| dc.description.abstract | Bromocresol green (BCG) dye is extensively used in the textile and medical fields. However, the excessive and uncontrollable usage of this dye has resulted in contamination of water sources, endangering both aquatic life and human beings, hence the need for an urgent remedy. Geopolymers have been proposed as suitable adsorbents for removing various water pollutants due to their excellent adsorption capabilities compared to conventional treatment methods. The adsorption rates and capacities of geopolymers have been shown to depend on their structural, morphological, and compositional characteristics, which can be controlled by preparation conditions. Nonetheless, there is no available data on the influence of curing temperature on the structural, morphological and composition of solid waste incinerator fly ash (SWI-FA) based geopolymer adsorbents. Furthermore, although SWI-FA geopolymer-based adsorbent has been used in the removal of cationic, methylene blue dye from water, no study has reported on its interaction with an anionic dye. Thus, the aim of this work was to determine the suitability of SWI-FA for geopolymer production, and the effect of curing temperature on the composition, structural, morphological, and adsorption characteristics of SWI-FA based geopolymers for the removal of bromocresol green (BCG), an anionic dye from water. Alkali-activated SWI-FA geopolymer samples (GP30, GP50, GP70 and GP90) were prepared at curing temperature of 30 ° C, 50 ° C, 70 ° C, and 90 ° C, respectively. The SWI-FA and the geopolymers were characterized for morphology, elemental composition, functional groups, crystalline phases, and pH of point of zero charge (pHPZC). The effect of adsorption parameters namely initial dye concentration, pH, contact time, and temperature were examined. The EDS analysis results indicated presences of C, O, Al, Si, and Ca elements in SWI-FA, GP30, GP50, GP70, and GP90, and a variation in Si/Al ratio in the geopolymers as curing temperature was increased. The SEM results revealed morphological changes with curing temperature. The FT-IR analysis revealed a shift in the main band at 991 cm-1 due to asymmetric vibrations of Si-O-T (T=Al/Si) in SWI-FA to lower wavenumber in the geopolymers. The XRD analysis confirmed changes in crystalline phases of the geopolymers while the pHPZC of the geopolymers remained unchanged at 6.8. The kinetic data were best described by the pseudo-second order model (R2>0.99) while Langmuir isotherm model presented the best fit to the equilibrium data. From Langmuir isotherm model the maximum adsorption capacity for BCG dye uptake increased with curing temperature from 41.70 mg/g to 515.5 mg/g for GP30 and GP90, respectively. The thermodynamic parameters; namely enthalpy (ΔH ), Gibbs free energy (ΔG ), entropy (ΔS ) and activation energy (Ea) indicated that BCG adsorption processes is spontaneous, exothermic, physical (Ea kJ/mol and ΔH kJ/mol) and enthalpy-driven. The adsorption mechanisms controlling BCG adsorption onto the geopolymers included hydrogen bonding and strong electrostatic interactions. The results of this present study highlighted the opportunity of recycling SWI-FA and the potential of the synthesized SWI-FA based geopolymer adsorbents in effectively treating BCG dye-contaminated water. | en_US |
