Planted and unplanted recirculating constructed wetland mesocosms maintained under non-aerated (reducing) and aerated (oxidizing) conditions were compared in their ability to remove 1 mg/L As(V) from simulated domestic wastewater over a 33-week period. Our results show that non-aerated mesocosms removed more arsenic than aerated mesocosms from the wastewater. For the first time in the literature, we estimated what proportion of arsenic was found in gravel detached biofilms and incorporated this aspect into mass balance calculations. We discovered biofilms from both types of mesocosms contained a higher proportion of arsenic than plants, and that the proportion of arsenic found in non-aerated biofilms was higher than for aerated biofilms. We discovered that non-aerated biofilms contained a higher concentration of iron than aerated mesocosms and this was attributable to the formation of biogenic pyrite. There was no formation of arsenic adsorbing biogenic minerals found in biofilms from aerated mesocosms. Although some of the arsenic removal in both types of wetlands was attributable to adsorption onto substrates and biofilms, removal of arsenic in the non-aerated mesocosms was likely enhanced by the presence of pyrite in the non-aerated mesocosms where arsenic adsorbed to the pyrite and/or co-precipitation of arsenic occurred with iron oxides that formed on the surface of the pyrite. The removal of arsenic through the formation of orpiment and arsenian pyrite was also possible in the non-aerated mesocosms but these minerals were not directly detected.