I met my cousin's boyfriend tonight and he just happens to be a chemist, so I asked him some questions about activated carbon. Does carbon leach things back into the water? Yes. When the receptor sites fill up, things with a stronger affinity for the carbon will knock out things with a weaker affinity. High affinity things will remain locked up in the carbon indefinitely. Is what is leached any different than what is adsorbed? No, there is no real chemical reaction that takes place, so what is forced out is exactly the same as what was adsorbed. Can carbon spontaneously dump all that it has adsorbed? Yes, if heated to 700 C in an oxygen free environment. So under normal conditions? Absolutely not. How long is it effective for? It's not a matter of time. The carbon has a certain amount of receptor sites, so how long it takes to fill up is a function of the concentration and the rate of introduction of the impurities.