Characteristic And Mechanism Of Cr(?) Bio-reduction With Redox Mediators

With the widely use of chromium(Cr)in industry and its tremendous environmental damages,how to properly deal with the Cr(VI)pollution becomes a hot spot.The reducing ability of the most toxic Cr(VI)to less toxic Cr(III)has been demonstrated in a wide range of microbes,and this offers a cost-effective and eco-friendly substitution for traditional physical and chemical method.For the slow biological reaction rate,redox mediator can enhance it by accelerating electron transport rate,but the acceleration mechanism is not clear.Thus,this research isolated a high-efficiency Cr(VI)reduction strain firstly,and then the characteristics and mechanism of Cr(VI)bio-reduction with redox mediators were carried out in detail.A Cr(VI)reduction Mangrovibacter plantisponsor strain CR1,which has the quinone-respiration character,was isolated from tannery effluent sludge.Strain CR1 exhibited a high chromate Cr(VI)resistance with a minimal inhibitory concentration(MIC)of 32 mmol/Lol/L in LB medium.Its Cr(VI)reduction efficiency with 0.16mmol/Lol/L AQS was ehanced 6.8 folds.AQS as the electron acceptor in respiration chain of strain CR1 and was reduced to hydroquinone,which could happen with electron donor and strain CR1 in the reaction system.Cr(VI)reduction efficiency with five different dissolved quinones(anthraquinone-1-sulfonate,anthraquinone-1,5-disulfonate,anthraquinone-2-sulfonate(AQS),anthraqui-none-2,6-disulfonate(AQDS),anthraquinone-2,7-disulfonate)and free redox mediator were 28.2%,20.3%,98%,42.0%,25.4%and 18.7%respectively,AQS was the best redox mediator among them.The correlation between the reaction constant k and thermody-namic temperature T was expressed as Arrhenius equation lnk=-7662.9/T+27.931(R2=0.9486),and the activation energy Ea was 63.71 kJ/mol.During Cr(VI)reduction process with AQS,the pH value first increased and then decreased due to the two step of Cr(VI)reduction and Cr(III)precipitate.The cyclic voltammol/Letry characteristics of five quinones suggested that AQS had good redox activity,which benefited to its optimal acceleration effect.Cr(VI)reduction by strain CR1 with non-dissolved quinones were also evaluated in details.The result suggested that Cr(VI)reduction efficiency with anthraquinone,1-chloroanthraquinone,2-chloroanthraquinone,1,5-dichloroanthraquinone,1,8-dichloroa-nthraquinone,1,4,5,8-tetrachloroanthraquinone and control were 55%,90%,76%,70%,50%,19%and 15%respectively,and 1-chloroanthraquinone was the best redox mediator among them.The correlation between Cr(VI)reduction rate v and concentration of 1-chloroanthraquinone C was expressed as v=3.68+200.78C(R2=0.996).Six repeated experiments indicated that Cr(VI)reduction efficiency with 1-chloroanthraquinone kept at about 75%.During this reduction process,the stabilized Oxidation-Reduction Potential(ORP)was higher 300mV than that of control,meanwhile the high redox activity of 1-chloroanthraquinone among six non-dissolved quinones.These are the reason of its optimal acceleration effect.