Toxic Elements

(Potentially) Toxic Elements

Heavy metals: 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: aluminum (Al)

Non-metallic toxic elements: 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 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” elemental pollution is not just a recent

anthropogenic phenomenon e.g., aluminum occurs at 7-10% in soil & crustal

rocks; iron at ~5%

there are also surface metal-rich mineralizations

e.g., Ni, Co, & Cr in serpentine minerals

in addition, high concentrations of soluble metals occur in acidic environments

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 “total” concentration includes insoluble forms

as well as water-soluble ones this is often analyzed after a hot-acid, strong-

oxidant digestion of a sample

“available” elements are soluble in environmental water

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:

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 soil containing serpentine minerals are

naturally toxic to plants because of: 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

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 serpentine sites in California are ancient, and

have many endemic species and communities 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

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 it is another example of natural pollution

in this case, associated with high levels of selenium

seleniferous sites support hyperaccumulator species that are toxic to herbivorous mammals

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 large, old fish of many species may have a high

concentration of methylmercury (CH3Hg) in flesh and organs

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:

agricultural sources:

inorganic insecticides and fungicides

sewage sludge: various metals; Cd of greatest concern

Anthropogenic Emissions (cont’d)

mining:

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:

primary metal smelters: Sudbury, Wawa, Flin-Flon …

secondary smelters, e.g., Pb-battery recyclers metal refineries metal foundries manufacturing …

Anthropogenic Emissions (cont’d)

utilities fossil-fueled power plants (coal, bunker-C)

emissions of vapour-phase Hg; particulate V & Ni

municipal incinerators wide range of potential emissions

automobile emissions leaded gasoline

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)

additional sources of elemental emissions

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 they are locally differentiated populations of

wider ranging species 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 they are: Thiobacillus thiooxidans and T.

ferrooxidans the reaction products are sulphate (SO4

-2) and oxidized iron (Fe3+)

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:

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 the metal pollution is associated with:

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