Development and validation of the US EPA hexavalent chromium source emissions measurement method

Abstract

The US Environmental Protection Agency (EPA) has validated a sampling and analytical method for measuring hexavalent chromium, Cr(VI), in source emissions at municipal sewage sludge incinerators and hazardous waste incinerators. The sampling method uses an impinger train that is designed to continuously recirculate an alkaline impinger solution to the sampling nozzle, where the incoming Cr(VI) is stabilized. Recirculation of an alkaline solution in the sample nozzle and through the probe avoids dry surfaces common to the traditional EPA Method 5 and Method 13-type sampling trains. Hot, dry surfaces in a sampling train can contribute to the conversion of Cr(VI) to trivalent chromium, Cr(III), during sampling. Additional post-sampling procedures to minimize the conversion of Cr(VI) include purging the sample train with nitrogen to remove reactive gases, followed by filtration of the impinger solution to remove particulate matter and insoluble chromium (Cr) compounds. The impinger solution is then analyzed by ion chromatography coupled to a post column reactor (IC/PCR), employing a diphenyl carbazide (DPC) reagent. Depending on the sample matrix, an in situ preconcentration technique can be employed resulting in an in-stack detection limit of 10 ng/m 3. During the validation of this test method, a radioactive isotope of Cr(VI) was used to monitor any conversion that occurred to Cr(VI) during sampling and preparation prior to analysis. EPA-sponsored laboratory experiments and field tests of this Cr(VI) sampling train typically demonstrated over 80% recovery of Cr(VI), compared to the Method 5-type train where recoveries were less than 50%. This paper describes EPA's current measurement method for Cr(VI) source emissions, discusses results obtained with this method at municipal sewage sludge incinerators and a hazardous waste incinerator, and presents sampling techniques for sources with elevated acid gas levels.