Removal of arsenic from simulated groundwater using a pilot-scale constructed wetland treatment system

A pilot-scale constructed wetland treatment system (CWTS) was designed and built to produce biogeochemical conditions that promoted targeted processes for removal of arsenic from simulated groundwater. Two 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 sulfide and co-precipitation of arsenic with iron sulfide under reducing conditions. The two major objectives were to (1) assess arsenic removal performance and (2) determine the fate and distribution of arsenic in each series. Results indicate that arsenic removal performance was greater in series designed to promote oxidizing conditions than in series designed to promote reducing conditions (mean removal extent of 64 and 108 mug L-1, respectively). Arsenic removal performance was significantly greater (alpha = 0.05) in the oxidizing series amended with zero-valent iron (ZVI) than in the other series, with removal extents, efficiencies, and rate coefficients ranging from 6-79 mug L-1, 40-95 %, and 0.13-0.77 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 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 of this study demonstrate that a CWTS can be used successfully to decrease the concentration of arsenic in simulated groundwater to below the World Health Organization (WHO) drinking water quality guideline primarily by transferring arsenic from the aqueous phase to the sediment.