Advanced Powder Technology 24 (2013) 459–467

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Advanced Powder Technology

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Original Research Paper

Magnetic nanocomposites for environmental remediation

0921-8831/$ - see front matter � 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rightshttp://dx.doi.org/10.1016/j.apt.2012.10.012

⇑ Corresponding author.E-mail addresses: suying.wei@lamar.edu (S. Wei), zhanhu.guo@lamar.edu

(Z. Guo).

Jiahua Zhu a, Suying Wei b, Minjiao Chen a, Hongbo Gu a, Sowjanya B. Rapole a, Sameer Pallavkar a,Thomas C. Ho a, Jack Hopper a, Zhanhu Guo a,⇑a Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USAb Department of Chemistry and Biochemistry, Lamar University, Beaumont, TX 77710, USA

a r t i c l e i n f o

Article history:Received 3 June 2012Received in revised form 10 October 2012Accepted 25 October 2012Available online 12 February 2013

Keywords:Magnetic nanocompositesEnvironmental remediationIron oxideHeavy metal ions

a b s t r a c t

This article provides an overview of current research activities on the synthesis and applications of mag-netic nanocomposites, especially highlights their potential environmental remediations such as heavymetal (Cr, As, Pd, Hg) removal. After a brief introduction of the emergency situation of heavy metal pol-lution all over the world and current techniques designed to deal with these situations, different syn-thetic methods to fabricate various types of magnetic nanocomposites will be reviewed. The focus isto reveal the advantages of magnetic nanocomposites as an efficient adsorbent which is able to reducethe heavy metal concentrations well below the EPA requirement. At the same time, the conventional pro-cess can be redesigned to be an economic and energetic one without using extra energy to recycle theadsorbent, which is desired for future. This review mainly deals with the heavy metal removal using mag-netic nanocomposites, the adsorption behaviors of heavy metal ions on the surface of novel adsorbentsare well investigated including the concentration effect of both contaminants and adsorbents, adsorptionkinetics, solution pH effect with regards to real application.� 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Rapid industrialization has led to an increased discharge ofwastewater containing heavy metals (Cr, Cd, Hg, Pb, and As), whichhave detrimental effects on the environment. Among those heavymetal species, Cr(VI) is a commonly identified contaminant be-cause of its high toxicity and mobility [1]. The maximum permissi-ble limit of the total Cr in drinking water has been recommendedas 100 lg/L by the US Environmental Protection Agency (EPA)[2]. Arsenic is also known for its toxicity and carcinogenicity to hu-man beings [3–5], the contaminated water with arsenic is becom-ing a significantly increasing issue in drinking water throughoutthe world. Long-term exposure to arsenic can cause cancers ofbladder, lungs, skin, kidney, liver and prostate [6]. The shallowgroundwater, a major source of drinking water for many southand southeast Asian countries like Bangladesh, is contaminatedwith arsenic [7]. The USEPA issued a new legislation effective inFebruary 2002 with a required reduced maximum contaminant le-vel (MCL) of 10 ppb by the year 2006 [5]. India [8], New Zealand[9], Taiwan [10] and Vietnam [11] have the same 10 ppb level ofarsenic as a maximum permissible limit for the drinking water.Bangladesh [12], Nepal [13], Argentina [14], China [15], Chile

[16] and Mexico [17] have regulated 50 ppb of arsenic as a maxi-mum level of contamination in the drinking water. These new reg-ulations make water facilities face challenging financial burdens oreven incapable of reaching the newly proposed MCL for arsenic,unless affordable methods of arsenic removal are developed.Therefore, effective treatment with an economical and reliabletechnique capable of removing arsenic species is required to meetthis new MCL value.

Arsenic can exit in both inorganic and organic forms. In general,inorganic arsenic compounds are more toxic than organic arseniccompounds, and arsenite [As(III)] is considerably more mobileand toxic than arsenate [As(V)] [18]. Arsenate (i.e., HAsO2�

4 ) is theprimary anion in the aerobic surface water and arsenite (i.e.,H3AsO3 or H2AsO�3 ) is the primary species in the ground water.The actual valence states and chemical form, however, depend onthe redox environments in the water systems including pH, oxida-tion–reduction potential, and the presence of complexing ions[18,19]. Toxic arsenic compounds have been detected in water sup-ply wells in the United States [20] and abroad.

Until now, a number of technologies to remove heavy metalions, such as Cr(VI), As(III) and As(V) have been developed, includ-ing cyanide treatment [21], electro-chemical precipitation [22],reverse osmosis (RO) [23,24], ion exchange (IE) [25,26] and adsorp-tion [27–34]. However, chlorination of cyanides can result in highlytoxic intermediate and other toxic organochlorines. Thesecompounds together with residual chlorine create additional

reserved.

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environmental problems. Precipitation is considered to be the mostapplicable and economical approach. However, this technique pro-duces a large amount of precipitate sludge that requires additionalprocess for the further treatment. Though RO can effectively reducemetal ions, but its applications are limited by a number of disadvan-tages such as high operational cost and limited pH range [24]. IE is aconvenient method to treat the wastewater containing chromiumions, but only limited literatures have been reported on the removalof Cr(III) [25,26,35]. Operation cost is also higher than that of theother methods [23]. In addition to the aforementioned technolo-gies, Cr(VI) reduction by zero-valence Fe [36,37], Fe(II) [38,39],atomic hydrogen [40,41], dissolved organic compounds [42] andsulfur compounds [43] has been developed and the removal capac-ity is satisfactory with an extended treating time.

However, separating and recycling these materials turn out tobe a challenge especially when the particle size goes down tonanoscale and thus reducing the operation time in each cycle is ur-gently required in modern industry. Consequently, an alternativeadsorption is favorable and feasible because of its low cost andhigh efficiency [27–31,44–47]. Besides, adsorption can effectivelyremove heavy metals present in the wastewater at low concentra-tions [46,48]. Though activated carbon is one of the most usedadsorbents to purify polluted water [48–50], it still failed to reducethe concentration of contaminants at ppb levels [51]. Iron mineralshave been recognized as an effective media to remove various hea-vy metals such as As(III) and As(V) [4], Cr(VI) [30] and Pd(II) [52].More recently, iron and iron oxide nanostructures have beenproved as higher efficient materials for the heavy metal removalby reduction or adsorption [53–55]. However, there are two majorchallenges when using these nanomaterials. One comes from theeasy oxidation/dissolution of the pure Fe nanoparticles (NPs), espe-cially at acidic solution. The other is the difficulty to recycle theseNPs with such a small size, especially in a continuous flowing sys-tem. To protect the magnetic NPs against oxidation, a shell struc-ture is often introduced, including silica [56–59], polymer[52,60,61], carbon [27] and noble metals [62–64]. And to overcomethe latter challenge and to prevent the side pollutant of the re-leased nanomaterials, researchers are trying to embed these NPsinto an easily separable substrate, the most typical substrate is car-bon due to its low cost and high specific surface area [65,66].

With a large dimension in XY plane reaching several microme-ters and extremely thin thickness (nanometer) in Z-axis, graphenehas large specific surface area and possesses great advantages to bea perfect substrate for embedding NPs. Though magnetic carbonnanocomposites with large specific surface area enhanced the hea-vy metal removal and a magnet facilitated the recycling of the NPswith a reported 95% removal rate [27], a 2-h treatment was re-quired. Furthermore, there are few reports on the fabrication ofmagnetic graphene nanocomposites, especially those with greatpotential to be used in environmental remediation with fasttreatment.

The present review reports on the concepts and state-of-the-artof magnetic nanocomposites. The focus is on the synthetic meth-ods and applications of polymer based and carbon based magneticnanocomposites. The environmental remediation applications ofthese composite materials are highlighted.

2. Fundamentals of magnetic nanocomposites

2.1. Polymer-based and carbon-based magnetic nanocomposites

Targeting to various applications, efforts have been made to dis-perse the magnetic NPs in a matrix, e.g., to prevent superparamag-netic NPs from aggregating into large ferromagnetic species [67].With regard to different applications, the matrix could be chosen

including polymer, carbon, ceramic or even metal. In this review,we will focus on polymer and carbon based magneticnanocomposites.

Magnetic polymer nanocomposites can be defined as materialscomposed of an inorganic magnetic component in the form of par-ticles, fibers or lamellae with at least one dimension in the nano-meter range embedded in an organic polymer [68]. The merits ofnanocomposites are to integrate several component materialsand their properties in a single material. In magnetic polymernanocomposites, organic–inorganic synergies add new propertiesthat cannot be achieved in either single organic or individual inor-ganic components.

Magnetic carbon nanocomposites have the similar definition aspolymer nanocomposites but using carbon as the hosting matrix.Carbon has been one of the most studied materials owing to its un-ique mechanical, physical and chemical properties. Various mor-phologies of carbon have been developed, for example, activatedcarbon, fullerene (C60), carbon nanofibers, carbon nanotubes, ex-panded graphite, and graphene. Once these carbon materials com-bined with magnetic nanostructures, unique magnetic propertiesare often observed.

2.2. Synthesis of magnetic nanocomposites

2.2.1. In situ methodThe in situ method refers to the creation of nanostructures from

precursor with the presence of another phase material. The com-monly used procedure for synthesizing magnetic NPs has beenthe coprecipitation of Fe2+ and Fe3+ ions by a base, which often re-sults in a rather broad size distribution of the NPs. An alternativeapproach yielding highly monodisperse magnetic NPs has beenthe thermal decomposition of metal precursors, including metalcarbonyls (Co2(CO)8, Fe(CO)5, Ni(CO)4) and metal oleates. Whenthese metal carbonyls are decomposed in the presence of poly-mers, the magnetic polymer nanocomposites (PNCs) could be ob-tained with good particle distribution. Taking polypropylene (PP)as an example [69], PP was initially dissolved in xylene with aweight ratio of 1:10 (20 g: 207 mL) and refluxed at the boilingpoint (�140 �C) of xylene for around 2 h until PP was completelydissolved. Then specific amount liquid of Fe(CO)5 was injected intothe dissolved PP solution. The mixture solution turned from trans-parent to yellow immediately after the addition of Fe(CO)5 andthen gradually changed to black during the additional 3-h refluxingprocess under the nitrogen protecting conditions, indicating theformation of the NPs. Upon heating, Fe(CO)5 was decomposed toFe2(CO)9 and Fe3(CO)12 with a rapid formation of CO, reaching anequilibrium mixture of all the three carbonyls. The Fe3(CO)12 wasthen decomposed and finally formed the metallic NPs [70,71].The resulting PNCs with a uniform particle distribution could beobtained, Fig. 1.

Following the similar method, graphene based magnetic nano-composites could also be prepared using one-pot thermodecompo-sition method [74]. To be specific, graphene (1.0 g, containingsurfactant sodium dodecylbenzenesulfonate (SDBS)) was dispersedin dimethylformamide (DMF, 100.0 mL) using ultrasonication for30 min at room temperature. And then Fe(CO)5 (0.385 g) was in-jected to the graphene suspended DMF solution (DMF and Fe(CO)5

are intermiscible). The suspension was heated to the boiling tem-perature of �153 �C and refluxed for additional 4 h. During thisprocess, the iron precursor was transformed to iron NPs and ad-hered on the graphene sheets, which were partially oxidized fromthe outer surface due to the existence of residue oxygen in thesolution. Meanwhile, the dissipated SDBS in the solution wasassembled on the nanoparticle surface. Finally, the solid productswere removed from the suspension using a permanent magnet,and the residue solution was transparent, indicating that Fe(CO)5

J. Zhu et al. / Advanced Powder Technology 24 (2013) 459–467 461

has been completely decomposed. After the drying of solid prod-ucts in vacuum oven at 80 �C overnight and annealing at 500 �Cfor 2 h under H2 (5%)/Ar atmosphere, the organics including thebenzene and alkyl chains were removed, and the S and O were re-mained. Fig. 2 shows the microstructures of the NPs, which are ob-served to be uniformly dispersed on graphene sheet with anaverage diameter of 22 nm. The particles are core/double-shellsstructured with iron core, iron oxide inner shell and amorphousSi–O–S compound outer shell.

2.2.2. Ex situ methodEx situ methods normally blend pre-synthesized nanostruc-

tures with polymer using specific processings, such as ball milling,melt blending, and thermal curing. For example, superparamagnet-ic nickel ferrite/polypropylene nanocomposites have been pre-pared using a ball milling process by incorporating thesynthesized nickel ferrite NPs [75]. Melt blending is a typical pro-cess that always combined with extrusion, which is quite useful inprocessing magnetic PNCs [76]. Thermal curing is a common andsimple method for thermosetting polymers, e.g., epoxy. In thermo-setting polymers, the NPs should be well dispersed in the mono-mer first and then a well programmed thermal processing forcuring was applied. A typically curing process is illustrated inFig. 3. Compared to in situ method, nanoparticle agglomeration,the major challenge of preparing high performance PNCs, is rela-tively difficult to handle with due to the high surface energy of NPs.

2.3. Applications of magnetic nanocomposites

Magnetic nanocomposites have attracted wide interest for theirdiverse potential applications such as energy storage devices [77],electrochromic devices [78], electronics [79,80], microwaveabsorbers [81,82] and sensors [83]. To be specific, Fe3O4/grapheneoxide magnetic nanocomposites have been developed for colori-metric detection of glucose. The composites were demonstratedto possess intrinsic peroxidase-like activity and enhanced affinitytoward H2O2 [84]. The 3d transition metals Fe, Co, Ni acquire cat-alytic activities, therefore, their corresponding nanocompositesexhibited strong catalytic activity in hydrogen generation [85]and biocatalysis [86]. Recently, magnetic NPs have been incorpo-rated into collagen which exhibited a significantly enhanced oilabsorption capacity. This nanocomposite exhibited selective oilabsorption and magnetic tracking ability, allowing it to be usedin oil removal applications [87].

3. Magnetic nanocomposites for heavy metal adsorption

Heavy metal ionic pollution in drinking water has been a seri-ous issue for a long time. Researchers never stop their efforts tosearch for and develop novel adsorbents which have large capacity

Fig. 1. (a) TEM images of the PNCs with particle loading of 8 wt% and (b) SAED pattern owell with the (104), (113), (116) and (214) diffraction planes of the Fe2O3 [72,73]. (Re

of metal ions adsorption. Both material design and adsorptionkinetics have been systematically investigated. Hierarchicallystructured metal oxides have two or more levels of structure,which provide a high specific surface area, a high surface-to-bulkratio, and surface functional groups that can interact with, e.g.,heavy metal ions for higher adsorption capacitance of 5.3 mg/gfor As(III) and 5.4 mg/g for Cr(VI) [55]. The combination of ironminerals and carbon materials shows an enhanced removal perfor-mance than each individual component. For example, magnetite/graphene oxide composite was synthesized via a chemical reactionwith a magnetite particle size of 10–15 nm, and was developed forthe removal of cobalt(II) ions from aqueous solutions with a max-imum capacity of 22.70 mg/g at 343 K [88]. Other materials, forexample, polymer encapsuled magnetic NPs [52], iron oxide coatedsand [89], and Fe(II)–Fe(III) hydroxysalt green rusts [41] showgreat potential to be used as promising adsorbents. In followingsection, two typical adsorbents, carbon coated magnetic NPs andmagnetic NPs decorated graphene nanocomposites have been sys-tematically studied for Cr(VI) removal from waste water.

3.1. Graphene-based magnetic adsorbents

The magnetic graphene nanocomposites (MGNCs) were used toremove Cr(VI) and pure graphene was also studied for comparison.Fig. 4a and b shows the UV–vis absorption of the 1000 lg L�1

Cr(VI) solution and the solutions after treatment with grapheneand MGNCs under ultrasonication for 5 min. The quantification ofCr(VI) in the solution was using the colorimetric method with acharacteristic peak at 540 nm in the UV–vis absorption curve.The higher the Cr(VI) concentration in the solution, the strongerpeak intensity was observed. The solution with Cr(VI) concentra-tion of 1000 lg/L showed the strongest absorbance of 1.04 aftersubtracting the absorbance at 540 nm (0.12) of the base solutioncomprising the same volume of DI water, phosphoric acid and1,5-diphenyl carbazide (DPC). After treating with different concen-trations of graphene and MGNCs (0�3 g L�1) under ultrasonicationfor 5 min, the peak intensity decreased gradually with increasingthe adsorbent concentration indicating the reduced Cr(VI) amountin the solution. To evaluate the efficiency of MGNCs for Cr(VI) re-moval, removal percentage (RP%) is introduced as a criterion whichcan be calculated using the following equation:

RP ¼ C0 � Cr

C0� 100% ð1Þ

where Co is the initial Cr(VI) concentration of the solution and Cr

represents the remaining Cr(VI) existing in solution after theadsorption process. The Cr(VI) removal percentage treated by differ-ent concentrations of graphene and MGNCs is shown in Fig. 4c.

The pure graphene shows much lower removal efficiency thanthat of the MGNCs, only 44.6% of the Cr(VI) is removed from the

f the NPs. The plane distances of 2.60, 2.20, 1.70 and 1.39 Å are observed, which fitprinted from Ref. [69] with permission from The American Chemical Society.)

Fig. 2. TEM images of the magnetic nanocomposites with a nanoparticle loading of 10 wt%. (a) The NPs are uniformly distributed on the graphene sheet (inset column figureshows the particle size distribution with an average size of 22 nm), (b) enlarged magnification of image (a) shows core@shell structure of the NPs independent of the particlesize, top inset shows the core@double-shell structure of one nanoparticle, (c) HRTEM of a single particle showing an iron core surrounded by a double shell structure, wherethe top-left inset gives a closer view on the lattice fringes of inner shell showing an interlayer distance of 2.52 Å corresponding to the (311) plane of iron oxide, and thebottom right inset gives a closer view on the lattice fringes of the core showing an interlayer distance of 2.03 Å corresponding to (110) plane of iron. The characteristic inter-laminar spacing of graphite 3.40 Å is marked at the upright corner, and (d) selected area electron diffraction (SAED) pattern of the core@shell NPs. (Reprinted from Ref. [74]with permission from The American Chemical Society.)

Fig. 3. Schematic curing process on nanoparticle surface. (Reprinted from Ref. [80] with permission from The American Chemical Society.)

462 J. Zhu et al. / Advanced Powder Technology 24 (2013) 459–467

solution even with the highest testing concentration of 3 g L�1.However, the MGNCs exhibit a much higher removal efficiencythat 52.6% of the Cr(VI) is removed with a much lower MGNCs con-centration of 0.25 g L�1. The Cr(VI) removal is increased by about100% in the MGNCs decorated with the 10 wt% nanoparticle load-ing. It is surprising to observe that the Cr(VI) can be completely

removed while using 3 g L�1 MGNCs within 5 min (the peak at540 nm has been flattened).

The kinetics of the adsorption that describes the Cr(VI) uptakerate is one of the important characteristics which controls the res-idence time of adsorbate uptake at the solid-solution interface.Hence, in the present study, the kinetics of Cr(VI) removal was

Fig. 4. UV–vis absorption of the solution after treated with (a) different loadings ofgraphene (Gra), (b) different loadings of MGNCs and (c) Cr(VI) removal percentagebased on different loadings of Gra and MGNCs. ([Cr(VI)] = 1000 lg L�1, pH = 7,treatment time: 5 min). (Reprinted from Ref. [74] with permission from TheAmerican Chemical Society.)

Fig. 5. Kinetic absorption data plots of Cr(VI) by MGNCs: Cr(VI) removal rate Qt vs.time t (square line) and the transformed rate plot t/Qt vs. t (triangle line).([MGNCs] = 1 g L�1, [Cr(VI)] = 1000 lg L�1, pH = 7). (Reprinted from Ref. [74] withpermission from The American Chemical Society.)

J. Zhu et al. / Advanced Powder Technology 24 (2013) 459–467 463

carried out to understand the adsorption behavior of the preparedMGNCs. Fig. 5 shows the adsorption data of Cr(VI) over MGNCs atdifferent time intervals. Quantifying the changes in adsorption

with time requires an appropriate kinetic model and pseudo-first-order [90], pseudo-second-order [91], Elovich [92,93] andintraparticle diffusion [94] kinetic models are investigated andcompared. To evaluate the suitability of different models, the cor-relation coefficient (R2, close or equal to 1) is introduced. The high-er R2 value indicates a more applicable model to the kinetics ofCr(VI) adsorption.

With the highest correlation coefficient of R2 = 0.999 (fittingcurve is shown in Fig. 5, square symbol curve), pseudo-second-or-der model provides an excellent correlation for the adsorption ofCr(VI) on MGNCs. The correlation coefficients for the pseudo-first-order, Elovich and intraparticle diffusion models are 0.898,0.881 and 0.672, respectively, indicating that these models arenot suitable for describing the Cr(VI) adsorption on MGNCs. Thehigher adsorption rate constant kad (0.28 g mg�1 min�1) of MGNCsfrom pseudo-second-order model than that of aluminum magne-sium mixed hydroxide (<0.024 g mg�1 min�1) [95], pomegranatehusk carbon (<0.032 g mg�1 min�1) [96] and active carbon(<0.093 g mg�1 min�1) [97] indicates the much faster removal rateof the MGNCs.

Solution pH is one of the most important variables affecting theadsorption characteristics. The Cr(VI) removal efficiency by MGNCsin different pH solutions is shown in Fig. 6 with an initial Cr(VI)concentration of 1000 lg L�1 and MGNCs concentration of1 g L�1. At a fixed adsorbent concentration, the complete Cr(VI) re-moval was achieved under acidic condition when the pH is be-tween 1 and 3 rather than in neutral and basic conditions. It isworthwhile to mention that a complete removal can be achievedfrom a less concentrated MGNCs solution, such as 0.5 g L�1 andeven 0.25 g L�1 when the solution pH is controlled at 1, 2 and 3.Therefore, the MGNCs exhibit much higher adsorption capacity atlower pH value solutions. The most important Cr(VI) ion forms insolution are chromate (CrO2�

4 ), dichromate(Cr2O2�7 ) and hydrogen

chromate(HCrO�4 ) and these ion forms are related to the solutionpH and total chromate concentration [95,98]. The predominancediagram [95] of the chromium species based on the thermody-namic database [99,100] using both pH and total Cr(VI) as variablesindicates that the major species of Cr(VI) are CrO2�

4 and HCrO�4 . ForpH lower than 6.8, HCrO�4 is the dominant species and above 6.8only CrO2�

4 is stable. The results show that MGNCs are favorablefor the adsorption of HCrO�4 rather than CrO2�

4 .

Fig. 6. The effect of solution pH on Cr(VI) removal efficiency of MGNCs.([MGNCs] = 1 g L�1, [Cr(VI)] = 1000 lg L�1, treating time: 5 min). (Reprinted fromRef. [74] with permission from The American Chemical Society.)

Fig. 8. Kinetic adsorption data plots of Cr(VI) by SP20: Cr(VI) removal rate qt vs.time t (solid square) and the transformed rate plot t/qt vs. t (open square).[SP20] = 1 g L�1, [Cr(VI)] = 1.5 mg L�1, pH = 7. (Reproduced from Ref. [101] bypermission of The Royal Society of Chemistry.)

464 J. Zhu et al. / Advanced Powder Technology 24 (2013) 459–467

3.2. Carbon-shell magnetic adsorbent

Iron minerals have been demonstrated to be an effective adsor-bent to remove hazardous materials from wastewater [102]. Theeffect of initial Cr(VI) concentration on the removal efficiency ofCr(VI) at a solution pH (initial) of 7.0 using SP20 (SP20 is Fe2O3/Ccore-shell structure nanocomposites, the diameter of Fe2O3 isabout 19.2 nm and shell thickness is about 2 nm) is shown inFig. 7a. The SP20 concentration was maintained at a constant valueof 1 g L�1 and a short contact time of 10 min was employed in thisstudy. A maximum RP of about 99% was observed for an initialCr(VI) concentration of 0.4 mg L�1. Thereafter, the RP decreasedwith the increase of initial Cr(VI) concentration, Fig. 7a. This is be-cause the SP20 sites would eventually become saturated with ad-sorbed Cr(VI), at which point further addition of Cr(VI) to thesolution hardly increases the amount of adsorbed Cr(VI) signifi-cantly. Keeping the highest Cr(VI) concentration of 1.5 mg L�1 withthe same contact time of 10 min, different loading of SP20 is ap-plied to investigate the Cr(VI) RP, Fig. 7b. It is observed that theRP increases almost linearly with the increase of SP20 loadingdue to the increased adsorption sites with larger amount of SP20

Fig. 7. Cr(VI) removal percentage (a) from Cr(VI) solution of different concentrations [SP2time: 10 min [101]. SP20 is Fe2O3/C core-shell structure nanocomposites, the diameter o[101] by permission of The Royal Society of Chemistry.)

NPs. The maximum RP of 96.3% could be achieved at the SP20 load-ing of 2.5 g L�1.

Fig. 8 shows the adsorption data of Cr(VI) over SP20 at differenttime intervals. Quantifying the changes in adsorption with time re-quires an appropriate kinetic model, and pseudo-first-order [90],pseudo-second-order [91], Elovich [92,93] and intraparticle diffu-sion [94] kinetic models are investigated and compared.

The kinetic fitting results obtained from different models aresummarized in Table 1. With the highest correlation coefficientof R2 = 0.992 (fitting curve is shown in Fig. 8, open square curve),pseudo-second-order model provides an excellent correlation forthe adsorption of Cr(VI) on SP20. The R2 for the pseudo-first-order,Elovich and intraparticle diffusion models are 0.953, 0.976 and0.936, respectively, indicating that these models are less suitablethan that of the pseudo-second-order model for describing theCr(VI) adsorption on SP20.

The Cr(VI) removal efficiency by SP20 in different pH solutionsis shown in Fig. 9 with an initial Cr(VI) concentration of 1.5 mg L�1

and SP20 concentration of 1 g L�1. At a fixed adsorbent concentra-tion, the complete Cr(VI) removal was achieved under acidic con-dition when the pH is between 1 and 3 rather than in neutral

0] = 1 g L�1; (b) with different SP20 concentrations, [Cr(VI)] = 1.5 mg L�1. Adsorptionf Fe2O3 is about 19.2 nm and shell thickness is about 2 nm. (Reproduced from Ref.

Table 1The parameters obtained from different kinetic models. (Reproduced from Ref. [101] by permission of The Royal Society of Chemistry.)

Models Equation Parameters R2

Pseudo-first-order [90] logðqe � qtÞ ¼ log qe � k12:303 t k1 (min�1) qe (mg g�1) 0.953

0.089 1.342Pseudo-second-order [91] t

qt¼ 1

kadq2eþ t

qekad (g mg�1 min�1) qe (mg g�1) h (mg g�1 min�1) 0.9920.099 1.519 0.229

Elovich [92,93] qt ¼ 1b lnðabÞ þ 1

b lnðtÞ a (mg g�1 min�1) b (g mg�1) 0.9761.102 4.338

Intraparticle diffusion [94] qt ¼ kdif t0:5 þ C kdif (mg g�1 min�0.5) C (mg g�1) 0.936

0.073 0.711

⁄qt is the solid-phase loading of Cr(VI) in the adsorbent at time t, qe is the adsorption capacity at equilibrium, k1 is the rate constant of pseudo-first-order adsorption. Inpseudo-second-order model, kad is the rate constant of adsorption and h is the initial adsorption rate at t approaching zero, h = kadqe

2. a and b represent the initial adsorptionrate and desorption constant in Elovich model. Kdif indicates the intraparticle diffusion rate constant and C provides information about the thickness of the boundary layer.

Fig. 9. The effect of solution pH on Cr(VI) removal efficiency of SP20.[SP20] = 1 g L�1, [Cr(VI)] = 1.5 mg L�1, treating time: 10 min. (Reproduced fromRef. [101] by permission of The Royal Society of Chemistry.)

J. Zhu et al. / Advanced Powder Technology 24 (2013) 459–467 465

and basic conditions. With increasing solution pH to 5 and evenhigher, the RP decreases significantly. Within the solution ofpH = 11, only 5% RP is observed. It is worthwhile to mention thatthe SP20 is relatively difficult to separate from the liquid suspen-sion by centrifuge after adsorption in solutions with pH beingequal to 1 and 2, which is probably attributed to the etching effectof protons in acidic solution on iron oxide core. The dependence ofthe Cr(VI) removal on solution pH can be explained from the per-spective of surface chemistry at the interface. The surface of metaloxides is generally covered with hydroxyl groups, which vary inform at different pH levels. With an increase in pH, the uptake ofCr(VI) ions decreased, which is due to the higher concentrationof OH� ions present in the mixture that compete with Cr(VI)species.

4. Conclusions and outlook

In this review, the recent progress of the synthetic methods formagnetic nanocomposites manufacturing have been introducedand their environmental remediation applications have been high-lighted. Magnetic nanocomposites could be prepared from bothin situ and ex situ methods, while the particle distribution is easierto manipulate from the in situ method. The preparations of twokinds of magnetic nanocomposites, e.g., nanoparticles coated by acarbon shell and supported on carbon matrix (graphene), havebeen detailed in this review. And their Cr (IV) removal

performances from waste water were systematically studied forexploring the next generation of novel adsorbents. The revelentkinetics deals with the adsorption rate and maximum capacity ofthe newly developed adsorbents. At present, a variety of magneticnanocomposites has been developed to deal with the heavy metalions in waste water. However, there are still a large gap betweentheoretical studies and practical applications. Magnetic nanocom-posites show great potential to be used in such applications, notonly because they obtain large adsorption capacity, but also theconvenient recycling using a permanent magnet is definitely aneconomic and energetic process. To better use magnetic nanocom-posites in environmental remediation applications, further studiesare still required including: (1) developing facile and environmen-tally benign methods to synthesize magnetic nanocomposites withlarge saturated magnetization and high specific surface area; (2)designing anti-corrosive coating materials and structures on thesurface of magnetic nanostructures that prevent proton perme-ation to magnetic core. Meanwhile, the coating layer should beable to provide abundant adsorption sites for heavy metal ions;(3) efficient process design for heavy metal ions adsorption andthe consequent separation of adsorbents from liquid, as well asthe desorption and regeneration of adsorbents. All efforts come to-gether, the final heavy metal ion concentration should be well be-low the EPA regulation or even a complete removal. Overall, thepossibilities of magnetic nanocomposites including conductivepolymer based nanocomposites [103] serve as novel adsorbentsused in environmental remediation will be substantially larger inthe future.

Acknowledgements

This work is supported by NSF – National Science Foundation-Chemical and Biological Separation under the EAGER program(CBET 11-37441). We acknowledge the support from the NationalScience Foundation-Nanoscale Interdisciplinary Research Teamand Materials Processing and Manufacturing (CMMI 10-30755) toobtain the DSC and TGA.

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