Effect of KOH- and NaOH-modified fly ash on the adsorption of methylene blue and crystal violet dyes: a Box-Behnken design study

Abstract

Coal Fly Ash (FA) and modified fly ash (mFA), treated with either KOH or NaOH, were used to remove methylene blue (MB) and crystal violet (CV) dyes from aqueous solution. Several techniques, including Thermogravimetric analysis, X-Ray diffraction analysis, Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller, and Scanning Electron Microscopy, were used to characterise both FA and mFA. To optimise the adsorption process, the Response Surface Methodology (RSM) with the Box - Behnken design (BBD) was applied. Key parameters such as solution pH (4-10), initial dye concentration (200-300 mg/L), contact time (15-180 minutes), and temperature (30-50 degrees C) were varied to assess their impact on adsorption efficiency. The adsorption followed the Langmuir model and pseudo-second-order (PSO) kinetic model. The adsorption capacity of mFA for MB dye was 49.5 mg/g at 50 degrees C, while for CV dye, it was 495.5 mg/g at the same temperature. The adsorption mechanism involved electrostatic attraction, n-pi interaction, Yoshida hydrogen bonding, and hydrogen bonding. These results highlight the effectiveness of mFA as a high-capacity adsorbent for treating MB and CV dyes in water solutions.