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A Citrobacter sp. originally isolated from metal-polluted soil accumulates heavy metals via metalphosphate deposition utilizing inorganic phosphate liberated via PhoN phosphatase activity. Further strain development was limited by the non-transformability of this environmental isolate. Recombinant Escherichia coli DH5 alpha bearing cloned phoN or the related phoC acquired metal-accumulating ability, which was compared with that of the Citrobacter sp. with respect to removal of uranyl ion (UO2(2+)) from dilute aqueous flows and its deposition in the form of polycrystalline hydrogen uranyl phosphate (HUO2PO4). Subsequently, HUO2PO4-laden cells removed Ni2+ from dilute aqueous flows via intercalation of Ni2+ into the HUO2PO4 lattice. Despite comparable acid phosphatase activity in all three strains, the E. coli DH5 alpha (phoN) construct was superior to Citrobacter N14 in both uranyl and nickel accumulation, while the E. coli DH5 alpha (phoC) construct was greatly inferior in both respects. Expression of phosphatase activity alone is not the only factor that permits efficient and prolonged metal phosphate accumulation, and the data highlight possible differences in the PhoN and PhoC phosphatases, which are otherwise considered to be related in many respects.

A Citrobacter sp. accumulates uranyl ion (UO2(2+)) as crystalline HUO2PO4.4H2O (HUP), using enzymatically generated inorganic phosphate. Ni was not removed by this mechanism, but cells already loaded with HUP removed Ni2+ by intercalative ion-exchange, forming Ni(UO2PO4)2.7H2O, as concluded by x-ray diffraction (XRD) and proton induced x-ray emission (PIXE) analyses. The loaded biomass became saturated with Ni rapidly, with a molar ratio of Ni:U in the cellbound deposit of approx. 1:6; Ni penetration was probably surface-localized. Cochallenge of the cells with Ni2+ and UO2(2+), and glycerol 2-phosphate (phosphate donor for phosphate release and metal bioprecipitation) gave sustained removal of both metals in a flow through bioreactor, with more extensively accumulated Ni. We propose 'Microbially Enhanced Chemisorption of Heavy Metals' (MECHM) to describe this hybrid mechanism of metal bioaccumulation via intercalation into preformed, biogenic crystals, and note also that MECHM can promote the removal of the transuranic radionuclide neptunium, which is difficult to achieve by conventional methods.

The use of citrate as a chelating agent in decontamination operations is of environmental concern as it can mobilize toxic heavy metals if discharged into the environment. Many heavy metalcitrate complexes are recalcitrant to biodegradation. Citrate-utilizing strains of Pseudomonas aeruginosa and Pseudomonas putida were isolated from a mixed culture which had been maintained with EDTA as the carbon source for 2 years. Citrate (5 mM) was used as the sole carbon source in medium supplemented with 5 mM Cd, Zn, Cu, Fe, Co, or Ni. Removal of the metals from the medium was promoted by the incorporation of inorganic phosphate as a precipitant, with formation of nickel and cobalt phosphates con®rmed by X-ray powder diffraction analysis. The potential of P putida to biodegrade citrate in a nickel±citrate secondary waste was illustrated using a ®ll-and-draw reactor supplied with ef¯uent from a bioinorganic ion exchange column that had been used previously to concentrate nickel from aqueous solution.

Immobilized cells of a Citrobacter sp. removed neptunium and plutonium negligibly from solution using an established technique that used biologically-produced phosphate ligand (Pi) for metal phosphate bioprecipitation. Removal of these transuranic radionuclides was enhanced by prior exposure of the biomass to lanthanum in the presence of organophosphate substrate to form cell-bound LaPO4. Polyacrylamide gel-immobilized cells removed little Np and Pu per se, but preloaded LaPO4 promoted the removal of Np and Pu upon subsequent challenge in a flow-through column. Approximately 2 μg of Np was loaded per 1 mL, column, when the experiments were stopped after 10 mL, with maintenance of approximately 90% removal of the input metal. Transuranic element removal by this technique, generically described as microbially-enhanced chemisorption of heavy metals (MECHM), is via a hybrid of bioaccumulative and chemisorptive mechanisms.

Zirconium in aqueous¯ows was moderately biomineralized by immobilized Citrobacter N14 cells, in the form of gel-like deposits, probably comprising a mixture of zirconium hydrogen phosphate (Zr(HPO 4 ) 2 ) and hydrated zirconia (ZrO 2 ). The simultaneous presence of uranyl ion (UO P P ) did not facilitate zirconium deposition and the biomineralization of uranium itself as HUO 2 PO 4 was repressed by zirconium in the presence of excess inorganic phosphate, liberated enzymatically. Nickel (Ni 2+ ) was not signi®cantly removed from aqueous¯ows by sorption into cell-bound zirconium deposits, although cell-bound hydrogen uranyl phosphate (HUP) facilitated nickel removal via intercalative ion exchange into its polycrystalline lattice. A preformed layer of HUP also promoted zirconium removal, at 100% ef®ciency at pH 2.6, maintained over 38 column¯uid-volumes before saturation.

Immobilized cells of a Citrobacter sp. can remove heavy metals from wastewaters by deposition of metals with enzymatically liberated phosphate. Nickel is not removed effectively by this technique, but Ni2+ can be intercalated into cell-bound, crystalline HUO2PO4 previously deposited enzymatically. This technique for efficient removal of Ni from solution has been generically termed microbially enhanced chemisorption of heavy metals (MECHM). The nickel uranyl phosphate deposits bound to Citrobacter sp. cells immobilized in polyacrylamide gel (PAG) were analyzed using scanning transmission electron microscopy with electron probe X-ray microanalysis (EPXMA) and proton-induced X-ray emission analysis (PIXE). Both methods gave the molar ratios of nickel, uranium, and phosphorus in the deposits as close to 1:2:2 in all analyzed parts of the sample. EPXMA proved that the deposits were localized on the surface of cells inside PAG particles as well as those immobilized on the edge. Small deposits of nickel uranyl phosphate were also found in PAG between the cells, indicating the possible involvement of extracellular polymeric substances (EPS) in the creation of intercellular deposits. These findings confirm the mechanism of MECHM and show that this mechanism operates throughout the immobilized cell matrix. The use of two independent methods of solid-state analysis in a common sample provides validation of both techniques for the spatial and quantitative analysis of biomass-bound elements.

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