Reuse of wool dyeing wastewater after electrochemical treatment at a BDD anode

Abstract

The present study aimed to evaluate the feasibility of reusing dyeing wastewaters, treated by electrochemical oxidation (EO) with a boron-doped diamond anode, in new dyeing processes. A wool fabric was dyed with acid dye Telon® Blue RR, using Avolan® UL 75 and sodium sulfate as dyeing auxiliaries, and the wastewater obtained was treated by EO at different applied current densities and treatment times. The treated wastewater was then reused in the dyeing process, being the resulting wastewater submitted to a new EO treatment and a posterior reuse in another dyeing cycle. The successive dyeing processes were evaluated in terms of dye-uptake, colour difference (ΔE*), and colour fastness to washing. In all EO treatments, before and after the reuse, complete discolouration of the effluents was observed, alongside with partial organic compounds mineralization. For all the applied experimental conditions, salt content was recovered in full, eliminating the need of salt addition in the reused dyeing baths. The dyeing processes performed with treated wastewater presented increased dye-uptakes, with consequent darker colour of the dyed fabrics, suggesting that the amount of dye in the reuse dyeing baths could be reduced. Moreover, higher affinity between the dye and the wool fabric was observed when using treated wastewaters with lower organic load and higher mineralization degree. For these wastewater conditions, obtained at the highest applied current density and treatment time studied, a reduction of 15 % in dye concentration resulted in a ΔE* of 1.19, complying with the less restrictive ΔE* requirements.