Seachem Purigen is a synthetic absorbent that captures organic waste in the water column. It can absorb and trap nitrogenous organic wastes that would otherwise release ammonia, nitrite, and nitrate. Tannins from driftwood and other natural decor in the aquarium are also captured. Eventually, Purigen will turn dark brown to black, as shown in the photos below. But you don't have to discard it and buy more at this point, because Purigen can be regenerated and reused!
The Purigen packet on the left in the above photo was used for just a few weeks in an aquarium with some large pieces of driftwood. The brown color is due to it absorbing tannins from the wood in the water. The packet on the right was used in a different aquarium for several months.
Recharging Seachem Purigen
Basic directions for re-charging Purigen are available on Seachem's website. A screenshot of their instructions is included below:
I recharged two 100 ml bags of purigen using a solution of 2 cups Clorox bleach and 2 cups water in a plastic container. The bleach burns off organic material that has been captured by the Purigen. You can see the brown and green coloration was completely removed from these two bags after 24 hours.
After soaking for 24 hours in the bleach solution, I mix a solution of 2 cups tap water and 1 TSP of Seachem Safe. Safe is the same thing as Prime, but it comes in a dry powder form and 250 grams treats up to 60,000 gallons. Compare that to a 500ml bottle of Prime, which treats only 5,000 gallons. Per Seachem's instructions, you can use any equivalent dechlorinator, just be sure to use a large enough dose to remove all chlorine.
This is the first time the packets shown have been regenerated. After multiple uses, Purigen will begin to show staining or discoloration. I don't recommend re-using it more than 3 or 4 times.