Geopolymer beads and 3d printed lattices containing activated carbon and hydrotalcite for anionic dye removal

Abstract

The use of geopolymers as adsorbent materials has been increasingly investigated in the last few years, thanks to their highly intrinsic mesoporosity and ion exchange capability. As they consolidate following a condensation reaction from a liquid slurry, they can easily be used as a binder for widely known adsorbents, such as zeolites or activated carbons. Such combination is very promising for applications in the field of water treatment. Geopolymer slurries consolidate at room temperature and can be easily shaped using different fabrication techniques.

In this work, the performance of geopolymer composites with the addition of activated carbon and hydrotalcite for the removal of an organic anionic dye (Orange II) from water was investigated. Beads produced by freeze-drying were compared with lattices produced by Direct Ink Writing. For both beads and lattices, the geopolymer matrix had the following molar ratios: SiO2/Al2O3 = 3.8; Na2O/Al2O3 = 1. H2O/Al2O3 ratio was adjusted to the different process. The different fillers were added to the slurries up to an amount of 27%wt (dry basis).

Beads were produced by dripping the slurries into a liquid nitrogen bath and by freeze-drying the resulting beads for two days; the geopolymerization was finally completed at 75 ºC and 100% relative humidity. The slurries prepared for Direct Ink Writing contained also sodium bentonite as a rheological agent to enhance the pseudoplasticity of the ink. The lattices were printed with a nozzle with a diameter of 840 µm and consisted of a 0º–90º stacking of layers of filaments with 0.8 mm spacing, which resulted in a design porosity of 50 vol%; the geopolymerization was completed at 75 ºC for 2 days and 100% relative humidity.

Physical and chemical characterizations were performed on both beads and lattices (porosity, morphology and crystallinity, compressive strength); the removal efficiency of Orange II dye was evaluated focusing on the adsorption capacity and kinetics.