Hot and cold CO2-rich mineral waters in Chaves geothermal area (Northern Portugal)

In order to update the geohydrologic characterisation of Chaves geothermal area, coupled isotopic and chemical studies have been carried out on hot and cold CO₂-rich mineral waters discharging, in northern Portugal, along one of the major regional NNE-trending faults (the so-called “Verin-ChavesPenacova Depression”). Based upon their location, and chemical and isotopic composition, the analysed waters can be divided into two groups. The northern group belongs to the HCO₃/Na/CO₂-rich type, and consists of the hot spring waters of Chaves and the cold spring waters of Vilarelho da Raia. The δD and δ¹⁸O values show that these waters are of meteoric origin. The lack of an ¹⁸O shift indicates that there is no evidence of water/rock interaction at high temperatures. The southern group includes the cold spring waters of Campilho/ Vidago and Sabroso/Pedras Salgadas. Their chemistry is similar to that of the northern group but their heavier δD and δ¹⁸O values could be attributed to different recharge altitudes. Mixing between deep mineralised waters and dilute superficial waters of meteoric origin might explain the higher ³H activity found in the Vidago and Pedras Salgadas mineral waters. Alternatively, they could be mainly related to shallow underground flowpaths. The δ¹³C values support a deep-seated origin for the CO₂. The δ³⁷Cl is comparable in all the mineral waters of the study area, indicating a common origin of Cl. The ⁸⁷Sr/⁸⁶Sr ratios in waters seem to be dominated by the dissolution of plagioclases of granitic rocks.     

Geochemical study of fluids on Lesbos island, Greece

Sixty-five water samples and seven associated gas samples have been collected on Lesbos island. The lithology and structural setting have resulted in two main types of hydrological circulation: a shallow circulation hosting low-salinity cold waters and a deeper one, hosting high-salinity hot waters that often emerge in thermal springs near the coast. The cold waters are characterized by Ca(Mg)-HCO₃(SO₄) composition, while the thermal waters generally have an Na-Cl composition. The chemical features of the former can be explained by their circulation in the ophiolite-bearing phyllitic basement and volcanic rocks. Waters circulating in the ultramafic layers of the basement are richer in Mg than the waters whose circulation is mainly within marble levels or volcanic rocks. The Na-Cl thermal waters are characterized by salinities ranging from 1910 to 35,700 mg/kg. As indicated by previous hydrogeochemical and isotopic studies, the Na-Cl composition of the thermal waters on Lesbos is the result of mixing between shallow meteoric waters and marine waters. While interacting with the minerals of the geothermal reservoir, the saline waters retain the Na/Cl sea water ratio but become enriched in Ca²⁺ and depleted in Mg²⁺ with respect to sea water.Processes of hydrothermal alteration at depth are activated by a gas phase enriched in CO₂, which reaches the geothermal reservoir by rising along the deep fractures of the basement. Thermodynamic calculations based on hydrothermal alteration processes occurring at the estimated temperatures of the geothermal reservoir (about 120 °C) indicate that the thermal waters of Lesbos are in equilibrium with talc and dolomite.     

Oxygen and hydrogen isotopes in deep thermal waters from the south meager creek geothermal area, british columbia, canada

Deuterium and oxygen-18 (¹⁸O) have been measured in deep thermal, shallow thermal and non-thermal water samples collected at various times between 1982 and 1989 from the Meager Creek area, with the aim of assessing the origin of the thermal waters. The isotopic composition of the reservoir waters (δ¹⁸O = −13‰ and δD = −114.8‰) was calculated from data on post-flash deep thermal waters, using a two-stage steam loss model. The reservoir composition shows an oxygen shift of 2.4‰ relative to the local meteoric water line. The composition of the recharge, obtained by removing the oxygen shift, is isotopically heavier than the average local meteoric waters, suggesting that the recharge may be from an area to the west of Mt Meager where isotopically heavier ground-waters are likely to be found. The small δ¹⁸O shift of the deep high-temperature waters is indicative of dominance of fracture-related permeability in the reservoir. Analysis of the chemistry and the temperature of the waters from hot springs and shallow thermal wells suggests that these waters have evolved from the deep geothermal waters through dilution by meteoric waters and about 40°C adiabatic cooling (steam loss).     

Temperature and chemical changes in the fluids of the Obama geothermal field (SW Japan) in response to field utilization

Thermal waters from Quaternary volcanic rocks (predominantly andesites) discharge along faults in the Obama geothermal field of southwestern Japan. The chemistry of more than 100 thermal and ground water samples collected between 1936 and 2005 indicate that the Na–Cl hot spring waters are a mixture of “andesitic” magmatic, sea and meteoric waters. Mixing models and silica and cation geothermometry were used to estimate the SiO₂ and Cl composition and the temperature (∼200 °C) of the reservoir fluids deep in the geothermal system. The isotopic data (¹⁸O and D) are consistent with a mixed origin interpretation of the waters feeding the Obama hot springs, i.e. a large proportion of meteoric and sea waters, and a small magmatic component. Temperatures and chemical concentrations of the thermal waters were affected by the 1944–1959 salt production operations, but have recovered after closure of the salt factories; now they are similar to their pre-1940 values. In the future, the Obama geothermal field may be suitable for electric power generation, although heat and fluid extraction will require careful management to prevent or minimize reservoir cooling.     

Research of geothermal anomalies by means of hydrochemical data in the East Eifel area, West Germany

In the East Eifel volcanic area 70 mineral and thermomineral waters have been sampled and analysed with respect to their chemical and isotopic composition. The isotopic composition shows that all waters are of meteoric origin. Distinct anomalies of SiO₂ contents and Na⁺/K⁺ ratios were found around the Lake Laach caldera. Manuring and mixing processes with a deep circulating brine could be excluded as reasons for the observed shift of Na⁺/K⁺ ratios. A final connection between the observed anomalies in spring water composition and a low geothermal anomaly is not possible because no information is available about the rock in contact with the water. The attempt to calculate geotemperatures failed because of the high CO₂ contents of the waters. But the results show that the subsurface temperatures with which the waters come into contact are certainly below 100°C.     

Evidence of a contact metamorphic aureole with high-temperature metasomatism in the deepest part of the active geothermal field of Larderello,Italy

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists of rocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were thermally metamorphosed in the same place during the emplacement of granite intrusions of 3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target of future exploitation. The petrographic, geochemical and fluid inclusion data indicate that thermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent a recrystallisation at temperatures of 425–670 °C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varying proportions of H₂O and CO₂ that formed during the contact metamorphism, were present during this stage. The fluids present during the contact metamorphic event were responsible for a widespread B-metasomatism and local F-metasomatism. In some cases, high-temperature metamorphism of graphitic schists can control the composition of the aqueous-carbonic fluids. A late-stage, lower temperature hydrothermal activity was responsible for both the propylitic and sericitic alterations and for the replacement of the high-temperature mineral assemblage. Stable isotope (δ¹⁸O and δD) data on the thermally metamorphosed rocks and granites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing and dehydration metamorphic reactions can explain the isotopic values of these rocks. Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of the heavy isotopes from the rocks. Under contact metamorphism conditions the rocks were plastic and impermeable. A transition from plastic to brittle conditions occurred as a consequence of the cooling of the system at depths of <4 km. The brittle-plastic transition at Larderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in the range 400–450 °C. Deep-seated fluids probably occur in this zone, as suggested by the geophysical data.     

Noble gases in some mineral and warm waters in the Massif Central, France

Cold and warm (up to 82°C) mineral waters in the Massif Central were found to contain enrichments of radiogenic ⁴He, and in a few cases also ⁴⁰Ar, the ratio of radiogenic ⁴He/⁴⁰Ar being 4.4. Atmospheric Ne, Ar and Kr have been identified by their isotopic compositions, the absolute concentrations being positively correlated with the accompanying radiogenic (deep-seated) gases. This indicates that meteoric water was recharged deep in the system and mixed with the radiogenic gases. The concentrations of the atmospheric noble gases are lower than expected in recharge, indicating depletion at depth or during ascent. This might be caused by exhalation of the large gas contents characteristic of the studied waters, or steam separation at depth. The D and ¹⁸O values fit, however, the local meteoric line and rule out steam separation. In any case, the depletion of the atmospheric noble gases may be used to calculate the losses of the accompanying CO₂ and other reactive gases. A ‘reversed’ fractionation pattern is observed in the atmospheric noble gases, the retention being Ne > Ar > Kr. This places boundary conditions that must be explored further. Ionic geothermometry indicates that only low to medium-temperature fluids are involved.     

Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the palinpinon geothermal system, Philippines

Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of −8.5‰ and −54‰ in ¹⁸O and ²H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as “natural tracers” to determine the origin and mixing of different fluids in the reservoir.     

Hydrology,hydrochemistry and geothermal potential of El Chichón volcano-hydrothermal system,Mexico

Fluid and heat discharge rates of thermal springs of El Chichón volcano were measured using the chloride inventory method. Four of the five known groups of hot springs discharge near-neutral Na–Ca–Cl–SO₄ waters with a similar composition (Cl ∼ 1500–2000 mg kg⁻¹ and Cl/SO₄ ∼ 3) and temperatures in the 50–74 °C range. The other group discharges acidic (pH 2.2–2.7) Na–Cl water of high salinity (>15 g/L). All five groups are located on the volcano slopes, 2–3 km in a straight line from the bottom of the volcano crater. They are in the upper parts of canyons where thermal waters mix with surface meteoric waters and form thermal streams. All these streams flow into the Río Magdalena, which is the only drainage of all thermal waters coming from the volcano. The total Cl and SO₄ discharges measured in the Río Magdalena downstream from its junction with all the thermal streams are very close to the sum of the transported Cl and SO₄ by each of these streams, indicating that the infiltration through the river bed is low. The net discharge rate of hydrothermal Cl measured for all thermal springs is about 468 g s⁻¹, which corresponds to 234 kg s⁻¹ of hot water with Cl = 2000 mg kg⁻¹. Together with earlier calculations of the hydrothermal steam output from the volcano crater, the total natural heat output from El Chichón is estimated to be about 160 MWt. Such a high and concentrated discharge of thermal waters from a hydrothermal system is not common and may indicate the high geothermal potential of the system. For the deep water temperatures in the 200–250 °C range (based on geothermometry), and a mass flow rate of 234 kg s⁻¹, the total heat being discharged by the upflowing hot waters may be 175–210 MWt.     

Isotopic and chemical composition of parbati valley geothermal discharges, North-West Himalaya, India

The isotopic compositions of the waters discharged from Parbati Valley geothermal areas indicate a higher altitude meteoric origin, with discharge temperatures reflecting variations in the depth of penetration of the waters to levels heated by the existence of a ‘normal’ geothermal gradient. On the basis of mixing models involving silica, tritium, discharge temperatures and chloride contents, deep equilibration temperatures of 120–140°C were obtained for Manikaran, possibly reaching 160°C at even greater depth. Geothermometers based on sulfate-water ¹⁸O exchange and gas reactions point to similar temperatures. Exceptionally high helium contents of the discharges correspond to apparent crustal residence times of the waters in the order of 10–100 Ma; relative nitrogen-argon contents support a largely meteoric origin of the waters with a possible fossil brine, but no detectable magmatic component.     

Geochemical characteristics of the Yufuin outflow plume, Beppu hydrothermal system, Japan

Thermal water at Yufuin (Kyushu Island, Japan) is tapped through about 820 shallow wells and used mainly for hot-spring bathing purposes. Chemical and isotopic data for fluids from wells and fumaroles in Yufuin and Beppu indicate that the thermal activity at Yufuin represents a dilute, westward-flowing hydrothermal outflow plume from the Beppu hydrothermal system. Two other (eastward-flowing) outflow plumes have long been recognized at Beppu, but the Yufuin outflow plume is first recognized here. The Yufuin outflow plume is apparently a mixture of two end-member fluids: (1) deep high-temperature (250–300°C) fluid from the Beppu system having high chloride concentration (1400–1600 mg/L) and a δ¹⁸O value near −6.0%, and (2) meteoric water having low chloride concentration (≤7 mg/L) and a δ¹⁸O value near −9.2%.A permeable conduit for the vertical and lateral transport of deep fluid from the Beppu system is provided by the Yufuin Fault zone, which extends westward from the southern flank of Mt. Tsurumi volcano to the town of Yufuin. Stable isotope ratios and chloride concentrations for shallow groundwaters near the eastern end of the fault, at an elevation near 700 m, are consistent with those required for the low-chloride meteoric end-member of the Yufuin thermal waters. Recharge of this meteoric water, as well as mixing with the Beppu deep fluid, may occur along the Yufuin Fault. Enthalpy-chloride relations indicate additional conductive heating of the Yufuin waters, in the amount of 350–500 kJ/kg beyond that which can be accounted for by mixing between Beppu deep fluid and meteoric water. This could be a result of conductive heating with convection to a depth of 1–2 km. Estimates of the magnitude of the heat source for the Beppu hydrothermal system should take into account the heat being discharged at Yufuin.     

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.     

Rock-water interactions in the Fenton Hill, new Mexico, hot dry rock geothermal systems I. fluid mixing and chemical geothermometry

The chemistry of fluids circulated through an artificially-stimulated, hot dry rock (HDR) fractured geothermal reservoir system in granitic rock is described in terms of mixing phenomena, geothermometry, and approach to saturation with reservoir rock minerals. Based on the similar dynamic behavior of Na⁺, K⁺, Li⁺, CI⁻, and B species and other isotopic evidence, the presence of a concentrated in-situ pore fluid was identified. Mixing and displacement of this in-situ fluid with meteoric make-up water is responsible for the observed behavior of the major dissolved species in the circulated fluid of this HDR system.     

The role of CuO in promoting photocatalytic hydrogen production over TiO2

Low cost semiconductor photocatalysts that can efficiently harvest solar energy to generate H₂ from water or biofuels will be critical to future hydrogen economies. In this study, low cost CuO/TiO₂ photocatalysts (CuO loadings 0–15 wt.%) were prepared, characterized and evaluated for H₂ production from ethanol–water mixtures (80 vol.% ethanol, 20 vol.% H₂O) under UV excitation. TEM, XRF, EDAX, EPR, Raman, TGA, XPS and Cu L-edge NEXAFS data showed that at CuO loadings <5 wt.%, Cu(II) was highly dispersed over the TiO₂ support, possibly as a sub-monolayer CuO species. At higher loadings, CuO crystallites of diameter 1–2 nm were identified. The photocatalytic activity of CuO/TiO₂ photocatalysts was highly dependent on the CuO loading, with 1.25 wt.% CuO being optimal (H₂ production rate = 20.3 mmol g⁻¹ h⁻¹). Results suggest that sub-monolayer coverages of Cu(II) or CuO on TiO₂ are highly beneficial for H₂ generation from ethanol–water mixtures and support the development of a sustainable H₂ economy.     

Speciation of the gaseous phase of the HI section of the iodine sulphur thermochemical cycle by modeling and inversion of FTIR spectra

This paper presents a laboratory experiment for the study of the liquid–vapor equilibrium of HI–I₂–H₂O ternary mixtures that are involved in the iodine sulphur thermochemical cycle for the production of hydrogen. The apparatus is based on a heated tantalum pressurized vessel equipped with temperature and pressure transducers and carrying windows for optical soundings. It enables recordings of the infrared transmission of the vapor with a Fourier Transform Spectrometer (FTS). The spectra show the signatures of HI and H₂O through rovibrational absorption transitions in both the 4200–5800 cm⁻¹ (1.7–2.4 μm) and 6200–7800 cm⁻¹ (1.3–1.6 μm) regions. In order to analyze the FTS recordings, a spectra calculation software and a dedicated database of spectroscopic parameters have been built. Using these tools, the HI and H₂O amounts in the vapor are determined from least square fits of the experimental spectra in both regions. It is first shown that the measured and the calculated transmissions are in good agreement. Furthermore, the species amounts determined independently in the two spectral regions are very consistent, confirming the quality of the optical soundings.     

Dewatering of HIx solutions by pervaporation through Nafion membranes

The Sulphur Iodine (SI) thermochemical cycle is a promising route for hydrogen production from water. It requires the processing of a complex solution, termed HI_x, which contains HI, H₂O, very high concentrations of I₂, plus a range of iodide species. Process efficiency can be improved by dewatering the HI_x stream beyond its azeotropic point. This paper reports work aimed at achieving this by pervaporation through a membrane operating at the harsh process conditions involved. Nafion^®117 and Nafion^®212 are investigated using both a batch and a continuous flow arrangement operating with a range of HI–H₂O and HI–H₂O–I₂ solution concentrations, up to the expected SI cycle composition. Permeates of almost pure water and reasonable fluxes are seen, and good correspondence is observed between the results from the two rigs. Consideration of the solution-diffusion model suggests that sorption and desorption, as well as diffusion, play a limiting role in the permeate flux.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.     

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co₃O₄ supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H₂-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co₃O₄ loading and the presence of H₂O and CO₂ in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co₃O₄ catalysts in CO PROX reaction. Co₃O₄/Ce_0.9Mn_0.1O₂ with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H₂O and 10 vol.% CO₂ in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N₂ adsorption/desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co₃O₄ dispersibility and the redox behavior of the catalysts. The large reducible Co³⁺ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co₃O₄ and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.     

Hydrogen production by redox of bimetal cation-modified iron oxide

Pure hydrogen can be stored and supplied directly to polymer electrolyte fuel cell by the redox of iron oxide: Fe₃O₄ + 4H₂ → 3Fe + 4H₂O and 4H₂O + 3Fe → Fe₃O₄ + 4H₂. Four bimetal-modified samples were prepared by impregnation. The hydrogen storage properties of the samples were investigated. The result shows that the Fe₂O₃–Mo–Al sample presented the most excellent catalytic activity and cyclic stability. H₂ forming temperature and H₂ forming rate could be surprisingly decreased and enhanced, respectively. The average H₂ forming temperature at the rate of 250 μmol min⁻¹·Fe-g⁻¹ for Fe₂O₃–Mo–Al in the first 4 cycles could be decreased from 469 °C before the addition of Mo–Al to 273 °C after the addition of Mo–Al. The reason for it may be that the Mo–Al additive in the sample can prevent from the sintering of the particles and accelerate the H₂O decomposition due to Mo taking part in the redox reaction. The average storage capacity of Fe₂O₃–Mo–Al was up to 4.68 wt%.     

Influence of Pr substitution on defects,transport, and grain boundary properties of acceptor-doped BaZrO3

We report on effects of partially substituting Zr with the multivalent Pr on the conductivity characteristics of acceptor (Gd) doped BaZrO₃-based materials. BaZr_0.6Pr_0.3Gd_0.1O_3−δ was sintered 96% dense at 1550 °C with grains of 1–4 μm. The electrical conductivity was characterised by impedance spectroscopy and EMF transport number measurements as a function of temperature and the partial pressures of oxygen and water vapour. H₂O/D₂O exchanges were applied to further verify proton conduction. The material is mainly a mixed proton–electron conductor: the p-type electronic conductivity is ∼0.004 and ∼0.05 S/cm in wet O₂ at 500 and 900 °C, respectively, while the protonic conductivity is ∼10⁻⁴ S/cm and ∼10⁻³ S/cm. The material is expectedly a pure proton conductor at sufficiently low temperatures and wet conditions. The specific grain boundary conductivity is essentially equal for the material with or without Pr, but the overall resistance is significantly lower for the former. We propose that replacing Pr on the Zr site reduces the grain boundary contribution due to an increased grain size after otherwise equal sintering conditions.