They make activated carbon from coconut shells, walnut shells and a bunch of other things, stikbow.

400 degrees is the temp that gets thrown around a lot but, actually, that's 400 degrees C, or some 848 degrees F... which is 200 degrees F more than the melting point of aluminum! It's only about 350 degrees F hotter than the melting point of polyester fabric though.

Some studies show that minimal desorption with activated carbon can occur at temps as low as 100 degrees C. Even that's still a good deal hotter than a household clothes dryer can get.