Mercury Removal from Aqueous Solution Using ETS-4 in the Presence of Cations of Distinct Sizes

The removal of the hazardous Hg²⁺ from aqueous solutions was studied by ion exchange using titanosilicate in sodium form (Na-ETS-4). Isothermal batch experiments at fixed pH were performed to measure equilibrium and kinetic data, considering two very distinct situations to assess the influence of competition effects: (i) the counter ions initially in solution are Na⁺ and Hg²⁺ (both are exchangeable); (ii) the initial counter ions in solution are tetrapropylammonium (TPA⁺) and Hg²⁺ (only Hg²⁺ is exchangeable, since TPA⁺ is larger than the ETS-4 micropores). The results confirmed that ETS-4 is highly selective for Hg²⁺, with more than 90% of the mercury being exchanged from the fluid phase. The final equilibrium attained under the presence of TPA⁺ or Na⁺ in solution was very similar, however, the Hg²⁺/Na⁺/ETS-4 system in the presence of Na⁺ required more 100 h to reach equilibrium than in the presence of TPA⁺. The Hg²⁺/Na⁺/ETS-4 system was modelled and analyzed in terms of equilibrium (mass action law) and mass transfer (Maxwell-Stefan (MS) formalism). Concerning equilibrium, no major deviations from ideality were found in the range of studied concentrations. On the other hand, the MS based model described successfully (average deviation of 5.81%) all kinetic curves of mercury removal.