Kinetic of sorption of Sr²⁺ ions from aqueous solutions by synthetic inorganic sorbents

Abstract

Purpose: All chemical processes including adsorption occur in complicated mechanism and are reversible. After some time equilibrium has introduce. These complex mechanisms can be divide into a number of simple stages, which can be describe by relatively simple mathematical equation. In this study the adsorption of Sr²⁺ ions from aqueous solutions by synthetic inorganic sorbents Titanium Phosphate (PhTi) and Zirconium Silicate (ZrSi) was investigate.  To simulate the adsorption kinetic, two commonly used models Elovich and Diffusion kinetic models were apply. In addition to Elovich kinetic model experimental data of sorption must have a linear dependence in coordinates At vs ln t, investigating adsorption processes is chemisorption, and limiting step of adsorption is surface chemical reaction. In addition to Diffusion kinetic model, for example, intra particle diffusion - the limiting step of adsorption is the diffusion stage of Sr²⁺from solution to the surface of sorbent. Diffusion in micro porous carries the character of activated diffusion, which usually described by Weber and Morris. The experimental data is the plot in coordinate A_t vs t^½.

Methods: Titanium Phosphate (PhTi) and Zirconium Silicate (ZrSi) synthesized in Institute of Sorption and Endoecology Problems NAS of Ukraine, Kiyv. Porous radius and volume, as well as surface area were investigate in the same institute using low temperature adsorption-desorption of Nitrogen and BET adsorption equation. To study the effect of pH and agitation time on adsorption of Sr²⁺ ions, batch studies performed and unabsorbed ions of Sr²⁺ analyzed using complexonometric titration.

Results: For all the systems, which examined, the Elovich kinetic model provided better correlation of the experimental data. For ZrSi the stage of diffusion deep into the sorbent is also an important stage. This is due to the fact, that Zirconium Silicate is a micro porous material with a narrow distribution of pore radii. It was observe that maximum adsorption coefficients take place at pH ~ 11.

Conclusions: It was shown, that adsorption of Sr²⁺ ions strongly depend on time of interaction between solution and surface of sorbents; as well as solution’s acidity. The sorption coefficients of Sr²⁺ by Titanium Phosphate are higher than by Zirconium Silicate. To simulate the adsorption kinetics, two commonly used models Elovich and Diffusion were apply. For all the systems, the Elovich kinetic model provided better correlation. It was observe that maximum adsorption coefficients take place at pH ~ 11. The pH of the solution may change the surface charge of the adsorbent and increase the probability of surface complex formation of –OH­-Sr⁺ or –OH-Sr-OH compounds.