Simultaneous Enrichment And Extraction Of Arsenic And Cadmium In Paddy Soil By Rice

Dietary exposure to arsenic(As)and cadmium(Cd)through rice consumption has long been identified as a global health issue.Co-contamination and transfer of As/Cd in paddy soil-rice continuum makes this issue worse and could be linked to higher cancer risk.From soil cleanup perspective,however,rice plant provides a neglected export eliminating exactly rice-available As/Cd from paddies.In this study,rice was used as a remediation plant to explore the simultaneous enrichment and extraction effect of rice on arsenic and cadmium in paddy soil.In the first part,the DGT technique was used to reveal the enrichment and extraction mechanism of the effective As/Cd in paddy soil through field experiment and laboratory rhizotrons test.The rhizotrons test was used to further explore the key chemical factors controlling the availability of As/Cd in the rhizosphere.In the second part,the enrichment and extraction capacity of available As/Cd in paddy soil of different varieties of rice was investigated by using the rhizotrons.The main research contents and results are as follows:(1)During the growth process of field remediation rice(2019),the content of DGT-As in the rhizosphere of rice was 45.7-83.6% lower than that of native aquatic plants at tillering stage(flooded 38 days),and the DGT-As was at a lower level after harvest.However,the migration of Cd in soil at tillering stage was greatly inhibited by flooding,and it was significantly increased within the depth of 0-10 cm at heading and flowering stage.In the 0-20 cm depth range,rice showed an obvious advantage in unstable Cd depletion compared with native hypotophytes within two months after harvest.Soil Cd availability was the lowest after the removal of whole rice plants including roots,which was about 18% lower than that after the retention of roots.(2)In the rhizotrons experiment,rice reduced porewater As by 28.9-87.1%compared with the control.However,the content of Cd in porewater was always inhibited by flooding.After five weeks of flooding,temporary drainage was carried out to simulate traditional field drying,and the content of Cd in porewater increased sharply.In addition,Cd was continuously activated in rice during the whole growing period.With the extension of growing time,the acidification of rice rhizosphere reduced the active Cd in the soil at the harvest period by about 53%.(3)In the following year(2020),test rice will be planted to evaluate the effectiveness of soil cleaning.For all treatments,the concentration of As in white rice decreased from 0.23 mg/kg in 2019 to 0.12-0.16 mg/kg,and the employment of the rice plant for soil As extraction followed by whole plant removal resulted in the lowest grain As concentration in 2020,which was 21%lower than that in removal native indigenous hygrophytes treatment.For Cd,the concentration of Cd in white rice was significantly reduced by 24% after removal of the whole rice plant.It was verified that the removal of the whole rice plant including the root system after planting rice had the best remediation effect on the simultaneous enrichment and extraction of arsenic and cadmium from paddy soil.(4)Different rice varieties have different transport and uptake of As/Cd in soil.As remediation of As/Cd contaminated paddy soil,the accumulation of As/Cd extracted from hybrid rice is higher than that from conventional rice,and the extraction amount of As/Cd per plant from conventional rice was only 83%and 23% of that from hybrid rice,respectively Therefore,hybrid rice is more suitable than conventional rice for remediation of arsenic-cadmium compound contaminated paddy soil.