Degradation of azo dyes by laccase: biological method to reduce pollution load in dye wastewater

Laccases are the oldest with low substrate specificity enzymes used for degradation of various compounds, especially, dyes. In the present investigation, the cell-free extract of laccase enzyme is applied to degrade azo dyes used in leather processing. The enzyme degrades the azo dyes rapidly at optimum growth conditions of pH 7.0, temperature 37 °C and incubation duration of 72 h. Better production of the enzyme was achieved with dextrose, yeast extract, acetone, copper sulphate and orange peel. The molecular weight of laccase was found to be 63 kDa. The percentage of degradation is found to be 96.4 % for CI Acid black 210 and 92.2 % for the CI Acid black 234. Ultraviolet–visible spectral analysis indicates the presence of peaks in the visible region confirming the complete degradation of the dye sample. Fourier transform-infrared spectroscopy analysis shows the transformation of N=N into either N₂ or NH₃ and then into biomass. The mass spectra analysis shows conversion of azo dye to final product through various intermediates with their respective molecular weights. Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) analysis reveal the reduction of pollution load more than 92 % in the experimental process while maintaining BOD/COD ratio to about 30 %.