Effects Of Biostimulation On Hexavalent Chromium Contaminated Soil Remediation And Microbial Community Structure

There are tens of thousands of microorganisms in the soil,which can reduce the toxicity of pollutants through their metabolism or secretion of specific products.In the natural environment,due to the lack of corresponding conditions to stimulate the growth and metabolism of functional populations,it is difficult for indigenous microorganisms to play an optimal role to improve the environment.This study promoted the removal of Cr(VI)by indigenous microorganisms from contaminated sites through biostimulation and explored the effects of different stimulation conditions on the microbial community.Then,the relationship between the microbial community and the environment of the actual Cr(VI)-contaminated site was explored,and the optimal biostimulation conditions were applied to the remediation.Combined with the directional succession of microbial communities,the law of the effect of biological stimulation on microorganisms was revealed,which provided a theoretical basis for the remediation of different types of Cr(VI)-contaminated soils.First,the removal of Cr(VI)by indigenous microorganisms under biological stimulation conditions such as water content,ferrihydrite,electron donor,and electron shuttle was studied.The best single factor conditions were moisture content(30%),sodium lactate(6%),ferrihydrite(1.2%),humic acid(1.2%),and the highest removal rate of Cr(VI)could reach89.02% at 82 d.The concentration decreased from 2234.92 mg/kg to 245.50mg/kg.The response surface method was used to optimize stimulation conditions.At 24 days,the measured value of Cr(VI)removal rate was71.61%,indicating that the model fitting results were effective.Secondly,the influence of different biological stimulation conditions on the microbial community structure was studied,and the microbial succession status and internal mechanism during the Cr(VI)reduction process were found.Biostimulation drove microbial directional succession and enhanced the competitive advantage of functional flora.The type of electron donor has the greatest impact on the microbial community,and the addition of the electron donor will promote the increase of chromium/iron reducing bacteria in the soil,such as Bacillus,Delftia,and Virgibacillus,etc.The simultaneous addition of ferrihydrite and electron shuttle will make this effect more pronounced.The functional prediction of microorganisms after biostimulation showed that the reductase and detoxification enzymes in the soil jointly provided conditions for the microorganisms to reduce Cr(VI).Then,the microbial community composition of polluted soils in different regions,the correlation between physicochemical properties,and environmental factors and microbial communities were explored to provide a theoretical basis for remediation of Cr(VI).There were obvious spatial distributions of soil physicochemical properties,high concentrations of Cr(VI)had a greater impact on microbial diversity and abundance,and microorganisms in deep soil had stronger reduction potential to Cr(VI).Functional metabolism prediction showed that Cr(VI)-contaminated soils were rich in cytochrome c,flavin mononucleotide reductase,chromate reductase,and glutathione reductase proving that indigenous microorganisms themselves have great potential to reduce Cr(VI).Finally,the biostimulation scheme optimized by the response surface was used to remediate the contaminated soil in different areas.The index of Cr(VI),TCLP toxic leaching concentration,and hydrochloric acid-extractable Fe content before and after restoration,combined with the change of microbial community structure,provided a theoretical reference for the widespread use of biostimulation.The repair rate of all groups in the first 12 days was higher than that in the last 12 days,and they could maintain a good repair state.Except for aged TJ and QH soils,all soils reduced the TCLP leaching concentration of Cr(VI)below the limit(5 mg/L)within 24 d.Biostimulation was suitable for most types of soils,especially deep NM soils,reducing Cr(VI)by about 2000 mg/kg below the national background value(61 mg/kg).Biostimulation was suitable for most types of soils.Cr(VI)/Fe(III)reducing bacteria Halomonas,Salipaludibacillus,Pseudomonas,Microbacterium were the dominant bacteria after biostimulation of various soils,and their abundance was related to the microbial composition and physicochemical properties of the soil itself.