Green citric acid in the sorption process of rare earth elements

Abstract

In this study the effects of citric acid (CA) as a biodegradable complexing agent in the sorption process of La(III), Nd(III) and Ho(III) ions (single component systems) on various types of ion exchangers (chelating ion exchangers, cation exchanger, strongly and weakly basic polystyrene and polyacrylate anion exchangers) were investigated. The effects of pH, Ln(III):CA molar ratio, phase contact time and initial concentrations as well as temperature on the adsorption capacity were analysed. The optimal process conditions were also determined which varied depending on the ion exchanger and the metal ion. The process was fast in all systems (up to 60 min). The sorption data was analysed using the pseudo-first order (PFO), pseudo-second order (PSO), intraparticle diffusion (IPD), Boyd and Dumwald–Wagner (DW) kinetic models as well as the Langmuir, Freundlich, and Temkin isotherm models. Among the studied adsorbents, the highest adsorption capacity was obtained for the Purolite S957 chelating ion exchanger which was 162.04 mg/g for the La(III) complexes, 142.65 mg/g for the Nd(III) complexes and 180.26 mg/g for the Ho(III) complexes. Reusability of ion exchangers in the desorption studies was also evaluated as a sustainable approach. Moreover, dynamic experiments were performed using the columns set. The Thomas, Adams–Bohart, Yoon–Nelson and Wolborska models were applied to the experimental data to predict the dynamic behaviour of fixed bed columns. The physicochemical properties of adsorbents were characterized by scanning electron microscopy (SEM), pHpzc, and Fourier transform infrared spectroscopy (FTIR). The binary complexes of Ln(III) ions with CA at various metal:ligand ratios were investigated using the potentiometric method. The overall stability constants of the complexes were determined.