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First data on elevated natural hydrogen occurrence at a site in SE Sofia, Bulgaria. 2025, Rev. Bulg. Geol. Soc.
This study investigates the presence of naturally occurring hydrogen (H2) within the Sofia Region, focusing on its possible relation to fault-controlled gas migration. Measurements were conducted at the Botanical Garden of the Bulgarian Academy of Sciences, situated on the Vitosha fault – part of the Iskar fault zone. Hydrogen concentrations were determined using a Dräger X-am® 8000 detector at a depth of 0.8 m, following procedures previously applied for radon and soil gas permeability. Five sampling points were examined, including sites located directly on and away from the fault zone. Elevated hydrogen concentrations of up to 200 ppm were recorded along the fault, whereas peripheral points showed values between 15 and 25 ppm. These results indicate a clear hydrogen anomaly spatially correlated with the fault trace, suggesting ongoing gas migration through a hydraulically active structural zone. Our findings represent the first field evidence of natural hydrogen occurrence in the Sofia Region.
Electrical properties and conductivity mechanism for Ca2KH7(PO4)4·2H2O and Ca2(NH4)H7(PO4)4·2H2O
For the first time the acid salts Ca2KH7(PO4)4·2H2O and Ca2(NH4)H7(PO4)4·2H2O are studied as potential proton conductors by complex impedance spectroscopy. Their electrical and dielectric properties, as well as conductivity are examined as a function of frequency (10 Hz–10 MHz) at various temperatures (293 K–363 K). The complex impedance spectra showed semicircle arcs depending on temperature indicating conductivity through grain boundaries with semiconducting behavior. It was found that for the two salts in the investigated temperature range the ac conductivity follows Joncher's power law and dc conductivity follows the Arrhenius behavior with activation energy 0.552 eV for Ca2KH7(PO4)4·2H2O and 0.713 eV for Ca2(NH4)H7(PO4)4·2H2O. The results were confirmed with electric modulus analysis. The conductivity mechanism of the two compounds was also investigated and the experimental results were analyzed using various theoretical models. It has been established that the overlapping large polaron tunneling (OLPT) was the best model to explain the proton conduction of both acidic salts. Comparison of the conductivity with other phosphate proton conductors was made.
Characterization of novel nickel cobaltite based bifunctional catalysts for carbon free gas diffusion electrodes for Zn-air rechargeable batteries performance
Zinc air batteries are reputedly known as a reliable option for sustainable energy storage due to their high energy density, low cost, high efficiency and environmental friendliness. Indeed, some problems are encountered in practice and need to be overcome with the eventually slow oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The integration with an oxygen electrocatalyst was identified as the most critical component inevitably determining the operation of a rechargeable Zn-air battery, as its performance depends on the cathode material. Spinel-structured base metal oxides have attracted attention for long time due to their satisfactory electrochemical performance and stability for OER in alkaline solution. Herein, we report the performance of the recently developed carbon-free gas-diffusion electrode (GDE) - a mixture of catalyst (Co3O4, NiCo2O4), pure nickel and polytetrafluoroethylene (PTFE) hot-pressed onto a stainless-steel mesh that works as a current collector. The electrochemical characteristics were investigated, including volt-ampere characteristics, charge/discharge tests and electrochemical impedance measurements. X-ray and neutron diffraction combined with scanning electron microscopy were applied to improve the understanding of structural stability and recharge-related properties as an important step in the development of an innovative design of rechargeable zinc-air batteries with high performance carbon-free reversible gas diffusion electrodes. The experimental Zn-air cell tested with NiCo2O4 as a GDE ingredient was cycled at 17 mAcm−2 for 250 hours in an alkaline environment, and the promising performance and mechanical stability shown by the carbon free NiCo2O4-based GDE are addressed. The obtained results are completable with commercial primary Zn-air button cells (for example IEC PR44) which has a capacity of 600 mAh and maximum current of only 22 mA achieved by silver oxide or manganese dioxide-based gas-diffusion electrodes.
Bulgarian Mavrud Wine Under Nanofiltration and Reverse Osmosis: Evaluating the Composition After the Process
This work presents new results and conclusions on nanomembrane filtration and reverse osmosis of Mavrud red wine, produced in Bulgaria. The experiments were focused on lowering the alcohol content while preserving the valuable substances in the wine. Commercially available nanomembranes were used (Alfa Laval NF99HF, Alfa Laval RO99, NADIR NP030P). Two modes of nanofiltration (concentration mode and diafiltration mode, including constant volume diafiltration and two-step diafiltration) and reverse osmosis were employed for this study. The nanofiltration membranes (Alfa Laval NF99HF, NADIR NP030P) used for wine dealcoholization showed high separation effectiveness. Several wine components were recognized as indicators to be monitored during the process: carboxylic acids (citric, tartaric, malic, succinic, acetic); monosaccharides (glucose, fructose); alcohol (ethanol). The monitoring of the named compounds was performed with an HPLC-RID system on an H-charged ion exclusion analytical column. Based on the analysis of the collected samples, it could be stated that the alcohol content in the wine was lowered from 11.8% to 4.3 vol% of ethanol, when the sequential diafiltration mode of operation is used. Content change depends on the type of molecule; for example, in most cases the citric acid is strongly retained (Rej > 90%) by the membrane, whereas the acetic acid could permeate significantly (Rej < 20%). The obtained results present valuable information about the changes in the composition of the Mavrud wine which will aid in the preservation of the chemical composition and valuable substances in the event of future full or partial dealcoholization of this wine variety.
Corrosion mechanism of austenitic stainless steel in simulated small modular reactor primary water chemistry. Metals 15 (2025) 875
In the present paper, impedance spectroscopy was employed to study the corrosion and anodic oxidation of stainless steel (AISI 316L at 280 °C/9 MPa) in contact with the boron-free primary coolant of a small modular reactor at two levels of KOH concentration. Analysis of impedance spectra with a distribution of relaxation times revealed contributions from the oxide layer and its interface with the coolant. Glow-Discharge Optical Emission Spectroscopy (GDOES) was used to estimate the thickness and elemental composition of the formed oxides. A quantitative interpretation of the impedance data using the Mixed-Conduction Model allowed us to estimate the kinetic and transport parameters of oxide growth and dissolution, as well as iron dissolution through oxide. The film thicknesses following exposure agreed with ex-situ analyses. The obtained corrosion and release rates were used for comparison with laboratory and industrial data in nominal pressurized water reactor primary coolants