腐植酸-水鐵礦對鎵和銦的錯合作用

Abstract

隨著半導體、光電和能源等科技的發展，鎵、銦兩個元素成為重要的原料而被使用於相關製程中並逐漸釋放至環境中。而排放入水體的污染物排放至環境，可能進入土壤或地下水系統造成環境污染，亦可能藉由食物鏈或飲水對人體產生風險，因此必須瞭解這些污染物進入環境後的物種型態與宿命。在土壤環境中，腐植酸對金屬的宿命扮演重要的角色；水鐵礦廣泛分布於環境中，對許多金屬有錯合能力。而本研究以腐植酸與水鐵礦，以及與水鐵礦共沉澱的腐植酸作為實驗材料，探討腐植酸和水鐵礦對鎵和銦的作用機制。在pH 4，與腐植酸錯合的鎵配位環境隨著反應時間增加趨於穩定；而在pH 7、9，液相中鎵金屬的比例逐漸減少。在鹼性條件(pH9)下，腐植酸主要是以羧基與鎵進行錯合。可以看到添加水鐵礦(包含共沉澱)，溶液中鎵的比例皆有下降，在添加水鐵礦後(包含共沉澱組)，添加固體的水鐵礦會影響腐植酸鎵的外圍電荷，推測鎵可能以表面錯合或吸附與水鐵礦反應。在pH4，銦與腐植酸形成的結構穩定。這部分推測與鎵的結果類似，主要受到腐植酸在酸性狀態下的型態影響。而中性和鹼性的部分與鎵不同，顯示鎵和銦與腐植酸錯合的機制有差異。其差異推測為鎵和銦不同絮聚能力所造成。以EXAFS結果顯示有無添加水鐵礦，銦可能皆以In(OH)3的物種與腐植酸反應。鎵和銦同為13族元素，但在與腐植酸的作用機制，以及此機制受水鐵礦的影響皆完全不同。因此未來需要更多研究探討腐植酸結構、官能基對鎵跟銦的反應機制，以評估鎵和銦污染後的環境宿命與風險。

Recent development of semiconductors and energy industries results in the release of trace elements such as gallium (Ga) and indium (In) into the environment. The contamination of Ga and In in soil and groundwater may lead the risk to human health via food chain. Hence, it is crucial to understand the fate of these trace metals in the environment. In soil environments, humic acid and ferrihydrite play an important role in the fate of metals in the soil environment. In this study, humic acid, ferrihydrite, and ferrihydrite-humic acid co-precipitates were investigated for the retention mechanisms of gallium and indium in aqueous solution. The complexation of gallium and humic acid tends to be stable along with the reaction time at pH 4, whereas the proportion of gallium in the aqueous phase gradually decreased at pH 7-9. Humic acid may form a smaller structure which results in increasing complexation of gallium and humid acid, causing flocculation over time. Under alkaline conditions (pH9), humic acid mainly complexed with gallium via the carboxyl group. The proportion of gallium in the solution was shown to be reduced after the addition of ferrihydrite (including co-precipitation). Ferrihydrite influenced the external charge of gallium humate. As the external charge of gallium was influenced by ferrihydrite, gallium is suggested to react with ferrihydrite by surface complexation or adsorption. Similarly, the complexation of indium and humic acid was shown to be stable at pH 4. However, the complexation of humic acid with gallium or indium varied in neutral and alkaline conditions, indicating that gallium and indium ions exhibit different complexation mechanism with humic acid and different flocculation abilities of the complexes. Indium was suggested to react with humic acid in the form of In(OH)3 with or without the presence of ferrihydrite (including co-precipitation) according to EXAFS results. Although, gallium and indium are both group 13 elements, their complexation mechanisms with humic acid and the influence of ferrihydrite on the complexation are completely different. Further studies are needed to elucidate the effect of the structure and functional group of humic acid on the complexation mechanisms of gallium and indium with humic acid for understanding the fates and potential risks of gallium and indium in the environment.