Removal of Herbicides from Water Using Heterogeneous Photocatalysis Case Study: MCPA Sodium Monohydrate

In this study, the herbicide MCPA sodium salt monohydrate (sodium (4-chloro-2 methylphenoxy) acetate has been studied as are presentative compound used in the agricultural field. Accordingly, direct photolysis and photocatalytic experiments under artificial irradiation simulating solar light in laboratorial conditions were performed. Photocatalytic experiments were performed using TiO2 dispersed powder and as an immobilized thin layer on the surface of blue glasses. The obtained results of photolysis showed a poor efficacy toward degradation of MCPA sodium monohydrate, with half-life (t1/2) 6931.5 min. While, the addition of TiO2 dispersed powder to the photocatalytic process enhances the process dramatically with (t1/2) equal to 36.5 min; furthermore, complete mineralization had been reached after approximately 4 hours, whereas the addition of TiO2 through immobilized system led to enhance the degradation rate with 2236 min. as t1/2. In spite of this, using TiO2 supported on glass substrates with more improvements could be a promising alternative to conventional TiO2 suspension, and provides a clean treatment method.


Introduction
The pollution of the aquatic environment by different contaminants such as (herbicides, fungicides, etc.) has gained increasing attention recently due to the frequent and wide uses of these chemicals in the agricultural activities [1]. Using pesticides has a lot of benefits such as increasing crop production, suppressing plant and animal pests and to protect agriculture products [2]. However, pesticides even if applied at the recommended doses, can induce environmental pollution especially for groundwater and surface waters [3] [4]. According to the European Water Framework directives (WFD), the amount of a single pesticide compound in drinkable water cannot exceed 0.1 µg·L −1 , and the total pesticide content must remain lower than 0.5 µg·L −1 [5]. Therefore, such contaminants should be removed by using nonconventional treatment processes to reduce the risk of water pollution. During last decade advanced oxidation processes (AOPs) have been classified as a promising way to decrease the content, or even to remove completely the pesticides from water [6] [7]. By using AOPs, the organic compounds can be completely mineralized to carbon dioxide and water mostly by hydroxyl radicals (HO . ) [8]. AOPs include several technologies such as photolysis, ozonation, photocatalysis, photo-Fenton and sonolysis [9]. Among AOPs techniques, heterogeneous photocatalysis is the most applied technique in the last decades regarding organic pollutants removal from water [10] [11].
As reported by Zhu et al. (2005), during the heterogeneous photo-catalysis process, dispersed solid particles of semiconductor efficiently absorb large fractions of the UV spectrum, and they generate chemical oxidants from dissolved oxygen or water in situ, these chemical oxidants activate the degradation of contaminants until the total mineralization [10].
Among the various semiconductors employed, TiO 2 is the most preferable material for the photo-catalytic process [11] due to its high photosensitivity, non-toxic nature, large band gap, chemical stability and lower cost [12]. In heterogeneous, photocatalysis titanium dioxide could be used in different forms, as a suspended powder or immobilized over glass substrates.
In this study, the herbicide MCPA sodium salt monohydrate (sodium  The aims of this work are to evaluate the efficacy of two oxidation processes (Photolysis and heterogeneous photocatalysis using a solar simulator light) towards the removal of MCPA sodium monohydrate from aqueous phase. As well, to evaluate the efficiency of using TiO 2 immobilized on blue glass slabs. This approach aims to circumvent the need for filtration to recover the catalyst from the reaction mixture and decrease the operational costs necessary to recover the powder.

Materials
The herbicide MCPA sodium salt monohydrate (sodium (4-chloro-2 methylphenoxy) acetate pure standard (99% purity) was purchased from Sigma-Aldrich Corporation (USA) and used as received. Formic acid and HPLC grade Acetonitrile were purchased from Aldrich and used as received. Water was Milli-Q quality. TiO 2 P-25 from Degussa (anatase/rutile = 3.6/1, surface area 50 m 2 /g, non-porous) was used for photo-catalytic experiments. Grafted TiO 2 thin blue glasses were obtained from Pilkington (UK) ( Figure 2). All the solutions were daily prepared in ultra-pure water from a Millipore purification system. The pH of the solution was monitored using a Basic pH Meter from Denver Instrument Company. In the experiments with heterogeneous was separated by filtration (through 0.2 μm membrane filters, Schleicher and Schuell, Germany. Cat. No. 10462200) before analysis of MCPA sodium salt monohydrate.

Equipments
Photolysis, photocatalysis with TiO 2 powder and photocatalysis with immobilized TiO 2 on thin blue glass experiments were accomplished using batch reactor system, this system consists of: 1) Suntest CPS + Solar Simulator (Heraeus Instruments, Germany) equipped with a xenon lamp, temperature sensor and water-cooling circuit. The xenon lamp was filtered by an optically stable borosilicate UV filter (Atlas Material Testing, France) delivering a light emission spectrum similar to that of the sun with a UV cut-off at 290 nm. The pyrex reactor that contains the solution placed inside the chamber of the suntest device. 2) Pyrex batch reactor. The capacity of the reactor is 300 ml. Its outer perimeter reactor is covered by aluminum foils and only the upper surface is exposed to radiation.
3) Magnetic stirrer device (Falc F20 Mini Magnetic stirrer. Progen Scientific, Merton. London) is employed to maintain a continuous stirring for the solution inside the pyrex reactor during the experiments.
A schematic drawing of the batch reactor system is shown in (Figure 3).

Analytical Method
In batch reactor system, samples of 2 mL were taken at determined time intervals. Samples of photocatalysis/TiO 2 powder experiments were filtered through a 0.2 µm filters to remove TiO 2 particles. Changes in the concentration of each drug were observed from its characteristic absorption at selected nm, using HPLC system through the following method:

Characterization of the Pilkington Active TM Blue Glass
The elemental analysis for the grafted TiO 2 thin Pilkington blue glasses was performed by using scanning electron microscope. Some cross sections obtained from the Pilkington Active TM Blue glass were analyzed. The thin sections were coated with a 30 nm-thick carbon films. Semi quantitative analyses of the elemental composition of the different layers were obtained using a Ge ED Oxford-Link detector equipped with a Super Atmosphere Thin Window. Operating conditions of the SEM were: 15 kV accelerating potential, 500 pA probe current and about 10 mm of working distance (WD).
Thin sections of glass were prepared by the Department of Health and Environmental Science, Bari University. Samples were embedded in resin epoxy plugs and then polished.

Photocatalysis
Photocatalysis processes were carried out by using commercial TiO 2 powder and TiO 2 immobilized on thin blue glass.

1) Photocatalysis with TiO 2 powder
Working solutions of MCPA sodium monohydrate (25 mg·L −1 ) with 200 mg·L −1 TiO 2 were prepared, 250 mL of the solution was placed into well-closed reactor, then placed in a radiation field inside the Suntest (solar simulator) device. At specific time intervals, samples of 2.0 mL were taken and immediately filtered through a 0.2 μm filter, and analyzed by HPLC system according to the analysis method mentioned above. The aqueous solution containing drug was mixed continuously with magnetic stirrer during the experiment.
2) Photocatalysis with immobilized TiO 2 on thin blue glass As in the previous sections, solution of 25.0 mg·L −1 was prepared, and then 250 ml transferred to the glass pyrex reactor, before that the blue glass had been placed vertically on the perimeter of the inner wall of the glass pyrex batch reactor, then transferred to the suntest CPS (solar simulator) device, and exposed to the solar irradiation with continues mixing using magnetic stirring, samples were taken (2 mL for each sample) at determined intervals then filtered and injected in HPLC system according to the analysis method mentioned above.

Kinetics Study
The knowledge of reaction order is essential for finding the accurate integrated rate equation. By trying to fit data of various integrated rate equations, it is possible to verify the reaction order. Kinetic parameters were calculated using integrated equations describing zero-, first-and second-(Langmuir-Hinshelwood) order equations. The determination coefficient (R 2 ) was used to check the best fit.
Kinetic parameters (reaction order (n), determination coefficient (R 2 ), half-life (t 1/2 ), kinetic constant (k) were obtained by linear regression of logarithmic concentration values determined as a function of time according to the following equations [15] [16]. Zero-order: Second-order: As shown in (Table 1), TiO 2 is a component present only on the glass surface along with other metal oxides such as iron oxide, while in the core glass it is absent and other metal oxides are countable. As reported in the Pilkington patent [17], cobalt oxide is present in low amounts (less than 75 µg/g) but it was not detectable by surface analysis used. Cobalt oxide may confer the blue color to the glass.

Preliminary Experiments
Preliminary experiments in the dark showed that the drug dissolved in aqueous solution with and without catalyst after 48 hours was stable and no reactivity was observed at all.  Table 2. But in general, the degradation rate is still substandard.

Photolysis and Photocatalysis Processes
S. Khalaf et al.   Table 2. Kinetic parameters of the photolysis, photocatalysis using TiO 2 and photocatalysis using TiO 2 immobilized system for MCPA sodium monohydrate. k: rate constant, R 2 : correlation coefficient, t 1/2 : half-life.    Initial mineralization rate constants (k) were determined, for all processes, from the slope of (ln CA (t) ) vs t (time) plots where CA (t) are MCPA sodium monohydrate concentration at time t. It is also presented the half-life times (t), i.e. the time necessary to reduce 50% of initial MCPA sodium monohydrate concentration for each process. The half-life times are determined by interpolation from experimental data ( Table 2).

Identification of Transient Photoproducts
Determining the transient photoproducts that resulted from degradation processes is very important, since some of these photoproducts could be more

Conclusion
The results exhibited that solar light alone was insufficient to achieve a complete and fast removal for MCPA sodium monohydrate herbicide. While using titanium dioxide as a catalyst either as a powder or in immobilized form accelerated the photodegradation rate. Although TiO 2 in the powder form gives faster degradation rate but this is requires a post treatment stages to recover the catalyst from the reaction mixture. For that using TiO 2 immobilized system over blue glass slabs could be promising alternative to traditional titanium dioxide powder although it was showed slow degradation rate. We believe that with more modifications in grafted TiO 2 thin blue glasses such as increasing the concentration of TiO 2 on the sheets surfaces or changing the amount of oxides in the blue glasses the effectiveness of these substrates will be improved. All photo-degradation processes have followed a first order kinetic with t 1/2 36.