Synthesis, characterization and application of acid and alkaline activated volcanic ash-based geopolymers for adsorptive remotion of cationic and anionic dyes from water

Abstract

In this work, two geopolymers were prepared by acid activation (GPAc) and alkaline activation (GPAl) approaches using volcanic ash (VA) as the precursor material. The geosorbents were characterized by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Fourier-transform infrared and Brunauer-Emmett-Teller (BET) surface area analyses and their comparative adsorptive potential for anionic (Eriochrome Black T/EBT) and cationic (methylene blue/MB) dyes from water is herein reported for the first time. GPAl and GPAc had 74.47 and 42.75 m2/g BET surface areas, respectively. Equilibrium adsorption data were described by the isotherm models in the order Freundlich > Sips > Langmuir model. Relative to acid activation (GPAc), alkaline activated geopolymer (GPAl) had the highest adsorption capacity for both MB and EBT, estimated at 952 mg/g and 251 mg/g, respectively, consistent with the BET surface areas. The adsorption rates were best described by the pseudo-second order (PSO) kinetic model. GPAl exhibited faster adsorption kinetics than GPAc as denoted by the adsorption half-life, t1/2. The thermodynamic functions indicated that the adsorption of both MB and EBT onto the geopolymers was spontaneous, exothermic and feasible. The adsorption was pH-dependent implying electrostatic interactions is the dominant driving mechanism of adsorption. The results show that alkaline activated geopolymers are better sorbents for dye removal than acid activated geopolymers under the examined conditions.