Biogenic activated carbon compositions disclosed herein comprise at least 55 wt% carbon, some of which may be present as graphene, and have high surface areas, such as Iodine Numbers of greater than 2000. Some embodiments provide biogenic activated carbon that is responsive to a magnetic field. A continuous process for producing biogenic activated carbon comprises countercurrently contacting, by mechanical means, a feedstock with a vapor stream comprising an activation agent including water and/or carbon dioxide; removing vapor from the reaction zone; recycling at least some of the separated vapor stream, or a thermally treated form thereof, to an inlet of the reaction zone(s) and/or to the feedstock; and recovering solids from the reaction zone(s) as biogenic activated carbon. Methods of using the biogenic activated carbon are disclosed.