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This study synthesizes fly ash-based geopolymers with varying sodium silicate/sodium hydroxide ratios to optimize their structure for methylene blue adsorption. Increasing the Na₂SiO₃ content enhanced polycondensation, resulting in a more compact gel structure and reduced porosity compared to samples with lower alkali activation. The optimized materials effectively removed methylene blue from water, adhering to the Langmuir isotherm model.

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Abstract

In this study, fly ash (FA)-based geopolymers were synthesized using varying proportions of sodium silicate/sodium hydroxide (Na₂SiO₃/ NaOH 10M) solution, ranging from 49% in the 51FA sample to 67% in the 33FA sample, used for the adsorption of methylene blue (MB) in water. Following curing at 60°C for 24 h, the porosity of the resulting geopolymers decreased, attributed to the enhanced polycondensation process driven by the increased Na₂SiO₃ content, which resulted in the formation of a more compact gel structure in the obtained geopolymer. The Weber–Morris model indicated that surface interactions with MB molecules were predominant in the 51FA sample, while pore-filling mechanisms were more pronounced in the 33FA geopolymer. Adsorption experiments revealed that all geopolymer samples conformed to the Langmuir isotherm model, with correlation coefficients approaching unity.

Key findings

  • Increasing Na₂SiO₃ content (from 49% to 67%) drives enhanced polycondensation, leading to a more compact gel structure and decreased porosity.
  • The 51FA sample primarily relies on surface interactions for methylene blue adsorption, whereas the 33FA sample exhibits a more pronounced pore-filling mechanism.
  • All synthesized geopolymer samples conform to the Langmuir isotherm model with correlation coefficients approaching unity.

Keywords

Fly ashGeopolymerSodium hydroxideValorisationAdsorptionCuring (chemistry)

Identifiers

DOI
10.23939/chcht20.01.010
Journal
Chemistry & Chemical Technology
Year
2026