Oleh : arifin_pararaja
The presence of heavy metal ions in the environment has been a matter of major concern due to their toxicity to human life. Unlike organic pollutants, the majority of which are susceptible to biological degradation, heavy metal ions will not degrade into harmless end-products. The treatment of aqueous wastes containing soluble heavy metal ions requires the concentration of metal ions followed by recovery for secure disposal. The Contamination expecially of water by toxic heavy metals is a world-wide environmental problem. Discharges containing Cadmium, in particular, are strictly controlled due to the highly toxic nature of this element and its tendency to accumulate in the tissues of living organisms. It presents also a risk on health: by oral way, the symptoms observed are: an episode of gastro-enteritis with epigastric cramps, the vomiting, the diarrhoeas and myalgias, or by ingestion way, an intoxication mortal was however observed following the voluntary ingestion. Lead is a metal ion toxic to the human biosystem, and is among the common global pollutants arising from increasing industrialization. The assimilation of relatively small amounts of lead over a long period of time in the human body can lead to the malfunctioning of the organs and chronic toxicity. The toxic effects of lead ions on humans, when present above the threshold level in the hydrosphere, are well documented. Among the heavy metal ions, Pb2+ and Cd is considered highly toxic and is kept in aqueous solution because of its ability to form complexes with the organic matter; therefore, the way in which this compound may be extracted from water using adsorbents and the factors that hinder such union are studied. Many techniques such as chemical precipitation, ion exchange, reverse osmosis, solvent extraction and adsorption are used for wastewater treatment (removal of metal ions). The choice of the type of treatment is done according on one side to the nature of the pollutants and to the other side to economical aspect of the process. Nevertheless, whatever the adopted solution, in one step at least, adsorption is used. The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are Carbon’s highly porous nature provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to. The large internal surface area of carbon has many attractive forces that work to attract other molecules. The resultant product has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place. The walls of the pores provide the surface layer molecules essential for adsorption. In a system where the adsorbate, that is a dissociated or protonated electrolyte, is under conditions in which the adsorption takes place; electrostatic interactions are produced, strongly influencing the metallic ions adsorption on activated carbons. Such interactions may be attractive or repulsive, depending on the adsorbent charge densities, ionic strength of the solution and kind of adsorbate The surface charge on the activated carbons is associated with the existence of heteroatoms and mainly, with the oxygen complexes content. Therefore, when an activated carbon is submerged in water, a charge produced by the dissociation of its functional groups occurs on its surface. The adsorption of heavy metals in carbonaceous materials thus mainly occurs in the acid sites, such as lactonic and carboxylic groups. The removal of Pb and Cd, therefore, is of great importance, especially in the protection of human health.
Key words : Heavy Metal, Cadmium, Lead, Adsorption, Activated Carbons.