Role of Nanomaterials in Water Treatment Applications a Review

Due to the infrequent characteristics which resulted from nanoscale size, such as improved catalysis and adsorption properties every bit well as high reactivity, nanomaterials accept been the subject of active research and development worldwide in contempo years. Numerous studies accept shown that nanomaterials tin can effectively remove diverse pollutants in water and thus have been successfully applied in h2o and wastewater treatment. In this paper, the about extensively studied nanomaterials, zero-valent metallic nanoparticles (Ag, Fe, and Zn), metal oxide nanoparticles (TiO_ii, ZnO, and iron oxides), carbon nanotubes (CNTs), and nanocomposites are discussed and highlighted in detail. Besides, future aspects of nanomaterials in h2o and wastewater treatment are discussed.

one. Introduction

Generally speaking, nanomaterials draw materials of which the structural components are sized (in at least i dimension) between 1 and 100 nm [1]. Due to the nanoscale size of nanomaterials, their backdrop, such as mechanical, electrical, optical, and magnetic properties, are significantly different from conventional materials. A broad range of nanomaterials have the characteristics of catalysis, adsorption, and high reactivity.

In the past decades, nanomaterials have been under active inquiry and development and have been successfully applied in many fields, such equally catalysis [2], medicine [3], sensing [4], and biology [5]. In particular, the application of nanomaterials in h2o and wastewater treatment has drawn wide attention. Due to their small sizes and thus large specific surface areas, nanomaterials take strong adsorption capacities and reactivity. What is more than, the mobility of nanomaterials in solution is high [6]. Heavy metals [7], organic pollutants [eight], inorganic anions [9], and bacteria [x] have been reported to be successfully removed by diverse kinds of nanomaterials. On the basis of numerous studies, nanomaterials show not bad hope for applications in water and wastewater handling. At present, the most extensively studied nanomaterials for water and wastewater treatment mainly include zero-valent metal nanoparticles, metal oxides nanoparticles, carbon nanotubes (CNTs), and nanocomposites.

2. Nanomaterials for Water and Wastewater Treatment

two.ane. Naught-Valent Metal Nanoparticles

two.1.1. Argent Nanoparticles

Silver nanoparticles (Ag NPs) are highly toxic to microorganisms and thus have strong antibacterial furnishings against a broad range of microorganisms, including viruses [11], leaner [ten], and fungi [12]. Every bit a good antimicrobial agent, silver nanoparticles have been widely used for the disinfection of h2o.

The mechanism of the antimicrobial effects of Ag NPs is not conspicuously known and remains nether debate. In recent years, several theories take been put forwards. Ag NPs accept been reported to exist able to adhere to the bacterial cell wall and later on penetrate it, resulting in structural changes of the cell membrane and thus increasing its permeability [thirteen]. As well, when Ag NPs are in contact with bacteria, free radicals can be generated. They accept the ability to damage the cell membrane and are considered to cause the decease of cells [xiv]. In addition, as DNA contains abundant sulfur and phosphorus elements, Ag NPs tin human action with it and thus destroy it. This is another explanation for the decease of cells caused by Ag NPs [15]. What is more, the dissolution of Ag NPs will release antimicrobial Ag⁺ ions, which can collaborate with the thiol groups of many vital enzymes, inactivate them, and disrupt normal functions in the cell [16].

With the development of nanotechnology, Ag NPs take been successfully applied in water and wastewater disinfection in recent years. Straight application of Ag NPs might cause some problems, such as their trend to aggregate in aqueous media that gradually reduces their efficiency during long-term use [17]. Ag NPs attached to filter materials have been considered promising for water disinfection due to their high antibacterial activity and cost-effectiveness [18].

Via the in situ reduction of silver nitrate, Ag NPs have been deposited on the cellulose fibers of an absorbent blotting paper sail (run across Figure 1). The Ag NPs sheets showed antibacterial properties towards suspensions of Escherichia coli and Enterococcus faecalis and inactivated bacteria during filtration through the sheet. Moreover, the silverish loss from the Ag NPs sheets was lower than the standards for silver in drinking h2o put forwards by Environmental Protection Agency (EPA) and World Health Organization (WHO) [xix]. Therefore, for water contaminated by bacteria, filtration through paper deposited with Ag NPs could be an constructive emergency water treatment. Besides, Ag NPs synthesized by chemical reduction have been incorporated into polyethersulfone (Foot) microfiltration membranes. The action of microorganisms nearby the membranes was observed to be remarkably suppressed. The PES-Ag NPs membranes exhibited stiff antimicrobial backdrop and held dandy potential in application for h2o handling [twenty].

In the by xx years, Ag NPs on ceramic materials/membranes have drawn substantial attention due to their disinfection and biofouling reduction for household (bespeak-of-use) water handling [21]. For instance, the addition of Ag NPs to ceramic filters constructed with clay and sawdust has turned out to be able to improve the removal efficiency of Escherichia coli. Information technology was also found that filters with higher porosity accomplished higher bacteria removal than those with lower porosity [22]. Besides, colloidal Ag NPs have been combined with cylindrical ceramic filters, which were fabricated upwardly of clay-rich soil with h2o, grog, and flour, in dissimilar quantities and ways (dipping and painting). It was proved that colloidal Ag NPs improved the filter operation and the filters can remove Escherichia coli in the rate betwixt 97.viii% and 100% [23]. Recently, the attachment of Ag NPs to ceramic membranes has been successfully predicted by Derjaguin-Landau-Verwey-Overbeek (DLVO) approximation methods [24]. Further studies on Ag NPs will promote their applications in h2o and wastewater treatment.

2.ane.2. Iron Nanoparticles

In contempo years, diverse cypher-valent metal nanoparticles, such as Fe, Zn, Al, and Ni, in water pollution treatment accept drawn wide research interest. The standard reduction potentials of Fe, Al, Ni, and Zn are listed in Table 1. Due to the extremely high reductive ability, nano-zero-valent Al is thermodynamically unstable in the presence of water, which favors the formation of oxides/hydroxides on the surface, impeding (completely) the transfer of electrons from the metal surface to the contaminants [25]. Compared with Fe, Ni has a less negative standard reduction potential, indicating a lower reducing ability. With a moderate standard reduction potential, nano-zero-valent Fe or Zn holds proficient potential to act as reducing agents relative to many redox-labile contaminants. Despite a weaker reduction power, Atomic number 26 possesses many prominent advantages over Zn for applications in water pollution treatment, including excellent adsorption properties, precipitation and oxidation (in the presence of dissolved oxygen), and low toll. Therefore, aught-valent fe nanoparticles have been the most extensively studied nil-valent metallic nanoparticles.

Metal Standard reduction potential () Fe Zn Al Ni

The information comes from [40].

As a result of the extremely pocket-sized size and thus large specific surface area, nZVI possesses good adsorption properties and strong reducing ability [26]. These characteristics contribute near to its first-class performance in the removal of contaminants. Nether anaerobic conditions, as shown in (1)-(2), Fe⁰ can exist oxidized by H₂O or H⁺ and generates Atomic number 26²⁺ and H₂, both of which are also potential reducing agents for contaminants. In the oxidation-reduction reaction between nZVI and contaminants, Fe²⁺ will exist oxidized to Fe^three+, which tin form Atomic number 26(OH)₃ with the increment of pH. As a mutual and effective flocculant, Fe(OH)₃ facilitates the removal of contaminants, for instance, Cr(Half-dozen) [27]. What is more than, ZVI can degrade and oxidize a variety of organic compounds in the presence of dissolved oxygen (Do) since ZVI transfers ii electrons to O_ii to produce H_twoO_two (see (iii)). The resultant H₂O₂ tin be reduced to H₂O by ZVI (see (four)). Moreover, the combination of H₂O₂ and Iron²⁺ (known as Fenton reaction) tin generate hydroxyl radicals () which have strong oxidizing ability towards a wide range of organic compounds (come across (5)) [28]:

With the effects of adsorption, reduction, atmospheric precipitation, and oxidation (in the presence of DO), nZVI has been successfully applied in the removal of a large range of contaminants, including halogenated organic compounds [29], nitroaromatic compounds [thirty], organic dyes [31], phenols [32], heavy metals [33], inorganic anions such as phosphates [34] and nitrates [35], metalloids [36], and radio elements [37]. What is more, enquiry on the awarding of nZVI in water and wastewater treatment is not limited to water or laboratory tests. In recent years, nZVI has also been practical in soil remediation [38] and already achieved pilot-scale and full-scale applications at real water contaminated field sites [39].

Despite many advantages, nZVI also has its ain disadvantages, such as aggregation, oxidation, and separation difficulty from the degraded system. To solve these issues, various modification approaches have been put forwards to raise the performance of nZVI in water and wastewater treatment. Mutual modification approaches mainly include doping with other metals, surface coating, conjugation with supports, encapsulation in matrix, and emulsification [41]. Doping with other metals is supposed to enhance the reactivity of nZVI [42]. Both surface coating and conjugation with supports tin can prevent aggregation and enhance the dispersibility of nZVI [43, 44]. Besides, both conjugation with supports and encapsulation in matrix facilitate the separation of nZVI from the degraded organization [45, 46]. In addition, the emulsification of nZVI is aimed at solving the delivery problem of nZVI in dumbo nonaqueous stage liquid (DNAPL) [47].

2.1.iii. Zinc Nanoparticles

Although nearly studies on contaminant degradation in water and wastewater handling by nix-valent metal nanoparticles take been focused on fe, Zn has also been considered as an alternative [48]. With a more negative standard reduction potential (Table 1), Zn is a stronger reductant compared with Atomic number 26. Therefore, the contaminant deposition rate of zinc nanoparticles may be faster than that of nZVI.

For the application of nano-cipher-valent zinc (nZVZ), most studies take been focused on dehalogenation reaction. Inquiry indicated that the reduction rates of CCl₄ by nZVZ were more significantly affected by solution chemistry than particle size or surface morphology. Past comparing the reactivity of various types of nZVI and nZVZ, it was institute that nZVZ could dethrone CCl₄ more apace and completely than nZVI under favorable weather [49]. Likewise, a study has been carried out to examine the deposition of octachlorodibenzo-p-dioxin (OCDD) in water with iv different goose egg-valent metal nanoparticles: zero-valent zinc (nZVZ), zilch-valent iron (nZVI), cipher-valent aluminum (nZVAL), and zero-valent nickel (nZVN). On the footing of experimental results, just nZVZ was able to efficiently dethrone OCDD into lower chlorinated congeners and thus became the starting time reported zippo-valent metal nanoparticles suitable for OCDD dechlorination under ambience weather condition [48].

Nonetheless, although several studies have demonstrated that contaminant reduction by nZVZ could be successful, the application of nZVZ is mainly limited in the degradation of halogenated organic compounds, especially CCl₄. The handling of other kinds of contaminants by nZVZ has rarely been reported upwards to at present. Therefore, pilot-calibration or full-scale applications of nZVZ have not been achieved at contaminated field sites nevertheless [49].

2.two. Metal Oxides Nanoparticles

ii.ii.1. TiO₂ Nanoparticles

As an emerging and promising technology, photocatalytic degradation has attracted great attention since 1972 when Fujishima and Honda [50] observed electrochemical photolysis of water on TiO_ii semiconductor electrode. In contempo years, photocatalytic degradation technology has been successfully applied in the contaminant degradation in water and wastewater. At the presence of low-cal and catalyst, contaminants can be gradually oxidized into depression molecular weight intermediate products and eventually transformed into CO_two, H₂O, and anions such as , , and .

The majority of common photocatalysts are metallic oxide or sulfide semiconductors, amidst which TiO_ii has been most extensively investigated in the past decades. Owing to its high photocatalytic activity, reasonable price, photostability, and chemical and biological stability [51–53], TiO₂ is the about infrequent photocatalyst to engagement. The large band gap energy (3.2 eV) of TiO₂ requires ultraviolet (UV) excitation to induce charge separation inside the particles. As shown in Figure 2, upon UV irradiation, TiO_two will generate reactive oxygen species (ROS) which can completely degrade contaminants in very short reaction time. Besides, TiO_ii NPs show picayune selectivity and thus are suitable for the degradation of all kinds of contaminants, such as chlorinated organic compounds [54], polycyclic aromatic hydrocarbons [55], dyes [56], phenols [57], pesticides [58], arsenic [59], cyanide [threescore], and heavy metals [61]. What is more, hydroxyl radicals generated under UV irradiation ( nm) enable TiO₂ NPs to damage the office and structure of various cells [62]. The photocatalytic properties of TiO_ii NPs are able to kill a wide array of microorganisms, such as Gram-negative and Gram-positive bacteria, likewise as fungi, algae, protozoa, and viruses [63].

However, TiO_ii NPs also have some disadvantages. As mentioned in a higher place, their large band gap energy makes them need the excitation of UV and the photocatalytic properties of TiO_ii NPs under visible light are relatively camouflaged. Hence, studies accept been conducted to meliorate the photocatalytic backdrop of TiO₂ NPs under visible lite and UV. For example, metal doping has been demonstrated to be able to improve the visible light absorbance of TiO₂ NPs [64] and increment their photocatalytic action under UV irradiation [65]. Among various metals, Ag has received much attending for metal doping of TiO_two NPs because it could enable the visible light excitation of TiO_two NPs [66] and greatly ameliorate the photocatalytic inactivation of leaner [67] and viruses [68]. Besides, modifications of TiO_ii NPs by nonmetal elements, such as Northward, F, S, and C, have also been found to be able to narrow the band gap significantly, raise adsorption in the visible region, and improve the degradation of dyes nether visible low-cal irradiation, especially under natural solar light irradiation [69].

Besides, the production process of TiO₂ NPs is rather complicated. What is more, it is difficult to recover TiO₂ NPs from the treated wastewater, specially when they are used in interruption. In contempo years, more and more than efforts have been devoted to surmounting this problem. Amongst them, the coupling of the photocatalysis of TiO₂ NPs with membrane engineering has attracted much attention and shown promise for overcoming the recovery trouble of TiO_two NPs. A wide range of membranes have been incorporated with TiO₂ NPs, such every bit poly(vinylidene fluoride) [seventy, 71], polyethersulfone [72, 73], polymethyl methacrylate [74], and poly(amide-imide) [75]. For instance, using N,N′-methylenebisacrylamide as the cross-linker and ammonium persulphate as the initiator pair, the polymerization of acrylamide in an aqueous solution was carried out to synthesize TiO_two/poly[acrylamide-co-(acrylic acrid)] blended hydrogel. Methylene blue was successfully removed by the photocatalysis of TiO₂ NPs. Moreover, due to the coupling with polymeric membranes, TiO₂ NPs could be easily separated from the treated organization through a simple filtration [76]. A detailed review on TiO₂ nanocomposite based polymeric membranes has been presented [77]. More recently, doped TiO_ii magnetic nanoparticles have been synthesized in a spinning deejay reactor to achieve a feasible recovery of the nanoparticles by a magnetic trap [78, 79]. The production process is continuous and thus suitable for industrial applications [79].

2.2.2. ZnO Nanoparticles

In the field of photocatalysis, apart from TiO₂ NPs, ZnO NPs have emerged as some other efficient candidate in water and wastewater treatment because of their unique characteristics, such equally direct and broad ring gap in the near-UV spectral region, stiff oxidation ability, and good photocatalytic belongings [eighty–82].

ZnO NPs are surroundings-friendly as they are compatible with organisms [83], which makes them suitable for the treatment of water and wastewater. Likewise, the photocatalytic adequacy of ZnO NPs is like to that of TiO₂ NPs because their band gap energies are well-nigh the same [84]. Nevertheless, ZnO NPs have the reward of low cost over TiO₂ NPs [84]. Moreover, ZnO NPs tin can adsorb a wider range of solar spectra and more light quanta than several semiconducting metal oxides [85].

Nevertheless, similar to that of TiO₂ NPs, the light absorption of ZnO NPs is too limited in the ultraviolet light region due to their big band gap energies. Besides, the awarding of ZnO NPs is impeded past photocorrosion, which volition effect in fast recombination of photogenerated charges and thus crusade depression photocatalytic efficiency [86].

To improve the photodegradation efficiency of ZnO NPs, metal doping is a common strategy. Various types of metal dopants have been tested, including anionic dopants, cationic dopants, rare-globe dopants, and codopants [87]. Besides, many studies have shown that coupling with other semiconductors, such as CdO [88], CeO_two [89], SnO₂ [xc], TiO_ii [91], graphene oxide (Get) [92], and reduced graphene oxide (RGO) [93], is a feasible arroyo to raise the photodegradation efficiency of ZnO NPs.

ii.2.3. Iron Oxides Nanoparticles

In recent years, in that location is a growing involvement in the use of atomic number 26 oxides nanoparticles for the removal of heavy metal due to their simplicity and availability. Magnetic magnetite (Fe_iiiO₄) and magnetic maghemite (γ-Fe₂O₄) and nonmagnetic hematite (α-Fe₂O_iii) are often used as nanoadsorbents.

Generally, due to the small size of nanosorbent materials, their separation and recovery from contaminated water are great challenges for water treatment. However, magnetic magnetite (Fe_threeO₄) and magnetic maghemite (γ-Fe_twoO₄) can be easily separated and recovered from the arrangement with the assistance of an external magnetic field. Therefore, they have been successfully used as sorbent materials in the removal of diverse heavy metals from water systems [94–96]. In order to increase adsorption efficiency and to avoid interference from other metals ions, fe oxides nanoparticles have been functionalized to tune their adsorption properties by adding various ligands (e.g., ethylenediamine tetraacetic acid (EDTA), L-glutathione (GSH), mercaptobutyric acrid (MBA), α-thio-ω-(propionic acrid) hepta(ethylene glycol) (PEG-SH), and meso-2,3-dimercaptosuccinic acrid (DMSA)) [97] or polymers (due east.g., copolymers of acrylic acid and crotonic acid) [98]. A flexible ligand shell has been reported to facilitate the incorporation of a wide array of functional groups into the shell and ensured the properties of Fe_iiiO₄ nanoparticles are intact [99]. Also, a polymer beat has been found to be able to prevent assemblage of particles and improve the dispersion stability of the nanostructures [98]. Polymer molecules could act as binders for metallic ions and thus became a "carrier" of metal ions from treated water [99].

Hematite (α-Fe₂O_iii) has been considered as a stable and cheap cloth in sensors, catalysis, and environmental applications [100]. Moreover, nanohematite has also been demonstrated to be an constructive adsorbent for the removal of heavy metal ions from spiked tap h2o [101]. 3D flower-similar α-Fe_iiO_three microstructures assembled from nanopetal subunits accept been synthesized for h2o handling utilise. The flower-like α-Fe_twoO_three could finer forbid further aggregation, and the enhanced surface area with multiple spaces and pores provided many active sites to interact with contaminants. The maximum adsorption capacities of the as-prepared α-Fe_iiO₃ for Every bit(V) and Cr(VI) were much higher than those of many previously reported nanomaterials [100].

two.3. Carbon Nanotubes

Carbon nanomaterials (CNMs) are a class of fascinating materials due to their unique structures and electronic properties which make them attractive for fundamental studies as well as diverse applications, especially in sorption processes. Their advantages for water and wastewater treatment are due to () peachy capacity to adsorb a broad range of contaminants, () fast kinetics, () large specific surface surface area, and () selectivity towards aromatics [half-dozen]. There are several forms of CNMs, such as carbon nanotubes (CNTs), carbon beads, carbon fibers, and nanoporous carbon [half-dozen]. Among them, CNTs have attracted the about attentions and progressed rapidly in recent years.

Carbon nanotubes are graphene sheets rolled up in cylinders with diameter as small equally 1 nm [102]. CNTs accept attracted nifty interest as an emerging adsorbent due to their unique properties. With an extremely large specific surface surface area and abundant porous structures, CNTs possess exceptional adsorption capabilities and high adsorption efficiencies for numerous kinds of contaminants, such as dichlorobenzene [103], ethyl benzene [104], Znⁱⁱ⁺ [105], Pbⁱⁱ⁺, Cu²⁺, and Cd²⁺ [106], and dyes [107]. According to their (super)structures, CNTs can be classified into ii types (Figure three): () multiwalled carbon nanotubes (MWCNTs), which comprised multiple layers of concentric cylinders with a spacing of about 0.34 nm between the next layers, and () single-walled carbon nanotubes (SWCNTs), which consist of unmarried layers of graphene sheets seamlessly rolled into cylindrical tubes [108]. In recent years, both MWCNTs [105–107] and SWCNTs [109] have been applied for the removal of contaminants in water.

(a)

(b)

To ameliorate the adsorption, mechanical, optical, and electrical properties, carbon nanotubes are often combined with other metals or types of support [110]. The functionalization increases the number of oxygen, nitrogen, or other groups on the surface of CNTs, enhances their dispersibility, and thus improves specific surface area [111–113]. For example, a study using CNTs as a back up for magnetic iron oxide has been reported by Gupta et al. [114]. Combining the adsorption properties of CNTs with the magnetic backdrop of iron oxide, a "composite" adsorbent was prepared to remove chromium from water. Apart from owning fantabulous adsorption properties, the "composite" adsorbent can exist easily separated from water via an external magnetic field.

In spite of the exceptional properties of CNTs, the evolution and applications of CNTs are mainly limited by their depression book of production and high cost. Too, CNTs cannot exist used alone without any supporting medium or matrix to grade structural components [102].

2.iv. Nanocomposites

Every bit mentioned above, every nanomaterial has its own drawbacks. For example, nZVI has the disadvantages of aggregation, oxidation, and separation difficulty from the degraded systems. The low-cal adsorption of TiO_two NPs and ZnO NPs is express in the ultraviolet light region due to their big band gap energies. Nanofiltration membranes are troubled by the problem of membrane fouling. Carbon nanotubes are mainly limited by their low book of production and loftier cost as well every bit the need for supporting medium or matrix. In club to overcome these bug and achieve better removal efficiency, information technology is a common and constructive strategy to fabricate nanocomposites for h2o and wastewater handling.

In recent years, the synthesis of various nanocomposites has go the virtually active subject in the field of nanomaterials. On the basis of numerous studies, much progress has been made throughout the earth. For example, via chemic deposition of nZVI on CNTs, a novel nanoscale adsorbent was prepared. According to the results, the adsorbent has good potential for quick and effective removal of nitrate in water. Too, due to its unique magnetic belongings, the adsorbent tin be hands separated from the solution past the magnet [115]. Besides, thin moving picture nanocomposite (TFN) nanofiltration membranes have been prepared via in situ interfacial incorporation of TiO_two NPs along with fabrication of copolyamide network on a polyimide support. To improve the compatibility of TiO_two NPs within the polymer matrix, both amine and chloride compounds were utilized to functionalize TiO₂ NPs. TFN membranes exhibited higher methanol flux and dye rejection in spite of lower swelling degree. The loading of TiO₂ NPs turned out to be a crucial factor on the NF membrane functioning [116].

In theory, ideal composites for real applications should exist continuous, bulk immobile materials of which the nanoreactivity is obtained by anchoring or impregnating a parent material construction with nanomaterials [117]. What is more, it is widely acknowledged that the handling of water and wastewater calls for nontoxic, long-term stable and depression-cost materials. To obtain desirable nanocomposites, farther research is still under manner.

3. Conclusions and Perspectives

In this paper, the most extensively studied nanomaterials, zero-valent metal nanoparticles (Ag, Fe, and Zn), metal oxide nanoparticles (TiO₂, ZnO, and atomic number 26 oxides), carbon nanotubes (CNTs), and nanocomposites were highlighted. Moreover, their applications in water and wastewater treatment were discussed in item. Considering the current speed of development and application, nanomaterials look extremely promising for water and wastewater treatment.

Still, further studies are still needed to accost the challenges of nanomaterials. Up to now, only a few kinds of nanomaterials have emerged commercially. Since depression production price is crucial to ensure their wide spread applications in h2o and wastewater handling, future inquiry should exist devoted to improving the economical efficiency of nanomaterials. Besides, with increasingly all-encompassing applications of nanomaterials in water and wastewater treatment, there are growing concerns on their potential toxicity to the environs and human health. Available information in the literature has revealed that several nanomaterials may take adverse effects on the environment and human wellness [118–120]. Nevertheless, standards for assessing the toxicity of nanomaterials are relatively insufficient at present. Hence, comprehensive evaluation of the toxicity of nanomaterials is in urgent demand to ensure their real applications. What is more, the evaluation and comparison of the performance of diverse nanomaterials in water and wastewater treatment are still brusk of uniform or recognized standards. It is difficult to compare the performances of different nanomaterials and figure out promising nanomaterials that deserve further development. Therefore, the performance evaluation machinery of nanomaterials in h2o and wastewater handling should be perfected in the hereafter.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this paper.

Acknowledgments

This inquiry is financially supported by National Natural Science Foundation of China (no. 21376165 and no. 51478308).

Copyright

Copyright © 2016 Haijiao Lu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in whatever medium, provided the original work is properly cited.

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Source: https://www.hindawi.com/journals/amse/2016/4964828/