Elsevier

Thin Solid Films

Volume 517, Issue 24, 30 October 2009, Pages 6441-6478
Thin Solid Films

Special Feature
Noble metal nanoparticles for water purification: A critical review

https://doi.org/10.1016/j.tsf.2009.03.195Get rights and content

Abstract

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 with the prosperity of human civilizations through their prominent use in jewellery and medical applications. The most important reason for the use of noble metals is the minimal reactivity at the bulk scale, which can be 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 events during last 200 years in the area of drinking water purification is presented. Realizing the molecular nature of contamination in drinking water, significant progress has been made to utilize the chemistry of nanomaterials 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 the future.

Introduction

The theme of this review is water, one of the essential companions of life on earth. During the phases of creation, evolution and continuity of life on earth, water remained as its most vital component. The molecular as well as macromolecular functions of making life possible are carried out using water. The earliest form of life appeared in sea water about 3.5 billion years ago and was transferred to land only 380 million years ago. Despite this transfer to land, an ocean, in terms of fluidic composition, continues to exist within us. Similarly, keeping a very high proportion of living organisms' body weight as water (∼ 70%), Nature has iterated the vitality of water for life. It is very appropriate to say that existence of life on Earth is largely owed to the 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 body. Clean and fresh water is essential for the existence of life. The evolution of civilization has always revolved around water. The Nile was the lifeline of the Egyptian civilization. The Indus Valley 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 signs of prosperity, health, serenity, beauty, artistry, purity and many other attributes. Leonardo Da Vinci had described water as “the vehicle of nature” (“vetturale di natura”).

For time immemorial, Nature has made noble metals part of our daily 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. This is a very important milestone for our societal progress — the reason being we spent sufficient time in making sure that what is being used by us, should be friendly to us. This has become a fundamental question while we are discovering newer materials every day. With noble metals, perhaps we know the answer.

Gold and silver belong to the family of “metals of antiquity”, having their history with mankind dating back to 6000 BC and 4000 BC, respectively. Gold's brilliance, intrinsic beauty and ability to preserve its shine for long periods made it the most documented metal in the human history. While there are no clear-cut references for the discovery of gold, it has always been associated with the gods, with immortality, and with wealth across the human civilizations.

The earliest evidence for the use of gold in jewellery comes from Sumer civilization (southern Iraq, 3000 BC). The use of gold as jewellery continued through different civilizations thereafter (tomb of King Tutankhaman [Egypt, 1300 BC], gold ornaments from Indus valley [Mohenjadaro, 3000 BC] and royal crowns from the Tillia tepe treasure [Scythian, 100 BC]). Another milestone in the history of noble metals was reached around 700 BC when Lydian merchants produced the first coins through use of gold–silver alloys called ‘electrum’. The use of noble metals for currency was further developed by the Roman Empire (100 AD) [1].

It was suggested that the use of gold, in the form of swarna bhasma (meaning, gold ash), for medicinal purposes started during the Vedic period (1000 BC-600 BC) in ancient India [2]. References also exist in the Chinese literature about the medicinal properties of gold and it was thought to act as the elixir of life [3]. The medicinal uses of gold were promoted after the work of Paracelsus (15th century) who first prepared gold colloid solution in modern times. The purple solution of gold was called Aurum Potable and it was strongly believed that this would impart rejuvenation to the human body, as historically gold was equated with the sun (“tears of the sun”). Antonii described the medical uses of colloidal gold in 1618, in the reportedly first documented literature on colloidal gold [4]. This was followed by an effort from Johann Kunckels, a German chemist, to explore the medicinal properties of the pink-solution of gold (1676) [5]. A few other subsequent efforts followed, highlighting the stable dispersion of gold [6], [7], [8], [9], [10]. The unusual color of metallic gold caught the attention of many researchers. As a consequence of the work accomplished so far on gold, Michael Faraday attempted a novel synthetic route to prepare colloidal gold in 1857 (Fig. 1) [11]. He synthesized a dark red solution of colloidal gold by the reduction of an aqueous solution of chloroauric acid using phosphorus in CS2. He realized that stability of colloidal gold against aggregation can be achieved using stabilizing agents. He also noticed the reversible color changes driven by mechanical compression (blue–purple  green) of thin films prepared with colloidal gold [12]. In 1890, the distinguished German bacteriologist Robert Koch discovered the use of gold cyanide for bacteriostatic action against tubercle bacillus, the causative agent for tuberculosis. Later, it was found that tubercle bacillus is also responsible for rheumatoid arthritis. This led to further investigations in modern medicinal uses of gold [16], [17], [18].

Silver has been equally popular for domestic use since the ancient times. As gold was associated with sun due to its color, the white brightness of silver became associated with the moon. The distinct optical properties of silver defined its name: the word silver is of Gothic origin meaning shiny white and the Latin name argentum originates from an Aryan root which means white and shining.

From the historical times, utensils were fabricated with silver lining. Silver vessels were utilized for the preservation of perishable items as well as for disinfection of water. The writings of Herodotus suggested that Cyrus the Great (Persia, 550–529 BC) consumed water boiled in silver vessels [19]. The account also mentions the use of silver as a precious booty, during the time of Pausanias (Sparta, 5th century BC). Silver was widely used as a disinfectant and anti-spoilage agent, across many civilizations (Greece, Rome, Phoenicia, Macedonia) [20]. Hippocrates, the Father of medicine, promoted the use of silver for early healing of wounds [20]. Alexander the Great (335 BC) was advised by Aristotle to store water in silver vessels and boil prior to use [21]. Evidence exists for the use of silver nitrate as an anti-bacterial agent, in the Roman pharmacopeias.

During the late 13th century, Lanfranc utilized silver tubes for introduction of food beyond fistula [22]. Thereafter, Paracelsus (1493–1541) pioneered the use of metals for medical applications. He linked silver with the development of brain activities in his hermetic and alchemical writings. During the mid-19th century, Joseph Lister and Marion Sims promoted the use of silver wire sutures, in order to reduce the incidences of septic complications [22]. During the late 18th century, Crede, a German obstetrician, popularized the use of prophylactic l% silver nitrate eye solution for the prevention of ophthalmia neonatorum [23]. Pioneering work in the study of anti-bacterial properties was carried out by Ravelin (1869) and von Nageli (1893) [20]. It was reported that even at low concentrations of 9.2 × 10  9 and 5.5 × 10  6 M, silver salt was toxic to Spirogyra and Aspergillus niger spores, respectively [20]. The term “oligodynamic” was coined to represent the activity even at low concentrations. A landmark work in the chemistry of silver was accomplished by Carey Lea in the late 19th century [24], [25]. Silver colloid was first prepared through the reduction of silver nitrate using ferrous sulfate and consequent protection of colloidal particles with citrate ions [24], [25]. A number of researchers worked on the anti-bacterial properties of silver salts: In the early 20th century, a porous metallic mesh of silver was prepared (“Katadyn silver”) and was utilized as an anti-bacterial water filter. The popularity of silver salts continued to grow through the development of new silver salts; e.g., silver sulfadiazine, silver citrate, silver lactate, etc. A number of products based on silver were also commercialized throughout the 20th century (Katadyn, Argyrol, Movidyn, Tetrasil, Alagon, etc.) [26]. Attempts were also made to immobilize silver in zerovalent form on activated carbon and subsequently use it for disinfection of water [27].

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 [28] and ethylene to ethylene oxide [29]. 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], [31], [32], [33], [34], [35], [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 groups: d-block metals belong to the moderately reducing group (Cd2+|Cd =   0.40 V, Fe2+|Fe =   0.44 V) whereas s-block metals belong to the strongly reducing group (Li+|Li =   3.05 V, Na+|Na =   2.71 V). However, there are exceptions to this rule: Gold (Au3+|Au = 1.5 V), Silver (Ag+|Ag = 0.80 V), Mercury (Hg2+|Hg = 0.87 V), Platinum (Pt2+|Pt = 1.2 V) and Palladium (Pd2+|Pd = 0.83 V). The category of metals exhibiting the exception, thus, can exist in the metallic state, without any oxidative effects of oxygen or water (O2|OH = 0.40 V). This property of existing in the metallic state renders the noble metals highly inactive for any chemical reactions. The nobility of certain transition metals is discussed in the next section.

Section snippets

Origin of nobility in certain transition metals

A summary of physical properties exhibited by noble metals is presented in Table 1. Some of the surprises for noble metals are: high conductivity, extreme hardness, large density and high electron affinity.

The factors affecting the standard oxidation potential of metals can be understood by the Born-Haber cycle:Eoxidation=ΔHs+ΔHh+IEwhere,

  • -

    Sublimation of a solid metal (ΔHs)

  • -

    Hydration of a gaseous ion (ΔHh)

  • -

    Ionization of a gaseous metal atom (IE).

Transition metals usually have high melting points

Drinking water purification — challenges

With the evolution of human civilization, our understanding of pure drinking water underwent dramatic changes. In early civilizations, the commonly practiced measure for purity was the taste of the water. Water was recognized as a symbol for the origin of life and for its medicinal value; it was not designated as a carrier of diseases. In the 17th century, Anton van Leeuwenhoek's discovery of the microscope started to change the perception of purity: We were empowered to see beyond the

Need for nanomaterials in water purification

In the course of our evolving understanding of the effect of water quality on health, standards for drinking water have been revised several times. For example, the allowed limits for well-known contaminants were revised periodically (e.g., lead), the definition of contaminant is becoming specific (e.g., maximum concentration of organic residues vs. pesticides) and newer contaminants (e.g., RDX) are being brought under regulation [68], [74]. In this course of evolution, it is likely that the

Noble metal nanoparticle based chemistry

The discovery of noble metal nanoparticle based chemistry has a fair amount of scientific history. The chemical synthesis of noble metal nanoparticles started with the pioneering work of Faraday [10]. Subsequent work by Turkevich [87] to develop a simple protocol, based on work by Hauser and Lynn [88], to synthesize colloidal gold, initiated the research on methods for nanoparticle synthesis. In addition to exploring the effect of temperature, reagent concentration and dilution, Turkevich also

Noble metal nanoparticle-based mineralization of pesticides

One of the areas of interest of our research group is the interaction of noble metal nanoparticles with organochlorine and organophosphorus pesticides. Specifically, the nanoparticles used for the study are gold and silver whereas the pesticides are endosulfan, malathion and chlorpyrifos. This study is a consequence of our finding on the possibility of degrading different halocarbons using noble metal nanoparticles [320].

It was discovered that mineralization of halocarbons happens in two steps:

Other nanomaterial based approaches for water purification

Iron-based nanoparticles: One of the most important nanomaterials studied for water purification is zero-valent iron (ZVI). Amongst the nano-absorbents popular today, ZVI is the most utilized, primarily because it covers the broadest range of environmental contaminants [312], [313]: halogenated organics, pesticides, arsenic, nitrate and heavy metals.

The mechanism of reactivity for ZVI is similar to the mechanism of corrosion (i.e. oxidation of iron) [68] and involves the generation of electrons

Commercial interests in drinking water purification

While the emerging applications of nanotechnology in drinking water purification have been outlined so far, it is vital that transformation of such technologies to commercial applications is emphasized. The need for quality and economic drinking water has always necessitated the utilization of newer technology breakthroughs for novel water purification products. Examples of nanotechnology based products in drinking water purification are described in Table 8. A number of technologies

Environmental implications of the use of noble metal nanoparticles

As discussed earlier, noble metal nanoparticle-based chemistry has immense advantages in the area of drinking water purification. The noble metal nanoparticles have multi-pronged use in drinking water purification (removal of organic compounds, heavy metals and micro-organisms). While the chemistry of noble metal nanoparticles is unique in the removal of many contaminants, their high energy surface results in system attempting to minimize the surface energy through protection or chemical

Summary and conclusions

Water has always been associated with the existence of life on earth. From early civilizations, water has been a central theme linked to economic development. The evolution of civilization has always revolved around water. Water is one of the purest signs of prosperity, health, serenity, beauty, and artistry. Similar to human civilization's association with water, a relationship has existed with noble metals. A number of examples from the historical civilizations have suggested the use of noble

Acknowledgements

We thank the Department of Science and Technology, Government of India for constantly supporting our research program on nanomaterials. We also thank World Gold Council for supporting a part of our program on noble metal nanoparticle-based drinking water purification. We thank Mr. K. R. Antony for the assistance in compiling the summary of different contaminants found in drinking water and the associated health effects.

List of acronyms and their definitions

USPHS
United States Public Health Service
USEPA
United States Environmental Protection Agency
CCL
Candidate Contaminant List (Published by USEPA)
WHO
World Health Organization
EU
European Union
TCE
Trichloroethylene, a chlorination byproduct commonly found in water
TTHM
Total Trihalomethane
FDA
Food and Drug Administration
RDX
Research and Development Explosive (1,3,5-Trinitroperhydro-1,3,5-triazine)
NPA
Asparagine–Proline–Alanine
TDS
Total Dissolved Solids
VOC
Volatile Organic Compound
UV
Ultraviolet
SERS

T. Pradeep is a professor of Chemistry at the Indian Institute of Technology, Madras. He received his PhD from the Indian Institute of Science, Bangalore and had post doctoral training at the University of California, Berkeley and Purdue University, Indiana. He held visiting positions in Purdue University, USA; Leiden University, Netherlands; EPFL, Switzerland; Institute of Chemistry, Taiwan; Pohang Institute of Science and Technology, South Korea; Institute of Molecular Science, Okazaki, Japan

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    T. Pradeep is a professor of Chemistry at the Indian Institute of Technology, Madras. He received his PhD from the Indian Institute of Science, Bangalore and had post doctoral training at the University of California, Berkeley and Purdue University, Indiana. He held visiting positions in Purdue University, USA; Leiden University, Netherlands; EPFL, Switzerland; Institute of Chemistry, Taiwan; Pohang Institute of Science and Technology, South Korea; Institute of Molecular Science, Okazaki, Japan and Hyogo University, Japan. He has 190 research papers and 12 patents to his credit. He is the author of ‘Nano: The Essentials’, McGraw-Hill and one of the authors of ‘Nanofluids: Science and Technology’, Wiley Interscience. He is an elected fellow of the Indian Academy of Sciences. One of the dreams of his research group is to develop an affordable all-inclusive drinking water purifier using nanotechnology. For more details please visit, http://www.dstuns.iitm.ac.in/pradeep-research-group.php.

    Anshup holds a B. Tech. degree from IIT Madras and is currently working with Professor T. Pradeep at the DST Unit on Nanoscience, Indian Institute of Technology Madras, Chennai, India. He studies application of nanomaterials in drinking water purification and desalination. He is also part of a team working on the development of nanomaterials-based affordable drinking water purifier. He is working with Professor Pradeep to establish InnoNano Research Private Limited, an organization focusing on nanomaterials research for commercial applications.

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