Toxic Elements

1. Toxic Elements

2. 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) (Potentially) Toxic Elements

3. Toxic Elements are Ubiquitous These elements and their compounds are naturally occurring and are ubiquitous in that they are present in all components of the environment • all organisms, soil, rocks, water, and the atmosphere contain these elements in at least a trace concentration • and so, there is a universal contamination with trace (or larger) amounts of these substances

4. Natural “Pollution” If chemicals occur naturally in the environment in high enough concentrations to cause toxicity to organisms, this may be viewed as “pollution” • elemental pollution is not just a recent phenomenon linked to anthropogenic emissions • e.g., Al occurs at 7-10% in soil & crustal rocks; Fe 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

6. “Total” and “Available” Exposure to “available” versus “total” chemical forms is a key aspect of metal and elemental toxicity • “total” elements include insoluble forms as well as water-soluble ones • this is often analyzed after a hot-acid, strong-oxidant digest of a sample • “available” elements are soluble in environmental water • they are mostly ionic species, plus metals chelated to organic compounds • available elements are often analyzed by an aqueous extraction • usually with a solution of ambient osmotic strength, e.g., 0.5 M CaCl2 • may also use an EDTA or citrate extraction

8. Toxicity Metals can cause toxicity in various ways, but the physiological mechanisms are most commonly associated with: • binding of the toxic element to specific enzymes, resulting in a change in their 3-D configuration and a loss of essential metabolic function • or binding to DNA, resulting in interference with genetic function

9. Natural Pollution Examples include: • surface mineralizations, which in extreme cases can exceed 10% metal • serpentine sites • volcanoes • forest fires

10. Serpentine Sites Serpentine minerals contain basic crystalline minerals, and are often associated with deposits of fibrous asbestos • serpentine minerals are naturally toxic growth substrates for plants because of: • an imbalance of Ca : Mg • low concentrations of available N and P • high Ni, Cr, and Co (generally 103s of ppm; sometimes >%) • serpentine sites may have local (or endemic) species & ecotypes • e.g., Sebertia acuminata of New Caledonia is a hyperaccumulator plant with ~25% Ni in its blue-coloured latex • such plants are useful in biogeochemical prospecting

11. Serpentine (cont’d) Serpentine sites may support an unusual flora and ecosystem • 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

12. An area of serpentine-influenced soil in western Newfoundland

13. Serpentine substrates are highly stressful to plants

14. 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 may support local hyperaccumulator species that are toxic to herbivorous mammals • e.g., many species of Astragalus or “locoweed,” which cause a disease known as “blind staggers” in cattle • these plants can contain up to 1.5% Se • 25 species of Astragalus in North America are seleniferous, out of a total of about 500 species in their genus

15. 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 their 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 a common observation in large, old fish in inland freshwaters • itis also a common observation in marine mammals • the Hg is apparently natural in origin • but this problem can be made much worse if there is also local anthropogenic pollution by mercury

18. 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

22. 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 produces H+ • industrial point-sources: • primary metal smelters: Sudbury, Wawa, Flin-Flon … • secondary metal smelters, e.g., Pb-battery recyclers • metal refineries • metal foundries • manufacturing

25. 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 it was banned in 1991 • MMT, a manganese compound, is now sometimes added

27. 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)

28. 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 as little as a few generations • if the selection gradient is steep enough, ecotypes can maintain themselves against gene flow, even over distances of only several meters

29. Metal-tolerant ecotype of the hairgrass, Deschampsia caespitosa, growing in polluted soil near Sudbury

31. Microbial Oxidation of Sulphur and Iron Compounds 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

32. 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

33. Acid-mine drainage

34. 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) • acidification, which makes metals more water-soluble

35. deposition of emitted particulates & gases

36. metal toxicity in soil is made much worse by acidification

37. slag is a metal-rich waste of the roasting process

38. a slag dump (dark substrate)

39. metal-containing tailings are disposed into a terrestrial basin

41. after the tailings dump is filled, it can be stabilized by covering it with vegetation