Improvements in Sampling for Cr (VI) in TSP

MEHDI AMOUEI TORKMAHALLEH (1), Robert A. Cully (1), Thomas M. Holsen (1), Philip K. Hopke (1)

Center for Air Resource Engineering and Science, Clarkson University

     Abstract Number: 91
     Last modified: October 29, 2009

Abstract
Chromium primarily exists in the trivalent and hexavalent states. Trivalent chromium (Cr (III)) is an essential micronutrient and plays a role in the control of glucose as well as the metabolism of lipids and proteins. Hexavalent chromium (Cr (VI)) is listed as a group 1 human carcinogen by the International Agency for Research on Cancer. Factors such as pH, temperature, humidity, ferrous ions and organic compounds can affect the reduction of Cr (VI) to Cr (III). Since hexavalent chromium is toxic, sampling particulate matter to measure the concentration of chromium (VI) in ambient air must be accomplished such that the reduction reaction of chromium (VI) to chromium (III) is avoided. Accurate exposure assessment requires that the chromium maintains its hexavalent state during sampling and subsequent storage prior to the application of any analytical methods. The current approach used in the National Air Toxics Network requires that all samples be retrieved from the field within one day after the sample has been collected to prevent Cr (VI) loss (up to 20% on the first day). However, retrieving filters within one day is often difficult for state and local agencies. To be a practical method, the filters need to be able to stay in the samples for up to 3 days. Thus, an improved sampling system is needed that will accurately sample and preserve Cr (VI) in ambient particulate matter.
Previous studies of reduction of chromium (VI) to chromium (III) suggest that the reduction reaction only takes place in the presence of liquid water. Therefore, water adsorption by filter and particulate matter on the filter during sampling time leads to loss of hexavalent chromium. Thus, the current sampling method could be improved by providing cooling and drying of the filter to retard reduction on the filter during sampling and non-sampling time. However, since cooling increases the relative humidity above the surface of the filter, the filter along with particles may adsorb water in different relative humidity. Hence, cooling may play a negative role by increasing the relative humidity during sampling time, and it can also play an effective role to stop the reduction reaction during non-sampling time. In this work the kinetics of water adsorption by particulate matter on the surface of the celluse filter in different relative humidity was investigated to understand whether cooling process is required in current sampling trains for sampling hexavalent chromium or not.