Determination of hexavalent chromium in ambient air: A story of method induced Cr(III) oxidation

The accuracy of the determination of Cr(VI) in ambient particulate matter remains a challenge from the point of view of minimal Cr species interconversion. Knowledge of this method induced oxidation and reduction is particularly relevant for the determination of Cr(VI) in ambient particulate matter, as the level of observed Cr(III) oxidation (average of 1.7% in this study) can contribute significantly to the monitored range of measured Cr(VI) in PM10. For Cr concentrations in PM10 > 10 ng Cr m−3, this method induced oxidation could lead to false positive exceeding of an air quality guideline value of 0.2 ng Cr(VI) m−3 in PM10. The median daily Cr(VI) concentration in PM10 measured over a monitoring period of more than 2 months at two locations close to a stainless steel factory amounted to 0.9 ng Cr(VI) m−3 and 0.27 ng Cr(VI) m−3. Average daily Cr(VI)/Cr ratios in PM10 of 3.5% and 2.6% were measured at these locations. The described monitoring for the determination of Cr(VI) in ambient air via alkaline impregnated filters is sensitive (method detection limit of 0.015 ng Cr(VI) m−3) and reproducible (precision of the method ∼25%). The average Cr(VI) recovery of 75% strongly indicates the effects of ambient sampling conditions and ambient particles on the Cr(VI) recoveries. The stability of the Cr(VI) and the Cr(III) spike on 0.12 M NaHCO3 impregnated filters observed with XANES, indicates that the alkaline extraction of the filter in combination with the sampled air matrix is likely to induce the Cr conversions. The XANES spectra shows further that a Cr-spinel is the predominant component of Cr in ambient air PM10 at the monitored locations.