Abstract:
BACKGROUND The hydrochemical characteristics of geothermal water include various information such as geological, structural, fracture, alteration, and environmental changes during its formation. It is particularly important to analyse various components in geothermal water accurately. Tibet is rich in geothermal resources, and some geothermal water contains colloidal particles. It is of great significance to study the morphology, composition and structure of colloidal particles for understanding the relationship between the material sources of geothermal fluids and colloidal particles. The generation and precipitation of colloidal particles can affect the turbidity, conductivity, and partial cation content of geothermal water. It is not easy to obtain naturally formed colloidal particles in geothermal water, so there are few reports on the analysis of colloidal particles in geothermal water.
OBJECTIVES To analyze colloidal particles in geothermal water accurately by multiple methods, and understand the relationship between colloidal particles and opals, and the impact of colloidal particles on the analysis of other elements in geothermal water.
METHODS By comparing the changes in the main components of the water samples before and after filtration, the composition of colloidal particles was inferred. The composition and structure of colloidal particles in the filtrate were characterized by scanning electron microscopy (SEM) and other instruments. The particle size of colloidal particles in geothermal water was measured by laser particle size analyzer, the morphology of colloidal particles was characterized by transmission electron microscope (TEM) and SEM, the characteristic spectrum peak of colloidal particles was measured by Fourier transform infrared spectroscopy (FTIR), the main composition of colloidal particles was analyzed by energy dispersive spectrometer (EDS), and the content of silicon in geothermal water was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) and ultraviolet-visible spectrophotometry (UV-Vis).
RESULTS The colloidal particles in geothermal water are colloidal silica particles. High levels of silica in geothermal water are present in the form of soluble silicic acid and colloidal particles. The average particle size of the colloidal particles is 80.83nm, which is related to the formation of cesium silica and the enrichment of light rare earth elements. The colloidal particle can be analyzed by ICP-OES method, but cannot show color with ammonium molybdate, resulting in a significant difference in the results of using UV-Vis and ICP-OES to determine silicon in this type of water. ICP-OES is a more suitable detection method for geothermal water with high silicon dioxide content. Silicon dioxide can be directly measured whether it exists in the form of metasilicic acid or colloidal particles.
CONCLUSIONS The research shows that the presence of colloidal particles increases the turbidity, but not the conductivity, of geothermal water. The results show that the high content of SiO2 in geothermal water co-exists in the form of metasilicate and colloidal particles, which can be analyzed by ICP-OES but cannot show color with ammonium molybdate. The content of SiO2 colloidal particles in geothermal water can be obtained by calculating the difference between SiO2 measured by ICP-OES and H2SiO3 measured by UV-Vis. SiO2 colloidal particles are relatively stable and not easy to precipitate. SiO2 colloidal particles will adsorb metal ions in geothermal water during the filtration process.
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