What should be done with a paper tissue that was used to wipe a blackboard on which Islamic materials were written? Should it be burned and thrown into the river?

If the material is from the Quran or Hadith, then what was wiped with it should be buried in a clean place or burned, out of reverence for the sacred things of Allah. The scholars have stipulated the obligation to respect the water used to wash the Quran and the dislike of pouring it in impure places or places that are trodden upon. Rather, it should be poured in a clean place like wells, the sea, or a clean, untrodden pit, because this wash-water has sanctity.

However, if the material is from the Islamic sciences other than the Quran and Hadith, then what was erased with it should also be respected, and it is disliked to pour it in unclean places, even if the prohibition does not reach the degree of the Quran.

If the material is not from the Quran or related to it, then there is no harm in discarding the tissue that was used to wipe it.