Photochemical transformation in aqueous solution and possible environmental fate of ethylenediaminetetraacetic acid (EDTA)

ECOTOXICOLOGY AND ENVIRONMENTALSAFETY 19,55-63 (1990)

Photochemical Transformation in Aqueous Solution and Possible Environmental Fate of Ethylenediaminetetraacetic Acid (EDTA)’

RUDOLF FRANK AND HERMANN RAU

Universitiit Hohenheim. Institut fir Chemie 130, Fachgebiet Ph.vsikalische Chemie, 7000 Stuttgart 70, Federal Republic of Germany

Received April IO, I989

INTRODUCTION

Ethylenediaminetetraacetic acid (EDTA) is used in a large variety of products, e.g., laundry detergents, cosmetics, photochemicals, and pharmaceuticals, and in indus- tries, e.g., textiles and galvanic and paper manufacturing. The content of EDTA in laundry detergents varies between 0.1 and 0.5%. For the FRG, therefore, a consumption of 1000 to 5000 t/u can be estimated for this purpose alone. Laundry detergents account for an estimated 20 to 30% of the total EDTA consumption which thus may amount up to 25,000 t/u.

Because EDTA is not volatile, it is released mainly with waste water streams. IE waste water treatment plants EDTA is neither transformed by microorganisms nor adsorbed to the sewage sludge (Gardiner, 1976; Means et al.. 1980). Taking into account the figures mentioned above and the amount of waste water of the FRG, concentrations of 50-500 pg/liter can be estimated for waste waters. Due to dilution, environmental concentrations in German rivers are expected to be lower by a factor of 1-3. Measured concentrations in natural waters were reported to range from 10 to 70 pug/liter, with a median value of 23 pg/liter (Dietz, 1984). These figures are confirmed in reports of the Arbeitsgemeinschaft Rhein-Wasserwerke e. V. (ARW, 1986) and the Eidg. Anstalt fur Wasserversorgung Abwasserreinigung und Gewasserschutz (EAWAG, 1987). Comparison of estimated and measured environmental concentrations confirms that most of the EDTA used is released to surface waters without chemical transformation.

Analytical determinations of EDTA do not provide information about the chemical bonding of EDTA in surface waters (Reichert et al.. 1980; Linckens and Reichert, 1982). Because it is a strong complexing agent, it seems appropriate to assume that it is present in the form of its metal complexes. Taking into account the pH-dependent effective complexing constants (Kober, 1979; Gmelin, 1987) and the concentrations of metals (including Ca and Mg) in surface waters (Bayerisches Landesamt, 1983; Landesanstalt, 1983), it can be estimated that most of the EDTA should be present in the form of Fe(II1) complexes. On the other hand, most of the iron in surface waters is present in the form of colloidal and amorphous iron hydroxides and therefore may not be complexed by EDTA. Calculations for nitrilotriacetic acid (NTA), therefore, lead to the conclusion that most of this compound in surface waters occurs in the form of its copper complexes (NTA, 1984). However, the Fe(III)-EDTA complex is

’ This work has been presented in part at the XII IUPAC Symposium on Photochemistry in Bologna, July 1988.

55 0 147-65 13/90 $3.00 Copyright 0 1990 by Academic Pres$ Inc. AI, ;A.1̂ ^F-̂ __̂ A..̂ *:̂ _ :.. ^_.. c-...- _̂ _̂ -.-J

56 FRANK AND RAU

the most stable EDTA complex (pH 7) and EDTA can react with Fe(II1) already at the waste water treatment plant. This possibility will be especially enhanced in the future when an increasing number of these plants will be equipped with phosphate eliminating stages.

As mentioned above, for the most part EDTA is not transformed by aerobic microorganisms in waste water treatment plants and transformation by microorganisms in nature is extremely slow also. For the removal of EDTA from surface waters photochemical reactions and reactions with other reactive species such as OH, singlet oxygen, H202, etc., therefore could be the only sinks.

In earlier publications, ethylenediaminetri-, di-, and monoacetic acid have been identified as photo products (Lockhart and Blakeley, 1975). Published quantum yields for FeEDTA range from 0.003 to 0.35 (Carey, 1973, 1975; Natarajan and Endi- cott, 1973). In these experiments, the FeEDTA concentrations have been much higher than environmental concentrations. Because these reports hint that the quantum yield depends on pH, excitation wavelength, and concentration it seemed appropriate to study the kinetics of the photoreaction in detail at concentrations similar to environmental concentrations. A further aim of this work is to estimate the possible environmental fate of EDTA and calculate its environmental half-life.

EXPERIMENTAL METHODS

EDTA sodium salt, analytical grade, was purchased from Merck and used without further purification. Water from an ion exchanger was doubly distilled in a quartz apparatus.

The Fe(II1) complexes form immediately after mixing of equimolar solutions. Most of the kinetics experiments were performed in 0.1 MNaC104 solutions but the results of single experiments in sodium acetate and in pure water did not deviate from the results in NaClO, .

For the irradiation with monochromatic light, single lines from a high-pressure mercury lamp (Philips HPK 125) were separated with precision interference filters (Schott UV-PIL Filter). The irradiation intensities were determined using the azobenzene actinometer (Gauglitz 1976; Gauglitz and Hubig, 1981). For uv measurements a spectral photometer (Zeiss DMR 10) was used.

Water samples from the Neckar river were taken from the surface. All samples were clear although the river appeared to be dark. The optical transmission of the samples was measured within approximately 30 min after taking the sample in a spectrophotometer2 No sedimentation of dissolved material could be observed visually.

RESULTS AND DISCUSSION

Reactive Species

The uv absorption spectrum of FeEDTA shows a maximum at 258 nm in acidic solution and at 242 nm in basic solution. The titration spectrum is shown in Fig. 1. The isosbestic point at 244 nm reveals that within the tested pH range (2.5- 10.5) and the high dilution (ca. 1 X 10e4 M) only two differently protonated forms of the FeEDTA complex can occur. The formation of dimers, which has been

’ The optical transmission data of the Neckar river are available as ASCII files. If you are interested in these data please send a formatted disc (5.25” MS-DOS) to the authors.

PHOTOCHEMICAL TRANSFORMATION OF EDTA 57

FIG. 1. Titration spectrum of FeEDTA. In acid (pH 2.5) solution the absorption maximum at 258 nm; in basic (pH 10.5) solution the maximum is situated at 242 nm.

is situated

observed previously (Schugar et al., 1969) and which can cause concentration- dependent quantum yields, is observed only at higher concentrations. At environmental concentrations, therefore, only photoreactions of the monomeric complexes are to be expected. In FeEDTA the EDTA trianion is active as a pentaden- tate ligand and the sixth coordination position in the Fe(II1) ion is occupied by a water molecule. In basic solutions this water molecule is assumed to lose its pro- ton (Schugar et al., 1969). The pK, value of the complex is 7.5. In natural surface waters, the neutral as well as the negatively charged complex can appear as the photoreactive species.

The ground state of the FeEDTA complex is a sextett. Hence, d-d absorption bands are not expected. This is confirmed by the location of the first absorption band in the spectrum at 258/242 nm. This absorption is attributed to a ligand to metal charge transfer state (Schugar et al., 1969).

Determination of Photochemical Quantum Yields

Quantum yields have been determined in dilute solutions ranging from 0.04 X lop4 to 1.7 X 1O-4 A4 at various pH values in oxygen-free (argon-saturated) and air-saturated solutions. The solutions were irradiated with monochromatic light at 3 13, 366, and 405 nm. A typical reaction spectrum is shown in Fig. 2. Absorption diagrams (Mauser, 1974; Rau, 1989 [appendix]) reveal that FeEDTA is the only light- absorbing substance in the wavelength interval depicted in Fig. 2. Therefore, the quantum yield can be determined using Eq. (1) (Mauser, 1974).

In A@, t) = In A(X, to) - 103Z,& l: ’ -fp^ dt.

58 FRANK AND RAU

0.8 _

0.6 _

250 3oo A/rim

FIG. 2. Reaction spectrum of FeEDTA at pH 4. Irradiation wavelength is 3 13 nm.

With

A(X, t) = Absorbance of FeEDTA at wavelength X and time t.

A’ = Absorbance of FeEDTA at the irradiation wavelength at time t.

IO = Irradiation intensity in mol photons cm-* set-‘. $ = Quantum yield of FeEDTA at the irradiation wavelength. t> = Molar, decadic absorption coefficient of FeEDTA at the irradiation wave-

length in M-’ cm-‘. t = Irradiation time in sec.

Figure 3 shows a typical diagram of In A(X, t) versus the integral of Eq. (1). The integral is stepwise numerically evaluated from the measured values of A’ and t. The quantum

200 LOO 600 800

FIG. 3. Plot of In A(258, f) versus the integral of Eq. (1).


TABLE 1

OPTICAL ABSORPTION COEFFICIENTS OF FeEDTA IN M-’ cm-’ AT THE IRRADIATION

WAVELENGTHSATVARIOUS~HVALUES

PH c313 .5366 t405

2.5-6.2 4300 4.0 4300 800 150 6.8 4030 7.55 3360 8.0 2950 450 75 8.4 2750 9.2 2590 9.5 2590

10.3 2550

yield is then determined from the slope of these diagrams and measured values of the irradiation intensity and the optical absorption coefficients. The absorption coefficients used for the calculations are given in Table 1. The quantum yield was determined as a function of pH and irradiation wavelength. The results are given in Figs. 4 and 5.

Due to the various errors which occur in determining the slope, the irradiation intensity, and the absorption coefficients the error bars in these figures are estimated by error propagation. Figure 4 shows the quantum yield as a function of pH when light of 3 13 nm is used for irradiation. In oxygen-free solution, a clear difference in the quantum yield of the protonated and deprotonated FeEDTA complex can be recognized. The quantum yield for the protonated complex is 0.133 and for the deprotonated complex 0.07. In the presence of oxygen, this difference is less pronounced, which can be due to various reasons. In the FeEDTA complex excitation by light causes an electron transfer from the ligand to the central Fe(II1). The primary photoproducts from this molecule can be an Fe(I1) ion and an alkyl-radical. Accord-

I 5 10 pH

FIG. 4. Quantum yield of FeEDTA versus pH. Irradiation wavelength is 3 13 nm. Open circles, oxygen- free (argon-saturated) solutions; filled circles, air-saturated solutions.

60 FRANK AND RAU

19 t ‘\

‘\ ‘\

‘\ 0.10 _ '1

'\ '\

+, "l,, '\

'\ _" 0. '\ 0.05 - '. '\

'\ '\ '. '\

'\ 4 '\

-\ '. '.

'.

-;

300 350 LOO A /nm

FIG. 5. Quantum yield of FeEDTA versus irradiation wavelength in oxygen-free solutions. Open circles, at pH 4; filled circles, at pH 8.

ing to Carey and Langford (1973, 1975) this radical can be stabilized by reaction with another FeEDTA. The reaction therefore could reach a limiting quantum yield of 2. This stabilization reaction becomes less probable in highly diluted solutions and certainly has no significance in environmental photochemistry. If the reaction pro- ceeds via the suggested path then lower quantum yields should be observed, if there are other molecules in the solution (radical scavengers) with which the primary alkyl- radical can react.

Typical radical scavengers such as ethanol, methanol, and 2-propanol in concentrations up to 0.3 A4 and excess EDTA lower the quantum yield only slightly by approximately 20%. But in the presence of these compounds the photo reaction becomes less reproducible and the resulting quantum yields show a larger scatter. The addition of saccharose does not change the quantum yield.

Oxygen also known as an effective radical scavenger lowers the quantum yield to a much larger extent as can be seen in Fig. 4 and the difference in the reactivity of the protonated and deprotonated complex is less pronounced in the presence of oxygen. Oxygen can change the quantum yield by at least two mechanisms. It can react with the primary alkyl-radical and thereby prevent the reaction of this radical with a further FeEDTA molecule and it can physically quench the excited FeEDTA complex and thereby prevent the primary photo reaction. It can further reoxidize the Fe(I1) formed in the primary reaction to Fe(II1). Because the Fe(II1) complexes with EDTA are much more stable than the Fe(I1) complexes, this can be true for ethylenediamine- triacetic acid and the various diacetic acids which are formed by the photoreaction also. At environmental concentrations, these reactions will be of no concern. But in laboratory experiments the presence of oxygen causes the experiments to become less reproducible. Comparison of the effects of the various radical scavengers on the quantum yield shows it is probable that the effect of oxygen is due to the physical quenching of the excited FeEDTA complex as well as to the reaction with the primary alkyl-radical. The importance of this effect with respect to an assessment and the estimation of environmental life times of EDTA will be discussed later.

Figure 5 shows the dependence of the quantum yield on the irradiation wavelength at pH 4 and pH 8. Contrary to organic molecules both complexes exhibit a depen-


dence of the quantum yield on irradiation wavelength with a clear decrease in the quantum yield with increasing irradiation wavelength. In organic molecules, photoreactions normally start from the lowest, clearly isolated singlet or triplet electronic states; this is known as Kasha’s rule. In metal complexes, there exist a variety of states which are situated close to one another in energy and which cannot be excited separately. Due to their electronic structure, metal complexes therefore more often exhibit a quantum yield which depends on the irradiation wavelength. However, for FeEDTA another possibility exists to explain this behavior (Natarajan and Endicott, 1973). Natrajan and Endicott suggest that in FeEDTA the photon may be absorbed either by the ligand or by a charge transfer absorption. Whereas the charge transfer state is photoreactive, the ligand excited state is not. According to this suggestion the dependence of the quantum yield should be due to an innermolecular filter effect.

Assessment of the Possible Environmental Fate of EDTA

The transformation of EDTA in natural waters can occur via the photoreaction of the FeEDTA complex. If the quantum yield of this complex, its optical absorption spectrum, the optical transmission of the natural water, and the solar irradiance are known, the rate of this transformation can be calculated. For the performance of these calculations the model of (Frank and Kliipffer, 1989) was used. Quantum yield and absorption spectrum of FeEDTA depend on the pH of the natural water. Oxygen content and the concentration of other substances of the natural water can influence the rate of the photochemical reaction. The differences in rate caused by these factors are of the same order of magnitude as those which are caused by the variations in the environment, mainly due to the variation of the solar it-radiance. Taking this into account and trying to get conservative values, for the performance of estimations, it seems to be appropriate to take optical absorption coefficients which belong to the FeEDTA spectrum at pH 7, quantum yields which belong to the same pH value, and an oxygen content which is near the air-saturated value. For the German river Neckar, the authors collected a relatively large number of optical transmission spectra over a relatively long time period. These transmission data were used for the performance of calculations. Different results for the same months reveal the variation of the environmental half-life which is due to the changes in the optical transmission of the river. The results of the calculations are compiled in Table 2.

For all calculations the starting concentration of FeEDTA was set at 8 X 10P6 h4. The depth of the natural water was set at 2 m and the flow rate to 0.5 msec~‘. The half-lives in Table 2 are given for averaged maximum, mean, and minimum solar irradiance in central Europe. The flow distance in Table 2 is the mean distance from the emission source within which the starting concentration decreases by a factor of 2. The results compiled in Table 2 show that FeEDTA can be transformed in rivers in central Europe within relatively short times by photochemical reactions during the summer season. In winter, transformation times become so long that most of the FeEDTA will reach the sea. Due to the higher optical transmission of the seawater, photochemical transformation rates in these waters seem to be higher than in rivers. However, the much higher depth of the sea is not taken into account in the calcula- tion given in Table 2. Comparison of calculated transformation rates and measured environmental concentrations in rivers is not yet possible to a large extent, because there are only a limited number of data sets available. The still limited published data of EDTA concentrations (EAWAG, 1987; ARW, 1986), which extend over a longer

62 FRANK AND RAU

TABLE 2

ESTIMATED HALF-LIFE OF FeEDTA IN THE GERMAN RIVER NECKAR IF NOT STATED OTHERWISE

Half-life/hr

Month, year Min Mean Max Flow distance/km

January 1988 1989

February 1988 March 1989 April 1988 May 1988 June 1988 July 1988 August 1987

1987 1988

September 1987 1988

October 1987 1988

November 1988 December 1988

Kieler F6rde March 1985

240 47 22

8

6 9

4 4 6

14 12 25 18 39 84

3 6 27

480 1600 98 410 46 190 16 70 12 46 10 38 13 51 10 34

5 18 6 20

10 31 24 87 19 70 46 200 33 150 87 400

180 770

860 180 82 29 21

:3 19 10 11 17 44 35 83 60

160 320

Note. For details see text.

time period, seem to reveal that higher concentrations occur during the winter season; this seems to be due not merely to the lower amount of water in the rivers during winter. The reason for the lower EDTA concentrations observed in summer, therefore, could be photochemical transformation.

Other abiotic transformation processes for FeEDTA could be reactions with OH radicals and singlet oxygen (‘0,). For the reaction with OH radicals, rate coefficients between 4.8 X IO8 and 15 X IO* M-’ set-’ have been reported (Farhataziz and Ross, 1977; Lati and Meyerstein, 1978). The steady-state concentration of OH radicals in natural waters can reach values of lo-l6 to 10-l’ M (Haag and Hoignh, 1985). That means that abiotic reactions with OH radicals could become important if the quantum yield of FeEDTA were as low as 10e5. This estimate is valid for FeEDTA, which has relatively low absorption coefficients in the solar spectrum. For other substances with higher absorption coefficients, this threshold would be even lower.

Rate coefficients for FeEDTA with singlet oxygen have not been published. However, a limiting value for rate coefficients of singlet oxygen with molecules containing electron- rich double bonds seems to be 10’ AC-’ set-’ (Wilkinson and Brummer, 198 1); for other molecules this value is still lower. The concentration of singlet oxygen in natural surface waters is ca. 1 O-l4 M(Scully and Hoigne, 1987). Reactions with singlet oxygen, therefore, will not become important as long as the quantum yield is higher than approximately 10m4; for substances with absorption coefficients in the solar spectrum higher than those of FeEDTA, this limiting value again would be even lower.


CONCLUSIONS

When EDTA is released with waste water streams, removal will not occur in the waste water treatment plants. In natural waters, EDTA in the form of its differently protonated Fe(II1) complexes can be transformed by photochemical reactions. In central Europe, these reactions can be relatively effective in summer but during winter most of the EDTA will reach the sea. Other abiotic processes are not likely to contribute to the degradation of EDTA.

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Photochemical transformation in aqueous solution and possible environmental fate of ethylenediaminetetraacetic acid (EDTA)