**Comparative Assessment of the Photocatalytic Efficiency of TiO2 Wackherr in the Removal of Clopyralid from Various Types of Water**

Biljana Abramović1, Vesna Despotović1, Daniela Šojić1, Ljiljana Rajić1, Dejan Orčić1 and Dragana Četojević-Simin2 *1Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Novi Sad, 2Oncology Institute of Vojvodina, Sremska Kamenica, Serbia* 

#### **1. Introduction**

164 Herbicides – Properties, Synthesis and Control of Weeds

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Many pyridine derivatives have found widespread application as herbicides. Because of their frequent use, chemical stability and resistance to biodegradation, they are encountered in waste waters, and, due to their hazardous effects on ecosystems and human health, their removal is imperative (Stapleton et al., 2006). With this in mind, we have recently paid significant attention to the study of the model compounds (Abramović et al., 2003; Abramović et al., 2004a, 2004b) and pyridine containing pesticides (Abramović et al., 2007; Šojić et al., 2009; Abramović & Šojić, 2010; Abramović et al., 2010; Guzsvány et al., 2010; Šojić et al., 2010a, 2010b; Banić et al., 2011).

Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid, CAS No. 1702-17-6, C6H3Cl2NO2, *M* = 192.00 g mol–1) (CLP) is a systemic herbicide from the chemical class of pyridine compounds, i.e., pesticides of picolinic acid. It has been used effectively for controlling annual and perennial broadleaf weeds in certain crops and turf. It also provides effective control of certain brush species on rangeland and pastures. The acidic form of CLP and three CLP salts (triethylamine, triisopropylamine, and monoethanolamine), which are very soluble in water, are commonly used in commercial herbicide products. Its chemical stability along with its mobility allows this herbicide to penetrate through the soil, causing long-term contamination of the ground water, as well as surface water supplies (Cox, 1998; Huang et al., 2004; Donald et al., 2007; Sakaliene et al., 2009). Due to these properties, CLP has recently been reported to occur in drinking water at concentrations above the Permitted Concentration Value of 0.1 μg L–1 for an individual pesticide (EU directive 98/83/EC). Although the occurrence of CLP in surface, ground and drinking waters has been widely reported, there are only a few studies concerning with its photocatalytic removal from water (Šojić et al., 2009; Šojić et al., 2010a, 2010b; Tizaoui et al., 2011). These studies showed that the degradation of this herbicide takes place most effectively in the presence of Degussa P25 as photocatalyst. However, several recent studies of photocatalytic activity reported that some cosmetic pigments (TiO2, Wackherr's

Comparative Assessment of the Photocatalytic Efficiency

**2.2 Photodegradation procedures** 

HCO

scavenger.

**2.3 Analytical procedures** 

**2.3.1 Kinetic studies** 

of TiO2 Wackherr in the Removal of Clopyralid from Various Types of Water 167

DDW Tap water Danube river

Parameter Water type

Table 1. The physicochemical characteristics of the analysed water types.

pH 6.5 7.3 7.8 El. conductivity at 25 0C (S mL–1) 2.9 516 365 TOC (mg L–1) 1.04 1.80 5.60 Carbonate hardness (odH) 0.37 13.06 8.36

<sup>3</sup> (mg L–1) 285 182

The photocatalytic degradation was carried out in a cell made of Pyrex glass (total volume of ca. 40 mL, liquid layer thickness 35 mm), with a plain window on which the light beam was focused. The cell was equipped with a magnetic stirring bar and a water circulating jacket. A 125 W high-pressure mercury lamp (Philips, HPL-N, emission bands in the UV region at 304, 314, 335 and 366 nm, with maximum emission at 366 nm), together with an appropriate concave mirror, was used as the radiation source. Irradiation in the visible spectral range was performed using a 50 W halogen lamp (Philips) and a 400 nm cut-off filter. The outputs for the mercury and halogen lamps were calculated to be ca. 8.8 × 10–9 Einstein mL–1 min–1 and 1.7 × 10–9 Einstein mL–1 min–1 (potassium ferrioxalate actinometry), respectively. In a typical experiment, and unless otherwise stated, the initial CLP concentrations were 1.0 mM, and the TiO2 Wackherr loading was 2.0 mg mL–1. The total suspension volume was 20 mL. The aqueous suspension of TiO2 Wackherr was sonicated (50 Hz) in the dark for 15 min before illumination, to uniformly disperse the photocatalyst particles and attain adsorption equilibrium. The suspension thus obtained was thermostated and then irradiated at a constant stream of O2 (3.0 mL min–1). During the irradiation, the mixture was stirred at a constant speed. All experiments were performed at the natural pH (~ 3.5), except when studying the influence of the pH on the photocatalytic degradation of the substrate. In the investigation of the influence of electron acceptors, apart from constant streaming of O2, H2O2, KBrO3 or (NH4)2S2O8 was added to the CLP solution to make a 3 mM concentration. Where applicable, ethanol (400 L) was added as a hydroxyl radical

For the LC–DAD kinetic studies of the CLP photodegradation, samples of 0.50 mL of the reaction mixture were taken at the beginning of the experiment and at regular time intervals. Aliquot sampling caused a maximum volume variation of ca. 10% in the reaction mixture. Each aliquot was diluted to 10.00 mL with DDW. The obtained suspensions were filtered through a Millipore (Millex-GV, 0.22 µm) membrane filter. The absence of the CLP adsorption on the filters was preliminarily checked. After that, a 20 µL sample was injected and analysed on an Agilent Technologies 1100 Series liquid chromatograph, equipped with a UV/vis DAD set at 225 nm (absorption maximum for CLP), and a Zorbax Eclipse XDB-

''Oxyde de titane standard'') are even more efficient than TiO2 Degussa P25 in the photodegradation of phenol (Rossatto et al., 2003; Vione et al., 2005) and herbicides with a pyridine ring (Abramović et al., 2011).

The aim of this work was to study the effect of water type (double distilled (DDW), tap and river water) on the efficiency of TiO2 Wackherr toward photocatalytic degradation of CLP. First of all, the study is concerned with the transformation kinetics and efficiency of photocatalytic degradation of CLP in DDW. The study encompasses the effects of a variety of experimental conditions such as the effect of the type of irradiation, catalyst loading, the initial concentration of CLP, temperature, pH, presence of electron acceptors, and hydroxyl radical ( OH) scavenger on the photodegradation kinetics in DDW. The results were compared to the most often used TiO2 Degussa P25. An attempt has also been made to identify the reaction intermediates formed during the photo-oxidation process of CLP, using the LC–ESI–MS/MS method. The cell growth activity of CLP alone or in the mixture with its photocatalytic degradation intermediates was evaluated *in vitro* in rat hepatoma and human fetal lung cell line, using colorimetric Sulphorhodamine B assay. Finally, the matrix effect of river and tap water on photocatalytic removal of CLP was also studied.
