Uranyl phosphates

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

See also Hydrotrioxides Hydrogen uranyl phosphates, 25 433-434 Hydrogeochemical cycle(s), 26 4-12. 

Complex uranium ores are often associated with phosphate-bearing minerals, and the presence of soluble phosphate has been found to adversely affect the recovery of the uranium. Studies such as those outlined above have established that, although the iron(III)-phosphonato complexes are more reactive, the decreased leaching rate in the presence of phosphate is due to the formation of insoluble, non-conducting layers of uranyl phosphate on the surface of the mineral. 

Chalcolite Cu(U02)2-8H20 Copper uranyl phosphate octahydrate... 

Sodium phosphate solution white precipitate of uranyl phosphate U02HP04, soluble in mineral acids but insoluble in dilute acetic acid. If precipitation is effected in the presence of ammonium sulphate or of ammonium acetate, uranyl ammonium phosphate, U02(NH4)P04 is precipitated. 

ADP = Ammonium Dihydrogen phosphate, NH4H2PO4 AlPO = A framework aluminophosphate HUP = Hydrogen uranyl phosphate KDP = Potassium Dihydrogen phosphate, KH2PO4 KTP = Potassium Titanyl Phosphate, KTi0P04 NASICON = Sodium Superionic Conductor. 

Uranyl Phosphates.—Well-defined ortho- and pyro-phosphates exist, but little is known of uranyl metaphosphate. Several complex uranyl phosphates have been described. 

Complex Uranyl Phosphates.—The following naturally occurring phosphates have been described (see pp. 273-4) ... 

Suzuki and Banfield (1999) classify methods of microbial uranium accumulation as either metabolism dependent or metabolism independent. The former consists of precipitation or complexa-tion with metabohcally produced ligands, processes induced by active cellular pumping of metals, or enzyme-mediated changes in redox state. Examples include precipitation of uranyl phosphates due the activity of enzymes such as phosphatases, formation of chelating agents in response to metal stress, and precipitation of uraninite through enzymatic uranium reduction. 

Figure 51 Phosphate coordination geometries and cocrystallized template from the structure-directed preparation of a layered uranyl phosphate (Francis, Drewitt et al. Chem. Commun. 1998, 279-280).

Figure 52 Coordination environment (a) about uranyl and within and (b) between uranyl phosphate layers in the first organically templated open-framework uranium phosphate, [Et2NH2]2[(U02)5(P04)4] (Danis,...

The structure of a-[(U02)(0H)2] [54] contains sheets of uranyl hexagonal bipyramids (Fig. 17), and the sheet anion topology consists only of hexagons. This is the only uranyl oxide hydrate that contains hexagonal bipyramids, although this coordination type is common in uranyl carbonates, uranyl nitrates, and uranyl phosphates of the phosphuranylite group. 

The uranyl phosphate chain in the structure of Cu2[(U02)(P04)2] is topologically identical to the uranyl arsenate chain found in walpurgite [104], Copper occurs in sixfold (4+2) coordination as elongated tetragonal bipyramids to form face-sharing chains that extend along the [010] direction. The chains of Cu polyhedra share vertices with adjacent phosphate tetrahedra of the parallel uranyl phosphate chains (Fig. 25). 

In the structure of autunite, Ca[(U02)(P04)]2(H20)n, (Fig. 31), Ca is in sevenfold coordination by H2O groups and is further coordinated by two O atoms of the uranyl phosphate sheet at somewhat longer distances. There are two additional H2O groups located in the interlayer where they are held in position only by hydrogen bonding [125]. 

A wider range of structural variation is exhibited by the autunite-type compounds that contain Ba. The barium uranyl phosphate decahydiate mineral uranocircite, Ba[(U02)(P04)]2(H20)io, is likely isostructural with its arsenate analogue, heinrichite, Ba[(U02)(As04)]2(H20)io, (Tables 12 and 13) [128, 131],... 

---