鐵氧化物這類的礦物易與水及土壤中的陰離子及陽離子進行表面錯合反應，進而吸附污染物質，對於污染物的控制有著相當大的作用。因此，本研究選用針鐵礦 (Goethite)、赤鐵礦 (Hematite)、纖鐵礦 (Lepidocrocite) 及水合鐵礦 (Ferrihydrite) 這四種土壤中較常見的鐵氧化物對於六價鉻進行吸附試驗，並探討其相關吸附模式。
研究結果發現針鐵礦、赤鐵礦、纖鐵礦及水合鐵礦均對於六價鉻具有吸附的能力，吸附所需平衡時間分別為180分鐘 (針鐵礦)、180分鐘 (赤鐵礦)、360分鐘 (纖鐵礦) 及720分鐘 (水合鐵礦)。吸附量則隨著pH值的上升而逐漸下降，於25℃、pH為2時，各種鐵氧化物對起始濃度為3 mM之六價鉻之平衡吸附量大小依序為水合鐵礦 (904.21 mmol/kg) ＞ 纖鐵礦 (304.53 mmol/kg)＞ 赤鐵礦 (261.69 mmol/kg) ＞ 針鐵礦 (176.02 mmol/kg)。
針鐵礦、赤鐵礦、纖鐵礦及水合鐵礦吸附六價鉻的過程較符合擬二階反應 (Pseudo-second-order reaction)，並且反應動力的階次不受到pH值所影響，擬二階反應速率常數 (k2) 依序為針鐵礦 (2.44×10-3 ~ 2.33×10-4 kg/mmol˙min-1) ＞ 赤鐵礦 (1.62×10-3 ~ 1.81×10-4 kg/mmol˙min-1) ＞ 纖鐵礦 (3.60×10-4 ~ 6.13×10-5 kg/mmol˙min-1) ＞ 水合鐵礦 (4.41×10-5 ~ 1.08×10-5 kg/mmol˙min-1)。在等溫吸附模式之探討上，Freundlich模式較能模擬針鐵礦、赤鐵礦、纖鐵礦及水合鐵礦對於六價鉻之吸附情形，其R值均大於0.940。
反應溫度的提高有利於本研究中鐵氧化物對於六價鉻的吸附，經由熱力學探討求出其自由能變化量 (△G) 為負值，屬於自發性反應，而焓變化量 (△H) 為正值，屬於吸熱反應。
添加硝酸鈉 (NaNO3) 改變溶液中的離子強度對於鐵氧化物吸附六價鉻的影響不大。然而，當水中含有硫酸鹽及磷酸鹽時，則會明顯的與六價鉻產生競爭吸附，並隨著硫酸鹽及磷酸鹽添加量的增加，六價鉻吸附量降低的程度愈明顯。
脱附實驗的結果發現，NaOH相較於EDTA二鈉鹽對於鐵氧化物脫附六價鉻有較好之效果，以0.01 N的NaOH作為脫附劑時，其六價鉻脱附率可達90 %以上。 The iron oxides in soil mineral could form complex with the anion and cation contaminants in aquifer and tightly bind to soil matrix surface to remediate the contaminated groundwater. Therefore, the predominant iron oxides in soil e.g. goethite, hematite, lepidocrocite and ferrihydrite were selected for the chromium(VI) adsorption studies. The equilibrium and kinetic parameters for the adsorption of chromium(VI) were calculated from batch experimental data.
The results indicate the variation of chromium(VI) adsorption capacity with iron oxides as a function of pH, decreasing with increment of pH. The sequence and equilibrium adsorption capacity for initial 3 mM chromium(VI) ion was ferrihydrite (904.21 mmol/kg) > lepidocrocite (304.53 mmol/kg) > hematite (261.69 mmol/kg) > goethite (176.02 mmol/kg) at 25°C and pH 2. The equilibrium time for the adsorption process was determined as 180 minutes for goethite, 180 minutes for hematite, 360 minutes for lepidocrocite and 720 minutes for Ferrihydrite in the same experimental condition.
The Cr(VI) adsorption with goethite, hematite, lepidocrocite and ferrihydrite followed the pseudo-second-order rate equation and did not affected with solution pH. The range of adsorption rate constants (k2) for iron oxides at different operation pH were determined as 2.44×10-3~2.33×10-4 (goethite), 1.62×10-3~1.81×10-4 (hematite), 3.60×10-4~6.13×10-5 (lepidocrocite), and 4.41×10-5~1.08×10-5 kg/mmol˙min-1 (ferrihydrite). The Langmuir and Freundlich isotherm models were applied to Cr(VI) adsorption data. The Freundlich model fit best the equilibrium isotherm data (R2 > 0.940) from goethite, hematite, lepidocrocite and ferrihydrite.
Increase of reaction temperature enhanced the Cr(VI) adsorption by iron oxides. Isotherm adsorption data were evaluated to determine the thermodynamic parameters for the adsorption processes. The free energy change (ΔG0) is negative and showed to be feasible and spontaneous reaction. The enthalpy change (ΔH0) is positive and found to be endothermic.
The Cr(VI) adsorption into iron oxides did not affect with ionic strength of solution by adding of NaNO3. However, the existence of sulfate and phosphate ions in solution would compete with Cr(VI) ion in adsorbing onto iron oxides. The adsorption competition increased with the increasing of sulfate and phosphate concentration.
The sodium hydroxide had better desorption efficiency of Cr(VI) ion from iron oxides than sodium-EDTA salt. The desorption yield of Cr(VI) ion was above 90 % at contacting of 0.01 N NaOH.
