Decolourisation and degradation of acid orange 7 using an acclimatised BAC-ZS mixed bacterial culture

Abstract

Formation of dark coloured auto-oxidation compounds had resulted in reduced efficiency of the sequential anaerobic-aerobic treatment system to decolourise sulphonated azo dyes. In view of this, a monosulphonated azo dye, Acid Orange 7 (AO7) was selected as a model dye to study the decolourisation of AO7 and its auto-oxidation compounds by a mixed bacterial culture, BAC-ZS. It consisted of three bacteria namely Brevibacillus panacihumi strain ZB1, Lysinibacillus fusiformis strain ZB2 and Enterococcus faecalis strain ZL. The decolourisation and degradation process was performed using the sequential facultative anaerobicaerobic system. Optimisation of the co-substrate showed that the combination of glucose (5 g/L) and yeast extract (3 g/L) was the best co-substrate for decolourisation; 98% of AO7 colour was removed within 2 h of facultative anaerobic phase. When the decolourised solution was further treated under the aerobic phase, auto-oxidation reaction resulted in heavy browning effect after 24 h of agitation. The browning effect had drastically decreased the decolourisation to 72%. However, continuous agitation up to 48 h successfully decolourised the auto-oxidation compounds as indicated by the increase in decolourisation up to 90%. Consequently, the decolourisation was accompanied by 73% decrease in Chemical Oxygen Demands (COD) and an increase of 94% of bacteria concentration (absorbance at 600 nm). It was also found that the initial pH 6.6 of AO7 solution dropped to pH 4.5 during facultative anaerobic decolourisation and increased to pH 7.7 at the end of aerobic treatment. The degradation of AO7 dye was determined and confirmed using the UV-Vis spectrophotometry and FTIR analysis. In addition, the formation of autooxidation compounds, 1,2-naphtholquinone and 1,4-benzoquinone were detected and monitored using HPLC analysis. Further phytotoxicity tests using Cucumis sativus confirmed detoxification of the final treated solution by BAC-ZS. Quantification of BAC-ZS using real-time polymerase chain reaction (RT-PCR) showed E. faecalis strain ZL was the dominant bacteria in the acclimatised BAC-ZS and throughout the AO7 treatment process. The annotatation of the draft genome of each bacteria revealed presence of genes coding for the azoreductases, dioxygenases and monooxygenases which played important roles in degradation and mineralisation of AO7 dye. In conclusion, the acclimatised BAC-ZS mixed bacterial culture has good potential to be used in the biological treatment of textile effluent.