Cadmium (II) distribution in complex aquatic systems containing ferrihydrite, bacteria and an organic ligand: The effect of bioactivity

Highlights: → Cd2+ and ferrihydrite did not inhibit H2Lp degradation by Comamonas spp. → In systems lacking ferrihydrite, Cd2+ sorption onto live bacteria exceeded dead bacteria for pH > 6. → With ferrihydrite present, no difference in Cd2+ sorption to live and dead bacteria was observed. → Cd2+ sorption to ferrihydrite and bacterial was successfully modeled by neglecting H2Lp degradation. - Abstract: The distribution of Cd2+ in the presence of phthalic acid (H2Lp), ferrihydrite and bacteria (Comamonas spp.) was investigated in biologically active systems involving H2Lp biodegradation. Tests showed that Cd2+ sorption onto bacteria, ferrihydrite and bacteria-ferrihydrite mixture increased with pH in all systems, irrespective of H2Lp degradation or not. The use of bacterial growth medium, Bushnell Hass Broth modified for low phosphate, had negligible effect on Cd sorption. In the presence of ferrihydrite, no difference was observed between Cd2+ sorption in the ferrihydrite-live bacteria and in the ferrihydrite-dead bacteria systems as ferrihydrite proved to be the dominant sorption phase. Cadmium sorption to ferrihydrite and to bacterial cells was described using the diffuse layer model and a nonelectrostatic 4-site model, respectively, which were developed for systems lacking H2Lp degradation. For systems experiencing H2Lp degradation this modeling approach predicted the general trend of Cd2+ sorption-edge shift and gave good fits to the observed sorption data. The results obtained demonstrate that Cd2+ sorption in the biologically active system was reasonably estimated by a model developed for biologically inactive systems, although uncertainty exists due to processes involving H2Lp biodegradation products and changes in the bacterial population.