Eco-friendly textile dyeing processes

Abstract

Environmental pollution is a major concern for textile industries. The generation of voluminous amounts of effluent and their disposal into the water bodies without proper treatment has lead to detrimental effects on environment. The textile effluent is characterized by high chemical oxygen demand (COD), biological oxygen demand (BOD), total dissolved solids (TDS), pH, and color. Fabric preparation steps like, desizing, scouring, bleaching, mercerizing, involve the use of various chemicals and plenty of water. Desizing process alone has been reported to account for about 50 % volume of effluent generated in textile industries. On the other hand, dyeing operation makes use of huge amount of dyes. During dyeing, in order to increase the amount of dye fixed to the cloth, auxiliary chemicals including sodium chloride and sodium carbonate are added in conventional process. In spite of this, large amount of dyes remain unconsumed in the process and find their way to the effluent along with the electrolytes added and pose serious threat to the environment by making the receiving water reservoirs unsuitable for agriculture and human consumption. The available end-of-pipe treatment procedures are either expensive or less efficient. Hence a large number of small-scale industries succumb to this problem. So finding an alternative eco-friendly process of textile production is of paramount interest. Here we review three potential eco-friendly systems in textile dyeing processes to minimize salt and water consumption. First, we review application of enzymatic processing in fabric preparation. Some of the enzymes involved in desizing, scouring and bleaching operations are amylases, pectinases, glucose oxidases, catalases, etc. Enzymes can eliminate use of strong alkali and subsequent water washes. It has been reported that 10 kg of enzyme can save up to 20,000 kg of water consumption per ton of yarn processed. Secondly, we review the use of bio-degradable organic salts like trisodium citrate, magnesium acetate, tetrasodium edate, sodium salts of polycarboxylic acids etc., as fixation and exhaustion agents. It is reported that total dissolved solids content in the spent liquor released from trisodium citrate dyeing process is about 40-65 % less than that of conventional sodium chloride dyeing. Finally, we review surface modifications of cotton to reduce the volume of effluent and total dissolved solids. Cationization of fibre surface results in salt-free dyeing process. Various monomers, polymers, dendrimers, chitosan etc. are used for cationization. The effects of other surface modification techniques like plasma treatment, corona discharge are also reviewed. It has been reported that plasma treatment of cotton fiber increases percentage exhaustion and K/S by about 10 % and 14 % respectively in reactive dyeing. These aspects can contribute to a more eco-friendly textile processing.