| Soil pollution is becoming a more and more serious problem in China.With the expansion of pollution area,environmental problems caused by the pollution can not be ignored.Lead(Pb)and Zinc(Zn)contaminated soil is among the large amount of soils polluted by heavy metals.It will be of significant meaning if remediated soils can meet the high demand of agricultural and forestry land and be reused effectively.In this thesis,the impact of different factors on the removal of Pb and Zn from contaminated soils by organic acid leaching enhanced by ultrasonic were investigated.Main research results are as follows.With the condition of liquid to solid ratio of 10:1,14 kinds of leaching agent including inorganic acid,chelation agents,antioxidants,low molecular weight organic acids,were compared.The impact of leaching agent concentration and leaching time were studied.It was concluded that the removal efficiency of Pb and Zn reached higher than 25.3% and 65.0%,after 4 h leaching with 0.1mol/L HCl,4 h leaching with 0.6 mol/L citric acid,4 h leaching with 0.1 mol/L Na2 EDTA,8 h leaching with 0.4 mol/L tartrate,and 8h leaching with 0.6 mol/L lactic acid,separately.The removal effect of 5 leaching agents on the removal of residual and oxidation contaminants were negligible.Maximum removal efficiency was found on acid extractable contaminants.Based on the optimized leaching parameters,the impact of leaching times and combined leaching process were studied.The leaching efficiencies of Pb and Zn were improved with the increased of leaching times.Combined leaching agent also increased the removal of Pb and Zn.The removal efficiencies of Pb and Zn had little change when the leaching order of citric acid and Na2 EDTA were changed.However,the removal efficiency of Pb was increased to 82.2% when Na2 EDTA was applied before tartaric acid or lactic acid,compared with the order of Na2 EDTA following tartaric acid or lactic acid.However,the leaching order had little impact on Zn removal.Based on the optimized leaching condition,the impact of ultrasonic frequency,power and time on the removal of heavy metals by ultrasonic enhanced organic acid leaching in Pb and Zn contaminated soils were studied.The change of Pb and Zn removal,physical and chemical properties of soils,and morphology of heavy metals were also detected.The removal efficiency of Pb and Zn decreased with the increase of ultrasonic frequency,increased with the increase of ultrasonic power.While with the increased of ultrasonic time,the removal efficiency of Pb and Zn increased significantly at first,then slowly increased and finally approached equilibrium.Under the optimized ultrasonic enhancement condition of 20 kHz,600 W,and 20 min,the removal of Pb by citric acid and tartaric acid reached 60.0% and 47.6%.while the removal of Zn reached 91.7% and 76.5%,separately.After the soils were treated by organic acid leaching enhanced by ultrasonic,the Pb and Zn concentrations met the requirement of environmental quality standard of agricultural soils(the 2nd draft).Acid extractable Pb and Zn were removed thoroughly,the removal of residual forms were also increased.Although the basic physical and chemical properties of soils were changed a little,the fertility of soils still met the standard of different land usage.The concentration of effective P,K and organic carbon in the soils were higher,when ultrasonic was applied,compared with single Na2 EDTA leaching.Meanwhile,the total N was reduced,although the ratio of effective N to total N was doubled in the soils treated with both ultrasonic and organic acid leaching.The treatment of ultrasonic enhanced organic acid washing technologies in Pb and Zn polluted soils had several advantages.The treatment process was not complicated,with high heavy metal removal efficiencies,and short washing duration.It solved the secondary pollution problems of washing agent effectively,with low soil fertilities loss,which set solid foundation for later safe utilization of the remediated soils. |
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