some inorganic (and problematic organic) compounds such as  hydrogen sulfide  (H 2 S), ammonia (NH 3 ), formaldehyde (HCOH), radioisotopes  iodine-131 ( 131 I) and  mercury  (Hg). This property is known as  chemisorption . A  gram  of activated carbon can have a surface area in excess of 500  m 2 ,