Instrumental Methods in Metal Ion Speciation

Imran Ali, Hassan Y. Aboul-Enein

• Publisher: CRC Press
• Year: 2006
• eBook ISBN: 9781000612059
• Topic: Medizin
• Subtopic: Umweltwissenschaft

The knowledge of metal ion speciation is essential for predicting the exact toxicities of metal ion species in the environment. Metal ions can exist in various oxidation states, each of which possesses different physical and chemical properties as well as exhibit varying toxicities. Often, toxicity data is unreliable because it is based on metal io

1 Introduction

1.1 IMPORTANCE OF THE ENVIRONMENT

“A sound mind exists in a sound body” is a well-known proverb in English literature, and the same is true for our environment. A neat and clean environment is essential for the proper growth, health, and persistence of human beings and other organisms. The conservation and protection of the environment are essential in the present industrialized and developing world. Overgrowth of the population in some countries is one of the most pressing problems of our age. The pollution problem of the environment has now reached a level that serves as a potential threat not only to the health of our environment but also to the entire population. The quality of our environment is deteriorating continuously due to the accumulation of undesirable constituents. The main sources of the contamination are industrialization, domestic activities, agricultural activities, atomic explosions, and other environmental and global changes. These activities and changes, if improperly controlled, can destroy the quality of our environment. Broadly, the environment is divided into three parts: the atmosphere, including the air sphere around the earth; the lithosphere, which consists of the earth; and the hydrosphere, which consists of all water bodies, including ocean, surface, and ground water. Hydrosphere and the atmosphere components of the environment are directly and readily available for contamination by pollutants. Therefore, the quality of these environmental constituents is deteriorating continuously, which is problematic and deserving of serious consideration. Again, the notorious pollutants find their way easily through water bodies, while the atmosphere is being contaminated by gases and volatile organic pollutants. Consequently, ground and surface waters at some places in the world are not suitable for drinking due to the presence of toxic pollutants. Similarly, the quality of the atmosphere in some cities and industrial areas of the world is not safe for human health. The importance of conservation and improvement of the environment is critical and urgent.^{1, 2, 3} Environmental awareness has grown dramatically in recent decades, and several nations are taking the lead in implementing laws related to the environment. In view of this, environmental authorities are asking for accurate data and information on pollution levels and for the improvement of measures to control the contamination of the environment.

1.2 ENVIRONMENTAL POLLUTANTS

Any undesirable and toxic chemical, commodity, organism, or other object present in the environment may be considered an environmental pollutant. The pollutant may be present in solid, liquid, or gas form. In all forms, the presence of toxic pollutants is serious and harmful to human beings and other useful organisms. Broadly, environmental pollutants may be categorized into chemical and biological classes. Chemical pollutants are organic and inorganic compounds; biological contaminants are toxic microbes. The presence of inorganic pollutants, especially toxic metal ions, is a serious issue, as metal ions may often be carcinogenic in nature.^{4, 5, 6, 7, 8, 9, 10, 11, 12} The route of these toxic metal ions to the human body is through water and other foodstuffs. Therefore, the monitoring of metal ions in water bodies and foodstuffs is essential and important. Prior to supplying water for drinking, bathing, agriculture, and other purposes, it is essential to determine the concentrations of these pollutants, if present. In addition, some toxic metal ions are also present in the atmosphere and indirectly affect our health; hence, the monitoring of air samples, especially of polluted areas, i.e., near roads, railway tracks, industries, and so forth, is critical. Trace elements date back to the early 20th century as many elements were recognized to exist at very low concentrations. During the following 60 years, all efforts have been made to focus on total trace element concentrations.

1.3 METAL IONS SPECIATION

Among the various metal ions, transition metals and some lanthanides and actinides are harmful and toxic. It is also interesting to observe that the transition metals, lanthanides, and actinides exist in different oxidation states. This is due to the presence of empty d orbitals in transition metal ions and f orbitals in lanthanides and actinides, respectively. Therefore, these metal ions occur in the environment in different oxidation states and form various species, for example, arsenic (As) as As(III), As(V), and organic arsenic species.¹³ The different oxidation states of some toxic metal ions are given in Table 1.1. The different oxidation states of a particular metal ion possess different physical and chemical properties. Mainly, these oxidation states differ in their redox potential, complexation, and hydration properties. It is very important to observe and note that these different oxidation states have different toxicities. Basically, speciation is a concept used frequently in biological science, and it was adopted by those in analytical chemistry, expressing the idea that the specific chemical forms of an element should be considered individually. The separation and identification of the different oxidation states of a particular metal ion is known as speciation. The concept of speciation dates back to 1954 when Goldberg¹⁴ introduced the concept of speciation to improve the understanding of the biogeochemical cycling of trace elements in seawater. Since then, this development has been growing exponentially to the point that research on trace element analysis is being conducted. Today, research is almost exclusively focused on trace element species. Kinetic and thermodynamic information together with analytical data make it possible to differentiate between oxidized versus reduced, complexed or chelated versus free metal ions in dissolved form. Of course, the analysis of the total concentration of metal ions is required and essential. There are many reports available in the literature on the analysis of metal ions present in our environment, but the data presented are not reliable. This unreliability is due to the different toxicities of various oxidation states of particular metal ions, which confuses the toxicity concept.^{15, 16} Due to these facts, the knowledge of metal ion speciation is essential for environmental and industrial chemists and scientists of other analytical laboratories. In view of these facts, attempts have been made to explain the term speciation in the following sections.

TABLE 1.1
Different Oxidation States of Some Toxic Metal Ions

Metal Ion	Oxidation States
As	As(III) and As(V)
Cd	Cd(I) and Cd(II)
Cu	Cu(I) and Cu(II)
Cr	Cr(III) and Cr(VI)
Co	Co(II) and Co(III)
Hg	Hg(I) and Hg(II)
Fe	Fe(II) and Fe(III)
Mn	Mn(I) and Mn(II)
Ni	Ni(II) and Ni(III)
Pb	Pb(II) and Pb(IV)
Sb	Sb(III) and Sb(V)
Se	Se(IV) and Se(VI)
Ti	Ti(II), Ti(III), and Ti(IV)
Zn	Zn(I) and Zn(II)

Templeton et al.¹⁷ described the speciation term as per the...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedications
• Preface
• About the Book
• Acknowledgments
• The Authors
• Table of Contents
• Chapter 1 Introduction
• Chapter 2 Metal Ion Species: Sources and Distribution
• Chapter 3 Metal Ion Species: Toxicities, Biotransformation, and Biodegradation
• Chapter 4 Sample Preparation
• Chapter 5 Speciation of Metal Ions by Gas Chromatography
• Chapter 6 Speciation of Metal Ions by Reversed-Phase High-Performance Liquid Chromatography
• Chapter 7 Speciation of Metal Ions by Ion Chromatography
• Chapter 8 Speciation of Metal Ions by Ion-Pair, Micellar Electrokinetic, Size Exclusion, Chiral, Capillary Electro-, and Supercritical Fluid Chromatographic Methods
• Chapter 9 Speciation of Metal Ions by Capillary Electrophoresis
• Chapter 10 Speciation of Metal Ions by Spectroscopic Methods
• Chapter 11 Speciation of Metal Ions by Electrochemical and Radiochemical Methods
• Chapter 12 Perspectives and Legislation of Metal Ion Speciation
• Index