Environment and Health
Metals occur in nature mainly as chemical compounds with oxygen (oxides) or with sulphur (sulphides); only very rarely are they found in pure form. Among these exceptions – besides the precious metals gold and silver – is copper.
As it is an essential trace element, the environmental impact of copper cannot be assessed in the same way as for man-made chemicals. The chemical form of copper is very important in determining its biological availability or bioavailability to organisms in the environment. The form, distribution, transport and potential uptake by organisms and the effects of copper in water, sediment and soil depend largely on the chemical and physical properties of the local environment and the bioavailability of different forms to each organism.
Humans and other organisms extract copper from their environment such as air, water and soil.
Bioavailability of copper
Bioavailability is the amount of a substance available to living organisms for absorption. The bioavailability of copper measures the amount of copper available to an organism’s sensitive receptor or organ. As the bioavailability of copper increases, organisms may absorb too much copper. If organisms absorb more copper than they can safely use and eliminate, undesirable results may occur.
The bioavailability of copper depends on several key factors, including:
- The chemical composition of different forms of copper.
- The concentrations of dissolved copper and copper adsorbed on particulate matter (i.e. suspended sediments).
- Chemical factors in the local environment, including acidity/alkalinity, hardness (Ca, Mg), other cations (Na, K), anions, complexing agents (such as bicarbonate, chloride, sulphate, sulphide), binders that prevent copper from being available and dissolved organic matter.
- Interactions with biological receptor sites (“biotic ligands”) such as gills on aquatic organisms.
Measurements of total copper concentration in the environment (i.e. surface waters, sediments, soils, etc.) cannot be used to predict risks to organisms. Only a small fraction of the total amount of copper is bioavailable to organisms and therefore potentially toxic. The bioavailability of copper is controlled by the local chemistry of the environment and the mutual interactions of these chemicals and copper with each organism.
Accurate predictive chemical-mathematical models such as the biotic ligand model (BLM) estimate the bioavailability of copper in different environmental media:
- Freshwater environments: Much of the metal released in freshwater environments is bound to particulate solids or dissolved organic matter and is not available for uptake by organisms. Only a relatively small proportion of total copper is available to aquatic organisms.
- Saltwater environments: The bioavailability of copper in saltwater bodies, including estuarine and marine environments, is determined by local water chemistry, just as in freshwater environments. In coastal saltwater bodies, variations in dissolved organic matter and, to a lesser extent, salinity largely determine copper bioavailability.
- Soils (terrestrial) and sediments (aquatic): Almost all copper in soils and sediments is bound to particulate matter. Exposure to organisms depends only on the small amount of bioavailable copper in pore water.
Bioavailability models using the measured chemistry of local environments can be used to set safe limits for organisms living in soils, sediments and marine waters.
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