Photocatalytic degradation of Methylene Blue using a mixed catalyst and product analysis by LC/MS

https://doi.org/10.1016/j.cej.2009.11.017Get rights and content

Abstract

Cr–Ti binary oxide with 10% molar Cr3+ content (Ti–10Cr) was prepared by sol–gel method and characterized by using transmission electron microscope (TEM) and X-ray diffraction analysis (XRD). The photoinduced degradation of Methylene Blue (MB) dye in the presence of Ti–10Cr was studied with UV–vis absorption. The absorption intensity of the dye solution monitored at 668 nm decreased with time upon irradiation in the presence of the catalyst. A maximum degradation efficiency of 70% of the dye was achieved in the presence of Ti–10Cr catalyst at pH 7. Photocatalytic degradation of the dye followed the pseudo-second-order-kinetics. Degradation product analysis was done using the LC–UV/vis–MS technique and the results showed that the dye degradation is initiated by demethylation of the molecule. The intermediates structure was confirmed by tandem mass spectrometric analysis.

Introduction

Many industries such as textile, plastics, and paper and pulp generate streams of waste effluents which contain considerable amount of organic dyes. When these compounds are discharged to the main water bodies without any prior treatment, they can cause havoc to the ecological balance in the environment as these molecules have carcinogenic and mutagenic properties towards aquatic organisms and thus pose threat to human life at the end of the food chain [1], [2]. The conventional water treatment methods (adsorption, sedimentation, coagulation, etc.) are now being supplemented by advanced oxidation processes (AOPs). The widely used AOPs include photolysis, Fenton method, photofenton, ozonolysis, sonolysis, and photocatalysis [3], [4], [5], [6]. All these techniques are versatile in nature and have their own pros and con. However in photocatalysis, one can use the available source of UV radiation from sunlight and in the presence of a suitable catalyst (such as semiconductors), conduct a chemical reaction to degrade the organic pollutant present in wastewater.

Various semiconductors have been tested for their efficiencies towards dye degradation. Some of these include TiO2, V2O5, ZnO, WO3, CdS, ZrO2 and their impregnated forms [7], [8], [9], [10]. Titanium dioxide mediated photocatalytic oxidation has been applied more extensively for dye studies. This is mainly because of its low cost, stable nature and its optical absorption in the UV region. The use of TiO2 has also guaranteed good results in detoxification of water samples loaded with molecules like anilines, alcohols, and organochlorides [11], [12]. The activity of the catalyst is generally enhanced by adding a secondary dopant to it.

Methylene Blue (MB) is an intensely colored compound which is used in dyeing and printing textiles and is a common water pollutant. In this study an attempt was made to investigate the effect of operational parameters on the photocatalytic degradation of MB in the presence of Cr–Ti binary oxide with 10% molar Cr3+ (Ti–10Cr) catalyst. The degraded products were analyzed by using the LC–UV/vis–MS technique.

Section snippets

Experimental

The dye under investigation namely Methylene Blue (MB), with a labeled purity of more than 90% was obtained from Sigma–Aldrich and used as such. Deionized water was used to make the dye solutions of desired concentration.

Results and discussion

In the present work, decoloration and kinetic behavior of Methylene Blue (MB) was investigated followed by product characterization using the LC–UV/vis–MS technique. The structure of the dye is given in Table 1. The λmax value for this dye is 668 nm. For decoloration studies, the λmax of the dye was chosen for further investigations. Initially, experiments were carried out in the absence and presence of either UV light or the catalyst alone. The results showed that either UV light or catalyst

Conclusion

Cr–Ti catalyst with 10% Cr3+ content (Ti–10Cr) was prepared by sol–gel method and used to degrade Methylene Blue (MB) dye in the presence of UV light. The photodegradation of the dye molecules in the presence of this catalyst followed the second-order-kinetics with a degradation efficiency of 70%. The degraded products were analyzed by using the LC–UV/vis–MS technique and it was found that the dye initially undergoes demethylation resulting in the formation of some intermediate products. The

Acknowledgments

The authors would like to thank the UAE University Scientific Research Affairs Unit for supporting this work through research project number 02-03-2-11/09.

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