Photocatalysis sunlight

Studies have appeared where photolysis in natural bodies of water under normal sunlight conditions has been examined. For example, metolachlor was slowly photodegraded by sunlight in lake water, with a half-life of 22 days in summer and 205 days in winter (28). Addition of a 5% solution of dissolved organic matter to the water extended the half-Hves two to three times longer, depending on the season (see PHOTOCHEMICAL TECHNOLOGY, photocatalysis). ... 

Procedure 10% aqueous solution of potassium iodide, KI, when exposed to sunlight, liberated I2 due to the photolytic decomposition and gave blue colour with freshly prepared starch solution. The intensity of blue coloured complex with the starch increased many fold when the same solution was kept in the ultrasonic cleaning bath. As an extension of the experiment, the photochemical decomposition of KI could be seen to be increasing in the presence of a photocatalyst, Ti02, showing an additive effect of sonication and photocatalysis (sono-photocatalysis) However, the addition of different rare earth ions affect the process differently due to the different number of electrons in their valence shells. 

Since the discovery of photoelectrochemical splitting of water on titanium dioxide (TiOj) electrodes (Fujishima and Honda, 1972), semiconductor-based photocatalysis has received much attention. Although TiO is superior to other semiconductors for many practical uses, two types of defects limit its photoeatalytic activity. Firstly, TiO has a high band-gap (E =3.2 eV), and it can be excited only by UV light (k < 387 nm), which is about 4-5% of the overall solar spectmm. Thus, this restricts the use of sunlight or visible light (Kormann et al., 1988). Secondly, the... 

Minero, C. Aliberti, C. Pelizzetti, E. Terzian, R. Serpone, N. Kinetic studies in heterogeneous photocatalysis. 6. AMI simulated sunlight photodegradation over titania in aqueous media A first case of fluorinated aromatics and identification of intermediates, Langmuir 1991, 7, 928. 

Photocatalysis and photosensitization processes are common in natural waters and are of special importance because they can stimulate a transformation of molecules that resist direct photolysis, such as transparent species or chromophores, the reactive states of which are inefficiently populated by sunlight absorption. 

Photocatalysis by transition metals shows an effective self-cleaning ability of soils and waters by the degradation and complete mineralization of the dissolved organic matter. Moreover, the oxidation processes are driven by sunlight and atmospheric oxygen and do not pollute the environment by risky side-products (see section 9.3, Chapter 9). 

For example, certain soil components can act as adsorption sites for organic materials and facilitate their photodecomposition upon exposure to sunlight. In the case that the soil component is a semiconductor, this process can occur through the formation of electron-hole pairs (i.e., photocatalysis, see Chapter 10). Photochemical processes may then affect soil components and composition. 

Photocatalysis processes are environmental friendly, heading for sustainable development because their driving force is sunlight, their oxidizing agent is atmospheric molecular oxygen and most photodegradations result in complete mineralization of the DOM without any pollution by risky side-products. 

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