Montanero - I can help with part of the answer. No doubt the best thing to use for absorbing chemicals in water is proper charcoal (activated charcoal) available in stores. This charcoal is probably higher purity.

However, in primitive villages in the third world I know people who use charcoal (from campfires) for the same purpose. It is routinely used when they wash out gourds - which are used for storing liquids. Gourds can be used for water storage, but they are also used for storing milk and the preparation of "sour milk" (i.e. milk aged for 12 hours and deliberately soured before drinking). When the gourds are cleaned after the milk preparation, the people wash them out with a mixture of water and charcoal (from the firepit) to absorb impurities. Subsequently, any water from the gourd can taste a bit "ashy", but is essentially harmless for drinking. So my guess is that you could use charcoal from a campfire as a crude method for absorbing impurities in water.

Pete #2