Accepted Manuscript Photolysis of inorganic chloramines and efficiency of trichloramine abatement by UV treatment of swimming pool water Fabian Soltermann, Tobias Widler, Silvio Canonica, Urs von Gunten PII:
S0043-1354(14)00150-X
DOI:
10.1016/j.watres.2014.02.034
Reference:
WR 10505
To appear in:
Water Research
Received Date: 13 December 2013 Revised Date:
12 February 2014
Accepted Date: 16 February 2014
Please cite this article as: Soltermann, F., Widler, T., Canonica, S., von Gunten, U., Photolysis of inorganic chloramines and efficiency of trichloramine abatement by UV treatment of swimming pool water, Water Research (2014), doi: 10.1016/j.watres.2014.02.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Photolysis of inorganic chloramines and efficiency of trichloramine
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abatement by UV treatment of swimming pool water
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Fabian Soltermann
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, Tobias Widler
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, Silvio Canonica† and Urs von Gunten*†,ǂ,‡
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Eawag, Swiss Federal Institute of Aquatic Science and Technology
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CH-8600, Dübendorf, Switzerland ‡
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Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich,
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CH-8092, Zürich, Switzerland ǂ
School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne (EPFL),
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CH-1015 Lausanne, Switzerland.
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*To whom correspondence should be addressed.
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E-mail:
[email protected] 17
Phone: +41-58-765-5270
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Fax: +41-58-765-5210
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Abstract
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Trichloramine, one of the three inorganic chloramines (mono-, di- and trichloramine), is a
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problematic disinfection by-product in recreational pool water since it causes skin and eye
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irritations as well as irritations of the respiratory tract. The most commonly used chloramine
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mitigation strategy in pool water is UV treatment. Experiments with membrane inlet mass
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spectrometry (MIMS) confirmed that inorganic chloramines are effectively degraded by UV
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irradiation with low-pressure (LP) and medium-pressure (MP) mercury lamps (apparent
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quantum yields (QY): NH2Cl = 0.50 (LP) and 0.31 (MP) mol einstein-1, NHCl2: 1.06 (LP) and
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0.85 (MP) mol einstein-1). Trichloramine showed the fastest depletion with a quantum yield
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slightly above 2 mol einstein-1 in purified (LP and MP) and pool water (MP). This high
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quantum yield can partly be explained by reactions involving •OH radicals (purified water) and
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the reaction of trichloramine with moieties formed during UV irradiation of pool water. The
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presence of free chlorine affects trichloramine degradation (QY: ~1.5 mol einstein-1) since it
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scavenges •OH radicals and competes with trichloramine for reactive species (e.g. organic
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amines).
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Measurements in a pool facility revealed that the installed UV reactors degraded trichloramine
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by 40–50 % as expected from laboratory experiments. However, trichloramine reduction in the
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pools was less pronounced than in the UV reactors. Model calculations combining pool
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hydraulics with formation / abatement of trichloramine showed that there was a fast
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trichloramine formation in the pool from the residual chlorine and nitrogenous precursors. The
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main factors influencing trichloramine concentrations in pool water are the free chlorine
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concentration and the UV treatment in combination with the recirculation rate through the water
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treatment system.
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Keywords: UV irradiation; pool water; chloramines; trichloramine; photolysis
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1. Introduction Disinfection of recreational pool water is necessary for hygienic safety reasons. Since the turn-
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over time of pool water is in the order of several hours, the presence of a residual disinfectant in
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the pool is statutory in most countries. Commonly, free chlorine is used for disinfection of pool
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water, with the consequence that numerous chlorinated disinfection by-products (DBP) are
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formed (Richardson et al., 2010). Inorganic chloramines (mono-, di- and trichloramine) belong
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to this group of undesired DBPs. Mono- and dichloramine are not known to have serious
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adverse health effects but to act as precursors for more potent DBPs such as nitrosamines
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(Schreiber and Mitch, 2006; Soltermann et al., 2013). In contrast, trichloramine can provoke
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skin and eye irritations and is suspected to cause inflammation of the respiratory tract and
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potentially also asthma (Bernard et al., 2003; Schmalz et al., 2011b; Parrat et al., 2012).
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Trichloramine is clearly more volatile than mono- and dichloramine, hence uptake via
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inhalation is the dominant pathway of exposure.
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The legal threshold for inorganic chloramines, also referred to as combined chlorine, in Swiss
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and German pool water is 0.2 mg L-1 as Cl2 (DIN, 2012; SIA, 2013). In Switzerland a
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regulatory value for trichloramine exists only for indoor pool air (0.2 mg m-3) but not for pool
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water (SIA, 2013). This standard was exceeded in 10−15% of the pool facilities in two studies
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considering a total of more than 100 facilities with different pool types in Germany and
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Switzerland (DGUV, 2009; Parrat et al., 2012). Data on trichloramine levels in pool water is
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very limited due to the lack of adequate analytical methods. Measured trichloramine
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concentrations reported in literature were mostly between 0.1–0.5 µM (0.02–0.1 mg L-1 as Cl2)
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but reached up to ~1 µM (0.2 mg L-1 as Cl2) (Gérardin and Subra, 2004; Weaver et al., 2009;
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Soltermann et al., submitted).
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A substantial fraction of trichloramine in pool water is assumed to originate from the pH-
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dependent reaction of free chlorine with urea (Blatchley and Cheng, 2010; Schmalz et al.,
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swimming pool conditions and there is no kinetic data available on the trichloramine formation
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from other nitrogenous precursors than urea. Even if other precursors have lower concentrations
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than urea, they can significantly contribute to the trichloramine formation if they react quickly
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with free chlorine. Measurement of trichloramine concentrations in various pool waters and on-
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site measurements revealed that the trichloramine concentration is strongly linked to residual
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chlorine and quickly changes with bather load while pH variations (7–7.5) and urea
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concentration play a minor role (Soltermann et al., submitted).
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Because combined chlorine is photosensitive, a common strategy for combined chlorine
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mitigation in pool water is UV treatment. This approach is based on various studies
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investigating the photolysis of combined chlorine with a focus on monochloramine (Cooper et
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al., 2007; Watts and Linden, 2007; Li and Blatchley, 2009; De Laat et al., 2010; Hansen et al.,
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2013). Li and Blatchley (2009) found that higher chlorinated chloramines are more
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photosensitive. The apparent photolysis rate constants depend on the irradiation wavelength due
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to the wavelength-dependence of the molar absorption coefficients and the quantum yields.
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Especially for di- and trichloramines, quantum yields > 1 hinted at the reaction of these species
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with photolysis products. Field studies confirmed that UV treatment with low-pressure (LP) and
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medium-pressure (MP) lamps reduced combined chlorine concentration in pool water by about
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50–70% while no clear effect on trichloramine concentration could be observed (Gérardin et al.,
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2005; Cassan et al., 2006; Kristensen et al., 2009). Furthermore, a significant increase of
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trihalomethane concentrations was linked to UV treatment in some studies (Gérardin et al.,
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2005; Cassan et al., 2006).
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In this study the photolysis (quantum yields, kinetics) of chloramines (NH2Cl, NHCl2, NCl3)
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with LP and MP irradiation was investigated. In addition, trichloramine photolysis was studied
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in different pool waters to elucidate matrix effects. Furthermore, on-site measurements of
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performance of MP UV lamps and their effect on trichloramine and combined chlorine
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concentrations in pool water. Based on this a model was developed to test the influence of
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various operational parameters on trichloramine concentrations.
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2. Materials and Methods 2.1.
Reagents
All chemicals were analytical grade and used without further purification. Sodium hypochlorite
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(6−14% active chlorine), dimethylamine (DMA), tert-butanol (t-BuOH), morpholine (Mor),
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ammonium chloride and ortho-phosphoric acid were obtained from Sigma-Aldrich. Sodium
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dihydrogenphosphate, sodium hydroxide pellets, hydrochloric acid and perchloric acid were
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purchased from Merck. Atrazine was obtained from Riedel-de Haën. Ultrapurified water was
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produced by a “barnstead nanopure” water purification system (Thermo Scientific). Ready-to-
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use DPD-, buffer- and KI-solutions for free and combined chlorine analysis were provided by
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SWAN Analytical Instruments AG (Hinwil, Switzerland). Chloramine stock solutions (NH2Cl:
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25 mM, NHCl2 and NCl3: ~2 mM) were produced as described elsewhere (Soltermann et al.,
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submitted).
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2.2.
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Analytical and methods
Chloramine analyses were performed with a membrane inlet mass spectrometer (MIMS 2000
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from Microlab, Aarhus). The inlet temperature was fixed at 40° C and the sample flow rate was
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about 8 mL min-1. In purified water, the signals of the ions (m/z) NH237Cl•+ (53), NH35Cl37Cl•+
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(87) and N35Cl37Cl•+ / N37Cl37Cl•+ (86 / 88) were used to quantify mono-, di- and trichloramine,
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respectively. Trichloramine in pool water samples was measured by analysing 88 m/z ion signal
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(Weaver et al., 2009). During the field study in the pool facility, signal 83 m/z was analysed,
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which presumably represents the trihalomethane concentration (ion HC35Cl35Cl•+ from
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chloroform and monobromodichloromethane) with potentially slight interferences from other
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substances. Since in Switzerland the filling water of the pools and the hypochlorite solution
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used for disinfection contain typically very small bromide concentrations, signal 83 m/z mainly
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represents chloroform (von Gunten and Salhi, 2003). Trichloramine calibrations were
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water for on-site trichloramine measurements. It was demonstrated that the pool water matrix
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affects trichloramine measurement with MIMS (Soltermann et al., submitted). During the field
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campaign in the pool facility, MIMS calibrations (performed every 2−4 days) revealed a
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continuous loss of sensitivity. Therefore, trichloramine concentrations were calculated with an
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interpolated slope. Further information on MIMS analysis (interpolation of calibration slope
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and baseline) is given in the supporting information (SI, section E).
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Free and combined chlorine measurements in the laboratory were performed with a DPD test
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kit (Chematest 20 S, SWAN Analytical Instruments AG, Hinwil, Switzerland). On-site
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measurements were conducted with both the test kit (Chematest 20 S) and an on-line
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measurement system (Analyzer AMI Codes II-CC, SWAN Analytical Instruments AG).
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Spectrophotometrical measurements were performed with a Cary 100 Scan (Varian), and pH
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was measured with a 632 pH-Meter (Metrohm). A high performance liquid chromatography
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(HPLC, Agilent 1100 series) coupled with a UV-visible diode array detector was used for
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atrazine analysis. Urea measurements were performed by the enzymatic hydrolysis of urea to
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ammonia with urease. Thereafter, ammonia was quantified according to the recommendations
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of the Federal Office of Public Health (FOPH) (FOPH, 2007) using a Beckman DU 640
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spectrophotometer.
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2.3.
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2.3.1 Photoreactor and actinometry
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UV irradiation experiments
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UV irradiation experiments were performed with a low-pressure (LP) (Heraeus Noblelight,
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TNN 15/32) and a medium-pressure (MP) mercury lamp (Heraeus Noblelight, TQ 150) in a
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temperature controlled (25 ± 0.2° C) merry-go-round photoreactor. To lower the fluence rate of
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the MP lamp independently of the wavelength by about 75%, two stainless-steel wire cloths
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were wrapped around the quartz cooling jacket. Atrazine (5 µM, buffered at pH 7.0 with 2 mM 7
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phosphate buffer) was used as chemical actinometer to regularly determine the UV lamp
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intensity. Thereby, the fluence rate of the UV lamp was calculated using equation 1:
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− =
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where the subscript λ indicates the wavelength of light, !"# (238−400 nm) is the photon fluence
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rate (einstein m-2 s-1) in the wavelength range of 238−400 nm, kp,atrazine is the pseudo first-order
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rate constant of atrazine photolysis (s-1), ɸatrazine is the quantum yield of atrazine photolysis
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(0.046 mol einstein-1 (Hessler et al., 1993), assumed to be λ-independent), fp,λ is the emission
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spectrum of the UV lamp (according to the manufacturer) normalized to the wavelength
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interval of 238−400 nm and εatrazine,λ is the spectral molar absorption coefficient of atrazine (m2
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mol-1). UV doses were calculated from !"# (238−400 nm) under consideration of the
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wavelength-dependent photon energy because the manufacturer did not quantitatively report a
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UV lamp emission for λ < 238 nm. However, there seems to be a slight light emission at λ