| In recent years, with the enhancement of the public’s awareness of environmental protection and the national emphasis on environmental pollution control, the management and remediation of heavy metal contaminated sites have attracted more and more attention. Compared with other repair techniques, passivation technology of heavy metal contaminated soils is better to meet the requirements of soil remediation from the time and cost, because of it’s short repair time, low repair cost and simple repair operation. Phytoremediation is a cheap, efficient and environmentally friendly remediation technology of contaminated soils. Therefore, in this study, we adopted combination of passivation technology and phytoremediation as the premise, the contaminated soil of Yangzonghai phosphogypsum stacking area as the research object and indoor pot experiments and outdoor field experiments as the research means. On the basis of the above,we tried to clarify the repair effects and mechanisms of the single and mixed passivating agents on the remediation of contaminated acidic soils, and select suitable passivating agents to repair heavy metal contaminated soil of Yangzonghai phosphogypsum stacking area, through studying the growth and heavy metal accumulation (As) of Vetiveria Zizanioides, the changes in the total amount of heavy metals and contents of available As and Cd in soil under the application of iron filing, superphosphate and fly ash.The findings of the study are as follows:(1)Results of field investigation:The average concentrations of Arsenic (As) and Cadmium (Cd) in soil at the study site exceeded Grade Ⅱ criteria of the national environmental quality standard for soils (GB15618-1995), with exceeding rates of 52% and 94%, respectively, which were 1.65 and 5.60 times as high as in corresponding for standard (As≤40 mg·kg-1, Cd≤0.30 mg·kg-1). the main form of As in the soil was presented as residual (73.91%), however, Cd were mainly presented as ion and manganese (39.07%) and exchangeable (27.81%). All plants did not show accumulative effects on As, Pb and Cu, Artemisia argyi Levl had better tolerance of As, with TF value of 3.716 and BF value of 0.226, and can be used as local dominant plant of As-contaminated soil to further research. Pharbitis nil (Linn.) Choisy would be used as a pioneer plant of Cd-contaminated soil, owing to the best enrichment capacity of Cd. Setairaviridis(L.)Beauv, Digitaria sanguinalis (L.) Scop and Poa annua L showed lowest accumulation and transfer factors in the results, but the three plants had highest density, short growth cycle, good adaptability, and roots proliferate, which are good to not only fix heavy metals into soil, but also prevent the migration and transformation of heavy metals.(2)Results of indoor pot experiments:This study selected iron filing, superphosphate and fly ash as suitable passivating agents from 11 kinds of passivating agents after simple soil experiments, and then indoor pot experiments of Vetiveria Zizanioides were conducted to study the effects of passivating agents (iron filing, superphosphate and fly ash) according to the L9 orthogonal test table. There was no obvious change on root length of Vetiveria Zizanioides, but impacted the plant heights significantly, after application of passivating agents (p<0.05). For example,5% superphosphate mixed with 5% fly ash significantly reduced the plant height by 33.2%. Although 1% iron filing mixed with 5% superphosphate and 5% iron filing mixed with 5% fly ash did not make root length or plant height increase, the leafy stems of Vetiveria Zizanioides were better than the other groups of passivating agents. 5% iron filing,5% superphosphate mixed with 1% fly ash,5% iron filing mixed with 1% superphosphate and 5% iron filing mixed with 5% fly ash significantly decrease the contents of As in upper part and underground part of Vetiveria Zizanioides, with declines of 85.1% and 95.4%,76.8% and 81.5%,71.5% and 92.1%, respectively. besides, the treatment of 5% iron filing,5% superphosphate mixed with 1% fly ash were the best for As reduction in Vetiveria Zizanioides. For contents of total As and available As in contaminated soils,1% iron filing mixed with 5% superphosphate and 5% iron filing,5% superphosphate mixed with 1% fly ash provided the best As immobilization for Vetiveria Zizanioides, with declines of 9.8% and 37.3%,17.0% and 34.6%, respectively. However, due to the excessive iron filings can often harden soil, destroy soil structure, and bring inconvenience to farming. So we selected 1% iron filing mixed with 5% superphosphate as the best passivating agents for this indoor pot experiment.(3)Results of outdoor field experiments:The treatments of passivating agents had no significant difference on the fresh weight, dry weight and contents of As in Vetiveria Zizanioides (P>0.05). but decreased the soil pH significantly,1% iron filing mixed with 5% superphosphate was most obvious.expect 5% superphosphate, other passivating agents treatments significantly increased the plant height of Vetiveria Zizanioides (P>0.05), the effects of superphosphate mixed with iron filing were better than their single treatments, and 5% iron filing mixed with 1% superphosphate was the best passivating agent for plant height, which followed by 1% iron filing mixed with 5% superphosphate. Furthermore,5% iron filings,1% iron filing mixed with 1% superphosphate and 5% iron filing mixed with 5% superphosphate Significantly reduced contents of available As in soils with declines of 35.2%,38.7% and 43.8%. on the whole, the effects of iron filing were better than superphosphate as passivating agent for As contaminated soils and the growth of Vetiveria Zizanioides, and dominated by low concentration. However, the consumption of iron filing mixed with superphosphate needed broad and further experimental study.
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