Assessment of H<Subscript>2</Subscript>S emission hazards into tunnels: the Nosoud tunnel case study from Iran

The presence of a gassy ground condition is an important problem in tunneling. In this study, the effects of groundwater H₂S and CH₄ emissions are investigated and characterized together with the factors that created these conditions in Nosoud tunnel in Iran. Through the geological investigations, the presence of these gasses was not detected prior to the construction of the tunnel. Groundwater sampling indicated that about 1 L of H₂S is released per 100 L of the water inflow into the Nosoud tunnel under normal conditions. However, the volume of the released gas was varying with the changes in the groundwater discharge rate. Thus, estimation of groundwater inflow into the tunnel is necessary for predicting the volume of gas emission. Based on the experience of the Nosoud tunnel excavations, there are several geological and hydrogeological factors that must be considered as the indicators of gas emissions during tunneling. Considering the importance of ground water gas emission into the tunnels located in gassy conditions, the present work was conducted to predict the H₂S seepage before the excavation using geological and hydrogeological indicators.     

Preferential dissolution of SiO<Subscript>2</Subscript> from enstatite to H<Subscript>2</Subscript> fluid under high pressure and temperature

Stability and phase relations of coexisting enstatite and H₂ fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO₂ grains were found at the margin of the heating hot spot, suggesting that the SiO₂ component dissolved in the H₂ fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH₄ and H₂O molecules formed through the reaction between dissolved SiO₂ and H₂. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO₂ components of enstatite in H₂ fluid, as well as that observed in the forsterite H₂ system and the quartz H₂ system, implies that H₂-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.     

Volatile (H<Subscript>2</Subscript>O,CO<Subscript>2</Subscript>, Cl,S) budget of the Central American subduction zone

After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H₂O, CO₂, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H₂O, 7.8–11.4 (9.3) Tg/Ma/m CO₂, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H₂O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (10⁵ years) average magma and K₂O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K₂O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K₂O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H₂O, 0.02–0.45 (0.17) Tg/Ma/m CO₂, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO₂-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H₂O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.     

H<Subscript>2</Subscript>O solubility in basalt at upper mantle conditions

This study presents a new experimental approach for determining H₂O solubility in basaltic melt at upper mantle conditions. Traditional solubility experiments are limited to pressures of ~600 MPa or less because it is difficult to reliably quench silicate melts containing greater than ~10 wt% dissolved H₂O. To overcome this limitation, our approach relies on the use of secondary ion mass spectrometry to measure the concentration of H dissolved in olivine and on using the measured H in olivine as a proxy for the concentration of H₂O in the co-existing basaltic melt. The solubility of H₂O in the melt is determined by performing a series of experiments at a single pressure and temperature with increasing amounts of liquid H₂O added to each charge. The point at which the concentration of H in the olivine first becomes independent of the amount of initial H₂O content of the charge (added + adsorbed H₂O) indicates its solubility in the melt. Experiments were conducted by packing basalt powder into a capsule fabricated from San Carlos olivine, which was then pressure-sealed inside a Ni outer capsule. Our experimental results indicate that at 1000 MPa and 1200 °C, the solubility of H₂O in basaltic melt is 20.6 ± 0.9 wt% (2 × standard deviation). This concentration is considerably higher than predicted by most solubility models but defines a linear relationship between H₂O fugacity and the square of molar H₂O solubility when combined with solubility data from lower pressure experiments. Further, our solubility determination agrees with melting point depression determined experimentally by Grove et al. (2006) for the H₂O-saturated peridotite solidus at 1000 MPa. Melting point depression calculations were used to estimate H₂O solubility in basalt along the experimentally determined H₂O-saturated peridotite solidus. The results suggest that a linear relationship between H₂O fugacity and the square of molar solubility exists up to ~1300 MPa, where there is an inflection point and solubility begins to increase less strongly with increasing H₂O fugacity.     

The effect of alkalis and polymerization on the solubility of H<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> in alkali-rich silicate melts

The effect of alkalis on the solubility of H₂O and CO₂ in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H₂O/(H₂O + CO₂) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H₂O solubility (melt equilibrated with pure H₂O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO₂ content in melts equilibrated with CO₂-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO₂ contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO₂ solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO₂ solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO₂ solubility at 500 MPa and 1,100–1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: \({\text{wt}}\% \;{\text{CO}}_{2} = - 0.246 + 0.014\exp \left( {6.995 \cdot \frac{\text{NBO}}{T}} \right) + 3.150 \cdot \frac{{{\text{Na}} + {\text{K}}}}{{\varSigma {\text{cations}}}} + 0.222 \cdot \frac{\text{Na}}{{{\text{Na}} + {\text{K}}}}.\) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz–fayalite–magnetite buffer and above.     

Phototrophic bacteria dominate consortia,potentially to remove CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>S from biogas under microaerophilic conditions

The use of microbial consortia to remove contaminants in industrial systems and in natural environments could be an alternative to the use of unique strains of microorganisms, since microbial consortia have greater robustness to environmental fluctuations. However, it is necessary to evaluate the relationship between the genetic structure and functionality of the consortia. In this work, the functional and structural stability over time of two bacterial consortia (C5 and C6) with the potential to remove CO₂ and H₂S from biogas was evaluated. Both consortia decreased the dissolved CO₂ by over 30% at the end of the incubation period, but C5 presented shorter removal kinetics (3.9 days) than C6 (6.4 days). Additionally, a chemical oxidation of H₂S could have occurred in the microcosms. Moreover, both consortia presented a stable genetic structure, measured by terminal restriction fragment length polymorphism profiles of the 16S rRNA gene, characterized by high homogeneity and prevalence of the genus Rhodopseudomonas throughout the incubation period, and an increasing abundance of Xanthobacter during the exponential phase of the growth curve in C5, which would account for the functionality of the consortia.     

TiO<Subscript>2</Subscript>-based shaped catalyst for the recovery of elemental sulfur from H<Subscript>2</Subscript>S and SO<Subscript>2</Subscript> gas streams

The Claus process has been used for the conversion of H₂S and SO₂ to elemental sulfur. These two sulfur compounds need special attention because they are very poisonous with negative impact on both the environment and human health. Here, highly active Fe–Ni/TiO₂ catalyst has been prepared and shaped by three different binders (bentonite, polyethylene glycol and carboxymethyl cellulose) into extrudes. Comparing the mechanical strength and surface area of prepared extrudes, the optimal shaped catalyst was selected with 20% of bentonite, 2% of PEG and 2% of CMC. The optimal catalyst was characterized by X-ray powder diffraction, temperature-programmed reduction, Brunauer–Emmett–Teller specific surface area, Barrett–Joyner–Halenda, scanning electron microscopy and energy-dispersive X-ray techniques and used for sulfur recovery process. The performance of this product for sulfur recovery via Claus process was excellent with the conversion of hydrogen sulfide of 76.77% and sulfur dioxide of 97.83%. The catalyst also provides high hydrolysis activity of CS₂ (83.06%). Therefore, a highly active TiO₂-supported shaped catalyst with 85.62% of conversion efficiency has been prepared successfully to convert the small amounts of H₂S, SO₂ and CS₂ to elemental sulfur.     

Experimental determination of liquidus H<Subscript>2</Subscript>O contents of haplogranite at deep-crustal conditions

The liquidus water content of a haplogranite melt at high pressure (P) and temperature (T) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (≤0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700–1000 °C, 0.6–1.5 GPa). To improve the high-P constraints on H₂O concentration at the granite liquidus, we performed experiments in a piston–cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi₃O₈)—orthoclase (Or: KAlSi₃O₈)—quartz (Qz: SiO₂)—H₂O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H₂O was homogenized well above the liquidus T, and T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T and then raised T until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H₂O are 935–985, 875–900, 775–800, 725–775, and 650–675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab_36.25Or_36.25Qz_27.5 (wt% basis) at 1.0 GPa is \(T = - 0.0995 w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 3} + 5.0242w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 2} - 88.183 w_{{{\text{H}}_{ 2} {\text{O}}}} + 1171.0\) for \(0 \le w_{{{\text{H}}_{ 2} {\text{O}}}} \le 17\) wt% and \(T\) is in °C. We present a revised \(P - T\) diagram of liquidus minimum H₂O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H₂O (<5.8 wt%) and higher temperature, our results plot on the high end of the extrapolated water contents at liquidus minima when compared to the previous estimates. As a consequence, amounts of metaluminous granites that can be produced from lower crustal biotite–amphibole gneisses by dehydration melting are more restricted than previously thought.     

Adsorption of methylene blue and crystal violet on low-cost adsorbent: waste fruits of <Emphasis Type="Italic">Rapanea ferruginea</Emphasis> (ethanol-treated and H<Subscript>2</Subscript>SO<Subscript>4</Subscript>-treated)

This study assesses the ability of two low-cost adsorbents made from waste of Rapanea ferruginea treated with ethanol (WRf) and its H₂SO₄-treated analog (WRf/H₂SO₄) for the removal of two cationic dyes methylene blue (MB) and crystal violet (CV) from aqueous solutions. The adsorbent was characterized by scanning electron microscopy, Fourier transform infrared spectrometry, thermogravimetric analysis, point of zero charge (pH_pzc), specific surface, and functional groups. The adsorption of dye onto the adsorbents was studied as a function of pH solution (2–12), contact time (up to 120 min) and initial concentration (20–120 mg/L), and temperature (25, 35, and 55 °C). The influence of these parameters on adsorption capacity was studied using the batch process. The response surface methodology (RSM) was used in the experimental design, modeling of the process, and optimizing of the variables and was optimized by the response involving Box–Behnken factorial design (15 runs). The results show that the data correlated well with the Sips isotherm. The maximum adsorption capacities of MB and CV onto WRf were found to be 69 and 106 mg/g, and onto WRf/H₂SO₄, the adsorption capacities were 33 and 125 mg/g, respectively. The kinetic data revealed that adsorption of cationic dyes onto the adsorbents closely follows the pseudo-second-order kinetic model. Regression analysis showed good fit of the experimental data to the second-order polynomial model, with coefficient of determination (R²) values for MB (R²?=?0.9685) and MB (R²?=?0.9832) for WRf and CV (R²?=?0.9685) and CV (R²?=?0.9832) for WRf/H₂SO₄ indicated that regression analysis is able to give a good prediction of response for the adsorption process in the range studied. The results revealed that waste from R. ferruginea is potentially an efficient and low-cost adsorbent for adsorption of MB and CV.     

Compressibility of strontium orthophosphate Sr<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> at high pressure

High pressure in situ synchrotron X-ray diffraction experiment of strontium orthophosphate Sr₃(PO₄)₂ has been carried out to 20.0 GPa at room temperature using multianvil apparatus. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a volume of V ₀ = 498.0 ± 0.1 Å³, an isothermal bulk modulus of K _T  = 89.5 ± 1.7 GPa, and first pressure derivative of K _T ′ = 6.57 ± 0.34. If K _T ′ is fixed at 4, K _T is obtained as 104.4 ± 1.2 GPa. Analysis of axial compressible modulus shows that the a-axis (K _a  = 79.6 ± 3.2 GPa) is more compressible than the c-axis (K _c  = 116.4 ± 4.3 GPa). Based on the high pressure Raman spectroscopic results, the mode Grüneisen parameters are determined and the average mode Grüneisen parameter of PO₄ vibrations of Sr₃(PO₄)₂ is calculated to be 0.30(2).     

Amphibole stability in primitive arc magmas: effects of temperature,H<Subscript>2</Subscript>O content,and oxygen fugacity

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO₂, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO₂, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H₂O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H₂O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H₂O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H₂O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H₂O pressure (\({P}_{\text {H}_2{\rm O}}\)), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and \(f_{\text {O}_2}\):

$$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * \Updelta {\rm NNO}\right]^{15.12} $$

This barometer gives a minimum \({P}_{\text{H}_{2}{\rm O}}\) recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H₂O pressures (500–900 MPa) and high pre-eruptive magmatic H₂O contents (10–14 wt% H₂O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).

     

Elastic behavior of vanadinite,Pb<Subscript>10</Subscript>(VO<Subscript>4</Subscript>)<Subscript>6</Subscript>Cl<Subscript>2</Subscript>, a microporous non-zeolitic mineral

The high-pressure behavior of a vanadinite (Pb₁₀(VO₄)₆Cl₂, a = b = 10.3254(5), c = 7.3450(4) Å, space group P6₃/m), a natural microporous mineral, has been investigated using in-situ HP-synchrotron X-ray powder diffraction up to 7.67 GPa with a diamond anvil cell under hydrostatic conditions. No phase transition has been observed within the pressure range investigated. Axial and volume isothermal Equations of State (EoS) of vanadinite were determined. Fitting the P–V data with a third-order Birch-Murnaghan (BM) EoS, using the data weighted by the uncertainties in P and V, we obtained: V ₀ = 681(1) Å³, K ₀ = 41(5) GPa, and K′ = 12.5(2.5). The evolution of the lattice constants with P shows a strong anisotropic compression pattern. The axial bulk moduli were calculated with a third-order “linearized” BM-EoS. The EoS parameters are: a ₀ = 10.3302(2) Å, K ₀(a) = 35(2) GPa and K′(a) = 10(1) for the a-axis; c ₀ = 7.3520(3) Å, K ₀(c) = 98(4) GPa, and K′(c) = 9(2) for the c-axis (K ₀(a):K ₀(c) = 1:2.80). Axial and volume Eulerian-finite strain (fe) at different normalized stress (Fe) were calculated. The weighted linear regression through the data points yields the following intercept values: Fe _a (0) = 35(2) GPa for the a-axis, Fe _c (0) = 98(4) GPa for the c-axis and Fe _V (0) = 45(2) GPa for the unit-cell volume. The slope of the regression lines gives rise to K′ values of 10(1) for the a-axis, 9(2) for the c-axis and 11(1) for the unit cell-volume. A comparison between the HP-elastic response of vanadinite and the iso-structural apatite is carried out. The possible reasons of the elastic anisotropy are discussed.     

Crystal chemistry of selenates with mineral-like structures. III. Heteropolyhedral chains in the crystal structure of [Mg(H<Subscript>2</Subscript>O)<Subscript>4</Subscript>(SeO<Subscript>4</Subscript>)]<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)

The crystal structure of a new compound [Mg(H₂O)₄(SeO₄)]₂(H₂O) (monoclinic, P2 ₁/a, a = 7.2549(12), b = 20.059(5), c = 10.3934(17) Å, β = 101.989(13), V = 1479.5(5) ų) has been solved by direct methods and refined to R ₁ = 0.059 for 2577 observed reflections with |F _hkl | ≥ 4σ|F _hkl |. The structure consists of [Mg(H₂O)₄(SeO₄)]⁰ chains formed by alternating corner-sharing Mg octahedrons and (SeO₄)^2? tetrahedrons. O atoms of Mg octahedrons that are shared with selenate tetrahedrons are in a trans orientation. The heteropoly-hedral octahedral-tetrahedral chains are parallel to the c axis and undulate within the (010) plane. The adjacent chains are linked by hydrogen bonds involving H₂O molecules not bound with M²⁺ cations.     

Hazards of mechanized tunnel excavation in H<Subscript>2</Subscript>S bearing ground in Aspar tunnel,Iran

Occurrence of hydrogen sulfide gas (H₂S) is one of the most important engineering geological hazards during tunneling. Its hazards and consequent challenges are very difficult and costly to solve. During site investigation, one of the tasks for engineering geologists is prediction and evaluation of the risk of H₂S gas in the underground spaces. In this study, water conveyance tunnel of Aspar, which was excavated in H₂S-bearing environments, is discussed. The tunnel is excavated in the hydrocarbon formations. Applied experiments suggest that geological formations pertaining to hydrocarbon resources are crucial in formation and reservation of H₂S gas. This paper briefly discusses hazards and geological sources of H₂S, as well as remedial measures for decreasing the risks and problems in excavation of the tunnel. To predict the risk of H₂S gas in the underground spaces, it is possible to use some precursors such as: sulfur springs, organic traces, organic argillaceous rocks, exposure of H₂S odor from fresh surface of rock and smell of H₂S during boreholes drilling. Controlling the inflow of groundwater into the excavation, diluting the concentration of H₂S, training the personnel and utilization of some proper safety equipment have been used to mitigate risks and problems in tunnel excavation.     

Experimental study of pure and mixtures of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> adsorption on modified carbon nanotubes

In this work 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane (TRI) was employed to functionalize MWCNT containing hydroxyl groups (OH-MWCNT), and the XRD, FTIR, TGA and CHNS elemental analysis techniques were used to characterize the resulted adsorbents. The characterization results for amine-MWCNT showed amine groups effectively attached to the surface of the MWCNT. The equilibrium adsorption capacity of pure CO₂ and CH₄ and their binary mixture on the pristine MWCNT, OH-MWCNT and amine-MWCNT was measured through a set of equilibrium adsorption experiments at 303.2 and 318.2 K. Capacities of all three types of adsorbents for CO₂ adsorption were higher than those for methane adsorption. Also, amine-MWCNT demonstrated better performance on CO₂ adsorption than the other two adsorbents, especially at low partial pressures. The capacity of amine-MWCNT for pure CO₂ adsorption was 2.5 and 4 times as much as those for pristine MWCNT and OH-MWCNT, respectively, at the temperature of 303.2 K and the pressure of 0.2 bar. The binary adsorption experiment revealed that CO₂/CH₄ selectivity for pristine MWCNT and amine-MWCNT in all molar fractions of CO₂ is about 1.77 and 7, respectively.     

Influence of chlorine substitution on adsorption of gaseous chlorinated phenolics on multi-walled carbon nanotubes embedded in SiO<Subscript>2</Subscript>

Multi-walled carbon nanotubes (MWCNTs) embedded in SiO₂ particles were prepared through the floating-catalyst chemical vapor deposition method. The parameters reaction time and flow rate of the carbon source (CH₄) were studied to obtain optimum conditions for MWCNT synthesis. The obtained MWCNTs were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy to confirm their morphology and crystallinity. The optimum conditions were a CH₄ flow rate of 100 ml/min in a H₂–Ar mixture at a flow rate of 500 ml/min and a reaction time of 20 min. Under these conditions, MWCNTs with average outer and inner diameters of around 50 and 10 nm, respectively, were obtained. SiO₂ particles with embedded MWCNTs were studied for their adsorption of gaseous chlorinated phenolic compounds (CPCs), with emphasis on the effect of number of chlorine substituents. The CPC compounds of 2-chlorophenol (CP) and 2,4-dichlorophenol (DCP) were compared against phenol (P). Adsorption of P and CPCs on the particles fit well the Langmuir isotherm. The adsorption capacities of P, CP, and DCP on SiO₂ particles with embedded MWCNTs were found to be 3.12, 13.83, and 44.25 mg/g, respectively. Desorption activation energy was determined by thermogravimetric analysis. Chlorine substitution on P changed the adsorption process from physical to chemical adsorption. The particles showed high potential for use as a pre-concentration unit for solid-phase microextraction.     

Crystal chemistry of selenates with mineral-like structures: VII. The structure of (H<Subscript>3</Subscript>O)[(UO<Subscript>2</Subscript>)(SeO<Subscript>4</Subscript>)(SeO<Subscript>2</Subscript>OH)] and some structural features of selenite-selenates

The crystal structure of a new compound, (H₃O)[(UO₂)(SeO₄)(SeO₂OH)] (monoclinic, P2₁/n, a = 8.6682(19), b = 10.6545(16), c = 9.846(2) Å, β = 97.881(17)°, V = 900.7(3) ų), was solved by direct methods and refined to R ₁ = 0.050. The structure contains two symmetrically different Se atoms. The Se1 site is coordinated by three O atoms as is characteristic of Se⁴⁺ cations. The Se2 site is coordinated by four O atoms and forms selenate anion SeO ₄ ^2? . The structure is based on selenite-selenate sheets [(UO₂)(SeO₄)(SeO₂OH)]^? linked by the interlayer H₃O^? ions. The sheets are parallel to (101). The structure is compared to that of schmiederite, Pb₂Cu₂(SeO₃)(SeO₄)(OH)₄.     

CO<Subscript>2</Subscript> mineralization of natural wollastonite into porous silica and CaCO<Subscript>3</Subscript> powders promoted via membrane electrolysis

CO₂ is a greenhouse gas, whose emissions threaten the existence of human beings. Its inherently safe sequestration can be performed via CO₂ mineralization, which is relatively slow under natural conditions. In this work, an energy-saving membrane electrolysis technique was proposed for accelerating the CO₂ mineralization of wollastonite into SiO₂ and CaCO₃ products. The electrolysis process involved splitting NH₄Cl into HCl and NH₃·H₂O via hydrogen oxidation and water reduction at the anode and cathode of the electrolytic system, respectively. In contrast to the chlor-alkali electrolysis, this method did not involve Cl^? oxidation and the standard potential of the anode was reduced. Additionally, NH₄Cl was used as the electrolyte instead of NaCl; as a result, the generation of NH₃·H₂O instead of NaOH occurred in the catholyte and the cathodic pH dramatically decreased, thus reducing the cathodic potential for hydrogen evolution. The observed changes led to a 73.5% decrease in the energy consumption. Moreover, after the process of CO₂ mineralization was optimized, SiO₂ with a specific surface area of 221.8 m² g^?1 and CaCO₃ with a purity of 99.9% were obtained.     

Pressure induced phase transition in Pb<Subscript>6</Subscript>Bi<Subscript>2</Subscript>S<Subscript>9</Subscript>

The crystal structure of Pb₆Bi₂S₉ is investigated at pressures between 0 and 5.6 GPa with X-ray diffraction on single-crystals. The pressure is applied using diamond anvil cells. Heyrovskyite (Bbmm, a = 13.719(4) Å, b = 31.393(9) Å, c = 4.1319(10) Å, Z = 4) is the stable phase of Pb₆Bi₂S₉ at ambient conditions and is built from distorted moduli of PbS-archetype structure with a low stereochemical activity of the Pb²⁺ and Bi³⁺ lone electron pairs. Heyrovskyite is stable until at least 3.9 GPa and a first-order phase transition occurs between 3.9 and 4.8 GPa. A single-crystal is retained after the reversible phase transition despite an anisotropic contraction of the unit cell and a volume decrease of 4.2%. The crystal structure of the high pressure phase, β-Pb₆Bi₂S₉, is solved in Pna2 ₁ (a = 25.302(7) Å, b = 30.819(9) Å, c = 4.0640(13) Å, Z = 8) from synchrotron data at 5.06 GPa. This structure consists of two types of moduli with SnS/TlI-archetype structure in which the Pb and Bi lone pairs are strongly expressed. The mechanism of the phase transition is described in detail and the results are compared to the closely related phase transition in Pb₃Bi₂S₆ (lillianite).