Monitoring the benefits of varying the template/monomer proportion in the synthesis of an ion-imprinted polymer for Ra(II) extraction

In our previous paper, an ion-imprinted polymer (IIP) was for the first time synthesized for Ra(II) extraction using Ba(II) as template ion since it has properties close to Ra(II), acetonitrile/dimethylsulfoxide (1/1, v/v) as porogen, vinylphosphonic acid as complexing monomer, styrene as co-monomer, and divinylbenzene as cross-linker. The present study aims at investigating the influence of the template ion/monomer proportion (1/4, 1/6, and 1/8) on sorbents properties. Polymers were packed in solid phase extraction (SPE) cartridges for characterization. A particular attention was paid to control the impact of this ratio on the retention of Ra(II) and interfering ions (i.e. specificity) on IIPs, their capacity, and their breakthrough volume. Although the IIPs 1/4 and 1/8 tended to be slightly more specific than IIP 1/6, the adsorption capacity of the IIP 1/4 was much lower (0.3 µmol g⁻¹) than the two others (3 µmol g⁻¹). Data modelling indicated that Sips and Redlich-Peterson models were best fitted, sign of a monolayer adsorption process into cavities not completely homogeneous in contrast to the Langmuir model. IIPs 1/6 and 1/8 showed both high breakthrough volumes (>80 mL for 100 ng of Ba(II) percolated on 100 mg of sorbents) and BET experiments demonstrated no impact of the monomer proportion on surface area of the IIPs 1/4, 1/6, and 1/8. The repeatability of the synthesis was evaluated on the IIP 1/6. The SPE profiles resulting from three independent syntheses of the IIP 1/6 were very similar and thermogravimetric and BET analyses also confirmed that the three IIPs had similar physical characteristics. The IIP 1/6 was applied to the extraction of Ba(II) from spiked mineral waters (Mont Roucous® and Volvic®). The Ba(II) extraction recovery was subjected to matrix effects but those were solved by adapting the amount of sorbent to the volume of percolation solution. The optimized procedure has been successfully applied to extract ²²⁶Ra for the first time with a dedicated IIP in a real sample: 5 mL of Mont Roucous® water were spiked with 80 mBq L⁻¹ of ²²⁶Ra and ²²⁸Ra which was used as tracer to perform quantification by isotope dilution. After extraction, the ²²⁶Ra activity (88 ± 7 mBq L⁻¹, coverage factor k = 2) and recovery on IIP 1/6 (93%) were determined using an Inductively Coupled Plasma Mass Spectrometer and the limit of quantification of the method was estimated at 19 mBq L⁻¹ (0.52 pg L⁻¹).