Decolorization and toxicity screening of selected reactive azo dyes under methanogenic conditions

Abstract

The objective of this research was to generate technical background for the development of a fixed-film anaerobic reactor to renovate reactive textile dyebaths and reuse the high salt-containing mixture in the dyeing process. Six commercial reactive azo dyes (Black-5; Red-2 and 120; Yellow-3, 15 and 17) were chosen for this study. Using an anaerobic, methanogenic culture enriched from municipal sewage sludge, batch assays were performed to evaluate potential toxicity of the selected dyes to the anaerobic microorganisms as well as to determine the anaerobic biodegradability of these dyes. Total gas and methane production were monitored. No significant toxic effects were observed at 300 mg/L concentration for all dyes tested. At the end of three consecutive feeding cycles all dyes exhibited overall color removals in the range of 77.8 and 97.1%. An assay intended to determine the toxic limit and kinetics of color removal of Red-2 revealed that up to 300 mg/L, this dye was not toxic and contributed to increased total gas and methane production over that of a control. However, higher concentrations of Red-2 (500, 1000 and 2000 mg/L) depressed gas production despite continued decolorization. These results suggest that color removal under the low redox conditions maintained by the methanogenic culture occurs irrespective of the culture activity level.