Discussion turned into argument with the conference keynote speaker, would it be appropriate to email an apology?
Academia is one of a few areas where it is still OK to have a discussion and even an argument. In fact, some research groups are involved in long-standing discussion / heated competition / argument. Academic discussion is considered a way to improve our joint understanding of the world and to further the scientific progress and knowledge.

However, as in any game, there are some rules to understand and follow. 


It is OK to disagree with someone's argument, but you should listen and do your best to understand it first. If you don't understand something, ask politely. If you still don't understand, ask someone for a third opinion. If necessary, allow yourself time to think before you continue the discussion.
It is very important to be polite and not to become personal.
Seniority per se does not matter so much: professors do make mistakes sometimes, and they know it. However, it is important to remember that most of professors are called professors for a good reason, and they are indeed experts in their field. Their words are not always infallible, but usually quite well thought through. It is OK to ask a professor to explain his/her statement, but it is good to show that you generally respect and trust them as colleagues and experts.


As long as you followed these rules, I think you are fine and there should be no hard feelings. Otherwise, it could be a good idea to write back with a sort of apology. Professors are people first, and as all people, they have feelings, which should be respected. So if you fill you hurt their personal feelings, an apology would be nice.