Interaction Processes between Biota and Natural Inorganic Systems: Environmental Applications

Bio-mediated and chemical processes for Arsenic uptake in calcite were investigated by means of synchrotron radiation techniques (XAS, CTR, RAXR) coupled with more traditional mineralogical analysis (XRD, XRF). The processes are presented here in two distinct works. Regarding the Arsenic uptake in bacterial calcite, our study began with the characterization of Bullicame Hot Springs (Viterbo, Italy) where naturally As-enriched travertines precipitate. From such hot springs bacteria were sampled and cultured in laboratory, selecting the strains which were more resistant to high As concentrations and were able to precipitate calcite. Among the 65 strains isolated, B.licheniformis was selected for bio-mediated calcite precipitation experiments both in liquid and solid medium. Bacterial calcites were characterized by means of XRD, XRF and XAS in order to investigate As trapping. The main outcomes regards the positive effect of Mg in incorporating As in calcite, the effect of As on calcite structure, and the role of bacteria and their living environment in regulating the As cycle. In particular we demonstrate that bacterial precipitation process allow Arsenite uptake in calcite differently form the chemical process. Medium properties and the presence of Magnesium have been observed to be critical parameters of this process. Regarding the chemical process we focused both on Arsenic and Mercury, two environmentally dangerous elements. Uptake experiments on calcite-(104) surface were performed by means of in situ experiments with liquid cell AFM and X-Ray Reflectivity techniques (CTR and RAXR), in order to investigate in depth the adsorption and incorporation during calcite-(104) epitaxial growth. Regarding Arsenic we observed that uptake process results efficient only for Arsenate which partially influences calcite surface structure, while Arsenite uptake is almost negligible. More in detail, our results suggest that Arsenate uptake passes through surface adsorption and subsequent ordering in calcite crystal lattice substituting carbonate during crystal growth. Regarding Mercury, we observed the effective uptake in calcite but our results do not clearly resolve if Hg2+ substitutes Ca2+ in calcite crystal structure. On the other hand we found that Mercury trapped in calcite seems to be unstable causing surface relaxation with mineral aging with re-ordering of surface structure.