[Problem to be Solved] Provided are an adsorbent to adsorb ruthenium from aqueous solution for recovery and/or reuse or removal of said ruthenium, and a method for purifying, for example, sea water and/or water containing sodium ions, magnesium ions, calcium ions, chlorine ions or other ions, polluted with a radioactive element, using said adsorbent. [Solution] The object is achieved through a ruthenium adsorbent comprising manganese in the form of oxides thereof. The adsorbent can further comprise at least one additional transition metal element other than manganese, preferably copper. The adsorbent soaked in water removes radioactive ruthenium or the like through adsorption, and thereby can purify, for example, sea water and/or water containing sodium ions, magnesium ions, calcium ions, chlorine ions or other ions.
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