- 學位論文
土壤和水鐵礦對鉬酸根離子的吸持反應以及對小麥和水稻之鉬有效性的作用
The retention of molybdate by soils and ferrihydrite and its effects on the availability of molybdenum to wheat and rice
摘要
鉬為動植物生長之必要微量元素，亦為新興污染物。由於過往從微量元素的角度不易探究鉬在土壤中的移動性及生物有效性，因此對此至今尚未完全了解。有鑑於土壤鉬污染對動、植物生長的潛在風險，本研究目的為探討影響鉬在土壤中吸持能力的主要土壤性質，以及對小麥和水稻的有效性。 吸持反應為影響營養元素與污染物在土壤中移動性及有效性的重要反應之一，因此本研究以十一種台灣土壤進行鉬酸根的吸附與脫附實驗，並以皮爾森相關性分析探討影響土壤鉬吸附量與釋出能力的主要土壤性質為何。結果顯示，鉬在土壤中的吸持能力受土壤酸鹼度、游離性鐵鋁氧化物含量以及可交換性鋁和鈣含量所影響。在酸性土壤中鉬酸根的吸持量較高，然而吸附於游離性鐵鋁氧化物上的鉬酸根較易被釋出；而在中鹼性土壤中，則可能形成鉬酸鈣沉澱使其釋出能力較低。雖然土壤鉬吸持能力與游離性鐵鋁氧化物含量呈正相關，而未與無定型鐵氧化物含量達顯著相關，然而隨時間增加，無定型鐵氧化物可能逐漸轉變為結晶性鐵氧化物，而可能進一步影響鉬在土壤中的移動性。故本研究進一步於25及75 ℃下進行水鐵礦老化試驗，並觀察水礦老化對鉬酸根吸持能力之影響。結果顯示，鉬酸根離子主要以四面體雙牙單核的內圈錯合型態吸持於水鐵礦上，並隨水鐵礦逐漸轉變為赤鐵礦的過程中，鉬酸根會首先以八面體雙牙單核的內圈錯合型態吸持於鐵氧化物上，再逐漸被固定到鐵氧化物的結構中，而使鉬酸根的移動性下降。 為探討上述土壤性質如何影響對植物鉬的有效性，本研究以平鎮系 (酸性黏質)、台南系 (酸性砂質)及太康系 (鹼性黏質)土壤進行盆栽試驗，並觀察鉬在小麥與水稻中的累積情形、物種組成，以及鉬在根圈土壤的空間分布。結果顯示，小麥的鉬累積量隨土壤pH上升而增加，顯示土壤酸鹼度為影響小麥鉬累積量的重要因子之一；而水稻對鉬的累積量則以台南系高於太康系，並與土壤溶液中鉬濃度隨土壤pH上升而增加的趨勢不吻合，顯示土壤溶液的鉬濃度未能反映對水稻的鉬有效性。由土壤連續萃取與根部鉬酸根物種的分析推測，造成台南系土壤水稻植體鉬累積較高的主要原因為土壤中無定型鐵氧化物比例較高，使得較高比例的鉬酸根以吸持於無定型鐵氧化物的物種存在於土壤中以及水稻根部，並在浸水還原的狀態下隨鐵氧化物還原溶解釋出，進而被水稻根部所吸收。此結果也顯示植物對鉬吸收所呈現的土壤鉬有效性，無法單純以土壤吸脫附反應以及土壤溶液的結果來評估，而需考慮土壤氧化還原狀態與植物特性等因子的影響。 為了評估土壤鉬污染在環境中的潛在風險，本研究建議後續可進一步研究：(1) 可交換陽離子對鉬由土壤膠體脫附的影響，以了解可交換性陽離子組成如何影響鉬在土壤中的移動性; (2) 鐵的氧化還原反應對鉬的物種形態的影響，以探討水田土壤中鉬的有效性；(3) 釐清控制鉬由土壤移動至根圈並進一步被植物吸收的主要反應與影響因子，以了解影響鉬在土壤中對植物有效性的主要機制。
並列摘要
Molybdenum (Mo) is a micronutrient to most living organisms and mainly presents as molybdate in the natural environment. It has also become one of the emerging contaminants in recent years. Due to the manufacture of high-tech products, Mo could potentially be released into adjacent agricultural fields, and further be uptake by plants and animals, causing a health risk to humans. However, the concentrations of Mo in soils are relatively low as a trace element, making it difficult to investigate the mobility and availability of Mo in soils so far. In view of the potential toxicity of Mo as a contaminant in the environment, it is crucial to understand the retention of Mo in soils and its availability to plants. Sorption is one of the key reactions that determine the mobility and availability of nutrients and contaminants in soils. In this regard, the effects of soil properties on molybdate sorption were studied using a wide range of soils in terms of soil properties. The results showed that the sorption of molybdate in soils was mainly dependent on soil pH, and the contents of free iron (Fe)/ aluminum (Al) oxides, and exchangeable Al and calcium (Ca). Higher content of molybdate was sorbed in acidic soils. However, the molybdate sorbed on free Fe/Al oxides could be partly released during desorption in acid soils, while the formation of CaMoO4(s) could increase the retention of molybdate in alkaline soils. While the sorption capacity of molybdate in soils was significantly correlated to the free Fe oxides contents but not to the poorly-crystalline Fe oxides content, the temporal transformation of poorly-crystalline Fe oxides to well-crystalline Fe oxides could affect the mobility of the sorbed molybdate. Therefore, the binding configurations of ferrihydrite-sorbed molybdate (FSM) during the transformation of ferrihydrite was examined with the aging experiment. The results showed that molybdate sorbed on ferrihydrite mainly via bidentate-mononuclear complexes as tetrahedron. During the transformation of ferrihydrite to hematite, the sorbed molybdate could be transformed into bidentate-mononuclear complexes of octahedral molybdate, and gradually be incorporated into the structure of Fe oxides, leading to the decrease of Mo mobility during aging. The effects of soil properties on soil Mo availability to crop plants were further examined with pot experiments. Wheat and rice plants were grown in Pc (acid-clayey), Tn (acid-sandy), and Tk (alkaline-clayey) soils. The results showed that the Mo accumulation in wheat plants increased with increasing soil pH (Pc < Tn < Tk), which was consistent with the result of the adsorption-desorption experiment. Meanwhile, the Mo concentrations in rice plants increased in the sequence of Pc < Tk < Tn. The Mo availability to rice plants could not be estimated with the batch experiment or the Mo concentration in soil solutions. Instead, the concurrent release of molybdate during the reductive dissolution of amorphous Fe/ manganese (Mn) oxides in soils and the formation of Fe/Mn plaques could play relatively important roles in determining the Mo accumulation in rice plants. The results also indicated that soil redox condition and the type of plants should be considered when evaluating the Mo availability in soils. To evaluate the potential risk of Mo contamination in the soil, the results of this study suggested further investigations include: (1) the effects of exchangeable cations on the desorption of molybdate on soil colloidal constituents, including clay minerals, humic substances, and oxides, to elucidate the release ability of molybdate in soils; (2) the effects of Fe redox cycles on the speciation and mobility of Mo in soils to elucidate the Mo availability in rice paddy soils; (3) the reactions and key factors that control the transportation of Mo from bulk soil to rhizosphere and plant roots to understand the mechanisms controlling soil Mo availability to plants.