Accumulation And Distribution Of Airborne Pb In Rice Plant And Prediction Model Of Pb In Polished Rice

Lead（Pb）is a common toxic heavy metal,which will enter the atmosphere with fossil fuel combustion,industrial production and other activities,causing lead pollution.Studying the migration and accumulation characteristics of atmospheric lead in agricultural ecosystems is of great significance to ensure the safety of agricultural products.This study selected rice as the research object.Firstly,the lead content in polished rice from rice producing areas in Zhejiang province was investigated,and the Pb stable isotope characteristics of the atmosphere and soil in various places were used to determine the contribution of airborne Pb to Pb enriched in polished rice.Secondly,the effects of lead in particulate matter from different emission sources and different particle size ranges on migration and enrichment of airborne Pb in rice were studied through pot trials in net house.Finally,field experiments were conducted to clarify the role of atmospheric lead at different growth stages of rice,predict the airborne Pb bioconcentration factor(K_a（rice）)of polished rice of different rice cultivars,and build a mathematical model of Pb in polished rice in the atmosphere-rice-soil ecosystem.The main conclusions were as follows:（1）The survey results of Pb content in rice in 9 regions of Zhejiang Province showed that the average Pb content in polished rice was 0.0265±0.0118 μg/g,which was in line with relevant national food safety standards（GB2762-2017）.Correlation analysis showed that there was a highly significant positive correlation between Pb content in polished rice and available Pb concentration in PM2.5（P<0.01）,and a significant positive correlation with available Pb concentration in soil（P<0.05）,but no significant correlation with total Pb concentration in soil（P>0.05）.Pb stable isotope traceability study indicated that the average contribution rates of the atmosphere and soil to Pb in polished rice were 69.37% and 30.63%,respectively.（2）There were significant differences in the Pb content of rice leaves in different emission source exposure groups.Pb in the waste incineration fly ash had the highest concentration in mesophyll.Pb in the coal-fired fly ash was mostly concentrated in the waxy layer of the leaves.And Pb in the vehicle exhaust fly ash was mostly fixed on the leaf surface.The migration performance of Pb in rice leaves from different emission sources was related to its Pb dissolution capacity.The results of soil culture experiments showed that there were significant differences in Pb content in rice leaves and grains in different particle size treatment groups.Coarse particles had better sedimentation performance,and its Pb had a higher bioconcentration factor in rice leaves.Pb in fine particles was highly bioavailable,and the airborne Pb enriched in rice grains mainly came from Pb in fine particles.（3）Through field experiments,the early exposure group and full-term exposure group were set to investigate the contribution of airborne Pb to rice in early and late growth stage.The result showed that the atmospheric exposure during the late growth stage resulted in the significant increase of Pb content in the aerial parts of rice plants.The Pb contribution of soil,air during the early and late growth stage to polished rice ranged from 17.86-27.33%,4.13-22.69%,and 49.98-73.73%,respectively,which indicating that the late growth stage was important for rice grains to absorb and enrich airborne Pb.（4）Field tests in 2018-2019 found significant differences in Pb content in polished rice of different rice types:Indica/Japonica Hybrid（0.0416±0.0086 μg/g）>Japonica Hybrid（0.0353±0.0093 μg/g）>Japonica（0.0281±0.0037 μg/g）.Source analysis of Pb in polished rice showed that the atmosphere was the main source of Pb in polished rice（75.64±10.44%）.Based on the grain size of different rice cultivars,this paper have established a prediction equation for the airborne Pb bioconcentration factor(K_a（rice）)of the grain,constructed a mathematical model of Pb in polished rice in the atmosphere-rice-soil ecosystem:C_rice=K_a（rice）C_al+K_sC_s,and verified the model by combining environmental sample data in the Yangtze River Delta.