Coordination and silica surface chemistry of lanthanides (III), scandium (III) and yttrium (III) sorption on 1-(2-pyridylazo)-2-napththol (PAN) and acetylacetone (acac) immobilized gels

Abstract

This article focuses on identifying the coordination and silica surface chemistry of PAN immobilized mesoporous silica gel in the adsorption of Rare earth elements (REE) in comparison to acetylacetone (acac) immobilized gel. PAN is used as an analytical reagent since it has both acidic (pKa 1.9) and basic (pKa 12.2) properties. A strategy for the adsorption of REE has been developed through the analysis on the adsorption of light rare earths (LREEs) and heavy rare earths (HREEs). In this study, the separation of REEs is achieved via the –N group of pyridine/ azo and –OH group of PAN modified silica, at different conditions. The REEs were studied based on different parameters such as pH, kinetics, metal loadings, binding sites, temperature etc. and distinct differences have been observed in separation between ‘d’ and ‘f’ block elements. Ionic radii, lanthanide contraction and REE tetrad effect impact the adsorption process for different pH and metal loading conditions. This eventually influences the binding mechanism as well. The affinity of the modified gel was towards LREEs at lower pHs and HREEs in higher regime. As a validation attempt, the developed separation strategy has been employed in investigating under natural water conditions for varying REE concentration and pH in the presence of competing ions. Silica gel with PAN (SEP) modifications performed superior to Acac modifications (SEA) in terms of REE adsorption under all conditions. The behavior was coherent for both single and multi-component systems where HREE adsorption showed temperature dependency. As SEP demonstrated adsorption even in the acidic range (pH 2), it has the potential to serve a wide range of applications involving acidic industrial waste such as mine water.