| Soil stabilization technology is widely used for the remediation of heavy metal pollution in soil due to its characteristics such as in-situ operability and highly efficient environmental protection,and the core of this technology is the application of environmentally friendly and highly efficient sustainable stabilizing materials.At present,there are many types of stabilizing materials in laboratories and markets,including soil conditioners,biochar materials,and industrial products.These materials provide a certain remedial effect on contaminated soil with arsenic and cadmium.However,the stabilized restoration is affected not only by the type of material,but also by climatic conditions,regions and other factors.Therefore,it is very worth studying and solving the problems which the efficient remediation of farmland contaminated by As、Cd and the applicability in seasonal freeze-thaw areas.This will provide basis to restoration technology of contaminated soil by As、Cd.Currently,remediation of contaminated soil by As、Cd with different stabilizing materials has been studied.But there are few reports on the effects of treatment of toxic soil by As、Cd during in the case of freeze-thaw alternation.The content of this study is what the changes of soil physicochemical properties and the stabilization effects of As、Cd were studied primarily under the simulated 15 cycles of freezing-thawing,and discussed posteriorly the stabilization effect of composite materials by organic and biochar and the soil physicochemical properties after application before and after freezing-thawing,and explore the changes of biochar structure,surface elements and conjugate（crystal form）in contaminated aqueous solution with As、Cd in freezing-thawing and unfreezing-thawing to reveal the mechanism of biochar stabilization and influence of freezing-thawing.（1）Experiment of screening soil arsenic-cadmium stabilization materials:In our experiment,19 kinds of common heavy metal stabilized materials were studied to explore the effects of freeze-thaw alternation on soil physicochemical properties and stability of arsenic and cadmium with 15 cycles of freezing-thawing process environment.The results showed:（1）the remediation materials increased p H and EC of compound contaminated soil.（2）Clay minerals,soil conditioner,biochar and microbial products had significant differences in stabling effect for As and Cd,which showed biochar>soil conditioner>clay minerals>microbial products.（3）Freeze-thaw alternation affected the remediation effect of stabilized materials.Only biochar materials（Musaceae biochar（MBC）,iron modified biochar（FBC2）and mixed ferromagnetic modified biochar（FBC1））were less affected by freeze-thaw.Among them,freeze-thaw improved the stabilization effect of MBC on As and Cd,which were14.2%（As）and 12.5%（Cd）respectively.In general,biochar（MBC）is befitting for the remediation of As and Cd contaminated soil in the cold area of northern China.（2）Composite material screening experiment:On the basis of pre-experiment and experiment（1）,primary plantain biochar（planto biomass direct firing,BC）and iron-mixed magnetic plantain biochar（plantain biomass fired after magnetic modification,FBC）were selected as stabilizers.Material.The organic acid-biochar mixed application method,that is,the organic acid was applied first and then the stabilized material was applied,was used to explore the changes of soil physicochemical properties,As、Cd stabilization effects and As、Cd graded forms under the combined application of organic acid-biochar and single application of biochar.The result showed that:（1）the application of materials affected the physicochemical properties of the soil.The single application of biochar increased the soil p H value,but the organic acid reduced the increase the amplitude of the soil p H value under the application of biochar.When the soil was treated for 21 days,the general rule of soil p H value was:BC>organic acid-BC>FBC>CK>organic acid-FBC.In addition,the applied materials significantly improved soil EC value and organic matter content;（2）the combined treatment effect of organic acid-biochar decreased first and the stabilized with the increase of the remediation period,while the single use of biochar increase at first and the stabilized with the change of period,but the stabilization effect of the combined materials,oxalic acid-biochar,was great better than the single biochar,citric acid-biochar.In terms of economic cost,mixed application of oxalic acid and magnetically modified biochar（OA-FBC）was the best choice,and could simultaneously stabilize 41.82%As and 33.79%Cd;（3）the proportion of As、Cd graded forms in contaminated soil were affected by different materials.Under the single application of FBC,the content of residual and oxidizable As、Cd increased,while the content of acid-extractable and reducible As、Cd decreased.Under the combined application of organic acid and biochar,especially the combined oxalic acid with ferromagnetic biochar,the proportion of acid-extractable As、Cd decreased and oxidizable As、Cd increased and residual As increased.（3）Stability test of composite materials under freeze-thaw alternating:Based on experiment（2）,this chapter studied the effects of freeze-thaw alternation on the physicochemical properties,biochar properties,As、Cd stabilization effects and As、Cd graded forms of soil remediation with composite materials under the simulated environment of different cycles of freeze-thaw alternation.The result was displayed to:（1）under the long-term freeze-thaw alternation,the single application of biochar increased the soil p H and decreased the soil p H value under the combined materials.In addition,the freezing-thawing increased the soil EC value of some treatment groups（SR5,SR8）to certain extent,but did’t affect the soil organic matter content;（2）long-cycle freeze-thaw alternation only significantly decreased the p H value of magnetically modified biochar（FBC）and the EC value of primary biochar（BC）;（3）the effect of stable remediation of As、Cd in soil was affected by the freeze-thaw alternation.Mixed application of oxalic acid and mixed iron magnetic biochar had the best effect of remediation on As、Cd simultaneously in the freeze-thaw environment.In a word,the application OA-FBC could simultaneously stabilize the soil heavy metal As、Cd,and it was also suitable for the freeze-thaw environment.（4）Experiment on the mechanism of As、Cd stabilization and freeze-thaw:In this chapter,under the simulated 15-cycle freeze-thaw environment,adsorption experiments were used to study the adsorption and immobilization of As、Cd in highly polluted aqueous solution by three biochars（namely BC,FBC,OA-FBC）.The change of the material（crystal form）revealed the stabilization mechanism of biochar and the mechanism of freeze-thaw effect.SEM-EDS and XRD image showed that:（1）biochar relies on different modes of action,including biochar adsorption properties,precipitation/co-precipitation,organic matter chelation and substance-to-substance interactions,to fix the heavy metal As、Cd on the pores or surfaces of biochar.The surface of magnetically modified biochar（FBC）is rich in iron oxides（Fe3O4,Fe2O3）,which is easy to combine with As to form iron-arsenic compounds（Fe（H2As O4）3·H2O）;（2）Due to the acid dissolution and self-decomposition of oxalic acid,the contact rate between As,Cd and elements such as Si、Ca、Al increases,which promotes the formation of cadmium complexes(Cd3Al2Si3O12)and cadmium precipitation Cd CO3;（3）Freeze-thaw not only increases the adsorption sites on the surface of carbon and the abundance of elements on the surface of biochar to improve the surface adsorption capacity of biochar,but also the physical effects of freeze-thaw will not destroy the stable As、Cd conjugates.Therefore,freeze-thaw promote the stabilization of As、Cd in soil. |
