Licking of the feet happens for two main reasons. One, it's a common place for dogs to scratch when they suffer from allergies. Two, it's soothing to them and they do it as a habit.
The first thing that you can try is drying their feet when they go outside. Dogs often get used to licking their feet to clean them after going to potty. If you dry the feet well, you might be able to stop this behavior.
There are a couple of other things that you can try. First, you need to see the vet to get an antibiotic in case there is an infection. This is often caused by the licking and it makes things worse. This antibiotic should not be short term. The dog needs it for at least 3-4 weeks. It sounds like your dog might also have a secondary bacterial or fungal infection. Your vet can give you treatment for this as well.
Second, I would get an anti-anxiety medication because licking the feet is often a compulsive disorder. Homeopet (www.homeopet.com) works great and is a natural medication.
You can also speak with the vet about getting an e-collar so that the will leave the feet alone long enough for them to heal.
Don't use lemon juice. That can be very irritating. I would wash the feet well with peroxide, even between the toes and then dry them well. Then apply some neosporine ointment.
You will want to see the vet as soon as you are able.
Let me know if you still have questions.