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Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids Chenyi Yuan, Mrinal Chakraborty, Silvio Canonica, Linda Kay Weavers, Christopher M. Hadad, and Yu-Ping Chin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03655 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016
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Environmental Science & Technology
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Isoproturon Reappearance after
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Photosensitized Degradation in the Presence
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of Triplet Ketones or Fulvic Acids
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Chenyi Yuan,a Mrinal Chakraborty,b,† Silvio Canonica,c Linda K. Weavers,d Christopher
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M. Hadad,b Yu-Ping Chin e,*
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a
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43210.
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b
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Ohio, 43210.
Environmental Science Graduate Program, The Ohio State University, Columbus, Ohio,
Department of Chemistry and Biochemistry, The Ohio State University, Columbus,
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c
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Dübendorf, Switzerland.
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d
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University, Columbus, Ohio, 43210.
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e
EAWAG, Swiss Federal Institution of Aquatic Science and Technology, CH-8600,
Department of Civil, Environmental and Geodetic Engineering, The Ohio State
School of Earth Sciences, The Ohio State University, Columbus, Ohio, 43210.
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ABSTRACT
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Isoproturon (IPU) is a phenylurea herbicide used to control broad-leaf grasses on grain
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fields. Photosensitized transformation induced by excited triplet states of dissolved
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organic matter (3DOM*) has been identified as an important degradation pathway for IPU
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in sunlit waters, but the reappearance of IPU in the absence of light is observed after the
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initial photolysis. In this study, we elucidate the kinetics of this photo-degradation and
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dark-reappearance cycling of IPU in the presence of DOM proxies (aromatic ketones and
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reference fulvic acids). Using mass spectrometry and nuclear magnetic resonance
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spectroscopic techniques, a semi-stable intermediate (IPUint) was found to be responsible
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for IPU reversion and was identified as a hydroperoxyl derivative of IPU. IPUint is photo-
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generated from incorporation of diatomic oxygen to IPU and is subjected to thermolysis
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whose rate depends on temperature, pH, the presence of DOM, and inorganic ions. These
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results are important to understand the overall aquatic fate of IPU and structurally similar
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compounds under diurnal conditions.
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TOC/ABSTRACT ART
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TEXT
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Introduction
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Direct or indirect (sensitized) photochemical reactions have been identified as important
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initiation steps in the natural attenuation of a variety of synthetic organic chemicals
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(SOCs) in shallow sunlit waters.1,2 However, sunlight intensity is spatiotemporally
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variable due to sun angle changes and cloud cover. As a result, the dominant
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transformation pathways may vary with sunlight intensity. Under sunny conditions,
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photolysis of SOCs generates unstable intermediates which, in turn, react by both
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photochemical and thermal routes. When light intensity becomes low or absent, the
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photo-generated intermediates can only react thermally, possibly forming different
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products compared to those formed at high light intensity. These thermally formed
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products may even include the parent compound. In this case, the intermediate-to-parent
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reversion leads to the persistence of the parent compound, which may not be accurately
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predicted based upon continuous photodegradation studies. A fast reversion from a short-
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lived intermediate will inhibit the overall degradation rate even in the presence of light,3
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whereas a slow reversion from a long-lived intermediate may result in a diurnal cycling
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of the parent chemical in the aquatic system, such as trenbolone acetate metabolites,4
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2,4,6-trimethylphenol,5 and substituted naphthalene.6
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Isoproturon (IPU,C12H18N2O, 3-(4-isopropylphenyl)-1,1-dimethylurea) is a compound
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that reappears thermally after indirect photolysis.7 As a substituted phenylurea herbicide,
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IPU is often applied to grain fields to kill broad-leaf weeds by inhibiting photosynthesis.
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However, IPU potentially promotes liver tumors in rats and impedes periphytic bacterial
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communities and aquatic invertebrates.8,9 Decades of extensive use of IPU in several
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European and Asian countries has led to occasionally elevated concentrations in surface
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water and groundwater.10-12 In sunlit waters, natural attenuation of IPU induced by photo-
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excited triplet states of dissolved organic matter (3DOM*) may be an important
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degradation pathway in addition to microbial degradation.7, 12 In a study using aromatic
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ketones (benzophenone and 3’-methoxy-acetophenone) as model 3DOM*
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photosensitizers, the reappearance of IPU in the dark after initial photolysis was first
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reported and proposed to result from a hydrophilic peroxide intermediate (IPUint).7
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However, the identity of IPUint, the connection between IPUint and IPU, and the detailed
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diurnal kinetics of photo-degradation and dark-reappearance of IPU were not further
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reported.
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The objectives of this paper were: (1) to confirm the cycling of IPU in the presence of
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model DOM compounds (aromatic ketones) and aquatic fulvic acids; (2) to quantify the
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significance and efficiency of this dark back reaction; and (3) to elucidate the diurnal
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transformation mechanism of IPU in aquatic systems by identifying the molecular
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structure of the reversible intermediate and by testing the effect of water constituents on
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the back reaction.
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Experimental
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Chemicals and analytical techniques. All chemicals were obtained from commercial
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sources and used without further purification. For a complete list of chemicals, see the
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Supporting Information (SI, Text S1 on page S5-S6). Suwannee River fulvic acid (SRFA)
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and Pony Lake fulvic acid (PLFA) were obtained from the International Humic
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Substances Society. Analytical techniques including High Performance Liquid
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Chromatography (HPLC) analysis are detailed in the SI (Text S1 on page S6-S8).
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Photochemical and sequential dark reaction kinetics. Similar experiments were
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independently designed and conducted in Columbus, OH and Dübendorf, CH.
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Methods used in Columbus. (a) Photochemical experiments. Solutions of 10 µM IPU and
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25 µM acetophenone (ACP) dissolved in pH 8 buffers (1mM NaHCO3) were sealed in
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quartz tubes (9 mm internal diameter) and irradiated in an Atlas Suntest CPS+ solar
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simulator equipped with a 500 W/m2 Xenon lamp and solar standard filter for 7 h at 25 ±
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2°C. A subset of the light experiments replaced NaHCO3 and ACP solutions with 5.5 or
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11 mgC/L PLFA or SRFA solutions and was adjusted to pH 8 with HCl or NaOH.
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Another subset used argon-saturated solution (See Text S1 on page S9). Control
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experiments in the absence of any photosensitizer were also conducted to measure IPU’s
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direct photolysis. (b) Experiments in the dark. Photolyzed solutions at selected time
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points were transferred to amber vials and kept in a dark room (22 ± 2°C) for 4 days. In
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selected ACP experiments to elucidate the dark reaction mechanism, adjustments
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including temperature adjustment with water bath/refrigeration and pH adjustment (acidic
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pH) with HCl immediately after photolysis were performed.
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Methods used in Dübendorf. (a) Photochemical experiments. A DEMA 125 merry-go-
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round photoreactor (Hans Mangels, Bornheim-Roisdorf, Germany) equipped with a
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Heraeus Noblelight TQ 718 medium-pressure mercury lamp (operated at 500 W), a
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borosilicate glass cooling jacket (10 °C) and a filter solution, was used. The experimental
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setup is described in detail elsewhere.13, 14 The photoreactor was operated using two
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different filter solutions. For irradiations of solutions containing 4-carboxybenzophenone
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(CBBP) as a photosensitizer, a 0.15 M sodium nitrate filter solution was used, whereby
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irradiation wavelengths
