Polymer-supported nanocomposites for environmental application: A review

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Chemical Engineering Journal 170 (2011) 381–394

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Chemical Engineering Journal

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olymer-supported nanocomposites for environmental application: A review

in Zhaoa,b, Lu Lva,b, Bingcai Pana,b,∗, Weiming Zhanga,b, Shujuan Zhanga, Quanxing Zhanga,b

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR ChinaNational Engineering Center for Organic Pollution Control and Resource Reuse (Suzhou Division), Suzhou High-Tech Institute of Nanjing University, Suzhou 215123, PR China

r t i c l e i n f o

rticle history:eceived 28 November 2010eceived in revised form 17 February 2011ccepted 25 February 2011

eywords:olymer-based nanocompositesnvironmental remediation

a b s t r a c t

Environmental nanotechnology is considered to play a key role in shaping current environmental engi-neering and science. Looking at the nanoscale has stimulated the development and use of novel andcost-effective technologies for catalytic degradation, adsorptive removal and detection of contaminantsas well as other environmental concerns. Polymer-based nanocomposites (PNCs), which incorporateadvantages of both nanoparticles and polymers, have received increasing attention in both academiaand industry. They present outstanding mechanical properties and compatibility owing to their polymermatrix, the unique physical and chemical properties caused by the unusually large surface area to volume

atalytic degradationdsorptionreen chemistry

ratios and high interfacial reactivity of the nanofillers. In addition, the composites provide an effectiveapproach to overcome the bottleneck problems of nanoparticles in practice such as separation and reuse.This article gives an overview of PNCs for environment application. A brief summary of the fabricationmethods of PNCs is provided, and recent advances on the application of PNC materials for treatment

t senpectiv

of contaminants, pollutanresearch trends and pros

. Introduction

Nanotechnology refers broadly to manipulating matter at thetomic or molecular scale and using materials and structures withanosized dimension, usually ranging from 1 to 100 nm. Due toheir nanoscale size, nanoparticles show unique physical and chem-cal properties such as large surface area to volume ratios or highnterfacial reactivity. Till now increasing nanoparticles have beenemonstrated to exhibit specific interaction with contaminants

n waters, gases, and even soils, and such properties give hopeor exciting novel and improved environmental technology [1–3].owever, the small particle size also brings issues involving mass

ransport and excessive pressure drops when applied in fixed bedr any other flow-through systems, as well as certain difficultiesn separation and reuse, and even possible risk to ecosystems anduman health caused by the potential release of nanoparticles

nto the environment. An effective approach to overcoming thebove technical bottlenecks is to fabricate hybrid nanocompositey impregnating or coating the fine particles onto solid particlesf larger size. The widely used host materials for nanocomposite
abrication include carbonaceous materials like granular activatedarbon [4–7], silica [8,9], cellulose [10,11], sands [12–15], and poly-ers [16–25], and polymeric hosts are particularly an attractive
∗ Corresponding author at: State Key Laboratory of Pollution Control and Resourceeuse, School of the Environment, Nanjing University, Nanjing 210046, PR China.

E-mail address: [email protected] (B. Pan).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.02.071

sing and detection and green chemistry are highlighted. In addition, thee in the coming future are briefly discussed.

© 2011 Elsevier B.V. All rights reserved.

option partly because of their controllable pore space and sur-face chemistry as well as their excellent mechanical strength forlong-term use. The resultant polymer-based nanocomposite (PNC)retains the inherent properties of nanoparticles, while the poly-mer support materials provide higher stability, processability andsome interesting improvements caused by the nanoparticle–matrixinteraction. The generally used nanoparticles include zero-valentmetals [26–33], metallic oxides [34–44], biopolymers [45–51], andsingle-enzyme nanoparticles (SENs) [52–58]. These nanoparticlescould be loaded onto porous resins [34–39,43,59–63], cellulose orcarboxymethyl cellulose [11,64–66], chitosan [33,67,68], alginate[37,47,69,70] etc. The choice of the polymeric supports is usuallyguided by their mechanical and thermal behavior. Other proper-ties such as hydrophobic/hydrophilic balance, chemical stability,bio-compatibility, optical and/or electronic properties and chem-ical functionalities (i.e. solvation, wettability, templating effect,etc.) have to be considered to select the organic hosts [16]. Herea review is provided on the recent progress in synthesis and char-acterization of these environmentally benign PNCs as well as theirperformance in environmental remediation, pollutant sensing anddetection, cleaner production and so on. Besides, the forthcomingdevelopment in the field was also briefly discussed.

2. Preparation of polymer nanocomposites

Various techniques have been developed and applied for prepa-ration of PNCs. According to the formation processes of thenanocomposites, they could be generally divided into two cate-gories. Those are direct compounding and in situ synthesis.

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382 X. Zhao et al. / Chemical Engineering Journal 170 (2011) 381–394

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Fig. 1. Schematic model for impregnation of supe

.1. Direct compounding

The direct compounding method has been extensively used toabricate PNCs because of its convenience in operation, compara-ively low cost and suitability for massive production. Nanofillersnd polymer supporters are prepared separately at first, and thenhey are compounded by solution, emulsion, fusion or mechani-al forces [17–23]. Nevertheless, direct compounding of polymersith nanofillers achieved only limited success for most systemsue to the difficulties in deciding the space distribution parameterf nanoparticles in or on the polymer matrix. The nanoparticlessually exhibit high tendency to form larger aggregates duringlending, which greatly diminishes the advantages of their smallimensions. In addition, polymer degradation upon melt com-ounding and phase separation of nanophase from the polymerhase is sometimes severe. These limitations should be consideredhen using the direct compounding method to prepare PNCs. Var-

ous surface treatments to nanoparticles have been adopted in theynthesis procedure, and the compounding conditions such as tem-erature and time, shear force and configuration of the reactor,an also be adjusted to achieve good dispersion of nanoparticlesn polymer matrices [24,25]. Sometimes, appropriate dispersantsr compatibilizers are added to improve the particle dispersionnd/or miscibility and adhesion between the nanoparticles and theatrix [20,71,72]. Yu et al. [71] reported an approach for synthesis

f cellulose/TiO2 nanocomposites in the presence of supercriticalarbon dioxide. It was found that supercritical carbon dioxide influ-nced the interactions between the molecular chains of cellulose,nd the titania particles were facilitated to access and impregnatento the crystalline structure of cellulose fibers through formation
f hydrogen bonds with abundant hydroxyl groups of cellulose,esulting in a great improvement in thermal stability. Fig. 1 depictssimplified process for impregnation of supercritical CO2 and TiO2anoparticles into cellulose.
al CO2 and TiO2 nanoparticles into cellulose [71].

2.2. In situ synthesis

The in situ synthesis approach is widely used to prepare PNCs,and many transition metal sulfide or halide nanoparticles canbe readily preloaded within polymeric phase through the in situsynthesis method. According to different starting materials and fab-rication processes, in situ synthesis can be generally classified asthree types, as illustrated in Fig. 2.

(1) Metal ions are preloaded within polymer matrix to serve asnanoparticle precursors first, where the ions are supposed todistribute uniformly. Then, the precursors are exposed to thecorresponding liquid or gas containing S2−, OH−, or Se2− toin situ synthesize the target nanoparticles [67,73–77]. Tonget al. [73] developed a sol–gel approach to prepare polyimide-TiO2 hybrid films from soluble polyimides and a modifiedtitanium precursor. The rate of the hydrolysis reaction of tita-nium alkoxide can be controlled by using acetic acid as amodifier. FTIR (Fourier Transform Infrared Spectroscopy) andXPS (X-ray Photoelectron Spectroscopy) indicated that TiO2particles were well encapsulated in polyimide matrixes withparticle size smaller than 60 nm.

(2) Another similar approach employs the monomers of the poly-meric hosts and the target nanofillers as the starting materials[78–83]. Usually, the nanoparticles are first dispersed into themonomers or precursors of the polymeric hosts, and the mix-ture is then polymerized under desirable conditions includingaddition of appropriate catalyst. Increasing attention is paidto this method because it allows one to synthesize nanocom-posites with tailored physical properties. A direct and well
dispersion of the nanoparticles into the liquid monomers orprecursors will avoid their agglomeration in the polymer matrixand thereafter improve the interfacial interactions betweenboth phases. Tang et al. [78,79], as an example, synthesized

X. Zhao et al. / Chemical Engineering Journal 170 (2011) 381–394 383

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ig. 2. Illustration of in situ synthesis process of PNCs. (a) Metal ions are preloaded wosts and the target nanofillers as the starting materials; (c) nanoparticles and polonomers of polymers with an initiator in proper solvent

nano-ZnO/poly(methyl methacrylate) (PMMA) composite byin-site emulsion polymerization. To avoid the aggregation inthe polymerization and to ensure their effective encapsulation,nano-ZnO particles were treated with the methacryloxypropy-ltrimethoxysilane (MPTMS) before they were added to thepolymerization system.

3) Also, nanoparticles and polymers could be prepared simulta-neously by blending the precursors of nanoparticles and themonomers of polymers with an initiator in proper solvent[84–88]. For example, Wan et al. [88] prepared UV-curable,transparent acrylic resin/titania nanocomposite films by con-trolled hydrolysis of titanium tetrabutoxide in Span-85/Tween80 reverse micelles and in situ photopolymerization of theacrylic monomers. AFM (Atomic Force Microscope) imagesshowed the inorganic domains (mean size 25.3–28.8 nm) wereuniformly dispersed in the polymeric networks.

.3. Other methods

Other techniques have also been investigated for preparingNCs in recent years, such as template synthesis, phase separa-ion, self-assembly, electro-spinning, etc. The template synthesis89–92], as the name suggests, uses a nanoporous material as aemplate to make nanoscale fillers of solid (a fibril) or hollow (aubule) shape. The most important feature of this method mayie in that nanometer tubules and fibrils of various raw materialsuch as electronically conducting polymers, metals, semiconduc-ors, and carbons can be fabricated. Cepak et al. [92] prepared

semiconductor–conductor tubular nanocomposite in a 60 mmhick alumina template membrane with 200 nm diameter pores.iO2 tubules were synthesized within the pores of the aluminaembrane via the sol–gel process before they were subjected

o thermal treatment. Polypyrrole wires were then grown insidehe semiconductor tubules by using the chemical polymerization

ethod. The conductive polymer enhanced the electrical conduc-ivity of the material, which promised a higher photo-efficiencyf the TiO2–polypyrrole nanocomposites as a photocatalyst. The
hase separation [93,94] consists of dissolution, gelation, andxtraction using different solvents, and freezing or drying resultsn nanoscale porous foams. Self assembly [95–98] is a process in
hich individual, pre-existing components organize themselves

olymer matrix to serve as nanoparticle precursors first; (b) monomers of polymericare prepared simultaneously by blending the precursors of nanoparticles and the

into desired patterns and functions. The well known self-assemblytechnique is the molecule-mediated one [96–98] and commonlyused for construction of various nanocomposite films with desir-able thicknesses. Instead of strong chemical bonds, nanoparticlesare often linked by weak hydrogen bond, van der Waals, and elec-tric/magnetic dipole interactions. Electrospinning has been widelyused to produce non-woven membranes of nanofibers [99–102]. Apolymer solution, such as cellulose acetate, was introduced intothe electric field, and the polymer filaments were formed fromthe solution between two electrodes bearing electrical charges ofopposite polarity. This process depends upon a number of param-eters, including the type of polymers, conformation of polymerchains, viscosity of solution, polarity, surface tension of the sol-vents, electric field strength and the distance between spinneretsand collectors.

3. Environmental applications of PNCs

Numerous PNCs have been applied in abatement of contam-inants from various environmental media like groundwater orindustrial effluents, gases, soils, and the main mechanisms respon-sible for their environmental application are catalytic degradationand adsorption. Also, PNCs are extensively explored for sensing anddetecting pollutants particularly at trace levels, as well as for greenchemistry to minimize the discharge of pollutants into the receivingenvironment.

3.1. Catalytic and redox degradation of contaminants

Nanoparticles have great potential as catalysts and redoxactive media due to their large specific surface area, highreactivity and shape-dependent optical, electronic and catalyticproperty, which have attracted many researchers to design highlyefficient photo/chem-catalytic materials for purification of con-taminated waters and gases. The common catalytic nanoparticlesinclude nanosized semiconductor materials (such as nano-TiO2[103–105], ZnO [106,107], CdS [68,108–110], and WO3 [111,112]),
zero-valence metal (such as Fe0 [26–28,30,31], Cu0 [32,33]and Zn0 [113,114]) and bimetallic nanoparticles (such as Fe/Pd[64,115–118], Fe/Ni [119,120], Fe/Al [121], Zn/Pd [122]). Theyusually serve as catalysts or redox reagents for degradation of


Table 1Several polymer-based nanocomposites (PNCs) for degradation of pollutants.

NPs Polymer matrix Target pollutant Preparation method Comments Ref.

TiO2 Polyhydroxybutyrate (PHB) Methylene blue Solvent-cast 96% of the MB solution wasdecolorized after one-hour solarillumination.

[17]

Polyaniline (PANI) Phenol Polymerization of aniline in thepresence of TiO2 NPs

Phenol aqueous solution wasdegraded after 5 hours undervisible light illumination.

[86]

Poly(dimethylsiloxane) (PDMS) Methylene blue andacetaldehyde

Sol–gel The MB and acetaldehyde weredecomposed when PDMScontent lower than 60% underUV irradiation.

[126]

Poly(3-hexylthiophene) (P3HT) Methyl orange Add TiO2 nanoparticles to theP3HT solutions

The degradation rate of MOreached the maximum of 88.5%in 10 h under visible light, whenthe molar ratio of TiO2/P3HT is75:1.

[127]

Poly(tetrafluoroethylene) Trichlorobenzene Electrophoretic deposition It required about 2 h forcomplete mineralization ofTCB(5 × 10−5M) under UV light,and Pt deposition on TiO2

enhanced the photodegradation

[128]

Poly(fluorene-co-thiophene)(PFT)

Phenol Add PFT solution into the TiO2

suspensionThe optimum ratio of PFT to TiO2was 1:100, and phenoldegradation efficacy reached74.3% after irradiating for 10 hunder visible light.

[129]

Poly(1-naphthylamine)(PNA) Methylene Blue In situ polymerization of1-naphthylamine monomerwith TiO2

MB dye degraded by ∼60% undervisible light illumination.

[185]

Polyethylene(PE) Methylene Blue Hot pressing Decomposition of about 0.3 mgMB was obtained per cm−2 in 1 hat ambient temperature from ansolution of 200 ppm undervisible light

[130]

Polythene and polypropylene Paraquat Hot pressing The optimum photocatalyticactivity with UV lamps andsunlight occurred when TiO2

coverage was about 8.8 × 10−5g/cm2 for polythene and 3.1×10−5g/cm−2 for polypropylene.

[131]

Nafion Victoria Blue R Mixing in solution After 24 h of UV irradiation,about 93.8% of the initial organiccarbon (50 mg/L VBR) had beentransformed into CO2.

[132]

CdS Chitosan Congo Red Simulating bio-mineralization 85.9% of degradation rate wasachieved within 180 min ofirradiation for 20 mg/L CRsolution at natural pH of 6.0 inthe presence of 1.5 g/L compositecatalyst under simulated visiblelight.

[68]

Fe0 Poly(vinyl pyrrolidone) (PVP) Bromate Electrospinning The composite fibers exhibited90% of activity of fresh bare Fe0,and maintained 78% of theactivity after 4 weeks storageunder the dry state.

[26]

PEG/nylon membrane Nitrobenzene In situ synthesis withFeSO4·7H2O as precursor

NB in groundwater wasdecreased by 68.9% in the first20 min, and by 15% from 20 to80 min.

[27]

Poly(methyl methacrylate)(PMMA)

Trichloroethene In situ synthesis with MMAand FeSO4·7H2O as precursors

TCE degradation rate constantwas 0.0034 h−1 anddechlorination efficiency was62.3%, with no significantdecrease after one monthair-expose.

[133]

Carboxymethyl Cellulose Cr(VI) In situ synthesis withFeSO4·7H2O as precursor

Cr(VI) reduction occurred withina broad pH range, and theremoval efficiency was 22% and94% for Fe0 nanoparticles andCMC-Fe0 after 60 min,respectively.

[134]

Alginate Trichloroethylene In situ synthesis of Fe0 inalginate bead from Fe3+

Removal efficiency of TCE was>99.8% within 4 h, the observedrate constants (kobs) was6.11 h−1.

[69]


Table 1 (Continued)


Resin Acid Blue 113 (an azo dye) In situ synthesis of Fe0 on theresin from FeCl3

With an initial dye concentrationof 100 mg/l and nano iron load of50.8 mg/g, the best removalefficiencies were obtained at 100and 12.6% for dye concentrationand total organic carbon,respectively.

[60]

Resin Nitrate In situ synthesis of Fe0 on theresin from FeCl3 resinfunctionalized with-CH2N+(CH3)3 (N-S) and-CH2Cl (Cl-S)

97.2% of nitrate was convertedinto ammonium whenintroducing 0.12 g N-S-ZVI into50 mL 50 mg N/L nitrate solution,while that for Cl-S-ZVI was 79.8%under identical Fe/N molar ratio.

[59]

Cationic exchanger Trichloroethylene In situ synthesis of Fe0 on CEMfrom Fe3+

About 36.2 mg/L of TCE wasremoved within initial 2 h andthe observed rate constant (kobs)and surface area normalized rateconstant (kSA) were 0.44 h−1 and35.77 L h−1m−2, respectively.

[125]

Chitosan Cr(VI) Mixing in solution The removal rate of Cr (VI) was82% over a 30-min period, whichincreased with increasingtemperature and NZVI dosagebut decreased with the increasein initial concentration of Cr (VI)and pH values.

[135]

Carboxymethyl cellulose Trichloroethylene Solvent-cast A significant impact in an 80 mg/lTCE aqueous solution wasobtained, with the extent ofdechlorination increasing whenthe iron loading increased, andthe synergistic effect ofpreconcentration of TCE by theCA was indicated.

[136]

Resin Cr(VI) and Pb(II) In situ synthesis withFeSO4·7H2O as precursor

For 100 mL of 0.50 mM metalsolution and 0.100 g of thecomposite, the initialremediation rates of Cr(VI) andPb(II) were 1.18 and 1.44 h−1,respectively, which were 30times higher than for iron filingsor powder on a (Fe) molar basis,and the reduction of Cr(VI) cameto 21 times greater over2 months.

[31]

Resin Cr(VI), Pb(II), TcO4− In situ synthesis with

FeSO4·7H2O as precursorThe initial rates of remediation,the total moles of contaminantreduced, and the lower initialcorrosion rate all point tosignificantly higher atomefficiency for iron nanoparticles.

[62]

Cu0 Resin Carbon tetrachloride In situ synthesis on thesulfonated polystyrene matrixfrom an aqueousCu(NO3)2·3H2O solution

The pseudo-first-order rateconstant normalized by thesurface-area and the massconcentration of nZVC (kSA) wasabout 0.021 l h−1m−2,approximately twenty times thatof commercial powdered ZVC(0.04 mm).

[32]

Chitosan Cr(VI) In situ synthesis withCu(SO4)2·5H2O as a precursor

After 1 day, the concentration ofCr(VI) solution with 1 gcomposites was decreased from50 mg/l to 2.21 mg/l (initial pH2.85), while when the initialconcentration was 5 mg/l, theremoval efficiency was greaterthan 99%.

[33]

Fe/Pd Sodium carboxymethylcellulose

Para-nitrochlorobenzene In situ synthesis withFeSO4·7H2O and K2PdCl6 asprecursors

At a dose of 0.2 g/L, thecomposites removed 100% ofp-NCB (C0 = 50 mg/L) within40 min, with a pseudo-first orderrate constant 4 times greaterthan that for the non-stabilizediron.

[64]


Table 1 (Continued)


Poly(vinylidene fluoride)(PVDF)

Trichloroacetic acid Reduction-deposition The TCAA was dechlorinatedwithin 180 min using 0.083wt% of Pd content, 0.997 g/L ofmetal loading, and 5 mg/L ofinitial concentration of TCAA,and kSA was 0.069 L min−1 m−2

[116]

Ni/Fe Cellulose acetate Trichloroethylene Solvent-cast The metal mass normalizedobserved reduction rateconstant was proportional tothe Ni content in the range of0–14.3 wt.%, which reached amaximum between 14.3 and21.4 wt.% and decreased withfurther increase in Ni content.

[139]

Pd/Sn Resin Trichloroethylene In situ reduction of Sn2+ to Sn0,and then deposit Pb0 throughreduction of Pb4+

The dechlorination rate wasenhanced by ∼2 orders ofmagnitude compared tocommercial powdered Sn,

[63]

a(g[[wotr[aoflPd

sIaionUwsd[[[babdtsTo

aa[mbdh

nanoparticles degraded trichloroethene (TCE) 17 times faster thantheir non-stabilized counterparts based on the initial pseudo-first-order rate constant.

large variety of environmental contaminants such as PCBspolychlorinated biphenyls) [30,123], aze dyes [68,105,106], halo-enated aliphatics [26–28,64,112,116], organochlorine pesticides33,113,114], halogenated herbicides [124], and nitroaromatics27,110]. However, the use of aqueous suspensions limits theiride applications because of the problems concerning separation

f the fine particles and the recycling of the catalyst. Immobiliza-ion of these nanoparticles onto polymer matrix such as porousesins [32,59–63], ion exchangers [125], and polymeric membranes26,27,126] has been available to solve the problems to consider-ble extent, serving for the reduction of particle loss, preventionf particles agglomeration, and potential application of convectiveow occurring by freestanding particles. Table 1 summarizes someNCs reported recently for catalytic removal of pollutants fromifferent environmental media.

As for the widely used catalyst nano-TiO2, they have been exten-ively studied for degradation of organic pollutants [103–105].llumination promotes an electron to the conduction band, leavinghole in the valence band. This process produces a potential reduc-

ng and oxidizing agent. Because of the high photocatalytic activityf titanium dioxide nanoparticles, the polymer substrates of theanocomposite catalysts are expected to be antioxidative underV or visible light illumination. Reported polymeric substratesere usually saturated carbon chain polymers or fluoropolymers,

uch as poly(dimethylsiloxane) (PDMS) [126], polyvinylpyrroli-one (PVP) [26], polyethylene (PE) [130], polypropylene (PP)131], poly(3-hexylthiophene) (P3HT) [127], polyaniline (PANI)86], poly(tetrafluoroethylene) [128], and Nafion [132]. Ameen et al.185] prepared poly 1-naphthylamine (PNA)/TiO2 nanocompositey in situ polymerization and observed an enhanced photocatalyticctivity for the degradation of methylene blue (MB) dye under visi-le light illumination. The high photodegrdation efficacy of the MBye may be attributed to the efficient charge separation of the elec-rons (e−) and hole (h+) pairs at the interfaces of PNA and TiO2, asuggested related to the slightly high red shift in UV–vis results.he schematic illustration of MB dye degradation over the surfacef PNA/TiO2 nanocomposites catalyst is shown in (Fig. 3).

Laboratory research has established that some nanoscale met-ls and bimetals such as Fe0, Cu0, Zn0, Fe/Pd, Fe/Ni, Pd/Zn, etc.re very effective in destroying various organic contaminants26–33,113–122] such as chlorinated methanes, brominated-

ethanes, trihalomethanes, chlorinated ethenes, chlorinatedenzenes, other polychlorinated hydrocarbons, pesticides andyes. The reactivity of these metal nanoparticles is usually veryigh, for example, nZVI can even self-ignite when exposed to air.

which was negatively affectedby groundwater constituentssuch as DO, nitrate and sulfide.

Thus, supporting the particles is requisite to preserve their chemicalnature by inhibiting oxidation until they can react with the targetedpollutants. Lin et al. [32] ever synthesized nanoscale zero valentcopper (nZVC) supported on a cation exchange resin to enhancethe removal of carbon tetrachloride (CCl4) from water (Fig. 4). Theuse of the cation exchange resin as a support effectively preventedthe reduction of surface area due to the agglomeration of nZVCparticles. Moreover, the cation exchange resin recycled the cop-per ions produced from the reaction between CCl4 and Cu0 bysimultaneous ion exchange. The decline in the amount of CCl4 inaqueous solution resulted from the combined effects of degradationby nZVC and sorption by the host resin. The pseudo-first-order rateconstant normalized by the surface-area and the mass concentra-tion of nZVC was approximately twenty times that of commercialpowdered ZVC. He et al. [64] reported a composite of parallelizediron (Fe/Pd) nanoparticles with sodium carboxymethyl cellulose(CMC) as a stabilizer. Compared to pristine Fe/Pd particles, theCMC-stabilized nanoparticles displayed markedly improved sta-bility against aggregation, chemical reactivity, and soil transport.Batch dechlorination tests demonstrated that the CMC-stabilized

Fig. 3. Schematic illustration of photocatalytic activity of PNA/TiO2 nanocomposite[185]. MB photodegrdation on the surface of PNA/TiO2 nanocomposite catalyst isoriginated from the energy level of PNA (�-orbital and �*-orbital) and TiO2 (con-duction band, CB, and valence band, VB) under visible light illumination.


Fg

3

neettatiire

ailNafA[TTwica

devmnteo

solution for repeated use without any noticeable capacity loss.

ig. 4. Application of Nanoscale zerovalent copper coated resin mesh in an above-round reactor and the associated reactions [32].

.1.1. Biocatalytic nanocompositesBiocatalysts such as enzymes or catalytic antibodies are natural

anoreactors regulated by biochemical factors in the micro-nvironment of living cells. Their high specificity and targetedffectiveness as well as environmental benignancy render themo be a more attractive choice than synthetic catalysts for pollu-ant degradation. However, the relatively short lifetime of enzymesnd their sensibility greatly dependent upon environmental condi-ions (pH, temperature, mechanical stress, etc.) tends to limit theirndustrial application. Improvement in enzyme stability becamemportant for practical applications because it can subsequentlyeduce the amount of enzyme required, prolong the lifetime ofnzymes, and increase the potential for the reuse of enzyme.

Researchers have experimented with various methods suchs immobilization, modification, and genetic modification for themprovement in enzyme stability, in which enzyme immobi-ization appeared to be one of the most successful approaches.atural and synthetic polymers [45–51] (polysaccharides, poly-crylamides, alginates, resins, chitosan, etc.) have been usedor bio-immobilization via loading or entrapment. For example,mberlite MB 150 and chitosan beads were used by Tripathi et al.

51] to immobilize �-Amylase from mung beans (Vigna radiata).he performance of free and immobilized enzymes was compared.he activity loss for free amylase after 100 days of storage at 4 ◦Cas ∼70%, whereas for Amberlite- and chitosan-based amylases,

t were 45% and 55% respectively under the identical experimentalonditions. Moreover, polymer-based amylase showed a residualctivity of 43% and 27% respectively after 10 uses.

Enzyme entrapment protects enzymes by preventing theirirect contact with the environment, thereby minimizing theffects of gas bubbles, mechanical sheer and hydrophobic sol-ents. Nevertheless, there still exist some drawbacks such asass transfer limitations and low enzyme loading. Recently, a

ew method of enzyme stabilization has been reported under
he name single-enzyme nanoparticles (SENs) [52–58], in whichach enzyme molecule was surrounded with a porous compositerganic/inorganic network of less than a few nanometers in thick-
Fig. 5. Diagram of a typical modification process for creating SENs.

ness (as illustrated in Fig. 5). Two tested proteases (�-chymotrypsinand trypsin) [52] were significantly stabilized and kinetic studieswith the SENs of chymotrypsin revealed that there was no seriousmass transfer limitation on the substrate.

3.2. Adsorption of pollutants

Adsorption techniques are widely used in water treatmentand gas purification as one of the most effective and simplestapproaches to removing toxic and recalcitrant pollutants. Manyenvironmentally benign inorganic particles, namely, metal (hydr)oxides (e.g., Fe(III) [140–142], Mn(IV) [142–144]) and M(HPO4)2(M= Zr, Ti, Sn) [145,146]) have been exploited as efficient adsor-bents for enhanced removal of targeted pollutants. When theparticle size gets into nanoscale, they appeared to be more efficientbecause of the extremely large surface area and high reactiv-ity. Till now increasing nanocomposite adsorbents were designedby impregnating the inorganic nanoparticles onto the conven-tional polymers, namely, alginate [37,38], cellulose [11,66], porousresins [43] and ion-exchangers [34–36,147–150], to avoid issuescaused by the ultra-fine particle size such as transition loss andexcessive pressure drops. Porous polymeric adsorbents or ionexchangers have proved to be ideal alternatives to fabricate simi-lar hybrid adsorbents when considering their excellent mechanicalstrength and adjustable surface chemistry of the polymeric sup-ports [34,147]. The immobilized charged functional groups boundto the polymeric matrix are believed to enhance permeation of inor-ganic pollutants of counter charges, which can be interpreted byDonnan membrane principle [35,36]. Table 2 summarizes severaltypical PNCs for adsorptive removal of contaminants mainly fromwaters.

A new hybrid adsorbent HMO-001, which was fabricated in ourlaboratory by impregnating nanosized hydrous manganese dioxide(HMO) onto a porous polystyrene cation exchanger resin (D-001),provided a nice example [149]. Basic structure and morphology ofHMO-001 is depicted in Fig. 6. Lead adsorption onto HMO-001 wastested and the maximum capacity of HMO-001 toward lead ion wasabout 395 mg/g. As compared to D-001, HMO-001 exhibited highlyselective lead retention from waters in the presence of competingCa2+, Mg2+, and Na+ at much greater levels than the target toxicmetal. Fixed-bed column adsorption of a simulated water indicatedthat lead retention on HMO-001 resulted in a conspicuous decreaseof this toxic metal from 1 mg/l to below 0.01 mg/l (the drinkingwater standard recommended by WHO). The exhausted adsorbentparticles were amenable to regeneration by the binary NaAc–HAc

Another typical example is hydrated ferric oxides, which canselectively bind anionic ligands (e.g., arsenate, phosphate) throughinner-sphere complex formation [154–156]. They are also envi-


Table 2Polymer-based nanocomposites (PNCs) for adsorption of pollutants.


Iron oxyhydroxide Cellulose As(III), As(V) In situ synthesis with FeCl3·6H2Oas precursor

Adsorption capacities are 99.6 mgfor As(III) and 33.2 mg for As(V) perg of adsorbent

[11]

Iron oxides Alginate Arsenic Dispersion of alginate beads inHFO solution

Arsenic removed from 50 to <10ppb within 230 BV for As(V), 45 BVfor As(III)

[37]

Fe(III) oxides Cation exchangeadsorbents

Se(IV); As(V) Contact the resin with a solutionof iron slats

Se(IV) removed from 100 to<0.5 ppm, As(V) removed from 100to <0.5 ppm

[151]

Hydrated ferricoxide

Polymeric anionexchangers

Phosphate Oxidizing anion - ferrous sulfatesolution on resin

P(V) removed from 100 to <5 ppbwithin 10,000 BV

[34]


Arsenic Precipitation of iron(III)hydroxides from fecl3

As(V) removed from 50 to <10 ppbwithin 4000 BV, As(III) removedfrom 100 to <10 ppb within 2000BV

[35]


Arsenic Precipitation of iron(III)hydroxides from FeCl3

As(V) removed from 23 to <0.5 ppbwithin 33196 BV

[39]

Polyacrylamide Pb(II), Hg(II), Cd(II) Precipitation of iron(III)hydroxides from FeCl3

Adsorption capacities are 211.4 mgfor Pb(II), 155.0 mg for Hg(II),147.2 mg for Cd(II) per g ofadsorbent

[40]



As(V) removed from 50 to <10 ppbwithin 4000 BV, As(III) removedfrom 100 to <10 ppb within 2000BV

[41]



As(V) removed from 300 to <10ppb within 3500 BV, As(V)removed from 20 to <10 ppbwithin 17,500 BV

[42]

Polystyrene adsorbents Arsenic Precipitation of iron(III)hydroxides from FeCl3

Arsenic removed from 100 to <10ppb within 60 BV

[43]

Fibrous polymeric ionexchangers

As(III), As(V) Precipitation of iron(III)hydroxides from FeCl3

Arsenic removed from 60 to <10ppb within 10000 BV

[44]

Polymeric cationexchanger

Pb(II), Cu(II), Cd(II) Precipitation of iron(III)hydroxides from FeCl3

Metal ions removal from 1 ppm to<5 ppb within 7000 BV

[152]

Hydrousmanganese oxide(HMO)

polymeric cationexchanger

Pb(II), Cd(II), Zn(II) Oxidation of the pre-loadedMn(II)

Kd increased by 20–800 times ascompared to host exchangers,sorption capacities increased by50–300%

[153]

Zr(HPO3S)2 polymeric cationexchanger

Pb(II), Cd(II), Zn(II) In situ synthesis of Zr(HPO3S)2

on the resin with ZrOCl2 andNa3PO3S

Pb(II) removed from 50–130 to <10ppb within 50,000 BV, Cd(II)removed from 80–140 to <3 ppbwithin 9000 BV

[148]

Zirconiumphosphate

polymeric cationexchanger

Pb(II) In situ synthesis of ZrP on theresin with ZrOCl2 and H3PO4

Pb(II) removed from 40 to<0.05 mg/L within 2000 BV

[147]

Fe3O4 alginate methylene blue, methyl orange Mixing About 50% of dye are adsorbedafter 10 min for MB (C0 = 1 and5 mmol/l) and 17 min for MO

[160]

M

rdshscheidscnC

3

nu

cyclodextrin Cu(II)

onmentally benign and cost effective. Like HMO, they cannot beirectly employed in fixed-bed columns or any other flow-throughystems due to the fine or ultrafine particles. Polymer-supportedydrated ferric oxides (D201-HFO) were then developed by using atrongly basic anion exchanger D201 as the host material to over-ome its inherent defect [36,150]. D201-HFO was found to exhibitigher capacity for arsenic or phosphate removal than the hostxchanger and a commercial sorbent Purolite ArsenX. Furthermore,t also presented fast kinetics, which was particularly crucial foreep removal of trace pollutants. Fixed-bed column experimentshowed that arsenic sorption on D201-HFO could result in a con-entration of this toxic metalloid element lower than 10 �g/l, theew maximum concentration limit set recently by the Europeanommission and imposed by US EPA and China.

.2.1. Magnetic nanocompositesMagnetic nanoparticles offer advantages over non-magnetic

anoparticles because they can be easily separated from waternder a magnetic field. Separation using magnetic gradients, the

(C0 = 1 mmol/l)ixing Adsorption capacity for Cu(II) is

47.2 mg per g of adsorbent at 25 ◦C[158]

so-called high magnetic gradient separation (HGMS), is a pro-cess widely used in medicine and ore processing. This techniqueallows one to design processes where the particles not only removecompounds from water but also can be easily removed, recycledor regenerated. Approaches have been proposed with magnetite(Fe3O4), maghemite (Fe2O3) and jacobsite (MnFe2O4) nanoparti-cles loading on or in the polymer matrix such as alginate beads.A series of magnetic alginate polymers were prepared and batchexperiments were conducted to investigate their ability to removeheavy metal ions [70,157–159] (Co(II), Cr(VI), Ni (II), Pb (II), Cu(II), Mn (II), La (III)) and organic dyes [160] (methylene blue andmethyl orange) from aqueous solutions. Magnetic particles in thenanocomposites allowed easy isolation of the beads from the aque-ous solutions after the sorption process.

3.3. Pollutant sensing and detection

Rapid and precise sensors capable of detecting pollutants atmolecular levels could enhance our understanding and protect-


(c) TE

ieatnt

TP

Fig. 6. Scanning electron micrographs of (a) D-001, (b) HMO-001,

ng the environment. Manufacturing, process control, compliance,cosystem monitoring, and environmental decision making wouldlso be significantly improved if more sensitive and less costly
echniques for contaminant detection were available. Actually,anoparticles are receiving growing attention for pollutants detec-ion owing to their specific characteristics of large specific surface
able 3olymer-based nanocomposites (PNCs) for pollutant sensing and detection.

Sensor type NPs Polymer matrix Preparation method

Gas sensor SnO2 Polystyrene/polyaniline(PSS/PANI)

In situ self-assembly

SnO2 Polyaniline (PANI) Hydrothermal method

TiO2 Polyaniline (PANI) Chemical polymerizatioand a sol–gel method

Iron Oxide Polypyrrole Simultaneous gelation apolymerisation

Pd Polyaniline Oxidative polymerizatioof solution with Pd NPs

Chemical sensor Au Chitosan Mix in solution

Humidity sensor TiO2 Polypyrrole In situphotopolymerization

Biosensor Au Chitosan One-step electrodeposit

M of HMO-001, and (d) schematic illustration of HMO-001 [149].

area and good biocompatibility. As a large fraction of the atoms arepresent at the surface in nanosized materials, the surface propertiessuch as the depth of the surface space charge region become pivotal.
When the particle size gets into nanorange, the coating layer takesover the bulk, and it becomes difficult to distinguish surface frombulk conduction [161–163]. These characteristics of nanosized par-
Detection target Comments Ref.

CO Sensitivity is <1 ppm [166]

Ethanol, acetone The response time at 90 ◦C andrecovery time were 23–43 and16–28 s for ethanol, 16–20 and35–48 s for acetone, respectively.

[186]

n Trimethylamine A quartz crystal microbalance(QCM) sensor

[187]

nd CO2, N2, CH4 Sensitivities show linerrelationship with pressures, andthe highest is to CO2 gas.

[188]

n Methanol The sensing mechanism has beenexplained on the basis of FT-IRspectroscopy

[169]

Zn2+, Cu2+ Use UV–visible absorptionspectrum to observe theconcentration levels of the analyte

[189]

Change in humidity The composite showed the highestsensitivity, smaller hysteresis andbest linearity when TiO2:pyrrole is0.0012 g/0.125 g

[190]

ion Glucose The biosensor exhibited a rapidresponse (within 7 s) and a linearcalibration range from 5.0 M to2.4 mM with a detection limit of2.7 M for the detection of glucose.

[191]


ents o

tstImtl[l(tsdSft2apva

3

i

TP

Fig. 7. Major stages of measurem

icle make the materials particularly appealing in applications ofensors [164]. Nevertheless, the drawback of nanoparticles in prac-ical application such as slow diffusion and aggregation still exists.mmobilization of nanoparticles by polymer matrix is one of the

ost efficient approaches to overcome such shortcomings. Sincehe chemical and physical properties of polymers may be tai-ored, they gained importance in the construction of sensor devices165]. Conductive polymer nanomaterials have attracted particu-ar interests as sensors for air-borne volatiles [166–169,186–190]alcohols, NH3, NO2, CO) because of large surface area, adjustableransport properties and chemical specificities, easy processing andcalable productions. Polythiophene based sensor has shown theetection of hydrazine gases at ppb levels [170]. Also, polyaniline-nO2/TiO2 nanocomposite ultra thin films have been fabricatedor CO gas sensing [166]. The range of the biosensor was foundo be 6.9 × 10−14 to 8.6 × 10−13 mol/l and the detection limit is.3 × 10−14 mol/l. A Pd–polyaniline nanocomposite was developeds a selective methanol sensor [169]. The synthesized nanocom-osite sensor showed high selectivity and sensitivity to methanolapors with rapid and reverse response. Some applications of PNCss various sensors are included in Table 3.

.3.1. BiosensorThe total effect of a biosensor is to transform a biological event

nto an electrical signal, as illustrated in Fig. 7. Biosensors have

able 4olymer-based nanocomposites (PNCs) for green chemistry.

Type NPs Polymer matrix

Polymer-basednanocatalysts

Cu0 Poly(N-vinyl-2-pyrrolidone)(PVP)

Au0 Anion exchanger

Ni/Pd Poly(N-vinyl-2-pyrrolidone)

Eco-friendly materials TiO2 Polyhydroxybutyrate (PHB

TiO2 Hyperbranchedpoly(�-caprolactone)(HPCL)

Energy Storage TiO2 Sulfonated poly(etherether ketone) (SPEEK)

Phosphorylated titanate Chitosan

f analytes with a biosensor [165].

found extensive applications in environmental pollution controlfor measuring toxic gases in the atmosphere and toxic solublecompounds in waters. By far the largest group of direct electron-transfer biosensors is based on co-immobilization of the enzyme ina conducting polymer, namely polypyrrole [171] and polyaniline[172,173]. For example, a cholesterol biosensor was fabricated byco-immobilize cholesterol oxidase, cholesterol esterase and per-oxidase onto electrochemically prepared polyaniline films [173].This polyaniline-based cholesterol biosensor has a response timeof about 240 s, an apparent Km value as 75 mg/dl and can be used toestimate cholesterol concentration up to 500 mg/dl. These polyani-line/cholesterol oxidase/cholesterol esterase films have a detectionlimit of 25 mg/dl with sensitivity of 0.042 �A mg/dl. The enzymefilms were found to be thermally stable up to 48 ◦C and have ashelf-life of about 6 weeks when stored at 4 ◦C.

3.4. Green chemistry

Pollution prevention by nanotechnology refers, on one hand, toa reduction in the use of raw materials, water or other resources andthe waste reduction or elimination, and on the other hand, to more
efficient use of energy or involvement in energy production. Greenchemistry is the design of chemical products and processes thatreduce or eliminate the use and generation of hazardous substances[174,175]. Nanomaterials can play a key role in the green chemistry
Preparation method Comments Ref.

In situ synthesis with Cu(II)as precursor

Promoting 1,3-dipolarcycloaddition reactions betweenterminal alkynes and azides tosynthesize 1,2,3-triazoles

[137]

Mixing For the reduction of 4-nirophenolto 4-aminophenol

[138]

Polyol reduction For hydrogenation of nitrobenzeneat 30 ◦C under an atmosphericpressure of hydrogen

[178]

) Solvent-cast A biodegradable and biologicallyrenewable film effective in thesterilization and decolorization oforganic dyes

[17]

Dip-coating An alternative strategy to thecurrent waste landfill and toxicbyproduct-emitting incineration ofPVC wastes

[180,181]

Solvent-cast Proton conductivity exceeded 10−2

S cm−1 at 80 ◦C, and the best DMFCperformance present when TiO2

was 5wt%

[182]

Solvent-cast The incorporation of 30% PTNT-6 hsimultaneously reduced themethanol crossover by 50% andincreased the proton conductivityby 64% in comparison with theplain CS membrane

[183]

eering

peT

mbicbtipaa

attueecnrtwc

amaaescdeNupD

3

eddnccatbrtimarsuria

X. Zhao et al. / Chemical Engin

rocess by minimizing the use of toxic chemicals, solvents, andnergy, and PNCs fall within the scope of such functional materials.able 4 illustrates some typical applications of PNCs in these fields.

For one thing, polymer-based nanocatalysts can make chemicalanufacturing more efficient and more environmentally benign

y providing higher selectivity for desired reaction products, help-ng to eliminate wasteful secondary reactions and reduce energyonsumption [137,138,176–178]. For example, polymer-protectedimetallic alloy nanocluster was widely used as a catalyst to con-rol the activity, selectivity and stability of certain reactions, whichs expected to minimize the consumption of chemical reagents androduction of hazardous substances [178]. Besides, some nanocat-lysts functioning at room temperature open the way for broadpplications of nanomaterials in many consumer products.

For another thing, polymers loaded with nanoparticles couldlso be used as benign alternatives for harmful materials, reducinghe energy requirement and waste generation [17,179–181]. Pho-ocatalytic nanoparticles like titanium dioxide have been widelysed to produce photo-degradable polymers, which helps to reducemission of toxic byproducts during polymer incineration. Kimt al. demonstrated suppression of dioxin emission for poly(vinylhloride) (PVC) incineration when titanium dioxide (TiO2)anocomposites were employed [180]. The GC (Gas Chromatog-aphy) results on the exhaust gases from incineration showed thathe emissions of dioxin and precursors were largely suppressedith the increasing content of TiO2 in PVC/TiO2 nanocomposite, as

ompared with those in the neat PVC without TiO2.In addition, PNCs may also be applied in energy production

nd storage. One example is the utilization of PNCs in directethanol fuel cells (DMFC) [182–184]. With characteristics such

s low working temperature, high energy-conversion efficiencynd low emission of pollutants, DMFCs may help solve the futurenergy crisis. However, an obvious limitation to DMFC is thelow reaction kinetics, which reduces the power output. Catalystsomposed of metal or metal oxide nanoparticles, supported on con-ucting polymers, are adopted to be an effective approach. Rheet al. [184] reported that the membrane electrode assembly usingafion/sulfonated titanate nanocomposite membranes exhibitedp to 57% higher power density than the assembly containing aristine Nafion membrane under typical operating conditions ofMFC.

.5. Conclusions and perspectives

The environmental applications of PNCs are interesting andndless. The widespread use of PNCs in photo/chem-catalysisegradation, adsorption of pollutants, and pollutant sensing andetection result in less pollution and benign products. Till now,umerous PNCs are available for environmental purpose, andurrently fabrication of new PNCs of high efficiency and lowost is still a hot topics. Also, we hope the polymeric hostsre available through cleaner processes instead of environmen-al unfriendly ones. In addition, further insights into the interplayetween the host polymers and the encapsulated NPs are stillequired. For example, how does the polymer chemistry affecthe dispersion and distribution of NPs? What is the effect of themmobilized NPs on the properties of the resultant PNCs such as

echanical performances? Certainly, the interaction between PNCsnd the targeted pollutants or the substrates should be furtherevealed, and modern analytical tools like XAS (X-ray absorptionpectroscopy) and AFM may help us to deeply understand the
nderlying mechanism at microscale levels. Besides, most of theecent achievements are still based on laboratory-level tests. Manyssues may need to be solved in the mass production and fieldpplication.
Journal 170 (2011) 381–394 391

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