Reactivities of Fe(II) on calcite: selenium reduction

Removal of toxic metals using iron sulfide particles: A brief overview of modifications and mechanisms

Growing mechanization has released higher concentrations of toxic metals in water and sediment, which is a critical concern for the environment and human health. Recent studies show that naturally occurring and synthetic iron sulfide particles are efficient at removing these hazardous pollutants. This review seeks to provide a concise summary of the evolution in the production of iron sulfide particles, specifically nanoparticles, through the years. This review presents an outline of the synthesis process for the most dominant forms of iron sulfide: mackinawite (FeS), pyrite (FeS2), pyrrhotite (Fe1-x S), and greigite (Fe3S4). The review confirms that both natural forms of iron sulfide and modified forms of iron sulfide are highly effective at removing different heavy metals and metalloids from water. Concurrently, this review reveals the interaction mechanism between toxic metals and iron sulfide, along with the impact of conditions for remedy and rectification. None the less, modifications and future investigations into the synthesis of novel iron sulfides, their use to adsorb diverse environmental pollutants, and their fate after injection into polluted aquifers, remain crucial to maximizing pollution control.

Solid-Water Interface Interaction of Selenium with Fe(II)-Bearing Minerals and Aqueous Fe(II) and S(-II) Ions in the Near-Field of the Radioactive Waste Disposal System

Selenium can be highly toxic in excess for both animals and humans. However, since its mobile forms can be easily adsorbed with ferric minerals, its mobility in the natural oxic environment is generally not an issue. Still, the removal and immobilization of the long-lived radioactive isotope ⁷⁹Se from the contaminated anoxic waters is currently a significant concern. ⁷⁹Se can be accessible in the case of radionuclides' leaching from radioactive waste disposals, where anoxic conditions prevail and where ferrous ions and Fe(II)-bearing minerals predominate after corrosion processes (e.g., magnetite). Therefore, reductive and adsorptive immobilizations by Fe(II)-bearing minerals are the primary mechanisms for removing redox-sensitive selenium. Even though the information on the sorptive interactions of selenium and Fe(II)-bearing minerals seems to be well documented, this review focuses specifically on the state of the available information on the effects of the redox properties of Fe(II)-bearing solid phases (e.g., ferrous oxides, hydroxides, sulfides, and carbonates) on selenium speciation via redox transformation and co-occurring coprecipitation.

Exploring the role of crystal habit in the Ostwald rule of stages

The crystallization of calcium carbonate is shown to be dictated by the Ostwald rule of stages (ORS), for high relative initial supersaturations (S CaCO 3 = [ C a 2 + ] [ CO 3 2 - ] / K SP, Calcite > 2500 ), under sweet (carbon dioxide saturated) and anoxic (oxygen depleted) solution conditions. Rhombohedral calcite crystals emerge after the sequential crystallization and dissolution of the metastable polymorphs: vaterite (snowflake-shaped) and aragonite (needle-shaped). However, the presence of certain cations, which can form trigonal carbonates (e.g. Fe2+ and Ni2+), in concentrations as low as 1.5 mM, triggers the emergence of calcite crystals, with a star-shaped crystal habit, first. These star-shaped crystals dissolve to yield needle-shaped aragonite crystals, which in turn dissolve to give the rhombohedral calcite crystals. The star-shaped crystals, formed at high SCaCO3 , possess higher surface free energy (therefore higher apparent solubility) than their rhombohedral counterparts. This sequence of dissolution and recrystallization demonstrates that the ORS does not only drive the crystal towards its thermodynamically most stable polymorph but also towards its most stable crystal habit.

A critical review on sulfur reduction of aqueous selenite: Mechanisms and applications

Selenite, which is extremely toxic at high concentrations, can easily be enriched in natural aquatic environments due to human activities, which causes great harm to ecosystems. Sulfur reduction can effectively reduce soluble selenite in large quantities to nontoxic solid elemental selenium, which plays a significant role in controlling the toxicity and cycle of selenium. In view of the bright prospects of the sulfur reduction reaction of selenite, this review comprehensively summarizes the continuous development in the sulfidation of selenite. First, the geochemical characteristics of aqueous selenium in different sulfur systems involving species distribution and various phase types at Eh-pH conditions were summarized. Second, sulfur reductions of selenite with chemical sulfide in natural water environments, sulfur reductase and extracellular polymer substances containing thiol groups in sulfate-reducing bacteria have been reviewed to further understand the corresponding mechanisms, rates and influencing factors. Furthermore, applications of sulfur reduction of selenium, including removal of selenium, enrichment of selenium, synthesis of selenoproteins and prevention of leakage of selenium, were also summarized. Finally, this review identified future research needs for the sulfidation of selenite for environmental applications.

The uptake of selenite in calcite revealed by X-ray absorption spectroscopy and quantum chemical calculations

Selenium (Se) is an important trace element in the environment, but the interaction of Se with calcite that may control the fate and geochemical behavior of Se is not fully understood. In this study, the molecular-scale mechanism for the uptake of selenite in calcite was investigated by a combination of laboratory experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and quantum chemical calculations. Results showed that selenite can be largely distributed to calcite at circumneutral pH. The local structure of Se in calcite obtained from EXAFS analyses, in combination with quantum chemical calculations, demonstrated that selenite can be incorporated into calcite by substituting for the carbonate, and that the geometric incompatibility of selenite could be accommodated by a slight expansion of crystal volume. The findings from this study suggest that calcite could be a potential Se sink, providing an important insight into the understanding of the mobility and geochemical behavior of Se in the subsurface environments particularly in the groundwater system.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Mechanism analysis of selenium (VI) immobilization using alkaline-earth metal oxides and ferrous salt

The immobilization of selenate (SeO₄^2-) using metal oxides (CaO and MgO) and ferrous salt as the immobilization reagents were examined by the leaching test and solid-phase analysis via XRD, XAFS, TGA, and XPS. The results indicated that nearly all of SeO₄^2- was reduced to SeO₃^2- in the CaO-based reaction within 7 days. Then, the generated SeO₃^2- was mainly sorbed onto the iron-based minerals (Fe₂O₃ and FeOOH) through the formation of both bidentate mononuclear edge-sharing (¹E) and monodentate mononuclear corner-sharing (¹V) inner-sphere surface complexes, suggested by PHREEQC simulation and EXAFS analysis. Differently, less amount of SeO₄^2- (approximately 45.50%) was reduced to SeO₃^2- for the MgO-based reaction. However, if the curing time increases to a longer time (more than 7 days), the further reduction could occur because there are still Fe(II) species in the matrix. As for the associations of Se in the solid residue, most of the selenium (SeO₃^2- and SeO₄^2-) was preferentially distributed onto the Mg(OH)₂ through outer-sphere adsorption. Definitely, this research can provide a deep understanding of the immobilization of selenium using alkaline-earth metal oxide related materials and ferrous substances.

The redox behavior of uranium on Beishan granite: Effect of Fe2+ and Fe3+ content

The Beishan granitic area in Gansu Province is a site with the greatest potential for a repository of high-level radioactive waste (HLW) in China. In this study, the redox behavior of uranium on Beishan granite was investigated at pH values from ~4.4 to ~9.2. Due to the presence of Fe²⁺-containing fluorannite, results showed that U(VI) was partially reduced by the granites from boreholes 2 (486 m) and 28 (670 m) at a relatively low initial pH whether Na₂CO₃/NaCl or native groundwater was used as a background electrolyte. Partial oxidation of UO₂ was observed when UO₂ contacted Beishan granite directly. Therefore, this incomplete reduction of U(VI) was mainly attributed to minor Fe³⁺ that was either originally contained in the granite or generated during U(VI) reduction. Consequently, aliovalent oxides (e.g., U₃O₈, U₃O₇, U₄O₉, etc.) should be the thermodynamically stable phase in Beishan granite. A mechanism involving the dissolution of Fe²⁺ from the granite structure followed by interfacial adsorption/reaction was proposed for the U(VI) reduction. This study demonstrates that Beishan granite has a good reducing capacity, which is suitable for the immobilization of redox-sensitive radionuclides. However, potential oxidation of spent fuel by Fe³⁺ in the granite should also been taken into account.

Stabilization and transformation of selenium during the Fe(II)-induced transformation of Se(IV)-adsorbed ferrihydrite under anaerobic conditions

Selenium (Se) is an essential nutrient for human beings at trace concentrations, but also a hazardous contaminant at high concentrations. As an important geological adsorbent, the transformation of 2-line ferrihydrite (Fh) strongly influences the geochemical behavior of selenium. However, little is known about the effect of the recrystallization of Fh on the fate of adsorbed Se(IV) in the reducing environments. We investigated the redistribution and transformation of Se(IV) during the recrystallization of Se(IV)-adsorbed Fh accelerated by Fe(II) under anaerobic conditions. Synchrotron based X-ray absorption near edge structure (XANES) spectroscopy was utilized to characterize oxidation state of Se. Results revealed that the adsorbed Se(IV) inhibited the Fe(II)-catalyzed recrystallization of ferrihydrite to goethite. Transmission electron microscopy (TEM) images showed that pH and the presence of Se(IV) had significant impacts on the morphology of the produced goethite. Approximately 30-75% adsorbed Se(IV) transformed to phosphate-unextractable form, indicating that the adsorbed Se transformed to more stable phase during the recrystallization of Fh. The XANES results indicated that a small fraction of Se(IV) was reduced to elemental Se. Our study demonstrated that the stability of adsorbed Se(IV) on ferrihydrite could be enhanced during Fe(II)-catalytic transformation of Fh under anoxic environments.

Effect of humic acid on Se and Fe transformations in soil during waterlogged incubation

Humic acid (HA) serves as electron donor and acceptor in the biogeochemical cycle of Fe and Se in soil. In anoxic condition, a series of redox reactions occur, including reductive dissolution of Fe oxides, decomposition of organic matters, and transformation of trace elements. Thus, this study demonstrates the effect of HA on Se and Fe transformations in soil during waterlogged incubation. Soils were incubated under anoxic condition for 56 days, and pH, redox potential (Eh), and Fe and Se concentrations were measured at specific reaction times (days 2, 4, 8, 15, 28, and 56 of incubation). Moreover, sequential extraction and X-ray photoelectron spectroscopy (XPS) were used to obtain Se and Fe transformations, respectively. High resolution transmission electron microscopy (HR-TEM) was used to observe the morphology properties of soil. Results indicated that 4% HA addition decreased the pH and inhibited Eh decline continuously, and HA addition inhibited the Fe and Se release from soil. The Se concentration in soil solution without and with 4% HA addition at the day 15 of incubation were 1.05 mg L^-1 and 0.30 mg L^-1, respectively. Moreover, the residual Se fraction in soil with HA addition was evidently more than that in soil without HA addition. XPS of Se3d and Fe2p revealed that the binding energy of the main peak shifted to low values and the peak shape varied with the increase in HA addition. XPS2p_3/2 and HR-TEM data indicated that the surface structure of Fe oxides in soil varied with the variations in anoxic incubation time and HA addition amount. HA addition would negatively influence Se and Fe release in soil solution and then reduce their bioavailability. This study aids in understanding the environmental behavior changes of Se and Fe when high HA concentrations enter the soils, especially wetland or paddy soil.

Facile modification of activated carbon with highly dispersed nano-sized α-Fe2O3 for enhanced removal of hexavalent chromium from aqueous solutions

Activated carbon-coated α-Fe₂O₃ nanoparticles (nFe₂O₃@AC) were synthesized by a facile impregnation method to enhance hexavalent chromium (Cr(VI)) removal from water. The SEM images confirmed that α-Fe₂O₃ particles ranging from 90 to 500 nm were dispersedly loaded on the AC, which successfully amended Cr(VI) removal. The nFe₂O₃@AC was able to remove Cr(VI) with a 3 times higher efficiency of 94% in comparison with the AC. After adsorption, Cr(VI) reduction coupled with AC oxidation and low soluble (Cr_xFe_1-x)(OH)₃ precipitates were eventually formed. The Cr(VI) removal process was pH-dependent and could be well fitted to pseudo second-order kinetics. The nFe₂O₃@AC could be easily regenerated by 0.1 M HCl and showed a good stability as an 80% Cr(VI) removal efficiency was recorded after 4 desorption-adsorption cycles. In addition, this composite had a promising potential for repeated utilization because the AC of the adsorbed nFe₂O₃@AC could be refreshed and remodified with nFe₂O₃ after stripping all the nFe₂O₃ and (Cr_xFe_1-x)(OH)₃ precipitates from its surface by 1 M HCl and a Cr(VI) removal efficiency of 86% could be achieved. Our results demonstrated that the use of nFe₂O₃ is an efficient and promising method to modify AC and enhance Cr(VI) removal form aqueous solutions.

Kinetics and mechanism of selenite reduction by zero valent iron under anaerobic condition activated and enhanced by dissolved Fe(II)

Batch test was conducted to investigate Se(IV) removal kinetics and mechanism by zero valent iron (ZVI) in presence of Fe(II) under anaerobic condition. Dissolved Fe(II) activated and enhanced Se(IV) reduction by ZVI, which also determined the removal efficiency, reduction rate, final corrosion products and their structures. Se(IV) was completely removed at initial Fe(II)/Se(IV) ≥ 1.0, and the specific rate constant significantly increased from 0.6 to 3.44 L h^-1 m^-2 with the augment of ratio from 1.0 to 1.4. At Fe(II)/Se(IV) < 1.0 (take 0.6 as an example), Raman, XPS, SEM-EDS and XRD results suggested that Se(IV) was reduced to amorphous Se(0) in forms of red suspended solids, amorphous FeSe and crystal maghemite (γ-Fe₂O₃) coated on ZVI surface. At Fe(II)/Se(IV) ≥ 1.0 (take 1.0 and 1.4 as examples), crystal FeSe and magnetite (Fe₃O₄) deposits formed on ZVI surface with a core-shell structure. Additionally, final pH increased due to Se(IV) reduction. This study suggested that traditional ZVI passivation problem could be overcome through the addition of excess dissolved Fe(II) under anaerobic condition, which also provided an alternative method to produce a reactive ammonia-free Fe₃O₄/ZVI/Fe(II) system.

Real-Time XANES Measurement of Se Reduction by Zerovalent Iron in a Flow-through Cell, and Accompanying Se Isotope Measurements

An anoxic flow-through cell experiment was conducted to examine mechanisms controlling the real-time reduction of selenate (Se(VI)) by zerovalent iron (ZVI), which is commonly used in permeable reactive barriers to treat dissolved contaminants including Se(VI). Changes in selenium (Se) isotope composition were examined by increasing the influent Se concentration over time, thus changing the proportion of Se removed from solution. At the conclusion of the experiment, an anoxic Se-free solution was pumped through the cell to assess the stability of the reaction products. At all stages, X-ray absorption data were obtained from the solid phase and Se isotope data from the aqueous phase. Reduced Se in the form of adsorbed Se(IV), Fe₂SeO₄, Se(0), and iron selenides accumulated on the ZVI over time. A linear regression function was fit to the δ^82/76Se values of the effluent, yielding an isotopic separation of 9.6‰. A Rayleigh curve was fit to the isotope data from the effluent samples collected during the rinse stage with an effective fractionation of 2.4‰. The results from this experiment can be used to elucidate the effect of multiple concurrent mechanisms on Se isotope behavior.

Retention and multiphase transformation of selenium oxyanions during the formation of magnetite via iron(ii) hydroxide and green rust

Environmental and health hazards associated with the trace element selenium are mainly related to the presence of the highly mobile selenium oxyanions selenite and selenate (oxidation states IV and VI). In this study, we investigated the immobilization of dissolved selenite and selenate during the formation of magnetite in coprecipitation experiments based on the progressive oxidation of an alkaline, anoxic Fe2+ system (pH 9.2). Up to initial selenium concentrations of 10-3 mol L-1 (mass/volume ratio = 3.4 g L-1), distribution coefficient values (log Kd) of 3.7 to 5.1 L kg-1 demonstrate high retention of selenium oxyanions during the mineral formation process. This immobilization is due to the reduction of selenite or selenate, resulting in the precipitation of sparingly soluble selenium compounds. By X-ray diffraction analysis, these selenium compounds were identified as trigonal elemental selenium that formed in all coprecipitation products following magnetite formation. Time-resolved analysis of selenium speciation during magnetite formation and detailed spectroscopic analyses of the solid phases showed that selenium reduction occurred under anoxic conditions during the early phase of the coprecipitation process via interaction with iron(ii) hydroxide and green rust. Both minerals are the initial Fe(ii)-bearing precipitation products and represent the precursor phases of the later formed magnetite. Spectroscopic and electron microscopic analysis showed that this early selenium interaction leads to the formation of a nanoparticulate iron selenide phase [FeSe], which is oxidized and transformed into gray trigonal elemental selenium during the progressive oxidation of the aquatic system. Selenium is retained regardless of whether the oxidation of the unstable iron oxides leads to the formation of pure magnetite or other iron oxide phases, e.g. goethite. This reductive precipitation of selenium induced by interaction with metastable Fe(ii)-containing iron oxide minerals has the potential to influence the mobility of selenium oxyanions in contaminated environments, including the behavior of 79Se in the near-field of nuclear waste repositories.

Selenium contamination, consequences and remediation techniques in water and soils: A review

Selenium (Se) contamination in surface and ground water in numerous river basins has become a critical problem worldwide in recent years. The exposure to Se, either direct consumption of Se or indirectly may be fatal to the human health because of its toxicity. The review begins with an introduction of Se chemistry, distribution and health threats, which are essential to the remediation techniques. Then, the review provides the recent and common removal techniques for Se, including reduction techniques, phytoremediation, bioremediation, coagulation-flocculation, electrocoagulation (EC), electrochemical methods, adsorption, coprecipitation, electrokinetics, membrance technology, and chemical precipitation. Removal techniques concentrate on the advantages, drawbacks and the recent achievements of each technique. The review also takes an overall consideration of experimental conditions, comparison criteria and economic aspects.

Influence of Fe(II) on the Se(IV) sorption under oxic/anoxic conditions using bentonite

⁷⁹Se, one of the key radionuclides for nuclear waste disposal, threatens the quality of the environment, as well as human health. Therefore, it needs to be permanently isolated from the biosphere. The aim of the study was to investigate the effects of Fe(II)/Fe(III) on the removal of ⁷⁹Se using bentonite in the pH range of 2.0-10.0 under oxic/anoxic conditions. Under oxic conditions, Se(IV) prefers to form inner-sphere complexes with Fe(III)-oxyhydroxide, derived from the oxidization of Fe(II) using oxygen. Interestingly, Se(IV) will interact with Fe(III) and form a poorly soluble ferric selenite at pH ∼4 under oxic conditions. Under anoxic conditions, however, the concentration of Fe(II) is closely related to the sorption process of Se(IV) on bentonite. When the concentration of Fe(II) was less than 1%, Fe(II) combined with the hydroxyl, forming Fe(OH)₂, which generated a disproportionation at pH ∼8 and formed a new sorbent, Fe₃O₄. However, when the concentration of Fe(II) was increased to 5%, reduction precipitation was the primary way to remove Se(IV) in aqueous solution. XANES (X-ray Absorption Near Edge Structure) spectra showed that higher pH values are beneficial for the formation of the final thermodynamic reduction product, Fe selenide. These results suggested that Fe(II) significantly affect the Se(IV) sorption. Overall, this study confirmed the significant role of Fe(II) on the retardation of ⁷⁹Se and on remediation for Se(IV) contamination in the hydrosphere.

Groundwater co-contaminant behavior of arsenic and selenium at a lead and zinc smelting facility

Co-contaminant behavior of arsenic (As) and selenium (Se) in groundwater is examined in this study at a former lead and zinc smelting facility. We collected water quality data, including concentrations of trace metals, major ions, and metalloid speciation, over a 15-year period to document long-term trends and relationships between As, Se, geochemical parameters, and other redox-sensitive trace metals. Concentrations of dissolved As and Se were negatively correlated (Kendall's Tau B correlation coefficient, r = -0.72) and showed a distinctive L-shaped relationship. High-concentration arsenic wells (>5 mg L-1) were characterized by intermediate oxidation-reduction conditions (75 < Eh < 275 mV), near-neutral pH (6.1-7.9), low Ca/Na ratios, elevated Fe and Mn concentrations, and high proportions of As(III) relative to total dissolved As. High-concentration Se wells (>500 μg L-1) were characterized by more positive Eh (305-500 mV), low Fe concentrations, and high proportions of As(V). Batch micocosm experiments showed that aquifer solids contain mineral surfaces and/or microbial communities capable of removing selenate from groundwater. Electron microprobe and Se K-edge X-ray absorption near-edge spectroscopic analyses demonstrated that Se was predominantly associated with elemental Se in the reduced aquifer solids. Factor analysis revealed three discernible groupings of trace metals. Group I includes U, Se, and nitrate-N, all of which are mobile under oxygenated to moderately oxygenated conditions. Group II includes elements that are mobile under Fe(III)-reducing conditions: Fe, total dissolved As, As(III), and ammonium-N. Group III elements (Mo, Sb, and V) showed mobility across the entire range of redox conditions encountered in site groundwater; As(V) clustered with this group of elements. Geochemical modeling suggests that As and Se species were in a state of disequilibrium with respect to measured parameters indicative of redox conditions, although predicted patterns of redox-controlled mobility and attenuation were confirmed. This analysis is important to better understand groundwater contaminant behavior in response to redox conditions ranging from oxic/suboxic to Fe(III)-reducing, but excluding sulfate-reducing conditions.

The enhancement roles of layered double hydroxide on the reductive immobilization of selenate by nanoscale zero valent iron: Macroscopic and microscopic approaches

Herein, we utilized nanoscale zero-valent iron loaded on layered double hydroxide (NZVI/LDH) to immobilize Se(VI) and evaluated the enhancement role of LDH in the NZVI reaction system. The structural characterization indicated that LDH could stabilize and disperse NZVI as well as prevent NZVI from oxidation, thereby increasing iron reactivity. Batch experiments displayed that, compared with those by NZVI, both extent and rate of Se(VI) immobilized by NZVI/LDH significantly increased, owing to the prominent synergistic effect ascribing from adsorption and reduction. Kinetics studies under a series of conditions showed that Se(VI) reaction could be well described by pseudo first-order model. The performance of Se(VI) immobilization was inhibited to a considerable extent by most of co-existing ions, Nevertheless, the presence of Cu²⁺ improved performance of NZVI/LDH due to its role as a catalyst or medium of charge transfer during reduction. XANES revealed that LDH acted as a promoter for complete reduction of Se(VI) into Se(0)/Se(-II) over a wide pH range, whereas EXAFS suggested that LDH acted as a scavenger for insoluble products, making more reactive sites exposure to Se(VI) for reduction. These results suggested that NZVI/LDH as a promising candidate exhibited potential application in remediation of wastewaters containing Se(VI).

Migration of 75Se(IV) in crushed Beishan granite: Effects of the iron content

The diffusion of selenite (labeled with ⁷⁵Se) in compacted Beishan granite (BsG) was investigated using the in-diffusion capillary method at pH values from ∼2.0 to ∼11.0 under oxic and anoxic conditions. The results indicate that the apparent diffusion coefficient (D_a) values of selenite in BsG always reached the minimum at approximately pH 5. Unexpectedly, the D_a values under oxic conditions are nearly one order of magnitude lower than those under the anoxic conditions. Further characterization reveals the existence of redox-sensitive Fe(II)-containing components, which can be responsible for the great difference in D_a values. Fe(2p) X-ray photoelectron spectroscopy (XPS) results show that more Fe(III)-oxyhydroxide coating is formed on the granite's surface under aerobic conditions than is formed under anaerobic conditions. Correspondingly, Se(3d) spectra indicate that more selenium is sorbed under oxic conditions, and the sorbed amount always reached the maximum at pH values from ∼4 to ∼5. A linear combination fit of X-ray absorption near edge structure (XANES) spectroscopy data revealed that Se(0) was formed under anoxic condition and that selenite preferred to form inner-sphere complexes with Fe(III)-oxyhydroxide. Overall, this study indicates that natural Fe-bearing minerals can greatly attenuate selenite diffusion and the retardation would be enhanced under aerobic conditions.

Solution and surface chemistry of the Se(IV)-Fe(0) reactions: Effect of initial solution pH

Aspects of solution and solid-phase reactions between selenite (Se(IV)) and nanoscale zero-valent iron (nZVI) were investigated. Experimental results on the effects of initial solution pH, formation and evolution of nZVI corrosion products, and speciation of selenium in nZVI were presented. In general, the rate of Se(IV) removal decreases with increasing initial pH. The observed rate constants of Se(IV) removal decreased from 0.3530 to 0.0364 min^-1 as pH increased from 4.0 to 10.0. Composition and morphology of nZVI corrosion products and selenium species were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results confirmed that Se(IV) was reduced to Se(0) and Se(-II) by nZVI. Lower solution pH favored further reduction of Se(0) to Se(-II). Amorphous FeOOH, magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃) and ferrous hydroxide (Fe(OH)₂) were identified as the main corrosion products. Under alkaline conditions, the corrosion products were mainly of Fe(OH)₂ along with small amounts of Fe₃O₄, while nZVI in acidic solutions was oxidized to mostly Fe₃O₄ and amorphous FeOOH. Furthermore, these corrosion products acted as intermediates for electron transfer and reactive/sorptive sites for Se(IV) adsorption and reduction, thus played a crucial role in the removal of aqueous Se(IV).

Immobilization of selenite from aqueous solution by structural ferrous hydroxide complexes

Selenium species before and after reaction with FHCs were studied and classified into five groups.

The influence of pH and reaction time on the formation of FeSe2 upon selenite reduction by nano-sized pyrite-greigite

The influence of pH and reaction time on the formation of FeSe2 by reductive precipitation of Se(IV) with nano-sized pyrite-greigite was investigated. Reductive precipitation is an effective method of attenuating the mobility of 79Se, which is foreseen to be a dangerous radioisotope for the geological disposal of high-level radioactive waste (HLW). The results indicated that Se(0)was formed at pH <4.05, whereas, at pH > 6.07, considerable amount of FeSe2 was formed along with Se(0). These observations are in agreement with the thermodynamic predictions reported in this work. Furthermore, the formation of FeSe2 was found to continue by increasing the reaction time, indicating that the Se(0) formed in the early reaction stage is gradually transformed to FeSe2 upon the depletion of aqueous Se(IV). Since FeSe2 has a stronger reactivity than pyrite, it was proposed that greigite, rather than pyrite, was responsible for the formation of FeSe2. The findings in this study are of interest for key geochemical processes governing the mobility of toxic 79Se in the environment in presence of iron sulfides.

Negative impact of oxygen molecular activation on Cr(VI) removal with core-shell Fe@Fe2O3 nanowires

In this study, we demonstrate that the presence of oxygen molecule can inhibit Cr(VI) removal with core-shell Fe@Fe2O3 nanowires at neutral pH of 6.1. 100% of Cr(VI) removal was achieved by the Fe@Fe2O3 nanowires within 60 min in the anoxic condition, in contrast, only 81.2% of Cr(VI) was sequestrated in the oxic condition. Removal kinetics analysis indicated that the presence of oxygen could inhibit the Cr(VI) removal efficiency by near 3 times. XRD, SEM, and XPS analysis revealed that either the anoxic or oxic Cr(VI) removal was involved with adsorption, reduction, co-precipitation, and re-adsorption processes. More Cr(VI) was bound in a reduced state of Cr(III) in the anoxic process, while a thicker Cr(III)/Fe(III)/Cr(VI) oxyhydroxides shell, leading to inhibiting the electron transfer, was found under the oxic process. The negative impact of oxygen molecule was attributed to the oxygen molecular activation which competed with Cr(VI) adsorbed for the consumption of donor electrons from Fe(0) core and ferrous ions bound on the iron oxides surface under the oxic condition. This study sheds light on the understanding of the fate and transport of Cr(VI) in oxic and anoxic environment, as well provides helpful guide for optimizing Cr(VI) removal conditions in real applications.

Fractionation of Selenium during Selenate Reduction by Granular Zerovalent Iron

Batch experiments were conducted using granular zerovalent iron (G-ZVI) with either ultrapure water or CaCO3 saturated simulated groundwater to assess the extent of Se isotope fractionation in solution under the anaerobic conditions characteristic of many aquifers. G-ZVI is a common remediation material in permeable reactive barriers (PRB) to treat Se-contaminated groundwater, and stable isotopes are a potential tool for assessing removal mechanisms. The solution composition, speciation of Se, and Se isotope ratios were determined during both sets of experiments. Dissolved Se concentrations decreased from 10 to <2 mg L(-1) after 3 d in the CaCO3 system and below 0.4 mg L(-1) after 2 d in the ultrapure water system. XANES analysis of the solid phase showed spectra consistent with the formation of Se(IV), Fe2(SeO3)3, FeSe, FeSe2, and Se(0) on the G-ZVI. Selenium isotope ratio measurements in solution in the CaCO3 and ultrapure water experiments showed enrichment of δ(82/76)Se values from -0.94 ± 0.07‰ and -1.93 ± 0.20‰ to maximum values of 6.85 ± 0.52‰ and 5.68 ± 0.20‰ over 72 and 36 h, respectively. The effective fractionations associated with the reduction of Se(VI) were 4.3‰ within the CaCO3 saturated water and 3.0‰ in ultrapure water.

Selenium distribution and speciation in plant parts of wheat (Triticum aestivum) and Indian mustard (Brassica juncea) from a seleniferous area of Punjab, India

The concentration, distribution, and speciation of selenium in different parts of wheat and Indian mustard, grown in a seleniferous area in Punjab, were investigated using synchrotron based (XAS) and classical acid digestion and extraction methods. The analyses revealed a high Se enrichment in all investigated plant parts, with Se levels in the range of 133-931 mg/kg (dry weight, dw). Such high Se enrichment is mainly due to the considerable amounts of easily available Se detected in the soil, which are renewed on a yearly basis to some extent via irrigation. Speciation analysis in soil and plants indicated selenate and organic Se as major Se species taken up by plants, with a minor presence of selenite. The analyses also revealed that the highest Se enrichment occurs in the upper plant parts, in agreement with the high uptake rate and mobility of selenate within plants. In both wheat and mustard, highest Se enrichments were found in leaves (387 mg/kg·dw in wheat and 931 mg/kg·dw in mustard). Organic species (dimethylselenide and methylselenocysteine) were found in different parts of both plants, indicating that an active detoxification response to the high Se uptake is taking place through methylation and/or volatilization. The high proportion of selenate in wheat and mustard leaves (47% and 70%, respectively) is the result of the inability of the plant metabolism to completely transform selenate to non-toxic organic forms, if oversupplied. Methylselenocysteine, a common Se species in accumulating plants, was detected in wheat, suggesting that, in the presence of high Se concentration, this plant develops similar response mechanisms to accumulator plants.

Insight into core-shell dependent anoxic Cr(VI) removal with Fe@Fe2O3 nanowires: indispensable role of surface bound Fe(II)

In this study, we investigated the anoxic Cr(VI) removal with core-shell Fe@Fe2O3 nanowires. It was found the surface area normalized Cr(VI) removal rate constants of Fe@Fe2O3 nanowires first increased with increasing the iron oxide shell thickness and then decreased, suggesting that Fe@Fe2O3 nanowires possessed an interesting core-shell structure dependent Cr(VI) removal property. Meanwhile, the Cr(VI) removal efficiency was positively correlated to the amount of surface bound Fe(II). This result revealed that the core-shell structure dependent Cr(VI) removal property of Fe@Fe2O3 nanowires was mainly attributed to the reduction of Cr(VI) by the surface bound Fe(II) besides the reduction of Cr(VI) adsorbed on the iron oxide shell via the electrons transferred from the iron core. The indispensable role of surface bound Fe(II) was confirmed by Tafel polarization and high-resolution X-ray photoelectron spectroscopic depth profiles analyses. X-ray diffraction patterns and scanning electron microscope images of the fresh and used Fe@Fe2O3 nanowires revealed the formation of Fe(III)/Cr(III)/Cr(VI) composite oxides during the anoxic Cr(VI) removal process. This study sheds a deep insight into the anoxic Cr(VI) removal mechanism of core-shell Fe@Fe2O3 nanowires and also provides an efficient Cr(VI) removal method.

Mechanism of selenite removal by a mixed adsorbent based on Fe-Mn hydrous oxides studied using X-ray absorption spectroscopy

Selenium cycling in the environment is greatly controlled by various minerals, including Mn and Fe hydrous oxides. At the same time, such hydrous oxides are the main inorganic ion exchangers suitable (on the basis of their chemical nature) to sorb (toxic) anions, separating them from water solutions. The mechanism of selenite adsorption by the new mixed adsorbent composed of a few (amorphous and crystalline) phases [maghemite, MnCO3, and X-ray amorphous Fe(III) and Mn(III) hydrous oxides] was studied by extended X-ray absorption fine structure (EXAFS) spectroscopy [supported by Fourier transform infrared (FTIR) and X-ray diffraction (XRD) data]. The complexity of the porous adsorbent, especially the presence of the amorphous phases of Fe(III) and Mn(III) hydrous oxides, is the main reason for its high selenite removal performance demonstrated by batch and column adsorption studies shown in the previous work. Selenite was bound to the material via inner-sphere complexation (via oxygen) to the adsorption sites of the amorphous Fe(III) and Mn(III) oxides. This anion was attracted via bidentate binuclear corner-sharing coordination between SeO3(2-) trigonal pyramids and both FeO6 and MnO6 octahedra; however, the adsorption sites of Fe(III) hydrous oxides played a leading role in selenite removal. The contribution of the adsorption sites of Mn(III) oxide increased as the pH decreased from 8 to 6. Because most minerals have a complex structure (they are seldom based on individual substances) of various crystallinity, this work is equally relevant to environmental science and environmental technology because it shows how various solid phases control cycling of chemical elements in the environment.

The reductive immobilization of aqueous Se(IV) by natural pyrrhotite

The interaction of Se(IV) with natural pyrrhotite was investigated at pH conditions ranging from acidic to nearly neutral. The results indicate that the reduction rate can be described in terms of a pseudo-first order reaction. At pH ∼4.0 to ∼5.0, the rate decreased with increasing pH. Unexpectedly, at pH ∼5.0, the rate increased with increasing reaction time. This response was also observed at pH ∼6.0. Two different reaction mechanisms were proposed to explain pyrrhotite oxidation by Se(IV). Because pyrrhotite is acid-soluble and can be attacked by both Fe(3+) and protons, direct reduction by the released aqueous sulfide dominates the reaction at low pH, whereas the cyclic oxidation of aqueous Fe(2+) adsorbed on pyrrhotite surfaces becomes predominant at high pH. Phosphate, which can be irreversibly bound to Fe(3+) intermediates even under acidic conditions, can significantly decrease the reaction rate by an order of magnitude at pH ∼4.5. In contrast to the thermodynamic calculations and the predicted prevalence of FeSe based on previous reports of aqueous Se(IV) reduction by synthetic mackinawite or troilite, only Se(0) was observed as the reaction product in this study. This observation confirmed that a slow reaction favors the formation of Se(0) rather than iron selenides.

Weak magnetic field significantly enhances selenite removal kinetics by zero valent iron

The effect of weak magnetic field (WMF) on Se(IV) removal by zero valent iron (ZVI) was investigated as functions of pH and initial Se(IV) concentrations. The presence of WMF significantly accelerated Se(IV) removal and extended the working pH range of ZVI from 4.0-6.0 to 4.0-7.2. The WMF induced greater enhancement in Se(IV) removal by ZVI at lower initial Se(IV) concentrations. The influence of WMF on Se(IV) removal by ZVI was associated with a more dramatic drop in ORP and a more rapid release of Fe(2+) compared to the case without WMF. SEM and XRD analysis revealed that WMF accelerated the corrosion of ZVI and the transformation of amorphous iron (hdyr)oxides to lepidocrocite. XANES analyses showed that WMF expedited the reduction of Se(IV) to Se(0) by ZVI at pH 6.0 when its initial concentration was ≤20.0 mg L(-1). Se(IV) dosed at 40.0 mg L(-1) was removed by ZVI via adsorption followed by reduction to Se(0) at pH 7.0 but via adsorption at 7.2 in the presence of WMF. Regardless of WMF, Se(IV) applied at 40.0 mg L(-1) was removed by reduction at pH 4.0-6.0. The WMF-induced improvement in Se(IV) removal by ZVI may be mainly attributable to the Lorentz force and magnetic field gradient force. Employing WMF to enhance Se(IV) removal by ZVI is a promising and environmental-friendly method since it does not need extra energy and costly reagents.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

The effect of granular ferric hydroxide amendment on the reduction of nitrate in groundwater by zero-valent iron

The feasibility of using granular ferric hydroxide (GFH) with zero-valent iron (Fe(0)) for its potential utility in enhancing nitrate reduction was investigated. The addition of 10gL(-1) GFH to 25gL(-1) Fe(0) significantly enhanced nitrate removal, resulting in 93% removal of 52.2mg-NL(-1) in 36-h as compared to 23% removal with Fe(0) alone. Surface analyses of the reacted Fe(0)/GFH revealed the presence of magnetite on the Fe(0) surface, which probably served as an electron mediator for nitrate reduction. Addition of GFH to Fe(0) also resulted in lower solution pH compared to Fe(0). The rate enhancing effect of GFH on nitrate reduction was attributed to the combined effects of magnetite formation and pH buffering by GFH. GFH amendment (100gL(-1)) significantly increased reduction capacity and longevity of Fe(0) to complete several nitrate reduction cycles before inactivation, giving a total nitrate removal of 205mg-NL(-1), while unamended Fe(0) gave only 20mg-NL(-1) before inactivation during the first reduction cycle. The overall result demonstrated the potential utility of Fe(0)/GFH system that may be developed into a viable technology for removal of nitrate from groundwater.

Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron

The kinetics of Se(IV) removal by zero valent iron (ZVI) open to the air as a function of pH and the involved mechanisms were investigated in this study. The specific rate constants of Se(IV) removal by ZVI decreased from 92.87 to 6.87 L h(-1) m(-2) as pH increased from 4.0 to 7.0. The positive correlation between the removal rate of Se(IV) and the generation rate of Fe(II) and the depression of Se(IV) removal in the presence of 1,10-phenanthroline indicated that both ZVI and adsorbed Fe(II) on ZVI surface contributed to the reductive removal of Se(IV). The soft X-ray STXM measurement confirmed the adsorption of Fe(II) on the surface of ZVI and freshly formed ferric (hydr)oxides. Se(IV) was removed by adsorption followed by reduction to Se(0) on ZVI surface at pH 4.0-7.0, as revealed by XANES spectra. A core-shell structure was observed when ZVI reacted with Se(IV)-containing solution for 3 h at pH 6.0. Se(IV) was reduced to Se(0) and co-precipitated with the freshly formed Fe(III), forming the shell surrounding the iron core. After reaction for 24 h, the generated Se(0) was surrounded by multiple layers of Fe(III) oxides/hydroxides. SEM images and XRD patterns revealed that the corrosion products of ZVI at pH 6.0 transformed from amorphous iron hydroxides to lepidocrocite (γ-FeOOH) as reaction proceeded. The final corrosion products of ZVI contained both lepidocrocite and goethite at pH 5.0 while they were X-ray amorphous at pH 4.0 and 7.0.

Field evidence of selenium bioreduction in a uranium-contaminated aquifer

Removal of selenium from groundwater was documented during injection of acetate into a uranium-contaminated aquifer near Rifle, Colorado (USA). Bioreduction of aqueous selenium to its elemental form (Se0) concentrated it within mineralized biofilms affixed to tubing used to circulate acetate-amended groundwater. Scanning and transmission electron microscopy revealed close association between Se0 precipitates and cell surfaces, with Se0 aggregates having a diameter of 50-60 nm. Accumulation of Se0 within biofilms occurred over a three-week interval at a rate of c. 9 mg Se0 m(-2) tubing day(-1). Removal was inferred to result from the activity of a mixed microbial community within the biofilms capable of coupling acetate oxidation to the reduction of oxygen, nitrate and selenate. Phylogenetic analysis of the biofilm revealed a community dominated by strains of Dechloromonas sp. and Thauera sp., with isolates exhibiting genetic similarity to the latter known to reduce selenate to Se0. Enrichment cultures of selenate-respiring microorganisms were readily established using Rifle site groundwater and acetate, with cultures dominated by strains closely related to D. aromatica (96-99% similarity). Predominance of Dechloromonas sp. in recovered biofilms and enrichments suggests this microorganism may play a role in the removal of selenium oxyanions present in Se-impacted groundwaters and sediments.

Interaction of aqueous Se(IV)/Se(VI) with FeSe/FeSe2: implication to Se redox process

Since reductive precipitation is considered as the most effective way to immobilize (79)Se, interaction of aqueous Se(IV)/Se(VI) with Fe(II)-bearing minerals has received extensive attention. In contrast to the thermodynamic calculations, as well as the prevalence of iron selenide phases observed in soil, sediments and ore deposits, most laboratory experiments have found that Se(0) was the reaction product. In this study, the interaction of Se(IV)/Se(VI) with FeSe/FeSe2 were investigated. The results demonstrate that FeSe and FeSe2 can be oxidized to Se(0) by Se(IV) with relatively fast kinetics, while reaction between Se(VI) and FeSe/FeSe2 only occurs under limited conditions (i.e. in the presence of high ferrous content and higher pH) with much slower kinetics, and there is no evident reaction in most case. Therefore, reduction of Se(IV) by Fe(II)-bearing minerals, in particular by natural occurring minerals, is envisioned to produce Se(0) at the early stage of experiments, rather than FeSe or FeSe2. Due to the formation of bulk Se(0) and its low solubility, the Fe-Se-O-H2O system will maintain redox disequilibrium in laboratory time-scale. This study also reveals that iron selenides, like iron sulfides, have strong reactivity toward Fe(3+). The findings in this study give insight into possible controls on Se redox process.

Electrochemical Removal Of Selenate From Aqueous Solutions

Removal of selenate from solution is investigated in batch electrochemical systems using reactive iron anodes and copper plate cathode in a bicarbonate medium. Iron anodes produce ferrous hydroxide, which is a major factor in the removal of selenate from solution. Iron anodes also generate a significant decrease in the oxidation-reduction potential (ORP) of the solution because it prevents generation of oxygen gas at the anode by electrolysis. The removal rates varied from 45.1 to 97.4%, depending on current density and selenate concentration. The transformation of selenate by the process is modeled based on a heterogeneous reaction coupled with electrochemical generation of ferrous and hydroxide. The rates are optimized at lower initial concentrations, higher electrical currents, and the presence of anions. Presence of dissolved oxygen does not cause any significant effects the removal of selenate.

Nanocomposite pyrite-greigite reactivity toward Se(IV)/Se(VI)

A nanopyrite/greigite composite was synthesized by reacting FeCl(3) and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30-50 nm sized particles containing 28% of greigite (Fe(2+)Fe(3+)(2)S(4)) and 72% pyrite (FeS(2)). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite-greigite solid phase at different pH-values and with or without the addition of extra Fe(2+). XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe(2)) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se(0), the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system.

Environmental selenium research: from microscopic processes to global understanding

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Effect of pH on aqueous Se(IV) reduction by pyrite

Interaction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free (<1 ppm) conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reduction of aqueous Se(IV) by pyrite is dominated by the following reaction: FeS2+3.5HSeO3−+1.5H+=2SO4(2−)+Fe2++3.5Se(0)+2.5H2O. The released Fe(II) was partitioned between the bulk solution and pyrite surface at pH≈4.5 and 4.8, with the Fe2+ density at pyrite-solution interface about 4 orders of magnitude higher than that in the bulk solution, while iron oxyhydroxide precipitated at pH≈6.6, resulting in the decrease of dissolved iron. In the Se(IV) concentration range of the experiments, aqueous Se(IV) reduction rate follows the pseudofirst order which is in the form of ln mSe(IV)=−k′t+ln mSe(IV)0, where k′ is apparent rate constant combining the rate constant k and pyrite surface area to mass of solution ratio (A/M). And the aqueous Se(IV) reduction rate constant for a standard system (k) with 1 m2 pyrite surface area per 1 kg solution was obtained to be 1.65×10(−4) h(−1), 3.28×10(−4) h(−1), and 4.76×10(−4) h(−1) at pH around 4.5, 4.8, and 5.1, respectively. The positive correlation between reaction rate and pH disagrees with the theories that protons are consumed when HSeO3− is reduced to Se0, and negative charge density on pyrite surface increases as pH increases. Thus, a ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product.