Effect of EDTA on Tomatoes Plants Seedlings Under Nickel Toxicity

CHAPTER ONE

1.0 INTRODUCTION 

1.1     NICKEL

1.1.1  Occurrence 

        Nickel (Ni) is a metallic element,with atomic number 28; atomic weight 58.69. Is a metal, which belongs to group VIIIB of the periodic table. The most important oxidation state of nickel is +2, although the +3 and +4 oxidation states are also known (Tundermann et al., 2005). Nickel resembles iron, cobalt, and copper in its chemical properties. However, unlike cobalt and iron, it is normally only stable in aqueous solution in the + 2 oxidation state (Kerfoot, 2002).

          Nickel is the earth's 22nd most abundant element and the 7th most abundant transition metal. It is a silver white crystalline metal that occurs in meteors or combined with other elements in ores. Two important groups of ores are: Laterites: oxide or silicate ores such as garnierite, (Ni, Mg)6 Si4O10 OH)8 which are predominantly found in tropical areas such as New Caledonia, Cuba and Queensland. Naturally occurring nickel is composed of five stable isotopes;

Ni,  58

Ni, 60

Ni, 61

Ni, 62

Ni and 64

Ni with 58 being the most abundant (68.077% natural abundance).

         Sulphides: these are ores such as pentlandite, which contain about 1.5%, nickel associated with copper, cobalt and other metals. They are predominant in more temperate regions such as Canada, Russia and South Africa. Canada is the world's leading nickel producer and the Sudbury Basin of Ontario contains one of the largest nickel deposits in the world.(Robert John Lancashire,2006).

      In 1899 Ludwig Mond developed a process for extracting and purifying nickel. The so-called "Mond Process" involves the conversion of nickel oxides to pure nickel metal. The oxide is obtained from nickel ores by a series of treatments including concentration, roasting and smelting of the minerals. In the first step of the process, nickel oxide is reacted with water gas, a mixture of H2 and CO, at atmospheric pressure and a temperature of 50 °C. The oxide is thus reduced to impure nickel. Reaction of this impure material with residual carbon monoxide gives the toxic and volatile compound, nickel tetracarbonyl, Ni(CO)4. This compound decomposes on heating to about 230 °C to give pure nickel metal and CO, which can then be recycled.(Robert John Lancashire,2006).

1.1.2    Properties

       Nickel is a hard silver white metal, which occurs as cubic crystals. It is malleable, ductile and has superior strength and corrosion resistance. The metal is a fair conductor of heat and electricity and exhibits magnetic properties below 345°C. Five isotopes of nickel are known. In its metallic form nickel is chemically unreactive. It is insoluble in cold and hot water and ammonia and is unaffected by concentrated nitric acid and alkalis. It is however soluble in dilute nitric acid and sparingly soluble in dilute hydrochloric and sulphuric acids( Robert John Lancashine,2006)

     Nickel has typical metallic properties; it can be readily rolled, drawn into wire, forged, and polished.  It is also