Chromium speciation analysis of waste water from PSI facilities

Hotlab Analytics

The Hotlab is a unique facility in Switzerland for the safe and hazard-free investigation of the properties of highly toxic, radioactive substances and materials. Through the Post-Processing Radioactive Materials group (NRM), the Hotlab also has expertise in waste treatment and is responsible for the disposal of active and inactive wastewater from all PSI facilities East of the Aare River. In the case of wastewater that is measured to be free of radioactive materials, the Hotlab facility follows the Swiss Water Protection Ordinance (814.201) to safely dispose of this material, including the differentiation between polluted and non-polluted wastewater.

Chromium exists in multiple oxidation states. Trivalent chromium (Cr(III)) is an essential nutrient for living creatures, while hexavalent chromium (Cr(VI)) is highly toxic and acts as a carcinogen, mutagen, and teratogen in biological systems1. Total Cr concentration measurements, therefore, are insufficient to comprehensively guide all waste-treatment decisions. Rather, a measured average daily Cr(VI) concentration under 0.1 mL/L (0.1 ppm) is also necessary, as described in the Swiss Water Protection Ordinance (814.201).

Accurate quantitative analysis of chromium speciation is a significant analytical challenge. The commonly used ICP-MS and ICP-OES cannot distinguish between different Cr oxidation states, since both species have identical atomic weight and ICP-OES emission spectra. A viable strategy for the relative quantification of Cr(VI) over total Cr is chromatographic separation followed by ICP-MS analysis of all aliquots containing Cr, using the infrastructure of the Analytics Radioactive Materials group (ARM). 

Ion chromatography instruments (IC) are relatively simple, consisting of an eluent for sample drag, pump systems that operate alternately while maintaining constant pressure, an injector, a separation column, a chemical suppressor, and a detector for generating the chromatograms2. The IC can replace conventional chemical methods, allowing for automated chemical purification and/or separation with minimal manipulation by the operator, generating improved technical performance.

Chromatographic separations were carried out using the Metrohm 940 Professional IC Vario system (Figure1), equipped with a Thermo Scientific Dionex AG-7 (2 mm i.D., 50 mm length) column, and utilizing 0.3M HNO3 as the eluent in an isocratic separation scheme adapted from3. Unlike many existing methods for Cr speciation that rely on extensive pretreatment (e.g., EDTA complexation)4, this workflow minimizes manipulation, reduces waste, avoids the use of organic complexants, and enables clean collection and transfer of Cr fractions. 

Metrohm 940 Professional IC Vario system
Metrohm 940 Professional IC Vario system (Figure1), equipped with a Thermo Scientific Dionex AG-7 (2 mm i.D., 50 mm length), automated sample processor, and dosing unit. The entire system is completely enclosed by a custom instrument intake extraction system, which allows for the safe manipulation of highly radioactive samples in other applications.

Using pure Cr(III), pure Cr(VI), and mixed certified standard solutions, we verified that the species remain stable during fraction collection (Figure2). Cr(VI) is completely removed from the column within the first 60 seconds, while Cr(III) is only removed between 120 to 180 seconds from the start of the sample processing. This allows the quantification of Cr concentrations by ICPMS in the first and third aliquots to be interpreted as Cr(VI) and Cr(III) concentrations, respectively. These results demonstrate that accurate Cr speciation information can be achieved without direct IC–ICP-MS coupling. Further work will evaluate the behavior of the different Cr species in complex matrices that approach the chemical composition of wastewater to confirm robust separation without additional purification.

Resulting fractions of the ion chromatography
Resulting fractions of the ion chromatography (IC) separation of Chromium. Cr(VI) is completely removed from the column within the first 60 seconds, while Cr(III) is only removed between 120 to 180 seconds from the start of the sample’s processing. This allows the quantification of Cr concentrations by ICPMS in the first and third aliquots to be interpreted as Cr(VI) and Cr(III) concentrations, respectively.

References

  1. Dupont, L. & Guillon, E. Removal of Hexavalent Chromium with a Lignocellulosic Substrate Extracted from Wheat Bran. Environ. Sci. Technol. 37, 4235–4241 (2003).
  2. Moura, A. V., Silva, J. D. S. da & Gubert, P. Ion Chromatography: Principles and instrumentation. Orbital Electron. J. Chem. 110–115 (2022) doi:10.17807/orbital.v14i2.15871.
  3. Kutscher, D., McSheehy, S., Wills, J. & Jensen, D. Speciation analysis of Cr (III) and Cr (VI) in drinking waters using anion exchange chromatography coupled to the Thermo Scientific iCAP Q ICP-MS. Application Note 43098. (2011).
  4. Chang, Y.-L. & Jiang, S.-J. Determination of chromium species in water samples by liquid chromatography-inductively coupled plasma-dynamic reaction cell-mass spectrometry. J. Anal. At. Spectrom. 16, 858–862 (2001).