Rapid Precipitation of Amorphous Silica in Experimental Systems with Nontronite (NAu-1) and Shewanella oneidensis MR-1
NAVAL RESEARCH LAB STENNIS SPACE CENTER MS SEAFLOOR SCIENCES DIRECTORATE
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Nanometer-size 50 nm precipitates of amorphous silica globules were observed in laboratory systems containing nontronite NAu-1, Shewanella oneidensis strain MR-1, and lean aqueous media. Their formation was attributed to the release of polysilicic acids at the expense of dissolving NAu-1, and subsequent polymerization and stabilization mediated by biomolecules. Rapid 24 h silica globule formation was confirmed in the immediate vicinity of bacterial cells and extracellular polymeric substances in all experimental systems that contained bacteria, whether the bacteria were respiring dissolved O2 or FeIII originating from NAu-1, and whether the bacteria were viable or heat-killed. Silica globules were not observed in bacteria- and biomolecule-free systems. Thermodynamic calculations using disilicic acid, rather than monomeric silica, as the primary aqueous silica species suggest that the systems may have been super-saturated with respect to amorphous silica even though they appeared to be undersaturated if all aqueous silica was assumed to be monomeric H4SiO4. The predominant aqueous silica species in the experimental systems was likely polysilicic acids because aqueous silica was continuously supplied from the concurrent dissolution of aluminosilicate. Further polymerization and globule formation may have been driven by the presence of polyamines, a group of biologically produced compounds that are known to drive amorphous silica precipitation in diatom frustules. Globules were likely to be positively charged in our systems due to chemisorption of organic polycations onto silica surfaces that would have been otherwise negatively charged. We propose the following steps for the formation of nanometer-size silica globules in our experimental systems i continuous supply of polysilicic acids due to NAu-1 dissolution ii polysilicic acid polymerization to form 50 nm silica globules and subsequent stabilization mediated by microbially produced polyamines...
- Physical Chemistry
- Geology, Geochemistry and Mineralogy