Application of geopolymers synthesized from incinerated municipal solid waste ashes for the removal of cationic dye from water.

In this study, municipal solid waste bottom ash (MSW-BA) and fly ash (MSW-FA) were used as a source of aluminosilicate to prepare geopolymer (GEO) adsorbents (GEO-MSWBA and GEO-MSWFA) for the removal of methylene blue (MB) from water. The effects of temperature, pH, and initial concentration on the MB adsorption onto GEO-MSWBA and GEO-MSWFA were evaluated. The adsorption isotherms parameters and thermodynamics were also determined. Detailed physical and chemical characterizations of the prepared adsorbents were carried out to further understand their impact on MB adsorption. The results from the scanning electron microscopy revealed a uniform granule-sphere like structure on both prepared geopolymers, which would facilitate the MB adsorption onto the adsorbents. The X-ray diffraction allowed observation of the microstructural transformations that occur after the alkaline activation. The surface areas of the GEO-MSWBA and the GEO-MSWFA were recorded as 32.78 m2/g and 4.5 m2/g, respectively. From the Fourier transform infrared, a stretching vibration of the aluminosilicate tetrahedral was observed, which indicated the success of geopolymerization. The prepared geopolymers showed a high capability of MB adsorption from an aqueous solution. The adsorption process was best suited and explained using the Langmuir isotherm model with a maximum adsorption capacity of 666.7 mg/g for the GEO-MSWBA (at 25°C) and 769.2 mg/g for the GEO-MSWFA (at 35°C). The positive value of the enthalpy (ΔHo) for the GEO-MSWBA suggested the reaction favored endothermic reaction while the negative value of entropy (ΔSo) indicated a solid/liquid random interaction. On the other hand, the negative ΔHo value for the GEO-MSWFA indicated the reaction followed an exothermic reaction causing energy to be released, the positive ΔSo value indicated a good affinity at the solid-liquid surface. The overall negative value for Gibbs free energy (ΔGo) for both adsorbents suggested the adsorption was spontaneous and feasible. It was also inferred that n- π interaction, direct and indirect hydrogen bond, and electrostatic interaction between the MB and the prepared geopolymers facilitated the adsorption process. The current study shows that the GEO-MSWBA and the GEO-MSWFA have a great potential of removing MB as a cationic dye from water without performing any sort of laborious pretreatments.