ENVR 203 Lecture Notes - Intellectual Disability, Cation-Exchange Capacity, Sustainable Forest Management

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26 Jan 2013

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Chapter 15
Heavy Metals
No specific definition; however some are defined by molecular weight or density; some are
mixed in terms with trace elements
Metals found in nature: in soil
They can be extracted from the ground as an ore and once extracted from nature, they enter a
cycle or incorporated in some products
However, when the life of that product is over, they may end up in a landfill or some other place
in the environment but never as an ore or in their previous state
They can be widely dispersed, mostly by human activities.
When in the environment, they can enter the human body like other chemicals.
What makes them different is that they can accumulate in some certain tissues
Another major characteristic is that they are toxic in very low levels (ppb or ppm)
How do we classify them?
o They are divided into three major groups (there are overlap of some elements)
o CLASS A, CLASS B, Borderline
o CLASS A: macronutrients; necessary for our physiological function in big amounts and
surround us in significantly big amounts (in food and water). What distinguishes is that
they have tendency to form ionic bonds. They are of very low toxicity.
o CLASS B: very toxic (e.g. mercury, lead, silver and gold). They are not essential for the
body. They can change states easily and also can exist as an organic form or an inorganic
form. What distinguishes them from Class A is that they have a tendency to form
covalent bonds.
o Borderline. They are necessary and are Chromium, Copper, Arsenic, Cobalt, Nickel, Zinc,
Manganese, and Iron. They are considered micronutrients.
Iron is important in the functioning of hemoglobin
Chromium is important in lowering blood sugar levels (used as a supplement)
o Toxicity: Class B> Borderline> Class A
Mechanism of Toxicity:
o Blocking essential functional groups such as proteins or enzymes, proteins can’t carry
anything. If a metal is combining with the protein, it may block essential functional
groups and the protein may become dysfunctional.
o Displace other metals (class B, borderline). For example Mercury can replace some of
the borderline elements in their functions and can disrupt the normal function
o Modifying the active conformation of biomolecules (twisting of molecules) (class B). It
may change the chemical arrangement of the molecule. The functioning of Class B is
usually responsible for this form of pain in the ass.
Coping Mechanisms
o Resistance: species develop mechanisms that do not uptake the metal (e.g. lead). This is
evident in some plants.
o Tolerance: the capacity of species to withstand high levels of metal
Internal detoxifying mechanisms (metabolize metal to a weaker state) (e.g.
methylation of arsenic in marine biota)
Binding of certain elements to non sensitive sites
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