Accurate Potentiometric Studies of Chromium‐Citrate and Ferric‐Citrate Complexes in Aqueous Solutions at Physiological and Alkaline pH Values

Influence of Organic Ligands on the Redox Properties of Fe(II) as Determined by Mediated Electrochemical Oxidation

Fe(II) has been extensively studied due to its importance as a reductant in biogeochemical processes and contaminant attenuation. Previous studies have shown that ligands can alter aqueous Fe(II) redox reactivity but their data interpretation is constrained by the use of probe compounds. Here, we employed mediated electrochemical oxidation (MEO) as an approach to directly quantify the extent of Fe(II) oxidation in the absence and presence of three model organic ligands (citrate, nitrilotriacetic acid, and ferrozine) across a range of potentials (E_H) and pH, thereby manipulating oxidation over a broad range of fixed thermodynamic conditions. Fe(III)-stabilizing ligands enhanced Fe(II) reactivity in thermodynamically unfavorable regions (i.e., low pH and E_H) while an Fe(II) stabilizing ligand (ferrozine) prevented oxidation across all thermodynamic regions. We experimentally derived apparent standard redox potentials, E_H^ϕ, for these and other (oxalate, oxalate₂, NTA₂, EDTA, and OH₂) Fe-ligand redox couples via oxidative current integration. Preferential stabilization of Fe(III) over Fe(II) decreased E_H^ϕ values, and a Nernstian correlation between E_H^ϕ and log(K_Fe(III)/K_Fe(II)) exists across a wide range of potentials and stability constants. We used this correlation to estimate log(K_Fe(III)/K_Fe(II)) for a natural organic matter isolate, demonstrating that MEO can be used to measure iron stability constant ratios for unknown ligands.

Structurally Dependent Electrochemical Properties of Ultrafine Superparamagnetic 'Core/Shell' γ-Fe2O3/Defective α-Fe2O3 Composites in Hybrid Supercapacitors

The paper presents a method for obtaining electrochemically active ultrafine composites of iron oxides, superparamagnetic ‘core/shell’ γ-Fe₂O₃/defective α-Fe₂O₃, which involved modifying sol-gel citrate synthesis, hydrothermal treatment of the formed sol, and subsequent annealing of materials in the air. The synthesized materials’ phase composition, magnetic microstructure, and structural, morphological characteristics have been determined via X-ray analysis, Mossbauer spectroscopy, scanning electron microscopy (SEM), and adsorption porometry. The mechanisms of phase stability were analyzed, and the model was suggested as FeOOH → γ-Fe₂O₃ → α-Fe₂O₃. It was found that the presence of chelating agents in hydrothermal synthesis encapsulated the nucleus of the new phase in the reactor and interfered with the direct processes of recrystallization of the structure with the subsequent formation of the α-Fe₂O₃ crystalline phase. Additionally, the conductive properties of the synthesized materials were determined by impedance spectroscopy. The electrochemical activity of the synthesized materials was evaluated by the method of cyclic voltammetry using a three-electrode cell in a 3.5 M aqueous solution of KOH. For the ultrafine superparamagnetic ‘core/shell’ γ-Fe₂O₃/defective α-Fe₂O composite with defective hematite structure and the presence of ultra-dispersed maghemite with particles in the superparamagnetic state was fixed increased electrochemical activity, and specific discharge capacity of the material is 177 F/g with a Coulomb efficiency of 85%. The prototypes of hybrid supercapacitor with work electrodes based on ultrafine composites superparamagnetic ‘core/shell’ γ-Fe₂O₃/defective α-Fe₂O₃ have a specific discharge capacity of 124 F/g with a Coulomb efficiency of 93% for current 10 mA.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Selenium Speciation in the Fountain Creek Watershed (Colorado, USA) Correlates with Water Hardness, Ca and Mg Levels

The environmental levels of selenium (Se) are regulated and strictly enforced by the Environmental Protection Agency (EPA) because of the toxicity that Se can exert at high levels. However, speciation plays an important role in the overall toxicity of Se, and only when speciation analysis has been conducted will a detailed understanding of the system be possible. In the following, we carried out the speciation analysis of the creek waters in three of the main tributaries—Upper Fountain Creek, Monument Creek and Lower Fountain Creek—located in the Fountain Creek Watershed (Colorado, USA). There are statistically significant differences between the Se, Ca and Mg, levels in each of the tributaries and seasonal swings in Se, Ca and Mg levels have been observed. There are also statistically significant differences between the Se levels when grouped by Pierre Shale type. These factors are considered when determining the forms of Se present and analyzing their chemistry using the reported thermodynamic relationships considering Ca²⁺, Mg²⁺, SeO₄²⁻, SeO₃²⁻ and carbonates. This analysis demonstrated that the correlation between Se and water hardness can be explained in terms of formation of soluble CaSeO₄. The speciation analysis demonstrated that for the Fountain Creek waters, the Ca²⁺ ion may be mainly responsible for the observed correlation with the Se level. Considering that the Mg²⁺ level is also correlating linearly with the Se levels it is important to recognize that without Mg²⁺ the Ca²⁺ would be significantly reduced. The major role of Mg²⁺ is thus to raise the Ca²⁺ levels despite the equilibria with carbonate and other anions that would otherwise decrease Ca²⁺ levels.

Chromium speciation in tannery effluent after alkaline precipitation: Isolation and characterization

It is difficult to completely remove Cr(III) from tannery effluent by alkaline precipitation due to the abundance of strong organic ligands. Thereby, the speciation of the residual Cr after alkaline precipitation is of crucial significance to guide the selection and design of further treatment process. For the first time, we revealed the speciation of the residual Cr with the aid of comprehensive analytical techniques. Results showed that the residual Cr(III) mostly located in two size ranges, i.e. the 13-100nm fraction and the <4nm fraction. Combined spectral analyses demonstrated Cr(III) was coordinated by carboxyl groups or hydroxyl groups in both fractions, while the complexation by nitrogen-containing groups was excluded by the total nitrogen and UPLC-MS analysis in the two fractions, respectively. Based on the comprehensive analyses, the structures of Cr(III) complexes in both fractions were proposed. Cr(III) cross-linked the carboxyl groups from polyacrylic acid chains to form the network gel structure in the 13-100nm fraction, while the complex structure of Cr(III) in the <4nm fraction was formed through hydroxyl-carboxyl chelation by masking agents such as tartrate and citrate. Although polyoxyethylene ether was abundantly present, it was responsible for the complexation of Cr(III) in neither fraction.

Uptake mechanisms for inorganic iron and ferric citrate in Trichodesmium erythraeum IMS101

Growth of the prevalent marine organism Trichodesmium can be limited by iron in natural and laboratory settings. This study investigated the iron uptake mechanisms that the model organism T. erythraeum IMS101 uses to acquire iron from inorganic iron and iron associated with the weak ligand complex, ferric citrate. IMS101 was observed to employ two different iron uptake mechanisms: superoxide-mediated reduction of inorganic iron in the surrounding milieu and a superoxide-independent uptake system for ferric citrate complexes. While the detailed pathway of ferric citrate utilization remains to be elucidated, transport of iron from this complex appears to involve reduction and/or exchange of the iron out of the complex prior to uptake, either at the outer membrane of the cell or within the periplasmic space. Various iron uptake strategies may allow Trichodesmium to effectively scavenge iron in oligotrophic ocean environments.

Metal speciation in health and medicine represented by iron and vanadium

The influence of metals in biology has become more and more apparent within the past century. Metal ions perform essential roles as critical scaffolds for structure and as catalysts in reactions. Speciation is a key concept that assists researchers in investigating processes that involve metal ions. However, translation of the essential area across scientific fields has been plagued by language discrepancies. To rectify this, the IUPAC Commission provided a framework in which speciation is defined as the distribution of species. Despite these attempts, contributions from inorganic chemists to the area of speciation have not fully materialized in part because the past decade's contributions focused on technological advances, which are not yet to the stage of measuring speciation distribution in biological solutions. In the following, we describe how speciation influences the area of metals in medicine and how speciation distribution has been characterized so far. We provide two case studies as an illustration, namely, vanadium and iron. Vanadium both has therapeutic importance and is known as a cofactor for metalloenzymes. In addition to being a cation, vanadium(V) has analogy with phosphorus and as such is a potent inhibitor for phosphorylases. Because speciation can change the metal's existence in cationic or anionic forms, speciation has profound effects on biological systems. We also highlight how speciation impacts iron metabolism, focusing on the rather low abundance of biologically relevant iron cation that actually exists in biological fluids. fluids. Furthermore, we point to recent investigations into the mechanism of Fenton chemistry, and that the emerging results show pH dependence. The studies suggest formation of Fe(IV)-intermediates and that the generally accepted mechanism may only apply at low pH. With broader recognition toward biological speciation, we are confident that future investigations on metal-based systems will progress faster and with significant results. Studying metal complexes to explore the properties of a potential "active species" and further uncovering the details associated with their specific composition and geometry are likely to be important to the action.

Iron(III) citrate speciation in aqueous solution

Citrate is an iron chelator and it has been shown to be the major iron ligand in the xylem sap of plants. Furthermore, citrate has been demonstrated to be an important ligand for the non-transferrin bound iron (NTBI) pool occurring in the plasma of individuals suffering from iron-overload. However, ferric citrate chemistry is complicated and a definitive description of its aqueous speciation at neutral pH remains elusive. X-Ray crystallography data indicates that the alcohol function of citrate (Cit4-) is involved in Fe(III) coordination and that deprotonation of this functional group occurs upon complex formation. The inability to include this deprotonation in the affinity constant calculations has been a major source of divergence between various reports of iron(III)-citrate affinity constants. However the recent determination of the alcoholic pKa of citric acid (H4Cit) renders the reassessment of the ferric citrate system possible. The aqueous speciation of ferric citrate has been investigated by mass spectrometry and EPR spectroscopy. It was observed that the most relevant species are a monoiron dicitrate species and dinuclear and trinuclear oligomeric complexes, the relative concentration of which depends on the solution pH value and the iron : citric acid molar ratio. Spectrophotometric titration was utilized for affinity constant determination and the formation constant for the biologically relevant [Fe(Cit)2]5- is reported for the first time.

Shewanella putrefaciens produces an Fe(III)-solubilizing organic ligand during anaerobic respiration on insoluble Fe(III) oxides

The mechanism of Fe(III) reduction was investigated using voltammetric techniques in anaerobic incubations of Shewanella putrefaciens strain 200 supplemented with Fe(III) citrate or a suite of Fe(III) oxides as terminal electron acceptor. Results indicate that organic complexes of Fe(III) are produced during the reduction of Fe(III) at rates that correlate with the reactivity of the Fe(III) phase and bacterial cell density. Anaerobic Fe(III) solubilization activity is detected with either Fe(III) oxides or Fe(III) citrate, suggesting that the organic ligand produced is strong enough to destabilize Fe(III) from soluble or solid Fe(III) substrates. Results also demonstrate that Fe(III) oxide dissolution is not controlled by the intrinsic chemical reactivity of the Fe(III) oxides. Instead, the chemical reaction between the endogenous organic ligand is only affected by the number of reactive surface sites available to S. putrefaciens. This report describes the first application of voltammetric techniques to demonstrate production of soluble organic-Fe(III) complexes by any Fe(III)-reducing microorganism and is the first report of a Fe(III)-solubilizing ligand generated by a metal-reducing member of the genus Shewanella.