Laboratory testing procedures to understand pesticide fate in surface waters

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Pesticide hydrolysis

This section presents an extended summary of a previously published paper [Masbou et al., 2018]. This work started before the PhD and was published during. Specific results from this publication are part of the interpretation of S-metolachlor degradation at the SWI. While hydrolysis of triazine (atrazine), acylalanine (metalaxyl) and chloroacetanilide pesticides (alachlor, acetochlor, butachlor and S-Metolachlor) were investigated in this study, the results presented here volontarily focus on chloroacetanilide pesticides. Sections on the enantio-selectivity of metalaxyl hydrolysis and isotope fractionation of transformation products are not presented. Only hydrolysis degradation rates, TPs formation and induced isotope fractionation are reported in this manuscript, for the sake of consistency with other chapters of this thesis.
Masbou, J., Drouin, G., Payraudeau, S., Imfeld, G. (2018). Carbon and nitrogen stable isotope fractionation during abiotic hydrolysis of pesticides. Chemosphere, 213, 368–376.
Compound-specific Stable Isotope Analysis (CSIA) has been recently established as a tool to study pesticide degradation in the environment. Among degradative processes, hydrolysis is environmentally relevant as it can be chemically or enzymatically mediated. Here, CSIA was used to examine stable carbon and nitrogen isotope fractionation during abiotic hydrolysis of legacy or currently used chloroacetanilide herbicides (Acetochlor, Alachlor, S-Metolachlor and Butachlor). Degradation products analysis and C-N dual-CSIA allowed to infer hydrolytic degradation pathways from carbon and nitrogen isotopic fractionation. Carbon isotopic fractionation for alkaline hydrolysis revealed similar apparent kinetic isotope effects (AKIEC = 1.03 – 1.07) for all pesticides, which were consistent with SN2 type nucleophilic substitutions. Reference values for abiotic versus biotic SN2 reactions derived from carbon and nitrogen CSIA may be used to untangle pesticide degradation pathways and evaluate in situ degradation during natural and engineered remediation.

Hydrolysis of chloroacetanilides and isotopic fractionation

Hydrolysis of chloroacetanilides was only observed under alkaline conditions (pH=12) and DOC addition did not influenced hydrolysis at pH = 7 (see Table II.3). All chloroacetanilide molecules underwent hydrolysis at 30◦C with similar degradation kinetics (k = 0.038 ± 0.003 d−1 for Acetochlor, k = 0.038 ± 0.004 d−1 for Alachlor, k = 0.027 ± 0.005 d−1 for Butachlor), with the exception of S-Metolachlor (k = 0.006 ± 0.001 d−1 at 30◦C and insignificant hydrolysis at 20◦C). Hydrolysis kinetics for Acetochlor, Alachlor and Butachlor in experiment at 30◦C were 3 to 4 fold faster compared to those in experiments at 20◦C. Except for S-Metolachlor, for which no degradation products were identified, mass spectra interpretation and comparison with literature [Zheng and Ye, 2001] indicates the formation of the hydroxylated products N-(2,6-diethylphenyl)-2-hydroxy-N-(metoxymethyl) acetamide, N-(ethoxymethyl)eN- (2-ethyl-6-methylphenyl)-2- hydroxyacetamide and N-(2,6-diethylphenyl)-2-hydroxy- N- (butoxymethyl)acetamide as degradation products for Alachlor, Acetochlor and Butachlor hydrolysis, respectively. Inspection of mass spectra obtained by both GCMS and LC-MS running in full-scan mode did not allow to detect other degradation products. The occurrence of hydroxylated degradation products in all experiments revealed nucleophilic substitution at the C −Cl bond position of the chloroacetanilide pesticides. Our results are consistent with those obtained during alkaline (NaOH 2 N) hydrolysis of chloroacetanilides [Carlson et al., 2006]. In few cases, however, an additional pathway involving amide cleavage was observed, that we could not exclude for S-Metolachlor under alkaline hydrolysis as no degradation products were unequivocally identified.
Acetochlor, Alachlor and Butachlor displayed similar carbon isotopic fractionation during abiotic hydrolysis (ǫC = −4.0±0.8‰, ǫC = −4.9±0.4‰and ǫC = −3.7±0.7‰, respectively), whereas ǫC for S-metolachlor was lower (−2.8 ± 0.6‰) (see Table II.3). In comparison, ǫC values have been obtained for biotic degradation of metolachlor (ǫC ≈ 0‰, degradation ≤ 30%), alachlor (ǫC = −2.0±0.3‰, degradation ≤ 60%) and acetochlor (ǫC = −3.4±0.5‰, degradation ≤ 65%) in a constructed wetland [Elsayed et al., 2014] or for S-metolachlor biotic degradation (ǫC = −1.5 ± 0.5‰, degradation ≥ 80%) in soil incubation experiments [Alvarez-Zald´ıvar et al., 2018]. Small differences in isotopic fractionation during chloroacetanilide hydrolysis and biotic degradation may reflect distinct mechanisms, which is supported by the formation of different degradation products. For instance, oxanilic (OXA) and ethanesulfonic acids (MESA) prevailed in wetlands [Maillard et al., 2016] but were not detected in this study. Weak nitrogen isotopic fractionation (δ15N ≈ 1.5‰) for Alachlor, Acetochlor and Butachlor did not allow to derive ǫN values given the analytical precision (≈ 1.0‰). In contrast, S-Metolachlor displayed small but significant ǫN = −2.0 ± 1.3‰. Since S-Metolachlor contains a single nitrogen heteroatom, nitrogen isotope effect is position specific and secondary.

Mechanistic insights from AKIE values and C − N isotope plots

AKIEC values derived from isotopic fractionation for both alkaline and acidic hydrolysis (Table II.3) ranged from 1.043 to 1.073 for all pesticides, which is consistent with primary isotope effect during SN −2 type mechanisms (AKIEC from 1.03 to 1.07 [Elsner et al., 2005]). Despite the occurrence of primary isotope effects and AKIEC calculation that limit isotopic dilution effects, carbon isotope fractionations lack of sensitivity to tease apart hydrolysis mechanisms. factors (ǫC and ǫN – ‰) as well as AKIEC and AKIEN values for Acetochlor, Alachlor, Butachlor and S-Metolachlor at pH = 2, 4, 7, 9 and 12 and T = 20◦C and T = 30◦C. Results indicate mean ±σ. The results obtained for the experiments with DOC (5 mgC.L−1, 10 mgC.L−1, 20 mgC.L−1) at pH=7 are not displayed as they did not differ from the condition without DOC (i.e. DT50 ≥ 200 days, and ǫ insignificants). n.s. stands for not significant.

Environmental implications

Abiotic hydrolysis of Acetochlor, Alachlor, S-Metolachlor, Butachlor, Metalaxyl, and Atrazine at most common environmental pH (4 < pH < 9) is a slow process (DT50 > 200 days) associated with insignificant isotopic fractionation (δ13C ≤ 0.5‰). Abiotic hydrolysis of studied pesticides thus may be neglected in most CSIA studies when pH of surface and groundwater systems range between 6 and 8.5, although hydrolysis may also occur locally at water-mineral interface in aquifers [Smolen and Stone, 1997]. In contrast, significant hydrolysis occurred at pH > 9. For instance, at 30◦C chloroacetanilide degradation ranged from a few days (Acetochlor, pH = 12) to several months (S-Metolachlor, pH = 12). Such alkaline conditions may locally occur in carbonate-rich minerals drainage (pH ≈ 10 − 12, [Khoury et al., 1992,M¨oller and Bau, 1993]). Most importantly, the combination of degradation products analysis and C-N dual-CSIA allowed to elucidate degradation mechanisms of chloroacetanilide pesticides. These first reference isotopic fractionation values for hydrolysis of anilide pesticides can be compared in the future with 15N and 13C fractionation patterns during microbial degradation, and eventually used to tease apart degradation pathways in the field.

Pesticide photodegradation in agriculturally impacted surface waters

From submitted paper: Drouin, G., Droz, B., Leresche, F., Payraudeau, S., Masbou, J. & Imfeld, G., Direct and Indirect Photodegradation of Atrazine and S-metolachlor in Agriculturally Impacted Surface Water and Associated C and N Isotope Fractionation, Environmental Science & Technology. Limited knowledge of photodegradation-induced isotope fractionation hampers the application of compound-specific isotope analysis (CSIA) to trace pesticide degradation in surface waters. Here, we investigated carbon and nitrogen isotope fractionation during direct and indirect photodegradation of the herbicides atrazine and S-metolachlor in synthetic water mimicking agriculturally impacted surface waters containing nitrates (20 mg.L−1) and dissolved organic matter (DOM – 5.4 mgC.L−1).
Atrazine and S-metolachlor were quickly photodegraded by direct and indirect mechanisms (half-lives < 5 and < 7 days, respectively). DOM slowed down photodegradation while nitrates increased degradation rates. Transformation products analysis showed that oxidation mediated by hydroxyl radicals predominate during indirect photodegradation. UV light (254 nm) caused significant C and N isotope fractionation, yielding fractionation factors ǫC = 2.7±0.3 and 0.8±0.1‰ and ǫN = 2.4±0.3 and −2.6 ± 0.7‰ for atrazine and S-metolachlor respectively. In contrast, photodegradation under simulated sunlight led to negligible C and slight N isotope fractionation. As the radiation wavelength influenced the direct photodegradationinduced isotope fractionation, the use of simulated sunlight is recommended to evaluate photodegradation mechanisms in the environment. Since C and N isotope fractionation patterns for atrazine and S-metolachlor photodegradation differed from those reported for biodegradation and hydrolysis, CSIA offers new opportunities to distinguish between photodegradation and other dissipation pathways in surface waters.

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Table of contents :

R´esum´e en Fran¸cais
CHAPTER I – General introduction 
1.1 Pesticide use, worldwide occurrence, threats and regulation
1.2 State of the art: pesticide persistence in surface waters
1.3 Multi-scale and multi-tool approaches to assess pesticide fate at the SWI.
1.4 Research questions and overall approach of the thesis
1.5 Graphical abstract of the thesis work
1.6 Publications and author contributions
CHAPTER II – Pesticide degradation pathways at the sediment-water interface 
2.1 Analytical section: Sample processing, Quantification and Compound- Specific Isotope Analysis
2.2 Pesticide hydrolysis
2.2.1 Introduction
2.2.2 Material and Methods
Chemicals and Solution Preparation
Experimental Setup
2.2.3 Results and Discussion
Hydrolysis of chloroacetanilides and isotopic fractionation
Mechanistic insights from AKIE values and C−N isotope plots
2.2.4 Environmental implications
2.3 Pesticide photodegradation in agriculturally impacted surface waters
2.3.1 Introduction
2.3.2 Material and method
Chemicals and preparation of solutions
Experimental Section
Analytical section
2.3.3 Results & Discussion
Effects of the Hydrochemistry on the Photodegradation
Rates under Simulated Sunlight
Formation of Phototransformation Products
C and N Isotope Fractionation to Trace Atrazine and S-
metolachlor Photodegradation
2.3.4 Environmental implications
2.4 Pesticide oxic and anoxic biodegradation in water-sediment systems
2.4.1 Introduction
2.4.2 Material and method
Sediment sampling and characteristics
Experimental set-up
Control experiments
Pesticide extraction, quantification and CSIA
Suspected screenings of transformation products
Phase-transfer and biodegradation modelling
2.4.3 Results and discussion
Dissipation kinetics and isotope fractionation
Water–sediment phase-transfer and implications for interpreting
degradation kinetics
Water–sediment phase-transfer and implications for interpreting
isotope signature
Pesticide degradation pathways
2.4.4 Environmental implications for water–sediment studies
CHAPTER III – Pesticide transport at the sediment-water interface 
3.1 Introduction
3.2 The FRT numerical model
3.2.1 Governing equations
3.2.2 Numerical resolution
3.3 Tracer recirculation in bench-scale river channel
3.3.1 Experimental setup
3.3.2 Conservative and sorptive tracer experiments
3.3.3 Data acquisition
3.3.4 Experiment conditions
3.4 Model validation vs experiment
3.4.1 Numerical simulations: Conceptual model, parameterization and domain discretization
3.4.2 Comparing numerical and experimental results
3.5 Pollutant transport at the sediment-water interface
3.5.1 Velocity field in the bench-scale river channel and transport processes
3.6 Transport processes governing mass exchange in the bench-scale river channel
3.6.1 Understanding and predicting the effect of sorption on mass exchange at the SWI
3.7 Towards a comprehensive understanding of hyporheic process at the SWI
3.8 Implication and perspectives for conservative and sorptive pollutant transport at the SWI
3.9 Investigation of the influence of water flow on pollutant degradation
3.9.1 Material and Methods
Adjustments to the bench-scale river channel
Experimental conditions
Adjustments to the FRT model
3.9.2 Results and discussion
Influence of water flow on the depth of the oxic sediment layer
Influence of water flow on caffeine degradation at the SWI
3.10 Implications and perspectives for pollutant degradation at the SWI
CHAPTER IV – S-metolachlor dissipation alongside a river reach 
4.1 Introduction
4.2 Material and methods
4.2.1 Study site
4.2.2 S-metolachlor application
4.2.3 Soil, water and sediment collection
4.2.4 Chemical analyses
4.2.5 Data analysis
4.3 Results and discussion
4.3.1 Hydrological response and AVB hydrological functioning
4.3.2 S-metolachlor sources and export to the AVB
4.3.3 S-metolachlor dissipation in the AVB: transport dominates over degradation
4.4 Environmental significance and outlook for CSIA use in rivers
CHAPTER V – General conclusions 
5.1 Introduction
5.2 Laboratory testing procedures to understand pesticide fate in surface waters
5.2.1 General discussion
5.2.2 Implications and perspectives
5.3 A robust and parsimonious mathematical framework tailored to the studied system
5.3.1 General discussion
5.3.2 Implications and perspectives
5.4 CSIA from molecular to river scales to characterize pesticide fate at the SWI: potential, limits and perspectives
5.5 Perspectives
5.6 Implications for water resource managers
CHAPTER A – Supporting information to chapter II 
A.S1 Photodegradation
A.S1.1 List of chemicals
A.S1.2 Main Properties of Selected Pesticides and Origin of the Substances Used
A.S1.3 Chemical Composition of Irradiation Solutions
A.S1.4 Organic Matter Photobleaching
A.S1.5 PNA/Pyr Actinometer System
A.S1.6 Irradiation conditions and correction of degradation rates .
A.S1.7 Light Spectrum Homogeneity within the Light-proof Box
A.S1.8 Atrazine and S-metolachlor transformation products
A.S1.9 Optical properties of chemicals and irradiation solutions
A.S1.10 Simulated sunlight characteristics
A.S1.11 Prediction of Degradation Rates and Identification of Dominant Photodegradation Mechanisms
General methodology
Calculation of Short-lived Reactive Intermediates Steady
State Concentrations
Identification of Main Photosensitizers
Calculation of Light Absorption Rates and Screening Factors
Detailed Results
A.S1.12 Presentation of Experimental Datasets
A.S1.13 Comparison of C and N isotope fractionation during atrazine
photodegradation, biodegradation and abiotic hydrolysis
A.S2 Biodegradation
A.S2.1 Appendices for material and methods
List of chemicals
Sediment sampling and characteristics
Hydrochemistry characterization
Batch sorption isotherm
Mass balance accounting for the phase-transfer process
A.S2.2 Appendices for results and discussion
Sediment characteristics
Control experiments
Batch sorption experiments
Pesticide dissipations
CHAPTER B – Supporting information to chapter III 
B.S1 Current developments at the SWI
B.S2 Sand characterization
B.S3 Characterization of FB291 adsorption from the batch equilibrium method – OECD
B.S4 Compensation of tracer concentrations for evaporation and ion release
B.S5 Scaling methodology of concentrations for inter-comparison of experiments
B.S6 Mesh sensitivity analysis
B.S7 Fit of modelled velocities with the log-law profile
B.S8 Darcy’s velocities calculation
B.S9 Characterization of the transport regime with Pe
B.S10 Characterization of the transport regime with Deff
CHAPTER C – Supporting information to chapter IV 
C.S1 S-metolachlor applications
C.S2 S-metolachlor and main TPs summary of physicochemical properties
C.S3 Hydrochemistry of the AVB river measured at S1 and S2
C.S4 Example of GC-IRMS chromatogram with soil sample
C.S5 Riverbed sediment texture
C.S6 Temporal evolution of S-metolachlor concentrations within top soils surrounding S1 and S2
C.S7 Test of isotopic signatures shift within the AVB river reach
C.S8 Averaged transit time and degradation within the AVB river reach
C.S9 Evidences of S-metolachlor release from the riverbed from monitoring of the regional water agency
C.S10 Temporal evolution of S-metolachlor concentrations in the riverbed sediment
C.S11 Excepted isotope fractionation with increasing residence time


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