Incorporating Comonomers into Polymeric Phosphate Ligands Can Tune the Affinity and Capacity for Rare Earth Element, La

Abstract

Phosphate ligands are widely used in ion-exchange and chelation separation materials (resins and membranes) to sequester heavy metals like lanthanides and actinides. In this work, the affinity, capacity, and kinetics of membranes functionalized with phosphate-containing copolymers for La3+ were characterized. In particular, the impact of the nonion complexing comonomers, 2-hydroxyethyl methacrylate (HEMA) and butyl methacrylate (BuMa), on the adsorption performance of ethylene glycol methacrylate phosphate EGMP was investigated. Membranes were coated with poly­(EGMP), poly­(EGMP-co-HEMA), or poly­(EGMP-co-BuMa) using thermally initiated free-radical polymerization and characterized using attenuated total reflectance Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and contact angle goniometry. Equilibrium adsorption data were modeled with the Langmuir isotherm and revealed several trends. At pH 4, the Langmuir constant (K L) which is an indicator of La3+ affinity is markedly higher when a non-H bonding “spacer” is present. Here, K L follows the trend: poly­(EGMP-co-BuMa) = 97 ± 18 mM–1 > poly­(EGMP-co-HEMA) = 6.0 ± 1.1 mM–1 > poly­(EGMP) = 2.3 ± 0.6 mM–1. Interestingly, the maximum capacity Q max (mmol La (mmol P)−1) follows a reverse trend: poly­(EGMP-co-HEMA) > poly­(EGMP) > poly­(EGMP-co-BuMA). Kinetic studies at pH 4 reveal that faster adsorption is observed for polymers with less intramolecular H-bonding. The impact of the spacing monomer on the affinity (K L) was pH-dependent where at pH 1, both poly­(EGMP-co-BuMA) and poly­(EGMP-co-HEMA) show the same K L. Furthermore, the polymer composition did not impact the adsorption kinetics at pH 1. Thus, the ability of the comonomer to serve as an H-bond donor did play a role at this pH. This work reveals the role of comonomers in polymeric adsorbents and suggests the ability to use them to tailor adsorbent performance.