applications of nanotechnology in wastewater treatment—a review

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ReviewJournal of

Nanoscience and NanotechnologyVol. 14, 613–626, 2014

www.aspbs.com/jnn

Applications of Nanotechnology in WastewaterTreatment—A Review

Tanujjal Bora and Joydeep Dutta∗

Water Research Center, Chair in Nanotechnology, Sultan Qaboos University,P.O. BOX 17, Al Khoud 123, Sultanate of Oman

Water on Earth is a precious and finite resource, which is endlessly recycled in the water cycle.Water, whose physical, chemical, or biological properties have been altered due to the addition ofcontaminants such as organic/inorganic materials, pathogens, heavy metals or other toxins makingit unsafe for the ecosystem, can be termed as wastewater. Various schemes have been adoptedby industries across the world to treat wastewater prior to its release to the ecosystem, and severalnew concepts and technologies are fast replacing the traditional methods. This article briefly reviewsthe recent advances and application of nanotechnology for wastewater treatment. Nanomaterialstypically have high reactivity and a high degree of functionalization, large specific surface area,size-dependent properties etc., which makes them suitable for applications in wastewater treatmentand for water purification. In this article, the application of various nanomaterials such as metalnanoparticles, metal oxides, carbon compounds, zeolite, filtration membranes, etc., in the field ofwastewater treatment is discussed.

Keywords: Wastewater, Water Contaminants, Wastewater Treatment, Sewage, Photocatalysis,Nanofiltration, Nanosorbent.

CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6132. Wastewater: Sources, Composition and Treatment . . . . . . . . . . . 614

2.1. Sources and Composition of Wastewater . . . . . . . . . . . . . . 6142.2. Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615

3. Nanotechnology for Wastewater Treatment . . . . . . . . . . . . . . . . 6163.1. Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6163.2. Nanofiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6193.3. Nanosorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620

4. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623

1. INTRODUCTIONWater covers almost two-thirds of Earth’s surface. Still,lack of clean water has been a global problem to humanityfor many years. Nature has its own mechanism to recy-cle water and provide an adequate quantity of clean waterto us. However, uncontrolled human population growthand unplanned industrialization have disrupted the natu-ral purification processes, leading to a shortage of potablewater.1 Nearly 90% of all diseases in most of the devel-oping countries are caused due to the consumption of

∗Author to whom correspondence should be addressed.

impure water. A major portion of the drinking watersources worldwide are found to be contaminated with var-ious toxins and pathogenic microbes, mostly due to therelease of untreated man-made wastes or wastewater tothese sources. Therefore, the proper treatment of waste-water prior to its release is very important for protectingour ecosystem. Industries across the world have adoptedseveral schemes to treat the wastewater before releasingit to the environment. Current wastewater treatment tech-nologies demand high capital investment, operation andmaintenance (O&M) cost, high energy requirements, andlarge plant areas. Consequently, industries from develop-ing countries are finding it very difficult to afford suchexpensive technologies because of their low profit margins,and thus the compliance to environmental legislation andstandards in these industries is relatively low.2 It impliesthat most of the wastewater enters into natural bodies ofwater without any treatment.In order to address these issues, it is the challenge for

research, development, and technology institutions to comeup with cost-effective alternative wastewater treatmenttechnologies with small area requirements. Nanotechnol-ogy offers the potential for the development of alternativetechnologies for wastewater treatment. Nanotechnology

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Applications of Nanotechnology in Wastewater Treatment—A Review Bora and Dutta

involves the use of nanoscale materials that are generallybetween a few nanometers (1 nm is one billionth of ameter) and ca. 100 nm along any or all dimensions.3

Nanotechnology, like microtechnology (1 �m is one mil-lionth of a meter), involves materials defined by a sizescale, but unlike micromaterials, nanomaterials are capableof possessing remarkable properties–properties that devi-ate dramatically from the bulk of the parent material.Nanomaterials typically have a high reactivity and degreeof functionalization, large specific surface area, and size-dependent properties, etc., which makes them suitable forapplications like wastewater treatment, as well as for waterpurification.4

This article mainly explores some recent advances andapplications of nanotechnology in wastewater treatment,emphasizing the potential use of these techniques toaddress various challenges faced by the current wastewa-ter treatment technologies. Several techniques introducedby nanotechnology, such as the applications of nanocata-lyst and nanomembrane filtration for wastewater treatmentare discussed. Research advances on various nanomaterialslike carbon compounds, metal oxides, metal nanoparticles,and zeolite used in these techniques are also reviewed.

2. WASTEWATER: SOURCES,COMPOSITION AND TREATMENT

2.1. Sources and Composition of WastewaterThe sources of wastewater can be broadly classified intotwo classes: residential and non-residential sources. Res-idential wastewater, also known as sewage, is discharged

Tanujjal Bora received his B.Tech. degree in Electronics and Communication Engineeringfrom North Eastern Regional Institute of Science and Technology, India in 2006. Hecontinued his postgraduate studies at the Asian Institute of Technology, Thailand andobtained M.Eng. (Microelectronics) in 2009, and Ph.D. (Nanotechnology) in 2012. Hisbroad research interests include applications of metal oxide nanostructured materials andunderstanding the device physics for solar energy conversion and water purification. He iscurrently a post-doctoral fellow in the Water Research Center at Sultan Qaboos University,Oman.

Joydeep Dutta is the chair in Nanotechnology and Professor at the Water Research Centerat Sultan Qaboos University, Oman and director of the Center of Excellence in Nanotech-nology at the Asian Institute of Technology (AIT), Thailand. His broad research interestsinclude nanomaterials in nanotechnology and application of nanomaterials in water andenergy nexus. He is a fellow of the Institute of Nanotechnology (IoN) and the Society ofNanoscience and Nanotechnology (SNN), senior member of the Institute of Electrical andElectronics Engineers (IEEE), USA, founding member of the Thailand NanotechnologySociety, and member of several other professional bodies. He is an award-winning author(Choice award for outstanding academic title of 2010 from the American Library Asso-ciation) of the book “Fundamentals of Nanotechnology.” He has two other textbooks andover 170 research publications, 11 chapters in Science and Technology reference books,

3 patents, and has delivered over 80 invited and keynote lectures.

from public residences and is much diluted. 99.90% ofsewage is composed of water and the rest, 0.10%, includessuspended solids, biodegradable dissolved organic com-pounds, inorganic solids, nutrients, metals, and pathogenicmicroorganisms.5 The suspended solids are mainly organicparticles, such as body wastes, food wastes, toilet paper,etc., and the inorganic solids are mainly composed of sed-iments, salts, and metals. The biodegradable organics areprimarily carbon-containing compounds, like proteins, car-bohydrates, and fats, which can be converted to carbondioxide (CO2� biologically. The sewage water also con-tains nutrients, such as nitrogen and phosphorus, whichshould be removed to avoid ecological as well as humantoxicity concerns. Figure 1 shows the typical composi-tion of sewage with some major contaminants present insewage.Non-residential wastewater is mainly discharged from

industries, agricultural fields, and commercial activities(e.g., hospitals, shops, restaurants, etc.). The composi-tion of non-residential wastewater depends on the typesof sources it comes from. For example, wastewater com-ing from textile industries mostly contains various organicdyes, whereas restaurants produce wastewater that mainlycontains fatty substances and grease. Moreover, wastewaterfrom various industries and agriculture contains high levelsof harmful chemical and biological pollutants. Heavy metalions are another part of wastewater from some industries,which are toxic to living organisms. Rain water is anothersource of non-residential wastewater that carries organicand inorganic pollutants from streets in addition to the pes-ticides and fertilizers from agricultural fields.

614 J. Nanosci. Nanotechnol. 14, 613–626, 2014



Bora and Dutta Applications of Nanotechnology in Wastewater Treatment—A Review

Figure 1. Typical composition of sewage water.

2.2. Wastewater TreatmentWastewater treatment is a process where the contami-nants or pollutants are separated from the aqueous phasewith the help of a number of physical and chemical pro-cesses, before the environmental release of the water.There are many ways of treating residential and non-residential wastewater;6 most commonly the wastewateris initially discharged to a municipal sewage treatmentplant. If designed and operated properly, a major portionof the pollutant can be removed in these treatment plants.The typical treatment process is mainly composed of threestages:5 preliminary, primary, and secondary stages.The preliminary treatment process involves removal of

large and/or heavy debris from the sewage. Screening andgrit removal steps are typically included in this stage.In the screening process large floating debris, such as rags(∼ 60%), paper (∼ 25%), and plastics (∼ 5%), are removedby using screens. The debris that is left during the screen-ing process, called screenings, is mostly contaminated withraw feces and hence is also odorous. Screenings are gen-erally collected in skips and used for land filling, or issometimes burned. Grit removal is the following step afterthe screening process. Grits are basically heavy inorganicparticles like sand, gravel, and other heavy particulate mat-ters, which are normally removed by settling in grit chan-nels. After separation, if sufficiently cleaned, grits can beused as filling materials or sent to a landfill.The effluent from preliminary treatment then goes to the

primary treatment process. The primary treatment involvesseparating a major portion of the suspended solids fromthe wastewater using a sedimentation process. The effluentis allowed to flow through sedimentation tanks, where themajority of the solid material gets settled at the bottom ofthe tanks, called sludge. Inside the tanks the effluent staysfor several hours, allowing the sludge to settle and a scumto form on the top. The scum is then skimmed off thetop, the sludge is removed from the bottom, and the par-tially treated wastewater moves on to the secondary treat-ment stage. The primary treatment generally removes upto about 40% of the biochemical oxygen demand (BOD;

these are substances that use up the oxygen in the water),around 80% to 90% of suspended solids, and up to 55%of fecal coliforms. While primary treatment removes a sig-nificant amount of harmful substances from wastewater, itis not enough to ensure that all harmful pollutants havebeen removed.The effluent from primary treatment normally contains

considerable organic materials and a relatively high BOD.To remove the organic matter and the residual suspendedsolids, the waste goes further, through a secondary treat-ment, which involves biological processes that can reducethe BOD level and most of the organic matters, as wellas a low amount of suspended solid material. In this pro-cess, the sewage undergoes strong aeration to encouragethe growth of aerobic bacteria and other microorganismsthat oxidize the dissolved organic matters to carbon diox-ide and water. In addition to the removal of organic mat-ters and BOD from sewage, some nutrients present inthe sewage, such as nitrogen and phosphorus, are alsoremoved in the secondary treatment stage using processeslike nitrification and luxury cell uptake.5 The main causesfor these nutrient removals are to avoid eutrophication anddepletion of the oxygen level of the receiving water bodyto which the treated sewage is discharged.Figure 2 schematically shows the complete process of

a typical wastewater treatment plant including all threestages; i.e., preliminary, primary, and secondary treatmentstages. Once the sewage is treated, it is very importantto make sure that the treated sewage is safe enough forrelease to the environment. Similarly, the sludge producedin the treatment processes contains various harmful pol-lutants; hence handling of the sludge after the treatmentis also essential. The treated sewage is usually disin-fected by using chlorination or UV (ultra-violet) disinfec-tion processes before environmental discharge. The sludgeis treated by using an anaerobic sludge digestion process,5

where growth of anaerobic bacteria is encouraged inside

Figure 2. Schematic representation of a typical wastewater treatmentplant showing the three basic stages of the wastewater treatment process.

J. Nanosci. Nanotechnol. 14, 613–626, 2014 615




a digester containing the sludge. These anaerobic bacteriadecrease the organic solids in sludge by degrading theminto soluble byproducts and gases, mostly methane andcarbon dioxide. The methane gas produced during the pro-cess can be then used as a fuel for heating the digesters,as well as for running other power equipment in the plant.The treated sludge is then used for land filling or sent tosolid waste handling plants.Apart from these common stages, some treatment plants

introduce a tertiary treatment stage to remove remainingorganic and inorganic matter and microorganisms fromthe secondary stage effluent using physical and chemicalprocesses.7 The water after tertiary treatment can meetthe standards of drinking water. However, this stage isextremely costly and seldom adopted by industries.

3. NANOTECHNOLOGY FORWASTEWATER TREATMENT

Nanotechnology is the application of nanoscience, whichis the study of nanoscale materials that exhibit remark-able properties, functionality, and phenomena due to theinfluence of small dimensions. Nanotechnology is basedon the manipulation, control, and integration of atomsand molecules to form materials, structures, components,devices, and systems at the nanoscale.3�8 In recent years,the development of various tools and techniques enabledby nanotechnology, especially in the area of water purifi-cation, opens up a new potential alternative to treat waste-water more efficiently and cost effectively.9�10 This ispossible since nanomaterials are small, highly reactive,more accurate, and most importantly, they can be pro-duced by environment-friendly techniques that are poten-tially cost effective. Some of the promising water treatmenttechniques/tools introduced by nanotechnology are:(i) Photocatalysis(ii) Nanofiltration(iii) Nanosorbents.

3.1. PhotocatalysisPhotocatalysis is a promising technique for water purifica-tion that uses a light active nanostructured catalyst mediumto degrade various pollutants present in the water. Photo-catalysis is a process defined as “change in the rate of achemical reaction or its initiation under the action of ultra-violet, visible, or infrared radiation in the presence of asubstance—the photocatalyst—that absorbs the light andis involved in the chemical transformation of the reactionpartners.”11 In a typical photocatalysis system, a semicon-ductor material is used as catalyst medium, which uponabsorption of a light energy higher than its bandgap energygenerates an electron–hole (e–h) pair. The photo-generatede–h pair then produces highly reactive oxidizing and/orreducing radicals, such as super oxides (O−

2 �, hydroxylions (OH•�, or other radicals, in water. These radicals then

degrade any organic/inorganic pollutant molecules presentin the contaminated water through some secondary reac-tions. The degradation of the water contaminants can alsooccur through direct transfer of the photo-generated elec-trons or holes from the catalyst surface to the contaminantmolecules. Figure 3 illustrates the process of photocatal-ysis that occurs on the surface of a nanostructured semi-conductor catalyst.Photocatalysis is a surface phenomenon and its general

mechanism is a complex process, which involves five basicsteps:12 (i) diffusion of reactants to the surface of the cat-alyst, (ii) adsorption of the reactants on the surface ofthe catalyst, (iii) reaction at the surface of the catalyst,(iv) desorption of the products from the surface of the cat-alyst and (v) diffusion of the products from the surface ofthe catalyst. The possible course of degradation of organicpollutants (OP) in water through photocatalysis is shownfrom Eqs. (1) to (10):13

CatalystLight−−→ h+

VB+ e−CB (e–h pair generation) (1)

OP→ OA−�aq�+OC+�aq�

(aqueous dissociation of the pollutant) (2)

H2O+h+VB → OH• +H+�aq�

(photo-splitting of water) (3)

O2+ e−CB → O−2

(electrophilic adsorption of dissolved O2� (4)

H++O−2 → HO•

2

(protonation of superoxide anion) (5)

OA−+h+VB → OA• (6)

OH−+h+VB → OH• (7)

Figure 3. Schematic representation depicting the photocatalysis pro-cess on the surface of a nanostructured metal oxide semiconductorphotocatalyst.





OA• +OH•/HO•2 → CO2+ Intermediates

+mineral acids+neutral sites (8)

OC++OH− → hydroxylated products (9)

h+VB+ e−CB → catalyst+heat

(e–h pair recombination) (10)

The activity of a photocatalyst is highly dependent onits ability to generate an e–h pair upon absorption of light.The photo-generated e–h pair in the semiconductor cata-lyst typically has a very small lifetime and it is essential toutilize these free electrons and holes for secondary reac-tions before their recombination (Eq. (10)). For example,the holes in valance band (VB) of TiO2 are good oxidizingagents with a redox potential of about +1�0 to +3�5 Vversus NHE (normal hydrogen electrode), while electronsin the conduction band (CB) are good reducing agentswith a redox potential of about +0�5 to −1�5 V versusNHE.14�15 Thus the holes degrade the surface adsorbedorganic molecules through oxidation, and similarly, elec-trons degrade them through the reduction process, eitherdirectly or through other indirect pathways, as mentionedabove. The position of the energy bands in various semi-conductor materials with respect to vacuum and electro-chemical scales are shown in Figure 4.The application of nanostructured semiconductor

materials for photocatalysis is more suitable compared totheir bulk counterparts, since most of the photo-generatedelectrons and holes are available at the surface of thenano-photocatalyst due to its high surface-to-volume ratio.

Figure 4. Position of energy bands in various semiconductor materials with respect to the vacuum and electrochemical scales. The scales marked as(a), (b) and (c) represent the vacuum, normal hydrogen (NHE), and saturated calomel (SCE) electrodes, respectively. Reprinted with permission from[15], C. G. Zoski, Handbook of Electrochemistry, Elsevier, Amsterdam (2007). © 2007, Elsevier.

For an efficient photocatalyst, the semiconductor shouldhave a wide bandgap in order to produce an e–h pairwith sufficient energy to carry secondary reactions, andthe recombination of an e–h pair should be as low aspossible. An ideal photocatalyst should exhibit the follow-ing properties: (i) high photoactivity, (ii) biological andchemical inertness, (iii) photostability, (iv) nontoxicity and(v) cost-effectiveness.16 Some examples of typically usednanostructured semiconductor photocatalysts are titaniumdioxide (TiO2�, zinc oxide (ZnO), ferric oxide (Fe2O3�,zinc sulfide (ZnS) and cadmium sulfide (CdS).17

The wide bandgap semiconductors absorb in the UVregion of the solar spectrum. However, the use of high-energy UV light sources to excite the catalysts may not bea cost effective solution in all cases. Therefore, research iscurrently focusing on the utilization of the visible portionof the solar spectrum to conduct photocatalysis. The solarenergy that hits the Earth’s surface contains almost 46%visible light, 47% infrared radiation and only 7% ultra-violet (UV) light. Several attempts have been reportedfor the modification of the wide bandgap semiconductorcatalysts to harvest the visible light region of the solarspectrum, and their visible light photocatalysis to degradevarious organic contaminants. Some of these techniquesinclude:(i) doping of the semiconductor catalysts with transitionmetals, such as manganese, copper, nickel, cobalt etc.,18–21

(ii) doping with non-metals, like nitrogen, sulfur, boron,halogens etc.,22–26

(iii) coupling with narrow bandgap semiconductors,27–29





(iv) sensitization of the nanostructured catalyst surface byvisible light active organic dyes and polymers,30–34

(v) creation of intermediate defect states within thebandgap of the semiconductor photocatalysts,35�36 and(vi) application of metal nanoparticles for surfaceplasmon-induced visible light photocatalysis.37–40

The advantages of using solar light for photocatalysis arethat the solar energy is free and abundantly available.Moreover, such visible light active photocatalytic systemswill be very useful for outdoor applications, like wastew-ater treatment processes. Some of the key areas in whichphotocatalysis can play a crucial role in the treatment pro-cess of wastewater are described below.

3.1.1. Removal of Organic ContaminantsPhotocatalysis has been widely used for the degradationof harmful organic contaminants in water into harmlessbyproducts, mostly carbon dioxide and water. Varioustypes of alcohols, carboxylic acids, phenolic deriva-tives and chlorinated aromatic contaminants have beensuccessfully degraded by the application of the pho-tocatalysis technique.16�41 Release of dyes from textileindustries into rivers is one of the most concerningissues in some developing countries. In this regard, semi-conductor metal oxides, such as TiO2, ZnO etc., haveshown great potential to photocatalytically degrade severaldyes in water.42–46 Photocatalysis has also been used todegrade natural organic matters or humic substances.47�48

Humic substances are naturally occurring yellow–brownorganic materials with high molecular weight.49 UsingTiO2 nanoparticles, Eggins et al.50 reported almost 50%reduction in humic acid concentration in drinking water.The observed reduction in humic acid concentration wasrecorded in about 12 minutes under the irradiation from amercury lamp. Bekbölet et al.51 also studied the photocat-alytic degradation of humic acid in water and found 40%and 75% reduction in TOC and color, respectively.

3.1.2. Removal of Inorganic ContaminantsInorganic contaminants, such as halide ions, cyanide, thio-cyanate, ammonia, nitrates and nitrites can be effectivelydecomposed with the help of photocatalytic reaction.52�53

The photocatalytic activity of TiO2 against silver nitrate(AgNO3� was studied by Ohtani et al.54 Similarly, Hidakaand co-workers have reported the photocatalytic removalof toxic Hg(II) and CH3Hg(II) chlorides from water usingTiO2 nanoparticles under simulated solar light (AM1.5).55

ZnO nanoparticles were used to remove toxic potassiumcyanide56 and Cr(Vi)57 ions from water using visiblelight. Recently Chen et al.58 reported the application ofCdS/Titanate nanotubes for the photocatalytic oxidation ofammonia in water. In another study, Lee et al.59 demon-strated over 80% photocatalytic conversion of ammoniato nitrogen under visible irradiation in about 40 minutesusing TiO2 nanoparticles as photocatalyst.

3.1.3. Removal of Heavy MetalsHeavy metal removal from wastewater is another chal-lenging area for treatment plants, since the amount canvary, depending upon the type of wastewater. For humanhealth and for maintaining water quality, removal of thetoxic heavy metals from the water body is of the utmostimportance. However, due to the rare availability and highcost of some metals, recovery of the metals is mostly pre-ferred to removal of the metals. Various heavy metals havebeen demonstrated to be recoverable using the photocatal-ysis technique.55�60–64 Recovery of gold(III), platinum(IV),and rhodium(III) using TiO2 dispersions have been shownby Minero and co-workers as early as in 1986.65 From amixture of gold(III), platinum(IV), and rhodium(1II) chlo-ride salts, the authors have successfully recovered morethan 90% of gold at pH value 0 under solar irradiation.Recovery of gold from samples containing cyanide ionswas also demonstrated by Serpone et al.66 along with thedegradation of CN− by employing two peroxides, H2O2

and S2O2−8 . Removal of cadmium (Cd) from wastewater

was investigated by Thurnauer and co-workers using nano-sized TiO2 particles.67 Using 253.7 nm wavelength lightirradiation, the authors reported more than 90% reduc-tion and recovery of the Cd onto the TiO2 surface. TiO2

photocatalyst was further investigated for the reduction oftoxic mercury (Hg2+� ions to its metallic state and itssuccessive recovery.55 A group of researchers from USAand Japan has developed activated carbon from sewagesludge and combined with TiO2 nanoparticles, which wereused to reduce Hg2+ ions followed by recovery of metallicHg(0).68 After combining the activated carbon and TiO2

nanoparticles, the researchers found a more than 70%removal rate for the metallic Hg(0) adsorbed onto the acti-vated carbon and TiO2 surface after photoreduction, whichwas recovered on a silver trap through heating. A TiO2-gold nanocomposite was recently used for the reductionof Cr(VI).69 Under UV illumination the composite hasshown 91% reduction in the Cr(VI) concentration com-pared to the 87% reduction with TiO2 nanoparticles only,which was mainly attributed to the broader light absorp-tion range contributed by the surface plasmon absorptionof gold nanoparticles and change in the e–h pair recombi-nation rate due to the presence of the gold nanoparticles.

3.1.4. Removal of MicrobesMost of the photocatalysts also show antimicrobial effectand prevent microbial growth. The process basicallyinvolves the destruction of the cell wall of the microbesby the highly reactive radicals generated during the photo-catalysis process, which eventually leads to the destructionof the microbes. Several harmful bacteria and viruses, suchas Streptococcus mutans, Streptococcus natuss, Strepto-coccus cricetus, Escherichia coli, Scaccharomyces cerevi-sisas, Lactobacillus acidophilus, etc., could be removed byusing heterogeneous photocatalysis.70 The algal blooms in





fresh water supplies and the consequent water contaminantMicrocystin toxins were shown to degrade on immobi-lized TiO2 catalyst.

71 TiO2 also inhibits Chlorella vulgaris(green algae), which has a thick cell wall. Similarly, zincoxide (ZnO) has also shown promising antimicrobial effectagainst Escherichia coli and Staphylococcus aureus.72–74

3.2. NanofiltrationFiltration is one of the most common and important stepsin water purification and wastewater treatment, whichinvolves a filter media or a membrane that separates thesolid part from the liquid. The various membrane-basedfiltration techniques along with the size and types of par-ticles that can be filtered out are illustrated in Figure 5.Nanofiltration (NF) is a pressure-driven membrane sep-aration technique and is rapidly advancing in the areaof water purification and wastewater treatment due toits unique charge-based repulsion property and high rateof permeation. Due to the lower pressure requirements(7–30 atm) compared to reverse osmosis (RO) processes(20–100 atm), NF is becoming more popular these days,being a lower energy consumption technique.75�76

The properties of the membranes used in NF lie betweenthe non-porous RO membranes and porous ultrafiltrationmembranes, and thus the transport in NF mainly occursthrough the solution diffusion mechanism; it is also due tothe size exclusion property of the membranes.77 Addition-ally some NF membranes possess a fixed surface chargethat provides selective binding of various contaminantsin the liquid, apart from the physical separation. Hencethe NF membranes are one step ahead of the RO mem-branes in terms of the separation mechanism. The processof water softening using the NF technique is illustratedin Figure 6. The pore sizes of the NF membranes are sosmall (typically in the range of 1–5 nm) that almost all thesolutes are efficiently rejected by the membrane. However,the surface charge property of the membranes allows themonovalent ions in the hard water to pass through, while

Figure 5. Membrane-based filtration techniques with effective size andtypes of particles typically removed by the membrane.

Figure 6. Schematic representation of water softening by nanofiltrationprocess.

retaining the multivalent ions. Size exclusion is the domi-nant rejection mechanism in NF for the uncharged species;whereas the ionic species get rejected by both size exclu-sion and electrostatic interaction.78–80

Application of NF in the area of wastewater treatmentis relatively new and the technique is gaining tremendousattention from various industries, such as textiles, pharma-ceuticals, the dairy industry, the petrochemical industry,and so on.81–84 Because of the unique filtration mecha-nism and availability of various types of membranes, NFis suitable to filter out almost all organic and inorganiccontaminants, including several harmful microbes fromwastewater.85�86 Most of the NF membranes are made ofsynthetic polymers due to their simple preparation process,high flexibility, and low cost. However, polymeric mem-branes have lower chemical resistance and a high rate offouling, and thus exhibit a short lifetime.87 On the con-trary, membranes made of inorganic ceramics have highchemical and thermal resistance and a longer lifetime; butwith the disadvantage of the high cost of fabrication and alack of flexibility. In this regard, newly developed nanoma-terials can play an important role in the fabrication of theNF membranes, as they can be synthesized cost effectivelyand can be made flexible as well.88 Some of the promisingnanomaterials and their applications in wastewater treat-ment processes are discussed below.

3.2.1. Carbon NanomaterialsCarbon nanomaterials are one of the most widely studiednanomaterials for membrane fabrication because of theirease of preparation, high mechanical robustness, and excel-lent rejection ability. Membranes based on hollow, one-dimensional carbon nanotubes (CNTs) have been reportedto have high solvent permeability and a high rejectionrate of the contaminants.89 The small pore diameter of theCNTs (typically in the range from 1 nm to 10 nm) allowsonly water to pass through, while blocking the chemicaland biological contaminants. The advantages of membranesbased on CNTs are that they are robust like the ceramic





Figure 7. Scanning electron micrographs of yeast cells immobilized ona CNT-based filtration membrane. The membrane can effectively removeheavy metal ions, like Fe2+, Cu2+, Co2+, Zn2+ and Mn2+. Reprinted withpermission from [100], H. Parham, et al., Carbon 54, 215 (2013). © 2013,Elsevier.

membranes and flexible like polymeric membranes. Addi-tionally, the permeation of water through CNTs isextremely fast.90–93 Recently Karan et al.94 reported anultrathin membrane composed of freestanding carbon nano-tubes with a pore diameter of ∼ 1 nm. Compared to thecommercially available membranes, the authors claim anincrease of the rejection rates for organic dyes by almostthree orders with their carbon nanotube-based membrane.Similarly, Srivastava et al.95 used the carbon nanotubesto eliminate several heavy hydrocarbons, and also demon-strated efficient removal of bacterial contaminants likeEscherichia coli and poliovirus from water. Apart fromsize-selective exclusion, CNTs have shown a strong abilityto adsorb several types of chemical and biological contam-inants present in the water.96–99 Zhu and co-workers fabri-cated a CNT/ceramic composite filter and demonstrated ahigh efficiency for yeast filtration (98%), and almost 100%heavy metal ion removal from water (Fig. 7).100

One atom-thick 2D graphene sheets have also beenused for the NF membranes. Graphenes can be producedmore cost effectively than CNTs, which shows similarthermal and chemical stability with superior flexibility.101

The potential application of graphene as the NF mem-brane for organic dyes and nanoparticles was reported byLi and co-workers.102�103 Very recently Han et al.104 usedgraphene for water purification, where they fabricated thegraphene membrane (thickness in the range from 22 nmto 53 nm) on a micro-porous substrate. The as-fabricatedNF membrane showed very high retention for organic dyemolecules present in water. However, the retention forionic substances was found to be moderate in this case.In another study, Nair et al.105 used graphene oxides to fab-ricate micrometer-thick NF membranes and demonstratedextremely high impermeability to any liquid, vapors, andgases, excluding water.

3.2.2. Metal OxidesMetal oxides are another cost-effective alternative for thefabrication of NF membranes. Additionally, most of themetal oxides show photocatalytic activity in the presenceof light,42 that helps to degrade various organic and inor-ganic contaminants, including several harmful microbespresent in water, making the membranes reactive instead ofbeing only a physical barrier. Leckie and co-workers fabri-cated a membrane with titanium dioxide (TiO2� nanowiremesh, which was used for the filtration of humic acid andtotal organic carbon (TOC) in wastewater.106 Combiningthe filtration and photocatalytic property of the TiO2-basedmembrane; the authors have claimed almost 100% removalof humic acid and more than 90% removal of TOC underultra-violet light irradiation. In a combination of TiO2 with�-alumina, the NF membranes also showed a significantretention rate for organic dyes.107 In order to improvethe corrosion resistance of the TiO2-based membranes,Gestel et al.108 reported the application of zirconium diox-ide (ZrO2� nanoparticles together with the TiO2 particlesand demonstrated highly stable membranes over a widerange of pH value (from 1 to 13). In a recent study, Alsalhyet al.109 reported enhancement in the water flux from 76to 107 (Lm−2 h−1 bar−1� in poly(phenyl sulfone) mem-branes after incorporation of zinc oxide (ZnO) nanopar-ticles into the membranes. The improvement in the fluxdensity is mainly attributed to the increased hydrophilicityof the membranes due to the presence of ZnO nanoparti-cles. However, the authors observed almost similar soluteretention activity for the membranes, with and without theZnO nanoparticles. Incorporation of silica (SiO2� nanopar-ticles into the polymeric NF membranes has also shownto improve hydrophilicity and permeation properties of themembranes, along with improved thermal stability.110–112

3.2.3. ZeolitesZeolites are microporous crystalline aluminosilicate mate-rials with pore sizes ranging from sub-nanometer tonanometer in scale. Zeolites have excellent thermal andchemical stability due to the inertness of the alumi-nosilicate crystals, and hence they are very suitable forapplications in RO and NF. Zeolite-filled polydimethyl-siloxane (PDMS) NF membranes (Fig. 8) have beenreported as an excellent NF membrane in terms of retain-ing several organic contaminants, thermal stability, andreducing the swelling of the PDMS network.113�114 Zeoliteshaves also been used to separate novel metals complexes115

and saturated/unsaturated hydrocarbons.116

3.3. NanosorbentsSorption is a process in which a substance, called sor-bate, adsorbs to another substance, called sorbent, bysome physical or chemical interactions. Sorbents are com-monly used in water purification and treatment as separa-tion media for removing organic and inorganic pollutants





Figure 8. Scanning electron micrographs of zeolite (ZSM-5)-filledPDMS NF membranes. Zeolite reduces the swelling of the PDMSmembrane and provides improved thermal stability to the membrane.Reprinted with permission from [114], L. E. M. Gevers, et al., J. Membr.Sci. 278, 199 (2006). © 2006, Elsevier.

from contaminated water. In general, the sorption processof pollutants in water on the sorbent surface occurs inthree steps: (i) transport of the pollutant from the water tothe sorbent surface, (ii) adsorption at the sorbent surfaceand (iii) transport within the sorbent. Nanoparticles havetwo important properties that make them very effective assorbents. They possess higher specific-surface areas thanbulk particles and can be functionalized easily with vari-ous chemical groups to increase their affinity toward thetarget contaminants. Moreover, nanosorbents have nano-sized pores, which help in the sorption of contaminants.Nanosorbents can also be reused by removing the absorbedpollutants, thereby regenerating them. For example, it hasbeen demonstrated that self-assembled 3D flowerlike ironoxide nanostructures can effectively remove heavy metalions and adsorb organic dyes present in contaminatedwater.117 Since the nanoparticles are magnetic, they can beeasily separated using a magnetic separation method, andthen can be regenerated by catalytic combustion at 300 �C.Some of the commonly used nanomaterials as absorbentsare described below:

3.3.1. Carbon NanosorbentsCarbon nanomaterials have been extensively used for theadsorption of various organic and inorganic pollutants inwater. Out of these nanomaterials, activated carbon isthe most popular carbon material due to its high adsorp-tion capacity, high thermal stability, excellent resistanceagainst attrition losses, and low cost. Granular activatedcarbon (GAC) was used for the removal of variousorganic contaminants as well as the odorous pollutantsfrom water.118–121 Adsorption of benzene and toluene fromindustrial wastewater on activated carbon was studied byAsenjo et al.122 and reported high adsorption capacity forbenzene (∼ 400–500 mg/g) and toluene (∼ 700 mg/g).Activated carbon was also found to be effective for the

removal of heavy metal ions, such as Hg(II), Ni(II), Co(II),Cd(II), Cu(II), Pb(II), Cr(III) and Cr(VI).123–125

In consort with the activated carbon, one-dimensionalcarbon nanotubes (CNTs) are also receiving a lot ofattention as an excellent adsorbent material, due totheir high specific surface area and good thermal andchemical stability, and specifically, the reactivity of theCNTs can be tuned by simply functionalizing the sur-face of the CNTs.126–129 The application of CNTs forthe adsorption of toxic 1, 2-dichlorobenzene along withlead and cadmium from water was studied by Luan andco-workers.130 The adsorption of these pollutants wasobserved to be greatly influenced by the morphology of theCNTs and their surface status, whereby treating with oxi-dants showed increasing adsorption capacity of the CNTsalong with their dispersibility. The adsorption capacity ofCNTs against dichlorobenzene was also found to be veryhigh.131 However, the adsorption capacity was observed todecrease when the CNTs were annealed at high tempera-ture (2200 �C) in an inert atmosphere resulting in defect-less CNTs with a smoother surface. Hence, the defectsin CNTs and their surface roughness are crucial for theadsorption process.131

In another work, Kuo et al.132 studied the adsorption oforganic dyes from water using CNTs, and found that theadsorption of dyes on the surface of the CNTs is drivenby a physisorption process. The adsorption rate and capac-ity of both CNTs and activated carbon are high and bothare thermally and chemically stable materials, which makethem suitable for a water treatment process. However, thecomplete separation of the CNTs and powdered activatedcarbons from the water is difficult due to their small sizes.To address this issue, integration of magnetic nanopar-ticles with the CNTs and activated carbon was foundextremely effective,133–136 since these nanosized compositeabsorbents can be easily separated from the aquatic phasewith the help of magnetic separation techniques.

3.3.2. BiosorbentsIt has been observed that some of the organic pollutantscannot be completely removed from the water body due tothe very low concentration of these pollutants, typically inthe range of picogram or nanograms per liter of water.137

For the efficient removal of such pollutants, biosorbentsare found promising, which are typically derived frombiological or agricultural materials. Compared to conven-tional absorbents, biosorbents have many advantages, likelow cost, high efficiency, low agricultural and biologicalsludge, no additional nutrient requirement, and they areregenerative as well. A DNA matrix composed of salmonmilt DNA hydrogel beads was developed by Liu et al.138

and the matrix was successfully used for the selectiveadsorption of dioxin derivatives. The regeneration of theDNA beads after adsorption of dioxins can be achievedsimply by rinsing the beads with hexane. Several stud-ies have been reported for triolein-embedded biosorbents





to remove organic contaminants from water.139–143 Theadvantages of using triolein includes its high accumula-tion capacity (105–107� for trace concentrations of organicpollutants in water144 and its low membrane solubilityand permeability because of the large molecular mass of885.45 Da.145

Biosorbents have been applied for the sorption of heavymetal ions from water146 Chitosan-based sorbents havealso shown promising results with highly efficient adsorp-tion capacity for metal ions, where the adsorption of themetal ions occurs through chelation on the amino acidgroups of chitosan.147�148 In another study, Guo et al.149

developed a biosorbent from black liquor, a waste frompaper industries, and investigated the sorption capacity ofheavy metals. The order of the sorption affinity of thebiosorbent against various heavy metal ions was reportedas Pb(II)> Cu(II)> Cd(II)> Zn(II)> Ni(II). Biosorbentsprepared from various other agricultural materials andwastes have also been used for heavy metal removal fromwater.150–152

3.3.3. Metal Oxide NanosorbentsThe common oxides used as adsorbents are mostly oxidesof iron (Fe), manganese (Mn), silicon (Si), titanium (Ti)and tungsten (W). As adsorbent materials, metal oxideshave the advantages of being low-cost materials and can befunctionalized easily to tune their adsorption capacity andselectivity. Nanosorbents based on Fe-oxides have beenrecently explored for the removal of several organic pollu-tants in water.117�153–155 Because of the magnetic nature, theFe-oxide nanosorbents can be magnetically separated fromthe aqueous phase.156 The Fe-oxides also showed excellentadsorption capacity for heavy metal ions.157�158

Nanostructured tungsten oxide (WO2� has also dis-played very high adsorption capacity for organic dyesin water.159 Talleb and co-workers have developed azinc-aluminum layered double hydroxide nanosorbent andsuccessfully applied it for the removal of reactive yel-low 84 dyes from several textile wastewater effluents.160

In another study, a group of researchers explored the sorp-tion efficiency of lead ions on seven natural and syn-thetic Mn- and Fe-based oxides.161 The study concludedthat Mn-oxides are more efficient lead adsorbent than Fe-oxides, where the specific chemical (bonding) interactionsdominate the sorption phenomena beyond the influenceof electrostatic mechanisms. Similarly, Wang et al. usedalumina (Al2O3� as nanosorbent and studied the sorptionmechanism of europium ion (Eu(III)) as a function of pH,humic acid (HA) concentration, and ionic strength.162 Theauthors reported that the sorption of Eu(III) on aluminais strongly dependent on pH values and independent ofthe ionic strength. However, they have observed a neg-ative effect for HA concentration on the sorption of themetal ion at higher pH values (beyond pH 8). Applicationsof non-metallic oxide, like silica (SiO2�, as nanosorbents

have also shown promising results in removing organicpollutants and heavy metals from wastewater.163–167

3.3.4. Zeolites as SorbentsZeolites have high specific surface area and high ionexchange capacity, making them an attractive adsorbent forwater treatment. Most of the zeolites occur naturally andcan also be produced commercially. Hexadecyltrimethy-lammonium (HDTMA)-modified zeolite was developed byDong et al.168 and used for the adsorption of phenol deriva-tives from water. The HDTMA molecules form a bilayermicelle at the surface of the zeolite increasing the adsorp-tion capacity of the nanosorbent. Degradation of pyridineand quinoline in wastewater using zeolite was studied byBai et al.169 The authors developed a biologically modifiedzeolite composed of mixed bacteria for the degradationof pyridine and quinoline. The biodegradation of pyridineand quinoline produces ammonium ions in water, whichwere then adsorbed by the zeolite. Hence, simultaneousbiodegradation of pyridine and quinoline and adsorption ofproduced ammonium ions occurs in the biologically mod-ified zeolite.Zeolites have also been used for the adsorption of heavy

metal ions.170–174 Perry and co-workers have studied theadsorption of lead and cadmium using two natural zeo-lites: chabazite and clinoptilolite.175 Using the two naturalzeolites pretreated with NaOH, the authors demonstratedvery high adsorption capacity for lead (Pb) and cadmium(Cd), with metal removal efficiency of more than 99%.The high porosity of zeolite gives it a higher adsorptioncapacity, and the photocatalytic reduction ability of zeoliteaids in reducing high valence metal ions to lower valencemetal ions, thus decreasing their toxicity.

3.3.5. Nano Zero Valent Iron (nZVI)Nano zero valent iron (nZVI) is an emerging nanomaterialfor the removal of various organic and inorganic pollutantsfrom water. The highly reactive nZVI has a short lifetime,and hence several modifications of nZVI have been stud-ied, like surface-modified nZVI, emulsified nZVI (bettermiscibility), bimetallic nZVI (higher reactivity), and nZVIon carbon support (better distribution). Heavy metals areeither reduced at the nZVI surface (e.g., Cu2+, Ag2+� ordirectly adsorbed to the nZVI surface where they are ren-dered immobile (e.g., Zn2+, Cd2+�. The magnetic nature ofnZVI enables the simple separation of the particles fromwater with the help of a magnet. nZVI shows remark-able adsorption and precipitation for As(III) and As(V)ions,176–178 where it was found that the adsorption occursthrough weak electrostatic interaction between the sorbateand the binding sites.179�180 Gupta et al.181 reported theapplication of nZVI encapsulated chitosan nanospheres forthe removal of arsenic from an aqueous medium. Withina wide range of pH (from 2 to 9), the authors have foundno significant interference from major anions like sul-fate, phosphate, and silicate on the adsorption behavior of





Figure 9. TEM micrographs of (a) ordered mesoporous carbon (OMC)and (b) nZVI deposited on the surface of OMC. The nZVI/carbon com-posite can effectively remove nitrobenzene from an aqueous solution asshown in (c). Reprinted with permission from [191], X. Ling, et al.,Chemosphere 87, 655 (2012). © 2012, Elsevier.

arsenic on nZVI, and removed total inorganic arsenic fromcontaminated ground water.Apart from arsenic, nZVI is being used to success-

fully treat various metallic ions in aqueous solutions, suchas Cr6+, Cu2+, Pb2+, Ba2+, As3+, As5+, Co2+ and soon.178�182–186 Additionally, nZVI is capable of removing orrecovering dissolved metals from solution (e.g., Cr(VI),U(VI)),187 as well as it is effective against several organicand inorganic contaminants (Fig. 9).188–191

4. CONCLUDING REMARKSCurrent wastewater treatment methods can control theorganic and inorganic wastes from water. But, these meth-ods are energy intensive and uneconomical because of theinability to completely purify water, as well as the inabilityto reuse the retentates. Nanotechnology can greatly influ-ence the domain of wastewater treatment in the comingfuture. Nanotechnology focuses on improving the exist-ing methods by increasing efficiency of the processes andenhancing the reusability of nanomaterials, thus saving thecost of operation of the plant or processes. Nanomaterialsare endowed with unique properties like high surface-to-volume ratio, high reactivity and sensitivity, having theproperty of self-assembling on substrates to form films,high adsorption, etc. that makes them suitable for watertreatment processing. Owing to these powerful properties,

nanomaterials are effective against various organic andinorganic pollutants, heavy metals, as well as againstseveral harmful microbes present in contaminated water.Nanomaterials can be engineered to efficiently harvestsolar energy, which is freely available, and thus can beused as visible light photocatalyst to decontaminate watercost effectively. Nanomaterials will become an essentialcomponent of industrial and wastewater treatment systemsin the future as more progress is made in terms of econom-ically efficient and ecofriendly technology development.

Acknowledgments: The authors would like to ack-nowledge financial support from The Research Council(TRC) of Oman.

References and Notes1. C. Fishman, The Big Thirst: The Secret Life and Turbulent Future

of Water, Free Press, New York (2011).2. R. Helmer and I. Hespanhol, Water Pollution Control: A Guide

to the Use of Water Quality Management Principles, E&FN Spon,London (2002).

3. G. L. Hornyak, J. Dutta, H. F. Tibbals, and A. K. Rao, Introductionto Nanoscience, CRC Press, Taylor & Francis Group, New York(2008).

4. T. E. Cloete, M. De Kwaadsteniet, and M. Botes, Nanotechnologyin Water Treatment Applications, Caister Academic Press, Poole,U. K. (2010).

5. M. R. Templeton and P. D. Butler, Introduction to WastewaterTreatment, Bookboon, London (2011).

6. F. S. G. Einschlag, Waste Water—Evaluation and Management,InTech, Rijeka, Croatia (2011).

7. G. Tchobanoglous, F. L. Burton, and H. D. Stensel, Wastewa-ter Engineering: Treatment and Reuse, McGraw-Hill Education,Whitby, Canada (2003).

8. G. L. Hornyak, J. J. Moore, H. F. Tibbals, and J. Dutta, Funda-mentals of Nanotechnology, CRC Press, Taylor and Francis Group,New York (2009).

9. S. Baruah, S. K. Pal, and J. Dutta, Nanosci. Nanotechnol. Asia 2, 90(2012).

10. S. Baruah and J. Dutta, Sci. Technol. Adv. Mater. 10, 013001 (2009).11. A. D. McNaught and A. Wilkinson, IUPAC Gold Book, Blackwell

Scientific Publications, Oxford (1997).12. K. Pirkanniemi and M. Sillanpää, Chemosphere 48, 1047 (2002).13. V. Pareek and A. A. Adesina, Handbook of Photochemistry and

Photobiology: Inorganic Photochemistry, edited by H. S. Nalwa,American Scientific Publishers, Los Angeles (2003).

14. M. Grätzel, Heterogenous Photochemical Electron Transfer, Taylorand Francis, New York (1988).

15. C. G. Zoski, Handbook of Electrochemistry, Elsevier, Amsterdam(2007).

16. D. S. Bhatkhande, V. G. Pangarkar, and A. A. C. M. Beenackers,J. Chem. Technol. Biotechnol. 77, 102 (2001).

17. A. Hagfeldt and M. Graetzel, Chem. Rev. 95, 49 (1995).18. R. Ullah and J. Dutta, J. Hazard Mater. 156, 194 (2008).19. K. G. Kanade, B. B. Kale, J. O. Baeg, S. M. Lee, C. W. Lee, S. J.

Moon, and H. Chang, Mater. Chem. Phys. 102, 98 (2007).20. C. B. Fitzgerald, M. Venkatesan, J. G. Lunney, L. S. Dorneles, and

J. M. D. Coey, Appl. Surf. Sci. 247, 493 (2005).21. T. Fu, Q. Gao, F. Liu, H. Dai, and X. Kou, Chin. J. Catal. 31, 797

(2010).22. J. Graciani, A. Nambu, J. Evans, J. A. Rodriguez, and J. F. Sanz,

J. Am. Chem. Soc. 130, 12056 (2008).





23. G. Ji, Z. Gu, M. Lu, J. Zhou, S. Zhang, and Y. Chen, Physica B:Condens. Matter 405, 4948 (2010).

24. S. Rehman, R. Ullah, A. M. Butt, and N. D. Gohar, J. HazardMater. 170, 560 (2009).

25. S. Fujihara, Y. Ogawa, and A. Kasai, Chem. Mater. 16, 2965 (2004).26. Z. Tao, X. Yu, X. Fei, J. Liu, Y. Zhao, H. Wu, G. Yang, S. Yang,

and L. Yang, Mater. Lett. 62, 3018 (2008).27. L. Wu, J. C. Yu, and X. Fu, J. Mol. Catal. A: Chem. 244, 25 (2006).28. W. Ho and J. C. Yu, J. Mol. Catal. A: Chem. 247, 268 (2006).29. J. Liu, R. Yang, and S. Li, Rare Metals. 25, 636 (2006).30. J. Moon, C. Y. Yun, K. W. Chung, M. S. Kang, and J. Yi, Catal.

Today 87, 77 (2003).31. D. Chatterjee and S. Dasgupta, J. Photochem. Photobiol. C: Pho-

tochem. Rev. 6, 186 (2005).32. M. Cheng, W. Ma, C. Chen, J. Yao, and J. Zhao, Appl. Catal. B:

Environ. 65, 217 (2006).33. P. Lei, C. Chen, J. Yang, W. Ma, J. Zhao, and L. Zang, Environ.

Sci. Technol. 39, 8466 (2005).34. S. Sarkar, A. Makhal, T. Bora, K. Lakhsman, A. Singha, J. Dutta,

and S. K. Pal, ACS Appl. Mater. Interfaces 4, 7027 (2012).35. I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, and

K. Takeuchi, J. Mol. Catal. A: Chem. 161, 205 (2000).36. I. Justicia, G. Garcia, G. A. Battiston, R. Gerbasi, F. Ager,

M. Guerra, J. Caixach, J. A. Pardo, J. Rivera, and A. Figueras,Electrochim. Acta 50, 4605 (2005).

37. S. T. Kochuveedu, D.-P. Kim, and D. H. Kim, J. Phys. Chem. C116, 2500 (2011).

38. E. Kowalska, O. O. P. Mahaney, R. Abe, and B. Ohtani, Phys.Chem. Chem. Phys. 12, 2344 (2010).

39. T. Wu, S. Liu, Y. Luo, W. Lu, L. Wang, and X. Sun, Nanoscale3, 2142 (2011).

40. S. Sarkar, A. Makhal, T. Bora, S. Baruah, J. Dutta, and S. K. Pal,Phys. Chem. Chem. Phys. 13, 12488 (2011).

41. A. Mills, R. H. Davies, and D. Worsley, Chem. Soc. Rev. 22, 417(1993).

42. S. H. S. Chan, T. Yeong Wu, J. C. Juan, and C. Y. Teh, J. Chem.Technol. Biotechnol. 86, 1130 (2011).

43. S. Baruah, R. F. Rafique, and J. Dutta, Nano 3, 399 (2008).44. S. Baruah, S. S. Sinha, B. Ghosh, S. K. Pal, A. K. Raychaudhuri,

and J. Dutta, J. Appl. Phys. 105, 0743081 (2009).45. S. Baruah, M. A. Mahmood, M. T. Z. Myint, T. Bora, and J. Dutta,

Beilstein J. Nanotechnol. 1, 14 (2010).46. S. Danwittayakul, M. Jaisai, T. Koottatep, and J. Dutta, Ind. Eng.

Chem. Res. 52, 13629 (2013).47. A. Kerc, M. Bekbölet, and A. M. Saatci, Int. J. Photoenergy 5, 75

(2003).48. Y. I. Matatov-Meytal and M. Sheintuch, Ind. Eng. Chem. Res.

37, 309 (1998).49. P. MacCarthy, Soil Sci. 166, 738 (2001).50. B. R. Eggins, F. L. Palmer, and J. A. Byrne, Water Res. 31, 1223

(1997).51. M. Bekbölet and G. Özkösemen, Water Sci. Technol. 33, 189

(1996).52. M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann,

Chem. Rev. 95, 69 (1995).53. A. Mills, A. Belghazi, and D. Rodman, Water Res. 30, 1973 (1996).54. B. Ohtani, Y. Okugawa, S. Nishimoto, and T. Kagiya, J. Phys.

Chem. 91, 3550 (1987).55. N. Serpone, Y. K. Ah-You, T. P. Tran, R. Harris, E. Pelizzetti, and

H. Hidaka, Sol. Energy 39, 491 (1987).56. J. Doménech and J. Peral, Sol. Energy 41, 55 (1988).57. L. B. Khalil, W. E. Mourad, and M. W. Rophael, Appl. Catal. B:

Environ. 17, 267 (1998).58. Y. C. Chen, S. L. Lo, H. H. Ou, and C. H. Chen, Water Sci. Technol.

63, 550 (2011).59. J. Lee, H. Park, and W. Choi, Environ. Sci. Technol. 36, 5462

(2002).

60. K. M. Joshi, B. N. Patil, D. S. Shirsath, and V. S. Shrivastava, Adv.Appl. Sci. Res. 2, 445 (2011).

61. K. Tanaka, K. Harada, and S. Murata, Sol. Energy 36, 159 (1986).62. Y. Inel and D. Ertek, J. Chem. Soc., Faraday Trans. 89, 129 (1993).63. M. R. Prairie, L. R. Evans, B. M. Stange, and S. L. Martinez,

Environ. Sci. Technol. 27, 1776 (1993).64. D. F. Ollis, E. Pelizzetti, and N. Serpone, Environ. Sci. Technol.

25, 1522 (1991).65. E. Borgarello, N. Serpone, G. Emo, R. Harris, E. Pelizzetti, and

C. Minero, Inorg. Chem. 25, 4499 (1986).66. N. Serpone, E. Borgarello, M. Barbeni, E. Pelizzetti, P. Pichat, J. M.

Hermann, and M. A. Fox, J. Photochem. 36, 373 (1987).67. L. R. Skubal, N. K. Meshkov, T. Rajh, and M. Thurnauer, J. Pho-

tochem. Photobiol. A 148, 393 (2002).68. F.-S. Zhang, J. O. Nriagu, and H. Itoh, J. Photochem. Photobiol. A

167, 223 (2004).69. X. Liu, T. Lv, Y. Liu, L. Pan, and Z. Sun, Desalin. Water Treat.

51, 3889 (2013).70. A. Mills and S. Le Hunte, J. Photochem. Photobiol., A 108, 1

(1997).71. G. S. Shephard, S. Stockenström, D. de Villiers, W. J. Engelbrecht,

and G. F. S. Wessels, Water Res. 36, 140 (2002).72. S. Baruah, M. Jaisai, and J. Dutta, Catal. Sci. Technol. 2, 918

(2012).73. M. Jaisai, S. Baruah, and J. Dutta, Beilstein J. Nanotechnol. 3, 684

(2012).74. S. Ajaya, J. A. Alfredo, B. Sunandan, V. S. Oleg, and D. Joydeep,

Nanotechnology 22, 215703 (2011).75. J. Cadotte, R. Forester, M. Kim, R. Petersen, and T. Stocker,

Desalination 70, 77 (1988).76. J. E. Drewes, C. L. Bellona, and P. Xu, Comparing Nanofiltration

and Reverse Osmosis for Treating Recycled Water, IWA Publishing(International Water Assoc), London (2008).

77. W. R. Bowen and H. Mukhtar, J. Membr. Sci. 112, 263 (1996).78. J. H. Choi, S. Dockko, K. Fukushi, and K. Yamamoto, Desalination

146, 413 (2002).79. A. I. Schäfer, A. G. Fane, and T. D. Waite, Nanofiltration: Princi-

ples and Applications, Elsevier Advanced Technology, Oxford, UK(2005).

80. A. R. D. Verliefde, E. R. Cornelissen, S. G. J. Heijman, J. Q. J. C.Verberk, G. L. Amy, B. Van der Bruggen, and J. C. van Dijk,J. Membr. Sci. 322, 52 (2008).

81. W. R. Bowen, J. S. Welfoot, and P. M. Williams, AIChE J. 48, 760(2002).

82. W. R. Bowen and J. S. Welfoot, Chem. Eng. Sci. 57, 1121 (2002).83. V. Yangali-Quintanilla, A. Sadmani, M. McConville, M. Kennedy,

and G. Amy, Water Res. 43, 2349 (2009).84. V. Yangali-Quintanilla, S. K. Maeng, T. Fujioka, M. Kennedy, and

G. Amy, J. Membr. Sci. 362, 334 (2010).85. M. Thanuttamavong, J. I. Oh, K. Yamamoto, and T. Urase, Water

Sci. Technol. 1, 77 (2001).86. M. Thanuttamavong, K. Yamamoto, J. Ik Oh, K. H. Choo, and S. J.

Choi, Desalination 145, 257 (2002).87. B. Van der Bruggen, M. Mänttäri, and M. Nyström, Sep. Purif.

Technol. 63, 251 (2008).88. M. Endo, H. Muramatsu, T. Hayashi, Y. A. Kim, M. Terrones, and

M. S. Dresselhaus, Nature 433, 476 (2005).89. A. Noy, H. G. Park, F. Fornasiero, J. K. Holt, C. P. Grigoropoulos,

and O. Bakajin, Nano Today 2, 22 (2007).90. M. Majumder, N. Chopra, R. Andrews, and B. J. Hinds, Nature

438, 44 (2005).91. G. Hummer, J. C. Rasaiah, and J. P. Noworyta, Nature 414, 188

(2001).92. K. Falk, F. Sedlmeier, L. Joly, R. R. Netz, and L. R. Bocquet, Nano

Lett. 10, 4067 (2010).





93. R. Wan, H. Lu, J. Li, J. Bao, J. Hu, and H. Fang, Phys. Chem.Chem. Phys. 11, 9898 (2009).

94. S. Karan, S. Samitsu, X. Peng, K. Kurashima, and I. Ichinose,Science 335, 444 (2012).

95. A. Srivastava, O. N. Srivastava, S. Talapatra, R. Vajtai, and P. M.Ajayan, Nat. Mater. 3, 610 (2004).

96. B. Pan and B. Xing, Environ. Sci. Technol. 42, 9005 (2008).97. Q. Li, S. Mahendra, D. Y. Lyon, L. Brunet, M. V. Liga, D. Li, and

P. J. J. Alvarez, Water Res. 42, 4591 (2008).98. V. K. Upadhyayula, S. Deng, M. C. Mitchell, G. B. Smith, and

N. V. K. S. Ghoshroy, Water Sci. Technol. 58, 179 (2008).99. Y. C. Sharma, V. Srivastava, V. K. Singh, S. N. Kaul, and C. H.

Weng, Environ. Technol. 30, 583 (2009).100. H. Parham, S. Bates, Y. Xia, and Y. Zhu, Carbon. 54, 215 (2013).101. A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007).102. L. Qiu, X. Zhang, W. Yang, Y. Wang, G. P. Simon, and D. Li,

Chem. Comm. 47, 5810 (2011).103. X. Yang, L. Qiu, C. Cheng, Y. Wu, Z.-F. Ma, and D. Li, Angew.

Chem. Int. Ed. 50, 7325 (2011).104. Y. Han, Z. Xu, and C. Gao, Adv. Funct. Mater. 23, 3693 (2013).105. R. R. Nair, H. A. Wu, P. N. Jayaram, I. V. Grigorieva, and A. K.

Geim, Science 335, 442 (2012).106. X. Zhang, A. J. Du, P. Lee, D. D. Sun, and J. O. Leckie, J. Membr.

Sci. 313, 44 (2008).107. G. E. Romanos, C. P. Athanasekou, F. K. Katsaros, N. K.

Kanellopoulos, D. D. Dionysiou, V. Likodimos, and P. Falaras,J. Hazard Mater. 212, 304 (2012).

108. T. V. Gestel, H. Kruidhof, D. H. A. Blank, and H. J. M.Bouwmeester, J. Membr. Sci. 284, 128 (2006).

109. Q. F. Alsalhy, J. M. Ali, A. A. Abbas, A. Rashed, B. V. D. Bruggen,and S. Balta, Desalin. Water Treat. 51, 1 (2013).

110. L. Jin, W. Shi, S. Yu, X. Yi, N. Sun, C. Ma, and Y. Liu,Desalination 298, 34 (2012).

111. L. M. Jin, S. L. Yu, W. X. Shi, X. S. Yi, N. Sun, Y. L. Ge, andC. Ma, Polymer 53, 5295 (2012).

112. A. K. Singh, R. P. Pandey, A. Jasti, and V. K. Shahi, RSC Adv.3, 458 (2013).

113. L. E. M. Gevers, I. F. J. Vankelecom, and P. A. Jacobs, Chem.Comm. 2500 (2005).

114. L. E. M. Gevers, I. F. J. Vankelecom, and P. A. Jacobs, J. Membr.Sci. 278, 199 (2006).

115. D. Turlan, E. P. Urriolabeitia, R. Navarro, C. Royo, M. Menendez,and J. Santamaria, Chem. Comm. 24, 2608 (2001).

116. V. Nikolakis, G. Xomeritakis, A. Abibi, M. Dickson, M. Tsapatsis,and D. G. Vlachos, J. Membr. Sci. 184, 209 (2001).

117. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, andL. J. Wan, Adv. Mater. 18, 2426 (2006).

118. A. Dabrowski, Adv. Colloid Interface Sci. 93, 135 (2001).119. D. S. Chaudhary, S. Vigneswaran, V. Jegatheesan, H. H. Ngo,

H. Moon, W. G. Shim, and S. H. Kim, Water Sci. Technol. 47, 113(2003).

120. F. Cecen and Ö. Aktas, Activated Carbon for Water and Wastewa-ter Treatment: Integration of Adsorption and Biological Treatment,Wiley-VCH, Weinheim, Germany (2011).

121. J. L. Fairey, G. E. Speitel, and L. E. Katz, Environ. Sci. Technol.40, 4268 (2006).

122. N. G. Asenjo, P. Alvarez, M. Granda, C. Blanco, R. Santamaria,and R. Menendez, J. Hazard Mater. 192, 1525 (2011).

123. K. Kadirvelu, K. Thamaraiselvi, and C. Namasivayam, Bioresour.Technol. 76, 63 (2001).

124. M. Kobya, E. Demirbas, E. Senturk, and M. Ince, Bioresour. Tech-nol. 96, 1518 (2005).

125. M. A. Al-Omair and E. A. El-Sharkawy, Environ. Technol. 28, 443(2007).

126. E. Tamburri, S. Orlanducci, M. L. Terranova, F. Valentini,G. Palleschi, A. Curulli, F. Brunetti, D. Passeri, A. Alippi, andM. Rossi, Carbon 43, 1213 (2005).

127. V. C. Moore, M. S. Strano, E. H. Haroz, R. H. Hauge, R. E.Smalley, J. Schmidt, and Y. Talmon, Nano Lett. 3, 1379 (2003).

128. X. Hu, T. Wang, X. Qu, and S. Dong, J. Phys. Chem. B 110, 853(2005).

129. Y. Li, J. Ding, J. Chen, C. Xu, B. Wei, J. Liang, and D. Wu, Mater.Res. Bull. 37, 313 (2002).

130. Y. H. Li, Y. M. Zhao, W. B. Hu, I. Ahmad, Y. Q. Zhu, X. J. Peng,and Z. K. Luan, J. Phys.: Conf. Ser. 61, 698 (2007).

131. X. Peng, Y. Li, Z. Luan, Z. Di, H. Wang, B. Tian, and Z. Jia, Chem.Phys. Lett. 376, 154 (2003).

132. C.-Y. Kuo, C.-H. Wu, and J.-Y. Wu, J. Colloid Interface Sci.327, 308 (2008).

133. I. Šafarík, K. Nymburská, and M. Šafaríková, J. Chem. Technol.Biotechnol. 69, 1 (1997).

134. L. C. A. Oliveira, R. V. R. A. Rios, J. D. Fabris, V. Garg, K. Sapag,and R. M. Lago, Carbon 40, 2177 (2002).

135. P. Gorria, M. Sevilla, J. A. Blanco, and A. B. Fuertes, Carbon44, 1954 (2006).

136. X. Peng, Z. Luan, Z. Di, Z. Zhang, and C. Zhu, Carbon 43, 880(2005).

137. Z. L. Zhang, H. S. Hong, J. L. Zhou, J. Huang, and G. Yu,Chemosphere 52, 1423 (2003).

138. X. D. Liu, Y. Murayama, M. Matsunaga, M. Nomizu, and N. Nishi,Int. J. Biol. Macromol. 35, 193 (2005).

139. Y. Xu, Z. Wang, R. Ke, and S. U. Khan, Environ. Sci. Technol.39, 1152 (2005).

140. R. Ke, Y. Xu, Z. Wang, and S. U. Khan, Environ. Sci. Technol.40, 3906 (2006).

141. J. Ru, H. Liu, J. Qu, A. Wang, and R. Dai, J. Hazard Mater. 141, 61(2007).

142. J. Huo, H. Liu, J. Qu, Z. Wang, J. Ge, and H. Liu, Sep. Purif.Technol. 44, 37 (2005).

143. H. Liu, J. Qu, R. Dai, J. Ru, and Z. Wang, Environ. Pollut. 147, 337(2007).

144. C. T. Chiou, Environ. Sci. Technol. 19, 57 (1985).145. J. N. Huckins, M. W. Tubergen, and G. K. Manuweera,

Chemosphere 20, 533 (1990).146. W. S. Wan Ngah and M. A. K. M. Hanafiah, Bioresour. Technol.

99, 3935 (2008).147. E. Guibal, Sep. Purif. Technol. 38, 43 (2004).148. P. Miretzky and A. F. Cirelli, J. Hazard Mater. 167, 10 (2009).149. X. Guo, S. Zhang, and X.-q. Shan, J. Hazard Mater. 151, 134

(2008).150. H. Aydın, Y. Bulut, and Ç. Yerlikaya, J. Environ. Manage. 87, 37

(2008).151. U. Kumar and M. Bandyopadhyay, Bioresour. Technol. 97, 104

(2006).152. K. Conrad and H. C. B. Hansen, Bioresour. Technol. 98, 89 (2007).153. L. H. Luo, Q. M. Feng, W. Q. Wang, and B. L. Zhang, Adv. Mater.

Res. 287–290, 592 (2011).154. S. Zhang, H. Niu, Z. Hu, Y. Cai, and Y. Shi, J. Chromatogr. A

1217, 4757 (2010).155. S. Zhang, W. Xu, M. Zeng, J. Li, J. Li, J. Xu, and X.-K. Wang,

J. Mater. Chem. A 1, 11691 (2013).156. M. Iram, C. Guo, Y. Guan, A. Ishfaq, and H. Liu, J. Hazard Mater.

181, 1039 (2010).157. H. Hu, Z. Wang, and L. Pan, J. Alloys Compd. 492, 656 (2010).158. N. N. Nassar, J. Hazard Mater. 184, 538 (2010).159. S. Jeon and K. Yong, J. Mater. Chem. 20, 10146 (2010).160. H. Abdolmohammad-Zadeh, E. Ghorbani, and Z. Talleb, J. Iran.

Chem. Soc. 10, 1 (2013).161. S. E. O’Reilly and M. F. Hochella, Jr, Geochim. Cosmochim. Acta

67, 4471 (2003).162. X. Wang, D. Xu, L. Chen, X. Tan, X. Zhou, A. Ren, and C. Chen,

Appl. Radiat. Isot. 64, 414 (2006).163. S. A. El-Safty, A. Shahat, and M. Ismael, J. Hazard Mater.

201–202, 23 (2012).





164. Y. X. Zhao, M. Y. Ding, and D. P. Chen, Anal. Chim. Acta 542, 193(2005).

165. M. Anbia and S. Salehi, Dyes Pigm. 94, 1 (2012).166. F. Li, P. Du, W. Chen, and S. Zhang, Anal. Chim. Acta 585, 211

(2007).167. W. Yantasee, R. D. Rutledge, W. Chouyyok, V. Sukwarotwat,

G. Orr, C. L. Warner, M. G. Warner, G. E. Fryxell, R. J. Wiacek,C. Timchalk, and R. S. Addleman, ACS Appl. Mater. Interfaces2, 2749 (2010).

168. Y. Dong, D. Wu, X. Chen, and Y. Lin, J. Colloid Interface Sci.348, 585 (2010).

169. Y. Bai, Q. Sun, R. Xing, D. Wen, and X. Tang, J. Hazard Mater.181, 916 (2010).

170. M. J. Zamzow, B. R. Eichbaum, K. R. Sandgren, and D. E. Shanks,Sep. Sci. Technol. 25, 1555 (1990).

171. E. Maliou, M. Malamis, and P. O. Sakellarides, Water Sci. Technol.25, 133 (1992).

172. S. K. Ouki and M. Kavannagh, Waste Manage. Res. 15, 383 (1997).173. K. M. Ibrahim, T. NasserEd-Deen, and H. Khoury, Environ. Geol.

41, 547 (2002).174. M. Pansini, C. Colella, and M. De Gennaro, Desalination 83, 145

(1991).175. S. Kesraoui-Ouki, C. Cheeseman, and R. Perry, Environ. Sci. Tech-

nol. 27, 1108 (1993).176. L. C. R. Machado, F. W. J. Lima, R. Paniago, J. D. Ardisson,

K. Sapag, and R. M. Lago, Appl. Clay Sci. 31, 207 (2006).

177. S. R. Kanel, J.-M. Grenèche, and H. Choi, Environ. Sci. Technol.40, 2045 (2006).

178. X.-Q. Li and W.-X. Zhang, J. Phys. Chem. C 111, 6939 (2007).179. X.-Q. Li, D. Elliott, and W.-X. Zhang, Crit. Rev. Solid State Mater.

Sci. 31, 111 (2006).180. X.-Q. Li and W.-X. Zhang, Langmuir 22, 4638 (2006).181. A. Gupta, M. Yunus, and N. Sankararamakrishnan, Chemosphere

86, 150 (2012).182. Ç. Üzüm, T. Shahwan, A. E. Eroðlu, K. R. Hallam, T. B. Scott,

and I. Lieberwirth, Appl. Clay Sci. 43, 172 (2009).183. O. Çelebi, Ç. Üzüm, T. Shahwan, and H. N. Erten, J. Hazard Mater.

148, 761 (2007).184. S. Y. Chen, W. H. Chen, and C. J. Shih, Water Sci. Technol.

58, 1947 (2008).185. L. Ma and W.-X. Zhang, Environ. Sci. Technol. 42, 5384 (2008).186. N. Moraci and P. S. Calabro, J. Environ. Manage. 91, 2336 (2010).187. N. Melitas, O. Chuffe-Moscoso, and J. Farrell, Environ. Sci. Tech-

nol. 35, 3948 (2001).188. R. Mantha, N. Biswas, K. E. Taylor, and J. K. Bewtra, Water Env-

iron. Res. 74, 280 (2002).189. A. Babuponnusami and K. Muthukumar, Environ. Sci. Pollut. Res.

20, 1596 (2013).190. K. S. Lin, N. B. Chang, and T. D. Chuang, Sci. Technol. Adv. Mater.

9, 025015 (2008).191. X. Ling, J. Li, W. Zhu, Y. Zhu, X. Sun, J. Shen, W. Han, and

L. Wang, Chemosphere 87, 655 (2012).

Received: 14 August 2013. Accepted: 1 September 2013.


applications of nanotechnology in wastewater treatment—a review

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