Coal fly ash-slag and slag-based geopolymer as an absorbent for the removal of methylene blue in wastewater

The fly ash and slag from coal burning were attractive byproducts to prepare geopolymers for the adsorption of methyelen blue in wastewater due to their availability and low cost. Various mixing amounts between them were conducted during geopolymerization with a Na2SiO3/NaOH ratio of 10 M of 2.5 and a curing temperature of 60°C for 24 h. When the amount of coal slag in the geopolymer composition was increased from 0 to 51 %, the surface structures of the resultant geopolymers were much softer and more porous due to the lack of initial material, causing a reduction in the surface area of geopolymers to 119,23 m2/g for 0 % and 5,29 m2/g for 51 %. The adsorption amount of methylene blue performed at pH 12 showed different tendencies on the dependence of contact time for the indivually prepared geopolymer. The uptake amount decreased from 36.2 mg/g to 34.2 mg/g with the enhancement of coal slag in the geopolymer from 0 to 51 % after 180 minutes of immersion in methylene blue solution. In addition, the adsorption mechanism evaluated by FT-IR spectroscopy was observed to involve electrostatic forces formed by hydrogen bonding between hydroxyl groups (Si–OH) and nitrogen atoms in the structure of methylene molecules. This study indicated that coal slag could be a potential material to prepare geopolymers for removing dye pollutants.