Synthesis, characterization and uptake studies of diglycolamic acid and diglycolamide analogs adsorbents for extraction of rare earth elements

Tetraalkyldiglycolamides are known for their high extracting efficiency of rare earth elements (REEs) from nitric acid solutions (1-6 mol/L). Nevertheless, solid-phase extractants based on diglycolamides (DGA) are poorly studied for REEs extraction. This paper explores the design and synthesis of novel REEs adsorbents by grafting silica with diglycolic acid-based ligands. The newly developed solid-phase materials were investigated for their potential to effectively separate REEs from various media. The functionalization of the silica with different ligands was quantified, and the adsorbents' extraction capacities and selectivity for REEs over Calcium (Ca2+), Aluminium (Al3+) and Iron (Fe3+) were evaluated. Si-NH-DGA emerged as the most efficient adsorbent for Gd3+ extraction, exhibiting a remarkable capacity of 44 mg/g at pH 4-6. However, modifications involving ether oxygen substitution with a methylamino group or sulfur led to reduced extraction efficiencies. Si-NH-DGA-NMe2 demonstrated strong binding of REEs in nitric acid solutions (1-3 mol/L HNO3) with corresponding adsorption capacities for Y3+, La3+, Gd3+ and Yb3+ being 16.3, 14.2, 22.8, and 30.2 mg/g, respectively. Furthermore, it displayed remarkable selectivity for REEs over Ca2+. The equilibrium extraction for Gd occurred within 60 min for Si-NH-DGA and Si-NH-DGA-NMe2 under their respective optimal conditions. Disodium EDTA solution demonstrated a high stripping efficiency for Gd3+ in the Si-NH-DGA-NMe2 column extraction, yielding a significant concentration factor (CF = 85). In the same column, following five cycles of loading/stripping of Gd3+, there was an average capacity loss of 6%. Si-NH-DGA-NMe2 emerged as a promising material for solid-phase extraction from concentrated nitric acid solutions, characterized by high capacity and good selectivity towards middle and heavy REEs. This structure has the potential to be further optimized for the development of a solid-phase extraction column for REEs separation and purification. The findings also highlight the potential of Si-NH-DGA-NMe2 as a candidate for REEs adsorption for potential industrial applications and the purification of acidic waste solutions.