Anthraquinone-2,6-disulfonate AQDS

Dos Santos AB, Cervantes FJ, Van Lier JB (2004) Azo dye reduction by thermophilic anaerobic granular sludge, and the impact of the redoxmediator anthraquinone-2,6-disulfonate (AQDS) on the reductive biochemical transformation. Appl Microbiol Biotechnol 64 62-69... 

Tab. 12.8 Per cent reduction of three Fe oxides by Shewanella putrefaciens in the absence (-) and presence (-h) of anthraquinone-2,6-disulfonate (AQDS) (Zachara et al. 1998).

Anthraquinone-2.6-disulfonate (AQDS) (anode cathode) Phenazines (anode) Methyl viologen (anode cathode) Flavins (anode) Neutral red (anode cathode)... 

Indirect EET involves the use of so-called electron shuttles which physically transfer electrons from the cell to the electrode [80]. Commonly applied mediators include humic substances such as anthraquinone 2,6-disulfonate (AQDS) [104]. Furthermore, Thrash and Coates reviewed the electron shuttles used in BESs, and reported that the addition of a chemical shuttle can be expensive, toxic, and prone to wash-out of the system [81]. In addition to artificial redox mediators, some microorganisms are able to produce their own mediators such as secondary metabolites like phenazines [46, 92] and flavins [77]. Finally, primary metabolites such as sulfur species [105] and hydrogen gas [106] are also able to convey electrons toward electrodes. 

Chemical mediators or electron shuttles were routinely added to MFCs that resulted in electron transfer by bacteria and even yeast. In the earliest studies by Potter (1911) the yeast Saccharomyces cerevisae and bacteria such as Bacillus coli (later classified as Escherichia coli) were shown to produce a voltage, resulting in electricity generation. How that worked is not well known as there were no known mediators added to the cell suspensions, and E. coli Bond and Lovley 2003) and yeast are not known to produce electricity today in the absence of mediators. Since that time, a variety of chemicals have been used to facilitate the shuttling of electrons from inside the cell to electrodes outside the cell. These exogenous mediators include, for example, neutral red Park et al. 1999), anthraquinone-2-6,disulfonate (AQDS), thionin, potassium ferricyanide Bond et al. 2002), methyl viologen, and others Logan 2004 Rabaey and Verstraete 2005). 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

Photosensitized vectorial electron transfer from the (morpholine)ethenesulfonate anion (MES ) to 1,5-anthraquinone disulfonate (AQDS ) mediated by dibutyl (BTDB) or diethyl (BTDE) esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid solubilized in CTAB micelles has also been reported [73], The proposed mechanism of BTD-mediated electron transfer is shown in Figure 7, where reductive quenching of photoexcited BTD by interfacially adsorbed MES is followed by electron transfer from reduced BTD to the AQDS , recycling the sensitizer. 

Bacteria have been isolated using reduced anthraquinone-2,6-disulfonate (HjAQDS) as electron donor and nitrate as electron acceptor (Coates et al. 2002). The organisms belonged to the a-, p-, y-, and 5-subdivision of the Proteobacteria, and were able to couple the oxidation of H AQDS to the reduction of nitrate with acetate as the carbon source. In addition, a number of C2 and C3 substrates could be used including propionate, butyrate, fumarate, lactate, citrate, and pyruvate. 

Poly(l-vinylimidazole)i2-[Os-(4,4 -dimethyl-2,2-bipyridyl)2Cl2] and poly(vi-nylpyridine)-[Os-(Ai,Ai -methylated-2,2 -biimidazole)3] were reported for their efficient capability of mediating electrons transfer between bacterial cells to electrodes. With S. oneidensis, the osmium redox polymer modified anode showed a 4-fold increase in current generation and a significant decrease in the start-up time for electrocatalysis. Using an anode modified with electropolymerized polypyrrole, a dramatie improvement in energy output was noticed in the MFCs. MFCs with a polypyrrole/ anthraquinone-2,6-disulfonic disodium salt (PPy/AQDS)-modified anode... 

Improving the kinetic activity associated with the transfer of electrons to the anode could potentially improve MFC performance. To these ends, several different composites have been constructed and thoroughly tested. One study reported increased performance of graphite -Fe +, -Mn" + and -neutral red composites (Park and Zeikus, 2003). A subsequent study observed an increase in kinetic activity of composites 1.5 to 2.2 times that of those associated with graphite controls. The composites tested combined graphite with one or two of a selection of minerals (Mn +, Ni +, Fe304) and mediators (anthraquinone-l,6-disulfonic acid (AQDS), 1,4-napthoquinone (NQ)) (Lowy et al., 2006). Additional research on various other anode metal modifications has shown limited success (Logan, 2008). 

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