Titanium(IV) Phosphates : The Next Generation of Wastewater Sorbents

Sammanfattning: Treatment of industrial waters containing heavy metal ions is essential before being discharged into the environment. Consequently, European regulations have been established to control and limit the amount of heavy metals released. There is a need to develop efficient water treatment techniques that can remove contaminants with respect to these EU regulations.Ion-exchange is one of the processes that is being investigated due to fast kinetics, high treatment capacity and its ability to remove heavy metal ions present in trace amounts. Titanium phosphates (TiP) are a group of inorganic ion-exchangers that have demonstrated to be particularly selective towards transition metal ions in aqueous solutions. Two types of ion-exchange units are present in TiP material, which are –HPO4 and –H2PO4 groups. Their structural characteristic is highly dependent on the synthesis conditions, which include the source of titanium, temperature, reaction time and P2O5:TiO2 ratio. Most of the studies have been performed on amorphous TiP containing a mixture of both exchange units, with –HPO4 groups being predominant; as crystalline TiP and –H2PO4 based TiP  are often obtained in difficult conditions, high temperature (up to 250 °C) and/or long reaction time (up to 30 days) and/or using autoclave. Despite promising properties depicted in batch conditions, very few data in continuous flow systems (fixed-bed columns) have been reported.In this work, amorphous TiP composed of entirely –H2PO4 ion-exchange units (TiP1) was synthesized at mild conditions using a TiOSO4 solution and HCl/deionized water as post-synthesis treatments. The sorbent was characterized using a range of techniques (solid-state 31P MAS NMR, Raman, XRD, TGA, BET, Elemental analysis, EXAFS and XANES,) and tested in batch and column set-ups towards single and multi-component waters. The chemical formula of TiP1 was established as TiO(OH)(H2PO4)·H2O and it was found that the synthesis of TiP1 was also dependent on the TiO2/H2SO4 content in the primary titanium solution.The material displayed a high maximum exchange capacity of ca. 6.4 meq.g-1, expressed as the sodium uptake. The actual ion-exchange capacity towards divalent metal ions was calculated to be ca. 3.4 meq.g-1 in batch condition and up to 4.1 meq.g-1 in fixed-bed column, which is to date the highest recorded for TiP materials. Kinetics of the exchange processes have been studied and the equilibrium was reached within 5-20 minutes. Modeling of the breakthrough curves was achieved using the Thomas model, indicating that the rate driving forces of the processes follow second-order reversible kinetics. The TiP1 sorbent has shown to maintain a high selectivity towards heavy metal ions in multi-component systems (including closed-mine waters) when column studies were performed. The sorption behavior of TiP1 in batch experiments correlates very well with data obtained in fixed-bed column conditions, confirming that prediction of the sorption behavior on the basis of batch data is conceivable.Another important aspect of this work also involves the mild syntheses of crystalline α-TiP, Ti(HPO4)·H2O, and LTP (Linked Titanium Phosphate) composed of α-TiP and TiP1, where the structural characteristics of these materials were investigated using solid-state NMR, XRD, TGA, EXAFS and XANES.