Characterization of chromium species in urban runoff

Little is known about the presence of the element Cr in its toxic hexavalent form Cr(VI) in stormwater runoff from urban areas. Most studies report only total Cr concentration, i.e., including also the nontoxic Cr(III) molecular form. The objective of this study was to evaluate a field method based on cation-exchange for characterization of Cr species in urban stormwater runoff and soil leachate. We used a 0.05 mol L Na and Ca solution and a soil leachate as matrices and spiked these with Cr(III), Cr(VI), or both in the concentration range of 1 to 100 μg L. We then filtered the test samples through cation-exchange cartridges. In the Na-Ca salt matrix, the Cr(III) was retained 100% and recovery values of Cr(VI) were 86 to 100%. Furthermore, in such a matrix, each cartridge could be reused at least nine times without a drop in retention of Cr(III) or recovery of Cr(VI). In a soil leachate matrix, the method appeared less applicable. Apparently Cr(III) forms complexes with dissolved organic matter, allowing it to bypass the cartridge, resulting in incomplete Cr(III) retention and thus incomplete speciation. The complexes are formed rapidly after spiking, thus changes in the Cr-species distribution are likely to occur within a few hours when fresh samples are stored. Furthermore, we concluded that Cr(III) at neutral pH in pure solution or complexed with dissolved organic C can sorb irreversibly to polyethylene and polypropylene containers. Our findings show that there is still a need for a method that can be applied to urban runoff in the field. Furthermore, cartridge speciation methods should ideally be tested before being applied on solutions containing organic matter.