Introduction

Heavy metal pollution has become a serious global concern due to heavy metals' toxic, persistent, and non-degradable nature1. Pakistan is also afflicted with this problem2, which is estimated to become more severe. Vehicular traffic is the environment's key source of heavy metal emissions3. Vehicles release these heavy metals via fuel combustion, tire wear and tear, and corrosion of auto-body, engine parts, batteries, radiators, clutches, and brake lining4. Cadmium (Cd), copper (Cu), nickel (Ni), and lead (Pb) are the most common vehicular-released metals5. These metals contaminate the surrounding environment and adversely affect the roadside flora as they enter the plant body through root or leaf surface6, eventually entering animals through food chains. Among these metals, Pb is responsible for human reproductive, renal, and neural disorders7. Cadmium is also carcinogenic and may affect reproductive, renal, hepatic, and circulatory systems8. Excess of Ni causes dermal, cardiac, and respirational disorders9. Exposure to a high concentration of Cu causes gastrointestinal, neurodegenerative, and hepatic disorders in humans10.

Heavy metal contamination of the roadside environment is directly linked with traffic density11, which is increasing drastically with rapid industrialization and urbanization. The metal distribution along roads also depends on meteorological conditions12, road conditions13, type of vehicle and fuel used, and driving speed14,15. Despite monitoring and controlling all these factors, the extent of heavy metal pollution in roadside surroundings is increasing daily. Therefore, the plant species that can uptake these heavy metals and accumulate them in their different body organs are used, called phytoremediation16, which is economical, sustainable, and eco-friendly to protect the ecosystem. Several researchers assessed the metal contamination along roads, but information on traffic-related heavy metal contamination along Lahore-Okara road (N-5) and Okara-Faisalabad road in Punjab, Pakistan is very limited. The metal contamination along these roads may vary spatially and temporally due to heavy traffic loads, which could threaten agriculture. Furthermore, Calotropis procera and Nerium oleander are among the most commonly grown plant species on these roads. Thus, these plants were selected to monitor the metal contamination along these roads as both are naturally growing along these roads and require very low maintenance.So the present study was planned to: (a) Assess the vehicular released heavy metal contamination in roadside soil. (b) Identify the biomonitor/bioremediator plant species to deal with heavy metal pollution problems. (c) Study the spatial and seasonal variability in heavy metal pollution in roadside environment.

Material and methods

Study area

To assess the concentration of traffic-related metals (Cd, Pb, Ni, and Cu,) in roadside soil and bioremediation potential of plants, two busy roads i.e., Lahore-Okara road (N-5) and Okara-Faisalabad roads (OFR) in the Punjab Pakistan, were selected as study area (Fig. 1). Both roads vary in traffic density, vehicle type, and age of roads. The National Highway (N-5) was constructed in 1913 to connect Torkham to Karachi. A very busy section of this highway, Lahore-Okara road (129 km) was selected for study. Five sites (Chung, Manga Mandi, Bhai Phero, Pattoki, and Renala Khurd) were selected. The Okara-Faisalabad road (OFR) is a newly built road with less traffic volume than N-5. Five sites along this road, Satghara More, Bangla Gogera, Tandaliawala, Sataiana, and Khanuana, were selected randomly for the study.

Figure 1
figure 1

Sampling sites along “N-5” and OFR. Where, (1) Chung; (2) Manga Mandi; (3) Bhai Phero; (4) Pattoki; (5) Renala Khurd; (6) Satghara More; (7) Bangla Gogera; (8) Tandaliawala; (9) Sataiana; (10) Khanuana.

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Sample collection and heavy metal analysis

The case study was conducted in four seasons (summer, autumn, winter, spring). The soil samples and leaves of two commonly growing plant species (Calotropis procera A., and Nerium oleander L.) were collected from all sites (0–1 m from road edge) of both roads. Control soil and plant leaf samples were collected from 100 m away from road. Five plants of each species from every site were collected during four seasons. Five soil samples were collected from 0–1 m from road edge (0–10 cm deep) from each site along both roads. Vehicular soot (Cars, trucks, and buses) and fuel (diesel, petrol) from various fuel stations along both roads were also analyzed to determine metal contents.

The soil particles on collected leaf samples were removed with deionized distilled water for acid digestion. After drying at 65 °C in a hot-air oven, all samples were milled to powder using Wiley Mill. While, soil samples were first put through a sieve (2-mm) and then dried at 65 °C in an oven for 72 h. Then, using nitric acid and hydrogen peroxide, all air-dried samples (plant leaves and soil) were processed on a hot-block digester by following USEPA, 3050B method for analysis of metals17. The methodology of18 was followed for soot digestion, and19 for petrol and diesel digestion. The Cd, Ni, Pb, and Cu contents in all digested samples were analyzed using an atomic absorption spectrophotometer (AAS). For accuracy and precision in analysis, reference materials with accuracy = 100 ± 20%, reagent blanks, and internal standard solutions were used. Traffic density (average daily traffic/season) was also recorded at all study sites (Table 1). For statistical analyses, analysis of variance was performed with the program COSTAT (Cohort Statistical Software 2003, Monterey, California, USA). Means were compared with LSD test (α = 0.05) to differentiate between different sites, plants, and seasons for metal contamination.

Table 1 Average daily traffic (number of vehicles/day) at studied sites.
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Ethical approval

It has been confirmed that the experimental data collection complied with relevant institutional, national, and international guidelines and legislation with appropriate permissions from authorities of the Department of Horticulture, University of the Punjab, Lahore, Lahore 54300, Pakistan. This research did not contain any studies involving animal or human participants, nor did it occur in any private or protected areas. No specific permissions were required for corresponding locations.

Results

Heavy metal content in roadside soil

The mean concentrations of vehicular-released metals (mg/kg) in soils are given in Table 2. Metal (Cd, Pb, Ni, and Cu) content in the soil of all sites along both roads varied significantly. All studied metals were higher in roadside soils than in control site soil. In the present case study, metal content in roadside soil ranged between 2.20 and 6.83 mg/kg Cd, 4.56–15.29 mg/kg Pb, 31.58–101.26 mg/kg Ni and 31.58–101.26 Cu along N-5. Along Okara-Faisalabad road, the Cd, Pb, Ni, and Cu contents were found as 2.73–6.37, 4.53–14.28, 29.78–95.89, and 62.85–132.10 mg/kg, respectively. Among sites, Chung was a highly contaminated site along N-5; along OFR, metal contamination was highest at the Tandliawala site. The comparison of roads showed higher metal contamination along N-5 road. Among seasons, the highest metal contamination in roadside soil was recorded in summer and the least in winter.

Table 2 Metal contents (mg/kg) in the roadside soil along N-5 and OFR during different seasons (mean ± SD).
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Bioremediation potential of roadside plants

The amount of metals (Cd, Pb, Ni, and Cu) in roadside plant leaves was considerably higher than the control site plants during all seasons. Among sites, the maximum amount of metal in plant leaves was observed at Chung site along N-5 and Tandaliawala site along OFR (Fig. 2). The mean metal uptake in Calotropis procera was higher than Nerium oleander (Fig. 3). Among seasons, plant metal content was highest in the summer season (Fig. 4).

Figure 2
figure 2

Metal content in plants leaves collected from various sites along N-5 and OFR during different seasons where, (a) Cd contents in plant leaves, (b) Pb contents in plant leaves, (c) Ni contents in plant leaves, (d) Cu contents in plant leaves. N-5 Lahore to Okara road, OFR Okara-Faisalabad road.

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Figure 3
figure 3

Metal uptake in plant leaves where, (a) Cd contents in plant leaves, (b) Pb contents in plant leaves, (c) Ni contents in plant leaves, (d) Cu contents in plant leaves. N-5  Lahore to Okara road, OFR  Okara-Faisalabad road.

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Figure 4
figure 4

Metal uptake in plants leaves collected from various sites along N-5 and OFR during different seasons where, (a) Cd contents in plant leaves, (b) Pb contents in plant leaves, (c) Ni contents in plant leaves, (d) Cu contents in plant leaves. N-5  Lahore to Okara road, OFR  Okara-Faisalabad road.

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Metal content in fuel and soot

High amounts of Cd, Pb, Ni, and Cu were noted in diesel, petrol, and used motor oil (Table 3). The categorization of metal content in fuel was Cd < Pb < Ni < Cu. However, the metal contents in used motor oil were highest. High metal content was also found in the soot of various vehicles, though truck soot has the highest amount of Cd, Pb, Ni, and Cu i.e., 0.97, 8.55, 12.78, and 21.51 mg/kg, respectively.

Table 3 Metal content in fuel (petrol and diesel), used motor oil and soot (mean ± S.D.).
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Discussion

Heavy metal content in roadside soil

The mean concentrations of vehicular-released metals (mg/kg) in soils have been presented in Table 2. Significant differences existed in metal (Cd, Pb, Ni and Cu) content in the soil of all sites along both roads. All studied metals were higher in roadside soils than in control site soil. Among sites, all the metals were higher in concentration at Chung site along N-5, and along OFR it was highest at Tandliawala site. Among seasons, the highest metal contamination in roadside soil was recorded in summer and the least in winter. The roads with high traffic volume and speedy automobiles incorporate many metals into the environment that sooner or later get deposited on the roadside soil20. During this study, high metal contents were found along N-5 as compared to ORF. Many earlier studies also found higher metal contamination in roadside soils15,21,22,23.

Cadmium (Cd) in roadside environment come from burning fuel, corrosion of radiators and batteries, and wearing old tires24. In the present case study, Cd content in roadside soil ranged between 2.20 and 6.83 mg kg−1 along N-5 and 2.73–6.37 mg kg−1 along Okara-Faisalabad road. Many earlier researchers also found higher Cd content in roadside soil15,25, 26.

Lead (Pb) is a most important autoexhaust pollutant 27. Regardless of the ban on leaded fuel, it is still found in petrol. In the current study, the content of Pb in roadside soil was significantly higher in contrast to control site soil. These results conform with many earlier studies 26,28,29,30,31. The mean Pb content in roadside soil along N-5 ranged between 4.56 and 15.29 mg/kg, and along Okara-Faisalabad road it ranged 4.53–14.28 mg/kg. These results showed that the vehicles are the key source of Pb contamination in roadside soil32.

Nickel (Ni) concentration in roadside soil was significantly higher than control site soil. The mean Ni content in roadside soil along N-5 ranged between 31.58 and 101.26 mg/kg, and along Okara-Faisalabad road it ranged 29.78–95.89 mg/kg. Many earlier researchers also reported higher levels of metals in roadside soil33,34. The Ni in roadside soil might have come from engine oil, fuel, tire, and brake wear 12 and corrosion of nickel alloy bearings, valves, and shafts 35.

The amount of Cu in roadside soil (61.68–138.46 mg/kg along N-5 and 62.85–132.10 mg/kg along Okara-Faisalabad road) was significantly higher than in control site soil. These results parallel many former studies36,37,38. This showed that the traffic is one of the major sources of Cu contamination in soil 39. Automobiles released Cu into the surrounding environment from fuel burning, battery corrosion38 and brakes and tire wear40. The metal content (Cd, Pb, Ni and Cu) found during present study was higher than the permissible limit of these metals in the soil set by 41(Table 4).

Table 4 Permissible limits of heavy metal contents (mg/kg) in soil and plants.
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Heavy metal content in roadside plants

Plants that grow on heavy metal-polluted soil uptake those metals along with other essential minerals and accumulate them in different body parts16. The metal content (Cd, Pb, Ni, and Cu) found along N-5 and OFR were above the permissible limit of these metals in the plants set by WHO (Table 4). Compared to common plant species, metal accumulating plants typically exhibit unique and distinct compartmentation and build different complexes44. Metals like cadmium induces phytochelatins (PCs) formation in plants; however, it is bound by weak oxygen ligands as a substitute for strong sulfur ligands in metal hyperaccumulators45. This is distinctive of many metal accumulating plants44. In plants, the accumulation of different metals is also influenced by environmental factors and plant genotypes46. So, it is different in different plants, even under the same environments47. Plant physiological mechanisms play an important role in metal accumulating plants to alleviate metal toxicity. Cell binds the metal with the cell wall and immobilizes it, thus alleviating metal toxicity in plants48. Furthermore, metal that enters the cell bonds to various organic ligands to produce stable chelates that are subsequently taken up by the vacuole49. In the current study, the metal accumulation potential of Calotropis procera was significantly higher than that Nerium oleander. So, both plants could be used to monitor metal contamination, while Calotropis procera could be a better choice to remediate the metal-contaminated soil.

Spatial variations in heavy metal contents in roadside soil and plant leaves

The difference in metal contamination at different sites is directly related to the traffic density of that particular site. Higher metal content in soil and plant leaves at Chung and Tandaliawala was due to high traffic flow at these sites11. Furthermore, Pattoki site along N-5 was the least contaminated site among all other sites. This is because of high vegetation protection along this site, as many plants can naturally ameliorate metal-contaminated areas50. The higher concentrations of metals along N-5 are attributed to higher traffic volume along this road. Besides traffic density, several other factors i.e., road structure, type of vehicles, fuel, and age of roads51, also contribute to metal contamination near roads. The national highway (N-5) is a very old concrete road, and older roads retain significant amount of deposited metals in nearby soil26. Furthermore, concrete roads cause more metal contamination than asphalt roads52. The speed of vehicles along N-5 remains very high compared to OFR, which causes more tire wear and tear, resulting in high metal content in the surrounding environment26. The reason for less metal content along OFR is that it is newly constructed and less busy than N-5 road. However, high metal contents were recorded at the Tandaliawala site, which act as junctions/temporary bus stops for passenger vans. This is supported by the results of53, who found high metal content at traffic junctions and crossroads.

Seasonal variation in heavy metal content in roadside soil and plant leaves

Metal contamination in roadside soil and plant leaves was higher in the summer and lowest in winter. The high metal contamination in summer might be due to high traffic density and more rubber abrasion at high temperatures (average day temperature is 43.5 °C). Furthermore, metals are more bioavailable at high temperatures, and plant accumulation is higher in plants during summer3. Metals percolate deep into the soil due to heavy precipitation during late summer 54,56, 57 also observed seasonal variation in metal contamination.

Metal content in fuel and soot

Despite the ban on leaded fuel, Pb was still found in petrol, diesel, and used motors during the current investigation. Lead-containing petrol is always considered a major cause of Pb pollution. Metal content in automobiles’ fuel found during the present study was greater than those reported by55. They noted 0.04 μg/g Cd, 4.50 μg/g Pb, 0.22 μg/g Ni, and 7.00 μg/g Cu in used motor oil, while Cd and Cu were not detectable in unused oil and Pb was lower in concentration (2.00 μg/g) in unused oil. This indicates that metals are released from automobiles during different process 3,35 also reported metal content in soot of vehicles. The higher amount of metals in fuel and soot of vehicles proved our findings that the vehicles are the main contributors of metal pollutants in the environment.

Conclusion

Heavy metal (Cd, Pb, Ni, and Cu) concentrations in roadside soil and plant leaves were much higher than the standard permissible limits. They showed a strong positive connection with traffic density. This demonstrates that automobiles are the primary source of heavy metal contamination in the environment near roadsides. Residents living close to roads and agriculture may suffer as a result. For control to be effective, heavy metal contamination must be regularly monitored. The current study focuses on employing native plant species to reduce soil contamination with Cd, Pb, Ni, and Cu as a sustainable management strategy. Calotropis procera is a useful metal accumulator. Therefore, it could be used to reclaim the heavy metal-contaminated soils. Though this study offers valuable insights, it is imperative to recognize its limits. The study locations chosen are urban areas, and more research in other locations and climates could enhance the conclusions' generalizability. Furthermore, long-term monitoring and assessing their performance would provide a more thorough knowledge of the chosen tree species' potential as useful instruments for improving environmental quality. Altogether, our research highlights the critical role that native vegetation plays in reducing environmental pollution and offers insightful advice to environmentalists, legislators, and urban planners on how to create more sustainable and healthy urban settings. We can get one step closer to creating more resilient and environmentally friendly cities for the benefit of present and future generations by incorporating suitable tree species into urban landscapes.