探討不同時間尺度下奈米零價鐵在水溶液中對砷的降解機制

Unravelling Degradation Pathways of Arsenic by Nanoscale Zero-valent Iron in Aqueous Solution at Different Time Scales

張琬柔(Wan-Jou Chang)

指導教授 ： 劉雅瑄

國立臺灣大學/理學院/地質科學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201903972

摘要

奈米級零價鐵(nanoscale Zero-Valent Iron, nZVI)是目前用於地下水與土壤修復最廣泛應用的奈米材料之一，對水中污染物具有轉化與固定化作用，為學界與業界矚目的地下水中重金屬降解技術。本研究利用奈米零價鐵與水溶液中砷污染物進行不同變因之批次實驗，透過不同分析儀器分析不同反應時間長度之固體樣品，及不同時間解析度之X光吸收光譜分析不同時間尺度之反應溶液以得知反應過程中砷鐵變化，包含砷鐵反應初始之以秒為單位的極短時間尺度、水溶液中主要砷移除以小時為單位之中時間尺度，以及砷從水溶液中被移除後和鐵的鍵結關係轉化之以天為單位的長時間尺度分析。
　　以還原法製備奈米零價鐵，其顆粒大小範圍為20-50 nm，型態為核殼結構之球狀材料。批次實驗結果顯示，劑量與其降解砷溶液能力成正比，0.1 g/L之奈米零價鐵降解100 ppm三價砷溶液於24小時降解率為60%，劑量0.5 g/L以上則降解率超過95%；奈米零價鐵對三價砷之反應性較等量之五價砷佳；奈米零價鐵在無氧環境下可維持奈米零價鐵之核殼結構特性，故降解之效率較含氧環境佳；長時間反應下，砷並無脫附現象發生，且氧化還原反應持續進行；pH值於中性環境下時，劑量0.5 g/L奈米零價鐵於30分鐘即達到90%的砷移除率。SEM與TEM影像分析結果顯示，部分核殼結構之奈米零價鐵與砷溶液反應後，會形成其他二次礦物而產生片狀、針狀、花簇狀型態之氧化鐵。
　　原位X光吸收光譜實驗結果顯示，三價砷溶液與奈米零價鐵反應後會氧化為五價砷；奈米零價鐵之零價訊號隨時間增加而降低，二、三價訊號增強。當反應時間越長，固體樣品中氧化情形越明顯、砷－氧鍵結數量越多；乾燥反應物會使砷－氧、砷－鐵鍵長變短，不影響其氧化情形。於快速掃描X光吸收光譜原位實驗下，奈米零價鐵上的As(III)在極短時間內會被還原，其趨勢和批次實驗之結果相同，由此可知奈米零價鐵的還原能力對移除水溶液中的砷有正向的影響，且其能力與劑量成正比，若劑量越高，還原能力越強。
　　本研究結果指出，奈米零價鐵降解水溶液中的砷為一複雜之反應，包含氧化、還原、吸附、共沉澱、螯合等。可分為三個階段，第一階段奈米零價鐵快速將水溶液中砷移除，且瞬間與水反應產生還原性物質，提供強大的還原能力；第二階段奈米零價鐵及其氧化層影響固、液相上砷的轉化(transform)，同時鐵砷之間鍵結型態轉變；第三階段為奈米零價鐵與砷之型態持續緩慢地轉變，奈米零價鐵進而有效地將水溶液中的砷固定化(immobilize)於其顆粒上不再釋出至水中。

關鍵字

奈米零價鐵 ； 砷 ； 原位X光吸收光譜 ； 降解機制

並列摘要

High levels of arsenic in groundwater influence millions of human health around the world. Nanoscale zero-valent iron (nZVI) has the function of transforming and immobilizing pollutants in aqueous solution, it’s widely recognized as a material with a high potential agent for environmental friendly treatment of groundwater. In this study, the batch experiments of different variables and in-situ experiments were carried out to explore the reaction mechanism between arsenic and nZVI in aqueous solution.
Nano Zero-Valent Iron (nZVI) was syntesized by the reduction of ferric chloride with sodium borohydride, and which particle size ranges from 20 to 50 nm. nZVI is a spherical material having a core-shell structure. The batch experiment results show that the dosage is proportional to its ability to degrade arsenic solution. 0.1 g/L nZVI degrades 100 ppm As(III) solution at a removal efficiency of 60% at 24 hours, and the dosage of 0.5 g/L or more is better than 95%. nZVI is more reactive to As(III) than As(V). The efficiency of nZVI degradation in an anaerobic condition is better than that of aerobic condition due to the maintaince of core-shell structure of nZVI. 0.5 g/L nZVI reaches 90% degeadation in 30 minutes when the pH is in a neutral condition. The SEM and TEM images showed that some of the core-shell structure of nZVI reacting with arsenic solution were transformed to flakes, needles and clusters forms.
In situ X-ray absorption spectroscopy results showed that the As(III) solution reacted with nZVI would oxidize to As(V). The Fe(0) signal of nZVI decreases with time, and the Fe(II) and Fe(III) signals are enhanced. The longer the reaction time, the more obvious the oxidation situation in the solid sample and the more the arsenic-oxygen bond number. Under the quick XAS analysis, As(III) on nZVI will be reduced to lower valence state in a very short time, which is in accordance with the results of batch experiments. It can be speculated that the excellent degradation efficiency is contributed to reduction ability. The higher the dosage, the stronger the reducing ability, and the better the degradation efficiency.
The results of this study indicate that nZVI degrades arsenic in aqueous solution as a complex reaction involving oxidation, reduction, adsorption, coprecipitation, and chelation. It can be divided into three stages. The first stage, nZVI quickly removed arsenic from the aqueous solution and reacted with water to produce a reducing species in a very short time, which providing a powerful reducing ability. The second stage is that nZVI and its oxidate layer affected the transformation of arsenic both in the solid and liquid phases. The third stage was continuous slowly transform between nZVI and arsenic, nZVI was effectively immobilized arsenic in the aqueous solution on its particles and no longer released into the water.

並列關鍵字

nanoscale Zero-Valent Iron (nZVI) ； arsenic ； in-situ X-ray absorption spectroscopy ； degradation mechanism

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