eChapter Name: The Fate of Heavy Metals
9789391383817
eBook Name: THE CHEMISTRY OF SOIL PROCESSES
by L. H. P. Jones, S. C. Jarvis
11.1 INTRODUCTION
The heavy metals which, by definition, are elements having a density greater than five in their elemental form, comprise some 38 elements. However, the term usually, and here, refers to twelve metals that are used and discharged by industry, i.e. Cd, Cr, Co, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sn, Zn. Most of the present discussion will centre on those metals that may represent potential hazards to plants or animals; these are Cd, Cu, Hg, Ni, Pb, and Zn.
The heavy metals are widely distributed in the environment, in soils, in plants and animals and in most of their tissues. The concentrations of individual metals in living tissue are ordinarily very low and must be maintained within narrow limits to permit the optimum biological performance of most organisms. Some heavy metals are essential in trace amounts, namely Co, Cu, Fe, Mn, Mo, Zn for plants and, in addition, Cr, Ni, Sn for animals; Cd, Hg, and Pb, have not been shown to be essential for either plants or animals. Deficiencies of the essential metals have been frequently reported and substantial benefits have arisen from correcting these by addition to the soil and subsequent transfer from soil to plant is for most increasing concern about the entry into soil of some heavy metals and the possible adverse effects that they might have on plants and animals and, in turn, on human health. The main sources of these heavy metals are: (i) urban industrial aerosols, created by combustion of fuels, metal ore refining and other industrial processes; (ii) liquid and solid wastes from animals and man; (iii) mining wastes; or (iv) industrial and agricultural chemicals. Although in some circumstances there are direct atmospheric inputs to plants, addition to the soil and subsequent transfer from soil to plant is for most heavy metals the major route of entry into the living tissues of plants, animals, and man. The fate of heavy metals added to soils, including their mobility and reactions in the soil and their subsequent uptake and distribution in plants, is therefore of critical importance in relation to man’s health.