Fixed Bed Reactor Performance for Herbicide Degradation Under Solar Radiation

Photocatalysis has the best potential to replace the traditional wastewater treatment techniques such as activated carbon adsorption, chemical oxidation, and biological treatment. 
Photocatalysis is a combination of a catalyst semiconductor is titanium dioxide TiO2 with an ultraviolet light source as the sun. It has the added advantage of not introducing additives in the medium to be treated. However, when applied in slurry form, agglomeration of nanoparticle will lead to serious decrease in photocatalytic performance due to hinderance effect. Further, the present study has made an effort to use a support for the immobilization of catalyst for treating the pesticide polluted water by photocatalytic degradation. In the present paper, we present the performance of 2 kinds of Titanium dioxide (Commercial Media Ahlstrom and impregned TiO2 on glass) as photocatlytic support in fixed bed reactor for the degradation of Metribuzin herbicide under solar radiation. The degradation Performance was studied under various conditions such as substrate and pH solution. Solar photocatalytic degradation rate can reach 100% during 180 min of treatment with using TiO2 immobilized on commercial support media 1048.


Introduction
Organic chemical products such as, dyes, endocrine disruptors, pesticides and herbicides lead to serious environmental contamination. In particular, various types of pesticide are widely employed in agricultural practice. They are one of the major causes of pollution of surface and ground waters (Oyama et al., 2004). These compounds are toxic and carcinogenic in nature even at low concentration (Ozdemir, Sahinkaya, & Onucyildiz, 2008), several of them are resistant to conventional chemical and biological treatment methods. The search for effective means of removing these compounds is of interest to regulating authorities everywhere (Kositzi et al., 2004). Advanced oxidation processes (AOPs) have been proposed as an alternative for the treatment of this type of wastewater. Many researchers have concentrated on this goal pointing out that these processes, although making use of different reacting systems, are all characterised by the same chemical feature: production of OH radicals (•OH) (Malato t al., 2002). Among the different AOPs, heterogeneous photocatalysis using UV/TiO2 system appears as one of the most destructive technologies (Chekir, Laoufi, & Bentahar, 2014;Chen, Yang, Wang, & Lou, 2005;Gaya & Abdullah, 2008;Herrmann, 2005). The titanium dioxide photocatalyst is widely available, inexpensive, non-toxic, and shows a relatively high chemical stability (Augugliaro, Loddo, Palmisano, & Schiavello, 1995;Topalov, Molnar-Gabor, Kosanić, & Abramović, 2000;Duffy et al., 2004). However due to the difficulty of nanosized photocatalysts separating fromaqueous or gaseous pollutants after reactions in suspended reactor and to avoid the filtration step, fixed-bed system appeared in which the photocatalysts were immobilized on the walls of the reactor, on the supported substrates, or around the light source (Xu et al., 2008). The aim of the present work is to study the efficiency of two catalytic supports on the solar photocatalysis of metribuzin. In order to determine the optimal degradation conditions of this organic pollutant different parameters influencing photocatalysis were considered.

Chemical and reagents a) Pollutant
Selected pestecide used in this study was the metribuzin C8H14N4OS ( fig.1.) with a molecular weight of 214.32 (g/mol). Metribuzin is one of the nitrogen-containing herbicides in the family of triazine herbicides, is registered for the control of annual broadleaf weeds and grassy weeds that infest various crops, including potatoes, tomatoes ... etc. Metribuzin degradation was evaluated by measuring the absorbance with spectrophotometer UV-VISIBLE, type Shimatzu UV1800. The maximum absorption band was located at 293 nm. A correlation curve between Metribuzin concentration and the absorption was preestablished.

b) photocatalysts
A commercially available titanium dioxide PC-500 was used as a photocatalyst produced by Millenium Inorganic Chemicals. This photocatalyst is in 99% anatase form with BET specific surface area of 320 m 2 .g -1 and particle diameter betweem 5 and 10 nm. In this study, we used titanium dioxide in two forms: TiO2 PC-500 supported on cellulose "média 1048" and TiO2 PC 500 Millennium impregnated and fixed on glass.

Photoreactor
All the experiments were performed under natural sun light in a step photoreactor (Fig.2) specially developed for photocatalytic application installed at the Solar Equipment Development Unit (UDES) on the north of Algeria (latitude 36°.39'; longitude 2°.42') using natural sun-light irradiation. Solar ultraviolet radiation (U.V.) was measured by global UV radiometer (KIPP&ZONEN, CMP11) mounted on a plat form tilted 36° as the reactor.
The solar photocatalytic reactor used in this experiment consists of several parts: a tank, a pump, a spillway at the top and steps. The reactor was made of three (03) regular glass steps of the same dimensions (40x20x1cm) covered with the photocatalytic media or impregned solution (corresponding to 1.1 mg/ cm 2 ).

Effect of pH solution
pH is an important parameter in photocatalysis, which influences the surface charge of TiO2 and therefore adsorption of the pollutant. The pH solution was adjusted by adding NaOH or H2SO4. The role of pH on the rate of photocatalytic degradation was studied in the pH range 3-9.5 at constant pesticide concentration (10 mg L −1 ). The obtained Results for the two kind of photocatalyst support, with varying pH from 3 to 9.5, were illustrated in Fig. 3. Table 1 summarizes the calculated kapp and t1/2.

Media 1048
Fixed on glass The obtained results in fig. 3 show that after 300 minutes of photocatalytic treatment, the metribuzin removal is around 97% and 94% for acidic and basic pH respectively when the TiO2 photocatalyst is supported on tissues "media 1048". The results are almost similar when the catalyst is supported on glass plates. While for the free pH (7.0-7.2), the degradation is complete with a yield of 100% for both types of cellulosic support.

Effect of Metribuzin concentration
To confirm the effectiveness photocatalysis, it is also necessary to study the photocatalytic degradation rate for different initial pollutant concentrations. The effect of the initial metribuzin concentration on the rate of pollutant degradation was studied by varying the initial concentration from 2.5 to 20 mg/L. All experiments were carried out with both types of photocatalytic support: TiO2 PC-500 supported on cellulosic (media 1048) or impregned impregnated on glass plates. The obtained results (Fig.4) show that the Metribuzin elimination rate on media 1048 photocatalytic support is greater when the pollutant concentration is lower, whereas for the second glass support the influence of the pollutant concentration is not very noticed.  Table 2, metribuzin degradation follows the pseudo-first order kinetic model. The apparent rate constants for each pollutant concentration were determined by linear regression form (ln (C/C0) versus time). We note that these values increase slightly for the glass photocatalytic support, on the other hand, for the media 1048 the apparent constant decreases by increasing the initial concentration of pollutant.

Comparative study
A comparative study of the photocatalytic degradation of Metribuzin on the two photocatalytic supports: the TiO2 Tissue media 1048 and the TiO2 fixed on the glass plates is presented. Figure 5 shows the kinetics of metribuzin degradation under the same optimal operating conditions, namely: Q = 17.7 mL /s, C0 = 10 mg / L and free pH.

Figure 5.
Temporal evolution of Metribuzin elimination rate for both catalytic support under optimal conditions (C0=10 mg/L, free pH Q=17.7 mL/s) Figure 5 shows that the metibuzin elimination is more important with using TiO2 media 1048 as a photocatalytic support. The degradation is complete after only 180 minutes of treatment while on the glass support, the rate of abatement does not exceed 90% after 300 minutes of treatment. Table 3 summarizes the different kinetic constants.

Conclusion
The degradation of metribuzin on TiO2 fixed bed was investigated under the solar irradiation.
The results show that the photocatalytic process U.V./TiO2 seems to be very efficient on the removal of herbicide resistant to conventional techniques. Solar photocatalytic degradation rate can reach 100% during 180 min of treatment with using TiO2 immobilized on commercial support media 1048 This process was also developed using an impregnated TiO2 on glass plates, around 90% of degradation for more time (300min). For the similar degradation, the solar photocatalysis in fixed bed reactor can be recommended to avoid the filtration step, for its low energetic cost and a very good efficiency.