Adsorption of lanthanides on mordenite from nitrate medium

The adsorption of lanthanides (except for Pm) on mordenite was investigated under various solution conditions of nitrate ion concentrations ([NO−3]: 0.001–2 mol/dm3) and total lanthanide concentrations (0.0005 mol/dm3). Solutions of lanthanide nitrates were equilibrated with zeolite samples at 296 K. A concave tetrad effect was evident in the change of logKdlogKd values within the lanthanide series and an explanation by a comparison of covalence in LnO bonds existing in AlO(1/3Ln)Si species found in the zeolite phase and in Ln(H2O)3+x or Ln(NO3)n−3n complexes formed in the aqueous phase is presented. The decreasing trend in C1C1 and C3C3 coefficients, which are the function of E1E1 and E3E3 Racah f-interelectron repulsion parameters, is evidence of the magnification of covalence in LnO bond in the series SiO(1/3Ln)Al < Ln(H2O)3+x < Ln(NO3)n−3n. The adsorption isotherms of the lanthanides, registered for total lanthanide nitrate concentrations (0.0001–0.001 mol/dm3), were found to be a Freundlich type. The obtained adsorption capacity K and adsorption intensity n   parameters show odd–even changes in the lanthanide series, indicating the importance of covalence effects in the lanthanides adsorption mechanism. The mordenite samples loaded with lanthanide nitrates were investigated using diffusive reflectance infrared Fourier transform spectroscopy (DRIFT), X-ray diffraction (XRD), and scanning electron microscopy (SEM) methods. Negative correlations between E=([Al]−[Na])/[Ln]E=([Al]−[Na])/[Ln] and [Ln]/[Al], and negative correlations for [Ln] and [Na] on the zeolite surface, resulting from the interaction (ion exchange) of lanthanide ions with SiO(Na)Al and SiOHAl groups within the mordenite framework were found. DRIFT spectra showed the sensitivity of the framework vibration band TOT for Ln3+ ion interactions with negative oxygen of the mordenite structure.

Graphical abstractTetrad effect in the distribution constants logKdlogKd of the lanthanides in the system with mordenite (numbers denote the molar concentrations of nitrates in the aqueous phase).Figure optionsDownload full-size imageDownload as PowerPoint slide