The biosorption process is designed to remove metallic species, especially those originating from the nuclear reaction process in power plants, for further processing and recovery. The majority of these are the products of nuclear fission of uranium to form lighter elements. During the first several hundred years after the fuel is removed from a reactor, fission products are considered the most hazardous elements to living organisms in the environ­ment. Among these, radiostrontium (Sr-90) is one of the most abundant radioactive components of nuclear waste (Watson et al., 1989). Strontium can be highly mobile in both soils and groundwater systems (Dewiere et al., 2004) and it has a half-life of 28 years.

Due to its chemical similarity with calcium, it is easily incorporated into bone material in mammals. When incorporated in the organisms in this manner, it continues to irradiate localized tissues with the eventual develop­ment of bone sarcoma and leukaemia (Chen, 1997). The main disadvantages of using conventional adsorbents, such as zeolites and synthetic organic ion exchangers, for strontium removal from radioactive waste are: their decreas­ing efficiency at higher pH, and inhibited performance at high sodium concentration (Chaalal and Islam, 2001). Microbial adsorbents, on the other hand, have been shown to possess high capacities for the selective uptake of a range of metals and radionuclides from dilute metal-bearing solutions (Beveridge, 1989; Mullen et al., 1989; Chubar et al., 2008).

In the following sub-sections, the biologically mediated reactions at the surface of cells are explained and the operational conditions for the bioad­sorbents and effects of co-occurrence with other cationic species are exam­ined. Sulfate reducing bacteria are used as an example due to their demonstrated ability in adsorbing a range of metals including palladium (II) and calcium (II).