Application of a mixture design to optimize textile azo-dye decolorization using a bacterial consortium

Abstract

In recent years, the release of textile wastewaters (TWWs), without adequate treatment, into natural ecosystem has become a great threat and one of the main sources of environmental pollution.1 Apart the high amounts, TWWs have a complex and specific nature.2 TWWs are highly charged by synthetic dyes. One of the most frequently synthetic dyes used in dyeing units, is azo dyes.3 More than 10,000 dyes are available commercially and more than 7×105 tons of dyestuffs are produced annually.4 Azo dyes can be grouped as mono-, di-, or tri-azo, according to the number of azo bonds (-N=N-) in their structure.5 There are about 15% of dyes are lost due to inefficiency dyeing process in wastewater.6 The release of these dyes in large quantities into surrounding environment have serious concerns in aquatic life and public health especially as they are considered as recalcitrant compounds,7 stable towards light and temperature and resist to microbial attack.8,9 Indeed, removal of these toxic compounds remained mandatory. So, different physical and chemical methods have been employed but their continuous application put forth their limitations as they require high energy, time and cost, and generate large amount of sludge and toxic by-products.10 Bioremediation has been considered as a good alternative process to deal this threat. Different azo dyes decolorizing microorganisms have been reported, including bacteria,11,12 fungi13,14 and yeast.15,16 Since different textile processing steps require an excessive use of salt and sodium hydroxide, TWWs are characterized by alkaline pH and high salinity.1 Considering the specific nature of TWWs, fungal treatment remained inadequate in TWWs treatment as that are functional at low pH.17 Researchers have been focused on bacteria dye removal as that it is more adequate to support TWWs characteristics; particularly the focus has been on haloalkaliphilic bacteria. Bioremediation by bacteria can be carried out using two approaches either mono or mixed culture to exploit decolorization potency of each strain and also cooperative interactions between them.18,19