Fe OH Research Articles

New water soluble heterometallic complex showing unpredicted coordination modes of EDTA

A mesoporous 3D polymeric complex (I) having formula {[Zr(IV)O-μ3-(EDTA)Fe(III)OH]·H2O}n has been crystallized and characterized by various techniques. Single-crystal X-ray diffraction analysis revealed that complex (I) crystallized in chiral monoclinic space group Cc (space group no. 9) with unexpected coordination modes of EDTA and mixture of two transition metal ions. In this complex, the coordination number of Zr(IV) ion is seven where four carboxylate oxygen atoms, two nitrogen atoms, one oxide atom are coordinating with Zr(IV). Fe(III) is four coordinated and its coordination environment is composed of three different carboxylic oxygen atoms from three different EDTA and one oxygen atom of –OH group. The structure consists of 4-c and 16-c (2-nodal) net with new topology and point symbol for net is (336·454·530)·(36). TGA study and XRPD pattern showed that the coordination polymer is quite stable even after losing water molecule and –OH ion. Quenching behavior in fluorescence of ligand is observed by complexation with transition metal ions is due to n–π⁎ transition. The SEM micrograph shows the morphology of complex (I) exhibits spherical shape with size ranging from 50 to 280nm. The minimum N2 (SBET=8.7693m2/g) and a maximum amount of H2 (high surface area=1044.86m2/g (STP)) could be adsorbed at 77K. From DLS study, zeta potential is calculated i.e. −7.94 shows the negative charges on the surface of complex. Hirshfeld surface analysis and fingerprint plots revealed influence of weak or non bonding interactions in crystal packing of complex.

Removal behavior and mechanism of Co(II) on the surface of Fe–Mn binary oxide adsorbent

Abstract An amorphous Fe–Mn binary oxide adsorbent was developed using simultaneous oxidation and coprecipitation, and characterized by Brunauer–Emmett–Teller analysis, X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The removal behavior of the prepared absorbent for cobalt (II) was studied. Results showed that iron and manganese in adsorbent existed mainly in the +III and +IV oxidation states, respectively. The adsorbent exhibited the best depletion of Co(II) at pH 6–8, and the best removal ability at an ionic strength of 0.01 mmol L−1. The Freundlich isotherm model fitted the Co(II) adsorption data best. The FTIR spectra of the absorbent before and after Co(II) adsorption showed that the surface hydroxyl groups, Mn–OH and Fe–OH, existed at the surface and formed Mn–O–Co or Fe–O–Co. The iron oxide was the main adsorbent, and the manganese oxide also showed adsorption and oxidation ability. The XPS spectra confirmed that the Mn in the absorbent was reduced whereas the Fe showed no difference after reaction with Co(II). The XPS measurements of cobalt adsorbed on the adsorbent indicate that Co(II) has been oxidized to Co(III).

Iron oxide formation from FeCl2 solutions in the presence of uranyl (UO22+) cations and carbonate rich media

The mineral goethite (α-FeOOH) has previously been investigated as a thermodynamically stable repository for many potentially toxic metals (e.g., Cd, Pb, Cu). The substitution of uranium (U as uranyl, UO22+) for Fe, however, has been studied sparingly, and conclusive uranium incorporation into the goethite structure has been obscured by formation of ploymineralic Fe systems that all appeared involved in U’s sequestration. In this study, we investigated the formation of goethite from FeCl2 in the presence of seven nominal uranyl concentrations (0.2–4mole percent, U/(U+Fe)×100) and characterised the solids using quantitative X-ray diffraction (QXRD), X-ray absorption fine structure (XAFS) spectroscopy at the U LIII edge (17,166eV), diffuse reflectance infrared Fourier Transform (DRIFT) spectrometry and congruency of acid dissolution. Our findings show that U does indeed, however sparingly, incorporate into the goethite structure. The unit cell volume, ascertained from Rietveld models of XRD patterns, increased linearly as a function of U content, which could be ascribed to a linear increase of the unit cell length a and c. The highest U-for-Fe substitution was 0.48mole percent, however, most U-containing goethite samples showed substitution levels around 0.2mole percent, which was in good agreement with previous findings. DRIFT spectra showed a shift of the symmetric Fe–O and asymmetric Fe–OH stretch modes (τ–O and τ–OH bands, respectively) to lower frequency, which by Hooks analogy, can only occur if a heavier atom substitutes for Fe, i.e., U. The congruency of acid dissolution results showed that fractional U release was greater than the corresponding fractional Fe release into solution, suggesting that U was overall more soluble. In two of the synthates, however, (initial U mole percent of 2 and 4), the dissolution was congruent. XAFS data collected on a selected subset of samples showed the disappearance of the uranyl moiety with higher levels of U incorporation and acid extraction. In sample S6, non-linear least-square fits of the extended XAFS data demonstrated that the coordination environment around U atoms was sixfold occupied by O/OH atoms and was further coordinated by next nearest Fe neighbours that are 1.063±0.008times inflated in distance to the normal Fe–Fe distances in goethite without U substitution. Despite low levels of incorporation, U bound by goethite was recalcitrant to desorption/dissolution in increasingly acidic solutions thus warranting further research into the possibility of using iron oxides as a sink for U.

Calcium phosphate-forming ability of magnetite and related materials in a solution mimicking in vivo conditions

Iron-based compounds, especially magnetite (Fe3O4), can be a candidate of thermoseeds for hyperthermia therapy. When iron-based compounds are applied for bone tumor treatment, they should have a heat-generating property and a bone-bonding property. However, the bone-bonding property of iron-based compounds is still unclear. The bone-bonding property of materials is estimated by their bone-like apatite formation property in simulated body fluid (SBF). The method to estimate apatite forming ability of materials by utilizing SBF was introduced by Kokubo et al. We thus report fundamental research into the behavior of iron oxides and an iron oxyhydroxide namely: FeO, Fe3O4, α-Fe2O3, γ-Fe2O3, and α-FeOOH, in SBF. Calcium phosphate precipitation was found in Fe3O4 and α-Fe2O3 within 7 and 28 days after soaking in SBF, respectively, while FeO, γ-Fe2O3, and α-FeOOH did not. Our results indicate that Fe3O4 and α-Fe2O3 have a better potential bone-bonding property than FeO, γ-Fe2O3, and α-FeOOH. The induction of apatite precipitation in SBF can be attributed to the specific structure of Fe–OH groups on the surface of Fe3O4 and α-Fe2O3.

Structural and Chemical Evolution of Amorphous Nickel Iron Complex Hydroxide upon Lithiation/Delithiation

Development of novel electrode materials is essential to achieve high-performance lithium ion batteries. Here, we demonstrate that amorphous nickel iron complex hydroxides (Ni–Fe–OH) synthesized by a laser–chemical method can be used as a potential conversion anode material for lithium storage. Complementary characterizations, including ensemble-averaged X-ray absorption spectroscopy, spatially resolved electron energy-loss spectroscopy, and energy dispersive X-ray spectroscopy in a scanning transmission electron microscope, were performed to reveal the chemical and structural evolutions of the active hydroxide particles undergoing electrochemical cycling. The solid–electrolyte interphase (SEI) layer with a primary component of lithium fluoride (LiF) was found and remained robust on the particle surface during the charge/discharge processes, which suggests that the LiF-containing SEI layer plays a critical role in maintaining the stable capacity retention and good reversibility of the Ni–Fe–OH anode.

Kinetics, thermodynamics and competitive adsorption of lead and zinc ions onto termite mound

The elemental composition of termite mound was determined by XRF which revealed K, Ti and Mn as minor constituents while Ca and Fe as major constituents. The dominant functional groups of termite mound are Fe–OH, Fe–O and O–H. The Pb(II) and Zn(II) adsorption capacities (mg g−1) are in order of Pb(II) (13.07) > Zn(II) (12.40) > Pb(Pb/Zn) (11.72) > Zn(Pb/Zn) (7.62). The Langmuir adsorption isotherm fitted the adsorption data better than Freundlich isotherm. The adsorption process was best described by pseudo-second-order kinetic model for single and binary solutions, and the rate constants k 2 (g mg−1 min−1) are 0.036, 0.016, 0.024 and 0.015, and the calculated value of q e (mg g−1) is 12.33, 12.25, 11.52 and 7.84 for Pb(II), Zn(II), Pb(Pb/Zn) and Zn(Pb/Zn), respectively. The regression coefficient, R 2 values, for these solutions ranged between 0.9966 and 0.9978. The ΔH (kJ mol−1) values were positive for single and binary solutions in the order Pb(II) (32.0) > Pb(Pb/Zn) (30.8) > Zn(Pb/Zn) (28.0) > Zn(II) (19.0), while ΔS (kJ mol−1 K−1) are in the order of Pb(II) (0.103) > Pb(Pb/Zn) (0.097) > Zn(Pb/Zn) (0.082) > Zn(II) (0.06). The ΔG value for Zn(II) is positive in both single and binary systems, while that for Pb(II) was positive between 313–333 and 323–333 K for single and binary systems, respectively. The data show that the use of neglected termite mound for Pb(II) and Zn(II) removal from aqueous solutions is economically significant in wastewater treatment.

Reversible solid to solid transformation in a crystalline state gas–solid reaction under ambient conditions: Fe–N(pyridine) bond formation at the expense of Fe–OH2bond breaking and vice versa

The non-porous crystalline state of a trinuclear iron cluster containing compound [Fe3(μ3-O)(μ2-CH3COO)6(C5H5NO)2(H2O)]ClO4·3H2O (1) demonstrates a series of crystalline state reactions that can be stimulated by an external substrate, such as methanol or pyridine. This simple trinuclear {μ3-O} cluster [Fe3(μ3-O)(μ2-CH3COO)6(C5H5NO)2(H2O)]1+ exhibits reversible gas–solid reactions driven by the ligand exchange at one of its metal centres that involve Fe–OH2 bond breaking and Fe–N(pyridine) bond formation in the solid state in a reversible crystal to crystal transformation. The gas–solid interface reactions have been confirmed by IR spectroscopy and powder X-ray diffraction studies. A classical trinuclear iron {μ3-O} cluster with a slight modification in its basic molecular structure opens up new possibilities that can find applications in the development of multi-functional materials. The reversible solid to solid transformations in the present article have been established by various spectroscopic arguments.

Interplay of the H-bond donor-acceptor role of the distal residues in hydroxyl ligand stabilization of Thermobifida fusca truncated hemoglobin.

The unique architecture of the active site of Thermobifida fusca truncated hemoglobin (Tf-trHb) and other globins belonging to the same family has stimulated extensive studies aimed at understanding the interplay between iron-bound ligands and distal amino acids. The behavior of the heme-bound hydroxyl, in particular, has generated much interest in view of the relationships between the spin-state equilibrium of the ferric iron atom and hydrogen-bonding capabilities (as either acceptor or donor) of the OH(-) group itself. The present investigation offers a detailed molecular dynamics and spectroscopic picture of the hydroxyl complexes of the WT protein and a combinatorial set of mutants, in which the distal polar residues, TrpG8, TyrCD1, and TyrB10, have been singly, doubly, or triply replaced by a Phe residue. Each mutant is characterized by a complex interplay of interactions in which the hydroxyl ligand may act both as a H-bond donor or acceptor. The resonance Raman stretching frequencies of the Fe-OH moiety, together with electron paramagnetic resonance spectra and MD simulations on each mutant, have enabled the identification of specific contributions to the unique ligand-inclusive H-bond network typical of this globin family.

Catalytic decomposition of H2O2 over Fe-based catalysts for simultaneous removal of NOX and SO2

Simultaneous flue gas desulfurization and denitrification were achieved with OH radicals from the decomposition of H2O2 over hematite (Fe) as well as hematite supported on alumina (Fe–Al) and anatase (Fe–Ti). Under all conditions, SO2 achieved 100% removal, whereas NOX removal varies with the catalysts. The supporting of Fe over aluminum enhances the catalytic removal of NOX, whereas that of anatase presents negative effect. The NOX removal is determined by the decomposition rate of H2O2 into OH radicals over OH bonded with Fe (Fe–OH). The supporting of Fe over alumina enhances the content of Fe–OH and the points of zero charge (PZC) values, which are beneficial for the production of OH radicals. The supporting of Fe over anatase results in the formation of FeOTi, which cannot decompose H2O2 into OH radicals. Furthermore, H2O2 tends more to be reacted with TiOH to produce O2 over Fe–Ti. Finally, the enhancement mechanism of H2O2 decomposition over Fe-based catalysts is speculated. It has a contribution to the correct choice for supports and active ingredients of the catalyst in the future industrial applications.

X-Ray Diffraction and Vibrational Spectroscopic Characteristics of Hydroxylclinohumite from Ruby-Bearing Marbles (Luc Yen District, Vietnam)

A honey-yellow hydroxylclinohumite from ruby-bearing marbles of the Luc Yen district in northern Vietnam was characterized by electron microprobe (EPMA), single-crystal X-ray diffraction (XRD), micro-Raman, and Fourier-transform infrared (FTIR) spectroscopy. The studied crystals correspond to nearly ideal clinohumite with the structural formula 4[Mg2SiO4]·[(Mg,Fe,Ti)(OH,F)2] and roughly equal F and OH proportions. Crystal structure analysis showed Ti substitution for Mg only at the Mg3 site. A Fourier-difference map revealed one hydrogen site associated with ninth oxygen atom. The calculated O–H bond distance was shorter than that in other natural clinohumites. FTIR revealed bands corresponding to combination of OH-stretching with Mg–OH and/or Fe–OH bending modes, combinations of OH−and Fe–OH vibrations, combination of fundamental bands of the Si–OH bonding, combination of OH−and Si–OH vibrations, and first (2νOH) and the second (3νOH) overtones of the OH-stretching vibration mode. Two groups of OH-stretching vibration and FTIR absorption bands at 3390–3420 cm−1and 3560–3580 cm−1show reversible temperature-dependent shift. The low-frequency bands absent in pure synthetic hydroxylclinohumites are assigned to OH-planar defects caused by Ti-for-Mg substitution.

Synthesis of paramagnetic iron incorporated silica aerogels by ambient pressure drying

The iron incorporated silica aerogels were prepared under ambient pressure drying through a two-step sol-gel route by using tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and ferric nitrate as co-precursors, and hexamethyldisilazane (HMDZ) as a surface modification reagent. Compared with the silica aerogels modified by trimethylchlorosilane (TMCS) or HMDZ, the iron incorporated silica aerogels exhibited good integrity, paramagnetic behavior, and highly hydrophobicity which could be maintained up to 650 °C. However, the introduction of iron into silica aerogels resulted in more apparent volume shrinkage, higher density, lower porosity, and smaller pore diameter. It was found that a large amount of water was retained in the silica network, accordingly, the water containing iron oxides (Fe–OH) were more readily formed than water free iron oxides (Fe–O), which adversely affected the physical properties of iron incorporated silica aerogels.

A New Deactivation Mechanism of Sulfate-Promoted Iron Oxide

The deactivation of sulfate-promoted iron oxide in the esterification of acetic acid and n-butanol was studied. The sulfate-promoted iron oxide was used ten runs and 10 h, continually and accumulatively. After ten-run continual use of the catalyst, a considerable deactivation happened to it. The fresh and the deactivated catalysts were compared by means of many characteristic methods including FTIR, XRD, BET, SEM, TG–DSC, and NH3–TPD, to disclose some possible reasons for the deactivation of sulfate-promoted iron oxide in the esterification. Based on the comparative analyses of IR patterns of the fresh catalyst and the deactivated one, a deactivation mechanism is tentatively proposed. Namely, surface sulfate groups, which are originally coordinated to Fe3+ cations and can so induce and generate strong Lewis acidity of Fe3+ cations, may have been gradually turned into free sulfate groups and sulfate esters arisen from strong Lewis-acidic Fe3+ cations’ being hydrolyzed by H2O and their being alcoholyzed by n-butanol, which leads to a gradual destruction of the originally strong coordination between Fe3+ cations and surface sulfate groups, so leading to the acidity degradation of the catalyst, and so finally leading to the deactivation of it. Emphatically, in the proposed mechanism, the water produced from the esterification may play a key role on the deactivation of the catalyst, because it can directly hydrolyze some strong Lewis-acidic Fe3+ cations of the catalyst and indirectly promote the alcoholysis of them, to form weak Lewis-acidic Fe–OH species. The deactivated catalyst has a larger crystallinity, a smaller specific surface area, a smaller sulfate groups content, a weaker acidity than the fresh. All these phenomena, accompanying the deactivation of sulfate-promoted iron oxide, can be interpreted by the proposed deactivation mechanism very well.

Synthesis and microstructural properties of α-Fe1−xGaxOOH solid solutions

Isostructural iron oxyhydroxide α-FeOOH (goethite) and gallium oxyhydroxide α-GaOOH, as well as their solid solutions α-Fe1−xGaxOOH in a whole concentration range (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.30, 0.50, 0.80 or 1.00) were synthesized by precipitation in a weakly alkaline medium using an organic alkali tetramethylammonium hydroxide (TMAH) for pH adjustment. Lath-shaped α-Fe1−xGaxOOH particles common for goethite formed in alkaline media were observed in all samples containing more Fe than Ga, whereas fairly uniform ellipsoidal α-Fe1−xGaxOOH particles characteristic of α-GaOOH formed in alkaline media were observed in samples containing more Ga than Fe. An increased content of Ga3+ ions in α-Fe1−xGaxOOH led to a decrease in the unit-cell size (in line with the smaller radius of Ga3+ ions). A reduction in the hyperfine magnetic field and a collapse of the room temperature magnetic order in α-Fe1−xGaxOOH with x>0.2 were found (due to the dilution of the magnetic lattice brought about by the replacement of the magnetic Fe3+ ions by the diamagnetic Ga3+ ions). A shift in the position of (Fe,Ga)–O–H bending bands and Fe–O and Fe–OH stretching (lattice vibration) bands towards higher wave numbers (due to increased strength of the O–H⋯O hydrogen bond), a decrease in intensity of all absorption bands in UV–Vis–NIR spectra and a shift of Fe3+ ligand field transition bands to lower energies (due to an increase in the ligand field splitting energy), and an increase in dehydroxilation temperature (due to the higher stability of the Ga–OH bond in comparison with the Fe–OH bond) with the increased Ga3+ content were also found.

Spectophotometric studies on the competitive adsorption of boric acid (b(iii)) and chromate (cr(vi)) onto iron (oxy) hydroxide (fe(o)oh)

Various pollutants (e.g. boron and hexavalent chromium) are introduced into the aquatic environment from a variety of industrial operations causing damages to environment and affecting human health. Boron in irrigation water is of particular interest because it can have beneficial or toxic effect on plants, depending on its concentration. Pollution of the environment with hexavalent chromium (Cr(VI)) and associated toxicity to microorganisms, plants, animals and humans is of major concern. Indeed, chromium in environmentally significant concentrations is found near to tanneries and involves large volumes of wastewater. One of the most effective remediation technologies used for the removal of B(III) and Cr(VI) from aquatic systems and wastewater is their sorption on metal oxide surfaces. However, in order to understand better the mechanisms involved and improve the efficiency of remediation technologies further fundamental studies are needed. The present study is focused on the adsorption of H3BO3 and CrO42- onto Fe(O)OH at various ionic strengths (I = 0.0, 0.1 and 1.0 M NaClO4), under normal atmospheric conditions, at 22 ± 3 oC and pH 8. Additionally, competitions studies were carried out to investigate the ion-exchange mechanism and compare the individual affinities of H3BO3 and CrO42- for Fe(O)OH. The concentration of H3BO3 and CrO42- in solution was determined spectrophotometrically by means of azomethine-H and DPC (1,5-diphenylcarbazide), respectively. The results obtained indicate that the release of Cr(VI) and therefore its concentration in solution increases as the amount of B(III) is increased in the sorption system. This phenomenon is due to the replacement of Cr(VI) ions by ions B(III) on the surface of Fe(O)OH. Evaluation of the experimental data results in a value for the competition constant which is equal logK= -3.5 ± 0.2, indicating that the adsorbent surface has greater affinity for Cr(VI) than for B(III) species. The formation constant for the Cr(VI)-Fe(O)OH surface complex is calculated to be logßCr= 7.9 ± 0.2.

Removal of arsenic(III) from aqueous solution using a low-cost by-product in Fe-removal plants—Fe-based backwashing sludge

This study investigated the elemental composition and distribution of the Fe-based backwashing sludge (FBBS), and studied its adsorption behaviors and mechanisms towards arsenite [As(III)]. The characterization results of EDS, XPS, and XRF corroborated that the valuable constituents within FBBS are ferric oxhydroxide (γ-FeOOH) and sulfate inter-layered Fe hydroxide [Fe(SO4)OH]. The zeta-potential results indicated a pHZPC value of 7.7. The adsorption equilibrium could be reached within 18h, and the kinetics data were well described by the Elovich and Power models due to the heterogeneous surfaces of FBBS. The isotherm experimental results suggested that the maximum adsorption amount of As(III) was around 59.7mg/g (initial As(III)=1–120mg/L, pH=7.0, T=25°C), which is higher than most of other low-cost adsorbents. The uptake of As(III) onto FBBS would increase with an increase in temperatures, inferring that it is an endothermic process. The optimal initial solution pH for As(III) removal was around pH 8.0. The release of sulfate from FBBS after As(III) adsorption implied the occurrence of ligand exchanges, while the mechanism of Fe(III) precipitation might be also involved. The spectra of FTIR and XPS revealed that the surface hydroxyl groups played an important role in the adsorption of As(III), and the oxidation state of As(III) was not changed. Moreover, phosphates (>1mM) could strongly inhibit the removal of As(III). The desorption results indicated that the release of As caused by alkali or phosphate eluent should be avoided for the scrutiny of waste landfill.

Ab initio study of structurally bound water at cation vacancy sites in Fe- and Al-oxyhydroxide materials

The structure and energetics of proton-compensated cation vacancies in crystalline Fe- and Al-oxide and oxyhydroxide materials are investigated using ab initio methods. In this defect model, a vacant Me3+ cation site is charge compensated by the presence of three protons, forming hydroxyls with the O atoms surrounding the vacant cation site. Proton-compensated cation vacancies are chemically equivalent to excess hydroxyl content, and are also known as hydrogarnet defects, or Ruetschi defects. These defects can be considered a particular form of structurally bound water, as the formation of the defect can be written as the product of the ideal crystalline material and water. Proton-compensated cation vacancy defects are shown to cause lattice expansion in all calculated Fe and Al materials, and are shown to destabilize all materials relative to the ideal crystalline phases. The magnitude of the destabilization due to the vacancy defects is structure dependent, thus defect content can induce shifts in the relative stability between crystalline phases.In all of the Fe-oxyhydroxide materials, proton-compensated cation vacancy defects are shown to be slightly co-stabilized in the presence of nearby Al dopant atoms, likely due to the stronger nature of Al–OH bonding (relative to Fe–OH), or from the cancellation of the opposing lattice strains introduced by the two defect types when considered in isolation. FTIR data in the literature confirms that these two defect types (proton-compensated cation vacancies and Al substitutions) have been observed to occur in tandem.Infrared vibrational frequencies are calculated for the non-stoichiometric hydroxyl groups found at the vacancy defect sites and compared with those of stoichiometric OH groups found in ideal crystalline (oxy)hydroxides. The calculated O–H stretching modes of the defect hydroxyls have higher frequencies than the modes of stoichiometric hydroxyl groups found in Fe-oxyhydroxide materials, consistent with experimental FTIR observations.

Adsorption kinetics of silicic acid on akaganeite

As part of a series of studies on the interaction between ferric ions and silicic acid in the hydrosphere, the adsorption of silicic acid on akaganeite was investigated kinetically at various pH values. The adsorption of silicic acid increased with increasing pH over an initial pH range of 4–11.5. In the kinetic experiment, the Cl− was released from akaganeite much faster than silicic acid was adsorbed. From this result, we concluded that chloride ions bound on the surface of akaganeite are released and Fe–OH or Fe–O− sites are formed, which then acts as an adsorption site for silicic acid. The uptake mechanism of silicic acid by akaganeite is significantly different from that by schwertmannite, despite the presence of the same tunnel structure.

H-bonding networks of the distal residues and water molecules in the active site of Thermobifida fusca hemoglobin

The ferric form of truncated hemoglobin II from Thermobifida fusca (Tf-trHb) and its triple mutant WG8F-YB10F-YCD1F at neutral and alkaline pH, and in the presence of CN− have been characterized by resonance Raman spectroscopy, electron paramagnetic resonance spectroscopy, and molecular dynamics simulations. Tf-trHb contains three polar residues in the distal site, namely TrpG8, TyrCD1 and TyrB10. Whereas TrpG8 can act as a potential hydrogen-bond donor, the tyrosines can act as donors or acceptors. Ligand binding in heme-containing proteins is determined by a number of factors, including the nature and conformation of the distal residues and their capability to stabilize the heme-bound ligand via hydrogen-bonding and electrostatic interactions. Since both the RR Fe–OH− and Fe–CN− frequencies are very sensitive to the distal environment, detailed information on structural variations has been obtained. The hydroxyl ligand binds only the WT protein giving rise to two different conformers. In form 1 the anion is stabilized by H-bonds with TrpG8, TyrCD1 and a water molecule, in turn H-bonded to TyrB10. In form 2, H-bonding with TyrCD1 is mediated by a water molecule. Unlike the OH− ligand, CN− binds both WT and the triple mutant giving rise to two forms with similar spectroscopic characteristics. The overall results clearly indicate that H-bonding interactions both with distal residues and water molecules are important structural determinants in the active site of Tf-trHb. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 5. Halide Ion (F–, Cl–, Br–, I–) Adsorption

Halide anion (F(-), Cl(-), Br(-), and I(-)) adsorption and its impact on sodium adsorption at the hematite/water interface were studied by cryogenic X-ray photoelectron spectroscopy (XPS). Measurements were carried out on frozen, centrifuged wet hematite pastes that were previously equilibrated in 50 mM electrolytic solutions in the pH 2-11 range. XPS-derived halide ion surface loadings decreased in the order F(-) > I(-) ≈ Cl(-) > Br(-), whereas sodium loadings were in the order Na(F) > Na(I) > Na(Br) > Na(Cl). The greater sodium loadings in NaF and in NaI resulted from larger anion loadings in these systems. Bromide ion had the lowest loading among all halide ions despite having a charge-to-size ratio that is intermediate between those of Cl(-) and I(-). This unexpected result may have arisen from specific properties of the hematite/water interface, such as water structure and electric double layer thickness. Fluoride ion adsorption proceeded via the formation of hydrogen bonds with the surface hydroxo groups (e.g., ≡Fe-OH(2)···F(-) or ≡Fe-OH···F(-)). Surface-bound fluoride ions exert a greater charge-screening effect than the other halide anions, as demonstrated by considerably small zeta potential values. Fe-F bond formation was excluded as a possible interfacial process as the F 1s peak binding energy (684.2 eV) was more comparable to that of NaF (684.6 eV) than FeF(3) (685.4 eV). Overall, these findings motivate further refinements of existing thermodynamic adsorption models for predicting the ionic composition of hematite particle surfaces contacted with sodium halide aqueous solutions.

Adsorption Behaviors of Arsenic(V) onto Fe-Based Backwashing Sludge Produced from Fe(II)-Removal Plants

This study investigates the adsorption characteristics of As(V) onto the Fe-based backwashing sludge (FBBS), which was produced in the Fe(II) removal process. FBBS exhibits rough surfaces and shows high BET surface area of 148.41 m2/g. According to the results of EDS and XRD, the main constituents include sulfate inter-layered Fe hydroxide [Fe(SO4)OH], ferric oxhydroxide (γ-FeOOH), quartz (SiO2), and calcium carbonate (CaCO3). The adsorption kinetics data were well described by the Elovich model (r2 = 0.993), indicating the highly heterogeneous adsorption. The maximum adsorption capacity of As(V) increased from 40.04 to 88.76 mg/g as temperatures increased from 298 to 318 K, suggesting an endothermic process. The removal of As(V) was inhibited with elevated solution pH, especially from pH 7.0 to pH 10.0. Moreover, the removal of As(V) was enhanced with an increase in ion strength (0.01-1 M NaNO3), implying that the adsorption of As(V) was mainly through inner-sphere complexes mechanism.