Silvio Canonica

Evaluating Pesticide Degradation in the Environment: Blind Spots and Emerging Opportunities

The benefits of global pesticide use come at the cost of their widespread occurrence in the environment. An array of abiotic and biotic transformations effectively removes pesticides from the environment, but may give rise to potentially hazardous transformation products. Despite a large body of pesticide degradation data from regulatory testing and decades of pesticide research, it remains difficult to anticipate the extent and pathways of pesticide degradation under specific field conditions. Here, we review the major scientific challenges in doing so and discuss emerging opportunities to identify pesticide degradation processes in the field.

Effect of Dissolved Organic Matter on the Transformation of Contaminants Induced by Excited Triplet States and the Hydroxyl Radical

Dissolved organic matter (DOM) has recently been shown to reduce the transformation rate of various aqueous organic contaminants submitted to oxidation by excited triplet states, apparently by inhibiting the transformation of oxidation intermediates. The main goals of the present study were to evaluate in more detail the effect of concentration and type of DOM on the triplet-induced transformation rate of four selected organic compounds and to check for an analogous inhibition effect in the case of oxidation induced by hydroxyl radical. A marked inhibition by DOM of triplet-induced oxidation was observed for N,N-dimethylaniline (DMA) and the two antibiotics sulfamethoxazole (SMX) and trimethoprim (TRI), with DOM of terrestrial origin being a more effective inhibitor than DOM of aquatic origin. The results are important to understand the role of DOM both as a photosensitizer and as an inhibitor for the triplet-induced transformation of aquatic contaminants. In contrast, no DOM-induced reduction in second-order rate constant could be observed in competition kinetics experiments for the reaction of hydroxyl radical with a series of 15 organic compounds, covering several classes of aromatic contaminants, indicating that Suwannee River fulvic acid (SRFA) used as reference DOM does not affect this reaction mechanism.

Efficiency and energy requirements for the transformation of organic micropollutants by ozone, O3/H2O2 and UV/H2O2

The energy consumptions of conventional ozonation and the AOPs O(3)/H(2)O(2) and UV/H(2)O(2) for transformation of organic micropollutants, namely atrazine (ATR), sulfamethoxazole (SMX) and N-nitrosodimethylamine (NDMA) were compared. Three lake waters and a wastewater were assessed. With p-chlorobenzoic acid (pCBA) as a hydroxyl radical ((•)OH) probe compound, we experimentally determined the rate constants of organic matter of the selected waters for their reaction with (•)OH (k(OH,DOM)), which varied from 2.0 × 10(4) to 3.5 × 10(4) L mgC(-1) s(-1). Based on these data we calculated (•)OH scavenging rates of the various water matrices, which were in the range 6.1-20 × 10(4) s(-1). The varying scavenging rates influenced the required oxidant dose for the same degree of micropollutant transformation. In ozonation, for 90% pCBA transformation in the water with the lowest scavenging rate (lake Zürich water) the required O(3) dose was roughly 2.3 mg/L, and in the water with the highest scavenging rate (Dübendorf wastewater) it was 13.2 mg/L, corresponding to an energy consumption of 0.035 and 0.2 kWh/m(3), respectively. The use of O(3)/H(2)O(2) increased the rate of micropollutant transformation and reduced bromate formation by 70%, but the H(2)O(2) production increased the energy requirements by 20-25%. UV/H(2)O(2) efficiently oxidized all examined micropollutants but energy requirements were substantially higher (For 90% pCBA conversion in lake Zürich water, 0.17-0.75 kWh/m(3) were required, depending on the optical path length). Energy requirements between ozonation and UV/H(2)O(2) were similar only in the case of NDMA, a compound that reacts slowly with ozone and (•)OH but is transformed efficiently by direct photolysis.

Inhibitory effect of dissolved organic matter on triplet-induced oxidation of aquatic contaminants

Excited triplet states of organic chromophores, in particular aromatic ketones, are capable of inducing oxidation of a variety of organic compounds. These reactions probably play an important role in the degradation of organic contaminants in sunlit natural waters. The effect of dissolved natural organic matter (DOM) on the oxidation rate of twenty-two aquatic organic contaminants, including phenols, anilines, phenylurea and s-triazine herbicides, and some pharmaceuticals, was investigated using photoexcited benzophenone-4-carboxylate (CBBP) as the oxidant. For about half of the studied compounds, a decrease in depletion rate was observed in the presence of Suwannee River fulvic acid, used as a reference DOM. Also, depletion rates decreased with increasing DOM concentration, as verified for five selected compounds. Such an inhibitory effect of DOM on oxidation is attributed to its antioxidant properties, whereby oxidation intermediates of the contaminants are supposed to be reduced back to their parent compounds. The presented screening study shows that DOM may be a relevant factor for inhibiting the oxidation of many organic contaminants in surface waters and possibly in engineered water treatment systems.

Determination of the Reaction Quantum Yield for the Photochemical Degradation of Fe(III)-EDTA: Implications for the Environmental Fate of EDTA in Surface Waters.

The photochemical reaction quantum yield Φ of the Fe(lll)-EDTA complex at concentrations <1 μM was determined as a function of wavelength, pH, and temperature. The reaction quantum yield is independent of pH but clearly dependent on wavelength. At wavelengths of 313, 366, and 405 nm the reaction quantum yields (average values at 25 °C) are 0.082, 0.034, and 0.018, respectively. The temperature dependence (5-40 °C) of Φ is characterized by a small activation energy of 9 kJ mol -1 . The impact of the presence of oxygen on the reaction quantum yield of Fe(lll)-EDTA was also investigated. The determined Φ values were used to predict typical photochemical half-lives of this substance in the Glatt River. The validity of this prediction was confirmed by comparison of computed and measured kinetic data for the photochemical conversion of Fe(lll)-EDTA in Glatt River water. From the predicted half-lives of the Fe(lll)-EDTA complex, it can be derived that Fe-(III)-EDTA is quickly degraded in typical rivers and that EDTA concentrations found in the North Sea or the Black Sea consist of EDTA species that are not degradable by sunlight.

Phenolic Antioxidants Inhibit the Triplet-Induced Transformation of Anilines and Sulfonamide Antibiotics in Aqueous Solution

Recent studies have shown that dissolved organic matter (DOM) may inhibit the excited triplet-induced oxidation of several aromatic water contaminants, in particular those containing an aniline functionality. Such an inhibition was ascribed to antioxidant moieties of DOM. The present study was conducted with the aim of verifying whether well-defined antioxidants could act as inhibitors in analogy to DOM. Various substituted phenols exhibiting antioxidant character were able, at micromolar concentration, to slow down the photoinduced depletion of several anilines and sulfonamides in aerated aqueous solution containing 2-acetonaphthone as the photosensitizer. A concomitant accelerated degradation of the phenols in the presence of such contaminants was observed. This reinforces the hypothesis of reduction of oxidation intermediates of the contaminants by the phenols. Phenol (unsubstituted) was found to be a useful inhibitor even in the case of DOM-photosensitized transformations. Phenolic antioxidants are proposed as diagnostic tools to investigate the aquatic photochemistry of aromatic amines.

Quantitative structure-activity relationships for oxidation reactions of organic chemicals in water

Even in the absence of microbiological mediation, oxidation is one of the most important chemical processes contributing to the degradation of organic contaminants in the aquatic environment. The oxidants that are responsible for these reactions include hydroxyl radical, carbonate radical, organic oxyl and peroxyl radicals, peroxides, excited triplet states of organic chromophores, singlet molecular oxygen, ozone, chlorine dioxide, permanganate, and chromate. Some of these oxidants contribute to natural attenuation of organic contaminants, but many are of greater interest because of their role in engineered remediation technologies. Kinetic studies of these processes have lead to numerous quantitative structure-activity relationships (QSARs). Many of these QSARs are simple empirical correlations to common convenient descriptor variables like Hammett constants (sigma), half-wave oxidation potentials (E1/2), energies of the highest occupied molecular orbital (E(HOMO)) or rate constants for other oxidation reactions. However, several environmentally relevant, aqueous-phase oxidation reactions have been described with QSARs based on theoretical models for electron transfer that were developed by Marcus-Hush and Rehm-Weller. This review summarizes many of the reported QSARs for aquatic oxidations of organic compounds with emphasis on the interrelation between traditional empirical models and the potential for future development of QSARs based on theoretical models.

Chemical oxidation of dissolved organic matter by chlorine dioxide, chlorine, and ozone: effects on its optical and antioxidant properties.

In water treatment dissolved organic matter (DOM) is typically the major sink for chemical oxidants. The resulting changes in DOM, such as its optical properties have been measured to follow the oxidation processes. However, such measurements contain only limited information on the changes in the oxidation states of and the reactive moieties in the DOM. In this study, we used mediated electrochemical oxidation to quantify changes in the electron donating capacities (EDCs), and hence the redox states, of three different types of DOM during oxidation with chlorine dioxide (ClO2), chlorine (as HOCl/OCl(-)), and ozone (O3). Treatment with ClO2 and HOCl resulted in comparable and prominent decreases in EDCs, while the UV light absorbances of the DOM decreased only slightly. Conversely, ozonation resulted in only small decreases of the EDCs but pronounced absorbance losses of the DOM. These results suggest that ClO2 and HOCl primarily reacted as oxidants by accepting electrons from electron-rich phenolic and hydroquinone moieties in the DOM, while O3 reacted via electrophilic addition to aromatic moieties, followed by ring cleavage. This study highlights the potential of combined EDC-UV measurements to monitor chemical oxidation of DOM, to assess the nature of the reactive moieties and to study the underlying reaction pathways.

DNA degradation by the mixture of copper and catechol is caused by DNA-copper-hydroperoxo complexes, probably DNA-Cu(I)OOH.

Free hydroxyl radicals (free ·OH), singlet oxygen (1O2), or ·OH produced by DNA‐copper‐hydroperoxo complexes are possible DNA‐damaging reactive oxygen species (ROS) in the reaction system containing copper, catechol, and DNA. para‐Chlorobenzoic acid (pCBA) degradation studies revealed that CuCl2 mixed with catechol produced free ·OH. In the presence of DNA, however, inhibition of the pCBA degradation suggested that another ROS is responsible for the DNA degradation. Of a series of ROS scavengers investigated, only KI, NaN3, and Na‐formate—all of the salts tested—strongly inhibited the DNA degradation, suggesting that the ionic strength rather than the reactivity of the individual scavengers could be responsible for the observed inhibition. The ionic strength effect was confirmed by increasing the concentration of phosphate buffer, which is a poor ·OH scavenger, and was interpreted as the result of destabilization of DNA‐copper‐hydroperoxo complexes. Piperidine‐labile site patterns in DNA degraded by copper and catechol showed that the mixture of Cu(II) and catechol degrades DNA via the intermediate formation of a DNA‐copper‐hydroperoxo complex. Replacement of guanine by 7‐deazaguanine did not retard the DNA degradation, suggesting that the DNA‐copper‐hydroperoxo complexes do not bind to the guanine N‐7 as proposed in the literature. Environ. Mol. Mutagen. 36:5–12, 2000.

Quenching of excited triplet states by dissolved natural organic matter

Excited triplet states of aromatic ketones and quinones are used as proxies to assess the reactivity of excited triplet states of the dissolved organic matter ((3)DOM*) in natural waters. (3)DOM* are crucial transients in environmental photochemistry responsible for contaminant transformation, production of reactive oxygen species, and potentially photobleaching of DOM. In recent photochemical studies aimed at clarifying the role of DOM as an inhibitor of triplet-induced oxidations of organic contaminants, aromatic ketones have been used in the presence of DOM, and the question of a possible interaction between their excited triplet states and DOM has emerged. To clarify this issue, time-resolved laser spectroscopy was applied to measure the excited triplet state quenching of four different model triplet photosensitizers induced by a suite of DOM from various aquatic and terrestrial sources. While no quenching for the anionic triplet sensitizers 4-carboxybenzophenone (CBBP) and 9,10-anthraquinone-2,6-disulfonic acid (2,6-AQDS) was detected, second-order quenching rate constants with DOM for the triplets of 2-acetonaphthone (2AN) and 3-methoxyacetophenone (3MAP) in the range of 1.30-3.85 × 10(7) L mol(C)(-1) s(-1) were determined. On the basis of the average molecular weight of DOM molecules, the quenching for these uncharged excited triplet molecules is nearly diffusion-controlled, but significant quenching (>10%) in aerated water is not expected to occur below DOM concentrations of 22-72 mg(C) L(-1).