Biochemical process for the removal of uranium from acid mine drainages

A biochemical process has been assessed with a view to removing heavy metals from acid mine drainages in which the metal cation removed is accumulated in situ as insoluble metal phosphate on the surface of Citrobacter N14 cells (Roig et al., 1995). The localized presence of inorganic phosphate (Pi) is brought about via the hydrolysis of a “donor” organic phosphate added to the solution of metals with precipitation as MHPO4 bound to the cells. The present work explores the potential of immobilized Citrobacter biomass for the recovery of uranium from the acid drainage waters of the “Faith” mine exploited by ENUSA (Ciudad Rodrigo, Salamanca). A physicochemical characterization of the acid waste-water from ENUSA was carried out and flow injection analysis methods for the determination of uranium and Pi in such water were developed and improved. The efficiencies of chemical precipitation (by the addition of Pi to the acid water) with regard to bioinsolubilization (supplementing the water with an organic phosphate that is (later) hydrolysed to Pi) were investigated and compared. Additionally, the efficiency of chemical and biochemical precipitation as phosphates of uranium present in ENUSA acid drainage water were assessed. Furthermore, the relative importance of chemical precipitation (by the addition of Pi plus alcohol) was established. To do so, a series of mass balances for chemical precipitation and for bioinsolubilization of the metal phosphate was performed. Once the efficiency of the bioprocess as regards the removal of uranium when glycerol-2-phosphate is used as a substrate had been determined, a major question was forthcoming: the search for an efficient and much more economical substrate for the process. In this sense, sodium tripolyphosphate, one of the main components of many formulations of commercial detergents, proved to be a poor substrate, such that the quest for an appropriate substrate (cheap, by-product or waste) remains open.