The Effects And Mechanism Of Chemical Washing And Biochar Immobilization On The Remediation Of Cadmium Contaminated Paddy Soils

Cadmium（Cd）pollution in the soil is a serious threat to rice quality in China,necessitating the development of efficient and cost-effective remediation technologies to decrease the risk of Cd pollution in farmland.Chemical washing may effectively reduce Cd content in the soil,however,it has a significant influence on soil properties,including aggregate structure damage and nutrient element loss.After biochar post-treatment of the chemical-washed soil,soil aggregate characteristics may be restored partially,resulting in lower Cd bioavailability in soil and alleviation of Cd accumulation in rice.Meanwhile,biochar contributes to soil amendment by restoring soil properties and biological activity that have been destroyed by chemical washing.Chemical washing combined with biochar remediation technology has a wide range of applications.However,the mechanisms for the restoration of soil aggregates and release-control of Cd in the soil during the combined treatment are still unclear.Besides,the remediation effect of this technology on the soil-rice systems at a field scale needs to be further demonstrated.In this study,two typical Cd-contaminated soils in south China,Changsha and Daye,were selected as the research object.Laboratory and field experiments were conducted to elucidate the effects and mechanisms of chemical washing combined with biochar on the remediation of paddy soil.The changes in soil properties and the mechanisms of Cd reduction in rice were also discussed.The main conclusions are as follows:（1）Changsha soil(total Cd 1.20 mg kg^-1,available Cd 0.42 mg kg^-1)and Daye soil(total Cd 1.78 mg kg^-1,available Cd 0.52 mg kg^-1)were severely and moderately polluted,respectively,with Cd mainly distributed in exchangeable fraction,carbonate-bound fraction and Fe/Mn oxide-bound fraction.Principal component analysis showed that total Cd,cation exchange capacity,total manganese,and amorphous Fe/Mn were the main factors affecting available Cd content in the soil.The content of total Cd in Changsha soil increased with decreasing aggregate size（250-2000,53-250,2-53,and<2μm）,and the content of total Cd was 0.87,1.07,1.21,and 1.40 mg kg^-1,respectively.However,the content of total Cd in Daye soil decreased initially and subsequently increased as aggregate size decreased.In Changsha soil and Daye soil,Cd is mainly enriched in 53-250μm and 250-2000μm aggregates,accounting for 42.37%and 62.69%,respectively.（2）Fe Cl₃ washing significantly reduced the total Cd content in the soil,and the removal rates of total Cd in Changsha and Daye soils were 51.6%and 38.9%,respectively.The removal rate of Cd in soil increased significantly with increasing Fe Cl₃ concentration（0-50 m M）and washing time（2-360 min）（P<0.05）,but decreased significantly with decreasing p H（2-5）and liquid-solid ratio（0.5:1-5:1）（P<0.05）.After the soil aggregates were destroyed,Fe Cl₃ increased the washing efficiency of Changsha and Daye soils by16.2%and 8.2%,respectively.Soil aggregates were disintegrated by Fe Cl₃ washing.For example,the proportion of Changsha macroaggregates（250-2000μm）decreased by 33.67%from 64.01%to 42.46%,while that of small aggregates（2-53μm and<2μm）increased from 6.69%and 1.47%to 16.88%and 11.9%,respectively.The percentage increases were10.19%and 10.43%,respectively.On the one hand,Fe Cl₃ promoted Cd release in the soil not only by ion exchange,acidolysis,and complexation but also by disintegrating soil macroaggregates.Iron-manganese oxide-bound Cd became the component controlling soil Cd activity after washing,and manganese oxide-bound Cd was easily released by the dissolution of organic acids generated by plant roots.Citric acid was utilized to represent the low-molecular-weight organic acid.It was found that citric acid could promote the increase of available Cd in the soil of Changsha and Daye.Citric acid destroys the formation of birnessite by acidolysis,complexation,and reduction,and promotes the release of Cd.（3）Biochar application can effectively reduce the content of available Cd in the soil following Fe Cl₃ washing.The content of available Cd in Changsha and Daye soils decreased by 74.4%and 79.6%,respectively,and further decreased with biochar application.For example,after 5%biochar application,the proportion of large-sized aggregates（53-250μm）increased by 62.1%from 28.8%to 36.5%,while the proportion of small-sized aggregates（2-53 and<2μm）decreased by 53.1%and 83.9%,respectively.As a result,the mobility and bioavailability of Cd are reduced.In addition,biochar can promote the conversion of exchangeable Cd and carbonate-bound Cd to iron-manganese oxide-bound Cd and organic-bound Cd through ion exchange,complexation and precipitation,thus inhibiting Cd release.（4）After Fe Cl₃ washing combined with biochar remediation at the field scale,Cd content in rice grains could reach the standard and yield increased.The field experiment showed that after Fe Cl₃ washing（optimized solution:0.03 M,solid-liquid ratio 1.5:1,the soaking field for 1 day,tilling and stirring once,and sunning operation）,total Cd and available Cd contents in soil decreased from 1.23 and 0.41 mg kg-1 to 0.85 and 0.24 mg kg-1,respectively,decreased by 30.9%and 41.6%.After washing,2%biochar could further reduce the content of available Cd in the soil to 0.11 mg kg-1,and the further reduction rate was 60.4%.After Fe Cl₃ washing combined with biochar remediation,the total Cd content of brown rice in Zhuliangyou 189 and Zhuliangyou 929 decreased from0.44 and 0.32 mg kg^-1 to 0.12 and 0.04 mg kg^-1,respectively,lower than the national standard limit of 0.2mg kg^-1.Rice yield increased by 11.8%and 9.1%,respectively.Biochar significantly reduced the transport of Cd from rice root to grain,and the transport coefficient decreased by 28.3%-68.3%.Soil organic matter,available nitrogen,available phosphorus,and available potassium significantly increased by 12%-102%,59%-93%,1%-48%,and 19%-1347%,respectively,after biochar application.The remediation cost was（?）9858/mu.The remediation effect could be further improved if combined with agronomic management.For example,flooding management had a larger inhibitory effect on cadmium absorption,migration,and accumulation in rice.