Spatial Variability Of Soil Chromium-vanadium Contamination And The Effect Of Washing Removal On Microbial Diversity

Soil chromium-vanadium contamination has become a hot environmental issue of current concern.The geostatistics-based spatial characterization can achieve accurate assessment of soil heavy metals and microorganisms from point to surface,based on which the use of drench remediation can effectively reduce the content and chromium-vanadium toxicity,which is the key to address this pollution.Based on Arc GIS platform,this study evaluated spatial distribution of the basic physicochemical characteristics,chromium-vanadium and soil microbial communities near a vanadium processing plant in Panzhihua to investigate the effects of chromium-vanadium contamination on soil microbial diversity and community structure.The three low molecular organic acids with reducing ability,ascorbic acid（ASA）,lactic acid（LA）and glycolic acid（GA）,were selected to investigate their efficiency in the removal of chromium and vanadium from soil.Finally,the risk assessment was carried out before and after washing from the changes in basic physical and chemical properties,residual chromium-vanadium morphology,valence concentration and soil microbial community structure to provide a theoretical basis for the remediation of soil chromium-vanadium contamination by washing.The main conclusions are as follows:（1）Soils in the study area were alkaline with pH range of 7.46-8.23.SOM concentration in the area was at a high level,while N and P were at moderate or moderate to low levels.Total concentration of soil Cr and V were 1392.94±568.75 and 2096.42±681.72 mg kg^-1,respectively.Hexavalent chromium and pentavalent vanadium were 16.72±6.28 and 372.45±72.73 mg kg^-1,respectively,and the overall contamination level was serious.The results of the correlation analysis showed that soil pH and TP were significantly negatively correlated with Cr and V（P<0.05）.The concentration of basic physical and chemical properties of soils in the study area showed different distribution characteristics.The high values of total Cr and V and their effective state were mainly concentrated near the plant in the southeast corner,and showed a spatial variation characterized by a gradual decrease toward the west and north.（2）The bacterial community in the northern part of the study area had the highest number of species and the highest diversity.Areas of high fungal abundance were located in the eastern part of the northern part of the study area,while areas of high abundance were mainly located in the northeastern corner.The results of the correlation analysis showed that soil microbial diversity was more significantly influenced by soil properties and heavy metals.Among them,soil V was significantly negatively correlated（P<0.05）for soil bacterial diversity,while the diversity index of fungi was less affected.Patescibacteria and Nitrospirota of bacteria,Ascomycota and Blastocladiomycota of fungi were tolerant to soil chromium and vanadium.The bacterial phylum of Entotheonellaeota and Desulfobacterota were both more sensitive to soil Cr and V.The fungus Basidiomycota was more sensitive to soil Cr,while the Glomeromycota was more susceptible to V.They could be used as indicator organisms for different heavy metal contamination.（3）The wahing efficiencies of the three reagents were maximized at a concentration of 2 mol L^-1,with LA having the highest removal rates.The efficiencies of the three washing agents showed different characteristics with pH increased.The ASA,LA and GA showed a significant increase（P<0.05）during the first 15 min of washing and gradually stabilized with time.（4）Soil pH decreased by about 0.83-1.14 units after ASA,LA and GA washing.The concentrations of soil organic matter,total nutrients and fast-acting nutrients all showed significant decreases of different degrees after washing.All three agents were effective in removing several forms of soil Cr and V,effectively reducing the content of exchangeable and carbonate-bound states and reducing the effectiveness of heavy metals.After washing,the levels of Cr（VI）and V（V）in the soil were significantly reduced（P<0.05）,which greatly reduced the environmental risk of the contaminated soil.（5）The diversity and abundance of soil bacteria and fungi showed a significant decrease（P<0.05）after ASA and LA washing,while some enhancement was observed by GA.The diversity and abundance of soil bacteria and fungi showed a significant decrease after ASA and LA washing,while some enhancement was observed after GA washing.Most of the dominant bacterial and fungal phylum in the soil were less affected by washing.In the moderate taxonomies,soil fungi were largely unaffected by washing.The heavy metal tolerant bacterial phylum of Nitrospirota and Patescibacteria and the sensitive bacterial phylum of Entotheonellaeota showed some similar characteristics of the changes,both decreasing after ASA washing and increasing after GA washing.The rare phylum showed greater variability,with 4,6 and 4 for bacteria and 2,0 and 1 for fungi with more than 80%variability after washing.The result showed that the abundance of some bacterial and fungal populations increased after GA washing,so it has a certain function of soil microbial population restoration.