Abstract
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.
Highlights
Water pollution is one of the main concerns the World is currently facing
Since the mercury concentration in this tetrapropylammonium hydroxide solution (TPAOH) solution was only 3.94 × 10−5 mol m−3, which represents less than 0.8% of the initial mercury concentration (CA,0, ca. 5 × 10−3 mol m−3 ), this contribution was considered irrelevant
The normalized Hg2+ concentration in the fluid converges to the same value (CA /CA0 = 0.13 ± 0.01) regardless of the pH adjusting solution At equilibrium, and depending on the amount of Engelhard Titanium Silicate No 4 (ETS-4), 86 to 88% of Hg2+ was exchanged by the Na+ initially present in the solid. These results clearly demonstrate that ETS-4 is highly selective for Hg2+ cation and at equilibrium the ion exchange process is not affected by the presence of high amounts of Na+ in solution
Summary
Water pollution is one of the main concerns the World is currently facing. The continuous increase of the world population, the need for high quality drinking water, and water scarcity in many regions of our planet make water treatment processes a hot subject among researchers.Heavy metals are considered traditional water contaminants. Mercury is one of the most hazardous metal contaminants, since its presence in water, even at very low concentrations, causes severe harmful effects to biota. This happens because of bioaccumulation and biomagnification [2], which means that the concentration of Hg2+ in the biota is much higher than in water. For these reasons mercury was classified as a priority pollutant by the European Union [3] and is in the Top 3 of the priority list of hazardous substances established by the Agency for Toxic Substances and Disease Registry [4]
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