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Abstract

Anthropogenic trace organic contaminants (TrOCs) such as pharmaceuticals and pesticides negatively affect human and environmental health. Wastewater treatment plant effluent and combined sewer overflows (CSOs) are primary pathways for the introduction of TrOCs to water bodies. Most treatments available to reduce TrOC releases are costly and energy intensive. Constructed wetlands (CWs) are low cost, passive wastewater treatment technologies that have been implemented to treat a myriad of wastewaters including domestic effluent and CSOs and have gained scholarly attention in the last two decades as promising technologies for TrOC attenuation. Subsurface flow CWs, with minimal overland flow, treat TrOCs through parallel mechanisms including sorption and biodegradation. Amendments, materials added to the CW soil, improve CW performance by enhancing sorption and biodegradation. To address recalcitrant contaminants, we investigated an aqueous iron ligand amendment (Fe-EDTA) which can catalyze the Fenton reaction to produce hydroxyl radicals (HO·) in saturated subsurface environments that experience oxic-anoxic cycling. We identified vertical flow CWs – subsurface systems in which influent water flows top down – to best meet the environmental requirements for the amendment. We amended sealed, anoxic CW microcosms containing different soils and solutions with Fe-EDTA. We subsequently oxidized the microcosms and quantified their HO· yields as 43.32 ± 17.48 to 264.15 ± 24.59 µmol/kg. The magnitude of HO· production was directly related to the aging of the soil and to a lesser extent related to the background constituents in the solution. Our results show that iron amendments enhance abiotic and biotic degradation mechanisms and improve recalcitrant TrOC treatment in CWs. Iron-amended soil columns (n=5/9 columns fed 7 months with synthetic domestic wastewater) operated with a removal efficiency of 12 ± 19% of the most recalcitrant TrOC, 18% higher than the control columns. Operating the columns with periods of retention and discharge further improved this removal efficiency to 49 ± 7.6%, 35% higher than corresponding unamended columns. The TrOC removal was associated with the anoxic environmental conditions that occurred because of the amendment; more dissolved oxygen consumption was likely caused by heighted microbial activity in the amended systems.