Most people suggest you boil the branches, then soak them for a while. That means you get rid of the microorganisms, but don't risk putting bleach in your aquarium.
As it's a citrus, you might want to soak it for a week or more, to leach out any oils in the wood.
Someone may tell you not to because of the oils, and they may be right. I am not 100% sure on this one.