R ESEARCH ARTICLE doi: 10.2306/scienceasia1513-1874.2015.41.049 ScienceAsia 41 (2015): 49–54 Decolourization of an azo dye in aqueous solution by ozonation in a semi-batch bubble column reactor Che Zulzikrami Azner Abidin * , Muhammad Ridwan Fahmi, Ong Soon-An, Siti Nurfatin Nadhirah Mohd Makhtar, Nazzery Rosmady Rahmat School of Environmental Engineering, Universiti Malaysia Perlis (UniMAP), 02600 Arau, Perlis, Malaysia * Corresponding author, e-mail: zulzikrami@unimap.edu.my Received 25 Mar 2014 Accepted 13 Feb 2015 ABSTRACT: The oxidative degradation of the azo dye Reactive Red 120 by ozonation was investigated. The decolourization was carried out by bubbling ozone at the bottom of a bubble column reactor containing the dye solution. The colour, chemical oxygen demand, and total organic carbon removal were evaluated, and the contaminants were characterized based on the changes in UV-Vis and FT-IR spectra. It was observed that changes of UV-Vis spectra represent the disappearance of both azo and aromatic groups, which causes the colour removal. FT-IR analysis indicated that ozonation shifts the functional groups in the azo dye which results in decolourization, a decrease in aromaticity, and an increase in acidity. The results indicate that the chromophore is destroyed and partially mineralized to small fragments during ozonation. The alkaline pH was favourable to decomposition by ozonation, initiated by the formation of the hydroxyl radicals. The oxidation followed first-order kinetics and the completed decolourization confirmed the capability of ozonation to cleave the azo bond from the dye. KEYWORDS: chemical oxygen demand (COD), total organic carbon (TOC), decolourization kinetics INTRODUCTION Synthetic dyes have increasingly been used in the textile and other dyeing industries as they are easy and cheap to synthesize, generate greater colour variety than natural dyes, and are highly stable (i.e., resistant to light, temperature, and detergent) 1–3 . There are almost 700 000 t of approximately 10 000 types of dyes and pigments produced annually worldwide, of which about 20% was assumed to be discharged from textile industries 4 . The azo dyes like Reactive Red 120 represents the largest and most important class, that accounted for over 50% of all commercial dyes 5, 6 . Zollinger 7 described the success of azo dyes is due to the simple synthesis procedure, great structural diversity, high molar extinction coefficient, and medium-to-high fastness properties to both light and wetness. The release of these compounds into the en- vironment is however undesirable because many azo dyes and their by-products are toxic and mu- tagenic 8, 9 . Even though the concentration of dyes in wastewaters is low, it often receives attention due to strong colour visible even at very low dye concentrations. Residual colours are difficult to remove completely by physical, chemical, or biolog- ical treatment processes. Moreover, the wastewater containing residual colour and compounds that are not easily biodegradable may cause environmental concerns and aesthetically displease 10 . Without sufficient treatment, dyes are stable and persist in the environment for an extended period. The treatment of dye-containing wastewater poses considerable problems to the wastewater treatment industry. Treatments using physical, chemical, and biological procedures or its combi- nations are well established methods for colour removal 11 . Conventional physical or chemical treat- ment methods such as coagulation, flocculation, fil- tration, adsorption, ion exchange or ultra-filtration, although efficient, often produce sludge or solid waste which requires further disposal 12–15 . Biolog- ical treatment systems are often inefficient due to low biodegradability of dyes 13 . The chemical oxidation treatments involves the use of oxidants such as ozone, hydrogen peroxide, permanganate, chlorine, and oxygen, to change the chemical composition of a compound or a group of compounds such as dyes 16 . Among these oxi- dants, ozone is the most widely used as it is highly reactive and it can remove colour efficiently 17 . In a process called selective oxidation, ozonation can effectively decolourize the wastewater by break- ing the conjugated azo bonds associated with the www.scienceasia.org