Photocatalytic Treatment of Wastewater

Advanced Oxidation Processes (AOPs)

The treatment process is defined as the Advanced Oxidation Processes (AOPs) where the photocatalytic oxidation of toxic organic compounds takes place using semiconductors. The AOPs have been proven to be suitable for the treatment of an extensive range of organic compounds/materials. The Isoproturon (IP) is a herbicide which is found in ground and waste waters, and it is has reported as difficult to eliminate using the standard biological waste treatment methods, however, the AOP has shown promising results for its elimination (Parraa et al., 2002).

There are various semiconductors such as oxides of zinc, iron and cadmium which can act as the semiconducting catalyst, however, Titanium Dioxide (TiO₂) has been considered to be the most viable and effective due to its ability to degrade/break a wide range of organic contaminants/pollutants. Furthermore, the properties of Titanium Dioxide such as low toxicity, high reactiveness, high chemical stability and lower costs make it an ideal photocatalyst (Fujishima et al., 2000). In this study, the degradation of a model dye will be studied using fixed Titanium Dioxide and Zinc Oxide catalysts.

Methodology

The heterogeneous photocatalytic reaction takes place with the adsorption of the photons. Semiconductor such as Zinc Oxide or Titanium Dioxide is exposed to energy greater than a photon which consequently results in the initialization of the excitation stage, where the electron movement takes place between the valence band and the conduction band. This movement results in electron promotion in the conduction band (ECB_CB^-), creating a powerful reducing agent, and a positive-hole formation in the valence band (h_VB⁺), creating a powerful oxidizing agent (Boroski et al., 2009). The oxidizing (h_VB⁺) can react with the organic materials found in the waste waters and successful oxidization of these materials results in the formation of Carbon dioxide and water. The h_VB⁺ can also react with the water to produce hydroxyl radicals (OH), which can also help in oxidizing the organic compounds in water. The following equations present the chemical reaction processes;

ZnO + hv à e_CB^- + h_VB⁺        (Equation-1)

And,

h_VB⁺ + R à Intermediates à C0₂ + H₂O  (Equation-2)

And also,

H₂O + h_VB⁺ à OH^- + H⁺         (Equation-3)

Which consequently gives,

OH^- + R à Intermediates à C0₂ + H₂O   (Equation-4)

Where R is represented as an organic compound.

The presence of dissolved oxygen in water to be processed is also of critical importance as it assures continuous photocatalytic degradation of the organic compounds. The oxygen present is reduced constantly by the e_CB^-, which neutralized the effect of oxidation of organic materials and therefore avoids electron accumulation/recombination process on ZnO/ TiO₂. The following explains this process;

e_CB^- + O₂ à O₂^-            (Equation-5)

The radiation source to be used for photocatalysis can include the use of sunlight and artificial Ultra Violet (UV) radiation. For experimentation purposes, the UV lamps of mercury are most commonly is the most commonly used source of photocatalysis radiation.