Sequential Anaerobic-aerobic Biodecomposition of Azo Dyes by Microbial Fuel Cell (MFC)

Abstract

Textile industry is responsible for the release of 200,000 tons/year of dyes in environment worldwide, in which 60-70 % belong to azo dyes. The chemicals used to synthsize dyes are oftenly highly carcinogenic, lethal or even explosive. Therefore, the release of these toxic color compounds into the environment is a major concern. An extensive sequential anaerobic-aerobic based study has been carried out for the removal of model Acidic Orange 5 (AO5) azo dye and chemical oxygen demand (COD). A variety of electrochemical and biological techniques has been used to exploit bacterial metabolism in microbial fuel cell (MFC) as an anaerobic reactor. For aerobic biodecomposition of AO5 dye, a bacterial consortium comprised of native bacterial strains from textile waste water were used. Both processes are used sequentially because the partial decomposition of AO5 was carried out in anaerobic MFC, while aerobic decomposition was conducted by applying response surface methodology using bacterial species native to textile industrial wastewater. A single chamber anaerobic MFCwas designed by incorporating electrospun lignin (EL) anode and activated carbon Polytetrafluoroethylene (PTFE) porous cathode. Aerobic degradation process and COD removal process was also optimized for AO5. To the best of our knowledge a novel electrospun lignin based bioanode (US Patent No. 20140271443) has not been investigated for anaerobic bioelectrochemical degradation for complex ring structure of AO5 so far. In this study anaerobic degradation of AO5 and reduction in COD using EL bioanode based MFC was examined. A biofilm of Geobacter sulfurreducens culture was used in MFC. It oxidizes the acetate and generates electrons, which in turn reduce azo bond of AO5 as a side reaction. This reduction of AO5 produced colorless 2- aminobenzenesulfonic acid (2-ABS) and unidentified compounds. Electrochemical analysis revealed that higher dye concentration as well as too low and high current densities have influenced negatively on performance efficiency of MFC. Gradual increase in hydraulic retention time (HRT) increased the loss in polarization activity. Current density of 0.59 A/m² was recorded at a load of 100 Ω with at anolyte composition of 2 % auxiliary salts (2 % w/v, 1:1 ratio Na2SO4.10H2O and NaCl). Maximum power density was reached upto 0.12 W/m² at these conditions. Time dependent degradation of AO5 and resulting metabolites were identified by using UV-HPLC. Newly designed MFC was responsible for xii 81 % degradation and 58 % COD removal of AO5 containing synthetic wastewater. Moreover, the remaining degraded metabolites of AO5 are considered to be toxic and are unable to be decomposed further under anaerobic conditions of MFC and responsible for high COD values. Therefore, a model metabolite 2-ABS of AO5 was selected and applied for aerobical degradation by bacterial consortium. Bacterial consortium was developed by mixing the five bacterial strains native to textile wastewater were isolated and genetically identified (16S rRNA). These include Proteus mirabilis, Bacillus anthracis, Enterobacter hormaechei, Pseudomonas aeruginosa, and Serratia rubidaea. The statistical analysis of variables like temperature (28-42 °C), pH (5.0-8.0) and initial 2-ABS concentration (5-40 ppm) on COD removal and percentage degradation yielded a high regression coefficient (R2 = 0.98). Analysis of variance (ANOVA) of quadratic model is significant and predicted R 2 is consistent with the adj. R2 with adequate signal to noise ratio of 20.8. The maximum degradation (%) of the 2-ABS in the simulation solution reached to 95 % and COD to 90 %, which is well fitted with predicted response under the optimal growth conditions suggested by RSM. The complete decomposition for 2-ABS and optimization of aerobical process was achieved and confirmed by UV-HPLC analysis. Finally, the phytotoxicity analysis confirmed the effectiveness of the techniques used for treatment of AO5. The identification of model, development of model equation and optimization of 2-ABS degradation using native consortia of bacteria is the new approach for increasing AO5 degradation. So, this study showed that the anaerobic MFC followed by aerobic sequentional decomposition has a potential to up scale the treatment process to mineralize the dyes present in the textile wastewater