The oxidative dissolution of arsenopyrite (FeAsS) and enargite (Cu3AsS4) by Leptospirillum ferrooxidans

Arsenopyrite (FeAsS) and enargite (Cu₃AsS₄) fractured in a nitrogen atmosphere were characterised after acidic (pH 1.8), oxidative dissolution in both the presence and absence of the acidophilic microorganism Leptospirillum ferrooxidans. Dissolution was monitored through analysis of the coexisting aqueous solution using inductively coupled plasma atomic emission spectroscopy and coupled ion chromatography-inductively coupled plasma mass spectrometry, and chemical changes at the mineral surface observed using X-ray photoelectron spectroscopy and environmental scanning electron microscopy (ESEM). Biologically mediated oxidation of arsenopyrite and enargite (2.5 g in 25 ml) was seen to proceed to a greater extent than abiotic oxidation, although arsenopyrite oxidation was significantly greater than enargite oxidation. These dissolution reactions were associated with the release of ∼917 and ∼180 ppm of arsenic into solution. The formation of Fe(III)-oxyhydroxides, ferric sulphate and arsenate was observed for arsenopyrite, thiosulphate and an unknown arsenic oxide for enargite. ESEM revealed an extensive coating of an extracellular polymeric substance associated with the L. ferrooxidans cells on the arsenopyrite surface and bacterial leach pits suggest a direct biological oxidation mechanism, although a combination of indirect and direct bioleaching cannot be ruled out. Although the relative oxidation rates of enargite were greater in the presence of L. ferrooxidans, cells were not in contact with the surface suggesting an indirect biological oxidation mechanism. Cells of L. ferrooxidans appear able to withstand several hundreds of ppm of As(III) and As(V).     

Cu and Fe chalcopyrite leach activation energies and the effect of added Fe

The leaching kinetics of chalcopyrite (CuFeS₂) concentrate in sulfuric acid leach media with and without the initial addition of Fe³⁺ under carefully controlled solution conditions (E_h 750 mV SHE, pH 1) at various temperatures from 55 to 85 °C were measured. Kinetic analyses by (i) apparent rate (not surface area normalised), and rate dependence using (ii) a shrinking core model and (iii) a shrinking core model in conjunction with Fe³⁺ activity, were performed to estimate the activation energies (E_a) for Cu and Fe dissolution.The E_a values determined for Cu and Fe leaching in the absence of added Fe³⁺ are within experimental error, 80 ± 10 kJ mol⁻¹ and 84 ± 10 kJ mol⁻¹, respectively (type iii analyses E_a are quoted unless stated otherwise), and are indicative of a chemical reaction controlled process. On addition of Fe³⁺ the initial Cu leach rate (up to 10 h) was increased and Cu was released to solution preferentially over Fe, with the E_a value of 21 ± 5 kJ mol⁻¹ (type ii analysis) suggestive of a transport controlled rate determining process. However, the rate of leaching rapidly decreased until it was consistently slower than for the equivalent leaches where Fe³⁺ was not added. The resulting E_a value for this leach regime of 83 ± 10 kJ mol⁻¹ is within experimental error of that determined in the absence of added Fe³⁺. In contrast to Cu release, Fe release to solution was consistent with a chemical reaction controlled leach rate throughout. The Fe release E_a of 76 ± 10 kJ mol⁻¹ is also within experimental error of that determined in the absence of added Fe³⁺. Where type (ii) and (iii) analyses were both successfully carried out (in all cases except for Cu leaching with added Fe³⁺, <10 h) the E_a derived are within experimental error. However, the type (iii) analyses of the leaches in the presence of added Fe³⁺ (>10 h), as compared to in the absence of added Fe³⁺, returned a considerably smaller pre-exponential factors for both Cu and Fe leach analyses commensurate with the considerably slower leach rate, suggestive of a more applicable kinetic analysis.XPS examination of leached chalcopyrite showed that the surface concentration of polysulfide and sulfate was significantly increased when Fe³⁺ was added to the leach liquor. Complementary SEM analysis revealed the surface features of chalcopyrite, most likely due to the nature of the polysulfide formed, are subtly different with greater surface roughness upon leaching in the absence of added Fe³⁺ as compared to a continuous smooth surface layer formed in the presence of added Fe³⁺. These observations suggest that the effect of Fe³⁺ addition on the rate of leaching is not due to the change in the chemical reaction controlled mechanism but due to a change in the available surface area for reaction.     

Kinetics of the leaching of mechanically activated berthierite,boulangerite and franckeite

In this study, changes in surface area, morphology and leachability of antimony from mechanically activated berthierite—FeSb₂S₄, boulangerite—Pb₅Sb₄S₁₁ and franckeite—FePb₅Sn₃Sb₂S₁₄ by a high-energy planetary mill were investigated. It appears that a selective extraction of antimony from these complex sulphosalts in alkaline solution of sodium sulphide is positively affected by mechanical activation. The influence of milling on mineral particle size and shape was studied by scanning electron microscopy. The temperature dependencies of berthierite alkaline leaching were investigated in an interval of 323–363 K. Resulting experimental activation energies E _a were 0.11 and 6.78 kJ mol⁻¹ for mechanically activated berthierite due to a break of Arrhenius plot. The values E _a are characteristic for a process controlled by diffusion as the rate-controlling step of leaching reaction.     

Copper isotope fractionation in acid mine drainage

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ⁶⁵Cu values (based on ⁶⁵Cu/⁶³Cu) of enargite (δ⁶⁵Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ⁶⁵Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ⁶⁵Cu < 1‰). These mineral samples show lower δ⁶⁵Cu values than stream waters (1.38‰ ? δ⁶⁵Cu ? 1.69‰). The average isotopic fractionation (Δ_aq-min = δ⁶⁵Cu_aq − δ⁶⁵Cu_min, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in ⁶⁵Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δ_aq-min^o=-0.57±0.14‰, where min^o refers to the starting mineral) and no apparent fractionation for enargite (Δ_aq-min^o=0.14±0.14‰). Abiotic fractionation is attributed to preferential oxidation of ⁶⁵Cu⁺ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of ⁶⁵Cu_aq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ⁶⁵Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.     

Dissolution of Columbia River Basalt under mildly acidic conditions as a function of temperature: Experimental results relevant to the geological sequestration of carbon dioxide

Increasing attention is being focused on the rapid rise of CO₂ levels in the atmosphere, which many believe to be the major contributing factor to global climate change. Sequestering CO₂ in deep geological formations has been proposed as a long-term solution to help stabilize CO₂ levels. However, before such technology can be developed and implemented, a basic understanding of H₂O–CO₂ systems and the chemical interactions of these fluids with the host formation must be obtained. Important issues concerning mineral stability, reaction rates, and carbonate formation are all controlled or at least significantly impacted by the kinetics of rock–water reactions in mildly acidic, CO₂-saturated solutions. Basalt has recently been identified as a potentially important host formation for geological sequestration. Dissolution kinetics of the Columbia River Basalt (CRB) were measured for a range of temperatures (25–90 °C) under mildly acidic to neutral pH conditions using the single-pass flow-through test method. Under anaerobic conditions, the normalized dissolution rates for CRB decrease with increasing pH (3 ? pH ? 7) with a slope, η, of −0.15 ± 0.01. Activation energy, E_a, has been estimated at 32.0 ± 2.4 kJ mol⁻¹. Dissolution kinetics measurements like these are essential for modeling the rate at which CO₂-saturated fluids react with basalt and ultimately drive conversion rates to carbonate minerals in situ.     

Spatial interpolation of the deuterium and oxygen-18 composition of global precipitation using temperature as ancillary variable

To aid our research in geographical forensic provenancing and food authentication we have developed high resolution prediction maps of the annual mean deuterium and oxygen-18 composition of modern precipitation. The maps have a spatial resolution of 10′ (∼ 20 × 20 km at the equator) and cover the global land surface excluding the Antarctic. To achieve this, the relation between various temperature related variables and the isotopic composition of modern precipitation was explored using a combination of high resolution climate maps and global isotope records from the GNIP database. This revealed that the isotopic composition of precipitation is somewhat better correlated to the temperature during the coldest – often driest – period of year than the temperatures during the warmest – often wettest – period of year (especially below 0 °C). Although the reason for this effect is not directly clear, the temperature during the coldest quarter is used as ancillary variable in simple kriging with varying local means (SKlm). In SKlm, only the residual isotope values from the regression with T_cq are kriging interpolated, which are then added to the predicted isotope map based on T_cq. Because the ancillary variable explains the bulk of the isotopic variation (R² = 0.79–0.85), the deuterium and oxygen-18 maps mainly reflect the large scale global temperature pattern. More local isotope effects are accounted for by the interpolation of the residual values. This study furthermore shows that surface temperature better explains the global isotopic variation compared to a combination of latitude and altitude (R² = 0.68–0.69). Yet, at very low temperatures (< − 40 °C) our maps might underestimate the true isotope signal. The new isotope maps and the maps of the 95% confidence intervals can be downloaded from www.waterisotopes.org.     

Diffusion of HTO,Cl and I in Upper Toarcian argillite samples from Tournemire: Effects of initial iodide concentration and ionic strength

Diffusion parameters for HTO, ³⁶Cl⁻_, and ¹²⁵I⁻ were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I⁻ interaction with Upper Toarcian argillite and to verify the effects of initial I⁻ concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl⁻ and I⁻ diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers’ diffusive behaviour. The results show that the effective diffusion coefficient (D_e) and diffusion accessible porosity (ε_a) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01 eq L⁻¹ are: D_e = 2.35–2.50 × 10⁻¹¹ m² s⁻¹ and ε_a = 12.0–15.0% for HTO, and D_e = 14.5–15.5 × 10⁻¹³ m² s⁻¹ and ε_a = 2.5–2.9% for ³⁶Cl⁻. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (D_e) and rock capacity factor (α) obtained for ¹²⁵I are: D_e = 7.00–8.60 × 10⁻¹³ m² s⁻¹ and α = 4.3–7.2%. Such values make it possible to calculate low ¹²⁵I distribution ratios (0.0057 < R_D < 0.0192 mL g⁻¹) which confirm the trend indicating that the ¹²⁵I rock capacity factor increases with the decrease of the initial I⁻ concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S. = 0.11 eq L⁻¹). No evolution of HTO diffusion parameters was observed. The anionic tracers’ effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for ¹²⁵I and ³⁶Cl. Whatever the ionic strength of the synthetic solution used, ¹²⁵I exhibited D_e values two times lower than those of ³⁶Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying ¹²⁵I sorption and postulating that ³⁶Cl and ¹²⁵I would diffuse in the same porosity. In other terms, at Tournemire, ¹²⁵I sorption could be more pronounced than previously indicated.     

Mineralogical,geochemical, and microbial investigation of a sulfide-rich tailings deposit characterized by neutral drainage

Mineralogical, geochemical and microbial characterization of tailings solids from the Greens Creek Mine, Juneau, Alaska, was performed to evaluate mechanisms controlling aqueous geochemistry of near-neutral pH pore water and drainage. Core samples of the tailings were collected from five boreholes ranging from 7 to 26 m in depth. The majority of the 51 samples (77%) were collected from the vadose zone, which can extend >18 m below the tailings surface. Mineralogical investigation indicates that the occurrence of sulfide minerals follows the general order: pyrite [FeS₂] >> sphalerite [(Zn,Fe)S] > galena [PbS], tetrahedrite [(Fe,Zn,Cu,Ag)₁₂Sb₄S₁₃] > arsenopyrite [FeAsS] and chalcopyrite [CuFeS₂]. Pyrite constitutes <20 to >35 wt.% of the tailings mineral assemblage, whereas dolomite [CaMg(CO₃)₂] and calcite [CaCO₃] are present at ?30 and 3 wt.%, respectively. The solid-phase geochemistry generally reflects the mineral assemblage. The presence of additional trace elements, including Cd, Cr, Co, Mo, Ni, Se and Tl, is attributed to substitution into sulfide phases. Results of acid–base accounting (ABA) underestimated both acid-generating potential (AP) and neutralization potential (NP). Recalculation of AP and NP based on solid-phase geochemistry and quantitative mineralogy yielded more representative results. Neutrophilic S-oxidizing bacteria (nSOB) and SO₄-reducing bacteria (SRB) are present with populations up to 10⁷ and 10⁵ cells g⁻¹, respectively. Acidophilic S-oxidizing bacteria (aSOB) and iron-reducing bacteria (IRB) were generally less abundant. Primary influences on aqueous geochemistry are sulfide oxidation and carbonate dissolution at the tailings surface, gypsum precipitation–dissolution reactions, as well as Fe reduction below the zone of sulfide oxidation. Pore-water pH values generally ranged from 6.5 to 7.5 near the tailings surface, and from approximately 7–8 below the oxidation zone. Elevated concentrations of dissolved SO₄, S₂O₃, Fe, Zn, As, Sb and Tl persisted under these conditions.     

In situ atomic force microscopy measurements of biotite basal plane reactivity in the presence of oxalic acid

We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (E_a,app) for the process, via mass-loss calculations from image analysis. We obtain a value of E_a,app = 49 ± 2 kJ mol⁻¹, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of E_a,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to E_a,app = 140 kJ mol⁻¹, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of E_a,app represents the slow formation of etch pits into the (0 0 1) surface.     

Mineral control of arsenic content in thermal waters from volcano-hosted hydrothermal systems: Insights from island of Ischia and Phlegrean Fields (Campanian Volcanic Province, Italy)

This paper documents arsenic concentrations in 157 groundwater samples from the island of Ischia and the Phlegrean Fields, two of the most active volcano-hosted hydrothermal systems from the Campanian Volcanic Province (Southern Italy), in an attempt to identify the environmental conditions and mineral-solution reactions governing arsenic aqueous cycling. On Ischia and in the Phlegrean Fields, groundwaters range in composition from NaCl brines, which we interpret as the surface discharge of deep reservoir fluids, to shallow-depth circulating fluids, the latter ranging from acid-sulphate steam-heated to hypothermal, cold, bicarbonate groundwaters. Arsenic concentrations range from 1.6 to 6900 μg·l^− 1 and from 2.6 to 3800 μg·l^− 1 in the Phlegrean Fields and on Ischia, respectively. They increase with increasing water temperature and chlorine contents, and in the sequence bicarbonate groundwaters < steam-heated groundwaters < NaCl brines. According to thermochemical modeling, we propose that high As concentrations in NaCl brines form after prolonged water-rock interactions at reservoir T, fO₂ and fH₂S conditions, and under the buffering action of an arsenopyrite + pyrite + pyrrhotite rock assemblage. On their ascent toward the surface, NaCl brines become diluted by As-depleted meteoric-derived bicarbonate groundwaters, giving rise to hybrid water types with intermediate to low As contents. Steam-heated groundwaters give their intermediate to high As concentrations to extensive rock leaching promoted by interaction with As-bearing hydrothermal steam.     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.     

Retention of biosignatures and mass-independent fractionations in pyrite: Self-diffusion of sulfur

Self-diffusion of sulfur in pyrite (FeS₂) was characterized over the temperature range ∼500-725 °C (∼1 bar pressure) by immersing natural specimens in a bath of molten elemental ³⁴S and characterizing the resulting diffusive-exchange profiles by Rutherford backscattering spectroscopy (RBS). The temperature dependence of the sulfur diffusivity (D_S) conforms to D_S = D_o exp(−E_a/RT), where the pre-exponential constant (D_o) and the activation energy (E_a) are constrained as follows:

     

Arsenite sorption at the magnetite-water interface during aqueous precipitation of magnetite: EXAFS evidence for a new arsenite surface complex

The interaction of aqueous As(III) with magnetite during its precipitation from aqueous solution at neutral pH has been studied as a function of initial As/Fe ratio. Arsenite is sequestered via surface adsorption and surface precipitation reactions, which in turn influence the crystal growth of magnetite. Sorption samples were characterized using EXAFS spectroscopy at the As K-edge in combination with HRTEM observations, energy dispersive X-ray analysis at the nanoscale, electron energy loss spectroscopy at the Fe L₃-edge, and XRD-Rietveld analyses of reaction products. Our results show that As(III) forms predominantly tridentate hexanuclear As(III)O₃ complexes (³C), where the As(III)O₃ pyramids occupy vacant tetrahedral sites on {1 1 1} surfaces of magnetite particles. This is the first time such a tridentate surface complex has been observed for arsenic. This complex, with a dominant As-Fe distance of 3.53 ± 0.02 Å, occurs in all samples examined except the one with the highest As/Fe ratio (0.33). In addition, at the two highest As/Fe ratios (0.133 and 0.333) arsenite tends to form mononuclear edge-sharing As(III)O₃ species (²E) within a highly soluble amorphous As(III)-Fe(III,II)-containing precipitate. At the two lowest As/Fe ratios (0.007 and 0.033), our results indicate the presence of additional As(III) species with a dominant As-Fe distance of 3.30 ± 0.02 Å, for which a possible structural model is proposed. The tridentate ³C As(III)O₃ complexes on the {1 1 1} magnetite surface, together with this additional As(III) species, dramatically lower the solubility of arsenite in the anoxic model systems studied. They may thus play an important role in lowering arsenite solubility in putative magnetite-based water treatment processes, as well as in natural iron-rich anoxic media, especially during the reductive dissolution-precipitation of iron minerals in anoxic environments.     

Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I–III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (La_N/Yb_N = 1.4–4.4). Isotopically, the rocks of Group I have the initial ratios ⁸⁷Sr/⁸⁶Sr_(i) = 0.709351–0.707112, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512490–0.512588 and high positive ?_Nd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values ⁸⁷Sr/⁸⁶Sr_(i) = 0.709434–0.710092, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512231–0.512210 and for ?_Nd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values ⁸⁷Sr/⁸⁶Sr_(i) = 0.710751–0.709448, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512347–0.512411 and for ?_Nd270 the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1 > Sm_N/Yb_N < 2.5; 0.9 > La_N/Sm_N < 2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu* = 0.23–0.40). The felsic rocks show the initial ratios the values: ⁸⁷Sr/⁸⁶Sr_(i) = 0.704096–0.707805, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512012–0.512021 and for ?_Nd270 the negative values from −5.27 to −5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting.     

Microstructure characterization of mechanically activated hematite using XRD line broadening

The effect of dry milling in a vibratory mill on the structural changes and microstructural characteristics of hematite using different methods was investigated. We have described the line profile analysis (LPA) to extract the size of coherently diffracting domains and the lattice strain of activated hematite in a vibratory mill. The Warren–Averbach and Williamson–Hall methods were used as the main tools for characterization. The changes in the particle size, surface area and new phase formation of hematite concentrate were also investigated. It was concluded that the breakage and agglomeration of particles take place mainly at lower and higher levels of specific energy input, respectively. The pores in agglomerates remain accessible for the nitrogen gas. Milling of hematite increased specific surface area up to 18.4 m²/g. The hematite milled under various levels of specific energy input did not undergo a significant reaction or phase transformation during milling. The Williamson–Hall method confirms its merit for a rapid overview of the line broadening effects and possible understanding of the main causes. The anisotropic character of line broadening for deformed hematite as a function of specific energy input was revealed. Higher level of specific energy input favors the generation of small crystallite size, higher microstrain, BET surface area, amorphization and line breadth. The Warren–Averbach method suggested that the nanocrystalline hematite with grain sizes of 73.5–12.2 nm was formed by mechanical treatment using different milling intensities in the vibratory mill. The root mean square strain (RMSS) at L = 10 nm varies between 1.7 × 10^− 3 and 4.0 × 10^− 3 depending on the level of energy input. Limits in the applicability of Williamson–Hall method and reliability of the results are discussed in detail.     

Tissue N content and N natural abundance in epilithic mosses for indicating atmospheric N deposition in the Guiyang area,SW China

Tissue N contents and δ¹⁵N signatures in 175 epilithic mosses were investigated from urban to rural sites in Guiyang (SW China) to determine atmospheric N deposition. Moss N contents (0.85–2.97%) showed a significant decrease from the urban area (mean = 2.24 ± 0.32%, 0–5 km) to the rural area (mean = 1.27 ± 0.13%, 20–25 km), indicating that the level of N deposition decreased away from the urban environment, while slightly higher N contents re-occurred at sites beyond 30 km, suggesting higher N deposition in more remote rural areas. Moss δ¹⁵N ranged from −12.50‰ to −1.39‰ and showed a clear bimodal distribution (−12‰ to −6‰ and −5‰ to −2‰), suggesting that there are two main sources for N deposition in the Guiyang area. More negative δ¹⁵N (mean = −8.87 ± 1.65‰) of urban mosses mainly indicated NH₃ released from excretory wastes and sewage, while the less negative δ¹⁵N (from −3.83 ± 0.82‰ to −2.48 ± 0.95‰) of rural mosses were mainly influenced by agricultural NH₃. With more negative values in the urban area than in the rural area, the pattern of moss δ¹⁵N variation in Guiyang was found to be opposite to cities where N deposition is dominated by NO_x–N. Therefore, NH_x–N is the dominant N form deposited in the Guiyang area, which is supported by higher NH_x–N than NO_x–N in local atmospheric deposition. From the data showing that moss is responding to NH_x–N/NO_x–N in deposition it can be further demonstrated that the variation of moss δ¹⁵N from the Guiyang urban to rural area was more likely controlled by the ratio of urban-NH_x/agriculture-NH_x than the ratio of NH_x–N/NO_x–N. The results of this study have extended knowledge of atmospheric N sources in city areas, showing that urban sewage discharge could be important in cities co-generic to Guiyang.     

Ni(II) sorption on natural chlorite

Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10⁻⁸–10⁻⁶ M) in an Ar atmosphere using batch sorption with radioactive ⁶³Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (K_d = ∼10⁻³ cm³/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi²⁺, Chl_OHNi(OH)⁺ and Chl_OHNi(OH)₂, gave the best fit to the sorption results using FITEQL. The high K_d values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.     

Empirical prediction of the Pitzer’s interaction parameters for cationic Al species with both SiO2(aq) and CO2(aq): Implications for the geochemical modelling of very saline solutions

The Pitzer’s interaction parameters, λ_N–M, involving the Mth cationic Al species Al³⁺ or AlOH²⁺ or AlO⁺ and the Nth neutral species SiO_2(aq) (at temperatures of 25–300 °C) or CO_2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λ_N–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λ_N–M for both SiO_2(aq) and CO_2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO_2(aq)–Al³⁺ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO₂ solubilities in concentrated solutions of AlCl₃, 0.272 ± 0.010 (2σ), and Al₂(SO₄)₃, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO₂, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study.