Asymmetric metakaolin-based geopolymer membranes for microfiltration: synthesis and first characterizations

In this preliminary study, metakaolin-based geopolymer samples were synthesized. For the synthesis of support samples, casting and pressing shaping methods were considered. Moreover, the deposition of a layer by dipping was carried out. Support and selective layer samples produced by casting were made of an aqueous sodium silicate solution, sodium hydroxide, foaming and stabilizing agents, as well as metakaolin precursor, and supports obtained by pressing involved the addition of solid-state anhydrous sodium metasilicate and water to the metakaolin precursor. Both pressed and cast samples were cured at 70°C for 24h and then left at room temperature for 6 days. Initial characterization experiments were conducted on the support samples. Concerning pressed samples, increases in both pressure loads and water concentrations led to increases in their mechanical performance, and decreases in their hydraulic permeabilities and total open porosities. It was therefore concluded that increases in both pressure load and water content leads to a higher compaction of the sample prepared, resulting in lower open porosity and higher mechanical properties. The study of porosity and pore size distribution has revealed that the pressed support pressed sample with 12% of water content at 2 MPa has an open porosity of 40% and a modal pore size value of 23.45 µm and meets the typical porosity of a ceramic membrane support. Results concerning cast samples were less satisfying with respect to pressed samples.The addition of foaming agents resulted in a heterogeneous distribution of pores. The addition of stabilizing agent improved the formation of a more homogeneous pore size distribution. An increase in liquid/precursor ratio led to an increase of the total open porosity. Finally, the slurry prepared with a high liquid/precursor ratio (L/P > 100) proved to produce a selective layer with a thickness of about 100 μm and a relatively good adhesion.