Noble metal nanoparticles for water purification: A critical ...
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Abstract: explained by a number of concepts such as electrochemical potential, relativisitic contraction, molecular orbital. theory, etc. Recently, water quality has been associated with the development index of society. ... theory states that overlapping of electronic states of two atoms leads. to orbital ...
Thin Solid Films 517 (2009) 6441–6478
Contents lists available at ScienceDirect
Thin Solid Films
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Noble metal nanoparticles for water puriﬁcation: A critical review
T. Pradeep ⁎, Anshup
Department of Chemistry and Sophisticated Analytical Instrument Facility, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e i n f o a b s t r a c t
Available online 1 April 2009 Water is one of the essential enablers of life on earth. Beginning with the origin of the earliest form of life in
seawater, it has been central to the evolution of human civilizations. Noble metals have been similarly associated
Keywords: with the prosperity of human civilizations through their prominent use in jewellery and medical applications. The
Water puriﬁcation most important reason for the use of noble metals is the minimal reactivity at the bulk scale, which can be
Nanotechnology explained by a number of concepts such as electrochemical potential, relativisitic contraction, molecular orbital
theory, etc. Recently, water quality has been associated with the development index of society. A number of
chemical and biological contaminants have endangered the quality of drinking water. An overview of important
Heavy metal ions events during last 200 years in the area of drinking water puriﬁcation is presented. Realizing the molecular nature
Micro-organisms of contamination in drinking water, signiﬁcant progress has been made to utilize the chemistry of nanomaterials
Drinking water for water puriﬁcation. This article summarizes recent efforts in the area of noble metal nanoparticle synthesis and
the origin of their reactivity at the nanoscale. The application of noble metal nanoparticle based chemistry for
drinking water puriﬁcation is summarized for three major types of contaminants: halogenated organics including
pesticides, heavy metals and microorganisms. Recent efforts for the removal, as well as ultralow concentration
detection of such species, using noble metal nanoparticles are summarized. Important challenges during the
commercialization of nano-based products are highlighted through a case study of pesticide removal using noble
metal nanoparticles. Recent efforts in drinking water puriﬁcation using other forms of nanomaterials are also
summarized. The article concludes with recent investigations on the issue of nanotoxicity and its implications for
© 2009 Elsevier B.V. All rights reserved.
Take up one idea. Make that one idea your life — think of it, dream of water about 3.5 billion years ago and was transferred to land only
it, live on that idea. Let the brain, muscles, nerves, every part of your 380 million years ago. Despite this transfer to land, an ocean, in terms
body, be full of that idea, and just leave every other idea alone. This is of ﬂuidic composition, continues to exist within us. Similarly, keeping a
the way to success, that is the way great spiritual giants are produced. very high proportion of living organisms' body weight as water
(∼70%), Nature has iterated the vitality of water for life. It is very
- Swami Vivekananda, World's most-respected Vedanta thinker, appropriate to say that existence of life on Earth is largely owed to the
1863–1902 presence of water. It is vital to us, both as a universal solvent as well as
being an important component of metabolic processes within the
Vedanta Philosophy: Lectures by Swami Vivekananda, Kessinger body. Clean and fresh water is essential for the existence of life. The
Publishing, USA, 1996, Page 70. evolution of civilization has always revolved around water. The Nile
was the lifeline of the Egyptian civilization. The Indus Valley
1. Introduction civilization ﬂourished on the banks of the Indus river. There is no
aspect of our life that is not touched by water. Water is one of the clear
The theme of this review is water, one of the essential companions signs of prosperity, health, serenity, beauty, artistry, purity and many
of life on earth. During the phases of creation, evolution and continuity other attributes. Leonardo Da Vinci had described water as “the vehicle
of life on earth, water remained as its most vital component. The of nature” (“vetturale di natura”).
molecular as well as macromolecular functions of making life possible For time immemorial, Nature has made noble metals part of our daily
are carried out using water. The earliest form of life appeared in sea life. In being a part of numerous applications such as jewellery,
currencies, photographic ﬁlms and electrical conductors; noble metals
have made a mark for themselves as being our own household materials.
⁎ Corresponding author. Tel.: +91 44 2257 4208 (direct) 2257 5938/5942 (lab and
students); fax: +91 44 2257 0545/0509/4202.
This is a very important milestone for our societal progress — the reason
E-mail address: [email protected] (T. Pradeep). being we spent sufﬁcient time in making sure that what is being used by
URL: http://www.dstuns.iitm.ac.in/pradeep-research-group.php (T. Pradeep). us, should be friendly to us. This has become a fundamental question
0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
6442 T. Pradeep, Anshup / Thin Solid Films 517 (2009) 6441–6478
while we are discovering newer materials every day. With noble metals, Silver has been equally popular for domestic use since the ancient
perhaps we know the answer. times. As gold was associated with sun due to its color, the white
Gold and silver belong to the family of “metals of antiquity”, having brightness of silver became associated with the moon. The distinct
their history with mankind dating back to 6000 BC and 4000 BC, optical properties of silver deﬁned its name: the word silver is of
respectively. Gold's brilliance, intrinsic beauty and ability to preserve Gothic origin meaning shiny white and the Latin name argentum
its shine for long periods made it the most documented metal in the originates from an Aryan root which means white and shining.
human history. While there are no clear-cut references for the From the historical times, utensils were fabricated with silver lining.
discovery of gold, it has always been associated with the gods, with Silver vessels were utilized for the preservation of perishable items as
immortality, and with wealth across the human civilizations. well as for disinfection of water. The writings of Herodotus suggested
The earliest evidence for the use of gold in jewellery comes from that Cyrus the Great (Persia, 550–529 BC) consumed water boiled in
Sumer civilization (southern Iraq, 3000 BC). The use of gold as jewellery silver vessels . The account also mentions the use of silver as a
continued through different civilizations thereafter (tomb of King precious booty, during the time of Pausanias (Sparta, 5th century BC).
Tutankhaman [Egypt, 1300 BC], gold ornaments from Indus valley Silver was widely used as a disinfectant and anti-spoilage agent, across
[Mohenjadaro, 3000 BC] and royal crowns from the Tillia tepe treasure many civilizations (Greece, Rome, Phoenicia, Macedonia) . Hippo-
[Scythian, 100 BC]). Another milestone in the history of noble metals was crates, the Father of medicine, promoted the use of silver for early
reached around 700 BC when Lydian merchants produced the ﬁrst coins healing of wounds . Alexander the Great (335 BC) was advised by
through use of gold–silver alloys called ‘electrum’. The use of noble metals Aristotle to store water in silver vessels and boil prior to use .
for currency was further developed by the Roman Empire (100 AD) . Evidence exists for the use of silver nitrate as an anti-bacterial agent, in
It was suggested that the use of gold, in the form of swarna bhasma the Roman pharmacopeias.
(meaning, gold ash), for medicinal purposes started during the Vedic During the late 13th century, Lanfranc utilized silver tubes for
period (1000 BC-600 BC) in ancient India . References also exist in the introduction of food beyond ﬁstula . Thereafter, Paracelsus (1493–
Chinese literature about the medicinal properties of gold and it was 1541) pioneered the use of metals for medical applications. He linked
thought to act as the elixir of life . The medicinal uses of gold were silver with the development of brain activities in his hermetic and
promoted after the work of Paracelsus (15th century) who ﬁrst prepared alchemical writings. During the mid-19th century, Joseph Lister and
gold colloid solution in modern times. The purple solution of gold was Marion Sims promoted the use of silver wire sutures, in order to reduce
called Aurum Potable and it was strongly believed that this would impart the incidences of septic complications . During the late 18th century,
rejuvenation to the human body, as historically gold was equated with Crede, a German obstetrician, popularized the use of prophylactic l%
the sun (“tears of the sun”). Antonii described the medical uses of silver nitrate eye solution for the prevention of ophthalmia neonatorum
colloidal gold in 1618, in the reportedly ﬁrst documented literature on . Pioneering work in the study of anti-bacterial properties was
colloidal gold . This was followed by an effort from Johann Kunckels, a carried out by Ravelin (1869) and von Nageli (1893) . It was reported
German chemist, to explore the medicinal properties of the pink- that even at low concentrations of 9.2 × 10 − 9 and 5.5 × 10 − 6 M, silver
solution of gold (1676) . A few other subsequent efforts followed, salt was toxic to Spirogyra and Aspergillus niger spores, respectively .
highlighting the stable dispersion of gold [6–10]. The unusual color of The term “oligodynamic” was coined to represent the activity even at low
metallic gold caught the attention of many researchers. As a conse- concentrations. A landmark work in the chemistry of silver was
quence of the work accomplished so far on gold, Michael Faraday accomplished by Carey Lea in the late 19th century [24,25]. Silver
attempted a novel synthetic route to prepare colloidal gold in 1857 colloid was ﬁrst prepared through the reduction of silver nitrate using
(Fig. 1) . He synthesized a dark red solution of colloidal gold by the ferrous sulfate and consequent protection of colloidal particles with
reduction of an aqueous solution of chloroauric acid using phosphorus in citrate ions [24,25]. A number of researchers worked on the anti-
CS2. He realized that stability of colloidal gold against aggregation can be bacterial properties of silver salts: In the early 20th century, a porous
achieved using stabilizing agents. He also noticed the reversible color metallic mesh of silver was prepared (“Katadyn silver”) and was utilized
changes driven by mechanical compression (blue–purple → green) of as an anti-bacterial water ﬁlter. The popularity of silver salts continued
thin ﬁlms prepared with colloidal gold . In 1890, the distinguished to grow through the development of new silver salts; e.g., silver
German bacteriologist Robert Koch discovered the use of gold cyanide sulfadiazine, silver citrate, silver lactate, etc. A number of products based
for bacteriostatic action against tubercle bacillus, the causative agent for on silver were also commercialized throughout the 20th century
tuberculosis. Later, it was found that tubercle bacillus is also responsible (Katadyn, Argyrol, Movidyn, Tetrasil, Alagon, etc.) . Attempts were
for rheumatoid arthritis. This led to further investigations in modern also made to immobilize silver in zerovalent form on activated carbon
medicinal uses of gold [16–18]. and subsequently use it for disinfection of water .
As illustrated from the historical perspective, the properties of gold
and silver have been used continuously for a number of applications. Till
a century ago, the applications were largely restricted to their medicinal
value. A few applications of noble metals based catalysis were also
studied in the last century such as silver based catalysis of methanol to
formaldehyde  and ethylene to ethylene oxide . Till recently, all
of these applications have been based on properties of macroscopic form
of the noble metals. Excellent review articles have been written on each
subject area [16,30–36] and these are not part of the present review.
The origin of chemical reactivity for metals is related to their standard
reduction potentials. Metals are usually electropositive and have a
tendency to lose electrons depending on the corresponding ionization
energy. Reduction potential is thus correlated with the electropositive
nature of the metals i.e. a metal with high electropositive nature is likely
to exist as an ion in the solution phase and thus is a strong reducing
agent. Based on the reduction potential, metals usually belong to two
Fig. 1. (a) Faraday's colloidal suspension of gold . (b) High resolution transmission
groups: d-block metals belong to the moderately reducing group (Cd2+|
electron microscopic image of individual colloidal gold particles (at a magniﬁcation of Cd = −0.40 V, Fe2+|Fe = −0.44 V) whereas s-block metals belong to
107 ×), prepared according to Faraday's recipe . the strongly reducing group (Li+|Li = −3.05 V, Na+|Na = −2.71 V).
T. Pradeep, Anshup / Thin Solid Films 517 (2009) 6441–6478 6443
Table 1 Transition metals usually have high melting points leading to a
Overview of the basic properties of noble metals and a comparison with other transition high energy of sublimation. Similarly, owing to ﬁlling of the d-band,
the transition metal atoms are usually smaller in size (Table 1) and
a b c d e f g h i j k l thus have higher ionization energies (Table 1). This explains the lower
Copper FCC 1090 887 2.2 78 947 114 1440 5727 596 50 oxidation potential of transition metals with respect to s-block.
Gold FCC 1337 3129 2.5 223 890 174 2450 19,300 446 320 The noble nature of certain transition metals is explained using the
Silver FCC 1235 2435 1.9 126 731 165 25 10,490 631 430
relativistic contraction concept . The Dirac-Fock equation  was
Platinum FCC 2041 4098 2.3 205 870 177 392 21,090 94 72
Palladium FCC 1828 3236 2.2 54 804 169 37 12,023 95 72 solved for all atoms with atomic number less than 120 . The
Mercury RH 234 630 2.0 0 1,007 171 – 13,534 10 8 explanation for relativistic contraction arises due to the special theory
Ruthenium FCC 2607 4423 2.2 101 710 178 2160 12,370 132 120 of relativity which imposes a limit on the maximum speed of particles.
Rhodium FCC 2237 3968 2.3 110 720 173 1100 12,450 222 150 As the particle accelerates to a velocity nearer to the speed of light, a
Rhenium FCC 3459 5869 1.9 15 760 188 1320 21,020 52 48
Osmium FCC 3306 5285 2.2 106 840 185 3920 22,610 105 88
correction in the mass is required and is calculated as follows:
Iridium FCC 2739 4701 2.2 151 880 180 1670 22,650 189 150
Avg (NM) – 2010 3515 2 106 833 170 1451 15,751 234 137
Avg (TM) – 2277 4021 1.7 45 690 182 865 9027 77 61
m = m0 = 1 −ðv=cÞ2 :
Column headers: (a) Noble metal, (b) Lattice structure, (c) Melting point in K,
(d) Boiling point in K, (e) Electronegativity, (f) Electron afﬁnity in kJ/mol, (g) Ionization Here m is the corrected mass, m0 is the rest mass, v is the velocity
energy in kJ/mol, (h) Radius in pm, (i) Hardness in MPa, (j) Density in kg/m3, of the particle and c is the speed of light.
(k) Electrical conductivity in mho/cm, and (l) Thermal conductivity in W/mK.
According to non-relativistic quantum mechanics, the average radial
(Abbreviations — FCC: Face-centered cubic, RH: Rhombohedral, NM: Noble metals, TM:
Transition metals). velocity of the electron in 1 s shell is bvrN/c = Zα, where Z is the atomic
number and α is the ﬁne structure constant (α ≈ 1/137). The effect of
the relativistic contraction is explained using the example of mercury.
However, there are exceptions to this rule: Gold (Au3+|Au = 1.5 V), Silver The atomic number of mercury is 80, i.e. vr/c = 80/137 = 0.58; thus the
(Ag+|Ag = 0.80 V), Mercury (Hg2+|Hg = 0.87 V), Platinum (Pt2+| radial velocity of 1 s electron is 58% of the velocity of light.
Pt = 1.2 V) and Palladium (Pd2+|Pd = 0.83 V). The category of metals Based on the relativistic mass equation, the calculated electron
exhibiting the exception, thus, can exist in the metallic state, without any mass in the 1 s shell of a mercury atom is m = 1.23 me, where me is the
oxidative effects of oxygen or water (O2|OH− = 0.40 V). This property of rest mass of the electron. Thus, in the relativistic calculation, the mass
existing in the metallic state renders the noble metals highly inactive for of the electron increases signiﬁcantly. Assuming Bohr's model for
any chemical reactions. The nobility of certain transition metals is calculating the shell radius, the radius of the 1 s shell correspondingly
discussed in the next section. contracts signiﬁcantly (a0 = 4πε0h2/m2 = 5.1 × 10 – 11 m). In the case of
mercury, the relativistic radius is around 81% of the non-relativistic
2. Origin of nobility in certain transition metals radius. Similarly, the other s shells, up to the valence shell, undergo
relativistic contraction. A similar nature of contraction is experienced
A summary of physical properties exhibited by noble metals is by p-shells, but to a lesser degree. The contrary effect is felt by d and f-
presented in Table 1. Some of the surprises for noble metals are: high shells. The relativistic contractions of s and p orbitals lead to more
conductivity, extreme hardness, large density and high electron afﬁnity. effective nuclear screening. It leads to a reduction in the attraction
The factors affecting the standard oxidation potential of metals can between the nucleus and the electrons in the d and f shells.
be understood by the Born-Haber cycle: Based on the concepts of relativistic contraction/expansion, the
relativistic contraction of the 6 s shell in elements was studied (Fig. 2(a)).
Eoxidation = ΔHs + ΔHh + IE The ﬁrst phase of continuous increase in contraction (decreasing curve
proﬁle) is due to ﬁlling of 4f and 5d shells. An interesting feature appears
for gold ([Kr]5d106 s1): the pronounced local maximum, which is not
found until reaching fermium (Z= 100). This pronounced contraction is
- Sublimation of a solid metal (ΔHs) reﬂected in the noble characteristics exhibited by gold.
- Hydration of a gaseous ion (ΔHh) Recently, there have been further efforts to explain the origin of
- Ionization of a gaseous metal atom (IE). nobility in certain transition metals by the molecular orbital (MO)
Fig. 2. (a) The relativistic contraction of the 6s shell in the elements Cs (Z = 55) to Fm (Z = 100) . (b) The density of one-electron states (DOS) (solid lines) for H atomically
chemisorbed on the (111) surface of Ni, Cu, Pt and Au . The DOS is projected onto the atomic H 1s state. The surface d bands DOS (dashed lines) of the four clean metal surfaces are
shown for comparison. The dominant features are the H 1s-metal d bonding resonances at energies; between − 5 and − 10 eV. Also prominent are the H 1s-metal d anti-bonding
DOS peaks (indicated by arrows) directly above the metal d bands. These anti-bonding states cause repulsion in Cu and Au, where they are ﬁlled. As indicated by the grey-shading,
only states below the Fermi energy (which is the energy zero in all cases) are ﬁlled.
6444 T. Pradeep, Anshup / Thin Solid Films 517 (2009) 6441–6478
theory, widely used to explain the concepts of bonding. The under- metallic gold. Mie was the ﬁrst to provide an explanation for the
lying principle in MO theory is that stability of a structure is reached dependence of color on the metal particle size . This led to the
after attaining the noble gas conﬁguration. This helps to explain the understanding of the phenomenon called surface plasmon resonance
formation of covalent bonds (and resultant sharing of p-shell (discussed elsewhere in the article). The next major thrust to explore
electrons) in p-block elements as a method of attaining stability. the properties of nanomaterials in general, came from Richard
Applying a similar concept, ‘n’ metal atoms can be visualized to Feynman's oft-repeated talk — There is plenty of room at the bottom
share the valence shell electrons, leading to the formation of Mn .
species. However, due to the electropositive nature of metals, the There are two more exciting discoveries that furthered the interest
valence electrons are delocalized in the metal lattice. Therefore, the in nanoscale properties: (a) the size dependence of the melting point
valence electrons are shared across all the metal atoms present. From of gold discovered by Buffat and Borel  (melting point of 4 nm gold
MO theory, we know that sharing of electrons between ‘n’ atoms lead crystals is 700 K while the bulk value is 1337 K) and (b) the variation
to the formation of ‘n’ bonding and ‘n’ anti-bonding orbitals. MO in the reduction potentials of metals with size (reduction potential for
theory states that overlapping of electronic states of two atoms leads Au3+|Au (atom) = −1.5 V, Ag+|Ag (atom) = −1.80 V [45,46]).
to orbital orthogonalization (Pauli's exclusion). From an energy The objective of this review article is to provide a consolidated
standpoint, formation of the bonding orbital is positively favored view of the research efforts accomplished so far, in the area of noble
(due to its symmetric nature) whereas the formation of anti-bonding metal nanoparticles for drinking water puriﬁcation. The article begins
orbital is negatively favored (due to its anti-symmetric nature). In case with a review of challenges in drinking water puriﬁcation. Thereafter,
both bonding and anti-bonding orbitals are occupied, the energy noble metal nanoparticle based chemistry is discussed in detail:
required for orthogonalization will render the bond formation various synthesis protocols, origin of reactivity, methods for nano-
energetically hindered. In the case of metals, the highest occupied particle deposition on supports and novel reactions feasible at the
molecular orbital (HOMO) is called the Fermi level at absolute zero nanoscale. Through a case study – nanoparticles based chemistry for
temperature. The spacing, δ, between the adjacent energy levels in a removal of pesticides from drinking water – some of the challenges
band is given by the approximate relationship, δ ≈ EF/n, where EF is associated with the commercialization of novel technologies in the
the Fermi level energy and n is the number of atoms in the particle. It market are highlighted. Thereafter, a few of the other nanomaterial-
is important to quickly point out that properties differ at the nano based approaches for drinking water puriﬁcation are summarized. A
scale vs. bulk scale: In case of bulk material as n→, δ becomes brief overview of nanomaterial based commercialized technologies is
negligible leading to the formation of conduction and valence bands in presented. The article is summarized with a review of the environ-
metals whenever δ is smaller than the thermal energy. However, as mental implications of use of noble metal nanoparticles at the
one restricts n, the discrete nature of energy levels starts to appear commercial scale.
leading to dramatic changes in the properties at smaller size.
With the above understanding, let us brieﬂy look at the density of 3. Drinking water puriﬁcation — challenges
one-electron states for different transition metals. The density of
states (DOS) of a quantum-mechanical system is deﬁned as the With the evolution of human civilization, our understanding of
number of states at each energy level that are available to be occupied. pure drinking water underwent dramatic changes. In early civiliza-
To illustrate it with an example: the density of states for energy levels tions, the commonly practiced measure for purity was the taste of the
between the valence and conduction bands of an insulator is zero. water. Water was recognized as a symbol for the origin of life and for
The nobility of certain transition metals is explained through the its medicinal value; it was not designated as a carrier of diseases. In
dissociation of H2 on the surface of four transition metals (Fig. 2(b)) the 17th century, Anton van Leeuwenhoek's discovery of the
. It has to be understood that amongst all the transition metals, microscope started to change the perception of purity: We were
differences in properties arise largely due to the valence d-band empowered to see beyond the suspended particles e.g., the tiny
structure. Thus, the interaction between an adsorbate and a transition material particles to the micro-organisms. Following the discoveries of
metal surface can be described as a two-state problem (adsorbate Louis Pasteur (study of micro-organism based diseases) and John
state and valence d-band) leading to the formation of bonding and Snow (linking of cholera spread in London with the quality of water),
anti-bonding states. Thus, an upshift of d states should increase the our understanding of pure drinking water was changed .
adsorbate-metal interaction, as it would lead to the formation of an Interestingly, the ﬁrst governmental act was passed in 1852 and was
anti-bonding orbital closer to the Fermi level. The strong features titled, Metropolis Water Act of 1852. Access to pure water was being
appearing between −5 and −10 eV represent the formation of a recognized as a right to every human being.
bonding orbital through the interaction of hydrogen 1 s with the metal The era of water puriﬁcation had begun.
d band, and this formation is shown by all the metals considered. During the course of over 150 years, our understanding of water
However, the difference arises in the anti-bonding DOS peak as quality, its effects on health and methods for water puriﬁcation has
appearing for Au and Cu vs. other metals. In the case of Au and Cu, the undergone a sea-change. A chronological view of some of the important
anti-bonding DOS peaks appear below the Fermi energy level, leading milestones in water puriﬁcation is described in Table 2. A number of
to the formation of a H 1s-metal d anti-bonding orbital. The formation important events had happened prior to this period which signiﬁcantly
of an anti-bonding orbital in turn leads to a drastic reduction in the inﬂuenced the course of the last 150 years. The historical records suggest
stability of metal-hydrogen compounds (hydrides). Therefore, the that the importance of pure water was emphasized even during ancient
metal-hydrogen bond is highly unstable in the case of Au. On the civilizations. Early Sanskrit writings outlined several methods for
contrary, for other metals, the anti-bonding DOS peak appears above purifying water such as crude sand and charcoal ﬁlters (Sushruta
the Fermi energy level, leading to an empty anti-bonding orbital. From Samhita). The ﬁrst recorded use of ion-exchange appears in the Old
the DOS peak intensity for anti-bonding orbitals and metal d band, it Testament of the Holy Bible . Ancient civilizations started the use of
can be interpreted that Au-H bond is highly unstable even vis-à-vis aqueducts for creating efﬁcient water transport networks (Indus valley,
Cu–H. This leads to nobility of certain transition metals. Greek, Roman and American civilizations). Hippocrates, the father of
While the ﬁrst evidence of existence of non-macroscopic proper- medicine, linked the importance of water to overall well-being of the
ties for gold appeared many centuries ago (The Lycurgus Cup ), the human health. In early 1600s, Sir Francis Bacon scientiﬁcally tested the
ﬁrst explanation was offered by Faraday in 1857. Faraday attributed idea of a sand ﬁlter for desalination in 1627.
the origin of the wine-red color of gold solutions to the colloidal The severity of pure drinking water scarcity has to be looked at
nature of gold particles, which interact differently with light vis-à-vis from two aspects: ﬁrst, the quantity of available water and second, the
T. Pradeep, Anshup / Thin Solid Films 517 (2009) 6441–6478 6445
Table 2 - Regulatory coverage of the USEPA for safe drinking water has
Important milestones in the history of water puriﬁcation (1800–2007) from the increased over four times since its inception (in 1974), with
perspective of noble metal nanoparticles in water treatment (compiled from multiple
sources on the World Wide Web).
revisions in maximum limits for many contaminants
Year Milestone To provide a larger overview of the extent of the drinking water
1804 Setup of world's ﬁrst city-wide municipal water treatment plant contamination, a consolidated summary of major contaminants is
(Scotland, sand-ﬁlter technology)
outlined in Table 3. It is quite clear that drinking water contamination
1810 Discovery of chlorine as a disinfectant (H. Davy)
1852 Formulation of Metropolis Water Act (England) has reached global levels in terms of the size of the population
1879 Formulation of Germ Theory (L. Pasteur) impacted. A number of contaminants such as lead and pesticides are
1902 Use of chlorine as a disinfectant in drinking water supply affecting water supplies globally due to their widespread use;
(calcium hypochlorite, Belgium)
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