Electron paramagnetic resonance spectroscopy of Fe<Superscript>3+</Superscript> ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe³⁺ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E ₁′. However, the two broad signals do not correspond to any known Fe³⁺ centers in the quartz lattice, but are most likely attributable to Fe³⁺ clusters on surfaces. The absolute numbers of spins of the Fe³⁺ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E _a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe³⁺ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe³⁺ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.     

Solid solutions of (Mg,Fe<Superscript>2+</Superscript>)-cordierite: Synthesis,water content,and magnetic properties

The dependence of water concentration in synthetic (Mg, Fe²⁺)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe²⁺/(Fe²⁺ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H₂O molecules in the structural channels of alkali-free (Mg, Fe²⁺)-cordierite indicate that the H-H vector of some H₂O molecules (H₂O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe²⁺)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.     

High-pressure electronic absorption spectroscopy of natural and synthetic Cr<Superscript>3+</Superscript>-bearing clinopyroxenes

Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi2O6 and LiCrSi2O6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe2+(M1, M2) and intervalence Fe2+/Fe3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr3+ in the crystal structures. The positions of the spin-allowed bands of Cr3+, especially of the low energy one caused by the electronic 4 A 2g → 2 T 1g transition, are found to be in accordance with mean M1–O distances. The local relaxation parameter ε calculated for limCr 3+ → 0 from the spectra and interatomic á Cr - O ñ \left\langle {Cr - O} \right\rangle and á Mg - O ñ \left\langle {Mg - O} \right\rangle distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the “guest” ion, Cr3+, deviates greatly from the “diffraction” value, ε = 0, than in any other known Cr3+-bearing systems studied so far. Under pressure the spin-allowed bands of Cr3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO6 octahedral modulus, k\textpoly\textloc k_{\text{poly}}^{\text{loc}} , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi2O6 kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (2008). Such a strong stiffening of the structural octahedron, i.e. twice higher value of k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} comparing with that of k\textMg2 + \textloc k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} , may be caused by simultaneous substitution of Ca2+ by larger Na+ in the neighboring M2 sites at so-called jadeite-coupled substitution Mg2+ + Ca2+ → Cr3+ + Na+. It is also remarkable that the values of CrO6 octahedral modulus of NaCrSi2O6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement by Origlieri et al. (2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility of M1- and M2-sites, our k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} -value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa.     

Single-crystal EPR and DFT studies of a [BO<Subscript>4</Subscript>]<Superscript>0</Superscript> center in datolite: electronic structure,formation mechanism and implications

A natural datolite CaBSiO₄(OH) (Bergen Hill, NJ, USA), before and after gamma-ray irradiation (up to ~70 kGy), has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy from 10 to 295 K. EPR spectra of gamma-ray-irradiated datolite show the presence of a boron-associated oxygen hole center (BOHC) and an atomic hydrogen center (H⁰), both of which grow with increasing radiation dose. The principal g and A(¹¹B) values of the BOHC at 10 K are: g ₁ = 2.04817(3), g ₂ = 2.01179(2), g ₃ = 2.00310(2), A ₁ = −0.401(7) mT, A ₂ = −0.906(2) mT, A ₃ = −0.985(2) mT, with the orientations of the g ₁ and A ₁ axes approximately along the B–OH bond direction. These experimental results suggest that the BOHC represents hole trapping on the hydroxyl oxygen atom after the removal of the proton (i.e. a [BO₄]⁰ center): via a reaction O₃BOH → O₃BO^· + H⁰, where · denotes the unpaired electron. Density functional theory (DFT) calculations (CRYSTAL06, B3PW, all-electron basis sets, and 1 × 2 × 2 supercell) support the proposed structural model and yield the following ¹¹B hyperfine coupling constants: A ₁ = −0.429 mT, A ₂ = −0.901 mT, A ₃ = −0.954 mT, in excellent agreement with the experimental results. The [BO₄]⁰ center undergoes the onset of thermal decay at ~200°C and is completely annealed out at 375°C but can be restored readily by gamma-ray irradiation. Isothermal annealing experiments show that the [BO₄]⁰ center exhibits a second-order thermal decay with an activation energy of 0.96 eV. The confirmation of the [BO₄]⁰ center (and its formation from the O₃BOH precursor) in datolite has implications for not only understanding of BOHCs in alkali borosilicate glasses but also their applications to nuclear waste disposal.     

Structure of magnetite (Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>) above the Curie temperature: a cation ordering study

A pristine magnetite (Fe₃O₄) specimen was studied by means of Neutron Powder Diffraction in the 273–1,073 K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873 K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700 K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH⁰ = −23.2(1.7) kJ mol⁻¹ and ΔS⁰ = −16(2) J K⁻¹ mol⁻¹. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1–4):625–630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520–522, 1981).     

Hg<Superscript>0</Superscript>(liq)-Hg<Superscript>0</Superscript>(solution) equilibrium and solubility of elementary mercury in water

The solubility metallic mercury in water and its dominating forms were studied. The prevalence of the Hg_aq⁰ form in the high-temperature range was confirmed and the reaction constant Hg_liq⁰ ai Hg_aq⁰ (logK = logm = −8.01) at 25°C with the predominance of oxidized forms of mercury for the 20–80°C area of low temperatures was found.     

The Kinetics of the Interaction Between Iron(III)-Ethylenediaminetetraacetate and Peroxynitrite

The kinetics of the formation of the purple-colored species between Fe^III-EDTA and peroxynitrite were studied as a function of pH (10.4–12.3) at 22°C in aqueous solutions using a stopped-flow technique. A purple-colored species was immediately formed upon mixing, which had an absorbance maximum at 520 nm. The increase in absorbance with time could be fit empirically by a power function with two parameters a and b. The power equation determined was absorbance = a*t ^b , where a increased linearly with pH and the concentration of peroxynitrite, while b almost remained constant with a value of ~0.25. The molar extinction coefficient ε_520 nm for the colored species was determined as 13 M⁻¹cm⁻¹, which is much lower than ε_520 nm = 520 M⁻¹ cm⁻¹ for the [Fe^III(EDTA)O₂]³⁻, a purple species observed in the Fe^III–EDTA–H₂O₂ system. The results of kinetics and spectral measurements of the present study are briefly discussed and compared with those of the reaction between Fe(III)-EDTA and hydrogen peroxide.     

Magnetic phase diagrams of Mg-, Fe-, (Cu + Ti)- and Cu-substituted braunite Mn<Superscript>2+</Superscript>Mn<Subscript>6</Subscript><Superscript>3+</Superscript>SiO<Subscript>12</Subscript>

 The magnetic behavior of the Jahn-Teller structure braunite, (Mn²⁺ _1−yM _y )(Mn³⁺ _{6− x} M_x)SiO₁₂, is strongly influenced by the incorporation of elements substituting manganese. Magnetic properties of well-defined synthetic samples were investigated in dependence on the composition. The final results are presented in magnetic phase diagrams. To derive the necessary data, ac susceptibility and magnetization of braunites with the substitutional elements M = Mg, Fe, (Cu+Ti) and Cu were measured. Whereas the antiferromagnetic ordering temperature, T _N , of pure braunite is hardly affected by the substitution of nonmagnetic Mg, it is rapidly suppressed by the substitution of magnetic atoms at the Mn positions. Typically for a concentration (x, y) ≥ 0.7 of the substituted elements, a spin glass phase occurs in the magnetic phase diagrams. Additionally, for the braunite system with Fe³⁺ substitutions, we observe in the concentration range 0.2 < x< 0.7 a double transition from the paramagnetic state, first to the antiferromagnetic state, followed by a transition to a spin glass state at lower temperatures. The unusual change of the magnetic properties with magnetic substitution at the Mn positions is attributed to the peculiar antiferromagnetic structure of braunite, which has been resolved recently. Received: 19 April 2001 / Accepted: 6 September 2001     

Paragenesis of unusual Fe-cordierite (sekaninaite)-bearing paralava and clinker from the Kuznetsk coal basin,Siberia, Russia

Sekaninaite (X_Fe > 0.5)-bearing paralava and clinker are the products of ancient combustion metamorphism in the western part of the Kuznetsk coal basin, Siberia. The combustion metamorphic rocks typically occur as clinker beds and breccias consisting of vitrified sandstone–siltstone clinker fragments cemented by paralava, resulting from hanging-wall collapse above burning coal seams and quenching. Sekaninaite–Fe-cordierite (X_Fe = 95–45) is associated with tridymite, fayalite, magnetite, ± clinoferrosilite and ±mullite in paralava and with tridymite and mullite in clinker. Unmelted grains of detrital quartz occur in both rocks (<3 vol% in paralavas and up to 30 vol% in some clinkers). Compositionally variable siliceous, K-rich peraluminous glass is <30% in paralavas and up to 85% in clinkers. The paralavas resulted from extensive fusion of sandstone–siltstone (clinker), and sideritic/Fe-hydroxide material contained within them, with the proportion of clastic sediments ≫ ferruginous component. Calculated dry liquidus temperatures of the paralavas are 1,120–1,050°C and 920–1,050°C for clinkers, with calculated viscosities at liquidus temperatures of 10^1.6–7.0 and 10^7.0–9.8 Pa s, respectively. Dry liquidus temperatures of glass compositions range between 920 and 1,120°C (paralava) and 920–960°C (clinker), and viscosities at these temperatures are 10^9.7–5.5 and 10^8.8–9.7 Pa s, respectively. Compared with worldwide occurrences of cordierite–sekaninaite in pyrometamorphic rocks, sekaninaite occurs in rocks with X_Fe (mol% FeO/(FeO + MgO)) > 0.8; sekaninaite and Fe-cordierite occur in rocks with X_Fe 0.6–0.8, and cordierite (X_Fe < 0.5) is restricted to rocks with X_Fe < 0.6. The crystal-chemical formula of an anhydrous sekaninaite based on the refined structure is | \textK_0.02 |(\textFe_1.54^{2 +} \textMg_0.40 \textMn_0.06 )_{\Upsigma 2.00}^M [(\textAl_1.98 \textFe_0.02^{2 +} \textSi_1.00 )_{\Upsigma 3.00}^T1 (\textSi_3.94 \textAl_2.04 \textFe_0.02^{2 +} )_{\Upsigma 6.00}^T2 \textO₁₈ ]. \left| {{\text{K}}_{0.02} } \right|({\text{Fe}}_{1.54}^{2 + } {\text{Mg}}_{0.40} {\text{Mn}}_{0.06} )_{\Upsigma 2.00}^{M} [({\text{Al}}_{1.98} {\text{Fe}}_{0.02}^{2 + } {\text{Si}}_{1.00} )_{\Upsigma 3.00}^{T1} ({\text{Si}}_{3.94} {\text{Al}}_{2.04} {\text{Fe}}_{0.02}^{2 + } )_{\Upsigma 6.00}^{T2} {\text{O}}_{18} ].     

<Superscript>226</Superscript>Ra, <Superscript>232</Superscript>Th and <Superscript>40</Superscript>K activities in soils of Cuihua Mountain National Geological Park,China

Gamma activity from the naturally occurring radionuclides namely, ²²⁶Ra, ²³²Th, the primordial radionuclide ⁴⁰K was measured in the soil of Cuihua Mountain National Geological Park, China using γ-ray spectrometry technique. The mean activity of ²²⁶Ra, ²³²Th and ⁴⁰K were found to be 27.2 ± 6.5, 43.9 ± 6.2 and 653.1 ± 127.6 Bq kg⁻¹, respectively. The concentrations of these radionuclides were compared with the typical world values and the average activities of Chinese soil. The radium equivalent activity, the air absorbed dose rate, the annual effective dose rate, and the external hazard index were evaluated and compared with the internationally approved values. All the soil samples have Ra_eq lower than the limit of 370 Bq kg⁻¹ and H _ex less than unity. The overall mean outdoor terrestrial gamma dose rate is 66.3 nGy h⁻¹ and the corresponding outdoor annual effective dose is 0.081 mSv.     

Vulnerability of headwater catchment resources to incidences of <Superscript>210</Superscript>Pb excess and <Superscript>137</Superscript>Cs radionuclide fallout

Recent identification of elevated excess ²¹⁰Pb (≤302.6 mBq L⁻¹) and ¹³⁷Cs (≤111.3 mBq L⁻¹) activity in drinking water wells up to 20 m depth indicates some transport of airborne radionuclide fallout beyond soils in the Shaker Village catchment, Maine. Estimated airborne mass loading ²¹⁰Pb_ex fluxes of about 0.9 mBq m⁻³, canvass this headwater catchment and may be sufficient to pose risks to unprotected shallow wells. Inventories of ²¹⁰Pb_ex and ¹³⁷Cs in pond sediments indicate maximum median activities of 943 mBq g⁻¹ and 40.0 mBq g⁻¹, respectively. Calculated ²¹⁰Pb_ex fluxes in the catchment soils range from 0.62–0.78 Bq cm⁻² year⁻¹ and yield a mean residence time of near 140 years. Measured ¹³⁷Cs activity up to 51.1 mBq g⁻¹ occurs in sediments at least to 5 m depth. Assumed particle transport in groundwater with apparent ⁸⁵Kr ages less than 5 years BP (2005) may explain the correlation between these particle-reactive radionuclides and elevated activity in some drinking water wells.     

Relationships between carbon isotope evolution and variation of microbes during the Permian–Triassic transition at Meishan Section, South China

This paper investigates kerogen carbon isotopes, the difference between carbonate and kerogen carbon isotopes (Δ¹³C_carb-kero = δ ¹³C_carb − δ ¹³C_kero) and the difference between carbonate and n-C₁₉ alkane compound-specific carbon isotopes (Δ¹³C_carb-n-C19 = δ ¹³C_carb − δ ¹³C _n-C19) during the Permian–Triassic transition at Meishan, South China. The results show that kerogen carbon isotopes underwent both gradual and sharp shifts in beds 23–25 and 26–29, respectively. The differences between carbonate and organic carbon isotopes, both the Δ¹³C_carb-kero and Δ¹³C_carb-n-C19, which are mainly affected by CO₂-fixing enzyme and pCO₂, oscillated frequently during the Permian–Triassic transition. Both the variations of Δ¹³C_carb-n-C19 and Δ¹³C_carb-kero coupled with the alternation between cyanobacteria and green sulfur bacteria indicated by biomarkers. The episodic low values of Δ¹³C_carb-n-C19 corresponded to episodic blooms of green sulfur bacteria, while the episodic high values of Δ¹³C_carb-n-C19 corresponded to episodic blooms of cyanobacteria. The relationships between the variation of carbon isotopes and biota show that the microbes which flourished after the extinction of macroorganism affected the carbon isotope fractionation greatly. Combining the carbon isotope compositions and the pattern of size variation of the conodont Neogondolella, this paper supposes that anoxia of the photic zone at bed 24 was episodic and it would be caused by the degradation of terrigenous organic matters by sulfate reducing bacteria in the upper water column. Considered together with results from previous research, the high resolution variation of the biogeochemistry presents the sequence of the important geo-events during the Permian–Triassic crisis.     

On the nature of the excess heat capacity of mixing

The excess vibrational entropy (ΔS _vib^ex) of several silicate solid solutions are found to be linearly correlated with the differences in end-member volumes (ΔV _i ) and end-member bulk moduli (Δκ _i ). If a substitution produces both, larger and elastically stiffer polyhedra, then the substituted ion will find itself in a strong enlarged structure. The frequency of its vibration is decreased because of the increase in bond lengths. Lowering of frequencies produces larger heat capacities, which give rise to positive excess vibrational entropies. If a substitution produces larger but elastically softer polyhedra, then increase and decrease of mean bond lengths may be similar in magnitude and their effect on the vibrational entropy tends to be compensated. The empirical relationship between ΔS _vib^ex, ΔV _i and Δκ _i , as described by ΔS _vib^ex = (ΔV _i  + mΔκ _i )f, was calibrated on six silicate solid solutions (analbite–sanidine, pyrope–grossular, forsterite–fayalite, analbite–anorthite, anorthite–sanidine, CaTs–diopside) yielding m = 0.0246 and f = 2.926. It allows the prediction of ΔS _vib^ex behaviour of a solid solution based on its volume and bulk moduli end-member data.     

Arsenic removal from aqueous solutions by mixed magnetite–maghemite nanoparticles

In this study, magnetite–maghemite nanoparticles were used to treat arsenic-contaminated water. X-ray photoelectron spectroscopy (XPS) studies showed the presence of arsenic on the surface of magnetite–maghemite nanoparticles. Theoretical multiplet analysis of the magnetite–maghemite mixture (Fe₃O₄-γFe₂O₃) reported 30.8% of maghemite and 69.2% of magnetite. The results show that redox reaction occurred on magnetite–maghemite mixture surface when arsenic was introduced. The study showed that, apart from pH, the removal of arsenic from contaminated water also depends on contact time and initial concentration of arsenic. Equilibrium was achieved in 3 h in the case of 2 mg/L of As(V) and As(III) concentrations at pH 6.5. The results further suggest that arsenic adsorption involved the formation of weak arsenic-iron oxide complexes at the magnetite–maghemite surface. In groundwater, arsenic adsorption capacity of magnetite–maghemite nanoparticles at room temperature, calculated from the Langmuir isotherm, was 80 μmol/g and Gibbs free energy (∆G⁰, kJ/mol) for arsenic removal was −35 kJ/mol, indicating the spontaneous nature of adsorption on magnetite–maghemite nanoparticles.     

The influence of degradation of the swamp and alpine meadows on CH<Subscript>4</Subscript> and CO<Subscript>2</Subscript> fluxes on the Qinghai-Tibetan Plateau

CH₄ and CO₂ fluxes from a high-cold swamp meadow and an alpine meadow on the Qinghai-Tibetan Plateau, subject to different degrees of degradation, were measured over a 12-month period. Air temperature, soil temperature and moisture, and the depths of the water table and thawing-freezing layer were determined. For swamp meadows, the greater the degradation, the lesser the carbon efflux. CH₄ emissions at the nondegraded swamp meadow site were 1.09–3.5 and 2.5–11.27 times greater, and CO₂ emissions 1.08–1.69 and 1.41–4.43 times greater, respectively, than those from moderately and severely degraded sites. For alpine meadows, the greater the degradation, the greater the CH₄ consumption and CO₂ emissions. CH₄ consumption at the severely degraded alpine meadow site was 6.6–21 and 1.1–5.25 times greater, and CO₂ emissions 1.05–78.5 and 1.04–6.28 times greater, respectively, than those from the nondegraded and moderately degraded sites. The CH₄ and CO₂ fluxes at both sites were significantly correlated (R ² > 0.59, P < 0.05) with air temperature, soil temperature, and topsoil (0–5 cm depth) moisture, indicating these to be the main environmental factors affecting such fluxes.     

Effects of strong network modifiers on Fe<Superscript>3+</Superscript>/Fe<Superscript>2+</Superscript> in silicate melts: an experimental study

The effect of CaO, Na₂O, and K₂O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe³⁺/Fe²⁺ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO₂–TiO₂–Al₂O₃–FeO–Fe₂O₃–MgO–CaO–Na₂O–K₂O–P₂O₅ melts at air conditions.     

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.     

On the Stability of the <Emphasis Type="Italic">AlOSi</Emphasis>(<Emphasis Type="Italic">OH</Emphasis>)<Stack><Subscript>3</Subscript><Superscript>2+</Superscript></Stack> Complex in Aqueous Solution

Summary The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The measurements were performed as potentiometric titrations using a hydrogen electrode with OH ⁻ ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si _tot ] and [Al _t ot], and −log[H ⁺] were varied within the limits 0.3 < [Si _tot ] < 2.5 mM, 0.5 < [Al _tot ] < 2.6 mM, and 2 ≤ -log[H ⁺] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH) ₃ ²⁺ complex with a formation constant log β_1,1-1 = −2.75 ± 0.1 defined by the reaction Al ³⁺+Si (OH)₄ ↔ AlOSi(OH) ₃ ²⁺ +H ⁺ An extrapolation of this value to I=0 gives log β_1,1-1 = −2.30. The calculated value of log K (Al ³⁺+SiO(OH) ₃ ⁻ ↔ AlOSi(OH) ₃ ²⁺ ) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF ²⁺ and AlOH ²⁺ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH ⁻>SiO(OH) ₃ ⁻  > F ⁻     

Biosorption of cadmium(II) from aqueous solution onto <Emphasis Type="Italic">Hydrilla verticillata</Emphasis>

Biosorption is an effective method to remove heavy metals from wastewater. In this work, the biosorption of Cd(II) onto Hydrilla verticillata was examined in aqueous solution with parameters of initial pH, adsorbent dosage, contact time, initial Cd(II) concentration, temperature, and co-existing ion. Linear Langmuir and Freundlich models were applied to describe the equilibrium isotherms, and both of the two models were fitted well. The monolayer adsorption capacity of Cd(II) was found to be 50 mg/g at pH 6 and 20°C. Dubinin–Radushkevich isotherm model was also applied to the equilibrium data. The mean free energy of adsorption (11.18 kJ/mol) indicated that the adsorption of Cd(II) onto H. verticillata might be carried out via chemical ion-exchange mechanism. Thermodynamic parameters, including free energy (∆G ⁰), enthalpy (∆H ⁰), and entropy (∆S ⁰) of adsorption, were also calculated. These results showed that the biosorption of Cd(II) onto H. verticillata was a feasible, spontaneous, and exothermic process in nature. Desorption experiments indicated that 0.01 mol/L EDTA and HNO₃ were efficient desorbents for the recovery of Cd(II) from biomass. IR spectrum analysis suggested that amido, hydroxyl, C=O and C–O could combine strongly with Cd(II). EDX spectrum analysis suggested that an ion exchange mechanism might be involved.     

Metal contamination and filtering in soil from an iron (magnetite) mine–smelter complex in the critical Hudson Highlands watershed,New York

Organic material in metal contaminated soils around an abandoned magnetite mine–smelter complex in the critical Highlands watershed protects the groundwater and surface water from contamination. Metals in these waters were consistently below local and national water standards. Two groups of soil types cover the area: (1) Group A disturbed metal-rich soils, and (2) Group B undisturbed organic soils. Chromium and nickel were more elevated than other metals with Cr more widespread than Ni. In Group A, Cr correlated strongly with sesquioxides in the lower horizons (Fe₂O₃: r = 0.74, p < 0.025; Al₂O₃: r = 0.92, p < 0.005). In Group B, Cr correlated strongly (r = 0.96, p < 0.005) with soil organic matter (SOM) in the O-horizons. Ni–Cr (Group A: 52 and 70% in O- and lower horizons, respectively; Group B: ~100% in both horizons) and V–Cr correlations (78% only in Group A lower horizons) suggest similar retention mechanisms for these elements. Average soil \textpH_\textCaCl2 {\text{pH}}_{{{\text{CaCl}}_{2} }} for both groups ranged between 3.65 and 5.91, suggesting that soil acidity is determined by organic acids and solubility of Al³⁺ releasing H⁺ ions. SOM and sesquioxides contribute significantly to creating naturally occurring filtration systems, removing metals, and protecting water quality. High Ca, Fe, and Ti in Group A soils suggest slag and ash were mixed into the soils. Some low-Cr sources include magnetite, slag, and ash (100, 100 and 200 mg/kg, respectively). Constant ZrO ₂ :TiO ₂ ratios in the lower soils indicate soil formation from breakdown of underlying tailing rocks, contributing Cr to these layers.