Environ. Sci. Technol. 2001, 35, 3915-3923
Quantification of Dissolved Natural Organic Matter (DOM) Mediated Phototransformation of Phenylurea Herbicides in Lakes ANDREAS C. GERECKE, SILVIO CANONICA, STEPHAN R. MU ¨ LLER, MICHAEL SCHA ¨ RER, AND RENE ´ P. SCHWARZENBACH* Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), P.O. Box 611, CH-8600 Du ¨ bendorf, Switzerland
For many important classes of pesticides including phenylurea herbicides (PUHs) and triazines, photosensitized transformation may be the only relevant elimination process in surface waters. In this study, the dissolved organic matter (DOM) mediated phototransformation of PUHs has been investigated in laboratory and field experiments. The results indicate that, in surface waters, the photosensitized transformation of PUHs may be significant and occurs primarily by an initial one-electron oxidation most likely involving excited triplet states of DOM (3DOM*) constituents. Using isoproturon and diuron as model compounds, it is shown that for a given DOM, quantum yield factors determined in the laboratory at a few selected wavelengths can be used to quantify the overall DOMmediated phototransformation of a given PUH under sunlight irradiation. Furthermore, it is demonstrated that this process can be modeled for a given surface water, by applying the program GCSOLAR and a simple algorithm for cloud cover for quantification of average daily light intensities. Finally, the model has been successfully applied to predict vertical concentration profiles of isoproturon and diuron in a small lake in Switzerland. To our knowledge, this is the first study in which DOM-mediated phototransformation of organic pollutants has been quantitatively validated in the field.
Introduction Phenylurea herbicides (PUHs), such as isoproturon (IPU), diuron, chlorotoluron, and fluometuron (for structures see Table 1), are used in large quantities for various purposes, for example, as systemic herbicids in cereal crops (isoproturon, chlorotoluron) and cotton fields (fluometuron), as total herbicides in urban areas (diuron), or as algicides in paints and coatings (diuron). PUHs are frequently detected in surface waters at concentrations above 100 ng L-1 which is above the European drinking water limit (1) often taken as a quality standard for natural waters. Therefore, in Switzerland, programs have been initiated to reduce the input of PUHs and of other pesticides into surface waters (2). However, for the identification of the major sources of a given compound and for elucidating the success of measures taken to reduce these sources, these inputs must be quantified. * Corresponding author phone: ++41 (1) 823 5109; fax: ++41 (1) 823 5471; e-mail:
[email protected]. 10.1021/es010103x CCC: $20.00 Published on Web 09/05/2001
2001 American Chemical Society
As we have demonstrated in earlier work using atrazine as model compound (3), the input of a given pesticide into a lake can be determined from a time series of vertical concentration profiles, provided that all processes leading to elimination of the compound in the water column of the lake can be quantified. Considering the physical-chemical properties and reactivities of PUHs (see Table 2), it can be assumed that sedimentation, gas exchange, and abiotic hydrolysis should not be relevant elimination processes (see ref 4 for calculations). Furthermore, because PUHs absorb light only very weakly above 300 nm and exhibit only moderate quantum yields (see Table 2), direct photolysis is also predicted to be of minor importance. This leaves microbial degradation and indirect photolysis (“photosensitized” transformation) as the only remaining possible removal mechanisms for PUHs in surface waters. Therefore, in this work, the photosensitized transformation of PUHs has been studied in detail using IPU and diuron as model compounds. Photosensitized transformations of organic chemicals in surface waters are mostly initiated through light absorption by chromophores present in dissolved organic material (DOM) or by nitrate and the subsequent formation of reactive species. The occurrence of several reactive photooxidants, including the hydroxyl radical, the carbonate radical, singlet molecular oxygen, and solvated electrons, has been documented, and photostationary steady-state concentrations of these species have been measured or calculated (5, 6). Recently, details of the fast photooxidation of phenols by short-lived triplet states of DOM (3DOM*) have also been reported (7). It was postulated that 3DOM* reacts with phenols by electron abstraction and/or hydrogen transfer. These transformation mechanisms were also suggested for the three PUHs fenuron, monuron, and diuron based on irradiation experiments with humic acids at 365 nm (8, 9). However, to date, these reactions have not been quantified for any PUH, and, therefore, the environmental significance of this process is not yet clear. In this paper, we report results of laboratory and field measurements aimed to characterize and to quantify DOMmediated phototransformation of PUHs in sunlit surface waters. The major goals of this study were (i) to check the hypothesis that in surface waters photosensitized transformation of PUHs occurs primarily by reaction with excited triplet states of DOM constituents, (ii) to establish a model for prediction of photosensitized transformation of PUHs in surface waters, and (iii) to validate this model by field measurements in a lake. To this end, photoirradiation experiments under laboratory and sunlight conditions were carried out using aqueous solutions of Suwannee River fulvic acid (SRFA) as well as water (GSW) from Greifensee (a small eutrophic lake in Switzerland) with IPU and diuron as model PUHs. The results of these laboratory experiments together with the program GCSOLAR (10) (which allows to calculate the solar irradiation and the kinetics of photochemical reaction in a water body) served to quantify sensitized phototransformation of the two compounds in the water column of Greifensee. A one-dimensional vertical transport and mixing model including flushing and indirect photolysis as sole elimination processes was then used to simulate IPU and diuron concentration profiles in Greifensee. Comparison of simulated and actually measured concentration profiles were enabled to validate our approach to quantify DOMmediated phototransformation of PUHs in surface waters as well as to assess the relative importance of this process for the fate of such compounds in lakes. VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemical Structure, Hammett Constants, and Pseudo-First-Order Rate Constants for SRFA Sensitized Photolysis of a Series of PUHs Hammett constanta
substituent compound
no.c
-R1
metoxuron CGA 24482 CGA 16519 IPU CGA 17767 CGA 17092 fenuron chlorotoluron CGA 18414 fluometuron diuron
1 2 3 4 5 6 7 8 9 10 11
-OCH3 3,4-tetramethylene -CH2CH3 -CH(CH3)2 -CH(CH3)2 -C(CH3)3 -H -CH3 -CH(CH3)2 -H -Cl
R3
σ+ p
σm
σ+ p + σm
k°P,sens b [h-1]
-CH3 -CH3 -CH3 -CH3 -CH2CH3 -CH3 -CH3 -CH3 -CH2CH3 -CH3 -CH3
-0.78 -0.30d -0.30 -0.28 -0.28 -0.26 0 -0.31 -0.28 0 0.11
0.37 -0.07d 0 0 0 0 0 0.37 0.37 0.43 0.37
-0.41 -0.37 -0.30 -0.28 -0.28 -0.26 0 0.06 0.09 0.43 0.48
0.63 0.52 0.47 0.44 0.32 0.31 0.18 0.22 0.16 0.051 0.055
-R2 -Cl -H -H -H -H -H -Cl -Cl -CF3 -Cl
a Reference 19. b SRFA: 2.5 mg C L-1, λ > 320 nm. c Numbers in Figure 4. to -R1) -R2 ) -CH2CH3.
d
Hammett constant for 3,4-tetramethylene assumed to correspond
TABLE 2. Relevant Compound Properties Required for the Assessment of the Environmental Fate of Isoproturon (IPU) and Diurond property vapor pressure water solubility air-water partition constant octanol-water partition constant org. carbon - water partition constant decadic molar absorption coefficient quantum yield for direct photolysis second-order rate constant for reaction with HO‚ reaction with CO3•reaction with 1O2 pseudo-first-order hydrolysis rate constant
symbol
units
IPU
diuron
ref
p* sat Cw Kaw
Pa mol‚L-1 w , M Lw‚L-1 a
∼3 × 10-6 2.0 × 10-4 ∼4 × 10-8
∼1 × 10-6 2.8 × 10-4 ∼2 × 10-8
(20) (20) (20)
Kow
Lw‚L-1 o
∼3 × 102
∼4.4 × 102
(20)
Koc
Lw‚kg-1 oc
∼2 × 102
∼3.5 × 102
(20)
(λ)
M-1‚cm-1
Φ(λ)
(mol)‚ (einstein)-1
4.5 × 10-3 b
kHO‚ kCO3•k1O2 kh
M-1‚s-1 M-1‚s-1 M-1‚s-1 s-1
5.2 × 109 4.6 × 109 3 × 107 0.8 × 107 not reported < 1 × 10-9 < 1 × 10-9
see footnote a 1.4 × 10-2 c
this work (21), (22)
(23) (24) (25)
a / -1 cm-1]: λ ) 297.5:88/460; λ ) 300:60/302; λ ) 305:46/96; λ ) 310:34/27; λ ) 320:24/7; λ ) 330:20/5; λ ) 340:16/4; λ ) 350:13/
