界面活性劑修飾鎳鐵雙金屬降解五氯酚之研究

Degradation of pentachlorophenol by surfactant modified Ni/Fe bimetallic nanoparticles

林承翰(Cheng-Han Lin)

指導教授 ： 施養信

國立臺灣大學/生物資源暨農學院/農業化學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.02170

摘要

五氯酚(pentachlorophenol, PCP)過去曾廣泛做為除草劑、木材防腐劑、殺蟲劑、落葉劑與殺菌劑。由於不當的貯存和處置五氯酚，世界各地發現發現許多五氯酚污染場址。本篇研究主要探討界面活性劑修飾或未修飾鎳鐵雙金屬移除五氯酚之能力並探討鎳鐵雙金屬之劑量、鎳的添加量、介面活性劑、溫度、腐植酸與陰離子之影響。
增加鎳鐵雙金屬的劑量可促進移除五氯酚。最適當之鎳添加量為0.5 %。不同介面活性劑之實驗中，對PCP之移除則以十六烷基三甲基溴化銨 (cetyl trimethylammonium bromide, CTAB)修飾鎳鐵雙金屬為最佳。但是羧甲基纖維素 (carboxymethyl cellulose, CMC)與Triton X-100 (TX-100)皆會抑制鎳鐵雙金屬移除五氯酚。CTAB前修飾鎳鐵雙金屬(CTAB-Ni/Fe)主要以吸附作用移除五氯酚；CTAB後修飾鎳鐵雙金屬(Ni/Fe-CTAB)主要以降解作用移除五氯酚。4.8 g/L Ni/Fe-CTAB移除五氯酚之擬一階動力為0.30 min-1。藉由反應動力學可發現CTAB修飾之鎳鐵雙金屬可較未修飾之鎳鐵雙金屬快速吸附PCP，並較快還原脫氯產生氯離子。可能由於CTAB增進PCP吸附至鎳鐵雙金屬表面，增加鎳鐵雙金屬附近之PCP濃度，因此CTAB可顯著的增進鎳鐵雙金屬移除與降解水中之PCP。CTAB修飾之鎳鐵雙金屬移除PCP之反應速率隨溫度增加而增加，並計算出活化能為81.6 kJ/mole，顯示此反應為表面控制之作用。CTAB修飾之鎳鐵雙金屬於水中之分散性較未修飾之鎳鐵雙金屬佳、顆粒粒徑也較小且比表面積也較大，CTAB可能因此增進PCP吸附至鎳鐵雙金屬表面。此外由於PCP之酚基於此反應中帶負電，CTAB修飾之鎳鐵雙金屬為正電性，所以修飾之CTAB也促進PCP吸附至鎳鐵雙金屬表面。比較CTAB修飾之奈米鎳鐵雙金屬與未修飾之鎳鐵雙金屬之表面特性，在穿透式電子顯微圖中，未修飾之鎳鐵雙金屬的表面形成細緻緊密之氧化層，限制內部的零價鐵進一步氧化，而使鎳鐵雙金屬失去反應活性；但CTAB修飾之奈米鎳鐵雙金屬表面形成巨大疏鬆之氧化層，可使內部的零價鐵繼續氧化，因此保有鎳鐵雙金屬之反應活性。由X光近邊緣結構也證實反應後CTAB修飾之鎳鐵雙金屬之氧化態較未修飾之鎳鐵雙金屬多。
初始pH值對CTAB修飾之鎳鐵雙金屬移除五氯酚並沒有太大影響。腐植酸可促進CTAB修飾之鎳鐵雙金屬移除與降解PCP，當腐植酸濃度為0-0.5 mg/L，CTAB修飾之鎳鐵雙金屬降解PCP之作用隨著腐植酸濃度減少而增加；當腐植酸濃度超過0.5 mg/L，促進降解之作用則不明顯，這可歸因於腐植酸佔據鎳鐵表面之反應位置與抑制鐵腐蝕。表水與地下水中常見之陰離子對CTAB修飾之鎳鐵雙金屬移除五氯酚之效率並沒有太大影響，但會影響五氯酚之移除速率與降解比例。硝酸鹽會與五氯酚競爭鐵腐蝕所產生之電子。磷酸鹽、重碳酸鹽與硫酸鹽會與不定型之氧化鐵形成內錯合物並抑制鐵腐蝕。鐵與陰離子之複合物會吸附於鐵表面，與五氯酚競爭鐵腐蝕所產生之電子，並抑制CTAB修飾之鎳鐵雙金屬對五氯酚之降解作用。但是高濃度的硫酸鹽會腐蝕鐵的氧化層，增加CTAB修飾之鎳鐵雙金屬對五氯酚之反應活性。陰離子不只抑制CTAB未修飾之鎳鐵雙金屬移除五氯酚並抑制CTAB未修飾之鎳鐵雙金屬對五氯酚之降解作用。
本篇研究也完整的探討CTAB修飾或未修飾之鎳鐵雙金屬對五氯酚的還原脫氯過程。CTAB修飾之鎳鐵雙金屬降解五氯酚之主要副產物為酚；然而CTAB未修飾之鎳鐵雙金屬降解五氯酚之主要副產物為四氯酚。其中五氯酚中鄰位的氯比間位或對位的氯更容易被移除，這可能是由於五氯酚陰離子是以羥基吸附於鎳鐵雙金屬上，因此五氯酚中鄰位的氯較靠近反應位置而較易被脫除。綜合上述結果顯示CTAB修飾之鎳鐵雙金屬將有相當之潛力應用於PCP污染之場址。

關鍵字

五氯酚 ； 鎳鐵雙金屬 ； 界面活性劑 ； 降解作用

並列摘要

Pentachlorophenol (PCP) had been used widely as herbicide, wood preservative, pesticide, defoliants, and general biocide in the past. Several PCP contaminated sites were found around the world due to the improper storage and disposal. In this study, we investigated the removal of PCP by using Ni/Fe nanoparticles with or without surfactant. We also evaluated the effects of Ni/Fe dosage, Ni loading, surfactants, temperature, pH, humic acid, and common anions on the removal of PCP by Ni/Fe nanoparticles.
The increase of Ni/Fe dosage enhanced the removal of PCP. The most suitable nickel percentage was 0.5 %. The pseudo-first order rate constant of PCP was 0.009 min-1 with 4.8 g/L Ni/Fe nanoparticles. Among the selected surfactants, cetyl trimethylammonium bromide (CTAB) remarkably enhanced the removal of PCP by Ni/Fe nanoparticles. However, the addition of carboxymethyl cellulose (CMC) and Triton X-100 (TX-100) inhibited the removal of PCP by Ni/Fe nanoparticles. For Ni/Fe nanoparticles pre-modified with CTAB (CTAB-Ni/Fe), most of the removal of PCP is an adsorption process. However, most of the removal of PCP is a degradation process for the Ni/Fe nanoparticles post-modified with CTAB (Ni/Fe-CTAB). The pseudo-first order rate constant of PCP was 0.30 min-1 with 4.8 g/L Ni/Fe-CTAB. The observed enhancement of PCP removal and reduction by Ni/Fe-CTAB attributed to the increased sorption of PCP onto iron surfaces and the increased local PCP concentration in the vicinity of Ni/Fe surface by CTAB, accelerating the reduction of PCP. The activation energies of the removal reactions of PCP with Ni/Fe and Ni/Fe-CTAB were 47.2 and 81.6 kJ/mole, respectively. This indicated that the removal of PCP by Ni/Fe is surface-control mechanism. From removal kinetics, the increased sorption of PCP onto Ni/Fe surface with CTAB was observed as compared to that without CTAB. It resulted from the smaller the particle size and the larger surface area of Ni/Fe nanoparticles stabilized with CTAB. Furthermore, the electrostatic interaction between electronegative phenolate group of PCP and electropositive of Ni/Fe nanoparticles stabilized with CTAB increased sorption of PCP on the surface of Ni/Fe nanoparticles. The fast adsorption and sequential dechlorination mechanisms were observed for the removal of PCP on Ni/Fe-CTAB. In addition, the surface characteristics of Ni/Fe nanoparticles with or without CTAB indicated that the lower reduction of PCP seems to be related to the presence of a passive surface film on Ni/Fe nanoparticles without CTAB.
In Ni/Fe-CTAB system, the effect of initial pH on the removal of PCP was not significantly. Removal rate kinetic and degradation ratio of PCP increased with the increase of humic acid concentration up to 0.5 mg/L. However, the removal rate kinetic and degradation ratio of PCP decreased when the humic acid concentration increased from 5 mg/L to 20 mg/L. This could be attributed to that humic acid molecules cover the reactive sites on the Ni/Fe surface and inhibit iron corrosion. Common anions in surface water and groundwater do not affect the removal efficiencies of PCP by Ni/Fe-CTAB but affect the removal kinetics and the degradation ratio of PCP. Nitrate can compete for electron offer from iron with PCP. Phosphate, bicarbonate, and sulfate may inhibit the corrosion of iron due to form inner-sphere complexes with amorphous iron oxide. These iron-anion complexes adsorb on the iron surface and compete for the electrons offer from iron with PCP and consequently inhibit the degradation of PCP. However, a high concentration of sulfate could dissolve the iron oxide layer, leading to increase the reactivity. Without CTAB, anions in Ni/Fe system not only inhibited PCP removal kinetics but also decreased the degradation ratio of PCP.
The complete dechlorination pathways of PCP with Ni/Fe with or without CTAB were also presented. The major byproduct was phenol for Ni/Fe-CTAB system, but the major byproducts in Ni/Fe system without CTAB were tetrachlorophenols. Furthermore, the ortho position of chloride can be more earily reduced by Ni/Fe nanoparticle with or without CTAB than para- and meta-positions due to the generally adsorption of hydroxyl group in PCP anion structue on Ni/Fe nanoparticles. The CTAB modified Ni/Fe nanoparticles could have a high potential in the remediation of PCP contaminated sites.

並列關鍵字

pentachlorophenol (PCP) ； Ni/Fe bimetallic nanoparticle ； surfactant ； degradation

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