Article Photochemical Degradation on Soil Model Surfaces

Marie Siampiringue 1,2, Rajae Chahboune 1,2, Pascal Wong-Wah-Chung 3 and Mohamed Sarakha 1,2,*

1 Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, 10448, F-63000 Clermont-Ferrand, France; [email protected] (M.S.); [email protected] (R.C.) 2 Centre National de la Recherche Scientifique, UMR 6296, ICCF, BP 80026, F-63171 Aubière, France 3 Laboratoire Chimie Environnement (LCE), CNRS UMR 7376, Aix Marseille University, F-13331 Marseille, France; [email protected] * Correspondence: [email protected]; Tel.: +334-7340-7170

 Received: 15 December 2018; Accepted: 24 February 2019; Published: 5 March 2019 

Abstract: The phototransformation of carbaryl was investigated upon solar light exposure on three surfaces, silica, kaolin and sand, as soil models. By excitation with a Suntest set up at the surface of the three solid supports, the degradation of carbaryl followed first-order kinetics with a rate constant of 0.10 h−1. By using the Kubelka Munk model, the quantum yield disappearance at the surface of kaolin was evaluated to 2.4 × 10−3. Such a value is roughly one order of magnitude higher than that obtained in aqueous . The results indicated that the particle size and the specific surface area of the various models have significant effects. The photo-oxidative properties as well as the byproduct elucidation by liquid chromatography combined with diode arrays (LC-DAD) and liquid chromatography coupled mass spectrometry (LC-MS) analyses allowed us to propose the degradation mechanism pathways. The main products were 1-naphtol and 2-hydroxy-1,4-naphthoquinone, which arise from a photo-oxidation process together with products from photo-Fries, photo-ejection and methyl hydrolysis. The toxicity tests clearly showed a significant decrease of the toxicity in the early stages of the irradiation. This clearly shows that the generated products are less toxic than the parent compound.

Keywords: photodegradation; carbaryl; kaolinite; soil; solar irradiation

1. Introduction Carbaryl, 1-naphtyl N-methylcarbamate, also known by the trade name Sevin®, is distributed by the Company. It is used as a substitute for some and represents one of the most commonly used [1]. Carbaryl has been used against many harmful on different crops by contact [2–4]. It has also been employed in golf courses, lawns, and alandscapes for flea, lice or mosquito treatment [5,6]. Su ch wide range of applications is due to its ability to act as an inhibitor of , which is one of the main in the nervous system [7]. Thus, regarding the large amount that is spread, it has been clearly established that this compound can lead to bioaccumulation in food and water sources. Martin et al. have shown that carbaryl is one of the most commonly detected insecticides in surface waters in the U. S., while Walters et al. confirmed its presence in fish (7 µg L−1) and in rain runoff (1737 µg L−1)[8,9]. More recently, studies have established the presence of carbaryl and/or its hydrolyzed products in soils in relatively high amounts (16.3 ppm) [10–12]. Such results have prompted researchers to focus their attention on its fate in various environmental compartments [13–18].

Soil Syst. 2019, 3, 17; doi:10.3390/soilsystems3010017 www.mdpi.com/journal/soilsystems Soil Syst. 2019, 3, 17 2 of 14

It has been well described in the literature that, among the carbaryl transformation pathways in water, the hydrolysis process may play a significant role. Such efficient reaction leads to the formation of 1-naphtol as a primary product, in pure water as well as in artificial or natural waters within a couple of days [16–18]. Moreover, carbaryl may also undergo photochemical dissociation in the environment. In aqueous media, carbaryl photolysis occurs through the production of naphtoxyl radicals by homolytic scission and leads to several naphtoquinone derivatives [19]. Concerning studies on indirect photodegradation in natural waters, these highlight the role of hydroxyl radicals that were generated by nitrate ions and dissolved organic matter (DOM) [20]. Beside these studies on carbaryl behaviour in water under sunlight irradiation, to our knowledge, no work has been devoted to the phototransformation of carbaryl on soil surfaces. However, several studies have investigated its adsorption, interactions and biodegradation at the surface of clays and soils [21–26].

2. Materials and Methods

2.1. Chemical Carbaryl (1-naphtyl-N-methylcarbamate) (99.8%), coumarin (≥99%), 1-naphtol and 2-hydroxy-1,4-naphtoquinone (97%) were purchased from Sigma Aldrich and used without further purification. Kaolin and silica were purchased from Fluka. Sand (sand of Fontainebleau) was purchased from Labosi, and was cleaned twice with methanol (5 g in 20 mL) and dried at 80 ◦C for several hours prior to use. All the other chemicals were used as received without further purification.

2.2. Preparation of Solid Model Media Doped with Carbaryl

2.2.1. Kaolin Layer Samples The preparation of clay layers (2.5 cm × 1.5 cm) was performed on Pyrex glass according to the published procedure [27]. Clay slurry and the standard of carbaryl (12.5 mg in 25 mL) were prepared in methanol solution. Accurate volumes of slurry were poured on the glass and allowed to dry for 2 h at room temperature (roughly 20 ◦C). The layer thickness was determined considering the dried amount of clay, surface area and kaolin bulk density (1.8 g cm−3)[28]. Thus, layers of various thicknesses were varied from 25 to 200 µm by varying the slurry volume. Then, 0.5 mL of carbaryl solution was systematically and carefully poured on kaolin layer to obtain a constant concentration of carbaryl (roughly 0.25 mg per layer) after solvent evaporation.

2.2.2. Powder Samples Five grams of solid sample were added to 5.0 mL of a methanol solution of carbaryl (2.5 × 10−4 mol L−1), mixed vigorously and then dried to allow the evaporation of the organic solvent. Carbaryl concentration was maintained constant at 0.5 mg of carbaryl per gram of solid support. Three hundred milligrams of this powder were deposited on a Teflon mold supported by a glass slide to obtain a surface of roughly 1.0 mm thickness.

2.3. Characterization of Solid Supports Particle size distribution was performed using a laser diffraction particle size analyser model Mastersizer S from Malvern Instruments in dry mode. A standard size within the range 500 nm to 900 µm was selected. Samples were previously dried in oven for 48 h at 100 ◦C and left in a dessiccator at room temperature prior to analysis. A Beckman Coulter sorptiometer model SA 3100 was used to obtain the specific surface areas (Sspec) and pore-size distributions using Brunauer–Emett–Teller (BET) and Barret–Joyner–Halenda (BJH) models, respectively. Samples were previously degassed for 24 h under vacuum (10−4 torr at 100 ◦C) and adsorption measurements were realized with at −196 ◦C. Soil Syst. 2019, 3, 17 3 of 14

A Bruker AXS X-ray fluorescence (XRF) analyser model S4 Pioneer was employed to achieve the elemental composition of the samples. The apparatus was equipped with an Rh anode X-ray tube and an Si (Li) detector. The measurements were performed under vacuum with an analysis range from sodium to bromine using a standard semi-quantitative method. The samples were prepared as pressed pellets containing 9 g of H3BO3 as binder and 1 g of solid powder.

2.4. Irradiation Systems

2.4.1. Solar Simulated Irradiation The system of irradiation was a device that simulated solar radiation (Suntest-CPS+, Atlas, IL, USA) equipped with a Xenon lamp. A filter allowing transmission at λ > 290 nm was used. The power was fixed to provide about 550 W·m−2 and the samples’ temperature was controlled by a water flow (T = 20 ◦C).

2.4.2. Monochromatic Irradiation at 365 nm Irradiations were carried out in an elliptical stainless-steel cylinder equipped with three high-pressure lamps (black lamp, Mazda MAW 125 W). The lamps presenting an emission spectrum with a maximum located at 365 nm (97% of the emission) were symmetrically installed and the reactor. A water-jacketed Pyrex tube (diameter of 2.8 cm) containing a maximum of 50 mL solution was located in the centre. The solution was continuously stirred and cooled by a water flow at a temperature of 20 ◦C during the entire irradiation process.

2.5. Extraction and HPLC Analysis The concentrations of carbaryl in the solid samples were determined after the following extraction procedure: the entire irradiated solid sample was transferred into a vial and 4.0 mL of methanol were added. The solution was vigorously stirred for 10 min and then centrifuged at 13,500 rpm. The supernatant was then analyzed by HPLC (Waters) equipped with a diode array detector (Waters 990), two pumps (Waters 515) and an auto sampler (Waters 717). A C18 column Nucleodur 100-5 (250 mm × 4.6 mm, 5 µm) with a Nucleodur 100-5 C8 pre-column from Macherey Nagel was used. The mobile phase was a mixture of methanol and water (0.3% formic acid). The separation was obtained using a linear gradient MeOH/water (10/90, v/v) to 100% of MeOH within 20 min analysis time. The flow was fixed at 1.0 mL min−1. Thirty microliters of the supernatant were injected three times. Two different samples were systematically irradiated, and the concentration of carbaryl was the average of six independent analyses. It should be pointed out that, under our experimental conditions, the extraction efficiency of carbaryl under these conditions was higher than 90%. The photoproduct identifications were carried out by LC-MS experiments realized with a Waters HPLC system (Alliance 2695) coupled to a Q-Tof mass spectrometer equipped with a pneumatically-assisted electrospray ionization source (ESI). The HPLC system was also coupled with a Waters 996 diode array detector. Chromatographic separation was achieved by using an elution program from 95% water (with 0.2% acetic acid) and 5% methanol to 5% water (with 0.2% acetic acid) and 95% methanol after 15 min; the obtained isocratic conditions were maintained during 10 min. The flow rate was 0.2 mL min−1, the injected volume was 30 µL and the column was distributed by Waters (Xterra MS C18, 100 mm × 2.1 mm, 3.5 µm). The electrospray source parameters were in a positive mode: capillary voltage 3000 V (2100 V in negative mode), cone voltage 35 V, extraction cone voltage 2 V, desolvation temperature 250 ◦C, source temperature 110 ◦C, ion energy 2 V and collision energy 5 eV. The mass resolution was set to 103.

2.6. Toxicity Experiments The toxicity tests were undertaken using kaolin layer samples (thickness 100 µm) spiked with carbaryl (20.0 µmol g−1) and irradiated with the Suntest. After extraction with methanol followed by Soil Syst. 2019, 3, 17 4 of 14 solventSoil Syst. evaporation2019, 3, x FOR PEER and REVIEW dissolution in water, the pH of the carbaryl solution (initial concentration4 of 14 of 1.55 × 10−4 mol L−1) was adjusted to 7.0 using buffer solution. The toxicity was then evaluated La Vallee, France), two valves of injection (Rheodyne model 9010 and 7725) and a bio-analytical using thermal lens spectroscopy (TLS) detection with flow injection analysis (FIA) as described in column containing . To determine the enzymatic activity, an inhibition the literature [29]. The FIA system is an HPLC system equipped with a pump (Shimadzu LC-10Ai, test is performed. Enzyme inhibition was calculated according to Equations (1) and (2): Marne La Vallee, France), two valves of injection (Rheodyne model 9010 and 7725) and a bio-analytical column containing acetylcholinesterasea enzyme. To determinea+a+a the enzymatic activity, an inhibition A= r 123 test is performed. Enzyme inhibition wascalculated with a= according0 to Equations (1) and (2): (1) a0 3

ar a1 + a2 + a3 A = withI=1-A a0 = (2)(1) a0 3 where a0 is the initial activity of enzyme (average of three measurements a1, a2 and a3), ar is the activity I = 1 − A (2) of the enzyme after injection of carbaryl solution and A is the remaining activity, and I is the enzyme whereinhibition a0 is theactivity. initial activity of enzyme (average of three measurements a1, a2 and a3), ar is the activity of the enzyme after injection of carbaryl solution and A is the remaining activity, and I is the enzyme inhibition3. Results activity.

3.3.1. Results Photodegradation of Carbaryl on Kaolin

3.1. PhotodegradationIn order to determine of Carbaryl spectrophotometric on Kaolin features of carbaryl on solid supports, and prior to the photochemical studies, the diffuse reflexion spectra at various concentrations on a 200 µm kaolin layerIn were order recorded to determine and spectrophotometric analysed (Figure 1). features After ofsubtracting carbaryl on the solid spectrum supports, of andkaolin prior alone to the as µ photochemicalreported in the studies, literature the diffuse [30], the reflexion spectra spectra show at an various absorption concentrations maximum on aaround 200 m 290 kaolin nm. layer The werecomparison recorded with and the analysed UV absorption (Figure1 ).spectrum After subtracting of carbaryl the recorded spectrum in aqueous of kaolin solution alone as [19] reported indicates in thea similar literature shape [30 ],with the a spectra slight showbathochromic an absorption shift of maximum roughly 10 around nm. Such 290 nm. an effect The comparison is more likely with owing the UVto chemical absorption and/or spectrum physical of carbaryl interactions recorded between in aqueous carbaryl solution molecules [19] indicatesand kaolin a similar support. shape with a slight bathochromic shift of roughly 10 nm. Such an effect is more likely owing to chemical and/or physical interactions between carbaryl molecules and kaolin support.

0.8 λ = 290 nm max 25 0.6

0.4

20 0.2

Kubelka Munk function Munk Kubelka 0.0 15 01530 -1

C, µmol g i 36.8 µmol g-1 10 -1 (%Reflexion) 27.4 µmol g Δ 18.4 µmol g-1 5 13.7 µmol g-1 9.2 µmol g-1 0 250 275 300 325 350 375 400 λ, nm

Figure 1. Diffuse reflexion spectra of carbaryl on kaolin as a function of carbaryl concentration (200 µm Figure 1. Diffuse reflexion spectra of carbaryl on kaolin as a function of carbaryl concentration (200 thickness). The inset shows the evolution of the Kubelka Munk function f (R∞,carb). µm thickness). The inset shows the evolution of the Kubelka Munk function f (R∞,carb). The evalutation of the molar absorption coefficient ε of carbaryl on the kaolin layer was obtained usingThe the methodevalutation reported of the molar in the absorption literature [ 31coefficient]. For samples ε of carbaryl with a on thickness the kaolin higher layer than was 200obtainedµm, theusing Kubelka the method Munk modelreported can in be the used literature [32], with [31]. the For expression samples aswith follows: a thickness higher than 200 µm, the Kubelka Munk model can be used [32], with the expression as follows:

k 2ln(10)εcarb(λ) f(R∞,carb) = k+ 2ln(10)εcarb(λ)Ccarb f(R∞ ) =s +s C ,carbss carb

Soil Syst. 2019, 3, x FOR PEER REVIEW 5 of 14

Soil Syst. 2019, 3, 17 5 of 14 where R∞,carb is the reflexion of the spiked solid with carbaryl, k and s are the absorption and the scattering coefficients of kaolin, respectively, εcarb is the molar absorption coefficient of carbaryl at a λ wheregiven R wavelength∞,carb is the reflexion, and Ccarb of is the the spiked concentration solid with of carbaryl. carbaryl, k and s are the absorption and the ∞ scatteringAs clearly coefficients shown of in kaolin, the inset respectively, of Figure ε1,carb f(Ris,carb the) shows molar absorptiona linear relationship coefficient with of carbaryl Ccarb with at the a givenintercept wavelength k/s thatλ is, and in perfect Ccarb is accordance the concentration with published of carbaryl. theoretical k and s values [31]. Thus, the molarAs clearlyabsorption shown coefficient in the inset at λmax of Figure= 290 nm1, f(R can∞ be,carb deduced) shows from a linear the relationship slope. It was with evaluated Ccarb with to 7.6 the× 10 intercept6 mol−1 cm k/s2, whereas that is in in perfect aqueous accordance solution, withit was published found equal theoretical to 5.2 × 10 k6 and mol s−1 valuescm2 [19], [31 indicating]. Thus, thea small molar hyperchromic absorption coefficient effect. As at farλmax as the= 290 overlap nm can with be the deduced solar light from emission the slope. spectrum It was evaluated is concerned, to 7.6such× 10 an6 moleffect,−1 cm associated2, whereas with in aqueousthe bathochromic solution, it sh wasift, foundis clearly equal in to favor 5.2 × of10 6anmol increase−1 cm2 of[19 the], indicatingabsorbance a small and thus hyperchromic of the potential effect. Asphotodegradability far as the overlap of with carbaryl the solar when light deposited emission on spectrum the surface is concerned,of solid supports. such an effect, associated with the bathochromic shift, is clearly in favor of an increase of the absorbanceWith the andaim thus of studying of the potential the photochemical photodegradability behaviour of carbaryl of carbaryl when on deposited kaolin surface, on the surfacelayers of ofvarious solid supports. thicknesses were prepared and exposed to a solar light simulator (Suntest). It should be noted thatWith no thesignificant aim of studyingreaction theoccurred photochemical when the behaviour samples ofwere carbaryl kept on in kaolin the dark surface, and layers at room of varioustemperature thicknesses (T = 20 were °C). prepared Thus, under and our exposed experiment to a solaral conditions, light simulator no biotic (Suntest). degradation It should occurs be noted even thatthough no significant the irradiations reaction were occurred undertaken when the in samplesnon-sterilized were kept conditions. in the dark As andcan be at roomseen in temperature Figure 2, in (Tthe = 20 absence◦C). Thus, of irradiation, under our experimentalthe conversion conditions, of carbaryl no represents biotic degradation less than occurs 3% within even though1 hour. theThis irradiationsfigure also wereshows undertaken the evolution in non-sterilized of the concentration conditions. of carbaryl As can as be a seen function in Figure of irradiation2, in the absence time and offor irradiation, five different the conversionthicknesses: of 25 carbaryl µm, 50 representsµm, 100 µm, less 150 than µm, 3% 250 within µm. The 1 h. Thiscomplete figure disappearance also shows theof evolutioncarbaryl was of the observed concentration for the lowest of carbaryl thickness as a functionwithin roughly of irradiation 50 h of timelight andirradiation, for five while different only thicknesses:30% degradation 25 µm, was 50 µ observedm, 100 µ m,for 150a thicknessµm, 250 ofµm. 250 The µm. complete The kinetics disappearance clearly depend of carbaryl on the was light observedpenetration for the and lowest appear thickness to follow within first-order roughly kine 50tics. h of The light apparent irradiation, first-order while only rate 30%constants degradation (kZ) as a wasfunction observed of the for clay a thickness layer thickness of 250 µ arem. gathered The kinetics in Table clearly 1. dependThey appear on the to lightbe proportional penetration to and 1/Z appearwithin to the follow thickness first-order range kinetics. 25 µm ≤ TheZ ≤ apparent100 µm, as first-order highlighted rate by constants the calculation (kZ) as aof function the kZ × ofZ. theThis clayvalue layer is thicknessconstant within are gathered the given in Table layer1. Theythickness appear range to be and proportional evaluated toas 1/Z2.40 within h−1 µm. the For thickness higher Z rangevalues, 25 µthem rate≤ Z constants≤ 100 µm, rapidly as highlighted decreased, by likely the calculation owing to the of the very kZ low× Z. carbaryl This value diffusion is constant within withinkaolin the particles. given layer thickness range and evaluated as 2.40 h−1 µm. For higher Z values, the rate constants rapidly decreased, likely owing to the very low carbaryl diffusion within kaolin particles.

1.0 25 µm 50 µm 100 µm 150 µm 250 µm 0.8 50 µm in the dark

0.6 C/Co 0.4

0.2

0.0 0 20406080 Irradiation time, minutes

Figure 2. Photodegradation of carbaryl at the surface of kaolin for different thicknesses: 25 µm, 50 µm, Figure 2. Photodegradation of carbaryl at the surface of kaolin for different thicknesses: 25 µm, 50 100 µm, 150 µm, 250 µm, [carbaryl] = 0.5 mg g−1, irradiation with a Suntest setup. µm, 100 µm, 150 µm, 250 µm, [carbaryl] = 0.5 mg g−1, irradiation with a Suntest setup. Table 1. Apparent first-order rate constant of carbaryl disappearance as a function of the layer thickness: 25Tableµm, 50 1.µ Apparentm, 100 µm, first-order 150 µm, 250 rateµm, constant [carbaryl] of =carbar 5 µmolyl gdisappearance−1, Suntest irradiation. as a function of the layer thickness: 25 µm, 50 µm, 100 µm, 150 µm, 250 µm, [carbaryl] = 5 µmol g−1, Suntest irradiation. Z(µm) 25 50 100 150 250 kZ (h−1) Z (µm) 0.09625 0.049 50 0.024 100 150 0.011 250 0.006 Z −1 k × Z (h µkm)Z (h−1) 2.40.096 2.50.049 0.024 2.4 0.011 1.7 0.006 1.25 kZ × Z (h−1 µm) 2.4 2.5 2.4 1.7 1.25 The constant value observed for kZ × Z is in agreement with the published following formula [27]:

Z 0 k × Z = 1.443(Z 0.5k

Soil Syst. 2019, 3, 17 6 of 14 where kZ is the rate constant obtained for a given layer thickness, k0 is the photolysis first order rate constant at the kaolin surface and Z0.5 represents the depth for which the light intensity is divide by a factor of two. Thus, the photolytic first-order constant at the surface of the kaolin layer, k0, was estimated as 0.27 h−1 [27,28]. This value permitted the calculation of the quantum yield under Suntest irradiation (Φ) using the following equation [27,33]:

Z 0 0 k k = 2.303 × φi × I0(λ) × ε(λ)dλ ⇒ φi = R 2.303 × I0(λ) × ε(λ)dλ λ λ

Under our experimental conditions, the quantum yield of carbaryl transformation was evaluated as 2.4 × 10−3, which is roughly one order of magnitude higher than that obtained in water [19]. This result demonstrates that the photodegradation of carbaryl on the kaolin surface is much more efficient than in aqueous solutions. In order to gain a better insight into the photodegradation pathways at the surface of solid supports, further investigations were undertaken on silica and sand (sand of Fontainebleau).

3.2. Photodegradation of Carbaryl on Kaolin, Silica and Sand Table2 gathers the features of the three model supports that were used in the present work, namely kaolin, silica and sand. Considering the maximum distribution of particle diameter size, one can note that the particle size for sand is the largest, with a value of 310 µm, while the smallest value was obtained for kaolin (5 µm).

Table 2. Some features of the solid model supports.

Kaolin Silica Sand Particle size (µm) 5 10 310 2 Specific surface (m /g) 15 294 0.1 Pore size (nm) 30–100 20 40–150 Silicon (wt %) 54.2 99.1 95.4 Iron (wt %) 2.6 0.00 0.1 Titanium (wt %) 0.9 0.00 0.1

Moreover, using gas adsorption technique, we confirmed the well-known high specific area of silica (up to 294 m2 g−1) but also a very low specific area for the sand, while Kaolin presents an intermediate value of 15 m2 g−1. One can notice that porous distributions as well as the maximum diameter size of the pores depend on the support. The porous distribution is more homogeneous for silica and centered around 20 nm, whereas a heterogeneous distribution was observed with kaolin and sand. Two maxima were evidenced for kaolin and sand, at around 30 and 100 nm and around 40 and 150 nm, respectively. The chemical composition showed that the main component for the solid model supports is silicon with a percentage of 99, 95.4 and 54.2 for silica, sand and kaolin respectively. Nevertheless, it is worth noting that traces (<3%) of titanium and iron were also detected in kaolin and sand. The kinetics of carbaryl disappearance on the different supports, prepared as powder samples, are represented in Figure3. Soil Syst. 2019, 3, x FOR PEER REVIEW 7 of 14 Soil Syst. 2019, 3, 17 7 of 14

1.0

sand 0.9 Silica Kaolin

0.8

0.7 C/Co

0.6

0.5

0.4 0123456 Irradiation time, hours

Figure 3. Photodegradation of carbaryl at the surface of different supports: () sand, (N) silica and (•) kaolin,Figure [carbaryl] 3. Photodegradation = 0.5 mg g− 1of, Suntest carbaryl irradiation. at the surface of different supports: () sand, (▲) silica and (•) kaolin, [carbaryl] = 0.5 mg g−1, Suntest irradiation. This clearly shows that the degradation rates are in the order Rsand > Rsilica > RKaolin (Table3). The degradation at the surface of silica is lower than that in sand despite its highest surface This clearly shows that the degradation rates are in the order Rsand > Rsilica > RKaolin (Table 3). The area indicating that this parameter is not a crucial one. If the surface area could be in favor degradation at the surface of silica is lower than that in sand despite its highest surface area indicating of high photodegradability, the low particle and pore sizes could decrease the photodegradation that this parameter is not a crucial one. If the surface area could be in favor of high efficiency [34–36]. photodegradability, the low particle and pore sizes could decrease the photodegradation efficiency [34–36]. Table 3. Initial rate constants and degradation of carbaryl on different supports, [carbaryl] = 0.5 mg g−1, Suntest irradiation. Table 3. Initial rate constants and degradation of carbaryl on different supports, [carbaryl] = 0.5 mg g−1, Suntest irradiation. Kaolin Silica Sand −1 k (h ) 0.075Kaolin 0.092Silica Sand 0.12 % of conversion after 5 h 35 46 38 k (h−1) 0.075 0.092 0.12 % of conversion after 5 h 35 46 38 In fact, the high specific area is in favor of the regular distribution and dispersion of carbaryl as a monolayerIn fact, the on high the specific solid support area is in particle. favor of This the couldregular promote distribution the incident and dispersion light penetration of carbaryl by as reducinga monolayer the screen on the effect solid of support carbaryl andparticle. thus This lead tocould an increase promote of the the photodegradationincident light penetration rate. Such by anreducing effect of the the screen specific effect surface of carbaryl area has and been thus proved lead to toan beincrease an important of the photodegradation factor in the evolution rate. Such of thean degradationeffect of the specific process surface [34]. Moreover, area has regardingbeen proved the to high be an sand important particle factor size, one in the can evolution presume of that the lightdegradation scattering process could [34]. be favored Moreover, owing regarding to the interparticle the high sand spaces. particle Both size, phenomena one can presume imply a that deeper light lightscattering penetration, could be inducing favored the owing increase to the of interpar carbarylticle photodegradation spaces. Both phenomena as previously imply described a deeper with light differentpenetration, silica [inducing35–38]. The the low increase initial rateof constantcarbaryl withphotodegradation kaolin can be attributed as previously to low specificdescribed surface with areadifferent and small silica particle[35–38]. size. The Therefore,low initial therate carbaryl constant screen with kaolin effect iscan enhanced be attributed by the to hypothetical low specific multilayersurface area structure, and small while smallparticle interparticle size. Therefore, spaces andthe restrictedcarbaryl lightscreen scattering effect is could enhanced limit incident by the lighthypothetical penetration. multilayer structure, while small interparticle spaces and restricted light scattering could limit incident light penetration. 3.3. Photoinductive Properties of the Model Supports 3.3. ThePhotoinductive chemical composition Properties of ofthe the Model used Supports model supports showed that the amount of titanium and iron, probablyThe chemical as oxide composition forms, is of not the negligible, used model in particularsupports showed for kaolin that and the sand. amount The of involvement titanium and ofiron, such probably species as in oxide carbaryl forms, degradation is not negligible, through in photocatalytic particular for kaolin reactions and is sand. highly The conceivable involvement for of prolongedsuch species irradiation in carbaryl times. degradation In fact, light through absorption photocatalytic by these photocatalysts reactions is highly is known conceivable to produce for • highlyprolonged reactive irradiation times. species In (ROS) fact, light such absorpti as hydroxylon by radicals these photocatalysts (OH ). These is species known areto produce able to reacthighly with reactive many oxygen organic species compounds (ROS) with such high as hydroxyl rate constants radicals leading (OH•). to These their transformationspecies are able [to37 ,react38]. Aswith a consequence, many organic the compounds study of the with photoinduced high rate constants properties leading of the to solid their supports transformation was undertaken [37,38]. As to a highlightconsequence, the formation the study of of hydroxyl the photoinduced radicals. Experiments properties wereof the carried solid outsupports with solidwas suspensionundertaken in to aqueoushighlight solution the formation of coumarin of hydroxyl in aerated radicals. medium Experi andments under were irradiation carried out at 365 with nm. solid This suspension irradiation in aqueous solution of coumarin in aerated medium and under irradiation at 365 nm. This irradiation

SoilSoil Syst. Syst.2019 2019, 3,, 3 17, x FOR PEER REVIEW 88 of of 14 14

wavelength was selected to allow selective excitation of the solid support. OH• formation was wavelengthfollowed by was the selected selective to allow fluorescence selective excitation measurement of the solidat 453 support. nm of OH 7-hydroxycoumarin• formation was followed (7-HC) byproduced the selective from fluorescence the reaction measurement of hydroxyl radicals at 453 nm with of 7-hydroxycoumarin coumarin [39–41]. (7-HC) produced from the reactionUnder of hydroxyl our experimental radicals with conditions, coumarin it [is39 worth–41]. noting that the fluorescence increase is observed withUnder all supports, our experimental confirming conditions, the generation it is worthof hydroxyl noting radicals that the (Figure fluorescence 4). No increaserelationship is observed between withthe allpercentage supports, of confirming titanium and the iron generation species ofand hydroxyl hydroxyl radicals radical (Figure production4). No rate relationship can be observed. between In thefact, percentage 7-HC formation of titanium is lower and under iron species irradiation and hydroxyl with kaolin radical despite production its highest rate content can be in observed. titanium Inand fact, iron, 7-HC while formation 7-HC quantities is lower under produced irradiation with silica with and kaolin sand despite after 6 itsh are highest similar, content probably in titanium because andof minor iron, while disparities 7-HC in quantities chemical produced composition. with More silicaover, and sandno significant after 6 h arevariation similar, appears probably between because the ofinitial minor appearance disparities rates in chemical of 7-HCcomposition. on different supports. Moreover, The no results significant suggest variation that titanium appears and/or between iron thespecies initial present appearance in the rates used of 7-HCsolid supports on different have supports. very low The photoactivity, results suggest and that we titanium assume and/orthat the ironphotodegradation species present can in the be usedattributed solid supportsto other species. have very In fact, low as photoactivity, previously described and we assume in the literature that the photodegradationby Katagi for clays, can the be photoreactivity attributed to other can be species. due to In the fact, solid as support previously by describedan electron in transfer the literature process bywith Katagi molecular for clays, oxygen the photoreactivity [42]. The electron can betransfer due to re theaction solid with support oxygen by ancan electron lead to transferthe formation process of • witha superoxide molecular oxygenanion [(O42].2 − The) which electron can transfer be transformed reaction with in oxygenthe presence can lead of to theresidual formation water of ain •− • superoxidehydroperoxide anion radical (O2 ) (HO which2 ) canand be afterward transformed in in the presence peroxide of residual (H2O2) and water hydroxyl in hydroperoxide radical. In • radicaladdition, (HO this2 ) andset of afterward experiments in hydrogen corroborates peroxide the (Hkinetics2O2) and of carbaryl hydroxyl degradation radical. In addition,with comparable this set ofefficiency experiments for silica corroborates and sand the surfac kineticses, but of carbarylhigher than degradation for kaolin. with This comparable leads us to efficiency the conclusion for silica that andOH sand• radicals surfaces, are probably but higher involved than for inkaolin. the carbaryl This leadsphotodegradation us to the conclusion processes. that OH• radicals are probably involved in the carbaryl photodegradation processes.

300 Kaolin 250 Silica Sand

200

150

100

50 Fluorescence intensity at 453 nm, a.u. 0 0123456 Irradiation time, hours

Figure 4. Evolution of the fluorescence at 453 nm during the irradiation of different model supports: Figure 4. Evolution of the fluorescence at 453− nm1 during the irradiation of different model−4 supports:−1 () kaolin, (N) silica and (•) sand, (c = 1.0 g L ) in the presence of coumarin (9.4 × 10 mol L ),

λexc(kaolin,= 365 nm. (▲)) silica and (•) sand, (c = 1.0 g L−1) in the presence of coumarin (9.4 × 10−4 mol L−1), λexc = 3.4. Photoproducts365 nm. and Mechanisms

3.4.The Photoproducts irradiation and of Mechanisms the carbaryl at the surface of the three solid surfaces leads to the formation of several and similar by-products that were easily identified by liquid chromatography The irradiation of the pesticide carbaryl at the surface of the three solid surfaces leads to the and using diode array detector. As shown in Figure5, the Suntest irradiation at the surface of formation of several and similar by-products that were easily identified by liquid chromatography silica powder for 3 h, permitting a conversion of roughly 25%, gives rise to the formation of mainly and using diode array detector. As shown in Figure 5, the Suntest irradiation at the surface of silica two products, P1 and P2, with the retention times of 13.4 and 14.2 min, respectively. Since both of powder for 3 h, permitting a conversion of roughly 25%, gives rise to the formation of mainly two them are commercial compounds, they have been formally identified and quantified by injecting products, P1 and P2, with the retention times of 13.4 and 14.2 min, respectively. Since both of them standards. P1 corresponds to 2-hydroxy-1,4-naphthoquinone while P2 corresponds to 1-naphthol. are commercial compounds, they have been formally identified and quantified by injecting The latter compound was shown to be present as an impurity and decreased with irradiation time while standards. P1 corresponds to 2-hydroxy-1,4-naphthoquinone while P2 corresponds to 1-naphthol. 2-hydroxy-1,4-naphthoquinone was obtained from the early stages of irradiation and disappeared in The latter compound was shown to be present as an impurity and decreased with irradiation time its turn after 3 h irradiation. Under our experimental conditions, P2 may be considered as the major while 2-hydroxy-1,4-naphthoquinone was obtained from the early stages of irradiation and byproduct since it represents roughly 80% of carbaryl conversion. disappeared in its turn after 3 h irradiation. Under our experimental conditions, P2 may be considered as the major byproduct since it represents roughly 80% of carbaryl conversion.

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O O 0.035 O CH 0.035 carbaryl O 3 0.035 O NHCH3 carbaryl O O NH CH3 -5 0.035 carbaryl O NH CH 0.035 carbaryl O 3 6.0x10-5 O NH CH3 6.0x10 -5 0.035 carbaryl O 6.0x10-5 0.030 carbaryl O NH CH3 0.0350.030 O NH 6.0x10-5 0.030 CH3 6.0x10-5 0.030 carbaryl O NH 6.0x10 0.030 -5

0.0250.030 1-naphtyl-N-methyl carbamate -1 6.0x10

1-naphtyl-N-methyl carbamate -1 0.0300.0250.025 1-naphtyl-N-methyl carbamate -1 0.025 1-naphtyl-N-methyl carbamate -1 0.025 1-naphtyl-N-methyl carbamate -1 -5

1-naphtyl-N-methyl carbamate -1 0.025 P1 OH 4.0x10-5 0.020 P1 4.0x10 -5 0.0250.0200.020 P1 1-naphtyl-N-methyl carbamateOHOH -1 4.0x10-5 O P1 OH 4.0x10-5 1-naphtol

0.020 O P1 OH 4.0x10-5 1-naphtol1-naphtol 0.020 O P1 4.0x10 OH OH 2-hydroxy1-naphtol 1,4-naphtoquinone 0.0150.020 O -5 OH 2-hydroxy 1,4-naphtoquinone 0.015 O OH P1 4.0x10 2-hydroxy 1-naphtol 1,4-naphtoquinone 0.020 OH 1-naphtol 0.015 O OH 2-hydroxy 1,4-naphtoquinone

0.015 OH 1-naphtol2-hydroxy 1,4-naphtoquinone 0.015 O OH 1-naphtol 2-hydroxy 1,4-naphtoquinone

0.0100.015 1-naphtol1-naphtol -5 Absorbance (a.u) Absorbance O 0.010 OH 1-naphtol 2.0x10-5 2-hydroxy 1,4-naphtoquinone Absorbance (a.u) Absorbance 0.0150.010 O 2.0x10 -5 Absorbance (a.u) Absorbance O P2 1-naphtol 0.010 2-hydroxy-1,4 naphtoquinone L mol Concentration, 2.0x10-5 Absorbance (a.u) Absorbance O P2 1-naphtol 2.0x10 0.010 2-hydroxy-1,4 naphtoquinone P2 L mol Concentration, -5 Absorbance (a.u) Absorbance O L mol Concentration, 0.010 2-hydroxy-1,4 naphtoquinone P2 2.0x10-5 1-naphtol L mol Concentration, Absorbance (a.u) Absorbance 0.005 2-hydroxy-1,4 Onaphtoquinone P2 2.0x10 0.0100.005 2-hydroxy-1,4 naphtoquinone L mol Concentration, -5 Absorbance (a.u) Absorbance 0.005 O P2 Concentration, mol L mol Concentration, 2.0x10 0.005 2-hydroxy-1,4 naphtoquinone P2

0.005 L mol Concentration, 0.0000.005 2-hydroxy-1,4 naphtoquinone 0.0050.0000.000 0.0 0.000 0.00.0 0.000 0 2 4 6 8 10121416182022 0.0 0246 0.000 0 2 4 6 8 10121416182022 0.0 02460246 0.000 0 2 4 6 8 10121416182022 0.0 0246Irradiation time, minutes 0 2 4 6Retention 8 10121416182022 time (min) 0.0 0246Irradiation time, minutes 0 2 4 6Retention 8 10121416182022 time (min) 0246Irradiation time, minutes Retention time (min) 0246Irradiation time, minutes 0 2 4 6Retention 8 10121416182022 time (min) Irradiation time, minutes Retention time (min) Retention time (min) Irradiation time, minutes −1 Figure 5. HPLC chromatogram of a silica powder spiked with carbaryl (0.5 mg g−1 ) irradiated 3 h in Figure 5. HPLC chromatogram of a silica powder spiked with carbaryl (0.5 mg g −1)) irradiatedirradiated 33 hh inin Figure 5. HPLC chromatogram of a silica powder spiked with carbaryl (0.5 mg−1 g−1) irradiated 3 h in FigureFigure 5. HPLC 5.λ HPLCdetection chromatogram chromatogram of aof silica a silica powder powder spiked spiked with with carbaryl carbaryl (0.5 (0.5 mg mg g )g irradiated−1) irradiated 3 h 3 in h in Suntest,Suntest,Figure 5. λ detectionHPLC = =chromatogram 280280 nm.nm. of a silica powder spiked with carbaryl (0.5 mg g−1 ) irradiated 3 h in FigureSuntest, 5. λHPLCdetection chromatogram= 280 nm. of a silica powder spiked with carbaryl (0.5 mg g ) irradiated 3 h in Suntest, λdetection = 280 nm. Suntest,Suntest,λdetection λdetection= = 280 280 nm. nm. Suntest, λdetection = 280 nm. InIn additionaddition toto P1P1 andand P2,P2, otherother photoproductsphotoproducts lalabelledbelled asas P3,P3, P4P4 andand P5P5 werewere detecteddetected usingusing thethe InIn addition addition to to P1 P1 and and P2, P2, other other+ photoproducts photoproducts labelled labelled asas P3,P3, P4 and P5 P5 were were detected detected using using the sensitiveIn addition technique to P1 HPLC/ESI and P2, other+ /MS. photoproducts They all show la retentionbelled as P3,times P4 andlower P5 thanwere thatdetected of the using parent the sensitive technique HPLC/ESI+/MS. They all show retention times lower than that of the parent sensitiveIn addition technique to P1 HPLC/ESI and P2, other+/MS.+ photoproducts They all show la belledretention as P3, times P4 and lower P5 werethan detectedthat of theusing parent the thesensitive sensitive technique technique HPLC/ESI ret +/MS./MS. They They all all show show retention timestimes lowerlower thanthan that that of of the the parent parent compoundcompoundsensitive technique carbarylcarbaryl (t(tHPLC/ESIret == 12.712.7 +min),min),/MS. indicatingindicating They all thatshowthat theythey retention areare moremore times polar.polar. lower TheThe proposedthanproposed that structuresstructuresof the parent areare sensitivecompound technique carbaryl HPLC/ESI (tret = 12.7 min),/MS. indicatingThey all show that theyretention are more times polar. lower The than proposed that of structures the parent are compoundcompound carbaryl carbaryl (t (t=ret 12.7= 12.7 min), min), indicating indicating that that they they are are more more polar. polar. The The proposed proposed structures structures are are gatheredcompound in carbarylTable 4.ret (tret = 12.7 min), indicating that they are more polar. The proposed structures are compoundgathered in carbaryl Table 4. (t ret = 12.7 min), indicating that they are more polar. The proposed structures are gatheredgathered in Tablein Table4. 4. gatheredgathered in in Table Table 4. 4. Table 4. Proposed structure for photodegradation products of carbaryl on different support, Table 4. Proposed structure for photodegradation products of carbaryl on different support, TableTable 4. Proposed 4. Proposed structure structure−1 for photodegradation for photodegradation products pr ofoducts carbaryl of on carbaryl different on support, different [carbaryl] support, [carbaryl]Table 4. Proposed= 0.5 mg gstructure−1 , 1 h irradiation for photodegradation in Suntest, with pr oductsan electrospray of carbaryl ionization on different source support,(ESI) in [carbaryl][carbaryl]−1 == 0.50.5 mgmg gg−1,, 11 hh irradiationirradiation inin Suntest,Suntest, withwith anan electrospray ionization source (ESI) in = 0.5Table[carbaryl] mg g4. Proposed ,= 1 h0.5 irradiation mg structureg−1, 1 inh Suntest,irradiation for photodegradation with in an Suntest, electrospray with products ionizationan electrospray of carbaryl source ionization (ESI) on indifferent positive source support, mode.(ESI) in positive[carbaryl] mode. = 0.5 mgThe gelemental−1, 1 h irradiation composition in Suntest, was obtainedwith an electrosprayusing MassLynx ionization software. source The (ESI) mass in [carbaryl]positive mode.= 0.5 mgThe g −1elemental, 1 h irradiation composition in Suntest, was withobtained an electrospray using MassLynx ionization software. source The (ESI) mass3 in Thepositive elemental mode. composition The elemental3 was obtained composition using MassLynx was obtained software. using The massMassLynx resolution software. was set The to 10 mass. resolutionpositive mode. was set The to 10elemental3 . composition was obtained using MassLynx software. The mass resolution was set to 103. positiveresolution mode. was setThe to elemental103. composition was obtained using MassLynx software. The mass resolution was set to 103. resolution was set to 103 . Accurate Mass Elemental Mass Error Proposed resolutionProduct was tset minto 10 . Accurate Mass Elemental Mass Error Product rettret min Accurate+ Mass Elemental Mass Error Proposed Structure Product tretret min [MAccurate + H] (+m Mass/z) CompositionElemental Massppm Error ProposedStructure Structure Product tret min Accurate[M+H]++ (m/z) Mass CompositionElemental Massppm Error Proposed Structure Product tret min [M+H][M+H]+ (m/z)(m/z) Composition ppm Proposed Structure Product tret min Accurate[M+H]+ Mass(m/z) CompositionElemental Massppm Error Proposed OStructure ret [M+H]+ (m/z) Composition ppm O Product t min [M+H]+ (m/z) Composition ppm Proposed StructureO [M+H] (m/z) Composition ppm O O NH CH O O NH CH 3 O NH CH33 + + OO NH CH3 CarbarylCarbaryl 12.712.7 202.0875 202.0875 C12 CH1212HO12O2N2N++ +3.4+3.4 O NH CH3 Carbaryl 12.7 202.0875 C1212H1212O22N + +3.4+3.4 O NH CH3 Carbaryl 12.7 202.0875 C12H12O2N+ +3.4 O NH CH Carbaryl 12.7 202.0875 C12H12O2N+ +3.4 3 Carbaryl 12.7 202.0875 C12H12O2N+ +3.4 Carbaryl 12.7 202.0875 C12H12O2N +3.4 O O O O OH OH O OH 10 7 +3+ O OH P1P1P1 13.413.413.4 175.0379 175.0379 175.0379 C10 C CH10H7HO7OO3 3++ −−−9.39.3 OH P1 13.4 175.0379 C10H7O3+ −9.3 OH P1 13.4 175.0379 C10H7O3+ −9.3 P1 13.4 175.0379 C10H7O3+ −9.3 O + O P1 13.4 175.0379 C10H7O3 −9.3 O O OH O OH O OH OH + OH P2 14.2 145.0663 C10H9O+ + +6.6 OH P2P2 14.214.2 145.0663 145.0663 C CH1010HO99O+ +6.6 P2 14.2 145.0663 10 C10H9 9O+ +6.6 P2 14.2 145.0663 C10H9O+ +6.6 P2 14.2 145.0663 C10H9O+ +6.6 P2 14.2 145.0663 C10H9O +6.6 O O O O O O OH O OH + OO OH P3 11.0 189.0543 C11H9O3++ −4.6 O OH P3 11.0 189.0543 C1111H99O+33+ −4.6 O OH P3P3 11.011.0 189.0543 189.0543 C CH11HO9O3+ −−4.64.6 P3 11.0 189.0543 11 C119H9O3 3+ −4.6 O OH P3 11.0 189.0543 C11H9O+3 −4.6 P3 11.0 189.0543 C11H9O3 −4.6 OH O OH O OH O OH O + OH O NH CH P4 9.6 202.0862 C12H12O2N+ −3.0 NH CH 3 P4 9.6 202.0862 C12H12O2N+ −3.0 OH O NH CH33 P4 9.6 202.0862 C12H12O2N++ −3.0 NH CH3 P4P4 9.69.6 202.0862 202.0862 C CH12HO12ON2N+ −−3.03.0 NH CH3 P4 9.6 202.0862 12C1212H12O2 2N+ −3.0 NH CH3 + NH CH3 P4 9.6 202.0862 C12H12O2N −3.0 O O O O O NH CH O O NH CH 3 O NH CH33 + OO NH CH3 P5 9.4 218.0810 C12H12O3N++ −3.3 O NH CH3 P5 9.4 218.0810 C1212H1212O33N + −3.3 O NH CH3 P5 9.4 218.0810 C12H12O3N+ −3.3 O NHOH CH P5 9.4 218.0810 C12H12O3N+ + −3.3 OH 3 P5P5 9.49.4 218.0810 218.0810 C12CH1212HO12O3N3N −−3.3 OH P5 9.4 218.0810 C12H12O3N+ −3.3 OH OH OH

ProductProduct P3P3 withwith aa retentionretention timetime ofof 1111 minmin exhibiexhibitsts aa protonatedprotonated molecularmolecular ionion m/zm/z == 189189 Product+ P3 with a retention time of 11 min exhibits a protonated molecular ion m/z = 189 ([M+H]Product+ ). The P3 odd with mass a suggestsretention thetime loss of of 11 the min nitrog exhibien tsatom a protonated and guides molecular us to the lossion ofm/z the = NH- 189 ([M+H]Product+). TheP3 oddwith mass a retention suggests time the ofloss 11 of min the exhibitsnitrogen aatom protonated and guides molecular us to the ion lossm /ofz the= 189 NH- ([M+H]Product+). The P3 odd with mass a retention suggests thetime loss of of11 themin nitrog exhibients atom a protonated and guides molecular us to the ionloss m/zof the = NH-189 ([M+H]3 +). The odd mass suggests the loss of the nitrogen atom and guides us to the loss of the NH- CHCH([M+H]3 group.group.++ ). The ThisThis odd couldcould mass bebe suggests substitutedsubstituted the by byloss aa hydroxyl.hydroxyl.of the nitrog ThisThisen waswas atom coco nfirmedandnfirmed guides byby theusthe to exactexact the massmassloss of beingbeing the NH- m/zm/z ([M([M+H]CH + H]3 group.). The The This odd odd could mass mass be suggests suggests substituted the the lossby loss a of hydroxyl. of the the nitrog nitrogen Thisen atomwas atom co andnfirmed and guides guides by us the usto exact the to the loss mass loss of being the of theNH- m/z CH3 group. This could be substituted by a hydroxyl.+ This was confirmed by the exact mass being m/z CH= 189.0543,3 group. Thiswhich could corresponds be substituted to Cby11 Ha hydroxyl.9O3+ as elemental This was cocomposition.nfirmed by theProduct exact massP4 presents being m/z a NH-CH= 189.0543,3 group. which This couldcorresponds be substituted to C1111 byH99 aO hydroxyl.33+ as elemental This was composition. confirmed by Product the exact P4 mass presents being a CH= 189.0543, group.3 This which could corresponds be substituted to byC11 aH hydroxyl.9O3+ as elemental This was cocomposition.nfirmed by the Product exact mass P4 presentsbeing m/z a protonated= 189.0543, molecularwhich corresponds ion m/z = 202 to similarC11H9O to3++ thatas elementalof the parent composition. substrate but Product a retention P4 presentstime of 9.6 a m/=protonated z189.0543,= 189.0543, molecularwhich which corresponds corresponds ion m/z = 202to to similarC C1111HH9O9O 3to+3 thatasas elemental elementalof the parent composition. substrate but Product a retention P4P4 presentspresents time of a 9.6a protonated molecular ion m/z = 202 similar to that of the parent substrate but a retention+ time of 9.6 min,protonated indicating molecular the formation ion m/z of= 202 an isomersimilar ofto carabarylthat of the (m/z parent = 202.0862; substrate C but12H a12 Oretention2N+ ). Product time ofP5 9.6 is protonatedmin, indicating molecular the ionformationm/z = 202of an similar isomer to of that carabaryl of the parent (m/z = substrate 202.0862; but C1212 aH retention1212O22N+). Product time of 9.6P5 is protonatedmin, indicating molecular the formation ion m/z = of 202 an similar isomer to of that carabaryl of the parent(m/z = substrate202.0862; butC12H a12 retentionO2N+). Product time of P5 9.6 is min, indicating the formation of an isomer of carabaryl (m/z = 202.0862; C12H12O2N++). Product P5 is min,min, indicating indicating the the formation formation of of an an isomer isomer of of carabaryl carabaryl (m (m/z/z = = 202.0862; 202.0862; C C1212HH1212OO22N+).). Product Product P5 P5 is

Soil Syst. 2019, 3, 17 10 of 14 SoilSoil Syst.Syst. 20192019,, 33,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1414 ischaracterizedcharacterized characterized byby by aa protonatedprotonated a protonated molecularmolecular molecular ionion ion withwith with m/zm/zm / ==z 218218= 218 thatthat that correspondscorresponds corresponds toto the tothe the additionaddition addition ofof 16 of16 16units.units. units. ThisThis This moremore more likelylikely likely relatedrelated related toto thethe to the hydroxylatiohydroxylatio hydroxylationnn ofof thethe of the naphthalenenaphthalene structure,structure, structure, asas indicatedindicated as indicated inin thethe in theproposedproposed proposed structurestructure structure shownshown shown inin inTableTable Table 4.4.4 .TheThe The exacexac exacttt massmass waswaswas mm/zm/z/z ==218.0810,=218.0810, 218.0810, whichwhich corresponds corresponds toto C12H12O3N++ +as elemental composition. CC1212H12OO33NN asas elemental elemental composition. composition. AsAs clearlyclearlyclearly shownshownshown experimentally,experimentally, 1-naphtol1-naphtol1-naphtol waswas detecteddedetectedtected inin thethethe startingstartingstarting aqueousaqueousaqueous solutionsolutionsolution inin smallsmall quantitiesquantitiesquantities prior priorprior to toto irradiation. irradiation.irradiation. It ItIt is isis probably probablyprobably the thethe result resultresult of ofof the thethe well-known well-knownwell-known carbaryl carbarylcarbaryl hydrolysis hydrolysishydrolysis in theinin thethe dark darkdark and andand at at roomat roomroom temperature temperaturetemperature [40 [40–44].[40–44].–44]. During DuringDuring the thethe irradiation irradiationirradiation process processprocess of ofof carbaryl carbarylcarbaryl at atat the thethe surfacesurfacesurface ofof thethethe solidsolidsolid supports,supports,supports, thethethe amountamountamount ofofof 1-naphtol1-naphtol1-naphtol continuouslycontinuouslycontinuously decreased,decreased, likelylikely duedue toto anan efficientefficientefficient photochemicalphotochemicalphotochemical [ [45].45[45].]. Such SuSuchch reaction reactionreaction permits, permits,permits, via viavia a aa multistep multistepmultistep photo-oxidation phphoto-oxidationoto-oxidation processes, processes,processes, the thethe formation formationformation of theofof thethe secondary secondarysecondary product productproduct 2-hydroxy-1,4-naphthoquinone 2-hydrox2-hydroxy-1,4-naphthoquinoney-1,4-naphthoquinone which whichwhich accumulates accumulatesaccumulates well wellwell in inin the thethe medium mediummedium [[42][42]42] (Scheme(Scheme(Scheme1 1)1)).

HH O HH O OO

OO CHCH33 OHOH OO OH hνν OH hhνν h

OO22

1-naphtol1-naphtol OO 2-hydroxy-1,4-naphtoquinone 2-hydroxy-1,4-naphtoquinone SchemeScheme 1.1. FormationFormation ofof 1-naphtol1-naphtol andand 2-hydroxy-1,4-naph2-hydroxy-1,4-naphtoquinone2-hydroxy-1,4-naphtoquinonetoquinone byby excitationexcitation ofof carabarylcarabaryl atat thethe surfacesurfacesurface ofofof kaolin.kaolin.kaolin.

TheThe formationformationformation ofof productproduct P3P3 maymaymay alsoalsoalso bebebe obtainedobtainedobtained bybyby thethethe hydrolysishydrolysishydrolysis ofofof thethethe NC(=O)NC(=O)NC(=O) bondbond ofof carbarylcarbaryl whilewhilewhile product productproduct P4 P4P4 more moremore likely likelylikely are areare from fromfrom a a photo-Friesa photo-Friesphoto-Fries process processprocess involving involvinginvolving homolytic homolytichomolytic scissions scissionsscissions of C–Hofof C–HC–H and andand O–C(=O) O–C(=O)O–C(=O) bonds. bonds.bonds. Product ProductProduct P5 P5P5 generation generationgeneration may maymay involve involveinvolve hydroxyl hydroxylhydroxyl radicals radicalsradicals that thatthat were werewere shown shownshown to betoto bebe formed formedformed by byby excitation excitationexcitation of ofof the thethe clays claysclays [30 [30].[30].]. It ItIt may maymay also alsoalso be bebe formed formedformed by byby an anan electron elecelectrontron photoejection photoejectionphotoejection processprocessprocess viaviavia thethethe directdirectdirect excitationexcitationexcitation ofofof carbarylcarbarylcarbaryl andandand thethethe oxygenoxygenoxygen oxidationoxidationoxidation ofofof thethethe generatedgeneratedgenerated radicalradicalradical cationcationcation onon thethethe aromaticaromatic ringringring (Scheme(Scheme(Scheme2 2).).2).

OHOH

OO OO OO CHCH3 CH 3 CH33 OO OO NHNH OO OHOH CH OO NHNH CH33 OO NHNH ν hhνν hhν H O O e-- H22O O22 ++ ++ e P3P3 OHOH PhotoPhoto FriesFries P5 P5 OO CH CH33 OHOH O O O NH CH O NH CH33 NHNH HH

P4P4

SchemeScheme 2.2. MechanisticMechanistic schemescheme forfor thethe formationformation ofof productsproducts P3,P3,P3, P4P4P4 andandand P5.P5.P5. 3.5. Toxicity Assessment 3.5.3.5. ToxicityToxicity AssessmentAssessment In this part, we investigate the toxicity evolution of carbaryl on a solid support surface under InIn thisthis part,part, wewe investigateinvestigate thethe toxicitytoxicity evolutionevolution ofof carbarylcarbaryl onon aa solidsolid supportsupport surfacesurface underunder solar light exposure by TLS-FIA experiments. Analyses were realized on spiked kaolin layer samples solarsolar lightlight exposureexposure byby TLS-FIATLS-FIA exexperiments.periments. AnalysesAnalyses werewere realizedrealized onon spikedspiked kaolinkaolin layerlayer samplessamples

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(Z = 100 µm). Carbaryl disappearance kinetics as well as the evolution of AchE activity are shown in (Z = 100 µm). Carbaryl disappearance kinetics as well as the evolution of AchE activity are shown in Figure 6. Figure6.

1.0 100

carbaryl 0.8 % inhibition AchE 80 %Inhibition AchE

0 0.6 60

C/C

0.4 40

0.2 20

0 102030405060 Irradiation time (h)

Figure 6. Evolution of AchE inhibition (•) and carbaryl kinetics ( ) as a function of irradiation time, Figure 6. Evolution of AchE inhibition () and carbaryl kinetics () as a function of irradiation time, kaolin layer samples (Z = 100 µm), [carbaryl] = 20 µmol g−1, Suntest. kaolin layer samples (Z = 100 µm), [carbaryl] = 20 µmol g−1, Suntest. As clearly shown, carbaryl itself presents a relatively high toxicity since a total inhibition of AchE As clearly shown, carbaryl itself presents a relatively high toxicity since a total inhibition of AchE activity was obtained at the initial time [46–48]. Moreover, upon sunlight irradiation, the enzyme activity was obtained at the initial time [46–48]. Moreover, upon sunlight irradiation, the enzyme inhibition drastically decreases, more particularly within the first stages of irradiation. After 5 h inhibition drastically decreases, more particularly within the first stages of irradiation. After 5 h irradiation, corresponding to about 20% conversion, AchE activity reaches the lowest value (roughly irradiation, corresponding to about 20% conversion, AchE activity reaches the lowest value (roughly 20%) and no evolution is observed for a longer time even if the carbaryl concentration slowly decreases. 20%) and no evolution is observed for a longer time even if the carbaryl concentration slowly Such results clearly show that carbaryl byproducts generated on a solid surface are less toxic than the decreases. Such results clearly show that carbaryl byproducts generated on a solid surface are less initial product. toxic than the initial product. 4. Conclusions 4. Conclusions Throughout this study, we clearly demonstrated that carbaryl photodegradation at the surface of theThroughout three solid supportsthis study, of we silica, clearly kaolin demonstrated and sand is that effective. carbaryl The photodegradation degradation rate onat the the surface kaolin surfaceof the three revealed solid itself supports to be moreof silica, efficient kaolin than and in sand water, is probably effective. because The degradation of the major rate modifications on the kaolin of carbarylsurface revealed absorption itself properties to be more on efficient this surface than (bathochromic in water, probably and hyperchromic because of the effects). major modifications Comparable initialof carbaryl rate constants absorption were properties obtained foron the this degradation surface (bathochromic kinetics at the surfaceand hyperchromic of the three supports. effects). FromComparable the physicochemical initial rate constants characteristics, were obtained it appears for the that degradation the main kinetics parameters at the controlling surface of carbarylthe three photodegradationsupports. From the could physicochemical be the particle characteristics, size diameter andit appears the specific that the surface main area. parameters Large particle controlling size andcarbaryl high specificphotodegradation area promote could light be diffusion, the particle and size the screendiameter effect and decreases the specific respectively, surface area. allowing Large a moreparticle efficient size photodegradation.and high specific The area formation promote of hydroxyllight diffusion, radicals and with the the threescreen solid effect supports decreases was evidencedrespectively, in particularallowing fora more silica efficient and sand photodegra with higherdation. amounts The offormation radicals. Regardingof hydroxyl the radicals byproducts with formed,the three various solid supports mechanistic was pathways evidenced could in particul explainar the for degradation silica and sand of carbaryl. with higher Some ofamounts them are of comparableradicals. Regarding to that observed the byproducts in water, suchformed, as photohydrolysis, various mechanistic electron pathways photoejection, could hydroxylation explain the anddegradation the scission of ofcarbaryl. the naphthalene Some of ring. them Moreover, are comparable we clearly to give that evidence observed for thein fastwater, decrease such ofas carbarylphotohydrolysis, toxicity upon electron irradiation photoejection, on the three hydroxyl solid surfacesation and from the thescission early stagesof the ofnaphthalene the reaction. ring. Moreover, we clearly give evidence for the fast decrease of carbaryl toxicity upon irradiation on the Authorthree solid Contributions: surfaces fromM.S. (Mariethe early Siampiringue) stages of the and reaction. R.C. have been involved in the methodology; investigation and data curation; P.W.-W.-C. and M.S. (Mohamed Sarakha) have been involved in supervision, project administration, original draft; writing reviewing and editing. Author Contributions: M.S. (Marie Siampiringue) and R.C. have been involved in the methodology; Funding:investigationThis and research data receivedcuration; no P.W.W.C. external funding.and M.S. (Mohamed Sarakha) have been involved in supervision, Acknowledgments:project administration,The original authors draf wouldt; writing like to reviewing thank Martin and LEREMBOUREediting. for his help in the experiments in LC/MS. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: The authors would like to thank Martin LEREMBOURE for his help in the experiments in LC/MS.

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