In this study, we used a hydrothermal method to synthesize microspheres of Cu2(Mn1−xFex)SnS4 solid solution (X = 1, 0.8, 0.6, 0.4, 0.2, 0). The process was optimized to improve the crystallinity, morphology, and purity of the obtained materials. All samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The following conditions were optimized: A mixture of water and ethylene glycol at the ratio of 1:7 as the reaction medium, polyvinylpyrrolidone (PVP) as the surface ligand, and reaction temperature of 195 °C for 7 days. The product of synthesis precipitated in the form of aggregates of nanocrystals, which form homogeneous, often concentric microspheres with a diameter of 1–1.5 μm. The chemical composition of the product can be well controlled by the chemical composition of the reactants. The compound Cu2(Mn1−xFex)SnS4 forms a continuous series of solid solutions.

Cu2MnSnS4 (CMTS) is acknowledged as an alternative to traditional semiconductors. The structure and microstructure of synthetic CMTS depend on, among other things, the types of sulfur sources used. Traditionally obtained CMTS mostly has a tetragonal structure. In this study, the effect of using thiourea (Tu) or Na2S as a sulfur source on the product structure was compared using hydrothermal synthesis at 190 °C for 7 days (ethylene glycol with water in the presence of poly(vinylpyrollidone) was used as a solvent). When Tu was used, CMTS precipitated in the form of concentric microspheres, 1–1.5 µm in size, consisting of hexagonal (in the cores) and tetragonal (the rims) forms. Most probably, the rapidly formed hexagonal nucleus was later surrounded by a slower-forming rim with a tetragonal structure. In contrast, when Na2S was used as a precursor, microspheres were not formed and a fine crystalline material with a homogeneous tetragonal structure was obtained. This allowed for the choice of micromorphology and product structure during synthesis.

The small stalactites found on the ceiling at level I near the Sutoris shaft in the thirteenth-century historic salt mine in Bochnia, Poland, are mainly composed of mirabilite (Na2SO4·10H2O) followed by blödite (Na2Mg(SO4)2·4H2O). The unique presence of these two minerals in only one location in this old underground mine is attributed to contemporary precipitation from percolating solutions. This can be caused by a combination of at least two factors: a specific and stable microclimate (characterised by a low temperature, high humidity, and relatively strong air circulation which accelerates the processes of evaporation and crystallisation) and the specific chemical composition of the leaking solution (contains a low carbonate and high sulphate content, and characterised by acidic pH (4.8) and intermediate-mineralisation (174,308 mg/L)). The microclimate specified above can be linked to the long distance from the ventilation shaft that pumps the air from the surface to the mine, while the composition of the leaking solution as well as the hydrochemical modelling results obtained with PHREEQC can be directly related to the top anhydrite layer and the overlying secondary cap consisting mainly of claystone, anhydrite, and gypsum. In this study, the challenges underlying the preservation of mirabilite in the underground environment of the salt mine are discussed, in terms of both nature and mining law. Based on the results of detailed geological, mineralogical, and chemical research, appropriate solutions that can be practically applied for the management, preservation, and protection of the mirabilite efflorescence are proposed. The presence of this intriguing mineral, with appropriate protection, can be another geological attraction for tourists visiting this thirteenth-century UNESCO-recognised salt mine.

<p>The wide range of colored wastewater from industries in the aquatic environment poses a great threat to human and animal health and poses a great obstacle to the ecological ecosystem. Therefore, an effective, efficient, and environment-friendly treatment methods are being sought. Hydroxyl- phosphate and arsenate compounds have recently attracted attention for magnetic and photocatalytic applications in photoreactions with visible light.  They can be a promising alternative to TiO<sub>2</sub>, in which the photoabsorption spectrum is in the range of ultraviolet (UV) light owing to its large bandgap, which accounts for only 5% of the sunlight.</p> <p>Here, we present a detailed analysis of the properties of libethenite Cu<sub>2</sub>PO<sub>4</sub>OH - olivenite Cu<sub>2</sub>AsO<sub>4</sub>OH solid solution series, and their photocatalytic activities under visible light. Seven compounds of the solid solution series were successively synthesized by the wet chemical method at 70°C according to the results of Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) tests. The photocatalytic performance of the samples was thoroughly investigated for the degradation of methylene blue MB solutions under visible light and measurements by UV-vis spectroscopy.</p> <p>We demonstrated the useful photocatalytic activity of these complex structures for the degradation of methylene blue (MB) dye under visible-light irradiation. The substitution effect of [PO<sub>4</sub>]<sup>3− </sup>anions by [AsO<sub>4</sub>]<sup>3−</sup> results in changes in the bonds of the OH group, which are the origin of the photocatalytic properties of this material, altering the bond length and geometry.  In addition, these substitutions affected the morphology of the precipitating solids, which changed the surface area of the material. This way the substitution of As with P in the solid solution series affected the photocatalytic properties The MB degradation efficiency after 6h declines from ~ 84 % for the Cu<sub>2</sub>AsO<sub>4</sub>OH and Cu<sub>2</sub>PO<sub>4</sub>OH down to ~81% for intermediate member.  The present work provides insights leading to a better understanding of the photocatalytic performance of Cu<sub>2</sub>PO<sub>4</sub>OH and Cu<sub>2</sub>AsO<sub>4</sub>OH. Thanks to these results it may be beneficial to prepare more efficient photocatalysts based on this material for sunlight photocatalysis, which will also be helpful in designing and preparing novel technologies.</p> <p>This research was funded by NCN research grant no. 2021/41/N/ST10/03566</p>

<p>The most industrial activities, that require organic dye materials for their applications, release a remarkable fraction of effluent water. As a result, serious environmental problems emerge due to the toxicity of many compounds which these industrial processes produce. The development of inexpensive and green methods for degradation of such organic dye laden water constitutes a landmark for the mitigation or even the elimination of the industrial sewage. Among many strategies, photocatalysis is regarded as the most viable one due to its usage of sunlight to decomposing organic pollutants.</p><p>The unlimited applications of nanocrystalline semiconductor materials in all sorts of technological fields, whether photochemical, biological, photovoltaic or photocatalysis have pushed to develop a new assortment of materials featuring novel properties for advanced applications. Semiconductor properties of stannite Cu<sub>2</sub>FeSnS<sub>4</sub> make it a potential candidate for application in photocatalytic industry. In nature, it is a common sulfide mineral which is formed as a result of hydrothermal processes. Its crystal structure allows for numerous substitutions including replacement of Fe by Mn.</p><p>In this report, the photocatalytic activity of  Cu<sub>2</sub>FeSnS<sub>4</sub> (CFTS) and Cu<sub>2</sub>MnSnS<sub>4</sub> (CMTS) synthetic microspheres for degradation of environment polluting dye such as methylene blue (MB) has been explored under UV light illumination. The unique morphology of the as-synthesized nanomaterial is expected to play a major role in tailoring the optical and electrical properties for the possible photo-voltaic application. The photocatalytic activity shows the potential use of this material as an efficient photocatalyst for wastewater treatment.</p><p>Modified stannite was synthesized by hydrothermal method using reactions of metal salts and sulfur in hot ethylene glycol at presence PVP solution in an autoclave at 195<sup>o</sup>C. The crystallinity, structural features, morphology and chemical composition were investigated using XRD, Raman, FTIR, and SEM - EDS. Synthetic Cu<sub>2</sub>(Fe,Mn)SnS<sub>4</sub> solid solutions are composed of spheres 1 – 1.5 µm in size, with rough surface and concentric internal structure. The structure matches hexagonal stannite. The promising optoelectrical and photocatalytic properties of CFTS and CMTS microspheres make them a potential candidate for photovoltaics as well as for effective wastewater treatment, providing further study has to be carried out to improve specific properties.</p><p>Financial support for the research was provided by Polish Ministry of Higher Education grant No. DI2016 004946 under "Diamond Grant" program.</p>