Assessment And Remediation Of The Lead And Cadmium Polluted Soil In Rice Production Areas

Human activities such as sewage irrigation,industrial waste discharge and fertilization have resulted in a faster accumulating rate of heavy metals in paddy soil than the natural accumulation,which seriously threatens agricultural safety production and human health.As Chinese staple crop,rice production is closely related to national health.However,heavy metal pollution incidents in rice have occurred frequently in recent years,which has aroused widespread concern.The rice producing areas in China have a wide distribution from north to south,with significant differences in the physical and chemical properties and pollution levels of paddy soils.And the demand for passivators in the process of remediation of heavy metal contaminated paddy fields is also different.However,there is no standard calculation method for the amount of passivators applied.Therefore,accurate assessment of heavy metal pollution in rice producing areas in China and the establishment of a set of calculation methods for the application amount of passivators are essential to achieve green,efficient,and sustainable remediation of heavy metal-polluted soil.Thus,this study firstly takes the paddy soil in the rice producing areas of China as the research object,and the data were obtained from the National Science and Technology Basic Condition Platform.Then we combines the multi-surface model and Arc GIS software to calculate the content of free Pb and Cd in soil as well as Pb and Cd in rice in the rice producing areas of China,and divides the high-risk areas of Pb and Cd in rice safety production.Finally,it systematically evaluates the effects of different types of passivators on the remediation of paddy contaminated soil.Based on it,a soil remediation scheme for high-risk areas of rice safety production in typical provinces is proposed.The main conclusions are as follows :(1)The multi-surface model calculations showed that the free Pb and Cd contents of soils in the rice producing areas in China ranged from 0-14.58 and 0-1.50 mg/kg,respectively,and were higher in the south than in the north.The Pb and Cd contents in rice are between 0-475.58 and 0-154.11 mg/kg,respectively,and both show varying degrees of exceeding the standard.The areas with rice production that exceed the standard account for 32.29% and 15.54% of the total rice production area,respectively.The most polluted areas of Pb and Cd in rice were similar,mainly concentrated in parts of central and southern Hunan Province and western Yunnan Province.Some areas of provinces such as Jiangxi,Guangxi,Guangdong,Fujian,Zhejiang,and small areas of Liaoning,Anhui,Guizhou,and Sichuan have different degrees of excess Pb in rice.Some areas of Anhui,Jiangxi,Guangxi,and small areas of Liaoning and Hubei have varying degrees of Cd in rice exceedances.(2)Passivators such as lime,organic fertilizer,and biochar can effectively remediate Pb and Cd-polluted paddy fields,with a better remediation effect of Pb than Cd.Among the lime-based remediation agents,quicklime has the best remediation effect,followed by slaked lime,and then limestone.The amount of lime application was linearly positively correlated with the change of soil p H.The remediation effect is significantly better for soil with an initial p H of less than 5.50 than for soil with an initial p H of 5.50 ≤ p H < 6.50.The optimal remediation amount was 0.20 %.Among the organic fertilizer passivators,domestic waste organic fertilizer has the best remediation effect,while crop stubble organic fertilizer and agricultural and sideline product waste organic fertilizer are relatively poor,which is suitable for remediation of lightly polluted soil with heavy metals.The application amount is linearly positively correlated with the change of soil organic matter content.The order of remediation effect of biochar amendments is lignocellulose biochar > herbaceous plant biochar > wood biochar.The application amount is positively correlated with the change of soil p H and organic matter content.The recommended remediation amount is 3.00 %,which is the most widely used amendments.(3)Liaoning Province,Anhui Province and Hunan Province are the representatives of alkaline heavily polluted,weakly acidic medium-to-light polluted and acidic heavily polluted paddy soils.The calculation results of multi-surface model show that the Pb content of rice produced in Liaoning Province,Anhui Province and Hunan Province is between 0-0.23,0-0.97 and 0.01-475.48 mg/kg,and the Cd content is between 0-17.49,0-1.29 and 0-52.99 mg/kg,respectively.There are different degrees of risk of exceeding the standard.The spatial distribution of Pb and Cd content in rice shows that the northern part of the three provinces are all safe production areas for rice and can be directly planted with rice.Rice produced in the central and southern parts of Anhui Province has moderate exceedances of Pb and Cd,and it is supposed to apply 3% wood fiber biochar or 5%other biochar to fix the soil before rice production.Rice produced in the southwestern coastal area of Liaoning Province along the Bohai Sea is seriously beyond the standard of Cd,and rice produced in the central and southern parts of Hunan Province contained too much Pb and Cd.It would be better to adopt other remediation methods or implement planting structure adjustments in these areas.