| Summary: | 碩士 === 國立中正大學 === 應用地球物理研究所 === 101 === Tatun Volcano Group (TVG) is not active since late Pleistocene but the post-volcanic activities, such as hot spring and sulfur gas, still widespread around the volcanic area. Peihuang Creek is the main watershed system in TVG. The creek water is characterized by higher temperature, low pH values (3.0-4.5) and high SO4 content (60-400 ppm) due to the mixing of hotspring. This would promote the geochemical interaction between water and andesitic rocks and results in waters with highly enriched iron, aluminum and silica. These elements prefer to form suspended colloidal particles in water and adsorb heavy metals. Once the pH of water increases under oxidation condition, the colloid would precipitate in the form of ochre colored powder on the riverbed.
Five samples of ochre colored powder were sampled along the creek. The results of XRF demonstrate that the powder is mainly composed of iron(54.3%), silica(15.2%) and aluminum(8.1%). No clear crystal structure can be determined by XRD; therefore, the ochre colored powder is defined as amorphous Fe-Al hydroxide. According to previous study, this kind of iron, aluminum oxides can adsorb a large amount of heavy metals. Using digestion method, that the amorphous Fe-Al hydroxides do adsorb houndreds to thousands of ppm heavy metals. To evaluate the binding phase of heavy metals, the BCR and long-term sequential extraction procedure were conducted. Generally, it is believe that heavy metals are mainly bonded with reducible phase. In this study, on the contrary, the results of sequential extraction show most heavy metals were released from exchangeable and oxidizable phases.
In addition, more extraction procedures were conducted for simulating variable natural and anthropogenic environments, including pH-dependent extraction, surfactant extraction and humic acid extraction. The results show that heavy metals were hard to be extracted in high pH value because of the neutral pHzpc of Fe-Al hydroxides. The low solubility of humic acid also results in the removal of heavy metals from water. Conclusively, Heavy metals in tens to hundreds of ppb can be extracted under natural environments.
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