Water-extracts Of Plants For The Washing Removal Of Pb,Zn And Cd From Soil

Heavy metal contamination of soil is one of the current environmental issues.Compared with other traditional contaminated soil remediation technologies,soil washing can effectively and rapidly get rid of the heavy metals from contaminated soils.In order to effectively and environmental-friendly remove the soil heavy metals,this article selected three natural plant materials including Fagopyrum esculentum,Pilea pumila and Fordiophyton faberi to be made washing agents for repairing the multi-metal polluted soils.Exploring the influence of solution concentration and pH and washing time on removal efficiency.To compare the variation of metal fractions and basic nutrient soils before and after washing,and evaluate the influence of biological leaching on soils.To reveal the mechanism of soil washing combine with the infrared and scanning technologies.The main research results showed as follows:(1)Under the same condition of leaching parameters,the metal removal efficiency of the water extracts of Pilea pumila was higher than those of Fagopyrum esculentum and Fordiophyton faberi.Because the water extracts of Pilea pumila contains more effective groups such as hydroxyl phosphate,amino,carboxyl and carbonyl group,etc.which could have a reaction with heavy metal.(2)The concentration and pH of washing solution and reaction time all can significantly affect heavy metal removal efficiency in the control range.In this paper,the concentration of 50 g L-1 is the most reasonable utilization of resources,and the 50 g L-1 P.pumila washing solution could remove the Pb,Zn and Cd of 6.93%,33.98%and 38.99%for mine soil and 6.97%,23.93%and 57.79%for farmland soil.PH is a key controlling factor of the soil washing,as it affects the solubility of the metals directly.Considering the follow-up effect of acid washing for soil,and the removal efficiency of three metals,combining buffer of the tested soil the plants characteristic,pH3 is a reasonable choice.For single soil lead removal,the washing time as short as possible.As the tested soil was multi-metal contaminated,need comprehensive considering the effect of three kinds of metal removal,so 120 min is the most appropriate choice.(3)The distribution of three metal fractions in soil is different.The exchangeable and bound to carbonates Cd occupy the 62%of mined soil and 55%of farmland soil total Cd respectively.The bound to carbonates,iron and manganese oxides Pb take up the major,and Zn contains many residual fractions.After the first washing,the five kinds of forms of Cd in two soils were all reduced as it was relative easier to contacted and removed by washing agents.After twice washing,the percent of residual Pb markedly increased,and the total soil Pb was in a more stable state.The residual Zn in mined soil and farmland soil also increased the maximum 51.84%and 34.26%,and the bound to carbonates Zn reduced to 11.83%and 29.90%respectively.(4)The mechanism of plants washing soil heavy mental is combined by the physical and chemical mechanism.Physical mechanism is the use of water power to break the original soil structure from outer to inner,go deep into the inside of soil to contact with the primary ore objects,and at the same time,accompany on decomposition of soil original mineral from the inside-out.The chemical mechanism is the inorganic acidradical ion,hydrocarbons,lipid,phenols and other effective substances in plant washing solution to adsorption,ion exchange,REDOX,chelation etc.the heavy metals in the soil in the process of soil physical anatomy.At last the heavy metals are dissolved and transfer into the liquid,and eventually removed from the soil.(5)The biological washing had less disturb the soil matrix,and the plant residues in soil is a natural substance,can be resolved gradually to suppliy some nutrient elements and trace elements due to thewashing loss.After the biological washing,the contents of organic carbon,total nitrogen,available nitrogen and total potassium of soils were increased.The organic carbon of mined soil and farmland soil improved the largest 36.29%and 24.11%,as well as the total nitrogen(21.20%and 17.89%)and available phosphorus(17.15%and 13.99%).Although the available potassium of washed soils all have reduced in varying degree,compared with EDTA,biological washing reagent could cut the loss of 17.65 mg·kg-1 available potassium for mine soil and 8.32 mg-kg-1 for farmland soil.Most of these increases come from the residues of plants washing solution.(6)High concentrations of heavy metals can significantly inhibit the germination rate of seeds,as well as the root,stem elongation and growth.As a consequence,the various biological growth index of mine soil was significantly lower than farmland soil(p<0.05).Although the soil heavy metal content is reduced after washing,as the changes in the overall soil structure and partial acid,therefore the germination rate were lower than before,and the root length,stem length and biomass showed no significant changes(p>0.05).