Heavy metals (HM) are elements that are known to be dense, usually more than 5g/cm3. They can be classified into groups.
The necessary elements that play important role in metabolism of organisms and cells at low concentrations. Examples are; Nickel, Copper and Zinc. These elements become toxic at high concentrations or high doses.
The elements that cause adverse effects on organisms even at low doses or concentrations. Examples are; Cadmium, Lead and Mercury (Richmond, 2004).
The primary sources of heavy metals to the environment include; geological weathering, pollution from industrial processing, burning of fossil fuels and waste disposal leaching. Geochemical composition, agrochemical and industrial wastes are the main sources of aquatic environment pollution with heavy metals (Papafilippaki, 2008)
Properties of Heavy metals
- They are highly dense materials
- They have a lustrous appearance
- They are good conductors of electricity
- They generally enter chemical reactions as positive ions
- They are generally less water soluble; instead they have a tendency to adsorb onto particulate matter or bind with organic groups, called organometallic compounds, thereby forming lipophilic compounds and ions.
- They are distributed among plants, animals, as well as in sediments. (Walker et al., 2006).
- Most of them have a tremendous affinity for sulfur and disrupt enzyme function by forming bonds with sulfur groups in enzymes
- Heavy metals include essential elements such as iron as well as toxic metals such as cadmium and mercury
- Heavy metals may also precipitate phosphate bicompounds or catalyze their decomposition. (Envtal chemistry, 10th edition)
Heavy metals, at various doses are among the most harmful of the pollutants and are of particular concern because of their toxicities to humans. These elements are in general the transition metals and some of the representative elements, such as lead and tin, in the lower right-hand corner of the periodic table. Heavy metals are known to chemically bound to protein, carboxylic acid (-CO2H) and amino (-NH2) groups. Cadmium, copper, lead and mercury ions bind to cell membranes, hindering transport processes through the cell wall. (Envtal chemistry, 10th edition). Heavy Metals are naturally present in water, sediments, plants, animals and they can be released into the environment by many processes such as weathering of rocks, leaching of soil, forest fires and other natural factors. In addition to anthropogenic activity, which plays a major role in polluting the environment with heavy metals (Mason,1990).
Generally, air and water are the main pathways of chemicals’ entry and diffusion into the environment. Solid particles would disperse and carry to a faraway area, such as the distribution of lead into the North Sea (meaning that it was generated from land, transported by air, and absorbed in water and biota) of England coast that the sources came from a lot of places e.g. industries, hazardous waste disposals, lead-suspended air conditions, etc. (Cheevaporn, 2004). Not only air and water, but also soil and biota are the contaminant pathways of chemicals.
For example, DDT was originally synthesized by Othmar Zeidler, German Scientist, in 1874 and has been used since 1942 for agricultural purposes as pesticide (Edwards, 2002). After post harvesting, this chemical was normally washed by rain from soil to water or carried by wind from soil to air (may be dropped in a faraway water area). Additionally, it has been also absorbed and accumulated in biota (flora and fauna on land and in watershed) called bioaccumulation that is increasingly accumulated in each trophic level along food chains as biomagnifications (Boonsaner, 2006).
For instance, a given chemical that has its own physicochemical properties e.g. concentration, polarity (water solubility), vapor pressure (volatilization), Henry’s law constant (H) (gas solubility), octanol-water partition coefficient (Kow) (hydrophobicity index), organic carbon normalized distribution coefficient (Koc) (organic soil sorbent capability), sorption, diffusivity, etc. (Corsi, 1998) is polluted to different compartment, it will act or behave in different ways. The concentration or characteristic of this chemical could be increased, decreased or remained the same upon its fate. This is because the transportation or movement depends on properties of the chemicals themselves and properties of the environmental compartments (Walker et al., 2006)
