Simultaneous Extraction of Cr(III) and Cr(VI) with a Dithiocarbamate Reagent Followed by HPLC Separation for Chromium Speciation

A simultaneous extraction of Cr3+ and Cr6+ species in aqueous solution was developed. The extraction behaviors of ammonium pyrrolidinedithiocarbamate (APDC) for both Cr6+ and Cr3+ are carefully discussed in this report. X-ray diffraction and spectroscopic studies indicate that Cr6+ reacts with the ammonium pyrrolidinedithiocarbamate ligand to form two products, Cr(PDC)2(OPDC) and Cr(PDC)3, where OPDC represents an oxygen insertion between Cr and S atoms. A high concentration of APDC in the phthalate buffer under an elevated temperature (50°C) was applied in solvent extraction (SE) to increase the extraction efficiency of Cr3+, so that both Cr3+ and Cr6+ could be extracted by APDC simultaneously. The complex involving the oxygen insertion bonding Cr-O-S is separable from the normal Cr(PDC)3 complex chromatographically, thus allowing the quantification of Cr6+. The major product of Cr6+ has a structure of Cr(PDC)2(OPDC) and the minor product is Cr(PDC)3 with a ratio of Cr(PDC)2(OPDC)/Cr(PDC)3 being 8.5. The extraction conditions for Cr6+ and Cr3+ as well as the chromatographic separation of the complexes using high-performance liquid chromatography (HPLC) are reported. Potential applications for the chemical speciation of chromium by SE/HPLC in environmental aqueous solutions are also discussed.