Thermodynamics, kinetics and isotherm studies on the removal of methylene blue from aqueous solution by calcium alginate

Calcium alginate (CA) was used as an adsorbent to remove methylene blue (MB) from aqueous solution. The effect of initial dye concentration, contact time, temperature and solution pH on the adsorption of MB onto CA was investigated by batch experiments. The percentage removal of MB decreased with increasing temperature. Comparatively high adsorption capacities were shown over a wide pH range (pH 2-11). More than 93% of MB removal was obtained within 30 min for an initial dye concentration of 1,000 mg/L at an adsorbent dose of 4 g/L. The adsorption equilibrium was investigated by the Langmuir and Freundlich isotherms. The maximum adsorption capacity was 2,355.4 mg/g on the basis of the Langmuir isotherm. Thermodynamic parameters were evaluated, and revealed that the adsorption process was a spontaneous, exothermic and entropy-reduced process. Pseudo-first, pseudo-second and intra-particle diffusion kinetic models were applied to the experimental data, and the results showed that the adsorption was in good agreement with the pseudo-second order kinetic model. Desorption studies implied that CA could be a useful adsorbent for the removal of MB from aqueous solution.