| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 107 === 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.
|
|---|
