Nickel biosorption by free and immobilized cells of Pseudomonas fluorescens 4F39:: A comparative study

The biosorption of nickel ions on Pseudomonas fluorescens 4F39 free cells or immobilized cells in beads of agar (biobeads) has been studied in batch experiments to determine the effect of cell immobilization on the metal accumulation properties of both systems. Bacterial cells were immobilized in agar beads following the interphase technique. When free cells were used, the sorption equilibrium was reached in 5 min but with biobeads it took 24 hr as a consequence of metal diffusion. The pH of the Ni(2+) solution was found to be critical for Ni(2+) accumulation, the optimum being 8, although the magnitude of this effect was lower in immobilized cells. The equilibrium data have been analysed using the Langmuir adsorption model. The q(max) of free cells, immobilized cells and biobeads was 145, 37 and 7.6 mg Ni(2+)/g dry sorbent, respectively. The removal capacity of free cells and immobilized cells increased when the cell concentration decreased. The maximum removal efficiency of biobeads was obtained when the cell concentration was 1.43 mg dry cells/mL Ni(2+) solution. The agar concentration in biobeads affected the Ni(2+) accumulation, the optimum being 2%. Desorption of Ni(2+) with 0.5 mM dipicolinic acid was efficient. Cycles of accumulation/desorption resulted in a loss of non immobilized cells. An increase of the removal efficiency from the first cycle of accumulation/desorption was observed with biobeads.