Enhanced bio-decolourisation of acid orange 7 by Shewanella oneidensis through co-metabolism in a microbial fuel cell

Abstract

The decolourisation of acid orange 7 (AO7) (C.I.15510) through co-metabolism in a microbial fuel cell by Shewanella oneidensis strain 14063 was investigated with respect to the kinetics of decolourisation, extent of degradation and toxicity of biotransformation products.

Rapid decolourisation of AO7 (>98% within 30 h) was achieved at all tested dye concentrations with concomitant power production. The aromatic amine degradation products were recalcitrant under tested conditions. The first-order kinetic constant of decolourisation (k) decreased from 0.709 ± 0.05 h⁻¹ to 0.05 ± 0.01 h⁻¹ (co-substrate – pyruvate) when the dye concentration was raised from 35 mg l⁻¹ to 350 mg l⁻¹. The use of unrefined co-substrates such as rapeseed cake, corn-steep liquor and molasses also indicated comparable or better AO7 decolourisation kinetic constant values. The fully decolourised solutions indicated increased toxicity as the initial AO7 concentration was increased.

This work highlights the possibility of using microbial fuel cells to achieve high kinetic rates of AO7 decolourisation through co-metabolism with concomitant electricity production and could potentially be utilised as the initial step of a two stage anaerobic/aerobic process for azo dye biotreatment.

Introduction

The majority (60–70%) of the more than 10,000 dyes used in textile, leather and paper industries are azo dyes (Carliell et al., 1995, van der Zee et al., 2001). Up to 50% of the dye applied during industrial processing may be lost depending on dye type and may lead to the release of intensely coloured effluents (Brás et al., 2001).

The discharge of azo dye contaminated effluents is strictly regulated in many countries not only because of aesthetic reasons, but also because the dyes and their breakdown products can be toxic and mutagenic (Gottlieb et al., 2003, Hatice, 2010).

The application of azo dyes in industry demands that they are highly stable during washing, lightfast and not susceptible to degradation under natural conditions. Hence, azo dyes go through activated sludge systems unchanged. Anaerobic and microaerophillic decolourisation of azo dyes yielding aromatic amine degradation products is widely reported in literature. However, the kinetics of anaerobic biotransformation is very slow and the resulting metabolites are known to be recalcitrant under the same conditions but amenable to further biodegradation under aerobic conditions (Haug et al., 1991, Weber, 1991). Extensive work exists on oxidation of dyes using fungal species, especially white rot fungi e.g. Pleurotus ostreatus and Trametes versicolor (Fu and Viraragharan, 2001) or their enzymes – laccases, peroxidases (Teerapatsakul et al., 2008). Fungal cultures require rather long growth phases before producing high amounts of active enzymes. Physico-chemical treatment options such as advanced oxidation, coagulation, flocculation and adsorption have also been used but they are costly and produce high amounts of sludge that may cause disposal problems (Sarasa et al., 1998, Vandevivere et al., 1998, Alaton et al., 2002). Therefore, a two-step approach involving anaerobic–aerobic sequencing processes remains the only industrially feasible biological degradation option for azo dye treatment. However, intrinsic slow decolourisation kinetics of anaerobic biotreatment systems at the initial reductive decolourisation phase may present a limiting factor in such a two-step approach for azo dye biotreatment. Therefore, exploring novel approaches to enhance reductive decolourisation of azo dyes using cheap, unrefined co-substrate types is of importance if biological treatment of azo dye effluents is to be made industrially feasible.

AO7 is a common hair dye used in oxidative and non-oxidative hair dye formulations up to an on head concentration of 0.5% (European Commission COLIPA n° C15 SCCS/1382/10 report). It is also used in dying and printing of wool, silk and nylon and used in paper and leather industry as a colouring agent to a limited extent. Therefore, it is of importance to study innovative technologies that are capable of rapid biodegradation and biotransformation of this model azo dye.

Microbial Fuel Cells (MFCs) have been suggested as a potentially viable way of treating wastewater with the added advantage of harnessing electrical energy (Rabaey et al., 2010). MFCs utilise micro-organisms (e.g. Shewanella, Geobacter, Rhodoferax, undefined mixed cultures) to catalyse organic substrate oxidation at the anode and can produce electricity when connected to a load/resistor via an external circuit. Unlike anaerobic digestion systems, MFCs can utilise oxygen (which has a high redox potential) as the final electron acceptor at the cathode (Logan et al., 2006). Therefore, microbial metabolism and kinetics of azo dye degradation is expected to be faster compared to other anaerobic systems. The application of MFC technology for treatment of azo dye contaminated water is described only in a limited number of publications. Li et al. (2010) described the application of MFC systems in an anaerobic, aerobic sequential mode where the azo dye Congo Red (CR) was reductively decolourised in the anode and the decolourisation metabolites underwent further degradation in an aerated biocathode. The kinetics of CR degradation and the effect of CR concentration on degradation were however, not addressed in the aforementioned study. Another study carried out by Mu et al. (2009) demonstrated the possibility of reductively decolourising AO7 when the dye was employed in abiotic cathodes of MFC systems as the sole electron acceptor. However, the achieved rates of AO7 removal were very low when no external power supply was used.

The kinetics of azo dye decolourisation is important when up-scaling biological treatment methods to an industrially relevant scale. It could be expected, in view of the large sizes of azo dye molecules that their biodegradation rates would benefit from co-metabolism. Co-metabolism involves the transformation of a non-growth substrate in the presence of a growth substrate that is used as the primary carbon and energy source (Dalton and Stirling, 1982). The current study investigated the possibility of utilising the model azo dye AO7 as a co-metabolite of readily oxidisable substrates in the anode compartment of two chamber MFC systems. To the knowledge of the authors, the co-metabolic degradation of AO7 and its rapid decolourisation kinetics in MFC systems have not been previously reported in the literature.

A common concern regarding the biodegradation of azo dyes is that the degradation products may be toxic (Hatice, 2010). The toxicity profiles of reductively decolourised AO7 solutions have not been widely characterised in earlier studies that reported the phenomenon of anaerobic decolourisation of AO7.

The aim of this work was to investigate the decolourisation of AO7 in the anode of a MFC through co-metabolism. Of particular interest were the kinetics of decolourisation, extent of degradation and the toxicity of biotransformation products.