Primary Exploration On Bioavailability Of Lead And Prediction Model In Soil And Wheat(Triticum Aestivum L.) System

Lead(Pb)is one of the typical toxic heavy metals,which will enter into farmland with the activities like industrial emission and agricultural production,threatening the safety of agricultural products and people's health.The environmental and health risk of Pb is closely related to the speciation transformation and bioavailability of Pb.So,study on the bioavailability of Pb in soil-crop system is important for controlling Pb pollution in soil and ensuring the safety of agricultural products.In this study,21 types of farmland soils were selected as our analytical objects.And chemical extraction methods,diffusive gradients in thin-films technique(DGT),Fourier transform infrared(FTIR)were combined to explore the aging process of Pb in typical soils and its influence on soil chemical properties,investigating the transformation,accumulation and distribution of Pb in soil-wheat system and building prediction models of Pb bioavailability in soil as well.The main research results are as follows:(1)Changes of Pb speciation in different aging soils and its influence on the chemical properties of soil were clarified.As a whole,the concentration of EDTA extractable Pb and CaCl2 extractable Pb decreased with aging time and gradually stabilized in 6 months,especially for acid soil.Pseudo-second-order model expressed the dynamic process of available Pb in the aging process well,of which the fitted coefficient was above 0.99 and 0.83 for EDTA extractable Pb and CaCl2 extractable Pb.Moreover,after aging for 6 months,the concentration of CaCl2 extractable Pb changed fastest among all the extractable Pb,which decreased by up to 91%,followed by DGT extractable Pb,water soluble Pb and EDTA extractable Pb,while the concentration of HNO3 extractable Pb changed slowest,which decreased by up to 26%.Besides,the addition of exogenous Pb reduced soil pH,but having no effect on cation exchange capacity(CEC)and dissolved organic carbon(DOC).Meanwhile,The pH value decreased with aging time.And no significant differences were found for the CEC value,but DOC decreased significantly with aging time.(2)Accumulation and distribution of Pb in soil-wheat systerm was investigated.When the exogenous Pb concentration was between 125?350 mg/kg,more than 80%of Pb treatment groups showed no significant difference to the control groups in the height,fresh biomass and ear weight of wheat.The order of concentration of Pb in wheat was:root>leaf>stem>husk>grain.And the Pb concentration of grain in the control groups did not exceed the requirements in the food standard(0.2 mg/kg),while about 50%and 83.33%of the grains exceeded the standard in the 125-175 mg/kg and 250-350 mg/kg Pb treatment group.Pb concentration of grain was significantly negatively correlated with pH,indicating that soil pH value was the key factor affecting the accumulation of Pb in wheat.FTIR-spectra showed that exogenous Pb mainly interacted with hydroxyl(-O-H)and carbonyl(C=O)of soil.And hydroxyl(-O-H)or C-O of root participated in the adsorption of Pb.Hydroxyl(-OH)or amino(-NH2),carboxyl(COO-),C-O of stem and C=O of leaf interacted with Pb,too.(3)Prediction models of Pb bioavailability were built.When aging for 1 month,total Pb concentration,pH and texture could predicted the EDTA extractable Pb concentration well,of which the fitted coefficient was 0.957;However,for the soils before planting wheat,total phosphorus,soil organic matter and texture could predicted the EDTA extractable Pb concentration well,of which the fitted coefficient was 0.931.The different factors of models may be related to the aging time.Besides,total Pb concentration in soil and pH explained the variation of Pb concentration in 83.8%wheat grain,while EDTA extractable Pb concentration in soil and pH explained the variation of Pb concentration in 78.0%wheat grain.Then,we used data from modeling and literature to verify the model.And the predicted concentration was closed to the measured one when the total Pb content in soil was in the range of fitting data,which showed that the prediction model of the concentration of wheat grain was reliable.So,safety critical value of Pb in soil derived by the prediction model can be of great help for soil Pb risk assessment and soil standard formulation in China.