Abstract <p>The efficiency of the treatment of liquid radioactive media, mainly with pH 4.0–10.5, containing boric acid in combination with iron(III) hydroxo species, as well as complexing agents such as oxalic acid and EDTA, to remove ruthenium radionuclides by sorption onto a composite sorbent based on Zr, Ca, and Mg phosphates was determined. The highest distribution coefficients (<i>K</i>_d = 1.68 × 10⁵ cm³/g) and decontamination factor (<i>K</i>_dec = 360) were obtained in solutions containing 5 g/dm³ boric acid and 1 mg/dm³ Fe(III) at pH ≥ 9.0. The presence of EDTA, as well as an increase in the boric acid concentration to 10 g/dm³, leads to a decrease in the treatment performance of the sorbent. Increasing the aging time of model solutions leads to a significant decrease in the efficiency of solution decontamination from ruthenium radionuclides with the sorbent at pH ≥ 9.0. The sorbent was applied to the treatment of real boron-containing liquid radioactive waste at the Joint Institute for Power and Nuclear Research—Sosny complex for liquid radioactive waste reprocessing. The required purification criterion (¹⁰⁶Ru volume activity ˂100 Bq/dm³) is achieved after 2–3 purification stages at the main Fe(III) concentration in the purified liquid radioactive waste of 1 mg/dm³ and pH ≥ 9.</p>