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REVIEWARTICLE

Applications of nanomaterials in water treatment andenvironmental remediation

Gholamreza GHASEMZADEH (✉)1, Mahdiye MOMENPOUR2, Fakhriye OMIDI3,

Mohammad R. HOSSEINI4, Monireh AHANI5, Abolfazl BARZEGARI (✉)6

1 Department of Agriculture, Payame Noor University, Tehran 19569, Iran2 Department of Environmental Biodiversity, Lahijan Branch, Islamic Azad University, Lahijan 4491874551, Iran

3 Department of Fisheries, Agricultural Science & Natural Resources University, Gorgan 1439955471, Iran4 Department of Environmental Science, University of Pune, Pune 411007, India

5 Department of Agriculture, Takestan Branch, Islamic Azad University, Takestan 18610307, Iran6 Research Center for Pharmaceutical Nanotechnology, Tabriz University of Medical Sciences, Tabriz 5165665811, Iran

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2014

Abstract Nanotechnology has revolutionized plethora ofscientific and technological fields; environmental safety isno exception. One of the most promising and well-developed environmental applications of nanotechnologyhas been in water remediation and treatment wheredifferent nanomaterials can help purify water throughdifferent mechanisms including adsorption of heavy metalsand other pollutants, removal and inactivation of patho-gens and transformation of toxic materials into less toxiccompounds. For this purpose, nanomaterials have beenproduced in different shapes, integrated into variouscomposites and functionalized with active components.Nanomaterials have also been incorporated in nanostruc-tured catalytic membranes which can in turn help enhancewater treatment. In this article, we have provided a succinctreview of the most common and popular nanomaterials(titania, carbon nanotubes (CNTs), zero-valent iron,dendrimers and silver nanomaterials) which are currentlyused in environmental remediation and particularly inwater purification. The catalytic properties and function-alities of the mentioned materials have also been discussed.

Keywords photocatalysis, titania, silver, carbon nano-tube, zero-valent iron, dendrimer

1 Introduction

Nanomaterials are structures with dimensions of less than

100 nanometers. Owing to innumerable unique properties,including high surface to volume ratio and high catalyticactivities, nanomaterials have found myriad pharmaceu-tical, cosmetic, electronic, energy-related and finallyenvironmental applications. Currently, lots of attention isdrawn toward environmental nanotechnology- which ispossibly the most recent application of nanomaterials.Although these engineered nano-sized materials havecaused serious concerns regarding environmental contam-ination, they are presently being exploited as novel tools inenvironmental sensing and biomonitoring, grabbingpathogenic bacteria, treatment of waste water and so on[1]. Nanotechnology is currently being exploited inpollution sensing through various techniques such assurface-enhanced Raman scattering, surface plasmonresonance, and fluorescent, electrochemical or opticaldetection.Treatment of wastewater- which is contaminated by

metal ions, radionuclides, organic and inorganic com-pounds, pathogenic bacteria and viruses- is essential for ahealthy human life. Due to extended droughts, populationgrowth and, very recently, the more stringent health-basedregulations, this rush is even intensified through increasingdemands for clean water. Water purification may be amongthe most developed environmental applications of nano-materials. Innovations in the development of noveltechnologies to desalinate water are among the mostexciting and promising pursuits. Water treatment usuallyinvolves adsorption and/or photocatalysis of contaminantsand their reduction by nanoparticles (NPs) and bioreme-diation. Remediation is the process of pollutant transfor-mation from toxic to less toxic in water and soil.Water quality can be greatly enhanced using nanosor-

Received March 27, 2013; accepted September 3, 2013

E-mail: [email protected], [email protected]

Front. Environ. Sci. Eng. 2014, 8(4): 471–482DOI 10.1007/s11783-014-0654-0

bents, nanocatalysts and bioactive NPs. Besides, nanos-tructured catalytic membranes and NP-enhanced filtrationare interesting nanotechnology-derived products. Novelwater-purification strategies such as dendrimer-enhancedultrafiltration are always welcome; nanomaterials are verypromising in this regard. For instance, nanoporousactivated carbon fibers with an average pore size of 1.16nm have been shown to absorb benzene, toluene, xyleneand ethylbenzene [2]. Moreover, some nanomaterialspossess intrinsic antibacterial properties [3]. One of themost interesting features of nanomaterials (most especiallyNPs) is that their surface area is larger than bulk materials.Functionalization of nanomaterials with numerous chemi-cal groups may be an additional advantage whichcontributes to the target-specificity of these materials.Conventional water treatment techniques including

reverse osmosis, distillation, bio-sand, coagulation-floccu-lation and filtration are not capable of removing all heavymetal ions. Therefore, the need is felt for more robustmethods and membranes to be used in water purification.Nanomaterials can be employed to treat inorganic pollutedwater, dye wastewater, papermaking wastewater, pesticidewastewater and oily wastewater. In the simplest way,sorbents remove pollutants from contaminated water andare thus widely used as separation media. Furthermore,some nanomaterials possess excellent reductive capabil-ities and can hence be used to render toxic pollutants lesstoxic. One good example is zero-valent iron which hasbeen extensively employed for in situ or abovegroundchemical reduction of organic pollutants in contaminatedwater. The extra advantage of zero-valent iron is that uponenvironmental application, it does not produce intermedi-ate by-products which are usually observed whencommercial Fe powders are used [4].Six principal characteristics of nanomaterials have been

proposed to be discriminated in environmental studies:size, dissolution/solubility, surface area, surface charge andsurface chemical composition [5]. Information on the sizedistribution, crystal structure, morphology, agglomeration/dispersion, etc. may prove not only useful but alsoimportant [5]. For example, metal oxides with hierarchicalstructures possess a high surface area as well as highsurface-to-bulk ratio, and subsequently more surfacefunctional groups which make them better candidates forremoval of heavy metal ions such as As(V) and Cr(VI) [6].Cai et al. have implied that simple organic anions cansimply affect the degree of chemical self-transformation ofamorphous particles into crystalline shells, and theirsubsequent self-assembly into complex higher-orderarchitectures [7]. The crystallinity, specific surface area,and pore structure of the boehmite hollow core/shell andhollow microspheres-which have been prepared in thisstudy-could be manipulated by changing the concentrationof sodium tartrate and altering the reaction time. Thesefeatures significantly affected the adsorption capacity ofmicrospheres for Congo red and phenol in aqueous

solution. The adsorption capacity of the boehmite micro-spheres could be partly enhanced by increasing theconcentration of sodium tartrate [7]. Similarly, novelself-assembled, monodispersed, flower-like hierarchicallysuper-structured γ-AlOOH has been shown to possess alarge Brunauer, Emmett and Teller (BET) surface area of145.5 m2$g–1 and to quickly remove Pb(II) and Hg(II) ionsfrom aqueous solutions [8].In general, ideal materials for removal of heavy metal

ions possess some specific characteristics: they areinexpensive; they have distinctive heavy metal adsorptioncapability as well as the ability to transform high valenceions into low valence or zero valence ions to reduce theirtoxicity. Examples of the materials fulfilling these require-ments include nano-Fe oxides, silicate and porous zeolites.In this review article, we primarily focused on the

application of nanomaterials such as metal containing NPs,titania nanomaterials, carbonaceous nanomaterials (e.g.,carbon nanotubes (CNTs)), dendrimers and silver nano-materials in water purification. Finally, application ofdifferent NPs and other nanomaterials in building poly-meric and ceramic membranes were discussed.

2 TiO2 nanomaterials

TiO2 materials are relatively less expensive than othernanomaterials. High photosensitivity and availability, non-toxicity and environment-friendliness are among thefavorable characteristics of these materials [9]. Photo-catalytic properties of titania nanomaterials and especiallyNPs have been exploited in environmental research [10].Heterogeneous photocatalysis with TiO2 nanomaterialscan find potential application in degradation of organic andinorganic compounds [11]. For instance, Ti nanomaterialssuch as TiO2 and TiO2 thin films have been used fordegradation of atrazine and organochlorine pesticides,respectively [12,13]. Photocatalytic degradation of methylorange using ZnO/TiO2 composites has also been reported[14]. Photodegradation results in total oxidative andreductive catalysis of organic and inorganic pollutantsand their transformation into carbon dioxide, water andinorganic acids [15]. This interesting property can beexploited in production of self-cleaning surfaces, cleaningproducts, water purification, and remediation of contami-nated soil or even the deodorization of environments. Thehydroxyl radicals and superoxide ions generated uponirradiation of TiO2 nanaomaterials are able to react withmost biomolecules, which is why many exhibit bacter-icidal and virucidal activity [16]. Many studies haveindicated the formation of reactive oxygen species uponTiO2 exposure to ultraviolet (UV) radiation [17]. Otherstudies have reported the phototoxic effects of TiO2 orTiO2 NPs and its utility in water disinfection [9]. P25 TiO2

has found application in treatment of exhaust waste watercontaminated with organic and inorganic pollutants. The

472 Front. Environ. Sci. Eng. 2014, 8(4): 471–482

application of P25 for the disinfection of Escherichia coli(E. coli) in water depends mainly on the intensity ofincident light and the TiO2 dose used [18]. TiO2 presencecan also lead to a reduction in the number of bacterialcolonies due to the possible agglomeration of TiO2 withthe bacterial cells and subsequent sedimentation [15].Porous titania-prepared by sol-gel process using triblockcopolymer (Pluronic P123) as template-has been shown topossess even higher photocatalytic activity than P25,which can be the result of enlarged surface area and porousstructure [19].TiO2 NPs have been shown to adsorb metals such as Pb,

Cd, Cu, Zn, and Ni at pH 8 [20]. NPs could simultaneouslyremove multiple metals from both pH 8 solution andspiked San Antonio tap water. In another study, bothsuspended amorphous TiO2 and TiO2 immobilized onvarious adsorbents (calcium alginate/polyacrylamide, sol-gel/polyvinylalcohol, polysulfone and carboxylmethylcel-lulose) was used for recovery of Pb(II) and Cd(II) fromwastewater [21]. Of the tested immobilizing adsorbents,TiO2-alginate gel beads exhibited the highest uptake for Pb(II) and Cd (II) ions. Moreover, the beads increased thepossibility of regeneration and reuse of the system andshowed outstanding performance even after ten adsorp-tion/desorption cycles.Furthermore, Liang et al. have devised a microcolumn

packed with nano-TiO2 to achieve simultaneous precon-centration of Cu, Mn, Cr, and Ni for their subsequentmeasurement by inductively coupled plasma atomicemission spectrometry (ICP-AES) [22]. The devisedsystem was successfully applied for the determination oftrace elements in biologic samples and lake water.Not only UV-activated but also visible light-activated

TiO2 NPs have attracted considerable interest in the lastdecade [23]. For example, mesoporous Au/TiO2 nano-composite microspheres can promote visible-light photo-induced degradation of organic compounds [24]. Liu et al.have also synthesized a bismuth oxyiodine/TiO2 hybridNPs with outstanding photocatalytic performance undervisible light irradiation (Fig. 1) [25].Immobilized TiO2 devices have been exploited for

disinfection of natural water by solar photocatalysis [26].This device creates the possibility of eliminating 100% ofthe bacteria covered by international regulations withrespect to water for human consumption. Integration ofcarbonaceous nanomaterials by titania materials is beingincreasingly investigated as a means to increase photo-catalytic activity of TiO2 nanomaterials. Leary et al. haverecently reviewed the novel photocatalytic systems basedon TiO2 [27].In a very recent and interesting study, a novel TiO2-SiO2

nanocomposite was formed inside the pore structure of acarbonate stone, converting it into a self-cleaning buildingmaterial (Fig. 2) [28]. As a bonus, the resultant stone hashigher mechanical resistance and durability.In spite of all advances in the field, better knowledge is

Fig. 1 Bismuth oxyiodine/TiO2 hybrid NPs synthesized througha reverse microemulsion method, photocatalytically detoxifyorganic pollutants to CO2 and H2O under visible light irradiation.The photoactivity of the nanocomposite is highly dependent on themole ratio of BiOI/TiO2. The synergistic photocatalytic activity isattributed to the effective electron-hole separations at the interfacesof the two NPs, which facilitate the transfer of the photo-inducedcarriers. (Reprinted from Applied Surface Science, Vol 258, ZhangLiu, Xiaoxin Xu, Jianzhang Fang, Ximiao Zhu, Jinhui Chu,Baojian Li, Microemulsion synthesis, characterization of bismuthoxyiodine/titanium dioxide hybrid nanoparticles with outstandingphotocatalytic performance under visible light irradiation, Pages3771–3778, Copyright (2012), with permission from Elsevier)

Fig. 2 The TiO2-SiO2 nanocomposite was formed inside theporous structure of carbonate stone by simple spraying of a solcontaining silica oligomers and TiO2 NPs. These NPs cantransform organic pollutants to CO2 and H2O under UV lightand lend self-cleaning capability to the stone. (Reprinted fromApplied Surface Science, Vol 275, Luís Pinho, Farid Elhaddad,Dario S. Facio, Maria J. Mosquera, A novel TiO2-SiO2

nanocomposite converts a very friable stone into a self-cleaningbuilding material, Pages 389–396, Copyright (2012), withpermission from Elsevier)

Gholamreza GHASEMZADEH et al. Applications of nanomaterials in water treatment and environmental remediation 473

required about scaling up of TiO2-based photocatalyticreactors [29]. Various design aspects must be addressedbefore scaling-up multiphase photocatalytic reactors. Theuniform distribution of light and light intensity inside thereactor are among the most important parameters, sincelight penetration depth in catalyst containing suspension isapproximately 2 cm. Furthermore, a high surface area forcatalyst coating per unit of reactor volume is required. Thiswill necessitate technical developments to translate thelaboratory-scale experiments to the pilot-scale and finallyto large-scale level. In such reactors, installation of thesolid catalyst, minimization of light loss, reactant-catalystcontact, flow pattern, mass transfer, mixing, reactionkinetics and temperature need to be considered.

3 CNTs

Owing to their extraordinary physical, mechanical andelectrochemical properties, CNTs have received consider-able attention in recent years. Due to their outstandingadsorbance of organic and inorganic pollutants, research isnow gaining momentum to incorporate CNTs in severaldevices so that they can function as adsorbents anddetection matrices. Furthermore, surface functionalizationof these nanomaterials using negatively-charged functionalgroups can greatly enhance the sorption capacity.CNTs are a new class of synthesized carbonaceous

materials which originated from the discovery of the firstfullerene, the 60 carbon atom hollow sphere in 1985 [30].In 1991, the first CNT was constructed [31]. CNTs aresynthesized either from graphite using arc discharge andlaser ablation or from carbon-containing gas throughchemical vapor deposition. CNTs mainly involve eithersingle-walled CNTs (SWCNT) or multiwalled CNTs(MWCNT). These two types of CNTs differ in thearrangement of graphene sheets. While SWCNTs haveonly a single layer of graphene, MWCNTs are composedof several concentric layers of rolled graphene cylinders.On one hand, due to the exceptional electronic, conductiveand mechanical properties, CNTs have been used infabrication of biosensors and hydrogen storage systems;furthermore, CNTs have also found several other applica-tions including environmental remediation. On the otherhand, owing to large surface area as well as highattachment capacity, CNTs have been proposed as newadsorbents that can help remove environmental pollutants.CNTs are among strong sorbents for 1,2-dichlorobenzene[32], trihalomethanes [33], dioxin [34], pyrene, phenan-threne and naphthalene [35], peptone and α- phenylalanine[36], dichlorodiphenyltrichloroethane (DDT) [37], atrazine[38], butane [39], and other polar and nonpolar organicchemicals. MWCNTs oxidized by different concentrationof sodium hypochlorite have been shown to adsorbtoluene, ethylbenzene and xylene isomers [40]. Fugetsuet al. have successfully demonstrated the high sorptive

capacity of caged (in cross-linked alginate vesicles)MWCNTs for four water-soluble dyes including acridineorange, ethidium bromide, eosin bluish and orange G [41].These adsorptive properties sometimes correlate withporosity and surface area of CNTs [35]. CNTs are amongstrong adsorbents for hydrophobic organic compounds.This property may get back to the strong hydrophobicityand high surface area of CNTs.Li et al. have shown the ability of MWCNTs to absorb

heavy metals such as Pb(II), Cu(II) and Cd(II) ions [42].They demonstrated the superiority of metal-ion sorptioncapacities of the MWCNTs over those of powder activatedcarbon and granular activated carbon which are the mostcommonly used sorbents used in water purification. Zn(II)and Ni ions have also been shown to adsorb onto CNTs[43]. Peng et al. have fabricated a high-surface area (189m2$g–1) arsenate sorbent by cerium oxide (CeO2)supported on CNTs [44].Functionalized nanosorbents can target specific micro-

pollutants and contaminants with very low concentrations.In comparison with activated carbon, when CNTs arefunctionalized with hydrophilic –OH, –COOH and L-cysteine groups, they can adsorb higher amounts of lowmolecular weight and polar compounds [45].Furthermore, carbon NP CNTs have been used for

sorption of copper ions [46], Pb(II) [47,48], trace gold (III)[49], Hg(II) [50], Ni(II) [51], sulfamethoxazole [52],naphthalene [53], 2,4-dichlorophenol, 4-chloroaniline[53], pyrene and phenantrene [54]. Experiments onremoval of heavy metal ions from large volumes ofaqueous solutions using oxides, clay minerals andMWCNTs have demonstrated the superior properties ofMWCNTs in adsorption of heavy metals [55].CNTs also show some intrinsic antimicrobial activity.

The irrecoverable E. coli K12 membrane damage resultingfrom direct physical contact with highly-purified SWCNTaggregates leads to bacterial cell death [56]. Membranedamage subsequently results in compromised membranepermeability. This was confirmed by measuring the effluxof cytoplasmic materials into the solution. A 5- and 2-foldincrease in the concentrations of plasmid deoxyribonucleicacid (DNA) and ribonucleic acid (RNA) were noted insolutions in contact with SWCNTs [56]. SWCNTs werethus proposed as building blocks for antimicrobialmaterials. Other studies have also suggested that thephysical interaction of carbon-based nanomaterials withcells is the primary killing mechanism [57]. SWCNTs havebeen shown to possess antibacterial activity against Gram-positive and Gram-negative bacteria including E. coli,Pseudomonas aeruginosa (P. aeruginosa), Bacillus sub-tilis (B. subtilis), and Staphylococcus epidermis (S.epidermis) in their monocultures, in natural microbialcommunities of river water and in wastewater effluent [58].In this study, bacterial cytotoxicity was time-dependent;for example, in case of B. subtilis, a 5-fold increase intoxicity is noted upon an additional 3.5 h of incubation.


Nanomaterials have opened new research lines todevelop more efficient nanostructured reactive membranesfor water treatment and desalination. CNT-based filtershave been synthesized consisting of hollow cylinderscoupled with radially-aligned CNTwalls [59]. These filterswere successfully employed to remove bacteria (E. coliand S. aureus) and viruses (e.g., Poliovirus sabin 1) fromcontaminated water. After application, filters can becleaned using autoclaving or ultrasonication [59]. AnotherSWCNT- based filter has been developed for removal ofbacterial and viral pathogens [60]. In this filter, authorshave made use of several CNT characteristics: their smalldiameter, high surface area, porous structures and theirantibacterial properties. The filter promotes bacterialinteraction upon its contact with the membrane. The highthermal resistance of CNTs would allow for simple thermalregeneration of the filter for reuse. Furthermore, contrary toconventional MWCNT-based filters, which work throughsize exclusion or sieving and thus require high pressures tooperate, another favorable property of the latter SWCNTfilter can be operating at relatively low pressures.One big challenge in this area is the difficulty of design

and development of biocompatible CNTs. Most studiesabout toxicity of CNTs have shown that CNTs tend to bewater insoluble and toxic [61]. In some cases, hydroxyl,carboxyl and amine groups can be used to functionalizeCNTs in order to increase their water solubility andbiocompatibility. Application of different nanomaterials infabrication of membranes has been briefly reviewed in alater section of this review.

4 Zerovalent metals and metal containingNPs

Zerovalent metals are usually prepared through reducingsolutions of metal salts. The reductant type and thereduction conditions are the most important factorscontrolling the physical properties of these materials.Due to their large surface area and reactivity, zerovalentmetals are favorable compounds for effective detoxifica-tion of organic and inorganic pollutants in aqueoussolutions [62].Nanoparticulate zero-valent iron has found variety of

applications in remediation of water, sediments or soils forreduction and removal of nitrates. They have been appliedto transform and detoxify organochlorine pesticides andpolychlorinated biphenyls [62]. In aqueous solutions, thesenanomaterials have been shown to reduce numerousorganic pollutants such as chlorinated alkanes and alkenes,chlorinated benzenes, organic dyes and nitro aromatics intoless toxic by-products.Fe0 and its bimetallic NPs have been shown to reduce

redox active metal ions such as Cr(VI) to less toxic species[62]. Zhang et al. have reviewed the application ofnanoparticulate iron (including Fe0 particle and Fe0/Pd0,

Fe0/Pt0, Fe0/Ag0, Fe0/Ni0 and Fe0/Co0) in environmentalremediation [62,63]. Metalloporphyrinogens have beenimmobilized in sol-gel matrices to fabricate catalytically-active NPs for dehalogenation and subsequent detoxifica-tion of chlorinated organic materials in aqueous environ-ments [64]. Reductive debromination of polybrominateddiphenyl ethers [65] and removal of alachlor andpretilachlor by zerovalent iron have also been reported[66].Similar to CNTs, bimetallic Fe0/Pt0 NPs have been

incorporated into cellulose acetate films to producereactive membranes, which were successfully used foreffective reduction and degradation of chlorinated organics[67]. The only detected by-product was found to be ethane.Fe0 NPs have great potentials in environmental

remediation; however, they still suffer from severaldrawbacks: 1) the high reactivity impedes easy storage;2) the activity of NPs decreases in time; and 3) thesynthesized NPs can be aggregated [68]. Attempts toincrease the stability of these NPs by stabilizing them onstarch or sodium carboxymethyl cellulose (CMC) havebeen successful. Another method is to increase theeffective surface area by incorporating another metal inthe structure of the NP; e.g. bimetallic nanosizedzerovalent iron rapidly dechlorinates polychlorinateddibenzo-p-dioxins [69]. Surface modification with func-tional groups such as phosphonic acids, carboxylic acid,and amine can greatly enhance the stability of iron oxidecolloid suspensions [70]. Sterically-stabilized zero-valentiron NPs retain their dispersion, avoid aggregation andexhibit reduced sedimentation even through industrialapplication [71].Some metal-containing NPs exert size-dependent toxi-

city [72] and may have unfavorable biologic andenvironmental consequences. Currently, we have onlysuperficial information about the potential of NPs foraccumulation in environmentally-relevant species [73].Therefore, acquiring regulatory acceptance for using thesenanomaterials will be a future challenge.

5 Dendrimers

Dendrimers are relatively-monodispersed hyper-branchedwell-defined macromolecules with controlled compositionand 3-dimensional architectures. Dendrimers can be usedas nanoscale building blocks for production of morecomplex nano-structured materials, e.g., in dendrimer-encapsulated NPs; they may thus find various materialsengineering applications. Furthermore, since dendrimerscan be used to functionalize other materials to enhancerecovery of metal ions from water, it is worthy to mentionthem here.The structure of a dendrimer consists of a core, interior

branch cells and terminal branch cells. Dendritic structuresinclude low molecular weight structures such as dendrons


and dendrimers and high molecular weight structures suchas random hyperbranched polymers, dendrigraft polymersand the polymer brush. The structure of a dendrimersignificantly impacts its physiochemical properties. Poly(AMidoAMine) dendrimers (PAMAMs) or starburstdendrimers were the first synthesized dendrimers and areusually based on either an ethylenediamine (EDA) core orammonia core. Other dendrimers include poly(propyleneimine) (PPI), polyaryl ether and poly(amidoamine-orga-nosilicon) dendrimers (PAMAMOS). Dendrimers can beclassified as sub-nanoscopic, nanoscopic and super-nano-scopic. Dendrimer diameter roughly increases by incre-ments of up to 1 nm per generation, from about 1 to about10 nm [74]. The tenth generation of PAMAM dendrimershypothetically contains 6141 monomers, has a diameter ofabout 124 Å and a molecular weight of over 930000 g$M–1

[75]. Each generation denotes a branching cycle performedduring synthesis.Dendrimers may have two important environmental

implications: 1) as chemical sensors and 2) in heavy metalremoval and water purification. Dendritic polymersdemonstrate several specific properties which make themsuitable for exploitation in water purification, where theyact as high capacity and recyclable water-soluble ligandsfor grabbing toxic metal ions, radionuclides and inorganicanions [76]. Although dendrimers are nanomaterials,membranes which exploit dendrimers as building blocksare not nanomaterials anymore. At any rate, these materialsare important assets in water purification.Although nanofiltration and reverse osmosis membranes

are very effective in filtration of solutes of size 1000 Daand 1000–3000 Da, respectively, they require high pres-sures to operate. On the other hand, ultrafiltrationmembranes require lower pressure, but do not efficientlydiscard dissolved organic and inorganic solute with molarmass below 3000 Da [29]. The advent of dendriticpolymers has presented new opportunities to developeffective ultrafiltration processes for removal of toxic metalions, radionuclides, organic and inorganic solutes andliving entities such as bacteria and viruses from con-taminated water. The main advantage of such membranesis that after filtration, the bound ions can be recovered andthe dendritic polymer can be recycled. Furthermore,dendrimer-based membranes function in a comparativelysmaller operating pressure and with lower energy con-sumption. Cross-linked polystyrene-supported low-gen-eration diethanolamine-typed dendrimers exhibit goodadsorption capacities for Cu2+, Ag+ and Hg2+ ions [77].Dendrimers have been used to functionalize othermaterials such as SBA-15 mesoporous silica and silica-gel for recovery of metal ions [78].

6 Silver nanomaterials

Silver compounds and silver ions have well-known

applications in disinfecting medical devices and watertreatment. Successful applications of Ag(I) and silvercompounds as antimicrobial agents has brought aboutgreat interest in the potential utility of AgNPs as biocides e.g. against E. coli. Cellulose acetate fibers containingembedded AgNPs are also effective biocides against anumber of Gram-positive and Gram-positive bacteria suchas S. aureus, E. coli, Klebsiella pneumonia and P.aeruginosa [79]. Silver NPs can directly damage bacterialcell membrane. In general, the release of silver ions fromsilver nanomaterials, leads to bacteriocidal activity byincreasing membrane permeability, which can subsequentlyresult in loss of the proton motive force and induce de-energization of the cells, phosphate efflux, leakage ofcellular content, and disruption of DNA replication [80].Furthermore, silver NP-alginate composite beads have beendeveloped for point-of-use drinking water disinfection andare shown to possess excellent disinfection efficiency andsatisfactory bactericidal performance.

7 Other nanomaterials

Several types of nanomaterials have been used orconsidered for environmental remediation. Materials suchas MgO NPs have been reported as effective biocidesagainst Gram-positive and Gram-negative bacteria such asE. coli and B. megaterium and even B. subtilis spores. Theantibacterial effects were attributed to changes in theintegrity of the cell membranes exerted by the NPs [81].CuO NPs and copper oxide incorporated mesoporousalumina are effective materials for As(III) and As(V)adsorption [82].Nanoporous carbon xerogels have been used as sorbents

for organic contaminants. Nanostructured black carbonand nanostructured charcoal have been used as sorbents foraromatic compounds [83] and exfoliated graphite nano-platelets (xGnP) have been proposed for aqueous sorptionof phenolic compounds [84]. Copper silicate hollowspheres and hierarchical SiO2@γ-AlOOH spheres havebeen successfully developed and used as adsorbents inwater solution. These hierarchically structured metalnanooxides possess a high surface area, a high surface-to-bulk ratio, and surface functional groups, all of whichenable them to interact and remove heavy metal ions fromwater. Chitosan NPs with average size of 40–100 nm havebeen shown to absorb 398 mg$g–1 Pb(II) ions [85].Functionalized nanomaterials have also been exploitedfor recognition and adsorption of trace pollutants andtoxins. For example, smart RNA aptamers have beencovalently immobilized on graphene oxide nanosheets tospecifically recognize and adsorb trace peptide toxins indrinking water (Fig. 3) [86]. The fabricated RNA-grapheneoxide nanosheets can resist nuclease and natural organicmatter well, and specifically adsorb trace peptide toxin(microcystin-LR) in drinking water.


8 Environmental risk

There is general concern that engineered nanomaterialsmay pose risks to the environment. More importantly, NPspresent or introduced in water bodies can lead to secondarytoxic effects and potentially threaten human health. Thisissue necessitates awareness of the scientific community. Acritical challenge for the emerging nanomaterials is toensure their safety as well as potential health andenvironmental impacts. A major task for ecologicalresearchers is to determine toxicity thresholds for nano-materials and to investigate whether currently usedbiomarkers of harmful effects will also work in studyingenvironmental nanotoxicity.Thus, several research groups are investigating the

practicality of using natural nanomaterials as sorbents. Forinstance, allophone is an excellent sorbents for copper andsurface-modified smectite adsorbs naphthalene and 17β-estradiol [87]. Both of these nano-sized minerals are ofgeological and pedological origins and are found in soil. Anumber of other nanomaterials not discussed in detail inthis article, have been presented in Table 1.

9 Applications of nanomaterials inmembranes

Membranes play important roles in fabrication of devicesfor water purification and treatment. Reactive membranescan also contribute to inactivation of pathogenic bacteria,viruses and other potential pathogens.Regarding the association of the term “nanofiltration”

with the term “nanotechnology”, some controversies anddiscrepancies exist. As defined by the International Unionof Pure and Applied Chemistry (IUPAC), the term “nano”in nanofiltration refers to the size of the filtered particles.However, this is sometimes mistaken with the nanostruc-ture used in the membrane. Some membranes can bedefinitely classified in both membrane technology andnanotechnology such as NP-functionalized membranes,but these systems and membranes cannot be considered asnanostructured materials [88].Researchers have incorporated metal oxide NPs or

CNTs to improve the permeability characteristics,fouling-resistance and permeate quality in thesemembranes [89–92]. For example, alumina ultrafiltra-tion membranes synthesized from alumina NPs of 7–25 nm in size show selectivity toward synthetic dyes suchas Direct Red 81, Direct Blue 71 and Direct Yellow 71. Theselectivity and permeability of this membrane can bemanipulated and increased by doping alumina NPs withFe, Mn, and La [92]. The catalytic properties of somemetal-oxide NPS (especially TiO2), enables a membrane tocombine oxidative functionality with size exclusion.Chemical oxidation facilitates the decomposition oforganic compounds and improves permeate quality [93].As noted earlier, inactivation of bacteria and viruses uponcontact with carbon-based nanomaterials has already beendemonstrated [94]. Moreover, applications of carbon-based materials such as CNTs and fullerenes in thestructure of membranes can lead to a more favorableporosity [95]. One important area that needs to beaddressed by scientific community is the integration ofnovel nanostructured and reactive membranes into existing

Fig. 3 Graphene oxide nanosheet was stably functionalized with smart RNA aptamers through covalent bonds. The produced device canspecifically recognize and adsorb microcystin-LR, a trace peptide toxin in drinking water. (Reprinted from Journal of HazardousMaterials, Vol. 213–214, Xiangang Hu, Li Mu, Jianping Wen, Qixing Zhou, Immobilized smart RNA on graphene oxide nanosheets tospecifically recognize and adsorb trace peptide toxins in drinking water, Pages 387–392, Copyright (2012), with permission from Elsevier)


water purification systems which can be a challenging task.Some of the most routine and interesting nanomaterials

used in construction of nano-structured membranes have

been presented in Table 2. For a comprehensive review ofthe polymeric and ceramic membrane nanostructures inwater treatment please refer to [96].

Table 1 Other nanomaterials used for removal or transformation of water pollutants and the associated mechanism of action

nanomaterial composition target compound(s) mechanism of action properties

bimetallic Pd/Mg polychlorinated biphenyls electrochemical reduction appropriate oxidationpotential of Mg coupled with low cost,high effectiveness and environmental

friendliness

akaganeite-type nanocrystals As(V), Cd ions and Cr(VI) sorption inorganic adsorbent material

nano/microscale FeO and Fe3O4 2,4-dichlorophenoxyacetic acid (2,4-D) reductive transformation smaller negative impact on the ecolo-gical environment compared to Fe0 NPs

granular activated carbon/Fe/Pdbimetallics

polychlorinated biphenyls adsorption-mediated dechlorinationand electrochemical catalysis

adsorption-mediated dechlorination is aunique feature of this material

Bi0.5Na0.5TiO3 (BNT)micro/nanostructure

organic pollutants photodegradation high activity, high degradation effi-ciency for organic pollutants

cuprous ferrite (CuFeO2) heavy metal ions photocatalytic reduction p-type semiconductor characterized bya low optical gap- matched to the sunspectrum- long-term chemical stability

in neutral solution

CuCrO2 heavy metal ions such as Ni2+, Cu2+,Zn2+, Cd2+, Hg2+ and Ag+

photocatalytic reduction p-type semiconductor characterized bya low band gap- long-term chemical

stability

Co3O4 chlorinated compounds decomposition by dechlorination among the best dechlorination materials

nano-CeO2-modified CNTs (CeO2-CNTs)

As(V) Adsorption effective pH-dependent adsorbent forarsenate

ZnO NPs chlorinated phenols photocatalytic degradation little photocorrosion of ZnO- ZnO canbe reused

Table 2 Nanomaterial-enhanced membranes used in water remediationa)

membrane structure class properties gained by addition of nano-component

nano TiO2 polymeric membranes improvements in thermal stability, mechanical strength and mass transfer acceleration inexposure time, smoother surfaces

nano-alumina (Al2O3) Polymeric membranes significant differences in surface and intrinsic properties

silica NPs polymeric membranes superior thermal stabilityhigher separation efficiency and productivity flux

relatively large pore sizes as well as higher pore number density

zeolite polymeric membranes –

CNT polymeric membranes diffusivity

TiO2 ceramic membranes chemical resistance and high water permeability photocatalysis

silver NPs ceramic membranes mitigation of biofouling

iron oxide ceramic membranes functioning in sorbent catalysismore resistance to acids, corrosive media and oxidants than alumina-based NPs

improvement in water quality by significantly reducing the concentration of disinfectionby-product precursors

reduction in ozonation by-products such as aldehydes, ketones and ketoacids

Al2O3 (Alumoxane) ceramic membranes high water permeability, narrow size distribution and good porosity- increase inselectivity and increased flux

retention coefficients and flux values could be altered by chemical functionalization

ferric oxide materials ceramic membranes resistant to acid, corrosive media and oxidantadvantage of no involvement of hazardous chemicals during the fabrication procedure

CNT ceramic membranes unique structure, sorbent, electric and thermal conductivity

Note: a) Adapted and modified from [96]


10 Conclusions

Nanotechnology has been integrated with many biologicand biomedical systems and applications. Nanomaterialsare also emerging as novel and interesting tools inenvironmental risk assessment and monitoring and arefinding new applications in water treatment.The availability of such large quantities of nanomaterials

at economically viable prices for water treatment purposescan be a serious bottleneck for industrial applications.Upon efficacy and safety assurance, nanomaterials can bethe key to several water purification problems: 1) they canbe used for desalination of brackish water; 2) they can helprecover valuable and toxic metals and thus facilitate brinedisposal; 3) they can be used in the development of novelchlorine-free biocides and finally 4) they can eliminatewater contaminants. In near future, nanomaterials may turnout to be the essential and indispensible components ofwater purification and treatment systems and facilities.Further research can be focused on improving thefunctional properties of nanomaterials to meet the versatileneeds in both detection and treatment of pollutants.

Acknowledgements The authors would like to thank Dr. Amir Ata Saei,research scientist at Research Center for Pharmaceutical Nanotechnology atTabriz University of Medical Sciences for the editing of this manuscript.

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