Liquid purification or separation – Processes – Treatment by living organism
Patent
1995-11-13
1997-01-28
Upton, Christopher
Liquid purification or separation
Processes
Treatment by living organism
210617, 210912, 210632, 4352625, C02F 334
Patent
active
055974848
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of removing a metal from an aqueous solution and is more particularly concerned with a method involving enzymically-mediated metal precipitation (biomineralization). Such a method can be used, for example, for one or more of the following purposes: process, for example water which has been used in the treatment of precious metal ores, soils for the purpose of removing heavy metals from such soils, and be bound in an organic ligand complex, e.g, citrate.
It has been previously proposed in "An immobilised cell bioprocess for the removal of heavy metals from aqueous flows", L. E. Macaskie, J. Chem. Technol. Biotechnol., 49, 357-379 (1990), to effect heavy metal removal using Citrobacter sp. wherein the biomass is cultivated using glycerol-2-phosphate to prepare the biomass cells in the correct physiological state for metal removal. Metal removal relies upon the co-presentation of the metal with glycerol-2-phosphate (G2P) in an amount of up to 5 mM as a phosphate donor for metal accumulation by the immobilised cells. The G2P is enzymically cleaved to glycerol (a potential energy source for the cells) and inorganic phosphate, an efflux of which intercepts the incoming metal and results in the precipitation of crystalline heavy metal phosphate.
It is an object of the present invention to provide a method for the removal of a metal from aqueous solution with improved efficiency.
According to the present invention, there is provided a method of removing a metal having an insoluble phosphate (hereinafter called the "target metal") from an aqueous solution, said method comprising the steps of immobilising in a bioreactor a phosphatase-producing microorganism which has been cultivated using a growth medium containing an assimilable organic source of phosphorus; contacting the microorganism with an element, other than the target metal, having an insoluble phosphate (hereinafter called the "priming element") so as to deposit the phosphate of the priming element on cell surfaces of the microorganism; and subsequently passing the aqueous solution containing the target metal to be removed through the bioreactor in order to precipitate the target metal out of said solution.
We have found it particularly convenient to provide a prolonged period of contact between the priming element and the microorganism before passing the aqueous solution containing the target metal through the bioreactor. Most preferably, the priming element is dissolved or dispersed in a liquid (usually aqueous) medium which is maintained in contact with the immobilised microorganism in the bioreactor. Preferably, the liquid medium is contacted with the microorganism under stationary flow conditions for a period of time of at least 2 hours and more preferably at least 16 hours before the aqueous solution containing the target metal is passed through the bioreactor. The contacting step may be effected by first contacting the microorganism with a liquid medium containing the priming element in a first concentration followed by a storage step wherein the microorganism is maintained in contact with the liquid medium containing the priming element in a second concentration which is lower than said first concentration eg lower by a factor of 100.
Alternatively, the contacting step may be effected in such a way that the priming element is present in an outflow from the bioreactor. By ensuring that, during the contacting step, the priming element is present in the outflow from the bioreactor, the subsequent removal of the target metal from the aqueous solution can be optimised. Ensuring that the priming element is present in the outflow from the bioreactor can be achieved by (a) ensuring that the phosphatase activity of the bioreactor is relatively low or (b) ensuring that the phosphatase activity in the bioreactor is relatively high and at the same time using a high flow rate of the priming element through the bioreactor during the contacting step. The use of either of these techniques (a) and (b) would not be expected to opti
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Macaskie Lynne E.
Tolley Mark R.
British Nuclear Fuels PLC
Upton Christopher
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