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According to Dantas et al. (2010), water treatment nowadays by means of ultraviolet (UV) irradiation is an established method for drinking water disinfection and has received recognition as a promising method for wastewater purification in the last year. Besides, it is an important remark that the photolysis pathways play an important role on the final organic compound removal even when UV is combined with other oxidants such as ozone or hydrogen peroxide and also the ability of it to degrade organic compounds by direct photolysis of photolabile compound as a consequence of light adsorption during water treatment (Dantas et al., 2010).
Photolysis is a phenomenon when a solution is irradiated by ultraviolet light radiation by referring to Bayarri et al. (2007). The mechanism of photolysis is based on the fact that the chemical species undergo photochemical reactions in which molecules are broken down into smaller molecules, merely through the absorption of light (Bayarri et al., 2007). Photolysis also causes hydroxyl ion from water to undergo chemical changes and become a free radical (âË†â„¢OH) called hydroxyl radical (Joseph et al., 2011). Photolysis process needs to be initiated with the supply of sunlight or artificial lamps where high, medium and low pressure mercury lamps and xenon lamps are most common used in generating this UV radiation (Bayarri et al., 2007). However, these types of UV sources have low removal rates for many environmental contaminants (Liu et al., 2011). On the other hand, black, germicide, solar simulating lamps are also among the variety of artificial radiation sources when working in laboratory (Bayarri et al., 2007).
There are many types of photolysis such as laser photolysis, water photolysis, UV photolysis and so on (Masao & Ichiro, 2002). But, UV photolysis has been used mostly to eliminate chlorinated and nitrated aromatics, phenols, halogenated aliphatics, end products of metal finishing, oil, and steel processing, and other hazardous wastes present in water (Legrini et al., 1993).
2,5-dichlorophenol (2,5-DCP) is the target pollutant in this study. 2,5-DCP contains of a chlorine atom on the second and the fifth carbon positions on the phenolic ring and is belongs to chlorophenols (CPs) group. CPs are polar compounds which the polarity decreases with an increase in the number of chlorine substitutions on the benzene ring (Joseph et al., 2011). In the list of 126 Priority Pollutants by U.S. EPA, CPs are included as the well-known hazardous chemical and are currently used for a wide range of domestic, agricultural and industrial purposes (Matafonova et al., 2011). The presence and accumulation of CPs in aquatic systems and biological organisms pose a severe toxicological risk due to the high toxicity and low biodegradation properties (Liu et al., 2011). Therefore, various methods such as biodegradation, adsorption, incineration, catalytic hydrodechlorination, and photocatalysis have been applied to the degradation of CPs in wastewater (Liu et al., 2011).
2,5-DCP only dissolves very slightly in water and can be found in polluted sources such as wastewater which is toxic to aquatic organism where may be hazardous to the environment (International Labour Organisation, 2011). Hence, according to International Labour Organisation, special attention should be given to soil contamination as bioaccumulation may occur and harms aquatic organisms. There are three conventional technologies that are used to treat CPs. Firstly, physical treatments with simply phase transfer techniques where the pollutant is not mineralised or destroyed but transferred from one phase (liquid) to another (solid) and usually requires further treatment before disposal (Joseph et al., 2011). In chemical treatments, the costs are normally high and reaction by-products and other chemicals are released into the environment as a result of the treatment process (Joseph et al., 2011). In biological treatments, the decomposition of CPs has been proven to be inefficient due to chlorinated phenols are resistant to biodegradation due to its stable molecular structures and toxicity (Joseph et al., 2011).
On the other hand, kinetic study is about ability to undergo chemical transformation upon photon absorption as an intrinsic molecular property and may drastically differ among various compounds (Czaplicka, 2006). Besides that, the possibility of photo-induced transformation of the intermediate products of the primary reactions increases when dissolved organic material has absorbed a high proportion of photons and can act as a potential photosensitizer and also arises singlet oxygen, superoxide ions, hydroperoxyl radicals, hydroxyl radicals and peroxy organic radicals in the aquatic environment by referring to Czaplicka (2006). Under these circumstances, photodegradation processes may include direct photolysis and reactions with reactive species where the kinetics and mechanism of these reactions is highly dependent on experimental conditions such as the wavelength of irradiation, pH of solution, concentration of dissolved oxygen and the presence of sensitizers (Czaplicka, 2006).
Therefore, in this study, the aim is to do a kinetic study of photolysis in aqueous solution by determining the degradation of 2,5-DCP using UV-A, UV-B and UV-C lamps and also to determine which lamp is better in degradation of 2,5-DCP.
The objectives of this study are:
1. To study the effect of UV-A, UV-B and UV-C lamps in the removal of 2,5-DCP in
2. To compare the photolysis effect on the degradation of 2,5-DCP using UV-A, UV-B
and UV-C lamps with the function of time.
1.3 Scope of Study
In this study, the photolysis of 20 ppm 2,5-DCP using UV-A, UV-B and UV-C lamps respectively has been investigated. This study was conducted in lab-scale Batch Photo-reactor set up with some modification as describe by Ahmad et al. (2004).
Spectrophotometry has been a valuable tool in quantitative analysis as it has been widely use by preparing a series of solution with known concentration to measure the absorbance of the analyte to prepare a calibration plot which is also known as Beer-Lambert law plot (Harvey, 2000). The absorbance value of 2,5-DCP was measured using UV-Vis spectrophotometer. Then, the concentration of the 2,5-DCP solution can be determined using calibration curve. The degradation kinetics was then calculated using the first and second order kinetics model.