Adsorption of anionic and cationic dyes from aqueous solutions on fly ash-based porous geopolymer

Fly ash, solid waste from coal-fired power plant, had been utilized as raw material for porous geopolymer by alkaline activation and addition of hydrogen peroxide (H2O2) blowing agent. Porous geopolymer had higher surface area and total pore volume compared to fly ash and geopolymer without blowing agent, namely 45.511 m2 g-1 and 0.05131 cc g-1, respectively. Porous geopolymer was applied as adsorbent for anionic dyes Eriochrome Black T (EBT) and cationic dyes Methyl Violet (MV) from aqueous solutions. In this paper, factors affecting adsorption process such as adsorbent dosage, pH, time, and initial concentration were studied, in addition to adsorption kinetics and isotherm studies. Adsorbent dosage, time, and initial concentration factors had the same effect on the adsorption process for both EBT and MV dyes. The optimum removal efficiency was obtained at adsorbent dosage of 2 g L-1 and adsorption time of 90 minutes. The increase of the initial concentration of dyes would decrease the removal efficiency. For pH factor, adsorption of EBT dyes was better at pH of 2, while adsorption of MV dyes was better at pH of 10. Both adsorption of EBT and MV dyes by porous geopolymer followed pseudo-second-order kinetics model and Langmuir isotherm model with maximum adsorption capacity of 45.454 mg g-1 and 49.261 mg g-1, respectively.