Thursday, August 28, 2008

Charcoals: Electron Microscopy Study

I worked some connections here at NC State Univ. and got a co-conspirator to scan some of the charcoal samples I brought back from Thailand with an electron microscope. SEM images are so cool!

The images just below are of charcoal made from longan wood (Dimocarpus longan). There are loads of these of these fruit trees throughout southern China, southeast Asia and the region around Pun Pun Farm. They grow them from layering and the trees never develop a strong tap root. So they have to prop the limbs up or the trees will blow over in a storm. And they have to prune them often - so it's very common to make charcoal from the prunings.

You can really see the macropore structure in some of these shots (note the scale bars at the bottom of each image...a "um" is a micro-meter, 10^-6 meter, or 1/1000 of a millimeter, for those not conversant in scientific notation).

This charcoal is made from coconut husk. It was not the best kind of coconut to made charcoal from - the thin hard shells of young coconuts. Probably charcoal made from the thick, woody shells of mature coconuts, and/or the coir would be better for our purposes. But this stuff does show some interesting surface morphology.

We would have to use TEM (transmission electron microscopy) to see the smallest pores, at the molecular scale (1 millionth of a millimeter and smaller). Then we'd actually be looking at the crystalline structure of the charcoals - that would be totally sick. But these SEM (scanning EM) images are good for getting a since of how porous charcoal materials are at the micro-level. So you can see how a couple tablespoons of this stuff can have the surface area - at the molecular level - of a football field. Plenty of space, in other words, to adsorb contaminants like nasssty pesticides!

Below is charcoal made from rice husks. It's an interesting material, but probably not very good for water filtration purposes because of its naturally high ash content from all the silica and minerals that make up the husk. Also, rice husk charcoal is not made in a kiln - husks are just piled onto a fire and allowed to smolder. So the pyrolysis is pretty low temperature, and oxygen is not well excluded - thus it doesn't develop awesome porosity/surface area and so is not a great water filtration charcoal.

But still interesting to look at in SEM - in the image, the husk on the left shows the underside, on the right the outer side of the husk. Cool eh? Very different morphology inside and out!

I had a great chat the other day with my man the electron-microscopy ninja when he brought me these images. (He snuck our samples in between runs of "official" work.) I told him about the project and our efforts to design simple/cheap systems for drinking water purification and he is down like 4 flat tires. So I will keep sending him samples and he will sneak them in all clandestine like - so more cool images to come, ha!

Renegade science is so much fun!

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