| Heavy metals are the most concerned pollutants at present.Heavy metal ions in soil not only casue damage to the sounding but also pollute ground water.Pb is one of the typical pollution metal in soil and high content of Pb in paddy soil would cause serious damage to human health.Understandingthe transportation and transformation characteristics of Pb in paddy soil could better evaluate its risk and provideimportant theoretical basis for polluted soil remediation.In this study,incubation and soil column experiments under simulated acid rain condition were conducted to evaluate aging and exogenous Pb concentration effect on the fractions distribution and migration ability of Pb in artificial polluted paddy soil.Four extractants?HCL,DTPA,CaCl2 and M3?CH3COOH+NH4NO3+NH4F+EDTA+HNO3??were chose to extract available Pb to study the effect of concentration and aging time on the Pb availability in paddy soil.The main results are as follows:?1?Aging and exogenous Pb concentration could significantly affect the fractions distribution of Pb in paddy soil.With the increase of aging time,the content of HoAc soluble lead decreased,and the content of oxidizable lead decreased obviously after fifteen days,the content of residual lead also increased,while the content of reducible lead was stable.With the exogenous Pb concentration increased,the content of HoAc soluble lead increased,and the content of oxidizable lead decreased obviously after fifteen days,the content of residual lead also increased,while the content of reducible lead was stable.With the increase of concentration,HoAc soluble Pb ratio increased,the oxidizable and residual ratio decreased,reducible Pb ratio remained stable.The effect of aging lead to the decrease of Pb activity,to a more stable form of change;the increase in the concentration of exogenous lead would elivate the ratio of high reaction active lead,and thereby enhance the risk of lead pollution.?2?Aging time and exogenous Pb concentration could significantly affect the migration ability of Pb in paddy soil.The lead concentration in the leaching solution and the lead accumulation in the leaching were all increased as increase of aging time and the exogenous lead concentration.Most of lead ions were leached in the first half of the leaching experiment,and there was almost no lead ion leached at the end of the experiment.The leaching amount of the Pb from the polluted soil only accounted for a small part of their total content in soil?<1%?.According to the third level criterion of quality standard for ground water?GB/T 14848-9?in China,the maximum concentrations of Pb were apparently higher than the safety limits under the low concentration of exogenous lead pollution?200 mg/kg?,which apparently increase the risk of groundwater lead pollution.Paddy soil can relieve acidity caused by acid rain in a short period,and tend pH in leaching solution to 7.?3?There was obvious differences among four reagents in extracting available Pb under different concentrations.Available Pb concentration extracted by different reagents in the unpolluted paddy soil followed the order:M3>DTPA>HCL>CaCl2.For the polluted paddy soil,the sequence was HCL>DTPA>CaCl2>M3 when the exotic lead concentration was 200 mg/kg.Available Pb concentration ranked in the order:DTPA>HCL>CaCl2>M3,when the exotic lead concentrations were 500,1000 and 2000mg/kg,respectively.With the increase of exotic lead concentration,both the amounts of the available fraction and the activating rate of Pb in paddy soil increased.The changing trends of the four kinds of extracting agents were different in the aging process,and the CaCl2 extraction effect was consistent with the first-order exponential decay model.Increasing the concentration of exogenous Pb had little effect on the stability time of Pb availability. |
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