The eukaryotic diatoms are unicellular algae. They are well known for their filigree micro- and nanostructured cell walls which mainly consist of amorphous silica as well as various organic compounds. However, diatoms are also known to incorporate certain amounts of aluminium into their cell walls. Unexpectedly, enhanced Al concentrations in the Southern Yellow Sea were found to be correlated with a diatom spring bloom. Therefore, we have analyzed the influence of strongly enhanced Al concentrations in the culture medium upon the growth behaviour of the diatom Stephanopyxis turris (S. turris). The uptake and incorporation of Al into the cell walls was monitored. It turned out that S. turris survives aluminium concentrations up to 105.5 μM (2.85 mg/l) in the culture medium. Under the applied conditions, this corresponds to an Al/Si ratio of 1:1. These large amounts of Al had to be offered in the form of bis-tris-chelates in order to prevent uncontrolled precipitation. Under these conditions, the Al/Si ratio in the cell walls could be increased up to about 1:15 as determined by ICP-OES, the highest amount of aluminium found in diatom cell walls yet. Structural characterization of the biosilica by ATR-FTIR and solid-state (27)Al NMR spectroscopy revealed that an amorphous aluminosilicate phase is formed where the aluminium exists as four- and sixfold-coordinated species.

Diatom biosilica may offer an interesting perspective in the search for sustainable solutions meeting the high demand for heterogeneous catalysts. Diatomaceous earth (diatomite), i.e., fossilized diatoms, is already used as adsorbent and carrier material. While diatomite is abundant and inexpensive, freshly harvested and cleaned diatom cell walls have other advantages, with respect to purity and uniformity. The present paper demonstrates an approach to modify diatoms both in vivo and in vitro to produce a porous aluminosilicate that is serving as a potential source for sustainable catalyst production. The obtained material was characterized at various processing stages with respect to morphology, elemental composition, surface area, and acidity. The cell walls appeared normal without morphological changes, while their aluminum content was raised from the molar ratio n(Al):n(Si) 1:600 up to 1:50. A specific surface area of 55 m2/g was measured. The acidity of the material increased from 149 to 320 µmol NH3/g by ion exchange, as determined by NH3 TPD. Finally, the biosilica was examined by an acid catalyzed test reaction, the alkylation of benzene. While the cleaned cell walls did not catalyze the reaction at all, and the ion exchanged material was catalytically active. This demonstrates that modified biosilica does indeed has potential as a basis for future catalytically active materials.

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Algae like diatoms are widely studied as a means to remediate anthropogenically contaminated sites. In the present study, CL (cathodoluminescence) and EDX (energy-dispersive X-ray) spectroscopy in an SEM (scanning electron microscope) were optimized for the detection of Eu(III) sorbed on diatom biosilica. The required stability of biosilica under a focused electron beam was extensively investigated. Using experimentally determined data of thermal properties, the temperature increase within biosilica exposed to an electron beam was simulated by finite element calculations based on results from Monte Carlo simulations of electron scattering. Complementary thermogravimetric studies substantiated a chemical stability of biosilica in a wide temperature range, confirming its suitability for long-lasting SEM investigations. In subsequent EDX measurements, characteristic Eu lines were detected. Eu was found to preferentially accumulate and aggregate on silica fragments. CL spectra were obtained for the Eu(III) reference material, EuCl3. For Eu-loaded biosilica, even parts without detectable Eu signal in the EDX spectra show significant Eu(III) signals in the CL spectra. This highlights the sensitivity of CL in studying f-element sorption. CL data showed that Eu(III) was distributed on the entire surface. In conclusion, this work demonstrates the merit of CL and EDX methods for sorption studies on biogenic materials.

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