Analyzing Soil Pollution: Heavy Metals in Setif City Region Using ICP-OES Technique

While heavy metals are critical to humans, human activities have caused dangerously high levels of some elements to build up in certain places. The buildup of heavy metals, primarily in soil, is a major environmental concern because it can release toxic metals into living organisms [1], [2]. Heavy metals are highly harmful to both human health and ecosystems, as they can accumulate in the food chain and lead to serious health issues. Understanding their sources and impacts is essential for developing effective remediation strategies and protecting public health. These substances are harmful to humans, animals, microorganisms, and plants [3], [4].

These metals are non-biodegradable and have a long biological half-life, meaning they do not decompose over time. Once these metals contaminate the soil, mitigating their harmful effects becomes particularly challenging. If these metals are available to plants, toxic metals can accumulate in vegetation, posing a threat to humans and animals consuming them through the food chain [5], [6], [7], [8].

Additionally, heavy metals can readily transfer into the atmosphere and groundwater. As a result, metal pollution increases the risk of exposure, whether through inhalation or ingestion of contaminated environments. Soil plays a crucial role as a substrate for many industrial, agricultural, and urban activities [9], [10]. Trace metals, also known as heavy metals, enter the soil through natural processes such as weathering and erosion of parent rocks and mineral deposits. Moreover, human activities such as industrial operations, urban areas (heating, wastewater, sewage sludge), transportation (roads, waterways), and agriculture (use of fertilizers and herbicides) also contribute to the release of these metals into the soil [11], [12], [13], [14].

Pollution can be classified into two distinct categories. The first is point-source pollution, usually localized to a specific area and often caused by agricultural, industrial, and urban activities. The second category is non-point source pollution, which spreads regionally, with the primary vector often being atmospheric (thermal fumes, metallurgical factories, etc.) [15], [16], [17], [18]. The increasing levels of heavy metals play a significant role as either nutrients or toxic elements in the biosphere [19], [20], [21].

The accumulation of heavy metals can reach toxic levels depending on the state of the environment [22], [23]. Zinc (Zn), Copper (Cu), and Cadmium (Cd) were selected as representative heavy metals, as their levels in the environment provide a reliable indicator of environmental pollution [24], [25]. These metals play a crucial role in the environmental ecosystem. Several techniques are used to analyze heavy metals in soils, including X-ray Diffraction (XRD), Total X-ray Fluorescence (TXRF), and Anodic Stripping Voltammetry (ASV), as well as Atomic Absorption Spectrophotometry (AAS). XRD, in particular, allows the determination of the mineralogical composition of samples by identifying crystalline phases, offering essential information to assess the interactions between heavy metals and soil [26], [27].

Despite the global concern regarding heavy metal contamination, research focusing on Setif, Algeria, remains limited. Setif is one of the most important agricultural and industrial regions in northeastern Algeria, where rapid urbanization, fertilizer use, and industrial activities may increase the risk of soil pollution. However, few studies have systematically quantified heavy metal concentrations in Setif’s soils, and even fewer have employed advanced spectroscopic methods. In particular, the combined application of ICP-OES for quantitative metal analysis and XRD for mineralogical characterization has not been adequately addressed in this region. This study aims to fill this gap by assessing the levels of Zn, Cu, Ni, and Cd in Setif soils using ICP-OES, while also considering the mineralogical background obtained through XRD.

The study’s goal was to evaluate the use of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to determine the concentrations of Zn, Cu, Ni, and Cd in sixteen soil samples collected from the city of Setif [28].

Setif is one of the largest cities in northeastern Algeria. Located at an altitude of 1,080 meters, Setif lies on the High Plateaus separating the northern and southern Atlas Mountains, approximately 300 kilometers from Algiers and about 100 km from the Mediterranean coast. As the third-largest city in Algeria after Algiers and Oran, the city is known for its rich history, vibrant culture, and significant agricultural contributions. With a diverse population, Setif also boasts historical landmarks such as the ancient ruins of Timgad and offers captivating scenic views of the surrounding mountains.

The city covers an area of 127.3 km$^2$ and had a population of 1,686,845 inhabitants as of October 2022. Setif's geographical location results in a continental climate, with relatively cold winters and hot summers.

As a crossroads city, Setif is distinguished by notable urban expansion, driven by a combination of spatial and socioeconomic factors [29].

As illustrated in Figure 1, a total of 16 surface soil samples (0–5 cm) were collected across the city of Setif during March and April 2024, representing various land uses, including industrial, residential, and recreational sectors [30].

Table 1 presents the GPS data and descriptions of the sampling sites. Using a stainless-steel scoop, surface soil samples were systematically collected using a regular 3 × 3 km grid across the study area, placed in polyethylene bags, and properly labeled. GPS devices were used to record the precise longitude and latitude of each sampling site.

Air-dried surface soil samples were homogenized using a pestle and mortar. The homogenized samples were then passed through a 2 mm mesh sieve and stored in polyethylene bags [31], [32].

Physical and chemical properties, including pH, OM, and CaCO$_3$, were determined in the samples [33].

Following two hours of stirring, the pH of the soil samples was measured using a pH meter in a 1:5 soil–water suspension [34].

The commonly used modified Walker–Black (WB) method was applied to determine OM content [35].

The Bernard calcimetry technique was employed to measure the CaCO$_3$ content.

Room-temperature XRD data were recorded for the sixteen samples using a PANalytical diffractometer.

All measurements were performed over an angular range of 5°–80° ($2 \theta$), with a step size of 0.025° ($2 \theta$) and a counting time of 5 seconds per step [36].

Based on the EPA Method 3051A developed by the U.S. Environmental Protection Agency (2007) [37], the soil samples underwent laboratory digestion using a microwave-assisted aqua regia digestion procedure. The process involved refluxing the samples. The digestion block was gradually heated from ambient temperature to 180$^{\circ} \mathrm{C}$, and the procedure continued until approximately 1 mL of acid remained.

After dilution of the digestion products with 10 mL of distilled water, the solutions were filtered through cellulose filter paper with a 0.45 $\mu$m pore size [37].

An inductively coupled plasma optical emission spectrophotometer (ICP-OES, Horiba Jobin-Yvon Ultima 2 CE) was used for the analysis of the elements Zn, Cu, Ni, and Cd [38]. Table 2 presents the specific instrumental operating parameters.

A crucial factor regulating metal chemical behavior, as well as other major soil processes, is pH. Table 3 shows the relatively wide variation in soil pH values in the Setif area, which range between 8.00 and 8.47. As shown in Figure 2, most of the soils exhibited pH values above 8.0. The presence of carbonates in the soil helps to explain its alkalinity [39], [40].

The values for OM and CaCO$_3$ are summarized in Table 4.

For OM, the minimum, maximum, and average values were 0.28, 2.32, and 1.15, respectively. The abundance of plant and animal waste present in the topsoil layer likely accounts for the observed variation in OM content [41].

As shown in Figure 3, the minimum, maximum, and average values for CaCO$_3$ were 20.22, 44.94, and 33.59, respectively [42].

Every diffractogram was analyzed to identify the phases present in the soils of the Setif region. The results revealed the presence of elements including Cd, Zn, Cu, and Ni in all samples, as well as calcite and quartz [43]. In addition, elements such as Fe, Ge, Se, and Mo were detected in several diffractograms.

Figure 4 illustrates the phase identification of sample 05.

The following phases were identified: the calcite phase, with characteristic peaks at $2 \theta=23.06^{\circ}, 29.39^{\circ}, 31.43^{\circ}$, and $35.95^{\circ}$, corresponding to reflections (102), (104), (006), and (110) (ICDD-PDF No. 96-900-9669), and the quartz phase, with characteristic peaks at $2 \theta=20.83^{\circ}, 26.60^{\circ}, 36.50^{\circ}$, and $39.41^{\circ}$, corresponding to reflections (100), (101), (110), and (102) (ICDD-PDF No. 96-901-0145).

Additionally, the presence of elements Cd, Zn, Cu, and Ni was observed in all diffractograms, with varying relative intensities. Cd is characterized by peaks at $2 \theta= 12.53^{\circ}, 19.88^{\circ}, 20.83^{\circ}$, and $23.06^{\circ}$, corresponding to reflections (-203), (82-3), (51-5), and (44-4) (ICDD-PDF No. 96-723-7903) associated with the C$_{38}$Cd$_2$N$_{12}$O$_8$ phase.

Zn and Cu exhibit peaks at $2 \theta=11.77^{\circ}$, $20.83^{\circ}, 25.29^{\circ}$, and $29.39^{\circ}$, corresponding to reflections (110), (112), (122), and (222) (ICDD-PDF No. 96-900-1251) attributed to the Cu$_{9.36}$Fe$_{3.56}$Ge$_{1.62}$S$_{16}$Zn phase. Finally, nickel (Ni) is identified by peaks at $2 \theta=16.23^{\circ}, 19.88^{\circ}, 20.83^{\circ}$, and $23.06^{\circ}$, corresponding to reflections (-111), (1-13), (0-14), and (005) (ICDD-PDF No. 96-710-0027) related to the C$_9$H$_4$N$_{0.5}$NiS$_{11}$ phase.

The mineralogical composition also influences metal mobility. Calcite, due to its alkaline buffering capacity, enhances the retention of heavy metals through adsorption and precipitation, whereas quartz, being chemically inert, exhibits limited retention capacity, potentially facilitating the mobility of metals in soil.

Table 5 summarizes the minimum, average, and maximum concentrations of the investigated trace elements (Zn, Cu, Ni, and Cd) in the soils [44].

With an average concentration of 407.06 mg kg$^{-1}$, Zn levels in the soils of Setif City range from 184.55 to 689.25 mg kg$^{-1}$; the regulatory limit is 300 mg kg$^{-1}$. Concentrations reached up to 689.25 mg kg$^{-1}$ at locations P07, P08, P09, P10, P15 , and P16 , resulting in 68.75% of the sampling sites exceeding this limit, as illustrated in Figure 5.

With an average concentration of 55.85 mg kg$^{-1}$, Cu levels in the soils of Setif City range from 36.12 to 86.42 mg kg$^{-1}$. The regulatory limit is 100 mg kg$^{-1}$. Although certain sites, such as P16, exhibit relatively high values, reaching 86.42 mg kg$^{-1}$, none of the samples exceeded this limit, as shown in Figure 6.

With an average concentration of 32.21 mg kg$^{-1}$, Ni contents in the soils of Setif City range from 15.92 to 47.75 mg kg$^{-1}$. Nickel has a regulatory limit of 50 mg kg$^{-1}$. Although sites P15 and P16 approach this threshold, with values of 47.75 mg kg$^{-1}$ and 39.81 mg kg$^{-1}$, respectively, as shown in Figure 7, none of the sites exceeded this limit.

With an average concentration of 0.16 mg kg$^{-1}$, Cd contents in the soils of Setif City range from 0.01 to 0.30 mg kg$^{-1}$. Cadmium has a regulatory limit of 2 mg kg$^{-1}$. With concentrations between 0.01 mg kg$^{-1}$ and 0.30 mg kg$^{-1}$, all samples remain well below this limit, as shown in Figure 8 [45].

The Igeo is used to assess the degree of soil or sediment pollution by trace metals (ETMs). It is calculated by comparing the measured metal concentration with its natural geochemical background value [46], [47], [48], [49]. The index was proposed by Müller in the 1970s and is calculated using the following formula:

$ \text { Igeo }=\log _2(\mathrm{Cn} /(1.5 \times \mathrm{Bn})) $

where,

• Cn is the concentration of the metal in the sample,

• Bn is the geochemical background concentration (natural level in the soil), and

• 1.5 is a correction factor accounting for natural variations in the geochemical background.

Table 6 presents the values of the geo-accumulation index (Igeo) for Zn, Cu, Ni, and Cd at the sixteen sampling points (P01 to P16). The Igeo index is commonly applied to evaluate the degree of soil or sediment contamination relative to a natural geochemical reference level.

Pollution levels are classified according to predefined thresholds as follows: an Igeo value of $\leq 0$ indicates no pollution, whereas values between 0 and 1 correspond to unpolluted to lightly polluted conditions. When Igeo values range between 1 and 2, pollution is considered moderate, while values between 2 and 3 indicate moderate to high pollution. An Igeo value between 3 and 4 reflects strong pollution, and values ranging from 4 to 5 indicate very high pollution. Finally, an Igeo value greater than 5 denotes extremely high pollution [50], [51].

For Zn, the Igeo values range from 1.05 to 2.95, with an average of 2.12, indicating moderate to high pollution. Approximately 31.25% of the sampling points exhibit moderate pollution, such as P02 (1.46) and P06 (1.32), while 68.75% of the points, including P16 with an index of 2.95, show moderate to high pollution.

For Cu, the Igeo values range from 0.12 to 1.38, with an average of 0.72. Copper pollution levels are lower than those of zinc, with 93.75% of the points showing no to light pollution, whereas 6.25%, such as P16 (1.38), exhibit moderate pollution.

Regarding Ni, the Igeo values are negative, ranging from -3.52 to -1.94, with an average of -2.57, indicating the absence of nickel pollution across all sampled areas. These negative values, observed at 100% of the sampling points, indicate that nickel concentrations are below natural background levels, reflecting no contamination.

Finally, for Cd, the Igeo values range from -4.91 to 0, with an average of -1.21. The majority of the sampling points (93.75%) show negative values, indicating no pollution. However, 6.25% of the points, such as P02, exhibit an index value of 0, suggesting the presence of very low cadmium traces without actual contamination [52], [53], [54], [55].

Figure 9 illustrates the Igeo for the four investigated metals.

The Cf is used to evaluate the level of pollution by comparing the concentration of heavy metals or other contaminants in environmental samples, such as soil, sediments, or water, with their natural background concentrations [56], [57], [58], [59]. It helps identify the degree of contamination by relating the measured metal concentration to its natural reference value.

The contamination factor is calculated using the following formula:

$ \mathrm{Cf}=\mathrm{Cn} / \mathrm{Bn} $

where,

• Cn is the measured concentration of the element in the sample, and

• Bn is the geochemical background concentration of the element in uncontaminated soil.

Table 7 presents the Cf values for the investigated trace metals (Zn, Cu, Ni, and Cd) measured at the sixteen sampling points (P01 to P16). The contamination factor is commonly used to assess each metal’s impact relative to its natural background level, thereby determining the intensity of pollution.

A Cf value below 1 indicates low pollution, whereas values between 1 and 3 suggest moderate pollution. Significant pollution is indicated by Cf values ranging from 3 to 6, while values greater than or equal to 6 indicate extremely high pollution [60], [61].

For Zn, the Cf values range from 3.10 to 11.56, with an average of 6.83, indicating pollution levels ranging from moderate to very high. Approximately 31.25% of the sampling points exhibit moderate to high contamination, such as P03 (5.89) and P05 (5.78), while 68.75% of the points, including P16 (11.56), show very high contamination.

For Cu, the Cf values range from 1.62 to 3.89, with an average of 2.51, indicating moderate contamination. Approximately 93.75% of the sampling points exhibit moderate pollution, except for P16 (3.89), which shows moderate to high contamination.

For Ni, the Cf values are relatively low, ranging from 0.13 to 0.39, with an average of 0.26, indicating negligible to very low pollution across 100% of the sampling points.

Finally, for Cd, the contamination factor values range from 0.05 to 1.50, with an average of 0.80. Cadmium pollution levels are generally low, with 68.75% of the points showing low contamination and 31.25%, such as P02 and P11, exhibiting moderate contamination [62], [63], [64].

Figure 10 presents the Cf for the investigated metals.

The dominance of Zn contamination is most likely linked to anthropogenic sources, including the use of phosphate fertilizers, industrial emissions, and urban traffic. In contrast, Ni and Cd exhibit negligible pollution, reflecting their lower anthropogenic inputs and greater geochemical stabilization under alkaline conditions. Cu contamination, although moderate, may also originate from agricultural practices and industrial activities.

Generally alkaline in character, the soils under investigation exhibit pH values ranging from 8.00 to 8.47. With an average value of 1.15, OM contents range from 0.28 to 2.32, most likely derived from plant and animal litter present in the topsoil layers. The minimum, maximum, and average values for CaCO$_3$ are 20.22, 44.94, and 33.59, respectively.

Although some values, particularly for zinc, exceed regulatory limits at sites such as P09, P14, and especially P16, suggesting potential contamination sources that should be monitored, heavy metal concentrations (Zn, Cu, Ni, and Cd) generally comply with established regulatory standards.

With concentrations generally below contamination limits, nickel and cadmium exhibit no significant pollution. The Igeo for the four metals indicates moderate to high zinc pollution in certain areas, while copper shows comparatively low contamination.

The Cf results further identify zinc as the most significant contaminant, with particularly high values at P16. Cu displays moderate contamination, with elevated values at specific locations, whereas nickel shows negligible pollution and cadmium exhibits low to moderate contamination. Overall, zinc pollution represents the greatest environmental concern, followed by copper and cadmium, while nickel remains minimally affected.

This study confirms that Zn is the primary contaminant, likely originating from fertilizer application, industrial activities, and traffic emissions. Ni and Cd show limited pollution due to their greater stability under alkaline soil conditions. Cu exhibits moderate contamination, potentially associated with agricultural practices and industrial sources.