Of Ch-1 Bacteria Detoxify Chromium Slag Mechanism Of A Preliminary Study

Chromate ore processing residue(COPR) is a kind of poisonous slagresulting from chromium industry. Harmfulness of the Cr(Ⅵ) in theCOPR to the environment and human beings has become an urgentproblem for our society to solve. Two serious pollute accident occurredby COPR in 2003 has attended much attention. The necessity of treatingCOPR has become the common knowledge, but, up to now, an idealmethod has not yet been set up, At present, Summarily speaking, thereare two general ways namely dry-detoxification and wet-detoxification.But because of the high cost, low slag using amount and narrow applyextent, the pollution of the COPR is still not solved.For this, an isolated strain Ch-1 bacteria with reducing highconcentration Cr(Ⅵ) in alkaline solution was obtained and the techenicsof detoxification on COPR by the bacteria has been implemented. Theresearch work on mechanism of the detoxification on COPR by Ch-1bacteria kept still unknown, though.On the basis of the physicochemical state of the slag and also manyinvestigator's studies, the mechanism of the detoxification on COPR byCh-1 bacteria is studied in this paper .The result of the research show:①Reductase rather than the metabolite plays a key role in the reductionof Cr(Ⅵ) by Ch-1 strain according to the results of the shake flaskleaching test on COPR and the Cr(Ⅵ) solution, and the reductase wasmainly the result of reduction of Cr(Ⅵ). The whole detoxification tothe COPR was a combination of leaching and reduction of hexavalentchromium. The bacteria Ch-1 promoted the leaching of hexavalentchromium and increased the leaching ratio obviously. The key todetoxification on COPR by bacteria is how to increase the leachingratio of the acetous hexavalent chromium, which to some extentdepended on the damage extent to the acetous hexavalent chromiumsoloid phases that is affected by the damage extent to the crystalphase,leaching time,control to the particle size,leaching conditionand other factors.②The slag percolation column-leaching experiment was done underdifferent pH values. It is obtained that: as decreasing of pH, Ca²⁺, Mg²⁺,Al³⁺,Fe³⁺ in chromium slag was dissolved and released tosolution gradually of which the Ca²⁺ and Mg²⁺ ions increased mostobviously. The quantity of HCl consumed by the metal ions fits well tothe practically consumed. XRD showed that periclase,Ca₁₂Al₁₄O₃₃and Ca₄Al₂SO₁₀·16H₂O dissolved gradually under acid condition(pHbelow 6). As solution acidity increased, the leaching behavior of Cr(Ⅵ)ion was controlled by both adsorption-desorption action on the surfaceof the slag and reduction by organic substance, which kept theconcentration of Cr(Ⅵ) decreasing after pH lowed to 6. Throughcolumn-leaching experiment, it was showed that increasing theliquid-to-solid ratio somewhat helped to increase Cr(Ⅵ) leaching ratio,but it made Cr(Ⅵ) leaching rate decrease which prolonged theleaching time to some extent. The leaching rate curve wasexponentially fitted separately when liquid-to-solid ratio were 2,5 and10.③XRD,SEM and EDX were applied to analysis the change ofphase,chemical component and the surface topography after the slagwas leached by bacteria Ch-1. It is showed that the phase compositionof COPR didn't change obviously after bioleaching, the effect tochromium slag by bacteria was manifested in increasing leaching ratioof Cr(Ⅵ). The content of chromium in the slag decreased distinctlyafter the bioleaching. The total leaching ratio of Cr(Ⅵ) in the samplewas 58.15%, in which aqueous Cr(Ⅵ) was 98.82% and acetous Cr(Ⅵ)was 45.92%, respectively. The content of periclase (MgO) wasdecreased which was an indication of the promotion on the solution ofpericlase by bacteria, of which the reason may be that the magnesiumion is the nutrient element needed in bacteria growth.④The biochemical reaction kinetics of Ch-1 bacteria reducing Cr(Ⅵ)was fitted by applying Monod equation. It was analyzed that themetabolism of Cr(Ⅵ) by Ch-1 bacteria was a direct reduction processby bacteria's enzyme. The reductase had a strong tendency to combinewith Cr(Ⅵ), which was analyzed by the kinetic parameters fitted. Theparameters on optimum experimental condition were K_c=40.0,K_m=1.205×10^-10, Rc=1.87×10^-9, respectively. The fitting value, which was close to the actual value, can basically reflect the change ofbacteria's biochemical reaction rate correlating to the change of theenvironment outside. This model can preferably estimate the rate ofenzyme reaction using Ch-1 bacteria removing Cr(Ⅵ) fromchromium-containing wastewater and the theoretical reductionquantity of enzyme reaction when using bacteria treating chromium,therefore, to estimate the optimum condition of the reaction.