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Development and Optimization of Ambient Hexavalent Chromium Measurement Methods

by
Lihui Huang
M.S., Tsinghua University - 2006



Thesis Advisor: Zhihua (Tina) Fan, Ph.D.
Graduate Program in Environmental Science/Exposure Science

Conference Room C
EOHSI Building
Piscataway

Monday, June 25, 2012
10:00 a.m.


Abstract

The overall goal of my dissertation research was to develop and optimize a sampling and analytical method for the measurement of soluble and total Cr(VI) in ambient air, respectively, so that the health risks associated with inhalation exposure to ambient Cr(VI) can be well defined.

The method for the measurement of soluble Cr(VI) in ambient air, which was previously established in our lab, was optimized. The modified filter cleaning and pretreatment procedures significantly improved the method sensitivity. The method detection limit (MDL, determined with field blanks) and analytical detection limit (ADL, determined based on the USEPA definition) of the optimized method were 0.06 ng/m3 and 0.03 ng/m3, respectively. This method yielded 116 ± 14% recovery for soluble Cr(VI) in a Cr(VI)-certified material (SQC 012).Soluble Cr(VI) in a reference urban PM (NIST 1648) was determined with this method to be 1.75 ± 0.24 mg/kg (%CV = 14%). The stability of soluble Cr species during extraction, storage and sampling was assessed. Both Cr species were stable during storage at -10șC for up to 120 days. Soluble Cr(VI) was stable during extraction but 0 – 17% soluble Cr(III) was converted. The chamber and field study results suggested that SO2, PM matrix and humidity could affect the stability of soluble Cr species during sampling. To correct potential errors in Cr(VI) measurements due to instability, Speciated Isotope Dilution Mass Spectrometry approach should be employed in the measurement method.

My thesis research also developed a method to measure total Cr(VI) in ambient air, which includes soluble and insoluble Cr(VI). This method included collection of PM using a Teflon filter, microwave extraction with 3% Na2CO3-2% NaOH and Cr(VI) detection with IC-UV (1,5-diphenylcarbazide colorimetry at 540 nm). The MDL was 0.33 ng/m3, which was lower than current standard measurement methods for airborne total Cr(VI)(e.g. NIOSH 7605, NIOSH 7703 and OSHA ID 215). This method yielded 119.5 ± 10.4% and 106.3 ± 16.7% recovery for Cr(VI) in two Cr(VI)-certified reference materials, SQC 012 and SRM 2700, respectively. Total Cr(VI) in a reference urban PM, NIST 1648a, was determined with this method to be 26.0±3.1 mg/kg (%CV = 11.9%).

The two methods were applied to 3 locations, i.e. Meadowlands, Elizabeth Trailer, Rahway, with mixed Cr emission sources and a suburb area, Piscataway, in New Jersey. In the 3 locations with mixed Cr emission sources, the mean concentrations were 1.05 – 1.41 ng/m3 (winter) and 0.99 – 1.56 ng/m3 (summer) for total Cr(VI); 0.11 - 0.19 ng/m3 (winter) and 0.18 – 0.37 ng/m3 (summer) for soluble Cr(VI). In the suburb area, the mean concentrations were 1.07 ng/m3 (winter) and 0.99 (summer) for total Cr(VI), 0.03 ng/m3 (winter) and 0.12 ng/m3 (summer) for soluble Cr(VI). The mean excess cancer risks from exposure to soluble and total Cr(VI) were 1.9 – 2.7 and 11 – 18 per one million population in the studied locations.


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