Study On Remediation Technology Of Biogas Solid Residue Cooperated Ferrous Sulfate In Hexavalent Chromium Contaminated Soil

The remediation of hexavalent chromium contaminated soil is of great significance to protect the ecological environment.The chemical reduction method of ferrous sulfate is widely used in the remediation of Cr(Ⅵ)contaminated soil due to its advantages of fast reaction speed,high efficiency,low cost and easy availability.However,in practical engineering application,it had been found that ferrous sulfate as reducing agent faced problems such as dosage,secondary pollution,soil acidification and so on.In addition,a large number of organic wastes such as biogas solid residue are produced during anaerobic digestion of food waste,which has not been scientifically and reasonably utilized as resources.Based on the above technical difficulties,this paper carried out an experimental study on the remediation of Cr(Ⅵ)contaminated soil with biogas solid residue and ferrous sulfate.The remediation effect of biogas solid residue cooperated with ferrous sulfate on Cr(Ⅵ)and the change of soil physical and chemical properties were analyzed through indoor simulation experiments.The remediation effects of water content,Fe(Ⅱ)/Cr(Ⅵ)molar ratio,biogas solid residue addition and Cr(Ⅵ)initial concentration of contaminated soil were studied by single factor experiment.The chemical stability of Cr(Ⅵ)and total chromium in the soil after remediation was studied through TCLP leaching experiments.The existing forms of chromium in the repaired soil and the changes of soil heavy metal pollution risk were determined through continuous extraction experiments.The phytotoxicity of the repaired soil was analyzed by phytotoxicity experiments and pot experiments,and the microbial community structure and characteristics in the soil were analyzed by highthroughput sequencing.The research results of this paper can provide technical reference for the scientific management of Cr(Ⅵ)contaminated soil.The main conclusions of this study are as follows:(1)The remediation effect of biogas solid residues cooperated with ferrous sulfate on Cr(Ⅵ)contaminated soil was studied through stabilization experiments.The results showed that the content of Cr(Ⅵ)in soil decreased from 1000 mg/kg to 0.18 mg/kg after 30 days,which met the requirements of the first category of land screening value standard≤3.0 mg/kg in the "Soil environmental quality Risk control standard for soil contamination of development land "(GB36600-2018).After remediation,the soil pH did not decrease significantly and there was no acidification phenomenon.The redox potential showed a downward trend,which was conducive to maintaining the chemical stability of the repair effect.When the water content was 40%,the molar ratio of Fe(II)/Cr(VI)was 2.5:1 and the addition amount of biogas solid residue was 4%(dry basis),the remediation of contaminated soil with Cr(VI)content≤2000 mg/kg can meet the requirements of remediation standards.After the biogas solid residue cooperated with ferrous sulfate stabilization treatment,the chromium in the soil was transformed into a more stable form,and the environmental risk level was reduced from high risk to low risk.The diversity and relative abundance of microbial communities in the soil increased,and bacterial genera with chromium tolerance and Cr(VI)reduction ability,such as Bacillus and Sphingomonas,were relatively abundant.(2)The remediation effects of different types of reducing agents on simulated chromium slag contaminated soil(Cr(VI)=10000 mg/kg)were compared and studied.The results showed that ferrous sulfate used alone did not meet the requirements of the remediation standard,and the soil acidification phenomenon was obvious when the Fe(II)/Cr(VI)molar ratio was 8:1.However,using biogas solid residue cooperated with ferrous sulfate,when the water content was 40%,the Fe(II)/Cr(VI)molar ratio was 3:1,and the biogas residue addition amount was 4%(dry basis),the Cr(VI)content in the soil dropped to 5.37 mg/kg after 60 days of remediation,which met the requirements of the second category of land screening value standard≤5.7 mg/kg,and the TCLP leaching concentration met the "Identification standards for hazardous wastesIdentification for extraction toxicity "(GB5085.3-2007)standard.(3)The growth of ryegrass and the accumulation of chromium after the remediation of biogas solid residue cooperated with ferrous sulfate were studied through pot experiments.The results showed that the growth of ryegrass was the best when the molar ratio of Fe(II)/Cr(VI)was 3:1 and the addition amount of biogas solid residue was 4%,and the physical and chemical properties of the soil did not change significantly.Chromium in the ryegrass plants mainly accumulated in the roots.The analysis of microbial community structure showed that the diversity of soil microbial community increased,the relative abundance increased,and the abundance of microbial metabolic function genes related to Cr(Ⅵ)reduction also increased.