Have you tried radioactive waste?
But in all seriousness, you can't fix any areas of the plant that are already dead or dying... depending on how severe this is, you may have to start over <.< >.>
don't let the plant try to heal the dying areas, that will probably kill it, cut them off.. if there isn't a good branch you might be screwed. though i do know that some trees can grow new branches at internodes in times of stress, i just don't know if kratom is one of those trees.
what you are hoping is that the offending minerals are concentrated where the plant naturally tries to put them, close to the new growth. if they are, you can cut the plant back and hope it recovers. if they aren't, because you mixed your fert too hot AND used it twice, and the plant could not transport all of the fert to the new growth, you're screwed, and your plant is already dead, it just doesn't know it yet.
there is not to my knowledge a way to extract minerals from a live plant, and have a live plant at the end of the process...