砷污染土壤中不同化學處理方法對水稻及青梗白菜吸收砷的影響

Abstract

砷普遍存在於環境中，當土壤遭受污染時，砷可能因植物吸收或淋洗作用進入地下水而對人類產生影響。本篇選用化學固定法復育砷污染土壤。化學固定法係在土壤中加入改良劑，改良劑與污染物產生吸附、錯合或沉澱反應，藉此降低污染物之生物有效性及移動性。然土壤中砷以含氧陰離子存在，與一般重金屬性質並不相同，適用於重金屬復育之改良劑並非適合砷污染場址。因此本研究於仁德及平鎮土系土壤外加0, 20 及 40 mg As(V) kg-1，以砷酸鈉作為砷劑來源，並研究以下六種處理：(1) 對照組，(2) 3% 堆肥，(3) 0.5% 針鐵礦，(4) 0.5% 硫酸亞鐵，(5) 0.5% 硫酸亞鐵混合石灰，及 (6) 0.5% 硫酸亞鐵加 0.1% 過錳酸鉀混合石灰。試驗期間測定土壤中水溶性砷，以序列萃取法評估砷在土壤環境中的風險，並進行水稻 (Oryza sativa L.) 及青梗白菜 (Brassica chinensis L. CV. Ching-Geeng) 盆栽試驗，計算土壤-植體間之砷傳輸係數，以評估上述各改良劑復育砷污染土壤之效益。另以 0.2% 檸檬酸銨、0.1 M 醋酸銨及 0.05 M 磷酸一銨萃取土壤生物有效性砷含量，與植體砷含量檢定相關性，預測植體砷含量。 仁德土壤各處理結果顯示隨砷的添加水稻明顯受到砷毒害，在外加 40 mg As(v) kg-1 處理種植兩個月後死亡。堆肥處理之有機質與砷形成可溶性錯合物，提高砷移動性，水稻盆栽試驗亦明顯受到毒害。而青梗白菜盆栽試驗各處理作物生長良好，添加 40 mg As(V) kg-1 堆肥處理可明顯降低植體砷含量 (67%) 並明顯提高產量 (209%)，但總吸收砷量與對照組無顯著差異。堆肥處理並無法降低砷在土壤-作物間的傳輸。添加堆肥可促使非特異性吸附相之砷含量增加，砷污染地區施用堆肥將造成砷移動的風險。針鐵礦處理與對照組無異，僅在外加 40 mg As(V) kg-1 針鐵礦處理降低青梗白菜植體砷含量 (84%) 並提高產量 (145%)。最理想之改良劑為硫酸亞鐵，可減緩砷對水稻毒害，降低青梗白菜植體砷含量 (87%) 並提高產量 (183%)。硫酸亞鐵處理明顯降低土壤非特異性吸附相砷，轉而存在於無定形鐵氧化物吸附相，即硫酸亞鐵形成無定形鐵氧化物吸附砷，降低砷在環境中流佈的風險。硫酸亞鐵混合石灰以及硫酸亞鐵加過錳酸鉀混合石灰處理，與單獨施用硫酸亞鐵處理無異甚至更差，因 pH 上升而減少砷在鐵氧化物表面的吸附量。若施用硫酸亞鐵後土壤酸化程度仍在作物生長範圍，可不需外加石灰調整 pH 值。 平鎮土壤本身 pH 低，鐵氧化物含量高，即使外加 40 mg As(V) kg-1 砷劑 (總砷： 48.7 mg kg-1)，土壤生物有效性砷極低，作物除未受砷毒害外，在低生物有效性砷情況下，砷有促進水稻生長之功能，而改良劑復育效果不明顯。供試平鎮土壤之總砷含量已接近臺灣土壤污染管制標準 (60 mg kg-1)，作物卻能正常生長；相反地，仁德土壤外加 40 mg As(V) kg-1 處理 (總砷：46.6 mg kg-1)，水稻生長明顯受砷毒害影響，導致不稔甚至死亡。不同土壤總砷量相近，但水稻生長情形卻有極大差異，顯示土壤中鐵氧化物含量對砷生物有效性有極大的影響。 醋酸鈉、檸檬酸銨及磷酸一銨萃取有效砷含量分別大於某一濃度時，水稻即受到砷毒害。外加改良劑不同，即使萃取生物有效性砷含量相近，水稻生長情形亦可能有不同之結果，代表改良劑除了與砷作用之外，同時也影響水稻之生理表現。生物有效性砷萃取與植體砷含量之相關性顯示，水田環境因土壤砷型態變動大，不易以單一萃取法預測糙米砷含量；而在旱作砷型態單純，醋酸鈉、檸檬酸銨及磷酸一銨萃取之生物有效性砷含量，可準確預測不同處理下青梗白菜植體砷含量，皆有潛力作為生物有效性砷含量之萃取試劑。

Arsenic is a ubiquitous element in the environment. Once soil contaminated by As, it could produce health risk by food chain or leaching into the groundwater. In this study, chemical stabilization remediation technique was selected to clean up the As-contaminated soils. Chemical stabilization is the soil was added the amendments to bind or precipitate the metals to reduce the mobility or bioavailability of contaminants in the soil. Due to As is present by oxyanion in the soil which is different from the properties of general heavy metal, the soil amendments applied to heavy metal contaminated sites are doubtful for As-contaminated sites. The objectives of this study are: (1) to evaluate the risk of As in the environment by determining the water soluble As and As fractionation in soil; (2) to evaluated the effects of different chemical treatments on the As concentration of rice or Chinese cabbage grown on As-contaminated soils; and (3) to understand the relationships between the soil bioavailable As concentration and the As concentration of brown rice or Chinese cabbage. Two representative soils of Taiwan, surface soils of Jente and Pinchen soil series, were used for this study. The total As concentration of two studied soil is about 4 and 9 mg As kg-1. Soils were added with sodium arsenate (Na2HAsO4•7H2O) solution to reach the level of 0, 20, 40 mg As(V) kg-1, respectively, and then the soils were againg by cycling of drying and wetting for three months to prepare the As(V)-contaminated soils. The chemical treatments included: (1) control, (2) adding 3% compost, (3) adding 0.5% goethite, (4) adding 0.5% ferrous sulfate, (5) adding 0.5% ferrous sulfate plus calcium carbonate, and (6) adding 0.5% ferrous sulfate, 0.1% potassium permanganate and calcium carbonate. The amended soils were aging for another 3 months by cycling of drying and wetting before they were planted with rice (Oryza sativa L.). The amended soils were dried and mixed for experiments of Chinese cabbage (Brassica chinensis L. CV. Ching-Geeng). The sequential extraction procedure (SEP) of As fractionation of each treatment was conducted. The soil bioavailable concentration of As extracted by 0.1 M ammonium acetate, 0.2% ammonium citrate or 0.05 M monoammonium phosphate was analyzed to compare with the As concentrations of brown rice and Chinese cabbage. Results of Jente soil indicated that the rice growth were inhibited with increasing of soil As concentration and the rice died after transplanting two months for As-spiked 40 mg As(V) kg-1 treatment. Organic matter of compost was proposed to form the aqueous complexes with As in soil and it significantly increased the water soluble and bioavailable As concentration. The restriction of rice growth by As toxicity is also significantly found. There is no any As toxicity for Chinese cabbage growing in soil spiked with 40 mg As(V) kg-1 treatment and it decreased 67% of As concentration in the upper part of cabbage and also increased 209% of dry weight compared with the control treatment. The total As uptake per pot are not significant differences between compost and control treatment. The result of As SEP in soil showed that compost treatment can increase the non-specific bounded As fractionation and increased the health risk to the environment. Geothite treatment can only reduce the As concentration by 84% and increase the dry weight by 145% of Chinese cabbage compared with control treatment for soil-spiked with 40 mg As(V) kg-1. Ferrous sulfate treatment is the most effective amendment to reduce the As toxicity and uptake in rice, in terms of reducing the 87% of As concentration in leaf and increasing 183% of dry weight of Chinese cabbage soil-spiked with 40 mg As(V) kg-1 treatment. Ferrous sulfate treatment can reduce the non-specific bound-As fractionation which is shown the increase on the amorphous hydrous Fe and Al oxide-bounded As fraction. Ferrous sulfate treatment can adsorbe with As by forming amorphous hydrous iron oxide to reduce As risk in the environment. There are no any significant differnces between ferrous sulfate treatment and ferrous sulfate mixed with other materials such as calcium carbonate or potassium permanganate. The low bioavailable As of Pinchen soil was produced by low soil pH and high iron oxide content in soil for soil-spiked with 40 mg As(V) kg-1. The growth condition and grain production do not suffer soil As toxicity and were increasd by 328% of grain yield. As can promote the rice growth under soil-spiked with 40 mg As(V) kg-1. grain production and As concnentration of grain were not significant differences among all the tretament. The significant differences of rice growth in different soils are dependent on the iron oxide content in two soils. The soil amendments for As-contaminated soils can not only interact with As ion but also effect on the rice growth condition. It is difficult to predict the As concentration in brown rice by chemical extraction methods because the As forms in rice growing condition are not stable for As specific forms. In the upland for cabbage production, As ion was proposed as As(V) form by different extraction solutions, such as ammonium acetate, ammonium citrate and monoammonium phosphate, to satisfy predict the As concentration in Chinese cabbage by different soil amendments.