Xiaoming Wang

Terrestrial organic carbon (OC) dynamics critically influence the global carbon budget. Humification is a key pathway for stabilizing OC by transforming labile compounds into macromolecular polymers. High-valent manganese oxides (e.g., Mn(IV)O2) are well-known as efficient catalysts of abiotic humification, whereas the role of Mn(II) in OC transformation remains poorly understood. Here, dissolved Mn2+ drives oxidative polymerization and nitrogen-enriched humic-like product formation in the presence of common soil precursors─catechol, glycine, and glucose─through a synergistic interaction with molecular oxygen (O2). Mechanistically, Mn2+ mediates sequential single-electron transfers from catechol to O2, forming reactive intermediates that potentially couple with Maillard precursors to yield nitrogen-enriched macromolecules. FT-ICR-MS reveals that CHON species in lignin-like and highly condensed aromatic regions collectively account for 75.6% of total product relative abundance. This process is accompanied by the generation of reactive oxygen species (ROS), which could further enhance molecular diversity. Across 14 soil and freshwater DOM samples, 13.1–54.7% of assigned product formulas overlap with those in the corresponding DOM samples. Collectively, these shared formulas account for 78% of the total relative abundance of product formulas, indicating molecular-formula-level compositional similarity. These findings provide mechanistic and molecular-level insight into the underexplored role of Mn2+ in abiotic humification.

The environmental persistence of glyphosate and its transformation into aminomethylphosphonic acid (AMPA) necessitate a mechanistic understanding of its abiotic degradation in natural environments. Manganese (Mn) oxides are ubiquitous redox-active minerals capable of oxidizing glyphosate; however, how Mn oxide mineralogy and pH regulate degradation kinetics, pathway selectivity, and interfacial redox processes remains unclear. Here, we investigated glyphosate oxidation by three representative Mn oxides, layered birnessite, disordered-layered vernadite, and tunnel-structured cryptomelane, across pH 3–7. Vernadite exhibited the highest and most sustained oxidative reactivity, cryptomelane showed rapid but strongly pH-dependent oxidation, and birnessite displayed slower yet persistent reactivity. Mineral structure controlled product selectivity: under near-neutral conditions, layered Mn oxides favored glycine formation through C–N bond cleavage adjacent to the phosphonate group, whereas tunnel-structured cryptomelane promoted AMPA accumulation through cleavage near the carboxyl group. Acidic conditions (pH 3 and 5) enhanced AMPA accumulation across all Mn oxides, suggesting increased persistence risks. Spectroscopic and kinetic analyses demonstrated that glyphosate oxidation involved surface complexation with Mn(IV), interfacial electron transfer, and propagation through transient, labile Mn(III) intermediates. These findings reveal that Mn oxide mineralogy and pH jointly govern bond-selective glyphosate oxidation and AMPA accumulation, advancing mechanistic predictions of glyphosate fate in soil and aquatic environments.

Manganese (Mn) redox cycling critically regulates soil organic carbon (SOC) turnover, especially in the forest litter layer. However, the molecular-level mechanisms underlying the Mn oxide-mediated decomposition and polymerization of dissolved organic matter (DOM) from litter remain poorly understood. Integrating excitation-emission matrix spectroscopy (EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated molecular mechanisms of δ-MnO₂-driven transformations of natural leaf litter-derived DOM samples across an ambient pH gradient (4, 6, 8). Results reveal that δ-MnO₂ simultaneously drives decomposition and polymerization of DOM, with pH modulating their relative contributions. At higher pH (pH 8), δ-MnO₂ facilitates aromatic condensation and coupling involving Mn(IV) → Mn(III)-mediated single-electron transfer, forming highly aromatic humic-like polymers and reducing electron-donating capacity, thereby enhancing oxidative resistance. Under lower pH (pH 4), δ-MnO₂ drives depolymerization and aromatic ring-opening by selectively cleaving oxygen-rich aromatic species. This process is dominated by multielectron transfer, with Mn(IV) reduced to Mn(II) via successive electron donation from DOM. Additionally, initial DOM composition impacts its transformation pathway. Specifically, high-aromaticity, high-molecular-weight fractions preferentially decompose, whereas low-aromaticity DOM tends to polymerize. These findings highlight the dual roles of Mn oxides in stabilizing organic matter, driving both the breakdown and reassembly through pH-regulated redox reactions.

The transformation of phosphate (P)-bearing ferrihydrite is critical for regulating iron (Fe) speciation and P availability in anaerobic soils and sediments. However, the effects of low P/Fe molar ratios (<0.1) on Fe(II)-catalyzed ferrihydrite transformation and the associated fate of P remain poorly understood. Here, we systematically investigated the influence of varying P/Fe molar ratios (0-0.05) and P incorporation modes (adsorption vs coprecipitation) on the Fe(II)-catalyzed transformation of dispersed and aggregated ferrihydrite and evaluated the resulting redistribution and availability of P. As the P/Fe ratio increased from 0 to 0.01, the predominant transformation products shifted sequentially from goethite to goethite-magnetite-lepidocrocite mixture and then to lepidocrocite, while higher P loading (P/Fe = 0.05) strongly inhibited transformation. Coprecipitated P exerted stronger inhibitory effects than adsorbed P. Mechanistically, P modified transformation pathways by enhancing Fe(II) sorption, suppressing labile Fe(III) generation, and retarding Fe oxides crystallization. Compared with dispersed ferrihydrite, aggregated particles exhibited lower surface reactivity toward Fe(II), favoring magnetite and lepidocrocite formation over goethite. These mineralogical transitions substantially altered P distribution: initially confined within ferrihydrite micropores, P became homogeneously incorporated into goethite and magnetite but preferentially accumulated on lepidocrocite surfaces. Ultimately, Fe(II)-catalyzed ferrihydrite transformation promoted P occlusion within mineral matrices, substantially reducing its availability. These findings elucidate coupled Fe-P transformation pathways and provide new mechanistic insights into P stabilization under reducing conditions.

The simultaneous sorption of cations and anions at the mineral-water interface can substantially alter their individual sorption characteristics; however, this phenomenon lacks a mechanistic understanding. Our study provides direct spectroscopic and modeling evidence of the molecular cosorption mechanisms of the cadmium ion (Cd²⁺) and phosphate (P) on goethite and layered manganese (Mn) oxide of birnessite, through in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR), P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and surface complexation modeling. Phosphate synergistically cosorbed with Cd on goethite predominantly through P-bridged ternary complexes (≡Fe-P-Cd) and electrostatic interactions at wide pH conditions. Likewise, P and Cd exhibited synergistic cosorption on birnessite by forming P-bridged ternary complexes (≡Mn-P-Cd) and weak competitive sorption at the layer edge sites. As pH and Cd loading increased, the surface P species transitioned from a binary complex to a ternary complex and/or Cd₃(PO₄)₂ precipitate for both goethite and birnessite. Compared to that in solution at pH 8, the formation of Cd₃(PO₄)₂ was inhibited by the presence of goethite and birnessite, ascribed to the specific adsorption of P and Cd, more pronounced in birnessite due to the stronger sorption of Cd at its vacant sites. The discovered cosorption mechanisms of P and Cd have important implications for understanding and predicting their mobility and availability in Cd-contaminated settings.

Ferrihydrite is one of the most reactive iron (Fe) (oxyhydr)oxides in soils, but the adsorption mechanisms of glyphosate, the most widely used herbicide, on ferrihydrite remain unknown. Here, we determined the adsorption mechanisms of glyphosate on pristine and Al-substituted ferrihydrites with aggregated and dispersed states using macroscopic adsorption experiments, zeta potential, phosphorus K-edge X-ray absorption near-edge structure spectroscopy, in situ attenuated total reflectance Fourier transform infrared spectroscopy coupled with two-dimensional correlation spectroscopy, and multivariate curve resolution analyses. Aggregation of ferrihydrite decreases the glyphosate adsorption capacity. The partial substitution of Al in ferrihydrite inhibits glyphosate adsorption on aggregated ferrihydrite due to the decrease of external specific surface area, while it promotes glyphosate adsorption on dispersed ferrihydrite, which is ascribed to the increase of surface positive charge. Glyphosate predominately forms protonated and deprotonated, depending on the sorption pH, monodentate-mononuclear complexes (MMH₁/MMH₀, 77-90%) on ferrihydrites, besides minor deprotonated bidentate-binuclear complexes (BBH₀, 23-10%). Both Al incorporation and a low pH favor the formation of the BB complex. The adsorbed glyphosate preferentially forms the MM complex on ferrihydrite and preferentially bonds with the Al-OH sites on Al-substituted ferrihydrite. These new insights are expected to be useful in predicting the environmental fate of glyphosate in ferrihydrite-rich environments.