Hydrostatic Pressure Effect on the Transport of Fluoride in a Loose-Pore Geothermal Reservoir.

The concentration of fluoride (F-) in many geothermal waters worldwide exceeds the World Health Organization's drinking water guideline of 1.5 mg/L, highlighting a widespread water quality issue in geothermal systems. It is essential to predict the mobility of F- in geothermal reservoirs compared to that of chloride (Cl-). Column experiments were conducted at 45°C under hydrostatic pressures from atmospheric pressure to 12 MPa to simulate geothermal reservoir conditions, with the computer code CXTFIT 2.1 used to fit the data and determine transport and adsorption parameters of Cl- and F-. The effect of hydrostatic pressure on F- transport was evaluated. Results showed that the Convection-Dispersion Equation (CDE) accurately fits the breakthrough curves of Cl- and higher pressures enhance its dispersion transport. In contrast, F- transport under atmospheric pressure shows rate-limited non-equilibrium, with the Two-Site Model (TSM) fitting F- curves better. However, when pressure exceeds 6 MPa, equilibrium adsorption becomes more pronounced, and the equilibrium CDE better characterizes F- transport. Higher pressure also increases the retardation factor (R) of F-, suggesting more instantaneous adsorption sites under these conditions. The release of hydroxide (OH-) during F- adsorption increases pH values in the effluent notably. Moreover, greater pH fluctuations occur with increasing pressure due to the enhanced adsorption of F- by the packed matrix. The findings would be helpful for understanding the solute transport and adsorption mechanism of fluoride under varying hydrostatic pressures in groundwater and geothermal systems.