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Chapter 3: Literature Review
One of the most pollutants of the environment is heavy metal ionized in water, as it accumulates in living tissues and causes many devastating effects. Many industries are held accountable for polluting the environment with the heavy metal ions contained in their waste-waters. Possible sources for these chemicals could be industrial discharge and domestic wastewaters from rivers as well as transportation and shipping activities are from seawater samples. Some effluents generated by industries are petroleum refineries, non-ferrous metal works, aircrafts platin and so on (J. Chil. Chem. (2007)). N. F. Gray (2004) in their study about Biology of wastewater has suggested that the toxicity of metals has been examined by using biochemical oxygen demand (BOD) inhibition test. Metals were found to decrease in toxicity in the following order; mercury, silver, copper, chromium, iron, aluminum, cadmium, cobalt and nickel, tin and zinc. Likewise many researchers and also U.S. GEOLOGICAL SURVEY CIRCULAR 1133 (1995) have classified copper and silver among the most toxicity metal as shown in table 1.1. These heavy metals are dangerous to man and aquatic life as well; therefore they should typically be given priority to remove from waste water before waste disposal.
The aim of this project is to devise a way for the development of ligands to chelate with copper and silver ions in order to remove to toxic metal from wastewater.
Historical and Background of "Metals-ligands association"
In order to understand the phenomenon of metals-ligands associations, it is necessary to have brief theoretical chemistry acknowledge about their history. These studies do not have a precise discovery but Dwyer and Mellor (1964) claimed that they are made soon after the birth of structural organic chemistry.
However, O.Costisor and W.Linert (2004) had interpreted that in 1840, Ettling studied a dark green crystalline product from the reaction of cupric acetate, salicyaldehyde and aqueous ammonia. Few years later in 1869, Schiff found that the salicylaldimine complex caused by reaction of performed metal-salicyladehyde with ammonia. Pfeifer and co-workers have realized the systematic study of Schiff's base complexes in the period 1930-1940 and notices the role of metal ions. Here, the term 'ligand' is defined as complex formation of metal ions with organic molecules.
Nature of ligands
In aqueous form, metals are not only present as "free" ions but also some other ions called as ligands are able to interact with them to form complex compounds. Such as the most essential ligands in natural waters and in industrial effluents are Cl-, HS- or H2S and OH-. However, NH3, F-, S2O32-, Sx2-, SO32-, CN-, SCN-, PO43- and also organic ligands can have an important influence on the complexation of metals (Bohumil Volesky (1990)).
In ligands field theory, many chemists have investigated various interest properties of metal complexes that would determine the suitability of organic reagents for its use in gravimetric or volumetric analysis as well as spectrophotometric methods for the estimation and identification of heavy metal (Muhammad, Karamat (1994)). The interaction between the softness and hardness of ligands and metals ions is term as Lewis acid-base concept. (Dr. Arlene Courtney (2001)).
As John Olmsted and Gregory M. Williams (1997) interpreted in their study of molecular science chemistry that metal cations are electron deficient. Thus, ligands by having lone pair of electrons, they will donate to form bonds to the metal ion. Briefly, Metals are Lewis acids that accept electron pairs from donating ligands that act as Lewis bases.
The development activities of a ligand to form a metallic complex depend in several parameters such as concentration, temperature, ionic strength, acidity and potential of the solution (Bohumil Volesky (1990)). Therefore; 'ligand' is defined as anions, molecules, clusters, polymers and small particulates to which a metal cations binds and also called as coordinate compound.
Heavy metal ligands
N. F. Gray (2004) and also Michael H. Gerardi (2006) implied in their study that heavy metal such as cadmium (Cd), copper (Cu), lead (Pd), mercury (Hg), silver (Ag) and zinc (Zn) are well-known, toxic metals. These metals can occur in variety of waste especially wastewater that cause acute or chronic toxicity in term of discharge into water bodies or onto land without any treatment together forming the source of heavy metals to ground water. This problem could successfully be solved by the complexation of the metal ions with suitable ligands.
Normally, Heavy metals ions are very stable by producing metal-complexes. This is possible because an acceptor metal cations form coordinate bond with electron-donation. Bohumil Volesky (1990) has suggested these processes as Lewis acid-Lewis base neutralization under the theory of hard and soft acids and bases (HSAB). He also explained these impressions as the principle of electron mobility or polarizability.
As such, Tabassum, Hamida (1994) give detail that the atoms, ions, molecules which act as the ligands have the property to stabilize the lower oxidation states. Furthermore this property is related with the fact that these ligands possess vacant pi orbitals in addition to the lone pairs, to form a type of pi bonding that supplements the sigma bonding arising from the lone pair donation, high electron density on the metal atom is thus delocalized on the ligands. This ability of the ligands to accept electron density into low lying empty pi orbitals can be called as pi acidity in the lewis sence. Such ligands form the bonds with the metals by using sigma orbitals and exercise their pi orbitals acidity by using their pi orbitals whose nodal plane includes the axes of the sigma bond.
Complexation and Chelation
When a metal ion creates a complex with a ligand, a ring structure will result. This ring structure changes the character of the metal ion - and the reactions that it could normally undergo. The term that is designated to this chemical reaction is Chelation. The ligand that reacts with the metal is given the term chelate or chelating agent.