The Study Of Natural Polymers In Remediating Lead And Cadmium Polluted Soil

With the rapid development of society and economy,environmental pollution becomes increasingly severe.Soil heavy metal pollution has already gained great attention due to the long existence,great difficulty for treatment and wide range of harm.Heavy metals in soil could be accumulated by soil microorganism and plants.It could finally be harmful to human health.Among various kinds of soil remediation techniques,In-Situ solidification/stabilization becomes popular due to the fact that it is cost-effective as well as time-effective with high efficiency.It could achieve the goal of lowering the risk of heavy metals in soil by applying amendments which could react with heavy metals through adsorption and etc.so as to promote the transformation of heavy metals from higher bioavailable forms to lower bioavailable forms.The commonly used amendments in In-Situ solidification/stabilization are some inorganic and organic materials.However,the use of inorganic amendments may cause the change of soil properties,while some organic materials such as compost may contain toxic elements which are harmful to the soils.Recently,natural polymers have gained great attention for they are environmental friendly,recyclable and abundant in nature.These natural polymers are rich in oxygen-containing functional groups which are effective to the removal of heavy metals.Thus,in our study,we choose lignin(Ln)which is abundant in nature as the amendment.At the same time,another two natural polymers carboxymethyl cellulose(CMC)and sodium alginate(SA)were selected and modified by chemically crosslinking to improve their stability.The soil samples were collected from the vicinity of mine places which have been polluted by heavy metals.Top soil samples were collected from the vicinity of mine places which have been polluted by heavy metals of lead(Pb)and cadmium(Cd).The surface soils at a depth of 0-20cm were collected and simulated soils polluted by different heavy metal pollution levels were also employed,We study the effects of three natural polymer-based amendments on the changes of soil heavy metals in different forms through a series of extraction techniques including TCLP,DTPA and Tessier extraction procedures during the incubation periods.At the same time,for the further understanding of remediation mechanisms,we also investigate the adsorption performances of Ln,CMC and SA in aqueous solutions including the effects of solution pH,temperature,different initial metal concentrations as well as the contact time.The main results are as follows:(1)For the treatment of real soil samples,the application of organic amendments could cause a slight increase in soil pH values by 0.078-0.535.The increased soil pH value could promote the dissociation of acidic oxygen-containing functional groups on the molecular structure of natural polymers,which could strengthen their abilities of complexation with heavy metals.(2)Comparing with the untreated soil,the use of aforementioned natural polymer based stabilizers caused a decrease of 1.23%-76.90%of Pb and 1.68%-40.83%of Cd concentrations in exchangeable form.Heavy metal concentrations in carbonate-bound increased slightly as the soil pH increased.While FM-bound forms decreased a little as the soil environment changed(such as soil pH values and redox potentials).There was an evidently increasing trend in organic-bound and residual forms of heavy metals,indicating that the application of organics promotes the transformation of heavy metals from exchangeable form to organic-bound and residual forms.o(3)TCLP and DTPA extraction experiment indicate that the application of natural polymers could decrease the extraction concentrations of Pb and Cd.However,the TCLP concentration of Pb and Cd are still much higher than the standard levels of 5 mg·L-1 for Pb and 1 mg·L-1 for Cd.These indicate that Pb and Cd could have some negetive effects on the environment.The abundant functional groups on the surface of organic stabilizers could form stable metal-organic complexes,thus decreasing the TCLP-concentrations.However,within the experimental application ratio,Ln,CMC and SA show limited effects on metal stabilization.(4)For the study of correlation of Pb and Cd in different forms,we found that a positive correlation with the total concentration of heavy metals in exchangeable and carbonate-bound forms(F1?F2)while a negative correlation with the organic-bound form.Those indicate that the application of organic stabilizers could decrease the concentration of Pb and Cd in exchangeable form,but the TCLP concentration would still be higher when the total amount of heavy metals in FI and F2 forms were high.At the same time,Ln,CMC and SA could promote to transfer Pb and Cd from exchangeable form to organic-bound ones and be stable under TCLP extraction conditions.(5)For the incubation of simulated soil samples,Pb and Cd enter the soil as ions but they could not stay as ions all the time.During the incubation periods,heavy metals could react with various soil elements such as soil collide,minerals and soil organic materials and transfer to other forms.Compared with the blank samples,the application of lignin cause a decrease of 4.93%-17.68%and 11.05%-12.92%for Pb and Cd respectively in exchangeable form.There are small fluctations in carbonate and FM bound forms.While an increase of 8.01%-27.13%and 20.14%-28.84%was found for Pb and Cd in organic-bound form.There is also a slight increase of heavy metals in residual form.TCLP and DTPA extraction experiments indicate that the application of lignin could decrease the extraction concentrations of Pb and Cd.However,the TCLP concentration of Pb and Cd are still much higher than the standards because of the higher dose of metal ions entering into the soil.(6)The three natural polymers mentioned above were also employed in removing heavy metals in solution for further exploring their soil remediation mechanism.The results indicate that the removal efficiency of those three natural polymers for Pb and Cd increase as the initial solution pH increase and then reach a plateau.Secondly,the isothermal adsorption could be best described by Langmuir model since it bears a higher correlation coefficient(R2)value and the simulated adsorption capacity is close to the ones obtained in experiment,indicating that the adsorption process was a single layer chemical adsorption.For the kinetics study,the three stabilizers could quickly remove Pb and Cd ions and obtained equilibrium within 15mins.Both pseudo-first order model and pseudo-second order model fitted the kinetics data well since the simulated adsorption capacity is close to the ones obtained in experiment and the(R2)values were much higher.The selective and sequential adsorption behaviors of Ln,CMC,and SA for Pb and Cd further indicated that those natural polymer based stabilizers bear greater affinity to Pb over Cd.Natural polymers are environmentally friendly and widely available containing abundant oxygen-containing functional groups.Among all these natural polymers,lignin bears great advantages since it is the precursor of humic substances and the waste of papermaking effluent.At the same time,lignin has distinct three-dimensional net structure.These significant features make lignin a good choice for the remediation of heavy metal polluted soil.