孔雀草非洲鳳仙與美女櫻對污染土壤鎘鉛之植生萃取研究

Abstract

植生復育 (phytoremediation) 為利用植物可收獲之部位以移除土壤中污染物的技術，可大量累積重金屬於植體的地上部，另外對於生物有效性較低的重金屬，可在土壤中施用可促進其生物有效性的螯合劑 (如強鍵結能力螯合劑EDTA) 以增加植生萃取的效果。由前人研究指出孔雀草 (French marigold, Tagetes patula)、美女櫻 (Garden verbena, Verbena bipinnatifida) 及非洲鳳仙 (Impatiens, Impatiens walleriana) 為具有累積重金屬鎘、鉛之潛力且適應台灣氣候生長之植物物種。本研究目的為選用此三種植物分別種植於人工污染鎘、鉛土壤中，測試其對鎘、鉛污染土壤之耐受程度及累積能力；及施用強鍵結能力之螯合劑EDTA以增加其植生萃取之能力。本研究在國立臺灣大學人工氣候室進行試驗，控制植物生長日/夜溫度為30℃/ 25℃。土壤重金屬處理濃度分別為對照組、鎘濃度20 mg kg-1、40 mg kg-1及80 mg kg-1，及對照組、鉛濃度1000 mg kg-1及2000 mg kg-1，四重複，種植35天後收割；並在收穫前七天添加不同濃度EDTA於鎘濃度為20 mg kg-1及鉛濃度為1000 mg kg-1。種植期間，以土壤水分採集器 (RSMS) 收集第0、7、14、21、28及35天的土壤溶液，探討土壤溶液中重金屬隨時間之變化。植體以HNO3 / HClO4法分解，樣品溶液以原子吸收光譜儀 (Hitachi 180-30型) 測定土壤溶液及植體中之鎘、鉛含量。結果顯示孔雀草可忍受程度達土壤Cd 40 mg kg-1，其植體地上部Cd濃度達366± 41 mg kg-1，但非洲鳳仙忍受程度可達土壤80 mg kg-1，其植體地上部Cd濃度高達1,230 ±101 mg kg-1，已達鎘超累積植物之標準 (100 mg kg-1) 之4-12倍。美女櫻與非洲鳳仙對土壤鉛的忍受程度為1,000 mg kg-1，其植體地上部Pb濃度分別為114 ±6 mg kg-1與131 ±28 mg kg-1，雖未達超級累積植物之濃度，但已比一般植物有特別明顯較高之鉛吸收能力。於土壤鉛濃度為1,000 mg kg-1下，施用2.5及5.0 mmol EDTA kg-1可顯著增加種植於非洲鳳仙及美女櫻的土壤溶液中鉛的濃度 (p < 0.05)，也顯著增加美女櫻及非洲鳳仙之植體地上部Pb濃度2-3倍，且兩種施用EDTA的濃度皆可顯著增加美女櫻之植體地上部Pb總移除量2-3倍 (p < 0.05)。

Phytoremediation is a technique by harvesting the plants which can remove large amounts of heavy metals from the contaminated sites. To enhance the mobility and bioavailability of the metals by applying chelating agents (e.g. EDTA) and to accelerate the effect of phytoextraction, french marigold (Tagetes patula), garden verbena (Verbena bipinnatifida) and impatiens (Impatiens walleriana) had been approved that they not only have potential removal of Cd and Pb in the site but also adapt to the climate condition of Taiwan. The objectives of this study are to understand the maximum tolerance and potential accumulation of Cd and Pb of these three plants growing in the Cd- and Pb-contaminated soils. Different EDTA concentrations are also applied to enhance the phytoextraction of Cd and Pb. The pot experiments were conducted in the phytotron of National Taiwan University, which controlled at 30 ℃ in the day and 25 ℃ in the night, respectively. The treatments of heavy metals are control, 20, 40 and 80 mg Cd kg-1, 1000 and 2000 mg Pb kg-1. Four replicates were conducted for each treatment. All plants were harvested at 35th day after transplanting into the pots. The chelating agent, EDTA, was applied with different concentrations into 20 mg Cd kg-1 and 1000 mg Pb kg-1 at the 28th day after transplanting, which were harvested at the 7th day after applying EDTA. Soil solutions were also collected by rhizon soil moisture sampler (RSMS) at the different days to monitor the changes of metal concentrations in the soil solutions. The plant samples were digested by the HNO3/HClO4 digestion method. The concentrations of Cd and Pb in the solution samples and plants were determined by atomic absorption spectrometry (Hitachi 180-30 type). The results indicated that the potential toxic concentrations of Cd for french marigold and impatiens were 40 and 80 mg kg-1 and the maximum Cd concentrations in the shoots of two plants were 366 ±41 mg kg-1 and 1230 ±101 mg kg-1, respectively. The Cd concentrations in the shoot of these two plants have reached 4-12 folds of the threshold value, 100 mg kg-1, in hyperaccumulators of Cd. The potential toxic concentrations of Pb for garden verbena and impatiens were 1000 Pb mg kg-1 and the maximum Pb concentrations in the shoots of two plants were 114 ±6 mg kg-1 and 131 ±28 mg kg-1, respectively. Although they do not reach the threshold value, 1000 mg kg-1, of hyperaccumulators of Pb, the Pb concentrations in garden verbena and impatiens are much higher than that of other normal plants. The concentration of Pb in soil solutions of garden verbena and impatiens increased significantly after applying 2.5 and 5.0 mmol EDTA kg-1 into the soils contaminated by 1000 Pb mg kg-1 (p <0.05). In addition, applying EDTA significantly increased the Pb concentrations in the shoot of garden verbena and impatiens by 2-3 folds and the total Pb removal by garden verbenas’ shoots is 2-3 folds (p <0.05) as well.