Distribution of heavy metal concentrations in the soil particle size fraction
Supplementary Table S3a,b show the minimum, maximum, and average concentrations of HMs in the different particle size fractions of urban and suburban soils. The HMs in the different fractions of urban and suburban soils were regularly distributed. Among the particle size fractions, the total content of most of the investigated HMs tended to increase as the size of the soil particles decreased. The highest content was generally pronounced for the particle size fraction of < 0.63 μm. The highest amount of metals that accumulated in the fine particles of < 63 μm could be explained by their high reactivity and their affinity toward HMs45. According to some researchers, HMs are often accumulated in the fine fraction, such as clay particles that act as metal sorbents, which is mainly due to their high surface area and negative surface charge40,45. It was generally observed that the total HMs concentrations in the mined soils of Mahad AD’Dahab were higher than those in the industrial activity-impacted soils of Riyadh. In this context, urban soils are often contaminated with HMs owing to anthropogenic sources. Previously, it has been reported that mining activities can result in significant metal accumulation in environmental compartments46. Alike, several studies found that mining operations are significant sources of HMs contamination in soils and higher contents of HMs in mining-impacted soils result from long-term activities47,48.
The Distribution factor (DF) has been widely used to assess the distribution of HMs and environmental risks in the different particle size fractions5,40. Supplementary Fig. S3a,b show the minimum, maximum, and average DFs for HMs in the different particle size fractions of urban and suburban soils. These obtained DF values indicated a greater metal accumulation in the finer fraction (< 63 µm) compared with that of larger size fraction (63–250 μm). Our data were in line with previous findings on the preferential partitioning of HMs to fine soil particle size fractions5,40,49. This can be explained by the larger surface area of the fine particles, which enhances the adsorption capacity of the fine fraction. Additionally, finer soil particles can have higher contents of secondary clay minerals, which are very strong sorbents for HMs49,50. On the contrary, the coarser fractions of sand and silt can have a higher content of the primary mineral quartz (e.g., SiO2), thereby leading to lower sorption capacity. It could be concluded that the fine particle fractions, especially the clay fraction, accumulated higher concentrations of HMs than the coarse fractions, thereby causing potential harm to human health and the environment50.
Supplementary Table S4 shows the Pearson correlation between the HM concentration in dust and soil < 63 µm over all the investigated areas. The Pearson correlation showed that there was a significant correlation between the total concentration of most HMs in dust and soil < 63 µm. The Pearson correlation was 0.55 for Cd, 0.67 for Cu, 0.53 for Fe, 0.66 for Mn, 0.52 for Pb, 0.55 for Ti, and 0.44 for Zn.
Heavy metals content in soil and dust samples
Table 1 shows the minimum, maximum, and mean concentrations of total HMs in the bulk soil samples collected from the investigated sites in the urban and suburban areas of Riyadh and Mahad AD’Dahab. The results indicated that the HMs content varied according to the metal type, sampling site, and study area. Generally, it was observed that the total HM concentrations in urban areas were higher than those detected in suburban areas. Among the two localities, the soil samples collected from Mahad AD’Dahab sites had the highest metal concentrations. For instance, the average HMs concentration in soil samples collected from urban Riyadh and Mahad AD’Dahab amounted to 5410 and 14,100 for Al, 0.012 and 0.197 for Cd, “nd” and 0.273 for Co, 9.08 and 13.2 for Cr, 0.702 and 18.5 for Cu, 4710 and 12,900 for Fe, 81.5 and 308 for Mn, 4.41 and 8.37 for Ni, 5.68 and 13.9 for Pb, 117 and 588 for Ti, and 13.7 and 54.3 for Zn (all in mg kg–1), respectively.
In the Riyadh area, the highest total concentrations of most HMs were found at site 16 (with the exception of Cu, Cd, and Co). In the Mahad AD’Dahab area, the highest total concentrations of Cd, Cu, Pb, and Zn were detected at site 3, which is close to the mining area. Moreover, the highest concentrations of Al, Co, Cr, Fe, Ni, and Ti were recorded at site 2.
In Saudi Arabia, quality guidelines for soil HMs have not been established. Therefore, in the current study, the concentrations of HMs in soil samples were compared with other guidelines, including the common range in the earth’s crust, the average concentrations in world soils, the average shale values, and the Dutch optimum and Act target values, as shown in Table 1. The concentrations of most investigated HMs were lower than their corresponding values of the common range in soil according to Ref.51. However, in urban Mahad AD’Dahab sites, the average concentrations of Cd, Pb, and Zn were higher than their corresponding values of the common range (0.06, 10, and 50 mg kg–1, respectively). Moreover, the maximum and average concentrations of Cu in urban Mahad AD’Dahab sites were higher than the average concentration in world soils.
Supplementary Table S5 shows the average metal concentrations in dust samples in relation to season. Generally, the highest average values of most of the HMs were detected in the spring season in urban and suburban areas of Riyadh and suburban areas of Mahad AD’Dahab. However, in urban areas of Mahad AD’Dahab, the highest average values of most HMs were found in the winter season. The variation in the order of the highest metal levels between these two seasons (winter and spring) could be explained by changes in meteorological conditions.
Table 2 shows the comparison of the obtained average metal concentrations with those reported for other countries. The comparison of HMs levels in the collected dust samples with those of other countries showed that the average content of HMs in the investigated sites in the current study were lower than those of most other cities (e.g., Riyadh (Saudi Arabia), Khamees-Mushait (Saudi Arabia), Jeddah (Saudi Arabia), Middle and South of Iraq, Kermanshah (Iran), Rafsanjan SE (Iran), Shangqing (China), Xi’an (China), Hong Kong (China), Oslo and Madrid). However, they were higher than those measured in Luanda (Angola) (especially for Al, Co, Cr, Ni, Ti, and Zn).
These results indicated that soil and dust samples were contaminated with Cd, Pb, Cu, and Zn. The highest levels of these HMs in top 3 cm soil layer could be due to the accumulation of HMs-laden dust particles. Jian et al.52 has previously reported the contamination of topsoil potentially hazardous elements such HMs. It was suggested that industrial and agricultural activities, traffic emissions and natural sources were responsible for topsoil contamination52. Likewise, Wang et al.53 collected samples from roadside dust and found that 90% of dust contained higher levels of HMs such as Cd, Cu, Hg, Pb, As, and Zn, thus posing serious health risks to ecosystem and human health. Similar results were reported by Cai et al.54 for soil samples, which reported the contamination of soil by Cd, Pb and Zn through agricultural practices and traffic activities.
Pollution and ecological indices and sources of HMs in soil and dust samples
The calculated data pertaining to the PI, Igeo, EF, Ei, and RI parameters for all HMs in soil and dust samples from urban and suburban areas of Riyadh and Mahad AD’Dahab are presented in Tables 3, 4, 5 and 6. It was generally observed that the values of pollution and ecological indices of all the analyzed HMs were higher in dust samples vs. soil samples. The calculated data of PI for all HMs of soil and dust samples from urban and suburban areas of Riyadh and Mahad AD’Dahab are presented in Table 3. The results showed that all soil samples collected at Riyadh sites had maximum, minimum, and mean PI values < 1, indicating a low level of pollution with HMs54. However, the soil samples collected at urban sites of Mahad AD’Dahab had maximum PI values of 2.0 for Cd, 1.4 for Pb, and 1.1 for Zn, indicating a moderate level of pollution with these HMs. For dust samples, compared with soil samples, the average PI values for Zn were higher than 3, suggesting a high pollution level (Table 3). The urban dust… 1,>
63 µm)> 0.63 μm.>