Synthesis of diphenylphosphinodithioic acid (DPPDTA) and application in an extraction chromatography resin for radiochemical separations of arsenic and selenium

Abstract

Arsenic (As) and selenium (Se) are neighboring metalloids that frequently appear together in radiochemical processing schemes used for the production of radioactive As isotopes used for nuclear medicine, national security, and environmental research. However, their similar aqueous chemistry complicates these processes, often necessitating stringent oxidation state control or highly acidic conditions to achieve effective separation. Previous efforts using thiol (SH) and thioether (R-S-R)-based extraction chromatography (EXC) resins demonstrated high selectivity for Se ⁴⁺ but were limited by rapid ligand degradation and instability in acidic conditions. Here, we investigate diphenylphosphinodithioic acid (DPPDTA), an aryl-substituted ligand bearing thiophosphoryl ( P  =  S ) and SH functional groups, as a next-generation extractant for As/Se radiochemical separations. A novel EXC resin was prepared with DPPDTA synthesized using a modified Friedel-Crafts reaction. Uptake characteristics (distribution coefficients, kinetics, and column separation behavior) of the radionuclides selenium-75 and arsenic-73 on the DPPDTA resin were assessed in nitric (HNO ₃ ), hydrochloric (HCl), and sulfuric (H ₂ SO ₄ ) acid solutions. Triplicate column studies in 1 M HNO ₃ , 0.01 M HCl, and 1 M H ₂ SO ₄ produced mean ⁷³ As recoveries of 97.1–99.9 %, with <0.1 % cumulative ⁷⁵ Se elution. The chemical stabilities of DPPDTA and the prepared EXC resin were analyzed with acid contact experiments and evaluated using high-resolution mass spectrometry. In HNO ₃ , DPPDTA was unstable and underwent oxidative degradation to diphenylphosphinothioic and diphenylphosphinic acids, accompanied by sulfur leaching over the tested acid concentration range. In HCl and H ₂ SO ₄ , DPPDTA exhibited improved stability consistent with stabilization by chloride and sulfate, but degradation products were still identified.