| Heavy metal-pollution in soil is one of worldwide environmental issues. The removal of heavy metals in soil is imperative but very difficut. Phytoextraction, an in situ, low-input, environmental benefit, has been proposed as an effective technology to remediate contaminated soils. Hyperaccumulators have great potential for absorbbing excessive heavy metals and have played an important role in soil remediation.This research was conducted by pot and filed experiments. First of all, plants with different metal-hyperaccumulative characteristics, namely Solanum nigrum and Pteris cretica var. nervosa, were monocropped or intercropped in multiple-metal contaminated field soils in this study. This line of study aims to investigate, compared with monocropping method, whether intercropping method could improve metal uptake by these two tested plants. The effects of ethylene diamine disuccinate (EDDS), ethylene diamine tetraacetate (EDTA) and nitrilotriacetic acid (NTA) on uptake of heavy metals (Pb, Cd and As) by plants growing in an intercropping system were also investigated using the same soil. Each chelant was applied to soil pots at the rate of 0,1.5,3,6,12 mmol·kg-1, respectively. Additionally, a heavy metal leaching experiment was designed to assess the environmental risk of the application of 0,3 mmol·kg-1 EDTA, EDDS and NTA. Secondly, S. nigrum and P. cretica were respectively monocropped in soil spiked by Pb, Cd, As at the concentration of 0,600,900,1200 mg·kg-1,0,2,5,10 mg·kg-1 and 0,50,100,200 mg·kg-1, respectively, aiming to investigate the effects of Pb, Cd, As multiple-contaminated soils on metal uptake by the two tested plants. Finally, a field experiment was carried out to estimate the potential for phyremediation under different planting conditions using three hyperaccumulators, namely Tephrosia candida, S. nigrum and P. cretica var. nervosa.Results of pot experiments showed that:1. Compared with monocropping method, intercropping method increased Cd concentration 1.3 times and As concentration 1.4 times in shoots of S.nigrum and P. cretica, respectively. This result indicated that intercropping method could significantly improve heavy metal uptake by hyperaccumulators. Much higher tolerance to chelators was observed in P. cretica than that in S. nigrum in the intercropping system. The application of 3,6,12 mmol·kg-1 EDTA to soil contaminated with heavy metals significantly increased soil Pb, Cd bioavailability and concentrations of Pb in two intercropped plants. The maximum Pb concentrations reached 27.58 mg·kg-1 and 26.17 mg·kg-1 in S.nigrum and P. cretica, respectively, which were 3.3 and 3 folds higher than that in the corresponding control plants to which no chelators were applied. Under the conditions of the intercropping system, the application of 3,6,12 mmol·kg-1 EDTA also significantly increased Cd concentration in the shoot of P. cretica. EDTA was more effective than NTA in increasing soil Pb, Cd bioavailability, but was less effective than NTA in increasing soil As bioavailability. The application of 1.5 and 3 mmol·kg-1 NTA significantly increased soil As bioavailability as well as the concentrations of As in shoots of two tested plants.2. The concentrations of Pb, Cd in the soil solution after 14 days, following treatment with 3 mmol-kg"1 EDTA, were respectively 21.12 mg·L-1 and 0.51 mg·L-1, which significantly exceeded the control of Pb(1.48 mg·L-1), Cd (0.20 mg·L-1) and were 422 and 51 folds higher than the standard of Pb (0.05 mg·L-1) and Cd (0.01 mg·L-1) in "Environment Quality Standard for Groundwater " (GB14848-93) (GradeⅢ). This result indicated that applying 3 mmol·kg-1 EDTA to Pb-, Cd-, and As- contaminated soils probably leads to leaching risks of Pb, Cd in soil.3. In orthogonal experiment of monocropped P. cretica, arsenic in soil could significantly increased the concentrations of As in shoots of P. cretica. Pb, AS, Cd in soil affect As uptake in shoots of P. cretica in the order of As>Pb>Cd. T4 (0 mg·kg-1 Pb,10 mg·kg-1 Cd,200 mg·kg-1 As) treatment reached the maximum As concentrations that was (1027.03±56.23) mg·kg-1 in shoot of P. cretica. In addition, Pb in soil also significantly affected the concentrations of As in shoots of P. cretica, but Cd induced no significant effects. Therefore, despite P. cretica being a As hyperaccumulator, its phytoeatraction potential is very limited in Pb-, As- contaminated soils.4. In orthogonal experiment of monocropped S. nigrum, Cd, As in soil could significantly affect the concentrations of Cd in shoots of S. nigrum. Pb, AS and Cd in soil affect Cd uptake in shoots of S. nigrum in order of Cd>As>Pb. T7 (600 mg·kg-1 Pb,5 mg·kg-1 Cd,200 mg·kg-1 As) treatment reached the maximum Cd concentrations that was (84.84±0.88) mg·kg-1 in shoot of S. nigrum.Results of filed experiment showed that:1. The average concentrations of Cd, As in soil at the studied site exceeded "Environment Quality Standard for Soils" (GB 15618-1995) (GradeⅢ). A significantly positive correlation was observed between Pb, Cd and As bioavailability and total concentrations in soils.2. The maximum concentrations of Pb, Cd and As reached (32.96±5.65) mg·kg-1, (6.99±0.25) mg·kg-1 and (326.98±93.99) mg·kg-1 respectively in shoots of T. candida, S.nigrum and P. cretica under 7 different planting conditions, which suggested the uptake of heavy metals by tested plants were limited in multiple-metal contaminated field soils.3. Monocropped S. nigrum (C) under all treatments reached the maximum uptake of Pb, Cd and As in shoots of S.nigrum, which were 1004.97 g·(ha·a)-1,152.04 g·(ha·a)-1 and 1534.47 g·(ha·a)-1, respectively. Cd, As uptake efficiency in shoots were 2.82% and 1.41% under monocropped S.nigrum (C) conditions, respectively. Monocropped S.nigrum (C) method was most effective to remove heavy metals from multiple-metal contaminated field soils. |
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