Functional behavior of bio-electrochemical treatment system with increasing azo dye concentrations: Synergistic interactions of biocatalyst and electrode assembly
Graphical abstract
Introduction
Azo dyes represent the largest class of synthetic dyes applied in textile processing, wherein huge quantities of water is required that usually ends up as wastewater during its processing (Ong et al., 2010, Cui et al., 2014, Balapure et al., 2015). Azo dyes are characterized by one or more azo groups (R1–N = N–R2) and aromatic rings generally substituted by sulfonated, benzene or naphthalene groups. These contain many different substituent’s, such as chloro (Cl), methyl (CH3), nitro (NO2), amino (NH2), hydroxyl (OH) and carboxyl (COOH) groups (Pandey et al., 2007). Presence of these azo groups makes the dye molecules recalcitrant and when discharged into the natural water bodies cause adverse impact on the aquatic ecosystem and are reported to be carcinogenic to humans (Venkata Mohan et al., 2002, Venkata Mohan et al., 2012, Azizi et al., 2015). Hence, treatment of azo dyes and their metabolites is necessary prior to their final discharge into the environment. Several physicochemical methods such as adsorption, chemical precipitation, photolysis, chemical oxidation/reduction and electrochemical treatment have been used for the removal of dyes from wastewater (Khan et al., 2014).
However, these methods have inherent drawbacks of being economically not feasible, as they require more energy and might not be able to completely remove the recalcitrant azo dyes (Robinson et al., 2001, Ong et al., 2010, Sreelatha et al., 2015a). Therefore, biological decolorization and degradation is an eco-friendly and cost-effective alternative to chemical treatment processes (Rai et al., 2005, Venkata Mohan et al., 2012, Venkata Mohan et al., 2013). Although conventional biological processes have the potential to treat azo dyes to some extent, it cannot be used as a standalone process and may be used in concurrence with current technologies such as bioelectrochemical treatment systems to achieve effective treatment. Bioelectrochemical treatment (BET) is a promising technology for the treatment of complex wastewaters along with bioelectricity generation (Venkata Mohan et al., 2010a, Venkata Mohan et al., 2010b, Velvizhi et al., 2014). BET systems use electrodes as solid electron acceptors for bacterial respiration and exploit microbial catabolic activities to generate electrons (e-) and protons (H+) by degrading organic molecules (Venkata Mohan et al., 2009, Sreelatha et al., 2015a). The reducing equivalents induce a potential difference, which acts as the net driving force for bioelectrogenic activity and complex pollutant removal (Velvizhi and Venkata Mohan, 2015, Khan et al., 2014, Butti et al., 2016).
Previous study evidenced that BET has the potential to treat azo dye based wastewater which was operated with 50 mg dye/l and concluded the presence of electrode assembly and anaerobic consortia as biocatalyst offered the advantage of dual benefits such as dye degradation along with bioelectricity generation (Sreelatha et al., 2015b). In this context, an attempt was made in this communication to evaluate the influence of increasing dye concentrations on BET performance. Synergistic interaction of electrode assembly as a solid electron acceptor and the functional variations in biocatalyst behavior were observed with respect to increase in dye load operations. Apart from azo dye mineralization, the bioprocess performance was evaluated in terms of bioelectricity generation, azo reductase and dehydrogenase enzyme activities.
Section snippets
Azo dye
Acid Black 10B an acid application group of dye (4-amino-5-hydroxy-3-[(4-nitrophenyl) azo]-6-(phenylazo)-2,7-naphthalene disulfonic acid disodium salt (C22-H14N6O9S2Na2; MW, 616.49; CAS No. 1064-48-8) was used for the present study. The simulated dye wastewater (SDW) was prepared by dissolving desired amount of dye concentrations (i.e. 100, 200, 300, 500 mg dye/l) in designed synthetic wastewater [DSW (g/l): glucose-3.0, NH4Cl-0.5, KH2PO4-0.25, K2HPO4-0.25, MgCl2-0.3, CoCl2-0.025, FeCl3-0.025,
Color removal
Initially, BET system was evaluated with designed synthetic wastewater (DSW) followed by lower dye concentration (50 mg dye/l) and the corresponding results were discussed in comparison with anaerobic treatment (Sreelatha et al., 2015b). As a continuation of the study, BET system was evaluated with higher dye load concentrations (100, 200, 300 and 500 mg dye/l) keeping the organic load constant (1.36 kg COD/m3-day). Maximum performance was observed at 300 mg dye/l documenting the color removal
Conclusions
Function of solid electrode as intermediate electron acceptor for the degradation of azo dye wastewater was evaluated in BET with increasing dye concentrations. BET documented positive influence on increasing dye concentration till 300 mg dye/l, which was reduced marginally thereafter. Presence of electrode assembly aided in development of bio-potential and contributed for enhanced dye degradation with simultaneous bioelectricity generation. Multi-spectrum and enzymatic profiles illustrated the
Acknowledgements
The authors wish to thank the Director, CSIR-IICT for support and encouragement. Research was supported by Department of Biotechnology (DBT: National Bioscience Award research grant-BT/HRD/NBA/34/01/2012 (vi) and CSIR (XII five year network project: SETCA, CSC-0113). GV duly acknowledges CSIR for providing the research fellowship.
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