QUANTIFICATION OF AMBIENT INSOLUBLE HEXAVALENT CHROMIUM WITH DOPED CARBON- DOTS AND FIELD MEASUREMENTS

Abstract

Hexavalent chromium Cr(VI) is a known carcinogen and apart from being carcinogenic, its exposure duration has adverse health effects on humans such as perforation of the nasal septum, asthma, bronchitis, pneumonitis, and lung cancer. Insoluble Cr(VI) persists to be more toxic than soluble Cr(VI) since when deposited in the lungs, it acts as a strong oxidizer that can breakdown cell membranes to produce reactive radicals that further causes changes in cell function or mutations. The deliquescence of Cr(VI) under atmospheric conditions (pH ~4), suggested the presence of insoluble/sparingly soluble Cr(VI) species such as PbCrO4, BaCrO4, CaCrO4, and (NH4)2CrO4 that can precipitate in solution. Hence, the need to quantify Cr has necessitated the development of advanced materials that can serve as sensors to detect total Cr(VI) (soluble + insoluble) in environmental samples. The main objective of this research was to develop a functionalized method to quantify insoluble atmospheric Cr(VI) concentrations in PM. The method development involved the use of a fluorescent carbon material (N,S-CDs) as a sensor towards Cr(VI) in ambient PM. Laboratory results yielded good recoveries of Cr(VI) with mean (relative standard deviation) values of 106.0% (5.3%), 102.3% (3.6%), 96.4% (1.9%), and 101.7% (2.2%) for PbCrO4, BaCrO4, CaCrO4, and (NH4)2CrO4, respectively. The application of the proposed fluorescent method on field PM samples resulted in a method detection limit (MDL) of 0.32 ng/m3 for total Cr(VI) quantification. This MDL is much lower than the NIOSH 7605 (50 ng/m3), OSHA ID-215 (3 ng/m3), and ASTM D 5281-92 (0.2-1.0 ng/m3) methods. The total Cr(VI) concentrations of ambient PM collected in Aktobe city in the fall and winter seasons had mean (S.D) of 5.30 ± 2.16 ng/m3 and 2.26 ± 1.80 ng/m3, respectively. Insoluble Cr(VI) values were 4.80 ± 1.96 and 2.19 ± 1.75 ng/m3, respectively for the fall and winter seasons. Size-segregated sampling in Astana showed total Cr(VI) in size fraction < 0.25 μm as the highest with a maximum value of 9.77 ng/m3 in summer. These results showed that total Cr(VI) concentrations on warmer days were significantly higher than on cooler days due to factors including higher temperature, ozone, and NO2 concentrations on warmer days, and higher VOCs concentration on cooler days. These observations and findings demonstrate that gas-solid reactions of Cr(III) and Cr(VI) can control the speciation of atmospheric Cr, especially below the deliquescence relative humidity (DRH) point (76%).