Diffusion and Precipitation Processes in Iron-Based Silica Gardens

URN to cite this document:: urn:nbn:de:bvb:355-epub-377131
DOI to cite this document:: 10.5283/epub.37713

Glaab, Fabian ; Rieder, Julian ; Garcia-Ruiz, Juan Manuel ; Kunz, Werner

; Kellermeier, Matthias

License: Creative Commons Attribution Non-commercial 3.0
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Date of publication of this fulltext: 18 Sep 2018 06:31

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Abstract

Silica gardens are tubular structures that form along the interface of multivalent metal salts and alk. solns. of sodium silicate, driven by a complex interplay of osmotic and buoyant forces together with chem. reaction. They display peculiar plant-like morphologies and thus can be considered as one of the few examples for the spontaneous biomimetic self-ordering of purely inorg. materials. Recently, we could show that silica gardens moreover are highly dynamic systems that remain far from equil. for considerable periods of time long after macroscopic growth is completed. Due to initial compartmentalization, drastic concn. gradients were found to exist across the tube walls, which give rise to noticeable electrochem. potential differences and decay only slowly in a series of coupled diffusion and pptn. processes. The effect of the nature of the metal cations on the dynamic behavior of the system has been studied. The authors have grown single macroscopic silica garden tubes by controlled addn. of sodium silicate sol to pellets of iron(II) and iron(III) chloride. In the following, the concns. of ionic species were measured as a function of time on both sides of the formed membranes, while electrochem. potentials and pH were monitored online by immersing the corresponding sensors into the two sepd. soln. reservoirs. At the end of the expts., the solid tube material was furthermore characterized with respect to compn. and microstructure by a combination of ex situ techniques. The collected data are compared to the previously reported case of cobalt-based silica gardens and used to shed light on ion diffusion through the inorg. membranes as well as progressive mineralization at both surfaces of the tube walls. These results reveal important differences in the dynamics of the three studied systems, which can be explained based on the acidity of the metal cations and the porosity of the membranes, leading to substantially dissimilar time-dependent soln. chem. as well as distinct final mineral structures. The insight gained in this work may help to better understand the diffusion properties and pptn. patterns in tubular iron (hydr)oxide/silicate structures obsd. in geol. environments and during steel corrosion.

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