Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments

Hydroxyl radicals (circle OH) play a crucial role in the fate of redox-active substances in the environment. Studies of the circle OH production in nature has been constrained to surface environments exposed to light irradiation, but is overlooked in the subsurface under dark. Results of this study demonstrate that abundant circle OH is produced when subsurface sediments are oxygenated under fluctuating redox conditions at neutral pH values. The cumulative concentrations of circle OH produced within 24 h upon oxygenation of 33 sediments sampled from different redox conditions are 2-670 mu mol circle OH per kg dry sediment or 6.7-2521 mu M circle OH in sediment pore water. Fe(II)containing minerals, particularly phyllosilicates, are the predominant contributor to circle OH production. This production could be sustainable when sediment Fe(II) is regenerated by the biological reduction of Fe(III) during redox cycles. Production of circle OH is further evident in a field injection-extraction test through injecting oxygenated water into a 23-m depth aquifer. The circle OH produced can oxidize pollutants such as arsenic and tetracycline and contribute to CO2 emissions at levels that are comparable with soil respiration. These findings indicate that oxygenation of subsurface sediments is an important source of circle OH in nature that has not been previously identified, and circle OH-mediated oxidation represents an overlooked process for substance transformations at the oxic/ anoxic interface.