Environ. Sci. Techno/. 1995, 29, 2814-2827
Degradson of
FRANZ GUNTER KARI AND WALTER GIGER* Swiss Federal Institute for Environmental Science and Technology (MWAG), CH-8600Dubendorf; Switzerland
Laboratory experiments, field measurements, and mathematical modeling were used to evaluate the processes responsible for the fate of ethylenediaminetetraacetate (EDTA) in the River Glatt, a tributary to the River Rhine. Direct photolysis was identified as the most important process for the degradation of EDTA. With respect to environmental conditions, direct photolysis acts only on the Fe(lll)-EDTA complex. Thus, the extent to which EDTA is degraded in rivers is directly related to its speciation. Approximately 50% of EDTA present in the River Glatt is discharged in the form of Fe(lll)-EDTA from sewage treatment plants. Available Fe(lll)-EDTA in the River Glatt is photolyzed within approximately 1 d during summer daylight conditions (global irradiation of about 800 W h m-2 at noon). The results of a spike experiment were used for the calibration of the light conditions within the water column. It showed that a great deal of sunlight was absorbed by water plants, which led to a decrease of the photolysis rate of Fe(1ll)EDTA in the River Glatt of 85%. The predictions of the chosen model showed good general agreement with data from field investigations.
Introduction Ethylenediaminetetraacetate (EDTA)is a substance with a long tradition of being investigated in the laboratory and in technical systems as well as in field surveys on water quality. Many of these studies were aimed to understand the processes that determine the occurrence and behavior of this substance in wastewater treatment plants (1-3), in surface waters (4-14, in subsurface environments (1214), and in drinking waters (15-18). Sources for EDTA are industrial uses, such as industrial cleaning, photographic industry, textile and paper manufacturing, and pharmacy, but it is not used as a phosphate substitute in laundry detergents in Europe where about 30 000 t was consumed in 1987. Because of its typical application as a metal sequestering agent in aqueous * Corresponding author e-mail address:
[email protected];Fax: +411-823 50 28.
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 11, 1995
solution, EDTA occurs widely as a water pollutant after its discharge from wastewater effluents into natural waters. In recent studies, no degradation of EDTA was observed in biological and chemical steps of wastewater treatment plants ( 2 , 3 ) . This leads to typical EDTA concentrations of 3-30 pg L-' (10-100 nM) in Swiss aquatic environments (5, 7). In Germany,in contrast to Switzerland,EDTAis still used in household detergents, so that riverine EDTA concentrations can even be 2-3 times higher (4, 6). Many studies have been conducted on EDTA to investigate processes such as microbial degradation, sorption, dissolution, hydrolysis, and photolysis (direct and sensitized). Typically, these studies were performed at concentrations much higher than environmental levels. Despite the vast amount of detailed work, studies are lacking that explain the relatively high EDTA concentrations in natural surfacewaters. No investigationhas been published that calculates in situ transformation rates for EDTA in riverwaters. Reasons for this might be that costs for carrying out extensive field studies and analysis for EDTA are high and that literature data about relevant processes in the aquatic environment concerning EDTA scatter within some range or are contradictory. Earlier reports have already shown that the direct photolysis of the Fe(II1)-EDTA complex is probably the main sink for EDTA in the environment (19-21). Recent studies provided additionalinsight into the processes, which are possibly decisive for the behavior of EDTA in natural surface waters (3, 10, 11). Kari and Giger (3) showed that considerable amounts of EDTA pass through sewage treatment plants in the form of Fe(II1)-EDTAwithout being chemically or biologically transformed. It was demonstrated that chemical equilibrium calculations are not a useful tool for predicting the EDTA speciation in effluents from wastewater plants since kinetically hindered metal exchange processes cause a slow approach to chemical equilibrium. In the effluent-receivingRiver Glatt, the Fe(111)-EDTA complex proved to be a very robust complex, exchanging with other metals with a half-life of about 20 d (10). The River Glatt has a short water residence time from source to mouth of approximately 1 d. Therefore, if high amounts of Fe(II1)-EDTA are initially present in this shallow river, the initial speciation will determine the fate of EDTA, because photolytic half-lives of less than 4 h (for direct photolysis) throughout the whole year have been predicted for the Fe(II1)-EDTA complex for this river (11). Under typical environmental conditions in surface waters, Fe(II1)-EDTA is the only metal-EDTA complex that is susceptible to sunlight irradiation. So far,only monitoring-type EDTA concentrations data and mass flows were available for surface waters. With the exception of a comparison of measured and calculated data for the conservative transport of EDTA in the River Glatt ( 3 ,these data, together with partly conflicting literature data, have been insufficient to prove an actual elimination of EDTA via direct photolysis in rivers. This paper focuses on the determination of in s i t u rates of the photochemical transformation of photolabile EDTA species in the River Glatt. This was achieved by following a comprehensive investigation concept in which laboratory data (10, 1I), previous results from field investigations (3),and field data
0013-936)(/95/0929-2814$09.00/0
@ 1995 American Chemical Society
from several investigations conducted under various seasonal conditions at the River Glatt were combined with computer modeling. After esthnatingthe rate-determining processes for the elimination of EDTA in the River Glatt, this combination allowed a quantitative evaluation of the fate of EDTA in this riverine environment. This methodological procedure and integration can improve our understanding andassessment ofthe behavior ofchemical substances in the environment.
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sampling locations @ Rumlang @ I- Niederqiatl @ Hwhlelden @ Rheinsfelden @ STP Niederglan @ STPBOlach
Experimental Section D d p t l o n o f t h e Watershed. AU field investigationswere conducted at the River Glatt (Switzerland),which originates at the outlet of Greifensee and is a tributary to the River Rhine. The flow distance from source to mouth is 36 km. In 1992, the average water discharge at a gauge in Rheinsfelden was about 7.5 m3 s-l; the water residence timewithintheriveris 15-20hdependingonflowvelocity. The river bed slope reaches from 1to 7 960,resulting in an averageflowvelocityofapproximately0.5ms-I. The bank is lined with trees to a certain extent so that the light conditions in the water column range from shadow to full exposure to sunlight. The River Glatt receives water from several small tributaries of which the most important are Chriesbach, Leutschenbach, andFischbach. TheGlattValleyisadensely populated area intensively used for agricultural and industrial purposes. Several sewage treatment plants (at f l o w h 1.4, Fiikmden; 6.3. Diibendorf;9.8, Zurich-Glatt; 12, Opfikon; 22.3, Niederglatt; 25.4, Biilach) discharge wastewater into the River Glatt. At low discharge, about 30% of the water at the river mouth consists of treated sewage. Sampllngpmgramfor EDTA. Samplesforanalysesfor EDTA and for the determination of the speciation of EDTA were taken at various sites in the river and in effluents of sewage treatment plants. Sampling sites for the river were at Riimlang (km 13.2, downstream),Niederglatt (km 21.51, Hochfelden (km27.2). and Rheinsfelden (km 35.2, just before entering into the Rhine). Wastewater effluent sampleswere taken fromthe sewage treatment plants (SIT) at Opfikon (km 12. not always sampled), Niederglatt (km 22.3). and Biilach (km25.4). The locations of the sampling SitesaregiveninFigure1. Thearrangementofthesampling locations was made to facilitate the modeling of the field dam betweenriversamplingsites1and2aswellasbetween river stations 5 and 6 are no additional external sources for
EDTAbesidetheEDTAloadcomingfromthepreviousriver section. The effluentsof the STPs at Opfikon, Niederglan, and Biilach were included since they deliver significant loads of EDTA into the river. To allow application of a one-dimensional river model, it was necessary that the effluents of the STPs be homogeneously mixed witbin the river cross section at the downstream river sampling site. For station Riimlang, this was secured by measurement of across-sectionalconcenuation profile. To locatethe station at Hochfelden, we used a formula given in ref 22 for estimating the distance required for complete crosssectional mixing. The samples from the river sites normally were I-h composite samples (1 grab sample per IO-min interval). They were taken in the middle of the river using timeproportional portable samplers (portablepriority contaminant samplers, no. 4900, Manning Products, Texas) and comprised time periods of at least 24 h. The STP effluent
tracer introduchon
sewage treatment plant (STP)
0
5
10 km
FIGURE 1. Map of tha Glatt Valley Watershed, Swikerland. Shown a n tha sewaga tmabnant plants (STPs). which discharge into the River Glatt. and the sampling locations. The size of the dots representing the STPs is propo~onalto their water outflow.
samples were 2-h composite samples from time-proportional sampling devices and were also taken at least during 24 h. Grab sampleswere used fortheinvestigationofEDTA speciation. After collection, the composite samples were transferred to 250-mL opaque PE bottles containing 2.5 mL of formaldehyde (37%), transported to the laboratoly, and stored at 4 OC until analysis. Analytleal Procedure for,EDTA. Propylenediaminetetraacetate PDTA)wasaddedtothesamplesasanintemal standard. Analysis was done with sample volumes of 25 mL by closely following procedures described by Schafher and Giger (23)and Schiirch and Diibendorfer (2¶. Quantilicationwasdoneby externalcalibrationmesviainternal standardization with PDTA. The reproducibilityoftheanalytical procedurewastested through3-folddeterminationsofriver and effluentsamples. The relative variation coefficient for average results was 55% for EDTA concentrations >10 pg L-' (-35 nM). Recovely rates of spiked samples were between 89 and 107%. Precision of the EDTA analysis was tested regularly by means of standard addition and was within deviations of 5 5 % . Detection limits were at 0.2 pg L-I (-1 nM) for sample volumes of 25 mL. DetendnatlonofPhotolabUePe(lII)-EDTA. Fe(1IOEDTA is the only environmentallyrelevant EDTA complex that is susceptible to sunlight (25). Throughout the text, we refer to Fe(1ID-EDTA forpractical reasonsas photolabile EDTA, because it is degraded in sunlight. The s u m of all other species, which are stable in sunlight, is denoted as photostable or photopersistent EDTA. In order to distinguish the fraction of photolabile from total EDTA, we have developed the followingprocedure: One aliquot of a grab sample is analyzed for total EDTA; another one is analyzed after irradiation under sunlight (summer conditions) for 2 h or, in case of winter or overcast conditions. for 10 min with a medium-pressure mercury lamp (A 2 313 nm, 852 W m-* for 313 nm 5 A 5 436 nm). The irradiations took place as quicklyas possible after the sampling (timeperiod ~ O L29,
NO.17,1995 I ENVIRONMENTALSCIENCE 5 TECHNOLOGYm zais
< 4 h). The irradiation leads to the complete photolysis of ambient Fe(II1)-EDTA complex species under both conditions (25). The fraction ofphotolabile Fe(II1)-EDTAspecies was calculatedby subtracting the EDTA concentration after irradiation from the total concentration of EDTA in the original sample. Proceeding this way does not allow an elucidation of the actual EDTA speciation at the moment of sampling, but serves as an indirect, operational method to distinguish between EDTA fractions that undergo degradation processes under sunlight and fractions that do not. ParticleMTater Distribution Ratio. For determining the particlelwater distribution ratio of some metal-EDTA complexes, we added 10 pg of Ca-EDTA, Zn-EDTA, and Cu-EDTA, respectively, to a suspension consisting of 5 g of Glatt sediment material ( 3 x lo8 M-l s-l. Assuming a steady-state concentrationof lo2 in the magnitude of M in surface waters (511,half-livesof more than 133 d can be calculated for the investigated EDTA species. Hydroxyl radicals (OH') react unselectively with many organic compounds and are the most likely photooxidant, among others, to react with EDTA species. In natural waters, hydroxyl radicals are mainly formed by photolysis of nitrate (52) or by reaction of H202 with Fe(I1) (53). Hydroxyl radicals are scavengedby NOM, bicarbonate, and carbonate (54). Resulting near surface steady-state concentrations of OH' in surface waters are expected to be M under the given conditions of the River Glatt (55). Rate constants k, of 4-35 x lo8M-' s-l have been reported for the reaction of OH' with EDTA and various metalEDTA complexes (56).Therefore, half-lives of 2.3-20 d can be calculated for the reaction of EDTA species with OH' in the River Glatt. Direct Photolysis. A prerequisite for direct photolysis of a compound is its ability to absorb light at 1> 290 nm. Free EDTA does not fulfill this condition, but some MeEDTA complexes do. Nevertheless, most of the environmentally relevant EDTA species (Ca-EDTA, Zn-EDTA, CuEDTA, Ni-EDTA, Mn-EDTA) will not be photolyzed to significant amounts because the reaction quantum yields for their photochemical degradation and/or their concentration in surface waters are too low. Among all EDTA species, only the Fe(II1)-EDTA complex photolyzes in sunlight according to (29)
-( d [ F e ; y l )
i= Qik,[Fe-EDTA]
(9)
where @A is the reaction quantum yield of the photochemical degradation of Fe(II1)-EDTA at wavelength 1, and kd is the specific rate of light absorption by the substance at wavelength 1. Average values for @A at wavelengths 313,366, and 405 nm have been reported to be 0.082, 0.034, and 0.018, respectively (11). Thus, the transformation rate of Fe(II1)EDTA under sunlight is fast. Half-lives (integrated over a full day) of a minimum of 30 min for summer and up to 270 min for winter conditions have been predicted for the River Glatt (11). These predictions relied on the determination of the photochemical reaction quantum yield of Fe(111)-EDTA at concentrations < 1 pM as a function of pH, wavelength, and temperature. Consequently, the photochemical half-life of Fe(II1)-EDTA is short in comparison to typical water residence times of the River Glatt, and Fe(111)-EDTA photolysis should be observable within a field investigation.
Relative Importance of Various Processes. A comparison of the efficiency with which all of the relevant processes eliminate EDTA from the aqueous phase shows that the behavior and fate of EDTA in the River Glatt,beside transport processes, should be determined mainly by fast degradation of Fe(II1)-EDTA species through direct photolysis. Within the water residence time of the River Glatt, chemical processes other than direct photolysis, although probably important for the long-term behavior of EDTA, e.g., in the sea, do not play a significant role for the transformation of EDTA over the distance from Lake Greifen to River Rhine. An efficientphotolysis of EDTA implies the availability of significant amounts of EDTA speciespresent as Fe(II1)-EDTA. In this context, the EDTA speciation is the key parameter deciding whether EDTA is eliminated from the aqueous phase within short times or not and can serve as an explanation for the relatively high EDTA concentration found in major European rivers. Kari and Giger (3)reported that -50% of the EDTA being discharged from some STPs along the Glatt Valley are Fe(II1)-EDTA species which, under favorable sunlight conditions, are expected to quickly photolyze afterwards in the River Glatt. But hitherto, in situ photodegradation of Fe(II1)-EDTA has never been investigated or proved. Considering the direct photolysis to be the most important and only elimination process for Fe(II1)-EDTA in the River Glatt, the various transport and transformation processes governing the behavior of photopersistent (eq 10) and photolabile (eq 11) EDTA species can be summarized as
where kp is the first-order rate constant (time dependent) for direct photolysis of Fe(II1)-EDTA speciesunder sunlight, the subscripts stable and labile denote photopersistent and photolabile EDTA species, fiabile is a fractional, timedependent constant, and L is a source/sink term with respect to EDTA species. Sources and Sinksfor Fe(II1)-EDTA in the River Glatt. For reliable model calculations,it is necessary to know the river internal sinks and sources for EDTA species. Beside the EDTA flowing at Rumlang into the river section of interest (x = 0, start of model calculations) and the EDTA input from the STPs Niederglatt and Biilach, no other sources, e.g., EDTA polluted exfiltrating groundwater, for EDTA are considered to be significant for the flowing distance of 13.2 km (station Riimlang) to 35.5 km (mouth into the Rhine). As already said in the paragraph about dilution, infiltration of the Glatt into the groundwater and consequentlyexport of EDTA is not consideredto influence the mass balance of EDTA in the river. Nevertheless, processes have to be taken &to account that shiftthe Fe(II1)-EDTA/ total EDTA ratio within the river. VOL. 29, NO. 11, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
2819
@
Fe(ll1)EDTA
4 %
02-
02
EDTA break-down products
....................
00
02
04 06 08 rd.tmeamretlm
10
12
FIGURE 3. la) Scheme of the hypothetical photoredox cycle of Fe leading to the formation 01 Fellll)-EDTA out of photostable EDTA C$+,etc. lb) Photolyricdegradation species. Mestandslor cd+.W+, of EDTA as a function of the relative exposure time to sunlight. (m) Glan watersample 10.45pmlilteredlspiked with290pg 1.' Fe(1ll)EDTA and 290 p g 1.' Na2H2EDTk4 h of total exposure time. (0) sample (0.45 pm filtered) from the settling tank of the STP Dplikon. unspiked: 20 h of total exposure time.
In other words, do processes take place which lead to an increase of Fe(II1)-EDTA species at the expense of photostableEDTAspeciesordoes theamount ofFenm-EDTA decreasealong the river because photostable EDTA species
areformednametalexchangeofdissolvedmetalionswith Fe? Fe(lI1)-EDTA is not the dominant EDTA species under the given conditions in the Glatt based on chemical equilibrium calculations (10, dominant are Zn-EDTA and Ca-EDTA). But Fe(II1)-EDTA is a very robust complex dissociatingwithhalf-lives of about 20 d to formphotostable EDTA species (10. 25). This means that dissociation of photolabile EDTA species is not an internal sink for Fe(111)-EDTAand consequentlyhasnoimpact onjabil.within the field sampling period. On the other hand, there is the question whether dissolved iron(II1) or iron(II1)hydroxo complexes, formed by oxidation of photochemically generated Fe(I1). could outcompete the metalionsofphotostableEDTAcomplexes via metal exchange processes. This process, possibly starting with the photochemical degradation of ambient Fe(II1)-EDTA, would result in a production of photolabile Fe(lII)-EDTA species, perhaps following a hypothetical scheme that is shown in Figure 3a. The concentration of Fe(l1) species in natural waters is proportional to the amount of ambient light (57,58). In summer, sunlight irradiates the earth's surface at -60 W m2in a wavelength range A = 320-400 nm. Therefore,we investigated whether summer sunlight is sufficient for initiating and maintaining an efficiently working Few)/ Fe(II1) redox cycle under given conditions as established 2820. ENVIRONMENTAL SCIENCE k TECHNOLOGY I VOL. 29.
NO. 11.1995
in Figure 3a. A sample from the River Glatt was taken, filtered through 0.45-pm filters, and spiked with 29Opg L-I (-1 pM) free EDTA and 345 pg L-' Fe(1IO-EDTA (-1 phQ (Note: During GC analysis both species are transformed to identical EDTA propylesters). Then this pretreated sample was exposed to summer UuIy) noon sunlight for 4 h. Another sample taken from the settling tank of the STP Optikon was filtered through 0.45-pm filters and irradiated under June sunlight for 20 h on two consecutivedays. The results showingEDTAconcentrationsplottedas a function of relative sunlight exposure time are presented in Figure 3b. In either case, the photolabile Fe(II1)-EDTA species weredegradedwithinateperiod 5 2 h. The photostable EDTA species, however, could not be attacked by the photochemically formed Fe"',, species and were consequently not photolyzed to any further extent. Therefore, precipitation processes of Fe(I1O species outweigh lightinduced reduction processes. There are various reasons for the failure of additional production of Fe(II1)-EDTA Mineral surfaces of colloids (still in the sample after fiItration) can serve as oxidation centers for dissolved Fe(11) species (59). Dissolved FeZ+ ions are removed from solution by adsorption to the negatively charged surface sites (at pH = 7-8) and k e d to these sites by oxidation to Fe(1IO. Another possibility would be that dissolved Fe(I1) is alreadyoxidized(andprecipitated)toiron(I1O insolution, which then is deposited at the mineral surfaces of colloids. In such a manner, k e d particulate Fe(lI1) could be redissolved by light-induced reduction processes at the solid's surface. But, in the absence of organic ligands (syntheticalor natural),such reductive dissolutionprocesses do not contribute to the production of Fe(I1) in solution (57,60, 61). With light of higher intensities (363W m-2 coming from amedium-pressuremernuylamp)resultinginhigherFe"l, steady-stateconcentrations,the possibility of this process was already shown (25). In homogeneous solution, an effective Fe(II)/Fe(III)redox cycle could be maintained becausedissolvedFeUI0intheformofFe(OH)2+canreadily be reduced to Fe(II), which in turn is oxidized again (62). Another source for Fe(l1I)-EDTA would be the phenomenon that photolabile EDTA results from adsorption of Ca-EDTA, Zn-EDTA, and Cu-EDTA species to natural Glatt sediment which, after metal exchange with Fe(1II) from the particle surface, are desorbed as Fe(lI1)-EDTA into solution (25). This phenomenon could also be proved hy other investigators using a groundwater aquifer (12.13) or synthetically produced iron oxide-coated sand (14) as reacting surfaces. For the River Glatt, this kind of source for Fe(II1)-EDTA can be neglected because of small &
vaIuesofEDTAcomplexesandthefactthattypicalsediment is rather scarce in this river and thus the amount of surface sites available for adsorption/desorption reactions with photostable EDTA species. Summarizing, the intensity of sunlight at the earth's surface is not sufficient to maintain a permanent Fe(lI)/ Fe(II1) redox cycle, which would lead to the formation of FeUID-EDTA species out of photostable EDTA species; neither are the phenomena leading to formation of Fe(II1)-EDTA after adsorption of photostable EDTA species to particles containing Fe. The mass balance ofFe0-EDTAin a River Glatt section is assumed not to be influencedby the described processes of metal exchange, which simplifiesthe model calculations.
Protocol for Field Investinations
I
FIGURE 4. Schematic protocol for modeling experimental data derived from field investigationsat the River Glatt The dimensionless friction slope is denoted by S; 0 is the water discharge (d s-l).
Modeling Approach. After having discussed the various processes that affect the behavior of EDTA in the Glatt, we can conclude that the sourcelsink term for EDTA can reasonably be neglected in eqs 10 and 11 and that the fractional constantfiabileis therefore not influenced by this term. For evaluation and modeling of the field investigation data, in all cases we used the procedure presented in Figure 4. After measuring concentration-time series of EDTA (including the STPs) and breakthrough curves of the dye tracer at several stations along the Glatt, the data were fed into RIVERSIM together with data about water flow in the Glatt and discharge from the STPs Niederglatt and Biilach. The river hydraulics were calibrated using the tracer experiment for defining the friction slope according to Darcy-Weisbach,at which the River Glatt was divided in 11 physically reasonable reaches. If deviations were found between the measured dischargeand the model predictions at Rheinsfelden (seeparagraph about dilution),we defined a hypothetical diffuse lateral inflowq that was varied within the model until the water balance at this station was equalized (deviation .C 5%).
A consistent water balance is a necessary prerequisite for reasonable transport calculations since substance concentrations are determined by ambient water volumes. EDTA transport calculations were based on the concentration-time series at Riimlang as input function (x = 0, start of simulation). Since we inferred that direct photolysisis the onlyprocess that acts on photolabile EDTA species, time- and depthdependent photolysis rate constants had to be provided to RIVERSIM. During the field investigation, several water samples were collected along the river and filtered (0.45 pm), and the absorbance was measured to get the average light attenuation coefficient of the water body, which was supplied to GCSOLAR. Then photolysis rate constants were calculated as a function of daytime and depth (0.0-1.4 m, steps of 10-l m) for clear sky conditions. An example of an output is given in Figure 5. For further use in RIVERSIM, the calculated rate constants were transferred into a function being continuous in time and depth as discussed below. Sincethe photolysis rate constantsfor Fe(II1)-EDTA at depth z = 0 m (water surface) are only dependent on the incident fluence rate and therefore on daytime, they can be approximated by a Gauss function (see shape of Figure 5, valid for clear sky):
where kp,z=O(t) is the photolysis rate constant at the water surface, tis daytime, and mi are the function parameters. The extent of light attenuation in a water body is a function of the light attenuation coefficient and the depth of the water body at a given angle of light incidence. It is thus independent of the amount of incident sunlight. Because GCSOLAR provides depth-integrated photolysis rate constants as output, we had to perform another interpolation in order to take the depth dependence of kp into account. For this purpose, we built the ratios k , , d kp,z=i (i = 0.1,0.2, ...,1.4 m) valid for noontime which then were plotted as a function of depth. Interpolationwas done by a polynome of order 8 to assure that the regression coefficient r 2 0,999.
k,,,,,,,,(z)
= P,(z)k,,,(t = noontime)
(14)
where P&) is the polynome of order 8 as a function of depth interpolating the ratios kp,z=~lkp,z=i (i = 0.1, 0.2,..., 1.4 m). According to eqs 13 and 14, the photolysis rate rp of Fe(II1)-EDTA in the River Glatt then can be calculated as a function of daytime and depth: kp
= kp(tPZ)&ab,,c = kz=o(t)Ps(z)&abdeC
(15)
The kinetics of direct photolysis of Fe(II1)-EDTA were programmed as subroutine RATES in this manner, which was coupled to RIVERSIM. Field Experiments. In this section, we first discuss the results of the field experiment where we injected 6 kg of dissolved Fe(II1)-EDTA into the Chriesbach to see how fast direct photolysis acts on the Fe(II1)-EDTA complex. The conclusions with respect to light availabilityin the river then are included in the model calculations for the other field investigations presented here. VOL. 29, NO. 11, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY m 2821
_r
4
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time of day
24
120em -e 30cm -0- 60cm -m- Wcm -AFIGURE 5. F-hotalvsis rate constants of Felllll-EDTA in the River Glattas ahnction &depth endtime of day ascaiculated by GCSOlAR for July 10, 1991. Lime o f day
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FIGURE7. Hourly sums of global radiation and duration of sunshine duringthe periods where field investigations were conducted. Data were receivedfmm gagingstation Klotenofthe Swiss Meteomlogical Service.
. 1
d a y
Hmhfelden
+ Rhcinsfeldcn
FIGURE 6. Concentration profiles of EDTA at the various sampling locations on July 29-3.1992. after the introduction of 6 kg of Fe(Ill)-EDTA intothe Chriesbach. Hatched areas represent the portion of Fe(lll)-EOTA coming fmm the injected pulse.
FemD-JiDTA Tracer Experiment. The breakthrough of the Fe(II1)-EDTA pulse on July 29-30, 1992, at the different river sampling sites is shown in Figure 6. Almost no clouds could be observed in the sky during these two days (Figure 7). Since we introduced the pulse into the Chriesbach during night hours and because of low light intensities in the morning, it reached station RUmlang almost intact. But within a flow distance of 8 km and with increasing light intensity, the pulse was diminished significantly. At stations Hochfelden and Rheinsfelden, no effects originating from the injection could be observed. Two things can be learned from the EDTA concentration profiles for the present. The Fe(1II)-EDTA complex is chemically robust to metal exchange processes within the river as was predicted by recent investigations (10). According to chemical equilibrium calculations, these processeswould haveledtotheformationofphotostableEDTA species and thus to a stabilization of the EDTA pulse. Despite the vast amount of light-absorbing macrophytes in the Glatt, ambient light intensities in the river are sufficientforafast andeffectivephotolysisofFe(II1)-EDTA resulting in arrival at the EDTA background level, which consists of photostable EDTA species. Since we expected that the actual light conditions in the Glatt water body were not equivalent to the ones used by GCSOIAR for calculation of the photolysis rate constants, weusedtheEDTAchemographsatRiimlangandNiederglatt for determination of in S i N degradation rates. To this end, the chemographs at Riimlang and Niederglatt were split 2822. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29.
NO. 11. 1995
40 60 0 20 40 60 irradiation time [ min 1 FIGURE 8. Temporal course of EDTA concentrations in irradiated grab samples taken from the River Glatt on July 29.1992. at times the Fe(lll)-EDTA pulse went through lsee Figure 7 for comparison). (a) Station RUmlang. sample taken at 93. Ibl Station Niederglatt, sample taken at 1515. 0
20
into two parts (in analogy of investigating floods in hydrology),the upper part representing the Fe(II1)-EDTA originating from the spike into the Chriesbach (hatched areas in Figure 6 ) ,the lower part representing the "natural" EDTA background consistingof a mixture of photolabile as well photostable EDTA species. In order to determine the approximate amount of photostable EDTA species with respect to the baseline, we took a grab sample at stations Rlimlang and Niederglatt during the times the pulse went through. The samples were filled into quartz tubes and irradiated for 60 min under sunlight. The results are given in Figure 8. In both cases, the final EDTA concentration was 11pg L-'. This means that the amount of photostable EDTA species did not increase between Riimlang and Niederglatt and that the fraction of photolabile species within the baseline with respect to total EDTA was in the range of -20-40%. Having this in mind and coming from the chemograph at Riimlang. we performed model calculations for the EDTA chemograph at Niederglatt with and without thespiked Fe(III)-EDTApulse, assumingthat about 30% of the baseline EDTA were photolabile and that photolysis took place at a rate as predicted by GCSOIAR. A comparison of measured data with prediction for (non)conservative transport of the baseline (Figure Sa) shows that best consistency is achieved by allowing 20-40% of EDTA to be present as photolabile species.
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,I 24
lime of day
!4GURE 9. (a) Comparison af measured and predicted EDTA concentrations(withoutlha Fa(lll)-EDTA pulse)assuming different percamage potlions of Fe(lll)-EDTA from total EDTA of the background concentration profile at RUmlang. Ibl Model prediction for station Niederglan includingthe Fellll)-EDTA pulsefordiflerent scenarios. I, measurement; II. 0% of total EDTA are photolabile, conservative simulation; 111. 100% of the EDTA baseline are photolabile; IV,lO%oftheEDTAbaselineat Rumlangarephotostable. The Fe(lll)-EDTA pulse is taken to be photolabile to 100%. The differencesbetween measurement and prediction at the beginning of the field sampling is caused by numerical relaxation of the algorithm used by RIVERSIM. (c)Comparison of EDTA concentrations at RUmlang on July 29. 1992, with model calculations assuming different portions of Fe(lll)-EDTA from the baseline total EDTA at station RUmlang. Shading affects of aquatic plants are taken into account by a lumped parameter refering to the flow distance from RUmlang to Niederglan. It corrects photolysis rate constants calculated by GCSOLAR by a factor of 0.15. If we include the Fe(II1)-EDTA pulse into the model calculations (Figure 9b), we can confirm that a significant degradation of Fe(II1)-EDTA between Riimlang and Niederglatt took place, but not as quickly as predicted by GCSOJAR photolysis kinetics. If we take 100% of EDTA to consist of photolabile species, even no EDTA should have been found at Niederglatt during the daytime. Hitherto, we did not take into consideration the light-absorbing effect of the “macrophyte carpet” in the River Glatt. Yet,
the results from Figure 9b clearly indicate that there are some “shade effects” slowing down photolysis of Fe(II1)EDTA in the water body. Since we do not know the actual speciation of EDTA (dependent on EDTA speciation of STP discharges) and the actual light conditions in the Glatt as a function of time and location, we introduced a lumped factor to correct for the shading effects of the macrophytes and trees. This factor was assumed to be independent of daytime and to be valid for the whole river distance investigated. Beside the fraction of photolabile EDTA species the light available in the water body also was varied in the model calculations now by adjusting the kp values. Best conformity between measured and predicted EDTA concentrations was achieved by model calculations using photolysis rate constants, which were lower by 85%than the predicted kpvalues (Figure9c). The calculations show that about 30-60% of the total EDTA at RUmlang must have existed as photolabile species with varying portions during the day. These altering portions are caused by the STP discharges, which are temporally superposed at the river sites and have their own emission characteristics with respect to different EDTA species due to the various EDTA sources within the catchment areas of the STPs. As an important preliminary result, we conclude from this field investigation that much less light is available in the River Glatt, due to light-absorbing water plants, than is predicted by GCSOLAR. Despite this fact, the actual amount of light the water body obtains is sufficient for photolysis of considerable quantities of Fe(lI1)-EDTA. Due to high nutrient levels in the river, the seasonal occurrence of aquatic plants in the Glatt is more or less in a steady state. Therefore, the same correction factor, as derived in this field experiment, was maintained for the investigations discussed below. Field Investigation A (December18-19,1991). This field investigation was dominated by meteorological conditions characterized by fog and drizzling rain in the morning and evening hours and low sunlight intensities during the day (see Figure 7). The concentration-time series of EDTA at the various field sites are presented in Figure 10 together with model calculations for station Niederglatt. The high concentrations at Riidang, supposedly caused by a EDTA pulse discharged from the STP Opfikon, could be found with a certain time shift at the downstream stations. Because of additional EDTA inputs dischargedfromtheSTPsNiederglattandBiilach,theEDTA concentrations at Hochfelden and Rheinsfelden are increased once more. The similar shape of the time-shifted concentration profiles is already a hint at degradation processes that did not take place. That no significant amounts of EDTA had been photolyzed was confirmed by a comparisonof the measured datawith model calculations assuming conservative behavior of EDTh In our opinion, thereasonforthisisnotto beseeninthelowlight intensities during December 18, because at the time when the pulse passed the station Niederglatt enough light was available to effectsome photolysis of ambient FelIII)-EDTh Probably the prevailing portion of the EDTA pulse consisted of photostableEDTAspecies. Unfortunately,wedo not know the fraction of Fe(III)-EDTA species included in the pulse since we did not subject any of the samples to irradiation with light. Field l m t i g a t i o n B (July10- 11,1991). Concentration profiles of EDTA at three river sites during this time period VOL. 29. NO. i t . 1995 i ENVIRONMENTAL SCIENCE &TECHNOLOGY
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t i m e of day
+ RUmlang
--
0 Niederglan -A-
Hochfelden +Rheimfeiden
120
L2 I W
-0- measurement
--
model without degradation
5 -L?
-
--c RUmlang
+ Niderglatl
o Rheinsfelden
16
12
'Z
E
8
f3
4
2
0
s
8
10 12 14 16 18 20 22 24 2 4 t l m e of d a y
6
8
10 I2
time of day
FIGURE 10. Temporal course ofthe EDTA concentrations atthe field investigation sites on December 18-19. 193l. together with a comparison of measured data and conservative model calculations for the station at Niederglatt
are shown in Figure lla. The meteorological conditions were stablewith cloudless skyduringthe day. At first sight, the EDTA concentrations in Niederglatt and Rheinsfelden were lower during the day than was to be expected from the concentration profile at Riimlang. During the night, the concentrations at Niederglatt and Rheinsfelden increased to a level that was comparable to the one we found in Riimlang. The results of the model calculations with variation of the fractions of photolabile EDTA species are presented in Figure 1lb.cforstations NiederglattandRheinsfelden.Good conformitybetweenmeasurement and prediction is found if we assume that 30-5096 of total EDTA at Riimlang consisted of Fe(II1)-EDTA species. This model prediction could be confirmed by an immediate irradiation of two grab samples that were taken at 830 in Riimlang and at 11:30 in Niederglatt. During 2 h of sunlight exposure, the concentrations of EDTA were reduced from 7.8 to 4.4 pg L-' and from5.8 to2.4pgL-' intheRiimlangandNiederglatt samples, respectively. This corresponds to fractions of photolabile EDTA species of 44 and 5946,respectively. For the site at Rheinsfelden, we performed model calculations for a conservative EDTA transport. one consideringonlythe concentration profile at Riimlangand the other additionally including the EDTA input from the STPs Niederglatt and BWach. The difference between the two model calculationsofconservativetransport thus represent the EDTA load of the STPs arriving at Rheinsfelden. The measured concentrations at Rheinsfelden were lower in any case than the predictions for conservative transport. This means that all of the EDTA load discharged from the STPsduringthistimeconsistedofphotolabileFe(II1)-EDTA species. Enough time was available for photolysis of these species, which therefore did not take any influence on the EDTA concentration profile at Rheinsfelden. Pleld InvestigationC (March 5-6,1991). Meteorological conditions during this field study were quite wuying 2824. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29.
NO. 11,1995
O J ~ , , , , , , , , , , , , , , , , , , , , , , , , , , , , 16 18 20 22 24 2 4 6 8 10 I2 14 16 18 20 22 l i m e of d a y
I
FIGURE 11. Results and interpretation of the field study on July 10-11. 199l. la) Concentration profiles of EDTA at the field inwsigatbn sites. (b) Comparison of EDTA consenhations at Niederglan with model predictions assuming different portions of total EDTAat RUmlangto be presentas Fe(lll)-EDTA.(clComparison of EDTA concentrations at Rheinsfelden with model predictions assuming different portions 01 total EDTA ai RUmlang to be present as FellWEDTA. I.consewative model simulation considering only the EDTA load at RUmlang (EDTA loads coming from STPs are excluded); II. like 1, but now with regard of the EDTA loads coming from the STPs Niederglan and BUlach.
on March 5, the sky was overcast all day, in the night fog came up, and on March 6 there was sunshine here and there in the morning. The concentration profiles at the investigation sites are shown in Figure 12a. High EDTA concentrations, higher than previously reported for rivers in the literature, were found at Rheinsfelden caused by an EDTA pulse leaving the STP Niederglatt during the night (Figure 12b). Modeling the concentration profile at Rheinsfelden is rendered more difficult by the changing meteorological conditions and the circumstance that we did not examine the speciation of the EDTA pulse. discharged from the STP, by irradiation experiments. Since the EDTA of the effluent of the STP Niederglatt showed good correlation with dissolved (0.45 pm Ntered) Fe during the time the pulse was discharged (31,we can reasonablyassume that the pulse comprised nearly 100%of photolabile EDTA species. This assumption facilitatesmodeling calculationssince we can proceed in a similar manner as we did for the Fe(II1)EDTA spike experiment. First, we established the fraction of photolabile EDTAcontainedin the concentration profile at Riimlang by comparing measurement and prediction for the river station Niederglatt. According to these
under complete cloud cover compared to the one under a clear sky.
-
Conclusions
L
I00
6
s"
50
2
0
12
8
20
I6
24
4
I2
8
16
:
20
lime of day
--c Riimlang
8
--L
12
16
o Niederglatl o Rheinsfelden
20
24
4
8
12
16
2
time of day
-n- STP Opfikoon
&
STP Niederglatt
-A-
STP BUlach
200
M
3 160
85 120
-5 2
8
g Y
80 40
o
16 18 20 22 24 2 4 6 8 IO I2 14 16 18 20 22 24 time
of day
pho(o1ysirrncmnnant I lCO% II 20% 1 10% 0 5 % of hat p d e l e d by GCSOLAR
0%
FIGURE 12. Results and interpretation of (he field study on Marl 5-6.l99lEn.(d Concerntion profiles of EDTAatthefield inwrtigati sites. (b) EDTA Concentrations in the effluents of various sewal
treatment plants during the same time period. (c) Comparison EDTA concentrations at Rheinsfelden with model calculationsusi different correctionfactorsfor the photolysis rate constants comput by GCSOLAR.
calculations, afractionof30%Fema-EDTAspecies seemi to be reasonable. With respect to variation of lip conditions, the calculations reacted sensitively whi decreasing kp values to portions smaller than 5% of d ones predicted by GCSOLAR. With these marginal con( tions, the model calculations were performed for the si at Rheinsfelden (Figure 12c). A comparison of measuri datawiththeresultsofmodelpredictionsshowthat,despi unfavorable light conditions, a significant degradation EDTA took place along the river within a distance approximately 13 km. Best conformity of measureme and calculationwas achieved by assuming that photoly! converted Fe(I1I)-EDTA at rates being only 5% of th calculated by GCSOLAR In this way, beside the lig absorption by aquatic plants, the photolysis rates we additionallyreduced by the overcastconditions. Ourresi is quite consistent with the results of other investigatior where it was found that the radiation coming down to tl earth's surface is reduced by 50% under entire overcz conditions (s3).Wmterle et al. (64observed a diminutiq of 50% for the photolysis rate of a chemical actinomet
By performing field investigations to elucidatwe chemodynamic behavior of EDTA, we verified for the first time that Fe(II1)-EDTA species are photolyzed rather fast in shallow surface waters. With the use of the computer programs GCSOLAR and RIVERSIM. we have been able to calculate in situ transformation rates for the photoconversionofFe(II1)-EDTAintheRiver Glatt. Transformation rates are 85% smaller than under direct sunlight since the light conditions in the water body are determined by the large presence of hydrophytes, which strongly retain light from the water column. Yet, light conditions should not be the limiting factor for the degradation of photolabile EDTA species as they move downstream. The extent of EDTA degradation by sunlight is coupled to its speciation. Our model calculationssubstantiatethattheconcentrations found at Rheinsfeldenare a consequence of the speciation with which EDTA is discharged from sewage treatment plants (3). Therefore,EDTA speciation in effluentsof STPs and rivers cannot be predicted by means of chemical equilibriumcalculations(3.6).For along-tern assessment, we can estimate that the EDTA concentrations found in large European streams are residual and should consist of photostable EDTA species like Ca-EDTA and Zn-EDTA that do not degrade under ambient sunlight. What we cannot evaluate is the amount of Fe(IIl)-EDTA that is released into the aqueous phase from particulate surfaces after transformationofphotostableEDTA speciesthat have been adsorbed to that Fe-containing surfaces. With the kinetics found for such reactions (14,25, 65).these metal exchangeprocesses,ifwe considertimescalesofdays, might contribute as an internal source for photolabile EDTA and thus are favorableforthe overall degradationof EDTA. Also, research has to be dedicated to the influence that heterogeneous photoredoxreactions exert on the photooxidation of metal-EDTA complexes in the presence of iron(I1I) (hydr)oxides(66). The investigative approachwe used clearly demonstrates that comprehensive concepts are inevitably necessary for a reasonable understanding and assessment of the occurrence and fate of organic chemicals in the environment. These concepts comprise the combined use of precise analytical methods and laboratory studies to identify the relevant processes and field investigationsand mathematical modelingtoquantifythepertinent processesthat govern the chemodynamicsof environmentalchemicalsin aquatic systems. Some illustrative examples where investigations strictly obeyed these concepts are available (7.28.67).The only use of single parts of this comprehensive concept is not sufficient for a final assessment of the environmental behavior of chemicals and can even be misleading. In a recent study investigating the fate of EDTA in the River Main (9),fitting procedures were used to account for unknown input and elimination. An approximate half-life of 4 d was calculated for EDTA in March 1990 (sunny weather),whereas no significant degradation was found in November 1990 (doudyweather). Withsimplecalculations, it would have been possible to calculate photolysis rates that are significant within the flow distance investigated even for cloudy November conditions. By not doing so, EDTA speciation was not identified to he the key factor for overall degradation of EDTA, although equilibrium calcuVOL. 29. NO. 11,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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lations were performed to assess the potential of EDTA to remobilize heavy metals out of the sediment. In our opinion, such kind of mathematical modeling should only be used to draw more information from monitoring-type studies to initiate new impulses of thinking. Interpretation of a substance’s environmental behavior on the basis of computer simulation of monitoring studies is especially dangerous in those cases where recommendations for a substitution of this or another substance are derived from such model calculations. Another study (21)yielded questionable results by calculating environmental half-lives of EDTA on the basis of experimentally determined photochemical reaction quantum yields of Fe(II1)-EDTA, assuming that the Fe(111)-EDTA complex is most prevailing in typical surface waters at pH = 7. We hope that we have demonstrated the usefulness of comprehensive investigation concepts with respect to the assessment of the environmental behavior of chemicals. Presenting our approach and using the photochemical degradation of EDTA in surface waters as an example, we wanted to show that many factors can be fate determining. These factors may not be elucidated either by laboratory or by field studies alone. It is necessary to apply both and also to combine them with model calculations. In this way, the results of our studies can be extrapolated to assess the fate of EDTA in other aquatic systems. With respect to structurally similar compounds, e.g., nitrilotriacetic acid, diethylenetriaminepentaaceticacid, etc., the detailed discussion of relevant processes for the fate of EDTA in surface waters should help support the identification of the fate-determining processes of these related compounds.
One author (F.G.K.) was recipient of a graduate student fellowship by the Daimler-Benz Foundation, Ladenburg, Germany. We very much appreciate the help of many people during the field investigations. T. Field, J. Field, A. Minet,and M. Nellen were tireless co-workersguaranteeing a successful course of the studies even during night hours. We would like to thank the operators of the different sewage treatment plants for their help in sample collection. Preliminary experiments and analytical work carried out by C. Schaffner,helpful comments by P. Reichert,W. Simon, and M. Ulrich,andimprovements ofour Enghsh by B. James are acknowledged. We are indebted to Mr. Luder from the Swiss Hydrological Service and Mrs. Felix from the Swiss Meteorological Service, who promptly provided the necessary data.
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