Adsorption behavior of Th(IV) onto illite: Effect of contact time, pH value, ionic strength, humic acid and temperature

•Adsorption mean free energy indicated the adsorption process to be chemisorptions.•The humic acid (HA) studied did not affect Th(IV) adsorption on illite.•Th(IV) adsorption on illite is reversible at low concentration, whereas it is irreversible at high concentration.•The adsorption mechanism is outer-sphere surface complexation and ion exchange with Na+/H+ at low pH.

The adsorption experiments of Th(IV) on illite as a function of time, pH, ionic strength, temperature and humic acid (HA) were investigated using batch experiments. The results showed that adsorption of Th(IV) on illite was strongly dependent on contact time, pH, and temperature and independent on humic acid(HA). The Th(IV) adsorption on illite increases with increasing pH (pH < 4.5) and temperature, but decreases with increasing ionic strength at pH < 4.5. Th(IV) adsorption data were successfully described by the pseudo second-order kinetic model and the intraparticle diffusion equation. Langmuir adsorption isotherm model simulated the adsorption process better than Freundlich model. The thermodynamic parameters of enthalpy, entropy and free energy change were calculated and thermodynamic parameters revealed the spontaneity and exothermic nature of adsorption Th(IV) on illite. Th(IV) of adsorption mechanism on illite is outer-sphere surface complexation and ion exchange with Na+/H+ on illite at low pH, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH.