Reductive release of Fe mineral-associated organic matter accelerated by oxalic acid

Significant role of sludge-derived organic matter in enhancing Cr (III) immobilization by jarosite: Insights from molecular selective adsorption and co-precipitation

The high mobility of chromium (Cr) caused by acid mine drainage (AMD) poses a serious threat to the aquatic environment. Weakly crystalline minerals can passivate Cr and modify its environmental behavior. Dissolved organic matter (DOM) plays an important role in synergizing the in-situ immobilization of Cr by minerals. However, few studies have explored the molecular-level impact of DOM on Cr immobilization by inducing weakly crystalline minerals transformation. Here, we investigated the effects and mechanisms of sludge-derived DOM (with rich molecular diversity) on Cr immobilization by jarosite (KJ, widely present in AMD). The results showed that sludge-derived DOM significantly changed the speciation distribution of Cr and Fe. The maximum enhancement of Cr immobilized via adsorption and precipitation was 63.43 % and 161.89 %, respectively. Synchronous fluorescence analysis showed that the fractionation of high-oxygenated aromatic compounds promoted Cr adsorption on KJ. FT-ICR-MS spectra suggested that low-oxygenated aromatic and aliphatic compounds promote Cr immobilization in secondary ferrihydrite by capturing free Cr, followed by coprecipitation with dissolved Fe ions. The molecular-level finding in this study will facilitate the deep understanding of the molecular selectivity of sludge organics in remediating heavy metal-contaminated environments, and is of great significance for realizing the resource utilization of enormous waste sludge.

Soil C mineralization and C-Fe coupling: a trade-off between Fenton reaction and microbial activity under straw mulching in banana orchards

Straw mulching influences soil carbon (C) cycling by adding organic matter and modifying the soil structure, with soil iron (Fe)-associated C (Fe-OC) and reactive oxygen species (ROS) playing critical roles in this process. However, the dynamics in soil Fe-OC and ROS following mulching in banana orchards remain unclear. This study investigated the impact of 3 years of straw mulching on soil Fe-OC, ROS, and soil C mineralization. The results showed that Fe-OC content decreased significantly in the first year of straw mulching, attributed to a weakened Fenton reaction, reduced reactive Fe oxides, and increased soil pH. In the subsequent two years, increased dissolved organic carbon (DOC) mitigated the negative impact on Fe-OC, preventing further significant reductions. Meanwhile, straw mulching significantly lowered soil ROS (O2·- and ·OH) levels over the three years, contrary to the hypothesis that it would increase ROS by promoting microbial biomass. This reduction was linked to increased soil pH and a weakened Fenton reaction. Moreover, straw mulching significantly increased soil C mineralization in the second and third years but had no significant effect in the first year. Despite the potential negative impact of reduced ROS, straw mulching promoted soil C mineralization by increasing soil microbial activity and C sources. The distinct responses of soil C mineralization to straw mulching in the first year compared to the following two years suggest that the overall impact of straw mulching on this process is determined by the trade-off between the Fenton reaction and microbial activity. These results indicate that straw mulching significantly influenced soil Fe-OC, ROS, and C mineralization by modulating the Fenton reaction, microbial activity, and soil physicochemical properties, with specific changes varying depending on the duration of mulching. These insights enhance our understanding of how straw mulching drives soil C cycling in agricultural ecosystems.

EDDS application destabilizes soil organic matter in phytoremediation: Insights from quantity and molecular composition of dissolved organic matter

The stability of soil organic matter (SOM) is crucial for metal transport and carbon cycling. S,S-ethylenediaminedisuccinic acid (EDDS) is widely used to enhance phytoremediation efficiency for heavy metals in contaminated soils, yet its specific impacts on SOM have been underexplored. This study investigates the effects of EDDS on SOM stability using a rhizobox experiment with ryegrass. Changes in soil dissolved organic matter (DOM) quantity and molecular composition were analyzed via Fourier transform ion cyclotron resonance mass spectrometry. Results showed that the use of EDDS increased the uptake of Cu, Cd and Pb by ryegrass, but simultaneously induced the destabilization and transformation of SOM. After 7 days of EDDS application, dissolved organic carbon (DOC) and nitrogen (DON) concentrations in rhizosphere soils increased significantly by 3.44 and 10.2 times, respectively. In addition, EDDS reduced lipids (56.3%) and proteins/amino sugars-like compounds (52.1%), while increasing tannins (9.11%) and condensed aromatics-like compounds (24.4%) in the rhizosphere DOM. These effects likely stem from EDDS's dual action: extracting Fe/Al from SOM-mineral aggregates, releasing SOM into the DOM pool, and promoting microbial degradation of bioavailable carbon through chain scission and dehydration. Our study firstly revealed that the application of EDDS in phytoremediation increased the mineralization of SOM and release of CO2 from soil to the atmosphere, which is important to assess the carbon budget of phytoremediation and develop climate-smart strategy in future.

Important role of Fe oxides in global soil carbon stabilization and stocks

Iron (Fe) oxides can interact with soil organic carbon (SOC) to form Fe-bound organic carbon (OC-Fe), which strongly promotes SOC protection, mitigating global climate change. However, the global patterns and factors controlling OC-Fe are unclear. Here, we conducted a meta-analysis of 3,395 globally distributed soil profiles to reveal the role of Fe-Al oxides in global soil carbon stabilization and stocks. The global OC-Fe stock in topsoil is 233 PgC, accounting for 33 ± 15% of the total SOC stock. A substantial OC-Fe deficit (difference between OC-Fe and OC-Femax) was observed at the equator and at mid-latitudes. Our findings suggest that mineral factors should be incorporated into soil carbon models to improve model predictions. Although there are uncertainties in current OC-Fe extraction method, the global distribution of OC-Fe and OC-Femax constitutes a vital resource for future research targeting carbon cycling issues and offers innovative strategies for global soil carbon sequestration initiatives.

Migration and transformation behaviors of potentially toxic elements and the underlying mechanisms in bauxite residue: Insight from various revegetation strategies

Revegetation is a promising strategy for large-scale bauxite residue disposal and management, potentially influencing the geochemical stability of potentially toxic elements (PTEs) through rhizosphere processes. However, the geochemical behaviors of PTEs and the underlying mechanisms during bauxite residue revegetation remain unclear. This study examined the migration and transformation behaviors of PTEs and their underlying mechanisms in the bauxite residue-vegetation-leachate system under various revegetation strategies, including single and co-planting of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), over a 100-day microcosm experiment. The results showed significant decreases in pH, EC, Na, Al, and Cr levels in the leachate under various revegetation strategies, with slight increases in Cu, V, As, and Pb. Over time, the pH, EC, Na, Cr, Cu, V, Pb, and As levels in the leachate decreased, while those of Al, Fe, Mn, and Zn increased. The mean pH, EC, and concentrations of Na, Al, Fe, and Cr in the leachate of the revegetated treatments decreased by 6%-8%, 21%-33%, 2%-4%, 19%-27%, 7%-22%, and 15%-26%, respectively, while the mean concentrations of Mn, V, Zn, and As increased by 47%-134%, 26%-46%, 39%-47%, and 3%-10%, respectively, compared to the unamended treatment. Co-planting generally exhibited a greater impact on leachate components compared to single planting. Available contents of Al, Cr, and Pb decreased by 81%-83%, 57%-77%, and 55%-72%, respectively, while those of other PTEs increased in the revegetated bauxite residue. Co-planting significantly reduced the availability of PTEs compared to single planting. Except for Na and Mn, the bioaccumulation and transportation factors of PTEs in both vegetation species remained below 1 under various revegetation strategies. The migration and transformation behaviors of PTEs in the bauxite residue-vegetation-leachate system were mainly influenced by pH and nutrient levels. These findings provide new insights into the migration and transformation behaviors of PTEs during bauxite residue revegetation.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Chemodiversity of Dissolved Soil Organic Matter from Amazon Rainforest as Influenced by Deforestation

Many biogeochemical processes are modulated by dissolved organic matter (DOM), but the drivers influencing the chemodiversity of DOM compounds in Amazonian soils are poorly understood. It has also been theorized whether deforestation controls the decline of DOM. In this study, we collected soil samples from thirty sites across different regions of Brazil's Legal Amazon, and we investigated the trade-offs among soil physical-chemical properties and DOM chemodiversity. We employed optical spectroscopy, Fourier transform ion cyclotron resonance, and multivariate analysis. Our results indicated that, despite variations in land use and soil physical-chemical properties, factors such as the deforested site, geometric mean diameter, weighted average diameter, and soil organic carbon were the main influencers of DOM chemodiversity variation. These findings highlight the importance of considering DOM chemodiversity as closely related to land use and its potential use in developing deforestation models for predicting soil quality decline in Brazil's Legal Amazon.

Effects of BC on metal uptake by crops (availability) and the vertical migration behavior in soil: A 3-year field experiments of crop rotation

Biochar (BC) has been substantiated to effectively reduce the available content of heavy metals (HMs) in soil-plant system; however, the risk of biochar (BC)derived dissolved organic matter (DOM) induced metal vertical migration has not been well documented, especially in the long-term field conditions. Therefore, this study investigated HM vertical migration ecological risks and the long-term effectiveness of the amendment of biochar in the three successive years of field trials during the rotation system. The results revealed that biochar application could increase soil pH and DOM with a decrease in soil CaCl2 extractable pool for Pb, Cu, and Cd. Furthermore, the results indicated a significant decrease in acid phosphatase activities and an increase in urease and catalase activities in the soil. Cucumber was shown to be safe during a three-year rotation system in the field. These results suggest that BC has the potential to enhance soil environment and crop yields. BC derived DOM-specific substances were identified using parallel factor analysis of excitation-emission matrix in deep soil (0-60 cm). The study incorporated HM concentration fluctuations in deep soils, providing an additional interpretation of DOM and co-migration of HMs.The environmental risk associated with the increase in DOM hydrophobicity should not be ignored by employing BC for soil HM remediation applications. The study enhances understanding of biochar-derived DOM's migration and stabilization mechanisms on heavy metals, providing guidelines for its use as a soil amendment.

Enhanced sequestration of uranium by coexisted lead and organic matter during ferrihydrite transformation

The dynamic reactions of uranium (U) with iron (Fe) minerals change its behaviors in soil environment, however, how the coexisted constituents in soil affect U sequestration and release on Fe minerals during the transformation remains unclear. Herein, coupled effects of lead (Pb) and dissolved organic matter (DOM) on U speciation and release kinetics during the catalytic transformations of ferrihydrite (Fh) by Fe(II) were investigated. Our results revealed that the coexistence of Pb and DOM significantly reduced U release and increased the immobilization of U during Fh transformation, which were attributed to the enhanced inhibition of Fh transformation, the declined release of DOM and the increased U(VI) reduction. Specifically, the presence of Pb increased the coprecipitation of condensed aromatics, polyphenols and phenols, and these molecules were preferentially maintained by Fe (oxyhydr)oxides. The sequestrated polyphenols and phenols could further facilitate U(VI) reduction to U(IV). Additionally, a higher Pb content in coprecipitates caused a slower U release, especially when DOM was present. Compared with Pb, the concentrations of the released U were significantly lower during the transformation. Our results contribute to predicting U sequestration and remediating U-contaminated soils.

Returning ryegrass to continuous cropping soil improves soil nutrients and soil microbiome, producing good-quality flue-cured tobacco

The widespread and continuous cultivation of tobacco has led to soil degradation and reduced crop yields and quality. Green manure is an essential organic fertilizer that alleviates obstacles to continuous cultivation. However, the plant-soil microecological effects of green manure on flue-cured tobacco cultivation remain unclear. Thus, a positioning trail including two treatments, chemical fertilizer application only (treatment NPK) and chemical fertilizer application with turning ryegrass (treatment NPKG) was conducted, and the effect of ryegrass returning on the soil physicochemical properties, soil microbiome, crop yield, and quality of flue-cured tobacco in continuous cropping soil were investigated. Results showed that returning ryegrass to the field increased the thickness of soil humus layer from 13 cm to 15 cm, reduced the humus layer soil bulk density to 1.29 cm3/g. Ryegrass tilled and returned to the field increased soil organic matter content by 6.89-7.92%, increased rhizosphere soil available phosphorus content by 2.22-17.96%, and converted the soil non-exchangeable potassium into potassium that was available for plant absorption and utilization. Ryegrass tilling and returning to the field increased the potassium content of middle leaves of flue-cured tobacco by 7.69-10.07%, the increased potassium content in flue-cured tobacco was accompanied by increased total sugar, reducing sugar, and the ratio of reducing sugar to nicotine, which facilitated the harmonization of the chemical composition of cured tobacco leaves. Moreover, the increased number of markedly improved operational taxonomic units enhanced the complexity of the soil bacterial community and its compactness after ryegrass tillage and their return to the field. The available potassium, available phosphorus, total potassium content, pH, and sampling period of the rhizosphere soil had considerable effects on the rhizosphere microbial. Ryegrass tilling and returning to the field changed the soil microbiome, which increased the abundance of bulk soil Proteobacteria, rhizosphere soil Fibrobacterota, and microbes with anti-pathogen activity (Lysobacteria, Sphingomonas, Chaetomium, and Minimedusa); and reduced the abundance of pathogenic fungi Neocosmospore genus in the soil. In brief, ryegrass returned to the field, improved soil microecology and restored soil nutrients, and established a new dynamic balance of soil ecology, thereby improving the quality of cultivated land and the quality of flue-cured tobacco.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Nutritional Composition and Antioxidant Activity of Gonostegia hirta: An Underexploited, Potentially Edible, Wild Plant

Wild, edible plants have received increasing attention as an important complement to cultivate vegetables, as they represent an easily accessible source of nutrients, mineral elements, and antioxidants. In this study, the tender stems and leaves of Gonostegia hirta, an edible species for which only scarce data are available in the literature, are thoroughly evaluated for their nutritional profile, chemical characterization, and antioxidant activity. Being considered as an underexploited, potentially edible plant, the nutritional composition of Gonostegia hirta was identified, and several beneficial compounds were highlighted: sugars, potassium, calcium, organic acids, fatty acids, phenolics, and flavonoids. A total of 418 compounds were identified by metabolomic analysis, including phenolic acids, flavonoids, amino acids, lipids, organic acids, terpenoids, alkaloids, nucleotides, tannins, lignans, and coumarin. The plant sample was found to have good antioxidant capacities, presented by DPPH, FRAP, ABTS⁺, hydroxyl radical scavenging capacity, and its resistance to the superoxide anion radical test. In general, Gonostegia hirta has a good nutritional and phytochemical composition. The health benefits of Gonostegia hirta as a vegetable and herbal medicine is important for both a modern diet and use in medication.

Effects of dissolved organic matter molecules on the sequestration and stability of uranium during the transformation of Fe (oxyhydr)oxides

Amorphous ferrihydrite (Fh) is abundant in aquatic environments and sediments, and often coprecipitates with dissolved organic matter (DOM) to form mineral-organic aggregates. The Fe(II)-catalyzed transformation of Fh to crystalline Fe (oxyhydr)oxides (e.g., goethite) can result in the changes of uranium (U) species, but the effects of DOM molecules on the sequestration and stability of U during Fe (oxyhydr)oxides transformation are poorly understood. In this study, the associations of DOM molecules with U during the coprecipitation of DOM with Fh were evaluated, and the effects of DOM molecules on the kinetics of U release during Fe (oxyhydr)oxides transformation were investigated using a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), X-ray photoelectron spectroscopy (XPS), and kinetic experiments. FT-ICR-MS results indicated that, in addition to phenolic and polyphenolic compounds with higher O/C ratios, portions of phenolic compounds with lower O/C ratios and aliphatic compounds were also contributed to UO₂²⁺ binding when Fh coprecipitated with DOM. In comparison, phenolic and polyphenolic compounds with higher O/C ratios and condensed aromatics were preferentially retained on Fe (oxyhydr)oxides during the transformation. XPS results further suggested that the coprecipitated DOM molecules facilitated the reduction of U(VI) to U(IV) during the transformation, possibly through providing electrons or acting as electron shuttles. The kinetic experiment results indicated that the transformation processes accelerated U release from Fe (oxyhydr)oxides, but the coprecipitated DOM molecules slowed down U release. Our results contribute to understanding the behaviors of U and predicting the sequestration of U in the environment.

Effect of humic acid on bioreduction of facet-dependent hematite by Shewanella putrefaciens CN-32

Interfacial reactions between iron (Fe) (hydr)oxide surfaces and the activity of bacteria during dissimilatory Fe reduction affect extracellular electron transfer. The presence of organic matter (OM) and exposed facets of Fe (hydr)oxides influence this process. However, the underlying interfacial mechanism of facet-dependent hematite and its toxicity toward microbes during bioreduction in the presence of OM remains unknown. Herein, humic acid (HA), as typical OM, was selected to investigate its effect on the bioreduction of hematite {100} and {001}. When HA concentration was increased from 0 to 500 mg L^-1, the bioreduction rates increased from 0.02 h^-1 to 0.04 h^-1 for hematite {100} and from 0.026 h^-1 to 0.05 h^-1 for hematite {001}. Since hematite {001} owned lower resistance than hematite {100} irrespective of the HA concentration, and hematite {100} was less favorable for reduction. Microscopy-based analysis showed that more hematite {001} nanoparticles adhered to the cell surface and were bound more closely to the bacteria. Moreover, less cell damage was observed in the HA-hematite {001} treatments. As the reaction progressed, some bacterial cells died or were inactivated; confocal laser scanning microscopy showed that bacterial survival was higher in the HA-hematite {001} treatments than in the HA-hematite {100} treatments after bioreduction. Spectroscopic analysis revealed that facet-dependent binding was primarily realized by surface complexation of carboxyl functional groups with structural Fe atoms, and that the binding order of HA functional groups and hematite was affected by the exposed facets. The exposed facets of hematite could influence the electrochemical properties and activity of bacteria, as well as the binding of bacteria and Fe oxides in the presence of OM, thereby governing the extracellular electron transfer and concomitant bioreduction of Fe (hydr)oxides. These results provide new insights into the interfacial reactions between OM and facet-dependent Fe oxides in anoxic, OM-rich soil and sediment environments.