Electrochemical treatment of acid dye systems: Sodium meta-bisulfite addition to the Andco system

Electrochemical treatment was shown to be successful in removing from solution two acid dyes used in the carpet industry. The Andco Environmental Processes laboratory scale iron electrode system was shown to decolorize solutions of Acid Red 337 (azo) and Acid Blue 40 (anthraquinone). High performance liquid chromatography (HPLC) with diode-array detection was used to quantify dye removal and provide qualitative information about degradation products. Both degradation and adsorption contribute to removal of Acid Red 337; adsorption appears to be the primary mechanism responsible for removal of Acid Blue 40. Addition of sodium meta-bisulfite to the Andco system was shown to enhance removal of both dyes. It is postulated that the dithionite and/or sulfoxylate radical anion that may be formed during the reduction of bisulfite is responsible for the more efficient cleavage of the azo linkage in Acid Red 337, causing further degradation. The mechanism for enhanced removal of Acid Blue 40 by addition of meta-bisulfite is most probably due to reduction of the anthraquinone dye to a hydroquinone, which in its leuco form adsorbs better to the iron solids generated in the electrochemical treatment process.