The adsorption capacity of natural (D1) and chemically structure-modified diatomite (DMA) in the removal of fluorine ions from highly concentrated fluorine solutions (up to 0.3 mol/L) under static conditions at room temperature is studied. The effect of different parameters—solution pH, initial fluorine concentration, sorbent weight, and particle surface charge density—is examined to determine the adsorption properties of DMA under different process conditions. It is shown that the solution pH plays a crucial role in the removal of fluorine from solutions. An efficient removal of fluorine occurs at a pH of 4.5–5.5. Under equilibrium conditions, upon the saturation of the DMA surface with fluorine ions, the adsorption capacity of DMA achieves 58 mmol/g of sorbent; this value is 5.5 times higher than that of unmodified D1. Fluorine adsorption isotherms for DMA samples are derived; equilibrium adsorption data are modeled using a twostage Langmuir model; it is shown that the experimental and calculated data on fluorine adsorption are in good agreement: correlation coefficient R² for the D1 and DMA samples is 0.9952 and 0.9687, respectively. The fluorine adsorption mechanism is studied. X-ray diffraction and chemical analyses, FTIR spectroscopy, potentiometric titration, and adsorption–desorption experiments reveal that the diatomite–NaF–H₂O system is characterized by the occurrence not only of physical adsorption and ion exchange but also of the chemical bonding of the fluorine ions with the active sites of the sorbent surface, i.e., the formation of weakly soluble fluorine compounds with Al on DMA and with Ca on D1 (AlF₃, Na₃AlF₆, СаF₂).