| Soil washing has become one of the research hotspots of soil heavy metal remediation due to its advantages of quick effect and complete removal of contaminants.The key of soil washing lies in the selection of eluant.Traditional eluents such as HCl、H2SO4 and EDTA have the characteristics of high leaching efficiency but great damage to soil.Therefore,the exploration of environmentally friendly leachate with high removal efficiency but little damage to soil is the key of soil washing.In order to find an environment-friendly eluent,this paper used amino acid salt and biogas residue extract as eluents to leach two Cd-contaminated soils with different pollution levels(1.25 mg·kg-1 Cd in soil 1 and 16.33 mg·kg-1 Cd in soil 2)from Xiangtan,Hunan.The effects of eluent concentration,washing time,liquid-soil ratio and p H on eluent efficiency were investigated.The mechanism of leaching was preliminarily studied.The effects of leaching on soil physical,chemical and biological properties were studied in order to provide a new theory and method for soil heavy metal remediation.The main research results are as follows:（1）By comparing the leaching efficiency of 12 amino acid salts,phosphoglycine（[Gly][H2PO4]）was selected as the eluent.When the[Gly][H2PO4]concentration was 0.3mol?L-1,the liquid/solid ratio was 4,the washing time was 60 min,the removal rate of Cd in soil 1 and soil 2 were 55.40%and 45.08%respectively.The second-order kinetic equation could fit the leaching process better.The removal efficiency of the two eluents for each species of Cd was as follows:acid soluble state>reducible state>oxidizable state>residual state.The removal mechanism of Cd from soil by[Gly][H2PO4]mainly includes acid dissolution,coordination,ion exchange and enhanced desorption and dissolution of Cd.Cd in the waste solution was removed by adding Ca（OH）2.When the concentration of Ca（OH）2 was 20 g·L-1,the removal rate of Cd in the waste solution reached 96.57%.（2）The biogas residue was extracted with alkaline Na4P2O7(0.1 mol?L-1KOH+0.1 mol?L-1Na4P2O7?10H2O),and the extracted solution was used to wash the two soil samples.The concentration of eluent was expressed by the carbon content in the extract.When the leaching p H was 8,the concentration of eluent was 4586 mg·L-1,the ratio of liquid to soil was 4,and the washing time was 120 min,the Cd removal rate of soil 1 reached up to 57.73%.When the leaching p H was 9,the concentration of eluent was 5715 mg·L-1,the ratio of liquid to soil was4,and the leaching time was 120 min,the Cd removal rate of soil 2 was up to 54.66%.The second-order kinetic equation could fit the leaching process better.The removal mechanism of Cd from soil by biogas residue extract is mainly due to the coordination between the electron-donating groups of carboxyl group and Cd to form a water-soluble complex.When the concentration of Ca（OH）2 was 40 g·L-1,the removal rate of Cd from the waste solution was73.16%.（3）Soil p H decreased after[Gly][H2PO4]leaching while soil p H increased after biogas residue extraction leaching.Both of the two kinds of eluents reduced the soil particle size,increased the soil viscosity,and increased the soil total nitrogen,total phosphorus,available potassium and organic matter contents.The enzyme activity decreased first and then increased.On the 15th day after leaching,the enzyme activity exceeded that of the original soil.The germination rate,fresh weight,dry weight and plant height of rice were all higher than those of the original soil.In conclusion,[Gly][H2PO4]and biogas residue extract,as leaches,not only have high leaching efficiency,but also have little influence on soil physical and chemical properties after leaching,which can be used as environmentally friendly eluant for further study. |
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