Factors Influencing Imazapyr Herbicide Removal from Wastewater Using Photocatalytic Ozonation

This study investigates the degradation of imazapyr herbicide from wastewater by photocatalytic ozonation using TiO2 as a semiconductor. Effects of operational parameters on imazapyr removal efficiency including TiO2 dosing, initial herbicide concentration and pH were also studied. Obtained results showed that more than 90% of removal efficiency representing the disappearance of imazapyr was maintained until 7 μM in the presence of 200 mgL–1of UV100-TiO2. Otherwise, the degradation of imazapyr followed the first-order kinetics with a photocatalytic rate constant of 0.247 min–1, and complete degradation was achieved within 20 min using photocatalytic ozonation for 5 μM of Imazapyr at pH 7.


Introduction
Persistent organic pollutants such as pesticides have attracted global environmental concerns in recent decades due to its adverse impacts on the environment and public health.] Imazapyr, 2-(4-methyl-5-oxo-4-propan-2-yl-1Himidazol-2-yl) pyridine-3-carboxylic acid, being a hetero aromatic molecule, 3 is a non-selective herbicide which belongs to the imadazolinone family.4] Therefore, it is likely suspected to contaminate groundwater. 5Elimination of imazapyr present in drinking water by treatment with ozone has been demonstrated to be unsuccessful since half of the initial compound remains in water after the process.The photocatalytic oxidation of imazapyr has been studied previously using commercial TiO 2 as well as newly synthesized mesoporous TiO 2 materials. 6However, more studies are still needed to better understand the effect of the oxidative approaches on the control of this herbicide.
In recent years, various technologies have been developed and tested in laboratory scales or pilot plants to remove various recalcitrant organic pollutants from wastewater in order to minimize the potential health risks associated with exposure to these chemical pollutants. 4] The photocatalytic ozonation have been studied by many studies worldwide and the high efficiency of this treatment has been explained by a synergistic effect between ozonation and photocatalysis.The photogenerated electrons can react with ozone molecules generating ozonide radicals while decreasing the possible recombination of electronehole pairs.Pizarro et al., 5 and Ibrahimi et al., 6 reported that among six different advanced oxidation processes, photocatalytic ozonation was the most efficient for completing mineralisation of 4-chloronitrobenzene.These promising techniques are used for the treatment of contaminated water and wastewater to evaluate their capability in the decomposition of pollutants such as pesticides and to assess the treatment efficiencies of these combinations. 72] Understanding the effects of these factors on the photocatalytic degradation efficiency have an importance when designing a sustainable and efficient technique for wastewater treatment.
Based on this background informations, the present study aimed at performing a comprehensive evaluation of the capacity of photocatalytic ozonation to remove imazapyr from wastewater.
The laboratory studies reported in this paper investigates the capacity and the applicability of TiO 2 -photocatalytic ozonation in removing imazapyr herbicide from wastewater.Operational parameters such as TiO 2 initial concentration, initial herbicide concentration and pH were also investigated.

Material and Methods
Imazapyr herbicide, (95%) was purchased from American Cyanamid Company.TiO 2 Hombikat UV100 (99%) was purchased from Sachtleben Chemie in powdered form.The chemical structure of imazapyr herbicide is shown in figure 1. Potassium indigo trisulfonate was purchased from Riedel-de Hahn AG.The other chemicals used in the experiments were purchased from Riedel-de Hahn AG.They were all of analytical grade and used without further purification.All solutions were prepared using Milli-Q water at room temperature collected from Milli-Q apparatus (Millipore, Bedford).
Sample analysis from the study of the photocatalytic degradation of imazapyr was carried out using a mass spec-trometry coupled to electrospray ionization system (ESI) for a qualitative and quantitative analysis.MS analysis was performed on Bruker Esquire 3000 plus mass spectrometer equipped with an ESI interface and an ion trap (Bruker-Daltonics Analytik GmbH Bremen, Germany).The ESI probe tip and capillary potentials were set at 2.5 kV and 25 V, respectively.The concentration of aqueous ozone was determined by Spectrometric method indigo trisulfonate according to Sanchez et al., 13 and mouradi et al. 17 The absorption analysis of the indigo was monitored by Shimadzu UV-160A UV-Vis Spectrophotometer at 600 nm.
Titanium dioxide (100 % anatase, average particle size of 10 nm and BET Method-Brunauer, Emmett and Teller [BET] surface of > 250 m 2 g −1 ) was used without any pre-treatment.
In a typical reaction, 200 mg/L of TiO 2 were added to 500 mL of 5 µM of imazapyr solution in a double walled cylindrical photoreactor (figure 2).Then, the solution was exposed to ultrasonic treatment for 2 minute in order to suspend the catalyst.Immediately afterwards, the stirring was started and maintained over 1h in the dark to ensure the adsorption equilibrium between the solution and the catalyst particles.The imazapyr degradation experiments were carried out in a simple photocatalyzed ozonation reactor, as presented in figure 2, with combining photocatalysis and ozonation.
The irradiation experiments were carried out under light generated by a medium pressure mercury lamp at 150W in a Duran cell (λ > 300 nm), placed inside the reactor.A cooling water system was set up to prevent overheating of the lamp and of the solution.Samples were taken every 10 min and filtered two times in order to remove all the catalyst.The photocatalytic ozonation degradation of Imazapyr herbicide was investigated in an aqueous suspension 1: 3 acetone/water mixture in the presence of pure titanium used as a catalyst.Imazapyr showed important  with m/z 284) and their degradation was followed as a function of time.
For this series of experiments the catalyst and the ozone doses were kept constant at 200 mg/L and 10 mg/L respectively.The irradiation time was fixed at 10 min for all samples after the adsorption step under dark conditions.

Effect of pH Influence on Imazapyr Degradation
Knowledge of the kinetics required to assess the efficiency of systems engineered for the oxidation of a variety of pollutants.Reliable kinetic studies require obvious substrate decay measurements.Thus, for comparison of the efficiency of these treatment processes, kinetic studies of imazapyr decomposition were carried out.
In all experimental runs, imazapyr concentration was found to decrease with irradiation time.A first order kinetics fitting of the thus obtained concentration vs. time plots allows to calculate the respective first order rate constants.Based on the exponential decay of concentration of imazapyr, the photoactivity profile was fitted assuming a first order reaction.
(1)  In which C is the concentration of imazapyr at time t, Co is the initial concentration, and k is the observed rate constant.
Many authors have reported that the kinetic behavior of photocatalytic reaction can be described by a modified Langmuir-Hinshelwood model, Atitar et al., 18 and Djerdj et al. 19 The influence of pH on the effectiveness of imazapyr degradation by photocatalytic ozonation is shown in figure 5.The degradation experiments were carried out at pH values of 3, 7, and 10.Imazapyr removal rate reached a maximum at pH 7 with a first order rate constant of 0.247 min −1 .However, for pH 3 and 10, the photocatalytic activity decreased appreciably.The rates constant at pH 3 and pH 10 are 0.107 min −1 , 0.134 min −1 respectively (Table1).The study of pH influence on the photocatalytic ozonation process would be helpful to understand its underlying mechanism and would help obtaining a higher degree of removal.Based on the exponential decay of imazapyr concentration, the degradation by photocatalytic ozonation profile was fitted assuming a first order reaction model.][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The results, shown in figure 5, demonstrates that the optimal conditions for imazapyr degradation with photocatalytic ozonation is neutral pH.8][19][20][21][22][23][24][25][26][27] The efficiency of photocatalytic ozonation is mainly attributed to the formation of more reactive and non-selective hydroxyl radicals in the oxidation medium, which react with almost all organic molecules at a rate of 10 6 -10 9 M -1 s -1 . 18-25-37The photogenerated electrons can react with ozone molecules generating ozonize radicals while decreasing the possible recombination of electron hole pair, so better electron-whole separation.In addition to the synergistic effects which occur during photocatalytic ozonation compared to simple photocatalysis in presence of oxygen.Bougarrani et al., 36 and Wang et al., 24 also reported similar results for the mineralization of aniline, dibutyl phthalate, respectively.

Conclusions
This study demonstrated the ability of photocatalytic ozonation using TiO 2 as semiconductors for the degradation of imazapyr herbicide as organic pollutants.Imazapyr degradation is strongly influenced by the operating parameters such as TiO 2 concentration, initial imazapyr concentration and pH.Under optimized conditions (TiO 2 dose of 200 mg/L, 5µM of initial imazapyr concentration and pH 7), up to 95% of imazapyr removal was achieved within 20 min with removal rate constant of 0.247 min −1 .Results of the present investigation suggest that photocatalytic ozonation using TiO 2 is efficient for the removal of imazapyr form wastewater.The high efficiency of photocatalytic ozonation could be explained by a synergistic effect between ozonation and photocatalysis.The photogenerated electrons can react with ozone molecules generating ozonize radicals while decreasing the possible recombination of electron hole pair, so better electron-whole separation.
Therefore, the application of photocatalytic ozonation under the optimal conditions is recommended for organic pollutants treatment such as imazapyr herbicides in order to promote environmental and human health protection.

Table 1 :
Rate constant of imazapyr degradation with photocatalytic ozonation for different pH.