Coal and oxygenation of water

I just finished a science fair experiment that studied coal, activated charcoal and hornwort's abilities to produce dissolved oxygen in fresh water. The findings were that the hornwort and the coal produced 42% dissolved oxygen in the water while the activated charcoal produced 21%. Throughout all of this my control started and ended at 0%. I just tried looking up how the coal would have oxygenated the water and I found nothing. Does anybody know how this is possible?