| Chromite ore processing residue(COPR)is a great environmental problem in China.The accumulative deposit has attained 600 million tons.The hexavalent chromium containing in COPR is one of the eight most harmful chemicals for the health of human and one of three carcinogenic metals.Therefore,treatment of Cr(Ⅵ)contamination of COPR is always import subject of environmental protection allover the world.In this study,an effective chromate reduction bacteria strain was isolated,which could reduce high concentrations of Cr(Ⅵ)to nontoxic trivalent chromium to detoxify Cr(Ⅵ).This strain shows a widely practical potential in chromate contamination treatment with biological methods.Based on the stress mechanism of microbe living in extreme environment,a strain with high chromate reduction performance was isolated from the site contaminated seriously by Cr(Ⅵ)leached out from chromite ore processing residue heap of Changsha chromate factory.The optimum environmental conditions for the growth of strain CRB1 were at temperature 35℃,pH 8.0,salt concentration between 0.1～0.5 mol/L. It was an intermediate temperature,Alkaliphiles and halophile.Under the optimum conditions,the generation time was 117min,Cr(Ⅵ)less than 1000 mg/L inhibited bacterial growth lightly,and controled electric potential between -900～300 mv could enhance its growth.Biochemical charasteristics and 16S rRNA sequencing indicated strain CRB 1 was a new species of Leucobacter.The sequence of its RNA has been submitted to Genbank database.Characteristics of chromate reduction of Leucobacter sp.CRB 1 were studied systemically.The results indicated that the optimum conditions for Cr(Ⅵ)reduction during bacterial growth were at pH 9.0 and 30℃, and O2 was necessary for the reaction.Addition of sodium lactate in culture media could enhance reduction rate,and optimum concentration was 4g/L.During reaction,more inoculums meant shorter process time and higher Cr(Ⅵ)concentration would prolong reduction process. When chromate reduction was carried out with resting cells of strain CRB1,generation of reaction demanded bacterial cells absolutely,and the metabolism products were not able to reduce Cr(Ⅵ).The optimum conditions for this reduction system were similar with growing cells reduction system.A comprehensive evaluating method was building firstly and the conception of chromate reduction capacity was promoted in this work.Under optimum conditions,the minimum inhibit concentration of Cr(Ⅵ)to CRB 1 was 1180 g/L,the maximum reduction volume of growing cells was 1820 mg/L,and 2180 for resting cells.The maximum reduction rate was 5.45 mg/L.min,and reduction capacity was 2490 mg/L.The reduction of Cr(Ⅵ)by Leucobacter sp.CRB1 was a direct reduction catalyzed by chromate reducing enzyme.To explain the mechanism of this process,scanning electron microscope,translated transmission electron microscope,EDXA and EPR were utilized to observe the appearance of bacterial cells before and after Cr(Ⅵ) reduction and analyze the elemental composing of precipitate product during the process.The results demonstrated that the reduction took place outside bacterial cells,and the major element composing precipitate was trivalent chromium implied the reduction product was Cr(OH)3.The chromate reduction enzyme was located on the outer membrane of CRB1 cells.Sodium lactate acted as electron donor during reduction process.HPLC analysis suggested that acetate and a little pyruvicate were produced during Cr(Ⅵ)reduction and pyruvicate was a intermediate product.Every sodium lactate could supply 4 electrons to Cr(Ⅵ)and was oxidized to sodium acetate at last.Kinetic models of chromate reduction with growing and resting cells were built based on chemical dynamics and enzymatic dynamics respectively.Under optimum conditions,as the Cr(Ⅵ)concentrations were low,reduction with resting cells was zero order reaction;as the Cr(Ⅵ)concentrations were higher than 2000 mg/L,reduction was first order reaction.Process with growing cells was also showing characteristics of zero order reaction.The dynamic model based on the substrate inhibiting Monod equation was also able to express the bacterial Cr(Ⅵ)reduction.However,this method was less accurate than chemical dynamic model.Enzymatic kinetic results implied that the redox reaction included two basic reactions,oxidization of sodium lactate and reduction of hexavalent chromium,and each basic reaction was single substrate reaction catalyzed by chromate reduction enzyme.Column leaching detoxification experiments of chromite ore processing residue(COPR)with Leucobacter sp.CRB1 were carried out in our laboratory based on the contamination circumstances of Changsha chromate factory and characteristics of COPR.Under the optimum conditions,the leachate after detoxification process reached national discharge standard of waste water(GB 8978-1996),and the leaching toxicity of detoxified COPR was also lower than national standard of solid waste(GB 5085-1996).During the bacterial detoxification process, the surface of COPR was corrodes by bacteria CRB1,and some substance were decomposed by bacterial functions.In detoxified COPR, Ca12Al14O33and Ca4Al2SO10·16H2O were disappeared and the content of MgO was decreased largely.The hexavalent chromium embedded in the crystal lattice of these substances was released to the solution; consequently,leaching and detoxification efficiency were enhanced.
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