 1;60 1;80 500 ·nm

Excitation wavelength: 280 nrn

2. Procedure

According to the prescriptions of sampling, a 50 ml oil-containing waste water sample is prepared for extraction. Waste water to be investigated should be processed as soon as possible.

The course of determination is as follows:

Extraction: In a 100 ml separating funnel, 50 ml of waste water is shaken for 1 minute "\\'ith 3 X 5 ml portions of n-heptane Qf grade "alt", further purified by distillation. To further the separation of the extract, in the first step 4 ml of 2 n sulfuric acid ispbured into the separating funnel, and cautiously

admixed. In the second extraction step, no further sulfuric acid is needed.

The extract is collected in a 25 mlnormal flask, and the flask is filled up

,~ith n-heptane to the mark. If the solution cannot be immediately measured,

it should bek~pt~iP. a d~rk, con place. - .

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90

80 .

70

60

50

40

30

20

10

MINERAL OIL C01YTENT IN WASTE WATERS 175

300 .3JO 3ltO 360 380

1. Light gas oil 2. Heav!;J gas oil

3. Light paraffin distillate 4. /1edium paraffin distillale

5 Heavy paraffin distillate 6. Gaudr.an

7. t1irferaf ail

A,[xC = 260 nm

400 420 It!tO 460 480 nm Fig. 6. Emission spectra of mineral oil and mineral oil fractions.

Fluorescent light intensity is measured at an excitation wavelength of 260 nm or 280 nrn, and at an emission wavelength of 330-335 nm.

Mineral oil can be used as standard comparing solution. If the oil content of the waste water investigated is higher than 5 ppm, the extract in n-heptane must be diluted.

Oil concentration can be calculated with formula (1), if the fluorescent light intensity of the n-heptane solution of 10 fig/ml mineral oil concentration is adjusted to 100% on the instrument.

I.E·

c= 1.07.--ppm 10.V

c is the oil concentration of the waste water in ppm, I is the-fluorescent light intensity,

E is the volume of the extract in ml, V is the volume of the waste water in ml.

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176

f 70

60

50

40

30

20

10

300 320 3ltO

I. KAsA and G. BAJN6CZY

360 380 400

1. Light gas all 2. Heavy gas oil

3. Light paraffin distillate

1,. l1edium paraffin distillate 5. Heavy paraffin disliflate

>.6.. Goudran 7. Mineral oil

.AE}(c=,24D nm

420 ^1,60 ⁴⁸⁰

100 ⁸⁰

/

20

o

² ³ ^{4 5} ⁶ ⁷ ⁸ ⁹ ¹⁰ ^ppm

500n:"!?

Fig. 8. Calibration curve for the quantitative determination of mineral oil. i'_Em= 280 nm;

lEx = 335 nm

With the empirical factor 1.07, extraction losses are taken into consideration.

At the given parameters, linearity in the organic solvent between concentration and light intensity is valid in the 1-10 ppm oil concentration range (Fig. 8).

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.lfISERAL OIL COSTEST IN WASTE WATERS 177 If the type of the oil causing the contamination is known, the standard deviation of the method is 12 % and the standard error of mean

+

4.4% in the measurement of a waste water with 3 ppm oil concentration.

Summary

The mineral oil content of various industrial waste waters can be extracted easily and

,~ith good efficiency with an appropriate organic solvent, thus e. g. with n.heptane. The oil content of the organic extract can be well determined by spectrofluorometry. The fluorescent emission spectra of mineral oil fractions have been studied at various excitation wavelength with the aim to determine that pair of excitation and emission wavelengths, at which the fluorescent light intensity of the single fractions differs least from each other.

It was found that the determination can be carried out in the case of different fractions with the smallest error, when the excitation wavelength is 260-280 nm, and the emission wavelength 330-335 nm. In the organic solvent, the relationship between oil concentration and fluorescent light intensity is linear in the 1-10 ppm concentration range.

If the type of oil causing the contamination is known, and an identical type of oil can be used for the plotting of the calibration curve, the standard deviation of the method is

±12%, and the standard error of mean ±4.4% in the measurement of a waste water with 3 ppm oil concentration.

References

1. KEIZER, P. D., GORDON, D. C.: J. Fish. Res. Board Can. 30, 1039 (1973).

2. LEVY, E. M.: Water, Air and Soil Pollution 1, 144-148 (1972).

3. GORDON, D. C., MICHALIK, P. A.: J. Fish. Res. Board Can. 28, 1912-1914 (1971).

4. LEVY, E. M., WALTON, A.: J. Fish. Res. Board Can. 30, 261-267 (1973).

5. BAUER, K., DREISCHER, H.: Fortschr. der Wasserchem. 10, 31-34 (1968).

6. NIETSCH, B.: Gas. Wasser, Wiirme 10, 66 (1956).

7. SEMENOV, A. D., STRADOMSKAJA, A. G., ZURIN.~, L. F.: Gig. Sanit. 35, 65 (1970).

Dr Imre K . .\.SA

Gabor BAJNOCZY } H·1521 Budapest