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Lecture 5

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University of Toronto Scarborough
Environmental Science
Michael Petit

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 • 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 • 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 • 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 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 • 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) Mercury • Neutral form of mercury is toxic • Where can we find mercury other than the thermometer? In dental fillings • In recent years we have had studies on it, whether we should stop using it yet we still do not have a final answer nor strong conclusion • Some studies show there are adverse affects, others say no – dentists still use it • Batteries can have mercury in metallic form • Some industries – industrial products • Liquid in pure form, not significant toxic threat • Organic and inorganic forms are both toxic • Bioaccumulation is the accumulation of some chemical in the food chain (means that the top predators will have the highest concentration) • What about tuna? Tuna is a big fish that eats small fish • Canned tuna is safer to eat than fresh tuna – if adults eat tuna they should eat it a maximum of once a week while kids should eat it once a month • How many times should pregnant women eat canned tuna? Avoid it, if possible never because mercury is extremely harmful to the fetuses • What are the symptoms if someone gets toxic from the mercury? Can be extremely severe, like tremor of the hands (shaking), memory loss (many things connected with the neural dysfunctions with the brain and nerves), insomnia, and sometimes extremely dangerous deliriousness and mental illness • Fetuses, children are sensitive to mercury which especially affects their nervous system by means of the buildup of the metal in the mother’s body What are the wood preservatives? • Case study of how the wood preservatives can contaminate the environment • Firstly, what are the wood preservatives? Basically they are pesticides; they are used to prevent decay of the wood and prolong their life in service • We can reduce the number of trees that are cut annually if we make the preservatives more efficient • Means that these preservatives for example protect wood for 20 years if we prolong that 50 years means that we will save more trees • There are a bunch of different wood preservatives – one of the most often used and most effective preservative is CCA-C • CCA-C contains all three chromate-copper-arsenate in different percentages (this one is the most often seen in Canada and U.S.) • All of these oxides are dissolved in water and they are used as a solution to treat wood • How is the wood treated? Slide shows one kind of cylinder called a retort – it will teach us later on the impact and adverse affects on our health • These retorts are equipment that are used for vacuum-pressure impregnation of the wood – simply wood is placed inside these tube and first treated with vacuum; all the air is sucked out of this retort • Then under very high pressur
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