不同土壤中鉈對水稻幼苗的生長與吸收的影響

Abstract

新興汙染物 - 鉈是一種具有極高毒性的金屬元素，通常在自然環境中的濃度極低。由於近年來半導體與光電儀器產業的興起，鉈作為原料被運用於製程，可能導致周圍區域遭受鉈汙染。若被農地中的作物吸收造成糧食汙染，將對人體健康造成巨大威脅。然而目前臺灣缺乏對於鉈的環境管制標準，同時少有研究探討鉈在土壤中的宿命與其生物有效性。為了了解鉈汙染可能造成之前在影響，本研究分別以水耕試驗探討植物的鉈有效性與鉈毒害，以土耕試驗探討不同試驗土壤對植物鉈吸收之影響。試驗皆選用處於三葉齡的台梗九號水稻幼苗作為植物材料，以Tl(I)NO3作為鉈處理的添加物種。水稻幼苗水耕試驗以0.01、0.05、0.1、0.2、0.5、0.7、1.0、2.0 mg L-1為處理濃度，暴露九天後分析其生長情形和植體鉈濃度。結果顯示，水稻幼苗經九天的鉈暴露後其生長勢與生質量受到顯著抑制，抑制程度隨著處理濃度越高而上升，同時葉綠素含量也下降，植體開始呈現萎黃。根部與地上部鉈濃度可以分別在短期間內吸收至高達1953 mg/kg 與633 mg/kg，累積係數ECR與ECS高達約2500與500，顯示了鉈具有高生物有效性。 土耕試驗則添加1、5、10、20、50 mg kg-1於平鎮系、坡堵系、臺南系、鹿港系、和美系、豐樂系、關山系七種供試土壤，以盆栽試驗的方式培植水稻幼苗，結果顯示水稻在土耕系統中的累積係數相比於水耕試驗急遽下降，顯示土壤的存在限制了鉈的吸收。水稻幼苗的鉈吸收量在各土系中不同，顯示鉈有效性的高低受不同土壤特性所影響。對照供試土壤的土壤基本性質，發現主要由土壤pH值、土壤陽離子交換容量、無定型鐵氧化物含量影響植物的鉈吸收。土壤陽離子交換容量本身即為土壤重金屬吸附代表，pH值越高的土系其土壤的pH依賴型電荷量越高，對鉈的吸附能力越強；同理無定型鐵氧化物含量高的土壤在pH值下形成更多pH依賴型電荷，進而增加土壤對鉈的吸附，限制植體的吸收。有鑑於一價鉈離子與鉀離子的離子半徑大小的相近，以富含蛭石的關山系(Ks)與多由高嶺石組成黏粒成分的平鎮系(Pc)兩土壤作為依據，比較其孔隙水中鉈濃度與孔隙水中鉈濃度與水稻鉈吸收情形，發現土壤中所含有的2:1型黏土礦物可能以限制鉀有效性相同的方式影響鉀的移動性。水耕與土耕試驗結果皆顯示，溶液中的鉀/鉈比例上升時，大量的鉀會與鉈競爭植體根部細胞膜的同一傳輸通道，進而抑制植體的鉈吸收。而鉀與鉈的相似性，使得土壤孔隙水中鉈的生物有效性得評估得以參照鉀的修正式，以[Tl]/([Ca]+[Mg])1/2作為修正後的活性比值可以與植體鉈濃度相關性更好的直線關係。而以鉈鉀吸收競爭作為修正的活性比值[Tl]/[K]也有同樣結果，顯示除了受鉈本身的劑量濃度影響外，溶液中其他的陽離子組成也會影響鉈的生物有效性。

Thallium(Tl) is a highly toxic metal element, which naturally presents in the environment at extremely low concentrations. With the development of semi-conductor and electro-optical industries, Tl has been applied in the production processes, which results in Tl contamination in surrounding areas. After Tl is absorbed by crops in the fields, the Tl-containing foods will pose a huge threat to human health due to the high toxicity of Tl. However, little is known about the fate of Tl in soil and the Tl uptake of plants. To better understand the potential effect of Tl contamination, rice seedlings (Oryza sativa L.) were grown in hydroponic and soil cultivation systems containing Tl(I)NO3 of various concentrations. In hydroponic experiment, Tl concentrations in the nutrition solution were 0.01, 0.05, 0.1, 0.2, 0.5, 0.7, 1.0, 2.0 mg L-1 . After exposure of 9 days, the growth and Tl concentration of rice seedlings were measured. The result of hydroponic experiment showed that the growth and biomass of rice seedlings were inhibited under Tl exposure, and the degree of inhibition increased as the Tl concentration increased. At the same time, the chlorophyll content decreased, and chlorosis of plant tissue appeared. In short period of exposure, Tl concentration in root and shoot of rice went up to 1953 mg/kg and 633 mg/kg at maximum respectively, and enrichment coefficient of root and shoot (ECR and ECS) went up to about 2500 and 500 respectively. Both of the facts show high bioavailability of Tl in rice seedlings. Pot experiments were carried out with 7 test soils spiked with 1, 5, 10, 20, 50 mg kg-1 of Tl The enrichment coefficient of Tl in rice seedling sharply decreased in pot experiment when compared with hydroponic experiment, which showed that the negative effect of soil on the Tl uptake by plants. The Tl uptake of rice seedlings were significantly different from each test soil, which suggested the variations in Tl availability in different soils. Soil pH, CEC, amorphous iron oxides content and the composition of exchangeable cations were found to be the key factors in determining Tl uptake. CEC is always considered as the ability of adsorbing heavy metal ions. Soils with higher pH made higher pH-dependent charge, which enhanced Tl adsorption capacity. Soils with high amorphous iron oxides in soil tends to form more pH-dependent charge at high pH, which enhanced Tl adsorption capacity and also restricted Tl uptake. In view of the similarity of ionic radii of Tl(I) and K+, we compare the difference between Kuanshan series (vermiculite-abundant soil) and Pinchen series (Kaolinite abundant soil). The difference of Tl concentration in pore water and the Tl uptake of rice seedlings between those two soil may be caused by 2:1 type clay mineral in soil, which may affect Tl mobility in the same mechanism as K+. Both hydroponic and pot experiments showed that the increase of K/Tl ratio in solution causes the reduction of Tl uptake in plant, attributed to the competition of K+ with Tl(I) for the same transporter on root sell membrane. The similarity between Tl(I) and K+ also provide a reference to predict Tl bioavailability with Tl concentration in pore water. The modified activity formula, [Tl]/([Ca]+[Mg])1/2, proved to be better correlated with Tl concentration in rice seedlings. Another modified activity formula, [Tl]/[K], takes K/Tl competition into consideration and also gives a better correlated with Tl concentration in rice seedlings. Besides Tl dosage, the cations composition in soil solution will also effect the bioavailability of Tl.