OK, I imagine that many of you have heard all about a nasty flood of red mud which has occurred in Hungary. When I say red mud do bear in mind that this is nothing to do with politics it is not a communist mud, rather it is a mud which is red in colour which is a waste product from the processing of aluminium ore. What has happened is that a pool of liquid waste at the Ajkai Timföldgyár plant has leaked, the mud is a waste product which is left behind from the Bayer process. The Bayer process is an ore processing method in which bauxite is digested with sodium hydroxide (lye), the mixture is filtered and then aluminium hydroxide is obtained. The aluminium hydroxide is then calcined to form high purity alumina which is then used for the production of aluminium.
Always be careful to understand that alumina might sound like a US way of saying aluminium but alumina is aluminum oxide while aluminium is aluminium metal. It becomes more complex because all aluminium metal which has been exposed to the air has a thin coating of alumina on it.
So what has happened is that the red mud which contains all the things which do not dissolve in caustic soda (sodium hydroxide) has been responsible for the pollution event. A chemical analysis of the red mud has been published.(http://english.mal.hu/engine.aspx?page=showcontent&content=Vorosiszap_HIR_EN)
Here are the major metal compounds which are in the waste.
I might post some details of the solid state chemistry of these metal oxides.
I am aware that some bauxite ores contain other toxic elements, it is interesting to note that C. N. Grant, G. C. Lalor and M. K. Vutchkov, Journal of Radioanayltical and Nuclear Chemistry, 2005, 266, 385-388 published a paper in which they report what elements are in bauxites from the west indies. These workers discovered that rare earths, radium, uranium and thorium were all present in the ore. Along with a lot of other metals, it will be interesting to note what the trace metals in the red mud will do. In the long run the iron, aluminum and titanium oxides which are the bulk metals in the red mud should not do too much harm. The caustic nature of the mud will decline with time, but toxic metals such as As and the radioactive metals may remain in soil which has been contaminated with the red mud.
The radioactive elements in the mud (V. Jobbágy, J. Somlai, J. Kovács, G. Szeiler and T. Kovács, Journal of Hazardous Materials, 2009, 172, 1258-1263) can make it unsafe for use as a building material. I know that today recycling is important and it would be nice to recycle waste into things like house bricks. But it is important to make sure that the recycled product is safe, I hold the view that if the recycled product is not safe then it is better not to recycle. In this paper which I have just cited it has been shown that while pure red mud might be unsafe for house bricks it a mixture of 20% red mud and 80 % clay is within the legal limits and that by adding saw dust to the bricks that the fired bricks are much less able to release radon into the air.
It is important to understand that this radioactivity has nothing to do with the Nuclear industry, the NORM or naturally occurring radioactive material tends to contain plenty of long and medium loved alpha emitters while much of the radioactivity in many wastes from the nuclear industry is shorter lived beta/gamma emitting waste. I hold the view that alpha emitters are much more nasty than beta emitters. To add to the trouble the NORM also tends to contain beta/gamma emitters so you can not simply say keep the natural radioactivity in a plastic sack.
The classic medium lived isotopes in nuclear waste are Cs-137 (beta / gamma), Cs-134 (beta / gamma) and Sr-90 (beta only). These are all beta/gamma emitters which have half lives of 30 years or less, some nuclear wastes do contain alpha emitters such as Pu-239, Pu-238, Am-241 etc but the amounts of these alpha emitters tends to be much smaller than the short lived beta/gamma isotopes.
The classic isotopes in NORM tend to be K-40 (beta/gamma), Ra-226 (alpha), Rn-222 (alpha), Th-232 (alpha), U-238 (alpha) and their decay products. The radium/radon is very tiresome as the radium is a calcium mimic which gets into bones while the radon and its decay products tend to get into the lungs. It is thought that the second largest cause of lung cancer in the UK is radon which comes out of the ground and building materials.
But this brings on an interesting question, should we allow the disposal of radioactive materials by the dilution with non radioactive matter. I would be interested to know if human exposure would be reduced if the red mud was used to make building materials for roads and non residential buildings while the normal clay was used (without the addition of red mud) for making homes, schools, hospitals and other buildings where people spend a lot of time.