不同電荷性質土壤對鉈之吸附及脱附機制

Adsorption and Desorption Mechanisms of Thallium(I) by Soils with Different Charge Properties

林虹妘(Hung-Yun Lin)

指導教授 ： 王尚禮

國立臺灣大學/生物資源暨農學院/農業化學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201700370

摘要

鉈是一種具有高毒性的元素且其毒性對人類具有致死性，主要污染來源為礦物冶煉與科技產業工廠排放。鉈在土壤中的宿命至今尚未有文獻探討，而土壤膠體電荷為決定吸附反應的重要因子之一，為了瞭解土壤與鉈之間的交互作用，本研究利用永久電荷土壤(Cp、Btw、Ta-2)及pH依賴性電荷土壤 (Lv、Ce、Pu)對Tl (I)進行等溫吸附反應及脫附反應，探討不同電荷特性土壤對Tl (I)吸附及脫附行為之影響。本研究結果顯示，永久電荷土壤對Tl (I)之吸附量明顯大於pH依賴性電荷土壤對Tl (I)之吸附量，且Ta-2對Tl (I)之最大吸附量與陽離子交換容量之比值 (Qmax/CEC)為1.06，由此推測應是Ta-2含有大量錳氧化物，使得Tl (I)氧化形成Tl(OH)3沉澱或Tl (III)與錳形成難溶之錯合物，而提高了Tl (I)之吸附量。另一方面，本研究將不同電荷特性土壤對Tl (I)之脫附實驗結果利用Freundlich方程式擬合，並計算其遲滯係數(nd/na)，結果顯示永久電荷土壤相對pH依賴性電荷土壤具有較高的遲滯現象，其主要原因為土壤組成的不同，在永久電荷土壤中，Tl (I)可藉由離子交換方式被固定在2:1型黏土礦物層間，且四面體帶電土壤之遲滯現象較八面體帶電土壤明顯，其歸因於Tl (I)吸附位置與土壤帶電位置之距離，所導致Tl (I)吸附鍵結力的不同；而pH依賴性電荷土壤則是利用高嶺石和氧化物作為其主要吸附位置，且土壤pH較低，因此pH依賴性電荷土壤對Tl (I)之吸附量較低，且反應具有可逆性。
研究結果發現，在土壤吸附Tl (I)的過程中，一部分的Tl (I)可能被土壤中的黏土礦物固定，然而Tl (I)的脫附通常具有可逆性，因此Tl (I)在土壤中可以輕易地被淋洗至地下水層中，進而增加環境污染的風險。

關鍵字

鉈 ； 永久電荷土壤 ； pH依賴性土壤 ； 吸附 ； 脫附 ； 遲滯現象

並列摘要

Thallium is highly toxic but its fate in soil has not been well understood. In Taiwan, elevated levels of Tl in soils have been detected due to its application in the electronic industries. To understand the interaction mechanism of Tl with soils, this study conducted Tl(I) adsorption and desorption experiments for six soil samples, which included permanent-charge and pH-dependent-charge soils. The Tl(I) adsorption capacities of permanent-charge soils were higher than those of the pH-dependent charge soils. The ratios of Tl(I) adsorption maximum to CEC were determined to be 17 – 38% for the variable-charge soils and approximately 60% for the permanent-charge soils. For Ta-2 the ratio of Tl (I) adsorption maximum to CEC was 1.06, which was assumed that Tl (I) may be oxidize to Tl (III) by the manganese oxides. Tl (III) froms inner-sphere complex at manganese oxides or Tl(OH)3 precipitate to increasing the adsoption of Tl (I). The adsorption/desorption isotherms of each soil were both fitted to Freundlich equation to obtain the corresponding n values, which were subsequently used to calculate hysteresis coefficient (nd/na). The values of the hysteresis coefficients of all the soils increased with the initial Tl(I) concentration. The permanent-charge soils exhibited a higher hysteresis coefficient than the pH-dependent charge soils. These results indicated that a fraction of Tl(I) may be retained by the clay minerals in the soils. However, because the desorption behaviors of Tl(I) were generally reversible, Tl(I) is expected to be leached readily in soil, indicating a high environmental risk of Tl(I) pollution in soil.

並列關鍵字

Thallium ； Permanent-charge soil ； pH-dependent charge soil ； Adsorption ； Desorption ； Adsorption hysteresis

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