以管柱淋洗實驗探討一價鉈在不同性質土壤中的吸持性

Abstract

有鑑於鉈為一種劇毒的環境污染物，長期暴露於高濃度鉈的環境下，生物將有致死風險。尤其高科技產業其產製過程所排放出的鉈逐漸累積於土壤中，適逢強降雨，鉈會受到雨水之淋洗作用而移動到地下水層及河流，將間接對人體健康造成嚴重的威脅，不容小覬。此外，土壤的異質性亦會對鉈的移動性產生不同程度的影響，亦即不同基本性質的土壤會對鉈產生不同程度的滯留，故而，瞭解並掌握鉈在土壤中的移動性誠乃目前土壤管理的重要課題。然而，綜觀過去有關鉈離子與土壤間交互作用之相關研究，主要均著重於探討單一土壤因子改變時，對鉈離子與土壤間靜態吸附及脫附過程的影響，卻鮮少完整地探究關於多種不同之土壤因子對鉈離子在土壤中動態傳輸的影響。準此，本研究選用二十種基本性質差易大的土壤，以管柱淋洗實驗模擬自然環境下鉈在土壤中的移動情形，探究何種土壤基本性質主導鉈在土壤中的傳輸。此外，由於競爭型陽離子會與鉈離子競爭土壤上的結合位，並交換出原本吸附在土壤中的鉈離子，間接使鉈離子重新釋放到環境中。因此，本研究更應進一步以管柱淋洗實驗探討四種競爭型陽離子對鉈離子脫附效應之影響。研究發現，土壤pH值、可交換性鉀離子、可交換性鈉離子、可交換性鎂離子、可交換性鈣離子以及游離氧化錳是主導鉈離子在土壤中傳輸的最關鍵因素。在土壤pH介於3.6~7.2的範圍內，上述六種土壤因子較高時，土壤對鉈離子有較高的滯留能力，從而降低鉈離子釋出到環境的機會。此外，由競爭型陽離子對鉈離子脫附效應的實驗結果明白顯示，鉈離子在脫附初期的濃度將會遠遠超過原通入土壤的濃度，尤其是鉀離子存在時，不僅造成脫附初期的鉈離子濃度遠遠超過原通入土壤的鉈溶液濃度，甚至能脫附大部分被土壤吸附的鉈離子。本研究針對鉈離子的傳輸特性進行系統性的研究，說明了土壤pH值、可交換性鉀離子、可交換性鈉離子、可交換性鎂離子、可交換性鈣離子以及游離氧化錳是控制鉈離子移動性的主要因素，同時競爭型陽離子會對鉈離子的脫附行為造成影響。

Thallium (Tl) is a highly toxic environmental pollutant. The deposition of Tl from anthropogenic activities such as the production process of mining and electronic industries has increased the levels of Tl in the environment. Once it enters the food chain, it will pose a serious threat to human health. The soils with different soil properties will have different effects on the transport of Tl(I). Thus, in this study, a column leaching experiment was used to simulate the transport behavior of Tl(I) in soils in the natural environment and explore which soil properties play a key role in determining the transport of Tl(I) in soils. Moreover, the competing cations, such as K+, Na+, Ca2+ and Mg2+, would compete with Tl(I) for binding sites on the soils, leading to the desorption of Tl(I) adsorbed on the soils. Therefore, a column leaching experiment was further used to investigate the effect of competing cations on the desorption of Tl(I). The results indicated that soil pH, exchangeable K, exchangeable Na, exchangeable Ca, exchangeable Mg and free manganese oxide content are the most critical factors in determining Tl(I) transport in soils. In the range of soil pH 3.6~7.2, the higher the soil pH, exchangeable K, exchangeable Na, exchangeable Ca, exchangeable Mg and free manganese oxide is, the stronger Tl(I) retardation in soils. Moreover, the competing cations play the important role in the desorption of Tl(I). At the initial stage of desorption, the Tl(I) concentration drastically increased, even higher than the inlet Tl(I) solution. Among the four competing cations, K+ is the strongest. When K+ is present, Tl(I) concentration would greatly increase at the initial stage of desorption and the adsorbed Tl(I) on the soils would almost be desorbed by K+ in the end of desorption phase. This study systematically explores the transport behavior of Tl(I) in soils. These findings can provide detailed information for soil remediation, and even establish a strategy for soil management, protecting human beings from being threatened by Tl.