3-1 Mineralogy of soil and airborne dust
The mineralogical composition of dust samples analyzed by X-ray powder diffraction (XRD) for soil and airborne particles deposited in containers at different heights are shown in Fig. 2 and Table 1. The main phases of the soil are quartz, albite, calcite, orthoclase and chlorite. Comparison of the XRD pattern of soil and all dust samples shows that the amorphous phase is the main and most common phase in the dust fractions deposited (Fig. 2). Quartz and albite are among the crystalline mineral phases of the collected dust samples. While a higher amount of calcite was found in 6m and 15m height samples, however, its content in the 21m level was very low. Mineralogical analysis of the collected dust samples at different heights ascertained that the amount of calcite in samples was decreased by increasing the sampling heights from 6m to 21m (Fig. 2). The presence and increase in the amorphous phase in the samples might be attributed to poorly crystalline aluminosilicates or clay minerals in the aggregates, rock crushing processes, abrasion and sieving. The presence of non-crystalline particles in airborne dust makes it difficult to determine the main sources of dust pollutants in Tehran province. Quartz and albite are common as the main phases in all samples, so they are more likely to be found in areas near the source of such minerals than other compounds. Calcite was decreased and occurred as a minor phase at 21m height in the mine and so at the nearest residential area. The presence of calcite was more likely in coarse particles. There is a high probability of solubilization or transformation processes to take place on calcite also the size of calcite and its relatively high density and low hardness as compared with quartz can be a reason of why the calcite was nearly absent in samples from 21m height. Quartz and albite, can be considered as the best indicators of the studied mine sites in the region and alluvium of Tehran. The low solubility of these minerals in aquatic and acidic solutions is an important factor in their respiratory toxicity resulting from their bioaccumulation (and bio-durability) in the lung, as well as their environmental toxicity (Krueger et al., 2005; Avramescu1et al., 2017).
Table 1. Mineralogical phases of soil and dust samples in the study area.
Locations
|
quarry pit soil sample (SQ)
|
6m
Mine site
|
15m
Mine site
|
21m
Mine site
|
Major Phases
|
quartz,
albite, calcite
|
amorphous
|
amorphous
|
amorphous
|
Minor Phases
|
orthoclase chlorite
|
quartz, calcite, albite,
|
quartz, calcite,
albite
|
quartz, albite
|
Trace phases
|
-
|
-
|
-
|
calcite
|
3-2 Size fractions and morphology of dust collected at different heights:
The weight percent of the samples collected at different heights decreased gradually from lower to higher heights (Table 2).
Table 2. Weight and Weight percent of collected samples in containers at each height using the SPHS sampling setup
Height above the ground (m)
|
6m
|
9m
|
12m
|
15m
|
18m
|
21m
|
Total
|
Weight of samples (g)
|
0.74
|
0.59
|
0.58
|
0.47
|
0.32
|
0.24
|
2.95
|
Weight percent of samples (%)
|
25.38
|
20.06
|
19.76
|
15.84
|
10.96
|
7.97
|
100
|
Weight percent of samples (%)
|
81.04%
|
18.93%
|
100%
|
The Feret's diameter (Df) of the largest particle deposited was 101.17μm. The amount of ≤10µm particles collected at different heights in either container was 85% of the total deposited sample. 40% of the particles were ≤2.5μm and 2.5% of particles were ≥40 µm (Table 3).
Table 3. Mass percentage of all dust particles based on the size in sample sections (both container and adhesive tape in all heights) of the SPHS method in the study area
Size of dust particles
(µm)
|
Numerical percentage of sediment particles in containers (%)
|
Numerical percentage of particles on adhesive tapes (%)
|
0-0.1
|
0.00
|
6.73
|
0.1-1
|
18.50
|
20.43
|
1-2.5
|
18.09
|
24.04
|
2.5-5
|
22.85
|
27.40
|
5-10
|
17.47
|
12.50
|
10-20
|
11.45
|
5.77
|
20-40
|
8.71
|
3.13
|
40-60
|
1.93
|
0.00
|
60-80
|
0.51
|
0.00
|
80-102
|
0.50
|
0.00
|
The highest recorded wind speed during the sampling campaign was 8 m/s, so that according to this study, particles of 50nm to 100µm in size, can travel up to 21 meters at this wind speed. The comparison of sediment particle size distribution collected dust samples at each height in containers in SPHS is depicted in Fig. 3. The distribution of particles between 2.5-5 prevailed in samples from 9 m is somehow irregular and at height of 6m the particles between 0.1-1 and 2.5-5 are dominant, whereas the number of particles between 1-2.5 is relatively low. The abundance in particle size can be attributed to the closer distance to the source of dust generation (i.e., crusher). In short, the dust stream and the amount of settled dust can be affected by the power of crusher, type, density, crystallization, hardness and moisture content of feed minerals, humidity and the characteristics of atmospheric currents of the surrounding environment and the sampling location. There are more than 60 sand and gravel quarries in this region. If wind speed reaches and/or exceeds 8 m/s, it can lift potentially airborne submicron particulates in these mine sites and transfer them to longer distances. Hence, the health impact of these particles should also be taken seriously.
The results show that the percent number of the particulates collected on adhesive tape in the range of ≤ 5µm, ≤ 1µm, and ≤ 0.1 µm were 78.6%, 27.16%, and 6.73%, respectively. The percent of particles in the dust sampling containers were respectively about 59.44%, 18.5% and 0%. Therefore, it can be suggested that capturing method is effective in size distributions of particles in nano and fine sizes. According to this study, the adhesive tape can be an accurate and easy way for dust sampling in comparison with dust falling in containers.
Physical processes in mine such as vibration of the conveyor belt, sieving, trucks movement on the dirt roads, crushing, and powerful local wind streams are factors that can separate and re-suspend the fine and ultra-fine clay particles into the air. Most of the time nano and ultrafine particles are in the nucleation mode and subsequently cluster to form larger particles (Reijnders et al., 2018). These particles due to the presence of electrostatic and intermolecular forces and their high surface area, can be accumulated, aggregated, and agglomerated to each other to make larger particles. This may create some errors in the estimation of the percentage of the fine and nanoparticles which is very important in toxicological studies (Bakand et al., 2012).
The comparison of the particle morphology by two methods in SPHS shows that the airborne particles are similar to each other in two sampling methods at different heights and mainly are non-spherical aggregates of irregular grains, rounded irregular, prismatic and rhombic forms (Fig. 4 and Fig.5).
A major problem to health risk evaluation of particles is the collection efficiency for nanoparticle sizes, particularly for airborne measurements (Kumar et al., 2010) then it is suggested to use dynamic light scattering (DLS) and other new methods to a better determination of percentage of nano-sized deposited or agglomerated and aggregated forms of particles and their characterizations (Knippertz and Stuut, 2014).
3-2 Geochemical data of dust collected at different heights in the SPHS method:
Because of the low mass (weight concentration) of fine particles, the EDX analysis is ideal for their analysis (Table 4, Fig. 4: [g] and [h], Fig. 6). The total amounts of silicon and calcium in total EDX samples are in good agreement with the coarse dust particles. In fine particles, there is a slight decrease in silica. Calcite in the XRD pattern was a main phase at 6m and a rare phase at 21 m, this change is associated with the elemental analysis and Ca/Al ratio result (Figure 6). Comparison of Si/Al ratios of dust samples indicates that the Si/Al content in all particles deposited at different altitudes is much higher than the Earth's crust (3.83) (Taylor and McLennan, 1995) and the Si/Al ratio could be attributed to 4-10 to these mines. This result is related to quartz crystalline contents in these samples. Quartz has low solubility and then could remain in the atmosphere without any interaction with other aerosols or gases/liquids. Si/Al ratios fall mainly into a range between 2 and 7, pointing to mixtures between quartz and aluminosilicates in agreement with the mineralogical data. These trends in the Si/Al ratio are not observed in other studies (Scheuvens et al., 2013). Iron is higher in the coarse particles up to 18 meters. In most samples, Fe/Al and Ca/Al are more than the Earth's crust (Taylor and McLennan, 1995). Iron-bearing particles often seem to be positioned at the surface of silicate particles probably due to natural processes and anthropogenic sources. For magnesium, this ratio is similar to that of the Earth's crust. The Geo-accumulation index and Enrichment factor for this mine (Menhaje-Bena et al., 2021) have shown that this soil is not contaminated by this element.
This data can be helpful to decipher the alteration and processing of mineral dust and its mixing with different pollution aerosols.
Table 4: The study of changes in the silica, calcium, magnesium and iron ratio to aluminum in total particles by total EDX at different heights
Height
|
21m
|
18m
|
15m
|
12m
|
9m
|
6m
|
Si/Al
|
4.37
|
4.21
|
5.28
|
4.26
|
4.93
|
5.52
|
Ca/Al
|
1.86
|
2.05
|
2.47
|
1.78
|
2.09
|
2.87
|
Mg/Al
|
0.17
|
0.24
|
0.13
|
0.52
|
0.12
|
0.13
|
Fe/Al
|
0.70
|
0.51
|
0.79
|
0.56
|
0.78
|
0.81
|