Volume 3 Number 6 December 2016 Pages 1225–1522 ......water treatment Volume 3 Number 6 December 2016 Pages 1225–1522 Environmental Science Nano PERSPECTIVE Cite this: Environ

rsc.li/es-nano

ISSN 2051-8153

Environmental ScienceNano

PERSPECTIVEPaul Westerhoff et al.Overcoming implementation barriers for nanotechnology in drinking water treatment

Volume 3 Number 6 December 2016 Pages 1225–1522

EnvironmentalScienceNano

PERSPECTIVE

Cite this: Environ. Sci.: Nano, 2016,

3, 1241

Received 19th June 2016,Accepted 14th September 2016

DOI: 10.1039/c6en00183a

rsc.li/es-nano

Overcoming implementation barriers fornanotechnology in drinking water treatment

Paul Westerhoff,*a Pedro Alvarez,b Qilin Li,b

Jorge Gardea-Torresdeyc and Julie Zimmermand

Nanotechnology enabled water treatment is a promising approach to enhance the effectiveness and effi-

ciency of purifying water in many areas of the world. Nanotechnology offers significant opportunities to

revolutionize approaches towards drinking water treatment by enhancing the multifunctionality and versa-

tility of treatment systems while reducing reliance on stoichiometric chemical addition (thus minimizing as-

sociated waste streams), shrinking large facilities with relatively long hydraulic contact times and minimizing

energy intensive processes. The unique material properties that emerge at the nano-scale enable solutions

to treat pollutants in water for which existing technologies are inefficient or ineffective. This perspective

describes the rationale, opportunities and barriers for translating this nascent technology from promising

bench-scale discoveries to full-scale commercialization and production of safe drinking water.

The need for technology innovation

Water treatment facilities rely on several treatment stages toremove multiple contaminants, which can be chemical ormicrobiological. Facilities treating fresh water rivers andlakes need to remove natural and anthropogeniccontaminants in addition to particulate matter, includingpathogenic microbes, clay and other debris, which typicallyrequire additional processes compared with groundwatersystems containing dissolved pollutants. Traditional, yet stillstandard practice today, drinking water treatment unitprocesses rely on chemical coagulants, gravity sedimentationand sand or membrane filtration to remove particulate

matter from surface waters. Chemical coagulants also offerthe potential to remove a few inorganic dissolved tracemetals (e.g., arsenic). The physical removal of pathogens isaugmented by oxidants to inactivate them and prevent theirreplication via addition of chemicals (chlorine, ozone) orexposure to ultraviolet light. However, there are nodisinfectants that are effective against all pathogens, andchemical disinfection can result in formation of disinfectionby-products (DBPs), some of which are known human carcin-ogens such as chloroform. To minimize formation of harmfulorganic DBPs, water utilities enhance coagulation processes(e.g., with higher coagulant dosages or acid/base addition tomodulate pH) or add non-selective sorbents (e.g., activatedcarbon, ion exchange) to remove natural organic matter(NOM) that can serve as DBP precursors. Despite theseknown challenges, innovation is generally slow in the munici-pal water market because of the large size, high capital cost,strict regulatory environment and need for proven technol-ogy. Advanced technologies are now even capable ofconverting treated sewage into drinking water that surpassesall regulatory guidelines, but high cost and operational exper-tise requirements will limit their implementation toaugmenting net water production in large cities. Desalination

Environ. Sci.: Nano, 2016, 3, 1241–1253 | 1241This journal is © The Royal Society of Chemistry 2016

a School of Sustainable Engineering and The Built Environment, Arizona State

University, Box 3005, Tempe, AZ 85287-3005, USA. E-mail: [email protected];

Tel: +480 965 2885bCivil and Environmental Engineering, Rice University, 6100 Main Street MS 519,

Houston, TX 77005, USAc Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968-

0513, USAdChemical & Environmental Engineering & Forestry & Environmental Studies,

Yale University, 9 Hillhouse Avenue, New Haven, CT 06511, USA

Nano impact

The unique properties that arise at the nano-scale (magnetism, high surface area, selective surface reactivity, surface catalysis, rapid ion delivery, photo-catalysis, plasmonic resonance, dielectric properties, electrical conductivity, super hydrophobicity, strength, etc.) can be used to purify drinking water.While discoveries of these processes are reported in the literature, actual products and processes have been slow to mature beyond the bench-scale intolarger-scale, constructed systems or consumer point of use devices for purifying drinking water. This perspective article helps identify barriers that can beovercome to enable nanotechnology for water treatment.

Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00

.

View Article OnlineView Journal | View Issue

http://crossmark.crossref.org/dialog/?doi=10.1039/c6en00183a&domain=pdf&date_stamp=2016-11-30

http://dx.doi.org/10.1039/c6en00183a

http://pubs.rsc.org/en/journals/journal/EN

http://pubs.rsc.org/en/journals/journal/EN?issueid=EN003006

1242 | Environ. Sci.: Nano, 2016, 3, 1241–1253 This journal is © The Royal Society of Chemistry 2016

technologies rely on energy-intensive processes such as highpressure reverse osmosis or thermal distillation. The largestdesalination plants are located in coastal regions and supplywater to large cities. Despite pervasive use of technology topurify drinking water, consumers have become convincedthat the all-purpose tap water currently provided by munici-palities may not meet their personal aesthetic or perceivedhealth requirements, and consequentially bottled water andpoint-of-use treatment units (under the sink, refrigerator,etc.) are multi-billion dollar markets in developed and devel-oping countries alike.

Water treatment plants (WTPs) are now efficient at pro-ducing safe water for use throughout society, where water isnot only used for drinking, but also for a wide array of do-mestic uses (washing clothes and dishes, bathing, toilets,outdoor irrigation), commercial uses (heating, cooling towers,sanitation at restaurants, etc.), industrial uses (bottling facili-ties, manufacturing facilities, etc.) and other societal uses(fire fighting, water features, etc.). The fraction of potable wa-ter used for drinking, food preparation and washing dishes issmall (<10 L per-day) compared against the net per capitatap water production (∼200 to 500 L per-day) in developedcountries. Yet in most locations, all the water is treated tothe same high degree of water quality.

Despite the reports and potential of emerging technolo-gies,1,2 many people do not have access to municipal watersupplies or water known to be in compliance with federal reg-ulations. For example, over 40 million people in the UnitedStates rely on other sources (e.g., wells) that are not regu-lated. Furthermore, over 84% of all water systems in the USAserve fewer than 500 people and account for 79% of all viola-tions of federal maximum contaminant level regulations. So-called point-of-entry (POE) water treatment systems for thesecommunities still need to meet the entire per capita water de-mand (beyond just drinking water demand), but small com-munities often lack capital or technical expertise to operatesophisticated water treatment systems. Many of these sys-tems treat groundwater and face very specific water qualitychallenges. For example, over 3000 water utilities in the USAviolate the EPA regulation for nitrate in drinking water, andothers violate regulations for a limited number of organicpollutants (e.g., pesticides). In emerging mega-cities and de-veloping communities across the globe water delivery may beintermittent or require augmentation by on-site storage (in-cluding rainwater harvesting). A growing number of middleclass families throughout China and India are purchasingpoint-of-use water treatment devices for ∼$200 (US) each thatinclude individual or combinations of processes such as acti-vated carbon, reverse osmosis and UV-disinfection.

Simply scaling down Victorian age coagulation, sedimenta-tion and filtration processes from municipal to communityor point-of-use scale faces many challenges, from use of con-centrated and hazardous chemicals to managing complex re-sidual waste streams. As such, POU systems (and increasinglyPOE systems) are relying on more selective technologychoices (selective sorbents, advanced separation systems or

disinfection schemes). Managing DBPs illustrates an interest-ing contrast between controlling their formation at large cen-tralized WTPs (e.g., through improved NOM precursor re-moval) and having consumers use POU devices that removesome regulated DBPs (e.g., trihalomethanes by sorption to ac-tivated carbon) but may not remove unregulated DBPs (e.g.,nitrosamines). The POU marketplace (>$20 billion annually)can rapidly incorporate new technologies because of its highsales volume, lower capital cost, small scale and relativelyrapid turnover of replacement parts (i.e., change out of sorp-tion systems may be once every three months or every fewthousand liters). Innovation and technology turnover in thelarger and more fiscally conservative municipal water treat-ment plants have been much slower. Examples of innovationdo exist. For example, although used for decades in thewastewater industry, consideration of low-pressure ultravioletirradiation for microbial inactivation in drinking water wasnot seriously considered until both a major outbreak of wa-terborne illness in the USA and tightening regulations on in-organic and organic DBPs spurred on research, technologyvalidation and sensors. 20 years later, regulatory acceptanceof UV light for primary drinking water disinfection was com-mon. This long cycle time in innovation is becoming a recog-nized barrier for investment and rapid innovation in themunicipal-level drinking water sector.

We postulate that the POU marketplace offers a very goodopportunity for development and implementation of noveltechnology because of its growing customer demand, compet-itiveness among industrial companies, relatively rapid prod-uct turn-over, willingness to innovate technologies, and rela-tively low per unit capital costs. Furthermore, residentialPOU systems offer the opportunity to innovate new watermanagement approaches, from complete in-home wastewaterreclamation and reuse to increased rainwater harvesting totreatment and use of brackish or saline groundwater. Insome locations across the rural western USA, especially onnative American lands, rural communities must haul waterover 10 miles because local well water is too saline. Develop-ing low-energy POU technologies to purify the local saline wa-ter could reduce the need to haul water. Innovations in thePOU marketplace can and will impact larger and more capitalintensive municipal water treatment facilities by validatingtechnologies. Early adaptors of new technology will likely in-clude larger progressive municipalities or utilities facing fewoptions because of existing technology limitations or limitedaccess to alternative water sources.

Promulgation of new regulations (e.g., lowering arsenic orDBP MCLs) has also spurred innovation and adoption oftechnologies. Smaller rural communities relying on POE sys-tems often have the most limitations on alternative watersources, but they must also meet increasingly stringent drink-ing water regulations. Therefore, POE systems will also likelybecome early adopters of new technologies. This was demon-strated in the USA after tightening regulation on arsenic indrinking water disproportionately affected smallgroundwater-reliant communities, and several new sorption

Environmental Science: NanoPerspective

Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00

. View Article Online



technologies using selective nano-structured sorbents of ironor titanium dioxide were rapidly developed, approved by reg-ulators and are now widely used. Because of these social,technology and regulatory drivers, consumers and even somesmall communities are increasing reliant on POU systems tohelp meet the growing demand for high quality drinkingwater.

Why nanotechnology?

While nanotechnology is not the only approach to innovatePOU technologies, it does offer the opportunity to decreasethe size and cost of POU systems and improve their pollutantselectivity. This enhances flexibility to match treated waterquality to the intended use, resulting in more effective use oftreatment capacity. Nanotechnology also offers the opportu-nity to mediate reactions and processes where chemical or re-actor based technology counterparts simply do not exist. Ex-cellent review articles describe how the novel properties ofnanoparticles, nanomaterials or nanoscale processes can beincorporated into water treatment processes.2–14 For example:iron nanoparticles can sorb arsenic and be removed in mag-netic fields; titanium dioxide nanoparticles can be photocata-lytic to produce powerful oxidants (HO radicals) or selectivelytransfer electrons to water pollutants (e.g., nitrate) to reducethem to innocuous by-products; platinum series metal nano-particles with high surface area can transform chlorinated or-ganics to innocuous by-products; high surface area nano-silver can release silver ions to disinfect membrane surfacesand prevent biofouling sunlight photolysis of fullerenes pro-duces reactive oxygen species capable of inactivating virusand some bacteria; low friction and narrow pore diametercarbon nanotubes can be used to separate H2O from saltions; and unique aspect ratio of graphene oxide can createanti-fouling bacterial surfaces.

Most nanotechnology research for water treatment appli-cations remains on the bench-top where a single process isoften demonstrated in very simple water matrices rather thancomplex real-world waters that contain low levels of multiplepollutants in the presence of competing ions, foulants (iron,silica, etc.), particulates, and NOM. There are few examples ofintegrated intentionally nano-enabled commercialized watertreatment systems. While some known examples exist, otherexamples new to the market are slow to disclose their preciseunique nano-material. Building on the previously mentioned,successful commercialization of nano-sorbents for arsenictreatment is an interesting example. Arsenic treatment in theUSA largely impacted POE systems where groundwater wasthe source for a publically owned water distribution sys-tems.15,16 Although decades of research showed the ability tosynthesize nano- to micron-iron (hydr)oxides and titanium di-oxide (rutile, anatase and amorphous phases) that readilycomplex arsenate ion on the sorbent surface, there were fewcommercially-available media that could be readily placedinto packed beds and operate for long periods of time with-out leaching metals or leading to excessive head loss. The

regulation spurred innovation to create iron-, titanium andother metal oxide based nano-structured and agglomeratednanoparticle based sorbents for arsenic removal. In one case,nano-scale iron (hydr)oxide was produced in situ withinmacroporous ion exchange beads.17–21 The spherical beadsembedded with irregularly shaped nano-iron could be easilyplaced in packed bed columns and operated at hydraulic load-ing rates consistent with practical experience. Design consul-tants, tank/pump equipment manufacturers and regulatorscould easily understand technology principles, thereby leadingto rapid adoption. A second nano-adsorbent also entered themarket for use in a packed bed configuration, presumably forthe same reasons including operational ease. This product in-volved nano-scale titanium dioxide sintered together into ahighly porous aggregate, which could be milled and sieved to asize commensurate with the required hydraulic conditionsin packed bed systems. Other metal oxide media entered themarketplace, and after a variety of testing, those that becamecommercially viable also received certification of claims by theNational Sanitation Foundation International (NSF Interna-tional). This certification is essential for all materials and sys-tems used in municipal water supplies and are commonly ad-vertised on POU devices. All these metal oxide materials inpacked beds potentially underwent some “attrition” duringuse (normal operations or backwashing to waste), where re-lease of nanoparticles could occur. The NSF International cer-tification centered on claims that the system removed arsenicand did not release any contaminants that potentially violateEPA pollutant regulations. There was no special regulationthat required monitoring or and unique concern about iron ortitanium nanoparticle release from the systems. It would ap-pear that the benefit of addressing challenging problems (i.e.,human health risk benefits of reducing arsenic levels in drink-ing water) outweigh concerns about potential hazards or pub-lic perception about adopting nanotechnology for water purifi-cation, even though the potential release of nanoparticles wasnot rigorously tested. Thus, there are examples of deployedsystems that are nano-enabled and accepted by the public andregulatory agencies, and these successful stories show oneroute of technology development in response to variousdrivers. This path depends upon regulatory drivers, takes sig-nificant time and does not represent a pro-active technologyleadership route for technology innovation. Now, around theglobe there are hundreds of examples of nanotechnologybased solutions to water quality challenges that have been dis-covered, but few nanotechnologies have yet to be integratedinto functional drinking water treatment systems.

Approaches towards nano-enabledwater treatment systems

In the context of this perspectives article, a system is definedas a single module or series of modules that removes pollut-ants and produces clean drinking water that meets regulatoryrequirements and consumer aesthetic and perceived safetyexpectations. A module may consist of an existing reactor

Environmental Science: Nano Perspective

Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




platform (e.g., packed bed, polymeric membrane, gravity sedi-mentation system, etc.) or a new reactor design platform nec-essary for deploying nanotechnology. Nanotechnology refersto the use of pre-synthesized objects, or in situ production ofobjects, with at least one dimension below 100 nm. We do notconsider the use of polymers or sorbents simply having poresbelow 100 nm as nanotechnology because many examples ofthese are already in use today for water purification (e.g., ionexchange resins, granular activated carbon, nanofiltration andreverse osmosis). Five overarching science questions will beaddressed that could guide the safe development of nano-enabled water treatment systems:

1. Which novel nano-properties could be used for waterpurification?

2. What features of new reactor designs are required to en-able use of nano-scale properties?

3. Does embedding or attaching nanomaterials into scaf-folding decrease their unique functionality?

4. How can we harness solar energy directly to reducecosts of water purification?

5. What safety concerns must be addressed to commercial-ize nano-enabled water technologies?

Which novel nano-properties could be used for waterpurification?

Nanomaterials offer engineers a unique array of propertiesdifficult or inefficient to obtain using traditional chemicals

or bulk materials (Table 1). Rather than adding bulkchemicals to the entire flow stream, the localized surface reac-tions allow treatment at targeted locations, greatly enhancingtreatment efficiency. Example applications include, amongmany others, localized reactions can achieve disinfection atsurfaces to prevent biofouling by adding nano silver or gra-phitic materials,22–26 disinfection based upon nano-materialshapes,27 heating for thermal distillation or oxidation/regen-eration of organic pollutants on surfaces,28,29 selective sorp-tion of trace organic or inorganic pollutants.30 Many nano-materials have catalytic properties. Potable drinking watersystems currently do not apply catalytic processes, and nano-technology enables the use of both low cost catalysts (e.g.,titanium dioxide activated by light31–33) as well as higher valuecatalysts (e.g., palladium, platinum, or gold activated by hydro-gen peroxide) to degrade oxo-anions or chlorinatedorganics.2,34–38

The large surface area provided by nano-structures has al-ready been incorporated into many sorbents (e.g., granularactivated carbon for organics removal or granular ferric(hydr)oxides for arsenic removal), but these materials are tra-ditionally produced in “top-down” processes that involve syn-thesizing micron to millimeter sized materials and crushing—and potentially re-agglomerating—them into pellet-sizedmedia for use in packed bed treatment systems. Nano-technology allows for “bottom-up” design of sorbents, wheremorphology, pore sizes and surface composition can be

Table 1 Examples nanomaterial applications for drinking water treatment

Nanoscale property Opportunities in water purification systemsComparison with current technology for drinking watertreatment

Magnetism Separation of small reactive particles used ascatalysts or sorbents

Replaces gravity separation or high pressure filtration

High surface area &selective surfacereactivity

High ratio of reactive atoms on the outside ofnanomaterials relative to their total mass improvesunit efficiency; achieves highly selective sorption ofsingular or multiple contaminants

High surface area can be attained using nano-porous materialsbut have internal mass transport limitations; high surface areamaterials can be precipitated in situ by continuous addition ofchemicals (e.g., alum)

Surface catalysis Allows for multiple elements to be placed in closeproximity; allows use of small quantities of costlyelements

Catalysis is not currently used in drinking water treatment

Ion delivery Localized deposition of anti-bacterial nanomaterialsto prevent biofouling

Practices chemical disinfection of the entire water body ratherthan localized surface release of anti-bacterial agents

Photocatalysis New materials allow for controlled production ofdifferent reactive oxygen species or aqueouselectrons

Titanium dioxide with low-pressure Hg lamps are only commer-cialized technology

Photo-thermal Super-heating via light to enable solar membranedistillation

Thermal membrane distillation heats entire body of water ratherthan localized surface heating

Dielectric properties Heating induced by microwaves for self-cleaningsurfaces or regeneration of sorbents

Self cleaning surfaces are not currently available, andreactivation of granular activated carbon occurs ex situ to packedbeds in large gas fired kilns

Electricallyconductive

Functionalized MWCNT electrodes in capacitivedeionization processes

Metallic electrodes have low adsorption capacity compared withMWCNTs

Super hydrophobic Controlling hydrophobicity of surfaces withgraphene can reduce biofouling or improvemembrane flux

A narrow range of polymers exist to modulate hydrophobicity ofmembrane or other surfaces

Strength Electrospinning of nanofibers offers newopportunities to synthesize polymers and metal(hydr)oxide composites

Hydraulically rigid ion exchange beds, activated carbonagglomerates or other porous sorbents are used in packed beds;thin film polymeric membranes are supported on more rigidsupports; cationic polymers added to strengthen metalhydroxide flocs


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




controlled, and then materials can be used directly or ag-glomerated into pellet-sized media. Specific surface area maynot be the key determinant for adsorption within the nano-range; instead, available surface area based on the number ofreactive sites is a more important characteristic, as shown forselenium ion sorption to nano-hematite.30 The prior exampleof agglomerated nano-TiO2 media for arsenic removal is oneexample. Zeolites, which have been explored for trace organicremoval in potable water treatment systems,39,40 have poten-tial to be integrated into other reactor designs that benefitfrom their molecular sieve properties. Thus, differences be-tween “top-down” and “bottom-up” material preparation isone important aspect of nano-technology. In some cases, theeconomics of manufacturing bottom-up products have lim-ited adoption of some water treatment products, but thenanotechnology revolution is moving such synthesis pro-cesses from custom design to bulk scale manufacturing.

What features of new reactor designs are required to enableuse of nano-scale properties?

Two basic process design approaches can be taken to utilizenanoparticles. First, nanoparticles can be freely dispersed inbulk water much like a chemical additive would be, with thedifference being that a method must be in place to recoverthe added nanoparticles. For example, the Purifics PhotoCat®system uses ∼1 g L−1 TiO2 photocatalyst slurry through banksof low-pressure mercury lamps (∼254 nm), and it is then re-covered by a ceramic membrane for recirculation and re-use.41 Purified water permeates through the membrane, anda high cross-flow velocity prevents membrane fouling. The ce-ramic membrane resists abrasion by the TiO2 slurry. Whilethe primary TiO2 nanoparticles are small (∼20 nm), the com-mon catalyst material (P25 – TiO2) is larger and photo-aggregates such that under normal operation the concentra-tion of titanium in the permeate is below most ICP-MS detec-tion limits of 2 to 10 μg L−1.41 This system type has beenshown to remove trace organics, hexavalent chromium, DBPprecursors, virus and Cryptosporidium.42–44 Other types ofmembranes have been used to retain and recycle particleslurries (e.g., the Cristal process® combines filtration mem-branes and powdered activated carbon), but this has notbeen applied to nanoparticles. Instead of recirculating parti-cles, other processes build up porous layers of iron oxide,PAC or silica particles on membrane surfaces to prevent accu-mulation of foulants directly at the membrane surface.45 An-other example is magnetite core nanoparticles, which couldbe removed from flowing water using magnetictechnology.46–50 While research exists on such reactors, nowater treatment magnetic separation reactors are currentlycommercially available. In these examples, it is advantageousto separate, recover and reuse nanoparticles, which differsfrom the single use application of traditional chemicals.There may be applications where solely nanoparticle removalis appropriate. For example, the template assisted nucleation(TAN) process produces calcite nanoparticles on the surface

of beads in a supersaturated solution of calcium carbonate,and hydraulic forces in a fluidized bed release the nano-particles into solution.51 Applying this technology in coolingtowers suggests that the nanoparticles cause less, or easier toclean, scale formation on heat exchange systems, and similarapproaches might be replacements for chemical intensivelime softening processes prior to membrane filtration, thusavoiding formation, separation and dewatering of sludges.

The second basic design approach is to embed nano-particles into macroscale hierarchical structures. This wouldinclude nanoparticle coatings on membranes; glass fiber fil-ters or packed bed media; precipitation of nanomaterialswithin pores of ion exchange, activated carbon or othermacroporous sorbents; attachment of nanomaterials to reac-tor surfaces; agglomeration of individual nanoparticles intopellets or porous ceramics (ceramic filters, carbon block); orintegration of pre-formed nanoparticles into polymers as theyare casted into membranes, fibers or other structures. Severalarsenic treatment media fall into this second basic design ap-proach. Hybrid ion exchange media contain iron (hydr)oxideprecipitated nanoparticles within pores of ion exchangebeads and are commercially available for drinking watertreatment. The engineered nanoparticles are initiated withsorption of permanganate in the beads before adding ironsalts, resulting in formation of iron nanoparticles within themedia. During operation, there is no release of iron nano-particles from the media. Other media are commercially pro-duced by sintering and re-agglomerating TiO2 using inor-ganic or organic binders. Fig. 1 shows additional techniquesyet to be commercialized for integrating functional nano-materials into hierarchical structures suitable for use inreactors.

Does embedding or attaching nanomaterials into scaffoldingdecrease their unique functionality?

Adding freely dispersed nanoparticles to water will almostcertainly require energy (magnetic, pressure) to separatethem from flowing water. While nanoparticle slurries haveadvantages associated with high particle concentrations andpollutant diffusion mass transfer limitations, the risk ofnanoparticles in finished drinking water may necessitateattaching or embedding nanoparticles into macroporous hi-erarchical structures (i.e., scaffolding). Scaffolding may beinert polymers or other materials or may be active struc-tures themselves (e.g., activated carbon, ion exchange poly-mers, etc.). Scaffolding nanoparticles may overcome barriersassociated with nanoparticle release, but it may also resultin new barriers associated with mass transport and/or theability to apply external stimuli (e.g., light) to nano-materials. While scaffolding nanoparticles may result in aloss of active surface for pollutant reactions, some evidenceexists suggests that immobilizing nanoparticles may en-hance performance by preventing agglomeration of nano-particles in slurry-form and by maintaining highly accessi-ble surface area.52


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




Nanomaterials have high surface area that can enablerapid reaction kinetics, but this is only advantageous if targetpollutants can readily come into contact with their surfaces.Pollutants such as arsenic need to diffuse through stagnantfilms and then into pores of millimeter diameter hybrid(nanoparticle) ion exchange/adsorption media. These systemsare often limited by intraparticle mass transfer rates53–57

whereas faster arsenic removal can occur when nanoparticles(5–25 nm diameter) are freely dispersed in water.57,58 In othercases, diffusion limitations may not be between a target pol-lutant and a nanoparticle reactive surface. Instead, the limita-tion is from diffusion of ions from a nanoparticle into solu-tion (e.g., dissolution of silver ions from nano-silver fordisinfection purposes). Embedding the silver nanoparticlewithin a low water permeability polymer limits the rate ofredox-mediate dissolution of Ag0 to Ag+, and diffusion of Ag+

back into solution. Proper polymer selection, surface attach-ment of nanoparticles, or creation of pores within polymer

may all influence the mass transfer rate of Ag+ delivery backinto solution. In contrast to influencing pollutant or ionmass transport, embedded nanoparticles themselves can leadto certain types of mass transport limitations. For example,the wettability (i.e., hydrophobicity) or tortuosity of pores inzeolites impregnated in non-permeability polymers influ-ences the water flux rate through this type of new low-pressure membrane system.59 Nano-silver particles attachedto the surface of ceramic membranes reduced bacterialgrowth.7,60 A final example relates to embedding pre-formednanoparticles into electrospun polymeric fibers for theintended use of pollutant sorption. Adding metallic nano-particles into polymer precursors produces fibers with nano-particles well dispersed in the fiber, but the polymer coatsthe nanoparticles and prevents their access to pollutants inwater.61 However, use of carbon nanoparticles surprisinglyproduces pores that allow access by pollutants in water tosorb on these nanoparticle surfaces.61 Alternative to this

Fig. 1 Example techniques to embed nanomaterials into hierarchical structures for enable purification of drinking water. A) LED launching lightinto a fiber optic coated with nano-catalysts combined with a membrane and photo-electric cathode; B) nanoparticle decorated surface on asuper paramagnet magnetite core; C) amine functionalize fullerene covalently bound on silica to attract and photoactively inactivate virus via ROSproduction; D) mixed matrix membrane with nano-zeolites embedded in a non-permeable polymer thin film; E) conducting MWCNTs into a capac-itive deionization system; F) homogeneous distribution of multiple highly selective for sorption nanoparticles embedded within carbon block; G)embedded carbon nanoparticles in an electrospun layer above a hydrophobic membrane to generate solar drive water vapor production; H) super-hydrophilic or antimicrobial functionalized nanoparticles deposited on the surface of conventional polymeric membranes; I) nanoparticles (whiteregions) imaged in a network of polymeric electrospun fibers.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




single-step synthesis process, fibers can undergo post-treatment after spinning and be decorated with nanoparticleson the outside of fiber.62

Many nano-materials of interest require external stimuli tobecome functional. This includes light (UV, visible, solar) forphotolysis, upconverting or photothermal processes, magneticfields for recovery of paramagnetic nanoparticles, microwavesfor materials with high dielectric constants, electrical currentfor conductive materials, or hydrogen gas for reductive pro-cesses. Reactors containing nanomaterials that depend on ex-ternal stimuli face similar mass transport limitations for mo-lecular diffusion or water flux, and they require configurationsto deliver the external stimuli. For example, solar light medi-ated processes require large surface areas and potentially shal-low reactor depths to allow light to contact nanoparticle sur-faces on the base of the reactor. Use of radial emitting low-pressure or medium-pressure mercury lamps with nano-particles coated on the internal surface of reactors have beendeveloped,63 but the reactors suffer from relative low masses ofphotocatalysts and are prone to surface fouling by inorganicions (e.g., iron).64 Solar light concentrated with reflective mir-rors or artificial light sources can be delivered into reactors viafiber optics. Coating high surface area fiber optics with photo-catalysts induces a change in refractive index on the fibers thatcauses light absorption by the photocatalyst to produce reactiveoxidants capable of oxidizing pollutants in the outer waterphase.65–67

Managing light irradiation of photocatalysts is a barrier inboth fixed film and slurry-based reactors. In dense slurries(e.g., >1 gTiO2 L

−1) light penetration may be <1 mm into theslurry solution, creating a small reactive zone around the lightsource. Decreasing the slurry concentration can increase lightpenetration, but may be offset by less efficient surface reac-tions. Slurry-based systems also have the barrier of requiringhighly efficient separation of freely dispersed nano-photocatalysts in water (e.g., ceramic membranes), fromtreated water which should not contain any nanoparticles.

Some have sought to overcome the barrier of using slurriesand ceramic membranes, but creating ceramic coatingscontaining photo-active surfaces.7,68 Others envision electro-spinning titanium precursors followed by calcination to pro-duce fibers of anatase or rutile that are a few microns wide andcan operate as a reactive filter when integrated with artificiallight.69,70 A second example involves a physic–chemical reduc-tive process, which is uncommon in drinking water treatmentdespite the presence of many oxidized pollutants (e.g., nitrate,perchlorate, MTBE, TCE). Slurry and embedded/attached cata-lytic nanoparticle (e.g., Pd/Pt) reactor designs are discussed inthe literature,71–73 and all face barriers of effective hydrogengas delivery through water to the catalyst surface.

How can we harness solar energy directly to reduce costs ofwater purification?

For smaller drinking water systems located off the water-grid,there is significant interest in developing solar-driven water

purification processes. Photovoltaic panels convert a rela-tively limited portion of the solar energy spectrum into elec-tricity, where subsequent transformation and equipment in-efficiencies (e.g., pumps to pressurize membranes, energy topower UV lamps, etc.) mean that a relatively small percentage(<10%) of the solar energy is available for water purification.In contrast, direct solar-driven processes aim to capture abroader solar spectrum or modulate absorption properties ofnanomaterials to utilize visible light rather than only UVlight. For example, traditional membrane distillation (MD)uses petroleum or solar energy to heat water and create atemperature gradient driving force across a hydrophobicmembrane for water vapor molecules to diffuse through andthen condense on the back-side of the membrane.74,75 Thediscovery that light absorbing black nanoparticles induceplasmonic resonance processes that generate localizedheating and a new mechanism for low-temperature light-in-duced steam generation offers new opportunities for MDdesign.28,76–78 This may lead to low-energy direct solar MDunits to desalinate brackish groundwater or even seawater byimmobilizing the light-absorbing nanoparticles at the surfaceof the MD membrane, resulting in localized solar heatingthat creates a large thermal gradient at the membrane sur-face and generates vapor that diffuses rapidly across themembrane (ref). Separately, significant progress has beenmade to synthesize visible light active photocatalysts or up-converting materials.79–89 Less effort has gone into design ofsolar-driven photocatalytic systems, but research to date in-cludes outdoor reactors using raceways coated with photo-catalysts, solar light captured via reflective-concentrating sur-faces and launched into fiber optics coated withphotocatalysts, or direct solar irradiation in batch bottles(e.g., modified SODIS).81,90–95 Two major barriers for scale-upof direct solar driven processes are light management (deliv-ery into reactors containing water) and the amount of solarenergy (∼1.4 kW m−2) available for reactions.

What safety concerns exist around nano-enabled watertechnologies?

Any new technology potentially faces public perception andregulatory acceptance obstacles. Getting ahead of and recog-nizing the importance of these obstacles will be critical forthe success of nano-enabled water treatment technologies.Release of nanomaterials from water treatment systems intodrinking water is arguably the most pressing risk. Recallingthat risk is a function of hazard and exposure, mitigatingthese concerns could be achieved through either of thesecharacteristics. However, few studies on nanomaterial inges-tion via drinking water exposures have been performed, butsome papers explore the potential occurrence levels or risksof nanomaterials in surface water sources entering watertreatment plants.96–99 The public may be more concernedabout the use of nanotechnology in drinking water treatment,as opposed to consumer products like textiles based upon re-cent social science research.100–102


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




In terms of addressing inherent hazard, safe-by-designprinciples of nanomaterials can be undertaken. The goal is toachieve or exceed the functional performance of current ma-terials and the technologies they enable while minimizing in-herent hazard to avoid risk to human health and the environ-ment at all stages of the nanomaterial and water treatmentunit process life cycle. This emphasizes the importance ofestablishing structure–property–function (SPF) and structure–property–hazard (SPH) relationships to guide the rational de-sign of nanomaterials.103 First and foremost, the use of ele-ments with known human toxicity (e.g., nickel, cadmium,chromium) should be avoided. Instead, nanomaterials of be-nign, low-cost, and earth-abundant compositions should beused. For example, iron nanoparticles synthesized fromcoagulant-grade ferric chloride, which is already approved foruse at water treatment plants, can be integrated into an insitu regeneration process for granular activated carbonpacked beds that alleviate the need to remove, transport andthermally regenerate GAC off-site.29,104 Next, it is importantto consider that shape and aspect ratio of nanomaterials in-fluence both treatment efficiency and potential toxicity. Al-though surface area is not the sole determinant of functionalefficacy for nanoscale materials, the shape of nano cupric ox-ide (CuO) influences antimicrobial activity,27 which could bedesirable to prevent surface membrane fouling. CuO nano-sheets have the highest surface reactivity, electrochemical ac-tivity and antimicrobial activity. While less active than thenanosheets, CuO nanoparticles (sphere-like shape) demon-strate enhanced reactivity compared to the bulk CuO.27 Incor-porating multi-walled carbon nanotubes (MWCNTs) into wa-ter treatment systems for sorption or electrochemicalreactions may be desirable, and it has been shown that sur-face functionalization can influence both functional perfor-mance and bioactivity. That is, surface carboxyl groups andthe redox activity of carbonyl groups promote enhancedMWCNT reactivity, both in terms of electrochemical and anti-microbial activity.105 A few examples exist where life cycle as-sessment and safe by design concepts have been integratedinto development of nano-enabled water treatment unit pro-cesses,30,106 but the sheer complexity in nanomaterial proper-ties relative to the supposedly known properties of chemicaladditives may lead to uncertainties in the actual benefits ofnanotizing processes or products.107 Thus, by designingnanomaterials using SPF and SPH, it is possible to utilizenanomaterials that realize the desired performance withlower potential human health risks. When this informationis unknown or this strategy is not viable because those nano-particles that provide the intended function also cause theundesirable hazard, exposure control is necessary to addresssafety concerns.

The first consideration with respect to exposure is to de-sign reactors that contain nanoparticles to either completelyretain nanoparticles via separation systems (e.g., magneticseparation) or physical barriers (e.g., membranes). Becausecomplete retention/separation may be difficult, an alternativeapproach is to embed nanoparticles in macro-scale structures

where they cannot be released into water. When designing tominimize exposure, it is essential to have validated analyticaltechniques to quantify nanomaterials in water at levels of rel-evant concern. Drinking water regulations exist for some ele-ments used in nanomaterials (Table 2) as primary (MCL) orsecondary (SMCL) maximum contaminant levels and wouldserve as absolute values not to exceed. Some nanomaterialsthat undergo dissolution (e.g., Ag0, ZnO) have been showngenerally to pose less adverse hazards than the ions they re-lease (Ag+, Zn2+), but concern about delivery of the nano-particles into cells and causing localized risks remains debat-able. Most carbon-based engineered nanoparticles containelemental carbon rather than organic carbon. Total organiccarbon (TOC) is indirectly involved in drinking water regula-tions as part of the Surface Water Treatment Rule, whereTOC of 1 to 6 mg L−1 is relevant. Some carbon-basedengineered nanoparticles can be detected by liquid-TOC ana-lytical methodologies. Turbidity is a measurement of visiblelight scattering and is used in drinking water regulations as asurrogate for pathogens. High concentrations of engineerednanoparticles can be detected as turbidity. Filtered drinkingwater turbidity must be less than 0.3 NTU in at least 95% ofsamples and at no time exceed 5 NTU. Surface and groundwaters contain natural nanoparticles (clays, cell fragments,iron (hydr)oxides, etc.) in the 1–100 nm range and are on theorder of 108 to 1012 particles per liter of 10 nm particles.Physical processes at WTPs conservatively remove >99% ofparticles in this size range, but clearly it would be difficult todistinguish a few engineered nanoparticles mixed in with theheterogeneous sources of natural nano-scale colloids. Whilea few techniques exist to count particles in the 1–100 nm sizerange, the drinking water industry currently only widely usesparticle counters in the 1–100 μm size range, where monitor-ing Giardia cysts or Cryptosporium oocysts in the 4 to 20 μmrange is a current focus.

The above regulatory frameworks provide the least restric-tive potential limits for nanoparticles in water, and analysesfollowing standard methods could be applied. More ad-vanced techniques exist to differentiate size and concentra-tion for some elements and nanoparticles. For example,single-particle ICP-MS methods are not yet available in com-mercial laboratories but have been demonstrated in researchlaboratories to work for many of the elements in Table 2.108

Thermal combustion and microwave based analyticalschemes can detect graphitic nanomaterials in water at con-centrations between 0.01 to 0.1 mg L−1, but their use remainslimited to only a few research labs. Fullerenes and fullerolscan be quantified using LC/MS methods.109 To improve de-tection at low concentrations, techniques such as cloud pointextraction and solid phase extraction can be used topreconcentrate the nanoparticles.110–112 Third-party organiza-tions such as NSF International certify that drinking waterchemicals, coatings, etc. do not exceed maximum levels ofimpurities and certify treatment processes removals. Similarcertifications will be a barrier for nano-enabled processes togain industry and consumer acceptance. Of course, potential


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




concerns exist regarding proper disposal at the end of life forany residuals or devices, and it remains unclear to what ex-tent disposal of nano-containing materials may be subject tostricter regulations than other types of wastes.113–115 Overall,barriers for safe use of nanomaterials include both readily ac-cessible instrumentation and selection of relevant or accept-able levels of nanoparticles in finished drinking waters.

The path forward to realize newparadigms in producing drinkingwater

It is important to ensure that the added benefits of usingnew materials or processes exceed associated risks and to en-sure that any risk of new technology is dwarfed by the knownrisks posed by priority pollutants in drinking water. Withnearly two decades of research on risks posed by engineerednanomaterials, the authors believe it should now be possibleto safely use nanotechnology for water purification. However,scaled-up production of nanomaterials, system-level design,operational experience and regulatory frameworks for safeuse of nano-based water treatment technologies remain intheir infancy and offer a rich environment for fundamentaland applied research.

Although there are few commercialized nano-enableddrinking water treatment technologies on the market today,nanotechnology holds promise for transitioning away fromcontinuous and stoichiometric chemical dosing that is com-monly practiced for municipal scale water treatment (e.g., co-agulants, acids, bases) or even POU scale (e.g., NaCl for ionexchange softeners) to materials that can be activated by ex-ternal, non-chemical stimuli (e.g., light, microwaves, etc.) orenable catalytic reactions that do not consume chemical re-

agents. Furthermore, nanotechnology offers the opportunityfor compact design because of the high and selective surfacearea available at the nanoscale and the ability to integratemultiple functionalities into a single process (e.g., reactivemembranes). Nanotechnologies for water treatment may re-main only laboratory exercises unless a viable commercializa-tion strategy develops to address potential unintended conse-quences and perceived risks. Because municipal watertreatment systems are large-scale, treating tens of millions ofgallons per day, they are chemical intensive and costly. Con-sequently cities are risk-adverse with respect to implementingnew technologies directly from the bench-scale. Testbeds andfield demonstrations alone are likely inadequate to convinceregulators and utilities of the scalability, robustness andsafety of nano-enabled drinking water systems. We believe apath to commercialization lies through niche markets ini-tially that can readily recognize the benefits of nanotechnol-ogy. These markets could include consumer-focused underthe sink systems (e.g., point of use) because of the large mar-ket segment, high level of global competition, rapid innova-tion cycle times (6 to 24 months), and desire for product dif-ferentiation. This application focus should improve themanufacturing capability of nanomaterials for the water sec-tor, driving down material costs as synthesis transitions fromreagent-grade materials in labs to bulk-purity materials atcommercial scale. Proven successes on this scale could, over5–10 years, provide sufficient confidence in the technologyand allow scale up to municipal level drinking water treat-ment systems.

Acknowledgements

This work was partially funded through the Nano-EnabledWater Treatment Technologies Nanosystems Engineering

Table 2 Regulatory implications for elements commonly used in engineered nanoparticles

Element Example nanomaterials containing element Regulatory level

Aluminum Al2O3 SMCL = 0.05 to 0.2 mg L−1

Boron BN No regulatory level but included on contaminantcandidate list 2 (CCL2) (<1 mg L−1)a

Cadmium CdSe MCL = 0.005 mg L−1

Carbon SWCNT, MWCNT, graphene,graphene oxide, C60, C3N4

Not directly regulated

Copper CuO Action level = 1.3 mg L−1

SMCL = 1.0 mg L−1

Gold Au0 No regulatory level (<20 ppt)a

Iron Fe2O3, Fe3O4 SMCL = 0.3 mg L−1

Nickel NiO Regulated until 1995 with an MCL = 0.1 mg L−1

Palladium Pd0 No regulatory level (<50 ppt)a

Platinum No regulatory level (<50 ppt)a

Silica SiO2, zeolites No regulatory level (5 to 50 mgSiO2 L−1)a

Silver Ag0 SMCL = 0.1 mg L−1

Titanium TiO2 No regulatory level (0.001 to 0.01 mg L−1)a

Vanadium BiVO4 No regulatory level but included on CCL3(0.001 to 0.01 mg L−1)a

Zinc ZnO SMCL = 5 mg L−1

a Provides commonly-occurring range in surface waters for non-regulated elements as a comparison.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




Research Center by the National Science Foundation (EEC-1449500).

References

1 S. Kumar, et al., Nanotechnology-Based Water TreatmentStrategies, J. Nanosci. Nanotechnol., 2014, 14(2), 1838–1858.

2 X. Qu, P. J. J. Alvarez and Q. Li, Applications ofnanotechnology in water and wastewater treatment, WaterRes., 2013, 47(12), 3931–3946.

3 A. S. Adeleye, et al., Engineered nanomaterials for watertreatment and remediation: Costs, benefits, andapplicability, Chem. Eng. J., 2016, 286, 640–662.

4 R. D. Ambashta and M. Sillanpaa, Water purification usingmagnetic assistance: A review, J. Hazard. Mater.,2010, 180(1–3), 38–49.

5 S. Baruah, M. N. Khan and J. Dutta, Perspectives andapplications of nanotechnology in water treatment, Environ.Chem. Lett., 2016, 14(1), 1–14.

6 M. M. Khin, et al., A review on nanomaterials forenvironmental remediation, Energy Environ. Sci., 2012, 5(8),8075–8109.

7 J. Kim and B. Van der Bruggen, The use of nanoparticles inpolymeric and ceramic membrane structures: Review ofmanufacturing procedures and performance improvementfor water treatment, Environ. Pollut., 2010, 158(7),2335–2349.

8 Z. Ma, et al., Evaluation and removal of emergingnanoparticle contaminants in water treatment: a review,Desalin. Water Treat., 2016, 57(24), 11221–11232.

9 D. Mohan and C. U. Pittman Jr., Arsenic removal fromwater/wastewater using adsorbents - A critical review,J. Hazard. Mater., 2007, 142(1-2), 1–53.

10 V. K. K. Upadhyayula, et al., Application of carbonnanotube technology for removal of contaminants indrinking water: A review, Sci. Total Environ., 2009, 408(1),1–13.

11 B. Van der Bruggen and C. Vandecasteele, Distillation vs.membrane filtration: overview of process evolutions inseawater desalination, Desalination, 2002, 143(3), 207–218.

12 S. Wang, et al., Adsorptive remediation of environmentalpollutants using novel graphene-based nanomaterials,Chem. Eng. J., 2013, 226, 336–347.

13 X. Qu, et al., Nanotechnology for a Safe and SustainableWater Supply: Enabling Integrated Water Treatment andReuse, Acc. Chem. Res., 2013, 46(3), 834–843.

14 M. A. Lazar, S. Varghese and S. S. Nair, PhotocatalyticWater Treatment by Titanium Dioxide: Recent Updates,Catalysts, 2012, 2(4), 572–601.

15 K. Alfredo, C. Seidel and J. A. Roberson, Reviewing theoccurrence data used in the revised Arsenic Rule, J. - Am.Water Works Assoc., 2014, 106(3), 67–68.

16 E. McGavisk, J. A. Roberson and C. Seidel, Usingcommunity economics to compare arsenic compliance andnoncompliance, J. - Am. Water Works Assoc., 2013, 105(3),47–48.

17 D. Chakraborti, et al., Arsenic calamity in the Indiansubcontinent - What lessons have been learned?, Talanta,2002, 58(1), 3–22.

18 L. Cumbal, et al., Polymer supported inorganicnanoparticles: characterization and environmentalapplications, React. Funct. Polym., 2003, 54(1–3), 167–180.

19 L. Cumbal and A. K. Sengupta, Arsenic removal usingpolymer-supported hydrated iron(III) oxide nanoparticles:Role of Donnan membrane effect, Environ. Sci. Technol.,2005, 39(17), 6508–6515.

20 A. M. Cooper, et al., The effect of carbon type on arsenicand trichloroethylene removal capabilities of iron (hydr)oxide nanoparticle-impregnated granulated activated car-bons, J. Hazard. Mater., 2010, 183(1–3), 381–388.

21 P. Sylvester, et al., A hybrid sorbent utilizing nanoparticlesof hydrous iron oxide for arsenic removal from drinkingwater, Environ. Eng. Sci., 2007, 24(1), 104–112.

22 F. Perreault, A. F. de Faria and M. Elimelech,Environmental applications of graphene-based nano-materials, Chem. Soc. Rev., 2015, 44(16), 5861–5896.

23 M. S. Rahaman, et al., Control of biofouling on reverseosmosis polyamide membranes modified with biocidalnanoparticles and antifouling polymer brushes, J. Mater.Chem. B, 2014, 2(12), 1724–1732.

24 M. Ben-Sasson, et al., In situ formation of silvernanoparticles on thin-film composite reverse osmosis mem-branes for biofouling mitigation, Water Res., 2014, 62,260–270.

25 J. D. Schiffman, et al., Biocidal Activity of Plasma ModifiedElectrospun Polysulfone Mats Functionalized withPolyethyleneimine-Capped Silver Nanoparticles, Langmuir,2011, 27(21), 13159–13164.

26 M. S. Mauter, et al., Antifouling Ultrafiltration Membranesvia Post-Fabrication Grafting of Biocidal Nanomaterials,ACS Appl. Mater. Interfaces, 2011, 3(8), 2861–2868.

27 L. M. Gilbertson, et al., Shape-Dependent Surface Reactivityand Antimicrobial Activity of Nano-Cupric Oxide, Environ.Sci. Technol., 2016, 50(7), 3975–3984.

28 M. L. Brongersma, N. J. Halas and P. Nordlander, Plasmon-induced hot carrier science and technology, Nat.Nanotechnol., 2015, 10(1), 25–34.

29 C. A. Chiu, et al., In-situ regeneration of saturated granularactivated carbon by an iron oxide nanocatalyst, Water Res.,2013, 47(4), 1596–1603.

30 A. W. Lounsbury, et al., The role of counter ions in nano-hematite synthesis: Implications for surface area and sele-nium adsorption capacity, J. Hazard. Mater., 2016, 310,117–124.

31 C. McCullagh, et al., The application of TiO2 photocatalysisfor disinfection of water contaminated with pathogenicmicro-organisms: a review, Res. Chem. Intermed.,2007, 33(3–5), 359–375.

32 T. Yang, K. Doudrick and P. Westerhoff, Photocatalyticreduction of nitrate using titanium dioxide forregeneration of ion exchange brine, Water Res.,2013, 47(3), 1299–1307.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




33 K. Doudrick, et al., Photocatalytic nitrate reduction inwater: Managing the hole scavenger and reaction by-product selectivity, Appl. Catal., B, 2013, 136, 40–47.

34 L. A. Pretzer, H. J. Song and Y.-L. Fang, et al.,Hydrodechlorination catalysis of Pd-on-Au nanoparticlesvaries with particle size, J. Catal., 2013, 298, 206–217.

35 H. F. Qian, et al., Supporting palladium metal on goldnanoparticles improves its catalysis for nitrite reduction,Nanoscale, 2014, 6(1), 358–364.

36 L. A. Pretzer, et al., Hydrodechlorination catalysis of Pd-on-Au nanoparticles varies with particle size, J. Catal.,2013, 298, 206–217.

37 M. S. Wong, et al., Cleaner water using bimetallicnanoparticle catalysts, J. Chem. Technol. Biotechnol.,2009, 84(2), 158–166.

38 Q. Cheng, et al., Hexavalent chromium removal using metaloxide photocatalysts, Appl. Catal., B, 2015, 176, 740–748.

39 A. Rossner and D. R. U. Knappe, MTBE adsorption onalternative adsorbents and packed bed adsorberperformance, Water Res., 2008, 42(8–9), 2287–2299.

40 A. Rossner, S. A. Snyder and D. R. U. Knappe, Removal ofemerging contaminants of concern by alternativeadsorbents, Water Res., 2009, 43(15), 3787–3796.

41 H. O. N. Stancl, K. Hristovski and P. Westerhoff, HexavalentChromium Removal Using UV-TiO2/Ceramic MembraneReactor, Environ. Eng. Sci., 2015, 32(8), 676–683.

42 D. Gerrity, et al., A comparison of pilot-scale photocatalysisand enhanced coagulation for disinfection byproduct miti-gation, Water Res., 2009, 43(6), 1597–1610.

43 D. Gerrity, et al., Photocatalytic inactivation of viruses usingtitanium dioxide nanoparticles and low-pressure UV light,J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ.Eng., 2008, 43(11), 1261–1270.

44 H. Ryu, et al., Photocatalytic inactivation ofCryptosporidium parvum with TiO2 and low-pressure ultra-violet irradiation, Water Res., 2008, 42(6–7), 1523–1530.

45 M. M. Zhang, et al., Fouling and natural organic matterremoval in adsorben/membrane systems for drinking watertreatment, Environ. Sci. Technol., 2003, 37(8), 1663–1669.

46 C. T. Yavuz, et al., Magnetic separations: From steel plantsto biotechnology, Chem. Eng. Sci., 2009, 64(10), 2510–2521.

47 J. T. Mayo, et al., The effect of nanocrystalline magnetitesize on arsenic removal, Sci. Technol. Adv. Mater.,2007, 8(1–2), 71–75.

48 C. T. Yavuz, et al., Low-field magnetic separation of mono-disperse Fe3O4 nanocrystals, Science, 2006, 314(5801),964–967.

49 S. Lin, D. Lu and Z. Liu, Removal of arsenic contaminantswith magnetic gamma-Fe2O3 nanoparticles, Chem. Eng. J.,2012, 211, 46–52.

50 J. W. Farrell, et al., Arsenic Removal by NanoscaleMagnetite in Guanajuato, Mexico, Environ. Eng. Sci.,2014, 31(7), 393–402.

51 E. Ruiz-Agudo, et al., Template-Assisted Crystallization ofSulfates onto Calcite: Implications for the Prevention of SaltDamage, Cryst. Growth Des., 2013, 13(1), 40–51.

52 J. Lee, et al., C-60 Aminofullerene Immobilized on Silica asa Visible-Light-Activated Photocatalyst, Environ. Sci.Technol., 2010, 44(24), 9488–9495.

53 M. Badruzzaman, P. Westerhoff and D. R. U. Knappe,Intraparticle diffusion and adsorption of arsenate ontogranular ferric hydroxide (GFH), Water Res., 2004, 38(18),4002–4012.

54 K. D. Hristovski, H. Nguyen and P. K. Westerhoff, Removalof arsenate and 17-ethinyl estradiol (EE2) by iron (hydr)ox-ide modified activated carbon fibers, J. Environ. Sci. Health,Part A: Toxic/Hazard. Subst. Environ. Eng., 2009, 44(4),354–361.

55 K. Hristovski, et al., Simultaneous removal of perchlorateand arsenate by ion-exchange media modified with nano-structured iron (hydr)oxide, J. Hazard. Mater., 2008, 152(1),397–406.

56 K. Hristovski, P. Westerhoff and J. Crittenden, An approachfor evaluating nanomaterials for use as packed bedadsorber media: A case study of arsenate removal bytitanate nanofibers, J. Hazard. Mater., 2008, 156(1–3),604–611.

57 K. Hristovski, A. Baumgardner and P. Westerhoff,Selecting metal oxide nanomaterials for arsenic removalin fixed bed columns: From nanopowders to aggregatednanoparticle media, J. Hazard. Mater., 2007, 147(1–2),265–274.

58 S. Yean, et al., Effect of magnetite particle size onadsorption and desorption of arsenite and arsenate,J. Mater. Res., 2005, 20(12), 3255–3264.

59 M. L. Lind, et al., Influence of Zeolite Crystal Size onZeolite-Polyamide Thin Film Nanocomposite Membranes,Langmuir, 2009, 25(17), 10139–10145.

60 Y. H. Lv, et al., Silver nanoparticle-decorated porous ce-ramic composite for water treatment, J. Membr. Sci.,2009, 331(1–2), 50–56.

61 N. H. von Reitzenstein, X. Bi and Y. Yang, et al.,Morphology, Structure, and Properties of Metal Oxide/Polymer Nanocomposite Electrospun Mats, J. Appl. Polym.Sci., 2016, 133(33), 43811.

62 C. Y. Su, et al., Photocatalytic Process of SimultaneousDesulfurization and Denitrification of Flue Gas by TiO2-Polyacrylonitrile Nanofibers, Environ. Sci. Technol.,2013, 47(20), 11562–11568.

63 A. Fernandez, et al., Preparation and characterization ofTiO2 photocatalysts supported on various rigid supports(glass, quartz and stainless steel). Comparative studies ofphotocatalytic activity in water purification, Appl. Catal., B,1995, 7(1–2), 49–63.

64 K. Kabra, R. Chaudhary and R. L. Sawhney, Treatment ofhazardous organic and inorganic compounds throughaqueous-phase photocatalysis: A review, Ind. Eng. Chem.Res., 2004, 43(24), 7683–7696.

65 N. J. Peill and M. R. Hoffmann, Mathematical model ofa photocatalytic fiber-optic cable reactor for heteroge-neous photocatalysis, Environ. Sci. Technol., 1998, 32(3),398–404.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




66 N. J. Peill and M. R. Hoffmann, DEVELOPMENT ANDOPTIMIZATION OF A TIO2 COATED FIBEROPTIC CABLEREACTOR - PHOTOCATALYTIC DEGRADATION OF4-CHLOROPHENOL, Environ. Sci. Technol., 1995, 29(12),2974–2981.

67 M. R. Hoffmann, et al., Environmental Applications ofSemiconductor Photocatalysis, Chem. Rev., 1995, 95(1), 69–96.

68 H. Choi, E. Stathatos and D. D. Dionysiou, Sol–gelpreparation of mesoporous photocatalytic TiO2 films andTiO2/Al2O3 composite membranes for environmentalapplications, Appl. Catal., B, 2006, 63(1–2), 60–67.

69 M. J. Nalbandian, et al., Tailored Synthesis of PhotoactiveTiO2 Nanofibers and Au/TiO2 Nanofiber Composites:Structure and Reactivity Optimization for WaterTreatment Applications, Environ. Sci. Technol., 2015, 49(3),1654–1663.

70 S. Agarwal, A. Greiner and J. H. Wendorff, Functionalmaterials by electrospinning of polymers, Prog. Polym. Sci.,2013, 38(6), 963–991.

71 B. P. Chaplin, et al., Critical Review of Pd-Based CatalyticTreatment of Priority Contaminants in Water, Environ. Sci.Technol., 2012, 46(7), 3655–3670.

72 R. Dittmeyer, V. Hollein and K. Daub, Membrane reactorsfor hydrogenation and dehydrogenation processes basedon supported palladium, J. Mol. Catal. A: Chem.,2001, 173(1–2), 135–184.

73 J. C. Fanning, The chemical reduction of nitrate in aqueoussolution, Coord. Chem. Rev., 2000, 199, 159–179.

74 A. Alkhudhiri, N. Darwish and N. Hilal, Membranedistillation: A comprehensive review, Desalination,2012, 287, 2–18.

75 L. M. Camacho, et al., Advances in Membrane Distillationfor Water Desalination and Purification Applications,Water, 2013, 5(1), 94–196.

76 O. Neumann, et al., Compact solar autoclave based onsteam generation using broadband light-harvesting nano-particles, Proc. Natl. Acad. Sci. U. S. A., 2013, 110(29),11677–11681.

77 O. Neumann, et al., Solar Vapor Generation Enabled byNanoparticles, ACS Nano, 2013, 7(1), 42–49.

78 N. J. Hogan, et al., Nanoparticles Heat through LightLocalization, Nano Lett., 2014, 14(8), 4640–4645.

79 R. Asahi, et al., Visible-light photocatalysis in nitrogen-doped titanium oxides, Science, 2001, 293(5528),269–271.

80 J. A. Byrne, et al., A Review of Heterogeneous Photocatalysisfor Water and Surface Disinfection, Molecules, 2015, 20(4),5574–5615.

81 E. L. Cates, et al., Engineering Light: Advances inWavelength Conversion Materials for Energy andEnvironmental Technologies, Environ. Sci. Technol.,2012, 46(22), 12316–12328.

82 H.-I. Kim, et al., Harnessing low energy photons (635 nm)for the production of H2O2 using upconversionnanohybrid photocatalysts, Energy Environ. Sci., 2016, 9(3),1063–1073.

83 J.-H. Kim and J.-H. Kim, Encapsulated Triplet-Triplet Anni-hilation-Based Upconversion in the Aqueous Phase for Sub-Band-Gap Semiconductor Photocatalysis, J. Am. Chem. Soc.,2012, 134(42), 17478–17481.

84 J. Lonnen, et al., Solar and photocatalytic disinfection ofprotozoan, fungal and bacterial microbes in drinking water,Water Res., 2005, 39(5), 877–883.

85 G. Y. Lui, et al., Photovoltaic powered ultraviolet and visiblelight-emitting diodes for sustainable point-of-use disinfec-tion of drinking waters, Sci. Total Environ., 2014, 493,185–196.

86 S. Malato, et al., Photocatalytic decontamination anddisinfection of water with solar collectors, Catal. Today,2007, 122(1–2), 137–149.

87 W. Zhang, B. Jia and Q. Wang, et al., Visible-light sensitiza-tion of TiO2 photocatalysts via wet chemical N-doping forthe degradation of dissolved organic compounds in waste-water treatment: a review, J. Nanopart. Res., 2015, 17(5), 221.

88 R. Fagan, et al., A review of solar and visible light activeTiO2 photocatalysis for treating bacteria, cyanotoxins andcontaminants of emerging concern, Mater. Sci. Semicond.Process., 2016, 42, 2–14.

89 M. Pelaez, et al., A review on the visible light activetitanium dioxide photocatalysts for environmentalapplications, Appl. Catal., B, 2012, 125, 331–349.

90 J. Blanco, et al., Review of feasible solar energy applicationsto water processes, Renewable Sustainable Energy Rev.,2009, 13(6–7), 1437–1445.

91 E. L. Cates and J.-H. Kim, Bench-scale evaluation of waterdisinfection by visible-to-UVC upconversion under high-intensity irradiation, J. Photochem. Photobiol., B, 2015, 153,405–411.

92 D. A. Keane, et al., Solar photocatalysis for waterdisinfection: materials and reactor design, Catal. Sci.Technol., 2014, 4(5), 1211–1226.

93 K. G. McGuigan, et al., Solar water disinfection (SODIS): Areview from bench-top to roof-top, J. Hazard. Mater.,2012, 235, 29–46.

94 B. Sommer, et al., SODIS - An emerging water treatmentprocess, J. Water Supply: Res. Technol.–AQUA, 1997, 46(3),127–137.

95 D. Spasiano, et al., Solar photocatalysis: Materials,reactors, some commercial, and pre-industrialized applica-tions. A comprehensive approach, Appl. Catal., B,2015, 170, 90–123.

96 K. Tiede, et al., How important is drinking water exposurefor the risks of engineered nanoparticles to consumers?,Nanotoxicology, 2016, 10(1), 102–110.

97 K. D. Good, et al., Implications of EngineeredNanomaterials in Drinking Water Sources, J. - Am. WaterWorks Assoc., 2016, 108(1), E1–E17.

98 I. Lynch, Water governance challenges presented bynanotechnologies: tracking, identifying andquantifying nanomaterials (the ultimate disparatesource) in our waterways, Hydrol. Res., 2016, 47(3),552–568.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00




99 M. Troester, H. J. Brauch and T. Hofmann, Vulnerability ofdrinking water supplies to engineered nanoparticles, WaterRes., 2016, 96, 255–279.

100 K. S. Fielding, et al., Comparing Public Perceptions ofAlternative Water Sources for Potable Use: The Case ofRainwater, Stormwater, Desalinated Water, and RecycledWater, Water Resour. Manage., 2015, 29(12), 4501–4518.

101 A. Capon, et al., Comparative analysis of the labelling ofnanotechnologies across four stakeholder groups,J. Nanopart. Res., 2015, 17(8), 13.

102 A. Capon, et al., Perceptions of risk from nanotechnologiesand trust in stakeholders: a cross sectional study of public,academic, government and business attitudes, BMC PublicHealth, 2015, 15.

103 L. M. Gilbertson, et al., Designing nanomaterials tomaximize performance and minimize undesirableimplications guided by the Principles of Green Chemistry,Chem. Soc. Rev., 2015, 44(16), 5758–5777.

104 A. Bach and R. Semiat, The role of activated carbon as acatalyst in GAC/iron oxide/H2O2 oxidation process,Desalination, 2011, 273(1), 57–63.

105 L. M. Gilbertson, et al., Toward safer multi-walled carbonnanotube design: Establishing a statistical model that re-lates surface charge and embryonic zebrafish mortality,Nanotoxicology, 2016, 10(1), 10–19.

106 M. Gifford, et al., Reducing Environmental Impacts ofMetal (Hydr)Oxide Nanoparticle Embedded Anion ExchangeResins Using Anticipatory Life Cycle Assessment, Environ.Sci.: Nano, 2016, DOI: 10.1039/C6EN00191B.

107 R. Hjorth, et al., The applicability of chemical alternativesassessment for engineered nanomaterials, Integr. Environ.Assess. Manage., 2016, DOI: 10.1002/ieam.1762.

108 S. Lee, et al., Nanoparticle Size Detection Limits by SingleParticle ICP-MS for 40 Elements, Environ. Sci. Technol.,2014, 48(17), 10291–10300.

109 B. F. G. Pycke, et al., Beyond nC(60): strategies foridentification of transformation products of fullereneoxidation in aquatic and biological samples, Anal. Bioanal.Chem., 2012, 404(9), 2583–2595.

110 M. D. Bezerra, M. A. Z. Arruda and S. L. C. Ferreira, Cloudpoint extraction as a procedure of separation and pre-concentration for metal determination using spectro-analytical techniques: A review, Appl. Spectrosc. Rev.,2005, 40(4), 269–299.

111 S. M. Majedi, H. K. Lee and B. C. Kelly,Chemometric Analytical Approach for the Cloud PointExtraction and Inductively Coupled Plasma MassSpectrometric Determination of Zinc OxideNanoparticles in Water Samples, Anal. Chem., 2012, 84(15),6546–6552.

112 T. M. Benn, et al., Evaluation of extraction methods forquantification of aqueous fullerenes in urine, Anal. Bioanal.Chem., 2011, 399(4), 1631–1639.

113 S. A. K. Read, et al., Foresight Study on the RiskGovernance of New Technologies: The Case ofNanotechnology, Risk Anal., 2016, 36(5), 1006–1024.

114 W. C. Walker, C. J. Bosso and M. Eckelman, et al.,Integrating life cycle assessment into managing potentialEHS risks of engineered nanomaterials: reviewing progressto date, J. Nanopart. Res., 2015, 17(8), 344.

115 C. E. H. Beaudrie, M. Kandlikar and T. Satterfield, FromCradle-to-Grave at the Nanoscale: Gaps in US RegulatoryOversight along the Nanomaterial Life Cycle, Environ. Sci.Technol., 2013, 47(11), 5524–5534.


Publ

ishe

d on

16

Sept

embe

r 20

16. D

ownl

oade

d on

07/

12/2

016

16:0

8:00



Documents

Volume 3 Number 6 December 2016 Pages 1225–1522 ......water treatment Volume 3 Number 6 December 2016 Pages 1225–1522 Environmental Science Nano PERSPECTIVE Cite this: Environ