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%).