Photo Catalytic Degradation Review

1828 words - 8 pages

INTRODUCTION
In Post-industrial era majority of the water bodies (surface water, ground water and river bodies) and industrial water .Most of them are toxic endocrine disrupting, causing genetic defects or causes cancer to humans, animals and aquatic life in general.

There are various ways for the treatment of wastewater, which are categorized as Physical which includes treatment by screening, sedimentation, floatation and filtration of the contaminants present in the water; The chemical method which comprises of precipitation, coagulation, carbon filtering and disinfection. The most widely used technique is a combination of physical and chemical processes which include activated carbon ...view middle of the document...

This review paper gives an summary of the understanding of the mechanism of heterogeneous, homogeneous photocatalysis & various equipment’s used in the process of photocatalysis ie (through photochemical AOP). The paper also analyses the demerits of the aforementioned process and the alternatives used to overcome them.

Chapter 2
Heterogeneous photo catalysis Mechanism
The semiconductor TiO2 has been widely employed as a photocatalyst for inducing a series of reductive and oxidative reactions on its surface. The catalyst acts as if it has got a distinct lone electron characteristic in its outer orbital. The mechanism of this type of photocatalysis can be summarized as below:-

When photon energy (hν) of greater than or equal to the band gap energy of TiO2 is illuminated onto its surface, usually 3.2 eV (λ < 400 nm) the lone electron will be photo excited to the empty conduction band in femtoseconds. The excitation leaves behind an empty unfilled valence band, and thus creating the electron-hole pair.
In the absence of electron scavengers, the photoexcited electron recombines with the valence band hole in nanoseconds with simultaneous dissipation of heat energy.

Thus, the presence of electron scavengers (like dissolved oxygen) is vital for prolonging the recombination and successful functioning of photocatalysis. The electron scavenger like dissolved oxygen gets reduced to superoxide in the conduction band and the pollutant (P) gets consecutively gets oxidized to (P+).
Without the presence of water molecules, the highly reactive hydroxyl radicals (OH.) could not be formed and impede the photodegradation of liquid phase organics. This was evidenced from a few reports that the photocatalysis reaction did not proceed in the absence of water molecules.

The reduced superoxide ion (O2-) and the hydroxyl radical formed after the hydrolyzing of water help in the degradation of the effluent (P+) to convert it into carbon dioxide and water. In case of aromatic pollutant compounds first hydroxylation by electrophilic aromatic substitution mechanism takes place. Further oxidation leads to ring opening which in turn converts itself into carboxylic acids and aldehydes. In certain cases these carboxylic acids undergo decarboxylation reaction to form harmless carbon dioxide and water. It is also worthwhile to note that the amount of successive oxidation of pollutant depends on the time for which reaction takes place as more reaction time leads to more surface activation of semiconductors which in turn forms more radical species which facilitate the degradation of the pollutant.

The physical mechanism of any heterogeneous photocatalytic process follows that of adsorption.
Firstly the mass transfer of the effluent from the bulk phase (liquid) to that of the semiconductors surface takes place.in the second step the pollutant gets adsorbed to the photon activated surface of the semiconductor. Then photocatalytic...

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