THE RISK OF EXCEEDANCE OF THE WHO HEALTH-BASED VALUE OF 70µG/L MO.

CHAPTER ONE

INTRODUCTION

1.1       What is Molybdenum?

Molybdenum is a naturally-occurring metal that can be found in small amounts in rocks and soils. It is also present in plants, animals and bacteria. The element was discovered in 1778 by Carl Wilhelm Scheele. This element is a silvery-white (Fig. 2) high melting metal and classified as a ‘Transition Metal’ and is located in group 6 and period 5 of the periodic table, it has an atomic number of 42 and atomic mass of 95.96 amu as seen in Fig.1. Transition metals are generally described as ductile, malleable and able to conduct electricity and heat.

Fig. 1: Expression of atomic mass and atomic number of molybdenum (en.wikipedia.org)

Molybdenum is obtained from Molybdenite MoS₂, Wulfenite PbMoO₄, PowelliteCa (Mo, W)O₄. Its crustal abundance is estimated to be around 1.5 mg/kg or less where an estimate of 1.4 mg/kg is for the upper crust and 0.6 mg/kg is for the lower crust. There are seven naturally-occurring stable isotopes: ⁹²Mo, ⁹⁴Mo, ⁹⁵MO, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo and ¹⁰⁰Mo of which ⁹⁸Mo is the most abundant at 23.78%, followed by ⁹⁶Mo at 16.653%, ⁹²Mo at 15.84% and ⁹⁵Mo at 15.72% of the total mass.

Molybdenum is an essential trace element in human, animal and plant nutrition. Both deficiencies and excesses can cause health problems, so there is considerable interest in the level of Mo and its activity in the environment. For example, 25µg day^-1Mo is required by humans (WHO, 1996); symptoms of Mo deficiency include coma and night blindness. Conversely, excessive Mo reduces the uptake of copper in the human body and leads to skeletal deformities (WHO, 1996). In plants, Mo has a role in nitrogen fixation, but high Mo levels in vegetation can induce Cu deficiency in grazing animals.

1.2       Properties of Molybdenum

Key properties include:

• Symbol of Element : Mo
• Atomic number : 42
• Atomic mass : 95.94 amu
• Melting point : 289 k (2623 ⁰C , 4753 ⁰F)
• Boiling point : 4912 k (4639 ⁰C , 8382 ⁰F)
• Number of protons/ electrons : 42
• Number of neutrons : 54
• Crystal structure : cubic
• High density of 10.2 g/cm³
• Phase at STP is solid
• Good thermal conductivity
• Good electrical conductivity
• Good hot strength
• Good strength and ductility at room temperature.

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