Sorption performances of TiO(OH)(H2PO4)·H2O in synthetic and mine waters

Abstract

The sorption properties toward Cu²⁺, Zn²⁺, Ni²⁺, Mn²⁺ and Co²⁺ ions, in synthetic and industrial waters with pH of 3.9–7.2, and the chemical stability of a titanium phosphate ion-exchanger synthesized at mild conditions and containing solely –H₂PO₄ groups, TiO(OH)(H₂PO₄)·H₂O (TiP1) are investigated. TiP1 displays the highest Na⁺ uptake (6.3 meq. g⁻¹) among TiP ion-exchangers and a maximum sorption capacity of ca. 1.55 mmol g⁻¹ (i.e. 3.1 meq. g⁻¹) for the studied ions, which is higher than the ones reported for exchangers composed predominantly of –HPO₄ groups. The sorption isotherms were best described by the Temkin model while the Langmuir and the Freundlich models appear to be insufficient in describing all data. TiP1 shows fast kinetics with an equilibrium reached within 10–20 minutes and diffusion processes play a role in the initial period of sorption that is overpowered by chemisorption reactions in the overall rate controlling step. The selectivity order of the metal ions on TiP1 is determined as: Cu²⁺ > Zn²⁺ ≫ Mn²⁺ > Co²⁺, Ni²⁺, following the order of stability of MOH⁺ complexes and the corresponding activation parameters for a water molecule exchange in [M(H₂O)₆]²⁺ ions. The surface sorption data are in good correlation with the EDS data for these systems, supporting the idea of chemical sorption with no metal hydroxide precipitation. Additional sorption studies show that the quality of industrial waters after sorption reaches the EU recommendation for drinking water. The faster kinetics and the higher exchange capacity reveal that the presence of –H₂PO₄ groups strongly enhances the sorption properties of titanium phosphate sorbents.