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Environ Sci Pollut Res DOI 10.1007/s11356-017-8765-3

REVIEW ARTICLE

Current advancements of magnetic nanoparticles in adsorption and degradation of organic pollutants Mazhar Ul-Islam 1 & Muhammad Wajid Ullah 2,3 & Shaukat Khan 2 & Shehrish Manan 4 & Waleed Ahmad Khattak 2,5 & Wasi Ahmad 1 & Nasrullah Shah 6 & Joong Kon Park 2

Received: 29 November 2016 / Accepted: 7 March 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract Nanotechnology is a fast-emerging field and has received applications in almost every field of life. Exploration of new synthetic technologies for size and shape control of nanomaterials is getting immense consideration owing to their exceptional properties and applications. Magnetic nanoparticles (MNPs) are among the most important group of nanoparticles thanks to their diverse applications in medical, electronic, environmental, and industrial sectors. There have been numerous synthetic routes of MNPs including thermal decomposition, coprecipitation, microemulsion, microwave assisted, chemical vapor deposition, combustion synthesis, and laser pyrolysis synthesis. The synthesized MNPs have been successfully applied in medical fields for therapy, bioimaging, drug delivery, and so on. Among environmental aspects, there has been great

intimidation of organic pollutants in air and water. Utilization of various wastes as adsorbents has removed 80 to 99.9% of pollutants from contaminated water. MNPs as adsorbents compared to coarse-grained counterparts have seven times higher capacity in removing water pollutants and degrading organic contaminants. This study is focused to introduce and compile various routes of MNP synthesis together with their significant role in water purifications and degradation of organic compounds. The review has compiled recent investigation, and we hope it will find the interest of researchers dealing with nanoparticles and environmental research. Keywords Magnetic nanoparticles . Synthetic techniques . Organic pollutants . Water purification

Responsible editor: Guilherme L. Dotto * Nasrullah Shah [email protected] * Joong Kon Park [email protected] 1

Department of Chemical Engineering, Dhofar University, Şalālah, Oman

2

Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, South Korea

3

Department of Biomedical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China

4

National Key Laboratory of Crop Genetic Improvement, College of Plant Sciences and Technology, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China

5

Liquid Fill Department, Tetrosyl Ltd., Landshire, Bridge Hall Lane, Bury, UK

6

Department of Chemistry, Abdul Wali Khan University Mardan, Mardan, Pakistan

Introduction Nanomaterials have been the focus of immense interest since their discovery (Ul-Islam et al. 2014a). Their small size makes them easier to penetrate, bind, and deliver to the site of interest. There has been enormous research related to nanoparticle synthesis and their corresponding applications. With advent of time, new, cheap, and advanced synthetic routes are developing. Besides, their applications have been expanded to almost every field of research such as photovoltaics, photocatalysis, photoelectrochromics, sensor development, cell imaging, drug/gene delivery, and cancer therapy (Ullah et al. 2016; Khan et al. 2015; Ul-Islam et al. 2014b). For example, the application of smart nanocarriers for drug delivery has shown promising results in the treatment of various cancers owing to their targeted chemotherapeutic nature (Ali et al. 2016; Ali 2011a). To this end, several novel agents have been developed for cancer chemotherapy. Nanoparticles play a vital role in the diagnosis and imaging of brain tumors and allow early

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detection of pre-cancerous cells. These further provide realtime, longitudinal, and noninvasive monitoring and imaging of the effects of treatment (Ali 2011b). Among nanoparticles, magnetic nanoparticles have gained much interest owing to their fine magnetic behavior that results in multiple applications in medical, electronics, conducting, environmental, and many industrial fields (Chen et al. 2011). Besides global warming and energy crises, problems associated with industrialization such as industrial waste management, wastewater purification, and degradation of organic compounds need to be thoroughly addressed (Ullah et al. 2014). Scientists have devoted immense interest to the growing concern of environmental pollution for decades (Chen et al. 2011). Various sensitive and effective methods for the determination of different pollutants in the environment have been explored and developed (Zhao et al. 2013). Traditionally, several techniques such as solid-phase extraction (SPE), micro-solid-phase extraction (MSPE), liquid–liquid extraction (LLE), dialysis, and various chromatographic methods such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and gas chromatography-mass spectrometry (GC-MS), etc. have been used for the removal of contaminants (Ali et al. 2008). However, these methods often fail to provide the desired selectivity and sensitivity for the determination of pollutants on trace level (Tong and Chen 2013). Magnetic solid-phase extraction (MSPE) is a highly sensitive mode of SPE based on magnetic adsorbent (Huo and Yan 2012). In this method, the magnetic adsorbent is dispersed in the sample. The adsorbent adsorbs the target analyte and then separate it through the application of an external magnetic field (Sha et al. 2008). MSPE method is simple, time saving, and low cost as it avoids the column passing, filtration, and centrifugation steps and increases the interfacial area between sorbents and analytes, while the magnetic adsorbent can be easily recycled (Wang et al. 2013a). Adsorption is simple, efficient, inexpensive, and universal method where pollutants are adsorbed on the solid surface. This strategy is commonly used for water purification and other recycling technologies (Ali 2012a; Ali and Gupta 2006). To this end, several adsorbing materials such as activated carbon capturing organic and inorganic pollutants from contaminated water source are used (Ali 2010). Besides, several wastes such as fruit wastes, coconut shell, bark and other tannin-rich materials, wood-type materials, rice husk, petroleum wastes, fertilizer wastes, chitosan and seafood processing wastes, seaweed, and algae, etc. have been used as adsorbents which removed 80 to 99.9% of pollutants from contaminated water (Ali et al. 2012a). The role of adsorption technology at laboratory, pilot, and process scales for treatment of contaminated water has been comprehensively overviewed (Ali 2014). Magnetic nanoparticles (MNPs) are normally applied as adsorbent in MSPE analysis due to their high surface area, diversity of shape, size, and functionality, and good

separation ability (Meng et al. 2011). These properties make MNPs as suitable adsorbent in wastewater treatment, removal of trace organic pollutants, and also separation of toxic metals (Heidari and Razmi 2012). Among the MNPs, Fe3O4 is the most popular due to their low price and toxicity (Wang et al. 2013b). However, bare Fe3O4 agglomerates in the sample solution (Faraji et al. 2010) Fe3O4 and needs to be modified and functionalized, for better suspension stability and improved selectivity before specific analytical application (Iida et al. 2007). Various substances including polymers, silica, metal oxides, carbon, and ionic liquids have been used to functionalize Fe3O4 for desired applications. Various functions and potential applications of magnetic nanoparticles have been depicted in Fig. 1. This paper provides a review on functionalized magnetic nanoparticles for the removal of organic pollutants from the environment. We will discuss in detail the various synthetic routes of MNPs and their effective applications in water treatment and degradation of organic pollutants.

Synthesis of MNPs MNPs have been synthesized through a number of methods including thermal decomposition, co-precipitation, microemulsion, solvothermal, microwave assisted, chemical vapor deposition, combustion synthesis, laser pyrolysis synthesis, etc. Compilation of all these methods will far exceed the scope of this review; therefore, we will only focus on some of the useful synthetic methods for MNPs such as co-precipitation, thermal decomposition, hydrothermal reactions, and sol–gel method. Co-precipitation This approach is commonly applied for the synthesis of ferrites and metal oxides. For the synthesis of iron oxide NPs, base is added to aqueous Fe2+/Fe3+ solutions at neutral pH and ambient or elevated temperatures. The chemical nature of salt used (e.g., chlorides, nitrates, sulfates), temperature, pH, ionic strength of the media, and Fe2+/Fe3+ ratio controlled the composition as well as the size and shape of the synthesized MNPs. Co-precipitation method is usually used for the synthesis of ferrites in aqueous medium. The chemical reaction is as follows: M2þ þ 2Fe3þ þ 8OH− →MFe3 O4 þ 4H2 O

ð1Þ

where M may be Fe2+, Mn2+, Zn2+, Ni2+, etc. Another co-precipitation method employs partial oxidation of Fe(OH)2 suspension with various oxidizing agents (Tartaj et al. 2003). Spherical iron oxide NPs with uniform size were obtained through the reaction of Fe(OH)2 with nitrate ions

Environ Sci Pollut Res Fig. 1 Applications of magnetic nanoparticles in different fields

(mild oxidant) in the presence of a base (Tartaj et al. 2003). In another method, NH4OH was applied instead of NaOH, and hydrazine was used as oxidation resistant. This method resulted in an increase in the crystallinity and saturation magnetization of the synthesized Fe3O4 NPs (Drofenik et al. 2011). Thermal decomposition One of the easiest methods for the synthesis of MNPs is the thermal decomposition of organometallic precursors (Gozuak et al. 2009). Surfactants are applied to avoid the agglomeration of the NPs. Metal acetylacetonates, {M (acac) n } (M = metal ion; acac = acetylacetonate), and metal cupferronates (MxCupx) (M = metal ion; Cup = Nnitrosophenylhydroxylamine are the organometallics normally applied. The thermal decomposition of organometallic precursors initially leads to the synthesis of MNPs, which are further oxidized to monodispersed metal oxides. The shape and size of the synthesized MNPs depend on the ratios of organometallic compounds, solvents and surfactants, temperature, and time. Hydrothermal reactions Also called as solvothermal reactions, they are performed at around 2000 psi and 200 °C in aqueous media in special reactors. This method is low cost, involves low reaction temperatures, no need of further calcinations, and is also environmentally friendly (Gan et al. 2010). Two

different hydrothermal approaches are used for the synthesis of ferrites: (1) hydrolysis and oxidation and (2) neutralization of metal hydroxides (Gozuak et al. 2009). Another recently developed hydrothermal route is the PEG-assisted technique applied extensively for the synthesis of MNPs. Again, the reaction time and temperature are the important factors affecting the shape and size of the MNPs.

Sol–gel method Sol–gel, a wet chemical method, is widely used in ceramic engineering and materials chemistry. This method involves a solution (sol) acting as a precursor for a network structure (gel) of polymers or ceramic particles. This method has been applied for the synthesis of TiO2, SiO2 particles, etc. (Selvan et al. 2007). ZrO2 nanopowder was synthesized from a transparent sol of ZrOC 2O 4 ·2H 2O which, in turn, was obtained by mixing ZrOCl2·8H2O, 1% PEG, and oxalic acid. The transparent sol was autoclaved at 600 °C for 4 h resulting in a white gelatinous powder which was calcinated to synthesize the ZrO NPs (Kim et al. 2006). The sol–gel method can be efficiently used for the synthesis of multicomponent NPs with binary or ternary components using double or mixed alkoxides. CoFe2O4 and NiFe2O4 NPs have been synthesized through calcination at 450 and 400 °C, respectively (Lee et al. 1998).

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Fig. 2 Schematic presentation of synthesis of multifunctional magnetic nanoparticles

Synthesis of multifunctional magnetic nanoparticles (MFMNPs) The advancement made in nanoscience has made it possible to mingle different functional nanomaterials including NPs to a single entity. In the recent decade, many multifunctional magnetic nanomaterials have been reported including core–shell, dumbbell, and multifunctional hybrid nanostructures. These multifunctional magnetic nanoparticles (MFMNPs) are resulted as a combination of MNPs with quantum dots (QDs) or metallic NPs. The recombinant MFMNPs show a combination of magnetic and optical properties broadening their applications in various fields. The synthetic scheme of multifunctional magnetic nanoparticles is shown in Fig. 2. MFMNPs can be further categorized into various groups as under.

Core–Shell NPs These are the most extensively studied multicomponent NPs. Core–shell diamagnetic FePt–Fe3O4 NPs were resulted when Fe(acac)3 was reductively decomposed in the presence of FePt NPs (Murray et al. 2000). Another promising MFMNP is Fe3O4–Au consisting of magnetic Fe3O4 core and metallic Au shell (Yu et al. 2005). The synthesis involves the reduction of HAuCl4 in the presence of Fe3O4 NPs in a CHCl3 solution of oleylamine. Oleylamine acts as reducing agent and as surfactant. The in situ reduction of HAuCl4 results in a coating on the Fe3O4 NPs; the coating can grow thicker with the addition of extra HAuCl4 in the reduction medium [32]. The MNPs synthesized through this method can serve for the synthesis of Fe3O4–Au–Ag NPs by simple addition of AgNO3 to the reduction medium after Au coating.

Fig. 3 a Growth of Au–Fe3O4 dumbell NPs (a) and the SEM (b) and TEM (c) images. b Preparation of nanoassemblies for multifunctional nanoplateforms. Figure has been modified from references (Yu et al. 2005; Kim et al. 2011)

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Dumbbell NPs Dumbbell NPs are usually the result of sequential growth of the second component on the seed NPs in only one crystal plane. The single-plane growth of the second component is achieved through tuning of the seed to precursor ratio and by keeping the precursor concentration below the nucleation threshold (Murray et al. 2000). Dumbbell Au–Fe3O4 is extensively studied. Their synthesis involves the decomposition of Fe(CO)5 on the seed Au NP surface as shown in Fig. 3 (Yu et al. 2005). The HAuCl4 ratio to oleylamine and its injection temperature provide control over the size of Au NPs, while the Fe(CO)5 to Au ratio controls the size of Fe3O4 NPs. The dumbbell structure is formed by the growth of (111) plane of the Fe3O4 in the (111) plane of Au (Yu et al. 2005). Dumbbell Pt–Fe3O4 NPs have also been synthesized by nucleation and growth of Fe on Pt NPs (Wang et al. 2013b). Multicomponent hybrid magnetic nanoparticles Multicomponent hybrid magnetic nanoparticles (MCHMNPs) are fabricated through self-assembling of multifunctional NPs. The intermolecular forces hold various NPs in the assembled nanostructure (Kim et al. 2008). The successful incorporation of hydrophobic Fe3O4, poly(D,L-lactic-co-glycolic acid) (PLGA), and doxorubicin (DOX) into the hydrophobic Pluronic F127 {(EO)97(PO)69(EO)97} micelles was achieved through sonication. The hydrophobic suspension of Fe3O4, DOX, and PLGA in CH2Cl2 was added into the aqueous Pluronic F127 followed by ultrasonication in a probe-type ultrasonicator. The mixture was stirred at room temperature to evaporate the organic solvent, and the obtained NPs were washed several times with DI water, resulting in PLGA NPs incorporated by MNPs and DOX (Yang et al. 2008). Magnetic nanoparticles for wastewater purification Several contaminants such as urban runoffs, industrial and domestic effluents, oil spills, etc. are immensely affecting the water and aquatic organisms. Besides, organic contaminants due to their acute toxicities and carcinogenic nature are the major cause of water pollution (Ali et al. 2012a). These contaminants may include the polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and heavy metals like mercury, lead, manganese, etc. The presence of these types of contaminants in drinking water has proven to be carcinogenic and a serious risk to both humans and aquatic life. Several strategies such as Bpump-and-treat technology^ and Bimmobilization technology^ have been developed to remediate this issue. However, such technologies are economically unfavorable due to extended operating hours and higher costs. These issues can be addressed through the development of in situ

technologies. The selection of an appropriate method and material for wastewater treatment is a highly complex process that should meet certain standards, quality, efficiency, and cost-effectiveness (Oller et al. 2011). In general, four conditions must be considered during the selection of wastewater treatment technologies: (1) treatment flexibility and final efficiency, (2) reuse of treatment agents, (3) environmental security and friendliness, and (4) low operating costs (Oller et al. 2011; Zhang et al. 2011). The preparation and characterization of nanoparticles are the first and foremost steps in water treatment by nanotechnology (Ali 2012a). Currently, the nanoparticles representing nanotechnology are developing with a nice pace and showing promising results for wastewater remediation due to high treatment efficiency and low cost. Technology becomes more promising, cost-effective, and simple when operated with magnetic nanoparticles. Magnetism is a unique physical property that helps in water purification by directly influencing the physiological properties of water. These nanoparticles are injected into the contaminated water, and loaded nanoparticles can be removed simply through a magnetic field (Jung et al. 2004). Further, an extensive database has been developed for the toxicity of magnetic nanoparticles that can help develop and apply such nanoparticles with minimal toxicity for water and wastewater treatment applications (Gehrke et al. 2015). This section describes the potential benefits and recent advancements of nanotechnology with special emphasis on magnetic nanoparticles for the wastewater purification. The detection and separation of water pollutants such as arsenic using the magnetic nanoparticles (magnetite Fe3O4) have been well established in groundwater remediation (Gupta et al. 2010). The application of magnetite (Fe3O4) in purification of wastewater can be broadly classified into two categories: (1) the use of magnetic nanoparticles as a nonabsorbent or immobilization carrier for removal efficiency enhancement (i.e., adsorptive/immobilization technologies) and (2) the use of magnetic nanoparticles as photocatalysts to break down or to convert contaminants into a less toxic form (i.e., photocatalytic technologies). Besides groundwater remediation, the recovery of magnetic nanoparticles serves as an ideal feature to increase the osmotic pressure of draw solutions used in the forward osmosis. Contrary to reverse osmosis, the forward osmosis draws water from a low osmotic pressure to one with a higher osmotic pressure using the osmotic gradient (Kim et al. 2011). The use of magnetic nanoparticles in the removal of contaminants from wastewater offers several advantages such as higher removal capacity, fast kinetics, and high reactivity for contaminant removal due to their extremely small size, high surface area-to-volume ratio, surface modifiability, excellent magnetic properties, and great biocompatibility (Tang and Lo 2013). In the recent years, there has been an increased trend in applying the engineered magnetic nanoparticles for the remediation and wastewater treatments that

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led to the elevated public concerns. These concerns necessitate the exploration of mechanism of how these engineered nanoparticles interact with the contaminants as well as surrounding environment during their application. Currently, the most widely used nanoparticles, magnetic nanoparticles, nano zero-valent iron (nZVI), magnetite (Fe 3 O 4 ), and maghemite (g-Fe2O3) nanoparticles have received considerable interest in research for engineering applications in the treatment of contaminated water, wastewater, and subsurface environments (Shen et al. 2009). Furthermore, several studies have shown the application of magnetite (Fe 3 O 4 ) and maghemite (g-Fe2O3) nanoparticles for the removal of heavy metals from contaminated water (Xu et al. 2012). The efficacy of magnetic nanoparticles to remove contaminants from wastewater is readily affected by several factors such as background ions, humic substances, pH, etc. (Tang and Lo 2013). It is a known fact that the aquatic chemistry and groundwater composition vary from site to site; therefore, the strategy development of magnetic nanoparticles must consider several operation parameters. Studies have shown that background electrolytes insignificantly affect the contaminant removal performance of magnetic nanoparticles under conditions where the affinity of pollutants toward the nanoparticles was higher than the background electrolytes (Chowdhury and Yanful 2010). For example, Chowdhury and Yanful found that the background electrolytes did not significantly affect the As(V) removal by magnetite (Fe3O4) and maghemite (gFe 2 O 3 ) nanoparticles (Chowdhury and Yanful 2010). However, some researchers found that some particular ions such as arsenic compete with target pollutants. In such situations, the arsenic exists as anions in a normal aquatic environment; some common anions, such as sulfate and chloride, showed little effect on the removal of As(V) (Bitema et al. 2007; Chowdhury et al. 2010). Several studies have shown that the removal capacity and reactivity of magnetic nanoparticles are highly dependent on their size (Rivero-Huguet and Marshall 2009). In a study, Shen et al. (2009) reported that the contaminants’ removal capacity of magnetite (Fe3O4) nanoparticles (8 nm) was almost seven times higher than that of coarse-grained counterparts (50 nm) (Shen et al. 2009). Similarly, the reactivity of ZVI was improved to be 50–90 times higher from the native ones when the particle size was reduced around 500 to 100 (Lin et al. 2008). However, some researchers have also reported that the contaminants’ removal capacity normalized by surface area is not much different when the particle size changes (Tratnyek and Johnson 2006). This could be due to the fact that all physical or chemical reactions take place at the liquid–solid interface. Besides, several other parameters such as high ionic strength and extreme pH are also of immense concern. Although this technology is emerging very rapidly and offering the possibility of an efficient removal of pollutants and germs, no real successful application of

magnetic nanoparticles for wastewater treatment has been reported yet. However, it can be assumed that a magnetic nanoparticle-based system can prove to be a very useful, cost-effective, simple, convenient, and efficient strategy in the near future for large-scale separation of all kinds of contaminants ranging from very large to tiny particles compared to very sophisticated filtration membrane. Modern technologies are developing specifically tailored surface-modified magnetic nanoparticles that can effectively uptake a trace of metal contaminants. Such nanoparticles have a magnetic core that facilitates their recovery, a shell providing stability, protection from oxidation, and a surface where the contaminants specific ligands are attached. Such nanoparticles can be operated in a continuous manner and are target specific.

Degradation of organic compounds through magnetic nanoparticles Several organic pollutants are produced for multiple purposes; however, many of them are hazardous; hence, their detection, degradation, and removal are necessary. For example, chiral pollutants are conventionally detected in the environmental and biological matrices through chiral analysis and capillary electrophoresis (Ali et al. 2015, 2013). Common sources of chiral pollutants are pesticides, biphenyls, polychlorinated hydrocarbons, and some drug residues and thus are widely distributed in water, soil, sand, air, and biota (Ali et al. 2012b). The use of nanoparticles for this purpose is one of the important research areas nowadays. The use of bare magnetic nanoparticles, maghemite, magnetite, and ferrite nanoparticles have been reported as novel nanoadsorbents for dye separation. The separation of anionic dyes by maghemite is achieved by the electrostatic attraction between the positively charged surface of the maghemite and the sulfonate group. The separation of anionic dyes Acid Red 27 and Congo Red by maghemite nanoparticles has been reported (Afkhami and Moosavi 2010). Similarly, the removal of cationic dyes such as Neutral Red, acridine orange, or methylene blue by iron oxides has also been reported (Giri et al. 2011; Qadri 2009). Magnetic nanoparticles impregnated onto maize cobe were used for the removal of methylene blue dye (Tan et al. 2012). Acridine orange dye was removed by the use of γFe2O3 magnetic nanoparticles. The particles showed a maximum sorption capacity of 59 mg/g for the as-mentioned dye (Qadri 2009). Various organic materials degraded by magnetic nanoparticles are summarized in Fig. 4. The magnetic nanoparticles coated with some organic or inorganic substance are also utilized for dyes and other organic substance removal. Acidic Acid Orange 10 and Congo Red and basic dyes methylene blue and acridine orange are

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Fig. 4 Illustration of various organic materials degraded by magnetic nanoparticles

removed by using amino-functionalized silica-coated magnetic nanoparticles (Zhou et al. 2013). Carboxylic-functionalized silica-coated MNPs, respectively, have been reported (Fu et al. 2011). Fe3O 4 magnetic nanoparticles modified with 3glycidoxypropyltrimethoxysilane and polylysine were used for the removal of anionic dyes, including methyl blue, orange I, amaranth, and Acid Red 18 from aqueous solution (Zhang et al. 2015). Wang et al. successfully utilized magnetic iron oxide–cyclodextrin–graphene oxide nanocomposites as an adsorbent for the removal of malachite green from aqueous solution (Wang et al. 2015). Xiao et al. introduced methyl orange imprinted chitosan–TiO 2 core–shell composite for photocatalysis of methyl orange in dual-dye systems in aqueous solution and got efficient results for methyl orange in dual-dye systems of methyl orange/sunset yellow and methyl orange/Rhodamine B (Xiao et al. 2015). Three cationic dyes, i.e., methylene blue, crystal violet, and malachite green, were removed from aqueous solution by using magnetic nanoparticles prepared with Fe3O4 and N-benzyl-O-carboxymethylchitosan, an amphiphilic chitosan derivative (Debrassi et al. 2012). Similarly, magnetic colloidal nanocrystal clusters modified with different amounts of poly(4-styrenesulfonic acid-co-maleic acid) sodium were utilized for the removal of cationic dye methylene blue from aqueous solution (Song et al. 2015). Zelmanov et al. have reported the degradation of phenol and ethylene glycol by using Fe3O4 magnetic nanoparticles as catalysts (Zelmanov and Semiat 2008). Zhang et al. synthesized Fe3O4 magnetic nanoparticles and used to remove phenol and aniline from aqueous solution (Zhang et al. 2009). MNPs are also employed for detection and degradation of pesticides. Gold nanoparticles (AuNPs) have been utilized for the detection of chlorpyrifos, malathion, and organophosphorous pesticides (Barahona et al. 2013; Lisha and Pradeep 2009). AuNPs have been used for monohydroxy PAHs using

AuNPs (Wang et al. 2009a). Functionalized AgNPs were used for the detection of optimal pesticides and organochlorine pesticide endosulfan (Guerrini et al. 2008). Iron nanoparticles have been used for the degradation of RDX in the presence or absence of a stabilizer additive (Naja et al. 2008). PAHs were also detected by metal-enhanced fluorescent method using silver nanoparticles (AgNPs) (Zhang et al. 2012). Dichlorodiphenyltrichloroethane (DDT) and related compounds in river water sample were detected by using sensors incorporating magnetic NPs (Mauriz et al. 2007). Fe3O4@phenol formaldehyde resin core–shell nanospheres loaded with AuNPs were utilized for detection of thiols in living cells (Yang et al. 2012). Detection of gallic acid and tannic acid was made by using chitosan-capped Ag NPs (Zhaohui et al. 2013). Besides using alone, magnetic nanoparticles are also utilized in making magnetic core–shell nanocomposites and were used for detection, extraction, or degradation of organic contaminants. Fe3O4–poly(4-MS-DVB-GMA) matrix grafted with poly(amidoamine) PAMAM dendrimer immobilized with AuNPs was used as a catalyst for the reduction of Rhodamine B (Murugan and Jebaranjitham 2015). Magnetic Fe3O4–MnO2 core–shell nanocomposites were used for degradation of 4-chlorophenol (Liu et al. 2015). TiO2 shell was coated with super paramagnetic Fe3O4 nanoparticles, and the resultant composite was used for the degradation of organic contaminants (Xin et al. 2014). Zhou et al. utilized Fe3O4@SiO2 MWCNTs for the removal of pentachlorophenol from aqueous solution (Zhou et al. 2014). Zhao et al. prepared surfactant-modified flowerlike-layered double hydroxide-coated magnetic nanoparticles for pre-concentration of phthalate esters from environmental water samples (Zhao et al. 2015). Chen et al. coated the magnetic hollow Fe3O4 nanoparticles with a polystyrene layer to selectively absorb lubricating oil from aqueous media (Chen et al. 2013). Luo et al. used an integrated catalyst of Pd supported on magnetic Fe3O4 nanoparticles for degradation of organic contaminants (Luo et al. 2014). Zhou et al. incorporated Fe, Ni, and Fe/Ni nanoparticles in commercial polystyrene cation exchange resin, to form different composites. The resultant composites were used to dechlorinate trichloroethylene in the aqueous solution (Zhou et al. 2016). Wang et al. prepared estrone imprinted core–shell magnetic nanoparticles which exhibited a much higher specificity and saturation magnetization for biochemical separation of estrone (Wang et al. 2009b). Wu et al. prepared Fe3O4@mSiO2, a magnetic mesoporous microsphere loaded with tyrosine for phenolic compound detection and degradation (Wu et al. 2011). Core–shell magnetic microspheres were obtained by encapsulating Fe 3 O 4 nanocrystals in mesoporous silica through a packing approach and utilized for removal of DDT from aqueous media (Tian et al. 2009).

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Conclusions and future prospects

References

MNPs have a great potential to be used in organic compound degradation. Its unique characteristic of having high surface area can be used efficiently for removing toxic metal ions, disease-causing microbes, and organic and inorganic solutes from water. Decomposition of organic compounds is one of the major problems which the world is facing today. The organic compound contamination not only affect the environment and human health, but it has also impacts on economic and social values. There are various ways used commercially and noncommercially to fight this problem which is advancing day by day due to technological progress. Nanotechnology has also proved to be one of the finest and advance ways for the degradation of organic compounds. However, these nanopaticles are extremely toxic and thus should be well-coated with biocompatible ligands. Moreover, synthesis and surface engineering of MNPs involve complex chemical, physical, and physicochemical multiple interactions; it is another challenge to understand the synthetic mechanisms detailed. The magnetic properties and function of naked or surface functionalized MNPs depend upon their physical properties: the size and shape, their microstructure, and the chemical phase in which they are present. Therefore, how to improve the stability and availability of functionalized MNPs in extreme environmental conditions, how to develop an efficient and orderly magnetic micro assembly or nanoassembly structures, and how to realize large scale or industrial synthesis, these problems are urgent to be solved for obtaining ideal functionalized MNPs. For all that, we still believe that the surface functionalization and modification of MNPs to introduce additional functionality will attract more and more attention. Furthermore, multifunctional magnetic oxide composite nanoparticle systems with designed active sites will promise for various applications, such as catalysts, magnetic recording, bioseparation, biodetection, etc. The future work in this area must be focused on the research of the toxicity and degradability of organic compound and preparation of the MNPs via green chemistry for reducing the environmental pollution as much as possible. Successful development in this area will aid the growth of the various scientific researches or industrial applications as well as improve the quality of life in the population.

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Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Education, Science and Technology (NRF-2014-R1A1A2055756).

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