Heavy metal is any metallic chemical element that has a relatively high density and with high toxicity, even at low concentrations. Examples include: mercury (Hg), Cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) Cobalt (Co), Nickel (Ni), Antimony (An), Vanadium (V), Zinc (Zn), Platinum (Pt), Palladium (Pd), Rhodium.
Heavy metals are natural components of the Earth’s crust and can enter a water supply by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater.
SOURCES AND ORIGIN OF HEAVY METALS
The Earth’s crust is a natural reservoir for heavy metals which accumulate in different proportions in the different rock types, based on the physic-chemical processes involved in the formation of these rock
Average abundance of some heavy metals in the earth’s crust and rocks (all values in parts per million)
Heavy metals accumulate in soils that have experienced matter differentiation. Here, precipitation, usually in the form of rain percolates the A horizon of the soil profile, leaches clay sized particlates and mobilizes them in the lower B horizon where the dominant clay minerals and Fe oxyhydroxides adsorb the potentially toxic metals, building up contaminant concentrations.
High porosity and permeability sedimentary rocks can invaded by ore metal bearing hydrothermal fluid, important ore deposits of many potentially toxic metals (eg Pb, Zn and U) can form.
High temperatures, pressure and presence of hydrothermal fluids, especially due to contact metamorphism may result to remobilization and concentration of chemical elements resulting in heavy metal enrichment in some metamorphic rocks. Important metal bearing ore deposits originate during contact metamorphism.
Volcanic eruptions and volatilization of metals such as Hg and Se from ore deposits and soils tend to send some heavy metals into the atmosphere. These elements then fall back as gases, aerosols and particulate that precipitates and become part of the earth surface.
Uncased Bore holes drilled to supply water can be the source of heavy metal into the environment. The water from the reservoir washes through rocks which may host some amounts of heavy metals. The water will then transfer these metals to the earth’s surface.
fungicides and herbicides are used to protect crops but at the same time enter the soil. The biocides may contain potentially toxic metals such as As and Hg. These metals can further intrude an ecosystem from soil by water infiltration into unconfined aquifers or through run offs to fluvial systems.
No matter how selective the picking of the ore bearing rock is, some ore remains in the waste rock together with accompanying minerals such as FeS2 (pyrite) which is not recovered from mineral processing. The waste is commonly disposed on the earth’s surface in piles of rock called tailings.
Tailings are exposed to weathering in 2 stage process which can cause environmental intrusions either singly or in union. One is the generation of acid mine drainage from oxidation of pyrite in the waste rock and the second is the mobilization of potentially toxic metals in the surface environment stimulated by the oxidizing weathering environment under acidic conditions from pyrite oxidation.
Solid waste such as manure from animal husbandry, coal ash from electricity generating and tailings from mining contain high concentrations of potentially toxic metals. For example solid animal waste if used directly in soil conditioning will pose a bacterial danger and can develop high concentrations of heavy metals which effectively concentrate in soils as waste dry out and decompose
Emissions from smelting of ores releases millions of tons of heavy metals into the atmosphere annually as gases, aerosols and particulate. Heavy metal bearing emissions originate from the burning of fossil fuel-fired electrical generating activities and other industrial processes and from the use of fossil fuels.
It is liquid waste flowing out of a factory, farm, or household into a water body such as a river, lake or lagoon. Industrial effluent discharge heavy metals into water courses, lakes, lagoons, estuaries, oceans and other wetlands. As a remediation method, effluents should be passed through a channel to a septic tank for treatment before disposing it to the environment.
FACTORS INFLUENCING MOBILITY HEAVY METALS IN SOILS
PH: PH is generally acknowledged to be the principal factor governing concentrations of soluble and plant available metals (Brallier et al., 1996). Metal solubility tends to increase at lower pH and decrease at higher pH values.
Temperature: Temperature exerts an important effect on metal speciation, because most chemical reaction rates are highly sensitive to temperature changes (Elder, 1989).
An increase of 10o C can double biochemical reaction rates, which are often the driving force in earth surface conditions for reactions that are kinetically slow, and enhance the tendency of a system to reach equilibrium.
Redox: Low redox potential in any environment can promote sulfate reduction and sulfide mineral deposition.
During diagenesis, much of the non-silicate-bound fraction of potentially toxic metals such as arsenic, cadmium, copper, mercury, lead, and zinc, can be co-precipitated with pyrite, form insoluble sulfides, and become unavailable to biota
Particulate size: Particulate size and resulting total surface area available for adsorption are both important factors in adsorption processes and can affect metal bioavailability
Small particles with large surface-area-to mass ratios allow more adsorption than an equivalent mass of large particles with small surface-area-to-mass ratios.
Organic matter content: Organic matter accumulates at the soil surface, mainly as a result of decomposing plant material.
The decay of some food often leads to the release of fulvic acid.
Some of the solid phase organic matter in soils appear to be mobile and a significant quantity of Pb appears to be transported through soils with high organic matter
Slope: slope affects the amount of sediment yield and runoff volume. Thus the amounts of Cd transported from the contaminated soil in various phases. Sheet flow lacks sufficient velocity to transport sand particles, which consequently, during sheet flow, clay sized particles are transported in preference to sand particles, resulting in a selective enrichment of the sediment with clay and Cd
METAL UPTAKE PATHS INTO ORGANISMS
Plant uptake of trace elements is generally the first step of their entry into the agricultural food chain. Movement of elements from the soil to the plant root, elements crossing the membrane of epidermal cells of the root, transport of elements from the epidermal cells to the xylem, in which a solution of elements is transported from roots to shoots, and possible mobilization, from leaves to storage tissues used as food, in the phloem transport system. After plant uptake, metals are available to herbivores and humans both directly and through the food chain.
EFFECTS OF HEAVY METALS
It produces bone defects in humans and animals
Long – term exposure can cause kidney and liver damage
At high levels copper causes anemia, liver and kidney damage
Causes kidney problems
At high concentrations it can result in toxic biochemical effects in humans such as problems in the synthesis of hemoglobin, effects on the kidneys, and reproductive system.
Long – term exposure can cause decreased in body weight, heart, and liver damage.
Damage to the circulatory tissue and nervous system
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