Outline of Titanium (IV) based H2PO4 Ion-exchangers: Kinetics and Sorption models

Decontamination of industrially polluted waters has been enclosed in REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) since 2007, when it entered into force in the European Union, which emphasized the need to search for more effective sorbents. Many studies on inorganic ion-exchangers have been carried out due to the scientific interest towards their high mechanical and thermal stabilities, high resistance to oxidation and high sorption capacity regarding transition and radioactive metal ions. Titanium phosphates (TiP) in particular, have revealed very good cation-exchange properties in relation to different transition metal ions in aqueous solutions. It has been demonstrated that their physicochemical properties as well as their structural characteristics can be altered by carefully choosing the conditions of synthesis: the temperature, acidity, Ti(IV) speciation in solution and reaction time. Since the ‘classical’ crystalline TiP ionites have been divided into two main groups: alfa type; α-TiP [α-Ti(HPO4)2•H2O] and gamma type; γ-TiP [Ti(H2PO4)(PO4)•H2O] with different functional groups (–H2PO4, –HPO4 and –PO4) present, the researchers have focused on synthesis of various metastable TiP with different functional properties. In this work, three different synthetic routes for TiP ion-exchangers were explored in order to obtain a sorbent composed solely of –H2PO4 exchange units. The ─H2PO4 groups were expected to considerably increase the pH working range of the sorbents and to nearly double the theoretical exchange capacity of TiP ion-exchangers containing mostly –HPO4 functional groups. Among the synthesized ion-exchangers (TiP1, TiP2 and TiP3), TiP1 has shown very good sorption characteristics and therefore, most of the studies were performed on it. TiP1 was synthesized at mild thermal conditions using cobalt(II) ions as a modifying agent and HCl-washes as post-synthetic treatments. This sorbent was characterized by different spectroscopic techniques and its chemical formula was established to be: TiO(OH)(H2PO4)•H2O. The sorption capacity of TiP1, estimated about 4.8 meq.g-1, is higher than the reported exchange capacity for various amorphous TiP. The sorption characteristics of TiP1 towards divalent ions such as Cu2+, Zn2+, Ni2+, Mn2+ and Co2+, were investigated in this work. The results of the sorption experiments (in the 1-20 mmol.L-1 range) were fitted to the Langmuir and Temkin models with the latter one being somewhat preferred for most of the metal ions studied. This indicates that interactions between adsorbed molecules cannot be neglected. It was also found that the kinetics of the ion-exchange process was very fast and the equilibrium was reached within 10 minutes. The kinetic data were modeled using the pseudo-second order reaction rate and the obtained curves were consistent with chemisorption being the rate limiting step of the reactions. The selectivity order of the metal ions studied towards TiP1 here was found to be: Cu2+ > Zn2+ > Mn2+ > Co2+ > Ni2+. The TiP1 sorbent has also shown to be a very good cation-exchanger when batch experiments were performed using heavy metals polluted waters from closed mines, supplied by Boliden AB. These studies delineated that TiP1 has displayed exclusive sorption capacities and imminent ion-exchange kinetics. It has been distinctly shown that a modest change in the synthesis could facilitate the fabrication of titanium phosphate ion-exchangers with improved and versatile sorption properties.Godkänd; 2015; 20151102 (myltru); Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Myléne Trublet Ämne: Gränsytors kemi/Chemistry of Interfaces Uppsats: Outline of Titanium (IV) based H2PO4 Ion-exchangers: Kinetics and Sorption models Examinator: Professor Oleg Antzutkin, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Diskutant: Professor Ingmar Persson, Kemi och bioteknologi, Uppsala Tid: Torsdag 10 december 2015 kl 13.00 Plats: F341, Luleå tekniska universite