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Kinetics and mechanism of Paraquat's degradation: UV-C photolysis vs UV-C photocatalysis with TiO2/SiC foams.

In this study, the photolytic and photocatalytic removal of the herbicide paraquat is investigated under UV-C (254 nm). For photocatalytic experiments, SiC foams were used with P25-TiO2 nanoparticles deposited by dip-coating. The foams were characterized by scanning electron microscopy and paraquat's degradation under UV-C photolysis or photocatalysis, followed by UV-vis spectroscopy, total organic carbon analyzer, LC-MS and ion chromatography. After 3 h of reactions by photolysis and photocatalysis, 4% and 91% of TOC removal were observed. An analysis of degradation by-products showed a similar degradation pathway with pyridinium ions observed by LC/MS and carboxylic acids (succinate, acetate, oxalate and formate) detected by ion chromatography. In conclusion, these two different photo-degradation processes are able to remove paraquat and produce similar by-products. However, the kinetics of degradation is rather slow during photolysis and it is recommended to combine the UV-C lightning with a TiO2 photocatalyst to improve the mineralization rate.


Introduction
Pollution is a worldwide problematic concerning water, air or soil quality. Now, it is observed in many countries that intensive agriculture causes troubles related to the use of harmful pesticides and/or herbicides contaminating surface waters (rainwater runoff), soils (crops) and air (spray application). In this study, the attention is focused on Paraquat (or Gramoxone), an herbicide used in more than 130 countries over the last decades [1].
This compound is mainly used for crops preparation by spray application. Unfortunately, its use is correlated to Parkinson's disease for farmers or residents living close to those crops and exposed during spray application [2,3]. Some precautions should thus be taken in order to avoid the use of such compound.
Paraquat is also applied for aquatic weed control from 0.5 to 1.5 ppm [4] and unfortunately, it was shown that concentrations as low as 0.1 ppm resulted in abnormal tail flexure on Xenopus Laevis [5]. Consequently, recommendation guidelines suggest Paraquat concentration lower than 0.1 ppb in Europe or 10 ppb in Canada [6].
The search for new treatment methods able to degrade such recalcitrant compound is under way. Currently, advanced oxidation processes (AOPs) are very attractive thanks to the production of highly oxidizing hydroxyl radicals. Among these processes, photocatalysis is particularly suitable because it does not require the handling of expensive chemicals such as H2O2 and hence, it can be operated autonomously. The principle of photocatalysis is based on the use of a photocatalyst (semiconductor), usually TiO2, activated by UV-light to produce electron-hole pairs (equations 1), reacting with water (holes-equation 2) and dissolved dioxygen (electrons-equation 3) to produce A C C E P T E D M A N U S C R I P T respectively the hydroxyl radical OH  and the superoxide radical O2 -.
Historically, photocatalysis was operated with TiO2 nanoparticles in suspension but the recovery of the photocatalyst was highly challenging. In order to solve this, supported photocatalysis and photocatalytic membrane reactors (PMR) were developed [13].
Unfortunately, the use of PMRs is rather expensive compared to supported photocatalysis because of the complex/energy consuming ultrafiltration membrane system. The advances in the field of supported photocatalysis come from the development of various supports [14] such as glass [15][16][17]), stainless steel [18,19], silicon carbide [20] or alumina [16].
In order to improve the poor exchange surface of the supported photocatalyst and the effluent in comparison with the TiO2 nano-slurries, it is recommended to use threedimensional supports such as beads [21,22,16], tubes [23,24] or foams [25][26][27].
Among 3D-support materials, silicon carbide foams are highly suitable due to their high chemical resistance, outstanding thermal stability and their macro-porosity providing a high internal surface enabling the immobilization of large amounts of photocatalyst, usually by dip-coating [26]. The development of such technology is still very recent and much progress is required especially for the development of photocatalytic reactors integrating TiO2/SiC foams.
The photolytic/photocatalytic degradation of paraquat was previously investigated by many groups [28][29][30][31]. However in these last works, photolysis and photocatalysis are not compared regarding oxidation by-products, especially under UV-C light sources (UV-C are the rays with a wavelength between 100 and 280 nm). The objectives of our work, is to show that UV-C lights can play a significant role in paraquat's removal. On the other hand, we have attempted to determine the paraquat degradation mechanism by measuring the Total Organic Carbon (TOC) during irradiation as well as the concentration of paraquat and its degradation by-products by UV-Visible spectroscopy and LC-MS (Liquid chromatography-mass spectrometry).

Reactants and chemicals
Paraquat dichloride hydrate (98%), anhydrous ethanol and acetone were provided from Sigma Aldrich. TiO2 nanoparticles (Aeroxide P25) were provided from Evonik. TiO2 P25 is a fine white powder with hydrophilic character caused by hydroxyl groups on the surface. It consists of aggregated primary particles. The aggregates are several hundred nm in size and the primary particles have a mean diameter of approx. 21 nm. Particle size and density of ca. 4 g/cm³ lead to a specific surface of approx. 50 m²/g. The weight ratio of anatase and rutile is approximately 80 / 20.

Preparation of SiC foams
SiC foams were kindly provided by the SICAT company (Germany) and their synthesis is described in detail elsewhere [32]. These foams were shaped as cylinders (diameter = 35 mm, length = 50 mm) and each foam weighs approximately 10 g (figure 1). Before dip coating, the foams were ultrasonicated in acetone, ethanol and water. Finally they were dried at 110°C overnight in an electric oven.

Deposition of P25 on SiC foams by dip-coating
Immobilization of TiO2-P25 photocatalyst was performed by preparing a slurry (100 g/l of P25 in anhydrous ethanol, Aldrich) under ultrasonication for 30 minutes. The reasons that explain the high efficiency of TiO2-P25 are not yet fully known. It is likely that the main reasons are the high hydroxylation of its surface combined with a lower recombination rate of electron/hole pair. There is also the creation of a heterojunction between anatase and rutile phases. SiC foams were then immersed in the suspension at 8 mm/s and five dipping cycles were performed for each foam (approximately 10 g without TiO2) with 10 min drying between consecutive immersions. After that, each foam was dried overnight at 110°C and heated for 4 hours at 450°C with a heating ramp of 5°C/min under air in a Nabertherm oven. For each foam, 16 ± 1 wt% of TiO2 was immobilized.

SEM analysis
SiC foams coated by TiO2 P25 were characterized by scanning electron microscopy at 20 kV with a Zeiss apparatus model EVO 50 equipped with a tungsten filament cathode. For analysis, a small fragment of a TiO2/SiC foam was inserted in the SEM with a carbon adhesive tape from Soquelec.

Photocatalytic reactor
The photocatalytic reactor is presented in detail on figure 2.

Probe molecule and UV-vis spectroscopy
To evaluate the photocatalytic performances of the reactor, Paraquat was used as a probe molecule. The concentration (5 to 40 ppm) of this compound was followed by UV-vis spectroscopy at 257 nm (maximum absorption wavelength) with a UV-Visible apparatus (Agilent Cary 50).

Total Organic Carbon (TOC) analysis
Total organic carbon measurements were determined by the non-purgeable organic carbon method, on a Shimadzu VCPH apparatus with a very low sensitivity (detection limit = 0.05 mg Carbon per liter).

LC-MS analysis
By-products of degradation were identified by LC-MS with a TSQ Quantum Access apparatus from Thermo Scientific. Sample injection was performed for 6 minutes with 20 µl/min mixed with the eluant and injected at 0,2 ml/min with an eluant composed of 87.5 % of solution A (water + 30 mM of ammonium formate + 0.1 % of formic acid) and 12.5 % of solution B (acetonitrile + 30 mM of ammonium formate + 0.1% formic acid). The analyte and eluant were then injected in a chromatographic column: Hypersil Gold from Thermo Scientific (100mm x 2,1 mm) heated at 35°C.

Ion chromatography
Organic acids were detected by ion chromatography with an ionic resin (Ion PAC AS11-HC at 30°C) set on an Integrion HPIC apparatus from Thermo. Elution was programmed with a concentration gradient of KOH between 1 and 30 mM. A cationic suppressor is then used to remove cations and then a conductivity detector was used for concentration determination at 35°C. Detection limits were in the range 0.001 to 0.002 mg/l for each compound.

A. Characterization of TiO 2 coated SiC foams
The dip-coating technique allows to obtain highly homogeneous and evenly coated TiO2/SiC foams. A picture of a foam before and after dip/coating in the TiO2 suspension is presented as additional information (figure 1). After annealing at 450°C, the coated foam was characterized by scanning electron microscopy ( figure 3). Macro-pores were observed in the range 3 to 5 mm. The coating is quite homogeneous but the presence of cracks is highlighted at the microscale due to the strains during the drying step (dipcoating) or the annealing step at 450°C. This suggests that the deposition method must be improved for future works.

B. Comparison between photolysis, photocatalysis and adsorption in the dark a. Degradation followed by UV-visible spectroscopy
The comparison between the three main processes involved (adsorption in the dark, UV-C photolysis and UV-C photocatalysis) is presented in figure 4. The photolysis experiment was performed without TiO2/SiC foams inside the reactor. It is found that UV-C photolysis and pollutant adsorption have a small effect on Paraquat removal. The removal of paraquat using the adsorption process (in the dark) was 30% after 3 h of treatment, whereas 50% of paraquat was removed using direct UV-C photolysis process.
By comparison, when UV-C irradiations were combined with a TiO2-supported  There is a direct relationship between paraquat absorbance of UV radiation and its degradation by photolysis under UVC. Due to the overlapping between the emission spectra of the UV-C lamp (254 nm) and the absorption spectra of Paraquat (max. absorption wavelength = 257 nm), electronic transitions must occur in Paraquat molecules. However, it is difficult to establish a consistent ratio because paraquat removal by UVC irradiation is much faster than for other wavelengths (unpublished results). In addition, the paraquat degradation by-products play a role in the degradation processes for example, via screening the UV radiation. Paraquat has high molar absorptivity in the UV region. On the other hand, in direct photolysis of paraquat at 254 nm, low concentrations of H2O2 are formed. The photolysis of the hydrogen peroxide does not compete with paraquat because the primary quantum yield of H2O2 at 254 nm is very high. It is thought that H2O2 formed could undergo some photolysis. If this is the case, hydroxyl radicals would form and oxidation of atrazine would be due to both direct photolysis and OH° radical reactions. These two reasons explain the decrease in Paraquat concentration during photolysis.

b. Degradation followed by TOC analyzer
In order to compare the efficiency of photolysis and photocatalysis towards mineralization, organic matter (paraquat + by-products) was followed by TOC analysis.
Results are presented in figure 4. Photolysis is able to degrade paraquat selectively, but TOC analysis reveals that less than 5% TOC is mineralized after 180 minutes of UV-C photolysis. On the other hand, photocatalysis is very efficient because 90 % of organic matter is removed after 180 minutes due to the ability of this process to produce the hydroxyl radicals allowing paraquat and its by-products to be converted to CO2, inorganic salts and water.

C. By-products formation and proposed mechanism of paraquat degradation a. Evolution of pyridinium compounds
In order to understand which compounds were mineralized by photocatalysis, paraquat by-products were analyzed by LC-MS and ion chromatography.   Paraquat has an atomic mass of 186 with z = 2 and hence, an m/z ratio appears at = 93.
Besides, the peak at m/z = 186 also appears because the reduced form of paraquat molecules is stabilized by resonance [28].
The kinetics of appearance/degradation of each m/z compound is shown in figure 6 with

. Evolution of short-chain carboxylic acids
Other by-products were detected by ion chromatography, especially short-chain organic acids. These compounds were detected as carboxylated anions at different elution time:

c. Proposed degradation pathway
Thanks to LC-MS and ion chromatography, a paraquat mechanism of degradation is proposed as shown in figure 8. Paraquat molecules are first converted to similar molecular structures bearing bipyridinium group (monoquat, reduced paraquat or paraquat monopyridone). Then fragmentation occurs and leads to lower molecular weight compounds. 4-carboxy-1-methylpyridinium ion and short chain carboxylic acids appears to be the major by-products. Mineralization experiments suggest that only photocatalysis is able to degrade these species to the ultimate products of oxidation, especially CO2.

Figure 8:
Proposed mechanism for Paraquat degradation by photocatalysis or photolysis under UV-C radiation.

Conclusion
This paper presents a critical comparison between UV-C photolysis and UV-C photocatalysis for the degradation of paraquat molecules. Experiments reveal that photolysis is able to degrade this compound but cannot lead to its mineralization. Besides, photocatalysis is very effective for removing paraquat and its by-products as shown by mineralization studies. The degradation pathway was evidenced through LC-MS and ion chromatography analysis. It appears that similar by-products (pyridinium ions or shortchain carboxylic acids) were formed during both types of treatments (photolysis or photocatalysis) apart for maleate that was only produced during photolytic degradation.
The elevated degradation efficiency of photocatalysis is attributed to the production of the OH  radical allowing a complete mineralization. Despite the poor efficiency of photolysis, short-chain carboxylic acids were produced and we suggest that a coupling with a biological reactor could be an interesting perspective to remove paraquat.

Acknowledgements
The authors wish to thanks the Région Alsace (France) and the INRS (Québec) for the PhD research grant of Cédric Marien, and SiCat Company for the samples of SiC foams.