Modeling the mechanism involved during the sorption of methylene blue onto fly ash

Batch sorption experiments were carried out to remove methylene blue from its aqueous solutions using fly ash as an adsorbent. Operating variables studied were initial dye concentration, fly ash mass, pH, and contact time. Maximum color removal was observed at a basic pH of 8. Equilibrium data were represented well by a Langmuir isotherm equation with a monolayer sorption capacity of 5.718 mg/g. Sorption data were fitted to both Lagergren first-order and pseudo-second-order kinetic models and the data were found to follow pseudo-second-order kinetics. Rate constants at different initial concentrations were estimated. The process mechanism was found to be complex, consisting of both surface adsorption and pore diffusion. The effective diffusion parameter Di values were estimated at different initial concentrations and the average value was determined to be 2.063×10−9cm2/s. Analysis of sorption data using a Boyd plot confirms the particle diffusion as the rate-limiting step for the dye concentration ranges studied in the present investigation (20 to 60 mg/L).