Fluence-based rate constant Application of Nitrate-Nitrite and Atrazine Actinometry for Determination of UV Fluence in Drinking Water and Reuse Applications Dr. Aleksey Pisarenko1,‡, Erica Marti1,2, Robert Young1,3, Dr. Eric Dickenson1,*  1-Southern Nevada Water Authority, 2-University of Nevada Las Vegas, 3-Colorado State University Introduction The use of nitrate-nitrite as a chemical actinometer for the determination of UV fluence during drinking water treatment has recently been reported (IJpelaar, Harmsen et al. 2006). In wastewater treatment, UV radiation is used for disinfection as well as for destruction of trace organic compounds susceptible to direct photolysis, such as N-Nitrosodimethylamine. A common measure of UV radiation is expressed as fluence in units of mJ/cm2, which may be difficult to measure in pilot-scale or full-scale UV reactors, where spiking a chemical at sufficient concentration prior to UV treatment is impractical. Actinometry provides a direct measure of UV fluence without knowledge of precise geometrical configuration of the UV reactor. Nitrate ion is also commonly present in natural and reclaimed waters, allowing the determination of UV fluence. Results The predicted formation of nitrite, based on the literature Φ, initial [NO3-], and bench-scale collimated beam UV reactor parameters, was overlaid with the experimental data in various water matrices, shown by Figure 1. Without the addition of an •OH scavenging agent such as t-butanol, or ethanol, formation of nitrite in DI water spiked with nitrate ion was higher than predicted, as indicated by the slope. In more complex matrices such as drinking water and treated wastewater, this effect of enhanced nitrite formation was even more pronounced. These results suggest that effective • OH scavenging alone may not affect nitrite yield, since DOM will readily scavenge •OH. Recent studies suggest that small amounts of • OH may be produced from photolysis of DOM, while other studies suggest that DOM acts as a sensitizer in nitrite formation. At higher fluences, such as ones used during advanced oxidation, the same trend of enhanced nitrite formation is observed as depicted by Figure 2. The enhanced nitrite formation was shown in bench-scale and pilot-scale reactor experiments with an enhanced nitrite yield of 1.2-3.0 times greater than literature values would suggest. Table 1 provides a summary of experimentally determined quantum yields of nitrate during exposure to 254 nm light, produced by the commonly used low pressure mercury lamps in water treatment. The experimental UV fluence values based on atrazine degradation were within ~15% of the radiometer readings, as can be shown by Figure 3. There were no apparent effects of DOM and water matrix on atrazine photolysis, as compared to nitrate. Implications Though, it is convenient that nitrate can be present at significant concentration in both drinking water and reclaimed water matrices, these results indicate that nitrate should be used with caution for determination of UV fluence regardless of the water matrix. Furthermore, presence of significant DOM appears to further exacerbate this effect. Therefore, atrazine provides a more robust alternative for determination of UV fluence in drinking and reclaimed water matrices during bench-scale work. Atrazine may be present in reclaimed water at detectable amounts, thus allowing determination of fluence without addition. Nitrate-Nitrite Actinometry. The formation pathway of nitrite during UV exposure of nitrate ion is complex and can depend on various water quality parameters, such as dissolved organic matter (DOM) concentration and other constituents that react with intermediate radicals produced during photolysis of nitrate. Nitrate undergoes photolysis by monochromatic light in the range of 205-300 according to the following pathways, as shown by Equation 1 and 2 (equations adopted from Goldstein and Rabani, 2008): At 254.7 nm, nitrate has a known Quantum Yield (Фλ) of 0.065 and a relatively low molar absorptivity coefficient (ελ) of 3.42 L•mol-1•cm-1 (Goldstein and Rabani 2008). Atrazine Actinometry. Atrazine, a well characterized actinometer may also be present in reclaimed waters and provide an alternative to nitrate system. Atrazine has been well characterized in previously reported studies, with a known Quantum Yield (Фλ) of 0.046 and molar absorptivity coefficient (ελ) of 3860 L•mol-1•cm-1 at 254 nm (Canonica, Meunier et al. 2008). Atrazine undergoes a direct photo-degradation following first-order kinetics. A fluence-based rate constant, k’, can be determined by plotting ln(C/Co) vs. UV dose in mJ/cm2. However, k’ can also be calculated based on the following Equation 3: (3) k´ = Фλ• ελ• ln(10) / 10•Uλ Where, Uλ is the energy carried by 1 mol of photons at specific wavelength λ. At 254 nm this equates to 47,528 J/Einstein (Bolton and Stefan 2002). Objectives The main objective of this study was to compare the measurements of UV fluence by nitrate-nitrite, atrazine, and radiometry in natural and reclaimed waters. In addition, this study examined the effects of DOM on the nitrite yield and discussed its practical uses for determination of UV fluence at disinfection and advanced oxidation doses. Figure 1. Formation of nitrite ion in various water samples. Figure 2. Formation of nitrite ion at higher fluences. [NO3-]o=0.9 mM [NO3-]o=11.2 mM Table 1. Experimental nitrate-nitrite quantum yields and fluence-based rate constants. Nitrate/Nitrite   Literature This study (Goldstein and Rabani 2008) DI water CRW DW WW-A WW-B Quantum Yield Φ 0.065 0.088 ± 0.003 0.076± 0.003 0.196 ± 0.004 0.145 ± 0.041 0.147 ± 0.024 Fluence-based rate constant k', (mJ/cm2) x 10-5 1.35 1.82 1.57 4.08 3.00 3.05 Current Work Assess •OH exposure using p-chlorobenzoic acid as a probe during UV/NO3 and compare to UV/DOM in various water matrices. Isolate the effects of •OH on nitrite yield by introducing a scavenging agent such as t-butanol. Determine conditions at which nitrate/nitrite may be used reliably as an actinometer. Acknowledgements The authors thank Janie Zeigler, Dan Gerrity, Beck Trenholm, and Yue Wang for participation in this study and SNWA’s laboratory staff for assisting with sample analysis. Figure 3. Determination of fluence by atrazine and radiometer. Methods The Project Team utilized a bench-scale collimated beam apparatus and used a UV radiometer to supplement determination of atrazine degradation studies in pilot-scale operation. Various correction factors that are specific to the water quality (mainly absorbance at specific wavelength) and to the UV apparatus were determined based on previously reported methodology (Bolton and Stefan 2002) and applied for accurate estimation of target UV dose.  Sample absorbance at 254 nm was measured using a Perkin-Elmer Lambda 45 UV-VIS Spectrometer, consistent with Standard Method 5910 B. Analysis of inorganic water constituents was performed based on Standard Methods. For determination of atrazine, samples were analyzed by an LC-MS/MS method in positive ion mode, using external calibration, and multiple reaction monitoring (MRM). References Bolton, J. R. and M. I. Stefan (2002). "Fundamental photochemical approach to the concepts of fluence (UV dose) and electrical energy efficiency in photochemical degradation reactions." Research of Chemical Intermediates 28(7-9): 857-870. Canonica, S., L. Meunier, et al. (2008). "Phototransformation of selected pharmaceuticals during UV treatment of drinking water." Water Research 42(1-2): 121-128. Goldstein, S. and J. Rabani (2008). "Polychromatic UV Photon Irradiance Measurements Using Chemical Actinometers Based on NO3- and H2O2 Excitation: Applications for Industrial Photoreactors." Environmental Science & Technology 42(9): 3248-3253. IJpelaar, G. F., D. Harmsen, et al. (2006). Fluence Monitoring in UV Disinfection Systems: Development of a Fluence Meter. Denver, CO, Awwa Research Foundation and Kiwa Water Research. Changes in nitrite and atrazine concentration as a function of UV exposure were monitored in various water matrices. Samples included laboratory DI water, Colorado River Water (CRW), finished drinking water (DW), non-nitrified and nitrified wastewaters (WW-A and WW-B, respectively). For DI water, CRW, DW, and WW-A samples, nitrate ion was fortified to 11.32 mM (158.5 mg-N/L), while atrazine was spiked at 0.1 µM (20 µg/L) or less. For WW-B only atrazine was spiked, since nitrate ion was present at 0.9 mM (12.7 mg-N/L). ‡-Aleks.Pisarenko@snwa.com *- Eric.Dickenson@snwa.com