While replacing a couple of cartridges on my PRO-10, I noticed a little icon of a trash-bin with an X over it.
So I looked it up...
Turns out Canon has a free recycling program. Just drop your spent cartridges at any FedEx Office store.
I've done recycled toner where they want you to put the spent toner in the same box that the new toner came in and ship it back. But this doesn't look like it has any requirements other than just drop the cartridges off and FedEx & Canon will handle the rest -- no cost to you.