短鏈有機酸對零價鐵移除土壤中五氯酚之影響

Abstract

五氯酚是一種過去廣泛應用的農藥，因此造成嚴重的土壤汙染，尤其在南台灣有一五氯酚污染場址即台南安順廠。在本研究中，探討四種短鏈有機酸鹽類(乳酸鈉、酒石酸鈉、草酸鈉和檸檬酸鈉)對於五氯酚在土壤中的吸脫附之影響，以及短鏈有機酸對零價鐵移除五氯酚的影響。在廠區之鹼性土壤中，由於短鏈有機酸提供羧基陰離子(-COOH)的數目不同，短鏈有機酸鹽類抑制五氯酚在土壤中的吸附依序為檸檬酸鈉 > 草酸鈉 > 酒石酸鈉 > 乳酸鈉。造成此現象的主要原因為短鏈有機酸陰離子可以陰離子交換的方式競爭五氯酚陰離子在土壤的吸附位置，進而減少五氯酚陰離子與土壤之間的交互作用。檸檬酸鈉濃度從100 mM 降低到1 mM，五氯酚的吸附減少量差異不大；但隨著濃度從100 mM增加到1000 mM，五氯酚的吸附量反而增加，其主要原因為高濃度的檸檬酸鈉會增加土壤的疏水性，進而促使五氯酚的吸附。另外，在酸性環境下，由於五氯酚為分子態，100 mM 檸檬酸反而會增加五氯酚在土壤的吸附量。至於脫附等溫曲線實驗的結果，隨著檸檬酸鈉的濃度從1 mM增加到100 mM，五氯酚在土壤中之吸附下降量大致上有增加。但經脫附效率之計算，顯示1~100 mM從土壤中的脫附量增加不大。總結來說，1 mM檸檬酸鈉即可用來降低五氯酚在土壤中吸附與促進脫附。另一方面，在溶液pH = 8.0時，五氯酚無法被微米級零價鐵降解。然而在酸性環境下，五氯酚仍不能被微米級與奈米級零價鐵所移除，但短鏈有機酸可以促進微米級與奈米級零價鐵降解五氯酚。在所有選擇的短鏈有機酸(乳酸、酒石酸、草酸和檸檬酸)當中，草酸促進奈米級零價鐵進行五氯酚的脫氯效果最佳。由於草酸的高pH緩衝能力，可減少奈米零價鐵的反應表面形成鐵的氧化物或氫氧化物。此外，從掃描式電子顯微鏡和X光繞射圖顯示反應中草酸會與二價鐵離子強烈地錯合，形成草酸鐵錯合物，降低溶液中二價鐵離子的濃度，也減少鐵離子沉積到奈米零價鐵的表面形成鐵氧化物之鈍化層，使得奈米零價鐵能持續有可反應之表面，所以草酸可增加奈米零價鐵對五氯酚降解的效率。經由副產物分析可證實草酸所增加奈米零價鐵對五氯酚之降解作用為還原脫氯機制。綜合檸檬酸鈉脫附土壤中的五氯酚與草酸增進奈米零價鐵對五氯酚的降解，將有助於土壤中五氯酚有機汙染物的整治與復育。

Pentachlorophenol (PCP) has been widely used as a pesticide and led to serious soil contamination, especially An-shun factory in Tainan which is the most serious PCP contaminated site in Taiwan polluted by China petrochemical development corporation. In this study, the effects of four short-chain organic acids (SCOAs) on sorption/desorption of PCP in soil and the dechlorination of PCP by zero valent iron (ZVI) in the presence of SCOAs were investigated. In alkaline soil conditions, SCOAs inhibited the sorption of PCP in the order: trisodium citrate > disodium oxalate > disodium tartrate > sodium lactate because of possessing more carboxylate functional groups. The sorption amount of PCP decreased similarly in the presence of 1 mM to 100 mM of trisodium citrate but increased as the concentration grows to 1000 mM. Desorption amount of PCP generally increased as an increase in trisodium citrate equilibrium concentration. After the desorption evaluation, 1 mM trisodium citrate is enough to decrease PCP sorption and increase PCP desorption. Furthermore, PCP could not be removed by microscale ZVI (MZVI) and nanoscale ZVI (NZVI) at pH 8.0 and pH 3.0, respectively, but SCOAs could promote their removal slightly at pH 8.0 and dramatically at pH 3.0, respectively. With the strong complexation of SCOAs with iron oxide or hydroxides, these carboxylic acids could inhibit the deposition of iron(hydro)oxides on the surface of iron and even refresh iron surface. Among these selected SCOAs, oxalic acid enhanced the largest removal of PCP with NZVI in an acid condition. Scanning electron micrograph (SEM) and X-Ray diffraction (XRD) patterns showed oxalate-iron complexes formed out of NZVI particles, indicating oxalic acid could strongly complex with ferrous ions produced from the dechlorination of PCP in solution and then diminish the formation of ferrous (hydro)xides to deposit on NZVI; as a consequence, less passivation layers and even fresh NZVI surface generated. Dechlorination process was proposed for the reactions of PCP with NZVI in the presence of oxalic acid according to the identification of degradation byproducts. Therefore, oxalic acid can lead to the highest dechlorination efficiency of PCP by NZVI. Combing the desorption improvement by adding trisodium citrate and the dechlorination enhancement by amending oxalic acid, PCP can be removed from soil efficiently. These findings can facilitate the remediation of organic pollutants such as PCP.