硒氧陰離子與二元氧化物系統界面關係

Abstract

由鋁或鐵氧化物混合非結晶態的二氧化矽成的二元氧化物系統去除硒氧陰離子的研究相當少。本研究利用 Al3+ 或 Fe3+ 與無定形二氧化矽在不同pH 值合成二元氧化物系統，再藉由巨觀和微觀尺度的實驗，包括比表面積、零電點 (zero point of charge, ZPC)、溶解度、傅立葉變換紅外線光譜 (FT-IR) 、固態核磁共振技術(27Al 和29Si MAS NMR) 和 X 光吸收光譜 (XAS) 來研究鋁-矽與鐵-矽之間的交互作用。巨觀實驗說明，Al(III)/SiO2 系統中的 Al(III) 可以阻塞二氧化矽表面，使得矽無法溶出；但 Fe(III)/SiO2 系統並沒有此現象，使得Fe(III)/SiO2 系統表面零電點較 Al(III)/SiO2 系統低。FT-IR 、 MAS NMR 和XAS都說明了，鋁的化學性質改變二氧化矽的表面，然而，鐵並沒有。二元氧化物系統進行對於吸附硒酸根與亞硒酸根的巨觀等溫、動力和脫附等實驗，以及微觀的XAS。吸附硒酸根與亞硒酸根的等溫與動力學結果適合以 Langmuir 等溫模式和擬二級動力學模型來說明其過程。熱力學參數則說明：硒酸根與亞硒酸根吸附在二元氧化物系統表面的過程為放熱和自發反應。根據 XAS 圖譜說明：硒酸根與亞硒酸根吸附在二元氧化物系統表面都形成化學吸附的內層表面錯合物。

Removal of selenium oxyanions by binary oxide systems, Al- or Fe-oxide mixed with X-ray noncrystalline SiO2, was previously not well understood. This study evaluates that reactions of Al or Fe with amorphous silica prepared at different pH values by macro- and micro-scale experiments including BET surface area, electrophoresis, stability, Fourier transform infrared (FT-IR), solid-state nuclear magnetic resonance techniques (27Al and 29Si magic-angle spinning (MAS) NMR), and X-ray absorption spectroscopy (XAS). Macro-scale experiments showed that Al(III) could block an silicon oxide surface group to suppress dissolution of silica, but not Fe(III). The negative charge of silica have no influence on the surface charge (i.e., pHzpc) of Al-Si binary oxide, nonetheless, the pHzpc of Fe-Si binary oxides was reduced by silica. Based on FT-IR, 29Si MAS NMR, and XAS spectra, Al has changed the chemical properties of silica, such as bonding vibration and electron distribution, however, Fe did not. Micro-scale experiments (i.e., XAS) demonstrated that Al(III) should be a stronger association with silica than Fe(III) on silica. The binary oxide systems were characterized for macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and the pseudo-second-order kinetic model. Thermodynamic parameters indicated that the processes of selenite and selenate adsorbed on the binary oxide systems are exothermic and spontaneous reactions. Based on simple geometrical constraints, selenite on the both binary oxides systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO2 forms stronger complexes than that on Al(III)/SiO2.