Adsorption affinity of heavy metals onto bacteria may be related to the electronegativity of metal ions as described by other studies (Merdy et al., 2009; Seco et al., 1997), which reported that attraction between metals and bacteria is higher with greater electronegativity. The selectivity in both single and multi-component solutions is higher for Pb2+ than for Cu2+. In addition, the high ionic radius of Pb2+ (1.12 ´ A)˚ must induce a quick saturation of adsorption sites due to steric hindrance compared with Cu2+ (0.70 ´ A). ˚ These facts could explain the reactivity of Cu and Pb with bacteria surface sites. Pb2+ adsorbs onto calcite and, thus, the Pb2+ ions move into Ca2+ sites, despite the large ionic radius of Pb2+ relative to Ca2+ (Sturchio et al., 1997).