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Photocatalytic degradation of Metronidazole with illuminated TiO2 nanoparticles
© Farzadkia et al.; licensee BioMed Central. 2015
- Received: 15 September 2014
- Accepted: 15 April 2015
- Published: 21 April 2015
Metronidazole (MNZ) is a brand of nitroimidazole antibiotic, which is generally used in clinical applications and extensively used for the treatment of infectious diseases caused by anaerobic bacteria and protozoans. The aim of this investigation was to degrade MNZ with illuminated TiO2 nanoparticles at different catalyst dosage, contact time, pH, initial MNZ concentration and lamp intensity. Maximum removal of MNZ was observed at near neutral pH. Removal efficiency was decreased by increasing dosage and initial MNZ concentration. The reaction rate constant (k obs ) was decreased from 0.0513 to 0.0072 min−1 and the value of electrical energy per order (EEo) was increased from 93.57 to 666.67 (kWh/m3) with increasing initial MNZ concentration from 40 to 120 mg/L, respectively. The biodegradability estimated from the BOD5/COD ratio was increased from 0 to 0.098. The photocatalyst demonstrated proper photocatalytic activity even after five successive cycles. Finally, UV/TiO2 is identified as a promising technique for the removal of antibiotic with high efficiency in a relatively short reaction time.
- Titanium dioxide
Recently, several different types of emerging contaminants in water systems are known as new environmental hazards those need to be treated with suitable methods . As various pharmaceutical compounds have been used since the 1950s due to rapid population growth and development of medical science, several pharmaceutical compounds have been found in surface water, ground water and effluents from wastewater treatment plants. Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) has been widely used to treat infections caused by anaerobic bacteria, bacteroides and protozoa [2-4]. Residual concentrations of metronidazole (MNZ) in surface waters and wastewater are 1 ~ 10 ng/L [5,6]. As MNZ is non-biodegradable and highly soluble in water, it can be accumulated in the aquatic environment [7,8]. Elimination of MNZ from water system is an important issue considering its toxicity, potential mutagenicity and carcinogenity [7,8]. In order to remove MNZ, many techniques such as adsorption [9,10], reduction with nanoscale zero-valent iron particles , biological methods [12,13], ozonation technology , photolysis , Fenton and photo-Fenton processes , heterogeneous photocatalysis [15,17,18] and electro-Fenton process with a Ce/SnO2–Sb coated titanium anode  have been applied.
Adsorption is widely used method for the treatment of wastewater containing toxic organic compounds. However, it just transfer contaminants from water to a solid phase without any degradation [9,10,19,20]. Biological method is also known as one of the suggested techniques. However this method generally requires long periods for treatment [12,21]. Oxidation is a promising process but sometimes it is regarded as a limited process due to the formation of intermediates with higher toxicity than the parent compound [5,8,22]. Therefore near complete mineralization of MNZ is the most relevant option. For this purpose, advanced oxidation process (AOP) is regarded as a promising option to treat wastewater containing MNZ due to a complete mineralization of parent material as well as lack of selectivity [7,23]. Generally AOPs involve generation of hydroxyl radicals through UV/photocatalyst, UV/H2O2 and UV/O3 processes [24-26]. Among these methods, photocatalytic reaction using TiO2/UV can treat non-biodegradable organic compounds to biodegradable species [23,24,27]. Considering characteristics of the AOP, it can be used as pre- or post-treatment process in wastewater treatment because of its installation easiness in conventional wastewater treatment facilities [23,24,27].
Therefore, in the present work, P-25 TiO2 was selected as a catalyst in the photocatalytic removal of MNZ. Effects of several operational parameters including pH, TiO2 dosage and MNZ concentration on photocatalytic degradation of MNZ were investigated. Kinetic parameters for the photocatalytic degradation were obtained by application of the Langmuir–Hinshelwood (L–H) model. Finally, electrical energy per order (EEo) was obtained to evaluate cost-efficiency of the processes used in this research.
Experimental procedure and analysis
All experiments were repeated three times and the average values with error percents were reported.
Effect of TiO2 dosage
Biodegradability of MNZ was evaluated in this work. To measure the biodegradability, BOD5 and COD values were measured before and after UV irradiation and the ratio of BOD5/COD was used as a biodegradability indicator. After 3 h reaction time, removal efficiency of COD was above 97.6% at all catalyst dosages and the ratio of BOD5/COD increased from 0 to 0.098 as the dosage increased from 0.5 to 3 g/L. This result indicates that MNZ can be changed to more biodegradable products.
Effect of pH
Effect of initial MNZ concentration
Kinetic study and electrical energy determination
Pseudo-first order kinetic parameters and E Eo values for the photocatalytic degradation of MNZ at different initial MNZ concentrations (catalyst dose = 0.5 g/L and pH =7)
E Eo (kWh/m 3 )
1/k obs (min)
k obs (1/min)
[MNZ] 0 (mg/L)
The E Eo values for the removal of MNZ ([MNZ] 0 = 80 mg/L, catalyst dose = 0.5 g/L and pH =7)
E Eo (kWh/m 3 )
UV 8 W-alone
UV 8 W/TiO2
UV 125 W/TiO2
Comparison of different MNZ removal processes and reusability test
Based on the above experiments and analysis, mechanism of the photocatalysis could be proposed as following:
Comparison of photocatalytic degradation of MNZ
Catalyst dosage (g/L)
[MNZ ] 0 (mg/L)
Removal efficiency (%)
k obs (min −1 )
From the application of TiO2 for the photocatalytic degradation of MNZ in aqueous solutions, a maximum removal of MNZ was observed at neutral pH. Removal efficiency was decreased by increasing TiO2 dosage and initial MNZ concentration. Electrical energy per order was increased and reaction rate constant was decreased with increasing initial MNZ concentration. Photocatalytic activity was maintained even after five consecutive runs. Finally, UV/TiO2 is identified as a promising technique for the removal of MNZ with high efficiency in a relatively short reaction time.
The authors thank the Iran and Zahedan Universities of Medical Sciences, Iran for all of the support provided. Also, authors would like to thank Mr. Bonyani for the HPLC analysis in the laboratory of nutrition department.
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