Study On Risk Assessment And Safe Utilization Technology Of Heavy Metal In Farmland Of Typical Rapid Economic Development Area

The national soil pollution investigation bulletin shows that heavy metals have become the main pollutants in farmland soils in our country.In recent years,the excessive heavy metals in farmland soils and agricultural products has become increasingly prominent with the processing of industrialization and urbanization in the Yangtze River Delta Region,which poses a potential threat to human health safety.Therefore,there is an urgent need to carry out the researches on risk assessment and safe utilization technology of heavy metal in farmland.In this thesis,the soil-crop systems of farmland were selected as the research object in the typical rapid economic development area to assess the risk of heavy metal pollution and analyze the source and affecting factors.On this basis,the typical farmland with heavy metals exceeding the risk screening values for soil contamination of agricultural land were selected in the study area.Then laboratory passivation tests,pot experiments and field experiments were conducted out using a new passivation agent(calcium aluminate chloride).The main findings are as follows:(1)Mean value of cadmium(Cd),lead(Pb),copper(Cu),zinc(Zn),and chromium(Cr)in the topsoil of the study area were greater than their respective soil background values in southern Jiangsu.Except for Pb,the contents of Cd,Cr,Cu and Zn in some soil samples were greater than the the risk screening values for soil contamination of agricultural land of China.Among the total sampling sites,there were 36% sampling sites with the Cd in soils exceeded the risk screening value.The mean content of Cd in wheat exceeded the food safety limit standard in China(GB 2762-2017),and the mean contents of heavy metal in rice and vegetable were below the limit standard.(2)The selected 5 heavy metals were higher in the north and lower in the south with patchy distribution in the study area,the potential point source pollution of Cd has appeared at the industrial and residential boundary in the north-central region,with the levels of Cd from 2.64 to 3.64 mg/kg.Except for Cr,the contents of other heavy metals in the surface soils were higher than those in the deep soils.The soil p H is opposite and appear acidification trend in the study area.The available phosphorus and available potassium were abundant in surface soils.These results indicated that the accumulations of heavy metals were highly affected by both human activities and chemical background.(3)The distributions of heavy metals and nutrients in soils were also affected by different land uses.The content of heavy metals in soil in industrial and residential areas were significantly different from those in agricultural areas(p<0.05).Three sources were obtained by PMF model: atmospheric deposition of industrial and traffic,agricultural and natural sources,with the contribution rates of 36.32%?34.59% and 29.09%,respectively.(4)In general,the soil of the study area was light to moderate contaminated with heavy metals and shown as industrial soil > residential soil > agricultural soil > fallow soil,with the Cd as the prominent pollutant and higher.There were 41.2% collected rice and 85.39% wheat samples pose a potential risk for human consumption,but the overall risk within the acceptable level of risk,collaborative monitoring of farmland soil and crops is still needed.(5)The results indicated that the passivation material Ca-Al-LDHs effectively passivated the heavy metals of different levels of Cd pollution in farmland soil through adsorption,ion exchange and coordination.Compared to the control,the pot experiment results showed that when the passivating agent amount is 0.5%,the Cd in the medium Cd pollution level of soil effectively reduced by 0.018 mg/kg(9.52%),but the remediate effect of Cr was not ideal.Two field experiments showed that the optimum passivation ratio of small leafy vegetables was 0.5%,the passivation efficiency of celery is higher than those of leafy vegetables,but the optimal ratio is difficult to define.The results of the second round of repair of the small leafy vegetables and celery were decreased and unstable.The passivation efficiency of celery was higher than those of leafy vegetables,and the second round passivation effect of both vegetables was weakened and unstable.