Environ. Sci. Techno/. 1995, 29, 2992-3000
Photodegradation of EDTA in the Presence of Lepidocrocite G E 0RGI 0S KARAM ETAXAS, STEPHAN J . H U G , A N D B A R B A R A SULZBERGER* Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Dubendorf, Switzerland
Based on laboratory experiments combined with kinetic modeling, w e propose a conceptual model for the photodegradation of initially uncomplexed EDTA in the presence of y-FeOOH (lepidocrocite), as follows: Free EDTA becomes adsorbed a t the surface of y-FeOOH and is initially photooxidized as a surface species. Thereby, y-FeOOH is reductively dissolved. Our results suggest that photooxidation of adsorbed EDTA, coupled to reductive dissolution of y-FeOOH, occurs through photolysis of the FeIllEDTA surface complex. The photochemically formed Fe(ll) then catalyzes the thermal dissolution of the solid phase in the presence of EDTA. This process results in production of dissolved FelIlEDTA, which is subsequently photolyzed. Hence, in these heterogeneous systems, initially uncomplexed EDTA is photooxidized via two pathways: (i)photooxidation a t the surface of y-FeOOH and (ii) photolysis of dissolved FelIlEDTA that is formed in the Fe(ll)-catalyzed dissolution of y-FeOOH. Which pathway predominates depends on the relative rates of Fe(ll) oxidation and of Fe(ll)catalyzed formation of dissolved FelIlEDTA. At pH 3, photooxidation of EDTA occurred predominantly through photolysis of dissolved FeIllEDTA, whereas a t pH 7, photooxidation of adsorbed EDTA was more important in our aerated heterogeneous systems, because of the faster Fe(ll) oxidation at pH 7, compared to pH 3. Our results indicate that not only dissolved Fell'EDTA but also FeIl'EDTA surface complexes are efficiently photolyzed.
Introduction Ethylenediaminetetraacetate(EDTA)is a powerful chelating
agent being widely used for the complexation of metal ions interfering with industrial processes, e.g., photographic developing, paper production, and textile dyeing. Since its biodegradation is very slow, little EDTA is removed by wastewater treatment. Considerable EDTA concentrations have been found in sewage effluents, surface waters, groundwaters, and drinking waters (1-4). The environmental concern surrounding the presence of EDTA in natural waters is its potential to mobilize heavy metals * . 4 ~ t h o rto whom correspondence should be addressed.
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bound to surfaces of particulate and colloidal matter. Furthermore, complexation of Fe(II1) by EDTA has implications for Fe(II1)hydrolysis equilibria (3,a major concern for the uptake of iron by phytoplankton (6, 7). The fate of EDTA in natural waters will depend on its speciation. Although FeII'EDTA complexes are among the most stable in the aquatic environment (81, equilibrium calculations of EDTA speciation predict that dissolved Felt'EDTAspecies may be unimportant compared to other metal EDTA complexes, because of the insolubility of iron(II1) (hydrloxides at near-neutral pH (9). For example, equilibrium calculations of EDTA speciation under the conditions of the Glatt River, Switzerland, suggest that ZnEDTA is the most important EDTA species in the pH range between 7.5 and 8.5 (9). (The abbreviation "EDTA" is used here independently of the degree of protonation. For this reason, the charges of EDTA and of the metal EDTA complexes are not specified.) In seawater, the major EDTA species are its Ca and Mg complexes (5). In order to predict EDTA speciation in a natural water system, not only the thermodynamics but also the kinetics of EDTA complex formation have to be considered because of the slow exchange kinetics of metal EDTA complexes (5, 9, 10). Therefore, the fate of EDTA in natural waters will depend not only on the water composition but also on the chemical form of the EDTA input. If its release into a surface water is in the form of dissolved FeII'EDTA, e g , from sewage treatment plants where iron(II1)salts are used for phosphate precipitation, it is, in part, photochemically degraded by photolysis of Fe"IEDTA (2,1I ) , a photoredox reaction with relatively high quantum yields (12, 13). The question arises about the fate of EDTA if its input to a natural water body does not occur as FellIEDTA. Uncomplexed EDTA has a strong tendency to adsorb at the surface of iron(II1) (hydr)oxides (14). Photoreductive dissolution of these oxides by EDTA as a reductant has been demonstrated (15, 16). In this paper, we discuss photooxidation of EDTA in the presence of lepidocrocite (y-FeOOH), an iron(II1) hydroxide commonly found in natural surface waters. Specifically, we investigate the effects of pH and of 0 2 on the pathways of EDTA photooxidation in such heterogeneous aquatic systems.
Experimental Section Materials. All chemicals were analytical or HPLC grade, and the solutions were prepared with bidistilled water. pH measurements were carried out with a combined glass electrode (Metrohm) standardized with pH buffer solutions (Merck). Ethylenediaminetetraacetate (EDTA) was obtained as a 0.1 M Na2EDTA standard solution from Merck AG (Titrisol). 1ron"'EDTA solutions were prepared by mixing solutions of Na2EDTA and FeC13-6H20 in a molar concentration ratio of 1:l. Lepidocrocite (y-FeOOH)was prepared as described by Schwertmann and Cornel1 ( 1 7). The specific surface area as determined by BET measurements was 78 m2 gl. X-ray powder diffraction patterns examined with a high-resolution Guinier-NONIUScamera (Mark I Y and Feal radiation were identical with a pure lepidocrocite standard.
0013-936X/95/0929-2992$09.00/0
E 1995 American Chemlcal
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Analytical Methods. The concentrations of dissolved Fe(I1) were determined colorimetrically with ferrozine by measuring the absorbance of the iron(I1)-ferrozine complex at 562 nm according to a modified method described by Stookey (18). A total ofO.1-1.0 mL ofthe sample was added to 60 pL of acid solution (3.6 M H2SO4). Then 40 pL of the ferrozine reagent (20 mM), 100 pL of 2 M NH4F [to mask Fe(III)],and 1OOpL of acetate buffer (193 g of ammonium acetate and 170 mL of 25% NH40H/500 mL of solution) were subsequently added. Bidistilled water was added to give a final volume of 1.5 mL. Absorbance at 562 nm was measured using a 5-cm small-volume cell. IronII'EDTA concentrations were determined by HPLC with U V absorption detection at 258 nm. As the mobile phase, an M tetrabutylammonium bromide, aqueous solution of containing 5% by volume of acetonitrile, was employed. A pH of 3.5 of the mobile phase was maintained by using formate buffer. The concentrations of formaldehyde (CH20) were determined colorimetrically as described by Smith and Erhardt (19). This method is based on the reaction of formaldehyde with acetylacetone and ammonia (Nash reagent) to form 3,5-diacetyl-1,4-dehydrolitidine (Hantzsch reaction). The formed compound has an absorbance with a maximum at 415 nm [ E (415 nm) = 8100 M-1 cm-']. After the addition of 1 mL of the Nash reagent to 1 mL of the sample, the mixture was kept at 60 "C for 45 min for the Hantzsch reaction to be completed. The absorbance at 415 nm was measured after the solution had cooled to room temperature, using a 5-cm photometric small-volume cell. Experimental Procedure. An experimental setup suited for the study of photoredox reactions with heterogeneous systems (20)was used. The light source was a 1000-Whighpressure xenon lamp (OSRAM). The experiments were carried out with polychromatic light. Lenses of optical BK7 glass were used, which transmit light above 300 nm. As an IR filter, a 10-cm H20 cell was employed, which cuts off light above 950 nm. The incident intensity was about 0.5 kW m-2, as measured by a compensated thermopile. The thermopile measured the integrated light intensity between 300 and 950 nm. Over the course of the experiments, the indicated light intensity of 0.5 kW m-2 remained stable within f20%. All the experiments were carried out at constant temperature (25 i~ 1 "C) by using a Pyrex glass vessel with a water jacket and a Pyrex glass bottom window. The reactionvolume was typically300mL,and the irradiated surface areawas 50 cm2(Pyrexwindow). The ionic medium used was M NaC104. In the experiments under N2, the suspensions were purged with N2 that had previously passed a Jones' reductor. The suspensionswere vigorously stirred throughout the irradiation experiments in order to prevent settling of the particles. The y-FeOOH suspensions were allowed to equilibrate at pH 3 or pH 7 for 3 h in the dark at 25 f 1 "C prior to the addition of Na2EDTA and the initiation of irradiation. The pH was adjusted with HC104 or NaOH. For the determination of the concentrations of dissolved Fe(II), dissolved FeII'EDTA, and CH20, typically 5-mL aliquots were removed from the batch reactor and filtered through 0.2-pm pore size membrane filters (Sartorius). For the determination of the Fe(I1)concentrations, 0.1-1.0 mL of the filtered sample was mixed with 60 pL of 3.6 M H2S04, immediately after filtration (without protection from air). Each experiment was repeated two
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FIGURE 1. Concentrations of CHlO (O), dissolved Fe(ll) (m), and dissolved Fe"'EDTA ( 0 )in the course of irradiation of an aerated y-FeOOH suspension at pH 3, to which uncomplexed EDTA had been added prior to irradiation (time zero), yielding an initial concentration of M. Experimental conditions: 0.05 g L-l y-FeOOH; ionic strength, M (NaC104); temperature, 25 "C; light source, white light from a 1OOO-W high-pressure xenon lamp; incident light intensity, ,/ % 0.5 kW m-*.
to three times. The experiments showed good reproducibility. Kinetic Modeling. For the mathematical simulation of the time courses of the concentrations of dissolved FeiI1EDTA, dissolved Fe(II), and CH20, we used the program ACUCHEM (21). This program can calculate the concentrations of reactants versus time in a complicated chemical system by finding a numerical solution to the system of nonlinear differential equations defined by the kinetics problem (22). Best fits of the kinetic constants,whichwere not accessible experimentally or whose values could not be taken from the literature, were obtained by combining ACUCHEM with a MATLAB program (The Math Works, Inc., Natick, MA 01760).
Results and Discussion Figure 1shows the concentrations of CH20,dissolved Fe(II), and dissolved FeIIIEDTA as a function of time upon irradiation of an aerated lepidocrocite suspension at pH 3 to which uncomplexed EDTA had been added prior to irradiation (time zero in Figure l),yielding an initital EDTA concentration of M. Formaldehyde is one of the photooxidation products of EDTA. Its concentration increased with time according to a sigmoidal function. Formaldehyde formation was accompanied by production of dissolved Fe(I1). The CH20 and Fe(I1) concentration versus time curves indicate first autocatalytic and then firstorder kinetics. Autocatalytic photoproduct formation is a known phenomenon in heterogeneous photoredox systems involving iron(II1) (hydrloxides (1620, 23). It occurs from the catalytic effect of the photoproduced Fe(I1) on the thermal dissolution of an iron(II1) (hydr)oxide in the presence of a ligand like EDTA or oxalate (24-26). This process results in the formation of dissolved Fe(II1) complexes, as can be seen in Figure 1: within 60 min of irradiation, about 80% of the initially added uncomplexed EDTA was present as dissolved FeII'EDTA. After this initial phase, the FeII'EDTA concentration decreased according to first-order kinetics due to photolysis of FeII'EDTA with formation of CH20 and Fe(I1). VOL. 29. NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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