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Inorganic engineered nanoparticles in drinking water
treatment: a critical review
Konstantinos Simeonidis, Stefanos Mourdikoudis, Efthimia Kaprara, Manassis
Mitrakas, Lakshminarayana Polavarapu
To cite this version:
Konstantinos Simeonidis, Stefanos Mourdikoudis, Efthimia Kaprara, Manassis Mitrakas, Lakshmi-
narayana Polavarapu. Inorganic engineered nanoparticles in drinking water treatment: a critical
review. Environmental Science : Water Research and Technology, Royal Society of Chemistry, 2016,
2, pp.43-70. �10.1039/C5EW00152H�. �hal-01195548�
1
Inorganic engineered nanoparticles in drinking water treatment: A
critical review
Konstantinos Simeonidis, *
a
Stefanos Mourdikoudis, *
bc
Efthimia Kaprara,
d
Manassis
Mitrakas,
d
Lakshminarayana Polavarapu*
ef
a
Department of Mechanical Engineering, University of Thessaly, 38334, Volos, Greece
b
Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, F-75005, Paris, France
c
CNRS, UMR 8233, MONARIS, F-75005, Paris, France
d
Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124,
Thessaloniki, Greece
e
Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-
Maximilians-Universität München, D-80799, Munich, Germany
f
Nanosystems Initiative Munich (NIM), D-80799, Munich, Germany
Abstract
This review summarizes recent research in the field of inorganic engineered nanoparticles
development with direct or potential interest for drinking water treatment. The
incorporation of engineered nanoparticles in drinking water treatment technologies against
the removal of heavy metals, microorganisms and organic pollutants appears as a very
dynamic branch of nanotechnology. Nanoparticles owe their potential on the high specific
surface area and surface reactivity compared to conventional bulk materials. Depending on
the mechanism of uptake, nanoparticles can be designed to establish high selectivity against
specific pollutants and provide the required efficiency for application. However, despite
early encouraging results, nanoparticles meet a number of limitations to get promoted and
become part of large-scale water treatment plants. The most important is their availability in
the required large quantities and their efficiency to fulfil the strict regulations for drinking
water consumption and environmental safety. Both deal with the particles preparation cost
and the cost of treatment operation with respect to the increase of supplied water price for
the consumers. Under this view, this work attempts to evaluate reported studies according
to their possibility to meet reliable requirements of water technology and also suggests an
experimental approach to allow validation of tested nanoparticles.
Keywords: engineered nanoparticles, drinking water treatment, heavy metals, antimicrobial
activity, organic pollutants
1. Introduction
Following the principles and the discoveries related to the evolution of nanosciences
2
during the last two decades, a wide variety of technological fields have been promoted.
1
The
impact was immediate and more obvious to the called high-technology applications where
the demand for dimensions decrease combined with the novel electronic, optical, magnetic
and mechanical properties of nanomaterials resulted in the development of new devices and
methods.
2
,
3
The expansion of nanotechnology, in these first stages, is mainly referred to the
field of electronics and health sciences
4
,
5
,
6
,
7
,
8
whereas its incorporation in more traditional
fields of technology was limited. For instance, the adoption of nanomaterials in conventional
everyday products (clothes, shoes, cosmetics, dyes), industrial, agricultural and
environmental protection processes
9
,
10
,
11
encounters more skepticism based on the large
quantities demands combined with their relatively high cost, the need for redesign and
reconstruction of process lines and the uncertainty arising by the fate and the effect of
nanomaterials to the direct or indirect environmental receptors.
12
,
13
,
14
,
15
,
16
Nevertheless, numerous products nowadays claim their innovation on the addition of
nanomaterials which improve their physical properties. In addition, there is a significant and
intense research effort on the development and optimization of nanomaterials aiming in
antimicrobial or catalytic activities with high potential for environmental applications.
17
,
18
The decontamination of flue gases from heavy metals and aromatics,
19
,
20
the treatment of
municipal and industrial wastewater for the removal of various pollutants
21
,
22
and the
purification of drinking water
23
,
24
or recirculating blood
25
,
26
are the major sections of
investigation. Among them, the treatment of natural water for drinking purposes appears as
the most challenging field directly dealing with human nutrition and health.
27
In this field,
any applied method should also comply with the extremely low pollutants concentrations
met in natural water and the strict international legislation for human health and
environmental safety. In particular, nanomaterials have been tested as media for
purification, disinfection, removal of heavy metals, degradation of organic compounds and
pharmaceuticals.
28
,
29
,
30
,
31
Since the beneficiary use of nanomaterials in drinking water treatment is considered as
an achievement of high importance, they should be designed to maintain the highest
possible specific surface area in order to maximize surface reactivity, effective contact and
uptake capacity. For this reason, the class of engineered nanoparticles, and more
specifically, inorganic nanoparticles which combine relatively enhanced purification
properties and high stability in water, should be preferred for water treatment. Inorganic
engineered nanoparticles optimized for water treatment usually act as reaction catalysts
causing the degradation, oxidation and reduction or as adsorbents which form strong bonds
with specific compounds in a non-reversible way. Therefore, apart from the requirement for
high surface area, the selectivity of nanoparticles for specific water purification processes
should stand on the chemical affinity, the surface charge density and the electron transfer
ability. However, the main drawbacks for nanoparticles use in water treatment are not
directly related to their efficiency but to technical, economical and safety limitations which
complicate the replacement of conventional methods.
This review presents the recent laboratory research related to the application of
engineered nanoparticles consisting of inorganic phases in water treatment and their
classification in the fields of heavy metal removal, antimicrobial activity and organic
3
compounds degradation. Reported results are discussed not only according to their potential
for application in different drinking water treatment processes but also from a critical
consideration of the possibility to scale-up in technologically viable methods and become
competitive to existing techniques and conventional materials. As one of the main
limitations in the effort to evaluate the efficiency of nanomaterials from different authors is
the absence of a unified procedure that enables direct comparison of results, this work
suggests an experimental methodology working with reliable conditions and parameter
ranges of drinking water treatment and generating proper indices for the validation of
performance.
2. Synthesis methods of nanoparticles
A wide variety of methods have been used to produce nanoparticles based on traditional
and modern chemical or mechanical procedures. Chemical methods usually generate
nanoparticles in dispersions following the gradual size increment of small nuclei after the
deposition of atoms or ions released by a chemical reaction (bottom-up).
32
Nanoparticles are
formed as a result of oversaturation of soluble phases when a change in their solubility
occurs. Depending on the source of solubility modification, chemical methods are classified
in precipitation (acidity variations),
33
thermal decomposition (high temperatures),
34
solvothermal (high pressure),
35
sonication (supersonics)
36
and electrodeposition (redox
potential).
37
On the opposite, in mechanical preparation routes, nanoparticles are obtained
after splitting large-dimension materials in smaller units (top-down). High-energy ball-milling
is the main mechanical preparation method for size reduction and preparation of single- or
multiple-phase nanoparticle systems.
38
Finally, spray techniques may produce nanoparticles
in the vapor phase by thermal or laser assisted chemical reactions.
39
In general, structural and chemical stabilization of nanoparticles are the most important
requirements for a successful synthetic approach. The existence of such features ensures
high surface-to-volume ratio, sufficient resistance against phase changes (e.g. oxidation) and
appearance of nanoscale effects. For this reason, high quality nanoparticles preparation
methods are usually based on the use of surfactants or inorganic coatings to ensure good
isolation and a series of size separation and classification procedures to minimize
polydispersity. However, most of these processes are not compatible to environmental
applications, being even less compatible to water purification for drinking purposes.
As already mentioned, due to the high volumes of water to be treated, water purification
demands proportionally high availability in nanoparticle quantities when those are qualified
for the application. Therefore, the preparation cost for nanoparticles may dominate the
overall cost of the treatment process. This implies that synthesis methods based on
expensive reactants or working at high temperatures are not so favorable. In addition, the
high toxicity and the incompatibility to aqueous processes is a serious drawback for the
adoption of methods using organic metal precursors, reactants or solvents. It should be
mentioned that in terms of industrial production, the accomplishment of strict
environmental conditions does not only concern the obtained nanoparticles and their
application in water treatment but their large-scale production line as well.
4
Since water treatment reactions usually take place in active sites on the surface of the
solid, another limitation in the preparation approach is related to the need for keeping the
surface free of surfactants or coating layers which are usually employed to isolate or protect
nanoparticles. On the contrary, the formation of nanoparticles without surfactants facilitates
agglomeration anda consequentloss in effective specific surface area. Such restrictions
indicate that in principle only low-cost, easily scalable, aqueous compatible and potentially
environmentally friendly methods may provide nanoparticles suitable for water technology.
According to previous analysis, it is concluded that chemical precipitation and mechanical
size reduction should be initially considered and developed to obtain nanoparticles for water
treatment. More expensive methods should be followed for secondary and selective stages
when small quantities of nanoparticles are required.
3. Applications in water treatment
3.1. Removal of heavy metals
The presence of heavy metals in aqueous systems is considered a major worldwide
problem related to many harmful effects on the health of humans and other life forms.
40
,
41
,
42
The main threats are associated with the consumption of elements such as arsenic, lead,
chromium, mercury, antimony, cadmium and nickel some of which appear in the form of
soluble oxy-ions in natural water. Traditional removal methods for heavy metals are
classified as relatively selective (chemical coagulation/filtration, adsorption) and non-
selective (nanofiltration, reverse osmosis). Among these methods, adsorption is considered
as a one of the most promising methods because metal-loaded adsorbents are more
compact and generally form stronger bonds. For this reason, the use of consumable
adsorbents is nowadays the dominant trend, since it is the simplest removal method. The
qualification of the proper adsorbent for an individual heavy metal is based on a number of
conditions defined by the uptake mechanism of its species. High chemical affinity,
stabilization of positive or negative surface charge and incorporation of ion or electron
exchange potential are described as possible directions of optimization. A large variety of
nanostructured materials, usually in the form of inorganic engineered nanoparticles, has
been studied as adsorbents for the removal of heavy metals. The main research efforts
concern the use of inorganic nanoparticles such as zero-valent iron (ZVI), iron oxides (Fe
3
O
4
,
γ-Fe
2
O
3
) and oxy-hydroxides (FeOOH), some other metal oxides (Al
2
O
3
, TiO
2
,
MnO
2
, ZrO
2
,
ZnO, MgO, CeO
2
) and metals or alloys (Au, Ag, Pd). Few of them were already promoted as
commercial products in water treatment technology. The goal of this process is the
reduction of residual concentration below the regulation limit set by international
organizations. Depending on the established risk of each heavy metal, its concentration
must comply with a different tolerance limit. For instance, the regulation limit for arsenic in
E.U. countries is 10 μg/L while the corresponding one for mercury is 1 μg/L.
43
Iron-based nanoparticles are the most widely applied systems for the uptake of heavy
metals in water.
44
The combination of properties like chemical affinity to targeted oxy-ions,
surface charge and redox potential together with their stability and low-cost enable their
use for various cases. In addition, the magnetic behavior of phases such as Fe
3
O
4
, γ-Fe
2
O
3
and ZVI facilitates their recovery after application. In particular, ZVI nanoparticles are
