界面活性劑結合鎳鐵雙金屬降解土壤中五氯酚之研究

Abstract

五氯酚(Pentachlorophenol, PCP)為一具致癌性且氯酚類化合物中毒性高的化合物，並可做為木材防腐劑、殺蟲劑與殺菌劑，過去曾被廣泛及大量施用，且臺灣南部有一遭受PCP嚴重汙染的廠址。本篇研究主要探討奈米級鎳鐵雙金屬(Ni/Fe nanoparticles)結合界面活性劑整治受PCP污染的土壤溶液及土壤的能力。以批次的土壤溶液反應試驗，探討鎳金屬及陽離子型介面活性劑十六烷基三甲基溴化銨 (cetyl trimethylammonium bromide, CTAB) 的添加量對鎳鐵雙金屬降解PCP的影響。並探討鎳鐵雙金屬之劑量、初始pH值及界面活性劑對鎳鐵雙金屬降解土壤中PCP的影響。 在不同鎳含量與不同CTAB濃度試驗中，含有2 % 鎳的鎳鐵雙金屬於1臨界微胞濃度(critical micelle concentration, CMC) CTAB (Ni/Fe-CTAB)，可在五分鐘內迅速移除安順廠附近土壤之土壤溶液中的PCP，及於反應 24小時後脫除大部分PCP的氯離子。Ni/Fe-CTAB可以迅速的吸附土壤溶液中的PCP並增加PCP於其表面反應位置的濃度，因而促進了PCP的移除及還原降解反應。由於Ni/Fe-CTAB表面帶正電，其與鹼性環境下帶負電的五氯酚離子(PCP-)間的靜電吸引力可能促進其表面吸附的作用。在土壤的批次反應試驗中，多數的PCP可由土壤釋放至反應溶液中，顯示 PCP-與土壤表面間的靜電排斥力主導了PCP在此鹼性土壤中的吸著行為。然而，在土壤中加入陽離子型的CTAB會增加PCP在土壤中的吸附量，導致Ni/Fe-CTAB無法直接施用於土壤之中。CTAB帶正電的含氮官能基容易佔據土壤中膠體與有機質上帶負電的吸附位置，可能抑制PCP-與土壤表面間的靜電排斥力；吸附於土壤上的CTAB也可能形成新的有機介質供PCP吸附於土壤上。另外，使用33 g/L的鎳鐵雙金屬可於8小時內移除99 %液相中的PCP，但是此高劑量的處理並無法有效的降解吸附在土壤中的PCP。於土壤中添加100 mM 草酸可維持反應在酸性環境下進行，可能有效地增加鎳鐵雙金屬的反應性。然而PCP (pKa = 4.73)在酸性環境下會形成分子態並容易分配於土壤之中，限制了PCP與鎳鐵雙金屬進行接觸並反應的機會。在降低初始pH的處理下亦無法促進鎳鐵雙金屬在土壤中降解PCP，可能由於降低反應的初始pH亦會導致PCP在土壤中的吸 附量增加。因為PCP在土壤中的吸著行為明顯影響了鎳鐵雙金屬的利用性，所以採用三種界面活性劑分別是陽離子型的CTAB、非離子型的聚乙二醇辛基苯基醚(Triton X-100, TX-100) 以及陰離子型的十二烷基硫酸鈉(sodium dodecyl sulfate, SDS)對土壤中的PCP進行脫附的動力試驗。其中1 CMC陰離子型的SDS可脫附土壤中大部分的PCP，因帶負電的SDS可能促進土壤與PCP-間的靜電排斥力。結合1 CMC的SDS與鎳鐵雙金屬(Ni/Fe-SDS)相較於其他處理組亦顯示出較佳的PCP脫附及降解效率，但是SDS並不能促進PCP與鎳鐵雙金屬間的反應性。最後，發展出先以SDS對土壤中的PCP進行脫附，再利用Ni/Fe-CTAB降解脫附後溶液中的PCP之技術，Ni/Fe-CTAB能於15分鐘內移除大部分脫附後溶液中的PCP，並在8小時後降解70 %的PCP。於液相中及鎳鐵雙金屬上的PCP降解副產物的分析，指出鎳鐵雙金屬對PCP的降解作用為一脫氯反應。在反應24小時後，PCP形成低含氯數的降解副產物。因此結合SDS的脫附程序與Ni/Fe-CTAB的降解程序具有處理PCP汙染土壤的潛力。

Pentachlorophenol (PCP), a carcinogen and the most toxic chlorophenols, has been widely used as wood preservative, fungicide, insecticide, and general biocide. One PCP heavily contaminated site has been found in southern Taiwan. This study aimed to evaluate the feasibility of applying Ni/Fe nanoparticles (NPs) with surfactant to remediate PCP-contaminated soil solution and soil. The optimum of Ni contents and cationic surfactant cetyltrimethylammonium bromide (CTAB) for facilitating PCP degradation by Ni/Fe NPs were conducted in the soil solution from soil near An-shun site. Effects of Ni/Fe dosage, initial pH, and surfactants on the removal of PCP by Ni/Fe NPs in soil were evaluated, and the remediation for PCP-contaminated soil using Ni/Fe NPs with surfactants was optimized. The Ni/Fe-CTAB NPs with 2 % nickel and 1 critical micelle concentration (CMC) CTAB in various synthesized processes demonstrated a nearly 100 % removal of PCP in soil solution within 5 minutes and the nearly complete dechlorination of PCP after 24 hours. The enhancement of PCP removal and reduction by Ni/Fe-CTAB was mainly contributed to the speedy adsorption of PCP onto Ni/Fe surfaces, and thus the PCP surface-bound concentration increased in the solid-water interfacial region of Ni/Fe surface, promoting the degradation of PCP. The increased adsorption of PCP onto Ni/Fe surface might be due to the electrostatic attraction between electronegative phenolate group of PCP and positively charged Ni/Fe-CTAB NPs. The most PCP- was dissolved to the aqueous phase due to the low pKa (4.73) of PCP. Hence, the electrostatic repulsion between the ionized PCP (PCP-) and negatively charged Ma-Tzu-Gung soil may dominate the sorption behavior of PCP in this alkaline soil. However, Ni/Fe-CTAB cannot degrade PCP well in soil because the presence of CTAB enhanced the sorption of PCP in Ma-Tzu-Gung soil. Because the main active group of CTAB is the positively charged ammonium group ([(CH3)3NR)]+), and the negatively charged sites of the mineral surface and organic matters in soil could be easily occupied by CTAB. Thus, the electrostatic repulsion between the PCP- and negatively charged soil may be reduced by CTAB. In addition, the sorbed CTAB might increase the organic matter content in soil, which served as a new partition medium for HOCs. As a consequence, the Ni/Fe-CTAB NPs cannot apply to soil directly. On the other hand, more than 99 % dissolved PCP can be removed by 33 g/L Ni/Fe NPs within 8 hours. However, this high amount of Ni/Fe NPs did not facilitate the degradation efficiency of PCP remained in soil. Furthermore, 100 mM oxalic acid was sufficient to maintain the soil in an acid condition which promoted the reactivity of Ni/Fe NPs. Unfortunately, the acid condition enhanced the sorption of neutralized PCP in soil, and thus limited the accessibility of PCP by Ni/Fe NPs. The initially acid condition is also not helpful for the degradation of PCP using Ni/Fe NPs in soil because the decrease of initial pH enhanced the sorption of PCP in soil and limited the contact between Ni/Fe NPs and PCP molecules. The effect of PCP-soil sorption on the applicability of Ni/Fe NPs in soil played an important role. Desorption efficiencies for PCP spiked soil treated with DI water, 1 CMC CTAB, 1 CMC sodium dodecyl sulfate (SDS), and 1CMC Triton X-100 (TX-100) were 88.2 %, 1.35 %, 103 %, and 80.9 %, respectively. The negatively charged SDS could enhance the electric repulsion between PCP- and soil, which leads to fast PCP desorption from soil. In addition, the combination of SDS and Ni/Fe NPs (Ni/Fe-SDS) shows the better removal and degradation efficiency of PCP in PCP spiked soil compared to Ni/Fe-CTAB and Ni/Fe-TX-100 systems. However, the surfactant SDS did not show the obvious acceleration on the degradation of PCP using Ni/Fe NPs. Thus, the combination of SDS desorption process and further Ni/Fe-CTAB degradation process was performed to treat PCP spiked soil (50 mg/kg). As most PCP in the soil solution desorbed from PCP spiked soil by 1 CMC SDS can be removed by adsorption on Ni/Fe-CTAB NPs within 15 min, the accessibility between PCP molecules and Ni/Fe surfaces is much higher than other treatments with the same reaction time. As a result, near 100 % PCP were removed from soil and more than 70 % PCP were degraded by Ni/Fe-CTAB NPs within 8 hours. The dechlorination pathways were also presented. The degradation orders of PCP were tetrachlorophenols, trichlorophenols, dichlorophenols, chlorophenols, and phenol. Consequently, CTAB speeded the adsorption of PCP on Ni/Fe NPs, and then Ni/Fe surfaces proceeded to dechlorinate PCP to lower chlorinated phenols. After 24 hours, the byproducts are less toxic than PCP and more biodegradable in the environment. The combination of SDS desorption process and further Ni/Fe-CTAB degradation process has a high potential in the remediation of PCP contaminated soils.