Using of ecofriendly α-hydroxycarbonyls as reducing agents to replace sodium dithionite in indigo dyeing processes

Nowadays, in most indigo dyeing processes the reduction step is performed chemically by sodium dithionite. This is considered environmentally unfavourable because of the resultant contaminated wastewaters. So, there has been interest to find new possibilities which would be ecologically more attractive to reduce indigo. In this paper, a comparison between sodium dithionite and some ecofriendly α-hydroxycarbonyls was effectuated. The examples of glucose (such as an α-hydroxyaldehyde), acetol and acetoin (such as an α-hydroxycetone) were treated. This comparison was based on redox potential measurements taken in absence and in presence of indigo under several conditions of temperatures, molar concentrations of studied reducing agents and sodium hydroxide. A thermodynamic study was also released in order to evaluate more precisely the reducing properties of each reductant by calculating the equilibrium constant of reduction reaction “Kr”. The dyeing performances resulted from the reduction of indigo by each one of the studied reducing agents were appreciated by measuring the colour yield (K/S). The obtained results showed that α-hydroxycarbonyls could offer an environmentally safe alternative to sodium dithionite as a reducing agent in indigo dyeing processes.