ABSTRACT
The term “heavy metal,” in spite of its widespread usage among professionals and laymen, does not have a rigorous scientific basis or a chemical definition. Although many of the elements listed under “heavy metals” have specific gravities greater than five, major ex ceptions to this rule remain. In hindsight, this group should preferably have been referred to as “toxic elements,” for they are all included in the United States Environmental Protection Agency’s (USEPA’s) list of priority pollutants. Figure 1.1 shows the periodic table contain ing the heavy metals that are of significant environmental concern. For comparison, com monly occurring light alkali and alkali-earth metals have also been included in the same figure. Strictly from a chemical viewpoint, heavy metals constitute transition and post-tran sition elements along with metalloids, namely, arsenic and selenium. They are indeed signif icantly heavier (i.e., higher specific gravities) than sodium, calcium and other light metals. These heavy metal elements often exist in different oxidation states in soil, water and air. The reactivities, ionic charges and solubilities of these metals in water vary widely. For their short-and long-term toxic effects, the maximum permissible concentrations of these heavy metals in drinking water as well as in municipal and industrial discharges are closely regu lated through legislation. Nevertheless, barring the exceptions of cadmium, mercury and lead, heavy metals are also required micronutrients, i.e., essential ingredients for living cells. Toxicity effects of these elements are, thus, largely a function of concentration. These ele ments are beneficial and have nutritional values lower than some critical dosages but become inhibitory to toxic with an increase in concentration, as shown in Figure 1.2. The threshold toxic concentrations differ for each heavy metal and are governed primarily by the chemistry of each heavy metal in question and associated physiological effects. On the contrary, nonessentia! heavy metal elements are inhibitory at all concentrations.
