土壤對有機污染物的吸持作用，包括無機相的吸附及有機相的兩相分佈。探究吸／脫附機制的困難在於環境基質的複雜性，所以本研究摒除水分的干擾，針對土壤無機相的構造與其吸附作用進行探討。 實驗係透過量測BET表面積、孔洞體積、孔洞大小、孔徑分布、晶格間距及推測孔洞連結，並配合電子顯微鏡之表面影像而得土樣基本結構參數；再使用Cahn D-200電動天平並控制溫度，測量苯及正己烷在鈣─蒙特石與鈦─蒙特石上之等溫吸／脫附曲線。 結果顯示，鈣─蒙特石經由過渡金屬鈦離子取代後，表面積、孔洞體積及平均孔徑均增加、孔徑分布趨向不均勻化、晶格C層間距改變且氣態吸／脫附曲線不具遲滯迴圈，顯示了蒙特石的結構發生改變，也使得孔洞連結度變得更佳。鈦─蒙特石提供較多吸附位置，又無立體阻礙，所以在達單層飽和吸附量時，其吸附平衡所需的時間均較鈣─蒙特石為快，單層吸附量也較高，且等溫吸／脫附曲線沒有迴圈，無遲滯現象。實驗所得之吸附熱則受到吸附劑的孔洞及氣體分子在孔洞內之排列影響，對於孔徑體積較小的鈣─蒙特石，其吸附熱須考量吸附劑與吸附質間的作用以及吸附分子間之作用力。 The objective of this research is to evaluate the effect of soil structure on the adsorption/desorption of volatile organic compounds (VOCs). The migration and the fate of nonionic organic pollutants in soils are highly depended on their vapor-phase sorptive behavior. However, it is difficult to explicit the mechanism of adsorption/desorption due to complexity of environmental medium. In order to show the effects of soil structure on adsorption/desorption of VOCs, two kinds of montmorillnite with different exchange cation, calcium and titanium were individually used to examine the isotherms for the vapor uptake of benzene and n-hexane under 288K and 298 K. After exchanged with metal cation, the porous structure of the mineral samples was changed. These changes were explored by quantifying the BET surface area, total pore volume, micropore volume, pore size distribution, surface fractal dimension (calculated from N2 adsorption/desorption isotherms), c-spacing (by XRD) and surface image (by SEM) of mineral phase. The results demonstrate that Ti-montmorillnite has higher surface area, extensive pore size distribution, and better pore connection. The influences of temperature and soil properties were also investigated. A gravimetric adsorption apparatus was developed and used to generate adsorption/desorption isotherms of benzene and hexane on two dry soil samples at 288 and 298 K. Isosteric heats of adsorption were calculated and heat curves were established. Equilibrium isotherms were all Type II, characterizing vapor condensation to form multilayers. The sorption capacity of soils is positively correlated with specific surface area, pore size distribution and pore connection. Nevertheless, Ti-montmorillnite didn’t reveal hysteresis loop, which might be attributed to better pore size distribution and pore connection. Isosteric heats of adsorption on Ca-montmorillnite were influenced by the reaction between adsorbent and adsorbate as well as among adsorbate molecules. The results of this study are to further understanding of soil properties, as a basis for desorption predictions. Findings, apply not only to environment applications but also to theoretical development.
