Incorporation of Cu into Goethite Stimulates Oxygen Activation by Surface-Bound Fe(II) for Enhanced As(III) Oxidative Transformation

Unveiling the dual role of calcium peroxide in enhancing green rust-catalyzed arsenite oxidation and stabilization

Iron oxyhydroxide redox processes coupled with reactive oxygen species (ROS) generation play a critical role in the transformation of arsenic (As). However, low ROS generation limits the detoxification and stabilization of As(III). In this study, we propose a novel strategy by integrating Fe(II) rich green rust (GR) with calcium peroxide (CaO2) for the efficient transformation of As(III) under oxic conditions. The results demonstrate that the introduction of CaO2 greatly enhances the oxidation and immobilization of As(III) by GR. Quenching experiments reveal that H2O2, ·O2-, and 1O2 are the primary ROS responsible for the oxidation of As(III). Importantly, CaO2 significantly boosts the production of key ROS in the GR/CaO2 system. Additionally, the introduction of CaO2 facilitates the transformation of both non-specific and specific adsorbed As into non-extractable As, improving the stability of immobilized As. Our findings unveil that CaO2 serves a dual role in promoting GR-catalyzed As(III) oxidation and stabilization by enhancing ROS production and forming iron-arsenic-calcium complexes. Moreover, the application of GR/CaO2 in As-contaminated soil can rapidly reduce the risk of As leaching in different scenarios. Therefore, this study provides a new strategy using CaO2 coupled with iron redox process to enhance oxidation and stabilization of arsenic in water and soil.

Effect of Impurities on the Redox Properties of Goethite

Iron oxide minerals regulate the flux of electrons in the environment and are important hosts for trace and minor, yet critical, elements. Here, we present the first evidence of a direct link between the local coordination environments of Ni and Zn and the redox properties of their host phase goethite (α-FeOOH), the most abundant Fe(III) (oxyhydr)oxide at Earth's surface. We used aqueous redox measurements to show that the redox potential EH0, and hence the mineral's stability, follows the order: pure goethite ≥ Zn-goethite > Ni-goethite. Parallel X-ray absorption and scattering measurements demonstrate, using quantum-informed analysis, that the local coordination environment of the smaller impurity, Ni, causes more bulk strain energy than Zn, which nearly accounts for the difference in EH0 between Ni- and Zn-goethite. Our theory-informed, experimental study reveals how two common impurities affect the stability of goethite with implications for the biogeochemical reactivity of Fe(III) (oxyhydr)oxide in mediating elemental and electron fluxes in the environment.

Synergistic enhancement of arsenic removal by Fe-Mn binary sorbent through oxygen vacancy-mediated electron acceleration and catalytic oxidation

Defect-engineered metal oxides hold great potential for enhancing arsenic capture through improved As2O3 adsorption and oxidation processes. However, the mechanisms underlying this enhancement, particularly the electronic effects stimulated by oxygen vacancy (OV) and the intrinsic oxidation schemes, are poorly understood. In this work, experimental and theoretical studies reveal that OV generated through citric acid etching of Fe-Mn binary sorbents markedly boosts the oxidation of As(III) by iron and manganese under oxic environments. Electron paramagnetic resonance and adsorption characteristics demonstrate that selective arsenic adsorption occurs at OV, leading to a 58.9 % enhancement in confined adsorption activity. Electrochemical and electrostatic potential investigations demonstrate that OV generates a distinctive high-activity potential field on the surface, characterized by improved electrical conductivity and superior redox properties. Interfacial electron regulation enables OV to achieve directional electron redistribution for arsenic adsorption, driven by strengthened ionic and covalent bonds as well as bridged electron transport channels. The catalytic effect imparted by OV preferentially facilitates the conversion from physisorption to chemisorption, resulting in reinforced dynamic arsenic adsorption. Ultimately, the intrinsically intensified oxidative pathways for arsenic by OV were elucidated. Following the oxygen temperature programming desorption (O2-TPD) and Mars-Maessen mechanism, OV facilitates the migration of surface lattice oxygen and reestablishes transport channels for bulk lattice oxygen, effectively lowering the rate-determining step energy barrier (exceeding 2.066 eV) and promoting the oxidation of As3 + to As5+. These findings demonstrate that oxygen vacancy engineering can be effectively implemented in developing and utilizing efficient gaseous arsenic sorbents at both macroscopic and atomic levels.

Iron-mineral-induced visible-light-catalytic degradation of BDE-47 enhanced by low-dose persulfate: Kinetics, mechanisms, and intervention of environmental factors

Iron minerals and visible light are ubiquitous in the environment, which have crucial effects on the fate and migration of organic pollutants during in-situ remediation. In this work, the role of iron minerals in visible light-activated persulfate for BDE-47 degradation was systematically investigated. Under visible light irradiation, low-dose persulfate (PS) (10 μM) is capable of achieving the rapid elimination of BDE-47 in the presence of iron minerals, and goethite exhibited the best performance among selected iron minerals. In Goe/PS/vis system, the reactive species involved in the degradation of BDE-47 included ·OH, SO4·-, O2·-, h+ (photo-generated hole), and e-. The existence of O2 and S2O82- led to the synergy of ROS generation derived from h+/e-. Four oxidized intermediates of BDE-47 with lower estimated toxicity were identified by high-resolution mass spectrometry (HRMS), suggesting that PS-based in-situ remediation driven by goethite and visible light can achieve deep mineralization and detoxification of BDE-47. In addition, the Goe/PS/vis system adapts to various co-existing anions and NOMs. Co-existing heavy metals facilitated BDE-47 removal. Heavy metal ions adsorbed on goethite are reduced to zero valent metal by photoelectron during irradiation, which further promotes electron transfer. While heavy metals entering the goethite lattice in the form of isomorphous substitution introduce oxygen vacancy, further promoting BDE-47 degradation. These findings reveal the critical role of natural iron minerals in PS-based in-situ remediation for polluted water and soils under solar irradiation and provide new ideas for developing efficient strategies for the combined contamination of PBDEs and heavy metals.

Atomic Insights into the Heterogeneous Crystallization of Manganese (Oxyhydr)oxides on Typical Iron (Oxyhydr)oxides: from Adsorption to Oxidation to Crystallization

Heterogeneous crystallization of manganese (oxyhydr)oxides (MnOx) on iron (oxyhydr)oxides (FeOx) is crucial for the biogeochemical cycling of Mn, yet atomic-level insights into this process are important but relatively limited. Herein, we revealed the distinct adsorption, oxidation, and crystallization mechanisms of Mn on hematite (Hem), ferrihydrite (Fhy), and goethite (Gth). Gth exhibited highest ability in Mn(II) removal and oxidation, followed by Hem and Fhy. Manganite and hausmannite were the main MnOx products with distinct proportions, and morphologies cross the systems. MnOx growth mechanisms involve surface-induced nucleation, crystallization by particle attachment (CPA), and self-catalyzed growth. On Fhy, self-catalyzed growth was dominant; for Gth, surface-induced nucleation was prevalent, supplemented by CPA; and Hem combined all three mechanisms. These distinct mechanisms led to nanoparticles primarily of hausmannite on Gth and nanowires of manganite and hausmannite on Hem and Fhy, with those on Hem displaying lower aspect ratios. Differences in MnOx structure and morphology were attributed to Mn(II)-FeOx complexation, FeOx electronic band structure, and crystal structure mismatch between MnOx and FeOx, which respectively influenced the direct and indirect electron transfer and heterogeneous nucleation efficiency. This work advances our understanding of MnOx crystallization on FeOx at the nanoscale, explaining the diverse morphology and structure of MnOx in different environments.

Quantitative Enhancement of Arsenate Immobilization Induced by Vacancy Defects on Various Exposed Lattice Facets of Hematite

Defects are common features in hematite that arise from deviations from the perfect mineral crystal structure. Vacancy defects have been shown to significantly enhance arsenate (As) immobilization by hematite. However, the contributions from vacancy defects on different exposed facets of hematite have not been fully quantified. In this study, hematite samples with various morphologies were pretreated with sodium borohydride (NaBH4) to generate oxygen vacancy defects (OVDs), analyzed quantitatively using extended X-ray absorption fine structure (EXAFS) and thermogravimetric analysis (TG). Batch experiments revealed that the OVDs on different exposed facets showed significant variations in improving arsenate adsorption, i.e., the quantitative enhancement of arsenate adsorption amount per unit OVD concentration (ΔQm/Cdefect) followed the sequence of (110) facet (80.05 μmol/mmoldef) > (001) facet (31.85 μmol/mmoldef) > (012) facet (13.14 μmol/mmoldef). The underlying mechanism by which OVDs affect arsenate adsorption across different exposed facets of hematite was studied. The results reveal that the tremendous improvement of arsenate adsorption caused by OVDs on the (110) facet compared to (001) and (012) facets was attributed to their stronger bonding strength of As to under-coordinated Fe atoms, thus significantly promoting the immobilization of arsenate. The findings of this study enhance our ability to precisely understand the migration and fate of As while also aiding in the design of highly efficient iron mineral materials for mitigating As pollution.

Extent of As(III) versus As(V) adsorption on iron (oxyhydr) oxides depends on the presence of vacancy cluster-like micropore sites: Insights into a seesaw effect

Iron (oxyhydr)oxides are ubiquitous in terrestrial environments and play a crucial role in controling the fate of arsenic in sediments and groundwater. Although there is evidence that different iron (oxyhydr)oxides have different affinities towards As(III) and As(V), it is still unclear why As(V) adsorption on some iron (oxyhydr)oxides is larger than As(III) adsorption, while it is opposite for other ones. In this study, six typical iron (oxyhydr)oxides are selected to evaluate their adsorption capacities for As(III) and As(V). The characteristics of these iron minerals such as morphology, arsenic adsorption species, and pore size distribution are carefully examined using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), positron annihilation lifetime (PAL) spectroscopy, and X-ray absorption spectroscopy (XAS). We confirm a seesaw effect occurred in different iron minerals for As(III) and As(V) immobilization, i.e., at pH 6.0, adsorption of As(V) on hematite (0.73 μmol m-2) and magnetite (0.33 μmol m-2) is higher than for As(III) (0.61 μmol m-2 and 0.27 μmol m-2, respectively), for goethite and lepidocrocite it is almost equal, while As(III) sorption on ferrihydrite (5.77 μmol m-2) and schwertmannite (28.41 μmol m-2) showed higher sorption than As(V) (1.53 μmol m-2 and 12.99 μmol m-2, respectively). PAL analysis demonstrates that ferrihydrite and schwertmannite have a large concentration of vacancy cluster-like micropores, significantly more than goethite and lepidocrocite, followed by hematite and magnetite. The difference of adsorption of As(III) and As(V) to different iron (oxyhydr)oxides is due to differences in the abundance of vacancy cluster-like micropore sites, which are conducive for smaller size As(III) immobilization but not for larger size of As(V). The findings of this study provide novel insights into a seesaw effect for As(III) and As(V) immobilization on naturally occurring iron mineral.

Dual-Ligand-Driven Dark Reactive Oxygen Species Generation on Iron Oxyhydroxides: Implications for Environmental Remediation

The dark generation of reactive oxygen species (ROS), particularly hydroxyl radicals (·OH), is crucial in the oxidative transformation of various pollutants. However, the mechanisms behind this process are predominantly linked to direct O2 activation by reduced substances such as Fe(II) and natural organic matter. In this study, we introduce a previously overlooked dual-ligand mechanism that significantly amplifies ·OH generation on iron oxyhydroxides, facilitated by cysteine and pyrophosphate. Our findings reveal that these ligands collaboratively boost ·OH generation by 99.5-125.7% compared to Fe(II) alone. This enhancement occurs through a two-step electron transfer (ET) process, where cysteine transfers electrons to O2 through iron oxyhydroxides. The complexation of pyrophosphate with iron oxyhydroxides further reduces the thermodynamic barriers, notably promoting this ET process and significantly improving the electron utilization efficiency for O2 activation by the electron donor cysteine. Such a process has shown its great potential for effectively driving the oxidative transformation of various pollutants, including As(III), dichlorophenol, and carbamazepine. These findings offer valuable insights for nature-based pollutant mitigation in soil and subsurface environments.

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Aluminum substitution stabilizes organic matter in ferrihydrite transforming into hematite: A molecular analysis

The association of soil organic matter (SOM) with iron (Fe) oxyhydroxides, particularly ferrihydrite, plays a pivotal role in the biogeochemical cycling of carbon (C) in both terrestrial and aquatic environment. The aging of ferrihydrite to more crystalline phases can impact the stability of associated organic C, a process potentially influenced by aluminum (Al) substitution due to its abundance. However, the molecular mechanisms governing the temporal and spatial distribution of SOM during the aging process of Al-substituted Fe oxyhydroxides remain unclear. This study aims to bridge this knowledge gap through a comprehensive approach, utilizing batch experiments, solid characterization techniques, and atomic force microscopy (AFM) based peak-force quantitative nanomechanical mapping (PF-QNM). Batch experiments revealed that humic acid (HA) was released into the aqueous phase during aging, with Al inhibiting this release. Various solid characterization methods collectively suggested that Al hindered the crystalline transformation of ferrihydrite and significantly preserved HA on the surface of newly formed hematite, rather than it being occluded within the interior of the new minerals. Results from 3-Dimensional fluorescence spectroscopy (3D-EEM) and Fourier-transform infrared spectroscopy (FTIR) indicated that the structure of HA remained constant, with the carboxyl-rich and hydroxyl-rich portions of HA fixed at the mineral interface during the aging period. Furthermore, we developed AFM-based PF-QNM to both quantify and visualize the interactions between Fe oxyhydroxides and HA, demonstrating variations in HA affinity among different Fe oxyhydroxides and highlighting the influence of the Al substitution rate.

Nanoconfined Fe(II) releaser for long-term arsenic immobilization and its sustainability assessment

Ferrous (Fe(II))-based oxygen activation for pollutant abatements in soil and groundwater has attracted great attention, while the low utilization and insufficient longevity of electron donors are the primary challenges to hinder its practical applications. Herein, we propose a nanoconfined Fe(II) releasing strategy that enables stable long-term electron donation for oxygen activation and efficient arsenic (As) immobilization under oxic conditions, by encapsulating zero-valent iron in biomass-derived carbon shell (ZVI@porous carbon composites; ZVI@PC). This strategy effectively enhances the generation of reactive oxygen species, enabling efficient oxidation and subsequent immobilization of As(III) in soils. Importantly, this Fe(II) releaser exhibits strong anti-interference capability against complex soil matrices, and the accompanying generation of Fe(III) enables As immobilization in soils, effectively lowering soil As bioavailability. Soil fixed-bed column experiments demonstrate a 79.5 % reduction of the total As in effluent with a simulated rainfall input for 10 years, indicating the excellent long-term stability for As immobilization in soil. Life cycle assessment results show that this Fe(II) releaser can substantially mitigate the negative environmental impacts. This work offers new insights into developing green and sustainable technologies for environmental remediation.

Facet-dependent activation of oxalic acid over hematite nanocrystals under the irradiation of visible light for efficient degradation of pollutants

Naturally occurring hematite has been widely studied in the Fenton-like system for water pollutant remediation due to its abundance and non-toxicity. However, its inadequate catalytic activity results in difficulty in effectively degrading pollutants in the catalytic degradation system that it constitutes. Thus, we constructed a photochemical system composed of hematite with {001} facet of high activity facet and low-cost and non-toxic oxalic acid (OA) for the removal of various types of pollutants. The removal rate for the degradation of metronidazole, tetracycline hydrochloride, Rhodamine B, and hexavalent chromium by hematite nanoplate with the exposed {001} facet activating OA under visible light irradiation was 4.75, 2.25, 2.33, and 2.74 times than that by the exposed {110} facet, respectively. Density functional theory (DFT) calculation proved that the OA molecule was more easily adsorbed on the {001} facet of hematite than that on the {110} facet, which would favor the formation of the more Fe(III)-OA complex and reactive species. In addition, the reactive site of metronidazole for the attraction of radicals was identified on the basis of the DFT calculation on the molecular occupied orbitals, and the possible degradation pathway for metronidazole included carbon chain fracture, hydroxyethyl-cleavage, denitrogenation, and hydroxylation. Thus, this finding may offer a valuable direction in designing an efficient iron-based catalyst based on facet engineering for the improved activity of Fenton-like systems such as OA activation.

Unveiling the Mechanism and Kinetics of Pollutant Attenuation by Free Radicals Triggered from Goethite in Water Distribution Systems

Investigating the fate of persistent organic pollutants in water distribution systems (WDSs) is of great significance for preventing human health risks. The role of iron corrosion scales in the migration and transformation of organics in such systems remains unclear. Herein, we determined that hydroxyl (•OH), chlorine, and chlorine oxide radicals are generated by Fenton-like reactions due to the coexistence of oxygen vacancy-related Fe(II) on goethite (a major constituent of iron corrosion scales) and hypochlorous acid (HClO, the main reactive chlorine species of residual chlorine at pH ∼ 7.0). •OH contributed mostly to the decomposition of atrazine (ATZ, model compound) more than other radicals, producing a series of relatively low-toxicity small molecular intermediates. A simplified kinetic model consisting of mass transfer of ATZ and HClO, •OH generation, and ATZ oxidation by •OH on the goethite surface was developed to simulate iron corrosion scale-triggered residual chlorine oxidation of organic compounds in a WDS. The model was validated by comparing the fitting results to the experimental data. Moreover, the model was comprehensively applicable to cases in which various inorganic ions (Ca2+, Na+, HCO3-, and SO42-) and natural organic matter were present. With further optimization, the model may be employed to predict the migration and accumulation of persistent organic pollutants under real environmental conditions in the WDSs.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 μmol L-1 combined with total Fe(II) increased to 188.79 μmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Biochemical mechanisms underlying iron plaque-mediated phosphorus accumulation and uptake in rice roots

The iron oxyhydroxides of iron plaque on the surface of rice root are crucial for the uptake of nutrition elements, especially phosphorus (P), but the effects of iron oxyhydroxides of iron plaque on the accumulation and uptake of P remain largely unknown. In this study, we investigated the regulatory mechanism of iron plaque on P uptake in rice via hydroponics of whole plant and simulation of iron oxyhydroxides-coated suspension cells in rice. The hydroponic experiment results showed that the presence of iron plaque increased the P content in rice shoots. The simulation experiment results further confirmed that after iron plaque coating, the P contents in the whole cell and on the cell wall were significantly increased from 5.16 mg/g and 2.73 mg/g to 8.85 mg/g and 5.27 mg/g, respectively. In addition, our data also showed that iron plaque coating led to an increase in cell surface potentials from -380 ± 40 mV to -200 ± 30 mV, thus promoting the adsorption of more P. Taken together, this study demonstrated that the iron plaque coating increased the surface potential of the cells, thus enhancing cellular P enrichment, eventually promoting P efficient adsorption in rice. Deciphering these regulatory mechanisms provide an insight into P biogeochemical cycling at the soil-plant interface and offer theoretical basis and practical references for the improvement of P bioavailability in rice production.

Direct Nanoscale Imaging Reveals the Mechanism by Which Organic Acids Dissolve Vivianite through Proton and Ligand Effects

The coprecipitation of iron (Fe) and phosphorus (P) in natural environments limits their bioavailability. Plant root-secreted organic acids can dissolve Fe-P precipitates, but the molecular mechanism underlying mobilizing biogenic elements from highly insoluble inorganic minerals remains poorly understood. Here, we investigated vivianite (Fe3(PO4)2·8H2O) dissolution by organic acids (oxalic acid (OA), citric acid (CA), and 2'-dehydroxymugineic acid (DMA)) at three different pH values (4.0, 6.0, and 8.0). With increasing pH, the vivianite dissolution efficiency by OA and CA was decreased while that by DMA was increased, indicating various dissolution mechanisms of different organic acids. Under acidic conditions, weak ligand OA (HC2O4- > C2O42- at pH 4.0 and C2O42- at pH 6.0) dissolved vivianite through the H+ effect to form irregular pits, but under alkaline condition (pH 8.0), the completely deprotonated OA was insufficient to dissolve vivianite. At pH 4.0, CA (H2Cit- > HCit2- > H3Cit) dissolved vivianite to form irregular pits through a proton-promoted mechanism, while at pH 6.0 (HCit2- > Cit3-) and pH 8.0 (Cit3-), CA dissolved vivianite to form near-rhombohedral pits through a ligand-promoted mechanism. At three pH values ((H0)DMA3- > (H1)DMA2- at pH 4.0, (H0)DMA3- at pH 6.0, and (H0)DMA3- and one deprotonated imino at pH 8.0), strong ligand DMA dissolved vivianite to form near-rhombohedral pits via ligand-promoted mechanisms. Raman spectroscopy showed that the deprotonated carboxyl groups (COO-) and imino groups were bound to Fe on the vivianite (010) face. The surface free energy of vivianite coated with OA decreased from 29.32 mJ m-2 to 24.23 mJ m-2 and then to 13.47 mJ m-2 with increasing pH, and that coated with CA resulted in a similar pH-dependent vivianite surface free-energy decrease while that coated with DMA increased the vivianite surface free energy from 31.92 mJ m-2 to 39.26 mJ m-2 and then to 49.93 mJ m-2. Density functional theory (DFT)-based calculations confirmed these findings. Our findings provide insight into the mechanism by which organic acids dissolved vivianite through proton and ligand effects.

Cooperative roles of phosphate and dissolved organic matter in inhibiting ferrihydrite transformation and their distinct fates

Phosphate and dissolved organic matter (DOM) mediate the crystalline transformation of ferrihydrite catalyzed by Fe(II) in subsurface environments such as soils and groundwater. However, the cooperative mechanisms underlying the mediation of phosphate and DOM in crystalline transformation of ferrihydrite and the feedback effects on their own distribution and speciation remain unresolved. In this study, solid characterization indicates that phosphate and DOM can collectively inhibit the crystalline transformation of ferrihydrite to lepidocrocite and thus goethite, via synergetic effects of inhibiting recrystallization and electron transfer. Phosphate can be retained on the surface or transformed to a nonextractable form within Fe oxyhydroxides; DOM is either released into the solution or preserved in an extractable form, while it is not incorporated or retained in the interior. Element distribution and DOM composition analysis on Fe oxyhydroxides reveals even distribution of phosphate on the newly formed Fe oxyhydroxides, while the distribution of DOM depends on its specific species. Electrochemical and dynamic force spectroscopic results provide molecular-scale thermodynamic evidence explaining the inhibition of electron transfer between Fe(II) and ferrihydrite by phosphate and DOM, thus influecing the crystalline transformation of ferrihydrite and the distribution of phosphate and DOM. This study provides new insights into the coupled biogeological cycle of Fe with phosphate and DOM in aquatic and terrestrial environments.

New Barrier Role of Iron Plaque: Producing Interfacial Hydroxyl Radicals to Degrade Rhizosphere Pollutants

Iron plaque, as a natural barrier between rice and soil, can reduce the accumulation of pollutants in rice by adsorption, contributing to the safe production of rice in contaminated soil. In this study, we unveiled a new role of iron plaque, i.e., producing hydroxyl radicals (·OH) by activating root-secreted oxygen to degrade pollutants. The ·OH was produced on the iron plaque surface and then diffused to the interfacial layer between the surface and the rhizosphere environment. The iron plaque activated oxygen via a successive three-electron transfer to produce ·OH, involving superoxide and hydrogen peroxide as the intermediates. The structural Fe(II) in iron plaque played a dominant role in activating oxygen rather than the adsorbed Fe(II), since the structural Fe(II) was thermodynamically more favorable for oxygen activation. The oxygen vacancies accompanied by the structural Fe(II) played an important role in oxygen activation to produce ·OH. The interfacial ·OH selectively degraded rhizosphere pollutants that could be adsorbed onto the iron plaque and was less affected by the rhizosphere environments than the free ·OH. This study uncovered the oxidative role of iron plaque mediated by its produced ·OH, reshaping our understanding of the role of iron plaque as a barrier for rice.

The fate of arsenic during the crystallization process of FeIII oxyhydroxides: Effect of reaction media, pH value, and Fe/As molar ratio under relatively low arsenic loading

Understanding the nature of arsenic (As) adsorbed on FeIII oxyhydroxides, and the subsequent behavior of As during the crystallization process, is critical to predicting its fate in a range of natural and engineered settings. In this work, As adsorbed on FeIII oxyhydroxides formed in the different reaction media at different pH values were characterized with X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), and extended X-ray absorption fine structure spectroscopy (EXAFS) to determine how As is redistributed during the crystallization process. Results showed that at pH 12, a quarter of the added As was still left in the liquid phase with the formation of goethite and hematite as the major and minor product. The concentration of As was found to be the lowest at pH 4 which is independent of the reaction media, indicating the importance of pH value in the crystallization process of the As adsorbed FeIII oxyhydroxides. Under acidic conditions, sulfate and chloride media favored the formation of goethite and hematite, respectively. Arsenic can indeed be incorporated into the structure of the formed goethite at pH 4. The morphology of the formed products changed to rhombus-like particles if both goethite and hematite appeared as the later as the dominant product.

Electrodeposited copper enhanced removal of 2,4-dichlorophenol in batch and flow reaction in Cu@CC-PS-MFC system

Combination of microbial fuel cell (MFC) and advanced oxidation process (AOP) is promising for pollutant removal. In this paper, Cu0-loaded carbon cloth cathode by electrodeposition (Cu@CC-PS-MFC) was applied to enhance 2,4-dichlorophenol (2,4-DCP) degradation based on persulfate (PS) activation in microbial fuel cell. Cu0 exhibited a typical structure of face-centered cubic metal polyhedron on carbon cloth. The removal of 2,4-DCP by Cu@CC-PS-MFC (75.6%) was enhanced by more than 50% compared to CC-PS-MFC (49.2%) after 1 h of reaction. 30 mg/L 2,4-DCP in Cu@CC-PS-MFC was completely removed and achieved a high mineralization (80.6%) after 9 h of reaction under optimized condition with low dissolved copper ion concentration (0.615 mg/L). Meanwhile, more than 90% removal of 2,4-DCP was stably achieved with flow operation condition (hydraulic residence time of 7.2 h). The change of copper valent state Cu0/Cu2O/CuO was the main mechanism of PS activation with main reactive species of O•H and O21. The bioanode of MFC enhanced the in-situ regeneration of ≡Cu+ and ≡Cu0 on the catalyst surface by transporting electrons, which was believed to contribute to good catalyst lifetime and excellent 2,4-DCP removal. Electrodeposited copper contributes to the enhanced degradation of 2,4-DCP with energy recovery at the same time which can further broaden the application MFC.

Enhanced arsenic(III) sequestration via sulfidated zero-valent iron in aerobic conditions: Adsorption and oxidation coupling processes

Sulfidated zero-valent iron (S-ZVI) has shown significant potential for the removal of arsenic(III). However, little attention has been paid to the mechanism of As(III) sequestration enhancement and how the phase transformation for S-ZVI strengthens this process in aerobic conditions. In this work, sulfidated ZVI was created by ball-milling (S-ZVIbm) and liquid-mixing (S-ZVIlm) of ZVI with elemental sulfur(S0) to investigate the performance and mechanisms of As(III) sequestration in air-saturated water. Sulfidation was found to significantly enhance the As(III) removal rate constant, which was 2.8 ∼ 6.7 times (S-ZVIbm) and 3.1 ∼ 17.1 times (S-ZVIlm) higher than that without sulfidation. FeS was identified as the predominant sulfur species in the S-ZVI samples using S K-edge XANES spectra. The enhanced electron transfer and ZVI corrosion after sulfidation were verified via electrochemical tests. XANES and Mössbauer spectra suggested that lepidocrocite(γ-FeOOH) was the predominant corrosion product generated on the ZVI surface with the presence of oxygen, and DFT calculations further confirmed the improved performance of γ-FeOOH for As(III) sequestration. Besides, As(III) oxidation occurred dominantly on the heterogeneous surface rather than in solution, and the As(III) sequestration pathway of adsorption followed by oxidation was proposed. This study provides new insight into the enhanced As(III) sequestration by S-ZVI in aerobic conditions.

The key roles of Fe oxyhydroxides and humic substances during the transformation of exogenous arsenic in a redox-alternating acidic paddy soil

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Production of reactive oxygen species from oxygenation of Fe(II)-carbonate complexes: The critical roles of carbonate

Hydroxyl radicals (•OH) production upon the oxygenation of reduced iron minerals at the oxic/anoxic interface has been well recognized. However, little is known in the influencing environmental factors and the involved mechanisms. In this study, much more •OH could be efficiently produced from oxygenation of Fe(II) with 20-200 mM carbonate. Both carbonate concentration and anoxic reaction time play a critical role in •OH production. High carbonate facilitates the formation of Fe(II)_{high reactivity}, i.e., surface-adsorbed and structural Fe(II) with low crystalline that is reactive toward O₂ reaction for •OH production, while long anoxic reaction time enables the transfer from Fe(II)_{high reactivity} to Fe(II)_{low reactivity}, i.e., Fe(II) at interior sites with high crystalline, that is hardly oxidized by O₂. Furthermore, the degradation pathway of p-nitrophenol (PNP) is highly dependent on the carbonate concentration that low carbonate facilitates •OH oxidation of PNP (80.2%) while high carbonate enhanced O₂^•- reduction of PNP (48.7%). Besides, carbonate also influences the structural evolution of Fe mineral during oxygenation by retarding its hydrolysis and following transformation. Our finding sheds new light on understanding the important role of oxyanions such as carbonate in iron redox cycles and directing contaminant attenuation in subsurface environment.