The mechanism for degrading Orange II based on adsorption and reduction by ion-based nanoparticles synthesized by grape leaf extract
Introduction
Iron-based nanoparticles (Fe NPs) have received attention for their environmental remediation properties both in soils and wastewater, from which a wide range of contaminants such as heavy metal ions, azo dyes, nitrate, chlorinated organic compounds, nitro-aromatic compounds have been removed [1]. The advantages of Fe NPs used for environmental remediation include high reactive surface area and a high degradation rate. However, despite their effectiveness in environmental remediation, there are several limitations associated with the chemical synthesis of Fe NPs. The aggregation of Fe NPs into chain-like structures is one of their characteristics, and is responsible for reducing the surface area to volume ratio. The stability of iron nanoparticles against aggregation is improved by imparting electrostatic repulsion [2]. Evidently, the chemical method using borohydride reduction is expensive and involves utilizing hazardous chemicals. Because the chemical synthesis of Fe NPs is expensive, site remediation becomes an unwanted financial burden and as such environmental technologies typically exhibit a relatively low market value [3]. Consequently, it is vital to develop an alternative eco-friendly and sustainable remediation strategy.
Recently, the green synthesis of Fe NPs using plant extracts has demonstrated a simple, cost-effective, and eco-friendly solution to environmental remediation. In fact this green synthesis method has attracted much attention due to its ability to be implemented in scaled-up industrial production of well-dispersed metal nanoparticles [4]. In the green synthesis of Fe NPs, plant extracts’ biomolecules can act as the capping and reducing agent for bioreduction of Fe2+ to form Fe NPs [5]. Consequently, synthesizing Fe NPs using plant extracts is generally cost-effective, biocompatible, non-toxic, and eco-friendly. Besides, plant leaves are largely available and this process reduces plant waste and creates a value-added product such as Fe NPs [6]. To date, several plant extracts such as green tea, sorghum bran and eucalyptus have been successfully employed in green synthesis of Fe NPs and used to degrade various contaminants [7], [8], [9], [10]. However, the reactivity of Fe NPs still needs to be better understood, with particular reference to their stability against aggregation, morphology, and size distribution features.
Applications of green synthesized Fe NPs for environmental remediation have been recently reported [9], [10], [11]. For example, Fe NPs produced by Eucalyptus globules were applied in the effective adsorption of Cr(VI) [12]. Fe NPs made from the extract of green tea leaves have also been utilized as a Fenton-like catalyst in the degradation of aqueous cationic and anionic dyes [9]. Recent studies show that Fe NPs synthesized using eucalyptus and green tea extracts proved to be effective for degrading total N and monochlorobenzene [10], [11], thus providing evidence that Fe NPs synthesized by plant extracts can be used for reductive and oxidative degradation of organic contaminants.
However, the Fe NPs’ mechanism for degrading contaminants by green synthesis is still unclear because the degradation pathways are linked to improving the degradation efficiency [11]. To date, only a few reports have confirmed the possible compositional structure of Fe NPs and whether these compositions exert an influence on the pollutant remediation and degradation mechanism [13]. It has been reported that chemically synthesized Fe NPs enable reduction, adsorption, and/or both processes simultaneously occurring in remediation applications [14]. For this reason, it is pertinent to examine the chemical composition of Fe NPs synthesized by plant extracts and explore the possible degradation mechanism for remediating pollutants.
In our previous report [15], we showed that Fe NPs, mainly composed of Fe0 and oxides and/or hydroxides of iron on their surface, constitute functional nanomaterials. In this study we proposed that the main removal mechanism for Orange II by green synthesized Fe NPs involved two steps: firstly, the initial adsorption process by some biomolecules and iron hydroxides or/and oxides on the surface of Fe NPs; secondly, the subsequent reduction process by Fe0. To support our assumption, kinetic studies of the reduction and adsorption processes were conducted. Furthermore, X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) were used to analyze the surface composition of Fe NPs, with specific reference to adsorption and reduction. Finally, liquid chromatography–mass spectrometry (LC–MS) was conducted to find evidence of degraded products in order to support the proposed degradation mechanism.
Section snippets
Materials
Ferrous chloride (FeCl2, purity > 99%), acid Orange II (C16H11N2NaO4S, purity > 99%) and methanol were all purchased from Sigma–Aldrich Co. (Australia), and they are of analytical grade. De-ionized water used in all experiments was obtained from Milli-Q Elga System (pH is 5.8 ± 0.2).
Synthesis of Fe NPs
The grape leaf extract was prepared by extracting 1.0 g of finely grinded grape leaf powder (grape leaves were collected from Adelaide, South Australia and ground into powder after being dried) in 50 mL of methanol at
Comparison of the efficiency of Fe NPs, Fe2+ and extract in removing Orange II
When Fe NPs prepared by grape leaf extract are used for removing Orange II, it is not clear whether biomolecules from the leaf extract do influence the reactivity of Fe NPs. To clearly confirm the reactivity of Fe NPs, Fig. 1 compares the removal percentage of Orange II using Fe NPs, grape leaf extract and Fe2+ under the same conditions. Approximately 82.0% of Orange II was removed by Fe NPs within 60 min, which was much higher than that using grape leaf extract (only 2.0%) and ferrous solution
Conclusion
In summary, green synthesized Fe NPs using grape leaf extract were used for the removal of Orange II in solution. The results revealed that both adsorption and reduction processes were involved in this removal process. Kinetics study demonstrated that this removal process was predominantly physical in character. TEM and XPS verified the core–shell structure and possible surface composition of Fe NPs. The confirmed composition of the shell, mostly composed of biomolecules and hydrous iron
Acknowledgements
Part of this project is financially supported by the CRC for Contamination Assessment and Remediation of Environment (Project: 4.1.6-11/12), Australia. Miss Fang Luo is supported by the International Postgraduate Research Scholarship (IPRS) program.
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