(527e) Tuning La(III)-Binding Strength in Membrane Adsorbers Using Heterogeneous Polymer Brushes

Radiochromatography plays a key role in the selective extraction of lanthanides and actinides to separate fission products, medical isotopes and to concentrate radioactive waste. These separations are challenging as they often involve separating chemically similar neighbors on the periodic table. Radiochemists still rely on resin-based chromatography columns which are known to suffer from diffusion limitations—leading to long purification times and high elution volumes. An alternative to resins that may reduce separation time is membrane adsorbers (MAs) which are macroporous membranes coated with polymeric ion-binding ligands. Organophosphorus ligands, especially phosphonates and phosphonic acids, are widely used in radiochemical separations due to their high polarizability and affinity for the f-block elements. The selectivity and affinity of the organophosphorus ligand can be tuned by controlling the hardness of the phosphoryl oxygen. In typical adsorption media, ligand hardness (and ion-affinity) can only be changed by synthesizing a new brand new ligand. In this work, we aim to tune ion-affinity by changing the local chemical environment around the ligand by altering the composition of the polymer brush. Tuning the binding strength of ligands through copolymerization would be a simple, scalable tool for designing new chromatography media.

This work describes a simple MA system that has been designed to study the effect of co-monomers on the hardness of an amphoteric phosphonate (O-PO(OH)₂) ligand, ethylene glycol methacrylate phosphate (EGMP). Membrane adsorbers with varying film compositions were synthesized by UV-initiated free radical polymerization. Monomers in this study include: EGMP (La³⁺ binding), 2-hydroxyethyl methacrylate (HEMA: does not bind La³⁺, H-bond donating), and butyl methacrylate (BuMa: does not bind La³⁺, does not H-bond). Membrane surface chemistry was characterized using Fourier-transform infrared spectroscopy and contact angle measurements, before and after functionalization. The degree of grafting was calculated gravimetrically and the composition of the copolymer brushes was determined by digesting the membranes and performing total phosphorous measurements. The effect of brush composition on membrane capacity, affinity and binding kinetics were studied by performing batch adsorption experiments. The degree of grafting (mg polymer grafted/g membrane) for p(EGMP), p(EGMP-co-HEMA) and p(EGMP-co-BuMa) was 0.3 mg/g, 0.12 mg/g and 0.09 mg/g, respectively. Equilibrium binding capacities for La³⁺ were modeled using the Langmuir isotherm and normalized to the mass of EGMP per membrane, for comparison. MA capacity for La³⁺ (mg La³⁺/g of EGMP) increases with decreasing H-bonding of the co-monomer following the trend: p(EGMP) = 91.2 mg/g < p(EGMP-co-HEMA) = 200.7 mg/g < p(EGMP-co-BuMa) = 653.8 mg/g. Even though p(EGMP) has the highest number of binding sites, as measured by degree of grafting, it has the lowest capacity. The data imply that the high degrees of inter- and intra-molecular interactions limit the MA capacity. Understanding the effect of polymer composition and how to control ligand hardness will guide the future design of chromatography materials.