Porous geopolymer spheres as novel pH buffering materials

The production of biogas from anaerobic digestion is emerging as an alternative to fossil fuels due to its lower carbon footprint. However problems such as low methane yield and process instability arise when using low pH waste streams, hindering the technology widespread. In this study, porous metakaolinbased and biomass fly ash-based geopolymer spheres were produced using a simple and low cost technology. Afterwards, the pH buffer capacity of the produced spheres was evaluated. Results demonstrate that the geopolymers alkalis leaching can be controlled by the content of fly ash in the geopolymers, higher content promoting greater leaching. The fly ash-based spheres showed superior pH buffer capacity than their metakaolin-based counterparts, with a narrow pH fluctuation (1.6-1.9) over the 30 days measurements. The maximum leaching of hydroxyl ions from the geopolymer spheres (0.0317 mol/dm(3) g) demonstrates the high pH buffer capacity of this innovative material, suggesting their use as pH regulator in anaerobic digestion. (C) 2016 Elsevier Ltd. All rights reserved.