3D-printed red mud/metakaolin-based geopolymers as water pollutant sorbents of methylene blue

This work reports a novel management strategy for a hazardous waste derived from the alumina industry, known as bauxite residue or red mud (RM). Herein, and for the first time, RM-containing porous structures were prepared by direct ink writing (DIW) and then used directly to extract methylene blue (MB) from synthetic wastewater. The lattices were prepared using a 50:50 wt% blend between RM and metakaolin (MK). The incorporation of RM was evaluated by comparing with the MK-based printed structure. Despite the high residue amount, the lattices showed high compressive strength (10.7 MPa), high open porosity (62.40%), very high specific surface area (55 m2/g), and excellent stability throughout the tests (leaching, sorption, and thermal regeneration). In the batch adsorption tests, the impact of contact time, and dye concentration was evaluated. At the optimized conditions ([MB]0 = 50 mg/g; contact time: 48 h) the RM/MK-based structures showed a MB uptake of 19.96 mg/g, ranking them amongst the best performing bulk-type (not powders) geopolymer adsorbents. Furthermore, lattices were successfully regenerated and reused (up to ten cycles) without compromising their performance. Their excellent performance was also corroborated under continuous-flow column experiments. These promising results demonstrate the potential valorisation of a hazardous waste in wastewater treatment.