Hydrothermally synthesized potassium hexaniobates powders were used for cobalt ion adsorbent under hydrothermal conditions to evaluate adsorbents for the removal of soluble corrosion-products, mainly Co-60 under pressurized water reactor coolant conditions. Selectivities for transition metal ions increased in the order; Ni2+ < Mn2+ < Co2+ << Fe2+ whereas light brown precipitate took place in the case of Fe2+ ions. The Co2+ adsorption capacities at 473 K for the K4Nb6O17 hydrothernially synthesized at 573, 673 K and the conventional solid-state synthesized K4Nb6O17 were 2.04, 2.25 and 1.98 mmol/g, respectively. Co uptake on the K4Nb6O17 hydrothermally synthesized at 673 K was highest among the K4Nb6O17 adsorbents examined. Co adsorption capacities at 473 K were exceeded compared to the Co-exchange capacity calculated from the composition. In addition, Co desorption amounts from the adsorbent decreased with an increase in the adsorption temperature and the formation of cobalt niobate (Co2Nb5O14) was confirmed by the XRD pattern. Therefore, Co can be fixed in the niobate matrix under hydrothermal conditions. (C) 2007 Elsevier B.V. All rights reserved.