Elemental Distribution Behavior of Sulfonic Acid Cation-Exchange Resins and Applications to High-precision Isotope Analysis

(1) The cleaning effect of 6mol/L nitric acid in regenerating AG^{50W-X8 resin is good, but Th remains in the resin.}

(2) Hydrofluoric acid can significantly change the distribution behavior of high field strength elements, some transition metals and rare earth elements in AG^{50W-X8 cations.}

(3) In the mixture of HCl-HF acid, with the increase of HCl concentration, the distribution coefficient of rare earth elements has a trend of first increasing and then decreasing.

Significance: With the development of analytical instruments, significant breakthroughs on high-precision isotope studies have been made in recent years. The dissolution of geological samples leads to solutions with complex matrices that jeopardize the accuracy and precision of the data, i.e. the so called “matrix effect”. Chemical purification of the elements of interest from the sample matrices can suppress and minimize matrix effects, and therefore provides an improved analytical precision for various isotopes on instruments such as, multiple collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). Ion exchange resins have been widely used for the separation and purification of target elements, in which cationic resins are widely used in the development of metal stable isotopes. Distribution coefficients (K_d) are essential for ion exchange chromatography because they provide information on elements distributed between the solution and resin for particular acids; hence enabling the choice of acid and strength that are suitable for effective separation of certain elements in solution. However, previous studies of the K_d on typical cationic exchange resins is incomplete, some alkali metals and transition metals have not been included in previous studies. Also, direct comparison of the distribution coefficients for various elements in different acids is lacking.

Methods: Two sets of experiments were performed, one was an equilibrium time test, and the other was to determine K_d in different molarity (0.1mol/L to 6mol/L) of HCl and HNO₃ and their mixtures with 0.2mol/L HF.

　　For the equilibrium time test, 400 mg of Bio-Rad AG^® 50W-X8 200-400 mesh cation resin was accurately weighed into 30mL PFA beaker and 10mL 0.5mol/L HNO₃ added. Then, 200μL of 10mg/L multi-element standards was added into the beaker. The initial concentration of analytes added was ~200ng/g so that they could be precisely analysed by ICP-MS before and after the partitioning. Small aliquots of the solutions were collected in PFA beakers at 0.5, 1, 5, 10, 30min and 1, 2, 4, 8, 24h. The solutions were then dried and reflux with 2% HNO₃ added, and finally analyszed on ICP-MS. After the concentrations of each analyte were determined, precise K_d were calculated using the equation of K_d=C_solid/C_solution=(C_B−C_A)×V/(C_A×w), where C_B and C_A are the elemental concentrations in μg/mL of solution before and after equilibration, V is the total volume of solutions in mL, and w is the weight in gram of dry resin.

　　For K_d dertermination, experiments were performed at room temperature (23℃) using the batch method. First, 400mg of Bio-Rad AG^® 50W-X8 200-400 mesh cation resin was accurately weighed into 30mL PFA beakers and 10mL solutions of HNO₃, HCl, a mixture of HNO₃-HF and HCl-HF of varying strengths from 0.1mol/L to 6mol/L were added. Then, 200μL of 10mg/L multi-element standards in different type of acid were added into the corresponding beakers. The mixtures were shaken for 5 to 10min every 2h and left to for 24h to achieve equilibrium. The solutions were collected in pre cleaned PFA beakers by filtering the resin mixture through empty 10mL Bio-Rad polypropylene columns. Afterwards, the solutions were evaporated to dryness on a hotplate at 60℃ and refluxed with 10mL 2% HNO₃ for ~5h with the cap sealed at 60℃. Finally, the solutions were transferred to 7mL PP tubes and were analysed by ICP-MS and precise distribution coefficients were calculated for each element in each solution. Blank samples were prepared using the same procedure, without adding any multi-element standards. The blank solutions were used to determine the background of reagents, resin and containers.

Data and Results: The distribution coefficients of almost all elements on AG^®50W-X8 cation exchange resin in hydrochloric acid and nitric acid were negatively correlated with the molarity (0.1mol/L to 6mol/L). For most elements, the calculated K_d in 6mol/L HNO₃ were slightly lower than in 6mol/L HCl, suggesting that using 6mol/L nitric acid to regenerate AG^®50W-X8 cation exchange resin was slightly better than 6mol/L hydrochloric acid. Significantly lower K_d for high field strength elements (HFSE) such as Zr and Hf was observed in hydrofluoric acid, and may be due to the strong ligand capacity of Cl^-. Some transition metals, metalloids, and non-metallic elements, such as Mo, W, Re, Ir, Sb, Ge, As, Se, Te, etc., would form oxygen-containing anions in acid solutions, and hence not adsorb in cation exchange resins with low K_d.

　　In the mixed acid of HNO₃-HF and HCl-HF, the partition coefficient of most elements in the mixed acid also decreased rapidly with the increase of acidity. The addition of hydrofluoric acid can significantly reduce the distribution coefficients of some elements in dilute hydrochloric acid and nitric acid, including Be, Al, Sc, Fe, Sn, Th, U and HFSE (Ti, Zr, Hf). The distribution coefficient of rare earth elements (REEs) in HNO₃-HF mixed acid was similar to that in nitric acid, but in HCl-HF mixed acid, as the concentration of hydrochloric acid increased (from 0.1mol/L HCl-0.2mol/L HF to 6mol/L HCl-0.2mol/L HF), K_dREE showed a trend of first increasing and then decreasing. This trend of change indicates that in a mixture of hydrochloric acid and 0.2mol/L hydrofluoric acid, an increase in hydrochloric acid concentration will reduce the coordination ability of hydrofluoric acid to rare earth elements, and the proportion of complex compounds formed by rare earth elements will decrease. This leads to a rapid increase in K_dREE as the concentration of hydrochloric acid in the mixed acid increases in the 0.1-0.5mol/L HCl-0.2mol/L HF mixture. Subsequently, as the concentration of hydrochloric acid further increased, [H₃O]⁺ rapidly increased to the dominant position in the mixed acid solution, leading to a rapid decrease in K_dREE, ultimately resulting in an arched distribution coefficient curve of rare earth elements in HCl-HF mixed acid.