Biofilm-immobilised Citrobacter sp. removed uranyl ion from flows supplemented with glycerol 2-phosphate. The metal uptake mechanism was mediated by the activity of a cell-surface bound phosphatase that precipitated liberated inorganic phosphate with uranyl ion as HUO2PO4 . 4H(2)O at the bacterial surface. A modified integrated form of the Michaelis-Menten equation is proposed to describe the removal of metal ion by a columnar bioreactor, where the efficiency of metal removal is semi-quantitatively related to the input flow rate, the total enzyme loading (E(0)) and the bioreactor activity. With biofilm-immobilised bacteria, E(0) was further divisible (split) into subparameters of phosphatase titre per bacterium and total biomass surface area. Varying the split E(0) and the reaction temperature modified the bioreactor performance. The immobilised bacteria retained high metal loads without loss in steady-state activity. Accumulated metal was recovered as a concentrated solution.