Stabilization And Remediation Of Cadmium And Arsenic Contaminated Soil In Cold Region By Modified Biochar

Heavy m et al pollution in soils is a common environmental problem in China,among which cadmium（Cd）and arsenic（As）are two toxic elements in contaminated soils,which are difficult to be rehabilitated.At present,there are many types of remediation technologies for heavy m et als in soil,among which solidification and stabilization remediation technology is studied more and applied most widely.Biochar and its modified materials are a kind of high-quality stabilization and remediation materials,especially iron modified biochar and magnetic biochar have become common biochar materials in heavy m et al contaminated soil,but their simultaneous stabilization effects on cadmium and arsenic still need to be further improved.Freeze-thaw action is a kind of abiotic stress commonly existing in cold areas,It has a unique influence on soil physical and chemical properties and element migration,and then changes the stabilization state of Cd and As after remediation in seasonal frozen soil areas.However,there are relatively few studies on the properties and remediation effects of remediation materials during freeze-thaw process,and the influence and mechanism of freeze-thaw process on the stability of Cd and As in soil are still unclear.Based on this,biochar,iron-modified biochar,magnetic biochar and ferrorich magnetic biochar were prepared from corn straw core in this study.The changes of micro-morphology and surface elements of modified biochar materials were studied by scanning electron microscope and energy dispersive spectroscopy.By applying four kinds of biochar materials with different doses,the best dosage ratio of cadmium and arsenic synchronous stabilization and the best biochar materials were screened out.A series of freezing and thawing experiments were carried out on the contaminated soil treated with the best biochar material for 30 days to explore the effects of freezing and thawing frequency and water content on the stable remediation of cadmium and arsenic with biochar materials,in order to provide theoretical and technical support for the stable remediation of cadmium and arsenic in cold contaminated soil.The paper draws the following results:（1）Biochar（BC）,Fe-modified biochar（F-BC）,magnetic biochar（MBC）and ferrorich magnetic biochar（F-MBC）all have stabilization and remediation effects on Cd and As in soil,among which F-MBC has the best remediation effect,and the available Cd and As contents have decreased by more than 45% and 55% respectively.Secondly,the dosage of 5% BC and MBC,BC had a better effect on Cd stabilization,which could reduce the available Cd by 43.3%,while MBC had a better effect on As restoration,which could reduce the effective As content by 36.3%.（2）The pH value of soil increased during the first freeze-thaw,and then decreased significantly（F1 ～ F12）;The EC value of soil increased significantly after12 freeze-thaw cycles.Under freezing and thawing,the soil pH value increased significantly with the increase of soil water content from 15% to 70%,while the change of EC value showed an opposite law.（3）Freeze-thaw process significantly increased the availability of Cd in soil,but decreased the availability of As.After 12 freeze-thaw cycles,the content of available Cd in soil was significantly higher than that without freeze-thaw treatment.The content of available As in soil decreased significantly after 12 times of freezing and thawing,and the content of available As in frozen and thawed soil was significantly lower than that in unfrozen and thawed soil.Under the condition of 30% moisture content,the content of available Cd and As in soil after freezing and thawing process was significantly higher than that of 15% and 70% moisture content treatments;However,under the condition of 70% water content,the available Cd content decreased to the lowest,and there was no significant difference between freeze-thaw treatment and non-freeze-thaw treatment.Under the condition of 15% and 70%moisture content,the available As of freeze-thaw treatment is lower than that of non-freeze-thaw treatment.（4）Freezing and thawing can change the surface structure of F-MBC and reduce the content of Fe,O and other elements on its surface,which leads to the decrease of As adsorption by F-MBC.After the initial freeze-thaw cycle,F-MBC could significantly increase the soil pH value,and with the continuation of freeze-thaw cycle,the soil pH value gradually decreased and gradually flattened;F-MBC significantly increased the EC value of soil,and showed a significant upward trend with the continuation of freeze-thaw cycle.（5）After adding F-MBC to soil,the content of available Cd in soil decreased with the increase of water content.Under the condition of 30% and 70% water content,the content of available Cd in freeze-thaw treatment decreased by 15.13%and 6.9% respectively compared with that in non-freeze-thaw treatment.Under the conditions of 30% and 70% moisture content,the available As content of freeze-thaw treatment was significantly higher than that of non-freeze-thaw treatment,and increased by 27.43% and 10.07%,respectively.（6）After 12 freeze-thaw cycles,the content of available Cd in soil increased by15.42%,but with the continuation of freeze-thaw cycles,the content of available Cd in non-freeze-thaw treatment was significantly higher than that in freeze-thaw treatment;In the process of stabilized As remediation,the content of available As decreased with the continuation of freeze-thaw cycle.In F1,F3 and F12,the content of available As in freeze-thaw treatment was significantly lower than that in non-freeze-thaw treatment.To sum up,F-MBC can still achieve simultaneous stabilization and remediation of Cd and As contaminated soil after freezing and thawing process,which shows that F-MBC as stabilization and remediation material in cold areas can provide long-term effective support for remediation of Cd and As contaminated soil in seasonal freezing and thawing areas.