Application of infrared spectroscopy and its theoretical simulation to arsenic adsorption processes

Accurate detection and analysis of arsenic pollutants are an important means to enhance the ability to manage arsenic pollution. Infrared (IR) spectroscopy technology has the advantages of fast analysis speed, high resolution, and high sensitivity and can be monitored by real-time in situ analysis. This paper reviews the application of IR spectroscopy in the qualitative and quantitative analysis of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite (FH), hematite, goethite, and titanium dioxide. The IR spectroscopy technique cannot only identify different arsenic contaminants but also obtain the content and adsorption rate of arsenic contaminants in the solid phase. The reaction equilibrium constants and the degree of reaction conversion can be determined by constructing adsorption isotherms or combining them with modeling techniques. Theoretical calculations of IR spectra of mineral adsorbed arsenic pollutant systems based on density functional theory (DFT) and analysis and comparison of the measured and theoretically calculated characteristic peaks of IR spectra can reveal the microscopic mechanism and surface chemical morphology of the arsenic adsorption process. This paper systematically summarizes the qualitative and quantitative studies and theoretical calculations of IR spectroscopy in inorganic and organic arsenic pollutant adsorption systems, which provides new insights for accurate detection and analysis of arsenic pollutants and arsenic pollution control. PRACTITIONER POINTS: This paper reviews the application of infrared spectroscopy in the qualitative and quantitative analyses of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite, hematite, goethite, and titanium dioxide, which can help identify and evaluate the type and concentration of arsenic pollutants in water bodies. In this paper, theoretical calculations of infrared spectra of mineral adsorbed arsenic pollutant systems based on density functional theory reveal the adsorption mechanism of arsenic pollutants in water at the solid-liquid interface and help to develop targeted arsenic pollution control technologies. This paper provides a new and reliable analytical detection technique for the study of arsenic contaminants in water bodies.

Insight into the Sensing Behavior of DNA Probes Based on MOF-Nucleic Acid Interaction for Bioanalysis

Adsorption of DNA probes onto nanomaterials is a promising strategy for bioassay establishment typically using fluorescence or catalytic activities to generate signals. Albeit important, there is currently a lack of systematic understanding of the sensing behaviors building on nanomaterial-DNA interactions, which greatly limits the rational method design and their subsequent applications. Herein, the issue was investigated by employing multifunctional metal-organic frameworks (MOFs) (FeTCPP⊂UiO-66) as a model that was synthesized via integrating heme-like ligand FeTCPP into commonly used MOFs (UiO-66). Our results demonstrated that the fluorescently labeled DNA adsorbed onto FeTCPP⊂UiO-66 was quenched through photoinduced electron transfer, fluorescence resonance energy transfer, and the internal filtration effect. Among different DNA structures, double-stranded DNA and hybridization chain reaction products largely retained their fluorescence due to desorption and conformational variation, respectively. In addition, ssDNA could maximally inhibit the peroxidase activity of FeTCPP⊂UiO-66, and this inhibition was strongly dependent on the strand length but independent of base composition. On the basis of these discoveries, a fluorescence/colorimetric dual-modal detection was designed against aflatoxin B1 with satisfactory performances obtained to further verify our results. This study provided some new insights into the sensing behaviors based on MOF-DNA interactions, indicating promising applications for rational bioassay design and its performance improvement.

Rapid Redox Cycling of Fe(II)/Fe(III) in Microdroplets during Iron-Citric Acid Photochemistry

Fe(III) and carboxylic acids are common compositions in atmospheric microdroplet systems like clouds, fogs, and aerosols. Although photochemical processes of Fe(III)-carboxylate complexes have been extensively studied in bulk aqueous solution, relevant information on the dynamic microdroplet system, which may be largely different from the bulk phase, is rare. With the help of the custom-made ultrasonic-based dynamic microdroplet photochemical system, this study examines the photochemical process of Fe(III)-citric acid complexes in microdroplets for the first time. We find that when the degradation extent of citric acid is similar between the microdroplet system and the bulk solution, the significantly lower Fe(II) ratio is present in microdroplet samples due to the rapider reoxidation of photogenerated Fe(II). However, by replacing citric acid with benzoic acid, no much difference in the Fe(II) ratio between microdroplets and bulk solution is observed, which indicates distinct reoxidation pathways of Fe(II). Moreover, the presence of ^•OH scavenger, namely, methanol, greatly accelerates the reoxidation of photogenerated Fe(II) in both citric acid and benzoic acid situations. Further experiments reveal that the high availability of O₂ and the citric acid- or methanol-derived carbon-centered radicals are responsible for the rapider reoxidation of Fe(II) in iron-citric acid microdroplets by prolonging the length of HO₂^•- and H₂O₂-involved radical reaction chains. The results in this study may provide a new understanding about iron-citric acid photochemistry in atmospheric liquid particles, which can further influence the photoactivity of particles and the formation of secondary organic aerosols.

Biochar-derived dissolved black carbon accelerates ferrihydrite microbial transformation and subsequent imidacloprid degradation

The effects of an electron shuttle (dissolved black carbon (DBC) derived from biochar) on the microbial reduction of ferrihydrite and subsequent imidacloprid (IMI) degradation were studied. The results showed that DBC addition enhanced the microbial reduction of Fe(III) in ferrihydrite and increased the quantity of Fe(II) released into the liquid phase. The electron transfer capacity of DBC was significantly influenced by the content of redox-active oxygen-containing functional groups (e.g., quinone, hydroquinone, and polyphenol groups), which was dependent on the pyrolysis temperature. The electrochemical characteristics of DBC resulted in enhanced electron transfer, which promoted Fe(III) reduction and mediated the microbial transformation of ferrihydrite. The microbial transformation of ferrihydrite resulted in the formation of secondary minerals such as siderite and vivianite. The IMI degradation efficiency was related to the Fe(III) reduction rate and the pyrolysis temperature used in DBC production, and the degradation pathways were nitrate reduction and imino hydrolysis induced by the Fe(II) generated from the reduction of Fe(III) in ferrihydrite. The results obtained in this study provide new data for understanding the multifunctional roles of biochar-derived DBC in the redox and transformation processes of iron minerals induced by iron-reducing bacteria, the related biogeochemical cycles of iron and the fate of pollutants.

Humic acid and fulvic acid facilitate the formation of vivianite and the transformation of cadmium via microbially-mediated iron reduction

The effects of humic acids (HA) and fulvic acids (FA) on the fate of Cd in anaerobic environment upon microbial reduction of Cd-bearing ferrihydrite (Fh) with Geobacter metallireducens were investigated. The results showed that HA and FA could promote the reductive dissolution of Fh and the formation of vivianite. After incubation of 38 d, vivianite accounted for 47.19%, 59.22%, and 48.53% of total Fe in biological control batch (BCK), HA and FA batches (C/Fe molar ratio of 1.0), respectively, by Mössbauer spectroscopy analysis. In terms of Cd, HA and FA could promote the release of adsorbed Cd during the initial bioreduction process, but reassuringly, after 38 d the dissolved Cd with HA and FA addition batches were 0.58-0.91 and 0.99-1.08 times of the BCK, respectively. The proportions of residual Cd in HA batches were higher than FA and BCK batches, indicating that HA was better than FA in immobilizing Cd. This might be because the quinone groups in HA could act as electron shuttle. This study showed that HA facilitated the transformation of vivianite better than FA, and Cd can be stabilized by resorption or co-precipitation with vivianite, providing a theoretical support for the translocation of Cd in sediment-water interface.

Insights into the influence of Fe(III) on the interaction between roxarsone and humic acid using multi-spectroscopic techniques

The influence of Fe(III) on the interaction between roxarsone (ROX) and humic acid (HA) was investigated by multi-spectroscopic techniques. The fluorescence quenching experiment indicated that the fluorescence intensity of HA-ROX was quenched by Fe(III) through a static quenching process. Synchronous fluorescence spectra provided further information concerning the competitive combination between ROX and Fe(III) for HA. The results of the dialysis equilibrium experiment confirmed the existence of Fe(III) (0.05-0.1 mmol/L) promotes the combination of HA and ROX. Binding mechanisms were further characterized by FTIR spectroscopy, and the carboxyl functional group is involved in the binding process of HA/Fe/ROX. In addition, acidic and neutral conditions are more conducive to the combination of ROX and HA/Fe than alkaline conditions. The above discussion is of great significance in understanding the environmental fate of ROX under the coexistence of Fe(III) and HA.

Transformations of Ferrihydrite-Extracellular Polymeric Substance Coprecipitates Driven by Dissolved Sulfide: Interrelated Effects of Carbon and Sulfur Loadings

The association of poorly crystalline iron (hydr)oxides with organic matter (OM), such as extracellular polymeric substances (EPS), exerts a profound effect on Fe and C cycles in soils and sediments, and their behaviors under sulfate-reducing conditions involve complicated mineralogical transformations. However, how different loadings and types of EPS and water chemistry conditions affect the sulfidation still lacks quantitative and systematic investigation. We here synthesized a set of ferrihydrite-organic matter (Fh-OM) coprecipitates with various model compounds for plant and microbial exopolysaccharides (polygalacturonic acids, alginic acid, and xanthan gum) and bacteriogenic EPS (extracted from Bacillus subtilis). Combining wet chemical analysis, X-ray diffraction, and X-ray absorption spectroscopic techniques, we systematically studied the impacts of C and S loadings by tracing the temporal evolution of Fe mineralogy and speciation in aqueous and solid phases. Our results showed that the effect of added OM on sulfidation of Fh-OM coprecipitates is interrelated with the amount of loaded sulfide. Under low sulfide loadings (S(-II)/Fe < 0.5), transformation to goethite and lepidocrocite was the main pathway of ferrihydrite sulfidation, which occurs more strongly at pH 6 compared to that at pH 7.5, and it was promoted and inhibited at low and high C/Fe ratios, respectively. While under high sulfide loadings (S(-II)/Fe > 0.5), the formation of secondary Fe-S minerals such as mackinawite and pyrite dominated ferrihydrite sulfidation, and it was inhibited with increasing C/Fe ratios. Furthermore, all three synthetic EPS proxies unanimously inhibited mineral transformation, while the microbiogenic EPS has a more potent inhibitory effect than synthetic EPS proxies compared at equivalent C/Fe loadings. Collectively, our results suggest that the quantity and chemical characteristics of the associated OM have a strong and nonlinear influence on the extent and pathways of mineralogical transformations of Fh-OM sulfidation.

Coagulation characteristic and mechanism of Fe(III) salts toward typical Cr(III) complexes in wastewater treatment

Cr(III) complexes are typical pollutants in various industrial wastewater and pose a serious threat to the ecosystem and humans. The coagulation process is commonly used in water treatment plants, yet its removal characteristic and mechanism toward Cr(III) complexes have been rarely reported. In this study, the Fe(III) coagulation process was adopted for the evaluation of Cr(III) complex removal in terms of Cr residual concentration as well as floc size. The results showed that Fe(III) with a dose of 0.8 mM removed more than 80% of total Cr for Cr³⁺ and Cr(III)-acetate, whereas poor removal rate (~ 50%) was obtained for Cr(III)-citrate under the same conditions. Neutral and alkaline conditions facilitated Cr(III)-acetate removal by Fe(III) coagulation, while limited influence was observed for Cr(III)-citrate with various pH. The main removal mechanism of Cr(III)-acetate was precipitation. Cr(III)-citrate elimination largely relied on the adsorption property and sweeping effect of Fe floc. Moreover, Cr(III)-acetate was easier to be separated from a solution since the generated floc sizes were 270 μm. Flocs that formed in the Cr(III)-citrate treatment were only 0.3 μm, resulting in separation difficulties during the coagulation process. The presence of Cr(III)-acetate and Cr(III)-citrate caused a significant decline in membrane flux. This study provided fundamental knowledge of Fe coagulation treatment in Cr(III) complex-containing wastewater.

Synergism of Fe and Al salts for the coagulation of dissolved organic matter: Structural developments of Fe/Al-organic matter associations

Dissolved organic matter (DOM) is distributed ubiquitously in water bodies. Ferric ions can flocculate DOM to form stable coprecipitates; however, Al(III) may alter the structures and stability of Fe-DOM coprecipitates. This study aimed to examine the coprecipitation of Fe, Al, and DOM as well as structural developments of Fe-DOM coprecipitates in relation to changes in Fe/Al ratios and pHs. The results showed that the derived Fe/Al/DOM-coprecipitates could be classified into three categories: (1) at pH 3.0 and 4.5, the corner-sharing FeO₆ octahedra associated with Fe-C bonds with Fe/(Fe + Al) ratios ≥0.5; (2) the Fe-C bonds along with single Fe octahedra having Fe/(Fe + Al) ratios of 0.25; (3) at pH 6.0, the ferrihydrite-like Fe domains associated with Fe-C bonds with Fe/(Fe + Al) ratios ≥0.5. At pH 3.0, the Fe and C stability of the coprecipitates increased with increasing Al proportions; nonetheless, pure Al-DOM coprecipitates were unstable even if they exhibited the maximum ability for DOM removal. The associations of Al-DOM complexes and/or DOM-adsorbed Al domains with external surfaces of Fe domain or Fe-DOM coprecipitates may stabilize DOM, leading to lower C solubilization at pH 4.5. Although the preferential formation of Fe/Al hydroxides decreased Fe/Al solubilization at pH 6.0, adsorption instead of coprecipitation of DOM with Fe/Al hydroxides may decrease C stabilization in the coprecipitates. Aluminum cations inhibit DOM releases from Fe/Al/DOM-coprecipitates, promoting the treatment and reuse efficiencies of wastewater and resolving water shortages. This study demonstrates that Al and solution pH greatly affect the structural changes of Fe-DOM coprecipitates and indirectly control the dynamics of Fe, Al, and C concentrations in water.

Effects of montmorillonite on the adsorption of Fe(II) by ferrihydrite and its phase transformation at different pH

Recently, there has been a clear understanding of the mechanism and influencing factors of ferrihydrite (Fh) phase transformation catalyzed by Fe(II); however, these factors mainly belong to environmental conditions and exogenous substances. And there is a lack of research on the effect of soil composition and structure on the phase transformation of Fh. Therefore, this study investigated the effects of montmorillonite (Mt) on the adsorption of Fe(II) and phase transformation of Fh under near-neutral pH. The initial rates ([Formula: see text]) of Elovich equation demonstrated the addition of Mt inhibited the adsorption of Fh but simultaneously accelerated the initial adsorption, thus increasing the adsorption of the system (e.g., 22.09-25.03 mg/g as increased Mt under pH 6.5) due to its high surface charge density. Increased pH enhances the surface charge density by promoting the deprotonation of the surface group (Fe-OH, Al-OH, and Si-OH) and consequently increases adsorption of Fe(II) (e.g., 17.97-22.09 mg/g as increased pH of pure Fh). Based on the previous method of extracting labile Fe(III), we found that pH promotes the initial formation of labile Fe(III) by increasing electron transfer and promoting recrystallization caused by bridging condensation, via increased -OH. Although Mt inhibits the adsorption of Fh, it promotes the formation of labile Fe(III) by increasing the system adsorption and bond with Fh. The results of the analysis of variance showed both pH and solid ratio influence significantly on the maximum adsorption (p = 6.81 × 10^-9 and 2.54 × 10^-3) and the conversion ratios of labile Fe(III) (p = 3.43 × 10^-24 and 9.16 × 10^-43).

Photoinduced transformation of ferrihydrite in the presence of aqueous sulfite and its influence on the repartitioning of Cd

The photoinduced transformation of ferrihydrite is an important process that can predict the geochemical cycle of Fe in anoxic environments as well as the fate of trace elements bonded to Fe minerals. We report that the photooxidation of sulfite by UV irradiation produces hydrated electrons (super-reductants), which significantly promote ferrihydrite reduction to Fe(II), and SO₃^•- (a moderate oxidant), enabling its further oxidation to more crystalline Fe(III) products. The experimental results show that the concentration of sulfite was key in influencing the rate and extent of surface-bound Fe(II) formation, which ultimately determined the distribution of individual products. For example, fitting of the Mössbauer spectroscopy data revealed that the relative abundances of mineral species after 8 h of treatment in the UV/sulfite systems were 41.9% lepidocrocite and 58.1% ferrihydrite at 2 mM SO₃^2-; 41.8% goethite, 28.2% lepidocrocite, and 29.1% ferrihydrite at 5 mM SO₃^2-; and 100% goethite at 10 mM SO₃^2-. The combined results of the chemical speciation analysis and the Cd K-edge EXAFS characterization provided compelling evidence that Cd was firmly incorporated into the structure of newly formed minerals, particularly at high sulfite concentrations. These findings provide an understanding of the role of UV/sulfite in facilitating ferrihydrite transformation and promoting Cd stabilization in oxygen-deficit soils and aquatic environments.

Iron isotopic fractionation driven by low-temperature biogeochemical processes

Iron is geologically important and biochemically crucial for all microorganisms, plants and animals due to its redox exchange, the involvement in electron transport and metabolic processes. Despite the abundance of iron in the earth crust, its bioavailability is very limited in nature due to its occurrence as ferrihydrite, goethite, and hematite where they are thermodynamically stable with low dissolution kinetics in neutral or alkaline environments. Organisms such as bacteria, fungi, and plants have evolved iron acquisition mechanisms to increase its bioavailability in such environments, thereby, contributing largely to the iron cycle in the environment. Biogeochemical cycling of metals including Fe in natural systems usually results in stable isotope fractionation; the extent of fractionation depends on processes involved. Our review suggests that significant fractionation of iron isotopes occurs in low-temperature environments, where the extent of fractionation is greatly governed by several biogeochemical processes such as redox reaction, alteration, complexation, adsorption, oxidation and reduction, with or without the influence of microorganisms. This paper includes relevant data sets on the theoretical calculations, experimental prediction, as well as laboratory studies on stable iron isotopes fractionation induced by different biogeochemical processes.

Influence of Humic Acids on the Removal of Arsenic and Antimony by Potassium Ferrate

Although the removal ability of potassium ferrate (K₂FeO₄) on aqueous heavy metals has been confirmed by many researchers, little information focuses on the difference between the individual and simultaneous treatment of elements from the same family of the periodic table. In this project, two heavy metals, arsenic (As) and antimony (Sb) were chosen as the target pollutants to investigate the removal ability of K₂FeO₄ and the influence of humic acid (HA) in simulated water and spiked lake water samples. The results showed that the removal efficiencies of both pollutants gradually increased along the Fe/As or Sb mass ratios. The maximum removal rate of As(III) reached 99.5% at a pH of 5.6 and a Fe/As mass ratio of 4.6 when the initial As(III) concentration was 0.5 mg/L; while the maximum was 99.61% for Sb(III) at a pH of 4.5 and Fe/Sb of 22.6 when the initial Sb(III) concentration was 0.5 mg/L. It was found that HA inhibited the removal of individual As or Sb slightly and the removal efficiency of Sb was significantly higher than that of As with or without the addition of K₂FeO₄. For the co-existence system of As and Sb, the removal of As was improved sharply after the addition of K₂FeO₄, higher than Sb; while the latter was slightly better than that of As without K₂FeO₄, probably due to the stronger complexing ability of HA and Sb. X-ray energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the precipitated products to reveal the potential removal mechanisms based on the experimental results.

Facet Dependence of Biosynthesis of Vivianite from Iron Oxides by Geobacter sulfurreducens

Vivianite plays an important role in alleviating the phosphorus crisis and phosphorus pollution. The dissimilatory iron reduction has been found to trigger the biosynthesis of vivianite in soil environments, but the mechanism behind this remains largely unexplored. Herein, by regulating the crystal surfaces of iron oxides, we explored the influence of different crystal surface structures on the synthesis of vivianite driven by microbial dissimilatory iron reduction. The results showed that different crystal faces significantly affect the reduction and dissolution of iron oxides by microorganisms and the subsequent formation of vivianite. In general, goethite is more easily reduced by Geobacter sulfurreducens than hematite. Compared with Hem_{100} and Goe_L{110}, Hem_{001} and Goe_H{110} have higher initial reduction rates (approximately 2.25 and 1.5 times, respectively) and final Fe(II) content (approximately 1.56 and 1.20 times, respectively). In addition, in the presence of sufficient PO₄^3-, Fe(II) combined to produce phosphorus crystal products. The final phosphorus recoveries of Hem_{001} and Goe_H{110} systems were about 5.2 and 13.6%, which were 1.3 and 1.6 times of those of Hem_{100} and Goe_L{110}, respectively. Material characterization analyses indicated that these phosphorous crystal products are vivianite and that different iron oxide crystal surfaces significantly affected the size of the vivianite crystals. This study demonstrates that different crystal faces can affect the biological reduction dissolution of iron oxides and the secondary biological mineralization process driven by dissimilatory iron reduction.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Molecular investigation of the multi-phase photochemistry of Fe(III)-citrate in aqueous solution

Iron (Fe) is ubiquitous in nature and found as Fe^II or Fe^III in minerals or as dissolved ions Fe²⁺ or Fe³⁺ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of Fe^III into natural aquatic systems, organic carboxylic acids efficiently chelate Fe^III to form [Fe^III-carboxylate]²⁺ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of Fe^III-carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of Fe^III-citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of (φ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe³⁺-citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j₁ ∼ 0.12 min^-1 and j₂ ∼ 0.05 min^-1, respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe-organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe-carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of Fe^III-citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments.

Generic CD-MUSIC-eSGC model parameters to predict the surface reactivity of iron (hydr)oxides

The surface reactivity of iron (hydr)oxides plays a crucial role in controlling their interfacial reactions, for which various surface complexation models have been developed. The diversity of mineralogical properties of iron (hydr)oxides has resulted in a redundancy of model parameters, which hampers the modeling of iron (hydr)oxides in soils and sediments, where goethite, hematite and ferrihydrite dominate the iron (hydr)oxide mass fraction. To capture their combined surface reactivity, optimized generic protonation parameters of the Charge Distribution-Multisite Complexation (CD-MUSIC) extended-Stern-Gouy-Chapman (eSGC) model were derived by reanalyzing literature datasets and tested with some newly synthesized iron (hydr)oxides. It was observed that the proton and monovalent ion affinity constants of the different iron (hydr)oxides were located in a narrow range. For the singly- and triply-coordinated hydroxyl sites the obtained generic log(affinity constants) were 8.3 and 11.7 for the protonation reaction and -0.5 for the reaction with the monovalent background ions. Their combination with fixed site densities of singly-/triply-coordinated hydroxyl sites of 3.45/2.70, 5.00/2.50, and 5.80/1.40 sites/nm² for goethite, hematite, and ferrihydrite, respectively, provided good results. The Stern layer capacitances of the inner and outer Stern layers were set equal and could be acquired by an empirical correlation with the sample specific surface area (SSA). The CD-MUSIC-eSGC model with the generic model parameters enables good quality predictions of the proton reactivity of iron (hydr)oxides in 1:1 electrolyte solutions regardless of the sample heterogeneity. The advantages of the generic CD-MUSIC-eSGC model are twofold: (1) protonation of iron (hydr)oxides can be described without making use of spectroscopic measurements and proton titrations, and (2) the model calculations are greatly simplified.

Enhanced Direct Electron Transfer Mediated Contaminant Degradation by Fe(IV) Using a Carbon Black-Supported Fe(III)-TAML Suspension Electrode System

Iron complexes of tetra-amido macrocyclic ligands (Fe-TAML) are recognized to be effective catalysts for the degradation of a wide range of organic contaminants in homogeneous conditions with the high valent Fe(IV) and Fe(V) species generated on activation of the Fe-TAML complex by hydrogen peroxide (H₂O₂) recognized to be powerful oxidants. Electrochemical activation of Fe-TAML would appear an attractive alternative to H₂O₂ activation, especially if the Fe-TAML complex could be attached to the anode, as this would enable formation of high valent iron species at the anode and, importantly, retention of the valuable Fe-TAML complex within the reaction system. In this work, we affix Fe-TAML to the surface of carbon black particles and apply this "suspension anode" process to oxidize selected target compounds via generation of high valent iron species. We show that the overpotential for Fe(IV) formation is 0.17 V lower than the potential required to generate Fe(IV) electrochemically in homogeneous solution and also show that the stability of the Fe(IV) species is enhanced considerably compared to the homogeneous Fe-TAML case. Application of the carbon black-supported Fe-TAML suspension anode reactor to degradation of oxalate and hydroquinone with an initial pH value of 3 resulted in oxidation rate constants that were up to three times higher than could be achieved by anodic oxidation in the absence of Fe-TAML and at energy consumptions per order of removal substantially lower than could be achieved by alternate technologies.

Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs

Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, Fe^II is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5-7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of Fe^II oxidation at all of the investigated DO of 0-5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30-40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10-20% when Fe^II oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of Fe^II oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of Fe^II oxidation and Fe^III hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.

Iron Vacancy Accelerates Fe(II)-Induced Anoxic As(III) Oxidation Coupled to Iron Reduction

Chemical oxidation of As(III) by iron (Fe) oxyhydroxides has been proposed to occur under anoxic conditions and may play an important role in stabilization and detoxification of As in subsurface environments. However, this reaction remains controversial due to lack of direct evidence and poorly understood mechanisms. In this study, we show that As(III) oxidation can be facilitated by Fe oxyhydroxides (i.e., goethite) under anoxic conditions coupled with the reduction of structural Fe(III). An excellent electron balance between As(V) production and Fe(III) reduction is obtained. The formation of an active metastable Fe(III) phase at the defective surface of goethite due to atom exchange is responsible for the oxidation of As(III). Furthermore, the presence of defects (i.e., Fe vacancies) in goethite can noticeably enhance the electron transfer (ET) and atom exchange between the surface-bound Fe(II) and the structural Fe(III) resulting in a two time increase in As(III) oxidation. Atom exchange-induced regeneration of active goethite sites is likely to facilitate As(III) coordination and ET with structural Fe(III) based on electrochemical analysis and theoretical calculations showing that this reaction pathway is thermodynamically and kinetically favorable. Our findings highlight the synergetic effects of defects in the Fe crystal structure and Fe(II)-induced catalytic processes on anoxic As(III) oxidation, shedding a new light on As risk management in soils and subsurface environments.

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

The dark production of reactive oxygen species (ROS) coupled to biogeochemical cycling of iron (Fe) plays a pivotal role in controlling arsenic transformation and detoxification. However, the effect of secondary atom incorporation into Fe(III) oxyhydroxides on this process is poorly understood. Here, we show that the presence of oxygen vacancy (OV) as a result of Cu incorporation in goethite substantially enhances the As(III) oxidation by Fe(II) under oxic conditions. Electrochemical and density functional theory (DFT) evidence reveals that the electron transfer (ET) rate constant is enhanced from 0.023 to 0.197 s^-1, improving the electron efficiency of the surface-bound Fe(II) on OV defective surfaces. The cascade charge transfer from the surface-bound Fe(II) to O₂ mediated by Fe(III) oxyhydroxides leads to the O-O bond of O₂ stretching to 1.46-1.48 Å equivalent to that of superoxide (^•O₂^-), and ^•O₂^- is the predominant ROS responsible for As(III) oxidation. Our findings highlight the significant role of atom incorporation in changing the ET process on Fe(III) oxyhydroxides for ROS production. Thus, such an effect must be considered when evaluating Fe mineral reactivity toward changing their surface chemistry, such as those noted here for Cu incorporation, which likely determines the fates of arsenic and other redox sensitive pollutants in the environments with oscillating redox conditions.

Influence of citrate/tartrate on chromite crystallization behavior and its potential environmental implications

The ferrite process has been developed to purify wastewater containing heavy metal ions and recycle valuable metals by forming chromium ferrite. However, organic matter has an important influence on the crystallization behavior and stability of chromite synthesized from chromium-containing wastewater. We focused on the influence and effect mechanism of two typical organic acid salts (citrate (CA) and tartrate (TA)) on the process of chromium mineralization. It was found that the presence of organic matter leads to the increase of the residual content of Cr in CA system (0.50 mmol/L) and TA system (0.61 mmol/L) in the solution, and the removal of chromium was mainly due to the surface adsorption of Fe(III) hydrolysate. The decreased crystallinity of mineralized products is ascribed to the completion of organic compounds with Fe(II) and Fe(III), which hinders the formation of ferrite precursors. There was bidentate and monodentate chelation between -COO- and metal ions in the CA system and TA system respectively, which resulted in a stronger affinity between CA and iron. This study provides the underlying mechanism for Cr(III) solid oxidation by the ferrite method in an organic matter environment and is of great significance to prevent and control chromium pollution in the environment.

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.

Role of Al substitution in the reduction of ferrihydrite by Shewanella oneidensis MR-1

Substitution of aluminum under natural environmental conditions has been proven to inhibit the transformation of weakly crystalline iron (oxyhydr)-oxides towards well crystalline iron oxides, thereby enhancing their long-term stability. However, exploration on the role of aluminum substitution in bacteria-mediated iron oxides transformation is relatively lacking, especially in the anaerobic underground condition where iron (oxyhydr)-oxides are easy to reduced. In this study, we selected four different levels of substitution aluminum prevalent in iron oxides under natural conditions, which are 0 mol%, 10 mol%, 20 mol%, and 30 mol% (mol Al/mol (Al + Fe)) respectively. With the presence of Shewanella oneidensis MR-1, we conducted a 15-day anaerobic microcosm experiment in simulated groundwater conditions. The experiment data suggested that aluminum substitution result in a decrease in bio-reduction rate constants of ferrihydrite from 0.24 in 0 mol% Al to 0.17 in 30 mol% Al. Besides, when containing substituted aluminum, secondary minerals produced by biological reduction of ferrihydrite changed from magnetite to akaganeite. These results were attributed to the surface coverage of Al during the reduction process, which affects the contact between S. oneidensis MR-1 and the unexposed Fe(III), thus inhibiting the further reduction of ferrihydrite. Since iron (oxyhydr)-oxides exhibit a strong affinity on multiple kinds of pollutants, results in this study may contribute to predicting the migration and preservation of contaminants in groundwater systems.

Immobilization of As(III) by gibbsite and catalytic oxidation to As(V): Profound impacts of doping and unraveling of associated mechanisms

As(III) is highly toxic, and its adsorption and oxidation to As(V) by minerals represent two effective approaches to remediate As(III)-contaminated sites. Gibbsite, one of the most abundant natural minerals, shows decent adsorption for As(III), and in this study, mechanisms of As(III) immobilization and oxidation by gibbsite with different dopants (M = Fe(III), Mn(III), Mn(IV)) are addressed by periodic DFT calculations. Influences of Fe(III) content and Mn oxidation state are also inspected. Although a majority remain structurally similar to those of pristine gibbsite, new adsorption configurations emerge due to doping: Inner-sphere complexes with M - As bonds for all doping, bidentate binuclear complexes for double Fe(III) doping, and physisorption with weak O_Mn-As interactions for Mn(IV) doping. As(III) adsorption affinities are significantly altered by doping and rely on dopants, while inner-sphere complexes with M-O_As bonds are always lowest-energy except doping Mn(III) that prefers trigonal bipyramidal coordination and impedes As(III) chemisorption. Doping causes strong M-3d and O_As-2p orbital interactions that facilitate As(III) adsorption whereas disappear for pristine gibbsite. Double Fe(III)- and Mn(IV)-doped gibbsite materials are effective for As(III) oxidation to As(V), and mechanisms differ significantly although all are characterized by dual electron transfers. Activation barriers for the most favorable reaction paths amount to 1.02 and 1.26-1.31 eV, respectively. Physisorbed and outer-sphere As(III) complexes exhibit comparable reactivities as chemisorbed complexes that become focus of literature reports, and may also be involved during interfacial and environmental reactions. Results rationalize experimental observations available, and provide significantly new insights that conduce to manage As-associated pollution and design efficient As(III) scavengers and oxidation catalysts.

Enhanced removal of high-As(III) from Cl(-I)-diluted SO4(-II)-rich wastewater at pH 2.3 via mixed tooeleite and (Cl(-I)-free) ferric arsenite hydroxychloride formation

An advanced cost-saving method of removal of high-As(III) from SO₄(-II)-rich metallurgical wastewater has been developed by diluting the SO₄(-II) content with As(III)-Cl(-I)-rich metallurgical wastewater and then by the direct precipitation of As(III) with Fe(III) at pH 2.3. As(III) removal at various SO₄(-II)/Cl(-I) molar ratios and temperatures was investigated. The results showed that 65.2‒98.2% of As(III) immobilization into solids occurred at the SO₄(-II)/Cl(-I) molar ratios of 1:1‒32 and 15‒60 °C in 3 days, which were far higher than those in aqueous sole SO₄(-II) or Cl(-I) media at the equimolar SO₄(-II) or Cl(-I) and the same temperature. SO₄(-II)/Cl(-I) molar ratio of 1:4 and 25 °C were optimal conditions to reach the As removal maximum. Mixed aqueous SO₄(-II) and Cl(-I) played a synergetic role in the main tooeleite formation together with (Cl(-I)-free) ferric arsenite hydroxychloride (FAHC) involving the substitution of AsO₃^3- for Cl(-I) for enhanced As fixation. The competitive complexation among FeH₂AsO₃²⁺, FeSO₄⁺ and FeCl²⁺ complexes was the main mechanism for the maximum As(III) precipitation at the SO₄(-II)/Cl(-I) molar ratio of 1:4. Low As(III) immobilization at high temperature with increased Fe(III) hydrolysis was due to the formation of As(III)-bearing ferrihydrite with the relatively high Fe/As molar ratio at acidic pH.

Enhanced immobility of Pb(II) during ferrihydrite-Pb(II) coprecipitates aging impacted by malic acid or phosphate

Metastable ferrihydrite is omnipresent in environments and can influence the fate of Pb(II) during ferrihydrite transformation. Ferrihydrite is rarely pure and often coexists with impurities, which may influence the mineralogical changes of ferrihydrite and Pb(II) behavior. In this work, we investigated the effect of malic acid or phosphate on Pb(II)-ferrihydrite coprecipitates (Fh-Pb) transformation and the subsequent fate of Pb(II) during the 10-day aging of Fh-Pb. Results showed that both malic acid and phosphate retarded Fh-Pb transformation and prevented the release of Pb(II) from Fh-Pb back into solutions. Pb(II) was beneficial to goethite formation by inhibiting hematite formation while both malic acid and phosphate inhibited goethite formation since they could act as templates of nucleation. Besides, malic acid and phosphate improved the proportion of non-extracted Pb(II) during Fh-Pb transformation, indicating that Pb(II) was incorporated into secondary minerals. Pb(II) could not replace Fe(III) within the crystal lattice due to its large radius but was occluded into pores and defect structures within the secondary mineral lattices. This work can advance our understanding of the influences of malic acid and phosphate on Pb(II) immobility during Fh-Pb aging.

Agglomerated serum albumin adsorbed protocatechuic acid coated superparamagnetic iron oxide nanoparticles as a theranostic agent

Iron oxide nanoparticles have been one of the most widely used nanomaterials in biomedical applications. However, the incomplete understanding of the toxicity mechanisms limits their use in diagnosis and treatment processes. Many parameters are associated with their toxicity such as size, surface modification, solubility, concentration and immunogenicity. Further research needs to be done to address toxicity-related concerns and to increase its effectiveness in various applications. Herein, colloidally stable nanoparticles were prepared by coating magnetic iron oxide nanoparticles (MIONPs) with protocatechuic acid (PCA) which served as a stabilizer and a linkage for a further functional layer. A new perfusion agent with magnetic imaging capability was produced by the adsorption of biocompatible passivating agent macro-aggregated albumin (MAA) on the PCA-coated MIONPs. PCA-coated MIONPs were investigated using infrared spectroscopy, thermogravimetric analysis and dynamic light scattering while adsorption of MAA was analysed by transmission electron microscopy, Fourier-transform infrared spectroscopy and x-ray diffraction methods. Magnetic measurements of samples indicated that all samples showed superparamagnetic behaviour. Cytotoxicity results revealed that the adsorption of MAA onto PCA-coated MIONPs provided an advantage by diminishing their toxicity against the L929 mouse fibroblast cell line compared to bare Fe₃O₄.

Engineering of 3D graphene hydrogel-supported MnO2-FeOOH nanoparticles with synergistic effect of oxidation and adsorption toward highly efficient removal of arsenic

Iron-manganese-based adsorbent has been regarded as a promising candidate for arsenic purification from water, especially the inorganic As(III), due to its inherent advantage of low cost and large-scale producibility. However, the nanoparticle aggregation, metal leaching and insufficient removal efficiency remain the main challenges in the practical applications of the granular adsorbents. In this work, we develop a universal strategy for the fabrication of an active Fe(III) oxyhydroxide-Mn(IV) oxide/3D graphene oxide (GO) gel composite via a simple hydrothermal reaction. The successful immobilization of Fe-Mn oxyhydroxide/oxides on the interconnected GO gels was intuitively confirmed by the transmission electron microscopy and atomic force microscopy. The combinative characterizations of the X-ray absorption near edge structure and X-ray photoelectron spectroscopy clearly reveal the electron transfer from Fe atoms to Mn atoms. The optimized Fe-Mn/GO composites possess the superior performance with the removal efficiency of over 90% for As(III) at pH 7.0 and ∼97% for As(V) at pH 5.0 and the As(III, V) levels (100 μg l^-1) are reduced to below the WHO guideline of 10 μg l^-1. The sorption isotherm and kinetic experiments on the As removal were also carried out. The post characterizations are employed to better unveil the oxidation-adsorption mechanism. Notably, the application of Fe-Mn/GO composites in the treatment of As-simulated natural water demonstrated a stable and continuous operation for over 20 days and an effluent concentration of arsenic as low as the 10 μg l^-1 in a specially designed flow reactor.

Iron redox cycling in layered clay minerals and its impact on contaminant dynamics: A review

A majority of clay minerals contain Fe, and the redox cycling of Fe(III)/Fe(II) in clay minerals has been extensively studied as it may fuel the biogeochemical cycles of nutrients and govern the mobility, toxicity and bioavailability of a number of environmental contaminants. There are three types of Fe in clay minerals, including structural Fe sandwiched in the lattice of clays, Fe species in interlayer space and adsorbed on the external surface of clays. They exhibit distinct reactivity towards contaminants due to their differences in redox properties and accessibility to contaminant species. In natural environments, microbially driven Fe(III)/Fe(II) redox cycling in clay minerals is thought to be important, whereas reductants (e.g., dithionite and Fe(II)) or oxidants (e.g., peroxygens) are capable of enhancing the rates and extents of redox dynamics in engineered systems. Fe(III)-containing clay minerals can directly react with oxidizable pollutants (e.g., phenols and polycyclic aromatic hydrocarbons (PAHs)), whereas structural Fe(II) is able to react with reducible pollutants, such as nitrate, nitroaromatic compounds, chlorinated aliphatic compounds. Also structural Fe(II) can transfer electrons to oxygen (O₂), peroxymonosulfate (PMS), or hydrogen peroxide (H₂O₂), yielding reactive radicals that can promote the oxidative transformation of contaminants. This review summarizes the recent discoveries on redox reactivity of Fe in clay minerals and its links to fates of environmental contaminants. The biological and chemical reduction mechanisms of Fe(III)-clay minerals, as well as the interaction mechanism between Fe(III) or Fe(II)-containing clay minerals and contaminants are elaborated. Some knowledge gaps are identified for better understanding and modelling of clay-associated contaminant behavior and effective design of remediation solutions.

Fe-g-C3N4/reduced graphene oxide lightless application for efficient peroxymonosulfate activation and pollutant mineralization: Comprehensive exploration of reactive sites

To overcome the shortcomings of homogeneous Fe ion activating peroxymonosulfate (PMS), such as high pH-dependence, limited cycling of Fe(III)/Fe(II) and sludge production, graphite carbon nitride (g-C₃N₄) is chosen as a support for Fe ions, and reduced graphene oxide (rGO) is employed to facilitate the electron transfer process, thereby enhancing catalysis. Herein, a ternary catalyst, Fe-g-C₃N₄/rGO, is first applied under lightless condition for PMS activation, which exhibits ideal performance for contaminant mineralization. 82.5 % of the total organic carbon (TOC) in 100 mL of 5 mg/L bis-phenol A (BPA) was removed within 20 min by the optimal catalyst named 30%rFe_0.2CN, which shows a strong pH adaptability over the range of 3-11 compared with a common Fenton-like system. Moreover, the highly stable Fe-g-C₃N₄/rGO/PMS catalytic system resists complex water matrices, especially those with high turbidity. To unveil the mechanism of PMS activation and pollutant degradation, the physicochemical properties of the as-prepared catalysts are comprehensively characterized by multiple techniques. The Fe(III) contained in both the Fe-N group and α-Fe₂O₃ component of 30%rFe_0.2CN not only directly reacts with PMS to produce sulfate radicals (SO₄^-) and hydroxyl radicals (OH), but also combines with PMS to form the essential [Fe(III)OOSO₃]⁺ active complex, thereby generating superoxide radicals (O₂^-) and singlet oxygen (¹O₂). Among the various reactive oxidizing species, ¹O₂ plays an important role in pollutant removal, which is additionally generated by the CO moiety of the catalyst activating PMS as well as PMS self-oxidation, indicating the dominance of the non-radical pathway in the pollutant degradation process. Due to the advantages of high efficiency, wide pH adaptability and stability, the proposed lightless Fe-g-C₃N₄/rGO/PMS catalytic system represents a promising avenue for practical wastewater purification.

Development of zinc ferrite nanoparticles with enhanced photocatalytic performance for remediation of environmentally toxic pharmaceutical waste diclofenac sodium from wastewater

Diclofenac sodium is an anti-inflammatory drug commonly used to cure pain in various treatments. The remarkable potential of this pain-killer leads to its excessive use and, therefore, a persistent water contaminant. Its presence in aqueous bodies is hazardous for both humans and the environment because it causes the growth of harmful drug-resistant bacteria in water. Herein, we present a comparative study of the ZnO and ZnFe₂O₄ as photocatalysts for the degradation of diclofenac sodium, along with their structural and morphological studies. A simple co-precipitation method was used for the synthesis of ZnO and ZnFe₂O₄ and characterized by various analytical techniques. For instance, the UV-Vis study revealed the absorption maxima of ZnO at 320 nm, which was shifted to a longer wavelength region at 365 nm for zinc ferrite. The optical band gaps obtained from the Tauc plot indicated that the incorporation of iron has led to a decreased band gap of zinc ferrite (2.89 eV) than pure ZnO (3.14 eV). The metal-oxygen linkages shown by FTIR indicated the formation of desired ZnO and ZnFe₂O₄, which was further confirmed by XRD. It elucidated the typical hexagonal structure for ZnO and spinel cubic structure for ZnFe₂O₄ with an average crystallite of 31 nm and 44 nm for ZnO and ZnFe₂O₄, respectively. The micrographs obtained by SEM showed rough spherical particles of ZnO, whereas for ZnFe₂O₄ flower-like clustered particles were observed. The photocatalytic investigation against diclofenac sodium revealed the higher degradation efficiency of ZnFe₂O₄ (61.4%) in only 120 min, whereas ZnO degraded only 48.9% of the drug. Moreover, zinc ferrite has shown good recyclability and was stable up to five runs of photodegradation with a small loss (3.9%) of photocatalytic activity. The comparison of two catalysts has suggested the promising role of zinc ferrite in wastewater remediation to eliminate hazardous pharmaceuticals.

Preparation of lignite-loaded nano-FeS and its performance for treating acid Cr(VI)-containing wastewater

In this study, lignite-loaded nano-FeS (nFeS@Lignite) was successfully prepared by ultrasonic precipitation, and its potential for treating acid Cr(VI)-containing wastewater was explored. The results showed that the 40--80-nm rod-shaped nFeS was successfully loaded onto lignite particles, and the maximum adsorption capacity of Cr(VI) by nFeS@Lignite reached 33.08 mg∙g^-1 (reaction time = 120 min, pH = 4, temperature = 298.15 K). The adsorption process of Cr(VI) by nFeS@Lignite fitted the pseudo-second-order model and the Langmuir isotherm model, and thermodynamic results showed that the adsorption process was an endothermic process with an adsorption enthalpy of 28.0958 kJ·mol^-1. The inhibition intensity of coexisting anions on Cr(VI) removal was in the order of PO₄^3-  > NO₃^-  > SO₄^2-  > Cl^-, and the increase of ionic strength resulted in more pronounced inhibition. Electrostatic adsorption, reduction, and precipitation were synergistically engaged in the adsorption of Cr(VI) by nFeS@Lignite, among which reduction played a major role. The characterization results showed that Fe²⁺, S^2-, and Cr(VI) were converted to FeOOH, S₈, SO₄^2-, Fe₂O₃, Cr₂O₃, and Fe(III)-Cr(III) complexes. This research demonstrates that nFeS@Lignite is a good adsorbent with promising potential for application in the remediation of heavy metal-contaminated wastewater.

Catalytic activity and mechanism of typical iron-based catalysts for Fenton-like oxidation

Heterogeneous Fenton-like systems were exploited for the degradation of Reactive Red X-3B (RR X-3B) using iron-carbon composite, sponge iron, chalcopyrite and pyrite as catalysts. The effect of operational variables on the catalytic activity and metal leaching behavior of catalysts was evaluated and the catalytic mechanism was discussed. The experimental results showed that under the optimum conditions, chemical oxygen demand (COD) removals by Fenton-like systems could reach 89.91%, 86.84%, 80.11% and 60.02% with iron-carbon composite, sponge iron, chalcopyrite and pyrite, respectively. Micro-electrolysis of iron-carbon composite and sponge iron resulted in higher COD removal at acid pH range. Electron Paramagnetic Resonance (EPR) analysis and quenching tests showed that ^•OH was the main reactive oxygen species responsible for the degradation of RR X-3B. A large amount of Fe²⁺ leached from iron-carbon composite and sponge iron, which served as a homogeneous Fenton catalyst during the degradation of RR X-3B. In contrast, much lower amount of Fe²⁺ was leached from chalcopyrite and pyrite, and surface catalysis of the minerals played more important role in the generation of ^•OH. Surface characterization and density functional theory (DFT) calculation results illustrated that ≡Fe(II) was the primary surface catalytic site during the reaction. The reduction of ≡Fe(III) and ≡Cu(II) can be facilitated by sulfides on the mineral surface. The Fenton-like systems catalyzed by iron-based materials exhibited higher H₂O₂ utilization and COD removal than classical Fenton system. With the lower metal leaching concentration and stable surface property, chalcopyrite and pyrite may be more practical applicable from a long-term catalytic activity point of view.

Simultaneous elimination and detoxification of arsenite in the presence of micromolar hydrogen peroxide and ferrous and its environmental implications

Experiments for simultaneous elimination and detoxification of microgram level of As(Ⅲ) in the presence of micromolar H2O2 and Fe(Ⅱ) which are frequently encountered in natural water were conducted. The results showed that the molar ratio of oxidant to As(III) plays important role in As(III) oxidation under the experimental conditions. The extent of As(Ⅲ) oxidation with single H2O2 or Fe(Ⅱ) ranged from 7.9 % to 60.3 % and 22.2-46.6 %, respectively. Treatments with H2O2/As(Ⅲ) molar ratios in the range 150: 1-750: 1 or Fe(Ⅱ)/As(Ⅲ) molar ratios in the range 37.5: 1-375: 1 were more favor for As(Ⅲ) oxidation respectively, and increasing oxidant concentration did not result in complete As(Ⅲ) oxidation. As(Ⅲ) was completely oxidized and eliminated following the precipitation of ferric hydroxides in 5 reaction minutes when H2O2 and Fe(Ⅱ) coexisted in the reaction system. The interface characterization for the reacted precipitates after the experiment were conducted by using a high-resolution field emission scanning electron microscopy (SEM) coupled with an EX-350 energy dispersive X-ray spectrometer (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that As(Ⅴ) was the merely arsenic species and As oxide primary situated in the subsurface layer of the reacted precipitates, whereas Fe was more concentrated in the outermost surface layer. Our research showed that H2O2 and Fe(Ⅱ) at natural level may exert significant influence on arsenic mobilization in natural water. Considering the much more toxic of As(Ⅲ) than that of As(Ⅴ), the research also provide us an environmental friendly choice in the elimination and detoxification of microgram As(Ⅲ) in drinking water.

Thiosulfate driving bio-reduction mechanisms of scorodite in groundwater environment

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite.

Carbon supported "core-shell structure" of Fe nanoparticles for enhanced Fenton reaction activity and magnetic separation

Effectively facilitating Fe³⁺/Fe²⁺ cycles and expanding its operating pH range are keys to optimizing the traditional Fenton reaction. In this exploration, we used chitosan and ferrous sulfate as precursors to prepare a multicomponent magnetic Fe/C Fenton-like catalyst, which exhibited extraordinary catalytic properties and excellent stability performance in a pH range of 4~8. Besides, it could be easily separated from the solution by a magnet. The characterization showed that the supported Fe species include troilite-2H (FeS), lepidocrocite (FeOOH), and pyrrhotite-6T (Fe_{1 - x}S) with a unique "core-shell structure." The presence of reductive iron sulfide core in the system can accelerate the reduction of Fe(III). Meanwhile, the graphite-like structure formed after calcination can adsorb and enrich priority pollutants near the active site through π-π coupling and strengthen electron transfer, which endows its high catalytic performance and enables it invulnerable to dissolved organic compounds.

Speciation analysis and formation mechanism of iron-phosphorus compounds during chemical phosphorus removal process

Iron (Fe) salt was applied extensively to remove phosphorus (P) in wastewater treatment plants (WWTPs). Exploring the formation mechanism of iron-phosphorus compounds (FePs) during the chemical P removal (CPR) process is beneficial to P recovery. In this study, the performance of P removal, FePs speciation analysis and the kinetics of P removal under different conditions (pH, Fe/P molar ratio (Fe/P_mol), type of Fe salt, dissolved organic matters) were comprehensively investigated. More than 95% of P was removed under the optimal conditions with pH = 4.7, Fe/P_mol = 2, FeCl₃ or polymeric ferric sulfate (PFS) as the coagulant. The FePs formation mechanism was considerably influenced by reaction conditions. Iron-phosphate compounds were the dominant FePs species (>76%) at pH < 6.2, while more iron oxides were formed at pH ≥ 6.2 with decreased P removal efficiency. When the initial Fe/P_mol was 2, iron-phosphate compound was the only product that was formed by the reaction between PO₄^3- and Fe(III) or Fe(II) ions directly. More iron oxides were generated when the initial Fe/P_mol was 1 or 3. At Fe/P_mol = 1, the Fe(III) was hydrolyzed to form iron oxides and trapped PO₄^3-, while at Fe/P_mol = 3, iron-phosphate compounds were produced firstly and the remaining Fe(III) was hydrolyzed to form iron oxides. The pseudo-second-order kinetic model simulated the chemical P removal process well. The reaction rate of P with Fe(II) was slower than that with Fe(III), but complete removal was still achieved when the reaction time was more than 30 min. Poly-Fe salt exhibited a fast P removal rate, while the removal efficiency depended on its iron content. Organic matters in wastewater with large molecular weight and multiple functional groups (such as humic acids) inhibited P removal rate but hardly affect the removal amount. This study provides an insight into CPR by Fe salts and is beneficial for P recovery in WWTPs.

Waste toner-derived porous iron oxide pigments with enhanced catalytic degradation property

'Wealth from Waste' is an emerging concept, since it leads an effective waste treatment and waste recyclability. On the other hand, cost effective production iron oxide (IO) nanomaterials is still needed to develop, owing to their wide applications. Herein, we proposed a simple direct calcination method to prepare porous IO (Fe₃O₄ and Fe₂O₃) nanomaterials from waste toner powder. Characterization techniques reveal that a structural change happened from Fe₃O₄ to γ-Fe₂O₃ and γ-Fe₂O₃ to α-Fe₂O₃ at the calcination temperature of 500 °C and 700 °C respectively. Consequently, optical (band gap) and magnetic parameters of IO samples were significantly varied. The pigment characteristics of the IO samples were evaluated using Commission Internationale de l'Eclairage (CIE) analysis. IO900 sample has shown good brown-red coloration (L* = 43.11, a* = 13.26 and b* = 5.69) and it also exhibited good stability in acidic and basic conditions. Practical applicability of IO pigments were also tested by mixing with plaster of paris (PP) powder. Further, porous IO samples were also used as catalysts in the reductive degradation of methyl orange (MO) dye in presence of excess sodium borohydride (NaBH₄). IO, prepared at 900 °C exhibited ∼99.9% reduction efficiency within 40 min. Recycling experiments indicated that IO900 possess good stability up to seven cycles. The present porous IO samples will become potential in pigment and environmental remediation.

Biogenic ferrihydrite-humin coprecipitate as an electron donor for the enhancement of microbial denitrification by Pseudomonas stutzeri

Nitrate pollution of groundwater has become an increasingly serious environmental problem that poses a great threat to aquatic ecosystems and to human health. Previous studies have shown that solid-phase humin (HM) can act as an additional electron donor to support microbial denitrification in the bioremediation of nitrate-contaminated groundwater where electron donor is deficient. However, the electron-donating capacities of HMs vary widely. In this study, we introduced ferrihydrite and prepared ferrihydrite-humin (Fh-HM) coprecipitates via biotic means to strengthen their electron-donating capacities. The spectroscopic results showed that the crystal phase of Fh did not change after coprecipitation with HM in the presence of Shewanella oneidensis MR-1, and iron may have complexed with the organic groups of HM. The Fh-HM coprecipitate prepared with an optimal initial Fh-HM mass ratio of 14:1 enhanced the microbial denitrification of Pseudomonas stutzeri with an electron-donating capacity 2.4-fold higher than that of HM alone, and the enhancement was not caused by greater bacterial growth. The alginate bead embedding assay indicated that the oxidation pathway of Fh-HM coprecipitate was mainly through direct contact between P. stutzeri and the coprecipitate. Further analyses suggested that quinone and organic-complexed Fe were the main electron-donating fractions of the coprecipitate. The results of the column experiments demonstrated that the column filled with Fh-HM-coated quartz sand exhibited a higher denitrification rate than the one filled with quartz sand, indicating its potential for practical applications.

Influence of the Interdomain Interface on Structural and Redox Properties of Multiheme Proteins

Multiheme proteins are important in energy conversion and biogeochemical cycles of nitrogen and sulfur. A diheme cytochrome c4 (c4) was used as a model to elucidate roles of the interdomain interface on properties of iron centers in its hemes A and B. Isolated monoheme domains c4-A and c4-B, together with the full-length diheme c4 and its Met-to-His ligand variants, were characterized by a variety of spectroscopic and stability measurements. In both isolated domains, the heme iron is Met/His-ligated at pH 5.0, as in the full-length c4, but becomes His/His-ligated in c4-B at higher pH. Intradomain contacts in c4-A are minimally affected by the separation of c4-A and c4-B domains, and isolated c4-A is folded. In contrast, the isolated c4-B is partially unfolded, and the interface with c4-A guides folding of this domain. The c4-A and c4-B domains have the propensity to interact even without the polypeptide linker. Thermodynamic cycles have revealed properties of monomeric folded isolated domains, suggesting that ferrous (FeII), but not ferric (FeIII) c4-A and c4-B, is stabilized by the interface. This study illustrates the effects of the interface on tuning structural and redox properties of multiheme proteins and enriches our understanding of redox-dependent complexation.

Visible Light Accelerates Cr(III) Release and Oxidation in Cr-Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)-Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photo-oxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO₂ under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)-Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between Sb^VO₆ and Fe^III(O,OH)₆ octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

Iron Isotopes in Acid Mine Drainage: Extreme and Divergent Fractionation between Solid (Schwertmannite, Jarosite, and Ferric Arsenate) and Aqueous Species

Examination of stable Fe isotopes is a powerful tool to explore Fe cycling in a range of environments. However, the isotopic fractionation of Fe in acid mine drainage (AMD) has received little attention and is poorly understood. Here, we analyze Fe isotopes in waters and Fe(III)-rich solids along an AMD flow-path. Aqueous Fe spanned a concentration and δ⁵⁶Fe range of ∼420 mg L^-1 and + 0.04‰ at the AMD source to ∼100 mg L^-1 and -0.81‰ at ∼450 m downstream. Aqueous As (up to ∼33 mg L^-1) and SO₄^2- (up to ∼2000 mg L^-1), like aqueous Fe, decreased in concentration down the flow-path. X-ray absorption spectroscopy indicated that downstream attenuation in aqueous Fe, As, and SO₄^2- was due to the precipitation of amorphous ferric arsenate (AFA), schwertmannite, and jarosite. The Fe(III) in these solids displayed extreme variability in δ⁵⁶Fe, spanning +3.95‰ in AFA near the AMD source to -1.34‰ in schwertmannite at ∼450 m downstream. Similarly, the isotopic contrast between solid Fe(III) precipitates and aqueous Fe (Δ⁵⁶Fe_ppt-aq) dropped along the flow-path from about +4.1 to -1.1‰. The shift from positive to negative Δ⁵⁶Fe_ppt-aq reflects divergence between competing equilibrium versus kinetic fractionation processes.

Redox Potentials of Magnetite Suspensions under Reducing Conditions

Predicting the redox behavior of magnetite in reducing soils and sediments is challenging because there is neither agreement among measured potentials nor consensus on which Fe(III) | Fe(II) equilibria are most relevant. Here, we measured open-circuit potentials of stoichiometric magnetite equilibrated over a range of solution conditions. Notably, electron transfer mediators were not necessary to reach equilibrium. For conditions where ferrous hydroxide precipitation was limited, Nernstian behavior was observed with an E_H vs pH slope of -179 ± 4 mV and an E_H vs Fe(II)_aq slope of -54 ± 4 mV. Our estimated E_H^o of 857 ± 8 mV closely matches a maghemite|aqueous Fe(II) E_H^o of 855 mV, suggesting that it plays a dominant role in poising the solution potential and that it's theoretical Nernst equation of E_H[mV] = 855 - 177 pH - 59 log [Fe²⁺] may be useful in predicting magnetite redox behavior under these conditions. At higher pH values and without added Fe(II), a distinct shift in potentials was observed, indicating that the dominant Fe(III)|Fe(II) couple(s) poising the potential changed. Our findings, coupled with previous Mössbauer spectroscopy and kinetic data, provide compelling evidence that the maghemite/Fe(II)_aq couple accurately predicts the redox behavior of stoichiometric magnetite suspensions in the presence of aqueous Fe(II) between pH values of 6.5 and 8.5.

Studies on the Efficiency of Iron Release from Fe(III)-EDTA and Fe(III)-Cit and the Suitability of These Compounds for Tetracycline Degradation

Iron ions can be used to degrade tetracycline dispersed in nature. Studies of absorption and fluorescence spectra and quantum chemistry calculations showed that iron is more readily released from Fe(III)-citrate than from Fe(III)-EDTA, so Fe(III)-citrate (Fe(III)-Cit) is more suitable for tetracycline (TC) degradation. At 30 °C, a severe degradation of TC by Fe(III)-Cit occurred as early as after 3 days of incubation in the light, and after 5 days in the dark. In contrast, the degradation of TC by Fe(III)-EDTA proceeded very slowly in the dark. By the fifth day of incubation of TC with Fe(III)-Cit in darkness, the concentrations of the former compound dropped by 55% and 75%, at 20 °C and 30 °C, respectively. The decrease in tetracycline concentrations caused by Fe(III)-EDTA in darkness at the same temperatures was only 2% and 6%, respectively. Light increased the degradation rates of TC by Fe(III)-EDTA to 20% and 56% at 20 °C and 30 °C, respectively. The key role of the light in the degradation of tetracycline by Fe(III)-EDTA was thus demonstrated. The TC degradation reaction showed a second-order kinetics. The rate constants of Fe(III)-Cit-induced TC degradation at 20 °C and 30 °C in darkness were k = 4238 M^-1day^-1 and k = 11,330 M^-1day^-1, respectively, while for Fe(III)-EDTA were 55 M^-1day^-1 and 226 M^-1day^-1. In light, these constants were k = 15,440 M^-1day^-1 and k = 40,270 M^-1day^-1 for Fe(III)-Cit and k = 1012 M^-1day^-1 and 2050 M^-1day^-1 at 20 °C and 30 °C; respectively. A possible reason for the higher TC degradation rate caused by Fe(III)-Cit can be the result of its lower thermodynamical stability compared with Fe(III)-EDTA, which we confirmed with our quantum chemistry calculations. Two quantum chemistry calculations showed that the iron complex with EDTA is more stable (the free energy of the ensemble is 15.8 kcal/mol lower) than the iron complex with Cit; hence, Fe release from Fe(III)-EDTA is less effective.

Red Flags and Adversities on the Way to the Robust CE-ICP-MS/MS Quantitative Monitoring of Self-Synthesized Magnetic Iron Oxide(II, III)-Based Nanoparticle Interactions with Human Serum Proteins

The growing interest in superparamagnetic iron oxide nanoparticles (SPIONs) as potential theranostic agents is related to their unique properties and the broad range of possibilities for their surface functionalization. However, despite the rapidly expanding list of novel SPIONs with potential biomedical applications, there is still a lack of methodologies that would allow in-depth investigation of the interactions of those nanoparticles with biological compounds in human serum. Herein, we present attempts to employ capillary electrophoresis-inductively coupled plasma tandem mass spectrometry (CE-ICP-MS/MS) for this purpose and various obstacles and limitations noticed during the research. The CE and ICP-MS/MS parameters were optimized, and the developed method was used to study the interactions of two different proteins (albumin and transferrin) with various synthesized SPIONs. While the satisfactory resolution between proteins was obtained and the method was applied to examine individual reagents, it was revealed that the conjugates formed during the incubation of the proteins with SPIONs were not stable under the conditions of electrophoretic separation.

WS2 significantly enhances the degradation of sulfachloropyridazine by Fe(III)/persulfate

The use of antibiotics has become an indispensable part of the production and life of human society. Among them, sulfonamide antibiotics widely used in humans and animals are considered to be one of the most crucial antibiotics. However, antibiotics are difficult to degrade naturally, leading to an accumulation in the environment and a potential hazard to human health. In this paper, WS₂ as a co-catalyst could reduce trace Fe(III) to Fe(II) which exhibited a great activating ability to PS through the exposed W(IV) active sites, and formed the Fe(III)/Fe(II) cycle to degrade sulfachloropyridazine (SCP) continuously. This paper systematically discussed the degradation of SCP under different conditions in the PS/WS₂/Fe(III) system, including the amount of WS₂, Fe(III) concentration, PS concentration, initial pH, natural organic matter (NOM) and common anions (NO₃^-, Cl^-, HCO₃^-, HPO₄^2- and H₂PO₄^-). The experimental results showed that PS/WS₂/Fe(III) system possessed a strong degradation ability for SCP in a wide pH range. NO₃^- and Cl^- could promote the degradation of SCP a little. HCO₃^-, HPO₄^2- and H₂PO₄^- could significantly inhibit the degradation of SCP. The main types of free radicals that degraded SCP were explored. In addition, the stability and reusability of WS₂ were examined, and two possible degradation pathways of SCP were proposed.

Insights into the underlying effect of Fe vacancy defects on the adsorption affinity of goethite for arsenic immobilization

Goethite is a commonly found iron (hydr)oxide in soils and sediments that has been proven to possess abundant defects in structures. However, the underlying impact of these defects in goethite on arsenic immobilization remains unclear. In this study, goethite samples with abundant, moderate, and sparse defects were synthesized to evaluate their arsenic adsorption capacities. The characteristics of the defects in goethite were investigated by extended X-ray absorption fine structure (EXAFS), high angle annular dark field-scanning transmission electron microscopy-energy dispersion spectrum (HAADF-STEM-EDS) mapping, vibrating-sample magnetometry (VSM), and electron spin resonance (ESR). The characterization analysis revealed that the defects in as-synthesized goethite primarily existed in the form of Fe vacancies. Batch experiments demonstrated that the adsorption capacities of defect-rich goethite for As(V) and As(III) removal were 10.2 and 22.1 times larger than those of defect-poor goethite, respectively. The origin of the impact of Fe defects on arsenic immobilization was theoretically elucidated using density functional theory (DFT) calculations. The enhanced adsorption of goethite was attributed to the improvement of the arsenic affinity due to the Fe vacancy defect, thus considerably promoting arsenic immobilization. The findings of this study provide important insight into the migration and fate of arsenic in naturally occurring iron (hydr)oxides.

Enhanced generation of reactive oxygen species by pyrite for As(III) oxidation and immobilization: The vital role of Fe(II)

The migration and conversion of arsenic in the environment usually accompany by the redox of iron-bearing minerals. For instance, the oxidation of pyrite can generate reactive oxygen species (ROS) affecting the species of arsenic, but the types and roles of ROS have been unclear. This paper demonstrated the vital role of Fe(II) in the pyrite for the formation of ROS. Results showed that exogenous addition of Fe(II) significantly enhanced the removal rate of As(III) by pyrite. 2,2'-bipyridine (BPY) decreased the oxidation of As(III) by complexing with Fe²⁺ in solution, whilst EDTA enhanced the oxidation of As(III) by boosting the autoxidation of Fe²⁺. In addition, neutral pH is superior for the oxidation of As(III) and removal of total arsenic. Importantly, Methanol, SOD enzyme and PMOS inhibited 54%, 28% and 17.5% of As(III) oxidation, respectively, which indicated O₂^•- and •OH were the main contributors to As(III) oxidation, and Fe(IV) contributed a small part of As(III) oxidation. The content of As(V) in the FeS₂-Fe²⁺-As(III) system was higher than that in the FeS₂-As(III) system, further confirming the vital role of Fe(II) for As(III) oxidation. Lepidocrocite was produced in a single Fe²⁺ system, which was not detected in the FeS₂-As(III) system. Thus, the presence of mineral surfaces changed the oxidation products of Fe²⁺ and accelerated the oxidation and immobilization of As(III). FA (Fulvic Acid) and HA (Humic Acid) accelerated the oxidation of As(III), but the oxidation of As(III) by pyrite was inhibited to a certain extent, with increasing phenolic hydroxyl groups in phenolic acid. Our findings provide new insight into the oxidative species in the pyrite-Fe(II) system and will help guide the remediation of arsenic pollution in complex environmental systems.