Rutile/TiO<Subscript>2</Subscript>II phase equilibria

The transition between rutile and α-PbO₂ structured TiO₂ (TiO₂II) has been investigated at 700–1,200 °C and 4.2–9.6 GPa. Hydrothermal phase equilibrium experiments were performed in the multi-anvil apparatus to bracket the phase boundary at 700, 1,000, and 1,200 °C. The equilibrium phase boundary is described by the equation: P (GPa)=1.29+0.0065 T ( °C). In addition, growth of TiO₂II was observed in experiments at 500 and 600 °C, although growth of rutile was too slow to bracket unambiguously the equilibrium boundary at these temperatures. Water was not detected in either rutile or TiO₂II, and dry synthesis experiments at 1,200 °C were consistent with the phase boundary determined in the fluid-bearing experiments, suggesting that the equilibrium is unaffected by the presence of water. Our bracket of the phase boundary at 700 °C is consistent with the reversed, dry experiments of Akaogi et al. (1992) and the reversals of Olsen et al. (1999). The new data suggest that the phase boundary is linear, in agreement with Akaogi et al. (1992), but in striking contrast to the phase diagram inferred by Olsen et al. (1999). The natural occurrence of TiO₂II requires formation pressures considerably higher than the graphite–diamond phase boundary.     

Photocatalytic performance of TiO<Subscript>2</Subscript> and WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanoparticles coated on urban green infrastructure materials in removing nitrogen oxide

This study investigated the performance of UV light active TiO₂ and UV–visible light active WO₃/TiO₂ nanoparticles as air purifying materials that can be potentially applied to urban green infrastructures such as rain gardens and pervious pavements. Using a laboratory-scale continuous gas flow photoreactor, the removal efficiency of gaseous nitrogen oxide (NO _x ) by two different photocatalytic nanoparticles coated on natural zeolites and pervious concrete blocks was evaluated. The results showed that the TiO₂- and WO₃/TiO₂-coated zeolites are excellent photoactive materials providing enhanced air purification function (~95% removal efficiency of NO _x ) under UV and UV–visible light irradiation, respectively. In contrast, both of the TiO₂- and WO₃/TiO₂-coated pervious concrete blocks showed a measurable NO _x removal (~60%) only under UV irradiation, whereas the visible light activity of the WO₃/TiO₂-coated concrete block was significantly reduced (~20%) mainly due to the decrease in the photocatalytic reaction sites for visible light. This study revealed the potential utility of photocatalytic nanoparticles in improving urban air quality, in the form of the surface component of various urban infrastructures.     

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.     

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.     

Equation of state of synthetic qandilite Mg<Subscript>2</Subscript>TiO<Subscript>4</Subscript> at ambient temperature

Using a diamond-anvil cell and synchrotron X-ray diffraction, the compressional behavior of a synthetic qandilite Mg_2.00(1)Ti_1.00(1)O₄ has been investigated up to about 14.9 GPa at 300 K. The pressure–volume data fitted to the third-order Birch–Murnaghan equation of state yield an isothermal bulk modulus (K _T0) of 175(5) GPa, with its first derivative \(K_{T0}^{{\prime }}\) attaining 3.5(7). If \(K_{T0}^{{\prime }}\) is fixed as 4, the K _T0 value is 172(1) GPa. This value is substantially larger than the value of the adiabatic bulk modulus (K _S0) previously determined by an ultrasonic pulse echo method (152(7) GPa; Liebermann et al. in Geophys J Int 50:553–586, 1977), but in general agreement with the K _T0 empirically estimated on the basis of crystal chemical systematics (169 GPa; Hazen and Yang in Am Miner 84:1956–1960, 1999). Compared to the K _T0 values of the ulvöspinel (Fe₂TiO₄; ~148(4) GPa with \(K_{T0}^{{\prime }} = 4\)) and the ringwoodite solid solutions along the Mg₂SiO₄–Fe₂SiO₄ join, our finding suggests that the substitution of Mg²⁺ for Fe²⁺ on the T sites of the 4–2 spinels can have more significant effect on the K _T0 than that on the M sites.     

Discrimination of TiO<Subscript>2</Subscript> polymorphs in sedimentary and metamorphic rocks

Investigation by Raman spectroscopy of samples from different geological settings shows that the occurrence of TiO₂ polymorphs other than rutile can hardly be predicted, and furthermore, the occurrence of anatase is more widespread than previously thought. Metamorphic pressure and temperature, together with whole rock chemistry, control the occurrence of anatase, whereas variation of mineral assemblage characteristics and/or fluid occurrence or composition takes influence on anatase trace element characteristics and re-equilibration of relict rutiles. Evaluation of trace element contents obtained by electron microprobe in anatase, brookite, and rutile shows that these vary significantly between the three TiO₂ phases. Therefore, on the one hand, an appropriation to source rock type according to Nb and Cr contents, but as well application of thermometry on the basis of Zr contents, would lead to erroneous results if no phase specification is done beforehand. For the elements Cr, V, Fe, and Nb, variation between the polymorphs is systematic and can be used for discrimination on the basis of a linear discriminant analysis. Using phase group means and coefficients of linear discriminants obtained from a compilation of analyses from samples with well-defined phase information together with prior probabilities of groupings from a natural sample compilation, one is able to calculate phase grouping probabilities of any TiO₂ analysis containing at least the critical elements Cr, V, Fe, and Nb. An application of this calculation shows that for the appropriation to the phase rutile, a correct-classification rate of 99.5% is obtained. Hence, phase specification by trace elements proves to be a valuable tool besides Raman spectroscopy.     

Heat capacity and entropy of rutile (TiO<Subscript>2</Subscript>) and nepheline (NaAlSiO<Subscript>4</Subscript>)

 From heat capacities measured adiabatically at low temperatures, the standard entropies at 298.15 K of synthetic rutile (TiO₂) and nepheline (NaAlSiO₄) have been determined to be 50.0 ± 0.1 and 122.8 ± 0.3 J mol⁻¹ K, respectively. These values agree with previous measurements and in particular confirm the higher entropy of nepheline with respect to that of the less dense NaAlSiO₄ polymorph carnegieite. Received: 23 July 2001 / Accepted: 12 October 2001     

Hydrothermal fabrication of WO<Subscript>3</Subscript>-modified TiO<Subscript>2</Subscript> crystals and their efficiency in photocatalytic degradation of FCF

WO₃-modified TiO₂ polyscale crystals were fabricated successfully using the hydrothermal technique. The as-prepared samples were characterized using powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, photoluminescence spectroscopy and UV–vis spectroscopy. The photocatalytic application of these synthesized samples was confirmed by photocatalytic degradation of fast green dye solution under sunlight and UV irradiation. The degradation efficiency was analyzed by measuring the parameters such as percent transmittance, chemical oxygen demand and percent decomposition of the dye solution. It was noted that the photodegradation efficiency of the samples varies with added amounts of WO₃ content. The highest photodegradation efficiency was obtained using 2WT sample where the pace of decomposition was 70.5% under UV light and 81.3% under sunlight.     

The Role of CaCO<Subscript>3</Subscript> Reactions in the Contemporary Oceanic CO<Subscript>2</Subscript> Cycle

The present analysis adjusts previous estimates of global ocean CaCO₃ production rates substantially upward, to 133 × 10¹² mol yr^?1 plankton production and 42 × 10¹² mol yr^?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO₃ production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO₃ mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO₃ sediment accumulation rates remain about the same as they have been for some years: ~20 × 10¹² mol yr^?1 on shelves and 11 × 10¹² mol yr^?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 10¹² mol yr^?1 (~50 %) of shelf benthonic carbonate production and 122 × 10¹² mol yr^?1 (>90 %) of planktonic production. Most CaCO₃ production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO₂ release to CaCO₃ precipitation (CO₂↑/CaCO₃↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO₂ associated with CaCO₃ reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 10¹² mol C yr^?1, at a time scale of 10²–10³ years. The CaCO₃-mediated influx of CO₂ is approximately offset by CO₂ release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO₃ and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.     

Oxyvanite,V<Subscript>3</Subscript>O<Subscript>5</Subscript>, a new mineral species and the oxyvanite-berdesinskiite V<Subscript>2</Subscript>TiO<Subscript>5</Subscript> series from metamorphic rocks of the Slyudyanka Complex,southern Baikal region

Oxyvanite has been identified as an accessory mineral in Cr-V-bearing quartz-diopside meta- morphic rocks of the Slyudyanka Complex in the southern Baikal region, Russia. The new mineral was named after constituents of its ideal formula (oxygen and vanadium). Quartz, Cr-V-bearing tremolite and micas, calcite, clinopyroxenes of the diopside-kosmochlor-natalyite series, Cr-bearing goldmanite, eskolaite-karelianite dravite-vanadiumdravite, V-bearing titanite, ilmenite, and rutile, berdesinskiite, schreyerite, plagioclase, scapolite, barite, zircon, and unnamed U-Ti-V-Cr phases are associated minerals. Oxyvanite occurs as anhedral grains up to 0.1–0.15 mm in size, without visible cleavage and parting. The new mineral is brittle, with conchoidal fracture. Observed by the naked eye, the mineral is black, with black streak and resinous luster. The microhardness (VHN) is 1064–1266 kg/mm² (load 30 g), and the mean value is 1180 kg/mm². The Mohs hardness is about 7.0–7.5. The calculated density is 4.66(2) g/cm³. The color of oxyvanite is pale cream in reflected light, without internal reflections. The measured reflectance in air is as follows (λ, nm-R, %): 440-17.8; 460-18; 480-18.2; 520-18.6; 520-18.6; 540-18.8; 560-18.9; 580-19; 600-19.1; 620-19.2; 640-19.3; 660-19.4; 680-19.5; 700-19.7. Oxyvanite is monoclinic, space group C2/c; the unit-cell dimensions are a = 10.03(2), b = 5.050(1), c = 7.000(1) Å, β = 111.14(1)°, V = 330.76(5)ų, Z = 4. The strongest reflections in the X-ray powder pattern [d, Å, (I in 5-number scale)(hkl)] are 3.28 (5) (20\(\bar 2\)); 2.88 (5) (11\(\bar 2\)); 2.65, (5) (310); 2.44 (5) (112); 1.717 (5) (42\(\bar 2\)); 1.633 (5) (31\(\bar 4\)); 1.446 (4) (33\(\bar 2\)); 1.379 (5) (422). The chemical composition (electron microprobe, average of six point analyses, wt %): 14.04 TiO₂, 73.13 V₂O₃ (53.97 V₂O_3calc, 21.25 VO_2calc), 10.76 Cr₂O₃, 0.04 Fe₂O₃, 0.01 Al₂O₃, 0.02 MgO, total is 100.03. The empirical formula is (V _1.70 ³⁺ Cr_0.30)_2.0(V _0.59 ⁴⁺ Ti_0.41)_1.0O₅. Oxyvanite is the end member of the oxyvanite-berdesinskiite series with homovalent isomorphic substitution of V⁴⁺ for Ti. The type material has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

Low-temperature heat capacities of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and spinels of the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution

We present results from low-temperature heat capacity measurements of spinels along the solid solution between MgAl₂O₄ and MgCr₂O₄. The data also include new low-temperature heat capacity measurements for MgAl₂O₄ spinel. Heat capacities were measured between 1.5 and 300 K, and thermochemical functions were derived from the results. No heat capacity anomaly was observed for MgAl₂O₄ spinel; however, we observe a low-temperature heat capacity anomaly for Cr-bearing spinels at temperatures below 15 K. From our data we calculate standard entropies (298.15 K) for Mg(Cr,Al)₂O₄ spinels. We suggest a standard entropy for MgAl₂O₄ of 80.9 ± 0.6 J mol⁻¹ K⁻¹. For the solid solution between MgAl₂O₄ and MgCr₂O₄, we observe a linear increase of the standard entropies from 80.9 J mol⁻¹ K⁻¹ for MgAl₂O₄ to 118.3 J mol⁻¹ K⁻¹ for MgCr₂O₄.     

Pharmaceutical residues in aquatic environment and water remediation by TiO<Subscript>2</Subscript> heterogeneous photocatalysis: a review

The occurrence of pharmaceutical compounds in the natural water sources has been reported as early as in the year 1980. Until now, the presence of pharmaceutical compounds in the aquatic environment has been frequently reported in the literature. Moreover, increasing evidence suggests that these contaminants have posed a threat to both humans and ecosystems. In this regard, the present review paper seeks to offer an overview of this environmental issue of pharmaceutical pollution where the subject matters to be reviewed include the effects, sources and mitigation strategies of pharmaceuticals in the aquatic environment. Besides, a review of the fundamentals and mechanisms of heterogeneous photocatalysis technology is also presented in this paper. Heterogeneous photocatalysis is a rapidly expanding technology which has been extensively investigated and applied in wastewater treatment for the remediation of persistent pollutants such as pharmaceutical compounds during the last decade. Furthermore, the ideal photocatalyst titanium dioxide (TiO₂), which can collaborate and perform well in the photocatalysis treatment process, is also discussed. The advantages and limitations associated with the application of this treatment method are summarized and discussed in details. Finally, this review paper focuses on the future trend of the photocatalysis technology and identifies the barriers and lacking parts which need to be resolved in the near future.     

Ab-initio calculations of the proton location in topaz-OH,Al<Subscript>2</Subscript>SiO<Subscript>4</Subscript>(OH)<Subscript>2</Subscript>

The position of hydrogen in the structure of topaz-OH was determined by means of ab-initio quantum-mechanic calculations. Static lattice energy calculations predict the existence of four non-equivalent positions of protons, which are characterized by O4–H1... O1, O4–H2... O2, O4–H3... O3 and O4–H4... O4 hydrogen bonds. The distribution of the protons between positions of local equilibrium is controlled by the proton–proton avoidance rule and the strength of the hydrogen bonds. The most favourable configuration of hydrogen atoms is achieved for adjacent protons, which form O4–H3... O3 and O4–H4... O4 hydrogen bonds, respectively. The thermal excitation of atoms at a temperature of 55 K is large enough for the hydrogen atoms occasionally to change their positions to form O4–H1... O1 and O4–H2... O2 bonds. At ambient pressures and higher temperatures the protons are in a dynamic exchange between the allowed positions of local minima. As a consequence, for nearly room-temperature conditions, the dynamic change between different structural configurations leads to the violation of all possible symmetry elements and with that to space group #E5/E5#1. The flipping of the protons between different sites is achieved by simple rotation of the OH-dipole and does not produce any significant distortion of the framework of topaz, whose symmetry remains that of the space group Pbnm. Therefore, no reduction of symmetry has been observed in former X-ray studies on topaz-OH. Calculated IR absorption spectra of topaz-OH were found to be in good agreement with measured spectra. According to the calculations, the two favourable configurations of protons might correspond to the measured peak splitting within the OH-stretching range. An experimentally observed low-frequency band at 3520 cm^–1 was assigned to the OH-stretching of the O4–H3... O3 bond, while the band at 3600 cm^–1 was attributed to OH-stretching of the O4–H4... O4 hydrogen bond. The broad peak in FAR-IR frequency range at 100–150 cm^–1 is attributed to the stretching of H3... O3 and H4... O4 contacts. The rate of proton exchange at 670 K among different sites was estimated by ab-inito molecular dynamic simulations. The calculations predict that flipping of adjacent protons between O4–H3... O3 and O4–H4... O4 bonds at 670 K occur at a rate of about 1.96 THz.     

Photocatalytic degradation of imidacloprid in aqueous suspension of TiO<Subscript>2</Subscript> supported on H-ZSM-5

The composite of TiO₂ and zeolite H-ZSM-5 has great photocatalytic ability for organic contaminants over a very large specific surface area and highlighted adsorption capacity. To describe abiotic degradation of imidacloprid, the photoinduced degradation of the pesticide imidacloprid in aqueous solutions, in the presence of TiO₂ supported on H-ZSM-5 as photocatalyst, was performed. The study focused on the comparison of the imidacloprid degradation between photolysis and photocatalysis. The experimental results showed that the degradation of imidacloprid was more rapid in the condition of photocatalytic than that of photolysis or TiO₂-only. The identification of possible intermediate products during the degradation was investigated by the high-performance liquid chromatography coupled with electrospray time-of-flight mass spectrometry (HPLC/TOF-MS). The main photocatalytic products were identified as chloronictinic acid, 1-[(6-chloro-3-pyridinyl) methyl]-2-imidazolidinone and 1-[(6-chloro-3-pyridinyl) methyl]-N-nitroso-2-imidazolidimine.     

The crystal structure of alumoklyuchevskite,K<Subscript>3</Subscript>Cu<Subscript>3</Subscript>AlO<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript>

The crystal structure of the unstable mineral alumoklyuchevskite K₃Cu₃AlO₂(SO₄)₄ [monoclinic, I2, a = 18.772(7), b = 4.967(2), c = 18.468(7) Å, β = 101.66(1)°, V = 1686(1) Å] was refined to R ₁ = 0.131 for 2450 unique reflections with F ≥ 4σF _hkl. The structure is based on oxocentered tetrahedrons (OAlCu ₃ ⁷⁺ ) linked into chains via edges. Each chain is surrounded by SO₄ tetrahedrons forming a structural complex. Each complex is elongated along the b axis. This type of crystal structure was also found in other fumarole minerals of the Great Tolbachik Fissure Eruption (GTFE, Kamchatka Peninsula, Russia, 1975–1976), klyuchevskite, K₃Cu₃Fe³⁺O₂(SO₄)₄; and piypite, K₂Cu₂O(SO₄)₂.     

The nature of blue luminescence from natural benitoite BaTiSi<Subscript>3</Subscript>O<Subscript>9</Subscript>

It is proved that blue luminescence from benitoite is connected with intrinsic luminescence centers, namely isolated TiO₆ octahedra. The metastable level ³T_1u is the emitting level at low temperatures with a long decay time of 1.1 ms. At higher temperatures an energy level with higher radiation probability must be involved in the emission process, and this level is situated at 0.06 eV higher than the lowest level. These two levels may be connected with ³T_1u level splitting or with closely spaced ³T_1u and ³T_2u levels. Decay time shortening and thermal quenching are connected with nonradiative decay within the TiO₆ luminescence center, while energy migration does not take place at least up to room temperature.