Recovery of Lithium from Aqueous Solution using Protonated Potassium Polytitanate

Potassiumpolytitanate(PPT) with different degree of protonation is proposed as a new kind of inorganic adsorbent for lithium ions recovery from natural aqueous solutions. The phase and chemical composition, as well as the particle size distribution of the sorbents are studied using XRD,XRFanalysis, and laser diffraction. The efficiency of lithium ions recovery and the adsorption process kineticsfor are investigatedusing standard solutions of lithium chloride. The sorption capacity and kinetic characterisitcsof the studied materials are estimated using pseudo-first and pseudo-second-order models. Keyword Potassium Polytitanate, Protonation, Lithium, Kinetics, Sorption

Protonated forms of potassium polytitanate (PPT) are similar in their composition and structure with metatitanic acid and can become a more attractive alternative due to low cost of synthesis and extremely high sorption capacity. At the same time, the structural characteristics of the protonated PPT can be varied by adjusting the protonation conditions. It is assumed that asubstitution degree of the potassium ions by hydronium ions presented in the protonated form of PPT will affect the amount of lithium ions extracted from aqueous solution, and, as a consequence, influence the composition and structure of lithiated products obtained during lithium ions recovery.
Thus, in this work, it is proposed to use protonated potassium polytitanates as a new inorganic ion exchangers, effective for extracting lithium from aqueous solutions. Amorphous potassium polytitanate (PPT) due to its high imperfection and layered structure is an effective ion exchanger and sorbent for ions of various metals. However, aqueous dispersions of PPT have high pH value, and in the neutral or acidic media potassium polytitanate particles uncontrollably changes their chemical composition due to replacement of metal ions by protons / hydronium ions.That is why; the goal of this work is to recognize the effect of the protonation degree, controlled by pH value of preliminary treatment, on efficiency of lithium extraction from aqueous solution of its salt.
II. EXPERIMENTAL Protonated potassium polytitanates were obtained by the treatment of aqueous dispersion of the parent potassium polytitanate (PPT), characterized by strong alkaline reaction (pH ~ 10.5), with aqueous solution of sulphuric acid, till obtaining a stable pH value equal to2; 3; 4; 5; 6; and 7. The obtained powders were centrifuged, dried at 40 o C for 4 h and grinded up. The protonated forms of the produced PPT derivatives were marked as PPTP2, PPTP3, PPTP4, PPTP5, PPTP6, PPTP7, in accordance with pH value of their aqueous dispersions, and further characterized by XRD (diffractometer ARL X'TRA), laser diffraction (particle size analyzerLaser Analyzerte 22 Micro Tec plus) and X-ray fluorescence analysis (X-ray fluorescence spectrometer SPECTROSCAN MAX-GV).
The efficiency of Li (I) ions extraction by protonated potassium polytitanates was calculated in accordance with (1): where С 0 and С t are the initial and current concentration of Li (I) in the solution. An amount of the adsorbed Li (in mmol/g) at equilibrium (q e ,) and during the process (q t ) were calculated using the formulas (2) and (3): where C o , C t and C e correspond to [Li] in the initial, currentand equilibrium condition, respectively; V is a volume of the solution (l); m is a weight of the PPT powder used as the adsorbent (g). The obtained experimental data were processed using the kinetic equations corresponding topseudo-first-and pseudo-second-order models (equations4 and 5): where k 2 is pseudo-second-order rate coefficient, g/mmol·min. Onlyweak and wide reflections corresponding to K 2 Ti 2 O 5 and crystalline forms of TiO 2 (for PPTP3 and PPTP4) could be identified. The appearance of TiO 2 crystals at low pH can be explained by intensive K + ↔ H 3 O + ion-exchange taking place at these conditions favouring formation of unstable titanium acid, which decomposes producing crystallized titanium dioxide. In the case of samples protonated at higher pH values, the reflectionscan be considered as corresponding to nanoscale partially protonated forms of the potassium titanate, According to the results of elemental analysis (Table I), a regular decrease from 8.3 wt.%(PPTP7) down to 1.1 wt.% (PPTP2) in the potassium content was noted.
An average particle size of the protonated potassium polytitanates does not exceed 12 μm, and a minimal value of this one was obtained for PPTP4 (Table I).  The ex coefficien 2.0 to 5.0 stage. In [16].

IV. CONCLUSION
The obtained results confirm that the protonated forms of the amorphous layered potassium polytitanate (PPTP) represent a new effective inorganic sorbent for the extraction of lithium ions from aqueous solutions. The PPTP samples obtained during the treatment of the parent potassium polytitanate (pH~10.5) in the sulfuric acid aqueous solution at pH value varied from 2.0 to 7.0 retain an amorphous structure of the initial PPT but are characterized with increased specific surface area. The highest adsorption capacity is recognized for the PPTP powders modified at pH~4. It is assumed that these conditions favor obtaining the PPT powder characterized with optimal combination of the specific area and average size of particles. The kinetics of the Li ions sorption by PPTP is in a good agreement with the pseudo-second-order model. This one indicates that in the adsorption process can be considered as ion-exchange chemical reaction and allows to estimate the equilibrium adsorption capacity of PPTP4 equal to 1.1 mmol/g. The obtained characteristics correspond to the best similar materials mentioned in the literature, have relatively low cost and very high rate of Li extraction. The last is considered as a result of large interlayer distance in the structure of protonated potassium polytitanates.