中文摘要
水及土壤中的陰離子及陽離子常會與鐵氧化物這類型的礦物進行表面錯合反應，利用鐵氧化礦物來吸附污染物質，對於控制污染物有相當大的作用。本研究選用土壤中較常見的鐵氧化礦物纖鐵礦 (Lepidocrocite) 及水合鐵礦 (Ferrihydrite)進行磷酸鹽的吸附試驗，並應用吸附模式作探討。
研究發現纖鐵礦及水合鐵礦對於磷酸鹽均有吸附能力，纖鐵礦及水合鐵礦所需平衡吸附時間分別為360與720分鐘 。磷酸鹽被吸附量也隨著pH值下降而增加，在pH為2、溫度25℃時，對於磷酸鹽起始濃度2 mM的平衡吸附量為水合鐵礦 (678.96 mmol/kg) > 纖鐵礦 (505.07mmol/kg)。
纖鐵礦與水合鐵礦吸附磷酸鹽皆符合擬二階反應，反應速率常數k2實驗值範圍分別為纖鐵礦(4.02×10-4 ~ 2.08×10-5 kg/mmole˙min-1) 與水合鐵礦 (2.69×10-4 ~ 6.91×10-6 kg/mmole˙min-1) 。
由熱力學參數求得自由能的變化量(△G)為負值是屬於自發性反應，而焓的變化量(△H)為正值且小於10 kcal/mol屬於吸熱反應及物理性吸附。
經脱附實驗結果發現，只有0.1N NaOH對於纖鐵礦有較好脱附效果(高達99%)，由此判斷纖鐵礦應為外部錯合反應，而水合鐵礦應為內部錯合反應。
關鍵詞：磷酸鹽、纖鐵礦、水合鐵礦、吸附 Abstract
The anions and cations from water and soil were easily to form complex reaction at the surface of minerals containing iron oxide which are very effective to consider as adsorbents applied to pollutants control system. In this study, the common soil containing iron oxide minerals, lepidocrocite and ferrihydrite, were selected to test phosphates adsorption, and examined by adsorption models for discussion.
This study evidenced that both lepidocrocite and ferrihydrite illustrated good adsorption ability for phosphates. The adsorption equilibrium time for lepidocrocite and ferrihydrite were 360 and 720 minutes, respectively. The phosphate adsorption capacity decreased with lowering pH value. Moreover, under the experimental conditions of pH 2, 25 ℃, and the initial concentration of phosphate = 2 mM, the equilibrium adsorption capacity of the ferrihydrite (678.96 mmol/kg) was greater than lepidocrocite (505.07 mmol/kg).
The phosphates adsorption kinetics of lepidocrocite and ferrihydrite followed the pseudo second-order reaction. The rate constant (k2) of lepidocrocite was in the range of 4.02 × 10-4 ~ 2.08 × 10-5 kg / mmole • min-1, and it of ferrihydrite was in the range of 2.69 × 10-4 ~ 6.91 × 10-6 kg / mmole • min-1.
Based on the thermodynamic parameters, the negative value of free energy (△G) indicates the reactions of phosphates adsorbed onto iron oxides might be spontaneous. Moreover, the enthalpy change (△H) was positive and less than 10 kcal / mol, which reveals their adsorption might be endothermic and physical reactions.
Desorption study was also conducted to examine the experimental results. Applied 0.1N NaOH to lepidocrocite showed the best desorption result, reached up to 99% desorption efficiency. It is noted that the lepidocrocite was outside complex reacting with phosphates, whereas the ferrihydrite was internal.
