Fate and distribution of arsenic in a process-designed pilot-scale constructed wetland treatment system

The fate and distribution of arsenic in simulated groundwater was determined in a pilot-scale constructed wetland treatment system (CWTS) designed to promote specific biogeochemical processes for arsenic removal. Two CWTS series were designed to promote co-precipitation and sorption of arsenic with iron oxyhydroxides under oxidizing conditions, and two series were designed to promote precipitation of arsenic with sulfide and co-precipitation of arsenic with iron sulfide under reducing conditions. Measured conditions in the CWTS were within ranges favorable for the targeted processes. Arsenic removal was significantly greater (α = 0.05) in an oxidizing series amended with zero-valent iron (ZVI) than in the other series, with removal extents, efficiencies, and rate coefficients ranging from 6 to 79 μg L−1, 40 to 95%, and 0.13 to 0.77 d−1, respectively. The majority of inflow arsenic retained in the first reactor of each series partitioned to the sediment (88-99%), while the remainder partitioned to Typha latifolia. A greater percentage of inflow arsenic was retained in the sediment of the first reactor of the two oxidizing series (20 and 13%) than in the first reactor of the two reducing series (6 and 7%). Addition of ZVI enhanced arsenic removal from the aqueous phase in both oxidizing series and reducing series and increased the percentage of inflow arsenic partitioned to sediment. A vertical concentration gradient developed over time in the sediment, with 74-85% of sediment-bound arsenic accumulated in the upper 6 cm and the remaining percentage below 6 cm. Results from this study demonstrate that a CWTS can decrease the concentration of arsenic in simulated groundwater to below the World Health Organization (WHO) drinking water quality guideline of 10 μg L−1 primarily by transferring arsenic from the aqueous phase to the sediment.