| 摘要: | 土壤是一複雜物質，由無機相與有機相兩大部分所組成，在吸附污染物時，可視為雙重吸附劑，其中無機相部份之吸附特性和一般傳統吸附劑類似，而有機相部份則是靠分配程序(partitioning)來進行吸附。由於土壤可同時進行吸附及分配程序，因此其於自然環境中對有機污染物之作用機制較一般無機固體吸附劑來得複雜，此兩種作用也決定了有機污染物在環境中的傳輸與宿命。 本研究乃以分子量相近、分子結構差異甚大之有機化合物(同樣含有6個碳而分子結構不同之扁平結構的苯、直鏈的正己烷及椅型的環己烷)為吸附質，探討有機化合物之立體效應對土壤吸/脫附之影響，以明確掌握分子結構對吸/脫附的影響，同時以含有高有機質之Florida Peat(有機質含量占86.4%)為吸附劑，進行至較高P/Po之氣態吸/脫附實驗，並比較低相對壓力及高相對壓力下之吸附圖象，以明確掌握其吸附機制。另以含有機質與無機質的紗帽山土(有機質含量占27.3%)做為氣態吸/脫附實驗之吸附劑，以釐清土壤無機相與有機相之吸附作用。綜合不同有機質含量的研究結果，比較無機相與有機相之吸附作用，進一步釐清其吸/脫附機制及影響因子，並掌握土壤無機相與有機相對不同有機污染物的吸附選擇性。 藉由Pseudo-first order kinetic model 、Pseudo-second order kinetic model、Intraparticle diffusion model 及The Elovich rate equation四組動力學模式來瞭解土樣進行吸/脫附之傳輸途徑。以土壤無機相結構而言(鈣-蒙特石、鈦-蒙特石)，四種動力學模式之線性關係均相當不錯，又以Intraparticle diffusion model模式之SSE值較小。初步判斷土壤無機相結構對有機化合物吸附動力行為較趨於孔洞之擴散。以土壤有機相結構而言(Florida Peat)，四種動力學模式之線性關係亦均相當不錯，在低相對壓力時以Intraparticle diffusion model模式之SSE值較小；但在中、高相對壓力時，則以The Elovich rate equation更適宜。初步推論，土壤高有機相結構對有機化合物吸附動力行為，瞬間以無機相之吸附(adsorption)為主，慢慢的轉為有機相的分配作用(partitioning)為主。 吸附直鏈型的正己烷與椅型的環己烷時，含土壤無機相與有機相的紗帽山土吸附量大於高有機相之Florida Peat。亦即吸附脂肪族碳氫化合物時，土壤無機相之吸附作用較有機相之分佈作用佔優勢。吸附含苯環之苯時，由於土壤有機質中含有aromatic compounds會產生π-π鍵作用力，使苯易分佈於土壤中。亦即吸附含苯環之有機化合物時，土壤有機相之分佈作用較無機相之吸附作用佔優勢。吸附具極性的水時，由於土壤有機質本質含有許多親水性官能基，使水分子易分佈到土壤中。亦即吸附極性有機化合物時，土壤有機相之分佈作用以極性化合物較佔優勢。 以高土壤有機質含量之Florida Peat吸附有機化合物，吸附量順序：Water> Benzene >>Hexane>Cyclohexane。因土壤有機質中含有會產生π-π鍵作用力的aromatic compounds及親水性官能基，易使水分子及苯分佈至土壤中；另一個可能的原因為水及苯具有較高的溶解度參數，會產生較強的聚合力，使水分子及苯易溶入土壤有機質內，因此造成極大之吸附量。至於非極性之環己烷與正己烷，其吸附量均相當低，尤其是環己烷，吸附量很低，脫附率亦相對的低，推論其椅型的立體結構是造成其難被吸/脫附的主因。 The effects of soil structure and chemical properties on the adsorption/desorption of volatile organic compounds were evaluated. The migration and the fates of nonionic organic compounds in soils are found to be highly depended on their vapor-phase sorptive behavior. However, it is difficult to explicit the mechanism of adsorption/desorption due to the complexity of environmental medium. Vapor-phase adsorption/desorption isotherms of water, benzene, hexane, and cyclohexane on dry soil with different soil organic matters, such as Ca-montmorillonite, Ti-montmorillonite, Shamon Mountain Soil and Florida Peat, were gravimetrically measured under 15°C, 20°C and 25°C. The surface area, pore structure, and adsorption/desorption characteristic were analyzed to show the soil structure and chemical properties effect on the adsorption/desorption of VOCs. After exchanged with metal cations, the porous structure of the soil mineral fraction was significanting changed. The results demonstrate that Ti-montmorillnite possess higher surface area, extensive pore size distribution, and better pore connection. Both the surface area and the pore structure of soil were characterized based on the classical and fractal analyses of the nitrogen adsorption isotherms. The surface fractal dimension D was calculated from their nitrogen isotherms using the fractal version of FHH (Frenkel-Halsay-Hill) equation. The results revealed that a smaller metal cation on the clay may slightly increase D values as a result of the increase in the BET surface area and the decrease in the pore size. The adsorption capacity of Florida Peat is greater than that of the Shamon Mountain Soil for the sorption of water and benzene, owing to the Florida Peat contains aromatic groups. Conversely, the mineral fraction was significant for aliphatic compounds, and the soil organic matter was quite significant for aromatic compounds. The steric structure of molecular effect on VOCs adsorption for soil, following the order：the plane form-benzene>the chain form-hexane>the chair form-cyclohexane. The experimental data were examined by the four sorption kinetic model：the pseudo-first order equation, the pseudo-second order equation, the intraparticle diffusion model and the Elovich rate equation. According to the sum of the errors squared (SSE), it showed that the intraparticle diffusion model fitted the data well, and the Elovich rate equation fitted the Florida Peat data well at relatively high pressure. |
