[TNew (OP)]

I work for an environmental company and are looking at methods to remove contaminants from soil. Anyone have any ideas?

[CliffordK]

It would depend a lot on the contaminant.  Radioactive material is often dug up and sequestered somewhere.  Plants may leach some of the radioactive materials out of the topsoil, but then one would still have to dispose of the organic matter.  Nearby water sources may be filtered.

Some acids or bases might be effectively treated in place simply by neutralizing them.

Organic and explosive residue contaminants might be dug up and incinerated.  I'm not sure about oils and fuels.  Perhaps one would also dig them up and incinerate if possible.  There may also be some microbes that would feed off of the oil.

[CliffordK]

The idea of oxidizing organic chemicals in the ground using supercritical fluids  is interesting, but I would wonder how easily the methods would scale up for large-scale applications.

[Mazurka]

The most common approach to contaminated land in the UK is to look at the "source", the "pathway"  & the "receptor" involved.  If this chain can be broken there is generally not a problem.

Appropriate remediation techniques to remove the contmaination depend on the nature of it. 

The cheapest way is often to remove the conaminated material to landfill and fill the void with clean inert waste (such as other excavation waste) or a recycled product (if there are specific engineering / stabillity issues for subsequent development.

It may be possible to wash the soil by injecting clean water and pumping out (and treating) contmainated water.

It may be necessary to use a thermal or catalytic treatment where soil is passed through a specific machine/ process.

Bio-remediation can be very effective - Phytoremediation using specific plants is particualrly effective with shallow heavy metals contamination - There is archeological evidence that the Romans used gorse to improve their recovery of gold from their mines...
   

[CliffordK]

There is archeological evidence that the Romans used gorse to improve their recovery of gold from their mines...

Gorse?
In Oregon, that is one of the most miserable invasive species, perhaps originally imported as a living barbed wire fence. 

However, it is a good point that plants will extract some minerals from soil, and then one may be able to extract the minerals from the plant matter, perhaps through incineration to reduce it to a small amount of ash (which might then be reprocessed).  That is, if the target mineral would not be lost in the smoke.

[Mazurka]

Yes, gorse.
At the Dolaucothi gold mine in west Wales, which was worked on and off since Roman times, there has been (as a consequence) considerable archeological investigations.  When clearing gorse out of a shallow valley feature, a flight of stone steps, fed by a system of leets connecting various stone cisterns was discovered.    The mine is in an area of ancient woodland where you would not ordinarily expect gorse.  Further invesitgation found fire pits (of around the right date) where the gorse was burnt.  In the ash deposits small particles of gold were found.
It is postulated that the romans tipped crushed rock at the top of the steps and used water to wash it down.  The larger pieces would tend to get deposited near the top, and the finer grains taken up by the gorse (at the bottom) whcih could then be extracted by burning it.   
   

[Mike2Osborne]

Environmental contaminants is mostly caused by the bad treatment that we do to the environment. I saw this post one time about environmental remediation - <dodgy link removed>