Effect of Oxidants on the Photocatalytic Degradation of Methylene Blue and Congo Red Under Sunlight

Abstract— As the need for clean drinking water grows on a daily basis, the need for an effect and cost friendly solution for the removal of dyes from water is essential. There are numerous methods for the removal of dyes from waste water. Each of these methods have its own advantages and disadvantages.

Photocatalytic degradation of dyes is a promising method. The addition of a strong oxidant will increase the photocatalytic effects of catalysts. The radiation was simulated sunlight in a light proof chamber that was held at constant temperature of 25oC. The photocatalyst that was used in these experiments are commercial zinc oxide (ZnO) powder. Zinc oxide can be excited under sunlight. The experiments was conducted with two different dyes; Congo red, a diazo dye and methylene blue, a cationic dye. The oxidants that were chosen are liquid hydrogen peroxide (H2O2), peroxydisulfate (S2O82-) which will dissociate from the salt, potassium persulfate (Na2S2O8) and Potassium peroxymonosulfate (KHSO5), also referred to as Oxone. The Oxone is supplied as a triple salt with the following formula 2KHSO5·KHSO4·K2SO4.

Compared to the congo red, the zinc oxide degrade the methylene blue much better. The addition of most oxidants greatly increased the degradation effect of the photocatalyst. The performance of the photocatalyst will be negatively impacted if the concentration of oxidants is too high.

From the mentioned oxidants, S2O82- performed the best followed by KHSO5. The peroxide performed the worst due to over saturation of OH¬-1 ions. Due to the ionic nature of the dyes, the ZnO adsorped much more congo red in comparison to methylene blue. The experiments will support the fact that photocatalytic degradation of dyes under sunlight and with the addition of an oxidant, is a very effective process for the removal of dyes from contaminated water.

Keywords- photocatalyst, oxidan ts, sunlight, ZnO,, H2O2, S2O82- and KHSO5

INTRODUCTION

Water also plays a very important role for the physiology of human being as well as for the industrial sector. Most water that runs through an industrial process gets contaminated with chemicals and metals and has to go through expensive treatment processes. Amongst these, contaminant dyes are the most revealing. Improving the effectiveness of these methods and minimize the discharge of contaminants, will increase profits and also benefit the environment.

It is estimated that over 700 000 tons of synthetic dyes are annually produced worldwide [1]. According to Ogugbue & Sawidis [2], the textile industry consumes most of the dyes. Approximately 200 000 tons, almost 30%, of dyes are lost due to the ineffective dying process and most of the lost dyes are disposed as waste water. The dying of textile also consumes a lot of clean water and the ratio of dye to water is around 1:30. It is clear that