Pollution Pattern And Bioremediation By Microbes Of Chromium Smelting Slags With Re-Dissolution After Wet Detoxification

Chromium and its compounds are the important industrial raw materials,they are widely used in metallurgy along with other industrial applications such as electroplating,leather tanning,pigments and alloying.However,a large amount of chromium slag with unorganized storage and discharge during the production process of chromium salt will form a lot of "chromium slag mountains",and cause serious chromium pollution in the surrounding environment.In China,chromium slag yards of leaving over by history have the characteristics of large quantity and heavy pollution.Cr(?)and Cr(?)are the main forms in the chromium slag,of them,Cr(?)could lead to serious disease due to its high toxicity,high mobility and strong bioavailability,thus,the essence of bioremediation of chromium slag is to remove Cr(?).At present,the key technologies for removing Cr(?)of the chromium slag are urgently required in our country.In this study,the chromium slag after wet detoxification from a chromium salt factory in Qinghai Province was taken as the research object.Studied on the problem of secondary pollution in the contaminated sites owing to re-dissolution of Cr(?),and causing serious"returning to yellow" phenomenon.The economical,environmental-friendly,and sustainable microbial remediation technology was proposed to be applied to treat chromium slag after wet detoxification in this study.Firstly,the pollution pattern of the contaminated sites were determined.On this basis,it was important to screen the functional microbes with high Cr(?)removal efficiency for further research.Secondary,the mechanisms of Cr(?)removal by functional microbes were revealed in the study.Finally,the application of microbial technology in chromium contaminated sites was systematically studied,and confirmed the feasibility of bioremediaiton,which provided the experimental foundation and process ideas for bioremediation of similar contaminated sites.The main findings were as follows:(1)Breeding of four functional microorganisms with high Cr(?)removal efficiency.Firstly,through sites investigation of chromium slag yard,pollution pattern of the chromium slag was mastered,the pollution level was evaluated.Some indigenous microbial communities with the capacity of removing Cr(?)were determined by high-throughput technology.Secondary,the functional microbes were screened,and identified as Stenotrophomonas maltophilia,Ochrobactrum sp.,Bacillus megaterium and Pseudomonas putida.Finally,by comparing the Cr(?)removal ability of 4 pure bacteria and compound microbes,the results showed that the removal rate of compound microbes was the best,therefore,the compound microbes were selected for the subsequent process optimization study.(2)Obtaining the optimization of microbial culture conditions for removing Cr(?).Firstly,the optimal values of every factor were obtained by "change-one-factor-at-a-time"method.In the Plackett-Burman design,the contribution of different factors for Cr(?)removal presented as following:carbon source>inoculation size>temperature>pH>salinity>nitrogen source>rotation rate.Finally,a Box-Behnken model of Response Surface Methodology for optimization of microbial culture conditions was designed to aim to predict removal rate of Cr(?)under the interaction of sensitive factors.The maximum removal rate over 89.01%and microbial optical density(OD600)over 1.36 were obtained under the optimal culture conditions of carbon source of 1.8 g/L,and inoculation percent(v/v)of 10%when pH at 8.0 and temperature at 30?.(3)Revealing the mechanism of Cr(?)remediation by functional microorganisms.It was determined that biosorption,bioreduction,and biomineralization occur simultaneously in the Cr(?)removal process.The contribution for Cr(?)removal rate varied greatly in conjunction with the different mechanisms.The detailed information presented as following:bioreduction(69.61%)>biosorption(19.16%)>biomineralization(11.23%).The analysis of XPS spectrum of products indicated the Cr(?)in the aqueous solution was converted into precipitated Cr(?)by catalysis of chromium reductase under the action of functional microbes.In addition,the Cr(?),unstable intermediate product,was detected by electron paramagnetic resonance technology in the process of reducing Cr(?)to Cr(?).For biosorption,the analysis of three-dimensional fluorescence spectroscopy showed that the extracellular polymeric substances(EPS)from microbial growth and metabolism was mainly compose of proteins and fulvic acids,and gradually accumulated with time in the biosorption system.According to the FTIR spectrum analysis,a large number of functional groups from EPS previded the abundant binding site for adsorption of Cr(?).The characterization of precipitation products by the scanning electron microscope and X-ray diffraction patterns showed that the products were gradually accumulated,and the well-crystallized peaks gradually were detected with time of the biomineralization.(4)Analysis the feasibility of bioremediation for removing Cr(?)by the functional microorganisms.In this study,according to result of shake flask tests,column experiment and field application,the feasibility of microbial technology has been proved by analyzing the macro and micro variation of chromium slag in the process of bioremediation by functional microorganisms.From a macro perspective,during the process of bioremediation,the redox potential decreased from oxidation zone(above 200 mV)to reduction zone(below-100 mV)providing favorable conditions for the reduction of Cr(?).Simultaneously,the diffusion of Cr(?)was effectively limited with the permeability decreasing to 0 in the bioremediation system.The above variation caused the increase of Cr(?)removal rate of the system.The variation of chromium occurrence state indicated that chromium in the slag was gradually converting into steady-state from non-steady state with time of bioremediation,and the stable chromium reached more than 95%by 355 days.The leaching toxicity of chromium slag showed that the Cr(?)concentration in the leaching solution was only 3.1 mg/L in the end of bioremediation,which was far below the maximum allowable emission standard of 5 mg/L.From a micro perspective,firstly,the increasing of OD600 value in the system provided sufficient biomaterials for bioremediation of Cr(?).Moreover,microbial community succession indicated that the inoculated functional microbes,as the dominant species,gradually had a relatively high abundance,and constructed a relatively stable microbial community structure during the bioremediation process.The above research proves that bioremediation technology for removing Cr(?)has realized from laboratory exploration to field application,and concludes that bioremediation aids to sustainable solve the problem of Cr(?)re-dissolution in the chromium slag,and keep down the "returning to yellow" of contaminated sites.