Production of hydroxyl radicals from Fe(II) oxygenation induced by groundwater table fluctuations in a sand column

Natural and artificial processes often cause the fluctuation of groundwater table, inducing the interaction of O₂ from the unsaturated zone with reduced components such as Fe(II) from the saturated zone. In light of previous findings that hydroxyl radicals (OH) can be produced from Fe(II) oxygenation, we hypothesize that OH could be produced during groundwater table fluctuations. Therefore, this study aims to measure the production of OH during water table fluctuations in a simulated sand column. Deoxygenated water in the absence and presence of 20mg/L Fe²⁺ (pH6.5) was fed into the sand column. Water table fluctuations were manipulated to observe O₂ entrapment, Fe²⁺oxygenation and OH production. Results showed that O₂ in the pore air was efficiently entrapped by the rise of water table at the tested rates of 0.16-0.34cm/min (or 0.10-0.20m/h), and the dissolution of entrapped O₂ into the pore water led to the oxygenation of Fe²⁺. Production of OH was presumably attributed to oxygenation of the Fe²⁺ adsorbed on Fe(III) oxyhydroxides generated in situ. In a total of 4cycles of fluctuations, the cumulative OH at all the elevations increased progressively, attaining 2.7μM in the zone near the water table in the 4th cycle. We suggest that OH produced from water table fluctuations could induce an overlooked pathway for contaminant transformation in the fluctuation zone.