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.

A novel electro-Fenton process was developed for wastewater treatment using a modified divided electrolytic system in which H2O2 was generated in situ from electro-generated H2 and O2 in the presence of Pd/C catalyst. Appropriate pH conditions were obtained by the excessive H+ produced at the anode. The performance of the novel process was assessed by Rhodamine B (RhB) degradation in an aqueous solution. Experimental results showed that the accumulation of H2O2 occurred when the pH decreased and time elapsed. The maximum concentration of H2O2 reached 53.1 mg/L within 120 min at pH 2 and a current of 100 mA. Upon the formation of the Fenton reagent by the addition of Fe2+, RhB degraded completely within 30 min at pH 2 with a pseudo first order rate constant of 0.109 ± 0.009 min(-1). An insignificant decline in H2O2 generation and RhB degradation was found after six repetitions. RhB degradation was achieved by the chemisorption of H2O2 on the Pd/C surface, which subsequently decomposed into •OH upon catalysis by Pd0 and Fe2+. The catalytic decomposition of H2O2 to •OH by Fe2+ was more powerful than that by Pd0, which was responsible for the high efficiency of this novel electro-Fenton process.

It has been documented that contaminants could be degraded by hydroxyl radicals (•OH) produced upon oxygenation of Fe­(II)-bearing sediments. However, the dependence of contaminant degradation on sediment characteristics, particularly Fe­(II) species, remains elusive. Here we assessed the impact of the abundance of Fe­(II) species in sediments on contaminant degradation by •OH during oxygenation. Three natural sediments with different Fe­(II) contents and species were oxygenated. During 10 h oxygenation of 200 g/L sediment suspension, 2 mg/L phenol was negligibly degraded for sandbeach sediment (Fe­(II): 9.11 mg/g), but was degraded by 41% and 52% for lakeshore (Fe­(II): 9.81 mg/g) and farmland (Fe­(II): 19.05 mg/g) sediments, respectively. •OH produced from Fe­(II) oxygenation was the key reactive oxidant. Sequential extractions, X-ray diffraction, Mössbauer, and X-ray absorption spectroscopy suggest that surface-adsorbed Fe­(II) and mineral structural Fe­(II) contributed predominantly to •OH production and phenol degradation. Control experiments with specific Fe­(II) species and coordination structure analysis collectively suggest the likely rule that Fe­(II) oxidation rate and its competition for •OH increase with the increase in electron-donating ability of the atoms (i.e., O) complexed to Fe­(II), while the •OH yield decreases accordingly. The Fe­(II) species with a moderate oxidation rate and •OH yield is most favorable for contaminant degradation.

Mackinawite (FeS) nanoparticles have been extensively tested for reducing contaminants under anoxic conditions, while the oxidizing impact induced by FeS under oxic conditions has been largely underestimated. In light of previous findings that hydroxyl radicals (·OH) can be produced from oxygenation of sediment Fe(II), herein we revealed that ·OH can be produced efficiently from FeS oxygenation at circumneutral conditions, yielding 84.7 μmol ·OH per g FeS. Much more ·OH was produced from the oxygenation of FeS compared with siderite, pyrite, and zerovalent iron nanoparticles under the same conditions. The oxidation of FeS was a surface-mediated process, in which O was transformed by the structural Fe(II) on FeS surface to ·OH with the generation of HO intermediate. A small proportion of Fe(II) was regenerated from the reduction of Fe(III) by FeS and S(-II), but this proportion did not significantly contribute to ·OH production. We further validated that the ·OH produced from FeS oxygenation considerably contributed to the oxidation of arsenic. As the change of redox conditions from anoxic to oxic is common in both natural and artificial processes, our findings suggest that the oxidizing impact induced by FeS at oxic conditions should be concerned due to ·OH production.

The important role of natural organic matter (NOM)-Fe colloids in influencing contaminant transport, and this role can be influenced by the formation, aggregation, and particle deposition dynamics of NOM-Fe colloids. In this work, NOM-Fe colloids at different C/Fe ratios were prepared by mixing different concentrations of humic acid (HA) with 10 mg/L Fe(II) under anoxic conditions. The colloids were characterized by an array of techniques and their aggregation and deposition behaviors were examined under both anoxic and oxic conditions. The colloids are composed of HA-Fe(II) at anoxic conditions, while they are made up of HA-Fe(III) at oxic conditions until the C/Fe molar ratio exceeds 1.6. For C/Fe molar ratios above 1.6, the aggregation and deposition kinetics of HA-Fe(II) colloids under anoxic conditions are slower than those of HA-Fe(III) colloids under oxic conditions. Further, the aggregation of HA-Fe colloids under both anoxic and oxic conditions decreases with increasing C/Fe molar ratio from 1.6 to 23.3. This study highlights the importance of the redox transformation of Fe(II) to Fe(III) and the C/Fe ratio for the formation and stability of NOM-Fe colloids that occur in subsurface environments with anoxic-oxic interfaces.