Photocatalytic Efficiency of Suspended and Immobilized TiO 2 P25 for Removing Myclobutanil, Penconazole and Their Commercial Formulations

: 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. TiO 2 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, TiO 2 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 TiO 2 /β-SiC material than with a suspension of TiO 2 which requires long and expensive filtration operations.


Introduction
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 [1]. 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%) [1]. 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 [2]. 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) [3] 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° [4] which have an oxidizing power greater than that of traditional oxidants such as H2O2 [5], Cl2 [6], or O3 [7]. These radicals are capable of partially (or totally) mineralizing most organic compounds. Among these AOPs, special attention has been paid to heterogeneous TiO2/UV photocatalysis [8,9], due to its ability to mineralize a wide range of recalcitrant organic pollutants at room temperature and at atmospheric pressure into harmless substances [10]. Several studies have been done using titanium dioxide in suspension [11,12]. However, the post-treatment recovery of TiO2 is a difficult process to achieve due to the nanometer-size photocatalyst and the cost this can entail. Therefore, filtration and re-suspension of TiO2 should be avoided in a wastewater treatment process. Therefore, the idea of immobilizing TiO2 on a suitable and chemically inert support has started to emerge because it could avoid the expensive process of phase separation [13,14]. 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 TiO2 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 TiO2 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 2 ·g −1 ). Previous studies have been carried out by removing recalcitrant organic compounds in the presence of the TiO2/β-SiC material [11,[15][16][17]. The objective of the present work is to compare the photocatalytic activity of TiO2 P25 in slurry (batch mode) with that of TiO2 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.

Photocatalytic Tests with TiO2 P25 in Slurry
Batch mode experiments were conducted at room temperature in a glass cylinder reactor with aqueous solutions (Penconazol/Topas, Myclobutanil/ Systhane TM , 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 TiO2 P25 powder [10] was introduced into 200 mL of fungicide solution (Penconazol/Topas, Myclobutanil/ Systhane TM , mixture) (between 10 mg·L −1 and 20 mg·L −1 ) up to a final concentration of 0.75 g·L −1 , 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).

Preparation of TiO2/β-SiC by Dip Coating Method
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 TiO2 P25 slurry (10 g TiO2 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. TiO2/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 −1 . The average wt% of TiO2 P25 per foam was about 7.5%, which corresponds to 1.5 g of TiO2 P25 fixed on the support [16]. 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.

Photocatalytic Tests with TiO2 P25 Supported onto β-SiC Foam
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 TiO2/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 TiO2/β-SiC photocatalysts (Figure 1). The irradiation wavelength was about 368 ± 20 nm with an irradiance of about 60 W·m −2 (radiometer, spectral range: 315-400 nm, HD 9021, Delta OHM, Italy). Fungicide solutions (10 mg·L −1 ) were circulated through the reactor system using a peristaltic pump (Master Flex, model 7520-47) at a flow rate of 26 mL·min −1 , with a residence time of 12 min. Before each irradiation, TiO2/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.

With the TiO2 P25 Photocatalyst Suspension
Photocatalytic elimination kinetics of Penconazol and its commercial formulation Topas, were determined with TiO2 P25 in suspension. Figure 2 shows the fungicide disappearance, as well as TOC variation as a function of irradiation time. According to Table 1, the apparent rate constant of Penconazol disappearance kPen is approximately five times greater than that (kTop) 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 TiO2 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 (Table 1). 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.  Morphologies of TiO2/β-SiC materials were characterized by SEM. SEM picture ( Figure 3A) of the TiO2 film on the support prepared without TTIP [19] shows cracks at the surface of the material. This is due to drying the material at room temperature (dipcoating 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 (Figure 3B), showing that the addition of TTIP to the TiO2 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.

B A
Kinetics of photocatalytic degradation of Penconazol (pure and in Topas) by the TiO2/β-SiC material are shown in Figure 4. 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 −1 and 0.64 mg·L −1 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 (Figure 4). 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 (Table 2).

With TiO2 P25 Suspension
Results of photocatalytic degradation of Myclobutanil in pure solution and in commercial formulation with TiO2 P25 in suspension were obtained by M'Bra et al. [11]. They are summarized in Table 3 and were obtained under the following optimal experimental conditions: Co = 10 ppm; pH = 6.7; CTiO2 = 0.75 g·L −1 ; Vsolution = 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 −3 min −1 ) than in the commercial Systhane formulation (21.2 × 10 −3 min −1 ). 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.
The authors have shown that Myclobutanil in pure solution degraded faster (5.24 × 10 −3 min −1 ) than in Systhane formulation (3.20 × 10 −3 min −1 ), 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 TiO2-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 TiO2 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. Table 4. Removal rate of Myclobutanil and TOC in pure solution and in Systhane TM formulation by the use of TiO2/β-SiC over 60 min of UV-A irradiation.

Photocatalytic Degradation of Fungicide Mixtures
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 [20] Figure 5A) and in mixtures of commercial Systhane and Topas formulations ( Figure 5B). 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 TiO2-P25 suspensions, mixtures of pure fungicides degrade faster than when present in their commercial formulations. Table 5 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.   Figure 6A) and of Systhane and Topas commercial formulations ( Figure 6B). 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 TiO2/β-SiC materials are irradiated, pure fungicide mixtures degrade faster than when present in their commercial formulations. Table 6 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 TiO2 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 (Tables 5 and 6).

Conclusions
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 TiO2-P25 suspended in water and the second with TiO2-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 TiO2 in suspension. This is a logical result because active sites of the supported photocatalyst are reduced compared to the suspended catalyst. However, TiO2/β-SiC material can be easily recovered and reused after fungicide degradation, whereas TiO2 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 TiO2/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.