Friday, October 04, 2013

Never complain about your stove again

Geologist have it pretty easy, in terms of lab safety.  Compared to chemists and biologists, we have to deal with a relatively low number of lethal chemicals, and our habits confirm this.  It is not coincidence the people call us rock-lickers.  But there are still some reagents which are genuinely dangerous, and command respect.

For most rock knockers, the chief among these is hydrofluoric acid, or HF.  HF is a volatile (evaporates easily) acid which is notorious for being a contact poison.  You don’t have to drink it for you to kill you, as it will diffuse through skin, and attack muscle tissue and bone inside your body.  If the muscle it attacks is your heart, then you die.  As a result, geologists are taught from a young age to observe strict safety protocols with HF: gloves, face shields, aprons, appropriate supervision and fume cupboards are all part of the drill.

But not all fluorine health effects are as dramatic.  excess fluorine consumption can often cause dental fluorinosis, a condition in which excess fluorine is deposited in the teeth, discoloring them.  In more severe cases, fluorine deposition in the bones can lead to osteofluorosis, which can cause disfigurement, deformity, and chronic pain.

One area in which osteofluorosis is distressingly common is Guizhou, China.  Over the past decade, this disease here was linked to the combustion of high Fluorine coal.  Studies showed tha the clay that was intermixed wit hthe coal was high in F, and a steady stream of recommendations has come along describing how this must be getting aerosolized in smoke ,and adhering to food, particularly corn and chilies hung up in houses to dry.

But something didn’t add up.  People were educated to wash their vegetables, to not breathe coal smoke, and still the disease persisted.  Finally, recent studies showed that the F was not adhering to the food products.  TOF SIMS showed that  it appeared inside uncut chilies, and sometimes was associated with silica- particulate matter which should not be able to penetrate food and is biologically inactive.

This was the key to a renewed investigation into the coal.  Which, as it turns out, was not just rich in fluorine, but also rich in pyrite- fools gold.  And all of a sudden, everything fell into place.

When burned, pyrite reacts exothermically with oxygen and water to form iron oxide and sulfuric acid:

2FeS2 + 8.5O2 + 4H2O -> Fe2O3 + 4H2SO4.

Sulfuric acid is not great to breathe, but it doesn’t cause fluorine poisoning.  It will, however, react with fluorite (the most common fluorine mineral like this:
H2SO4 + CaF2 -> CaSO4 + 2 HF

And there is the chemical that terrifies geochemists even in controlled lab spaces, HF, being generated in the household stove. It, in turn reacts with coal ash to form the toxic gas SiF4, which permeates plant and animal tissues and deposits silicon inside of vegetables. In short, domestic cooking stoves are generating incredibly toxic F-bearing gases inside the home. Not even your brother-in-law’s cooking is as hazardous as this.  This was the coolest talk from the session I was fortunate enough to chair this afternoon; there is a paper here.

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