Method validation and geochemical modelling of chromium speciation in natural waters

The study focuses on validating reference methods such as ICP-OES and ICP-MS for detecting ultra-trace levels of chromium in groundwater, where concentrations are typically very low. Additionally, it verifies a hyphenated technique, IC-ICP-MS, for determining naturally occurring Cr(VI) in tested waters. The validation process involved various chromium analysis variants, including isotopes ⁵²Cr and ⁵³Cr in ICP-MS and IC-ICP-MS techniques, along with specific emission lines in the ICP-OES technique. Statistical data processing revealed that the achieved limits of quantification for Cr in different techniques ranged from 0.053 µg/L to 1.3 µg/L, with the associated measurement uncertainty estimated between 14% and 19% (at a coverage factor k = 2, 95%). For speciation analysis, it was possible to quantitatively determine Cr(VI) at concentrations as low as 0.12 µg/L, with the measurement uncertainty ranging between 10% and 14%. The Kruskal-Wallis test indicated that for the 14 water samples analysed, there was no statistically significant difference in the results obtained using different analytical techniques (p > 0.05). The geochemical modelling approach applied enhances the understanding of chromium speciation in water samples, verifying the accuracy of speciation analysis and identifying specific ion forms in which Cr(III) and Cr(VI) occur. In the analysed water samples, the concentration of Cr(VI) ranges between 0.13 and 35 µg/L, with the primary form identified as the oxoanion CrO₄^2-. Importantly, statistical tests demonstrated no statistically significant differences between the total chromium concentration in water and the concentration of Cr(VI), indicating that the entire concentration of total chromium corresponds to Cr(VI) speciation.