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Results
Results
SEM Images
SEM Images
Scanning electron microscopy images from Dietrich et al. (2019). The set of images show extensive evidence of technogenic spherules of variable sizes.
Scanning electron microscopy images from Dietrich et al. (2019). The set of images show Pb-rich particles. Size distribution and chemical composition of spherules, as well as bulk concentrations of Pb, are not fully constrained.
HAADF-STEM Images
HAADF-STEM Images
Left: High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of aggregates of road sediment particles showing a wide range of textures including minerals, clay mineral aggregates, and technogenic spherules. (A) Shows individual and aggregated technogenic spherules. (B) Shows an isolated spherule in a mineral aggregate and a phyllosilicate clay mineral aggregate. (C) Shows spherules in an aggregate and one adhered to a conchoidal fractured grain. (D) Shows an example of an iron oxyhydroxide aggregate with textures consistent with goethite. (E) Shows aggregates with a larger spherule surrounded by smaller spherules. (F) and (G) Show spherules mixed in fine-grained mineral aggregates. (H) Shows a natural dolomite exhibiting rhombahedral cleavage. Collectively these images show that technogenic spherules are pervasive in road sediment samples, are intimately mixed, and partially explain some linear relationships of siderophile elements. This assertion is supported by STEM elemental maps on sample materials shown at right.
STEM-EDS Images
STEM-EDS Images
Above: Examples of scanning transmission electron microscopy—energy dispersive spectroscopy maps of road sediment form MDMK 2 near the steel plant. Images show complex and uneven distributions of elements in technogenic spherules and mineral aggregates. Chromium and molybdenum were repeatedly observed and arsenic was occasionally observed. The chemical textures partially explain chromium as a pollutant linked to the steel plant. The overall variation in the particles partially explains some variation in the bulk chemical concentrations observed.
HR-ICP-MS Correlations
HR-ICP-MS Correlations
Above: Cr and V concentrations showing a strong linear relationship. The trend is interpreted as largely reflecting contamination from steel manufacturing from technogenic spherules and other steel manufactured derived particles. Upper continental crust values are 92 ppm (Cr) and 97 ppm (V) Rudnick and Gao (2003), suggesting some points in the power portion of the plot may be geogenic background.
Above: Pb and Zn concentrations showing clustering relationship separated primarily on Pb concentrations. The trend is interpreted as largely reflecting variation in traffic with higher Pb associated with the steel plant and interstate. Upper continental crust values are 17 ppm (Pb) and 67 ppm (Zn) Rudnick and Gao (2003), indicating that the vast majority of analyses indicate pollution concentrations.
Histogram: Diameter if Spherules
Histogram: Diameter if Spherules
Left: A histogram showing the distribution of technogenic spherule diameter measured from HAADF images in micrometers. The X-axis is the binning of diameters in micrometers. A overall logarithmic decrease in diameter is observed indicating suggesting that spherules less than 212 μm dominate by population. Thus technogenic spherules may pose a risk for human health exposure and are likely to be remobilized by air and rainfall.
HR-ICP-MS & XRF Data Tables
HR-ICP-MS & XRF Data Tables
Above: Tabulated averages for HR-ICP-MS data with aggregate statistics shown.
Above: Comparison of XRF and HR-ICP-MS data showing large differences.
Discussion
Discussion
CONTEXT:
CONTEXT:
Road sediment is a medium that can be utilized to understand the extent of pollution in a community (Dietrich et al., 2019, 2022). It is important to quantify the nature and distribution of particles within the road sediment to do this. Road sediment is an environmental concern because there may be particles that are toxic and are small enough to become airborne in the wind and be inhaled, or be transported by precipitation runoff and deposited in water supplies. This particulate matter (PM), is common in road sediment and has a high risk of inhalation by humans. PM2.5-10 can be inhaled and absorbed through the lungs, while PM2.5 can be inhaled and absorbed into the bloodstream. Specifically, PM2.5, PM2.5-10 has the potential to contain harmful metals in the form of technogenic spherules. Previous work by Dietrich et al. (2019) showed the presence of technogenic spherules using scanning electron microscopy but the lower size range of these particles was not quantified. Additionally Dietrich et al. (2019) identified extensive Pb-rich particles but did not quantify the concentrations.
TEM INVESTIGATION:
TEM INVESTIGATION:
Our TEM data show that nanoscale technogenic spherules are common in road sediment. Technogenic spherules are commonly observed using multiple TEM techniques and the average dimension observed was 0.23 micrometers which is an order of magnitude below the PM 2.5 classification. Most technogenic particles are nanoparticles and these appear to be iron-rich. Siderophile elements such as Cr and V are very likely concentrated in these spherules and this is supported by the positive correlation of concentrations observed in HR-ICP-MS. Technogenic spherules are intimately mixed in sediment but are distinct from natural particles and should be recognizable in other environmental media and may be recognizable in tissue samples using TEM techniques. The limited data presented here provides context for others to do public health investigations in the context of technogenic spherule investigations. Additionally limited TEM data found presence of arsenic, and this was potentially linked to triangular crystals of which are trigonal or pseudo-trigonal in forms. Sparse occurrence of molybdenum (Mo) were observed in STEM-EDS maps which may be steel manufacturing related. More STEM-ED mapping is desired to better understand the diversity of composition of nanoparticulate and technogenic spherules.
BULK CHEMICAL INVESTIGATION:
BULK CHEMICAL INVESTIGATION:
Bulk chemical investigation with HR-ICP-MS indicate that Cr, V, Pb, and Zn have concentrations of concern amongst the sample population. The strong correlation of Cr and V is interpreted to reflect steel pollution. Pb, Zn, and Cu do not strongly correlate but form groups, largely defined by a population of higher lead samples. This is somewhat unexpected owing to the fact that chalcophile elements generally follow each other in urban settings (e.g. Davis et al. 2001). The grouping Pb, Zn, (and Cu) suggests perhaps high or legacy lead associated with traffic, exterior homes, traffic paint, or steel pollution near the steel plant, including the major roadways. Zinc also might be expected to occur in association with traffic. The results for Pb, Zn, and Cu point to a potential opportunity for future work to investigate Pb, Zn , and Cu isotopes to source this complex pollution. This might be challenging owing to detailed reservoir characterization needed (access to steels, access to coke and coal, geogenic background). Notable Pb, Zn, and Cu were not observed in TEM investigation of samples and this suggests that materials containing these are in larger (e.g., > 5 micrometer) size fractions.
HUMAN HEALTH CONCERNS: Pb & Cr
HUMAN HEALTH CONCERNS: Pb & Cr
The presence of Pb in any amount within road sediment is concerning because of its effects on both human health and the environment (e.g., ATDSR, 2020). Children are especially vulnerable to the ingestion of lead because of common engagement in hand-to-mouth activities and their more fragile nervous systems. Pb in road sediment also has the potential to be dissolved and leach into the groundwater system. Cr is recognized as a carcinogen in hexavalent form (Cr6+) (e.g., ATDSR, 2012) and the oxidation state of Cr in these materials is not constrained.
Compared to the XRF data, the HR-ICP-MS results indicate a significantly lower average ppm of Pb, copper (Cu), and zinc (Zn). Additionally, the results also demonstrate the variability of metal concentrations in different samples. A systematic investigation of XRF determinations and HR-ICP-MS on road sediments should be determined to better understand instrumental variation. XRF techniques have emerged as useful geochemical survey techniques, but clearly HR-ICP-MS in this case has produced better, more accurate data.
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