Effect of Al source and alkali activation on Pb and Cu immobilisation in fly-ash based “geopolymers”

Solidification/stabilisation technologies are attracting great interest from mining and energy industries alike, to solve their pressing waste disposal problems. “Geopolymers”, in particular, are becoming one of the more popular solidification/stabilisation methods since they can be applied to a variety of waste sources at low cost, yielding added-value products. However, the effect of Al source on the solidification/stabilisation of heavy metals within fly ash-based Geopolymers, has received little attention. This study examines the effect of variable Al source and alkali-activator on the final properties of fly ash-based Geopolymers as characterised by compressive strength testing, infrared and X-ray diffraction analyses. Leaching tests were performed to determine the efficiencies of Pb and Cu immobilisation, which were compared to the initial properties of the Al source (e.g. particle size, cation exchange capacity, total extractable cation concentration and suspension yield stress). It was observed that Pb was generally better immobilised than Cu. In addition, the total extractable cation concentration of the Al source greatly affected the efficiency of Pb immobilisation while the physical properties of the Al source (suspension yield stress and eventual compressive strength) determined the efficiencies of Cu immobilisation. For both metals, NaOH activation was the most favourable method for metal immobilisation, however, a clear mechanism of adsorption remains elusive.