| Abstract |
Geothermal energy systems offer an appealing, sustainable, and renewable energy source. However, a significant challenge faced by many geothermal reservoir waters is the occurrence of metal sulfide and metal silicate scaling and deposition, which pose serious obstacles to the efficient operation of geothermal installations. The formation, crystallization, and deposition of these inorganic salts primarily stem from their extremely low solubilities, characterized by low thermodynamic solubility constants (Ksp), and in some cases, their inverse solubility properties. This study focuses on several common types of geothermal scale, including iron sulfides, iron silicates (both ferrous and ferric), silica, iron oxy-hydroxides, and elemental sulfur. From a geology point of view, both iron silicates and silica (quartz) constitute a family of widely studied and well characterized geological formations. However, although these terms are commonly used by water treatment professionals to indicate any salt or deposit that contains both Si and Fe, the true identity of such deposits remains elusive.
In reality, these water-formed deposits actually resemble amorphous silica with Fe2+ and/or Fe3+ ions entrapped in its colloidal matrix and bear little resemblance to their geological counterparts. The aim of our approach in tackling such scaling issues is the systematic study of the influence of chemical additives in the presence of Fe2+ and/or Fe2+ ions and the correlation (when possible) of chemical structure with inhibition chemistry. Notably, this is the first mention of the distinct influence of ferrous vs. ferric ions on silica formation. Iron sulfide(s) (with a Ksp), value of 8.0 × 10-19 mol2/L) are among the metal sulfides that pose problematic issues. Their extremely low Ksp), values allow immediate precipitation at very low metal and sulfide concentrations. Classic threshold inhibition techniques, as well as dispersion approaches, are useful strategies to control their formation and/or deposition. This can be achieved by nucleation/crystallization inhibition at the early formation stages or by tackling their deposition on metallic surfaces by reducing scale adherence. A reliable protocol was established that ensured reproducibility and reliability. It was based on the following principles: (a) Visible iron sulfide precipitates should form rapidly, (b) these should appear as black solids at the bottom of the reaction vessel, and not dispersed throughout the solution volume (this is needed for application of dispersion mitigation methods), (c) reproducible results, (d) been in agreement with the geothermal water chemistry. The sulfur deposition in the geothermal system mainly consists of elemental sulfur (S0). Elemental sulfur is an oxidation product of H2S originating from the steam. Chemically and biologically, H2S in the steam can be transformed into elemental sulfur (S0), thiosulfate (H2S2O3), and sulfate (SO42-). Elemental sulfur (S0) is insoluble, while sulfate (SO42-) is highly soluble. To our knowledge, there are no reports on tackling elemental sulfur fouling. Elemental sulfur is highly water insoluble at ambient temperature. Thus far, achieving the threshold inhibition of elemental sulfur formation using chemical additives that stabilize sulfur and retain it soluble has been impossible. All dispersion efforts of sulfur have failed in our experiments. Currently, the only effective method for sulfur mitigation is mechanical cleaning of scaled surfaces, and no literature reports are known on the removal of sulfur by chemical means. These results are the first examples of sulfur removal from surfaces by use of chemical additives.
Based on a plethora of experimental data, a number of useful functional insights have been generated, which add to building a more complete and comprehensive picture of the mechanism of iron silicates, iron sulfides and sulfur formation and control.
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