| In recent years, the problem of heavy metal contamination in soil has been received closed attention. Heavy metals are easily accumulated and difficultly to be degraded in soil. Furthermore, they can enter into human body via soil-plant-(animal)-human system and threaten human health. It is difficult to remove heavy metals from soils containing high concentrations of metals quickly, economical and efficiently. Owing to protecting human health, it is worthwhile to investigate the speciation of heavy metals in soil-plant system in depth, screened out cultivars suitable for growing on metal-contaminated area, and used economic and effective improvement measures to reduce toxicity of heavy metals in plants.In this paper, firstly, we conducted a field survey to study the total concentrations of five heavy metals (As, Cd, Cu, Pb, Zn) and their speciation in soil and plants in a multi-metal mining area, Jijie Town, Gejiu City, Yunnan Province. Secondly, the plant cultivars with low metal uptake in their edible parts were screened out by pot experiments, and further distribution of heavy metals in plant was examined. Thirdly, the effect of different amendments on metal uptake of plants was studied to reduce the bioavailability of heavy metals.The main results of the present research are listed as follows:(1) Three heavy metals, Cd, Pb and As, contributed to the heavy metal pollution of farmland in the study area, and the bioavailable Cd and Pb in soil accounted for 39.12% and 36.28%, respectively. The clustering results from accumulation factors of vegetables showed that the leaves of pakchoi, peppermint and coriander had a strong accumulation for five heavy metals, and they were not suitable for planting in the area. In the case of metal speciation in the edible parts of the three typical vegetables containing high metal concentrations, the main forms of As was presented as ethanol-extractable and residue, Pb as NaCl-extractable and residue, Cd as acetic acid-extractable and residual, Cu as NaCl-extractable and Zn as acetic acid-extractable fractions. The toxic effect of As among five heavy metals was the highest. In addition, the comprehensive pollution indexes of vegetables followed the order of Pb> As> Cd> Zn> Cu. Although the proportion of soil bioavailable Cd was up to 39.12%, but its speciation in the edible parts of vegetables was mainly HAc-extractable and residual forms, which have a relatively low activity. The ratio of daily intake to the limiting value of provisional tolerable daily intake for Cd was only 1.86 for Cd, which was much lower than that for Pb (5.71) and As (5.12). In conclusion, Pb and As imposed a greater threat on the health of local residents compared to Cd.(2) A significant difference in plant height and biomass was observed among different cultivars within same vegetable. As for slightly arsenic-contaminated soil,27 cultivars were suitable to plant because their As concentrations in edible parts were lower than As-MPC. The concentrations of Pb in some cultivars were lower than Pb-MPC but all cultivars were over Cd-MPC. The pollution of Pb and Cd in soil is a serious issue, especially Cd.For the As-Pb-Cd combined pollution soil in the study, the metal concentrations of cultivars Lvxiu848(S3), Fengan(P4), Chuncheng3(All) and Liangma(Al) were relatively lower and are much safer than other cultivars.Results from metal speciation in cultivars with different metal-uptake ability in the same vegetable showed that:① for As, the low bioactivity forms were mainly presented in the leaves of low As uptake cultivar (S6) in spinach, and the similar result was also shown in low or high As uptake cultivars in asparagus lettuce, especially in the low As uptake cultivar A15. ②For Pb and Cd, the low bioactivity forms were mainly presented in the leaves of high metal uptake cultivar (S3) in spinach, but the opposite result was shown in low metal uptake cultivar (S6). The low bioactivity forms were mainly presented in low Pb uptake cultivar (S3) in aubergine. Therefore, in the selection of low metal uptake cultivars, the cultivars with high metal bioactivity forms should be paid much attention, although the total metal concentrations in them are very low.(3) Amendments addition to metal-contaminated soil could change metal bioavailability, and a majority of treatments could significantly decrease the contents of available metal, such as treatments 7,9 and 10 for As, treatments 7,9,10,13 and 14 for Pb, and treatments 9,10,11 and 13 for Cd.It is worth noting that treatments 9 and 10 could significantly reduce available contents of three heavy metals, indicating that the combination of middle level slaked lime (5 g kg-1), low level organic fertilizer(0-25 g kg-1), middle or high level fly ash (10-15 g kg-1) could effectively remediate As-Pb-Cd co-contaminated soil. Compared to the control, soil available As and As content in the ecible parts of pakchoi significantly decreased in treatments 9 and 10. Similar result was also shown in the case of Cd in treatments 10 and 13. Amendment addition could promote the growth, height and biomass of pakchoi, and the most effective was treatment 16. A majority of treatments could improve the quality of pakchoi. Results from metal speciation in plants showed that for As, the percent of Fethanoi+Fwater fractions was dominant in leaf and petiole; for Cd, FNaci+FHc fractions was superior to other forms in leaf and petiole. |
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