Studies on the effect of inorganic salts on decolouration of acid dye effluents by ozonation M. Muthukumar, N. Selvakumar* Department of Textile Technology, A.C. College of Technology, Anna University, Chennai, 600 025, India Received 21 July 2003; received in revised form 12 October 2003; accepted 5 November 2003 Abstract This paper deals with the effect of presence of inorganic salts namely, sodium chloride and sodium sulphate and their concentration on decolouration of acid dye effluents by ozonation. Studies have been conducted at various pH levels on synthetic dye effluents, having a concentration of 500 mmol, prepared using Acid Red 14, Acid Violet 12, Acid Brown 14 and Acid Blue 113. Salt concentration of 0, 5 and 15 g/l and pH of 3, 7 and 11 were used. It has been found that salt content in the effluent increases the decolouration time of the acid dye effluents and it is more at neutral pH than at acidic and alkaline pH. Higher the salt content more is the decolouration time and among the salts, sodium chloride requires more time than sodium sulphate. Diazo acid dyes are found to take more time for decolouration than monoazo dyes, irrespective of the nature and quantity of the salt and pH used. # 2004 Elsevier Ltd. All rights reserved. Keywords: Acid dyes; Inorganic salts; Sodium chloride; Sodium sulphate; Ozonation and colour removal 1. Introduction Salts find major usage in the application of variety of dyes on textiles. They can act as exhausting as well as retarding agents. For exam- ple, they act as exhausting agents in the applic- ation of direct dyes on cellulosic fibres, acid milling dyes on wool and reactive dyes on cotton and as retarding agents in the application of leveling acid dyes on wool and basic dyes on acrylic. Sodium chloride and sodium sulphate are the very com- monly used exhausting and retarding agents. The quantity of these agents required for dyeing varies with the chemical nature of the dye and the fibre used. In general, the quantity varies from 5 to 60 g/l. The addition of salt in the dye bath increases the pollution load on the effluent generated as well as has an effect on various effluent treatment processes. The survey of literature revealed that there are number of studies on ozone treatment of effluent, which were carried out in presence of additives. The effect of additives such as silicone type anti- foaming agents, salt, sodium silicate, surfactants, thickeners and urea in reactive dye bath, chelating agents such as ethylene diamine tetra acetic acid, diethylene triamine penta acetic acid in disperse dye bath and gar gum in acid dye bath on ozon- ation have been reported by different authors [1–4]. 0143-7208/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2003.11.002 Dyes and Pigments 62 (2004) 221–228 www.elsevier.com/locate/dyepig * Corresponding author. E-mail address: nselva@annauniv.edu (N. Selvakumar).