A friend of mine was in a similar situation with a dangerous chemical that he used for some hobby projects (an acid for glass etching)?
It wouldn't be accepted anywhere so he ended up calculating how diluted it needed to be and pouring it mixed with a lot of water down the sink.
For 5 litres of nicotine to be safe you'd need a lot of water though if you'd want to make this remotely ok for the environment...
What about mixing it with cat litter and then disposing it?