FULL PAPER * E-mail: fahim_uddin01@yahoo.com; Fax: 0092-021-9261330 Received August 9, 2009; revised October 30, 2009; accepted December 2, 2009. 748 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 748—754 Kinetics and Mechanism Study of Chemical Treatment of Methylene Green by Urea Ahmed, Tehseen a Uddin, Fahim* ,a Azmat, Rafia b a Department of Chemistry, University of Karachi, Karachi 75270, Pakistan b Department of Chemistry, Jinnah University for Women, Nazimabad, Karachi, Pakistan The kinetics and mechanism studies, for the reduction of methylene green (MG) by urea, in acidic and alkali media, were studied at λ max ＝652.8 nm by monitoring the depletion in MG concentration. The reaction was carried out by UV radiation, with variable dye concentration, reducing agent (urea), acid and base under different additive ions that are very common in dye waste water. The reduction followed pseudo first-order kinetics with respect to different anions, cations, dye, reductant and OH － ion concentrations. It was found that most of the cations tested showed the inhibitory effect on dye decoloration, due to the formation of insoluble precipitate and followed the order K ＋ ＞Na ＋ ＞Al 3＋ ＞Ca 2＋ ≈Mg 2＋ . Tested anions showed that the dye decoloration was significantly accelerated and followed the order Cl － ＞Br － ＞I － ＞ 3 NO - ＞ 2 4 SO - . A mechanistic model involving generation of a complex of dye with ions was proposed. Keywords methylene green (MG), urea, additive ion, precipitation, OH － ion, decoloration Introduction The release of colored compounds with the effluents from different industrial activities such as paper and pulp manufacturing, dying of cloths, leather treatment, printing, etc. into the environment is undesirable. This is not only because their color may affect the photosynthe- sis of aquatic plants but also they contain a substantial amount of organic dye macromolecules. Their break- down products may be toxic in nature and their removal from industrial effluents is a major environmental prob- lem. 1-5 In fact many of these dyes have shown hazardous properties and are toxic in nature due to aromatic rings present in them. Strong color is the most visual charac- teristic of textile waste water. Decoloration has become an integral part of the textile waste water treatment process. 6 Decolorization by organic reductant or adding decoloring chemicals is a common practice in the textile industry, which includes photolytic, photocatalytic and bio decoloration and degradation of various dye solu- tions. Advanced oxidation processes based on UV/H 2 O 2 and ozonation represent novel methods and show prom- ising results in degrading and reducing many organic dyes. Furthermore, these processes generally depend upon the generation of OH － radical, which ultimately increases the oxidation potential. Therefore, a non-biodegradable molecule breaks down into smaller ones, resulting in better color removal. These radicals can react with the dye molecule to undergo a series of reactions and as a result the dye molecule is finally re- duced or converted into simple smaller fractions. 7-9 A detailed literature survey has revealed that the oxidation of dyes with many oxidants was reported but no work has been carried out on aerobic reduction of MG with urea under the influence of different additive ions, which are the part of industrial waste effluent. Therefore in this study an attempt has been made to in- vestigate the reduction of MG with urea to decolorize the dye under different optimum conditions. Further- more, it also discusses the reduction kinetics and mechanism involving the role of different ions in bleaching of dye. Materials and methods All reagents obtained from E-Merck were used as received. The experiment was divided into different ses- sions, including preparation of solutions, kinetic meas- urements, data analysis and investigation of kinetic salt effect. All solutions were prepared in de-ionized water and diluted before use. Kinetics was monitored on a UV/visible Schimadzu 160A spectrophotometer. Preparation of sample solutions MG stock solution of 1×10 －4 mol•L －1 was prepared in 250 mL of deionized water. Dilutions of this stock solution were made with deionized water to obtain a series of dye solutions with varying concentrations of reagents. Kinetics measurements Kinetics was monitored by preparing three sets of