yea, you want to thoroughly rinse both the bio-char and the volcanic rock, the biochar moreso cuz it'll blast the ph up high for a bit
and as far as charging, yea, the process is exactly the same for biochar as it is rotted tree chunks
just make sure the tree chunks are from the inside of the rotted log, and not the bark, and also it should have the consistency of Styrofoam, kinda, only able to crumble
when they are charged and soaked it's almost like wood sponges.
another good use of that is its badass for compost piles, the bottoms in specific, cuz it keeps the compost from getting anaerobic, also from compacting, so it keeps the ammonia gasoff to a minimum, as well as keeping the pile more fungal oriented