19 Chemical Speciation: Environmental Consequences

Dr. Y. P. Singh

Contents

1. Introduction
2. Speciation of Iron
3. Speciation of Arsenic

            a.Inorganic Species in Water

           b.Organic Arsenic

4. Speciation of Mercury

        a. Speciation of mercury in aqueous systems

         b. Biotic Methylation of Mercury

5. Speciation of Chromium

6.Toxicity

        a.  Arsenic

        b. Beryllium

        c. Cadmium

        d. Copper

        e. Iron

        f.  Lead

        g. Manganese

        h. Mercury

        i. Nickel

        j. Selenium

        k. Vanadium

7. References

Introduction

The term speciation means identification of inorganic or organometallic species of an element/chemical in environment. The chemical reactivity and properties, toxicity of an element depends on the nature/ form of chemical entity or species. The speciation of a metal in natural water can change its kinetic and thermodynamic properties. For example Cu2+ in free ionic form is toxic to phytoplankton; while copper complexed to organic ligands is not toxic. The different forms of a metal in solution would be differently soluble. For example Fe(II) is soluble in aqueous solution whereas Fe(III) is nearly insoluble.

The change in form of chemical species may be due to one of the following reasons

1. Due to the difference in oxidation state

The common oxidation states of iron are +2 (ferrous) and +3 (ferric) and almost all the aqueous chemistry of iron is confined to these two +2 and +3 oxidation states. In natural waters ferrous form(Fe(II)) changes easily to ferric form(Fe(III)) due to air oxidation as shown below-

2. Due to the hydrolysis of ions

The hydrolysis of ions and other species is a common method of generation of new species. For example hydrolysis of acetate ions in water generates undissociated acetic acid, and solution will have two acetate species, viz., acetate ion and acetic acid molecule.

3. Due to the complex formation

Complex formation between metal ions and ligands is a common process for generating differently complexed species. For example, in an aqueous solution the presence of copper(II) cation and chloride ion can lead to several chlorocopper (II) complexes as in the following reactions.

4. Species as a weak acid or weak base

The weak acids and weak bases on dissociation produce a number of species depending upon the number of ionizable protons or hydroxide groups. Some examples are presented below.

Toxicity of the chemical species also depends upon the species present. For example Hg is not toxic but (CH3)2Hg is highly toxic. The mobility or transport of the element concerned in the environment depends upon the physical properties like, volatility and solubility etc. the mobility and transport of a toxic species effect the man and other organism. Arsenic mainly exists in environment as arsenite, As(III), arsenate, As(V), and organoarsenic. Only arsenite form of arsenic is toxic. The concentration of As in delicious sea food may be 10ppm or more. Arsenic in marine organism is present largely as an arsenobetaine, which is not toxic. Thus for the evaluation of toxic effect of an element, accurate determination of the total concentration of the element is not enough; the concentration of all the species of elements must be known in order to assess its impact on the organism and environment. Here we shall discuss the speciation of arsenic and mercury in detail in the following pages.

Speciation of Iron

The accepted rate law for the oxidation of Fe (II) represented by stoichiometric equation in water by air is (0

From laboratory studies, the rate law is: 

Depending on pH, acidic compounds and ions in solution, complexing agents, microorganisms the kinetics change. Moreover, the presence of surfaces has rate enhancing effect. In natural water there are several oxidants of iron(II) such as H2O2, MnO2, bacteria Thiobacillus ferroxidans, etc. The reducing agents are superoxide ion, photo-reduction in presence of solar light and others.

Since Fe2+ ions are readily oxidized to Fe3+, he ferrous iron, Fe2, is a fairly strong reducing agent.A sample of natural water containing both ferrous and ferric irons shall have a number of species governed by a large number of hydrolytic and complexation equilibria resulting in the formation of a very large number of species, some of which are: Fe2+ , FeIIL, Fe3+, Fe(OH)2+,Fe(OH)2+, Fe(OH)3, FeIIIL( L represents an inorganic or an organic ligand).

Speciation of Arsenic

Inorganic Species in Water

Arsenic is perhaps unique among heavy metalloids and oxyanion forming elements (e. g., As, Se, Sb, Mo, V, Cr, U and Re) in its sensitivity to mobilization at the pH value. In nature arsenic occurs in several oxidation states (-3, 0, +3 and +5), but in water it occurs mainly as oxoanion of +3 and +5 oxidation states. Arsenic in contaminated groundwater occurs largely as arsenite (As(III)) because oxygen or air are kinetically slow oxidant for As(III). Among metal oxides found in water, manganese oxides are capable of oxidize As(III) into As(V).The existence of particular form depends upon the redox potential and pH. Because the solubility, mobility, bioavailability and toxicity of arsenic depend on its oxidation state, studies of arsenic speciation and transformation among species are essential to understand its behavior in the environment.

Arsenate(V) and arsenite(III) are primary arsenic forms which are found in water and soil. These forms undergo chemical and/or microbiological mediated oxidation-reduction and methylation reactions in soil and water.

The speciation of arsenic species has been the subject of numerous studies. The speciation of arsenic(V) in aqueous media has been shown to be governed by the equilibria:

Three successive pKa values of H3AsO4 are: pKa1 = 2.19, pKa2= 6.94 and pKa3 = 11.5. At pH< 2, arsenic(V) remains almost exclusively as undissociated H3AsO4. Based on pKa values, in neutral or weakly alkaline media arsenic(V) exists as H2AsO4-. In strongly alkaline media, arsenate ions exist as AsO43-. Thus, the most common species of As(V) in neutral water is due to its stability in neutral and mildly alkaline conditions at negative E° values.

The trivalent arsenic species,As(III), are governed by the equilibria:

pKa values of arsenic(III) species arepKa1 = 9.20, pKa2 = 14.22 and pKa3 = 19.22. Due to the relatively high value of pKa of H3AsO4, arsenic(III) exists as undissociated H3AsO3 over a wide range of pH under reducing environment. At pH < 9.2, the dissociation of H3AsO3 is insignificant and it remains mainly in undissociated form.

In ground water, inorganic arsenic generally exists as As(V) (arsenate) and As(III) (arsenite). The trivalent arsenic is considered to be more labile and toxic for living organisms. The mobility of arsenic is mainly determined by the absorption capacity on the mineral surfaces, which is controlled by geochemical parameters such as pH, redox potential, ionic composition and mineral type.

pH and redox potential are two important factors which control arsenic

speciation as shown in Fig. 1

Fig. 1.(a) Arsenite and (b) arsenate speciation as a function of pH.

In ground water, the ratio of As(III) to As(V) varies due to variation in the abundance of redox active solids, such as organic carbon, MnO2, etc., activity of micro organisms. The extent of convection and diffusion of oxygen from the atmosphere is another important parameter. In anoxic ground water, [As (III [Astotal] ratio lies in the range 0.1 and 0.9.The oxidation of As(III) by dissolved oxygen is slow and becomes very slow in slightly acidic medium(pH ≤ 5). The rate of oxidation of As(III) does not depend on the concentration of dissolved oxygen.

Organic Arsenic

Proportion of organic form of arsenic is usually minor in surface as well as in ground water. The dominant forms of arsenic are mono and dimethyl arsenic acids. In both the cases, arsenic is present in pentavalent state. Organoarsenic is formed by methylation of inorganic arsenic catalyzed by microbial activity. Bioalkylation of arsenic involves both redox and methylation reactions, which produce organoarsenic compounds in the environment. Bacteria associated with methylation of arsenic are found in the soil, moulds fungi, sediment and also in animal and human beings. Organoarsenic compounds are found in water, fish, sea shells, mammal tissue and in marine plants.

The toxicity of Arsenic(III) is due to the greater sensitivity of enzyme –SH group with arsenite as in Eq.

Similar reactions occur with other As(III) compounds such as AsO3- or KAsO2.

The Bengal basin is considered to be the most acutely arsenic affected geological areaglobally. Indeed, the Bengal arsenic disorder is serious issue affecting the people and different other life forms. During 1980’s some cases of arsenical disaster, dermatosin, were reported in the district North 24 Parganas, Nadia, Murshidabad and Burdwan in West Bengal. By the end of December 2001, this problem spread in eight districts. Sources of arsenic in Bengal delta are geogenic and related to the sediment deposited by the river Gangas, Brahmputra and Meghna along with their tributaries and distributaries.

Speciation of Mercury

The speciation of mercury has been studied in detail. And the pollution of water by mercury was the main reason for the studies on speciation of mercury. There was serious poisoning by mercury owing to methyl mercury and dimethyl mercury species at Minamata Bay, Japan. The studies revealed that different compounds of mercury widely differ in toxicities and there is strong possibility of conversions of these compounds in the environment. In case of Minamata, the mercury was converted was to methylmercury by microorganisms.

Mercury exists in three oxidation states:

Hgo (elemental mercury), Liquid- poorly absorbed by ingestion and skin contact, so inert and non-toxic.

Hgo (elemental mercury), Vapour- highly toxic

Hg22+ (mercurous), Salts such as Hg2Cl2 insoluble in water and poorly toxic Hg2+ (mercuric), HgCl2 soluble in water, toxic

CH3Hg+,(CH3)2Hg sparingly soluble in water, highly toxic

Mercurous and mercuric mercury form numerous inorganic and organic chemical compounds. Organic Forms of mercury, particularly methylmercury, CH3Hg(II)X, where X is a ligand generally Cl- or OH, are the most toxic species. Mercury can form compounds not only with a methyl group but also with other alkyl groups of the type RHg+ and R2Hg. Airborne mercury is primarily inorganic mercury.

Mercury compounds found in natural environment can be also divided according to their reactivity, as follows.

The most reactive species: Hg2+, HgX+, HgX2, HgX3¯, HgX2-(where X = OH¯, Cl¯ or Br¯), Hg° bound to aerosol molecules and Hg2+ bound to organic acids (water-soluble).

The unreactive species: CH3Hg+, CH3HgCl, CH3HgOH, etc., HgS and Hg2+ bound to sulfur in the humic substances. The chemically unreactive species such as CH3Hg+ are bioavailable and subject to bioaccumalation and biomagnification..

Speciation of mercury in aqueous systems

In natural waters mercury concentration are very low. In surface water, mercury occurs in the form a hydroxyl and chloro complexes. In marine water the dominant mercury compounds are chloro complexes. The existence and migration of different mercury species in water environment is largely governed by redox conditions and the pH of the medium.

The reduction of mercury in ionic form Hg(II) to elemental mercury occurs through biological and chemical processes. The experimental evidence favors the biological processes involving the bacteria from the genus pseudomonas and other microorganism as the most important. Photochemical reaction are also believed to play an important role. Humic substances are capable of reducing mercury (II) photo chemically in the influence of solar light or otherwise. In waters with good sufficient dissolved oxygen elemental mercury is relatively easily oxidized by different reagents such as oxygen nitrates, nitrite, iron hydroxides, andiron(III), etc.

In low pH aqueous medium, the dominant mercury compounds are HgCl2 and CH3Hg2+,which are soluble. In mildly alkaline aqueous media, Hgo and (CH3)2Hg dominate. Under oxidizing atmosphere, the dominant mercury compounds are HgCl42-andHgOH+, while under reducing conditions CH3HgS and HgS2-are found. In polluted waters, phenyl mercury derivatives are found.

The major inorganic species in fresh water is Hg (OH)2which is formed at about pH 6. The other inorganic species are Hg (OH) and Hg (OH)2, which are formed in the pH range 2-6. Humic acid, which is an integral constituent of fresh water, binds mercury. So mercury -humic complexes are the principle mercury species in fresh water. However, such complexes are almost absent in sea water. In sea water, mercury exists mostly as HgCl(OH), HgCl2, HgCl3-, HgCl42- and HgCl3Br2-. HgCl42- is the major species in sea water due to high concentration of chloride ions. Low pH favors chloro complexes and high pH favors hydroxo species, namely, Hg (OH)2, Hg(OH)3- at .

Biotic Methylation of Mercury

Methylation of mercury takes place at bottom in sediments, solids and in water via biotic and abiotic pathways. This process is governed by many factors such as availability of inorganic mercury (II), activity of microorganisms, oxidizing/reducing conditions, pH, temperature, salinity and organic matter. Anaerobic conditions favor the biotic methylation of mercury, which is the main pathway; methylcobalaminacts as a donor of methyl groups.

Methylation in aerobic conditions is poor. In the aerobic process, the sulfate reducing bacteria help carry out methylation.

Hg2++ RCH3 CH3Hg + R

Fig. 2- Environmental Chemistry of Hg

The bacteria, which synthesize CH4, produce methylcobalamin as an intermediate. When the aqueous medium is neutral or alkaline, (CH3)2Hg. in formed. However, when the conditions become acidic, (CH3)2Hg converts into CH3Hg+. The dimethyl compound is volatile and escapes from bottom mud into water and air. Monomethylmercury (II) compound is soluble in water and it bioaccumulates in living organisms usually in body lipids. A summary of environmental chemistry of Hg is given in Fig. 2.

Speciation of Chromium

In natural systems chromium exists in two oxidation states, Cr(III) and Cr(VI). Chromium (III) plays an important role in some metabolic processes and it is responsible for reducing blood glucose in addition to insulin, so it appears to be an essential trace element species. On the other hand, Cr(VI) is highly toxic and carcinogenic in humans. Since the chemical reactivity depends up on chemical speciation and that the Cr(III) and Cr(VI) species interact differently with living organisms, the speciation of chromium is described here is important. The natural source of chromium is rocks. The chromium(III) is oxidized by oxidants such as Mn oxides, H2O2, gaseous O2, and Fe(III)

oxyhydroxides, etc., to Cr(VI). The equilibrium between chromate, CrO4- and dichromate, Cr2O72-, species is expressed as:

The equilibrium is highly pH dependent. It can be shown that at pH=11all chromium would be present in solution as CrO4 while at pH=1.2 Cr2O72 — would be dominant species.

In acidic medium it is present as: Cr3+, Cr(OH)2+ and Cr(OH)3butin presence of oxidants, e. g., MnO2, Cr(III) is oxidized to Cr(VI), which is highly toxic for biological systems and human carcinogen. In high concentrations, it causes nausea, skin ulcerations, and lung cancer. At high level, 100 pm, it becomes lethal.

Because of unique anti-corrosive and tanning properties, chromium finds widely used in chrome plating industries, leather tanneries, etc. Despite the official directive, the industries release the untreated toxic effluent into the environment. The percolation of the toxic substances into groundwater led to its sever contamination. Tata Environmental Research Institute, India, reported about 72% of the hazardous waste from industries generated each year in India is improperly disposed off. Kanpur houses a large number of leather tanneries, which use and the release as waste a huge amount of chromium sulfate. Another source of chromium pollution are the industrial units manufacturing basic chromium sulfate.

Toxicity

Intake of metallic compounds due to their non-biodegradable nature cause local irritation, tissue damage or systematic poisoning .Toxicity is defined as the degree to which a substance can damage an organism. Toxic effect of certain metals in certain oxidation states or forms and doses on life is known as metal toxicity. The toxicity of a substance depends on its form. For example chromium in the +6 oxidation state is highly toxic. On the other hand, some metals become toxic when they form poisonous soluble compounds. Often heavy metals are regarded as toxic, which is not true. Some metals are essential, such as iron.

The substance shows its toxic effect when the substance is taken in a certain amount. Thus, the amount of a substance that may be expected to produce a toxic effect is called toxic dose. The dose – response relationship of a substance is quantitatively expressed in LD50. The dose that proves to be lethal to 50% of the population of test animals is called LD50.

Arsenic

Arsenic oxides are volatile and during the metallurgy of some metals, these volatile compounds evaporate into atmosphere. Arsenic is the by-product of mining operations and manufacture of certain chemicals. A number of arsenic compounds are used as insecticides, fungicides and herbicides. Chemical waste and many sub-soils contain arsenic compounds naturally and so arsenic seeps into groundwater. Due to leaching of arsenic in ground water, the latter is contaminated. In some areas of West Bengal and Bangladesh, the groundwater contains high concentrations of arsenic. So the people of this area suffer from chronic arsenic poisoning. A dose of 100 mg arsenic is sufficient to cause acute poisoning.

The toxic action of arsenic is due to its attack on SH group of an enzyme. It then inhibits the enzyme action, which affect the metabolism and DNA repair. Enzymes generating cellular energy are adversely affected. Arsenic is similar to phosphorus. So it interferes in biochemical processes of enzyme involving phosphorus. For example phosphorylation is replaced by arsenolysis. Arsenic (III), which is highly toxic, attacks by coagulating proteins, complexation with co-enzymes and uncoupling the phosphorylation

Beryllium

Beryllium is one of the most toxic elements in existence. Almost all the known compounds of beryllium are toxic. The major sources of Be in the environment are the industrial combustion processes, coal power plants and manufacturing units. So we are exposed to slight levels of beryllium through the air we breathe, the foods we eat, and the water we drink. It is an important industrial metal with some useful applications. It forms alloys with other metals and is a key component in the aerospace and electronics industries. Beryllium is also used in the production of nuclear weapons.

The most common and harmful beryllium exposure occurs from inhalation. The toxicity of beryllium depends upon the duration, intensity and frequency of exposure (features of dose), as well as the form of beryllium and the route of exposure (i.e. inhalation, dermal, ingestion).It is a Class A EPA carcinogen to both animals and humans and its exposure can cause, an fatal lung disease which is better known as Chronic Beryllium Disease.

Cadmium

Cadmium is found only in +2 oxidation states. Cadmium is not an essential element for human body. It is an extremely toxic metal commonly found in industrial areas. Cadmium is used in electroplating, manufacture of paints, plastics, nickel-cadmium batteries and control rods for nuclear power industry, Cd-batteries, some industrial paints Cadmium batteries are another source of it. Other potential sources of cadmium in environment are mining waste, metal plating and water pipes. A major source of cadmium is its presence in zinc products because cadmium occurs in nature with zinc minerals due to similarity in their properties .When plants acquire zinc, cadmium is also taken in. Bioaccumulation of cadmium occurs in tissues.

Exposure to cadmium dust or fumes during industrial operations causes hypertension and cardiovascular problems which finally leads to the acute damage to the lungs of worker. It is suspected that probably Cd –poisoning occurs due to substitution of Cd in place of Zn in certain enzymes. It causes metabolic disorder.

Cadmium is class b metal and it binds to the thiol(-SH) group of thionein like protein. Enzyme bound cadmium accumulates in kidney, liver and reproductive organs. In Japan due to cadmium exposure, a specific disease calleditai-itai broke out. The disease showed painful symptoms of multiple-fracture arising from osteomalacia. In Japan, Cd-poisoning occurred when effluents from a zinc mine were discharged into Zintsu river. This river water was used for irrigating the rice fields. Those who consumed this rice developed a very painful skeletal disorder known Ouch Ouchor itaiitai disease. This was due to replacement of calcium in bones by cadmium.

Copper

Copper is an essential trace element due to its role in metalloenzymes for example cytochrome c oxidase, superoxide dismutase, ascorbic acid oxidase, ceruloplasminhemocynin etc. But when a soluble copper salt is consumed even in moderately low concentrations, it may cause vomiting and gastrointestinal irritation. Copper toxicity is due to its affinity for thiol group of enzyme protein .Whenever such groups are bonded to copper the activity of enzyme is lost. Copper toxicity may occur from eating acid foods cooked in uncoated copper cookware or from exposure to excess copper in drinking water or other environmental sources. It can also result from the genetic condition of Wilson’s disease in which the copper level control mechanism is damaged. Consequently, the copper is not excreted to the extent it is absorbed and hence it accumulates in liver, kidney and brain. Accumulation of copper severely damages central nervous system and causes cirrhosis of liver, which ultimately causes painful death.

Iron

Iron is an essential element. Toxicity of iron is caused by a large excess of iron intake and usually refers to an acute overload rather than a gradual one. No known cases of iron toxicity are known that are associated with iron exposure during mining.

Lead

Lead is a heavy metal. It is the most widely distributed toxic metals in the environment. It occurs naturally but it is not an essential element for human. Lead has wide variety of applications, such as in lead acid batteries, glazed ceramic wares, lead pipes used for water supply, lead based component of solder and leaded petrol lead(it is now banned in several countries due to environment concern). Smelters also release lead in environment.

When ingested or inhaled its increased levels in the body are harmful to humans and the children under the age of six are affected most. An average adult can excrete 2 mg of lead per day. The excess lead is transported to bone marrow and then stored in bones. Lead disrupts hemoglobin metabolism and causes anaemia. It is inhibitor of enzymes containing sulfhydryl group, resulting in the damage to the central nervous system. It casts detrimental on the neurological development of children by causing potentially permanent learning and behavior disorders. It interferes with a variety of body processes and is toxic to many organs and tissues including the heart, bones, intestines, kidneys, and reproductive and nervous systems Symptoms of lead poisoning include abdominal pain, confusion, headache, anaemia, irritability, and in severe cases seizures, coma, and death.

Manganese

Manganese is a naturally occurring element. It is found in rock, soil, water, and food. Its oxidation states vary from –3 to +7. The most common ones are +2 (e.g., manganese chloride [MnCl2]), +4 (e.g., manganese dioxide [MnO2]), and +7 (e.g., potassium permanganate [KMnO4]). Manganese and its compounds are found in environment as solids in the soil and as solutes or small particles in water. Manganese salts by and large are readily soluble in water. The phosphate and the carbonate compounds have low solubility. The manganese dioxide, MnO2 and manganese tetroxide, Mn3O4 have poor solubility in water.

It is released to air mainly as particulate matter. Size, density and wind speed determines their fate and transport. Two main forms of existence of manganese in the aquatic environment are Mn(II) and Mn(IV). Inter conversion between these two forms occurs through oxidation and reduction reactions that may be biotic or microbially mediated. pH and redox conditions largely govern the environmental chemistry of manganese. Mn(II) dominates at lower pH and red oxpotential. The proportion of colloidal manganese oxyhydroxides increases above pH 5.5.

Manganism or manganese poisoning is a toxic condition which results from the chronic exposure to manganese. It was first identified in 1837 by James Couper. Although, most of the manganese is eliminated by biliary excretion, manganese may affect liver function. And the damage to the liver may slow the process of biliary excretion. So the concentration of manganese increases in blood plasma. The long-term accumulation of manganese affects fertility and neurological functions. The interaction of manganese with iron disturbs the iron metabolism leading to neurological disorder.

Mercury

As discussed earlier the inorganic mercury is transformed into methyl mercury and dimethylmercury by the action of microorganism in presence vitamin B-12 coenzyme. Methyl mercury compounds enter into food chain through their uptake by aquatic plants and fishes.

The extent of the absorption of inorganic mercury is less than that of organic mercury. Because of the non-ionic nature of methyl mercury compounds easily penetrate the cell membrane and are readily distributed throughout the cells and tissues. It is not readily excreted and stays in the body. It bio-accumulates as it moves up the food chain. Mercury prefers to bind to thiol group(SH) of enzymes and proteins. Toxic effects include damage to the brain, kidney, and lungs. Mercury poisoning can result in several diseases, such as Hunter-Russell syndrome, and Minamata disease.

In late 1950s, environmental poisoning due to mercury occurred, because of the release of Hg-containing wastes by a chemical plant in Minamata bay. The methylmercury was formed and passed up the food chain to smaller fish and then to larger fish. Due to bioaccumulation the Hg became more concentrated. Hundred people who ate these fish got poisoned. About 40 people died and many deformed babies were born. The disease was named as Minamata disease. Symptoms typically include sensory impairment (vision, hearing, and speech), disturbed sensation and a lack of coordination. The type and degree of symptoms exhibited depend upon the individual toxin, the dose, and the method and duration of exposure

Nickel

Nickel is an essential trace element and plays important role in many biochemical reactions. It is widely distributed in the natural environment. The form and speciation of nickel dissolved in fresh or sea water is determined by pH, redox conditions, ionic strength, and the concentration and type of inorganic and organic complex-forming agents. In addition to pressure and temperature, the surfaces available for adsorption are also important.

The nickel in aqueous media is found as dissolved form as well as particulate. The bioavailability and toxicity of the nickel is strongly dependent up on the solubility of nickel compound, its chemical form and nickel oxidation state. The species of nickel present in natural and sea water are in nickel(II) state mostly. The species of nickel (II) in aqueous media are: Ni2+, NiOH+,Ni(OH)2, NiL( L = inorganic, organic and dissolved organic matter, humic and fulvic acids.

Inhalation of nickel dust or asbestos containing Nickel causes bronchial cancer. It causes dermatitis, respiratory disorder, reduces activity of cytochrome c oxidase isocitratedehydogenase of liver and malice dehydogenase of kidney.

Selenium

Selenium is an essential element. In trace amounts it is necessary as a nutrient. Inorganic selenium compounds such as sodium selenite or sodium selenate are involved together with vitamin E in the prevention of a many nutritional deficiency diseases. Selenium is necessary for the prevention of a number of selenium deficiency diseases in various species of livestock and poultry. Selenium deficiency in animals affects fertility and produces muscular dystrophy, liver necrosis, etc.

At the same time it is a toxic element. Excess of selenium causes cancers, deformation of hair and nails, giddiness, depression and nervousness. Thus, health problems can arise from both excess and deficiency of selenium. Indeed, range between essentiality and toxicity of selenium is narrow.

Selenium occurs naturally in soils, igneous and sedimentary rocks, and waters. Anthropogenic activities like coal mining, fuel refining, industrial uses in photocopy machines, electronics, glass manufacturing, chemicals, pigments, flame-proofing agents for textiles, sensitizer in photographic emulsions, are other environmental sources.

The speciation of selenium in natural environments is determined by many physical, chemical and biological factors, which determine the changes in its oxidation states. Selenium displays four different oxidation states, including elemental selenium(Se(0)). It is also a component of some organic compounds in natural environments. The principal oxidation sates and compounds exhibiting these are as follows.

Oxidation sate +6, found in selenates, is the highest oxidation state of selenium. Selenates are highly soluble in water. Selenite(III) is less available to organisms because of its affinity to sorption sites of sediment and soil constituents. In anaerobic conditions, elemental Se and selenides are the thermodynamically stable forms.

Selenite andselenatespecies are of major environmental concern. Generally, the order of toxicity is: selenite >selenate> dimethyl selenide. Biological transformations of selenium include methylation and reduction by metalloid resistant microbes. Biotransformation of selenates and selenites into the volatile dimethylselenide, dimethylseleneylsulfide, etc., occur.

It binds plasma protein and gets distributed into tissues. It can replace sulphur from cystine and methionine. Selenium causes pneumonia, degeneration of kidney and liver and cancer of throat and liver.

Vanadium

Vanadium is an essential element for the normal growth. But exposure to vanadium pentaoxide dust causes conjuctivities, nasopharyngitis and persistent cough. It inhibits synthesis of amino acids cholesterol, phospholipids and other lipids. Vanadium blocks sulfydryl activity of many other enzymes. It also reduces blood lecithin content and precipitates serum proteins.

References:

• International Atomic Energy Agency, Speciation Analysis of Arsenic, Chromium and Selenium in Aquatic Media, Proceedings of a final research coordination meeting held in Vienna, 26–29 April 2004
• G Ottonello , Aqueous Speciation and Isotopic Fractionation of Chromium: Environmental Implications, Research Project, MIUR-2002
• R. S. T. Basnayake(2001), Inorganic Selenium and Tellurium Speciation in Aqueous Medium of Biological Samples, M. S.Thesis, The Faculty of the Department of Chemistry, Sam Houston State University,Huntsville, Texas,

• Marla Sheffer,  Concise  International  Chemical  Assessment  Document  63, MANGANESE AND ITSMPOUNDS: ENVIRONMENTAL ASPECTS.WissenchaftlicheVerlagsgesellschaftmbH, Stuttgart,Germanyfile:///H:/Manganese%20And%20Its%20Compounds_Environmental%20Aspects%2 0(Cicads%2063,%202004).html

• P. Apostoli(2006), Elemental Speciation in Human Health Risk Assessment, World Health Organization,
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