You might want to do some reading up on preparing driftwood for an aquarium. I can't help you with the type of tree, although I wouldn't be too confident about a citrus tree.
Common preparation methods for "found" driftwood include: months of soaking, boiling for a few hours, salt or bleach treatments etc.
This is to kill off microorganisms in/on the wood, and to release tannic acid in the wood that would turn your tank water into a tea colour otherwise (not harmful, but not necessarily aesthetic).
Additionally, people tend to look for wood that has been dead for a number of years. Natural weathering will take care of much of the tannins in long-dead wood. Green wood is more likely to contain substances than old wood, and this makes me even more sceptical about a citrus tree.
But I'd recommend doing a browser search and finding what others have said. You may come across other forums where users have had no issues.
Do not neglect to treat your wood, no matter what you end up getting though, or it'll be more trouble than its worth once in your tank. There's many many articles on treatment, find what works best for your size of wood, and convenience of treatment.
