28 March 2011

Mine Reclamation, Disequilibrium, and Selenium Contamination

Posted by John Freeland

Phophate Mine (Source: Greater Yellowstone Coalition)

At an Idaho phosphate mine, a lack of understanding of mineral weathering leads to selenium contamination, a major court ruling, and public liability.

An article at Marten Law’s website, available here, focuses on a recent federal district court ruling that would make the government liable for contamination on public lands leased to private mining companies. The ruling, stemming from selenium contamination, may have serious implications for all mining operations on leased public lands – and who gets stuck with the cleanup costs. Mine leases on federal land are especially common in the western United States.

Will the court decision slow down applications and force more stringent environmental reviews and enforcement of mining regulations? We’ll have to wait and see.

There’s another science story, however, imbedded in the legal account of CERCLA regulations: mineral weathering, sulfide oxidation, sorption-desorption, and selenium toxicology. The Greater Yellowstone Coalition (GYC), an advocacy group, has this short video introduction to selenium toxicology:

Selenium Kills from GYC on Vimeo.

If your early chemistry courses were like mine, you worked problems with the assumption of standard pressure and temperature, and (and this is key) equilibrium. It wasn’t until later classes in soils and limnology that it became clear that the world we live in is not at equilibrium! Those who study arts and humanities probably regard this fact as painfully obvious.

The selenium Pourbaix diagram shown to the right illustrates varius stable forms of selenium under a range of pH and Eh (oxidation potential) conditions. I added the green box and text to show the range of conditions commonly found in soils. Variablility in oxygen and pH levels in soils is related to many factors including the soil parent materials, texture, landscape position, weather and climate, plant root functions, microbial activity, decomposition, soil fauna, and a host of other reasons.

As soils containing selenium undergo pH and Eh conditions due to varius biotic and abiotic influences, selenium becomes mobilized as once-stable compounds break down to form new compounds that are stable in the altered environment.

Typically during phosphate mining, minerals containing the reduced form of selenium, selenite, are brought to the surface where they react with water and oxygen to form oxidized selenium in a process similar to sulfide weathering, which causes acid mine drainage around coal and sulfide metallic mines. Selenate is more mobile in the environment than selenite, and also more toxic when ingested beyond very small doses.

More about the technical properties and toxicology of selenium is available here.

The challenge for phosphate mines, and many other types of mines, is to understand the mineralogy of the rocks they plan to excavate, and then accurately predict the weathering products that will appear once rocks formerly buried deep beneath the surface are exposed to weathering processes.

When weathering products contain toxic elements, such as selenium from phosphate mines, the responsible task is to either isolate the toxic materials from the environment or react the material with something to make it less toxic. Both approaches add costs to the mining operations, and, ultimately, force us to examine how we use the resources that we dig up from under gound.