| Soil heavy metal pollution affects agricultural production and safety of agricultural products seriously. On one hand, because of more people and less agricultural land in our country, utilization of contaminated soils is hard to avoid; On the other hand, with the improvement of living standards, the per capita paper consumption is growing quickly, as well as paper mill sludge production. Thus how to make the paper mill sludge resource utilization, especially applied to soil improvement with heavy metal pollution, is significant works that worth of exploring.On the basis of survey of heavy metal contents in soils (red soil, purple soil development), vegetables in vegetable garden surrounding the mine, assessment of potential human health risk. Applying of indoor analysis, simulation experiment and pot experiment methods, research the influences of paper mill sludge addition on Cd, Pb adsorption and desorption in red soil, purple soil and migration of Cd, Pb in soil-vegetable system. Several conclusions are achieved:(1) Heavy metal pollution in survey area containing18kinds of vegetable plot and80vegetable samples were analyzed and the results showed that:soils were polluted by heavy metals seriously, especially by Cd, and each contribution rate of heavy metals was Cd>Zn>Pb>Cu. According to comprehensive pollution index, leafy vegetables were polluted moderately or seriously, especially Amaranthus mangos-tanus. Root vegetables were polluted less than leafy vegetables. The vegetable’s enrichment capacity of different metal elements was different, i.e. Cd>Zn>Cu>Pb. And the abilities of heavy metal enrichment in Chrysanthemum coronarium, Amaranthus mangostanus, Apium grabeolens were higher than other vegetables. Heavy metal exposure on human risk assessment indicated that hazard quotient (HQ) of Cd and Pb for an adult through consumption of vegetables were higher than1, which suggested that it has high potential health risk for an adult by consuming contaminated vegetables.(2) The experiments of Cd2+, Pb2+adsorption and desorption by tested soils(red soil, purple soil), paper sludge indicated that isothermal sorption process of Cd2+and Pb2+by soils and sludge could be well described by Langmuir equation and Freundlich equation, and all coefficients(r) were higher than0.9. For tested soils, the purple soil had higher sorption capacity and less desorption capacity of Cd2+and Pb2+than red soil. Addition of paper sludge increased adsorption capacity of Cd2+, Pb2+and decreased desorption capacity greatly. Paper sludge effectively enhanced the adsorption capacities of the tested soils and capabilities of fixed Cd2+, Pb2+.(3) Paper addition had effects on residual fraction of Cd in potting soils (P<0.05). For tested soils, concentration of Cd forms from high to low were F4(Residual fraction)>F1(Exchangeable fraction)>F2(Reducible fraction)>F3(Oxidizable fraction). The content of Cd reducible fraction in purple soil was higher than red soil, while there was little difference about Cd oxidizable fraction. Paper addition had a greater impact on Pb forms in potting soils. Concentration of Pb forms in red soil, purple soil from high to low were F2>F4>F3>F1, and F4>F2>F3>F1, respectively. Paper sludge additon reduced the content of exchangeable fraction and reducible fraction and improved the content of residual fraction.(4) The content of Cd, Pb in leaves, roots of white radish had significant reduction with the paper sludge addition of1%,2%(P<0.05). After adding more than2%paper sludge, content of Cd in white radish still decreased slowly, while content of Pb was slightly rebounded. Add paper sludge significantly reduced transfer factor and enrichment coefficient of Cd in soil-vegetable system (P<0.05); while transfer factor of Pb had little change and enrichment coefficient reduced significantly (P<0.05).In conclusion, Paper sludge land application can not only ease the migration of heavy metal in soil-vegetable systems, but also provid conditions for the utilization of paper sludge. Though paper sludge can reduce the effectiveness of heavy metals to some extent and prevent migration of heavy metals in soil-crop system, more studies are needed to research whether paper sludge can reduce the concentration of heavy metal in some edible agricultural products to meet safety standards or not. |
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