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.

Cu-based catalysts have attracted much interest in CO 2 hydrogenation to methanol because of their high activity. However, the effect of interface, coordination structure, particle size and other underlying factors existed in heterogeneous catalysts render to complex active sites on its surface, therefore it is di cult to study the real active sites for methanol synthesis. Here, we report a novel Cu-based catalyst with isolated Cu active sites (Cu 1 -O 3 units) for highly selective hydrogenating CO 2 to methanol at low temperature (100% selectivity for methanol at 180 o C). Experimental and theoretical results reveal that the single-atom Cu-Zr catalyst with Cu 1 -O 3 units is only contributed to synthesize methanol at 180 o C, but the Cu clusters or nanoparticles with Cu-Cu or Cu-O-Cu active sites will promote the process of reverse water gas shift (RWGS) side reaction to form undesirable byproducts CO. Furthermore, the Cu 1 -O 3 units with tetrahedral structure could gradually migrate to the catalyst surface for accelerating CO 2 hydrogenation reaction during catalytic process. The high activity isolated Cu-based catalyst with legible structure will be helpful to understand the real active sites of Cu-based catalysts for methanol synthesis from CO 2 hydrogenation, thereby guiding further design the Cu catalyst with high performance to meet the industrial demand, at the same time as extending the horizontal of single atom catalyst for application in the thermal catalytic process of CO 2 hydrogenation.

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.

No abstract