Guojun Chen

Fe(II) can be oxidized by mixotrophic nitrate-reducing Fe(II)-oxidizing (NRFeOx) bacteria to Fe(III), leading to mineral precipitation. However, the effects of this process on the metabolism and reproductive ability of Fe(II)-oxidizing bacterial cells have not yet been quantified. In this study, we investigated the effects of Fe(II) on the mixotrophic NRFeOx bacteria Acidovorax sp. BoFeN1 by performing cell plate counts, chemical analyses, scanning electron microscopy (SEM) imaging, live/dead fluorescence staining, and proteomic analysis. The results showed that nitrate-reducing Fe(II)-oxidizing bacteria Acidovorax sp. BoFeN1 undergo a striking physiological state in which respiration persists while cell reproduction collapses after their oxidation of Fe(II). Across 0.1-10 mM Fe(II), colony-forming units dropped by up to 10³-fold, yet acetate consumption remained largely unaffected, and nitrate reduction was even slightly promoted. SEM-EDS revealed extensive Fe(III) encrustation on cells, while live/dead staining showed intact inner membranes where nitrate reduction occurs. Adding an Fe(III) chelator, citrate, restored reproductive capacity without notable changes in Fe(II) oxidation or nitrate reduction, indicating that it was the products of Fe(II) oxidation, the secondary Fe(III) minerals, that impaired cell reproduction. The phenotypes observed in this study were supported by proteomic analyses, which showed that Fe(II) increased the abundance of proteins involved in nitrate reduction and the respiratory electron transport chain. These findings uncover a mineral encrustation-driven uncoupling between respiration and reproduction, suggest how natural organic ligands may buffer NRFeOx bacterial communities in Fe-rich anoxic environments, and point to cellular-level levers for mitigating denitrification failures and N₂O risks in engineered and natural systems.

Manganese minerals are a vital container for cadmium (Cd) scavenging in anoxic soils and sediments, where the redox reaction between ferrous ions (Fe(II)) and manganese minerals critically controls its fixation and release. However, how the Fe(II)-mediated reductive transformation of manganese minerals affects cadmium binding speciation remains unknown. Here, the kinetics of birnessite transformation mediated by Fe(II) and its impacts on cadmium mobility across a range of Fe(II)/Mn ratios and pH were systematically investigated. During the redox reaction, Fe(II) oxidation into ferrihydrite and the reductive dissolution of birnessite into divalent and trivalent manganese were facilitated by higher Fe(II) loadings and pH, resulting in the formation of more Fe-Mn complex minerals. After the redox reaction, no other crystalline iron and manganese minerals were detected. Despite efficient immobilization, a portion of cadmium, initially bound to birnessite, was redistributed to newly formed ferrihydrite. Interestingly, greater cadmium immobilization by Fe-Mn complex minerals was observed at higher pH values and lower Fe(II) loadings. The cadmium immobilization was primarily attributed to its binding to adsorption sites and vacancies of minerals, with minor contributions from possible physical encapsulation and the formation of Cd-Mn coprecipitates. Shell-by-shell fitting revealed that cadmium formed double corner-sharing, edge-sharing, and triple-corner-sharing complexes with the Fe-Mn complex minerals. These findings provide novel insights into the redox reaction between Fe(II) and birnessite and the associated cadmium dynamics, advancing our understanding of iron, manganese, and cadmium geochemical cycling in anoxic environments.

The mobility of nickel (Ni), chromium (Cr), and lead (Pb) in basalt-derived paddy soils under redox-fluctuating conditions governs their accumulation in rice. However, the influence of redox-induced changes in soil components, particularly iron (Fe) (oxyhydr)oxides and organic matter (OM), on the mobility of these metals remains poorly understood. This study revealed that Fe redox cycling serves as the fundamental driving force controlling the release and immobilization of Ni, Cr, and Pb. Fe(III) reduction and OM complexation under anoxic conditions increased metal mobility, while Fe(II) oxidation and OM stabilization under oxic conditions reduced metal mobility. A kinetic model revealed that Ni and Pb mobility was predominantly controlled by Fe cycling, with Pb showing the strongest coupling (81.5-93.5% influence). For Cr, although Fe cycling remained the underlying driver, a greater proportion (82.6-83.6%) was associated with OM interactions. These findings advance risk assessment and inform redox-based remediation strategies for contaminated paddy soils.

Organic matter (OM) sequestrated by iron (Fe) and manganese (Mn) complex minerals may be an effective way to achieve organic carbon (OC) persistence in sediments and soils. However, the impact of Fe-Mn complex minerals from a redox reaction of Fe(II) and birnessite on OC dynamics remains unexplored. Herein, the immobilization and molecular transformation of fulvic acid (FA), mediated by a redox reaction of Fe(II) and birnessite, were investigated at pH 4.0-8.0 with 1-5 mM Fe(II). Higher pH and Fe(II) addition promoted Fe(II) oxidation coupled with the reductive transformation of birnessite. The coupled adsorption-oxidation of FA occurred, and FA sequestration efficiency increased with a decrease in pH and increasing Fe(II) concentrations. Fe(II) oxidation primarily formed ferrihydrite, and a small portion of lepidocrocite and feitknechtite was detected under higher pH and Fe(II) addition conditions. In mineral-organo composites, Fe/Mn-O, Fe-C, edge- and corner-sharing Fe-Fe, and Mn-Mn bonds constituted the coordination environment. FA was associated with Fe-Mn complex minerals to improve its stability, while C in FA distributed within nanopores and thin coaggregates exhibited a higher oxidation state. Compounds with high molecular weight, O/C, and aromatic and unsaturated structures, predominantly composed of lignin, tannin, and condensed hydrocarbons, exhibited stronger binding to Fe-Mn complex minerals under lower pH and higher Fe(II) addition conditions. These findings provide a fundamental understanding of heterogeneous Fe-Mn complex mineral formation coupled with OM sequestration, which has significant implications for understanding the long-term cycling of Fe, Mn, and C in natural settings.

Most natural soils and minerals are typically dry or semidry, and their surface characteristics and interfacial interactions are distinct from those of slurry systems. The surface acidity of dry soils and minerals cannot be measured by using traditional methods such as potentiometric and temperature-programmed desorption methods. In this article, we describe the development of an analytical method based on a combination of acid-base indicators and diffuse reflectance spectroscopy (DRS). The surface pH (pH_surf) values of dry clay minerals and metal oxides were obtained, which were 1.13-3.14 units lower than that of slurry systems. This result was confirmed by the pollutant degradation activity on the mineral surface with different water contents. The potentiometric titration and surface complexation models (SCM) were used to reveal the acidity characteristics of different minerals. The pH_surf values of soils from seven provinces in China were determined using three methods: (1) real soil samples with different colors were diluted in BaSO₄, and pH_surf values were determined using DRS; (2) the pH_surf values of soil samples were also calculated using the pH_surf vs slurry pH curves, which were constructed using the pH_surf of six metal oxides and clay minerals obtained using DRS and their slurry pH values; (3) the pH_surf values of soil samples were determined using the pH values through extension from the slurry pH vs soil-water ratio plot. The pH_surf values calculated using the former two DRS methods were consistent and provided a better measure of the surface acidity of dry soil than slurry pH and its extension method. This review presents an in situ method for the quantitative surface-acidity measurement of dry soils and minerals and provides insights into acidity and interface research in other low-moisture systems.

The application of remote sensing technology for water quality monitoring has attracted much attention recently. Remote sensing inversion in coastal waters with complex hydrodynamics for non-optically active parameters such as total nitrogen (TN) and total phosphorus (TP) remains a challenge. Existing studies build the relationships between remote sensing spectral data and TN/TP directly or indirectly via the mediation of optically active parameters (e.g., total suspended solids). Such models are often prone to overfitting, performing well with the training set but underperforming with the testing set, even though both datasets are from the same region. Using the Hong Kong coastal region as a case study, we address this issue by incorporating spatial covariates such as hydrometeorological and locational variables as additional input features for machine learning-based inversion models. The proposed model effectively alleviates overfitting while maintaining a decent level of accuracy (R² exceeding 0.7) during the training, validation and testing steps. The gap between model R² values in training and testing sets is controlled within 7%. A bootstrap uncertainty analysis shows significantly improved model performance as compared to the model with only remote sensing inputs. We further employ the Shapely Additive Explanations (SHAP) analysis to explore each input’s contribution to the model prediction, verifying the important role of hydrometeorological and locational variables. Our results provide a new perspective for the development of remote sensing inversion models for TN and TP in similar coastal waters.