Oxyhydroxide Research Articles

Porous and Partially Dehydrogenated Fe2+ -Containing Iron Oxyhydroxide Nanosheets for Efficient Electrochemical Nitrogen Reduction Reaction (ENRR).

The design and development of efficient catalysts for electrochemical nitrogen reduction reaction (ENRR) under ambient conditions are critical for the alternative ammonia (NH3 ) synthesis from N2 and H2 O, wherein iron-based electrocatalysts exhibit outstanding NH3 formation rate and Faradaic efficiency (FE). Here, the synthesis of porous and positively charged iron oxyhydroxide nanosheets by using layered ferrous hydroxide as a starting precursor, which undergoes topochemical oxidation, partial dehydrogenated reaction, and final delamination, is reported. As the electrocatalyst of ENRR, the obtained nanosheets with a monolayer thickness and 10-nm mesopores display exceptional NH3 yield rate (28.5µg h-1 mgcat. -1 ) and FE (13.2%) at a potential of -0.4V versus RHE in a phosphate buffered saline (PBS) electrolyte. The values are much higher than those of the undelaminated bulk iron oxyhydroxide. The larger specific surface area and positive charge of the nanosheets are beneficial for providing more exposed reactive sites as well as retarding hydrogen evolution reaction. This study highlights the rational control on the electronic structure and morphology of porous iron oxyhydroxide nanosheets, expanding the scope of developing non-precious iron-based highly efficient ENRR electrocatalysts.

Retention of Iodide by Chloride Green Rust and Magnetite.

Green rust (GR), a layered double hydroxide (LDH) containing Fe, and magnetite can be found in natural and engineered environments. The ability of chloride GR (GR-Cl) and magnetite to retain iodide as a function of various parameters was investigated. Sorption equilibrium is achieved within 1 day of contact time between iodide and preformed GR-Cl in suspension. pHm variations (7.5-8.5) have no significant influence, but the iodide sorption decreases with increasing ionic strength set by NaCl. Sorption isotherms of iodide suggest that the uptake operates via ionic exchange (IC), which is supported by geochemical modeling. The short-range binding environment of iodide associated with GR is comparable to that of hydrated aqueous iodide ions in solution and is not affected by pHm or ionic strength. This finding hints at an electrostatic interaction with the Fe octahedral sheet, consistent with weak binding of charge balancing anions within an LDH interlayer. The presence of sulfate anions in significant amounts inhibits the iodide uptake due to recrystallization to a different crystal structure. Finally, the transformation of iodide-bearing GR-Cl into magnetite and ferrous hydroxide resulted in a quantitative release of iodide into the aqueous phase, suggesting that neither transformation product has an affinity for this anionic species.

Biogenic amorphous ferric hydroxide as adsorbent for vanadium removal in drinking water production

Vanadium as toxic heavy metal is a drinking water relevant contaminant. However, there is a lack in treatment processes to meet regulatory requirements (e.g. 4 μg l−1 in Germany). This study introduces a novel treatment process — the vanadium adsorption onto biogenic amorphous ferric hydroxide (AFH). Basic mechanisms of adsorption onto AFH are described and compared to granular ferric hydroxide (GFH). Adsorption kinetics and pH dependent isotherms in drinking and ultrapure water, parametrization via the empirical Freundlich and Langmuir models, and bond type and strength assessments via sequential extraction are presented.AFH was generated in pilot waterworks in which Fe(II) and oxygen were dosed and subsequently Fe(II) microbiologically oxidized and precipitated in the filter bed. The backwash-water was collected and used for adsorption experiments. Sequential extraction was executed with vanadium loaded AFH produced in the pilot plant. AFH is identified as alternative adsorbent to GFH with similar affinity and capacity. The isotherms cover a concentration range from 10 μg l−1 to 4 mg l−1 and the Freundlich model showed a better fit with the experimental data than the Langmuir model. A bidentate mononuclear inner sphere complex is assumed for vanadium adsorption onto AFH, while a bidentate binuclear inner sphere complex is expected for GFH. Sequential extraction showed a strong bond between AFH and vanadium, which was only mobilized by the last extraction step the dissolution of iron particles. A treatment process – adsorption onto biogenic AFH – is suitable for effective vanadium removal and should be further investigated for technical implementation.

Radionuclides' Recovery from Seawater Using FIC and FIC A Sorbents.

To solve radioecological and oceanological problems (estimate the vertical transport, flows of particulate organic carbon, phosphorus biodynamics, submarine groundwater discharge, etc.), it is necessary to determine the natural values of the radionuclides' activity in seawater and particulate matter. For the first time, the radionuclides' sorption from seawater was studied using sorbents based on activated carbon modified with iron(III) ferrocyanide (FIC) and based on activated carbon modified with iron(III) hydroxide (FIC A-activated FIC) obtained by FIC sorbent treatment with sodium hydroxide solution. The possibility of trace amounts of phosphorus, beryllium, and cesium recovery in laboratory conditions has been investigated. Distribution coefficients, dynamic, and total dynamic exchange capacities were determined. The physicochemical regularities (isotherm and kinetics) of sorption have been studied. The results obtained are characterized via Langmuir, Freindlich, and Dubinin-Radushkevich isotherm equations, as well as pseudo-first and pseudo-second-order kinetic models, intraparticle diffusion, and the Elovich model. Under expeditionary conditions, the sorption efficiency of 137Cs using FIC sorbent, 7Be, 32P, and 33P-using FIC A sorbent with a single-column method by adding a stable tracer, as well as the sorption efficiency of radionuclides 210Pb and 234Th with their natural content by FIC A sorbent in a two-column mode from large volumes of seawater was assessed. High values of efficiency of their recovery by the studied sorbents were achieved.

Analysis of Uranium Sorption in a Laboratory Column Experiment Using a Reactive Transport and Surface Complexation Model

The transport and retardation of radioactive elements in hyper-alkaline conditions of radioactive waste repositories is a challenging field that is still poorly understood. In this study, the transport and attenuation of uranium in a column experiment was modelled by considering kinetic reactions, advection–dispersion and chemical/physical retardation processes. The modelling was first performed for three alluvium samples from Yucca Mountain in circumneutral pH to moderately alkaline conditions. Sorption of uranyl ({mathrm{UO}}_{2}^{2+} ({U}_{mathrm{VI}})) was found to strongly depend on the surface complexation model assumed, with no significant removal of {U}_{mathrm{VI}} by precipitation or ion exchange process. The surface/edge site reaction of Al-hydroxyl group in kaolinite was shown to have a high affinity for uranyl adsorption, while the hydrous ferric oxide edge on hematite adsorbed most of the uranyl ions. The model was then used to interpret uranium transport in a laboratory column filled with Hollington sandstone under hyper-alkaline (pH 13) conditions. The simulation results show that uranium adsorption on the Al-hydroxyl edge of kaolinite exceeds adsorption by the calcium silicate hydrate phase. This result may reflect the lack of surface complexation parameters for calcium silicate hydrate minerals. Hence, further studies are required in the field of surface complexation reactions for calcium silicate hydrate phases.Graphical

Adsorptive Removal of Selenium(IV) from Aqueous Solution by Ferrous Hydroxide Complex-Zero Nickel Composites

ABSTRACT Nanoscale ferrous hydroxide complex-zero nickel composites (FHC/Ni0) were fabricated by ball milling method for Se(IV) adsorption from aqueous solution. The effects of pH, solid-liquid ratio, time, temperature, initial concentration of Se(IV) on the adsorption of Se(IV) by FHC/Ni0 were investigated. FHC/Ni0 was characterized by SEM, XPS, XRD, FT-IR, BET and Zeta potential, and the mechanism of removing Se(IV) was analyzed. The results showed that FHC/Ni0 had a good removal effect on Se(IV). When pH was 5.0, the solid-liquid ratio was 0.15 g L−1, the reaction time was 40 min, the maximum adsorption capacity of Se(IV) by FHC/Ni0 could reach 216 mg g−1. The pseudo-second-order kinetic model and Langmuir had a good fit for the Se(IV) adsorption process of FHC/Ni0, indicating that the monolayer and chemical adsorption played a leading role in the adsorption process. XPS analysis further confirmed that the adsorption of Se on FHC/Ni0 was mainly due to the formation of stable complexes with a large number of functional groups containing Fe, Ni and S, meanwhile, the removal of Se(IV) by the material is achieved through a combination of chemical adsorption and reduction reactions. The research results can demonstrate the feasibility of FHC/Ni0 in Se(IV) removal and provide a theoretical basis for the treatment of wastewater containing Se(IV).

Insight into direct phosphorus release from simulated wastewater ferric sludge: Influence of physiochemical factors

Phosphorous recovery from chemical phosphorus removal (CPR) plants is limited. CPR plants that use iron salts for phosphorous removal generate iron phosphate (Fe-P) rich sludge. This study investigates the influence of pH and competing anions (chloride), on phosphorus and soluble iron release from lab simulated Fe-P sludge. Factors influencing recovery, such as sludge age and Fe/P molar ratio are also investigated. Results showed that alkaline treatment of Fe-P sludge was not effective in releasing Fe (<3%) but effective in releasing P, particularly at controlled pH value of 10 where the %P release was (90 ± 2%). Sludge ageing time did not affect P release from Fe-P sludge at ages less than 5 days. However, a remarkable decrease in %P release (i.e., 50% reduction) was observed at ages of 9 and 11 days for pH values of 9–10. Arsenic extraction of exchangeable surface bound P from Fe-P sludge and surface area determination of hydrous ferric oxide (HFO) under different aging times supported qualitatively the internalization of surface bound P during aging, which is a proposed mechanism responsible for the decrease in P release for older sludge. Chloride effect in releasing P from Fe-P sludge was negligible. The reduction in P release by aging was best described by a zero-order kinetic model. Modeling with the PHREEQC geochemical software did not always agree with the measured release of phosphorus from the Fe-P sludge. PHREEQC assumes all surface sites for phosphorus are exchangeable and has no mechanism to represent phosphorus trapped within HFO particles.

Дезактивація рідких радіоактивних відходів Чорнобильської зони відчуження: шляхи вирішення

The purpose of the study: on the basis of information analysis of modern approaches and experimental studies, to develop effective methods of decontamination of radioactively contaminated waters of the Chernobyl Exclusion Zone. Research materials and methods. The object of our research was liquid radioactive waste stored in the sumps of one of the decontamination points for equipment and vehicles (PuSO) - the "Dibrova" facility. Modern research methods are applied - scanning electron microscopy, X-ray diffractometry, laser sedimentography. The mass fraction of cesium, strontium, cobalt, and manganese was determined by the atomic absorption method using an AA-8500 atomic absorption spectrophotometer (Nippon Jarrell Ash Co Ltd, Japan). The results. The effectiveness of the proposed sorbents based on nickel-potassium ferrocyanide-modified iron oxide micro- and nanotubes and iron (III) hydroxide nanoparticles, the size of which is mainly 1-100 μm, is shown. The choice of decontamination algorithm depends on the composition of liquid radioactive waste to be cleaned. To increase the degree of strontium extraction while maintaining the high efficiency of cesium and transition metal extraction, it is advisable to apply preliminary plasma chemical treatment followed by the use of sorbents based on iron oxides/hydroxides modified by ferrocyanides. Conclusions. The possibility and expediency of using the proposed method for the decontamination of liquid radioactive waste accumulated in the Chernobyl exclusion zone and for the current control of the condition of the storage sites of accumulated radioactive waste and radiation monitoring are shown.

7Be Recovery from Seawater by Sorbents of Various Types.

For the first time, a comprehensive study of sorbents based on manganese dioxide was carried out for beryllium sorption from seawater in laboratory and expeditionary conditions. The possibility of using several commercially available sorbents based on manganese dioxide (Modix, MDM, DMM, PAN-MnO2) and phosphorus(V) oxide (PD) for 7Be recovery from seawater for solving oceanological problems was evaluated. Beryllium sorption under static and dynamic conditions was studied. The distribution coefficients and dynamic and total dynamic exchange capacities were determined. Sorbents Modix (Kd = (2.2 ± 0.1) × 103 mL/g) and MDM (Kd = (2.4 ± 0.2) × 103 mL/g) showed high efficiency. The dependences of the degree of recovery on time (kinetics) and the capacity of the sorbent on the beryllium equilibrium concentration in solution (isotherm) were established. The data obtained were processed using kinetic models (intraparticle diffusion, pseudo-first and pseudo-second orders, Elovich model) and sorption isotherm equations (Langmuir, Freindlich, Dubinin-Radushkevich). The paper contains results of expeditionary studies to evaluate the sorption efficiency of 7Be from large volumes of the Black Sea water by various sorbents. We also compared the sorption efficiency of 7Be for the considered sorbents with aluminum oxide and previously obtained sorbents based on iron(III) hydroxide.

New hard-template strategy for the synthesis of [formula omitted]-Fe[formula omitted]O[formula omitted] hollow-spheres

A new route to manufacture hematite (α-Fe2O3) hollow spheres with a diameter of approximately 165 ± 29 nm is presented. The proposal is based on the use of polystyrene spheres as a hard template and ethylene glycol as a solvent. In the intermediate stage of the synthesis, the PS@Fe(OH)3 sample was manufactured. For this sample, transmission electron microscopy and scanning electron microscopy confirmed the good uniformity of the polystyrene core coating, which has a thickness of approximately 12 ± 5 nm. EDS mapping confirmed the presence of iron and oxygen. The shell composition (α-FeOOH) was confirmed by Raman spectroscopy and FTIR. In the final stage of the synthesis, the Void@Fe2O3 sample was obtained. For this sample, core removal and the shell thickness (16 ± 5 nm) were confirmed by electron microscopy. EDS mapping confirmed the presence of iron and oxygen, in addition to indicating the complete removal of the polystyrene core. XRD and Raman measurements confirmed that the shell is constituted of hematite with traces of iron hydroxide. These results demonstrate the success of the adopted strategy for the synthesis of hematite hollow spheres.

Characterization of plate-shaped strontium ferrite particles synthesized via co-precipitate and heat treatment in molten potassium bromide flux

Plate-shaped hexagonal strontium ferrite particles were synthesized using potassium bromide as the molten salt flux. The co-precipitate of strontium and iron hydroxides was heated in the temperature range of 670–870 °C in the flux. Clear peaks based on hexagonal ferrite and weak peaks based on α-Fe2O3 were observed after the heat treatment at 670 °C. The peaks based on α-Fe2O3 almost disappeared after the heat treatment at 730 °C, and the peaks observed in the particles heated at temperatures higher than 730 °C were consistent with the standard data for SrFe12O19. The particle shape tended to change from a hexagonal plate shape to a rounded plate shape with increasing temperature while maintaining a particle diameter of approximately 100–200 nm. The coercive force was approximately 420–440 kA/m, while the magnetization at the applied field of 1353 kA/m was approximately 62 Am2/kg at heating temperatures higher than 750 °C.

Features of the course of stable exertional angina against the background of iron deficiency anemia in outpatient practice

Introduction. Iron-deficiency accompanies many diseases, including cardiovascular pathology, so timely diagnosis and correction of this condition is necessary. Aim. To study the features of the course of stable exertional angina against the background of iron deficiency anemia before and after correction with iron (III) polymaltose hydroxide in patients in primary health care using a computerized patient register. Materials and methods. 1210 patients with stable exertional angina aged 50–85 years (309 men, 901 women) were included. A special computer program was created for data analysis. In 500 out of 1210 patients, IDA was detected (385 women, 115 men). Mild anemia – 410 patients, moderate – 75, severe – 15. The main group – patients with stable exertional angina and Irondeficiency anemia (the first subgroup – patients treated with iron (III) polymaltose hydroxide and the second subgroup – patients who did not receive the drug iron (III) polymaltose hydroxide due to refusal of treatment or contraindications) and the control group – patients with stable exertional angina without Iron-deficiency anemia. All patients received standard therapy recommended for the treatment of stable exertional angina (the main group additionally received iron (III) polymaltose hydroxide). Results. In the first subgroup of the main group, after treatment with iron (III) hydroxide polymaltose, there was a statistically significant increase in hemoglobin levels by 11,7%, hematocrit by 5%, erythrocytes by 4%, serum iron by 112,8%, and an improvement in exercise tolerance by 48,7%, episodes of ST segment depression by 30,2%. In the second subgroup and the control group, there were no significant changes in the above parameters. Conclusion. Iron-deficiency anemia contributes to the severe course of stable exertional angina, which is confirmed by the high frequency of III-IV functional class exertional angina, a decrease in tolerance to exercise stress. Correction of Iron-deficiency anemia reduces the frequency of episodes of transient myocardial ischemia in the main group.

Nine‐year organic fertilization enhances poorly crystalline iron hydroxide formation and phosphorus availability in a temperate rice–wheat cropping system

AbstractPhosphorus (P) is typically associated with iron (Fe) hydroxides in paddy soils. Our study investigated the impact of different fertilization treatments on the availability of P regulated by Fe redox transformation in a rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping system in the mid‐low reaches of the Yangtze River. Five fertilization treatments were examined: control (CK), chemical fertilizer (CF), 35% CF (P) plus pig manure compost (CFM), 100% CF (P) plus straw (CFS), and 35% CF (P) plus pig manure compost and straw (CFMS). Nine‐year rice–wheat rotations increased the oxalate‐extractable poorly crystalline Fe hydroxides (i.e., Feo) by 33%–87% compared with the initial soil, while fertilization further accelerated this process. Compost application increased the proportion of labile P by 75%–108% and decreased the proportion of non‐labile P by 14%–22% compared with the single chemical fertilization treatment. Furthermore, organic fertilization increased the mass proportions of macroaggregates and macroaggregate‐associated labile P. The Feo was positively correlated with the content of labile P but negatively correlated with the proportion of non‐labile P. Moreover, the reductive dissolution of Fe hydroxides was accompanied by the transformation of P from NaOH‐extractable to NaHCO3‐ and H2O‐extractable phases. These results indicate that seasonal alternation of drying and wetting can progressively drive the redox transformation of Fe hydroxides and promote the formation of Feo, thereby affecting the availability of P. Therefore, we suggested that P fertilizer should be reduced in the rice season due to the reduction of Fe hydroxides, particularly in the compost‐amended soils in the temperate rice–wheat cropping system.

CHARACTERISTICS OF THE BINDING BETWEEN THE ENZYME PREPARATION RAPHANUS SATIVUS L. AND NATURAL LAYERED ALUMOSILICA IN THE PRESENCE OF IRON OXIDES

IR and X-ray analyses of bentonite modified with magnetite and enzyme established the dependence of enzyme immobilization on the presence of iron oxides. IR studies have proven that the binding of the enzyme to the carrier is adsorption in nature without stable chemical interactions. This type of binding does not significantly affect the change in the structure of the enzyme module. Characte­ristic bands of vibrations of amide groups of proteins were detected at 1637 cm-1 ( ν-vibrations of C=O bonds (amide I)), 713 cm-1 (δ – vibrations of O=C-N in the plane (amide IV)), which can participate in the sorption binding of the enzyme to bentonite and the SiO2 group, which is characterized by a weak connection of the outer part of protein with the formation of a surface structure, which disappears over time due to the diffusion of molecules into the pores of the mineral. Comparison of the basal distances in the samples established the following dependence: 12.43 Å bentonite+ Fe3O4+enz (48 h) &gt; 12.38; Å natural bentoni­te&gt; 12.31; Å bentonite+ Fe3O4+enz (2 hours) &gt; 11.91; Å bentonite+ Fe3O4 , which indicates a high probability of iron compounds entering the interlayer space due to the hydrolysis of magnetite with the formation of Fe-(OH)n bonds and unstable compounds. According to the research results, a scheme of the interaction of the enzyme with modified layered iron aluminosilicate is proposed: 1 – transport of the enzyme to the surface (phase distribution boundaries); 2 – deformation of the enzyme under the action of the surface as a result of a polycondensation reaction with the formation of water molecules; 3 – entry of water molecules into the interlayer space; 4 – hydrolysis of magnetite particles; 5 – extraction of iron hydroxide from the interlayer space due to the formation of aggregates with an enzyme molecule that has a related adsorption center.

Effect of Heat Treatment on the Passive Film and Depassivation Behavior of Cr-Bearing Steel Reinforcement in an Alkaline Environment

Using Cr-bearing low-alloy steel is an effective preventive measure for marine structures, as it offers superior corrosion resistance when compared to plain carbon steel. However, it remains unclear how quenching and tempering heat treatment, which is commonly applied to steel reinforcement in some specific environments to improve its mechanical properties, affects its corrosion resistance. In the present work, the impact of heat treatment on the passive film and depassivation behavior of the 0.2C-1.4Mn-0.6Si-5Cr steel are studied. The results reveal that quenching and tempering result in grain refinement of the Cr-bearing steel, which increases its hardness. However, this refinement causes significant degradation in its corrosion resistance. The critical [Cl−]/[OH−] ratio after quenching and tempering is determined to be approximately 6.6 times lower than that after normalization, and the corrosion rate is 1.6 times higher. After quenching and tempering, the passive film predominantly comprises iron oxides and hydroxides, with relatively high water content and defect density. Additionally, the FeII/FeIII ratio and film resistance are relatively low. In comparison, after normalization, the steel exhibits high corrosion resistance, with the passive film formed offering the highest level of protection.

Electrocoagulation with Fe-Al hybrid electrodes for the removal of arsenic, fluoride, and silica from natural groundwater

This communication is about the removal of arsenic (As), fluoride (F−) and hydrated silica (HS) from real underground water (initial concentration: 28 µg L−1 arsenic, 1.55 mg L−1F−, 155 mg L−1 HS, 35 mg L−1 sulfate, pH 7.4, and 470 µS cm−1 conductivity) by electrocoagulation (EC). The novel EC reactor consists of eight horizontally stacked cells using Fe and Al plates as sacrificial anodes with an upward flow. The abatement of As and the elimination at the same time of F− and HS was performed by analyzing the influence of the mean linear flow velocity (1.2 < u < 4.8 cm s−1) at different current densities (3 < j < 7 mA cm−2) imposed on the EC system. The best condition was carried out at j = 7 mA cm−2 and u = 1.2 cm s−1, achieving complete arsenic elimination, and a remaining F− concentration of 0.45 mg L−1, fulfilling the World Health Organization (WHO) guideline for arsenic (< 10 μg L−1) and fluoride (< 1.5 mg L−1). The remaining HS reached a value of 10 mg L−1. Spectroscopic techniques applied to the Fe-Al flocs evidenced the generation of iron oxides and hydroxides, as well as the formation of aluminosilicates.

Efficient removal of p-arsanilic acid and arsenite by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes

The widespread occurrence of p-arsanilic acid (p-ASA) in natural environments poses big threats to the biosphere due to the generation of toxic inorganic arsenic (i.e., As(III) and As(V), especially As(III) with higher toxicity and mobility). Oxidation of p-ASA or As(III) to As(V) followed by precipitation of total arsenic using Fe-based advanced oxidation processes demonstrated to be a promising approach for the treatment of arsenic contamination. This study for the first time investigated the efficiency and inherent mechanism of p-ASA and As(III) oxidation by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes. p-ASA was rapidly degraded by the Fe(II)/PAA process within 20 s at neutral to acidic pHs under different conditions, while it was insignificantly degraded by PAA oxidation alone. Lines of evidence suggested that hydroxyl radicals and organic radicals generated from the homolytic OO bond cleavage of PAA contributed to the degradation of p-ASA in the Fe(II)/PAA process. p-ASA was mainly oxidized to As (V), NH4+, and p-aminophenol by the Fe(II)/PAA process, wherein the aniline group and its para position were the most vulnerable sites. As(III) of concern was likely generated as an intermediate during p-ASA oxidation and it could be readily oxidized to As(V) by the Fe(II)/PAA process as well as PAA alone. The in-depth investigation demonstrated that PAA alone was effective in the oxidation of As(III) under varied conditions with a stoichiometric molar ratio of 1:1. Efficient removal (> 80%) of total arsenic during p-ASA oxidation by Fe(II)/PAA process or during As(III) oxidation by PAA process with additional Fe(III) in synthetic or real waters were observed, mainly due to the adsorptive interactions of amorphous ferric (oxy)hydroxide precipitates. This study systematically investigates the oxidation of p-ASA and As(III) by the Fe(II)/PAA and PAA processes, which is instructive for the future development of arsenic remediation technology.

Evolution of solid contents in vinasse waste during electrocoagulation process at various current densities: Experimental and kinetic analyses

Vinasse waste contained high solid contents. The goal of this study was to examine the effect of current densities (3.93, 4.72, 5.50 A/dm2) on the evolution of solid contents during electrocoagulation (EC) through experimental and kinetic analyses. Therefore, a new mechanistic model was proposed in this study. The results showed that the profiles of total solids (TS) mass had an interesting correlation with the liquid pH, in which at pH of 4.4–6, the TS mass increased and at pH of 6–7.3, the TS mass decreased during the EC process. The increase in TS mass at the beginning of the process was caused by the accumulation of iron hydroxy complexes and iron hydroxides. Besides the TS and liquid pH, this study also measured the total dissolved solids (TDS), total suspended solids (TSS), colour (absorbance value), scum, sludge, liquid temperature, liquid volume, dissolved Fe, and electrical voltage. All measured data were successfully predicted using the proposed mechanistic model with high accuracy (R2 = 0.9900–0.9985). By modelling, 14 important kinetic constant values were found.

Arsenic release and colloidal species characterization during microbial reduction of As-containing ferrihydrite

Ferrihydrite is a common iron hydroxide in mining environments and arsenic has high adsorption affinity with it. Biological anaerobic reduction of arsenical ferrihydrite represents an aqueous arsenic and colloids release process. Arsenical ferrihydrite was collected from Jiepaiyu As-(Au) Mine area and used to investigate aqueous arsenic (III and V) and the relationship with colloid fractions (Large: <10 μm, medium: <1 μm and small: <10 nm) under sulfate-reducing bacteria (Desulfovibrio vlugaris miyazaki). Different arsenical ferrihydrite solid to solution ratios were investigated by batch experiment. In anaerobic conditions, the highest concentrations of aqueous arsenic(III) and arsenic(V) released into solution per gram arsenical ferrihydrite were on days 2 and 3 for low ratio (9449 μg/L and 8035 μg/L, respectively) and high ratio conditions (13672 μg/L and 7760 μg/L, respectively). Arsenic(III) concentrations with colloids varies the same as for aqueous arsenic(III), increasing from 0 day to 3 days and then decreasing. These colloids contain total iron of 7–563 mg/L, arsenic(III) of 84–3534 μg/L and arsenic(V) of 5082–14520 μg/L. Ferrous (Fe2+) and sulfur (S2-) components are found in the colloids, which their mole ratio is 0.8–8.7. There is considerable difference among mobilization of various colloids. The transportation velocities of these colloids follow this sequence: large > medium > small. These results suggest that biological reduction changes arsenic species and colloid fractions, which affect geochemical mobilization of arsenic.

Identifying Fe as OER Active Sites and Ultralow-Cost Bifunctional Electrocatalysts for Overall Water Splitting.

Electrocatalysts based on Fe and other transition metals are regarded as most promising candidates for accelerating the oxygen evolution reaction (OER), whereas whether Fe is the catalytic active site for OER is still under debate. Here, unary Fe- and binary FeNi- based catalysts, FeOOH and FeNi(OH)x , are produced by self-reconstruction. The former is a dual-phased FeOOH, possessing abundant oxygen vacancies (VO ) and mixed-valence states, delivering the highest OER performance among all the unary iron oxides- and hydroxides- based powder catalysts reported to date, supporting Fe can be catalytically active for OER. As to binary catalyst, FeNi(OH)x is fabricated featuring 1) an equal molar content of Fe and Ni and 2) rich VO , both of which are found essential to enable abundant stabilized reactive centers (FeOOHNi) for high OER performance. Fe is found to be oxidized to 3.5+ during the *OOH process, thus, Fe is identified to be the active site in this new layered double hydroxide (LDH) structure with Fe:Ni = 1:1. Furthermore, the maximized catalytic centers enable FeNi(OH)x @NF (nickel foam) as low-cost bifunctional electrodes for overall water-splitting, delivering excellent performance comparable to commercial electrodes based on precious metals, which overcomes a major obstacle to the commercialization of bifunctional electrodes: prohibitive cost.