CURRENT RESEARCH Aerobic Photodegradat ion of Fe(III)- (Ethy lenedinit rilo)t et raacet at e (Ferric EDTA) Implications for Natural Waters Haines B. Lockhart, Jr.;
and Rose V. Blakeley
Health and Safety Laboratory, Eastman Kodak Co.,Rochester, N.Y. 14650
T o determine the extent of ferric EDTA removal from the environment by interaction with sunlight-i.e., photodegradation-the behavior of aqueous solutions of ferric1-14C-EDTA a t pH 4.5, pH 6.9, and pH 8.5 under irradiation from a wide spectrum 5500-W xenon arc lamp has been studied. Carbon dioxide, formaldehyde, N-carboxymethyl-N,N’-ethylenediglycine(ED3A), N,N’-ethylenediglycine (EDDA-N,N’), iminodiacetic acid (IMDA), N-carboxymethyl-N-aminoethyleneglycine (EDDA-N,N), N aminoethyleneglycine (EDMA), and glycine have been identified as the major photodegradation products. No nitrilotriacetic acid (NTA) was detected during photodegradation. The rate of photodegradation was pH dependent, being most rapid a t pH 4.5. At a light intensity of 4000-ft candles and an initial concentration of 0.0016M, Fe(II1)EDTA removal was complete after 24 hr of irradiation a t either pH 4.5 or pH 6.9, and after 32 hr at pH 8.5.
the method of Aspinall (8). Fe(III)-1-I4C-EDTA was prepared in situ by methods described below. Other chemicals used during these studies were reagent grade unless otherwise noted. All solutions were prepared in sterile distilled water. Static Photolysis of Fe(III)-1-14C-EDTA. Static photolysis of Fe(III)-1-14C-EDTA was performed in a modified 50-ml Pyrex Erlenmeyer flask. This was fitted with a downward curving 3/4-in.diameter side arm threaded to receive a scintillation vial containing a COz absorber of 1.0 ml of diethanolamine and 0.2 ml of water attached by a threaded Teflon sleeve obtained from Kontes Glass Co. This flask is shown in Figure 2.
HOOCCHp
CH2COOH
HOOCFHp
I(I-cH~-cH~-~
YH
H
HOOC~H,
HOOCCH, IMDA
ED3A
(iminadiacetic ocidl
(N-carboxymethyl- N. Y’-ethylenediglycinei
Fe(II1)-(ethylenedinitri1o)tetraacetate (ferric EDTA), through its use in photographic processing solutions and as a fertilizer, may be reaching the environment in a discharge of photographic wastes or in land runoff. Previous photodegradation studies on this very thermodynamically stable chelate have identified carbon dioxide, formaldehyde, and N-carboxymethyl-N,N’-ethylenediglycine(ED3A) as degradation products (1-4). This paper reports the identification of additional major degradation products N,N’-ethylenediglycine (EDDAN , N ’ ) ,N-carboxymethyl-N-aminoethyleneglycine(EDDAN , N ) ,iminodiacetic acid (IMDA), N-aminoethyleneglycine (EDMA), and glycine; the structures of these materials are given in Figure 1. These have been quantitated and their generation studied kinetically from pH 4 to pH 8, the pH range of natural waters ( 5 ) .
7
POOCCHp Hi-CH2-CHZ-i
FHpCOOH V-CH~-CHZ-Y H CHpCOOH EDDA-N N
CHpCOOH
EDDA-N,N’ ( U , N -ethylenediglycinei
( N-c~rboxymethyl-N-~mlnoethyle~~~lyclne)
HOOCYH, H I N-CHp-CH, -N H H EDMA
H
Y -CH&OOH
H Glycine
(N-aminoelnyleneglycine)
Figure 1.
Photodegradation products of Fe(ll1)-EDTA SERUM
/IIJ/SToPPER
Materials and Methods EDTA used in these experiments was obtained as the disodium salt from Eastman Organic Chemicals. EDDA-N,N’ was obtained from K & K Fine Chemicals. 1-14C-EDTA (specific activity 9.1 yCi/mg) was obtained from Mallinckrodt Chemical Works and diluted before use to a final specific activity of 0.011 pCi/mg and a concentration of 0.005M with NazEDTAm2HzO. ED3A was synthesized by the method of Genik-Sas-Berezowsky and Spinner (6), EDDA-N,N was prepared according to the method of Schwartzenbach et al. (7), and EDMA was synthesized by
/ Fe
Figure 2.
50ml
\
(m) - I - l 4 C - EDTA
C /-t
ABSORBER
Static photodegradation flask Volume 9, Number 12, November 1975
1035
E33A
4-amino butyric acid
(STDI
Time (minutes1
Typical gas chromatograph of the photodegradation products of Fe(ll1)-EDTA Figure 3.
Eight replicates were prepared for each assay. Five ml of the 1J4C-EDTA solution was added to each flask followed by 5 ml of pH 4.5 buffer (0.05M sodium acetate). Five ml of 0.005M FeC13 was then added to each flask, the flask sealed with a serum stopper, and irradiated a t 4000 ft-candles by a 5500-W xenon arc lamp fitted with a uv transmissive filter ( 1 ) . Flasks were removed successively a t 0, 4, 8, 24, 32, 48, 72, and 96 hr. Flask contents were adjusted to 20 ml with distilled water, and aliquots removed for gas chromatographic analysis, formaldehyde analysis, and bacterial counts. Similar methods were used to study Fe(III)-1-14CEDTA photolysis a t pH 6.9 (0.05M sodium phosphate) and pH 8.5 (0.05M ammonium chloride). Radioactive Counting. All samples were counted by liquid scintillation spectrometry on a Tri-Carb Scintillation Spectrometer in a cocktail of toluene: 2-methoxy-ethanol-2:l containing 0.5% P P O and 0.1% POPOP. The cocktail was added directly to the water-diethanolamine I4CO2 absorber in static photodegradation experiments. Coefficients of variation were f 6 % for all samples analyzed. Formaldehyde Analysis. Formaldehyde in photolysis solutions was assayed by the chromotropic acid method (9) of known specificity for formaldehyde (IO). Bacterial Counts. T o ensure the absence of bacterial interference during the photodegradation of Fe(111)-EDTA, portions of photolysis samples were plated on nutrient agar to analyze for viable bacteria. Preparation of N-Trifluoroacetyl-n-Butyl Esters of EDTA and Photodegradation Products. The method used for the preparation of N-trifluoroacetyl-n-butyl esters of Fe(II1)-EDTA and its photodegradation products was adapted from Zumwalt et al. ( 1 1 ) . One to 800 kg of EDTA, in 0.95 ml of water, was mixed with 1 ml of 0.0025M ferric chloride in pH 4.5 0.025M sodium acetate in a 2-dram vial. After adding 0.05 ml of 1M NaOH, the vial was capped and heated at 100°C for 30 min. After it cooled, the mixture was filtered through a 0.25-wrn filter, a 0.5-ml aliquot was transferred to a 2-dram vial, 50 wg of internal standard (4aminobutyric acid) and 0.25 ml of 6M HCl were added, and the mixture was taken just to dryness a t 6OoC. The n-butylation of the EDTA samples treated as described above was performed with 1.4 ml of 3 M HC1 (Air Product's electronic grade) in anhydrous n-butanol added to the reaction vial. The vial was capped with a Teflonlined screw cap, agitated vigorously on a vortex mixer for 30 sec, ultrasonically stirred for 5 min in an Ultramet I1 sonic bath, and then heated a t 100°C for 30 min. After cooling for 5 min, excess HC1-n-butanol reagent was removed a t 6OoC under nitrogen. n-Butyl esters of EDTA were acylated by the addition of 1 ml of 25% trifluoroacetic anhydride in CHzCl2 in the same vial. Vials were capped, 1036
Environmental Science & Technology
agitated on a vortex mixer for 30 sec, sonicated for 5 min, and left a t ambient temperature for 1 hr. Excess reagent was removed under nitrogen, the dry sample taken up in 0.2 ml of CHZC12 and agitated for 30 sec on a vortex mixer. Five-wl. portions were removed and injected onto a gas chromatographic column (see below). A standard curve of ratio peak areas (EDTAI4-aminobutyric acid) vs. rug EDTA was prepared. Standard curves of the N-trifluoroacetyl-n-butyl esters of ED3A, EDDA-N,N', EDDA-N,N, IMDA, nitrilotriacetic acid (NTA), EDMA, and glycine were prepared similarly. Aliquots from static photodegradation experiments with Fe(II1)-EDTA a t pH 4.5 and p H 8.5 were treated with 1M NaOH and derivatives prepared as above. Aliquots from photodegradation experiments performed a t pH 6.9 were treated with sufficient saturated Ca(OH)2 to precipitate both the Fe(II1) and (Po4)-3 from solution before derivatives were prepared. Duplicate samples were analyzed in all cases. Gas Chromatography. Gas chromatographic analysis of the N-trifluoroacetyl-n-butyl esters of Fe(II1)-EDTA and photolysis products was performed on a 4-ft X Ys in. i.d. glass column of 3% QF-1 on 100/120 mesh Gas-Chrom-Q using a Hewlett Packard Model 7610A gas chromatograph with flame ionization detectors, with nitrogen as the carrier gas. Oven temperature was programmed from 110-260°C a t 10°/min, starting a t 2-min postinjection and holding for 2 min a t 260OC. Injection temperature was 25OoC, and detector temperature was 300OC. Figure 3 shows a typical chromatogram of the photolysis products from Fe(II1)EDTA. Lower limits of detection (0.5 ml total sample size) were: EDTA and ED3A, 4 pg/ml; EDDA-N,N', EDDAN,N, IMDA, NTA, EDMA, and glycine, 2 pg/ml. Coefficients of variation were no greater than f 5 % for duplicate analyses of each compound tested. Combined Gas Chromatography-Mass Spectrometry. The N-trifluoroacetyl-n-butyl esters of EDMA and EDDA-N,N, as photodegradation products of Fe(II1)EDTA, were identified by fragmentation pattern analysis (12) on a Varian 1800 gas chromatograph coupled via a molecular separator to an AEI-MS-30 double beam mass spectrometer. The identities of the other photodegradation products were determined by comparison to reference materials whose mass spectrometric properties had been established by Belly et al. ( 1 3 ) .
Results Figure 4 compares the rates of l4COZevolution from the photodegradation of Fe(III)-1-14C-EDTA a t pH 4.5, pH 6.9, and pH 8.5. Photodecarboxylation of Fe(III)-1-14C-
Hours
Concentration of Fe(lll)-1-14C-EDTAand percent initial volatilized (4000 ft-candles) Figure 4.
14C
EDTA was pH dependent, with the greatest amount of 14C02evolution occurring when photolysis was performed at pH 4.5. Similarly, the rate of Fe(II1)-EDTA disappearance as determined by gas chromatography was most rapid at pH 4.5, as shown in Figure 4; no Fe(II1)-EDTA was detectable after 24 hr of irradiation at pH 4.5 or p H 6.9, and none was detectable after 32 hr of photolysis a t pH 8.5. Figure 4 also shows that significant photodecarboxylation was evident even after Fe(II1)-EDTA was no longer detectable at all the pH values tested. EDSA, EDDA-N,N', EDDA-N,N, EDMA, IMDA, and glycine were identified as photodegradation products by GC-MS or by comparison with known materials. Formaldehyde was identified as a photodegradation product a t all pH levels studied but was not quantitated since the amounts of formaldehyde in the C02 absorbers could not be measured. The zero time Fe(II1)-EDTA samples contained, by gas chromatographic measurements, 0.0016M EDTA, 0.0002M ED3A, and 0.00003M NTA. However, no additional NTA was detected during any gas chromatographic analysis run during the course of the experiment nor were any other unidentified peaks found on the gas chromatograms. Bacterial counts were less than 5/ml for all the samples analyzed. The changes in concentrations of the various photodegradation products of Fe(II1)-EDTA vs. time are given in Figure 5. After 96 hr of photolysis a t pH 4.5 no ED3A, EDDA-N,N', or EDDA-N,N was detectable; EDMA and lesser amounts of IMDA and glycine were the only photodegradation products remaining. All photodegradation products remained after 96 hr of Fe(II1)-EDTA photolysis at pH 6.9; at this pH, IMDA and EDMA were found a t approximately the same concentrations a t the end of the ex-
I
I
ED3A
EDDA-N, N
I-
i
periment. At pH 8.5, the photolysis of Fe(II1)-EDTA gives rise to all the photodegradation products found a t p H 4.5 and pH 6.9, but a t a slower rate than at the lower pH values. Comparisons of the concentrations of EDDA-N,N' and EDDA-N,N vs. time a t any p H value tested (Figure 5) indicated that within experimental error ( f 5 % ) there was no preference for the generation of one of these two EDDA isomers over the other as a photolysis product of Fe(II1)EDTA. The material balance in terms of 96 nitrogen atoms and % carbon atoms recovered after 96 hr of Fe(II1)-EDTA photodegradation a t the pH value studied is given in Table I. These calculations used the accepted photodegradation product stoichiometry during decarboxylation of C02: CHzO = 1:1 ( I , 3, 4 ) to account for any CH20 produced. The percent recoveries indicate that all major photodegradation products have been identified during these experiments. ~-
Table I. Ninety-six-Hr Materials Balance for Photodegradation Products of Fe(1ll )-1-14C-EDTA PH
% C atoms (CO, + CH,OP
% C atoms, GC
recovered
% N atoms, GC
4.5 6.9 8.5
54 42 35
40 54 56
94 96 91
94 92 90
a CH,O
value calculated from C O , data-ee
Total % C atoms
text.
Discussion and Conclusions These studies on the photodegradation of Fe(II1)-EDTA in aqueous solution have given insight into its fate when discharged to the environment. While photodegradation is rapid, the rate is pH dependent, with the most rapid degradation occurring at p H 4.5 where EDTA was undetectable after 24 hr a t 4000 ft-candles. At p H 8.5, the highest pH tested, complete removal of EDTA required 32 hr of irra: diation. This indicates a significant photosensitivity of Fe(II1)-EDTA over the pH range of natural waters. The overall reaction for the aerobic photodegradation of Fe(II1)-EDTA has been proposed ( I ) :
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