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Kinetics and equilibrium adsorption experiments were conducted to evaluate the adsorption characteristics of a cationic dye (methylene blue, MB) onto bio-sludge ash. Results showed that the ash could remove the dye effectively from aqueous solution. The adsorption rate was fast and about 80% of absorbed-MB was removed in 10 min. The adsorption kinetics could be expressed by the modified Freundlich equation and intra-particle diffusion model. It was found that both the initial MB concentration and ionic strength could affect the rate of adsorption. The effect of electrical double layer thickness on the adsorption kinetics was discussed. The equilibrium adsorption data were correlated well to the non-linear multilayer adsorption isotherm. The maximum adsorption capacities for MB were 7.3 × 10−6, 6.3 × 10−6, 5.0 × 10−6, and 3.5 × 10−6 mol/g, respectively, at temperature of 4, 14, 24, and 34 °C. Values of the first-layer adsorption energy, ΔG°, ranged from −6.62 to −7.65 kcal/mol, suggesting that the adsorption could be considered as a physical process, which is simultaneously enhanced by the electrostatic effect. The multilayer adsorption energy, ΔG°, ranged from −4.51 to −5.02 kcal/mol, suggesting that the adsorption was of the typical physical type. On the basis of the monolayer dye adsorption capacity, the specific surface area of this ash sample was estimated as 2.1–2.9 m2/g which is close to the value (3.7 m2/g) obtained via BET nitrogen gas adsorption measurements. |
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