Revisiting syntheses of Ti(iv)/H2PO4–HPO4 functional ion-exchangers, properties and features

Abstract

Amorphous titanium phosphate ion-exchangers are often of a “mixed type”, i.e., they contain a mixture of –HPO₄ and –H₂PO₄ active groups. Their synthesis requires careful handling to obtain the same proportion of active units and sorption characteristics. This article focuses on the influence of titanium sources and post-synthetic treatments on the uniform synthesis of amorphous TiP1 (TiO(OH)(H₂PO₄)·H₂O). It also describes a mild and straightforward method for obtaining crystalline α-TiP (Ti(HPO₄)₂·H₂O). Amorphous TiP1 was successfully synthesized using five sources of titanium providing that the content of titanium and H₂SO₄ in the primary solution was 60–110 g L⁻¹ and 400 ± 50 g L⁻¹, respectively. Observations revealed that organic and inorganic acids could also be comparably used in post-synthetic treatments to protonate the phosphate groups into –H₂PO₄ units. The Na⁺ uptake (up to 7.2 meq g⁻¹) and ion-exchange capacities towards divalent ions (up to 3.8 meq g⁻¹) of all the TiP1-type sorbents studied are among the highest reported for TiP systems. Despite differences in the surface characteristics, the TiP1 materials synthesized in this study displayed comparable sorption properties, supporting the fact that chemisorption is the governing factor behind the sorption processes. Crystalline α-TiP is obtained under similar mild synthesis conditions when the P₂O₅ : TiO₂ molar ratio is greater than 1 : 1, regardless of the titanium source. The possibility of using various types of TiOSO₄ as a titanium source for TiP1 and α-TiP syntheses is emphasized and all reported data are re-considered from a synthetic perspective.