Photocatalytic Efficiency of Suspended and Immobilized TiO2 P25 for Removing Myclobutanil, Penconazole and Their Commercial Formulations

M'Bra, Ignace Christian; Keller, Nicolas; Trokourey, Albert; Robert, Didier

doi:10.35534/prp.2023.10004

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Photocatalytic Efficiency of Suspended and Immobilized TiO₂ P25 for Removing Myclobutanil, Penconazole and Their Commercial Formulations

Photocatalysis: Research and Potential. 2024, 1(1), 10004; https://doi.org/10.35534/prp.2023.10004

Ignace Christian M'Bra ^1,2 Nicolas Keller ³ Albert Trokourey ⁴ Didier Robert ¹ ^*

¹

Institut de Chimie et Procédés Pour l’Energie, l’Environnement et la Santé (ICPEES), CNRS-UMR7515 Université de Strasbourg, Antenne de Saint-Avold, Université de Lorraine, 12 rue Victor Demange, 57500 Saint-Avold, France

²

Laboratoire de Thermodynamique et de Physico-chimie du milieu (LTPCM), UFR Sciences des Fondamentales et Appliquées (SFA), Université Nangui Abrogoua d’Abidjan, 02 BP 801 Abidjan 02, Côte d’Ivoire

³

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), CNRS-UMR7515-University Strasbourg, 25 Rue Becquerel, 67087 Strasbourg, France

⁴

Laboratoire de Chimie Physique, UFR Sciences des Structures de la Matière et de la Technologie (SSMT), Université Félix Houphouët Boigny d’Abidjan, 22 BP 582 Abidjan 22, Côte d’Ivoire

^*

Authors to whom correspondence should be addressed.

Received: 10 Mar 2023 Accepted: 29 Jun 2023 Published: 05 Jul 2023

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Abstract

Fungicide application in viticulture is a major source of surface and groundwater contamination. It is therefore essential to find solutions to stop this environmental pollution. Heterogeneous photocatalysis is an advanced oxidation method for the degradation and mineralization of organic pollutants in water. TiO₂ P25 photocatalyst in suspension has been used for removing the fungicides Myclobutanil and Penconazol, and their respective commercial formulations Systhane and Topas, in contaminated water. The apparent kinetic constants k_app of fungicides removal over 30 min batch treatment was higher for a mixture of pure molecules of Myclobutanil and Penconazol than for a mixture of their commercial formulations (17.5 × 10⁻³ by comparison with 10.3 × 10⁻³ min⁻¹ for Myclobutanil, and 10.0 × 10⁻³ by comparison with 2.80 × 10⁻³ min⁻¹ for Penconazol). TOC removal constants k_TOC were similar for the two mixtures, due to the presence of mineral and organic additives in the commercial formulations. To easily recover the photocatalyst after fungicide removal, TiO₂ P25 has been supported on β-SiC foam. Fungicides degradation was lower with supported photocatalysts than with the suspension of photocatalyst nanoparticles (NPs) because of a lower concentration of active sites on the supported photocatalyst than in the catalyst suspension. However, catalyst recovery and reuse after fungicide removal is obviously easier with TiO₂/β-SiC material than with a suspension of TiO₂ which requires long and expensive filtration operations.

Keywords

Fungicides; Photocatalysis; Myclobutanil; Penconazol; TiO₂/β-SiC

While the world is facing a drastic increase in water requirements, several water sources are contaminated with organic pollutants, and therefore unsuitable to consumption. Pollution generally arises from industrial, household and agricultural activities. In viticulture, the use of fungicides has tremendously increased in the recent years [<a href='#B1' class='html-bibr' title='1'>1</a>]. A study conducted under the aegis of the French Ministry for Ecological and Inclusive Transition reveals an alarming increase in sales of insecticides, fungicides, and herbicides for 10 years in France (for example, the sales of fungicides increased by 41%) [<a href='#B1' class='html-bibr' title='1'>1</a>]. Their use by several actors in the agricultural world is responsible for health and environmental problems. This is how in-situ, soils are found with high levels of these chemicals. Pesticides drainage into water tables and water currents strongly contributes to water pollution. Aquatic species and humans are exposed to these toxic pollutants.

It is therefore urgent to effectively treat water contaminated by fungicides, for making it suitable for consumption. The ideal and economical treatment would consist (after filtration, decantation of suspended particles and solid waste) in biological removal or even mineralization of fungicides. Unfortunately, this type of treatment has its limits because there are often recalcitrant organic products which cannot be eliminated this way [<a href='#B2' class='html-bibr' title='2'>2</a>]. Simple and inexpensive solutions are then strongly required for removing these types of pollutants. By keeping this in mind, the scientific community has effectively developed oxidation techniques called Advanced Oxidation Processes (AOP) [<a href='#B3' class='html-bibr' title='3'>3</a>] over the past twenty years. These technologies have already shown their effectiveness in the treatment of toxic and "biologically recalcitrant" organic pollutants. These processes are based on the in-situ formation of hydroxyl radicals HO° [<a href='#B4' class='html-bibr' title='4'>4</a>] which have an oxidizing power greater than that of traditional oxidants such as H<sub>2</sub>O<sub>2</sub> [<a href='#B5' class='html-bibr' title='5'>5</a>], Cl<sub>2</sub> [<a href='#B6' class='html-bibr' title='6'>6</a>], or O<sub>3</sub> [<a href='#B7' class='html-bibr' title='7'>7</a>]. These radicals are capable of partially (or totally) mineralizing most organic compounds. Among these AOPs, special attention has been paid to heterogeneous TiO<sub>2</sub>/UV photocatalysis [<a href='#B8' class='html-bibr' title='8'>8</a>,<a href='#B9' class='html-bibr' title='9'>9</a>], due to its ability to mineralize a wide range of recalcitrant organic pollutants at room temperature and at atmospheric pressure into harmless substances [<a href='#B10' class='html-bibr' title='10'>10</a>]. Several studies have been done using titanium dioxide in suspension [<a href='#B11' class='html-bibr' title='11'>11</a>,<a href='#B12' class='html-bibr' title='12'>12</a>]. However, the post-treatment recovery of TiO<sub>2</sub> is a difficult process to achieve due to the nanometer-size photocatalyst and the cost this can entail. Therefore, filtration and re-suspension of TiO<sub>2</sub> should be avoided in a wastewater treatment process. Therefore, the idea of immobilizing TiO<sub>2</sub> on a suitable and chemically inert support has started to emerge because it could avoid the expensive process of phase separation [<a href='#B13' class='html-bibr' title='13'>13</a>,<a href='#B14' class='html-bibr' title='14'>14</a>]. However, the surface of the photocatalyst is active only when illuminated by light. Thus, heterogeneous systems often suffer from mass transfer limitation due to the reduction in specific surface area of TiO<sub>2</sub> compared to homogeneous systems. However, it should be noted that catalysts immobilization on substrates remains currently a promising alternative in heterogeneous photocatalysis.

The use of β-form (β-SiC) silicon carbide cellular foams in 3D as a macroscopic support for TiO<sub>2</sub> P25, is the subject of our work. These foams have very interesting characteristics such as their good mechanical and thermal resistance, chemical inertness, excellent hydrodynamic properties, and above all a large specific surface area (20 m<sup>2</sup>·g<sup>−1</sup>). Previous studies have been carried out by removing recalcitrant organic compounds in the presence of the TiO<sub>2</sub>/β-SiC material [<a href='#B11' class='html-bibr' title='11'>11</a>,<a href='#B15' class='html-bibr' title='15'>15</a>,<a href='#B16' class='html-bibr' title='16'>16</a>,<a href='#B17' class='html-bibr' title='17'>17</a>]. The objective of the present work is to compare the photocatalytic activity of TiO<sub>2</sub> P25 in slurry (batch mode) with that of TiO<sub>2</sub> P25 supported onto β-SiC (recirculation mode), for the removal of a mixture of either pure molecules of Myclobutanil and Penconazol, or commercial formulations of these fungicides. To do this, we have built a photoreactor to remove and mineralize these organic compounds in water under irradiation of artificial UV-A lamps.

<em>2.1. Chemicals and Materials</em>

Penconazol (C<sub>13</sub>H<sub>15</sub>N<sub>3</sub>Cl<sub>2</sub>, 1-(2,4-dichloro-β-propylphenetyl)-1H-1,2,4-triazole according to IUPAC, 99.9%, Techlab), Topas commercial formulation (100 g·L<sup>−1</sup>, 10.2% Penconazol, Bayer-BASF), Myclobutanil (C<sub>15</sub>H<sub>15</sub>N<sub>4</sub>Cl, 99.9%, (RS)-2-p-chlorophenyl-2-(1H-1,2,4-ylmethyl) hexanenitrile from Techlab), SysthaneTM 20EW commercial formulation (100 g·L<sup>−1</sup> Myclobutanil from Bayer), were used as received. Aeroxide TiO<sub>2</sub> P25 nanoparticles were supplied by Evonik (size of primary nanoparticles ranged from 10 to 50 nm, largely distributed from 15 to 25 nm, and their BET surface area was 60.8 m<sup>2</sup>/g). The exact phase composition of TiO<sub>2</sub>-P25 has been determined to be anatase (77.1%), rutile (15.9%) and amorphous TiO<sub>2</sub> (7.0%) [<a href='#B18' class='html-bibr' title='18'>18</a>]. Ethanol anhydride (99.8%, FLUKA) and titanium tetraisopropoxide (TTIP, C<sub>12</sub>H<sub>28</sub>O<sub>4</sub>Ti, 97%, SIGMA ALDRICH) were used without puriﬁcation. Distilled water was used to prepare all the solutions.

<em>2.2. Photocatalytic Tests with TiO<sub>2</sub> P25 in Slurry</em>

Batch mode experiments were conducted at room temperature in a glass cylinder reactor with aqueous solutions (Penconazol/Topas, Myclobutanil/ SysthaneTM, mixtures) under UV-vis irradiation. The ATLAS Suntest CPS solar case simulating natural radiation and equipped with a xenon vapor lamp was used in all experiments. The photocatalyst TiO<sub>2</sub> P25 powder [<a href='#B10' class='html-bibr' title='10'>10</a>] was introduced into 200 mL of fungicide solution (Penconazol/Topas, Myclobutanil/ SysthaneTM, mixture) (between 10 mg·L<sup>−1</sup> and 20 mg·L<sup>−1</sup>) up to a final concentration of 0.75 g·L<sup>−1</sup>, and the slurry was homogenised by stirring. Before irradiation, suspensions were kept in the dark for 30 min to reach the adsorption equilibrium. During the irradiation procedure, 10 mL of solution were taken at regular intervals and ﬁltered (Whatmann, 0.45 mm). Concentrations of fungicides remaining after irradiation were determined with a LIBRA S12 UV-vis spectrophotometer (λ = 215 nm), and by TOC analysis (SHIMADZU TOC-L).

<em>2.3. Preparation of TiO<sub>2</sub>/β-SiC by Dip Coating Method</em>

Alveolar foams of β-SiC have been synthesized and supplied by SICAT company (Willstätt, Germany). Foam samples have a size of 9.5 cm (length) × 6 cm (width) × 1 cm (thickness), with a weight of about 20 g and a cell size of 4.5 mm. The foam was calcined at 1000 °C for 2 h to remove residual organic carbon. Each β-SiC foam sample was completely immersed in the TiO<sub>2</sub> P25 slurry (10 g TiO<sub>2</sub> P25 and 4 mL TTIP in 200 mL ethanol) for 5 min. Photocatalytic materials were dried at room temperature for 20 min, avoiding clogging of the cells. TiO<sub>2</sub>/β-SiC foams were introduced in an oven at 110 °C overnight to evaporate residual organic compounds. The temperature was then increased at 450 °C for 2 h at a rise rate of 5 °C·min<sup>−1</sup>. The average wt% of TiO<sub>2</sub> P25 per foam was about 7.5%, which corresponds to 1.5 g of TiO<sub>2</sub> P25 ﬁxed on the support [<a href='#B16' class='html-bibr' title='16'>16</a>].

Photocatalytic materials were characterized by scanning electron microscopy (SEM). SEM was performed in secondary electron mode on a JEOL-JSM-6700 microscope equipped with a ﬁeld emission gun and operating with an extraction potential, between 1 and 10 kV.

<em>2.4. Photocatalytic Tests with TiO<sub>2</sub> P25 Supported onto β-SiC Foam</em>

Photocatalytic experiments were performed in a polypropylene recirculation mode photoreactor measuring 20 cm (length) × 7.5 cm (width) × 2.5 cm (depth). Two samples of TiO<sub>2</sub>/β-SiC foam (10 cm (length) × 6 cm (width) × 1.5 cm (depth)) were introduced in the photoreactor to carry out the photocatalytic tests. The photoreactor was covered with quartz plates. Two UV-A lamps (Philips 18 W) were horizontally disposed 2 cm above the reactor to illuminate TiO<sub>2</sub>/β-SiC photocatalysts (<a href='#Figure1' class='html-fig html-figpopup'>Figure 1</a>). The irradiation wavelength was about 368 ± 20 nm with an irradiance of about 60 W·m<sup>−2</sup> (radiometer, spectral range: 315–400 nm, HD 9021, Delta OHM, Italy). Fungicide solutions (10 mg·L<sup>−1</sup>) were circulated through the reactor system using a peristaltic pump (Master Flex, model 7520-47) at a ﬂow rate of 26 mL·min<sup>−1</sup>, with a residence time of 12 min. Before each irradiation, TiO<sub>2</sub>/β-SiC was maintained for 1 h in the dark. Concentrations of fungicides remaining after irradiation were determined with a UV-vis spectrophotometer and TOC analysis without ﬁltration.<div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure1'><img data-large='/uploads/2023/07/10/2596c00505224d5c778ee531d01a7da3.png' data-original='/uploads/2023/07/10/2596c00505224d5c778ee531d01a7da3.png' src='/uploads/2023/07/10/2596c00505224d5c778ee531d01a7da3.png'><a class='html-expand html-figpopup' href='#Figure1'></a></div></div><div class='html-fig_description'><b><a href='#Figure1' class='html-fig html-figpopup'>Figure 1</a>.  </b>Experimental photocatalytic device with TiO<sub>2</sub>/β-SiC.</div></div>

<em>3.1. Photocatalytic Degradation of Pure Penconazol and of Topas Formulation</em>

3.1.1. With the TiO<sub>2</sub> P25 Photocatalyst Suspension

Photocatalytic elimination kinetics of Penconazol and its commercial formulation Topas, were determined with TiO<sub>2</sub> P25 in suspension. <a href='#Figure2' class='html-fig html-figpopup'>Figure 2</a> shows the fungicide disappearance, as well as TOC variation as a function of irradiation time. According to <a href='#Table1' class='html-table html-tablepopup'>Table 1</a>, the apparent rate constant of Penconazol disappearance <em>k<sub>Pen</sub></em> is approximately five times greater than that (<em>k<sub>Top</sub></em>) of Topas solution. After one-hour treatment, 86.74% of Penconazol are eliminated in the pure molecule solution whereas only 23.66% of the fungicide is removed from the commercial Topas formulation. Remember that Penconazol represents approximately 10.20% by mass of Topas solution, the remaining 89.80% comprising cyclohexanone and 2-methylpropanol. During the treatment, OH free radicals responsible for the molecular degradation, attack both Penconazol molecules and the additives. These additives compete on the active sites of the TiO<sub>2</sub> P25 photocatalyst. Therefore, degradation kinetics of Penconazol are slower in Topas formulation than in the pure molecule solution.

On the other hand, the comparison of TOC variations shows that during 60 min, the mineralization rate constant is approximately two times greater for Topas than for Penconazol (<a href='#Table1' class='html-table html-tablepopup'>Table 1</a>). As indicated above, the additives present in the Topas solution participate to the kinetics of molecules removal. Hence, TOC elimination is higher in the commercial formulation than in the pure molecule solution.<div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure2'><img data-large='/uploads/2023/07/10/31234e9aa1f86ebcb1637923c74f3a7e.png' data-original='/uploads/2023/07/10/31234e9aa1f86ebcb1637923c74f3a7e.png' src='/uploads/2023/07/10/31234e9aa1f86ebcb1637923c74f3a7e.png'><a class='html-expand html-figpopup' href='#Figure2'></a></div></div><div class='html-fig_description'><b><a href='#Figure2' class='html-fig html-figpopup'>Figure 2</a>.  </b>Photocatalytic degradation kinetics and TOC variation of a pure Penconazol solution and of Topas formulation using TiO<sub>2</sub> P25 photocatalysts in slurry.</div></div><div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table1' class='html-table html-tablepopup'>Table 1</a>. </b>Removal Rate of Penconazol and TOC in pure solution and in Topas formulation by TiO<sub>2</sub> P25 in slurry over 60 min of UV-A irradiation.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/9830335f5cc97dac60217339906c3c33.png' data-original='/uploads/2023/07/10/9830335f5cc97dac60217339906c3c33.png' src='/uploads/2023/07/10/9830335f5cc97dac60217339906c3c33.png'><a class='html-expand html-tablepopup' href='#Table1'></a></div></div></div>3.1.2. Photocatalytic Degradation with Supported TiO<sub>2</sub>/β-SiC Photocatalyst

Morphologies of TiO<sub>2</sub>/β-SiC materials were characterized by SEM. SEM picture (<a href='#Figure3' class='html-fig html-figpopup'>Figure 3</a>A) of the TiO<sub>2</sub> film on the support prepared without TTIP [<a href='#B19' class='html-bibr' title='19'>19</a>] shows cracks at the surface of the material. This is due to drying the material at room temperature (dip-coating phase) or to the calcination phase at 450 °C, because thermal expansion coefficients of the support and the catalyst are different. However, in the presence of TTIP, the surface is more homogeneous (<a href='#Figure3' class='html-fig html-figpopup'>Figure 3</a>B), showing that the addition of TTIP to the TiO<sub>2</sub> suspension improved the stability of the catalyst film on the support, since TTIP acts as a binder. Photocatalytic materials prepared with TTIP were used to perform all photocatalytic experiments for the removal of Penconazol and its commercial formulation Topas.<div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure3'><img data-large='/uploads/2023/07/10/6523447c1295e353a789908f650980cc.png' data-original='/uploads/2023/07/10/6523447c1295e353a789908f650980cc.png' src='/uploads/2023/07/10/6523447c1295e353a789908f650980cc.png'><a class='html-expand html-figpopup' href='#Figure3'></a></div></div><div class='html-fig_description'><b><a href='#Figure3' class='html-fig html-figpopup'>Figure 3</a>.  </b>SEM pictures of the surface of TiO<sub>2</sub> P25 ﬁlms coated on β-SiC alveolar foams with (<b>A</b>) without addition of TTIP and (<b>B</b>) addition of TTIP in TiO<sub>2</sub> slurry.</div></div>Kinetics of photocatalytic degradation of Penconazol (pure and in Topas) by the TiO<sub>2</sub>/β-SiC material are shown in <a href='#Figure4' class='html-fig html-figpopup'>Figure 4</a>. UV measurements at 215 nm performed for determining disappearance kinetics of the active molecule show similar adsorption percentages for Penconazol (24.59%) and Topas (22.19%), after one-hour adsorption in the dark. However, TOC determinations show a decrease of 2.52 mg·L<sup>−1</sup> and 0.64 mg·L<sup>−1</sup> TOC for Topas and Penconazol respectively, after one hour adsorption. This difference reflects the adsorption of the additives present in the commercial formulation. Over 4 h irradiation, Penconazol degrades faster than Topas (<a href='#Figure4' class='html-fig html-figpopup'>Figure 4</a>). However, after 2 h irradiation, the degradation rate of pure Penconazol decreases because of the competition existing between by-products and the molecule on the active sites of the photocatalyst. In the literature, Penconazol molecule is stable and hardly biodegradable in an aqueous medium. Therefore, the amount of TOC removed is four times lower than for Topas (<a href='#Table2' class='html-table html-tablepopup'>Table 2</a>).<div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure4'><img data-large='/uploads/2023/07/10/c69f4afa5492aaef07918ec4c9950379.png' data-original='/uploads/2023/07/10/c69f4afa5492aaef07918ec4c9950379.png' src='/uploads/2023/07/10/c69f4afa5492aaef07918ec4c9950379.png'><a class='html-expand html-figpopup' href='#Figure4'></a></div></div><div class='html-fig_description'><b><a href='#Figure4' class='html-fig html-figpopup'>Figure 4</a>.  </b>Photocatalytic degradation’s kinetics and TOC variation of a pure Penconazol solution and of Topas formulation by the use of TiO<sub>2</sub>/β-SIC.</div></div><div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table2' class='html-table html-tablepopup'>Table 2</a>. </b>Removal rate of Penconazol and TOC in pure solution and in Topas formulation by the use of TiO<sub>2</sub>/β-SiC over 60 min of UV-A irradiation.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/95a33d7c7f0b33d74cc1346eec035a8e.png' data-original='/uploads/2023/07/10/95a33d7c7f0b33d74cc1346eec035a8e.png' src='/uploads/2023/07/10/95a33d7c7f0b33d74cc1346eec035a8e.png'><a class='html-expand html-tablepopup' href='#Table2'></a></div></div></div><em>3.2. Photocatalytic Degradation of Myclobutanil in Pure Solution and in SysthaneTM Formulation</em>

3.2.1. With TiO<sub>2</sub> P25 Suspension

Results of photocatalytic degradation of Myclobutanil in pure solution and in commercial formulation with TiO<sub>2</sub> P25 in suspension were obtained by M’Bra et al. [<a href='#B11' class='html-bibr' title='11'>11</a>]. They are summarized in <a href='#Table3' class='html-table html-tablepopup'>Table 3</a> and were obtained under the following optimal experimental conditions: <em>C</em><sub>o</sub> = 10 ppm; pH = 6.7; <em>C</em><sub>TiO<sub>2</sub></sub> = 0.75 g·L<sup>−1</sup>; <em>V</em><sub>solution</sub> = 200 mL; Irradiation time = 120 min; UV-vis irradiation in batch mode.

The authors have shown that Myclobutanil degraded faster in a pure aqueous solution (41.4 × 10<sup>−3</sup>·min<sup>−1</sup>) than in the commercial Systhane formulation (21.2 × 10<sup>−3</sup>·min<sup>−1</sup>). Like for Penconazol, the percentage of TOC removed is lower with Systhane than with the solution of pure Myclobutanil, which is explained by the presence of organic additives in the commercial formulation.<div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table3' class='html-table html-tablepopup'>Table 3</a>. </b>Removal rate of Myclobutanil and TOC in pure solution and in Systhane formulation by the use of TiO<sub>2</sub> P25 in slurry over 60 min of UV-A irradiation.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/31292b7bf1ff566ac439ea552f54a2e4.png' data-original='/uploads/2023/07/10/31292b7bf1ff566ac439ea552f54a2e4.png' src='/uploads/2023/07/10/31292b7bf1ff566ac439ea552f54a2e4.png'><a class='html-expand html-tablepopup' href='#Table3'></a></div></div></div>3.2.2. Photocatalytic Degradation with Supported TiO<sub>2</sub>/β-SiC Photocatalyst

Results of photocatalytic degradation of Myclobutanil and its commercial formulation with TiO<sub>2</sub>/β-SiC are shown in <a href='#Table4' class='html-table html-tablepopup'>Table 4</a>. They were obtained under the following optimal experimental conditions: two TiO<sub>2</sub>/β-SiC samples; <em>C</em><sub>o</sub> = 10 ppm; pH = 6.7; Flow rate = 26 mL·min<sup>−1</sup>; <em>V</em><sub>solution</sub> = 500 mL; Irradiation time = 4 h; UV-A irradiation in the recirculation mode (<a href='#Figure1' class='html-fig html-figpopup'>Figure 1</a>).

The authors have shown that Myclobutanil in pure solution degraded faster (5.24 × 10<sup>−3</sup> min<sup>−1</sup>) than in Systhane formulation (3.20 × 10<sup>−3</sup> min<sup>−1</sup>), due to the presence of organic additives which compete with the degradation of Myclobutanil in Systhane formulation. Degradation rate constants are lower in the case of the TiO<sub>2</sub>-P25 photocatalyst supported on β-SiC than in suspension. This is explained on the one hand by the decrease in the available surface of supported TiO<sub>2</sub> particles compared to the suspension and on the other hand by a recirculation reactor system which causes a dead volume of non-irradiated fungicide solution.<div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table4' class='html-table html-tablepopup'>Table 4</a>. </b>Removal rate of Myclobutanil and TOC in pure solution and in SysthaneTM formulation by the use of TiO<sub>2</sub>/β-SiC over 60 min of UV-A irradiation.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/3e4638b586e446cfc7dcc59f1b75f0f2.png' data-original='/uploads/2023/07/10/3e4638b586e446cfc7dcc59f1b75f0f2.png' src='/uploads/2023/07/10/3e4638b586e446cfc7dcc59f1b75f0f2.png'><a class='html-expand html-tablepopup' href='#Table4'></a></div></div></div><em>3.3. Photocatalytic Degradation of Fungicide Mixtures</em>

Among the numerous fungicide treatments carried out on vineyards, there is often an association of two fungicides. It is therefore important to evaluate the performance of photocatalytic degradation process on mixtures of pure active molecules (Penconazol and Myclobutanil) but also on mixtures of their Topas and Systhane commercial formulations. We believe that our study will complement the work undertaken by Pichat et al. in 2004 [<a href='#B20' class='html-bibr' title='20'>20</a>].

3.3.1. Photocatalytic Degradation of Fungicide Mixtures with TiO<sub>2</sub> P25 Suspension

<a href='#Figure5' class='html-fig html-figpopup'>Figure 5</a> shows degradation kinetics of Myclobutanil and Penconazole mixtures in pure solutions (<a href='#Figure5' class='html-fig html-figpopup'>Figure 5</a>A) and in mixtures of commercial Systhane and Topas formulations (<a href='#Figure5' class='html-fig html-figpopup'>Figure 5</a>B). After 30 min adsorption, removal rates of the active compounds are 6.26% and 4.89% respectively for Myclobutanil and Systhane; 4.28% and 1.86% respectively for Penconazol and Topas. During irradiation in presence of TiO<sub>2</sub>-P25 suspensions, mixtures of pure fungicides degrade faster than when present in their commercial formulations. <a href='#Table5' class='html-table html-tablepopup'>Table 5</a> shows the various fungicide removal rates, as well as their apparent disappearance and mineralization rate constants. More TOC was removed from the commercial mixture than from the pure fungicide mixture. This result is due to the presence of additives in the commercial mixture, because the radicals attack not only the fungicides, but also the additives.<div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table5' class='html-table html-tablepopup'>Table 5</a>. </b>Removal rates of Myclobutanil/Penconazol mixtures and TOC in pure solutions and in mixtures of Systhane and Topas commercial formulations, by the use of TiO<sub>2</sub> P25 suspension over 60 min irradiation.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/1f757c58ffab41528a94b3b1be1b46a4.png' data-original='/uploads/2023/07/10/1f757c58ffab41528a94b3b1be1b46a4.png' src='/uploads/2023/07/10/1f757c58ffab41528a94b3b1be1b46a4.png'><a class='html-expand html-tablepopup' href='#Table5'></a></div></div></div><div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure5'><img data-large='/uploads/2023/07/10/9b7fbbfbd233ee949e9b9e5e18ef31a5.png' data-original='/uploads/2023/07/10/9b7fbbfbd233ee949e9b9e5e18ef31a5.png' src='/uploads/2023/07/10/9b7fbbfbd233ee949e9b9e5e18ef31a5.png'><a class='html-expand html-figpopup' href='#Figure5'></a></div></div><div class='html-fig_description'><b><a href='#Figure5' class='html-fig html-figpopup'>Figure 5</a>.  </b>Photocatalytic degradation kinetics of mixtures of (<b>A</b>) pure solutions of Myclobutanil-Penconazol and (<b>B</b>) Systhane-Topas commercial formulations, by the use of TiO<sub>2</sub> P25 suspension.</div></div>3.3.2. Photocatalytic Degradation of Fungicide Mixtures with Supported TiO<sub>2</sub>/β-SiC Photocatalyst

<a href='#Figure6' class='html-fig html-figpopup'>Figure 6</a> shows degradation kinetics of Myclobutanil-Penconazole in mixtures of pure solutions (<a href='#Figure6' class='html-fig html-figpopup'>Figure 6</a>A) and of Systhane and Topas commercial formulations (<a href='#Figure6' class='html-fig html-figpopup'>Figure 6</a>B). After one-hour adsorption, removal rates of the active compounds are almost similar for the two mixtures. Proportions of fungicide removed are 19.20% and 15.71% for Myclobutanil and Systhane, respectively, and 16.78% and 15.63% for Penconazol and Topas, respectively. When TiO<sub>2</sub>/β-SiC materials are irradiated, pure fungicide mixtures degrade faster than when present in their commercial formulations. <a href='#Table6' class='html-table html-tablepopup'>Table 6</a> shows the various rates of fungicide removal, as well as their apparent disappearance and mineralization rate constants. Results show a higher TOC removal in the commercial mixture than in the pure fungicide mixture. This phenomenon was observed with TiO<sub>2</sub> P25 nanoparticles in suspension. Thus, additives of the commercial mixture disrupt the action of OH° radicals on active compounds. Radicals attack not only fungicides but also additive compounds. Therefore, degradation and removal kinetics of TOC molecules decrease (<a href='#Table5' class='html-table html-tablepopup'>Table 5</a> and <a href='#Table6' class='html-table html-tablepopup'>Table 6</a>).<div class='html-fig-wrap'><div class='html-table_wrap_discription'><b><a href='#Table6' class='html-table html-tablepopup'>Table 6</a>. </b>Photocatalytic degradation kinetics of mixtures of (<b>A</b>) pure solutions of Myclobutanil-Penconazol and (<b>B</b>) Systhane-Topas commercial formulations, by the use of TiO<sub>2</sub> P25 suspension.<div class='html-figpopup-table html-figpopup-link'><img data-large='/uploads/2023/07/10/87c7320061a2ef28c6d69c481d8e6c3b.png' data-original='/uploads/2023/07/10/87c7320061a2ef28c6d69c481d8e6c3b.png' src='/uploads/2023/07/10/87c7320061a2ef28c6d69c481d8e6c3b.png'><a class='html-expand html-tablepopup' href='#Table6'></a></div></div></div><div class='html-fig-wrap'><div class='html-fig_img'><div class='html-figpopup html-figpopup-link' href='#Figure6'><img data-large='/uploads/2023/07/10/f4fdaab75d754c9794d37e6f57ad6696.png' data-original='/uploads/2023/07/10/f4fdaab75d754c9794d37e6f57ad6696.png' src='/uploads/2023/07/10/f4fdaab75d754c9794d37e6f57ad6696.png'><a class='html-expand html-figpopup' href='#Figure6'></a></div></div><div class='html-fig_description'><b><a href='#Figure6' class='html-fig html-figpopup'>Figure 6</a>.  </b>Photocatalytic degradation kinetics by the use of TiO<sub>2</sub>/β-SiC (<b>A</b>) of a Myclobutanil-Penconazol mixture and (<b>B</b>) of a Systhane-Topas mixture.</div></div>

The present work assessed the efficiency of photocatalytic degradation of active molecules of pure fungicides (Myclobutanil and Penconazol) in aqueous solution but also by their commercial formulations (Topas and Systhane). Two photocatalytic systems were used, the first with TiO<sub>2</sub>-P25 suspended in water and the second with TiO<sub>2</sub>-P25 supported on β-SiC foams, under UV-vis irradiation. Results showed slower degradation kinetics of the active molecules with the supported photocatalysts than with TiO<sub>2</sub> in suspension. This is a logical result because active sites of the supported photocatalyst are reduced compared to the suspended catalyst. However, TiO<sub>2</sub>/β-SiC material can be easily recovered and reused after fungicide degradation, whereas TiO<sub>2</sub> P25 recovery in slurry requires long and expensive filtration operations.

Fungicide removal with the supported photocatalyst led to a significant reduction in the concentration of pure active molecules with reasonable treatment times and irradiance (about 50% fungicide degradation in 2 h under our conditions). Concerning the processing of commercial formulations, degradation kinetics of the active molecules were slower due to the presence of mineral and organic additives. Nevertheless, our results have shown that it is possible to degrade and mineralize commercial formulations of fungicides. We think these results are encouraging for photocatalytic removal of fungicides removal at industrial scale, especially when using the TiO<sub>2</sub>/β-SiC material which can easily be recycled. Our future work will consist on the one hand in assessing interactions of the components of fungicide formulations on the photocatalytic performances of our system, and on the other hand in optimizing the process in order to reduce the processing time of commercial formulations of fungicides.

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