Phytoremediation Of Zn-Cd Contaminated Soil By Intercropping System Consisted Of Sedum Plumbizincicola And Cash Crops And Post-treatment Of The Harvested Plant Biomass

The contamination of farmland by heavy metals has become one of top concerns in China,since it has posed a serious threat to regional economic development and human health.Phytoextraction,a green technology to remediate the heavy metal contaminated soil by hyperaccumulating plants,is a promising and feasible way to reduce the metal concentrations in the soil and maintain its agricultural productivity.However,it is evident that a fairly long period would be required to reduce the metal concentration to an acceptable level by monoculture of hyperaccumulating plants.Moreover,the farmers can gain little economic benefit during this phytoextraction.There is therefore an urgent need to find out an innovative way to guarantee the farmers'income in the soil remediation process.In this study,the phytoextraction efficiency of a Cd/Zn mixed contaminated soil by intercropping system consisted of a hyperaccumulating plant(Sedum plumbizincicola)and cash crop(either Zea mays or Phyllostachys praecox)was evaluated via greenhouse and field trials.Meanwhile,the effect of biochar application on the plant growth and removal of heavy metals was also investigated.In addition,two the post-treatment methods of harvested biomass of hyperaccumulating plant were compared in an attempt to minimize the risk to cause the secondary contamination.More specific,the main contents of this thesis includes(i)a two-year field trial to investigate the phytoextraction effect by S.plumbizincicola intercropping with P.praecox at a Zn and Cd mixed contaminated farmland;(ii)a greenhouse study to evaluate the phytoextraction efficiency by S.plumbizincicola and Z.mays with different biochar application rates(0%,1%,2%,5%)as well as investigate the variation of metal concentrations in different plant tissues;and(iii)a protocol development to post-treat the harvested biomass of S.plumbizincicola via pyrolysis and incineration at different temperatures in an attempt to verify if the leaching toxicity and bioavailability of the metal in the residues could be reduced by risk assessment.The major outcomes from this study are summarized as follows:1.The results of the field trial demonstrated that the intercropping of S.plumbizincicola intercropping with P.praecox may significantly improve the phytoextraction efficiency.The required duration to reduce Zn and Cd concentrations to the thresholds for Grade II(GB15618-1995)soil in accordance with the National Soil Environmental Quality Standards,could be shortened from 7.75 and 9.92 years to 4.42 and 4.75 years,respectively.2.The results of greenhouse study revealed that(1)the presence of biochar inhibited the metals uptake by both sedum and maize,and stimulated maize growth with a maximum biomass obtained at 5%addition rate due to the immobilization effect of biochar on the reduction of heavy metal bioavailability;however,the growth of sedum was worsened compared with no biochar addition control;(2)the maximum phytoextraction effect was observed by the intercropped sedum and maize in the absence of biochar,which allowed the Zn and Cd concentrations in the soil to be decreased to the thresholds of Grade II within 4.73 and 2.77 years,respectively.The optimal agricultural benefit was achieved when maize was intercropped with sedum with 5%biochar addition.However,the required phytoextraction period had to be prolonged to 13.07 and 8.31 years accordingly.It was suggested that the biochar-aided intercropping system of sedum and maize provided a feasible strategy for the phytoextraction of Zn and Cd mixed contaminated soil as the agricultural benefit could be guaranteed at sacrifice of remediation time.It is hoped that the outcomes of this study may also be of help for the remediation research and engineering of metal contaminated farmland under current statues of rural economic development in China.3.This study has confirmed that both pyrolysis and incineration were effective ways to post-treat the biomass of harvested S.plumbizincicola.With the increase of temperature from 350-650~oC,the stability of Zn and Cd in the biochar and ash was strengthened via the transform from bioavailable speciations to aromatized ones since aromatization is preferable to occur at high temperature.Therefore,the bioavailability and leaching toxicity of the post-treat products were also decreased with the rising temperature.However,pyrolysis showed more ideal performance than incineration in reducing the risk of post-treated products in accordance with the risk assessment.