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สารฮิ วมิ กช่วยเพิ่ มความสามารถการลดสี ... 214 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 FIG. 5  Model for mechanism by which AQDS, a humic analogue stimulate sulfonated azo dyes reduction by Paenibacillus sp. strain A5 and NfsB such as flavin reductase (FRP) (61), may be also able to function under anaerobic conditions in the appropriate conditions as quinone-dependent azo reductases in many bacte- rial genera (43, 60). Indeed, the further study is necessary to identification the real enzyme system which is responsible for the ability of Paenibacillus sp. strain A5 to reduce AQDS and thus to reduce sulfonate azo dyes under anaerobic condition in the presence of AQDS or other quinoide mediators. In biological systems, quinones were also shown to accelerate azo dye reduction by anaerobically incubated aerobic biomass as well as granular sludge (29, 42, 43). Theoretically, feasible quinoide redox mediators should have redox between those of the two eventual half reactions, the reduction of azo dyes and the oxidation of a primary electron donor. Although standard redox potential (E0’) for the reduction of the sulfo- nated azo dyes to their constituent aromatic amines are not available, an indication can be derived from paragraphic data. Redox potentials of the azo compounds are approximately vary between –180 mV and –430 mV (19, 42). For bacterial azo dye reduction, i.e., coupled to the oxidation of organic primary electron donors by anaerobically incubated bacteria, the E0’ value of NAD(P)H, the cellular redox cofactors with the lowest electron potential (-320 mV), can be taken into account (54). It was recently suggested that quinoide re- dox mediators with standard redox potentials (E0’) between approximately –320 mV and –50 mV could in general function as effective redox mediators in the bacterial reduction of sulfonated azo dyes (42). Thus quinones with a rather negative redox potential such as AQDS are suitable as redox mediators for the anaerobic treatment of azo dyes.