Hydrogen uranyl phosphate (HUP) deposited enzymatically on Citrobacter N14 immobilized in polyacrylamide gel removed nickel ions from solution via intercalative ion-exchange into the HUP lattice. Using flow-through columns containing 100 mg dry weight of biomass and similar to 200-250 mg loaded uranium column saturation and breakthrough of Ni2+ occurred after ca. 600 ml, with a total of similar to 30 mg Ni2+ loaded per column, corresponding to a molar ratio of U:Ni of similar to 2:1, in accordance with the identity of the material as Ni(UO2PO4)(2), identified previously. Ni2+ was selectively desorbed using 100 mM sodium citrate-citric acid buffer over 140 ml or a short pulse (5 ml) of 500 mM citrate buffer followed by a water wash, giving a total recovery volume of 80 ml, with a total citrate concentration of similar to 30 mM in the wash solution of the latter. As an alternative eluant which gives no residual BOD NaCl (0.6 M) or seawater gave comparable recovery of Ni2+ to the 0.5 M citrate pulse, but with a Ni2+ recovery volume of 40-50 ml. The concentration ratio of Ni2+-deposition:desorption (vol:vol) was 3-4 fold better with chloride ion than with 100 mM citrate.