[William]

Treebeard "eats" long draughts of liquid. If you don't errata out a need to eat or drink, then I see no reason why elementals in bodies shouldn't have to sustain those bodies, which would require "eating and drinking" sufficiently pure versions of their element.

Thus an air elemental spirit could be bound indefinitely inside a bottle, say, while an embodied air elemental materially stuffed into a container would last no longer in there than anything else without its food sources. Then the body "dies" and you end up with the spirit, which would no longer be trapped unless the bottle was also prepped to trap spirits. (And of course morally you've just starved something to death, if perhaps not quite so unpleasantly as a more natively material creature.)

For what it's worth this also opens up the possibility of offering such spirits fine food and drink of the types they like: for instance, an air elemental might be more willing to work without coercion, in exchange for a regular offering of rare perfumes or the scent of many fresh roses.