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Lecture 5

ENVS4012 week 5 notes


Department
Environmental Sciences
Course Code
ENVS 4012
Professor
Wilfred
Lecture
5

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02/06/2008
Human Health and the Environment
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 gravity
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 products
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
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02/06/2008
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
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
How else can this happen? 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 toxic
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
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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)
Huge differences depend on form, element as well because different elements
from different forms
For example Zn is a charged electron such as Zn+2 are more bioavailable than
just neutral elemental thing
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
They can 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 form
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
Most of the metals are more toxic and more bioavailable
When we say bioavailable means more toxic
In Ph above 7 and neutral they are less bioavailable and less toxic
How does temperature affect the bioavailability? If you look at sea water and
find some concentration of mercury (Hg) what do you think whether it’ll be more
bioavailable in the coast of the Caribbean or here in Canada?
In warm waters is more available – that is how the temperature affects the
bioavailability (also seasonal)
Redox potential of solution – the actual amount of oxygen in water, soil, solution,
and so on
If the amount of oxygen is low most of the heavy metals are toxic
Heavy metals are in more toxic form in contaminated water because there is not
enough oxygen in the water (potentially more toxic forms for us)
Routes of exposure
Three major routes we see not only for metals but for all chemicals are
inhalation (dust or PM, fume, gas), ingestion (soil, food, plants accumulate
metals in roots and leafs), and through the skin
If we eat plants it can be a source of metals in our body
The higher concentration is going to be in the roots, then stem, and then leaves
– means that if we have contaminated soil it is not good to eat plants
What about potato? Potato is not a root botanically, it is the storage organ
(stores starch) reserved for plants
Thus pretty safe to eat potato, a little bit different from the root
Heavy metals will store in certain tissues, organs (liver, bones, kidney), damage
brain (neurological damage), carcinogens hard to detect for doctors (no specific
symptoms)
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