乾燥或水分未達飽和之天然土壤其與環境中有機污染物之作用,往往因土壤有機相含量較無機相為低,使土壤無機相主導整體吸附行為。在水溶液或水分達飽和之系統中，因土壤無機相對水分子之親和力較大，土壤無機相傾向於優先吸附水分子再與有機污染物作用，雖然土壤無機相對於有機污染物仍有微量吸持作用，但相較於有機相之分佈量仍屬偏低。過去許多學者曾針對天然土壤進行水相與氣態之研究，但天然土壤在酸鹼淋洗純化後仍含有無法去除之有機質，且土壤經純化後易導致無機相結構與表面特性改變，使單純具有土壤無機相之研究甚為艱難。本研究為克服土壤無機相純化上之缺點，因此選用三種與土壤無機相礦物晶體結構與化性組成雷同，且不含有任何有機質之人工合成物質:沸石、氧化鋁、矽膠，來模擬土壤無機相於水相、氣相中進行與環境常見有機污染物質:苯、正己烷、環己烷及甲苯之吸附實驗。結果顯示，乾燥系統中，吸附等溫線呈現BET模式中第II型曲線，吸附量與吸附劑比表面積大小呈正相關，並受吸附質分子形狀、極性及官能基影響。由BET吸附理論計算可得各吸附質及水分子對土壤無機相表面之第一層吸附熱，顯示水分子於吸附發生時相較於有機污染物優先被土壤無機相吸附，因此解釋水相系統中，等溫線呈現線性之原因為有機污染物以分離狀態與水分子呈現混合相分佈薄層，以凡得瓦力及官能基引力吸持於吸附劑表面之水膜上，使吸附熱小於凝結熱，並呈現非競爭線性等溫線。研究除證明水分子於水相吸附系統所扮演之角色，亦可得水相系統中吸附量僅與有機污染物水溶解度呈現負相關之結論。 The sorption phenomena of organic compound in dry or water- unsaturated soil are dominated by mineral/inorganic adsorption due to the relatively low soil organic matter contents. However, in water- saturated system, the partioning of organic compounds into soil organic matter as the major process because the adsorption of organic compounds by minerals is suppressed by water. The sorption of organic compounds by natural soil in dry/water- saturated system has been studied extensively. However there are still some controversies in purification of soil inorganic phase. To overcome the purification shortcomings of soil inorganic phase, three highly pure synthetic mineral oxides with no detectable organic carbon：Boiling chips、Aluminum oxide and Silica gell were selected as adsorbents; In addition, n-hexane、cyclohexane、benzene and toluene were selected as adsorbates for all experiments to evaluate the sorption phenomena between soil mineral phase and organic compounds in dry/water- saturated systems. In dry system，all isotherms are in the form of BET type II，total adsorption capacity was related to adsorbent surface area. Most notably, as stated, the extent of uptake for given compounds is affected by molecular steric structure、polarity as well as polar-group of adsorbate. The heat of first layer on adsorbent surface of all adsorbates and water was calculated by BET adsorption theory. The sorption data in aqueous system show that water molecule must be preferentially adsorbed by soil mineral phase and offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition and hence organic solutes are separated by water showing a partition flake. Adsorption occurs presumably by dispersion forces and polar-group interactions onto a water film above the mineral surface. This study not only proved the role of water in linear adsorption, but also revealed the negative relationship between Kd and water solubility of organic compounds.
