Enzymically-enhanced extraction of uranium from biologically leached solutions

Many wastes contain heavy metals which are toxic and refractory: further problems arise in the production and discharge of waste radionuclides which have additional radiotoxic effects on the biosphere. Currently the problem may be tackled in four ways: (i) direct chemical methods; (ii) electrochemical treatments; (iii) ion exchange and biosorption methods; (iv) intracellular sequestration by growing microbial cells. A hybrid approach exploits the advantages of processes (iii) and (iv) with the disadvantages of neither. In this context, a biotechnological process for removing and recovering heavy metals from aqueous solutions has been evaluated at low pH. Metal uptake relies upon the in situ cumulative deposition of insoluble metal phosphate tightly bound to the cell surface of Citrobacter N14. Localized high loading of phosphate is contributed via a phosphatase-catalysed hydrolysis of an organic phosphate ‘donor’ molecule added to the metal solution with precipitation of metals (M) as cell-bound MHPO4. The present work reports on the potential of this immobilized microbial biomass for uranium recovery from the dilute uranium acid drainage of ENUSA mine in the Ciudad Rodrigo district of Spain. A range of supports (organic and inorganic) and immobilization methods for Citrobacter cells have been screened. Finally, biofilm and entrapped cells on polyurethane foam and cells covalently immobilized on silanized and glutaraldehyde coated Al2O3 Raschig rings were chosen, characterized and evaluated for stability and suitability for large scale use. The physico-chemical monitoring and chemical composition of naturally bioleached waters from ENUSA mine were evaluated and improved methods for flow injection analysis (FIA) of uranium and inorganic phosphate were developed. Optimization studies of the operational conditions and performance of a plug flow bioreactor of immobilized Citrobacter for uranium removal at 30°C gave the following conclusions: (1) a working pH of 4•5 in the absence of any metal complexing agent (citrate) has been established, (2) a [glycerol 2-phosphate]/[UO22+] ratio of 39 is necessary for an optimum uranium removal; (3) at a flow rate of 50 ml/h (residence time of 1•4 h) the efficiency of removal was 50%.