Synthesis and characterization of phosphate Geopolymers of pumice and medical waste incinerator fly ash fractions: methylene blue adsorption and recyclability studies.

Abstract

Methylene blue (MB) is a toxic dye used mainly in the textile industry as a fabric colourant. Medical waste incinerator fly ash (MWI-FA) is a toxic waste with unresolved disposal challenges. The general objective of this study was to synthesize composite geopolymers of pumice with fractions of MWI-FA, and to evaluate the effect of the fractions on the physicochemical and adsorptive properties of the geopolymers for the abatement of MB from water. Four composites, GP-0, GP-10, GP-20 and GP-30 were synthesized from pumice, substituted with fractions of 0, 10, 20 and 30% w/w of MWI-FA respectively, followed by a phosphoric acid activation. The adsorptive performance of the geo-composites was evaluated for the abatement of MB, assessing the effect of the MWI-FA fractions on this performance. Material characterization revealed the formation of new functional groups such as –Si-O-P-O-T- and -Si-O-P- bonds, confirming geopolymer formation, and the evolution of new mineral phases tobermorite and heulandite, attributable to the incorporation of MWI-FA. Some physicochemical dissimilarities resulting from the incorporation of the fractions of the adjuvant were also observed. Furthermore, geopolymerization with 10% MWI-FA increased the specific surface area (SSA) of the precursors, but this diminished proportionately with increased MWI-FA fractions. The adsorption kinetics was best described by the pseudo-second order kinetic model, with the rate constant, (K2), increasing linearly with the rate of incorporation of MWI-FA. Thermodynamically, the results show that the adsorption process was enthalpy-driven, exothermic (negative ΔH values) and a physisorption process (ǀΔHǀ and Ea < 40 kJmol-1). Gibbs free energy, ΔG < 0, suggested a spontaneous and feasible adsorption process, with the adsorption spontaneity increasing linearly with temperature. The equilibrium data was best described by the Sips isotherm model. The maximum adsorption capacities of the geopolymers, ~31 mg/g, were indistinguishable despite the decline in SSA with increase in MWI-FA. MWI-FA provided new energetically favorable adsorption sites compensating the diminished SSA. Hot water showed good potential to regenerate the spent geopolymers twice, with up to 76.9% recovery. The adsorption of MB diminished considerably under saline conditions. The composite geopolymers provide an acceptable strategy for stabilization of up to 30% MWI-FA without compromising their chemical stability or adsorptive properties. The study recommends the incorporation of up to 30% MWI-FA, with sufficient aluminosilicate composition, in the pumice-based phosphate geopolymers for the sequestration of MB as a dual waste management strategy.