Competitive adsorption of Li, Na, K, Rb and Cs ions onto calcium alginate–potassium tetraphenylborate composite adsorbent

Abstract

In this study, the calcium alginate–potassium tetraphenylborate (Ca(ALG)₂–KB(C₆H₅)₄) composite adsorbent was synthesized using potassium tetraphenylborate (KB(C₆H₅)₄) as the adsorption-active component and calcium alginate (Ca(ALG)₂) as the matrix material. Different techniques were used for characterization of the adsorbent such as SEM, EDS, XRD, laser particle size analysis and moisture analysis. Competitive adsorption of Li, Na, K, Rb, Cs ions onto the adsorbent was investigated through kinetic curves, adsorption isotherms and column techniques at 25 °C. The equilibrium adsorption capacities were compared in the single-element and multi-element systems. The equilibrium adsorption amount was in the sequence Cs⁺ > Rb⁺ ≫ Na⁺ ∼ Li⁺, and Li and Na ions were hardly adsorbed. The separation factor was found to follow the order β_Cs/Li > β_Rb/Li > β_Na/Li under both the noncompetitive and competitive adsorption conditions. In the kinetic experiments for competitive adsorption, the adsorption reached equilibrium in about 24 h. The equilibrium sorption data were described by the Langmuir and Freundlich isotherm models and the results showed that the Langmuir model with determination coefficients of 0.997 and 0.981 for Cs⁺ and Rb⁺ respectively could describe the competitive system at room temperature more correctly. Lower breakthrough ratio and higher adsorption amount were observed for Cs⁺ in the column experiments for competitive adsorption. Ion-exchange was deduced as the mechanism for selective adsorption of Rb⁺ and Cs⁺ ions onto the composite on the basis of EDS and XRD analysis of the adsorbent before and after adsorption. The tetraphenylborate anion (B(C₆H₅)₄⁻) had different affinity to different alkali metal ions following the order Cs⁺ > Rb⁺ > K⁺ due to the order of the solubility product K_sp (CsBph₄) < K_sp (RbBph₄) < K_sp (KBph₄).