Effect of NaCl concentrations on the photodecoloration of reactive azo-dyes and their cotton dyeings

The color fastness of reactive cotton dyeings under simultaneous exposure to light and saline water, as well as light and perspiration, is one of the most important characteristics especially for summer-time sportswear. In order to illustrate the dual effects (i.e. favorable or adverse) of saline water on the photofading of reactive azo-dyes, the degradation mechanism and reaction kinetics of dye photodecoloration in the presence of various NaCl concentrations were investigated in simulated experiments. Experimental results indicated that the photodecoloration of dye solution was mainly attributed to the hydroxyl radical. Due to the quenching of the hydroxyl radical by the chloride ion, the dye photodecoloration rate decreased with low NaCl concentration (<10 g/L). Nevertheless, higher NaCl concentration (>50 g/L) was beneficial to produce reactive chlorine species, resulting in a significant increase of the photodecoloration rate. Moreover, reaction kinetic analysis demonstrated that the observed first-order rate constant for the photodecoloration of dye solution consistently increased with the increasing NaCl concentration.