Predicting The Speciation Distribution Of Cd And Pb In Soil And Accumulation Characteristics In Rice By Multi-surface Model

Heavy metals come from the soil and eventually return to the soil.With China's social development and population growth,the problem of soil heavy metal pollution is becoming more and more serious.In the rice-producing areas of China,Cd and Pb usually coexist in the soil,and its toxic effect is more complicated than single heavy metal pollution.Study on the species changes and transportation of compound heavy metals in the active soil solid phase-soil solution-rice system during rice growth,and clarification of the competition of multi-metals at the interface of soil active components and the mechanism of bioavailability.These are benefit for understanding the transportation and transformation process of heavy metals in farmland systems,at the same time provide reasonable support for the assessment of agricultural product safety risks and soil ecological risks of heavy metals in farmland systems.Therefore,in this research,the brown-red paddy soil under combined pollution of Cd and Pb was taken as the research object.The speciation changes of heavy metals and the effect of different concentrations of Pb on the bioavailability of Cd under drying-wetting cycles conditions in the rice growth process were studied by pot experiments.The multi-surface model predicted the heavy metal speciation and its responsed to environmental changes,and initially established the relationship between the concentration of heavy metal in soil solutions and the concentration of heavy metals in rice grains.The specific research content includes:?1?Studied the speciation changes of Cd and Pb in the soil under drying-wetting cycles conditions during rice planting and its influencing factors;?2?Usage of the multi-surface model to predicted the ion concentration of free heavy metal Cd and Pd under the redox reaction during rice planting;?3?Exploration of the correlation between the concentrations of Cd and Pb in various speciation and the concentration of heavy metals in grain.Studied on the effect of Pb on the bioavailability of Cd.The main results of the study are as follows:?1?The pH,Eh,DOC in the water layer and the upper and bottom soil layers under flooding conditions during the rice growth period had significantly different changes,but the values mentioned above of soil layer were just slightly changed compared with later flooding period.The pH of the water layer decreased first,then gradually increased and stabilized,and the pH of the upper and bottom layers of the soil changed in a consistent manner,when they stabilized.The Eh of water layer was stable during the entire flooding period,while the Eh of soil layer dropped sharply and tended to be stable.The application of organic pesticides in the later flooding period caused the DOC value to be higher than in the early stage.The DOC in the soil layers is no obvious change during the flooding period,and the DOC in the drainage period wsa slightly lower than that in the flooding period.?2?The concentration of NO₃^-,SO₄^2-,dissolved iron and manganese under drying-wetting cycles conditions all increased first then decreased and gradually returned to the concentration in initial flooding period.In the early stage of flooding,the concentration of NO₃^-in the water layer and the upper and bottom soil layers increased rapidly.It returned to the concentration of initial flooding period in about 60 days.The concentration of NO₃^-consistent with SO₄^2-increased within 20 days of flooding period.The concentration of dissolved iron in the upper and bottom soil layers reached the maximum value after 17 days.The concentration of dissolved iron in the water layer reached the maximum value after 40 days.The dissolved manganese in the bottom soil layers reached the maximum value at 17 days.The dissolved manganese in the upper soil layers and water layers reached the maximum after 35 days.?3?The concentrations of dissolved and free Cd and Pb in each treatment were basically the same,and the concentration gradually decreased with the flooding time.The concentrations of free Cd accounted for 71.32%of concentrations of dissolved Cd.The concentrations of free Pb accounted for 83.97%of concentrations of dissolved Pb.The results were related to the increase in pH,the precipitation of heavy metals with S^-and the co-precipitation of heavy metals with iron and manganese.?4?Predicted and measured the concentration of dissolved and free Cd and Pb in the compound pollution system.These concentration values were linearly related to the concentration of Cd and Pb in rice.The multi-surface model could predict the concentration of dissolved and free Pb and Cd in different concentrations of Pb and Cd.The concentration of Cd and Pb in rice increased with the increase of the concentration of Pb.Except for the concentration of Pb in the Pb 0 mg/kg treatment,the concentration of other treatments exceeded the food safety standards.The dissolved and free Cd concentrations in the soil at the early stage of flooding period were significantly correlated with the Cd concentration in the rice?p<0.01,R²?0.98?.It showed that the dissolved and free Cd and Pb concentrations in the soil could reflect its accumulation in rice grains,which could provide a reference for the establishment of a composite system of safety standards of agricultural products.