Adsorptive Removal of Methylene Blue by Mn-Modified Tourmaline

Adsorbent Mn-modified tourmaline was creatively prepared by wet impregnation in a MnSO4 solution and subsequent calcination procedure. The prepared Mn-modified tourmaline was used for the adsorptive removal of a cationic dye methylene blue (MB). The experimental data under different solution pH conditions were simulated using both linear and non-linear kinetic models. Linear pseudo-secondorder model and non-linear Elovich kinetic model were found to be more suitable to describe the adsorption kinetics. This indicates that the adsorption of MB onto the Mn-modified tourmaline is possibly a chemisorption process in which the rate-determining step is diffusion in nature. The adsorption isotherms were also investigated and the experimental data were fitted by both Langmuir and Freundlich models. Usinxg Langmuir model, the calculated maximal adsorption capacities for MB achieved 158.5 mg/g at 298 K. Thermodynamic analysis indicates that the changes of enthalpy and entropy of the adsorption processes are 55.6 KJ mol-1 and 199.0 J mol-1 k-1, respectively. The negative value of Gibbs free energy change and the positive value of enthalpy also indicates that the adsorption process is spontaneous and endothermic.