Arsenic and iron removal in horizontal subsurface flow constructed wetlands with alternative supporting media: key mechanisms

This work identifies the arsenic and iron removal mechanisms in a horizontal subsurface flow constructed wetlands system with zeolite and limestone as alternative supporting media. To do this, two sequential extraction procedures were performed in media samples obtained after the system treated highly acidic, metal and arsenic rich contaminated water. Advanced techniques were also employed to detect arsenic and metals in the media samples and discover possible changes in their composition. According to the arsenic fractioning revealed by the first procedure, which targets arsenic only, the main arsenic removal mechanisms depended on the media type. In limestone: coprecipitation with very amorphous iron oxyhydroxides and carbonates, since of the total extracted (982 mg As/kg), the fraction associated with these phases was 51.8 %; whereas in zeolite, of the total extracted (400 mg As/kg), the adsorbed fraction was 79.4 %. Conversely, the second procedure, which targets metals, indicated that the main mechanism was bonding to iron and manganese oxides in both media types, since this fraction was 78.8 % of the total extracted in zeolite (211 mg As/kg), and 68.9 % of the total extracted in limestone (272 mg As/kg) (average values). The iron fractioning indicated that iron was mainly removed by precipitation of very amorphous and amorphous iron oxyhydroxides in zeolite and limestone cells. Changes in the composition of both media were detected after being used as wetland media. To elucidate arsenic fractioning, sequential extraction procedures targeting arsenic instead of metals must be employed.