The Mediated Reduction Of Se/Te And Azo Dye By Humic-like Substances

Quinones have been used as analogues of humic substances. Although there existed a lot of studies on the stimulating effects of quinones for the bioreduction of azo dyes, nitroaromatics and heavy metals, little was known about the effects of quinones on the reduction of toxic metallic substances such as selenite and tellurite. Additionally, considering the toxicity of quinone, the application of low-concentration of quinone or humic substance seems to be a more environmentally friendly choice. This study focused on quinone-mediated bioreduction of selenite and tellurite, and mediated bioreduction of azo dyes by humics.The reduction of selenite and tellurite by Escherichia coli K12 was significantly stimulated by quinone redox mediators. In the absence of redox mediator,6.1% selenite and 9.0% tellurite (1 mmol/L) were reduced by E. coli K12 in 8 h, respectively. When 0.2 mmol/L 2-hydroxy-1,4-naphthoquinone (LQ),2-methyl-1,4-naphthoquinone (MQ), anthraquinone-2-sulfonate (AQS), or anthraquinone-2,6-disulfonate (AQDS) were supplemented, the reduction efficiency of selenite was increased 93.0%,2.1%,6.5% and 1.4%, respectively, and the reduction efficiency of tellurite was increased 87.4%,5.1%, 10.7% and 5.4%, respectively. The reduction rate increased with the increase of LQ concentration whereas no further increase of the reduction rate was observed when its concentration was higher than 0.2 mmol/L.The mediating effects of quinones on Acid Red 27 (AR27) decolorization by Shewanella oneidensis MR-1 were studied using 6 quinones (AQDS, AQS, anthraquinone-2-carboxylic acid (AQC), LQ,5-hydroxy-1,4-naphthoquinone (JQ) and 5,8-dihydroxy-1,4-naphthoquinone (DHNQ)). Stimulating effects were found with all the quinones at lower concentrations. However, the addition of higher concentrations of LQ, JQ and DHNQ inhibited the decolorization process.The mediating effects of 13 humic substances on AR27 decolorization were studied. The decolorization rate was stimulated to different extents by various humic substances and the optimal enhancing effect was observed with Elliott Soil humic acid (ES-HA). Better stimulating effects were found with humic acids compared to fulvic acids from the same sources and humic acids from terrestrial source generally demonstrated higher mediating capabilities. ES-HA was used to investigate the impacting factors of mediated decolorization of AR27. The decolorization rate increased with the increase of HA concentration. Lactate and formate were found to be better electron donors for AR27 decolorization. Competitive inhibition of AR27 decolorization was observed in the presence of dissolved oxygen, nitrate and nitrite. Soluble Fe(III) could stimulate the HA-mediated decolorization of AR27, whereas the stimulating effects of HA disappeared in the presence of insoluble Fe(III).The kinetics of two-step mediated AR27 decolorization were investigated using ES-HA, A-HA, AQDS and LQ, respectively and electron transfer from MR-1 to humic-like substance was identified as the rate-limiting step. Enzyme activity assays of membranous protein, cytoplasmic protein and periplasmic protein only found azoreductase activity with the membranous components and the addition of humic-like substances could stimulate the enzymatic reaction.