EESA10 - Lecture 5.doc

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Published on 28 Jun 2012
Lecture 5: Heavy Metals and Human Health
Heavy Metals
We say that heavy metals are all elements that have metallic characteristics but they also have a very
high atomic weight number one definition is based on atomic weight
The second definition says that it is based on the density of the element density of elemental forms
of these metal
They say that all elements that have density higher than 7g/cm3 belong to this group
Some other scientists say that this is not true we are going to base this definition on a specific
We actually don’t know or don’t agree with what heavy metals are and not only that
Some elements known as metalloids (not really truly metals)
Arsenic is it doesn’t have full characteristics but we still consider arsenic as a heavy metal
There is another term that is very often used and seen in literature; this is known as trace elements
It is different than metals trace elements can be any element found in environment of lower
concentration (very low sometimes hard to detect)
For us here in our class and for most of the people heavy metals are toxic only elements that are
toxic based on their toxicity
Where can we find them? They are coming from the ground part of normal geological ground core
of the planet
We humans extract them as ore and mine them as ore and then extract them from the ore in different
forms sometimes in a pure form or sometimes as a salt and incorporate them as different kinds of
After that, when we mine them, use them for some products, that product will finish their life cycle
and possibly can be reused or can be deposited if some depositions occur again under the ground
This closed circle of metals before metal finished circle it is very widely spread everywhere in
water, environment, food, air
Most of them are toxic especially in some certain concentrations because every metal is toxic in some
concentration (some of them in extremely low concentrations)
They affect different types of organs and accumulate in certain parts of our body selected tissues
Classification of metals
There are 3 basic groupings of the metals
First is class A not very toxic, low toxicity (i.e. K, Na, Mg, ,Ca, Al), are essential elements for
plants, for us, they contribute to many physiological processes in our body and they are needed in
significant amounts (that is why we call them macronutrients) and are essential
These elements tend to form ionic bond; are usually positively charged
Other class is class B is very toxic (i.e. Hg, Ti, Pb, Ag, Au), these elements are not essential (we
don’t need them for any process in our body), they just can interfere with the processes in our body
and cause some adverse effects
They have high electron negativity (opposite from class A), tendency to acquire electrons, form a
covalent bond (form pairs of electrons)
Third class are metals that we call borderline (somewhere in between these two)
All of them are micronutrients they are necessary for many physiological processes in our body
including plants (i.e. Cr, Cu, As, Co, Ni, Zn, Mn, Fe)
Toxicity: class B is the most toxic, then borderline, then class A is least toxic
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Mechanism of toxicity
1) Metal is attached to protein that means that protein can carry anything because the essential
functional groups that need to attach hormone or something else are already satisfied with the metal
Protein can carry anything means that proteins function is blocked
2) Especially for class B and borderline is that instead of some necessary metal important for our
physiological processes some other toxic metals from class B or borderline is attached
3) Changing molecule some organic compound such as biomolecule twisting that molecule changing
the conformation of molecule means that new molecule will be like what you see in the mirror
(different conformation)
Coping Mechanisms
Class B metals are really harmful for humans how do we cope with this? Not just humans, plants,
other organisms cope with this
First is resistance, simply do not uptake the metal (humans do not have this function, mechanism)
Some of the plants have a great mechanism to resist and even to uptake the metals (i.e. Pb, will grow
acceptably well on some soils)
What we do have is tolerance to some chemicals
There are two groups of chemicals being threshold and non-threshold chemicals
Threshold means that in a very low concentration we can tolerate them without experiencing any
adverse affects
Non-threshold means that just in very low concentration already we are going to experience some
adverse affects
Can happen by metabolizing some of these chemicals, that is also a possibility can metabolize part
of the metals and get less toxic forms (i.e. methilation of As in marine biota, it means forming organic
forms of arsenic are less toxic that inorganic forms)
Some other example is binding to non sensitive compound structures somewhere where it is not that
We and plants can develop and resist tolerance not only to one metal but more than one metal we
call this multiple tolerance (i.e. Cu, Pb, Zn, Cd)
Bioavailability of metals
Bioavailability is everything that is available to us
If you look at the environment and you see how many lead you have all around you not everything is
available for you that you can uptake or absorb
Not every form of the metal is bioavailable some are more available and others are less available
It depends on species, not plant or animal species species of metal (means form is it ionic form, or
stable neutral form, or is it organic or inorganic form)
differences depend on form, (element also because different elements from different forms
For example Zn is a charged electron such as Zn+2 are more bioavailable than just neutral elemental
Usually the charged electrons are more bioavailable
But it doesn’t meant that neutral species are bioavailable but also may be available somehow
especially when they form complexes
Form complexes with any kind of organic metal and as organic they can be bioavailable
Compare two elements arsenic and mercury arsenic is less toxic in organic form than in inorganic
On the other hand mercury is opposite it is toxic in any form
Mercury changes form very quickly, every form is very toxic even inorganic form
Ph of solution, water, soil, stomach can affect everything in acidic conditions (below 7) more 4, 4.5, 5
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