Ecological risk assessment of heavy metals from cement factory dust

Background: The release of metals from industrial factories is one of the most important sources of environmental pollution. The present study aimed to determine the concentration of heavy metals like cadmium (Cd), chromium (Cr), manganese (Mn), nickel (Ni), and lead (Pb) in dust around the cement factory. Methods: A total of 22 dust samples were collected from areas around the cement factory in Isfahan province in spring and summer and transferred to laboratory for chemical digestion. Risk index (RI), integrated pollution index (IPI), mean of contamination degree (mCd), and contamination factor (Cf i) were calculated to determine the contamination status. Results: The concentration of heavy metals in the falling dust around the factory was expressed as Cd <Ni <Pb <Mn <Cr. Pearson correlation showed that there is only a significant negative relationship between the concentration of Cd and the distance from the factory. By increasing the distance from the factory, the concentration of Cd in dust decreased. The results of falling dust analysis showed that Cr has a high-risk potential in two seasons of spring and summer and Cd has a middle level of pollution in spring. Conclusion: According to the results, the deposited dust of study area is considered as a polluted dust and it is at higher risk of pollution with Cd and Cr.


Introduction
A dust storm is a meteorological phenomenon and usually occurs when the wind speed exceeds the threshold value and particles are removed from the soil (1).In recent years, the rapid growth of industry and urbanization has produced dust, particulates, and million tons of pollutants which threat the environment and human health (2).One of the industries that produce alternative amounts of dust in most stages of the process is the cement industry (3).Dust particulates released from the cement industry, due to the size and composition with other air pollutants, in addition to soil contamination, have irreparable effects on plants (4).Generally, sedimentation of cement dust on plants reduces the energy stored in plant tissues, closes stomata, and decreases photosynthesis and the growth of plants (5).Depending on their origin and direction, these particulates have a high ability to carry heavy metals (6).Heavy metals are the most toxic pollutants that enter the environment through industrial and human activities (7,8).Cement industries as the most polluting industries in the world, are the main sources of heavy metals such as cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), manganese (Mn), copper, and zinc (9,10), which create heavy metals with different physical and chemical properties at each stage of production including preheating, clinkering, clinker cooling, crushing, and storing (11).Heavy metals are important due to their inherent microorganisms and their physiological effects on humans and other living organisms at low concentrations (12) and are considered as the most hazardous components of human-made contaminants because of their toxicity and environmental sustainability (13).Among all heavy metals, Cd, Cr, and Pb are of most concern due to high toxicity (14,15).Several studies have been conducted on the pollution caused by the cement industry and determined the concentration of heavy metals like Cd, Pb, copper, zinc, Ni, Cr, and cobalt in limestone, particulates, and surface soil around the factory (5,7,8,10,11,(16)(17)(18)(19).Their results showed that the origin of heavy metals in soil depends on the dust rich in heavy metals from cement production (16).Evaluation of the effect of heavy metals on cement soil and vegetation showed that lead mobility in surface soil is more than 20 cm depth.In addition, heavy metals poisoning plays a role in reducing plant height, burning leaf margins, reducing leaf growth, and mostly the wilting of plants (17).Assessment of the heavy metals content in the dust generated by the Sagamu cement factory and its attendant health effects, especially on the inhabitants of the area.The results detected that the concentration of Cd, Pb, and zinc in all four sample types of limestone, shale, soil, and dusts were generally higher than their global means.In addition, the results of medical reports on the health of residents of the study area showed that the incidence of some diseases like cancer, respiratory, lung, heart, skin, kidney, and liver diseases are constantly increasing.Furthermore, the incidence of respiratory, lung, and heart diseases have doubled in this period (8).The evaluation of Ilam cement factory dust effects on the concentration of the heavy metals in soils in Karzan area, showed that the maximum concentration of Pb (96.40 ppm) was observed in April 2015 (18).A study on Cd and Pb concentrations of soils around the Kurdistan cement factory in western Iran, showed that Pb and Cd concentrations were weakly correlated with each other, indicating that Pb and Cd in soils may originate from different pollution sources.No distinct spatial trends of Pb with its low accumulation in the soils demonstrate that Pb content was mainly influenced by soil factors.The spatial pattern of Cd showed that the cement factory emission has an impact on the soil's Cd content, since the highest level in an area close to the cement factory.Estimated integrated pollution index (IPI) showed that the soils around the factory were practically uncontaminated by metals (19).The most contaminated sites are found within the 0 to 3 km of the cement factory (20), so, the present study aimed to determine the concentration of heavy metals such as Cd, Cr, Mn, Ni, and Pb in dust around the factory and determine the contamination level using the contamination factor (C f i ), IPI, risk index (RI), and mean of contamination degree (mC d ) at 0-3 km around the factory.

Materials and Methods
The study area The cement factory is located at 53 °8 É and 32 °40 Ń in the east of Isfahan province with a hot and arid climate in the desert region with an altitude of about 1600 m above sea level.The study area is 3 km radius around the cement factory, which has no residential area and its distance to the city center is about 30 km.The closest residential areas are two villages of Benvid-e-Olia and Sofla located at 4 and 5 km from the factory in the north of the region, respectively.The dominant vegetation in this region includes Artemisia sieberi, Fortuynia bungei, and Pteropyrum aucheri.Figure 1 shows the location of the study area.The executive operation of the factory was started in 2011 and the production and exploitation of cement was started in 2004.In this factory, 240 employees are working in the production line and administrative and technical departments.The total area of the factory is about 16 hectares and its nominal capacity is about 600 tons of cement per day.

Analysis of wind speed and direction in the region
In order to investigate the wind changes, the seasonal wind rose was plotted at the time of the trap placement (March-September 2017).Wind speed and direction data obtained from Naein station were analyzed.The data format was changed by visual basic (version 7) and used as the input of WRPlot (version 7) to plot the wind rose.Figures 2 and  3 present the seasonal wind rose of six months during spring and summer 2017.As shown in these figures, the dominant wind is in the west and northwest direction in spring and northwest in summer, which is effective in spreading dust pollution of the cement factory.
Sampling and laboratory analysis Dust samples were taken seasonally in two seasons of spring and summer in 2017.The distribution of sampling points was designed to cover 11 sampling stations according to the topographic characteristics and the dominant wind direction up to 3 km from the factory (21,22).The dust deposition trap was used to collect falling dust with 27 cm in diameter and 10 cm in depth covered by the stainless steel anti-bird thorns and artificial grass into the trap.The traps were installed at a height of 120 cm above the ground level in sampling sites (Figure 4).The trapped dust samples were seasonally distilled by water twice, transferred to the laboratory, and heated at 70 ᵒ C for 48 hours.Then, 1 g of dust was weighed with a precision of ±0.001 to measure the total concentration of heavy metals in accordance with ISO 11446 international standard procedure.According to the extraction procedure of trace elements, 7 mL of concentrated chloride acid plus 2.5 mL of concentrated nitric acid (1:3 ratio) and then, 5 mL of diluted nitric acid (0.5 M) drop by drop were added to each sample.The adsorption vessel and condenser were installed and rinsed with a further 10 mL of nitric acid after reaction time.In the final step, after cooling the samples, 33.3 mL of diluted nitric acid was added and the solution was completely filtered by Whatman 42 filter paper and reached the volume of 50 mL by adding deionized distilled water.The concentration of heavy metals like Cd, Cr, Mn, Ni, and Pb was determined in digested solution by Analytic Jena 330 Flame Atomic Absorption Spectrophotometer (23).Data analysis Data were analyzed using SPSS (version 22).The descriptive statistics of variables were calculated.Then, paired t-test was used for comparison of the heavy metals in spring and summer.Pearson correlation coefficient was used to determine the relationship between the distance from factory and the amount of pollutant integration.The normality of data was also investigated using the Kolmogorov-Smirnov test.In addition, the spatial variations of heavy metals concentration around the cement factory were described using definite methods such as radial basis functions and local polynomial interpolation by ArcGIS software version 10.2.

The pollution indices used for heavy metals analysis
To determine the status of contamination in the study area in terms of pastures and environmental protection, the C f i , and in terms of integration, the IPI, RI, and mean of mC d were calculated.
Contamination factor and mean of contamination degree C f i can be used to indicate the environmental contamination of a specific metal.This factor is calculated by the Eq.(1) (24).
Where C i f represents the contamination factor (mg/kg), 0 1 i C − is the average metal concentration (mg/kg), and C i n is the concentration of the same metal in the reference sample (mg/kg).
In Eq. ( 2), Cd is used for evaluating the overall pollution of the environment and calculated by the sum of the total contamination factor for all metals as follows.Due to the limitations of the degree of contamination index, the mean of contamination degree was used as Eq.(3) (25). 1 (3) Where mC d represents the mean of contamination degree and n is the number of examined trace elements (mg/kg).The standard value of trace elements was considered based   on the standards of Soil and Water Research Institute of Iran (Table1 ).

Integrated pollution index
The pollution level was calculated using Eq. ( 5).In this equation, PI refers to the pollution index of the i-th pollutant, C i is the concentration of the i-th pollutant (mg/kg), B i represents the base concentration of pollutant of soil parent materials (mg/kg), and n is the number of pollutants (26).Table 1 shows the categories of level of IPI. ) In a recent study, the values of Iran's standard soil trace element were used for comparing rangeland and environmental protection land-use (27) as shown in Table 2 (28).

Risk index
The potential environmental risk factor has been calculated to assess the contamination of heavy metals in soil and the ecological and environmental effects of heavy metals.RI is calculated according to the Eq. ( 6) and ( 7): 1 Where E i r represents the risk factor for each metal, T r refers to the toxicity response to heavy metals (Table 3), and RI is the risk index.This index is calculated by the sum of several metals or various pollution factors under investigation (14).Table 4 shows the classification of the index and potential environmental risk levels.

Results
Descriptive statistics on the concentration of trace elements including Cd, Cr, Mn, Ni, and Pb as well as the amount of dust in spring and summer are shown in Table 5.According to this table, Cr had the highest range of changes in spring and summer and Cd had the least change.In addition, Cr had the highest concentration in dust and Cd had the lowest concentration.Trace elements concentration was detected from the lowest to the highest Cd< Ni< Pb< Mn< Cr, respectively (Table 5).The results of the paired t-test for comparison of the heavy metals concentration in spring and summer are shown in Table 6.As shown in this table, there is only a significant difference between the concentration of Cd and Ni in two seasons.
To determine the relationship between Cd, Cr, Mn, Ni, and Pb in dust in spring and summer and the distance from the factory, Pearson correlation coefficient was calculated.The results are presented in Table 7.As shown in this table, the relationship between the concentration    Very high E i r ≥320 of Cd and the distance from the factory was significant (P < 0.01), indicating that with a probability of 99%, by increasing the distance from the factory, the concentration of Cd decreases.The correlation between the distance from the factory and dust deposited Cd concentration was negative (r= -0.863, P < 0.01) in spring.There is also a significant negative correlation between the distance from the factory and concentration of dust Cd (r= -0.784, P < 0.01) in summer.
The spatial variations of heavy metals in deposited dust were interpolated by inverse distance weighting and polynomial function methods (Figure 5).According to this figure, high concentrations of Cd and Pb in the study area were measured in spring and summer 2017.Spatial distribution of other elements shows that the hot spots were located far away from the factory and almost in the north, northeast, and east of the factory.The heavy metals content of the sampling site No.11 shows the highest concentration of Cr.The dust deposition sampler No.11 was located near the tailings and mining waste materials of the factory.Table 8 indicates the results of the contamination factor (C i f ) of each heavy metal based on Iranian soil standard for pasture and environmental protection land-use.The results show that the dust deposited around the cement factory area by considering pasture land-use standards in terms of Cr is polluted.Cd is in the moderate level of pollution in spring and Cr is in high pollution level in spring and summer seasons, by assumed the standard reference of environmental protection.However, it has less contamination in terms of other metals.The risk factor of each heavy metals (E i r ) and integrated RI of the dust deposited around the cement factory are shown in Table 9.The Cr of falling dust causes high risk for pasture and environmental protection land-use in spring and summer while the Cd is in moderate risk for the protection of the environment land-use in spring.The results of RI also indicate that the overall risk of heavy metals in the dust deposited around the factory in both seasons is very high for pasture use and environmental protection.IPI was obtained 1746.44 and 1438.03 in spring and summer, respectively, for pasture land-use and 321.61 and 53926.32 in spring and summer, respectively, for environmental protection, indicating that the pollution level of falling dust around the factory is high.In addition, mC d was obtained 349.57and 287.84 in spring and summer, respectively, for pasture use and 13098.82and 10785.65 in spring and summer, respectively, for environmental protection, indicating that the dust is severely polluted.The results of RI, IPI, and mC d analysis indicated that measurement of the risks caused by heavy metals pollution in the dust deposited around the cement factory can be useful and each of these three indices can be a good alternative for each other in terms of pollution measurement in such environments.Cd and Cr has a main role in the pollution of dust deposited around the factory.

Discussion
According to the results, dust deposition of cement factory increases the ecological risk due to trace elements such as Cd and Cr.Cd content in the dust deposited around the factory increased ecological risk in spring but Cr increased ecological risk in two seasons of spring and summer.Cr is not only abundant in the earth's crust, it also enters the environment during the mining process and cement production.Mousavi et al evaluated the concentration of heavy metals in the dust caused by Karun cement factory and concluded that Cr had the highest concentration while Cd had the lowest concentration (16), which is consistent with the results of the present study and a study by Ogunkunle and Fatoba (10).There was no difference in the concentrations of Cr, Mn and Pb between two seasons of spring and summer.However, the concentration of Cd and Ni differed in two seasons.Cd concentration was higher in spring while Ni concentration was higher in summer.The analysis of the correlation between the concentration of heavy metals and the distance from the factory showed that there is only a correlation between the concentration of Cd and decreased distance from the factory, which indicates the man-made origin and the role of the cement factory in producing Cd as well as the effects of occupational exposure groups, soil, and surrounding plants.The overall contamination status of the study area, using integrating indices (IPI, mCd, and RI), showed that the pollution level of cement factory is high.Cd and Cr are  the main pollutants.Khaksarnejad et al investigated the contamination index of Cr, Ni, and aluminum in the dust of Ghaen cement factory and reported that the contamination index for all three elements was in the middle class and the IPI was in polluted level (29,30).
In addition, the concentration of Cr, Ni, and aluminum in the dust of Ghaen cement factory was assessed and it was concluded that the potential ecological risk of heavy metals in all stations was low, which is inconsistent with the results of this study (21,30).

Conclusion
The single and integrating indices of heavy metals of the cement factory deposited dust detected that the moderate to high risk of pollution.Most risk factors were related to Cr and Cd.Because of the high concentration of Cr in the dust around the factory, there is no significant correlation between the distance from the factory and Cr content of dust deposited at 3 km away from the factory.By increasing the distance from the factory more than 3 km, the concentration of Cr in falling dust decreases.It is concluded that the Cd pollution is due to cement production process and the Cr pollution is due to mining and transportation of the primary materials from the mine to the factory.Range management and animal grazing preservation are necessary surrounding the cement factory due to increase the heavy metals of Cr and Cd by falling dust.Cr concentration is high and the kind of oxidation of Cr (Cr-III or Cr-VI) around the factory is very important.Further studies are recommended to analyze plant species at different distance from the factory and soil to determine the level of contamination.

Figure 1 .
Figure 1.Location of the study area including the cement factory, sample points, and villages.

Figure 4 .
Figure 4. Dust deposition trap in a sample site.Figure 2. Wind rose of Naein weather station (spring 2017).

Figure 2 .
Figure 4. Dust deposition trap in a sample site.Figure 2. Wind rose of Naein weather station (spring 2017).

Table 1 .
Standard classification of C i

Table 2 .
Standard reference value of some soil heavy metals of Iran for different land-use * The mean of earth's crust was used as the reference standard of manganese.

Table 4 .
The potential ecological risk factor

Table 5 .
Descriptive statistics for heavy metals concentration (mg/kg) in the cement factory during spring and summer 2017

Table 6 .
The results of comparison of heavy metals concentration (mg/kg) in the dust using paired t-test during spring and summer 2017

Table 7 .
Correlation coefficients between heavy metals concentration (mg/ kg) and distance from the factory (m) during spring and summer Summer -0.784 a -0.181 ns 0.279 ns 0.459 ns -0.506 ns ns=not significant; a significant (P value < 0.01).