| Heavy metal and radionuclide contamination present significant environmental problems worldwide. Precipitation of heavy metals on membranes of cells that secrete phosphate has been shown to be a possible method of reducing the volume of these wastes, thus reducing the cost of disposal. Towards these goals, the polyphosphate kinase gene from Pseudomonas aeruginosa was overexpressed in its native host, resulting in the accumulation of 100 times the polyphosphate seen in wild type strains. Upon carbon starvation, this polyphosphate was degraded within 48 h and was eventually released as phosphate into the medium. Evidence shows that this degradation is performed via the action of glycogen phosphorylase, using polyphosphate as a source of phosphate.; Upon resuspension in 1 mM uranyl nitrate, nearly all polyphosphate was degraded within 48 h, resulting in the removal of nearly 97% of uranyl ion, compared to a control strain that removed 28%. Transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDXS) suggested that this removal was due to the precipitation of uranyl phosphate on the cell membrane, while time-resolved laser fluorescence spectroscopy (TRLFS) indicated that uranyl initially sorbed to the cell as a hydroxide prior to precipitation.; A naturally occurring polyphosphate cycling organism was identified in a wastewater treatment process known as Enhanced Biological Phosphorus Removal (EBPR). A consortium of wastewater treatment organisms was enriched in a laboratory-scale reactor performing EBPR. Organisms collected after the aerobic phase of this process released phosphate and precipitated greater than 98% of the uranyl from a 1.5 mM uranyl nitrate solution within 24 h when supplemented with an organic acid under anaerobic conditions. TEM, EDXS, and TRLFS were used to identify the precipitate as membrane-associated uranyl phosphate.; The engineered P. aeruginosa was effective against trivalent actinides. However, divalent heavy metal cations remained in solution at high phosphate concentrations, likely the result of chelating species released from the cells actively or during lysis. Phosphate release from cells was unaffected by the presence of doses of gamma irradiation lethal to the bacteria, giving further indication that the enzymes responsible for the degradation of polyphosphate exist prior to carbon starvation. In addition, applicability of such a system in radioactive wastes is possible. |
