Elsevier

Water Research

Volume 47, Issue 8, 15 May 2013, Pages 2613-2632
Water Research

Review
Magnetic nanoparticles: Essential factors for sustainable environmental applications

https://doi.org/10.1016/j.watres.2013.02.039Get rights and content

Abstract

In recent years, there has been an increasing use of engineered magnetic nanoparticles for remediation and water treatments, leading to elevated public concerns. To this end, it is necessary to enhance the understanding of how these magnetic nanoparticles react with contaminants and interact with the surrounding environment during applications. This review aims to provide a holistic overview of current knowledge of magnetic nanoparticles in environmental applications, emphasizing studies of zero-valent iron (nZVI), magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles. Contaminant removal mechanisms by magnetic nanoparticles are presented, along with factors affecting the ability of contaminant desorption. Factors influencing the recovery of magnetic nanoparticles are outlined, describing the challenges of magnetic particle collection. The aggregation of magnetic nanoparticles is described, and methods for enhancing stability are summarized. Moreover, the toxicological effects owing to magnetic nanoparticles are discussed. It is possible that magnetic nanoparticles can be applied sustainably after detailed consideration of these discussed factors.

Highlights

► This article reviews the contaminants removal mechanisms of magnetic nanoparticles. ► Desorption of contaminants mainly depends on removal mechanisms. ► Magnetic nanoparticles recovery with magnetic field requires further study. ► Stability and reactivity have to be balanced during surface modification. ► Type of bacteria and determination method greatly affect the toxicity results.

Introduction

In recent decades, various environmental challenges have been mitigated due to a boom in nanotechnologies and nanomaterials development. Nanoparticles have been widely used in environmental applications and have shown promising performance in pollutant removal or toxicity mitigation (Gavaskar et al., 2005; Tratnyek and Johnson, 2006). Among the most widely used nanoparticles, magnetic nanoparticles, mainly nano zero-valent iron (nZVI), magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles, have sparked an immense interest in research for engineering applications for treatment of polluted water or subsurface environments (Hu et al., 2004, Hu et al., 2005a, Hu et al., 2005b; Li et al., 2006a, Li et al., 2006b; Shen et al., 2009; Yantasee et al., 2007). One of the most famous examples is the injection of nZVI into subsurface forming reactive treatment zones (Gavaskar et al., 2005; Grieger et al., 2010). Heavy metals such as arsenic and chromium, and organic pollutants like chlorinated solvents, can be immobilized or reduced to less toxic species by nZVI. The effectiveness of nZVI is not just limited to laboratory findings. Many companies working on the manufacturing of nZVI and its application for environmental remediation have been established (Mueller et al., 2012). In the U.S., numerous practical experience of site remediation using nZVI has been reported (Comba et al., 2011; Su et al., 2012). In Europe, although only three full scale applications were reported, there were many pilot test projects with the application of nZVI for various contaminants (Mueller et al., 2012). According to a case specific cost comparison of pump and treat, permeable reactive barrier and nZVI technology on a site in New Jersey, it was estimated that the cost for treating TCE and PCE with the application of nZVI was far lower than the other two technologies (PARS Environmental, 2013).

Moreover, a number of studies have also shown application of Fe3O4 and γ-Fe2O3 nanoparticles on heavy metals removal from contaminated water (Hu et al., 2005a, Hu et al., 2005b; Hu et al., 2006; Xu et al., 2012). In addition to the great removal performance, some researchers demonstrated the feasibility of reusing magnetic nanoparticles by desorbing the contaminants and regaining the removal capacity in successive treatment cycles (Hao et al., 2010; Hu et al., 2005a, Hu et al., 2005b; Hu et al., 2005a, Hu et al., 2005b). However, there are numerous conditions influencing the applicability of magnetic nanoparticles in a sustainable treatment process.

These magnetic nanoparticles not only have a large removal capacity, fast kinetics and high reactivity for contaminant removal due to their extremely small particle size and high surface-area-to-volume ratio, but also one more important property, magnetism. This is a useful property for water and wastewater treatment systems. The high reactivity of the magnetic nanoparticles for pollutant removal, a compact and efficient water or wastewater treatment system can take advantage of this. It is expected that magnetic separation could be a more cost effective and convenient method for separating such tiny particles than sophisticated membrane filtration. Separation of magnetic nanoparticles from solution with a low-gradient magnetic field or a hand-held magnet has been frequently reported (Hu et al., 2004, 2007a; Yantasee et al., 2007). However, although ample experimental photographic results elsewhere (Bhaumik et al., 2011; Do et al., 2011; Li et al., 2010a, Li et al., 2010b; Liu et al., 2010; Shin et al., 2011; Zhang and Kong, 2011) showed the feasibility of separation and recovery of magnetic nanoparticles from water or wastewater, no successful real applications of magnetic particles for water or wastewater treatment have yet been reported. On top of the high surface free energy, the magnetism of nanoparticles has been suspected to enhance the aggregation of nanoparticles and reduce the removal capacity (Petosa et al., 2010; Phenrat et al., 2009a), so become a great hindrance to recover and reuse magnetic nanoparticles.

Additionally, with extensive use of nanoparticles for a wide spectrum of applications such as energy, electronics, and personal care products, nanoparticles have been released to the environment, heightening concern about the safety and toxicity of nanoparticles (Borm et al., 2006; Brar et al., 2010; Holsapple et al., 2005; Moore, 2006; Tsuji et al., 2006). Although nanoparticles occur in nature, such as those originating from volcanoes and combustion, artificially produced and released engineered nanoparticles have been of a comparatively tremendous amount (Hendren et al., 2011; Posner, 2009), threatening natural environments and wastewater treatment systems (Brar et al., 2010). Considering the direct applications of these magnetic nanoparticles in the treatment technologies, their release into the environment is inevitable, hence the assessment of their toxicity is of vital importance (Moore, 2006). To widely and wisely apply magnetic nanoparticles in subsurface and water treatment processes, several major concerns require further exploration.

In this article, influencing factors and mechanisms of contaminant removal by magnetic nanoparticles are reviewed, since their removal performance and associated factors are the major considerations. To reduce the treatment cost, the feasibility of reuse and recovery of magnetic nanoparticles are also discussed. In addition, concerns such as their aggregation affecting capacity and reactivity as well as their toxicity, are also addressed.

Section snippets

Removal of contaminants

The most popular magnetic nanoparticles are iron-based nanoparticles, namely nZVI, Fe3O4 and γ-Fe2O3. These nanoparticles are iron-based, but possess different chemical properties originating from the oxidation states of iron. Their capability and reactivity for contaminant removal are different. Yet, the removal performance can be influenced by various conditions, depending on the removal mechanisms (Fig. 1). Knowing the removal mechanisms of these magnetic nanoparticles can justify the

Desorption of contaminants

When considering the applicability of magnetic nanoparticles for treatment technologies, another major consideration is the reusability of magnetic nanoparticles. To maintain reactivity, physical and chemical properties of the nanoparticle products, the synthesis and manufacturing process of nanoparticles require meticulous control of conditions. The treatment cost of applying nanoparticles is relatively high, compared with traditional treatments. For instance, the price of 1 kg of γ-Fe2O3

Recovery of magnetic nanoparticles

One of the most fascinating characteristics of magnetic nanoparticles, showing advantage over other nanoparticles, is their magnetism. Abundant evidence showed that magnetic properties of particles alter when particle size reaches nano-scale (Sundaresan and Rao, 2009). In particular, the magnetic property of Fe3O4 and γ-Fe2O3 changed from ferromagnetic to superparamagnetic (Chatterjee et al., 2003). This property offers an encouraging option, satisfying requirements of high accessibility as

Aggregation

Nanoparticles with a high surface free energy tend to aggregate, achieving a stabilized state. The smaller particles have higher tendency of aggregation due to the lower energy barriers (Petosa et al., 2010). Formation of aggregate decreases the surface area of the magnetic nanoparticles. This reduces the removal capacity and reactivity, thereby limiting the treatment performance. Aggregation also undermines the effectiveness of magnetic nanoparticles during remediation because of the loss of

Toxicity

Nanoparticles surreptitiously enter the environment through water, soil, and air during various human activities. However, the application of nanoparticles for environmental treatment deliberately injects or dumps engineered nanoparticles into the soil or aquatic systems. This has resultantly attracted increasing concern from all stakeholders (Brar et al., 2010; Posner, 2009). The advantages of magnetic nanoparticles like their small size, high reactivity and great capacity, could become

Conclusions

Five major concerns about magnetic nanoparticles for environmental applications have been summarized and highlighted. The surface chemistry of engineered magnetic nanoparticles is complex and plays a crucial role in various interactions. Magnetic nanoparticles readily interact with other magnetic nanoparticles, contaminants, and microbial communities, thus further increasing the difficulty of investigation. Given the advantages of magnetic nanoparticles, a systematic understanding of surface

Acknowledgements

The authors wish to thank the Research Grants Council of the HKSAR Government for providing financial support under General Research Fund 617309 and FSGRF12EG28.

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