131 ISSN : 0974-9411 All Rights Reserved © Applied and Natural Science Foundation www.ansfoundation.org Journal of Applied and Natural Science 3 (1): 131-138 (2011) 2008 A P P L I E D A N D N AT U R A L S C I E N C E F O U N D A T I O N ANSF Fate of some heavy metals in soils: a review Ehi Robert Orhue* and Uzu Ogbonnaya Frank Department of Soil Science, Faculty of Agriculture, University of Benin, Benin city, NIGERIA *Corresponding author. E-mail: orhuerob@yahoo.com Abstract: This review revealed that heavy metals are naturally components of the soil orchestrated by weathering processes and that the abundance of these metals in our soil environment nowadays is due to numerous anthropogenic activities. These heavy metals are in two major categories namely the essential and non- essential ones. The essential heavy metals are needed in trace amount by living things for their physiological processes. But at higher concentration, it is hazardous in plants and animals. On the other hand, the non essential ones are dangerous to plants and animals even at low concentrations. This write-up further showed that soil pH, organic carbon, available P, oxides, effective cation exchange capacity, moisture content, oxidation-reduction state of the metals, ion exchange and sorption capacity of the metals are some of the soil factors influencing the fate and availability of the heavy metals. At lower levels of these soil factors, heavy metal ions are rendered free in the solution thereby making them available to plant and underground water. However, the most prominent soil factor influencing availability of these metals is the Soil pH. Raising some of these soil factors to appropriate levels have been discovered to reduce the availability of the heavy metals to mankind. Keywords: Heavy metal, Soil, Contamination, Ion exchange JA N S INTRODUCTION Heavy metals are natural components of the earth’s crust and have specific gravity of about 5gcm -3 (Duffus, 1980; Ademoroti, 1996 and Brady and Weil, 2002), and able to form sulphides (Atimanar and Aldoleya, 2004). Bond and Straub (1973) have earlier classified them as metals with atomic mass greater than the atomic mass of calcium. Heavy metals are classified into essential and non- essential metals. The essential metals are needed by living organisms in trace quantities for optimum performance of life processes and they include Zn, Fe, Co, Mo, Ni, etc. In the case of human body, Forstner and Prosi (1978) identified Fe, Co, Cu, Zn, and Mo as essential. Insufficient supply of these essential metals in an organism leads to problems associated with growth and ability to complete its life cycle, while sufficient supply results in optimum conditions and excess supply results in toxic effects and possibly death (Forstner and Wittmann, 1981; Young and Blevin, 1981). The non essential elements include As, Ag, Cd, Hg, Pb and the ability of numerous organisms to accommodate these non-essential metals are limited (Bryan, 1976). They may be tolerated at very low concentration but at higher concentrations, they become toxic. All heavy metals according to Young and Blevin (1981) are potentially harmful to most organisms at some levels of exposure and absorption. However the minimum tolerance levels for heavy metals concentration in plants, fish and man have been published or computed by World Health Organization (WHO, 1984). Metals are natural components in the soil brought about mainly by weathering of rocks, sometimes volcanic activity (Forstner and Prosi, 1978; Zollar, 1984 and Biney 1991). Contaminations of environment however, has resulted from industrial activities such as mining and smelting of metalli- ferrous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, and generation of municipal waste (Kabata-Pendias and Pendias, 1989). These heavy metals can be leached into surface water or groundwater, taken by plants, released as gasses into the atmosphere or bonded semi-permanently by soil components such as clay or organic matter. These artificial sources are known as anthropogenic sources. Therefore, the essence of this review was to identify some factors influencing the availability of heavy metals in soils. FATE OF HEAVY METALS IN SOILS In the environment, heavy metals are partitioned among the various environmental compartments such as water, soil, suspended solids and biota. Processes such as dilution, dispersion, sedimentation, adsorption and desorption govern their distribution (Biney et al., 1994). Miller (1997) reported that heavy metals exist in colloidal ionic particulate and dissolved phases. Elliot et al. (1986) and Connel and Miller (1984) reported that metals have a high affinity for humic acids, organic