Dynamic in situ detection in iRhizo-Chip reveals diurnal fluctuations of Bacillus subtilis in the rhizosphere.

Effective colonization by microbe in the rhizosphere is critical for establishing a beneficial symbiotic relationship with the host plant. Bacillus subtilis, a soil-dwelling bacterium that is commonly found in association with plants and their rhizosphere, has garnered interest for its potential to enhance plant growth, suppress pathogens, and contribute to sustainable agricultural practices. However, research on the dynamic distribution of B. subtilis within the rhizosphere and its interaction mechanisms with plant roots remains insufficient due to limitations in existing in situ detection methodologies. To achieve dynamic in situ detection of the rhizosphere environment, we established iRhizo-Chip, a microfluidics-based platform. Using this device to investigate microbial behavior within the rhizosphere, we found obvious diurnal fluctuations in the growth of B. subtilis in the rhizosphere. Temporal dynamic analysis of rhizosphere dissolved oxygen (DO), pH, dissolved organic carbon, and reactive oxygen species showed that diurnal fluctuations in the growth of B. subtilis are potentially related to a variety of environmental factors. Spatial dynamic analysis also showed that the spatial distribution changes of B. subtilis and DO and pH were similar. Subsequently, through in vitro control experiments, we proved that rhizosphere DO and pH are the main driving forces for diurnal fluctuations in the growth of B. subtilis. Our results show that the growth of B. subtilis is driven by rhizosphere DO and pH, resulting in diurnal fluctuations, and iRhizo-Chip is a valuable tool for studying plant rhizosphere dynamics.

Radial Oxygen Loss Triggers Diel Fluctuation of Cadmium Dissolution in the Rhizosphere of Rice.

Cadmium (Cd) contamination poses a significant global threat to human health, primarily through dietary intake, with rice serving as a major source. While Cd predominantly resides in bound states in soil, the physiological processes by which rice facilitates Cd absorption in the rhizosphere remain largely elusive. This study delves into the mechanisms governing Cd uptake by rice plants in the rhizosphere, emphasizing the impact of daytime and nighttime fluctuations in microenvironmental conditions. Employing a microfluidic chip setup, the research reveals that radial oxygen loss from rice roots triggers dissolution of Cd in the rhizosphere. Notably, Cd mobility exhibits distinct diurnal fluctuations, peaking at 44.0 ± 4.1 nM during the daytime and dropping to 8.3 ± 1.3 nM during the nighttime. Further investigations reveal that variations in dissolved oxygen and hydroxyl radical concentrations influence Cd release, while pH changes and microbial reduction reactions play crucial roles in Cd immobilization. These findings provide insights into the intricate processes governing Cd mobilization in the rice rhizosphere, highlighting the importance of regulating these processes for effective Cd adsorption control in rice crops and safeguarding public health.

Simultaneous Structural and Electronic Engineering on Bi- and Rh-co-doped SrTiO3 for Promoting Photocatalytic Water Splitting.

Activating metal ion-doped oxides as visible-light-responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)-molten (dopants) reaction to synthesize Bi- and Rh-codoped SrTiO3 (SrTiO3:Bi,Rh) particles. Our investigation reveals that SrTiO3:Bi,Rh manifests as single-crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well-stabilized Rh3+ energy state for visible-light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high-concentration and long-lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3:Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9% at 420 nm, setting a new benchmark among Rh-doped-based SrTiO3 materials. Furthermore, when integrated into an all-solid-state Z-scheme system with Mo-doped BiVO4 and reduced graphene oxide, SrTiO3:Bi,Rh enabled water splitting with an AQY of 7.1% at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid-molten reaction as a viable approach for this purpose.

Water Vapor Condensation Triggers Simultaneous Oxidation and Hydrolysis of Organic Pollutants on Iron Mineral Surfaces.

Increasing worldwide contamination with organic chemical compounds is a paramount environmental challenge facing humanity. Once they enter nature, pollutants undergo transformative processes that critically shape their environmental impacts and associated risks. This research unveils previously overlooked yet widespread pathways for the transformations of organic pollutants triggered by water vapor condensation, leading to spontaneous oxidation and hydrolysis of organic pollutants. These transformations exhibit variability through either sequential or parallel hydrolysis and oxidation, contingent upon the functional groups within the organic pollutants. For instance, acetylsalicylic acid on the goethite surface underwent sequential hydrolysis and oxidation that first hydrolyzed to salicylic acid followed by hydroxylation oxidation of the benzene moiety driven by the hydroxyl radical (•OH). In contrast, chloramphenicol underwent parallel oxidation and hydrolysis, forming hydroxylated chloramphenicol and 2-amino-1-(4-nitrophenyl)-1,3-propanediol, respectively. The spontaneous oxidation and hydrolysis occurred consistently on three naturally abundant iron minerals with the key factors being •OH production capacity and surface binding strength. Given the widespread presence of iron minerals on Earth's surface, these spontaneous transformation paths could play a role in the fate and risks of organic pollutants of health concerns.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals.

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

Integrative Active Sites of Cathode for Electron-Oxygen-Proton Coupling To Favor H2O2 Production in a Photoelectrochemical System.

The oxygen reduction process generating H2O2 in the photoelectrochemical (PEC) system is milder and environmentally friendly compared with the traditional anthraquinone process but still lacks the efficient electron-oxygen-proton coupling interfaces to improve H2O2 production efficiency. Here, we propose an integrated active site strategy, that is, designing a hydrophobic C-B-N interface to refine the dearth of electron, oxygen, and proton balance. Computational calculation results show a lower energy barrier for H2O2 production due to synergistic and coupling effects of boron sites for O2 adsorption, nitrogen sites for H+ binding, and the carbon structure for electron transfer, demonstrating theoretically the feasibility of the strategy. Furthermore, we construct a hydrophobic boron- and nitrogen-doped carbon black gas diffusion cathode (BN-CB-PTFE) with graphite carbon dots decorated on a BiVO4 photoanode (BVO/g-CDs) for H2O2 production. Remarkably, this approach achieves a record H2O2 production rate (9.24 µmol min-1 cm-2) at the PEC cathode. The BN-CB-PTFE cathode exhibits an outstanding Faraday efficiency for H2O2 production of ∼100%. The newly formed h-BN integrative active site can not only adsorb more O2 but also significantly improve the electron and proton transfer. Unexpectedly, coupling BVO/g-CDs with the BN-CB-PTFE gas diffusion cathode also achieves a record H2O2 production rate (6.60 µmol min-1 cm-2) at the PEC photoanode. This study opens new insight into integrative active sites for electron-O2-proton coupling in a PEC H2O2 production system that may be meaningful for environment and energy applications.

Natural Disinfection-like Process Unveiled in Soil Microenvironments by Enzyme-Catalyzed Chlorination.

Substantial natural chlorination processes are a growing concern in diverse terrestrial ecosystems, occurring through abiotic redox reactions or biological enzymatic reactions. Among these, exoenzymatically mediated chlorination is suggested to be an important pathway for producing organochlorines and converting chloride ions (Cl-) to reactive chlorine species (RCS) in the presence of reactive oxygen species like hydrogen peroxide (H2O2). However, the role of natural enzymatic chlorination in antibacterial activity occurring in soil microenvironments remains unexplored. Here, we conceptualized that heme-containing chloroperoxidase (CPO)-catalyzed chlorination functions as a naturally occurring disinfection process in soils. Combining antimicrobial experiments and microfluidic chip-based fluorescence imaging, we showed that the enzymatic chlorination process exhibited significantly enhanced antibacterial activity against Escherichia coli and Bacillus subtilis compared to H2O2. This enhancement was primarily attributed to in situ-formed RCS. Based on semiquantitative imaging of RCS distribution using a fluorescence probe, the effective distance of this antibacterial effect was estimated to be approximately 2 mm. Ultrahigh-resolution mass spectrometry analysis showed over 97% similarity between chlorine-containing formulas from CPO-catalyzed chlorination and abiotic chlorination (by sodium hypochlorite) of model dissolved organic matter, indicating a natural source of disinfection byproduct analogues. Our findings unveil a novel natural disinfection process in soils mediated by indigenous enzymes, which effectively links chlorine-carbon interactions and reactive species dynamics.

Field Quantification of Hydroxyl Radicals by Flow-Injection Chemiluminescence Analysis with a Portable Device.

Hydroxyl radical (•OH) is a powerful oxidant abundantly found in nature and plays a central role in numerous environmental processes. On-site detection of •OH is highly desirable for real-time assessments of •OH-centered processes and yet is restrained by a lack of an analysis system suitable for field applications. Here, we report the development of a flow-injection chemiluminescence analysis (FIA-CL) system for the continuous field detection of •OH. The system is based on the reaction of •OH with phthalhydrazide to generate 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence (CL) when oxidatively activated by H2O2 and Cu3+. The FIA-CL system was successfully validated using the Fenton reaction as a standard •OH source. Unlike traditional absorbance- or fluorescence-based methods, CL detection could minimize interference from an environmental medium (e.g., organic matter), therefore attaining highly sensitive •OH detection (limits of detection and quantification = 0.035 and 0.12 nM, respectively). The broad applications of FIA-CL were illustrated for on-site 24 h detection of •OH produced from photochemical processes in lake water and air, where the temporal variations on •OH productions (1.0-12.2 nM in water and 1.5-37.1 × 107 cm-3 in air) agreed well with sunlight photon flux. Further, the FIA-CL system enabled field 24 h field analysis of •OH productions from the oxidation of reduced substances triggered by tidal fluctuations in coastal soils. The superior analytical capability of the FIA-CL system opens new opportunities for monitoring •OH dynamics under field conditions.

Wavelength-dependent direct and indirect photochemical transformations of organic pollutants.

Sunlight-induced photochemical transformations greatly affect the persistence of organic pollutants in natural environment. Whereas sunlight intensity is well-known to affect pollutant phototransformation rates, the reliance of pollutant phototransformation kinetics on sunlight spectrum remains poorly understood, which may greatly vary under different spatial-temporal, water matrix, and climatic conditions. Here, we systematically assessed the wavelength-dependent direct and indirect phototransformations of 12 organic pollutants. Their phototransformation rates dramatically decreased with light wavelength increasing from 375 to 632 nm, with direct photolysis displaying higher wavelength-dependence than indirect photolysis. Remarkably, UV light dominated both direct (90.4-99.5 %) and indirect (64.6-98.7 %) photochemical transformations of all investigated organic pollutants, despite its minor portion in sunlight spectrum (e.g., 6.5 % on March 20 at the equator). Based on wavelength-dependent rate constant spectrum, the predicted phototransformation rate of chloramphenicol (4.5 ± 0.7 × 10-4 s-1) agreed well with the observed rate under outdoor sunlight irradiation (4.3 ± 0.0 × 10-4 s-1), and there is no significant difference between the predicted rate and the observed rate (p-value = 0.132). Moreover, rate constant and quantum yield coefficient (QYC) spectrum could be applied for facilely investigate the influence of spectral changes on the phototransformation of pollutants under varying spatial-temporal (e.g., season, latitude) and climatic conditions (e.g., cloud cover). Our study highlights the wavelength-dependence of both direct and indirect phototransformation of pollutants, and the UV part of natural sunlight plays a decisive role in the phototransformation of pollutants.

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation.

Reactive oxygen species (ROS) are widespread in nature and play central roles in numerous biogeochemical processes and pollutant dynamics. Recent studies have revealed ROS productions triggered by electron transfer from naturally abundant reduced iron minerals to oxygen. Here, we report that ROS productions from pyrite oxidation exhibit a high facet dependence. Pyrites with various facet compositions displayed distinct efficiencies in producing superoxide (O2• -), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). The 48 h •OH production rates varied by 3.1-fold from 11.7 ± 0.4 to 36.2 ± 0.6 nM h-1, showing a strong correlation with the ratio of the {210} facet. Such facet dependence in ROS productions primarily stems from the different surface electron-donating capacities (2.2-8.6 mmol e- g-1) and kinetics (from 1.2 × 10-4 to 5.8 × 10-4 s-1) of various faceted pyrites. Further, the Fenton-like activity also displayed 10.1-fold variations among faceted pyrites, contributing to the facet depedence of •OH productions. The facet dependence of ROS production can greatly affect ROS-driven pollutant transformations. As a paradigm, the degradation rates of carbamazepine, phenol, and bisphenol A varied by 3.5-5.3-fold from oxidation of pyrites with different facet compositions, where the kinetics were in good agreement with the pyrite {210} facet ratio. These findings highlight the crucial role of facet composition in determining ROS production and subsequent ROS-driven reactions during iron mineral oxidation.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet.

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Sequential anodic oxidation and cathodic electro-Fenton in the Janus electrified membrane for reagent-free degradation of pollutants.

Electrified membrane technologies have recently demonstrated high potential in tackling water pollution, yet their practical applications are challenged by relying on large precursor doses. Here, we developed a Janus porous membrane (JPEM) with synergic direct oxidation by Magnéli phase Ti4O7 anode and electro-Fenton reactions by CuFe2O4 cathode. Organic pollutants were first directly oxidized on the Ti4O7 anode, where the extracted electrons from pollutants were transported to the cathode for electro-Fenton production of hydroxyl radical (·OH). The cathodic ·OH further enhanced the mineralization of organic pollutant degradation intermediates. With the sequential anodic and cathodic oxidation processes, the reagent-free JPEM showed competitive performance in rapid degradation (removal rate of 0.417 mg L-1 s-1) and mineralization (68.7 % decrease in TOC) of sulfamethoxazole. The JPEM system displayed general performance to remove phenol, carbamazepine, and perfluorooctanoic acid. The JPEM runs solely on electricity and oxygen that is comparable to that of PEM relies on large precursor doses and, therefore, operation friendly and environmental sustainability. The high pollutant removal and mineralization achieved by rational design of the reaction processes sheds light on a new approach for constructing an efficient electrified membrane.

Long-Term Exposure of Graphene Oxide Suspension to Air Leading to Spontaneous Radical-Driven Degradation.

Understanding the environmental transformation and fate of graphene oxide (GO) is critical to estimate its engineering applications and ecological risks. While there have been numerous investigations on the physicochemical stability of GO in prolonged air-exposed solution, the potential generation of reactive radicals and their impact on the structure of GO remain unexplored. In this study, using liquid-PeakForce-mode atomic force microscopy and quadrupole time-of-flight mass spectroscopy, we report that prolonged exposure of GO to the solution leads to the generation of nanopores in the 2D network and may even cause the disintegration of its bulk structure into fragment molecules. These fragments can assemble themselves into films with the same height as the GO at the interface. Further mediated electrochemical analysis supports that the electron-donating active components of GO facilitate the conversion of O2 to •O2- radicals on the GO surface, which are subsequently converted to H2O2, ultimately leading to the formation of •OH. We experimentally confirmed that attacks from •OH radicals can break down the C-C bond network of GO, resulting in the degradation of GO into small fragment molecules. Our findings suggest that GO can exhibit chemical instability when released into aqueous solutions for prolonged periods of time, undergoing transformation into fragment molecules through self-generated •OH radicals. This finding not only sheds light on the distinctive fate of GO-based nanomaterials but also offers a guideline for their engineering applications as advanced materials.

Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions.

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41-59% and 55-77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63-78% of Cd was associated with Fe minerals, while 3-12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

Water Vapor Condensation on Iron Minerals Spontaneously Produces Hydroxyl Radical.

The hydroxyl radical (•OH) is a potent oxidant and key reactive species in mediating element cycles and pollutant dynamics in the natural environment. The natural source of •OH is historically linked to photochemical processes (e.g., photoactivation of natural organic matter or iron minerals) or redox chemical processes (e.g., reaction of microbe-excreted or reduced iron/natural organic matter/sulfide-released electrons with O2 in soils and sediments). This study revealed a ubiquitous source of •OH production via water vapor condensation on iron mineral surfaces. Distinct •OH productions (15-478 nM via water vapor condensation) were observed on all investigated iron minerals of abundant natural occurrence (i.e., goethite, hematite, and magnetite). The spontaneous •OH productions were triggered by contact electrification and Fenton-like activation of hydrogen peroxide (H2O2) at the water-iron mineral interface. Those •OH drove efficient transformation of organic pollutants associated on iron mineral surfaces. After 240 cycles of water vapor condensation and evaporation, bisphenol A and carbamazepine degraded by 25%-100% and 16%-51%, respectively, forming •OH-mediated arene/alkene hydroxylation products. Our findings largely broaden the natural source of •OH. Given the ubiquitous existence of iron minerals on Earth's surface, those newly discovered •OH could play a role in the transformation of pollutants and organic carbon associated with iron mineral surfaces.

Spontaneous Oxidation of Thiols and Thioether at the Air-Water Interface of a Sea Spray Microdroplet.

The transport of dissolved organic sulfur, including thiols and thioethers, from the ocean surface to the atmosphere through sea spray aerosol (SSA) is of great importance for the global sulfur cycle. Thiol/thioether in SSA undergoes rapid oxidation that is historically linked to photochemical processes. Here, we report the discovery of a non-photochemical, spontaneous path of thiol/thioether oxidation in SSA. Among 10 investigated naturally abundant thiol/thioether, seven species displayed rapid oxidation in SSA, with disulfide, sulfoxide, and sulfone comprising the major products. We suggest that such spontaneous oxidation of thiol/thioether was mainly fueled by thiol/thioether enrichment at the air-water interface and generation of highly reactive radicals by the loss of an electron from ions (e.g., glutathionyl radical produced from ionization of deprotonated glutathione) at or near the surface of the water microdroplet. Our work sheds light on a ubiquitous but previously overlooked pathway of thiol/thioether oxidation, which could contribute to an accelerated sulfur cycle as well as related metal transformation (e.g., mercury) at ocean-atmosphere interfaces.

Redox Oscillations Activate Thermodynamically Stable Iron Minerals for Enhanced Reactive Oxygen Species Production.

Reactive oxygen species (ROS) play key roles in driving biogeochemical processes. Recent studies have revealed nonphotochemical electron transfer from redox-active substances (e.g., iron minerals) to oxygen as a new route for ROS production. Yet, naturally occurring iron minerals mainly exist in thermodynamically stable forms, restraining their potential for driving ROS production. Here, we report that tide-induced redox oscillations can activate thermodynamically stable iron minerals for enhanced ROS production. •OH production in intertidal soils (15.8 ± 0.5 µmol/m2) was found to be 5.9-fold more efficient than those in supratidal soils. Moreover, incubation of supratidal soils under tidal redox fluctuations dramatically enhanced •OH production by 4.3-fold. The tidal hydrology triggered redox alternation between biotic reduction and abiotic oxidation and could accelerate the production of reactive ferrous ions and amorphous ferric oxyhydroxides, making thermodynamically stable iron minerals into redox-active metastable iron phases (RAMPs) with reduced crystallinity and promoting surface electrochemical activities. Those RAMPs displayed enhanced redox activity for ROS production. Investigations of nationwide coastal soils verified that tide-induced redox oscillations could ubiquitously activate soils for enhanced ROS production. Our study demonstrates the effective formation of RAMPs from redox oscillations by hydrological perturbations, which provides new insights into natural ROS sources.

Probing Molecular-Level Dynamic Interactions of Dissolved Organic Matter with Iron Oxyhydroxide via a Coupled Microfluidic Reactor and an Online High-Resolution Mass Spectrometry System.

The interactions between dissolved organic matter (DOM) and iron (Fe) oxyhydroxide are crucial in regulating the biogeochemical cycling of nutrients and elements, including the preservation of carbon in soils. The mechanisms of DOM molecular assembly on mineral surfaces have been extensively studied at the mesoscale with equilibrium experiments, yet the molecular-level evolution of the DOM-mineral interface under dynamic interaction conditions is not fully understood. Here, we designed a microfluidic reactor coupled with an online solid phase extraction (SPE)-LC-QTOF MS system to continually monitor the changes in DOM composition during flowing contact with Fe oxyhydroxide at circumneutral pH, which simulates soil minerals interacting with constant DOM input. Time-series UV-visible absorption spectra and mass spectrometry data showed that after aromatic DOM moieties were first preferentially sequestered by the pristine Fe oxyhydroxide surface, the adsorption of nonaromatic DOM molecules with greater hydrophobicity, lower acidity, and lower molecular weights (<400) from new DOM solutions was favored. This is accompanied by a transition from mineral surface chemistry-dominated adsorption to organic-organic interaction-dominated adsorption. These findings provide direct molecular-level evidence to the zonal model of DOM assembly on mineral surfaces by taking the dynamics of interfacial interactions into consideration. This study also shows that coupled microfluidics and online high-resolution mass spectrometry (HRMS) system is a promising experimental platform for probing microscale environmental carbon dynamics by integrating in situ reactions, sample pretreatment, and automatic analysis.

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

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 (H2O2) 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.

Assessing the quantum yield spectrum of photochemically produced reactive intermediates from black carbon of various sources and properties.

Black carbon (BC) is ubiquitous in sunlit waters and atomosphere. Recent studies revealed that under sunlight irradiation BC is photoactive on producing photochemically produced reactive intermediates (PPRIs), a group of key species in accelerating earth's surface biogeochemical processes and pollutant dynamics. Nevertheless, reported PPRIs productions from BC exhibit large inconsistency and the intrinsic capacities of BC in producing PPRIs remain poorly characterized. This work provided a wavelength-dependent quantum yields (QYs) assessment of four environmentally-relevant PPRIs (excited triplet state BC (3BC*), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (·OH)) from various BC. The QYs of all investigated PPRIs exhibit high dependence on incident light wavelength. For instance, the QYs of 1O2 dramatically decreased from 4.4% to 0.4% with light wavelength increasing from 375 to 490 nm and decreased to 0 above 490 nm. Suprisingly, PPRIs QYs only varied by 2.0-2.5-fold among BC prepared from different biomasses (i.e., pine needle, shell, straw, and wood), while the pyrolysis temperature and size of BC demonstrate higher impacts on the PPRIs QYs by up to 30.3- and 7.1-fold variations, respectively. Analyses on the physicochemical properties of BC demonstrate that QYs of 3BC* and 1O2 were linked to the optical properties of BC, while the QYs of H2O2 and ·OH were determined by multiple factors including the surface redox characteristics. Further, PPRIs productions from BC follow similar paths and efficiencies compared to those from natural organic matter. The revealed QYs of BC-derived PPRIs establish a key basis for evaluating PPRIs-mediated element cycles and pollutant transformation in natural waters, which are becoming increasingly important in the context of higher BC input from more frequent wildfires and artificial sources.