Sunlight-Driven Production of Reactive Oxygen Species from Natural Iron Minerals: Quantum Yield and Wavelength Dependence

Abstract

Photochemically generated reactive oxygen species (ROS) play numerous key roles in earth's surface biogeochemical processes and pollutant dynamics. ROS production has historically been linked to the photosensitization of natural organic matter. Here, we report the photochemical ROS production from three naturally abundant iron minerals. All investigated iron minerals are photoactive toward sunlight irradiation, with photogenerated currents linearly correlated with incident light intensity. Hydroxyl radicals ( • OH) and hydrogen peroxide (H 2 O 2 ) are identified as the major ROS species, with apparent quantum yields ranging from 1.4 × 10 −8 to 3.9 × 10 −8 and 5.8 × 10 −8 to 2.5 × 10 −6 , respectively. Photochemical ROS production exhibits high wavelength dependence, for instance, the • OH quantum yield decreases with the increase of light wavelength from 375 to 425 nm, and above 425 nm it sharply decreases to zero. The temperature shows a positive impact on • OH production, with apparent activation energies ranging from 8.0 to 17.8 kJ/mol. Interestingly, natural iron minerals with impurities exhibit higher ROS production than their pure crystal counterparts. Compared with organic photosensitizers, iron minerals exhibit higher wavelength dependence, higher selectivity, lower efficiency, and long-term stability in photochemical ROS production. Our study highlights natural inorganic iron mineral photochemistry as a ubiquitous yet previously overlooked source of ROS.

Water Vapor Condensation on Iron Minerals Spontaneously Produces Hydroxyl Radical

“…The findings of spontaneous production of • OH through water vapor condensation on iron minerals expand the natural source and spatial-temporal distribution of • OH. Our recent study revealed that under sunlight irradiation iron minerals can photochemically produce • OH (Figure ) with apparent quantum yield from 1.4 × 10 –8 to 3.9 × 10 –8 . As a supplement to the photochemical • OH in the daytime, water vapor condensation and • OH production on iron minerals could occur in the nighttime when temperature drops below the dew point (Figure ).…”

“…Our recent study revealed that under sunlight irradiation iron minerals can photochemically produce • OH (Figure 5) with apparent quantum yield from 1.4 × 10 −8 to 3.9 × 10 −8 . 42 As a supplement to the photochemical • OH in the daytime, water vapor condensation and • OH production on iron minerals could occur in the nighttime when temperature drops below the dew point (Figure 5). We expect this process to ubiquitously occur on iron minerals in the desert as well as in subsurface soils of arid/semiarid areas with high air permeability.…”

New Barrier Role of Iron Plaque: Producing Interfacial Hydroxyl Radicals to Degrade Rhizosphere Pollutants

“…3 In addition to adsorption, the iron minerals are able to produce hydroxyl radicals (•OH) with light irradiation to induce the oxidative transformation of pollutants. 7 The production of •OH by iron minerals is generally thought to be limited to environments with access to light. 7,8 Nevertheless, recent reports have shown that iron minerals are capable of reducing oxygen to produce •OH under dark conditions.…”

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation

“…Reactive oxygen species (ROS) have a pervasive presence in nature and play pivotal roles in regulating environmental processes . Traditionally, ROS productions have been attributed to photochemistry involving organic photosensitizers (such as natural organic matter and black carbon) and inorganic compounds (such as nitrate and iron minerals). Recent advances have shed light on non-photochemical ROS production through electron transfer from reduced substances [such as reduced natural organic matter (NOM) and iron minerals] to molecular oxygen (O 2 ). , One prominent example is pyrite, the most prevalent naturally occurring ferrous sulfide mineral widely distributed in nature, with 0.5–5% typical content in modern sediments, which undergoes oxidation in regions characterized by frequent hydrological purtabations and redox oscillations (e.g., wetland, rice paddy, coast, etc.).…”

“…Reactive oxygen species (ROS) have a pervasive presence in nature and play pivotal roles in regulating environmental processes. 1 Traditionally, ROS productions have been attributed to photochemistry involving organic photosensitizers (such as natural organic matter 2 and black carbon 3 ) and inorganic compounds (such as nitrate 4 and iron minerals 5 ). Recent advances have shed light on non-photochemical ROS production through electron transfer from reduced substances [such as reduced natural organic matter (NOM) and iron minerals] to molecular oxygen (O 2 ).…”