| Heavy metals, a kind of inorganic pollutants, has the characteristics of high concentration and low migration posing a serious threat to the natural ecological environment and human health. The soil, water and atmosphere polluted by heavy metals not only affects the growth and development of vegetable, but makes heavy metals accumulated in the root, stem and grain of vegetables. Finally these heavy metals will enter the body through the food chain and endanger human body health. In recent years, with rapid economic development of Shanghai, the continuous improvement of living standards, increasing demand for vegetables in the market, suburban areas appear a lot of vegetable production base and the vegetables grown by individual farmers enter the market one after another. These vegetables are not only faced with the serious heavy metal pollution risks but also consumed by lots of people with a potential safety hazard. Existing research shows that the atmospheric wet deposition is one of the important sources of heavy metals in soil and vegetables.This study is based on the National Natural Science Foundation (41271472), "Pollution effects of heavy metals in urban dry and wet deposition on soil-vegetable system". Shanghai suburbs was selected as the representative area, and elements of Hg, As, Cu, Pb, Zn, Cd and Cr were chosen as the main study objects. Through the investigation on heavy metal concentration in atmospheric wet deposition, vegetables simulation experiment in micro field, precipitation simulation test in greenhouse, this study clarified the process of heavy metals accumulation by edible vegetable part, the contribution of atmospheric precipitation for accumulating heavy metals by leaf and root of vegetables, and the migration mechanism of heavy metals in the system of soil-vegetable. A contribution model of atmospheric wet deposition was also built and the human health risk of heavy metals by the way of consuming vegetables was calculated. With all of these efforts, this study aimed to improve and develop the biogeochemical theory of heavy metals in the gas-soil-organism interface. The experimental analyses could be summed up with following conclusion.(1) The concentration of heavy metals in atmospheric wet deposition dust and rainwater from March to May in Minhang district was generally higher than that in Pudong district. Deposition flux of seven kinds of heavy metals ranked according to the order from high to low was Zn>Cu>Cr>As>Pb>Hg>Cd, and they were 5672.15, 672.38,332.26,128.42,90.08,22.92 and 21.31 μg·m-2·month-1 respectively. Deposition flux of seven kinds of heavy metals in Pudong district ranked according to the order from high to low was Zn>As>Cu>Cr>Pb>Cd>Hg, and they were 1754.76, 174.53,172.42,87.16,48.74,14.62 and 12.72μg·m-2·month-1 respectively. Deposition flux of Zn was the most in both districts and that of Hg and Cd were lesser.(2) Results of vegetables simulation experiment in micro field showed that concentrations of heavy metals in surface of soil out of greenhouse were much higher than that in greenhouse except As. Concentrations of Zn and Cr were higher than that in primary standard of soil environment but lower than that in standard of edible agricultural products environmental quality (HJ332-2006). The average levels of Cu, Pb, Cd, Hg and As were lower than that in primary standard of soil environment. Average heavy metals concentrations in vegetables out of greenhouse were higher than those in greenhouse which indicated that atmospheric wet deposition exerted a crucial effect on heavy metals in vegetable. Heavy metal concentrations in vegetables were beyond the limit of heavy metal pollution of vegetable enormously except Cu and As. As the vegetables growing, accumulation of Pb, Hg and Cu showed the specific laws. From the vegetable seedling stage to maturity, accumulation of Pb kept decreasing, accumulation of Hg increased firstly and then reduced, accumulation of Cu kept decreasing generally. The other heavy metals didn’t show obvious rule in the growing process of vegetables. Heavy metals in vegetable roots didn’t show obvious accumulation rules either.(3) Results of precipitation simulation test in Pudong district showed that concentrations of heavy metals in soil after simulative wet deposition was higher than that before, and this was most obvious for Hg. The ways of wet deposition had an effect on vegetable accumulating heavy metals in different parts. In the way of watering, concentrations of heavy metals in roots was higher than that in edible parts of vegetable over the soil. And this was on the contrary under the way of spraying. The concentrations of Pb, Hg, Cu and As in edible parts of vegetable under the way of spraying were much higher than those under the way of watering specially for Pb and Hg. Accumulation of heavy metals in vegetable roots by watering was much more than that by spraying.(4) Wet deposition could facilitate leafy vegetables accumulating many kinds of heavy metals (such as Zn, Cu, Pb and Hg). Meanwhile, it could also control vegetable accumulating some heavy metal such as Cd. Different pH value of wet deposition had an influence on edible parts of vegetable accumulating heavy metals. On the whole, the wet deposition of pH=4 was beneficial for edible parts of vegetable accumulating Pb and Hg. The wet deposition of pH=6.5 could mostly facilitate leaves accumulating Cr. Accumulation of Zn and Hg was more in edible parts of vegetable under the condition of normal rainwater (pH≈5.2). Wet deposition could facilitate roots of vegetables accumulating Pb, Cr and As, and control the accumulation of Zn and Hg. The roots of 4 kinds of vegetables accumulated Zn most under the condition of normal rainwater (pH≈5.2). With the wet deposition of pH=4, Pb was the most obvious. And the accumulation of Cd and Hg in roots was more under the wet deposition of pH=6.5. The enrichment factor of 7 kinds of heavy metals ranked according to the order from big to small was Cd>Zn>Hg>Cu>Cr>Pb>As.(5) Result of multiple linear regression analysis shows that Pb and Hg in the edible part of vegetables were more compliant with the Voutsa contribution model fitting. These two kinds of elements had obvious linear relationship with soil and atmospheric wet deposition. And the contribution from atmospheric wet deposition was more than that from soil. Cd and Hg in the roots of vegetables were fit for Voutsa contribution model. While the other heavy metals in vegetables didn’t fit the model very well for the reason that firstly, this paper only did the research on wet deposition ignoring the influence of dry deposition. Secondly, it was related to the complexity of atmosphere-soil-biological systems.(6) The single indexes of heavy metal potential pollution in soil at Zifeng Road were all less than 40, which indicated that the soil had a low ecological risk. RI, the Hakanson ecological risk index of all kinds of heavy metals, fell in the range of 52.256-86.468 being considered as a low ecological risk, too. The single indexes of heavy metal potential pollution in soil of Pudong district were also less than 40 on the level of low ecological risk. RI was in the range of 74.979-152.881 and it was higher than that at Zifeng Road. But RI of soil in Pudong district was no more than 150 which showed that the ecological risk was low, too. Heavy metal pollution index of edible part of vegetables at Zifeng Road ranked according to the order from big to small was Cr>Zn >Hg>Cd>Cu>As>Pb. The concentration of Zn, Cu, Pb, Cd, Hg and As were on the clean level. There was a slight pollution for Cr with the single indexes of heavy metal potential pollution between 1 and 2. Heavy metal pollution index of edible part of vegetables in Pudong district ranked according to the order from big to small was Cr >Zn>Hg>Cd>Pb>Cu>As. There were no pollution for Zn, Cu, Pb, Cd, Hg and As but slight pollution for Cr. The comprehensive pollution index of heavy metals in edible part of vegetable grown in Pudong were on the range of 0.218-4.164 and the average value was 1.038 which showed there was a slight pollution. |
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