Impact of crystalline and amorphous iron- and aluminum hydroxides on mechanisms of phosphate adsorption and desorption.

Fourier-transform infrared (FT-IR) spectroscopic experiments were carried out during phosphate adsorption on highly crystalline gibbsite, poorly crystalline 2-line-ferrihydrite and amorphous iron-aluminum-hydroxide mixtures in the molar ratio 1:0, 10:1, 5:1, 1:1, 1:5, 1:10 and 0:1. The OH stretching vibrational bands were utilized to analyze changes in structural and surface OH groups during adsorption, because the position of characteristic PO vibrational bands can shift depending on reaction conditions, pH or adsorbed phosphate content. Adsorption and desorption kinetics were studied at pH6 and different initial phosphate concentrations to achieve varying phosphate coverage on the mineral surfaces. For gibbsite the formation of AlHPO4 and Al2HPO4 can be assumed, while for ferrihydrite, a FeHPO4 or Fe2PO4 complex and the precipitation of FePO4 with longer equilibration time were proposed. Fe2HPO4 or a Fe2PO4 surface complex was deduced for Fe-hydroxides, an AlH2PO4 surface complex was identified for Al-hydroxide, and both displayed either hydrogen bonds to neighboring hydroxyl groups or hydrogen bonds to outer-sphere complexes. Fe:Al-hydroxide mixtures with high Al ratios showed a low phosphate desorption rate, while ferrihydrite and the Fe:Al-hydroxide mixtures with high Fe ratios had almost negligible desorption rates. It was concluded that within the weakly associated amorphous FeO(OH) materials, FePO4 precipitated, which was bound by outer-sphere hydrogen bonds. With high Al ratios, desorption increased, which indicated weaker phosphate binding of both inner-sphere and outer-sphere complexes and hence, either no or minor quantities of precipitate. Ferrihydrite showed a more rigid structure and a lower extent of precipitation compared to amorphous Fe-hydroxide.

Clay-associated microbial communities and their relevance for a nuclear waste repository in the Opalinus Clay rock formation.

Microorganisms are known to be natural agents of biocorrosion and mineral transformation, thereby potentially affecting the safety of deep geological repositories used for high-level nuclear waste storage. To better understand how resident microbial communities of the deep terrestrial biosphere may act on mineralogical and geochemical characteristics of insulating clays, we analyzed their structure and potential metabolic functions, as well as site-specific mineralogy and element composition from the dedicated Mont Terri underground research laboratory, Switzerland. We found that the Opalinus Clay formation is mainly colonized by Alphaproteobacteria, Firmicutes, and Bacteroidota, which are known for corrosive biofilm formation. Potential iron-reducing bacteria were predominant in comparison to methanogenic archaea and sulfate-reducing bacteria. Despite microbial communities in Opalinus Clay being in majority homogenous, site-specific mineralogy and geochemistry conditions have selected for subcommunities that display metabolic potential for mineral dissolution and transformation. Our findings indicate that the presence of a potentially low-active mineral-associated microbial community must be further studied to prevent effects on the repository's integrity over the long term.

Living Lithic and Sublithic Bacterial Communities in Namibian Drylands.

Dryland xeric conditions exert a deterministic effect on microbial communities, forcing life into refuge niches. Deposited rocks can form a lithic niche for microorganisms in desert regions. Mineral weathering is a key process in soil formation and the importance of microbial-driven mineral weathering for nutrient extraction is increasingly accepted. Advances in geobiology provide insight into the interactions between microorganisms and minerals that play an important role in weathering processes. In this study, we present the examination of the microbial diversity in dryland rocks from the Tsauchab River banks in Namibia. We paired culture-independent 16S rRNA gene amplicon sequencing with culture-dependent (isolation of bacteria) techniques to assess the community structure and diversity patterns. Bacteria isolated from dryland rocks are typical of xeric environments and are described as being involved in rock weathering processes. For the first time, we extracted extra- and intracellular DNA from rocks to enhance our understanding of potentially rock-weathering microorganisms. We compared the microbial community structure in different rock types (limestone, quartz-rich sandstone and quartz-rich shale) with adjacent soils below the rocks. Our results indicate differences in the living lithic and sublithic microbial communities.