Synthesis of pumice and medical waste incinerator fly ash based phosphate geopolymers for methylene blue dye adsorption: co-valorization, parameters and mechanism

• Published in: Materials advances Vol. 5; no. 21; pp. 8546 - 8563
• Main Authors: Onyango, Collins, Nyairo, Wilfrida, Kwach, Bowa, Shikuku, Victor, Sylvain, Tome, Dzoujo Tamaguelon, Hermann, Rüscher, Claus
• Format: Journal Article
• Language: English
• Published: 28.10.2024
• ISSN: 2633-5409; 2633-5409
• DOI: 10.1039/D4MA00779D

In this study, four geopolymer composites, GP-0, GP-10, GP-20 and GP-30, were synthesized from pumice, an abundant and inexpensive volcanic rock precursor, substituted with fractions of 0, 10, 20 and 30% by weight of medical waste incinerator fly ash (MWI-FA), respectively. The materials were characterized by standard methods (FTIR, XRF, BET surface area measurement, XRD, SEM-EDX and TGA). The materials were morphologically distinct and the specific surface areas (SSA) decreased with an increase in MWI-FA fraction. The adsorption performances of the geocomposites were evaluated in batch mode for the removal of methylene blue (MB), a toxic dye, from water. The study determined that the dye was optimally removed at circumneutral pH, 303 K temperature, 0.6 g/40 mL adsorbent dosage and 30 min contact time. The equilibrium data were best described using the Sips isotherm model. The geopolymers had ∼30 times higher adsorption capacities than pristine pumice. The maximum adsorption capacities of the geopolymers, ∼31 mg g −1 , were indistinguishable despite an increase in MWI-FA indicating that MWI-FA provided new energetically favorable adsorption sites compensating diminished SSA. The adsorption kinetics was best described using the pseudo-second order kinetic model wherein the rate constant ( K 2 ) increased with the MWI-FA fraction suggesting porosity structures with reduced tortuosity. Thermodynamically, the adsorption process was exothermic (Δ H < 0), physical (Δ H and E a < 40 kJ mol −1 ) spontaneous (Δ G < 0) and enthalpy-driven. Adsorption diminished in a saline environment. The exhausted adsorbent was recoverable and recycled twice using hot water before significant loss of adsorption potential. The composite geopolymers present a plausible strategy for stabilization of up to 30% MWI-FA without compromising the adsorptive properties for dye removal from water.