Synthesis of hierarchically structured materials: microporous diatoms and nanoporous hydroxyaluminosilicate

Diatomite is well known to remove particulate matter for a broad range of applications due to diatom skeleton structures. Hydrothermally treated diatomite has been demonstrated to be a feasible pathway to produce advanced materials for the removal of aqueous heavy metal ions. However, single-synthesis routes to obtain altered diatomite for simultaneous removal of dissolved and particulate contaminants are still missing. Therefore, hydrothermal experiments at pH 12.8 ([KOH] = 0.1 M), and T = 100 °C and different added Al concentrations (0 mM ≤ [Al] ≤ 11 mM) were conducted. Results reveal that in comparison with previous experiments (Höllen et al. in J Mater Sci Eng B 2(10):523–533, 2012) using less alkaline solutions slower dissolution of diatomite is triggering the formation of nanosized spherical particles (NSP) in the presence of diatom skeleton relicts. NSP ranged between 10 and 50 nm in size and were located within the micropores of the diatoms (100–200 nm). TEM-SAED analyses of NSP indicated nanocrystalline phases with distinct lattice planes of 0.45 and 0.26 nm, which could not be attributed to common aluminosilicates. In accordance with the small crystal size, no X-ray diffraction peaks occurred in the bulk reaction product using powder XRD analyses. Metal ion removal experiments at ambient temperature indicated its high removal capacities for aqueous Cu²⁺, Pb²⁺ and Zn²⁺ ranging from 10 to 99 % (e.g., ion sorption capacities of about 0.02–0.2 mval/g). The synthesized product was also successfully tested for its metal ion removal behaviour using a mine drainage solution containing Cd²⁺, Pb²⁺, Zn²⁺ and Sr²⁺. The developed synthesis path yields a hierarchically structured material based on combined micropores (about 100 nm) from the diatoms and NSP with nanopores (1–5 nm) which is promising for large-scale production to simultaneously remove dissolved and particulate contaminants from aqueous solutions.