Speciation and preconcentration of Cr^III and Cr^VI in waters by retention on ion exchange media and determination by EDXRF

Abstract

The preconcentration, separation and determination of Cr^III and Cr^VI have been achieved by using a combination of ion exchange media and energy dispersive X-ray fluorescence (EDXRF) spectrometry. The species in aqueous solution have been retained sequentially on activated alumina and Dowex 1-X8 for Cr^VI and three different cation exchangers, Dowex 50W-X8, Zerolite and activated alumina, for Cr^III and then determined directly on the adsorption media. The results obtained show that the Dowex resins are the best for the speciation of chromium in waters. The use of these resins allows linear ranges of over three orders of magnitude and a reduction of those matrix effects so often observed in EDXRF. The method was tested using a 100-fold diluted Ground Water and Waste Water Pollution Control Check Standard WP-15 which contains Cr^III and was spiked with Cr^VI. Full recoveries were achieved from this standard with precision values of 4% or less. The detection limits obtained for a preconcentration factor of 50 were 0.3 mg l^–1 for Cr^VI on Dowex 1-X8 and 0.4 mg l^–1 for Cr^III on Dowex 50W-X8. These limits were reduced to 0.03 and 0.04 mg l^–1, respectively, if a 500-times preconcentration factor was employed. The scope of the method was investigated further by analysing spiked samples of local natural waters (river and sea) for both Cr^III and Cr^VI species directly. While the advantages of using this methodology are clearly seen, some limitations are noted for samples of high saline content.

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