Toxic Elements
(Potentially) Toxic Elements
Heavy metals:
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cadmium (Cd)
chromium (Cr)
cobalt (Co)
copper (Cu)
lead (Pb)
manganese (Mn)
mercury (Hg)
nickel (Ni)
silver (Ag)
tin (Sn)
vanadium (V)
zinc (Zn)
Lighter metals:
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aluminum (Al)
Non-metallic toxic
elements:
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arsenic (As)
phosphorus (P)
selenium (Se)
Toxic Elements are Ubiquitous
Toxic elements and their compounds are
naturally ubiquitous – they are present
in all components of the environment
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all organisms, soils, rocks, water, and
the atmosphere contain these elements
in at least a trace concentration
as such, there is a universal
contamination with trace (or larger)
amounts of these potentially toxic
substances
Natural “Pollution”
If chemicals occur naturally in high enough
concentrations to poison organisms, this
may be viewed as “pollution”
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elemental pollution is not just a recent
anthropogenic phenomenon
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there are also surface metal-rich mineralizations
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e.g., aluminum occurs at 7-10% in soil & crustal rocks;
iron at ~5%
e.g., Ni, Co, & Cr in serpentine minerals
in addition, high concentrations of soluble metals
occur in acidic environments
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e.g., >1 ppm Al & Fe ions, which are toxic at these
levels
“Total” and “Available”
Exposure to “available” or “total” chemical forms is a
key aspect of metal and elemental toxicity
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“total” concentration includes insoluble forms as
well as water-soluble ones
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this is often analyzed after a hot-acid, strong-oxidant
digestion of a sample
“available” elements are soluble in environmental
water
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they are mostly ionic species, plus metals bound
(chelated) to organic compounds
available concentrations are often analyzed by an
aqueous extraction
 commonly using a solution of ambient osmotic
strength, e.g., 0.5 M CaCl2
 may also use an EDTA or citrate extraction
Toxicity
Metals may cause toxicity in various
ways, but the physiological
mechanisms are most commonly
associated with:
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binding of the toxic element to specific
enzymes, causing a change in their 3-D
configuration and a loss of essential
metabolic function
or binding to DNA, resulting in interference
with genetic functioning
Natural Pollution
Non-anthropogenic pollution may
be caused by:
 surface
mineralizations, which
in extreme cases may exceed
10% metal
 serpentine sites
 volcanoes
Serpentine
Serpentine deposits contain basic crystalline
minerals, and are often associated with
asbestos deposits
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soil containing serpentine minerals are
naturally toxic to plants because of:
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an imbalance of Ca : Mg
low concentrations of available N and P
high Ni, Cr, and Co (often 103s of ppm; sometimes
>%)
serpentine sites may have local (or endemic)
species & ecotypes
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e.g., Sebertia acuminata of New Caledonia is a
hyperaccumulator with ~25% Ni in its blue-coloured
latex
such plants are useful in biogeochemical prospecting
Serpentine (cont’d)
Serpentine sites may support an unusual
flora
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serpentine sites in California are ancient, and
have many endemic species and communities
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there are ~215 serpentine endemics in California,
comprising 14% of the serpentine flora
e.g., many species and endemics in the genus
Streptanthus
in contrast, serpentine sites in eastern Quebec
and western Newfoundland have no endemic
species; only ~8,000 years have passed since
their deglaciation
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but the Canadian sites have a distinct, stunted,
sparsely vegetated, ecosystem structure
An area of serpentine-influenced
soil in western Newfoundland
Serpentine substrates are
highly stressful to plants
Seleniferous Soil
Seleniferous soil is widespread in arid and
semi-arid ecosystems
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it is another example of natural pollution
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in this case, associated with high levels of
selenium
seleniferous sites support hyperaccumulator
species that are toxic to herbivorous mammals
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e.g., many species of Astragalus or “locoweed,”
which cause “blind staggers” in cattle
 these plants can contain up to 1.5% Se
25 species of Astragalus in North America are
seleniferous, out of ~ 500 species in the genus
Marine Mercury
Mercury pollution of fish & marine mammals
is a rather common phenomenon
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large, old fish of many species may have a high
concentration of methylmercury (CH3Hg) in flesh and
organs
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the WHO limit for Hg in fish for human consumption is 0.5
ppm f.w.
this limit is often exceeded in large, old, wild fish
high methylmercury is also common in large, old fish
in inland freshwaters
it is also frequent in marine mammals
the Hg is apparently natural in origin
but this problem can be made much worse if there
are local anthropogenic emissions of mercury
Anthropogenic Emissions
Anthropogenic sources of elemental
pollution are important in many places
and regions:
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agricultural sources:
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inorganic insecticides and fungicides
sewage sludge: various metals; Cd of
greatest concern
Anthropogenic Emissions (cont’d)
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mining:
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mine spoils; discarding of overburden and shaft
waste
waste tailings of the milling process
Acid-generating spoils; oxidation of S & Fe –> H+
industrial point-sources:
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primary metal smelters: Sudbury, Wawa, Flin-Flon …
secondary smelters, e.g., Pb-battery recyclers
metal refineries
metal foundries
manufacturing …
Anthropogenic Emissions (cont’d)
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utilities
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fossil-fueled power plants (coal, bunker-C)
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municipal incinerators
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emissions of vapour-phase Hg; particulate V & Ni
wide range of potential emissions
automobile emissions
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leaded gasoline
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tetraethyl-Pb was used as an “anti-knock”
additive since 1923, but banned in 1991
MMT, a manganese compound, is now sometimes
added
Anthropogenic Emissions (cont’d)
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additional sources of elemental emissions
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hide tanneries (Cr)
pulp & paper and chor-alkali factories (Hg)
photographic manufacturing & processing (Ag)
solid-waste disposal sites
electroplating and metal-product manufacturing
hydroelectric reservoirs (methyl-Hg)
Metal-Tolerant Ecotypes
Metal-tolerant ecotypes are local
populations of plants with a genetically
based, physiological tolerance of toxic
elements in their growth medium
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they are locally differentiated populations of wider
ranging species
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local endemic species are not an “ecotype”
if the selection pressure is strong enough, ecotypes
can evolve rapidly – in only a few generations
if the selection gradient is steep enough, ecotypes
can maintain themselves against gene flow, even
over a few meters
Metal-tolerant ecotype of the hairgrass,
Deschampsia caespitosa, growing in
polluted soil near Sudbury
Microbial Oxidation of S & Fe
Certain chemoautotrophic bacteria derive
energy from the oxidation of reduced
compounds of sulphur and iron
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they are: Thiobacillus thiooxidans and T.
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the reaction products are sulphate (SO4-2) and
oxidized iron (Fe3+)
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ferrooxidans
the reactions are highly acidifying and are the
cause of “acid-mine drainage” and similar
environmental problems that occur when
sulphide minerals become exposed to
atmospheric oxygen
Microbial Oxidation of S & Fe
(cont’d)
The following reactions are important:
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FeS2 + 7/2 O2 + H2O  2SO4-2 + Fe2+ + 2H+
Fe2+ + 1/4 O2 + H+  Fe3+ + 1/2 H2O
Fe3+ + 3 H2O  Fe(OH)3 + 3H+
overall: FeS2 + 15/4 O2 + 7/2 H2O  2 H2SO4 +
Fe(OH)3
2 SO4-2 and 1 Fe3+ and 2 H+ are generated per
FeS2 oxidized
Acid-mine drainage
Metal Pollution near Sudbury
The Sudbury area has been affected by
extreme pollution associated with emissions
of SO2 and metals, as well as acidification
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the metal pollution is associated with:
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emissions of particulates from roast-beds,
smelters, and refineries
the dumping of molten, metal-rich slag (waste
from roasting process)
the disposal of metal-rich tailings (waste from
milling process)
environmental acidification, which makes metals
more water-soluble
deposition of emitted
particulates & gases
metal toxicity in soil is made
much worse by acidification
slag is a metal-rich waste
of the roasting process
a slag dump (dark substrate)
metal-containing tailings are
disposed into a terrestrial basin
after the tailings dump is filled, it can be
stabilized by covering it with vegetation