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Electron release and transfer are pivotal to the efficiency of multiple biogeochemical and pollutant processes. Despite substantial efforts to develop electron-transfer characterization techniques, in situ visualization of electron transfer remains challenging. This study introduces an innovative strategy for mapping electron-transfer distance using nanoscale zerovalent iron (nZVI) as a case study. In this method, silver ions (Ag + ) are employed as electron traps, forming silver nanoparticles. These nanoparticles can then be immobilized in the agarose-solidified gel used as a heterogeneous porous medium to simulate the underground aquifer and visualized through photothermal imaging. In situ detection has unveiled remarkably extensive electron transfer, reaching distances of up to centimeters from the nZVI source. Nationwide groundwater investigations have further confirmed this long electron-transfer distance, ranging from 1.7 ± 0.4 to 9.6 ± 0.1 mm (agarose gel may not fully replicate the complex and heterogeneous conditions of groundwater environments). Importantly, the efficiency of electron transfer from nZVI was primarily influenced by factors such as nZVI dosage and groundwater matrix conditions, particularly the abundance of electron-shuttling materials like natural organic matter. Surprisingly, the inherent properties of nZVI had a lesser impact on electron-transfer distance. Our work highlights the long-distance electron transfer from nZVI, revealing a previously overlooked but ubiquitous nZVI remediation process.

Hydrogen peroxide (H 2 O 2 )-based advanced oxidation technology has emerged as a cost-effective and green solution for tackling soil pollution. Given the highly heterogeneous nature of soil, the effectiveness of H 2 O 2 remediation is significantly influenced by its diffusion distance in soils. However, the dynamics of H 2 O 2 diffusion and its effective range remain largely unexplored, primarily due to the lack of analytical methods for mapping H 2 O 2 in soils. This study introduces a precipitation-based fluorescent probe (PFP) method for in situ, high-resolution (micrometer scale) mapping of H 2 O 2 diffusion in soils. Using the PFP method, we visualized real-time H 2 O 2 diffusion in various types of soils, revealing distinct diffusion patterns with rates ranging from 0.011 to >0.56 mm min -1 . The observed differences in diffusion rates are associated with soil permeability. Additionally, soils exhibited a wide range of diffusion distances, from 0.22 to >11 mm in 20 min. Soil's reactivity for H 2 O 2 decomposition, a previously overlooked factor, is critical in determining the diffusion distance of H 2 O 2 . We further demonstrate that the efficacy of H 2 O 2 diffusion in soils is a pivotal factor in controlling pollutant degradation and soil remediation efficiency. These findings enhance our understanding of reagent diffusion processes in soil remediation, informing the optimization of more efficient soil remediation strategies.

Indirect photolysis driven by photochemically produced reactive intermediates (PPRIs) is pivotal for the transformations and fates of pollutants in nature. While well-studied in bulk water, indirect photolysis processes at environmental interfaces remain largely unexplored. This study reveals a significant acceleration of indirect photodegradation of organic pollutants at the soil-water interface of wetlands. Organic pollutants experienced ubiquitously enhanced indirect photodegradation at the soil-water interfaces, with rates 1.41 ± 0.01 to 4.27 ± 0.03-fold higher than those in bulk water. This enhancement was observed across various natural and artificial wetlands, including coastal wetlands and rice paddies. In situ mapping indicated that soil-water interfaces act as hotspots, concentrating both organic pollutants and PPRIs by 9.30- and 4.27-folds, respectively. This synchronized colocation is the primary cause of the accelerated pollutant photolysis. Additionally, the contribution of each PPRI species to pollutant photolysis and a coupled transformation pathway at the soil-water interface significantly differed from those in bulk water. For instance, the contribution of singlet oxygen to metoxuron photolysis increased from 10.1% in bulk water to 44.4% at the soil-water interface. Our study highlights the rapid indirect photolysis of organic pollutants at the soil-water interfaces, offering new insights into the natural purification processes in wetlands as "Earth's kidneys."

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 (H 2 O 2 ) 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 O 2 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.

The oxygen reduction process generating H 2 O 2 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 H 2 O 2 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 H 2 O 2 production due to synergistic and coupling effects of boron sites for O 2 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 BiVO 4 photoanode (BVO/g-CDs) for H 2 O 2 production. Remarkably, this approach achieves a record H 2 O 2 production rate (9.24 μmol min -1 cm -2 ) at the PEC cathode. The BN-CB-PTFE cathode exhibits an outstanding Faraday efficiency for H 2 O 2 production of ∼100%. The newly formed h -BN integrative active site can not only adsorb more O 2 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 H 2 O 2 production rate (6.60 μmol min -1 cm -2 ) at the PEC photoanode. This study opens new insight into integrative active sites for electron-O 2 -proton coupling in a PEC H 2 O 2 production system that may be meaningful for environment and energy applications.

Photochemical homolysis of hydrogen peroxide (H 2 O 2 ) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H 2 O 2 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 H 2 O 2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 10 3 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H 2 O 2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H 2 O 2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H 2 O 2 . This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H 2 O 2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO 2 and volatile organic compounds.

The application of sulfate radical advanced oxidation for organic pollutant removal has been hindered by some shortages such as the recycling difficulty of a powered catalyst, the low utilization efficiency of oxidants, and the secondary pollution (including soil acidification) after reaction. Herein, we fabricate a selective separation catalysis membrane (SSCM) for a highly efficient and environment-friendly persulfate-based advanced oxidation process. The SSCM comprises a top polydimethylsiloxane layer which is selectively penetrable to hydrophobic organic pollutants, followed by a catalyst layer with a magnetic nitrogen-doped porous carbon material, targeting the advanced oxidation of the selected pollutants. Compared with the catalyst in powder form, such SSCM devices significantly reduced the dosage of peroxymonosulfate by more than 40% and the catalyst dosage by 97.8% to achieve 80% removal of phenol with the coexistence of 20 mg L -1 humic acid (HA). The SSCM can extract target pollutants while rejecting HA more than 91.43% for 100 h. The pH value in the receiving solution demonstrated a significant reduction from 7.01 to 3.00. In comparison, the pH value in the feed solution varied from 6.05 to a steady 4.59. The results can be ascribed to the specific functionality for the catalyst anchored, natural organic matter isolation, and reaction compartmentation provided by SSCMs. The developed SSCM technology is beneficial for catalysts reused in remediation practices, saving oxidant dosage, and avoiding acidification of soil and water, thus having tremendous application potential.

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial ( Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H 2 atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H 2 treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Hydroxyl radicals ( • OH) are important reactive species that are photochemically generated through solar irradiation of chromophoric dissolved organic matter (CDOM) in surface waters. However, the spatial distribution within the complex three-dimensional structure of CDOM has not been examined. In this study, we used a series of hydrophobic chlorinated paraffins as chemical probes to elucidate the microheterogeneous distribution of • OH in illuminated CDOM solutions. The steady-state concentration of • OH inside the CDOM microphase is 210 ± 31-fold higher than the concentration in the aqueous phase. Our results suggest that the most photochemically generated • OH are confined into the CDOM microphase. Thus, illuminated CDOM behaves as a natural microreactor for • OH-based oxidations. By including intra-CDOM • OH, the quantum yield of • OH for CDOM solutions was estimated to be 2.2 ± 0.5 × 10 -3 , which is 2 orders of magnitude greater than previously thought. The elevated concentrations of photogenerated • OH within the CDOM microphase may improve the understanding of hydrophobic pollutant degradation in aqueous environments. Moreover, our results also suggest that • OH oxidation may play more important roles in the phototransformation of CDOM than previously expected.

We here present a novel Ti 4 O 7 -based electrode loaded with amorphous Pd clusters that achieve efficient anodic destruction of perfluorooctanoic acid (PFOA), a persistent water pollutant with significant environmental and human health concerns. These amorphous Pd clusters were characterized by the disordered, noncrystalline arrangement of Pd single atoms in close proximity, in contrast to crystalline Pd nanoparticles that have been often employed to tailor the electronic properties of an electrode. We found that the Ti 4 O 7 electrode loaded with amorphous Pd clusters significantly outperformed the Ti 4 O 7 electrode loaded with crystalline Pd particles due to enhanced electron transfer through dominant Pd-O bonds. Combined with the efficient binding of PFOA and its degradation intermediates to the fluorinated electrode surface, this electrode was capable of mineralizing PFOA and releasing fluoride as F - . The reaction pathway was found to proceed without involving reactive oxygen species and therefore was not quenched by common anions in complex natural water systems such as chloride ions.

Bacitracin is a mixture of nonribosomal peptides (NRPs) that is extensively used as an antibiotic in both human and veterinary medicine. Despite its widespread use over the past six decades, very few studies have addressed the environmental fate of bacitracin and zinc-bacitracin complexes. In this study, the photochemical transformation of bacitracin components (i.e., cyclic dodecapeptides) in the aquatic environment was investigated. A high resolution mass spectrometry (HRMS)-based approach enabled monitoring of the photochemical degradation kinetics of individual bacitracin components, investigation of the relative contribution of reactive oxygen species (e.g., singlet oxygen, (1)O2) in dissolved organic matter-sensitized photoreactions, and identification of oxidative modifications in bacitracin photoproducts. The results of this study support the hypothesis that indirect photochemical oxidation of the histidine (His) residue by (1)O2 is a major degradation pathway for bacitracin A, the most potent congener of the mixture. Furthermore, the photooxidation rate of bacitracin A with (1)O2 decreased upon bacitracin A coordination with Zn(2+), demonstrating that the photochemistry of metal-bound His is different from that of metal-free His. Overall, these results provide insight into the fate of bacitracin components in the aquatic environment and highlight the potential of utilizing this HRMS-based methodology to study transformations of other environmentally relevant NRPs.

Cysteine (Cys) plays numerous key roles in the biogeochemistry of natural waters. Despite its importance, a full assessment of Cys abiotic transformation kinetics, products and pathways under environmental conditions has not been conducted. This study is a mechanistic evaluation of the photochemical and nonphotochemical (dark) transformations of Cys in solutions containing chromophoric dissolved organic matter (CDOM). The results show that Cys underwent abiotic transformations under both dark and irradiated conditions. Under dark conditions, the transformation rates of Cys were moderate and were highly pH- and temperature-dependent. Under UVA or natural sunlight irradiations, Cys transformation rates were enhanced by up to two orders of magnitude compared to rates under dark conditions. Product analysis indicated cystine and cysteine sulfinic acid were the major photooxidation products. In addition, this study provides an assessment of the contributions of singlet oxygen, hydroxyl radical, hydrogen peroxide, and triplet dissolved organic matter to the CDOM-sensitized photochemical oxidation of Cys. The results suggest that another unknown pathway was dominant in the CDOM-sensitized photodegradation of Cys, which will require further study to identify.

Photooxidation is an important abiotic transformation pathway for amino acids (AAs) in sunlit waters. Although dissolved free AAs are well studied, the photooxidation of dissolved combined AAs (DCAAs) remains poorly investigated. This study is a systematic investigation of the effect of neighboring photostable AA residues (i.e., aliphatic, cationic, anionic, or aromatic residues) on the environmental indirect photochemical transformation of histidine (His) in His-containing oligopeptides. The pKa values of His residues in the studied oligopeptides were found to be between 4.3 and 8.1. Accordingly, the phototransformation rate constants of the His-containing oligopeptides were highly pH-dependent in an environmentally relevant pH range with higher reactivity for neutral His than for the protonated species. The photostable AA residues significantly modulated the photoreactivity of oligopeptides either through altering the accessibility of His to photochemically produced oxidants or through shifting the pKa values of His residues. In addition, the influence of neighboring photostable AA residues on the sorption-enhanced phototransformation of oligopeptides in solutions containing chromophoric dissolved organic matter (CDOM) was assessed. The constituent photostable AA residues promoted sorption of His-containing oligopeptides to CDOM macromolecules through electrostatic attraction, hydrophobic effects, and/or low-barrier hydrogen bonds, and subsequently increased the apparent phototransformation rate constants by up to 2 orders of magnitude.

Photochemical transformations greatly affect the stability and fate of amino acids (AAs) in sunlit aquatic ecosystems. Whereas the direct phototransformation of dissolved AAs is well investigated, their indirect photolysis in the presence of chromophoric dissolved organic matter (CDOM) is poorly understood. In aquatic systems, CDOM may act both as sorbent for AAs and as photosensitizer, creating microenvironments with high concentrations of photochemically produced reactive intermediates, such as singlet oxygen (1O2). This study provides a systematic investigation of the indirect photochemical transformation of histidine (His) and histamine by 1O2 in solutions containing CDOM as a function of solution pH. Both His and histamine showed pH-dependent enhanced phototransformation in the CDOM systems as compared to systems in which model, low-molecular-weight 1O2 sensitizers were used. Enhanced reactivity resulted from sorption of His and histamine to CDOM and thus exposure to elevated 1O2 concentrations in the CDOM microenvironment. The extent of reactivity enhancement depended on solution pH via its effects on the protonation state of His, histamine, and CDOM. Sorption-enhanced reactivity was independently supported by depressed rate enhancements in the presence of a cosorbate that competitively displaced His and histamine from CDOM. Incorporating sorption and photochemical transformation processes into a reaction rate prediction model improved the description of the abiotic photochemical transformation rates of His in the presence of CDOM.