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Degradation of chlorotriazine pesticides by sulfate radicals and influence of organic matter Holger V. Lutze, Stephanie Bircher, Insa Rapp, Nils Kerlin, Rani Bakkour, Melanie Geisler, Clemens von Sonntag, and Torsten C Schmidt Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es503496u • Publication Date (Web): 27 Oct 2014 Downloaded from http://pubs.acs.org on October 28, 2014

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Degradation of chlorotriazine

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pesticides by sulfate radicals and the

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influence of organic matter

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Holger V. Lutze1,2*, Stephanie Bircher1, Insa Rapp1, Nils Kerlin1, Rani Bakkour1, Melanie Geisler1, Clemens von Sonntag1,3 and Torsten C. Schmidt1,2,4

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Essen, Germany, (2) IWW Water Centre, Moritzstr. 26, D-45476 Mülheim an der Ruhr,

(1) University Duisburg-Essen, Instrumental Analytical Chemistry Universitätsstr. 5 D-45141

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Germany, (3) Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, P.O. Box

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101365, D-45470 Mülheim an der Ruhr, Germany, (4) Centre for Water and Environmental

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Research, Universitätsstraße 2, D-45117 Essen, Germany

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e-mail: [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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TOC art

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Abstract: Atrazine, propazine, and terbuthylazine are chlorotriazine herbicides that have been

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frequently used in agriculture and thus are potential drinking water contaminants. Hydroxyl

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radicals produced by advanced oxidation processes can degrade these persistent compounds.

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These herbicides are also very reactive with sulfate radicals (2.2–4.3 × 109 M-1 s-1). However,

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the dealkylated products of chlorotriazine pesticides are less reactive towards sulfate radicals

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(e.g., desethyl-desisopropyl-atrazine (DEDIA; 1.5 × 108 M-1 s-1). The high reactivity of the

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herbicides is largely due to the ethyl- or isopropyl group. For example, desisopropyl-atrazine

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(DIA) reacts quickly (k = 2 × 109 M-1 s-1), whereas desethyl-atrazine (DEA) reacts more

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slowly (k = 9.6 × 108 M-1 s-1). The tert-butyl group does not have a strong effect on reaction

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rate, as shown by the similar second order reaction rates between desethyl-terbuthylazine

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(DET; k = 3.6 × 108 M-1 s-1) and DEDIA. Sulfate radicals degrade a significant proportion of

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atrazine (63%) via dealkylation, in which deethylation significantly dominates over

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deisopropylation (10:1). Sulfate- and hydroxyl radicals react at an equally fast rate with

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atrazine (k (hydroxyl radical + atrazine) = 3 × 109 M-1 s-1). However, sulfate- and hydroxyl

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radicals differ considerably in their reaction rates with humic acids (k (sulfate radical + humic

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acids) = 6.6 × 103 L mg C-1 s-1; k (hydroxyl radical + humic acids) = 1.4 × 104 L mg C-1 s-1).

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Thus, in the presence of humic acids, atrazine is degraded more efficiently by sulfate radicals

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than by hydroxyl radicals.

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KEYWORDS: CHLOROTRIAZINE PESTICIDES, DEALKYLATION, RATE CONSTANTS, SULFATE

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RADICALS, HYDROXYL RADICALS, WATER TREATMENT

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Introduction

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Oxidation based on the sulfate radical (SO4●─) is of increasing interest as an oxidative water

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treatment. SO4●─ is a strong one-electron oxidant, but it also readily reacts by addition to C–C

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double bonds and by H-abstraction. Thus, it is capable of oxidizing a large number of

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pollutants such as arsenic [1], trichloroethene [2], tert-butylmethylether [3], endosulfan [4],

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and different derivates of phenol [5-7]. SO4●─ can be produced by photolysis [8, 9],

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thermolysis [10], or reduction of peroxodisulfate (S2O82─) by transition metals in their low

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oxidation states [5, 11-13]. Mechanisms of SO4●─ reactions differ somewhat from those of

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hydroxyl radical (●OH) reactions. SO4●─ reacts more readily by electron transfer than ●OH, 3

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but slower by H-abstraction and addition [14, 15]. Thus, reactivity, product pattern, and

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energy efficiency of SO4●─-based oxidation may also be different from ●OH-based oxidation.

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For example, perfluorinated carboxylic acids are inert toward ●OH but can be degraded by

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SO4●─ [16, 17]. This indicates that SO4●─-based oxidation may complement more common

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(●OH-based) advanced oxidation processes.

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The reaction of SO4●─ with atrazine has been described to be very fast (k (SO4●─ + atrazine) =

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3 × 109 M-1 s-1 [18]), similar to the reaction rate of ●OH (k (● OH + atrazine) = 2.4–3.0 × 109

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M-1 s-1 [19-21]). The herbicide is readily dealkylated by ● OH, however, the triazine ring

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system also seems to be attacked, but products have not been identified [21]. In the European

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Union, a drinking water standard of 0.1 µg L-1 has been defined for a particular pesticide and

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0.5 µg L-1 for the sum of all pesticides. Chlorotriazine pesticides continue to be an issue for

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the water suppliers and public health. Although the use of atrazine as a herbicide has been

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banned in the European Union since March 2004 (commission decision 2004/248/EC &

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commission decision 2004/247/EC), atrazine has frequently been found in European ground

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waters even in the year 2010. The same study also reports the detection of terbuthylazine [22].

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The mechanism of the ●OH reaction with atrazine has already been investigated in detail [19,

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21], and a mechanistic study on the reaction of atrazine with SO4●─ was published recently

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[23]. The present work provides a kinetic study on the reaction of different chlorotriazine

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herbicides and their products with SO4●─ and also provides further advancements in the

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mechanistic ideas on the reaction of atrazine with SO4●─ by providing product yields of main

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products. Furthermore, the efficiency of SO4●─-based oxidation to degrade atrazine in the

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presence of organic matter was investigated.

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Materials and methods

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All chemicals were commercially available and used as received: Acetaldehyde (p.a.) from

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Sigma-Aldrich; acetone (p.a.) from KMF Laborchemie Handels GmbH; argon (5.0) from Air

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liquid; atrazine (97.4%) from Sigma-Aldrich; 4-chlorobenzoic acid (pCBA) (99%) from

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Aldrich; desethyl-atrazine (DEA) (99%) from Sigma-Aldrich; desethyl-desisopropyl-atrazine

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(DEDIA) (97.8%) from Sigma-Aldrich; desisopropyl-atrazine (DIA) (95.4%) from Sigma-

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Aldrich; 2,4-dinitrophenyl-hydrazine (50% in water, p.a.) (DNPH) from Merck; ethanol

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(≥99.8% v/v) from Sigma-Aldrich; humic acids (Depur) (45–70%) from Carl Roth;

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hydrochloric acid (37% in water, p.a.) from Merck; methanol (p.a.) from Sigma-Aldrich; 4

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perchloric acid (60.4% in water, p.a.) from J.T. Baker; simazine (99%) from Sigma-Aldrich;

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sodium hydroxide (≥99.9%, p.a.) from Sigma-Aldrich; sodium peroxodisulfate (p.a.) from

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Sigma-Aldrich; sulfuric acid (98%, p.a.) from Merck; tert-butyl alcohol (≥99%) from Merck;

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terbuthylazine (98.8%) from Sigma-Aldrich; and desethyl-terbuthylazine (DET) (97.4%) from

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Sigma-Aldrich.

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SO4●─ rate constants for atrazine and the other chlorotriazines (terbuthylazine, propazine,

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DEA, DIA, DEDIA, and DET) have been determined by competition kinetics (Refer to the

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Supporting Information; SI) [24] using the following competitors: pCBA, k (SO4●─ + pCBA)

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= 3.6 × 108 M-1 s-1 [25], acetanilide, k (SO4●─ + acetanilide) = 3.6 × 109 M-1 s-1 [25], and

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atrazine, k (SO4●─ + atrazine) = 3 × 109 M-1 s-1 [18]. The caption for Table 1 indicates the

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choice of competitors for determining the different rate constants. SO4●─ were generated by

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thermolysis of peroxodisulfate (S2O82–) at 60°C. The second order reaction rates obtained at

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this temperature correspond to the temperature at which the reference compound was

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determined (refer to the SI). The reaction solutions were adjusted to pH 7 with sodium

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hydroxide. Unless otherwise specified, buffers were not added because they can also react

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with SO4●─ (e.g., k (HPO42– + SO4●─) = 1.2 × 106 M-1 s-1 [15]). This reaction leads to the

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formation of HPO4●─, which could also react with the chlorotriazines but at a different rate.

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Similar problems may arise with other buffers.

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In a SO4●─-based process, solutions slowly acidify because in the reaction of SO4●─ plus

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triazines, protons could be released (see below). This can be problematic since protonation of

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the substrates may change both the reaction kinetics and the mechanism. Thus, we maintained

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the pH at least 1 pH unit above the highest pKa value of the corresponding substrates.

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Because SO4●─ normally reacts slower with protonated species than with deprotonated species

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(compare reaction rates compiled in Neta et al. [15]), the maximal 10% contribution of these

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compounds in our study can be neglected. In the present experiments, the reaction time was

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sufficiently short so that the pH did not drop below this value. After heating up the sample,

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S2O82– was added to initiate the reaction (S2O82─ dose was 1 mM). After different reaction

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times, aliquots were withdrawn and chilled on ice. Methanol was immediately added (1 M in

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the sample, ≈4% v/v) for scavenging the SO4●─ that could be formed at low rates during

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storage time. It was observed that small amounts of chloride (2–30 µM) were present in all

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experiments. Since concentrations of Cl─ in the µM range can be introduced into samples by

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several means such use of standard electrodes for determination of pH (KCl as background 5

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electrolyte) and impurities in chemicals, it is very difficult to avoid the presence of Cl─. SO4●─

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reacts moderately fast with Cl─ (see below), and in that reaction ●OH are formed at neutral pH

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[26]. For minimizing a bias by ● OH in the competition experiments, tert-butanol was added.

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By that means, formation of ●OH was suppressed since ●OH reacts about 1000 times faster

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with tert-butanol than SO4●─ (k (SO4●─) = 4.0–9.5 × 105 M-1 s-1 [27, 28], k (●OH) = 6 × 108 M-

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1

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and 10 mM), while 5 µM of the triazines and competitors were added. In the case of atrazine

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(2–3 × 109 M-1 s-1 = k (●OH) [19, 21] = k(SO4●─) [18]), ●OH are largely scavenged by tert-

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butanol >95% (with 1 and 10 mM tert-butanol), whereas SO4●─ are scavenged by only 3–6%

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(at 1 mM tert-butanol) and 20–40% (at 10 mM tert-butanol) (calculated by competition

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kinetics (refer to the SI) and the reaction rates given above). In the case of the product studies,

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tert-butanol was not added, so as to prevent interferences in the product pattern. However, the

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substrate (atrazine) was dosed in higher concentrations (20–25 µM) compared with the

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competition kinetic experiments. Since atrazine reacts faster with SO4●─ than Cl─, the reaction

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of SO4●─ plus Cl─ is suppressed (average value obtained from rate constants provided by [30]

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is k (Cl─ + SO4●─) = 3.3 × 108 M-1 s-1). Given a maximal Cl─ concentration of 30 µM (see

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above), no more than 14% of the SO4●─ reacted with Cl─ yielding ●OH (calculated by

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competition kinetics). As will be explained below, this did not influence the outcome of

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product formation in the primary reaction of SO4●─ plus atrazine.

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Photochemical experiments were conducted using a merry-go-round photo reactor according

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to Wegelin et al. [31] equipped with quartz tubes for irradiating the sample (radiation source

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GPH303T5L/4, 15 W, Heraeus Noble Light; refer to the SI). Fluence rates were determined

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by atrazine actinometry in solutions containing humic acids according to Canonica et al. [32]

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and by uridine actinometry in pure water according to von Sonntag and Schuchmann [33].

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All compounds were separated by HPLC on a C-18 reversed phase column and detected by

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UV absorption. The following HPLC-systems were used: 1) Shimadzu: liquid chromatograph

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LC 20-AT, UV/Vis detector SPD 20A, auto sampler SIL 20A, degasser DGU-20A5,

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communication bus module CBM 20A, and column oven CTO-10AS vp. 2) Shimadzu: liquid

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chromatograph LC-10AT vp, diode array detector SPD M10A vp, auto sampler SIL 10 AD

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vp, degasser DGU 14A, system controller SCL-10A vp, and column oven CTO-10AS vp. 3)

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Agilent 1100 Series: quaternary pump G1311A, UV-Vis detector VWD G1314A, autosampler

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ALS G1313A, degasser G1379A, and column oven Colom G1316A.

s-1 [29]). For the individual experiments, two concentrations of tert-butanol were dosed (1

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For separation of the analytes reversed phase C-18 columns were used (5 µm particle size,

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250 × 4.6 mm from Knauer or Bischoff). For the mobile phase, various isocratic mixtures and

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gradients of water and methanol as well as water and acetonitrile were used. All compounds

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were determined by UV absorption at their maxima (chlorotriazine herbicides: 210–225 nm,

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pCBA: 235–239 nm, and acetanilide: 224 nm).

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Acetone and acetaldehyde were derivatized with 2,4-dinitrophenylhydrazine [34] and the

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hydrazones quantified by HPLC-UV. The experiments were carried out in air-saturated

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solution.

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Cl─ was analyzed by ion chromatography (Metrohm 883 basic) equipped with a conductivity

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detector coupled with ion suppression (anion separation column with quaternary ammonium

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groups: Metrosep A Supp 4 - 250/4.0 mm, particle size 9 µm; eluent: a solution of HCO3─

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(1.7-1.9 mM), CO32─ (1.8 mM) was mixed with acetonitrile (70% aqueous buffer : 30 %

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acetonitrile); flow: 1 mL min-1).

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Results and discussion

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Kinetics. The rate constants for the reaction of SO4●─ and ●OH with the investigated triazine

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herbicides and their dealkylation products are compiled in Table 1.

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Table 1: Rate constants for the reaction of SO4●─ and ●OH with s-triazines and competitors. n

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= number of replicates. SO4●─ / M-1 s-1 (4.2 ± 0.20) × 109 a) n = 4 (4.7 ± 0.10) × 109 c) n = 4 Average: 4.5 ± 0.30 × 109 3 × 109 [18] 2.6 × 109 [23] 1.4 × 109 [35] (2.0 ± 0.57) × 109 a) n = 4

●

(9.6 ± 0.17) × 108 a) n = 4

1.2 × 109 [20]

(1.5 ± 0.08) × 108 a) n = 4

isopropyl- >> tert-butyl group attached to the triazine ring.

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This indicates that in case of triazine herbicides, N-ethyl and to a somewhat lesser extent the

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N-isopropyl group is a key factor in the reactivity of SO4●─ towards s-triazine-herbicides. In

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that it finds its analogy in ●OH reactions, albeit SO4●─ display a more distinctive prevalence to

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the ethyl function.

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However, SO4●─ react remarkably faster with DEDIA than ●OH.

). Yet, the reaction of SO4●─ plus DEA displayed a distinctive decrease in rate, albeit it was N-isopropyl group, because the reactivity of DET and DEDIA was

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Product formation. Similar to the studies of Kahn et al. [23], the reaction of SO4●─ with

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atrazine led to dealkylation and thereby produced DEA and DIA as well the corresponding

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products acetaldehyde and acetone (Figure 1).

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Atrazinkonzentration / µM DEA DIA Acetaldehyd Aceton

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Concentration / µM

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20 15 10 5 0 0

5

10

15

20

25

30

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Time / min 211 212 213 214

Figure 1: Degradation of atrazine by SO4●─ and product formation in air-saturated aqueous solution. Initial pH = 7. [S2O82–] = 1 mM. T = 60°C. Atrazine (squares), acetaldehyde (open circles), DEA (filled circles), DIA (filled triangles), and acetone (open triangles).

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The formation of these products has also been observed as a result of

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degradation of atrazine [21]. There is, however, a remarkable difference. In the case of ●OH,

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the DEA to DIA ratio can be calculated to be ≈3, while with SO4●─ it was ≈10 (Table 2).

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Acetone at low turnovers was too uncertain to be reported due to a fluctuating background of

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acetone in the samples. At higher turnovers, acetone was mainly formed as a secondary

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product from SO4●─-induced dealkylation of the major product DEA (Figure 1). In contrast,

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acetone was formed substantially in primary reactions of ●OH with atrazine [21] (Table 2).

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Due to the possible presence of chloride in the µM range, one has to consider a contribution

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of ●OH (see above). As explained above ●OH could contribute to atrazine degradation by

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14%. When we consider that the reaction of ●OH plus atrazine yields 51% DEA and 20%

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DIA, then a 14% contribution of ●OH in atrazine degradation yields a maximum of 7% DEA

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and 3% DIA. A similar situation applies for yields of acetaldehyde and acetone. These minor

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influenc was neglected.

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Table 2: Product yields (%) of atrazine degradation in the SO4●─ and ●OH reactions. The primary yields were derived from a plot of atrazine degradation versus product formation by a linear regression (n = 4 replicates). *Detected but not quantified due to fluctuating background contamination. Product

SO4●─

Acetaldehyde Acetone DEA DIA Mass balance

57 ± 11% * 57 ± 4% 6 ± 0.8% 63%

OH [21] 53% 20% 51% 20% 71%

(DEA+DIA)

(DEA+DIA)

≈10

≈3

Ratio DEA:DIA

●

234 235

Desisopropylation occured to a minor extent (≈6%) in primary reactions of atrazine, reflecting

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the low concentration of DIA during atrazine degradation (Figure 1). The cleavage of the N-

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ethyl group was largely favored over the cleavage of the N-isopropyl group.

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Product yields shown in Table 2 indicate that ≈40% of the SO4●─ reactions gave rise to

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unidentified products. Such an incomplete mass balance has also been reported for ●OH

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reactions with atrazine [21] and in purine free-radical chemistry [14]. For the purines, N-

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centered and O-centered radicals that do not react with O2 play a major role, and in the

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present study it is envisaged that these produced a large number of dimers that escaped

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detection. It is conceivable that the missing fraction in our system was due to the production

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of such dimers. With three ring nitrogens and two exocyclic nitrogens, the number of

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potential dimers is very large; hence, the yield of each individual dimer may be very low and

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result in concentrations below the detection limit. A compilation of other possible degradation

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products in the missing fraction can be found in Kahn et al. [23].

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Reaction mechanism. In the reaction of SO4●─ with electron-rich aromatic compounds,

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electron transfer is a preferred reaction pathway. In the case of methoxy derivatives of

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benzene, the corresponding radical cations are generated as primary short-lived intermediates

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[36]. With aromatic carboxylic acids, the formation of phenyl radicals was observed by ESR,

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pointing to a decarboxylation of the originally formed radical cations [36]. For benzene

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lacking electron-donating groups, hydroxycyclohexadienyl radicals have been observed [37].

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A rapid reaction of water with the originally formed radical cations may account for this. The 11

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SO4●─ reacts with these benzene derivates as fast as it reacts with the triazine herbicides (≥1 ×

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109 M-1 s-1 [25]), and for atrazine it was suggested that SO4●─ also reacts via electron transfer

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[18, 23]. Based on this suggestion, a mechanism can be proposed resulting in dealkylation

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along with formation of carbonylic compounds and free amines. Furthermore, the below

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mechanistic prospects provide a possible explanation for the prevalence of deethylation in the

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reaction of SO4●─ plus atrazine. In the first step, electron transfer yields a radical cation

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(reaction 1).

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Kahn et al. [23] supposed one possible pathway to be the substitution of the chlorine atom by

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a hydroxyl group. However, products detected in the present study indicate that another

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reaction might occur. In the present study it is proposed that the primary radical cation will be

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in equilibrium with the corresponding N-centered radicals (reactions 2 and 4). The N-centered

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radicals are expected to undergo a (water-catalyzed) 1,2-H shift analogous to the well-

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documented 1,2-H shift of alkoxyl radicals [38], thereby forming C-centered radicals

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(reactions 3 and 5). The N-centered radicals do not react with O2, but the C-centered ones

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react rapidly (reaction 6, k = 3 × 109 M-1 s-1) [21].

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The resulting peroxyl radical subsequently loses O2●─ (reactions 7 and 8) or HO2● (reaction

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10) [21]. The subsequent acid and base catalyzed hydrolysis of the imine yields the carbonylic

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compound and the free amine (reaction 10) [19, 21].

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The N–H bond at the N-ethyl group is more acidic than that at the N-isopropyl group and

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deprotonation will occur more preferably at the H–N-ethyl site (reaction 4). Hence, the

280

formation of the C-centered radical and the subsequent reactions (see above) are favored at

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the ethyl side group. That is a conceivable explanation for the prevalence of deethylation in

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the reaction of SO4●─ with atrazine.

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The main reaction pathway of ●OH is abstraction of hydrogen located at the α-position of the

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alkyl groups (preferably at the ethyl group), thus resulting in a C-centered radical that is also

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formed in reactions 3 and 5 [21]. Kahn et al. [23] suggested that SO4●─ could also react via H-

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abstraction, in principle. However, reaction rates of H-abstraction by SO4●─ typically have a

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range of 107–108 M-1 s-1. Thus, the high reaction rate of >109 M-1 s-1 for atrazine, DIA,

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terbuthylazine, and propazine would be unusual. Furthermore, H-abstraction cannot explain 13

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the strong prevalence for deethylation of atrazine. With respect to H-abstraction, the side

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groups of atrazine (ethylamine- and isopropylamine group) are comparable to ethanol and 2-

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propanol that react with SO4●─ via H-abstraction at the α-C-atom [27]. Eibenberger at al. [27]

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determined the reaction rates of SO4●─ plus 2-propanol (k = 3.2 × 107 M-1 s-1) to be faster than

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SO4●─ plus ethanol (k = 1.6 × 107 M-1 s-1). This indicates that if the SO4●─ reacts with atrazine

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by H-abstraction, deisopropylation would be the dominant pathway instead of the observed

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strong prevalence for deethylation.

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Atra-imine from reaction 8 was observed in the present HPLC-DAD measurements. When the

297

analysis occurred ≤2 hours after the experiment, atra-imine was eluted as a dominant product

298

peak at a retention time between atrazine and DEA (Figure S2 (SI). For a quantitative view on

299

the evolution of atra-imine, its response factor was determined (refer to the SI).

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The evolution of atra-imine in atrazine degradation is shown in Figure 2. Even though atra-

301

imine was considered to be the precursor of DEA, the latter compound appeared in all

302

samples. This can be explained by partial hydrolysis of the imine before HPLC determination

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began (an approximate 2–3 h delay between experiment and analysis). The full yield of DEA

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was also determined after complete turnover of the atra-imine (i.e., after ≈72 h). The sum of

305

atra-imine and DEA determined in the measurements started after 2–3 h (see above)

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resembled the DEA yield after complete conversion (measurement 72 h after the experiment,

307

Figure 2 inset).

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12 y = 1.04x - 0.5

DEA (72 h)

10

Concentration / µM

20

8 6 4 2

15

0 0 10

2

4 6 8 10 DEA + Atra-imine

12

5

0 0

10

20

30

40

50

60

Time / min

309 310 311 312 313 314

Figure 2: Degradation of atrazine (open circles) and formation of products: atra-imine (gray circles), DEA (black triangles), and DEDIA (open triangles). Formation of DEA after 72 h (gray squares). Thermal activation of S2O82─, T = 60°C, initial pH 7. [atrazine]0 = 25 µM. [S2O82─]0 = 1 mM. Inset: correlation of the sum of atra-imine and DEA of the first measurement versus DEA formation after 72 h.

315 316

The hydrolysis of isopropyl imine was much faster than atra-imine, and immediately after the

317

reaction, DIA was present in full yield. The reason for the difference in hydrolysis rates of the

318

two imines is not fully understood. Full conversion of the imine to DEA was assured when

319

determining the total yields of desethyl products (Table 2).

320

The above mechanistic considerations can explain why in SO4●─-induced dealkylation of

321

atrazine, formation of DEA plus acetaldehyde was favored over the formation of DIA plus

322

acetone. Yet, in the ●OH-induced reaction, such a preference was also observed, albeit to a

323

smaller extent [21]. This requires that ●OH preferably abstract hydrogen at the α–C of the

324

ethyl side group than the isopropyl group.

325 326

Reactions in the presence of humic acids. Figure 3 shows the degradation of atrazine in the

327

presence of humic acids under conditions of direct photolysis, UV/H2O2, and UV/S2O82–.

328

Degradation by the UV/S2O82– treatment was more efficient than by UV/H2O2, and 15

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considerably more efficient than direct photolysis. The reason for this is the higher rate

330

constant of the reaction ●OH plus dissolved organic matter (DOM) compared with the

331

reaction SO4●─ plus DOM (see below).

concentration / µM

0.5 0.4 0.3 0.2 0.1 0.0 0

2

4

6

8

10

12

14

16

18

Time / min 332 333 334 335 336 337 338 339 340 341

Figure 3: Photochemical degradation of atrazine (0.5 µM) in the presence of humic acids (15 mg C L-1). [S2O82–] = [H2O2] = 1 mM. [phosphate] = 2.5 mM (a contribution of phosphate radicals in this system can be ruled out (refer to the SI). pH = 7.2. Radiation source: low pressure mercury lamp (15 W), Fluence rate (254 nm) = 58 µEinstein m-2 s-1 in pure water and 8.7 µEinstein m-2 s-1 in presence of humic acids (A254 = 1.65 cm-1). Direct photolysis (crosses). UV/H2O2 (closed circles). UV/S2O82– (open circles). Curved lines represent calculated best fit of atrazine degradation for k (SO4●─ plus DOC) and k (●OH plus DOC) according to equation 1 (refer to the SI). Symbols represent measured data. Observed degradation kinetics of atrazine: UV/S2O82–: 2.0 × 10-3 s-1; UV/H2O2: 8.8 × 10-4 s-1; UV: 3.6 × 10-4 s-1.

342 343

According to equation 1, the second order reaction rate of the reaction SO4●─ / ●OH plus

344

humic acids can be calculated using the first order degradation rate of atrazine given the

345

following information: fluence rate, first order degradation of atrazine by direct photolysis,

346

reaction rates with the corresponding radical plus atrazine, and the concentration of humic

347

acids (expressed as mg C L-1).

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Equation 1:

350

k´(atrazine) =

351

Where k´(atrazine ) is the first order degradation rate of atrazine / s-1, ε peroxide is the molar

352

absorption of H2O2 or S2O82─ / m2 mol-1, φ peroxide is the quantum yield of H2O2 or S2O82─ / mol

353

Einstein-1, H is the fluence rate / Einstein m-2 s-1, [ peroxide] is the concentration of H2O2 or

354

S2O82─ / M, k ( atrazine + radical ) is the second order rate constant of atrazine plus ● OH or

355

SO4●─ / M-1 s-1, k ( photolysis of atrazine ) is the rate of atrazine photolysis / s-1, [DOC ] is the

356

concentration of DOC / mg C L-1, k ( DOC + radical ) is the rate constant of the reaction ●OH

357

or SO4●─ plus DOC / L mg-1 C s-1.

358

The k (DOC + radical) is the only unknown and can either be derived by calculating the first

359

order degradation rate of atrazine (from experimental data) and solving the above equation for

360

k (DOC + radical), or by adjusting k (DOC + radical) and arriving at the best fit for the

361

experimentally determined degradation of atrazine. The latter method was used in the present

362

study for different DOC and peroxide concentrations (refer to the SI). Consequently, the

363

following rate constants were derived from the three individual experiments (concentration of

364

humic acids were 7.5 and 15 mg C L-1): k (SO4●─ + humic acids) = 6.6 ± 0.4 × 103 L mg C -1

365

s-1, and k (● OH + humic acids) = 1.4 ± 0.2 × 104 L mg C -1 s-1.

366

The value determined for ● OH was in accordance with a value determined for Suwannee

367

River humic acids (k = 1.9 × 104 L mg C-1 s-1 [39]; note that an average value for the reaction

368

of ●OH with NOM of 2.5× 104 L mg C-1 s-1 was reported by Schwarzenbach et a. [40]). This

369

confirms our experimental concept. Beside the lower reaction rate of SO4●─ toward DOC, the

370

higher molar absorption coefficient of S2O82─ and the higher quantum yield for radical

371

formation (refer to the SI) further increased the efficiency of atrazine degradation in

372

UV/S2O82─.

373

Beside DOC, HCO3─ is an important water matrix constituent, which might limit the

374

oxidation capacity of a radical-based oxidation system by scavenging (reaction rates of ●OH

375

and SO4●─ are given below).

376

Figure 4 compares the first order degradation rates of atrazine in UV/H2O2 with UV/S2O82─ in

377

the presence of humic acids at different concentrations of HCO3─ (2.5–10 mM). Atrazine was

2.303× ε peroxide × φ peroxide × H × [ peroxide ]× (k (atrazine + radical ) − k ( photolysis of atrazine))

[DOC]× k (DOC + radical )

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degraded twice as fast in UV/S2O82─ than in UV/H2O2, regardless of the presence of HCO3─.

379

This independence from alkalinity is further illustrated in Figure 4 b), showing that the ratios

380

of degradation rates in both processes do not correlate with the HCO3─ concentration. This is

381

in accordance with the similar reaction rates of the reactions ●OH and SO4●─ plus HCO3─

382

reported in the literature, cancelling the influence of HCO3─ ((k (●OH + HCO3─) = 0.85–1 ×

383

107 M-1 s-1 [29], recommended value 1 × 107 M-1 s-1[40], k (SO4●─ + HCO3─) = 2.8–9.1 × 106

384

M-1 s-1 [41, 42]).

385 0

3

a)

-0.5

b) 2.5 kobs(S2O82-)/kobs(H2O2)

ln(c/c0) (UV/S2O82-

-1 -1.5 -2 -2.5 -3 -3.5

y = 1.92x - 0.08

-4

2 1.5 1 0.5

-4.5 -5

0 -2.2

-1.7

-1.2

-0.7

-0.2

0

ln(c/c0) (UV/H2O2)

2

4 [HCO3─]

6

8

10

/ mM

386 387 388 389 390 391 392

Figure 4: a) Observed degradation rates of atrazine in UV/H2O2 and UV/S2O82─ in the presence of humic acids and different concentrations of HCO3─. b) Ratio of first order degradation rates for atrazine versus HCO3─ concentration (derived from a)). 15 mg C L-1 (circles). 7.5 mg C L-1 (triangles). [HCO3─] = 0, 2.5, 5, and 10 mM. [phosphate] = 2.5 mM in the case of experiments with 15 mg C L-1, and 1.25 mM in case of experiments with 7.5 mg C L-1. pH = 7.2, T = 25°C.

393 394

The lower reactivity of the organic matter in SO4●─ reactions compared with ●OH reactions

395

can be explained by the tendency that H-abstraction reactions in SO4●─ are comparatively

396

slow. The reactions of several alcohols, alkanes, and ethers with SO4●─ proceed via H-

397

abstraction, with reaction rates in the range of 107–108 M-1 s-1 [43, 44]. In contrast, H-

398

abstraction is about one to two orders of magnitude faster for ●OH reactions (compare with

399

●

400

react faster with ●OH but slower with SO4●─. This leads to a weaker scavenging of radicals,

OH rate constants in Buxton et al. [29]). Thus, saturated moieties of the organic matter might

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401

which could improve the degradation efficiency of pollutants. However, it has to be

402

mentioned that the higher selectivity of SO4●─ could be counterbalanced in the case of target

403

compounds reacting faster with ●OH than with SO4●─.

404 405

The present study shows that the mode of chlorotriazine degradation in SO4●─-based

406

oxidation is somewhat similar to ●OH (dealkylation), despite different predominant reaction

407

mechanisms (H-abstraction versus electron transfer). Furthermore, SO4●─ might even degrade

408

DEDIA, a final product in common advanced oxidation processes, because the reaction is

409

about one order of magnitude faster than with ●OH. The higher selectivity of SO4●─ in the

410

presence of organic matter and HCO3─ further indicates that the degradation of DEDIA by

411

SO4●─ might be possible. However, the energy demand has to be carefully estimated under

412

water purification conditions to allow meaningful comparisons with other oxidative treatment

413

options for water treatment, such as ozonation.

414 415

Acknowledgements

416

All authors are deeply thankful for the expert advice of Prof. Clemens von Sonntag. His great

417

knowledge, vast experience, and brilliant creativity left its mark in the present and in

418

countless other studies. He is a noble person and remains our ideal as both a scientist and a

419

human being. H.V.L. thanks the German Water Chemistry Society for generous financial

420

support.

421 422

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423 424 425 426 427 428 429 430 431 432 433 434

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