Remediation Effect And Influencing Factors Of High Concentration Chromium Contaminated Soil Leaching

It is an urgent issue to improve the removal efficiency of heavy metal contaminated soil,reduce the economic cost of leaching and repair,ensure the green feasible type of leaching and repairing,and reduce the damage of soil structure after leaching in chemical leaching to repair heavy metal contaminated soil.Therefore,from the three aspects of?repairing efficient,economically viable and environmentally friendly?,it is of great significance to explore and study the effects of different factors on soil composition and structure during heavy metal contaminated soil remediation.In this paper,citric acid,oxalic acid,fulvic acid,water,rhamnolipid and tartaric acid,which are economical and biodegradable,are used as eluents for repairing high-concentration Cr-contaminated soil.Considering the the of repairing effect and soil impact,the best compounding eluent for oscillating leaching and soil column leaching was determined.The repairing effect of different leaching schemes on high concentration Cr contaminated soil and its influencing factors were investigated.At the same time,the soil properties and the distribution characteristics of heavy metal Cr were analyzed before and after leaching.The main conclusions are as follows:1.In the experiment of shaking leaching of high concentration of Cr contaminated soil with different eluents and their rinsing conditions,the leaching effect of citric acid and oxalic acid on Cr-contaminated soil was better.The concentration is 0.5 mol/L,the ratio of solid to liquid is 1:10,the number of rinsing times is 1 time,and the rinsing time is at 4 h and 8 h of citric acid and oxalic acid,the removal efficiency of heavy metal Cr is higher,both of which are about 85%.In addition,the use of different eluents for particle size leaching did not improve the repair effect of contaminated soil.The six eluents are effective in reducing the water-soluble and exchangeable Cr content in the soil,that is,effectively removing the heavy metal Cr with high biological activity in the soil,which greatly reduces the environmental risk.2.In the experiment of shaking leaching of high concentration of Cr contaminated soil with different compound eluents,the removal rate of total Cr in contaminated soil was improved by the combined eluents compared with the single eluent.The order of removal of Cr by six compounding methods:0.5 mol/L oxalic acid+water>0.5 mol/L.Citric acid+water>2%rhamnolipid+0.5 mol/L citric acid>water+0.5 mol/L oxalic acid>2%rhamnolipid+0.5 mol/L oxalic acid>water+0.5 mol/L citric acid.Among them,0.5 mol/L oxalic acid+water and 0.5mol/L citric acid+water had the best removal effect of Cr in soils,which were 87.46%and 85.02%,respectively.After 0.5 mol/L oxalic acid+water leaching,the exchangeable state,the iron-manganese oxide bound state and the organic combined state of heavy metal Cr were the lowest,and the three forms of 0.5 mol/L citric acid+water is slightly higher.And the soil environment risks after rinsing in both formulations are low.3.The characterization and analysis of high concentration Cr contaminated soil before and after leaching were completed.The FTIR spectrum showed that after adding K₂Cr₂O₇,the composition of soil changed little,but after oxalic acid and citric acid leaching,the composition of soil changed,which indicated that the soluble organic matter in the soil may be lost during the leaching process.XRD spectrum analysis shows that the new characteristic peaks of Si-Cr-Fe-Mn form minerals and Cr?III?hydroxide appear,the peak intensity of characteristic peaks of Cr minerals with higher solubility such as aminocarbonate chromium compounds decline,and the peak position of some compounds shifts after leaching of citric acid and oxalic acid.The SEM-EDS spectrum analysis showed that the element types of the soil samples before leaching were more diversified;After the 0.5 mol/L citric acid+water and 0.5mol/L oxalic acid+water compound leaching,the content of C and O did not change significantly.The loss of soil salt ions and mineral elements caused by 0.5 mol/L citric acid+water leaching is small,and it can maintain the functional stability of soil.Microbial high-throughput gene sequencing showed that the soil samples treated with0.5 mol/L citric acid+water were the closest to the microbial community,COG gene content and frequency of the original soil,and the microbial distribution diversity was higher than 0.5 mol/L oxalic acid+water treatment.4.In the experiment of column leaching of high concentration of Cr contaminated soil by 0.5 mol/L citric acid+water and 0.5 mol/L oxalic acid+water,The amount of leaching was 1075 mL,the number of leaching was 1 time of 0.5mol/L citric acid+water,which‘s effect was the best,reaching 60.71%.The heavy metal Cr removed by 0.5 mol/L citric acid+water mixture mainly exists in water-soluble and acid-extracted state.After leaching,the heavy metal Cr in the soil is transformed into the iron-manganese oxide bound state and the organic combined state,and the column leaching also effectively reduces the environmental risk of heavy metal Cr-contaminated soil.In conclusion,the 0.5 mol/L citric acid+water compound leaching has a good removal effect on Cr-ontaminated soil.It has less damage to the physical and chemical properties and structure of the soil and is prone to degradation.Therefore,it is recommended to choose 0.5 mol/L citric acid+water shaking leaching as a repairing technical scheme for high concentration Cr contaminated soil.