Remediation Of Heavy Metal Contaminated Soil In Landfill By Two Enriched Plants Combined With Passivating Agents

In this experiment,the heavy metal(Cd,Cu,Pb,Zn)contaminated soil of simple landfill in Yanting County,Sichuan Province was taken as the research object,and the soils at the top(T),middle(M)and bottom(D)of the landfill were taken as the test soils,and lobelia(Solanum nigrum L.)and three-leaved ghostwort(Bidens pilosa L.)were selected from the local ecological environment as enrichment plants,,the sepiolite and phosphate rock powder were used as passivating agents,1% sepiolite was added to the contaminated soil for testing in autumn 2021 and spring 2022,respectively,followed by planting two plants for remediation of contaminated soil.After the enrichment plants were harvested,sepiolite(w/w: 1%),phosphate rock powder(w/w: 0.2%,0.4%,1%,2%)and their combination of passivating agents were added to the harvested soil for passivating culture test.By measuring the content of heavy metals in plants,the effective content of soil DTPA,the toxic leaching of TCLP,the speciation distribution of heavy metals,and the changes of soil pH and EC values,the accumulation of plants,the passivation rate of soil heavy metals and the risk evaluation coefficient were integrated to investigate the effect of combined remediation technology of plants and passivating agents on the remediation of heavy metalcontaminated soil in a simple landfill..The results showed that:(1)Solanum nigrum L.and Bidens pilosa L.were able to enrich soil heavy metals.For Cd elemental,the best enrichment effect was observed in the autumn for lobelia planted alone,which had the highest above-ground accumulation of 11.26 μg/pot,with an enrichment coefficient of 0.21 and a transfer coefficient greater than 1..For Cu,Pb,and Zn elements,there was no significant difference in the heavy metal content of each part of the two plants,with the highest aboveground accumulation of Pb and Zn by Bidens pilosa L.reaching 56.74 μg/pot and 1195 μg/pot,respectively.(2)Plants(Solanum nigrum L.and Bidens pilosa L.)with 1% sepiolite treatment significantly reduced the content of available heavy metals in soil,of which Cd and Zn decreased by 14.73%-47.95% and 7.40%-47.73%,respectively,compared with CK treatment.The treatment of plant and sepiolite reduced the proportion of Cd and Zn active forms(weak acid extraction and reducible form)by 1.19%-3.38% and 0.83%-10.40%,respectively,and the proportion of residue state increased by 1.32%-2.67% and 0.88%-12.32%,respectively,compared with CK.After treatment with plants and sepiolite,the toxic leaching amount and total amount of heavy metals in soil decreased,and the soil pH value increased,and the soil electrical conductivity decreased.(3)The addition of phosphate rock powder and sepiolite reduced the effective state content and toxic leaching of soil heavy metals in the post-harvest soil of plants(lobelia and clover),and the passivation rate increased with the addition of phosphate rock powder.The best passivation effect was achieved by adding 2% phosphate rock powder,1%sepiolite and 2% phosphate rock powder in post-harvest treatment of lobelia,where the passivation rates of Cd,Cu,Pb and Zn reached 52.21%,80.26%,66.52% and 60.44%,respectively;the passivation rates of Cd and Cu reached 46.68% and 71.23%,respectively,with the addition of 2% phosphate rock powder after harvesting of triticale.The addition of passivation agent after plant harvest reduced the proportion of extracted weak acid states of Cd by 0.32%-1.10%,and increased the proportion of less active components(oxidizable state + residue state)by 0.13%-2.26%.(4)Passivator addition after plant harvesting reduced the soil RAC risk evaluation coefficient.In lobelia post-harvest,the addition of 2% phosphate rock powder Cd and Zn passivation was the most effective,and its risk evaluation coefficient was reduced by 2.26%and 7.19%,respectively.The risk evaluation coefficients of Cd and Cu were reduced by0.56% and 5.54% with the addition of 1% sepiolite and 2% phosphate rock powder in the mixed treatment after harvesting of trillium,and the risk evaluation coefficients of Pb and Zn were reduced by 5.64% and 6.22% with the addition of 2% phosphate rock powder.In conclusion,Solanum nigrum L.has a good effect on the enrichment of heavy metals in soil.Adding 2% phosphate rock powder to soil after harvest can significantly reduce the content of heavy metals in soil,and the soil environment is less disturbed.Therefore,it is recommended to adopt the technical measure of planting lobelia first and adding 2%phosphate rock powder afterwards to remediate the heavy metal contaminated soil in landfill.