Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments

Abstract

Hydroxyl radicals (•OH) play a crucial role in the fate of redox-active substances in the environment. Studies of the •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 •OH is produced when subsurface sediments are oxygenated under fluctuating redox conditions at neutral pH values. The cumulative concentrations of •OH produced within 24 h upon oxygenation of 33 sediments sampled from different redox conditions are 2-670 μmol •OH per kg dry sediment or 6.7-2521 μM •OH in sediment pore water. Fe(II)-containing minerals, particularly phyllosilicates, are the predominant contributor to •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 •OH is further evident in a field injection-extraction test through injecting oxygenated water into a 23-m depth aquifer. The •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 •OH in nature that has not been previously identified, and •OH-mediated oxidation represents an overlooked process for substance transformations at the oxic/anoxic interface.

Biochar-Supported FeS/Fe3O4 Composite for Catalyzed Fenton-Type Degradation of Ciprofloxacin

“…The total aqueous Fe was examined by atomic absorption spectrometry (Shimadzu AA-7000, Japan). The accumulative production of ·OH was measured using benzoic acid as the probe, and the production of 1 mol of p-hydroxybenzoic acid (p-HBA) from this reaction was reported to consume 5.87 mol of ·OH [56]. The p-HBA was measured with HPLC using the mixture of acetonitrile/water (containing 0.15% acetic acid) = 60/40 (v/v) as the mobile phase at a flow rate of 1 mL/min, and using UV detection at 254 nm.…”

The Controls of Iron and Oxygen on Hydroxyl Radical (•OH) Production in Soils

“…This assumption likely results in conservative estimates of •OH production because the measured •OH during each period of the experiment is a net of •OH production and consumption given fast quenching and reaction rates of •OH with soil constituents [42]. In addition, considering that •OH was measured only from soil water flushed from the soils, •OH production from colloids or particles retained in the soils [7] was likely not detected. Thus, it is likely that more •OH was produced in the soil waters than detected during both periods of the experiment.…”

“…High Fe(II) concentrations suggest that Fe(II) production outpaces its consumption by O 2 (and thus outpaces the O 2 supply rate) during static, waterlogged conditions. However, the oxidative consumption of Fe(II) by O 2 is the source of •OH [2,3,7]. Thus, if the O 2 supply rate was similar to the Fe(II) production rate, then the •OH production could be higher compared to conditions when O 2 supply rates are lower than Fe(II) production.…”

“…Preliminary estimates indicated that on a landscape scale the oxidation of DOC to CO 2 by •OH in Alaskan Arctic soil waters is on the same order of magnitude as microbial respiration of DOC in the surface waters draining the same soils [2]. Thus, this iron-mediated abiotic oxidation of DOC may be an important component of local and regional carbon budgets in the Arctic [2,3] or at any terrestrial-aquatic interface with waterlogged soils and strong redox gradients [5][6][7]. However, the preliminary estimates of •OH's impact on DOC oxidation in arctic soil waters were based on two untested assumptions [2].…”

Insight into Different Mechanisms for Oxidation of Liquid and Gaseous Pollutants by Bi−NaBiO3 with or without Visible Light Illumination

“…NBT (2.5 × 10 À 2 mM), FFA (0.01 mM) and benzoic acid (50 μM) were selected as a molecular probe to test the concentration of * O 2 À , [44] 1 O 2 [45] and * OH. [46] 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57…”