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.

Automated summary

This study presents a new management strategy for bauxite residue or red mud (RM), a hazardous waste derived from the alumina industry. Porous structures containing RM were prepared for the extraction of methylene blue (MB) from synthetic wastewater. The structures exhibited high compressive strength, high open porosity, large specific surface area, and excellent stability. Batch adsorption tests showed that the RM/MK-based structures had a high MB uptake, ranking them among the best performing geopolymer adsorbents. Furthermore, the structures were successfully regenerated and reused without compromising their performance.