Desorption of Cd(II) from tourmaline at acidic conditions: kinetics, equilibrium and thermodynamics

The effects of different conditions such as time, pH, the initial concentration of the metal ions, temperature and additional cations like Ca(II) and Mg(II) ions on desorption of Cd(II) from tourmaline at acidic conditions were investigated. The results indicated that the desorption of Cd(II) depended significantly on all the above-mentioned parameters. The desorption kinetics closely followed the pseudo-second-order model. And the desorption of Cd(II) followed the Freundlich isotherm model better than Langmuir isotherm model. The maximum desorption capacity of tourmaline for Cd(II), evaluated using a Langmuir equilibrium model, were 2.74 and 13.17 mg/g at an initial pH 4.0 under temperature 25 and 35 °C after the 1440 min equilibrium time, respectively. Thermodynamic parameters indicated that the desorption processes were viable, spontaneous and endothermic in nature. Additionally, Mg(II) and Ca(II) ions can both facilitate the efficiency of desorption, and Mg(II) ions promoted the process of desorption more better than Ca(II) ions. Thus, this study showed that tourmaline was quite an effective adsorbent for the removal of heavy metals in the acidic condition because its desorption capacity was too low to cause secondary pollution. Tourmaline may be used as a stable adsorbent for removing pollutants from environment, especially soil.