Acid azo dye remediation in anoxic–aerobic–anoxic microenvironment under periodic discontinuous batch operation: Bio-electro kinetics and microbial inventory

Abstract

Functional behavior of anoxic–aerobic–anoxic microenvironment on azo dye (C.I. Acid black 10B) degradation was evaluated in a periodic discontinuous batch mode operation for 26 cycles. Dye removal efficiency and azo-reductase activity (30.50 ± 1 U) increased with each feeding event until 13th cycle and further stabilized. Dehydrogenase activity also increased gradually and stabilized (2.0 ± 0.2 μg/ml) indicating the stable proton shuttling between metabolic intermediates providing higher number of reducing equivalents towards dye degradation. Voltammetric profiles showed drop in redox catalytic currents during stabilized phase also supports the consumption of reducing equivalents towards dye removal. Change in Tafel slopes, polarization resistance and other bioprocess parameters correlated well with the observed dye removal and biocatalyst behavior. Microbial community analysis documented the involvement of specific organism pertaining to aerobic and facultative functions with heterotrophic and autotrophic metabolism. Integrating anoxic microenvironment with aerobic operation might have facilitated effective dye mineralization due to the possibility of combining redox functions.

Introduction

Synthetic dyes are aromatic organic colorants, which have potential industrial applications. Generally, dyes are classified based on their chemical and application classes. Among the chemical class, the azo dyes are more versatile and accounts to be more than half of the annual dye production. These dyes are characterized by the presence of diazotized amine coupled to an amine or a phenol and contain one or more azo linkages which are extensively used in industries such as textile, food, cosmetics, leather tanning and plastic for colorization (Chun et al., 2006, Ong et al., 2010). Azo dyes are also used in simple diazotization reaction during synthesis (Telke et al., 2008). About 10–90% of unfixed dyes during the dying process get discharged through the effluents (Abadulla et al., 2000). Release of these dyes into environment causes adverse impact on the aquatic ecosystem both physically and chemically. Azo dyes are also considered to be toxic to the aquatic biota and are reported to be carcinogenic to the humans (Yahagi et al., 1975).

Dye bearing effluents are typically characterized by residual color, excess salts and low biodegradability (high COD with relatively low BOD), which requires treatment prior to their discharge. The overall cost, regeneration, secondary pollutants, interference by other wastewater constituents and residual sludge generation associated with physico-chemical methods limits their usage in spite of their efficiency in dye removal (Davies et al., 2005, Venkata Mohan et al., 2002, Venkata Mohan et al., 2005a). Alternatively, biological processes are considered to be advantageous compared to the physico-chemical methods due to their eco-friendly nature (Davies et al., 2005, Katuri et al., 2009, Venkata Mohan et al., 2005a, Venkata Mohan et al., 2007a). However, biological treatment of dye containing wastewater is particularly challenging due to the recalcitrant and inhibitory nature of these compounds when they serve as microbial substrates (Venkata Mohan et al., 2007a). Azo dyes are recalcitrant to the microbial degradation due to their complex aromatic molecular structures. Strong electron withdrawing property of the azo group protects against oxygenases, which makes aerobic treatment processes less feasible (Carliell et al., 1995). Alternatively, anaerobic metabolic function facilitates reductive breakdown of azo dye molecule by cleaving the azo bond to the corresponding colorless aromatic amines. These aromatic amine residues from anaerobic decolorization resist further anaerobic degradation due to their mutagenic nature (Basibuyuk and Forster, 1997, Ong et al., 2005). On the contrary, aromatic amines could be mineralized in aerobic microenvironment by non-specific enzymes through hydroxylation and ring-fission of aromatic compounds (O’Neill et al., 2000, McMullan et al., 2001, Ong et al., 2005). The anaerobic process is seldom capable of degrading the dye molecule while anaerobic process alone cannot handle the complete mineralization of the dye molecule. To overcome the problem of recalcitrance of azo dye, combined anaerobic–aerobic treatment can efficiently remove color while simultaneously degrade the organic matter (Fu et al., 2001, O’Neill et al., 2000, Ong et al., 2005, Venkata Mohan et al., 2007a).

Periodic discontinuous batch reactor (PDBR) also called as sequencing batch reactor (SBR) has considerable feasibility to include anoxic microenvironment along with aerobic condition during the cycle operation. This integration facilitates a combination of reductive and oxidative steps in a single reactor. PDBR is a batch analog process contrary to the continuous process. From the process engineering point of view, PDBR can be distinguished by the enforcement of controlled short-term unsteady state conditions leading to stable steady state conditions in the long run (Wilderer et al., 2001, Venkata Mohan et al., 2005b). Operating at the steady state conditions as a function of either organic/nutrient load or reactor microenvironment thereby controls the distribution and physiological state of the microorganisms. The periodic operation also imposes selective pressures that can select a defined population which are able to degrade complex compounds (Buitron et al., 2004). This process is gaining high popularity for the biological treatment of industrial wastewater (Venkata Mohan et al., 2005b, Venkata Mohan et al., 2007b) and mineralization of azo based compounds (Fu et al., 2001, Ong et al., 2005, Buitron et al., 2004, Venkata Mohan et al., 2007a). Due to the flexibility of combining multiple microenvironments during operation, periodic discontinuous batch process might of significant interest to treat recalcitrant azo dyes. Henceforth, an attempt was made in this communication to evaluate the functional role of anoxic–aerobic–anoxic microenvironments during periodic discontinuous batch mode operation for the treatment of azo-dye bearing wastewater. An attempt was also made to comprehensively understand the process dynamics as a function of dehydrogenase activity, microbial community analysis (microbial inventory) and bio-electro kinetics analysis.