Gwendal Daniel
email: gdaniel@uoc.edu

Gerson Sunyé
email: gerson.sunye@ls2n.fr

Jordi Cabot
email: jordi.cabot@icrea.cat

Scalable Queries and Model Transformations with the Mogwaï Tool

Keywords: MDE, Scalability, OCL, ATL, Model Query, Model Transformation

Scalability of modeling frameworks has become a major issue hampering MDE adoption in the industry. Specifically, scalable model persistence, as well as efficient query and transformation engines, are two of the key challenges that need to be addressed to enable the support for very large models in current applications. In this paper we demonstrate Mogwaï, a tool designed to efficiently compute queries and transformations (expressed in OCL and ATL) over models stored in NoSQL databases. Mogwaï relies on a translational approach that maps constructs of the supported input languages to Gremlin, a generic NoSQL query language, and a model to datastore mapping allowing to compute the generated query on top of several datastores. The produced queries are computed on the database side, benefiting of all its optimizations, improving the execution time and reducing the memory footprint compared to standard solutions. The Mogwaï tool is released as a set of open source Eclipse plugins and is fully available online.

Introduction

Existing empirical assessments from industrial companies adopting MDE [START_REF] Whittle | The State of Practice in Model-Driven Engineering[END_REF] point to the limited support for managing large models as one of the factors limiting the success of MDE in industrial MDE processes. Indeed, existing modeling solutions were primary designed to handle simple, human-based modeling activities, and existing technical solutions are not designed to handle large models (potentially generated using model driven reverse engineering techniques [START_REF] Bruneliere | MoDisco: A model driven reverse engineering framework[END_REF]) commonly used nowadays. In particular, several studies have reported the scalability issues of the Eclipse Modeling framework (the de-facto standard framework for building modeling tools in the Eclipse community) and its default serialization mechanism XMI.

These limitations have led to the creation of several scalable model persistence frameworks built on top of different types of databases [START_REF] Barmpis | Hawk: Towards a Scalable Model Indexing Architecture[END_REF][START_REF] Daniel | Neoemf: a multi-database model persistence framework for very large models[END_REF][START_REF]The CDO Model Repository (CDO)[END_REF] combined with advanced mechanisms such as application-level caches [START_REF] Ujhelyi | Emf-incquery: An integrated development environment for live model queries[END_REF] and lazy-loading. While this new generation of model persistence techniques has globally improved the support for managing large models, they are partial solutions to the scalability problem in current modeling frameworks. In its core, all frameworks are based on the use of low-level model handling APIs that are focused on manipulating individual model elements and do not provide support for generic model query and transformation computation. This approach is clearly inefficient because (i) the API granularity is too fine-grained to benefit from the advanced query capabilities of the backend and (ii) an important time and memory overhead is necessary to construct navigable intermediate objects that are needed by the modeling API.

To overcome this situation, we developed Mogwaï, a scalable query and transformation framework for large models. Mogwaï consists of a translation component that maps model queries and transformations (expressed in OCL [START_REF] Omg | OCL Specification[END_REF] and ATL [START_REF] Jouault | ATL: A Model Transformation Tool[END_REF]) into expressions of a graph traversal language, Gremlin [START_REF] Tinkerpop | The Gremlin Language[END_REF], a multi-database graph traversal query language we use as our output language. Generated queries are then directly computed on the database side, bypassing the standard modeling API. This avoids the above mentioned problems and significantly improves the overall performance.

This paper complements our existing work [START_REF] Daniel | Gremlin-ATL: a scalable model transformation framework[END_REF][START_REF] Daniel | Mogwaï: a framework to handle complex queries on large models[END_REF] by introducing its modular architecture, additional tool implementation details, and includes a unified ModelDatastore component that allows to access multiple datastores transparently for both query and transformation computations.

The rest of the paper is organized as follows: Section 2 gives an overview of the Mogwaï infrastructure, Section 3 presents the architecture of our tool and its query processing engine, and Section 4 presents the tool implementation. Finally, Section 5 summarizes the key points of the paper.

Framework Overview

Figure 1 shows an overview of the Mogwaï framework that creates Gremlin scripts from input model queries and transformations. An initial Model Query or Transformation is parsed and sent to a Translation Engine, that selects the translation to apply and performs a systematic mapping of the input expressions' to Gremlin constructs. These constructs are then assembled into a Gremlin Script that is sent to the database for computation.

The Mogwaï Translation Engine relies on a Model Datastore Definition to produce the output gremlin script. This generic library provides an abstraction layer that decouples the computation from the low-level database access by adding modeling primitives to manipulate natively the data representing the model. As a result, the generated Gremlin Script is not tailored to a specific data store, and can be parametrized with a Model Datastore Implementation, that wraps the concrete Model Datastore to use (i.e. the backend storing the model). This architecture, originally defined for the Gremlin-ATL engine [START_REF] Daniel | Gremlin-ATL: a scalable model transformation framework[END_REF], has been integrated in the OCL engine [START_REF] Daniel | Mogwaï: a framework to handle complex queries on large models[END_REF] to allow to query models stored in multiple types of data storage solutions, and can be easily extended to support additional backends.

Internally, the framework defines two model-to-model transformations: OCL2Gremlin, that handles model queries expressed using the OCL language [START_REF] Omg | OCL Specification[END_REF], and ATL2Gremlin, that translates model transformations expressed in ATL [START_REF] Jouault | ATL: A Model Transformation Tool[END_REF]. Note that the modular archi- tecture of the framework allows to define additional translations to support alternative query and transformation solutions such as EOL [START_REF] Kolovos | The Epsilon Object Language (EOL)[END_REF] or QVT [START_REF] Omg | QVT Specification[END_REF].

The generated scripts can be returned to the modeler and used as stored procedures to execute in the future, or directly computed with a specific implementation of the Model Datastore library. Finally, the returned elements from the computation (if any) are reified into regular model elements thanks to the Model Datastore implementation.

Compared to existing query frameworks, Mogwaï does not rely on the default modeling API to compute model queries and transformations. In general, API based frameworks translate queries and transformations into a sequence of low-level API calls, which are then performed one after another on the persistence layer. While this approach has the benefit to be compatible with every API-based applications, it does not take full advantage of the database structure and query optimizations. Furthermore, each object fetched from the database has to be reified to be navigable, even if it is not going to be part of the end result. Therefore, the execution time and memory consumption of the API-based solutions strongly depends on the number of intermediate objects fetched from the database.

Architecture

Figure 2 describes the internal structure of the Mogwaï framework. The QueryProcessor is the core of the engine: it provides the process method, that takes as its input a MogwaiQuery and a set of ModelDatastores, and returns a QueryResult containing the result of the computation and additional monitoring information (such as the computed MogwaiQuery and the raw query execution time).

The QueryProcessor relies on an internal GremlinScriptRunner that provides utility methods to setup a Gremlin environment and execute queries. Note that the tool provides two implementations of the abstract QueryProcessor: the first one, ATLProcessor, is dedicated to ATL transformation computation, and the second one, OCLProcessor, handles OCL queries. Both processors rely on an internal model to model transformation responsible for mapping the constructs of the input languages to Gremlin. Note that this modular architecture could be easily extended to support alternative query and transformation languages. The Mogwaï's architecture is not tailored to a specific backend or model persistence technology, and can be used on top of any ModelDatastore implementation. However, the tool also provides an advanced integration into the NeoEMF platform (light-grey boxes) that speeds-up query computation and improves the tool's integration with existing EMF-based application by returning EMF-compatible objects. The MogwaiResource class extends the NeoEMF one with a simple API defining the query and transform methods. This resource embeds a set of QueryProcessors as well as a preset ModelDatastore implementation targeting the native API of the database storing the model. Query and transformations executed through the MogwaiResource return NeoEMFQueryResults, that contain database records that can be reified into navigable EMF elements if needed. Note that resulting model elements are created only from the results of the Gremlin script execution, removing the memory overhead implied by intermediate objects created during EMF-based computations.

Implementation

The Mogwaï tool is implemented as a set of open-source Eclipse plugins released under the EPL license. Source code and benchmark materials are fully available in the project's GitHub repository 4 . An Eclipse update site containing the last stable version of the framework is also available online 5 .

The OCL engine relies on Eclipse MDT OCL [START_REF]MDT OCL[END_REF] to parse the input queries, and the produced OCL models constitute the input of a set of 70 ATL [START_REF] Jouault | ATL: A Model Transformation Tool[END_REF] transformation rules and helpers implementing the mapping and the transformation process presented in detail in our previous work [START_REF] Daniel | Mogwaï: a framework to handle complex queries on large models[END_REF].

The ATL engine presents a similar architecture, and relies on the ATL parser to create a model from the transformation to compute. This transformation model is then sent to a high-order transformation mapping ATL constructs to Gremlin [START_REF] Daniel | Gremlin-ATL: a scalable model transformation framework[END_REF] (represented as a set of 80 rules and helpers).

Conclusion

We have showcased Mogwaï, a tool that generates Gremlin scripts from model queries and transformations in order to maximize the benefits of using a NoSQL backend to store and manipulate large models. Gremlin scripts are created using a set of model-tomodel transformations, and are parametrized with a specific Model Datastore, enabling their computation over a variety of backends compatible with the Gremlin language. The Mogwaï approach allows to bypass the existing modeling framework's API, improving the performance of query and transformation computations both in terms of execution time and memory consumption [START_REF] Daniel | Gremlin-ATL: a scalable model transformation framework[END_REF][START_REF] Daniel | Mogwaï: a framework to handle complex queries on large models[END_REF]. The tool development roadmap for Mogwaï includes adding support for more types of NoSQL backends, like documentoriented and column databases by providing the necessary translations from ATL and OCL to their native languages. In the demonstration, we show the required steps to initialize the Mogwaï engine with the transformation and compute it against the database storing the model (Fig-ure 5). In addition, we show how the output model can be stored in another data representation using a different Model Mapping Implementation. We also compare the execution time of computing the transformation using the regular ATL engine with Mogwaï, and show that using our approach can bring significant improvements in terms of execution time. Finally, the key points of the tool will be summarized and some remarks on the integration into existing modeling application will be provided. All the presented examples and models will be publicly available on the Mogwai GitHub repository. In addition, a video summarizing the key points of the demonstration is available online at https://youtu.be/_nTBPJMVRQY.

Fig. 1 .

 1 Fig. 1. Mogwaï Infrastructure

Fig. 2 .

 2 Fig. 2. Mogwaï Internal Structure

Listing 1 . 1 .Fig. 4 .

 114 Fig. 4. Running OCL Queries with Mogwaï

Listing 1 . 2 .

 12 Sample ATL Transformation module C l a s s 2 R e l a t i o n a l ; c r e a t e OUT : R e l a t i o n a l M M from IN : J a v a ; r u l e C l a s s 2 T a b l e { from c : J a v a ! C l a s s D e c l a r a t i o n t o o u t : R e l a t i o n a l M M ! T a b l e ( name ← c . name , c o l ← S e q u e n c e {key}->union ( c . b o d y D e c l a r a t i o n s ->s e l e c t ( b | b . o c l I s T y p e O f ( J a v a ! M e t h o d D e c l a r a t i o n ) ) ) , key ← key ) , key : R e l a t i o n a l M M ! Column ( name ← ' o b j e c t I d ' , t y p e ← keyType ) , keyType : R e l a t i o n a l M M ! Type ( name ← ' I n t e g e r ' ) } r u l e Method2Column { from m : J a v a ! M e t h o d D e c l a r a t i o n t o o u t : R e l a t i o n a l M M ! Column ( name ← m. name + ' C a l l C o u n t ' , t y p e ← t y p e ) , t y p e : R e l a t i o n a l M M ! Type ( name ← ' I n t e g e r ' ) }

https://github.com/atlanmod/Mogwai

atlanmod.github.io/Mogwai

http://git.eclipse.org/c/modisco

Appendix A Demonstration Overview

The demonstration presents two typical use cases where the Mogwaï framework significantly improves the execution time and memory consumption of an application computing OCL queries and ATL transformations on top of large models.

First, we briefly introduce the sample model, that is a real-world model obtained by applying model driven reverse engineering techniques on an existing code base. The manipulated model contains around 80 000 elements representing a Java application. Figure 3 shows an excerpt of the metamodel describing the sample. Note that the complete metamodel can be found in the MoDisco repository 6 .

Fig. 3. Excerpt of the MoDisco Java Metamodel

Then, we present a set of OCL queries to compute against this model. An example of such query is provided in Listing 1.1, that describes the protectedMethod query, which finds in the model all the MethodDeclarations that have a protected Modifier. We show in the demonstration how to initialize and configure the Mogwaï engine to translate and run the query from an existing Java application (Figure 4). In parallel, we show how the query is computed using the regular Eclipse MDT-OCL interpreter embedded with Eclipse, and emphasize the execution time and memory consumption differences.


Sabrina Halladja

Amina Amine-Khodja

Alexandra Ter Halle

Abdelaziz Boulkamh

Claire Richard
email: claire.richard@univ-bpclermont.fr

Photolysis of fluometuron in the presence of natural water constituents

Keywords: phenylurea herbicide, photosensitization, nitrate ions, fulvic acids, hydroxyl radicals Introduction

Phototransformation of the herbicide fluometuron (1 μM) in natural sunlight was investigated in neutral Milli-Q water and in synthetic waters containing either fulvic acids, nitrate ions or both in order to mimic reactions taking place in aquatic environments.

Fluometuron degradation followed a pseudo-first order kinetics. The reaction was faster in synthetic than in Milli-Q water. Fulvic acids (10 mg L -1 ) increased the rate of fluometuron photolysis by a factor 2.5 and nitrates (25 mg L -1 ) by a factor 15. Identification of major photoproducts was conducted under laboratory conditions using LC-ESI-MS. Numerous photoproducts were detected and tentatively characterized. In the presence of nitrates, hydroxylation of the aromatic ring with or without hydrolysis of CF 3 into CO 2 H and oxidation of the urea chain leading to demethylation were observed. In the presence of fulvic acids, hydroxylation of the aromatic ring was the major reaction route.

take place. In particular, dissolved natural organic matter (DOM) which absorbs a large portion of photons is a potential photosensitizer. Singlet oxygen, superoxide ion/hydroperoxyl radicals, hydroxyl radicals, excited triplet states and alkylperoxyl radicals were proved or proposed to be generated in natural waters under the influence of sunlight [START_REF] Zafiriou | Nitrate photolysis in seawater by sunlight[END_REF][START_REF] Zepp | Comparison of photochemical behavior of various humic substances in water : Sunlight induced reactions of aquatic pollutants photosensitized by humic substances[END_REF][START_REF] Hoigné | Aquatic humic substances as sources and sinks of photochemically produced transient reactants[END_REF][START_REF] Haag | Singlet oxygen in surface waters. Photochemical formation and steady state concentrations in various types of waters[END_REF][START_REF] Cooper | Sunlight-induced photochemistry of humic substances in natural waters : major reactive species[END_REF][START_REF] Vaughan | Photochemical formation of hydroxyl radical by constituents of natural waters[END_REF][START_REF] Canonica | Transformation kinetics of phenols in water : photosensitization by dissolved organic material and aromatic ketones[END_REF]. However, a part of these species are trapped by DOM itself. Nitrate ions that are present in surface waters at level varying from 0.2 to 25 mg L -1 generate the highly oxidizing hydroxyl radicals under light excitation [START_REF] Boule | Phototransformations induced in aquatic medium by Fe III , NO 3 -, NO 2 and humic substances[END_REF] for a review). Hydroxyl radicals that are unspecific oxidants are scavenged by a variety of aquatic components. The percentage of hydroxyl radicals trapped by pollutants is thus strongly dependent on the medium composition. Due to all these factors, studies can help to predict the fate of pollutants at the condition that they are undertaken under relevant experimental conditions.

In the present work, we focused on the phototransformation of the phenylurea herbicide fluometuron (FM). This compound is widely used for pre-and post-emergence control of weeds in fields of conventional cotton cultivars. It is persistent [START_REF] Stoeckel | Degradation of persistent herbicides in Riparian Wetlands[END_REF] and may pose some risks to aquatic organisms [START_REF] Muschal | Risk posed by pesticides to aquatic organisms in rivers of northern inland New South Wales, Australia[END_REF]. However, literature data are very scarce, especially concerning its photochemical reactivity. FM poorly absorbs solar light (see Figure 1) but indeed its direct photolysis in simulated solar light (λ > 290 nm) was reported [START_REF] Lam | Aqueous photochemical reaction kinetics and transformations of fluoxetine[END_REF]. The CF 3 group undergoes photohydrolysis into CO 2 H. FM was also included in a recent study [START_REF] Lam | PhotoFate: A new approach in accounting for the contribution of indirect photolysis of pesticides and pharmaceuticals in surface waters[END_REF] in which the contribution of indirect processes in surface waters under sunlight simulated irradiation conditions was evaluated. Nitrate ions at a level of 8x10 -4 M significantly enhanced FM degradation while humic acids gave unclear results.

The objective of the present work was to investigate the photolysis of FM in conditions approaching real ones. FM in the micromolar range was irradiated in natural solar light. The influence of DOM or/and nitrates on the phototransformation was assessed. In parallel, laboratories experiments were conducted to identify the main photoproducts on the basis of HPLC-ESI-MS analyses.

Material and Methods

Chemicals. FM (97.7 % purity) was purchased from Riedel de Haën (Saint Quentin Fallavier, France) and used as received. Elliott soil fulvic acids were purchased from International Humic Substances Society (IHSS). The salt used to add nitrate ions was NaNO 3 (Fluka, 99 %). All other reactants were of the highest grade available and used as received. Water was purified using a Milli-Q device (Millipore).

Preparation of solutions.

A stock solution of FM was prepared by dissolving 4 mg of substrate in 100 ml of Milli-Q purified water. Solubilization was achieved after a 4-day stirring. Stock solutions of fulvic acids and sodium nitrate were prepared in Milli-Q purified water. They were at a concentration of 40 mg L -1 and 50 mg L -1 , respectively. Stock solutions were then diluted in order to obtain the desired concentrations. Solutions were buffered in order to maintain constant the pH all along irradiations. A pH of 6.5 was chosen because natural aquatic medium is generally around neutrality. Phosphate buffers were used; their final concentration was equal to 10 -3 M. Deoxygenation of solutions was achieved by nitrogen bubbling for 20 minutes prior to irradiation and during irradiations. Solutions were oxygenated by pure oxygen bubbling.

Photoreaction setup. For kinetic purpose, FM (1 μM) was irradiated (i) in MilliQ water, (ii) in water containing fulvic acids (10 mg L -1 ) used as a surrogate of DOM, (iii) in water containing nitrate ions (25 mg L -1 or 3x10 -4 M) and (iv) in water containing both fulvic acids (10 mg L -1 ) and nitrate ions (3x10 -4 M). Irradiation experiments in natural sunlight were performed at Clermont-Ferrand (46° N, 3° E) in June 2006. Cylindrical quartz glass reactors (14-mm internal diameter) were filled with 14 ml of solutions. A headspace was left above solutions. Reactors were closed by a septum, attached on a rack inclined by about 15° from horizontal and exposed to solar light. Samples received 13 h of sunshine per day. Aliquots of 0.5 mL were removed simultaneously from all the solutions at selected intervals. At each sampling, solutions were re-saturated with air. Samples were immediately analyzed by HPLC.

Irradiations started at 9 am on the first day. The first samplings were made at 5 pm. Then, the next samplings were made at 9 am and 5 pm each day. Solutions were left outside overnights.

Dark control experiments showed no loss of FM by adsorption on glass surfaces and no transformation, during the time required for the irradiations, irrespective of the aqueous media (variations less than 3 %).

Irradiations under laboratory conditions were also carried out for photoproducts identification. Irradiations were performed in a device equipped with six TLAD 15W/05 fluorescent tubes emitting within the wavelength range 300-450 nm with a maximum of emission at 365 nm and in a Pyrex glass reactor (14 mm i.d). The device was cylindrical and equipped with reflecting inner walls. A ventilator was used as a cooling system. The reactor was placed in the centre of the device and was surrounded by the six fluorescent tubes. Light intensity was measured using p-nitroanisole/pyridine as a chemical actinometer [START_REF] Dulin | Development and evaluation of sunlight actinometers[END_REF]. PNA (10 -5 M) and pyridine (10 -4 M) were irradiated in the same conditions as samples in the polychromatic device and in solar light. In the polychromatic device, PNA loss was 12 % after 1 hour and 63 % after 5 h. In solar light, 56 % of PNA had disappeared after 4 h of exposure between 10 am and 2 pm. It can be deduced that the average light intensity delivered by the tubes of the polychromatic device was of the same magnitude order as that of solar light within the wavelength range 300-400 nm.

Analytical procedures. Loss of fluometuron and formation of photoproducts were monitored by HPLC-UV using a Waters apparatus equipped with two pumps (model 510), an autosampler, a photodiode array detector (model 996), a detector W2487 and a C 18 reversephase column (4.6 mm x 250 mm, Spherisorb S5 ODS2, 5 μm, Waters). Eluent was a mixture of water acidified with 0.1% of orthophosphoric acid and methanol (50 %/50 %) delivered at a constant flow of 1 mL min -1 . For the HPLC mass spectrometry analyses, samples containing fulvic acids were prepared as follows. Portions of 0.6 ml of irradiated solutions were passed over an Oasis BHL column conditioned beforehand with 1 ml of methanol and 1 ml of water acidified with formic acid 0.1 %. Fulvic macromolecules were recovered by passing 1 ml of acidified water through the column while fluometuron and its photoproducts by passing 1 ml of methanol. The HPLC-UV-MS analyses were performed using a Waters/Micromass LC/QTOF (Micromass, Manchester,UK). For the HPLC conditions, a Waters Alliance 2695 HPLC equipped with a photodiode array detector (DAD) was used. A reversed-phase column (C18 Hypersil ODS, 5 μm, 100 mm x 2.1 mm; Interchim, Montluçon, France) was used at a flow rate of 0.3 mL min -1 . The mobile phase was composed of acetonitrile (solvent A) and acidified water (formic acid, 0.4% v/v; pH 2.6) (solvent B). Gradient: 0-5 min, 5% A; 5-30 min, 5-95% A (linear); 30-34 min, 95% A; 34-35 min, 95-5% A; 35-40 min, 5% B (equilibrium period). The injection volume was 30 μL. The LC-ESI-MS worked both in positive and negative mode. The optimum voltages for the probe and ion source components (to produce maximum intensity) were 3 kV for the stainless-steel capillary, 35 V for the sample cone, and 2 V for the extractor cone. In the negative mode, voltages were switched, except the capillary voltage which was -2.1 kV for better sensitivity. UV spectra were recorded on a Cary 3 (Varian) spectrophotometer. A 1-cm path quartz cell was used for all the experiments. The reference beam blank was always Milli-Q water.

Results and Discussion

Kinetics of phototransformation in solar light

We first compared the profiles of FM loss in various conditions. In the absence of fulvic acids (FA) and nitrate ions, the consumption of FM (1 μM) in solar light was very slow: less than 30 % had disappeared after 6.7 d. The addition of fulvic acids (10 mg L -1 ) significantly increased the rate of FM consumption: about 50 % of FM had disappeared after 6.7 d. A larger enhancement of the reaction rate was observed in the presence of nitrate ions (25 mg L -1 ): a complete FM loss was obtained after 3.3 d. In the presence of both fulvic acids (10 mg L -1 ) and nitrate ions (25 mg L -1 ), FM disappeared more slowly than in the presence of nitrates alone. To determine rate constants, we plotted lnc 0 /c vs irradiation time, where c 0 is the initial FM concentration and c the concentration at t, taking into account that samples were irradiated for 8 h between 9 am and 5 pm and for 5 h between 5 pm and 9 am (Figure 2). In all cases, FM consumption followed pseudo-first order kinetics. The rate coefficients, k, and R 2 values are reported in Table 1.

These kinetic results bring insight into the photodegradability of FM in solar light. FM is hardly transformed in pure water due to the poor absorption of solar radiations. Using the rate coefficient given in Table 1 one computes a half-life of 175 h. This value is 2.6-fold higher than that of [START_REF] Lam | PhotoFate: A new approach in accounting for the contribution of indirect photolysis of pesticides and pharmaceuticals in surface waters[END_REF]. It shows that in the case of FM a one-day irradiation in June in our place is comparable to about 5 h of irradiation in the photosimulator used by these authors.

The chromophoric constituents of water (nitrate and fulvic acids) promoted FM phototransformation. The effect of fulvic acids was quite moderate. In the presence of fulvic acids (10 mg L -1 ), the rate coefficient was increased by a factor of 2.5 and the half-live reduced by the same factor. Nitrate ions (25 mg L -1 ) had a more pronounced influence increasing the rate coefficient and reducing the half-live by a factor of 15. In the presence of fulvic acids and nitrate ions, the rate of FM photodegradation was smaller by about 40 % than in the presence of nitrate ions alone. At the considered concentrations, the absorbance of nitrates is very small (around 0.002) while that of FA bigger (around 0.22) for a 1-cm pathlength (see Figure 1). The inhibiting effect of FA on nitrate photolysis through screen effect is difficult to evaluate due to the cylindrical shape of reactors. Based on a mean path-length of 0.7 cm, one would compute a rate reduction between 15 and 20 %. It represents 50 % of the measured inhibition. The remaining 50 % of inhibition are likely to be due to the scavenging of hydroxyl radicals by FA [START_REF] Brezonik | Nitrate-induced photolysis in natural waters: controls on concentrations of hydroxyl radical photo-intermediates by natural scavenging agents[END_REF][START_REF] Schindelin | Nitrate and natural organic matter in aqueous solutions irradiated by simulated sunlight: Influence on the degradation of the pesticides dichlorprop and terbutylazine[END_REF]Frimmel, 2000, ter Halle and[START_REF] Ter Halle | Simulated solar light irradiation of mesotrione in natural waters[END_REF].

In an attempt to delineate the role of hydroxyl radicals in the fulvic acids mediated phototransformation of FM, we studied the influence of 2-propanol added as a hydroxyl radical scavenger on the reaction (k = 1.9x10 10 M -1 s -1 , [START_REF] Buxton | Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution[END_REF]. These experiments were undertaken in laboratory conditions. In the absence of fulvic acids, the initial rate of FM phototransformation was equal to 2x10 -8 M h -1 . In the presence of fulvic acids (5 mg L -1 ), it raised to 5.2x10 -8 M h -1 . The addition of 2-propanol (0.015 M) reduced the latter rate by a factor comprised between 1.5 and 2, suggesting the involvement of hydroxyl radicals in the phototransformation reaction and showing the ability of FA to produce them under irradiation. We also studied the influence of oxygen on the reaction. After 5 h of irradiation, FM consumption was twice faster in air-saturated than in oxygen-saturated medium and 3-fold faster in nitrogen-saturated medium than in air-saturated medium. Thus, oxygen clearly inhibited the reaction.

Photoproducts identification

To achieve photoproducts characterization, we irradiated more highly concentrated FM solutions under laboratory conditions. The photolysis of FM (3x10 -5 M) in Milli-Q water yielded the acidic compound resulting from the hydrolysis of CF 3 into CO 2 H as previously reported [START_REF] Lam | Aqueous photochemical reaction kinetics and transformations of fluoxetine[END_REF]. Its UV absorption spectrum differs from that of FM: the far UV absorption band (λ max = 224 nm) shows a shoulder and the second maximum of absorption is located at 297 nm instead of 275 nm in the case of FM.

The irradiation of FM (3x10 -5 M) in the presence of fulvic acids (25 mg l -1 ) yielded two HPLC-UV detectable photoproducts, I and II (Figure 3A). The UV spectrum of I exhibited the same shoulder as that of the acidic product and a second maximum red-shifted by 27 nm (λ max = 297 nm). The UV spectrum of II resembled that of FM but the second maximum was red-shifted by 20 nm compared to it (λ max = 295 nm). The HPLC-mass analysis of I gave a first molecular ion at m/z = 223 in ES -mode and at m/z = 225 in ES + mode corresponding to the loss of 9 amu that may be explained by the hydrolysis of CF 3 into CO 2 H and the addition of an oxygen atom. Fragments at m/z = 179 [M-44] and at m/z = 134 [M-89] were obtained in ES -mode. These data are compatible with the structure proposed in Table 2.

The first fragment would correspond to the loss of the carboxylic group and the second to the cleavage of the terminal amine group -N(CH 3 ) 2 followed by loss of an H atom to yield an isocyanate as shown in Scheme 1.

Photoproduct II gave molecular ions at m/z = 247 and 249 in ES -and ES + modes, respectively, corresponding to the addition of an oxygen atom to FM. A fragment at m/z = 202 corresponding to M-45 was observed in ES -mode. The loss of 45 amu is likely to result from the departure of N(CH 3 ) 2 by cleavage of the terminal urea C-N bond and the elimination of H to form the isocyanate. It is compatible with structure given in Table 2.

The irradiation of FM (3x10 -5 M) in the presence of nitrate ions (25 mg l -1 ) yielded seven photoproducts detectable by HPLC-UV (see Figure 3 B). Based on HPLC retention times, UV absorption spectra and mass data, we could conclude that photoproducts I and II were present among the seven photoproducts. Two other compounds III and IV with a molecular ion at m/z = 223 in ES -mode were also detected. As they showed distinct retention times and UV maxima, but similar fragmentation at m/z =179 and 134 with different percentages of fragments, they are likely to be isomers of I. Photoproducts I, III and IV should differ from each other in the position of the hydroxyl group on the ring. The photoproducts V and VI that were eluted just before FM showed similar absorption spectra.

One of them, V, gave a molecular ion at m/z = 203 in ES -mode corresponding to the loss of 28 amu and a fragment at m/z = 160. The other one, VI, gave a molecular ion at m/z = 217 in ES -mode and the same fragment at m/z = 160 (see Scheme 2). These compounds could be assigned to demethylated products; VI would be the monodemethylated derivative and V the didemethylated derivative (see Table 2).

A molecular ion at m/z = 245 in ES -mode was also detected at a retention time longer than that of FM. This mass corresponds to the addition of 14 amu. Three fragments were observed at m/z = 217 [M-28], 188 [M-28-29] and 160 [M-28-29-28]. This fragmentation indicates that the aromatic ring was not altered. It is in favour of photoproduct VII that bears a carbonyl group and for which successive losses of CO, NCH 3 and CO are again possible.

Finally, we used the selected peak method to detect some specific photoproducts formation of which was expected. In the case of nitrate ions, we could find nitro derivatives at m/z = 276 in ES -mode corresponding to the addition of 45 amu (+NO 2 -H). In the case of fulvic acids, careful examination of chromatograms revealed the presence of peaks at m/z = 217 and 245 in ES -mode and m/z = 219 in ES + mode, showing that fulvic acids photosensitized transformation also yielded FM demethylation, but this pathway was very minor.

Mechanisms of reactions

FA and nitrates both accelerated the phototransformation of FM through different reaction mechanisms. Nitrate absorbs solar radiation in the actinic spectrum (ε 304 = 7.4 M -1 cm -1 ) and produces photoreactants through the two following primary processes [START_REF] Boule | Phototransformations induced in aquatic medium by Fe III , NO 3 -, NO 2 and humic substances[END_REF] :

NO 3 -+ hν → NO 2 -+ O NO 3 -+ hν → NO 2 • + O -• O -• + H + ↔ HO •
The formation of hydroxyl radicals is 9-fold more efficient than that of atomic oxygen.

As hydroxyl radicals are also much stronger oxidants than atomic oxygen, they are generally considered to be the main species involved in the nitrate-photoinduced transformations of organic compounds, even though nitration reactions were also reported to occur [START_REF] Machado | Phototransformation of phenolic derivatives in aqueous solution induced by excitation of nitrate and nitrite ions[END_REF][START_REF] Vione | Formation of nitrophenols upon UV irradiation of phenol and nitrate in aqueous solutions and in TiO 2 aqueous suspensions[END_REF]. Hydroxylation of the aromatic ring and oxidation/elimination of the methyl groups of the urea aliphatic chain confirm that FM was oxidized by hydroxyl radicals (Scheme 3).

The aromatic ring of FM is a site of attack for hydroxyl radicals. The OH-adduct radical is expected to yield the ring hydroxylated compound (photoproduct II). Photoproducts I, III and IV are also ring hydroxylated products, but, contrary to II, they bear the CO 2 H substituent instead of the CF 3 substituent initially present in FM. The formation of these products requires two steps. Either they are produced by photohydroxylation of the acid, or they result from the direct photolysis of photoproduct II. Photoproduct II which shows a maximum of absorption at 295 nm instead of 275 nm for FM absorbs solar light better than FM, and thus could undergo faster photolysis. In both cases, several isomers corresponding to the possible sites of attack for hydroxyl radicals are expected. As three hydroxylated acids were detected, three hydroxylated derivatives of FM should be produced too. We only detected one peak at the mass m/z = 247 in ES -. Either, the three isomers show the same retention time and are eluted altogether, or we have failed to detect two of them.

The second site of attack of hydroxyl radicals is the urea aliphatic chain. Three photoproducts were detected: the carbonyl derivative VII, the monodemethylated product VI and the didemethylated product V. The first step should be the abstraction of a H atom from CH 3 to form the CH 2

• radical that can add oxygen and finally give V, VI and VII. On the HPLC chromatograms and by total ion count (TIC), the area of products V, VI and VII were higher than those of products I-IV. It seems therefore that the attack of the urea terminal chain is the main reaction pathway. However, without titration of photoproduct to measure chemical yields, this argument must be taken with caution because ring hydroxylation and hydrolysis of CF 3 significantly shift maxima of absorption to longer wavelengths and photoproducts I-IV might not accumulate due to photoreactivity. In the chromatograms of solar-light irradiated solutions of micromolar FM, only photoproducts VI and VII were detectable.

Fulvic acids are known to produce several photoreactants under light excitation (oxidant excited states, singlet oxygen, radicals). The quite small photosensitizing effect of fulvic acids on FM shows that FM is not easily oxidized by these species. A part of the reaction can be attributed to hydroxyl radicals as confirmed by the detection of product VI as a minor by-product and the inhibiting effect of 2-propanol on the reaction. The two main photoproducts were I and II. Contrary to what was observed in the nitrate-mediated phototransformation, only one hydroxylated acid derivative was produced in the presence of fulvic acids. This result suggests that the FA mediated ring hydroxylation does not involve hydroxyl radicals. As an alternative, the hydroxylation reaction could involve the triplet excited state of FA ( 3 FA * ). 3 FA * reacts with FM either through energy transfer or through electron transfer. We found that the rate of FM phototransformation is accelerated upon deoxygenation of the medium by nitrogen bubbling as in the case of monuron [START_REF] Richard | Transformation of monuron photosensitized by soil extracted humic substances : energy or hydrogen transfer mechanism ?[END_REF]. This result confirms the involvement of 3 FA * in FM photodegradation but does not clear up the mechanism. In the case where an energy transfer reaction would occur, 3 FM * would be produced. Oxidation photoproducts are formed. It implies that 3 FM * is oxidized. As direct photolysis of FM leads to acidification of CF 3 substituent, one would conclude that singlet and triplet excited states of FM react differently. As an alternative, electron transfer from FM to 3 FA * could occur. It is the hypothesis proposed by [START_REF] Gerecke | Quantification of dissolved organic matter (DOM) mediated phototransformation of phenylurea herbicides in lakes[END_REF]. This hypothesis was also adopted in the case of fenuron, an analog of FM, because the FA mediated phototransformation of fenuron was found to be inhibited by removal of oxygen [START_REF] Richard | Transformation of monuron photosensitized by soil extracted humic substances : energy or hydrogen transfer mechanism ?[END_REF]. Thus, the mechanism of reaction of FM is not clear. Further addition of dioxygen on the ring should finally produce the hydroxylated products, either photoproduct II in the case where the oxidation step involves FM or photoproduct I if it involves the acid, the photoproduct of direct photolysis. As proposed above, II might also produce I by photolysis.

Conclusion

In conclusion, direct photolysis of aqueous FM is slow but photosensitizing processes may promote its transformation in surface waters. In the presence of hydroxyl radicals, both the lateral chain and the ring of FM are oxidized. Photoreactants deriving from fulvic acids privilege oxidation of the aromatic ring. Photoproducts of hydrolysis or hydroxylation of the aromatic ring absorb solar light better than FM itself. They are thus susceptible to undergo photolysis faster than FM. Photoproducts resulting from oxidation of lateral urea chain are more photostable. It would be now necessary to test the toxicity of each individual compound or that of the irradiated mixtures toward reference organisms in order to access the risk that may pose FM.

Captions for figures

Figure 1 : Absorption spectrum of (----) FM at 3x10 -4 M, ( __ ) FA at 10 mg L -1 , ( …. ) nitrates at 0.01 M and solar light emission reaching the earth surface in summer from [START_REF] Zepp | Rates of direct photolysis in aquatic environment[END_REF].

Figure 2 : Kinetics of FM (10 -6 M) phototransformation in solar light in various aqueous media. Plot of lnc 0 /c vs the irradiation time, where c and c 0 were the concentrations at t and t=0, respectively.

: in pH 6.5 MilliQ purified water : in water containing FA (10 mg L -1 )

: in water containing FA (10 mg L -1 ) and nitrates (3.10 -4 M)

: in water containing nitrates (3.10 -4 M)

Figure 3 : HPLC chromatogram of (A) a neutral solution of FM (3x10 -5 M) containing fulvic acids (25 mg L -1 ) at a conversion extent of 20 % and (B) a neutral solution of FM (3x10 -5 M) containing nitrate ions (25 mg L -1 ) at a conversion extent of 46 %. Both solutions were irradiated in simulated solar light. Photoproducts are numbered as indicated in Table 2.

  Scheme 1: Fragmentation of photoproduct I

Table 1 :

 1 Phototransformation of FM (10 -6 M) in solar light. Rate coefficients deduced from logarithmic decay, correlation values and half-lives considering that samples were irradiated for 13 h per day.

		k	R 2	t 1/2
	Conditions	(s -1 )		(h)
	FM in MilliQ water, buffered at pH 6.5	1.1×10 -6 0.972 175
	FM in presence of FA (10 mg L -1 )	2.7×10 -6 0.983	71
	FM in presence of nitrate ions (25 mg L -1 )	1.7×10 -5 0.991	11
	FM in presence of FA (10 mg L -1 )	1.1×10 -5 0.986	18
	and nitrate ions (25 mg L -1 )

Table 2 :

 2 Proposed structure of detected photoproducts, retention time in HPLC-UV, maxima of absorption, molecular ions and main fragments detected by LC-ESI-MS.

	formula				number Molecular	Condition	λ max	m/z and main
						weight	of	(nm)	fragments
							obtention	(%)
		N CO N H	CH 3	FM	232		242/275 ES -: 231
			CH 3
	F 3 C
		N CO N H		CH 3 CH 3	acid	208	water Pure	224/297 ES -: 207
	HO 2 C
	HO	N CO N H	CH 3 CH 3	I	224	FA or nitrates	222/324 ES -: 223 (179, 134) ES + : 225
	HO 2 C
	HO	N CO N H	CH 3 CH 3	II	248	FA or nitrates	242/295 ES -: 247 (202) ES + : 249
	F 3 C
	HO	N CO N H	CH 3 CH 3	III	224	Nitrates	230/311 ES -: 223 (179, 134)
	HO 2 C	isomer of I
	HO	N CO N H	CH 3 CH 3	IV	224	Nitrates	224/315 ES -: 223 (179, 134)
	HO 2 C	isomer of I
		H			V	204	Nitrates	241/280 ES -: 203
		N CO NH 2						(160)
	F 3 C
		N CO N H	CH 3 H	VI	218	Nitrates (minor) or FA	243/279 ES -: 217 (160)
	F 3 C