I want to generate CO2 for some purpose, I don't want to generate it by chemical reaction because I do not want to add CO2 to the environment so I thought of getting it back from some used activated carbon filters used for removing CO2 example 'removal from fruit storage halls etc.
For that purpose I bought Activated Carbon, produced CO2 by simply reacting citric acid, baking soda.and water. Somehow I managed to pass CO2 through Activated Carbon.
Now I am asking about how to get it back
You may perform carbon reactivation procedure to release the CO2 back to the environment from the spent carbon. Thermal reactivation of carbon required you to heat it in a high temperature reactivation furnace to over 800°C.
You need to liquefy CO2 gas to liquid (cooling) then dissolve it water to get it back
Whatever you do to recover CO2 from activated carbon, you will consume energy and generate additional CO2. If your goal is to remove CO2 from fruit storage halls, then read about carbon capture and sequestration technologies.You may start from https://en.wikipedia.org/wiki/Carbon_capture_and_storage