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Nanotechnology for water purification: Applications of nanotechnology methods in wastewater treatment

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  • Fare Labs Pvt Ltd
  • Massachusetts Institute of Technology The David H. Koch Institute for Integrative Cancer Research at MIT, BCH/Harvard Medical School At Dan Anderson/Robert Langer Lab

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Providing clean and affordable drinking water is one of the modern-times challenges. The world’s growing population causes water scarcity, and pollutants contaminate whatever water sources are left. Nanotechnology has provided innovative solutions for water purification. This chapter reviews nanotechnology-enabled water-treatment processes, showing how they transform our water supply and wastewater treatment. The following topics are discussed: different nanomaterials, properties, mechanisms, advantages compared to existing methods, limitations, research needs for commercialization, and toxicities of nanomaterials.
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33
Water Purification. http://dx.doi.org/10.1016/B978-0-12-804300-4.00002-2
Copyright © 2017 Elsevier Inc. All rights reserved.
2
NANOTECHNOLOGY FOR WATER
PURIFICATION: APPLICATIONS
OF NANOTECHNOLOGY
METHODS IN WASTEWATER
TREATMENT
Konda Reddy Kunduru*, Michael Nazarkovsky*,
Shady Farah*, Rajendra P. Pawar**, Arijit Basu*,
Abraham J. Domb*
*The Hebrew University of Jerusalem, Department of Medicinal Chemistry
and Natural Products, Institute for Drug Research, School of Pharmacy—
Faculty of Medicine, Jerusalem, Israel; **Deogiri College, Department of
Chemistry, Aurangabad, Maharashtra, India
1 Introduction
Water is the most important asset of human civilization, and
potable water supply is a basic human necessity. However, we are
far from meeting global demands; this problem will only increase
with time (Hillie and Hlophe, 2007). Demand escalates due to pop-
ulation growth, global climate change, and water-quality deterio-
ration (Ali and Aboul-Enein, 2004; Nemerow and Dasgupta, 1991;
Tchobanoglous and Franklin, 1991).
Only 2.5% of the world’s oceans and seas harness fresh water,
FW (salts concentration of <1 g/L). However, 70% of fresh water
is frozen as eternal ice. Only <1% of FW can be used for drink-
ing. Globally, >700 million people do not have access to potable
water (WHO, 2014). This problem is severe in developing nations
and sub-Saharan African countries. Therefore, water treatment
must be implemented in these affected places. Available tech-
nologies for water treatment are reaching their limits in provid-
ing sufficient quality to meet human and environmental needs
(Qu et al., 2013a). Therefore, reuse, recycle, and repurpose are the
“needs of the day.”
34 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
Water contaminants may be organic, inorganic, and biological.
Some contaminants are toxic and carcinogenic (Ali and Aboul-
Enein, 2006; Ali et al., 2009; Laws, 2000) and have deleterious ef-
fects on humans and ecosystems (Ali, 2012). Some heavy metals
are notorious water pollutants with high toxicity. Arsenic is one
of the deadliest elements, well known since ancient times. Other
heavy metals water pollutants with high toxicity are cadmium,
chromium, mercury, lead, zinc, nickel, copper, and so on; they
have serious toxicities (Ali, 2012). Nitrates, sulfates, phosphates,
fluorides, chlorides, selenides, chromates, and oxalates show
hazardous effects at high concentrations; these ions also change
the taste of water. For example, high levels of fluoride in water
causes fluorosis. Organic pollutants, such as pesticides, fertilizers,
hydrocarbons, phenols, plasticizers, biphenyls, detergents, oils,
and greases are associated with toxicities (Damià, 2005). Emerg-
ing contaminants include pharmaceuticals and personal care
products (PPCPs) (Carballa et al., 2007; Ellis, 2006; Mohapatra
et al., 2014). PPCPs are usually resistant against natural biodete-
rioration. The general origin of such compounds is household and
hospital water, which contains metabolized and nonmetabolized
(1) drugs, (2) drugs products, (3) additives to detergents, and (4)
packaging. PPCP concentration ranges from ng/L up to µg/L, in
water. Therefore, conventional water treatment provides unsat-
isfactory results, because treatment facilities are not equipped to
remove stable low-concentrated pollutants.
Produced water containing hydrocarbons necessitates mea-
sures that are fast and simple. For instance, according to Gouma
and Lee (2014), the petroliferous shale production in the USA will
amount to 5 MIO barrels/d by 2017. Hydraulic fracturing (frack-
ing) of shale or rocks are used to recover oil under high pressure.
The US’s mining-holes (1 MIO) produce 3.29 × 103 m3 of produced
water (frac-water) per year.
Aromatic hydrocarbons, such as benzene, toluene, ethylben-
zene, and xylenes, labeled as BTEX are the most predominant
components in produced water among other volatile contami-
nants. These volatile organic compounds can easily evaporate
when water is exposed to the atmosphere. BTEX chemicals are
toxic and have carcinogenic persistent contaminants. The cur-
rent maximum contaminant levels (MCL, µg/liter = ppb) for
BTEXs in potable water are: benzene 5; toluene 1,000; ethylben-
zene 700; xylenes (total) 10,000. Since customary procedures of
water purification (filtration, flocculation, and sedimentation)
do not remove organic compounds at low concentrations, elim-
inating BTEX pollutants is challenging. Working expenses de-
pend on various factors, but eventually are dictated by the cost
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 35
of the media and means used, the cost of their arrangement, and
distribution.
Microbes are responsible for several waterborne diseases.
Natural organic matter (NOM) is one of the principal character-
istics of water quality, and it determines the strategy of purifi-
cation. Sometimes NOM is represented by total organic carbon
and dissolved organic carbon (Matilainen et al., 2011). NOM is
omnipresent from remains of animal or plant origins. It negates
the effectiveness of certain techniques—flocculation, carbon ad-
sorption, or filtration. Even worse, NOM produces undesirable
oxidation products during chlorination treatment. NOM serves as
a breeding medium for a large variety of other microorganisms
stimulating growth of bacteria in an aquatic habitat. Given that
NOM is various, it is not possible to remove all related objects
from the environment, disfavoring water authorities. NOM pol-
lution in the surface and ground water makes it unfit for drinking
(Gledhill, 1987).
The global population is expected to reach 7.9 billion by 2020,
and therefore the world may encounter severe water scarcity.
Therefore, it is necessary to remove these pollutants from contam-
inated water to provide good health to the public (Dyson, 1996).
Different methods are available for water purification (Gupta
et al., 2011a,b; Gupta and Nayak, 2012; Saleh and Gupta, 2012;
Saleh et al., 2011). The most important methods are screening, fil-
tration, micro- and ultrafiltration, crystallization, sedimentation,
gravity separation, flotation, precipitation, coagulation, oxida-
tion, solvent extraction, evaporation, distillation, reverse osmosis,
ion exchange, electrodialysis, electrolysis, adsorption, setting-out,
centrifugal and membrane separation, fluidization, neutraliza-
tion and remineralization, reduction and oxidation, and so on
(Ali, 2012).
Any of the above methods can be combined depending on the
type of contaminated water and prospective purpose (Fig. 2.1)
(Stackelberg et al., 2004). These methods work well, but recent
notorious anthropogenic pollutants (result of modern human
life style) pose a challenge to purify/treat the contaminated wa-
ter. Table 2.1 summarizes major limitations associated with con-
ventional methods (Das et al., 2014). A literature survey reveals
no single method sufficient to remove all pollutants from water.
High operational costs prevent use of sophisticated techniques.
Moreover, efficiency of the treatment plants decreases during
the removal of contaminants. Thus, the existing technologies
are not “100%” effective to supply potable water (Upadhyayula
et al., 2009). New and improved technologies for water purifica-
tion are, therefore, extremely important.
36 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
2 Importance of Nanotechnology in Water
Purification
Nanotechnology provided innovative solutions for water
treatment. Nanomaterials are fabricated with features, such as
high aspect ratio, reactivity, and tunable pore volume, electro-
static, hydrophilic, and hydrophobic interactions, which are
useful in adsorption, catalysis, sensoring, and optoelectronics
Figure 2.1. Schematic diagram showing (A) location of stream sampling sites, surface-water intakes, and
drinking-water-treatment plant, and (B) physical and chemical processes used in drinking-water-treatment plant.
From Stackelberg, P.E., Furlong, E.T., Meyer, M.T., Zaugg, S.D., Henderson, A.K., Reissman, D.B., 2004. Persistence of pharmaceutical
compounds and other organic wastewater contaminants in a conventional drinking-water-treatment plant, Sci. Total Environ. 329,
99–113, Copyright © 2004, Elsevier.
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 37
(Das et al., 2014). Nanotechnology-enabled processes are highly
efficient, modular, and multifunctional in nature, and they pro-
vide high performance, affordable water and, wastewater treat-
ment solutions. Materials consisting of nanoobjects are durable
and instantiated by high specific surface (SBET). In other words, a
huge surface to volume ratio controls the interaction with pollut-
ants and/or bacteria (Qu et al., 2013b). Nanotechnology-enabled
processes for the water treatment constitute major challenges to
existing methods. Nanotechnology can also be extended to the
purification and utilization of unconventional water sources in an
economic way.
Table 2.1 Major Limitations Associated with
Conventional Water Purification Methods
Conventional
Methods Limitations
Distillation Mostcontaminantsremainbehindandrequirehighamountsofenergyandwater.Pollutantswith
boiling point >100°Caredifculttoremove
Chemical
transformation
Excessreagentsarerequired.Productmaybealow-qualitymixtureandcannotbereleasedinto
environment.Inactiveinharshconditions.Thisisnothighlyselectivemethod
Coagulation and
flocculation
Thisisacomplexandless-efcientmethodandrequiresalkalineadditivestoachieveoptimumpH
Biological
treatment
Microorganismsaresensitivetoenvironmentalfactorsanddifculttocontrol.Intermediates
damagethemicrobialcells.Thisisnotcosteffective.Timeconsuming
Ultraviolet
treatment
Expensivemethodandinactivatedbywatercloudinessandturbidity.Ineffectiveforheavymetals
andothernonlivingcontaminantsremoval
Reverseosmosis Thismethodremovesmineralsfromwaterwhichisunhealthy,andthetreatedwaterwillbe
acidic.Thismethodcannotremovevolatileorganics,chemicals,chlorine,chloraminesand
pharmaceuticals.Requireshighenergy
Nanolteration Thistechniquerequireshighenergy,andpretreatment.Limitedretentionforsaltsandunivalent
ions.Membranefoulingwilloccurwithlimitedlifetimeandexpensive
Ultraltration Thismethodwillnotremovedissolvedinorganics.Requireshighenergy.Susceptibleto
particulateplugginganddifculttoclean
Microltration Cannotremovenitrates,uoride,metals,sodium,volatileorganics,color,andsoon.Requires
regularcleaning.Membranefoulingwilloccur.Lesssensitivetomicrobes,especiallyvirus
Carbonlter Cannotremovenitrates,uoride,metals,sodium,andsoon.Cloggingoccurswithundissolved
solids.Susceptibletomold.Requiresfrequentchangingoflters
38 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
Treating industrial wastewater with nanomaterials is also im-
portant and widespread. The remediation technologies avail-
able now are effective, but they are costly and time consuming.
Nanotechnologies are advantageous in treating wastewater, since
they eliminate contaminants and help in the recycling process to
obtain purified water. This leads to reduction in labor, time, and
expenditure to industry solving various environmental issues
(Kanchi, 2014).
It should be noted that nanomaterials for purifying drinking
water must be environment-friendly and nontoxic. Unsafe par-
ticles can cause severe injury to vital organs upon contact with
the human body. Due to dimensional features nanoobjects may
translocate to various organs, which aggravates the danger of bio-
logical damage. Thus, before introduction into the industry, tox-
icity performance tests must strictly be included into safety data
sheets, SOPs, and other related normative documents. This chap-
ter reviews an overview of nanotechnology applications in water
and wastewater treatment.
3 Applications of Nanotechnology in Water
or Wastewater Treatment
Nanomaterials are typically less than 100 nm in dimension and
contain materials with novel and significantly changed physical,
chemical, and biological properties (Theron et al., 2008). Materi-
als of this scale contain novel size-dependent properties, which
are different compared to their larger counterparts. Desirable
nanomaterial properties, such as high surface area for adsorption
and high reactivity toward photocatalysis should also have good
antimicrobial properties for disinfection and also to control bio-
fouling, should have superparamagnetism for particle separation,
should contain optical and electronic properties, and should have
good sensing nature to measure water quality (Qu et al., 2013a).
Some nanotechnology applications for water and wastewa-
ter treatment are discussed in the following sections (Table 2.2)
(Gehrke et al., 2015).
3.1 Nanoadsorption
Adsorption is a surface process wherein pollutants are ad-
sorbed on a solid surface. Adsorption takes place in general by
physical forces, but sometimes this can be attributed to weak
chemical bonds (Faust and Aly, 1983). The efficiency of conven-
tional adsorbents may be restricted by their surface area, and
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 39
the lack of selectivity (Qu et al., 2013a). Usually nanoadsorbents
are used to remove inorganic and organic pollutants from wa-
ter and wastewater. The unique properties of nanoadsorbents,
such as small size, catalytic potential, high reactivity, large sur-
face area, ease of separation, and large number of active sites
for interaction with different contaminants make them ideal
adsorbent materials for the treatment of wastewater (Ali, 2012).
Carbon-based (Chowdhury and Balasubramanian, 2014;
Gao et al., 2011; Jurado-Sanchez et al., 2015; Lubick, 2009;
Sui et al., 2012) nanoadsorbents, metal-based (>99.5% purity)
(Das et al., 2012; Zhang, 2003) nanoadsorbents, polymeric nano-
adsorbents, magnetic or nonmagnetic (Arshadi et al., 2015; Chen
et al., 2014; Rafiq et al., 2014) oxide composite, and zeolites are
currently used nanoadsorent technologies in the treatment of
water (Gehrke et al., 2015).
Adsorption efficiency of target compounds hinges on the wa-
ter used. With the increasing concentrations of any substances
(natural or anthropogenic) in water, the capacity for the specific
compound is reduced because of competitive binding with the
surface sites.
Table 2.2 Overview of Different Nanomaterials in
Water and Wastewater Treatment
Nanomaterial Properties/Applications Limitations
Nanoadsorbents Havehighspecicsurfaceandverygoodadsorptioncapacity
Usedtoremoveorganicandinorganiccontaminants,and
bacteria
Highproductioncosts
Nanometals and
nanometal oxides
Containhighspecicsurfaceareaandshortintraparticle
diffusiondistance,compressiblewithoutchangeinthe
surfacearea,abrasionresistant,magnetic,andphotocatalytic
in nature
Usedtoremoveheavymetalsandradionuclides
Usedinmedialters,slurryreactors,powders,andpellets
Less reusable
Membranes and
membrane process
Highlyreliableandmostlyautomatedprocess.
Appliedinalleldsofwaterandwastetreatments
Requireshigh-energy
source
Photocatalysis PhotocatalyticactivityinUVandpossiblyvisiblelightrange,
lowhumantoxicity,highstability,andlowcost
Reactionselectivity
Disinfectionand
microbial control
Strongandwide-spectrumantimicrobialactivity,lowtoxicity
tohumans,easeofuse
Lackofdisinfection
residue
40 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
3.1.1 Carbon-Based Nanoadsorbents
3.1.1.1 Removal of Organic Contaminants
Carbon-based nanoadsorbents, such as carbon nanotubes
(CNTs) are cylindrical. CNTs are explored as substitutes for acti-
vated carbon. CNTs are categorized as single-walled nanotubes
and multiwalled nanotubes (MWCNTs) depending on their prep-
aration. CNTs contain a high specific surface area with highly
assessable adsorption sites. Their surface chemistry can also be
modified accordingly (Yang and Xing, 2010). The hydrophobic
surface of CNTs makes them form loose bundles/aggregates in
aqueous medium, which reduces the active surface area. These
aggregates are high-energy sites for the adsorption of organic con-
taminants in water (Pan et al., 2008). The reason for the adsorption
of bulky organic contaminants by CNTs is the availability of larger
pores in bundles and more accessible sorption sites (Ji et al., 2009).
CNTs can also adsorb polar organic molecules because of diverse
contaminant-CNT interactions in the form of hydrophobic effect,
ππ interactions (with polycyclic aromatic hydrocarbons), hydro-
gen bonding (with acids, amines, alcoholic functional groups),
covalent bonding, and electrostatic interactions (with positively
charged organic contaminant molecules, such as antibiotics)
(Chen et al., 2007; Ji et al., 2009; Lin and Xing, 2008; Yang and
Xing, 2010).
3.1.1.2 Removal of Heavy Metal Ions
Surface-oxidized CNTs using hydrogen peroxide, KMnO4, and
nitric acid are used in the removal of Cd2+ from aqueous solu-
tions (Li et al., 2003a). The oxidation of CNTs may have high ad-
sorption capacity for metal ions with faster kinetics. The surface
of oxidized CNTs contains functional groups, such as carbox-
ylic acid, hydroxyl, and carbonyls (Fig. 2.2) (Vukovic´ et al., 2010).
These groups have good adsorbing capacity for heavy metal ions
Figure 2.2. Functionalization of MWCNT for the removal of heavy metals (Vukovic´ et al., 2010).
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 41
when the pH is above the isoelectric point of the oxidized CNT
(Datsyuk et al., 2008; Li et al., 2003b; Musameh et al., 2011; Peng
and Liu, 2006; Lau et al., 2015; Liu et al., 2012). Many other stud-
ies have been reported that CNTs are very good adsorbing nano-
materials for heavy metal ions, such as Cu2+, Pb2+, Cd2+, and Zn2+
(Li et al., 2003a; Lu et al., 2006). A sponge made of CNTs with a
dash of boron showed a very good adsorbing capacity for oil from
water. These sponges are reusable once the oil is removed from
them, and they are promising in the removal of oil spills for oil
remediation (Hashim et al., 2012).
Even though CNTs have significant advantages, they have cer-
tain limitations even in water treatment by the adsorption method.
The use of CNTs on an industrial scale for wastewater treatment
plants is not expected due to high production costs (De Volder
et al., 2013). Toxicity of CNTs is well known since their discovery
in the early 1990s (Iijima, 1991), and many reports are available
regarding health concerns (Bottini et al., 2006; Muller et al., 2006).
Another limitation is that CNTs show a coagulation phenomenon
with some organic contaminants and algae, and lose their nanoi-
dentity/structure (Ali, 2012).
Regeneration of CNTs after their use as adsorbents can be
achieved by altering the pH toward acidic. The regeneration will
not change CNTs adsorbing capacity for metal ions. It has been re-
ported that Zn+2 adsorption by CNTs decreases only 25% after the
tenth regeneration, while activated carbon loses its activity almost
50% after its first regeneration (Lu et al., 2006).
In combination with graphene, aerogels of acid-treated
MWCNTs exhibit improved absorbance of heavy metals from
aqueous solutions (Fig. 2.3) (Sui
et al., 2012). To depend upon types
of MWCNTs used—pristine or acid
treated (c-MWCNT)—there is a sub-
stantial difference in absorbance
capacities. Aerogels containing ac-
id-pretreated MWCNTs represent
higher values of capacities in con-
trast to CNT-free graphene or pristine
MWCNT–graphene hybrid samples.
This was supposed to have been due
to electrostatic interactions revealed
by oxygen-based groups on the pore
surface of graphene-c-MWCNT
sample–carboxyl groups located on
the edges of graphene sheets and
walls of c-MWCNTs.
Figure 2.3. Binding capacities
of graphene–MWCNTs for
heavy metals (Pb2+, Hg2+,
Ag+, Cu2+). From Sui, Z., Meng,
Q., Zhang, X., Ma, R.Cao, B.,
2012. Green synthesis of carbon
nanotube-graphene hybrid aerogels
and their use as versatile agents for
water purification. J. Mater. Chem.
22, 8767–8771, Copyright © 2012,
Royal Society of Chemistry.
42 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
3.1.1.3 Metal-Based Nanoadsorbents
Metal-based nanoadsorbents, such as iron oxide, titanium di-
oxide, zinc oxide, and alumina are used in heavy metal removal
during water decontamination. They are effective and low-cost
materials. The mechanism of action is that the oxygen in metal
oxides complexed with heavy metals dissolves in contaminated
water (Trivedi and Axe, 2000). As the particle size decreases, the
adsorption capacity increases several fold. For example, 300 and
20 nm magnetite particles have similar degrees of adsorption for
As(III). When the particle size is reduced to less than 20 nm, the
specific area of the nanoparticles increases, and the adsorbing ca-
pacity for arsenic increases three times (Auffan et al., 2008). This is
because of the “nanoscale effect” of the magnetite, in which sur-
face structure creates new adsorption sites for metal ions (Auffan
et al., 2009).
Magnetic nanoadsorbents (MNPs), such as maghemite
(γ-Fe2O3), hematite (α-Fe2O3), and spinel ferrites (M2+Fe2O4,
where M2+: Fe2+, Cd2+, Cu2+ Ni2+, Co2+, Mn2+, Zn2+, Mg2+) are very
good adsorbing materials for the collection and removal of toxic
elements from contaminated water. Environmental benefits lie
in their magnetic nature. They can be easily separated from reac-
tion media by application of an external magnetic field. Ample
reports are available in the literature for the use of these MNPs for
the removal of a variety of elements, such as arsenic, chromium,
cobalt, copper, lead, and nickel in their ionic forms (Badruddoza
et al., 2013; Lei et al., 2014; Ngomsik et al., 2012; Tan et al., 2014;
Tu et al., 2012).
Such magnetic oxide nanomaterials modified by functional
groups—Si(CH2)3NH2/SiCH3 and (Si(CH2)3NH2/ SiC3H7n
are shown to act as adsorbents of undesired biopolymers from
water (Melnyk and Zub, 2012).
The iron oxide surfaces are covered with OH groups. This is
because of adsorbed H2O or structural features. Hence, the surface
is not inert:
Fe-OH + H+ FeOH2+ (2.1)
FeOH + OH FeO + H2O (2.2)
The functionality can be varied in relation to the nature of
both oxide and pH. In this respect FeOH+ or FeOH2+ (pH < 7) and
Fe(OH)2
0, Fe(OH)3
, FeO (pH > 7) are predominant on surfaces of
iron oxides (Chowdhury et al., 2012). Such dependences become
agents for different removals of chromium(IV) from water.
Maghemite nanotubes have been used for the removal of
Cu2+, Zn2+, and Pb2+ from water (Roy and Bhattacharya, 2012).
Fe(OH)20Fe(OH)3-
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 43
The use of magnetite nanorods was reported for the removal
of heavy metal ions, such as Fe2+, Pb2+, Zn2+, Ni2+, Cd2+, and Cu2+
from aqueous solutions (Karami, 2013). In the same study, when
compared to nanotubes, the nanorods had higher adsorption
capacity for Zn2+ and Pb2+ but lower adsorption capacity for
Cu2+. Water soluble magnetite superparamagnetic nanoparticles
had fast and selective for the adsorption of Hg2+ (Qi et al., 2014).
The study on the role of nanohematite as an adsorbent for the
removal of heavy metal ions was carried out using spiked tap
water. The nanohematite particle surface contains hydroxyl
groups, which allows for the adsorption of specific heavy met-
al ions. This model also proves that the adsorption of Pb2+,
Cd2+, and Cu2+ is endothermic, and Zn2+ is exothermic (Shipley
et al., 2013).
The application of paramagnetic ferrite-based nanoparticles
(CuFe2O4) makes possible adsorption of oxidized forms of As—
As(V ) in the form of arsenate salts from contaminated groundwa-
ter (Tu et al., 2012). This feature aids in a rapid recovery (20 s) by
force of magnetic field.
Another metal oxide, TiO2, was studied for the removal of arse-
nic. Luo et al. acquired wastewater from copper smelting industry
that contained an average concentration of 3310 mg L1 As(III),
24 mg L1 Cu, 5 mg L1 Pb, and 369 mg L1 Cd. The authors suc-
cessfully removed arsenic in less than 1 h to 100 mg L1 from the
initial concentration 3310 mg L1. They have used TiO2 for almost
21 successive cycles after its regeneration (Luo et al., 2010).
Magnesium oxide (MgO) was noticed due to its signaled re-
moval properties against inorganic micropollutants, such as
As(V ) and heavy metal ions. Flower-like nanoforms of MgO
(SBET = 72 m2/g) are high-efficiency adsorbents of Cd2+ and Pb2+
with maximum capacities of 1.98 and 1.5 g/g (pH = 7), respec-
tively. The XRD studies bear record to ion exchange mechanism
between both types of metal ions and Mg2+: MgO + M2+(Cd2+/
Pb2+) = PbO/CdO + Mg2+. The growth of Mg2+ during the ad-
sorption process does not, however, exceed the World Health
Organization limit (450 mg/L) (Cao et al., 2012). The adsorption
capacity of porous MgO nanowires toward As(V) was shown to
be at 384.6 mg/g (Jia et al., 2013). In this case, the ionic exchange
does not take place on the surface of MgO. Although the shape
of 1-D MgO had been saved, new “plates” of magnesium arse-
nate were formed on the surface of nanowires. Thus, a chemical
interaction occurs during As(V ) adsorption without any loss of
Mg2+ into water.
By conjugation of MgO and TiO2 into a dual nanocomposite,
the adsorbing effectiveness of metal oxides goes together with
44 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
strongly marked photocatalytic activ-
ity (Fig. 2.4). Videlicet, As(V) removal
via MgO and photocatalytic degrada-
tion of model compound (methylene
blue, MB) induced by TiO2 all comple-
ment each other despite low specific
surface area (SBET = 76 m2/g). The
highest capacity to adsorb As(V ) was
217.8 mg/g (almost 1.8 times less than
that of flower-like MgO). This is be-
cause of titanium phase introduction.
Presence of arsenic did not influence
the photocatalytic decomposition of
MB (100% oxidation in 25 min). But,
the dye reduced removal of arsenic
due to possible competitive adsorp-
tion of oxidized products of MB on
MgO surface.
ZnO nanoadsorbents were used to
remove Zn2+, Cd2+, and Hg2+ ions from
aqueous solutions in a study reported in the literature. These met-
al ions adsorbed onto ZnO nanoparticles at different concentra-
tions, however, Hg2+ had highest adsorbing capacity. The reason
may be that Hg2+ has the smallest hydrated ionic radii compared
to other metal ions in the study (Sheela et al., 2012). Alumina
nanoadsorbents can be prepared at low cost. They have high sur-
face area and good thermal stability. They have been used for the
removal of cadmium, chromium, copper, lead, and mercury metal
ions (Pacheco and Rodriguez, 2001).
Metal-based nanoadsorbents are one of the highly efficient
nanoadsorbents for the removal of heavy metal from water and
wastewater. They have their own advantages, such as faster kinet-
ics, high adsorption capacity, and preferable nanomaterials for
heavy metal removal (Hua et al., 2012).
Long reactive iron nanoparticles (10–100 nm) as reducing
materials demonstrate effectiveness as detoxicants of chlorine-
containing compounds (pesticides, organic solvents, and poly-
chlorinated biphenyls) (Fig. 2.5) (Zhang, 2003).
These contaminants, preadsorbed on zero-valent Fe, accept
electrons, which provokes the reduction process with formation
of ethene:
C2Cl4 + 4Fe0 + 4H+ C2H4 + 4Fe2+ + 4Cl
Monodisperse nanosilver bioconjugate particles (15 nm) syn-
thesized using fungal mycelia (Rhizopus oryzae) are efficacious
Figure 2.4. TEM images of
the necklace-like MgO/TiO2
heterojunction structures. From
Jia et al. (2014), Copyright © 2013,
Royal Society of Chemistry.
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 45
adsorbents and antimicrobal agents. The concentrations of stud-
ied pesticide (parathion and chlorpyrifos) were significantly re-
duced (85–99%) in contrast to gamma-hexachlorocyclohexane
(γ-BHC), whereas only 16% of absorbance occurred. In addition,
the amount of Escherichia coli (initial concentration—60 cell/mL)
was not sufficient to be detected after treatment. Such a difference
can be explained by means of the HSAB theory, that is, with the af-
finity of sulfur atoms of parathion and chlorpyrifos to Ag. Hence,
hard Cl-atoms in γ-BHC cannot redound to an effective adsorp-
tion onto nanosilver particles.
Metal-based nanoadsorbents can be regenerated by changing
the solution pH. Also, after several reuses and regeneration the ad-
sorbing capacity of these nanoadsorbents is not altered. However,
in some cases the adsorbing capacity is reduced after regenera-
tion due to the formation of aggregates by van der Waals and other
forces (Pan et al., 2008).
3.1.2 Polymeric Nanoadsorbents
Polymeric nanoadsorbents gained interest recently. They are
used either as a system into which inorganic nanosized materi-
als can be inserted or as a bed or template to prepare nanopar-
ticles (Khajeh et al., 2012). The most important advantage of the
Figure 2.5. Nanoscale iron particles for in situ remediation. From Zhang, W.-X., 2003. Nanoscale iron particles for
environmental remediation: an overview. J. Nanopart. Res. 5, 323–332, Copyright © 2003, Springer.
46 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
polymer-inorganic nanoadsorbents is their good adsorption
capacity and very good thermal stability over a wide range of
pH. Further, the resistance of polymeric groups and their link-
ages to acid and base hydrolysis is an added advantage (Kaya
et al., 2011; Khajeh et al., 2012). Some recent examples are
discussed. Fe3O4 magnetic nanoparticles (Fe3O4 MNPs) modi-
fied with 3-aminopropyltriethoxysilane (APS) and copolymers
of acrylic acid (AA) and crotonic acid (CA) as polymer shells
(Fe3O4@APS@AAco-CA MNPs) were prepared. The use of a poly-
mer shell prevented interparticle aggregation and improved
dispersion stability of the nanostructures. The polymer modi-
fied MNPs successfully removed heavy metal ions, such as Cd2+,
Zn2+, Pb2+, and Cu2+ from aqueous solution with high maximum
adsorption capacity at pH 5.5. This nanoadsorbent could be re-
usable in at least four cycles (Ge et al., 2012). In another study,
a bimetal doped micro- and nanomultifunctional polymeric
adsorbent for the removal of fluoride and arsenic(V) was de-
veloped (Fig. 2.6). The polymer was prepared by suspension
Figure 2.6. Metal-doped
phenolic polymeric beads for
the removal of fluoride and
arsenic ( Kumar et al., 2011).
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 47
polymerization. During polymerization Al and Fe salts were
incorporated to obtain a bimetal-doped nanoadsorbent. The
synthesized Fe-doped nanoadsorbents had very good adsorp-
tion for arsenic, compared to fluoride, whereas Al-doped nano-
adsorbents showed good adsorption toward fluoride compared
to arsenic (Kumar et al., 2011).
A polymeric hybrid sorbent (ZrPS-001) was fabricated for en-
hanced sorption of heavy metal ions, such as lead, cadmium, and
zinc ions from aqueous solution by impregnating Zr(HPO3S)2 (i.e.,
ZrPS) nanoparticles within a porous polymeric cation exchanger
D-001. The negatively charged groups bound to polymeric matrix
D-001, resulting in preconcentration and permeation enhance-
ment of target metal ions prior to sequestration. The nanoparticles
are expected to sequester heavy metals through an ion-exchange
process (Zhang et al., 2008).
Even though, polymeric nanoadsorbents are excellent ma-
terials with respect to their structures, pore sizes, and tunable
functional groups, for the removal of heavy metals from water/
wastewater the ability to make them selective for a given pollut-
ant is a difficult task. The adsorbing capacities of these materi-
als is rather low, and regeneration is required at high cost (Pan
et al., 2008).
3.2 Membranes and Membrane Process
A membrane is a porous thin-layered material that allows wa-
ter molecules to pass through it, but at simultaneously restricts
the passage of bacteria, viruses, salts, and metals. Membranes use
either pressure-driven forces or electrical technologies. Pressure-
driven membrane technology is a perfect method for water pu-
rification to any desired quality (Kumar et al., 2014). Membrane
separation processes are increasingly advanced methods for the
treatment of water and wastewater. Membranes separate sub-
stances depending on pore and molecule size. It is a reliable and
automated process for wastewater treatment (Gehrke et al., 2015).
The challenge of membrane technology is the inherent tradeoff
between membrane selectivity and permeability. This technique
requires high-energy consumption due to the pressure-driven
process. Fouling of membranes makes the process very complex
and also reduces the life time of membranes and membrane mod-
ules (Qu et al., 2013a). The performance of the membrane system
depends on the type of membrane material. Functional nanoma-
terial inclusion into membranes is definitely advantageous in the
improvement of membrane permeability, fouling resistance, me-
chanical, and thermal stability.
48 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
3.2.1 Nanofiber Membranes
Electrospinning is a simple, inexpensive, and efficient tech-
nique to fabricate nanofibers. These nanofibers contain high sur-
face area, porosity, and form nanofiber mats with complex pore
structures. The physical and chemical parameters of electrospun
nanofibers can be easily manipulated for different applications.
Nanofiber applications have been documented in the literature.
Preparation, characterization of nano fibers, and their suitability
for air and water filtration applications have been presented in
the literature (Ahmed et al., 2015; Balamurugan et al., 2011; Feng
et al., 2013; Nasreen et al., 2013; Subramanian and Seeram, 2013).
This class of membranes removes micron-sized particles from
water without any significant fouling (Ramakrishna et al., 2006).
Thus, nanofibers can be used in pretreatment prior to ultra fil-
tration or reverse osmosis. Electrospinning is a widely employed
technology in air treatment, not so common in wastewater treat-
ment (Ramakrishna et al., 2006).
An electrospun nanomembrane can eliminate bacteria or virus-
es by size exclusion. However, the utilization of these membranes
incurs difficulties because of smaller pore sizes for appropriate re-
moval of viral agents. At such a scale, size of pores will essentially
depress the onrush of water. To the contrary, a novel composite
cellulose-based membrane to remove microorganisms (99.9999%
removal of E. coli) maintaining the water stream was developed
(Sato et al., 2011). Positive charged fibers trapped the negatively
charged viruses. This rate of activity is in keeping with require-
ments specified by the National Sanitation Foundation Standard
(max. 2 CFU/mL).
Poly(acrylonitrile-co-glycidyl methacrylate) nanofibers (100 nm
in diameter) were surface functionalized with protein. Biofunc-
tionalization made these nanofibers slightly bigger in diameter to
126 nm. The bionanohybrid electrospun nanofiber membranes
undergo conformational change upon wetting during filtration
and increase of pH to above the isoelectric point of the protein.
This leads to the emergence of the hidden functional groups, mol-
ecules (thereby protein swelling) and nanosolids to be filtered.
Moreover, swollen protein makes a higher steric hindrance, facili-
tating the capturing of the nanosolids, such as metal ions (Elbahri
et al., 2012).
The marketed nanofiber filter, NanoCeram (Argonide Cor-
poration, Sanford, FL, USA) is a fiber with a small diameter and
large surface area (300–600 m2/g). This fiber is scalable to kilo-
gram through a sol–gel reaction. The resulting product is white,
free-flowing made up of nanofibers (2 nm) in diameter and
10–100 nm length. They aggregate and can be embedded in glass
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 49
and cellulose sheets, and may be used in ultra filtration. The fi-
bers are free flowing, but gets rid of dirt, bacteria, viruses, and pro-
teins through Columbic interactions. NanoCeram can be applied
in commercial/industrial water treatment as a microbiological
sampler or as a stand-alone filtration device (Karim et al., 2009).
Membranes made of hydrophobic nanofiber materials might be-
come very appropriate for separation of organic solvents, leading
to higher flux efficiency (Feng et al., 2013).
3.2.2 Nanocomposite Membranes
Nanocomposite membranes are promising filtration units; they
may be fabricated from mixed matrices and surface-functionalized
membrane. Mixed matrix membranes use nanofillers, and most are
inorganic. They are mixed to a polymeric or inorganic oxide matrix,
and have substantial surface area (Gehrke et al., 2015). Hydrophilic
metal oxide nanoparticles (Al2O3, TiO2, and zeolite), antimicrobial
nanoparticles (nano-Ag and CNTs), and (photo)catalytic nanoma-
terials (bimetallic nanoparticles, TiO2) are some of the nanomateri-
als used for such applications (Qu et al., 2013a).
Addition of hydrophilic metal oxide nanoparticles into the
membrane reduces fouling. Such addition of metal oxide nanopar-
ticles (alumina, silica, zeolite, TiO2, etc.) to polymeric membranes
increases membrane surface hydrophilicity, water permeabil-
ity, or fouling resistance (Bae and Tak, 2005; Bottino et al., 2001;
Maximous et al., 2010; Pendergast and Hoek, 2011; Pendergast
et al., 2010). Also, these inorganic nanoparticles enhance the me-
chanical and thermal stability of polymeric membranes by reduc-
ing the negative impact of compaction and heat on membrane
permeability). Also, these inorganic nanoparticles enhance the
mechanical and thermal stability of polymeric membranes by re-
ducing the negative impact of compaction and heat on membrane
permeability.
Nanocomposite membranes are made up of ordered meso-
porous carbons as nanofillers fabricated as thin-film polymeric
matrices. They are semipermeable, the top surface used in reverse
osmosis. Atmospheric pressure plasma converts hydrophobic
mesoporous carbons to a hydrophilic one. Small percentage of
hydrophilic mesoporous carbons increases the hydrophilicity; re-
sulting in increased pure water permeability (Kim and Deng, 2011).
Thin-film nanocomposites made from polyamide and nano-NaX
zeolite (40–150 nm) were reported. They were coated by interfa-
cial polymerization using trimesoyl chloride and m-phenylenedi-
amine monomers over porous polyethersulfone. This membrane
shows good permeability to pure water, leaving contaminants be-
hind the membrane (Fathizadeh et al., 2011).
50 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
Nano-Ag and CNTs prevent membrane biofouling. Inhibition
of bacterial attachment or biofilm formation was observed with
doping or surface grafting of nano-Ag on polymeric membranes
(Mauter et al., 2011; Zodrow et al., 2009) and also inactivation of
viruses (De Gusseme et al., 2011). However, authors do not men-
tion membrane long-term efficacy against biofouling. It has been
reported that direct contact of CNTs inactivate bacteria (Brady-
Estévez et al., 2008). High bacterial inactivation (>90%) was ob-
served with polyvinyl-N-carbazole-SWNT nanocomposite at 3
wt.% of SWNT (Ahmed et al., 2012). However, long-term filtration
experiments are required to determine the impact of fouling on
the antimicrobial activity of CNTs.
Photocatalytic nanoparticles inserted into membranes perform
a physical separation process, and the photocatalyst contaminate
degradation. Photocatalytic TiO2 nanoparticles were grafted on
metallic filters through dip coating. Combination if foul repellent
and photocatalytic nanocoatings degrades unwanted substances
forming a dense cake. However, such surface activation is limited
to inert materials, polymeric membranes would degrade during
the oxidation process (Gehrke et al., 2012).
Nanoscale metallic iron (nZVI) or zero-valent iron (ZVI) has
been extensively studied for two decades for the treatment of
ground water and also wastewater polluted with different or-
ganic and inorganic contaminants (Crane and Scott, 2012; Guan
et al., 2015). When these nano-ZVI and noble metal supported on
nano ZVI incorporated in to polymeric membrane system for the
degradation of contaminants, such as chlorinated compounds,
nano ZVI serves as electron donor and noble metals catalyze the
degradation reaction (Qu et al., 2013a; Wu et al., 2008).
3.2.3 Thin-Film Nanocomposite Membranes
Thin-film nanocomposite (TFN) membranes are a new cate-
gory of composite membranes prepared by an interfacial polym-
erization process. Nanoparticles are incorporated within the thin
layer of the polymer to improve the properties of the interfacially
polymerized layer (Lau et al., 2015). Nanomaterials, such as nano-
zeolates, nano-Ag, nano-TiO2, and CNTs were incorporated as
nanoparticles into active thin layers of thin film composite (Lau
et al., 2015; Qu et al., 2013a).
Zeolite-polyamide nanocomposite thin films were prepared by
interfacial polymerization, which results in reverse osmosis mem-
branes with improved permeability and interfacial properties
when compared to similarly formed pure polyamide thin films.
In this study water permeability increased up to 80% compared
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 51
to thin-film composite membrane, with the NaCl rejection large-
ly maintained up to 94% (Jeong et al., 2007). In another study,
zeolite-polyamide TFN membranes for reverse osmosis process
were prepared and achieved upon usage for salt rejection (32 g/L)
up to 99.4% compared to regular reverse osmosis membranes
(Kurth et al., 2011; Lind et al., 2010). This technology is now com-
mercially available; LG NanoH2O, Inc. successfully conducted a
field test of a TFN element and reported that the water flux of the
TFN membrane was twice the flux of the polyamide membrane
and achieved salt rejection >99.7% (Lau et al., 2015). The unique
crystal structure of the zeolite molecular sieve particle is respon-
sible for the improvement in the water flux through its internal
pore structure (Lind et al., 2009). Also, hydrophilicty and negative
charge of zeolite nanoparticles can cause greater affinity toward
water molecules and increased repulsions of anions due to cou-
lombic effects (Jeong et al., 2007).
In a study, incorporation of appropriate amounts of nano-TiO2
into the thin-film composite active layer resulted in increase in the
membrane rejection for salts while maintaining the permeabil-
ity. In another study, a TFN nanofiltration membrane was devel-
oped by the incorporation of aminosilanized TiO2 nanoparticles.
Aminosilanized TiO2 nanoparticles had good dispersion inside the
polyamide skin layer by reducing their surface energy. At ultra-low
concentration (0.005 wt.%), the functionalized TiO2 nanoparticles
improved NaCl rejection up to 54%. For higher concentrations of
TiO2 nanoparticle incorporation into thin layers, increased water
flux up to twofold was observed compared to thin film composite
membrane with negligible rejection loss (Rajaeian et al., 2013).
The incorporation of Ag nanoparticles into TFN membrane has
also been reported. Ag nanoparticles can deactivate microorgan-
isms during the filtration process and also reduce membrane bio-
fouling. TFN membranes containing Ag nanoparticles in the thin
layer improved water permeability and also demonstrated anti-
bacterial effects on the growth of Pseudomonas aeruginosa (Kim
et al., 2012).
CNTs also found application in TFN membranes. Introduc-
tion of carboxyl-functionalized MWCNTs into thin-film compos-
ite structure can improve both membrane antifouling property
and chlorine resistance (Zhao et al., 2014). The better antifouling
property of TFN membrane was due to the improved surface hy-
drophilicity and greater negative surface charge upon addition of
MWCNTs. Further, this membrane was reported to exhibit better
chlorine resistance when evaluated either in dynamic or immer-
sion mode, owing to the protection of amide linkage by electron-
rich MWCNTs.
52 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
3.2.4 Aquaporin-Based or Biologically Inspired Membranes
Aquaporins are pore-forming protein channels. They are
ubiquitous in living cells. Under specific conditions they regu-
late water flux to reject most ionic molecules (Gehrke et al., 2015;
Tang et al., 2013). They are selectively permeable to water; ideal
material for making efficient biomimetic membranes for wa-
ter purification. Aquaporins are unstable, so incorporated into
small vesicles embedded in a matrix (Gehrke et al., 2015; Tang
et al., 2013). Aquaporin Inside (Aquaporin A/S, Copenhagen,
Denmark), first commercial membrane with embedded aqua-
porins. They withstand pressures up to 10 bar, and water flux
rate >100 L/(hm2). One of the example of using this product is
in desalination (Gehrke et al., 2015). The experiments of (Kumar
et al., 2014) show that aquaporins have exceptional water perme-
ability. Their observations lead to postulate desalination mem-
branes with vastly improved performance. In a study, aquaporins
form self-assembled polymer vesicles forming dense hydrophobic
polymer layer. It provides a nanostructured selective barrier, with
significant mechanical strength (Xie et al., 2013). Recent literature
concerning membrane nanotechnologies has indicted that aqua-
porin-based bioinspired membranes offer the best chance for rev-
olutionary performance, but they were also seen as the furthest
from commercialization (Pendergast and Hoek, 2011).
3.3 Photocatalysis
Photocatalysis is an advanced oxidation process employed
in the treatment of water and wastewater. This technique is
based on the oxidative elimination of micropollutants and mi-
crobial pathogens (Friedmann et al., 2010; Gehrke et al., 2015).
Most organic pollutants can be degraded by heterogeneous
photocatalysis (Chong et al., 2010; Fujishima et al., 2008; Gaya
and Abdullah, 2008; Lazar et al., 2012). TiO2 is a validated pho-
tocatalyst as it is readily availability, safe, and inexpensive
(Qu et al., 2013a). When TiO2 is irradiated by UV light in the range
of 200–390 nm, electron-hole pair (e-h+)s is photoexcited. They
move into the conduction (CB) and valent (VB) bands, which re-
sults in charge separation for an effective photocatalytic func-
tion depending on redox potential of a substrate. Therefore, the
biodegradability of heavily decomposable substances can be in-
creased in a pretreatment step.
TiOh
eh
2(CB
)(
VB)
ν
+→ +
−+
(2.3)
+→
−−
Oe O
2(CB
)2
(2.4)
TiO2+hνe(CB)+h(VB)+
O2+e(CB)O2
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 53
+→⋅+
++
hHOH
OH
(VB) 2
(2.5)
OH + OH H2O2 (2.6)
+→⋅+ +
−⋅
OHOOHH
OO
22
22
(2.7)
+→
−⋅ +
OH OOH
2
(2.8)
OH + Organic matter + O2 CO2 + H2O (2.9)
Since, the band gap (Eg) of TiO2 undertakes broadening (Eg)
within the nanorange (d = 1 ÷ 12 nm), more possible organic sub-
strates can be involved in redox processes according to the neces-
sary criteria of photocatalysis:
EE
CB A/A
0
<
EE
VB D/D
0
+
>
where ECB and EVB, the energy edges of conduction and valence
bands, respectively; EA/A
0
and ED/D
0
+, the standard potentials in re-
spect to reduction and oxidation of an acceptor (A) or a donor (D).
Another requirement to a photocatalyst is correspondence of its
band gap to redox potential of H2O/OH couple (OH OH + e,
E0 = 2.8 V). This figure resolves the case for TiO2 (Eg = 3.2 eV).
Principally, persistent compounds, such as antibiotics or other
micropollutants may be eliminated through photocatalysis dur-
ing polishing. UV-A radiation is 5% of the total sunlight. This low
photon efficiency limits its industrial use (Gehrke et al., 2015).
In general, activation of TiO2 is carried out by a UV lamp, but a
sunlight source or visible light lamps are also permitted. KRONO
Clean 7000 (Kronos Inc., Cranbury, NJ, USA) a photocatalyst, band
gap shifted to lower energy; this renders using broader spectrum
in sunlight (Gehrke et al., 2015).
To enhance photocatalytic properties of titania, which consist
of improving activity or red-shift for energy saving, modification
techniques have been explored. For example, the consociation of
nanosilica (high thermal and chemical stability) and nanotitania
(n-semiconductor) gives rise to new active surface sites. Catalytic
properties of silica/titania nanocomposites considerably depend
on TiO2 content and distribution. Jung and Park (2000) stated
that photocatalytic activity of SiO2/TiO2 enhance significantly in
compared to TiO2. The highest performance was found at Ti/Si ra-
tio = 70/30. Maximal photocatalytic activity was observed in thin
composite films of SiO2/TiO2 at =C5mol.%
SiO2. It is the authors’
opinion that such a phenomenon deals with the size reduction
of crystallites accompanied with the expansion in the number
h(VB)++H2OHO+H+
O2+H2O2OH+HO+O2
O2+H+OOH
ECB<EA/A0
EVB>ED/D+0
EA/A0
ED/D+0
CSiO2=5 mol.%
54 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
of OH-groups on the film surface (Yu et al., 2002). According to
Gao and Wachs (1999), direct contact between TiO2 and SiO2 en-
ables generating of new characteristics and changes in reactive-
ness of the surface. Such changes are provoked by the formation
of Ti–O–Si bonds—their amount is directly proportional to TiO2
concentration.
Doping of nanotitania makes possible improvement in photo-
catalytic properties. Inherently, dopants can be metal ions (Choi
et al., 1994; Czech et al., 2015; Nazarkovsky et al., 2014), nonmet-
als (Takeshita et al., 2006; Zaleska et al., 2008); or other semicon-
ductors (Hou et al., 2007; Wu, 2004; Wu and Ritchie, 2008; Zhou
et al., 2008). For instance, tungsten trioxide and some fullerene
derivatives, such as Fullerol and C60 encapsulated with poly(N-
vinylpyrrolidone) and composites with TiO2 show photocatalytic
effects under visible light irradiation (Meng et al., 2011, 2013). But,
they generate 1O2 that contains low oxidation potential compared
to TiO2. Another study shows that to enhance the performance
of TiO2 nanoparticles, TiO2 nanotubes, and 25–40% doping with
noble metals are more efficient, reducing the e/h+ pairs recombi-
nation (Lazar et al., 2012). A recent review covers effect on water
purification by different operating parameters, such as TiO2 load-
ing, pH, temperature, dissolved oxygen, contaminant type, con-
centration, light wavelength, and intensity (Chong et al., 2010).
Many research groups have been investigating the combina-
tion of separation and catalytic processes using a membrane pho-
tocatalytic reactor to purify water and retain catalytic particles
(Azrague et al., 2007; Gehrke et al., 2015; Ollis, 2003). When using
highly efficient nanoparticles, suitable filtration system should
ensure complete removal of these toxic nanoparticles. Hence,
the technology is expensive and energy inefficient—use of high-
pressure pumps. A good solution is that photocatalytic nanopar-
ticle immobilization on defined materials uses suitable coating
methods, such as physical or chemical vapor deposition, and wet
chemical coating method. Microfilters provide both mechanical
filtration and chemical decontamination. Dirt particles and larger
microorganisms are eliminated by microfiltration membranes at
the same time that viruses, spores, and contaminants are also de-
graded (Gehrke et al., 2015).
In one study, TiO2 nanoparticles and bactericidal silver-coated
metallic filters were reported. The authors used dip coating, chem-
ical, and physical vapor deposition to produce photocatalytic thin
coatings, maximum thickness 500 nm. Photocatalyst surfaces were
less effective than corresponding nanoparticle suspensions, due
to lesser surface area (Gehrke et al., 2012). Purific Water (Holiday,
FL, USA) combined water treatment process with photocatalysis,
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 55
and ceramic membrane filtration. The filtration assembly has the
capacity of >4 million cubic meters per day. They were successful
in degrading 1, 4-dioxane (http://www.purifics.com/).
Photocatalysis shows a substantial potential as a low-cost, envi-
ronmental friendly, and sustainable water-treatment technology.
Yet, there are some technical challenges for its large scale applica-
tion, such as (1) catalyst optimization to improve quantum yield
or to utilize visible light; (2) efficient photocatalytic reactor design
and catalyst recovery/immobilization techniques; (C) better reac-
tion selectivity (Qu et al., 2013a).
3.4 Antimicrobial Nanomaterials in Disinfection and
Microbial Control
Current disinfection methods applied in the treatment of
drinking water can effectively control the microbial pathogens.
Research conducted in the past few decades, however, discloses a
dilemma between effective disinfection and formation of harmful
disinfection by-products (DBPs) (Li et al., 2008). The commonly
used chemical disinfectants in the water industry are chlorine,
chloramines, and ozone. They can react with other constituents
in the water and generate harmful DBPs. Most are carcinogenic
(Hossain et al., 2014). There were more than 600 DBPs, such as
halogenated DBPs , carcinogenic nitrosamines, bromate, and so
on, reported in the literature (Krasner et al., 2006). UV-disinfection
processes have come out as an alternative for oxidative disinfec-
tion, since they generate fewer DBPs, while the required high dos-
age for certain viruses, such as adenoviruses. All these limitations
urge the development of alternative methods that can enhance
the robustness of disinfection while avoiding DBP formation (Li
et al., 2008).
The ideal disinfectant should have the following properties
(modified from Hossain et al., 2014):
1. very-broad antimicrobial activity at ambient temperature
within short time;
2. cannot produce any harmful by-products during and after
their use;
3. does not affect human health;
4. inexpensive and easy apply for the intended use;
5. easy to store, highly soluble in water, and must not be corrosive
for any equipment or surface; and
6. amenable to safe disposal.
Materials, such as nano-Ag, nano-ZnO, nano-TiO2, CNTs, and
fullerenes exhibit antimicrobial properties without strong oxi-
dation; they have lower tendency to form DBPs (Li et al., 2008;
56 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
Hossain et al., 2014). Brief updates regarding these nanomateri-
als and their antimicrobial actions are discussed in the following
sections.
3.4.1 Antimicrobial Action of TiO2 Nanoparticles
TiO2 is a very common nanoparticle type to inactive microbes
in drinking water, surface water, wastewater, and other sources
(Brame et al., 2011; Dimitroula et al., 2012; Friedmann et al., 2010;
Hossain et al., 2014; Li et al., 2008; Liga et al., 2011; Markowska-
Szczupak et al., 2011; Matin et al., 2011; Ng et al., 2013; Pleskova
et al., 2011; Simon-Deckers et al., 2009). The antibacterial mech-
anism of TiO2 is due to reactive oxygen species (ROS) generation,
especially hydroxyl-free radicals and peroxides formed under
UV-A irradiation via oxidative and reductive pathways, respec-
tively (Kikuchi et al., 1997; Li et al., 2008). Generated ROS de-
struct the cell membrane, damaging DNA and protein, releasing
hazardous ions for cell malfunction, disrupting electron trans-
fer, and hampering respiration process (Hossain et al., 2014)
(Fig. 2.7).
Strong absorbance of UV-A furnish activation of TiO2 under
solar irradiation, significantly enhancing solar disinfection.
Figure 2.7. Different mechanisms of antimicrobial activities showed by nanomaterials.
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 57
However, TiO2-based solar disinfection is a very slow process
that may be a small fraction of UV-A in solar radiation. There-
fore, successful research on metal or nitrogen doping to im-
prove visible light absorbance of TiO2 or UV-A activity is critical
to the application of TiO2 solar disinfection. Bacterial death was
observed even in the dark by this nanomaterial, indicating the
involvement of other unknown mechanisms (Adams et al., 2006;
Li et al., 2008).
3.4.2 Antimicrobial Action of Ag Nanoparticles
Silver has been known for its antimicrobial action since an-
cient times. It has a wide range of industrial applications in
healthcare and external medicine (Hua et al., 2012). In recent
times nano-Ag has been widely used as an antimicrobial nanoma-
terial. They are the material of choice for water decontamination,
due to the following: significant and broad antimicrobial activ-
ity, safety, and easy to fabricate. Nanosilver releases silver ions
in water (Qu et al., 2013a; Xiu et al., 2011, 2012) binding to SH
groups in vital enzymes and damaging them (Liau et al., 1997;
Qu et al., 2013a). Silver ions interfere with DNA replication and in-
duce structural changes in the cell envelope (Feng et al., 2000; Qu
et al., 2013a). Toxicity of nano-Ag depends rate of release of silver
ions. Size, shape, coating, and crystallographic facet influences the
release kinetics of silver ions. Presence of ubiquitous ligands re-
duces its bioavailability, and reduces its toxicity (Qu et al., 2013a;
Xiu et al., 2011). In another study they show sublethal doses of
silver ions favors E. coli growth. Suggesting these strategy could
hit back if not designed properly (Qu et al., 2013a; Xiu et al., 2011).
Commercial devices, such as MARATHON and Aquapure systems
utilizing nano-Ag are already available. Nano-Ag was incorpo-
rated into ceramic microfilters as a barrier for pathogens, which
can be employed in remote areas of developing countries (Peter-
Varbanets et al., 2009).
3.4.3 Antimicrobial Action of ZnO Nanoparticles
Zinc oxide nanoparticles have been used in sunscreen lo-
tions, coatings, and paints due to their strong UV absorption ca-
pacity and transparency to visible light (Franklin et al., 2007; Qu
et al., 2013a). ZnO nanoparticles show very good antibacterial ac-
tivities on a broad spectrum of bacteria. However, the antibacte-
rial mechanism of ZnO nanoparticles is unclear, since researchers
have obtained opposite results, for example, the particles size ef-
fect was established in Jones et al. (2008), but that was not consis-
tant in Franklin et al. (2007). The reason was suggested to be the
58 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
photocatalytic generation of H2O2 responsible for antimicrobial
action of ZnO. Even though, both Zn+2 ion and ZnO nanoparticles
show antibacterial activity. Aquatic organisms can be highly sen-
sitive to dissolved zinc (Franklin et al., 2007). In view of its easy
dissolving nature, ZnO applications in drinking water purification
are limited.
3.4.4 Antimicrobial Action of Carbon Nanotubes
CNTs show antimicrobial property upon direct contact with
cells (Vecitis et al., 2010). Graphene and graphite materials also
exhibit similar mechanisms (Liu et al., 2012). Short, dispersed,
and metallic CNTs with small diameters are more lethal (Qu
et al., 2013a). CNTs effectively removes bacteria by size exclusion,
and viruses by depth filtration (Brady-Estévez et al., 2008). Re-
tained bacteria are quickly inactivated by CNTs; MWNTs directly
oxidize adhering bacteria and viruses. They decontaminate wa-
ter within seconds by using small intermittent voltage (Rahaman
et al., 2012; Vecitis et al., 2010).
3.5 Nano Antimicrobial Polymers
Polymeric nanoparticles kill microorganisms either by releas-
ing antibiotics, antimicrobial peptides, and antimicrobial agents
or by contact-killing cationic surfaces, such as quaternary ammo-
nium compounds, alkyl pyridiniums, or quaternary phosphoni-
um. Different antibacterial mechanisms are reported to show how
these cationic groups are able to disrupt bacterial cell membrane.
The main mechanism is that the hydrophobic chains of certain
lengths will penetrate and burst the bacterial membrane. It has
been shown that high levels of positive charge are capable of con-
ferring antimicrobial properties irrespective of hydrophobic chain
length, perhaps by an ion exchange mechanism between the bac-
terial membrane and the charged surface (Beyth et al., 2015; Jain
et al., 2014).
Nanopolymeric antimicrobial materials show long-term an-
timicrobial activity. They are nonvolatile and chemically stable.
They can bind to the surface of interest and hardly permeate
through biological membranes (Kenawy et al., 2007). Polyca-
tionic antimicrobials contain high surface density of active
groups, which result in higher antimicrobial activity. Quaterna-
ry ammonium compounds possess a broad spectrum of antimi-
crobial activity against both Gram-positive and Gram-negative
bacteria. Polyamines that have been reported as being highly ef-
fective antimicrobial nanoparticles are quaternary ammonium
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 59
polyethylenimines, which have a broad range of bacterial tar-
gets when incorporated into various polymeric matrixes (Shvero
et al., 2015).
3.5.1 Chitosan-Based Nanoparticles
Chitosan is obtained from chitin in shells. It show antibacterial
activity and is a natural polymer. The nanoparticles of chitosan
or its derivatives have a broad spectrum of antibacterial, anti-
fungal, and antiviral activities (Chirkov, 2002; Li et al., 2008; No
et al., 2002; Qi et al., 2004; Rabea et al., 2003). The nanoparticles
of chitosan are more effective against fungi and virus compared
with bacteria (Rabea et al., 2003). Among bacterial strains the anti-
microbial activity of chitosan is higher for Gram-positive bacteria
than for Gram-negative bacteria (Don et al., 2005; No et al., 2002).
Minimum inhibitory concentrations from 18 to 5000 ppm were re-
ported for the chitosan nanoparticles depending on the organism,
pH, molecular weight, degree of polymerization, and the pres-
ence of lipids and proteins (Don et al., 2005; No et al., 2002; Rabea
et al., 2003).
The main antimicrobial mechanisms of chitosan are the posi-
tively charged chitosan particles that interact with negatively
charged cell membranes, causing an increase in membrane per-
meability and eventually rupture and leakage of intracellular
components (Qi et al., 2004). The antibacterial action of chito-
san derivatives containing quaternary ammonium groups, such
as N,N,N-trimethyl chitosan, N-propyl-N,N-dimethyl chitosan,
and N-furfuryl-N,N-dimethyl chitosan are stronger than those of
chitosan, and they increase with decreasing pH (Ji et al., 2009; Li
et al., 2008).
Nanochitosan has potential drinking water disinfection ap-
plications as an antimicrobial agent in membranes, sponges,
or surface coatings of water storage tanks. Nanochitosan shows
superiority over other disinfectants due to its broader spectrum
of activity against bacteria, viruses, and fungi, and less toxicity
toward animals and humans. However, chitosan is an effective
disinfectant only at acidic pH; this is due to its solubility and the
availability of charged amino groups (Li et al., 2008; No et al., 2002;
Rabea et al., 2003). This limitation can be overcome by preparing
water-soluble chitosan derivatives.
3.5.2 Nanopolymers-Based Water Purification Systems
N-bromohydantoin/uracil-conjugated polystyrene beads
were prepared by the authors and studied controlled release of
bromine in water purification systems (Farah et al., 2015a,c).
60 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
Active bromine release from the beads into the running water
and the antimicrobial efficiency were examined against E. coli
and MS2. This resin exhibited excellent antimicrobial proper-
ties; 6- and 4-log reduction for E. coli and MS2, respectively, were
obtained for all tested points during 250 L. Moreover, beside
the effect of the N-haloamine structure on lasting disinfection
activities, bead’s nanomicro characteristics were found criti-
cal for oxidative halogen release control: rate stabilization and
modulation, extension and consequently influences antimicro-
bial activity. This aspect is important, since the rate of halogen
release influences antimicrobial activity and subsequently the
material usage for different applications, Fig. 2.8B and C (Farah
et al., 2015a). Bromine% of the reported materials after release
study indicated the capability of antimicrobial hypobromous re-
lease for extra hundreds of liters. N-bromo-dimethylhydantoin
with low crosslinked beads maintained their activity for 550 L.
This brominated resin should be considered in filters for
Figure 2.8. (A) N-bromo-hydantoin synthesis by conjugation to micronsize beads followed by bromination; (B) filters
loaded with N-bromo-hydantoin/5,5’-dimethylhydantoin-polystyrene beads; and (C) in filters operations: hypobromous
release from different beads having different nanomicro characteristics affecting oxidative halogen releasing profiles
from polymeric beads/surfaces (Farah et al. 2015a).
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 61
decontamination of drinking water (Aviv et al., 2015). These
materials were found to have great potential due to ability to
decontaminate large volumes of contaminated water, low costs,
and bromine rechargeability (Farah et al., 2015a,c) (Fig. 2.9).
Quaternary ammonium polyethylenimine
nanoparticles (QA-PEI NPs) of C8 chain alkylated,
and C18 modified were prepared (Fig. 2.10), and
these nanoparticles were embedded into polyeth-
ylene vinyl acetate and polyethylene methacrylic
acid coatings (Farah et al., 2013; Yudovin-Farber
et al., 2010). These coatings were tested for their
antibacterial activity against representative bacte-
ria, E. coli, P. aeruginosa, Staphylococcus aureus, and
heterotrophic plate count. Study was performed in
both static and dynamic modes. The authors antici-
pated that these QA-PEI NPs will find application in
water purification systems as disinfectants and for
Figure 2.10. Chemical structure
of C8/C18 alkylated QA-PEI
crosslinked nanoparticles.
Figure 2.9. Representative photos of the nanopores onto the brominated-hydantoin beads surface or inside were
found to play a critical role in oxidative halogen release control: rate stabilization and modulation, extension and
consequently influences antimicrobial activity ( Farah et al. 2015a).
62 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
preventing the accumulation of microorganisms onto device
surfaces as well as in bioadhesives and self-sterilizing surfaces
(Farah et al., 2015b).
4 Regeneration of Nanoparticles
Regeneration of nanoparticles in water purification is one
of the crucial aspects, since it controls the economy of water-
treatment technology. pH-dependent solvents play crucial roles
in regeneration of nanoparticles. They can also be achieved by
applying a separation device or immobilizing nanomaterials
in the treatment system. Membrane filtration is promising for
the regeneration and reuse of nanomaterials due to its chemi-
cal use. Ceramic membranes are always advantageous, as they
are more resistant to UV compared to polymer membranes (Qu
et al., 2013a). Raw water pretreatment is very important to re-
duce turbidity; otherwise the suspended particles are retained
by membranes, and they decrease the efficiency of the treat-
ment. Immobilization is another technique for nanomateri-
als. But, current immobilization techniques have not been very
successful. Development of simple and low-cost methods is re-
quired to immobilize nanoparticles without affecting their effi-
ciency. Magnetic separation is another option for the separation
of magnetic nanoparticles.
Nanomaterials coated on treatment system surfaces are re-
leased in a relatively quick and complete manner. Nanomaterials
embedded into a solid matrix have slow release until they are dis-
posed of. Identification of a nanomaterial release is a major tech-
nical hurdle for risk assessment, and it remains challenging. The
details regarding different detection techniques are mentioned in
the literature (Qu et al., 2013a; Silva et al., 2011; Tiede et al., 2008).
According to the literature, nanoparticles can be regenerated and
used for water treatment, which makes them economically vi-
able materials. The regeneration capacity of nanoparticles may be
considered an extra advantage for their popularity in wastewater
treatment. However, only very few methods are available for the
detection of nanomaterials in complex aqueous matrices. They
are highly sophisticated, expensive, and have many limitations.
Currently, rapid, sensitive, and selective nanomaterial analytical
techniques are in great demand.
Management of used nanoparticles and recovered pollutants
is one of the most important aspects. Everyone is aware of pollut-
ant hazards and nanotoxicology; proper disposal must be carried
out by the users. The best way is to recycle the nanoparticle and
exhausted nanoparticles may be in the manufacturing of bricks,
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 63
stones, and so on. Different regenerated materials may be recycled
for manufacturing various commodities. The recovered organic
contaminants should be treated as priority pollutants (Ali, 2012).
5 Safety, Toxicity, and Environmental Impact
of Nanomaterials
Due to our current poor understanding of the fate and behav-
ior of nanoparticles in humans and the environment, toxicity is
becoming one of the urgent issues of nanotechnology. The main
concerns related to nanotechnology are the hazardousness of
nanoparticles and the exposure to risk (Gardner and Dhai, 2014).
Biological and chemical effects on humans or the environment is
the first major issue. The second is of leakage, spillage, circulation,
and concentration of nanoparticles that might cause a hazard to
humans or the environment (Gardner and Dhai, 2014). Proper-
ties, such as size, shape, reactivity, and so on, are making these
nanomaterials very useful. The same properties can also make
them harmful to the environment and toxic to humans (Hillie and
Hlophe, 2007). The entry of nanoparticles into our body is possi-
ble through the skin, inhalation, ingestion, and so on. After reach-
ing the bloodstream, they can travel to various body parts, such
as the brain, heart, liver, kidneys, spleen, bone marrow, and ner-
vous system (Ali, 2012). The toxicity of nanoparticles is due to their
properties and can lead to high chemical reactivity and produc-
tion of ROS. Production of ROS is possible from CNTs and metal
oxides. The ROS generated will cause oxidative stress, inflamma-
tion, which results in damage to proteins, membranes, and DNA
(Nel et al., 2006). There is a possibility that nanoparticles can ad-
sorb on the body surface and alter the mechanisms of enzymes
and certain proteins (Hubbs et al., 2011). Nanoparticles show their
toxicity in the environment by agglomeration. Environmental risk
analysis of nanoparticles is mentioned in the literature (Grieger
et al., 2012). Knowledge of hazards and exposure risks of nanopar-
ticles to the ecology is less among the scientific community, so risk
assessment and management is crucial (Gardner and Dhai, 2014).
The challenge is to resolve problems before nanoparticle usage
starts on a large scale in water purification. There must be safe-
ty evaluation, large-scale production facilities, safe disposal of
wastes, and energy efficiency. These are the major challenges that
may cause delay in the large-scale application of nanotechnology
in water purification (Gardner and Dhai, 2014). However, the be-
havior of nanoparticles inside the body is still a major question
that needs to be addressed.
64 Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION
6 Limitations and Research Needs
Water/wastewater-treatment processes by nanotechnology
show great promise in laboratory studies. Some of these tech-
nologies are marketed, and others require significant research
before they can be scaled up. Their commercialization is chal-
lenging; we need to overcome many technical hurdles; make
them cost effective, and safe. Research is needed before full-scale
operation of nanotechnology for treating natural and wastewa-
ters. Studies should be conducted under realistic conditions to
assess the efficiency of available nanotechnology to validate the
nanomaterial-enabled sensing. Another research need is to mea-
sure the long-term efficiency of available technologies, which
are conducted on a laboratory scale. Commercialization of these
technologies is possible only by their long-term performance in
the treatment of water and wastewater. Also, adoption of an inno-
vative technology strongly depends on the cost effectiveness and
potential risks involved. At the moment, the cost of nanomaterials
is very high, with few exceptions, such as nano-TiO2, nano-scale
iron oxide, and polymeric nanofibers. Cost effectiveness can be
achieved by regeneration and reuse of these nanomaterials (Qu
et al., 2013b). Since these materials are nanoscale, risk assessment
and management is a challenge. Researchers should understand
the potential hazards of these materials in the treatment of water
and wastewater.
7 Conclusions
Today we need water purification technologies that provide
high-quality drinking water, remove micropollutants, and inten-
sify the industrial processes. Nanotechnology provides the oppor-
tunity; unique properties of nanoparticles are ideal candidate for
developing rapid water-treatment technology. Nanoparticles may
eliminate metal ions, anions, organic compounds, and microor-
ganisms. Nanoparticle doses required for the water treatment are
low, making their application relatively economical.
Different nanotechnologies are reviewed in this chapter. A few
technologies are in laboratory research stage, some reached to pi-
lot testing, and some are commercial. Among these technologies,
nanoadsorbents, nanomembranes, and nanophotocatalysts are
most promising. Although, these technologies have been com-
mercialized, their potential has not been reached for large-scale
use in wastewater treatment.
Risk assessment of recovered pollutants and exhausted
nanoparticles still remains significantly unexplored. Therefore,
Chapter 2 NANOTECHNOLOGY FOR WATER PURIFICATION 65
ecofriendly waste management methods are required to avoid
hazards and toxicities. The future of the nanoparticles in water
treatment is quite progressive, but it requires collaborative efforts
of academic and industrial resources to materialize a fast, eco-
nomical, and feasible water-treatment technology. It will be pos-
sible by working together to solve water contamination globally.
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Chapter
IntroductionChiral SelectorsMicellar Electrokinetic Chromatography (MEKC)Capillary Electrochromatography (CEC)Supercritical Fluid Chromatography (SFC)Thin Layer Chromatography (TLC)LC versus GCConclusions References