Removal of azo dyes from aqueous solutions using calix[4]arene and b-cyclodextrin Aydan Yilmaz a , Elif Yilmaz a , Mustafa Yilmaz a, * , Richard A. Bartsch b a Department of Chemistry, Selcuk University, 42031 Konya, Turkey b Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA Received 11 November 2005; received in revised form 9 January 2006; accepted 13 January 2006 Available online 9 March 2006 Abstract A calix[4]arene-based oligomer 5 and a b-cyclodextrin polymer 6 were synthesized by the condensation of p-tert-butylcalix[4]arene and b- cyclodextrin, respectively, with hexamethylene diisocyanate. In their batch sorption of selected azo dyes, the b-cyclodextrin polymer 6 was found to be a better azo dye sorbent than the p-tert-butylcalix[4]arene-based oligomer 5. In addition, oligomer 4 containing pendant calix[4]- arene units with crown-6 functionalities on the lower rim was synthesized and its complexation of the azo dyes was investigated by liquideliquid extraction. The calix[4]crown-6 oligomer 4 showed the highest affinity for the azo dyes. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: b-Cyclodextrin; Calix[4]arene oligomer; Azo dye; Calix[4]crown-6 1. Introduction Azo dyes represent a major group of all the dyes produced world-wide [1,2] and are important colorants having extensive applications in textiles, papers, leathers, gasoline, additives, foodstuffs and cosmetics [3]. Approximately 10e15% of the overall production of azo dyes is released into the environ- ment, mainly via wastewater [4]. This is very dangerous be- cause some of the azo dyes and their degradation products have a toxic, mutagenic or carcinogenic influence on living or- ganisms. Therefore, the azo dye-containing wastewater should be treated before discharge [2]. Reduction of azo dyes leads to the formation of aromatic amines. Aromatic amines are gener- ally not degraded and accumulate under anaerobic conditions [5], with the exception of a few aromatic amines characterized by the presence of hydroxyl and/or carboxyl groups [6]. Some aromatic amines pose health hazards to human beings either by direct contact and through the environment. The release of aromatic amines from consumer products, such as clothing and leather, has been reported [7]. Hence, it is necessary to en- sure that our clothing, footwear, and other personal wear items are free from carcinogenic amines. Although azo dye mole- cules are biologically inactive, microorganisms in skin and in the environment can cleave the azo linkages and release the respective amines [8e10]. In view of these findings, some member states of the European Union (EU) have intro- duced a ban on azo colorants in consumer goods, which might release any of 20 listed harmful aromatic amines. At present, the German method DIN 53316 is the most accepted analytical method for the determination of azo colorants in leather and dyes [11]. In recent years, much attention has been paid to chemical separation techniques and the design and synthesis of new ex- traction reagents for ions and molecules. This attention results from environmental concerns and efforts to save energy and enhance recycling at the industrial level. In this respect, supra- molecular chemistry has provided important avenues in the search for molecular structures that can serve as building blocks for the production of sophisticated molecules by an- choring functional groups oriented in such a way that they * Corresponding author. E-mail address: myilmaz@selcuk.edu.tr (M. Yilmaz). 0143-7208/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2006.01.011 Dyes and Pigments 74 (2007) 54e59 www.elsevier.com/locate/dyepig