Innovative application for bauxite residue: Red mud-based inorganic polymer spheres as pH regulators

• Published in: Journal of hazardous materials 2018-09, Vol.358, p.69-81
• Main Authors: Novais, Rui M., Carvalheiras, João, Seabra, Maria P., Pullar, Robert C., Labrincha, João A.
• Format: Article
• Language: English
• Subjects: Alkalis leaching; Bauxite residue; Biomass fly ash; Geopolymer; pH buffer capacity
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2018.06.047

[Display omitted] •Red mud-based geopolymer spheres (2.5 mm) were synthesised for the first time.•The aluminosilicate sources of the geopolymer spheres were 100% waste-based.•The spheres pH buffering ability in aqueous medium was evaluated.•High OH− leaching (0.0237 mol/dm3g) and narrow pH drop (2.4 pH units) was attained.•This innovative recycling strategy may significantly reduce red mud disposal. In this study, and for the first time, red mud (RM)-based geopolymer spheres were synthesised, with varying porosity and RM content, and then their use as pH regulators was evaluated. The aluminosilicate sources of these inorganic polymers were 100% waste-based, consisting of a mixture of RM and fly ash wastes. Geopolymer spheres containing up to 60 wt.% RM were successfully produced, while higher RM contents distorted the specimens’ spherical shape. Results showed that alkalis leaching from the spheres over time can be controlled by their porosity, while the RM content induces only minor changes to leaching. The RM-based spheres leached up to 0.0237 mol/dm3g of OH− ions from their structure, this being among the highest values ever reported for geopolymers. This allowed a continuous and prolonged pH buffer capacity with narrow pH decay over the 28 days (2.4 pH units), suggesting the use of the RM-based spheres as pH buffering materials in wastewater treatment and anaerobic digestion systems.