In order to apply a seeding method to the removal of silica from geothermal brines, the effects of temperature, initial seed concentration and initial silica concentration on the silica removal were experimentally examined. When silica gel seeds are added to a supersaturated silica solution, amorphous silica is precipitated on the pore surface of the seeds and the silica concentration in the solution decreases. The silica removal ratio, which means the ratio of the removed silica to the excess silica, increases with increasing initial seed concentration and increasing temperature. A high silica removal ratio is obtained for relatively low initial silica concentrations (≤0.7 kg-SiO₂·m^–3). When the initial silica concentration is higher, the silica removal ratio is expected to be enhanced by increasing initial seed concentration. The seeding method is suggested to be effective for the removal of silica in actual geothermal brines.

In this study, arsenate, As(V), and fluoride (F) were simultaneously removed from contaminated water using MgO, Mg(OH)₂, and MgCO₃ as Mg-based adsorbents, as existing studies only focus on their individual removal. The removal performance of As(V) and F followed the order MgCO₃ < Mg(OH)₂ < MgO. Under the test conditions, MgO and Mg(OH)₂ met the environmental standards for As and F (0.01 and 0.8 mg/L, respectively), but MgCO₃ did not. The As(V) removal performance was not significantly affected by an increase in the initial F concentration. It was concluded that As(V) was adsorbed and removed more preferentially than F by Mg-based adsorbents because a considerable amount of F remained even when the majority of As(V) was removed. Most arsenic (As)-adsorption data for MgO fit the Langmuir and Freundlich models, whereas those for Mg(OH)₂ did not fit either model well. Additionally, the As-adsorption data for MgCO₃ fit the Freundlich model but not the Langmuir model. Most of the F-adsorption data for the Mg-based adsorbents fit the Langmuir and Freundlich models. The removal mechanisms of As(V) and F using Mg-based adsorbents were assumed to be predominantly caused by ion-exchange and chemical-adsorption reactions on the adsorbent surface because no magnesium arsenate, magnesium fluoride, or magnesium hydroxide fluoride species were observed in the X-ray diffraction analysis. This research advances the sustainable As–F simultaneous treatment method using inexpensive adsorbents.

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