Remediation Of Heavy Metal Severely-contaminated Farmland By Soil Leaching Combined With Phytoextraction

The threat of heavy metal contaminated farmland soils has become a public concern.Due to the high bioavailability and migration risks of heavy metals in severely-contaminated soils,the toxic metals have a deep impact on the quality of crop products and immediately harm for the human health through food chain.It is therefore highly desirable to apply suitable remediation measures to severely polluted farmland soils.In this paper,we evaluated the efficience of combined soil leaching and phytoextraction to remediate severely contaminated soils by heavy metals.First,we investigated the effects of mild washing agents and explored the optimal conditions.And then we conducted the soil leaching and risk assessment of the treated soil.At last,Sedum plumbizincicola was utilized for phytoremediation in the treated soil after amendments.(1)Batch extraction experiments showed that comparing to single FeCl3 or citric acid(CTA),FeCl3+CTA increased heavy metal removal efficiency especially for the neutral soil.For acid soil,using FeCl3 could save the cost of extractants and achieve high removal efficiency.Using multi-step washing instead of single-step increased the metal extraction.The removal rate of Cd was higher than that of Zn,Cu or Pb because of the higher proportions of acid-soluble forms of Cd in the soils.Soil pH,CEC,exchangeable Ca and exchangeable Mg decreased significantly after soil washing.(2)Column leaching experiments were conducted to acid and neutral soils using 20mmol·L-1 FeCl3.The concentration of elements in leachate were monitored.In the process of acid soil leaching,the removal of Cd reached the maximum rate quickly and it reduced to about 40%of that maximum removal rate at the liquid-to-solid ratio was 3.0.At this point,we changed FeCl3 to water to rinse soil.After 2 volume water rinsing,the concentration of Cd,Cu,Zn,Pb,K,Ca,Mn,Fe and Cl in the leachate were very low.3 volume of FeCl3 and 2 volume of water removed 45%Cd,13%Cu,20%Zn and 24%Pb in acid soil.While the removal efficiency of heavy metal in neutral soil was much lower.Soil pH,available nitrogen,phosphorus and EC decreased significantly after soil leaching.(3)We conducted soil amendment and risk assessment by planting B.chinensis in the leached acid soil.Compared to the untreated soil,the process of leaching inhibited plant growth.The biomass of B.chinensis increased in various degrees after adding soil amendments and fertilizer in which adding lime and manure had the best effects.The concentration of soil CaCl2-Cd decreased significantly in the leached soil and further decreased from 0.33 mg·kg-1 to 0.14 mg·kg-1(decreased by 58%)by adding lime and manure.However Cd concentration in edible parts of B.chinensis exceeded Chinese maximum level.(4)Phytoextraction using hyperaccumulator Sedum plumbizincicola was conducted following leaching and amendment to extract the residual heavy metals in the soil.The biomass and Cd removal efficiency by S.plumbizincicola were improved by soil amendment:1%sepiolite,1%sepiolite + 0.5%manure and 0.18%lime + 0.5%manure.Among these amendments,0.18%lime + 0.5%manure had the highest biomass.In the treatment of adding 1%sepiolite,the remediation efficiency was highest and the cost was the lowest.Therefore,1%sepiolite was the optimal amendment.(5)We comprehensively evaluated soil leaching and phytoextraction to the severely-contaminated acid soil.Soil leaching removed 45.2%Cd and phytoextraction removed 35.2%Cd.Nevertheless,the joint use of soil leaching,amendment and phytoextraction removed 63.8%Cd.Meanwhile,the leached soil microbial community structure can be restored after phytoextraction.In summary,the soil extractant are different from soil types.FeCl3+CTA is suitable for neutral soil and FeCl3 is suitable for acid soil.The optimal remediation of Cd severely-contaminated red paddy soils is the combination of soil leaching,amendment and phytoextraction.The united technology avoids the problem of high cost and environment risks caused by single soil leaching.Meanwhile,this technique overcomes the shortage of long growth period of phytoextraction.The experiment results show that the united technique has bright future in remediation of Cd severely-contaminated red paddy soils in the South China mining area.