The analytical assessment of metal contamination in industrial soils of Saudi Arabia using the inductively coupled plasma technology

: In response to signi ﬁ cant demand for evaluating the presence of heavy elements in diverse industrial areas of Saudi Arabia, the study sought to analyze the concentration ratios of 16 elements across 11 di ﬀ erent arti ﬁ cial soil types in the industrial zones situated in Riyadh, Sudair, and Al-Ahsa. To achieve this objective, the research utilized the wet digestion technique and employed an ICPE-9000 spec-trophotometer for quantifying element concentrations. The study had a dual focus: initially, it sought to gauge the pollution attributed to heavy metals arising from enrichment processes, and second, it evaluated the geographical accumulation of pollutants in the soil. The results revealed varying concentration levels of heavy metals across the industrial areas under study. Speci ﬁ cally, the soil in the Yanbu region exhibited the highest concentration values for iron, manganese, zinc, chromium, nickel, copper, cobalt, cadmium, and beryllium. In contrast, the soil in the industrial area of Riyadh had the highest concentration values for lead, mercury (Hg), molybdenum (Mo), selenium (Se), and arsenic (As). Furthermore, the highest concentration of Cd was recorded in the soil of the Jubail region. Conversely, the soil in the Al-Ahsa region displayed the lowest concentration levels for these heavy metals. The conductivity of the synthetic soil ranged from 0.47 to 6.07 μ S·cm − 1 , accompanied by a pH range of 6.6 – 8.6. The results emphasized the ﬂ uctua-tions in element concentrations, indicating signi ﬁ cant implications for both environmental and human health. Notably, around 20% of the gathered samples showed concentrations of heavy elements like Mo, As, Hg, and Se that surpassed the allowable limits.


Introduction
Soil contamination with heavy elements represents a severe and enduring environmental challenge that has profound implications for human society [1][2][3][4][5][6].These heavy elements, which are inorganic pollutants and include heavy metals, are naturally occurring but are also generated as a result of various human works [6].The soil element ratio has been scientifically established to be a consequence of agricultural and industrial processes [7].Furthermore, the escalation in the levels of trace metals in the environment due to human activities is performed, recently [8].While these elements are essential for cellular functions, they can become toxic when present in excessive concentrations.In fact, some heavy metals exhibit toxicity even at exceedingly low levels [9].
Industrial processes contribute significantly to environmental pollution, which, in turn, poses significant problems for human populations [10,11].High concentrations of these metals can enter the human body through various means, such as suspended dust and direct contact with contaminated materials [12].Contaminated soils not only jeopardize the ecosystem but also become more problematic as environmental conditions undergo alterations [13].
Among the heavy metals, certain ones are particularly harmful, including arsenic (As), asbestos, cadmium (Cd), uranium, lead (Pb), thallium, mercury (Hg), antimony, manganese (Mn), barium, chlorine (Cl), zinc (Zn), beryllium (Be), bromine, and bismuth [14].Significantly, certain heavy elements have garnered acknowledgment as extremely toxic substances in contemporary contexts.The mining and utilization of these minerals in diverse industrial procedures have led to previously overlooked environmental and health concerns.The increased presence of these pollutants in the soil contributes to water pollution, particularly as environmental conditions undergo alterations [15][16][17][18].
The presence of heavy metals in coastal environments has a profoundly detrimental impact on their environmental quality [19][20][21][22].The coastal city of Jeddah, located along the Red Sea in Saudi Arabia, has experienced a significant increase in human activities over the past three decades, resulting in heightened pollution levels.The Red Sea coast, particularly in Yanbu, serves as a vital source of seafood and marine transportation within the Kingdom, making it closely linked to public health concerns.To assess element contaminations in the marine ecosystem, scientists often analyze minerals present in sediments as indicators of toxic elements [23].
Recently, significant industrial progress has taken place in Saudi Arabia, particularly in sectors like petrochemicals, oil, and gas production.Precise and dependable measurement of potentially harmful element ratios in soil or sediment requires the adoption of contemporary methodologies, such as microwave-assisted digestion of organic components.These approaches have been designed to break down metals present in the samples effectively.Many studies have utilized sophisticated technical instruments capable of applying high pressure to the samples for this specific purpose [15][16][17].
Herein, the research objective is to evaluate the concentration ratios of 16 different elements across 11 types of artificial soils located within industrial zones in Riyadh, Sudair, and Al-Ahsa regions of Saudi Arabia.To achieve this, the study utilized the wet digestion technique and employed an ICPE-9000 spectrophotometer to determine the concentrations of these elements.The study had a dual focus: first, it aimed to assess the presence of pollution due to heavy metals resulting from enrichment processes, and second, it investigated the geographical distribution of pollutants within the soil.
The results unveiled varying levels of concentrations for heavy metals within the industrial areas under investigation.Specifically, the soil in the Yanbu region exhibited the highest concentration values for elements such as Fe, Mn, Ni, Zn, Cu, Cr, Co, Cd, and Be.In contrast, the industrial soil in Riyadh had the highest concentration values for elements such as Pb, Hg, Mo, selenium (Se), and As.Moreover, the highest Cd concentration was detected in the soil of the Jubail region.Conversely, the soil in the Al-Ahsa region displayed the lowest concentration levels for these heavy metals.
This study offers a comprehensive examination of potentially heavy element pollution in the soil of diverse industrial regions in Saudi Arabia.The findings underscore the disparities in heavy metal concentrations, which can have significant implications for the environment.The development of effective strategies for monitoring and mitigating soil pollution in industrial areas is imperative for the preservation of ecosystems and the well-being of the public.

Soil materials
The 11 soil samples were gathered from various locations, encompassing three industrial areas located in Riyadh, Sudayr, and Al-Ahsa, Kingdom of Saudi Arabia.Additionally, soil samples were collected from two coastal areas situated in Jubail and Yanbu, each at varying distances from specific points of interest.Among the samples taken in Yanbu, some were obtained from marine environments, while others were collected from desert regions.This comprehensive sampling approach aimed to capture a diverse range of soil conditions and locations for further analysis and study.The soil samples gathered are manually ground for 15 min using a ceramic mortar to produce fine particles.This procedure is conducted at standard room temperature.

Methods and analyses of the data
Numerous analytical techniques exist for the detection and quantification of heavy elements.However, in the context of this research study, the chosen method was inductively coupled plasma (ICP) technology.This selection was made due to several distinct advantages that ICP offers over alternative methods.
One of the primary advantages of utilizing ICP technology is its ability to measure a wide array of elements efficiently and swiftly.This capability is particularly valuable when dealing with a research study that requires the analysis of a large number of elements within a relatively short time frame.The speed and versatility of ICP technology enable researchers to obtain comprehensive data sets promptly, facilitating more efficient data collection and analysis.
Additionally, ICP technology has a high level of sensitivity, particularly detecting trace amounts of elements.This increased sensitivity allows for the precise measurement of elements even when they are present in minuscule concentrations.This accuracy is of paramount importance in scientific research, where the detection of subtle variations in element concentrations can yield critical insights into various phenomena.Furthermore, ICP technology offers cost-effectiveness than some alternative methods.The efficient use of resources, coupled with the ability to analyze multiple elements simultaneously, makes ICP an economically advantageous choice for both quantitative and qualitative analyses.

ICP spectrometry conditions and parameters
This study relied on the application of ICP technology for both quantitative and qualitative analyses.This cuttingedge technology featured high-efficiency emission sources along with a charge-coupled device detector, ensuring precision and accuracy in the results obtained.To maintain the integrity of the analysis, high-purity inert argon gas was employed to prevent any potential spectral interference issues.The detector's temperature was carefully set at a specific −14.89°C to optimize its performance.The gas played a multifaceted role in the analytical process.First, there was a continuous flow of plasma gas at a rate of 10 L•min −1 .Second, an additional 0.6 L•min −1 of gas was introduced into the system.Third, a sample carrier gas was used at a rate of 0.7 L•min −1 .These controlled gas flows were essential to create the ideal conditions for the ICP.Furthermore, various key parameters were fine-tuned to ensure the optimal functioning of the ICP technology.The radio frequency power was set at precisely 1.2 kW, and atmospheric pressure was maintained at 450 ± 10 kPa.In terms of orientation, the system operated in the axial direction, with a rotation speed of 20 rpm.

Setting solutions standards
A standard multi-component solution was utilized.This solution of 1,000 ppm is synthesized through a 5% nitric acid (HNO 3 ) solution.
The soil samples were prepared usinga wet digestion method.To begin, 1 g of the sample and 4 mL of royal water, consisting of 3 mL of HNO 3 and 1 mL of hydrochloric acid, were added to the sample.The mixture was allowed to settle for 1 day.It was then cooked on a hot plate to 200°C until it vaporized completely.Following this, the sample was filtered and collected into a volumetric flask with a predetermined volume.To complete the process, deionized water was poured to the flask to reach the desired volume.

Heavy element analysis of samples 2.4.1 Chemical and physical characteristics of the samples
The pH level was measured, and the element concentration was calculated in mg•kg −1 .Additionally, electrical conductivity (EC) and total dissolved solid (TDS) values were measured in S•cm −1 and g•L −1 , respectively.Furthermore, the concentration of chlorine anions (Cl -1 ) was measured in g•L −1 .

Analysis of metals at distance or proximity to the source of pollution
Point pollution originates from stationary and fixed sources of pollution, often exemplified by factories and industrial facilities.In contrast, non-point pollution is associated with mobile and diffuse sources of contamination, such as fertilizers in agriculture.In this study, the objective was to assess the impact of proximity to pollution sources on the concentration of elements in the soil.To achieve this, soil samples were obtained from three distinct industrial areas located in Sudayr.
In particular, the research aimed to investigate how varying distances from these industrial sites, specifically distances of 50 and 100 m away from the factories, influenced the heavy metals ratios in the soil.This spatial analysis sought to understand the extent to which point pollution, generated by these industrial complexes, affected the surrounding soil.
By measuring and comparing potentially harmful element concentrations at different distances from the pollution sources, the study aimed to provide valuable insights into the spatial distribution of soil pollution.The choice of Sudayr, known for its industrial activities, served as an ideal location to examine the dynamics of point pollution and its impact on soil quality.

Statistical analyses and contamination assessment methods
Significant differences were detected through the utilization of analysis of variance (ANOVA).To evaluate the extent of mineral-induced soil contamination, it is essential to assess parameters such as the Pollution Load Index (PLI), fertilization factors (EF), and the Geographical Accumulation Index (I geo ).

Measurement of the concentrations of the elements
The method suggested below for pollution investigations involves the calculation of element concentrations through the PLI.The determination is carried out as follows [24]: where PLI represents the pollution level, P i stands for the position of the pollution index for an individual element I, and n represents all elements considered.The Pi is determined as follows: P i = C i /S i , and P i represents the contamination index for a specific element I, C i (element in the sample), and S i is the background concentration of the sample.
When the PLI value exceeds 1, it signifies a polluted site, while a PLI value less than 1 indicates a site without pollution.
In addition to that, the Geographical Index of Accumulation (I geo ) is investigated; the calculation for I geo [25] is as follows: where C n represents the measurement of all elements n in the soil, B n stands for the average element concentration n, which serves as a parameter for the geochemical background, and the constant 1.5 is employed to mitigate the potential impact of fluctuations in the soil background values.This index, I geo , provides insight into the accumulation of elements in relation to their background levels, helping to assess the degree of geographic accumulation in the soil.

Soil fertilization assessment
To evaluate the extent of human-induced pollutant deposition on the surface soil, enrichment factor (EF) indicators were estimated using the concentration of iron [26][27][28] in the topsoil as a reference element.The calculation for EF [29] is as follows: By calculating EF values based on the iron concentration in the topsoil, this method helps assess the element enrichment degree and provides insights into the potential impact of human-related pollutants on the soil.

Results and discussion
3.1 Heavy element analysis of samples

Chemical and physical properties of the samples
The physiochemical behavior and the mineral concentrations of soil samples are presented in Tables 1 and 2 and Figure 1.The pH values and TDS values in the five regions vary greatly ranging from 220 to 1,647 mg•L −1 with an average of 436 mg•L −1 .Chloride anion concentration ranges from 21 to 400 mg•L −1 .Additionally, the EC value ranges from 0.47 to 6.07 μS•cm −1 , with an average conductivity of 1.6 μS.
According to the European Commission classification, soils are categorized as non-saline when the EC value is below 2, salty within the range of 2-8, and brine within the range of 8-16.Soils with an EC value exceeding 16 are considered highly saline [30][31][32].The study results indicate the prevalence of non-saline soil in all regions except for the Riyadh region.

Mineral in the soil in the industrial area of Riyadh
The data presented in Table 3 investigate the potentially harmful element concentrations found in the soil of the first industrial city.This soil contains a range of mineral concentrations ranging from 0.16 to 5.5 ppm, as outlined in Table 4.It is noteworthy that Cu and Cr show a notably high concentration, reaching 9.5 ppm, while the Fe content is recorded at 900 ppm, a level that adheres to established safety and suitability standards [32].It is crucial to highlight that the average concentrations of As and Mo in this soil exceed the global averages for these elements in soil.Conversely, the average other metal ratios studied in this soil fall below the global average levels.
Moving on to the soil of the second industrial city, Table 4 provides an overview of the varying concentrations of elements detected.Among these elements, Fe stands out with the highest concentration, reaching 500 ppm, while vanadium (V) exhibits the lowest concentration at 0.13 ppm.Importantly, the average concentrations of these elements in the soil are all within globally permissible limits, except for Cd, Se, Mo, and Hg, which are recorded at 0.225, 4.7, 2.25, and 0.37 ppm, respectively.
The soil of the third industrial city shows significant variance in possibly element concentrations ranging from 0.165 to 13.5 mg•kg −1 .Notably, As and Fe values are 1,100 and 60 ppm, respectively.It is crucial to emphasize that all the measured potentially harmful element concentrations in this soil fall within globally accepted and specified limits.
However, there are notable exceptions to this pattern, particularly concerning the concentrations of Mo, Cd, As, Se, and Hg.These elements stand out due to their concentrations, which significantly exceed global averages.In fact, the values for these elements are 4, 4, 12, 20, and 17 times higher than the global averages, respectively.This raises concerns about potential environmental and health impacts associated with the elevated levels of these heavy metals in the soil.The findings highlight the importance of continued assessment of the soil quality and potentially heavy elements contamination in industrial areas, particularly in regions where certain heavy metals exceed permissible levels.Such data are crucial for implementing effective pollution control measures and safeguarding both the environment and human health (Figure 2).

Minerals in the soil in the industrial area of Sudayr
Table 3 presents the outcomes regarding mineral concentrations in the topsoil of the Sudayr region.It is noteworthy that all potentially heavy element concentrations in the soil of this region fall within the acceptable average concentration levels [33].However, exceptions arise in the case of Cd, Se, and Hg, where their concentrations surpass the global average by factors of 4, 16, and 12, respectively.This highlights potential concerns regarding elevated levels of these specific heavy metals in the soil, necessitating further attention and monitoring to mitigate potential environmental and health risks.

Minerals in the soil in the industrial area of the Al-Ahsa region
The findings presented in Table 3 demonstrate that the concentrations of all elements remain within safe limits, except cadmium, which exceeds the permissible limit by threefold.In addition to that, selenium and mercury concentrations have elevated with 18 allowable values.This notable increase in Se and Hg concentrations raises concerns about potential environmental and health implications, necessitating further investigation and potential remediation measures.

Minerals in the soil at the industrial area of Jubail
The diversity of mineral concentrations in the soil of the industrial area in Jubail is evident from the data presented in Table 3.Here, the concentrations of various elements range from 0.09 to 3.7 mg•kg −1 .Interestingly, it is worth noting that Co was not detected in this particular study area, setting it apart from the other regions under investigation.

Minerals in the soil at the industrial area of Yanbu
The results presented in Table 2 offer a comprehensive comparison of element concentrations obtained from various soil environments, including both desert and coastal areas.It is evident that the concentrations of Se, Mo, Hg, and Cd in the soil of the Yanbu desert site exceed the permissible limits set for soil quality.Similarly, the same observation holds for Cd, As, Hg, and Se in the soils at the Yanbu site along the Red Sea.Notably, the concentration levels of cadmium, selenium, and mercury in both desert and marine locations are significantly elevated, with values reaching 4, 16, and 12 times higher than the globally accepted average, respectively.Moreover, the concentration of Mo surpasses the permissible limit in the desert soil, whereas it remains below the safe threshold in the soil near the coastline.As concentration is also notably higher than the upper safe limit for minerals in the soil at the Yanbu Bahri site, while its concentration in the desert area does not exceed the permissible limit.Statistical significance was established at a threshold level of p > 0.05 and was calculated for both desert and coastal areas, highlighting the disparities in element concentrations between these environments.

Comparison of the concentration of elements in the soil at the coast sides of Yanbu and Jubail
Table 7 indicates that the levels of these metals are greater in the stems on the coastal side compared to the coastal area of Jubail.Additionally, it is worth noting that Co was not detected in the soil within the industrial zone of the Jubail region.

Analysis of metals at distance or proximity to the source of pollution
The findings presented in Table 5 and Figure 3 shed light on the concentrations of various elements within plants, particularly in relation to their proximity to the pollution source (the factory).Notably, it appears that the concentrations of these elements are generally higher in plants located closer to the pollution source, specifically at distances of 50 and 100 m.However, a critical observation reveals no statistically t difference in the increase of concentrations for all elements, except for silicon.Silicon, in particular, stands out as its concentration in plants located at a closer distance to the pollution source is notably elevated.It is worth noting that the concentration of silicon is approximately eight times higher than that observed at 50 m from the pollution source and an impressive ten times higher than what is recorded at a distance of 100 m from the source.To ascertain the statistical significance of these differences, a one-way ANOVA was conducted.The outcome of this analysis yielded a significance level value of 0.692, which is greater than the conventional significance threshold of 0.05.In practical terms, this implies that there  are no substantial differences in the concentrations of the elements based on their proximity or distance from the pollution source.In other words, the increase in element concentrations in plants near the source of pollution is not statistically significant.

Comparison of mineral concentrations in different regions
Table 2 and Figure 1a provide an overview of the data obtained from the analysis of mineral concentrations in  the soil across five industrial cities in the Kingdom.Notably, in the first industrial area of Riyadh, the soil concentrations for certain elements were observed to be higher compared to other industrial cities in different regions.For instance, the soil in the Jubail region recorded the highest concentration of cadmium, reaching 0.32 ppm.Conversely, the Al-Ahsa region exhibited the lowest concentration levels for most elements.This is related to its geographical location, which places it at a considerable distance from the primary sources of oil refining concentrated in the Jubail and Yanbu regions.Furthermore, the soil in the Yanbu region displayed the highest concentrations of nickel and copper, measuring 13.5 and 4.75 mg•kg −1 , respectively.These heightened concentrations could be attributed to oil refining and extensive industrial activities taking place in this region [33,34].
When assessing the concentrations of certain soil elements across all the studied industrial regions, it becomes evident that, for the most part, they fall within the global average concentrations for soil.However, there are significant exceptions to this pattern, particularly for Cd, Se, and Hg.These three elements stand out as their concentrations in the soil are notably higher than the global average.Cd, in particular, is recorded at levels five times greater than the world average for soil elements [35].Se and Hg, on the contrast, exhibit concentrations that are 20 and 21 times higher than the global average for soil elements, respectively.It is noteworthy that Riyadh and Yanbu are reported to have the highest concentration of the highly toxic Cd metal, with levels reaching 0.27 mg•kg −1 .Additionally, Riyadh's soil records the highest Se concentration at 6 mg•kg −1 , while the Sudayr area's soil exhibits the highest Hg concentration, at 0.65 ppm.Furthermore, the soil in Riyadh and Jubail has a Se concentration of 6.7 mg•kg −1 , surpassing the permissible safe limit [37,38].

Statistical analysis
Table 5 provides a succinct overview of the standard deviation values derived from the analysis of 11 soil samples.When delving deeper into Table 6, it becomes evident that the variance observed in the range of potentially heavy element distributions, in relation to their respective rates,   identified in soil from industrial areas in diverse countries across the globe, as detailed in Table 7.These results indicate that soil samples reflect anthropogenic contributions of these elements in the five regions, particularly in Al-Ahsa, suggesting the possibility of environmental mineral pollution from industrial activities.Industrial waste emissions in the study areas could potentially serve as sources of these elements [53].

Assessment of heavy element pollution
Table 9 and Figure 5 display the PI values for heavy minerals in the industrial zones of Riyadh, Sudayr, and Al-Ahsa.Table 10 provides a summary of the minimum, maximum, and average values for all heavy minerals detected in the cities under study.

Quantitative scale of the extent of mineral contamination
Tables 11 and 12, along with Figure 6, present a numerical scale that quantifies the degree of mineral contamination in the soil under investigation.The I geo values exhibit a range of variations.

Conclusions
In this research investigation, a highly sensitive and easily reproducible method was employed, which, despite its simplicity, offers the advantage of facilitating international comparisons to assess the presence of heavy elements responsible for soil contamination.Samples were collected and subjected to analysis in areas lacking prior information regarding pollution levels.Interestingly, among the collected samples, approximately 20% exhibited concentrations of heavy elements such as Mo, As, and Se that exceeded the permissible limits.One notable finding from the study is the stark contrast in the concentration of these elements between the soil samples collected along the Red Sea coast and those from the Arabian Gulf region.Specifically, the concentration of these elements in the soil along the Red Sea coast was significantly higher than what was observed in the Arabian Gulf soil.These findings shed light on the critical role played by soil components in the enrichment process of heavy elements.The research highlights the significance of understanding the distribution and levels of these elements in soil, particularly in areas where such information was previously lacking.The use of a sensitive and easily replicable analytical method allows for the assessment of soil quality and pollution levels on a global scale, facilitating cross-border comparisons and enabling a better grasp of environmental conditions.

Figure 1 :
Figure 1: (a) Heavy element concentrations in soil and (b) the iron concentration in the soil of the five industrial cities of the Kingdom of Saudi Arabia (very small standard deviation approximately <1.0%).

Figure 2 :
Figure 2: (a) Heavy mineral concentrations in soil and (b) iron element concentration in the soil of the three industrial areas in Riyadh city (very small standard deviation approximately <1.2%).

Figure 3 :Table 7 :-
Figure 3: (a) The Sudayr region metal concentrations at various distances from the factory.(b) Fe concentrations in the soil of the Sudayr region and at different distances from the factory (very small standard deviation approximately <1.1%).

Figure 4 :
Figure 4: Percentage of EF for heavy elements in the soil samples of study areas.

Figure 6 :
Figure 6: Differences of I geo values in samples of study areas.
reference is the reference element, which in this case is iron (Fe), used for normalization purposes.The EF values obtained are used to categorize the level of element enrichment in the soil, with different ranges indicating various degrees of enrichment.These categories are as follows:EF < 2 suggests minimal element deficiency. 2 < EF < 5 notify moderate enrichment.5 < EF < 20 notify significant enrichment.20 < EF < 40 notify strong enrichment.EF > 40 notifies of very high enrichment.
where EF stands for the enrichment factor, C x represents the target element being assessed, and C

Table 1 :
Physicochemical behavior of soil in some industrial areas in Saudi Arabia

Table 2 :
[33]age and common range of heavy element concentrations in soil (ppm)[33]

Table 3 :
Heavy metal concentrations (soil) for five industrial cities in Saudi Arabia

Table 4 :
Heavy mineral concentrations in the soil of three industrial areas in Riyadh city

Table 5 :
Heavy element concentrations in Sudayr soil and at various distances from the factory

Table 6 :
Metal descriptive statistics studied industrial areas

Table 8 :
Mean and range of EF in soil samples of study areas

Table 9 :
PI value for metal in soils

Table 8 and
Figure 4offer a synthesis of the minimum, maximum, mean, and standard deviation of various heavy metals identified in 30 soil samples collected from Riyadh Industrial Area, Sudayr, Al-Ahsa, Jubail, and Yanbu in the Kingdom of Saudi Arabia.Notably, Co exhibited the highest EF across all regions, with a median value of 937.3608, indicating severe contamination.This trend was also evident for Mo and Hg in all five areas, underscoring significant levels of contamination and fertilization.Cu exhibited the highest EF in the soils of Al-Ahsa and Jubail, indicating strong fertilization.In contrast, the soils of Riyadh, Sudayr, and Yanbu showed severe pollution and high fertilization for these elements.Lead (Pb) exhibited moderate fertilization in Yanbu soil, while Co was not detected in Jubail soil, showing moderate fertilization in Riyadh, Sudayr, and Yanbu and significant fertilization in Al-Ahsa soil.The level of vineyard soil fertilization was moderate in Al-Ahsa and Jubail, while it was classified as fertilized in Riyadh, Sudayr, and Yanbu.The degree of fertilization increased significantly in the soil of and Jubail.