| As the cornerstone of agricultural development and food security,soil is of selfevident importance to human production and life.In recent years,the rapid development of our economy and society has brought about heavy metal pollution in soil which can not be ignored,which is a serious threat to food safety and human health.How to reduce the absorption and accumulation of heavy metals in crops and achieve soil pollution remediation is a major issue in the sustainable development of agriculture.Soil heavy metal passivator,also known as soil conditioner,is an important means recommended by the Ministry of Agriculture to repair and control heavy metal pollution in farmland.However,most of the previous studies were on non-iron passivating agents,and mainly focused on a single heavy metal,and there were few studies on the pollution control of complex heavy metals.In this study,iron-based passivating agents were selected,and pot experiments were conducted on rice(flooded)and wheat(dry)soil to explore the effect and preliminary mechanism of different iron-based passivating agents on reducing heavy metal absorption and accumulation in rice and wheat,and further screen out two passivating agents with better effects.A constant temperature and humidity culture test was used to further clarify the passivation repair effect of the compound heavy metals under drought and flooding conditions.The main findings are as follows:1.Five kinds of soil passivating agents,including zero valent iron(Fe P),humic acid(H),iron base humic acid(Fe H),iron base medical stone(Fe M)and iron base shell powder(Fe B),were selected in rice soil pot experiment to study their effects on absorption and accumulation of As,Cd,Pb and Zn in rice and the changes of available heavy metal content in soil.The results showed that the heavy metal content of brown rice was significantly decreased by applying five kinds of iron passivating agents except for the insignificant reduction of Zn content.The contents of As and Pb were reduced by 14.7%-33.6%,43.9%-74.0%,and Cd content was reduced by 46.5%、50.0% by using Fe B and Fe M,respectively.In addition,MDA content in rice leaves decreased significantly at tillering stage by 18.0%-40.9%.Application of passivating agent significantly reduced the content of available As,Cd and Pb in soil by 21.0%-60.8%,8.2%-17.3% and 10.3%-40.4% respectively.A comprehensive comparison showed that passivating agent Fe M had the best control effect on the combined pollution of arsenic,cadmium,lead and zinc.2.In order to understand the after-effects of passivating agents,the absorption and utilization of arsenic,cadmium,lead and zinc in wheat after application of passivating agents in rice and the changes of available heavy metal content in soil were further studied.The results showed that the contents of Cd,Pb and Zn in wheat flour were reduced to 3.2%-12.9%,12.6%-36.9% and 17.8%-31.1%,respectively.However,there was no significant effect on arsenic absorption at maturity stage except on tillering stage of wheat.The analysis of heavy metal morphology in wheat soil showed that the proportion of residual Cd increased by 3.2%-23.6%,but the proportion of Fe and Mn oxides binding,carbonate binding and exchangeable states decreased by 3.4%-17.3%,3.8%-7.1% and 1.2%-6.3%,respectively.3.The effects of two passivating agents,humic acid(H)and ferri-based Maifan stone(Fe M),which performed well in pot experiments,on the availability of soil As,Cd,Pb and Zn and the morphology transformation of As and Cd under drought and flooding conditions were studied by constant humidity constant temperature soil culture experiment.The results showed that under drought conditions,the content of available As,Cd and Zn in soil decreased by 42.75%-59.47%,48.13%-49.45% and 13.37%-28.21%,respectively.Using Fe M reduced the content of available As,Cd and Zn in soil to 11.64%-37.73%,25.72%-26.17% and 19.12%-49.85%,respectively.The results of morphology analysis of arsenic and cadmium showed that the proportion of residual As increased by 5.25%-6.27%,while the proportion of crystalline Fe-Al hydrate oxide binding,amorphous,weakly crystalline Fe-Al oxide binding and obligate adsorption states decreased by 1.33%-2.37%,0.71-1.50%,and 3.55%-4.07%,respectively.Applying passivating agent increased the proportion of Cd in residual state by 2.58%-10.49%,and decreased the proportion of carbonate binding state by 0.75%-5.14%.Under flooding conditions,application of H increased the content of available As,Pb and Zn by 12.2%-13.3%,7.1%-45.7% and 16.9%-31.0%,and decreased the content of available Cd by 10.7%-18.5%,respectively.Using Fe M increased the soil available As,Pb and Zn by 23.4%-34.4%,77.6%-86.6% and 9.12%-14.0%,respectively,and reduced the content of effective Cd by 10.5%-15.0%.The results of As and Cd morphology analysis showed that the proportion of residual As increased by 5.4%-6.4%,while the proportion of crystalline Fe-Al hydrate oxide binding and obligatory adsorption states decreased by 1.0%-2.1% and 0.1%-1.1%,respectively.Applying 1%Fe M increased the proportion of residual Cd by 5.2%,and applying 0.2%Fe M and 1%Fe M reduced the proportion of Fe-Mn oxide binding state by 5.7%、7.4%,respectively.The results showed that under the experimental conditions,although the content of available As,Pb and Zn changed under flooding conditions,the passivation effect of iron based passivation agent on heavy metals in this experiment was positive according to the morphological analysis results. |
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