Tetrahydrofuran (THF)-assisted alkaline extraction to determine hexavalent chromium (Cr(VI)) in retardant polymers containing Sb(III)

Low recovery of hexavalent chromium (Cr(VI)) in polymers due to the reduction to Cr(III) by Sb(III) has been a big issue for the implementation of the Regulation of Hazardous Substances (RoHS) directive. In this work, we have developed a THF (tetrahydrofuran)-assisted alkaline extraction method to improve the extraction efficiency of Cr(VI) in the presence of Sb(III) by suppressing the reduction through the formation of an Sb(III)–THF adduct. The adduct formed was identified as Sb₂O₅Cl₅(THF)₃ using various analytical techniques, such as XRD, NMR, and MALDI-TOF-MS, and by molecular simulation through the calculation of ground state energy. The efficiency of the developed method was demonstrated by the analysis of in-house manufactured reference polymers containing Sb(III). The recovery was obtained in the range of 48.9–77.5%, which was about 19.6–31.0 times more efficient than the current RoHS method. Furthermore, when the THF solvent was mixed with 2-propanol (isopropyl alcohol, IPA) or 1-methyl-2-pyrrolidone (NMP), the recovery was significantly improved to over 95.2%, whereas a mixture with EDTA and NMP had a recovery of <20.0%.