1.1 INTRODUCTION
1.2.2 Toxicological Aspects of Heavy Metals
Heavy metals are found naturally in the earth's crust and can enter into the living organisms in very small amounts through the food, water and air. They are important as trace minerals for metabolic functions but, at greater amounts through biomagnification, they can lead to poisoning. Due to their mobility in aquatic ecosystems and they are toxic to higher life forms, heavy metals in surface and groundwater supplies have been considered as major inorganic contaminants in the environment even at very low concentrations.
1.2.2.1 Effects of Heavy Metals on Human Health
Some heavy metals are naturally found in the body and are essential to human health.
The human body need many friendly trace heavy metal elements. For example, iron prevents anemia, and zinc is a cofactor in over 100 enzyme reactions. Magnesium and copper are other familiar metals that, in minute amounts, are necessary for proper metabolism to occur. Other metals are xenobiotics, i.e., they have no useful role in human physiology (and most other living organisms) and, even worse, as in the case of Arsenic, Cadmium, Lead, Mercury and Nickel etc. may be toxic to the enzyme systems and metabolism of the human body even at trace levels of exposure. These heavy metals are taken into the body at very low concentrations via inhalation, ingestion, and skin absorption. They tend to accumulate in the food chain and in the body and can be stored in soft (e.g., kidney) and hard tissues (e.g., bone). When heavy metals are accumulated in human body tissue faster than the body’s detoxification pathway will build-up these toxins gradually in the human body. Being metals, they often exist in a positively-charged form and can bind on to negatively-charged organic molecules to form complexes. Overload of heavy metals in the adrenal glands reduce the production of hormones, which cause early aging, stress, decreased sex drive and aggravation of menopausal symptoms. Heavy metal overload can lead to unresponsiveness of diabetics to their medication and neurological diseases such as depression and loss of thinking power. It can also aggravate conditions such as osteoporosis and hypothyroidism. For obvious reasons, removing metals from the body safely has been a concern of physicians for many years. Even minute levels of toxic elements have negative health consequences, affecting nutritional status, metabolic rate and the integrity of detoxification pathways. The biological half-lives for heavy metals are generally long; the half-life for cadmium in the kidney is decades. Most heavy metals are readily transferred across the placenta, found in breast milk, and are well known to have serious detrimental effects on behaviour, intellect and the developing nervous system in children. For adults, silent symptoms of chronic, low level heavy metal accumulation in tissues can progress from a steady decline in energy, productivity and quality of life to accelerated cardiovascular disease, premature dementia and total debilitation. Details of toxic response on human due to some important heavy metals ions are presented in Table 1.2.
1.2.2.2 Effects of Heavy Metals on Aquatic Organisms
Aquatic organisms are adversely affected by heavy metals in the environment. The toxicity is largely a function of the water chemistry and sediment composition in the surface water system. The metals are mineralised by microorganisms, which are taken up by plankton and further by the aquatic organisms. Finally, the metals which are bio accumulated by aquatic organisms are several times biomagnified by fish and this fish is taken by human from metal contaminated water. Slightly elevated metal levels in natural waters may cause the following sub lethal effects in aquatic organisms:
• Histological or morphological change in tissues;
• Changes in physiology, such as suppression of growth and development, poor swimming performance, changes in circulation;
• Change in biochemistry, such as enzyme activity and blood chemistry;
• Change in behaviour; and
• Changes in reproduction (Connell et al., 1984).
Many organisms are able to regulate the metal concentrations in their tissues. Fish and crustacean can emit essential metals, such as copper, zinc, and iron that are present in excess. Some can also emit non-essential metals, such as mercury and cadmium, although this is usually met with less success (Connell et al., 1984). Research has shown that aquatic plants and bivalves are not able to successfully regulate metal uptake (Connell et al., 1984). Thus, bivalves tend to suffer from metal accumulation in polluted environments. In estuarine systems, bivalves often serve as biomonitor organisms in areas of suspected pollution (Kennish, 1992).
In comparison to freshwater fish and invertebrates, aquatic plants are equally or less sensitive to cadmium, copper, lead, mercury, nickel, and zinc. Thus, the water resource should be managed for the protection of fish and invertebrates to ensure aquatic plant survivability (USEPA, 1987). Metal uptake rates will vary according to the organism and the metal in question. Phytoplankton and zooplankton often assimilate available metals quickly because of their high surface area to volume ratio.
The ability of fish and invertebrates to adsorb metals is largely dependent on the physical and chemical characteristics of the metal (Kennish, 1992). With the exception of mercury, little metal bioaccumulation has been observed in aquatic
organisms (Kennish, 1992). Metals may enter the systems of aquatic organisms via three main pathways:
i. Free metal ions that are absorbed through respiratory surface (e.g., gills) can readily diffused into the blood stream.
ii. Free metal ions that are adsorbed onto body surfaces can also passively diffused into the blood stream.
iii. Metals which are adsorbed onto food and particulates may be ingested from free metal ion bearing water bodies (Connell et al., 1984). For example:
Chromium is not known to accumulate in the bodies of fish but high concentrations of chromium due to the disposal of metal products in surface waters can damage the gills of fish that swim near the point of disposal.