An evaluation of coal fly ash as an adsorbent for the removal of methylene blue from aqueous solutions: kinetic and thermodynamic studies

Abstract

In this study the effect of the dose and particle size of the adsorbent, initial dye concentration, initial pH, contact time and temperature were investigated for the removal of by means of fly ash (FA) methylene blue (MB) from an aqueous solution. The FA dose was found to be 2.0 g and the under 270 mesh sized particles were found to be effective particles for adsorption. The adsorption process reached its maximum value at 0.5 mg/L dye concentration and attained equilibrium within 10 minutes. The adsorption isotherm was found to follow the Langmuir model. The estimated adsorption free energy (ΔG^o), enthalpy change (ΔH^o), and entropy change (ΔS^o) for the adsorption process were −37.77 kJ mol⁻¹, −13.44 kJ mol⁻¹ and 122 J mol⁻¹ K⁻¹ respectively at 298 K. The maximum adsorption capacity is 0,12 mg g⁻¹ at 298 K and 0,07 mg g⁻¹ at 398 K. The adsorption process was exothermic, feasible and spontaneous. The positive value of ΔS^o shows the affinity of FA for MB while the low value of ΔG^o suggests a physical adsorption process.

Graphical Abstract

Keywords:

• Adsorption
• dye
• fly ash
• langmuir isotherm
• methylene blue