Regeneration of Dye-Saturated Quaternized Cellulose by Bisulfite-Mediated Borohydride Reduction of Dye Azo Groups: An Improved Process for Decolorization of Textile Wastewaters

Cellulosics modified to contain quaternary ammonium groups have a strong affinity for anionic dyes. Therefore, ion exchangers based on quaternized cellulose or lignocellulose can be used to remove textile dyes from wastewater. However, restoration of exchanger binding capacity is poor using conventional, low-cost regenerants. Experiments were conducted with two monoazo dyes, Orange II (Acid Orange 7) and Remazol Red F3B (Reactive Red 180), to determine whether reductive cleavage of dye azo bonds improves exchanger regenerability. Treatment with the redox couple KBH 4 /NaHSO 3 fully restored the binding capacity of Orange II-saturated quaternized cellulose. KBH 4 /NaHSO 3 treatment of quaternized cellulose saturated with Remazol Red F3B (hydrolyzed, unreactive form) restored 74% of the exchanger binding capacity, which increased to 95% with a subsequent wash with NaOH or NaClO 4 . High-performance liquid chromatography was used to confirm that KBH 4 /NaHSO 3 reductively cleaved dye azo bonds. Bisulfite was found to form a stable adduct with Orange II but to not cleave the dye's azo bond. The efficiency of dye azo bond reduction was the same for dye in solution and exchanger-bound dye. These results indicate that reduction of monoazo dyes is an efficient method by which to regenerate the dye binding capacity of quaternized cellulosics used to decolorize textile wastewater.