| Chelator-assisted phytoextraction is one of the effective techniques for the remediation of heavy metal contaminated soils,studying the potential leaching for heavy metals and the risk to the underground water will be of significance in both theory and practise.In the present study,a soil column cultivation-leaching experiment was conducted to investigate the Cd and Pb leaching char acteristics during assisted phytoextraction of Cd-Pb contaminated soils.The columns were planted with Zea mays “Zhengdan 958” seedlings and joitings,treated with three different chelators(EDTA,EDDS,and rhamnolipid)and simulated rainfall(pH 4.5 or 6.5,heavy rainwater)in three different particle-sized fractions of soil aggregates(< 1,2,and 4mm).In addition,a soil column-leaching experiment,an adsorption experiment on chelator-enhanced heavy metal and a dispersion experiment were conducted,constructing a mathematical migration model about chelating heavy metals based on CDE(Convection-Dispersion Equation).Finally,assessing the migration distance of chelating heavy metals based on the above model,and the risk to the underground water.The main results of this study were as follows.(1)Leaching of different chelators-enhanced(EDTA,EDDS,and Rhamnolipid)phytoextraction of Cd and Pb contaminated soil by Z.mays with artificial rainfall will be shown as follow.EDTA increased both Cd and Pb concentrations in the leaching solution,obviously,whereas EDDS and rhamnolipid increased Cd concentration but not Pb.The amount of Cd and Pb decreased as the leaching solution increased,the patterns as well matched LRM(linear regression models)well,with R-square(R2)higher than 90% and 82% for Cd and Pb,respectively.The maximum cumulative Cd and Pb in the leaching solutions were 18.44%,and 16.68%,respectively,which was amended by EDTA and acid rainwater(pH 4.5),and followed by: EDTA(pH 6.5),EDDS(pH 4.5),EDDS(pH 6.5),rhamnolipid(0.5 g·kg-1 soil,pH 4.5),and rhamnolipid(pH 6.5).(2)Leaching of EDTA-enhanced phytoextraction of Cd and Pb contaminated soil by Z.mays with artificial rainfall in different particle-sized fraction of soil aggregates will be shown as follow.The greatest uptake of Cd and Pb by Z.mays was observed after treatment with EDTA(2.5 mmol·kg-1 soil)and soil aggregates of < 1 mm;uptake decreased as the soil aggregate size increased.Simulated rainfall,especially acid rain(pH 4.5),after EDTA applications led to increased metal concentrations in the leachate,and EDTA significantly increased the concentrations of both Cd and Pb in the leachate,especially with soil aggregates of < 1 mm;metal leachate concentrations decreased as soil particle sizes increased.Concentrations of Cd and Pb decreased with each continuing leachate collection,and data were fit to linear regression models with coefficients of determination(R2)above 0.90 and 0.87 for Cd and Pb,respectively.The highest total amounts of Cd(22.12%)and Pb(19.29%)were observed in the leachate of soils treated with EDTA and artificial acid rain(pH 4.5)with soil aggregates of < 1 mm.The application of EDTA during phytoextraction method increased the leaching risk in the following order: EDTA2.5-1(pH 4.5)> EDTA2.5-1(pH 6.5)> EDTA2.5-2(pH 4.5)> EDTA2.5-4(pH 4.5)> EDTA2.5-2(pH 6.5)> EDTA2.5-4(pH 6.5).(3)Leaching of EDTA-enhanced phytoextraction of Cd and Pb contaminated soil by Z.mays(seedlings and jointing stages)with artificial rainfall will be shown as follow.Based on two stages of Z.mays,concentrations of Cd and Pb decreased with each continuing leachate collection,and data were fit to linear regression models with coefficients of determination(R2)above 0.87 and 0.83 for Cd and Pb,respectively.The highest total amounts of Cd(14.67%)and Pb(13.72%)were observed in the leachate of soils treated with EDTA and artificial acid rain(pH 4.5)at jointing stage.The application of EDTA during phytoextraction method increased the leaching risk in the following order: EDTA?-2.5(pH 4.5)> EDTA?-2.5(p H 6.5)> EDTA?-2.5(pH 4.5)> EDTA?-2.5(pH 4.5).(4)Based on the adsorption experiment on chelating heavy metals by the vadose zone soil,results showed that,soil adsorption of EDTA-Cd(Pb as background),EDTA-Pb(Cd as background)induced by phytoremediation from the surface soil could be simplified as a linear adsorption model,the adsorption distribution coefficient(Kd)were 0.0027 and 0.0176,respectively.Then through a soil column-leaching experiment,based on CDE model,the dispersion coefficient of soil was induced as 0.0095 cm2·h-1 using a method of trial and error.Further,constructing a mathematical migration model about chelating heavy metals based on CDE(Convection-Dispersion Equation).Finally,assessing the migration distance of chelating heavy metals based on the above model.Results showed that,for a period of 15 d phytoextraction forecasting,in order to match ? standard of groundwater quality standard in China,the migration distance of Cd and Pb in the leaching solution were 17 cm and 18 cm at seedlings;and were 18 cm at jointing stage.For 5 period of 75 d phytoextraction forecasting,in order to match ? standard of groundwater quality standard in China,the migration distance of Cd and Pb in the leaching solution were 69 cm at seedlings;and were 69 cm at jointing stage.In addition,under the extreme conditions of 10 and 15 cycles,the migration distances of the peak concentrations of heavy metals still could not reach the local groundwater(above 5 m),which showed that the chelating heavy metals induced by the chelator-enhanced phytoextraction of Cd-Pb contaminated soil wouldn't have any impact on the local underground water. |
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