| At present,domestic waste in some villages and towns in China is mostly treated by simple landfill,which leads to serious heavy metal pollution in the landfill and the nearby soil environment.In this study,the heavy metal contaminated soil of Yunxi Town simple landfill in Yanting County,Sichuan Province was taken as the research object.(1)Two levels of 0.5% and 1% of silicon fertilizer and three levels of 0.2%,0.4% and 1% of ground phosphate rock were set.The passivation culture test(45d and 90d)was carried out by compounding silicon fertilizer and ground phosphate rock,and two best combinations of compound passivators were selected;(2)Amaranthus hypochondriacus L.and Bidens pilosa L.were selected as remediation plants for a 120 days pot experiment to explore the effects of different plants on the remediation of heavy metals;(3)The selected compound passivator combination was applied to the soil after phytoremediation for 45 days.By measuring the contents of DTPA,TCLP and BCR of soil heavy metals before and after culture,as well as the changes of soil p H and EC value,combined with the fixation rate of heavy metals in soil and RAC risk evaluation coefficient,the passivator combined phytoremediation technology for repairing soil polluted by heavy metals Cd,Cu,Pb and Zn was explored.The results show that:(1)The passivation culture of the compound treatment of silicon fertilizer and ground phosphate rock can reduce the content of available Cd and Zn in soil DTPA.Among them,the passivation effect of the compound treatment of 1% silicon fertilizer and 0.2% ground phosphate rock(Si2P1),1% silicon fertilizer and 1% ground phosphate rock(Si2P3)are better.The highest fixation rates of Cd by the combination of two compound passivators were 25.51% and 30.79% respectively.For Zn,the fixation rates were 15.14%-46.16% and 24.17%-40.74%.BCR speciation classification showed that after adding the compound passivator,the proportion of weak acid extracted and reducible contents of heavy metals in soil decreased,the residual state increased,and the heavy metals transformed to the less active form.(2)After 120 days pot experiment,Amaranthus hypochondriacus L and Bidens pilosa L can enrich heavy metals.For heavy metal Cd,Bidens pilosa L had the best enrichment effect,the highest enrichment coefficient was 0.22,the content of Cd in soil decreased by 0.2243-0.5752 mg/kg,and the proportion of residual state increased by2.21%-2.36%.The enrichment effect of Amaranthus hypochondriacus L on Zn was better than Bidens pilosa L,and the enrichment coefficient was 0.49.After Amaranthus hypochondriacus L restoration,the proportion of weak acid extraction state of Zn in soil decreased by 1.30%-9.25%,and the proportion of residue state increased by10.86%-20.95%.(3)The fixation rate of four heavy metals in soil has been significantly improved by applying passivator to the soil after planting plants.And the soil p H is slightly higher than that of the original soil,with an increase range of only 0.10-0.62,which has little impact on the soil p H.After planting Bidens pilosa L and applying Si2P1 for passivation culture,the fixation rate of Cd in soil reached 87.07%;After planting Amaranthus hypochondriacus L and applying Si2P3 for passivation culture,the highest fixation rates of Cu,Pb and Zn in soil were more than 70%.Compared with the tested soil,the proportion of weak acid extracted Cd and Zn in the soil under the combined remediation measures decreased by 2.26%-3.25% and 6.27%-9.31% respectively,and the residual state increased by 7.71%-8.42% and 20.35%-24.08% respectively.In general,the fixation rates of the four heavy metals in the soil under the combined remediation measures were more than 60%,and the remediation effect was significantly higher than that only using compound passivator.(4)Through RAC risk assessment coefficient method,it is found that the tested soil is mainly polluted by Cd and Zn,and the pollution of Cu and Pb is relatively light.The RAC risk coefficients of Cd,Cu,Pb and Zn decreased by 3.56%-64%,3.54%-7.03%,37.00%-43.41% and 19.26%-22.30% respectively.Under the passivator combined with phytoremediation measures,the pollution level of heavy metal Cd in soil was reduced from moderate pollution to mild pollution,while Zn was reduced from severe pollution to moderate pollution.To sum up,combined with the magnitude of heavy metal fixation rate in soil and the results of soil RAC risk assessment coefficient.The passivation effect of combined remediation is better than that of only applying compound passivator.It is suggested to use the combination of Amaranthus hypochondriacus L and 1% silicon fertilizer plus 1%ground phosphate rock,Bidens pilosa L and 1% silicon fertilizer plus 0.2% ground phosphate rock as the joint scheme of soil heavy metal remediation. |
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