Surface chemistry of disordered mackinawite (FeS)

Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K_s^app = {Fe²⁺} · {H₂S(aq)} · {H⁺}⁻² = 10^+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10⁻³ mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10⁻³ mol/g FeS, determines surface charge changes. Total site density is 4 sites nm⁻². The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.     

Optical properties and crystal chemistry of synthetic rutile implanted with cobalt ions

Implantation of high-energy cobalt ions into plates of synthetic rutile has been studied, and absorption, luminescence, and luminescence excitation spectra have been recorded and interpreted. Long-wave luminescence (820 nm) of Ti _IV ³⁺ ions in rutile has been revealed; its intensity increased after the cobalt implantation. Analysis of luminescence and luminescence excitation spectra has allowed us to specify the scheme of electron energy levels of rutile and to establish the energy levels of impurity Ti³⁺ ions occupying vacant octahedrons with the C _2h symmetry in structure of the mineral.     

Chemical and structural characterization of As immobilization by nanoparticles of mackinawite (FeSm)

The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeS_m) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeS_m particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeS_m, suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeS_m. XPS also revealed that the FeS_m that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O ( three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p_3/2) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.     

Iron sulfide oxidation and the chemistry of acid generation

Acid mine drainage, produced from the oxidation of iron sulfides, often contains elevated levels of dissolved aluminum (AI), iron (Fe), and sulfate (SO₄) and low pH. Understanding the interactions of these elements associated with acid mine drainage is necessary for proper solid waste management planning. Two eastern oil shales were leached using humidity cell methods. This study used a New Albany Shale (4.6 percent pyrite) and a Chattanooga Shale (1.5 percent pyrite). The leachates from the humidity cells were filtered, and the filtrates were analyzed for total concentrations of cations and anions. After correcting for significant solution species and complexes, ion activities were calculated from total concentrations. The results show that the activities of Fe³⁺, Fe²⁺, Al³⁺, and SO₄ ²⁻ increased due to the oxidation of pyrite. Furthermore, the oxidation of pyrite resulted in a decreased pH and an increased pe+pH (redox-potential). The Fe³⁺ and Fe²⁺ activities appeared to be controlled by amorphous Fe(OH)₃ solid phase above a pH of 6.0 and below pe+pH 11.0. The Fe³⁺, Fe²⁺, and SO₄ ²⁻ activities reached saturation with respect to FeOHSO₄ solid phase between pH 3.0 and 6.0 and below pe+pH 11.0 Below a pH of 3.0 and above a pe+pH of 11.0, Fe²⁺, Fe³⁺, and SO₄ ²⁻ activities are supported by FeSO₄·7H₂O solid phase. Above a pH of 6.0, the Al³⁺ activity showed an equilibrium with amorphous Al(OH)₃ solid phase. Below pH 6.0, Al³⁺ and SO₄ ²⁻ activities are regulated by the AlOHSO₄ solid phase, irrespective of pe+pH. The results of this study suggest that under oxidizing conditions with low to high leaching potential, activities of Al and Fe can be predicted on the basis of secondary mineral formation over a wide range of pH and redox. As a result, the long-term chemistry associated with disposal environments can be largely predicted (including trace elements).     

Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide

Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate- and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the reactivity of Hg, including the propensity of inorganic Hg to be methylated, are poorly understood. Under abiotic conditions, using a laboratory flow reactor, bisulfide (HS⁻) was added at 40 to 250 μM h⁻¹ to 5 g L⁻¹ goethite (α-FeOOH) suspensions to which Hg(II) was adsorbed (30-100 nmol m⁻²) at pH 7.5. Dissolved Hg initially decreased from 10³ or 10⁴ nM (depending on initial conditions) to 10⁻¹ nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (β-HgS_(s)) formed, based on identification using Hg L_III-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. The apparent coordination of oxygens surrounding Hg(II), measured with EXAFS spectroscopy, increased during one flow experiment, suggesting desorption of monodentate-bound Hg(II) while bidentate-bound Hg(II) persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 nM or 30 nM), suggesting that byproducts of bisulfide oxidation and Fe(III) reduction, primarily polysulfide and potentially Fe(II), enhanced the dissolution of β-HgS_(s) and/or desorption of Hg(II). Rapid accumulation of Fe(II) in the solid phase (up to 40 μmol g⁻¹) coincided with faster elevation of dissolved Hg concentrations. Fe(II) served as a proxy for elemental sulfur [S(0)], as S(0) was the dominant bisulfide oxidation product coupled to Fe(III) reduction, based on sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. In one experiment, dissolved Hg concentrations tracked those of all sulfide species [S(-II)]. These results suggest that S(-II) reacted with S(0) to form polysulfide, which then caused the dissolution of β-HgS_(s). A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy, although there was no clear evidence that either surface-bound or mineralized Fe(II) strongly affected Hg speciation. Examination of interrelated processes involving S(-II) and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg reactivity models.     

Electron accepting capacity of dissolved organic matter as determined by reaction with metallic zinc

Information about the chemical electron accepting capacity (EAC) of dissolved organic matter (DOM) is scarce owing to a lack of applicable methods. We quantified the electron transfer from metallic Zn to natural DOM in batch experiments at DOC concentrations of 10–100 mg-C L^− 1 and related it to spectroscopic information obtained from UV-, synchronous fluorescence, and FTIR- spectroscopy. The electron donating capacity of DOM and pre-reduced DOM was investigated using Fe(CN)₆³⁻ as electron acceptor. Presence of DOM resulted in release of dissolved Zn, consumption of protons, and slower release of hydrogen compared to reaction of metallic Zn with water at pH 6.5. Comparison with reaction stoichiometry confirmed that DOM accepted electrons from metallic Zn. The release of dissolved Zn was dependent on pH, DOC concentration, ionic strength, and organic matter properties. The reaction appeared to be completed within about 24 h and was characterized by pseudo first order kinetics with rate constants of 0.5 to 0.8 h^− 1. EAC per mass unit of carbon ranged from 0.22 mmol g^− 1 C to 12.6 mmol g^− 1 C. Depending on the DOM, a calculated 28–127% of the electrons transferred from metallic Zn to DOM could be subsequently donated to Fe(CN)₆³⁻. EAC decreased with DOC concentration, and increased with aromaticity, carboxyl, and phenolic content of the DOM. The results indicate that an operationally defined EAC of natural DOM can be quantified by reaction with metallic Zn and that DOM properties control the electron transfer. Shortcomings of the method are the coagulation and precipitation of DOM during the experiment and the production of hydrogen and dissolved Zn by reaction of metallic Zn with water, which may influence the determined EAC.     

Surface chemistry and reactivity of biogenic silica

The surface chemistry of cultured diatoms was compared to that of biosiliceous material in Southern Ocean sediments, using potentiometric titrations and aluminum adsorption experiments. Aerosil 200, a well-studied synthetic amorphous silica, served as reference solid. Surface charge development and aluminum adsorption on cultured diatom shells were comparable to Aerosil. The surface chemical properties of biosiliceous material buried to depths of 15-25 cm in Southern Ocean sediments, however, deviated markedly from those of the cultured diatoms. In pH range 4-8.5, the surface charge density was systematically lower for biogenic silica from the sediments. In addition, the aluminum adsorption edge on the biosiliceous sediments was shifted to higher pHs by about 0.4 units. The results indicate that ionizable surface silanol groups on diagenetically altered diatom shells are less abundant and, possibly, less acidic than on freshly cultured diatoms. The observed differences in surface chemical structure are consistent with the progressive loss of reactivity, or aging, of biogenic silica which promotes its preservation in sediments.     

Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments

Pyrite (FeS₂) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O₂ after transport by bioturbation to the sediment surface. FeS₂ and FeS may also be oxidized within the anoxic sediment in which NO₃⁻, Fe(III) oxides, or MnO₂ are available as potential electron acceptors. In chemical experiments, FeS₂ and FeS were oxidized by MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked ⁵⁵FeS₂ occurred with MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS₂ and FeS oxidation with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS₂ as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO₃⁻, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe²⁺ and S⁰ as substrates and NO₃⁻ as electron acceptor, in the presence of ⁵⁵FeS₂, to test for co-oxidation of FeS₂, but an anaerobic microbial dissolution of ⁵⁵FeS₂ could not been detected. FeS₂ and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS₂ could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO₃⁻ as electron acceptor. O₂ and MnO₂, but not NO₃⁻ or amorphous Fe(III) oxide, are chemical oxidants for both FeS₂ and FeS.     

氢气还原富铁矿石制备零价铁富集磷

以褐铁矿、菱铁矿、赤铁矿和磁铁矿为原料,在600 ℃下氢气焙烧制备零价铁(ZVI).采用X射线衍射仪、X射线荧光光谱仪、比表面积和孔结构分析仪、扫描电镜、X射线光电子能谱、红外光谱表征了富铁矿石矿物组成及制备产物ZVI的形貌、比表面积和孔结构特征,考察了接触时间、初始磷浓度和pH值对磷富集的影响,并分析了pH值为3和5时,富铁矿石中Pb(Ⅱ)、Cu(Ⅱ)、Hg(Ⅱ)的释放情况,探讨了零价铁对磷的富集性能及作用机制.结果 表明,磁铁矿制备的ZVI中铁含量较高(Fe2O387.4％)、杂质较少、拥有大量的微纳米孔隙且矿石原样在15天内未检出Pb(Ⅱ)、Cu(Ⅱ)、Hg(Ⅱ)的释放,磁铁矿制备的ZVI表现出最好的磷富集效果,w(P)可达13.45 mg/g.使用NaOH (2.2 mol/L)溶液可以回收ZVI富集的磷,回收率可达到99.9％.表面络合、静电吸附和Fe氧化产生的Fe(Ⅱ)/Fe(Ⅲ)对磷的共沉淀作用是ZVI富集磷的主要机制.研究结果有助于推动富铁矿石的综合利用及其在磷的深度处理或磷资源循环利用方面的应用.     

The crystal chemistry and thermal stability of sol-gel prepared fluoride-substituted talc

Stable, colloidal sols were prepared from the addition of methanolic (Mg(OCH₃)₂) to one equivalent of H₂O₂ in methanol. Fluoride was quantitatively incorporated by treatment of these sols with HF. Stable sols were obtained when sols, prepared from the acid catalyzed hydrolysis of tetramethylorthosilicate (TMOS), were combined with these fluoridated magnesium sols. Solvent removal gave xerogels, which were calcined, and treated with stoichiometric quantities of water at 750 °C and 1.6 kbar. The resulting products were high-purity, single-phase talcs with fluoride substitution as high as 75 mole percent; the fluoride substitution is higher than any previously reported. Powder XRD analysis showed a non-linear decrease in d(060) spacing with increasing fluoride substitution, which is attributed to a decrease in the b-dimension of talc when fluoride replaced hydroxide. FTIR spectroscopy showed a non-linear decrease in v_O-H in talc with increasing fluoride substitution, which was attributed to an increase in hydrogen bonding of the OH groups and an increase in the electronegativity of the octahedral layer in talc with increasing fluoride substitution. The thermal stability of the talcs was studied using DT and TG, powder XRD, FTIR, and fluoride ion-selective electrode analyses. Synthetic talc without fluoride decomposed at 860 °C, whereas talc with 68% fluoride substitution showed essentially no decomposition when heated to 1060 °C. When heated to 1200 °C, 68% fluoride substituted talc formed amorphous material, enstatite, protoenstatite, norbergite, and chondrodite. The upper stability temperature of talc, taken as the maximum rate of the first endothermic event in its DTA profile, was dependent on the extent of fluoride substitution. Talc with 100% fluoride substitution is predicted to be stable up to ～1100°C.     

深部地球条件下铁和氧元素的化学变化

氧与铁分别为地球上丰度最高的元素与占地球质量最大的元素,它们所形成的化合物遍布我们的星球,具有-2价的氧控制着地球的氧逸度,随着深度的增加氧逸度逐渐降低,与铁所形成的氧化物中铁的价态也逐渐降低,O/Fe逐渐增加,近些年来科学家们发现深部地球条件下铁的一系列氧化物,其中具有高压黄铁矿结构的铁超氧化物FeO2具有最高的O/Fe,并且它还能够携带一定量的H(表示为Py-FeO2H,, x=0-1)。     

Mineralogy and chemistry of the iron ores of Nainarmalai,Tamilnadu

The magnetite-quartzites of Nainarmalai forms part of a large iron ore belt of Tamilnadu which occurs in a high grade granulite terrain. They are associated with basic granulites and gneisses. Mineralogical and chemical studies indicate their similarities with other metasedimentary iron ores.     

Rates of reaction of pyrite and marcasite with ferric iron at pH 2

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction:

$\text{FeS}{}_2\text{+ 14}\text{Fe}{}^{\mathrm{3+}}\text{+ 8}\text{H}{}_2\text{O}\text{= 15}\text{Fe}{}^{\mathrm{2+}}\text{+ 2}\text{SO}{}^{\mathrm{2?}}{}_4\text{+ 16}\text{H}{}^{\mathrm{+}}$

was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law:

where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and $\text{A}\text{M}$ is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater $\text{A}\text{M}$ ratio, they appear to be more reactive per unit mass than the higher temperature types.     

X射线光电光谱表征金属氧化物的酸碱性

本文探讨了氧配位的八面体及四面体的金属配合物中氧原子的价态变化及其酸碱性差异之间的关系。首先利用X射线光电光谱（XPS）分别推导出了氧原子在碱及碱土铝酸盐、铬酸盐、高铁酸盐、钼酸盐及钨酸盐中的价态,随后对氧原子的价态及氧化物的酸碱性质进行了比较。研究结果表明,氧原子的价态在这两种配位构型的化合物中没有显著的差别,其酸碱性的不同在于四面体配位的金属化合物中O2p态在非成键和成键的电子密度中高价带的分裂。     

Surface chemistry and reactivity of plant phytoliths in aqueous solutions

In order to better understand the reactivity of plant phytoliths in soil solutions, we determined the solubility, surface properties (electrophoretic mobilities and surface charge) and dissolution kinetics of phytoliths extracted from fresh biomass of representative plant species (larch tree and elm, horsetail, fern, and four grasses) containing significant amount of biogenic silica. The solubility product of larch, horsetail, elm and fern phytoliths is close to that of amorphous silica and soil bamboo phytoliths. Electrophoretic measurements yield isoelectric point pH_IEP = 0.9, 1.1, 2.0 and 2.2 for four grasses, elm, larch and horsetail phytoliths respectively, which is very close to that of quartz or amorphous silica. Surface acid–base titrations allowed generation of a 2-pK surface complexation model (SCM) for larch, elm and horsetail phytoliths. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 1 ≤ pH ≤ 8, were found to be very similar among the species, and close to those of soil bamboo phytoliths. Mechanistic treatment of all plant phytoliths dissolution rates provided three-parameters equation sufficient to describe phytoliths reactivity in aqueous solutions:$R(\text{mol}/{\text{cm}}^2/\text{s})=6?{10}^{?16}?a_{\text{H+}}+5.0?{10}^{?18}+3.5?{10}^{?13}?a_{\text{OH}?}^{0.33}$Alternatively, the dissolution rate dependence on pH can be modeled within the concept of surface coordination theory assuming the rate proportional to concentration of > SiOH₂⁺, > SiOH⁰ and > SiO^? species. In the range of Al concentration from 20 to 5000 ppm in the phytoliths, we have not observed any correlation between their Al content and solubility, surface acid–base properties and dissolution kinetics.It follows from the results of this study that phytoliths dissolution rates exhibit a minimum at pH ～ 3. Mass-normalized dissolution rates are similar among all four types of plant species studied and these rates are an order of magnitude higher than those of typical soil clay minerals. The minimal half life time of larch and horsetail phytoliths in the interstitial soil solution ranges from 10–12 years at pH = 2–3 to < 1 year at pH above 6, comparable with mean residence time of phytoliths in soil from natural observations.     

Formation of iron sulfide at faecal pellets and other microniches within suboxic surface sediment

Faecal pellet deposition and bioturbation may lead to heterogeneously distributed particles of localized highly reactive organic matter (microniches) being present below the oxygen penetration depth. Where O₂, NO₃⁻, and Fe/Mn oxyhydroxides become depleted within these microniches or where they exist in zones of sulfate reduction, significant localized peaks in sulfide concentration can occur. These discrete zones of sulfide evolution can cause formation of iron sulfides that would not be predicted by analysis of the ‘bulk’ sediment. Using a reaction-transport model developed specifically for investigating spherical microniches, and incorporating 3D diffusion, we investigated how the rate constants of organic matter (OM) degradation, particle porosity and niche lifetime, affect dissolved sulfide and iron concentrations, and formation of iron sulfide at such niches. For all of the modelled scenarios the saturation index for iron sulfide is positive, indicating favourable conditions for FeS precipitation in all niches. Those simulations within the microniche lifetime range of 2.5-5 days gave comparable concentration ratios of sulfide to iron in solution within the niche to experimentally observed values. Our model results provide insight into the mechanisms of preservation of OM, including soft tissue, in the paleo record, by predicting conditions that result in preferential deposition of precipitates at the edge of microniches. Decreases in porosity, shorter niche lifetimes and increases in OM degradation rate constants, all tend to increase the likelihood that FeS precipitation will preferentially occur at the edges of a niche, rather than uniformly throughout the niche volume.