Cadmium(II) removal from aqueous solution using microporous titanosilicate ETS-4

The ability of microporous titanosilicate ETS-4 to uptake Cd2+ from aqueous solutions has been investigated, assessing its potential in water remediation. In order to study the equilibrium and the kinetics of the process, batch stirred tank experiments have been carried out by contacting a fixed volume of solution with known masses of ETS-4. The evolution of the cadmium concentration with time has been monitored by inductively coupled plasma mass spectrometry. Concerning equilibrium, Freundlich isotherm performs accurately in the range of experimental conditions studied. The solid loadings measured in this essay surmount significantly the values found in literature for different ion exchangers in the same range of temperatures. A Nernst–Planck based model combining internal and external diffusion resistances has been used to describe the ion exchange process, where the convective mass transfer coefficient and the self-diffusivities of the counter ions are the unique parameters. The Nernst–Planck based model accomplishes good representations (average absolute deviation of 6.74%), even in the transition from the steep descent to the horizontal branch of the bulk concentration versus time curve. The results obtained using the commonly adopted pseudo first- and second-order models found in literature are worse (average absolute deviation of 215.7% and 12.11%, respectively), although more parameters are involved.