Metakaolin/red mud-derived geopolymer monoliths: Novel bulk-type sorbents for lead removal from wastewaters

Abstract

In this study, metakaolin and red mud were used as solid precursors to synthesize geopolymer monoliths coupling hierarchical porosity with suitable compressive strength (4.5 MPa) enabling their use as bulk-type (not powders) sorbents. Then, the lead removal ability of these novel materials was investigated under various conditions. The present work is one of the first investigations evaluating the use of red mud-containing geopolymer monoliths in the extraction of heavy metals from wastewater. Herein, metakaolin was employed as a reactive precursor to overcome the low reactivity of red mud and ensure the production of benign monolithic sorbents. The lead (II) sorption of the metakaolin/red mud sorbents was studied by varying the contact time, lead concentration, pH value and the volume of the solution. Results show that this unexplored approach, involving the use of a toxic waste to produce monolithic bodies able to treat lead-containing wastewaters, is not only feasible, but highly effective. The maximum lead removal capacity of the porous bodies reached 30.7 mg/g (at pH 5, C0 = 600 ppm) being amongst the highest values reported to date for bulk-type geopolymers. The monoliths were also successfully regenerated post-sorption without significantly affecting their performance, and this enables their reuse in multiple sorption cycles provided that a suitable desorption agent is used. These are promising results that might contribute towards the industrial deployment of clay derived geopolymers in wastewater treatment systems, while encouraging a novel and sustainable recycling strategy for the red mud waste.