|Everything in this world is potentially harmful if present in sufficiently large quantities and if handled, stored or disposed of in an inappropriate manner. PFA is no exception. In “reasonable” quantities, it is believed not to represent a major hazard to the environment. However there are risks if a large amount of it is dumped in one place, while the actual infilling operations are certainly damaging to the natural environment. In Radley’s case, some of the infilling in the past has visually improved the environment (Lakes A-D:compare with photos in 1982 and 2005); The situation with lakes G-P is questionable. Regarding future infilling, it is certainly a case of enough is enough.||
All coal contains low concentrations of radionuclides, principally those of the decay series of U238, U235, Th232 and also K40. Its combustion releases volatile radionuclides (which include Polonium210 and Radon226, which are alpha-emitters) and, more importantly, concentrates non-volatile heavy radionuclides in the resultant ash. From the figures given, the concentration factor is somewhere between 3 and 10. Moreover, because coal is formed from plant residues, and plants have the ability to sequester and thus concentrate heavy metals in their environment (Such a process, where a biological entity absorbs and retains, and thus concentrates, a substance, is called biomagnification.) there is no reason to suppose that the composition of ash residues should reflect the originally prevailing radionuclide and heavy metal concentrations in the original environment. This will vary accpording to the sources of the coal, and some coals and resultant ash can contain unexpectedly high radionuclide concentrations (here for example).
Indeed radioactive discharges from coal fired powered stations greatly exceed those from nuclear power stations of equivalent power. COMARE, the Committee on Medical Aspects of Radiation in the Environment, considered this in 1988 and commissioned a study by the the NRPB to investigate. As a result, COMARE, in their 3rd report in 1989, concluded that [atmospheric] discharges from coal-fired plants [in Oxfordshire and Berkshire] make a very small contribution to the total radiation dose received by people living around the sites. In their 10th report in 2005, COMARE revisit this issue and note that Didcot power station is now the major radiological polluter in the area with emissions exceeding those of Aldermaston and Burghfield combined, by a factor of two. A further report relating to Berkshire and South Oxfordshire is promised in the near future.
The radioactive content of the retained ash is likely to be somewhere in the region of 1000 times greater than that released into the atmosphere. This is not to say that the ash is likely to be dangerously radioactive. The activity is below the threshold for it to be considered radioactive waste and alpha emitters are only hazardous if ingested. In modest quantities, the ash represents a negligible hazard (though some concerns have been raised over its use of a building material in certain circumstances). One million tonnes of ash all in one place does however represent quite a lot of radioactivity (1 TBq). However, for the hazard to be realised, the ash would have to be subject to wholesale rapid dispersal, so burying it is a whole lot better than discharging it directly into the atmosphere.
It is worth noting that UK coal ash has lower than average content of U238 and Th232. Coal from international sources generally has 2-3 times higher concentrations of U238 and Th232 than UK coal, but lower concentrations of K40. The total radioactivity of fly ash is typically in the region of 1 Bq/g (irrespective of source) of which 65-90% is K40. This is about 10 times the average natural level. Also, 0.35 Bq/g of natural uranium in decay equilibrium represents a uranium content of about 2ppm by mass, or, put another way, one million tonnes of ash may contain up to about 2 tonnes of uranium.
A related concern is likely contamination by neurotoxic heavy metals such as lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr) and vanadium (V) which may be present in the ash in small but highly variable amounts. Also one would expect these to be concentrated by the same factor as the radioactivity (~ x10). However the potential for leaching of such contamination from fly ash is often considered to be low because of their low solubility and the alkaline nature of the leachate. However this may understate the long-term problem. Natural acidification over time combined with plant sequestration and biomagnification processes provide possible mechanisms whereby these contaminants, if present, may eventually enter the environment.
Chromium, vanadium and arsenic are water soluble and occur in leachate and supernatant outflow in small but significant amounts (typically combining to around 0.2 mg/litre).
PFA contains significant amounts of alkali (group I and II) metal oxides (Na2O, K2O, CaO, MgO) which become “slaked” into alkaline hydroxides on reaction with water. This is what accounts for the alkalinity of the PFA. Again, this may be highly variable even between coal batches from the same source and one can speculate whether this was responsible for the pollution incident in 2003. Also, it means that PFA that has been slaked or “conditioned” is less useful for use as a building material (see below). The PFA stockpiled in Radley’s lakes is therefore degraded for this purpose, if it were ever to be recovered (though it could be restored by recooking).
Regulatory and Epidemiological
PFA is a “Controlled Waste” in the UK and has no special requirements for its disposal at appropriately licensed facilities. It is included in the European Waste Catalogue but is not hazardous material as defined by the EC Hazardous Waste List.
It is also a “Green List” material for transfrontier shipment and for transportation it is classed as Non-Hazardous under the Classification, Packages and Labelling Regulations 1994.
Epidemiological evidence from power workers in the UK exposed to airborne fuel ash suggests, somewhat surprisingly, that it has no adverse effects whatsoever. However this may be partly attributable to particle shape and size which tend to be spherical in the 10 micron range. Smaller more irregularly shaped silicaceous particles, such as those that might arise from crushed cenospheres, should still be considered as potential carcinogens.
Other Links and Sources of Information
Other Information Relating to Potential Biological Hazards of PFA.
Cordes, K.B., Mehra, A., Farago, M.E. and Banerjee, D.K. 2000. Uptake of Cd, Cu, Ni and Zn by the water hyacinth, Eichhornia crassipes , (Mart.) Solms from pulverised fuel ash (PFA) leachates and slurries. Environmental Geochemistry and Health, 22, 297-316.
Some articles on the above website:
Cherry,D.S., Curry,R.J. and Soucek,D.J. Review of the Global Adverse Environmental Impacts to Ground Water and Aquatic Ecosystems from Coal Combustion Wastes Report prepared for Hoosier Environmental Council, Indiana
Harte, J., Holdren,C. and Schneider,C.S. 1991. Toxics A to Z: A Guide to Everyday Pollution Hazards.
Radioactivity is measured in one of two units, the Becquerel (Bq) which is the preferred (SI) unit in Europe. and the Curie (Ci) which is commonly used in the USA.
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Hazards of PFA
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