Not knowing what the boots are made of makes it hard to know how best to clean them.

If you have a steam genie/cleaner, then steam will clean them up without causing to much damage etc.  Steam is completely chemical free, yet kills 99.9% of germs, it is more enviornmentally friendly and is easier on the budget.  It is also more suitable to more delicate materials, and great for cleaning curtains with too as well as a general purpose cleaner.