The Impact of Continuous Heavy Metal Emissions from Road Traffic on the Effectiveness of the Phytoremediation Process of Contaminated Soils

Abstract

Heavy metals are among the most toxic and persistent environmental pollutants, accumulating in soils and living organisms. Phytoremediation, the use of plants to remove contaminants, is considered one of the promising methods for cleaning soils contaminated with metals. This study assessed the effectiveness of phytoremediation of heavy metals in soil using lettuce (Lactuca sativa) as a phytoaccumulative species. Despite the successful extraction of significant amounts of metals by the plants, post-harvest soil analysis revealed persistently elevated concentrations of elements such as iron (Fe), lead (Pb), and zinc (Zn). To clarify the reasons behind the limited improvement in soil quality, additional field investigations were conducted and identified a nearby highway as a continuous source of heavy metal emissions. In the next phase of the study, metal concentrations were analyzed in dust deposited along the highway, confirming their significant contribution to ongoing secondary soil contamination. The findings emphasize the importance of considering both environmental and anthropogenic factors when designing long-term phytoremediation strategies in urban and traffic-impacted areas.

1. Introduction

In response to the growing global population and the continuous desire of most of the world’s population to improve their standard of living, all economic sectors must undergo constant development. This development, however, is often not neutral to the environment and contributes—either directly or indirectly—to its degradation [1]. The expansion into and destruction of natural areas for industrial or construction purposes, the excessive and intensive exploitation of both non-renewable and renewable natural resources, as well as the direct emission of pollutants, are just some examples of environmental degradation [2,3].

Anthropogenic pollution, resulting from human activity, is deposited in environmental matrices such as water, air, and soil. Some pollutants are capable of migrating between these matrices or degrading over time, while others remain permanently bound within a given medium [4,5]. In the case of elements classified as heavy metals (HMs), their compounds in soil are stable and resistant to decomposition [6,7,8]. The presence of these pollutants in soil affects all organisms—those in direct contact with the contaminated layer (plants, microorganisms, small animals) and, over time, those without direct exposure to the affected substrate. Pollutants can be absorbed along with water and nutrients through plant root systems in contaminated areas, subsequently accumulating in plant tissues. Furthermore, over time, these harmful compounds may leach into groundwater and migrate into larger bodies of water, eventually affecting aquatic organisms [4,9,10].

In plants, there are numerous mechanisms for uptake and interaction with pollutants within the rhizosphere. Many plants exposed to metals in the root zone accumulate them in their tissues and then respond in various ways to the stress caused by increased metal concentrations, depending on the species [11]. Some plants can sequester pollutants in their cell walls and vacuoles without exhibiting significant signs of decline or disease related to metal uptake [11,12]. Other species may experience stunted or halted growth, reduced fruiting, poor health, or even death—leading to the gradual collapse of local ecosystems. Regardless of a plant’s tolerance to contaminants, the concentration of metals in soil directly impacts global food safety [13,14]. Crops intended for human consumption (either directly or as animal feed) may pose risks to consumers even if they grow and appear healthy in contaminated soil [15]. Pollutants accumulated by plants are passed on to consumers, where they are sequestered in cells and organs. This bioaccumulation is additive—metal particles do not break down or get excreted quickly, so prolonged exposure leads to their steady increase in the body. Heavy metals are harmful and toxic even at low concentrations [16,17]. Although some metals like iron (Fe), copper (Cu), and zinc (Zn) are essential for normal cellular and systemic functions, elevated concentrations exhibit strong toxicity [18,19]. Moreover, lead (Pb), cadmium (Cd), and mercury (Hg)—known as the “elements of death” [4]—have no known biological function in vertebrates. Even the smallest amounts of these metals in cells are toxic burdens that severely harm the organism [20,21]. Examples of damage from HM accumulation include nervous system disorders, DNA sequence disruptions, respiratory, circulatory, and immune system diseases, skin conditions, cognitive impairments, and increased cancer risks [21,22,23,24]. Toxic metals can enter organisms not only through consumption of contaminated food and water but also through inhalation of polluted air and direct skin contact [25].

Considering the continuously growing global population and the resulting need for increased food production, the quality and cleanliness of soils and agroecosystems are of critical importance to public health and safety [15,26]. A 2016 study indicated elevated heavy metal concentrations in soils across Europe and approximated that over 137,000 km² of agricultural land on the continent contains alarmingly high levels of toxic elements, necessitating monitoring and potential remediation measures [27]. Soil contamination is particularly prevalent in developing countries (e.g., Colombia, Bangladesh, South Africa), where lower environmental awareness and different priorities often lead to the marginalization of pollution issues and their migration in the environment [28,29,30,31].

Anthropogenic sources of heavy metal emissions are diverse and span many economic sectors, though they are most often linked to the extraction or processing of metals and the improper disposal of related waste. Some HMs are components of phosphate fertilizers used directly on agricultural fields (Cd, Zn, Mn) [32]; others are used in the production of dyes and paints (Zn, Cu, Pb, Ni, Hg) [33,34], batteries, accumulators, and electronic devices (Cd, Hg, Pb, Ni) [34,35]. Chromium (Cr), for instance, is widely used in tanning to stabilize leather fibers and in electroplating processes [36,37]. A significant amount of environmental lead (Pb) also originates from the combustion of leaded gasoline, which contained tetraethyl lead (C₂H₅)₄Pb and has since been phased out [38]. Furthermore, all metals and metalloids are released into the environment through mining, ore processing, smelting, and subsequent manufacturing [39,40,41].

Road transport constitutes a significant source of heavy metal (HM) emissions, primarily due to the wear of vehicle components and asphalt surfaces [42]. It is now well established that the main contributors to HM emissions are no longer exhaust gases, but rather the abrasion of tires, brakes, bodywork elements, and the resuspension of road dust. Tire wear is a major source of zinc (Zn), while brake systems and car body components contribute to the emission of metals such as iron (Fe) and copper (Cu). The use of paints, batteries, and the flow of oil and fuel through metallic engine parts are also linked to the release of lead (Pb), cadmium (Cd), and manganese (Mn) [43,44]. Additionally, asphalt road surfaces, which often contain fillers rich in heavy metals, can release these contaminants into the environment through surface wear [44]. Furthermore, the increasing share of electric vehicles, despite some of their environmental advantages, does not necessarily lead to a reduction in HM emissions [45,46]. These vehicles tend to be heavier than conventional combustion-engine cars, which results in greater friction and wear of components such as tires and brakes, thereby increasing the release of particulate matter into the air [47,48,49]. Some studies have shown that the concentrations of certain metals in roadside soils can be up to eight times higher in areas with high traffic volumes, particularly near intersections and in city centers [50]. Research also highlights that these concentrations are further elevated near highways, primarily due to the movement of heavy-duty vehicles [51], which also emit higher levels of metals like iron (Fe), antimony (Sb), and barium (Ba)—compared to passenger cars [52]. Other studies emphasize the high mobility of metallic particles, as they have been detected not only in roadside soil but also on the leaves of tall trees, indicating a strong potential for long-range dispersion [53].

One of the methods for removing toxins from soil is phytoremediation—a process involving the extraction or stabilization of contaminants from soil using plants cultivated on the polluted area [54,55]. Phytoremediation technologies are categorized based on the approach used to counteract a specific contaminant, which varies depending on both the plant species employed and the type of pollution present in the soil [56]. The most common phytoremediation techniques include:

• Phytoextraction—a method involving the uptake of contaminants by the root systems, followed by their accumulation within the plant’s cells. This is one of the most widely used bioremediation techniques, due to its low cost and the ease of managing post-process biomass [57,58,59].
• Phytostabilization—a method that stabilizes contaminants within and around the plant’s root zone, preventing or significantly limiting further migration. Certain plant species produce enzymes in the rhizosphere that are capable of stabilizing contaminant particles, including heavy metal ions. This method is typically used in areas where removing contaminants could create additional complications, especially when such areas pose no immediate threat to local organisms or are not intended for further land use [60,61].
• Phytodegradation and Phytoevaporation—techniques similar to phytoextraction but differing in that the contaminants are not directly accumulated within the plant’s tissues. Instead, certain plant species can uptake pollutants from the soil and subsequently break them down or transform them into other compounds within their cells. In phytodegradation, the resulting compounds usually exhibit minimal or no harmful effects on the plant. In phytoevaporation, these newly formed compounds are also released into the atmosphere. These methods are used for specific contaminant groups (e.g., selenium (Se) compounds) [62,63,64].

Phytoremediation technologies are generally considered socially acceptable, as they do not disrupt the physical or chemical properties of soil, nor do they degrade the environment or landscape. They are also relatively inexpensive compared to physical or chemical soil remediation methods. However, the main disadvantage is the slow pace of the process, which is dependent on plant growth and, consequently, weather and environmental conditions [65,66].

In the case of plants used for phytoextraction, attention is focused on species that can thrive on contaminated sites, tolerate the stress associated with pollutants in the rhizosphere, and exhibit a strong capacity for accumulating contaminants within their tissues. Plants that combine these characteristics are referred to as hyperaccumulators [67,68]. There are several definitions for hyperaccumulator plants, but it is commonly accepted that these are plants capable of concentrating a given contaminant above 1000 mg per kilogram of dry biomass [69], or for which the concentration ratio of the contaminant in plant tissues to that in soil exceeds 1 [70]. Examples of hyperaccumulators include species from the Brassicaceae (mustard family) and Asteraceae (daisy family), as well as some trees, perennials, and vegetables [71,72,73,74].

An increasing number of studies focus on the practical application of phytoremediation under real environmental conditions. In the case of phytoextraction, based on the current state of knowledge, hyperaccumulating plants—especially species from the Brassicaceae family—are most commonly used. In a 2023 study, broccoli (Brassica oleracea var. italica) was used both for the stabilization of chromium (Cr) in the rhizosphere and for effective extraction of lead (Pb) and cadmium (Cd) from contaminated soils [75]. Similar properties have been observed in plants from the Cannabis genus. Several studies indicate their high efficiency in hyperaccumulating zinc (Zn) and cadmium (Cd) in the aboveground parts [76,77], as well as their ability to stabilize chromium (Cr) in the root zone [78]. In other large-scale experiments, hemp was applied for copper (Cu) removal from soil—approximately 30% of the metal was extracted during a single growing season, with notable accumulation in seeds and young shoots. At the same time, the plants absorbed up to 90% of polycyclic aromatic hydrocarbons (PAHs) present in the soil [79]. Hemp has also been successfully used in the remediation of areas contaminated with arsenic (As) [80]. although its cultivation remains limited or regulated in many countries. In other field-scale studies, tobacco (Nicotiana tabacum) and sunflower (Helianthus annuus) were applied to remediate soils highly contaminated with zinc (Zn). After one growth season, the plants accumulated up to 70% of the initial Zn content in the soil [81]. Additionally, Lantana camara has been classified as a hyperaccumulator of cadmium (Cd), capable of accumulating several hundred mg/kg of this metal in its biomass [82].

Regarding phytostabilization technologies, various grasses have been reported to effectively immobilize heavy metals. For example, red fescue (Festuca rubra) has been used for the stabilization of cadmium (Cd) and lead (Pb) [61], while perennial ryegrass (Lolium perenne) has demonstrated effectiveness in stabilizing zinc (Zn), lead (Pb), and copper (Cu) [83,84]. Literature also mentions the use of white lupin (Lupinus albus) for the stabilization of arsenic and lead [85], and different species of ferns [86]. A significant portion of phytostabilization applications involves the use of trees and shrubs, such as Acer pseudoplatanus and Acer platanoides (for As, Cu, Zn, Cd) [87,88]; Jatropha species (for Pb, Zn, Ni) [89]; and Pinus radiata (for Cu) [90,91]. A practical example of phytostabilization can be found in the Silesian Voivodeship in Poland, where former industrial areas have been successfully revitalized and repurposed, including the Silesian Park of Culture and Recreation (Wojewódzki Park Kultury i Wypoczynku—WPKiW) or the Żabie Doły nature and landscape complex [92].

An analysis of the available literature reveals that most studies focus primarily on the effectiveness of the plants themselves and the use of supporting additives (such as chelators). However, the impact of constant sources of heavy metal emissions on the efficiency of phytoremediation processes is often overlooked. It is important to assess the extent to which the presence of such sources may disrupt or limit the plants’ ability to cleanse the environment. This would contribute to a better understanding of real-world environmental conditions and enable more effective planning of remediation strategies.

The initial aim of this study was to evaluate the effectiveness of heavy metal phytoextraction from soil using lettuce (Lactuca sativa), as well as to assess the capacity of diatomite (diatomaceous earth) to stabilize metals in the soil environment. However, preliminary analysis revealed the presence of a persistent source of heavy metal emission, which led to an extension of the research scope to include the identification of this source and an evaluation of its impact on the phytoremediation process.

2. Materials and Methods

The first part of the study focused on analyzing the level of soil contamination in the Silesian Voivodeship, specifically in the vicinity of industrial facilities and post-mining waste heaps, which historically served as dumping grounds for waste from local industrial and mining centers. Subsequent stages of the research addressed the potential for soil purification through phytoremediation, during which the growth and condition of plants cultivated in the degraded soil were observed in relation to the presence of heavy metals. The ambiguous results of the remediation process led to the formulation of a new hypothesis, necessitating additional investigations aimed at identifying the sources of heavy metal emissions.

2.1. Analysis of Contaminated Soil

In order to assess the extent of soil degradation caused by heavy metal contamination, several soil samples were collected from two locations in the Silesian Voivodeship: a zinc and lead smelter operating in the area of Miasteczko Śląskie, and a post-mining waste heap located on the border between the towns of Bytom and Radzionków. At each site, five surface soil samples (from a depth of 0–20 cm) were taken from five different points spaced approximately 5 m apart. About one liter of soil was collected from each point. The samples from each location were then thoroughly mixed to prepare a single composite sample representative of the respective area. The sampling locations are shown on the map below (Figure 1), as well as a sampling spatial distribution diagram (Figure 2).

The area surrounding the smelter is, for obvious reasons, continuously exposed to toxic metals through emissions from the industrial plant. The local vegetation displays reduced growth and poorer condition compared to plants found farther from the smelter. The soil in this area was characterized by a dry, loose, and sandy structure, with a visible presence of inorganic fractions. Soil samples were collected in May from publicly accessible areas, without disturbing the surrounding flora, and were then mixed to obtain an average sample. The soil from this location was also used as the medium for the remediation process described in the later part of the study.

The waste heap on the border of the aforementioned municipalities has served as a dumping ground for industrial waste from nearby metal extraction and processing facilities—dating back to the late 19th century—including a now-defunct zinc smelter and acid factory. Waste in the form of slags, asphalts, and other by-products remains visible on the site and continues to act as a source of contamination, including heavy metals. The area is predominantly overgrown with metallophyte species, such as Verbascum thapsus (common mullein) and Solidago canadensis (Canadian goldenrod). The plant species composition, their weak condition, and sparse morphology clearly indicate the presence of heavy metals in the soil (Figure 3). Wild illegal dumping grounds and larger, dark agglomerates of metallurgical waste—nearly devoid of vegetation and emitting a strong sulfuric odor—are also visible on the heap (Figure 4). Several soil samples were taken from this area, from sites with varying levels of vegetation density, and then averaged. Additionally, a control sample was taken from one of the dark agglomerates. Due to the fact that these samples were collected in early September, they were not used in the phytoremediation process described later in the text.

Additionally, for reference analysis, a commercially available compost-based horticultural substrate intended for vegetable and flower cultivation was purchased. Organoleptic evaluation revealed that this gardening soil was distinctly different from the samples collected near the smelter and waste heap. The commercial soil was moist, cohesive, and contained visible fragments of decomposed organic matter (e.g., leaf and grass residues). The analysis of this soil allowed for comparison of metal content between the contaminated soils and a widely available, standard-use substrate. This gardening soil was also used as a reference medium in the phytoremediation process.

To determine the concentration of individual metals in the collected substrates, the samples were first dried at 105 °C to constant weight and then prepared for the mineralization process. From each sample, 0.5 g of dried soil was weighed into Teflon reaction vessels, followed by the addition of 7 cm³ of 65% nitric acid (HNO₃) and 3 cm³ of 40% hydrofluoric acid (HF). After a brief period, the vessels were tightly sealed and placed into a Speedwave Xpert 2.1 microwave digestion system. Mineralization parameters were selected based on the manufacturer’s guidelines for the given sample type—approximately one hour with a temperature ramp-up to 200 °C. At the end of the digestion process, the vessels were cooled to room temperature, and the resulting solutions were filtered and transferred to volumetric flasks. The flasks were then filled with distilled water to a final volume of 100 cm³. The concentrations of selected metals—including copper (Cu), chromium (Cr), cobalt (Co), lead (Pb), nickel (Ni), zinc (Zn), cadmium (Cd), manganese (Mn), and iron (Fe)—were determined using atomic absorption spectroscopy (AAS), with a Hitachi Z-2000 spectrometer. The digested solutions were introduced into the instrument via a flame atomizer, where thermal decomposition generated free atoms capable of absorbing radiation at element-specific wavelengths. The light source for these measurements consisted of element-specific hollow cathode lamps, each emitting radiation at wavelengths corresponding to those absorbed by the respective atomized element. The spectrometer calculated the final metal concentrations based on absorbance values (as determined via pre-established calibration curves) and the known sample masses. A limitation of atomic absorption spectrometry is the requirement for a dedicated lamp corresponding to each target element. For each soil sample, the mineralization and metal quantification procedures were performed in triplicate. The averaged results, expressed on a dry weight basis, are presented in the following section (

Table 1. Averaged results of measurements of heavy metals content in soil samples.

Sample	Element Share [ppm]
	Cu	Cr	Co	Pb	Ni	Zn	Cd	Mn	Fe	Hg
Garden soil	30	0	0	6	0	114	0	93	3875	0.47
Smelter soil	31	4	4	510	10	367	6	53	4997	0.22
Heap soil	3411	18	14	570	46	3786	15	423	27,051	0.88
Dark aglomerate	16,612	27	10	1254	35	4290	5	140	31,591	1.71

).

In all subsequent analyses described in this article, the procedure for metal detection in other samples followed the same protocol; therefore, the details of digestion and measurement are not repeated further in the text.

2.2. Phytoremediation Process

In an effort to remediate the contaminated soil, a phytoremediation process was applied—specifically, the phytoextraction and subsequent accumulation of pollutants in plant tissues. An additional objective of the study was to observe the effect of heavy metals on the development and growth of cultivated plants. For the process, Lactuca sativa (lettuce), a member of the Asteraceae family, was selected as a potential pollutant accumulator due to its low cultivation requirements and rapid biomass gain during the vegetative season, which began in May and coincided with the sampling of soil from the vicinity of the zinc smelter.

In June, 30 lettuce seedlings from the same sowing batch were purchased from a single grower. The plants were divided into three groups of 10 and planted in separate pots. Each group was planted in a different soil substrate:

• Group 1 (G1)—Reference substrate, compost-based, dedicated to horticultural cultivation.
• Group 2 (G2)—Contaminated substrate collected from the vicinity of the zinc and lead smelter.
• Group 3 (G3)—Contaminated substrate from the same smelter area, with added diatomite.

Diatomite (diatomaceous earth), the additive used in Group 3, is a powdered, amorphous sedimentary rock used in agriculture as a natural pesticide, a source of silicon, and a stabilizer of heavy metals. Its presence in the soil was hypothesized to reduce the uptake of pollutants by the plants, and thereby mitigate the negative effects of metal presence on plant growth and condition. Diatomite was applied at a rate of 10 g per 0.4 dm³ of soil, corresponding to the volume of each planting pot. The content of heavy metals in the diatomite was also determined (

Table 2. Averaged results of measurements of heavy metals content in diatomite.

Sample	Element Share [ppm]
	Cu	Cr	Co	Pb	Ni	Zn	Cd	Mn	Fe	Hg
Diatomite	12	140	5	0	16	102	0	20	9022	0.01

).

The plants were cultivated outdoors in a sunny and well-ventilated area from June through early September. Soil moisture was regularly monitored to prevent complete desiccation—especially critical in the contaminated soils due to their loose, granular structure and limited water retention capacity. Routine maintenance included the removal of pests (e.g., caterpillars, slugs) and draining excess water from saucers after heavy rainfall. No pruning, shaping, or removal of wilted leaves was carried out, in order to avoid influencing plant development.

After the phytoremediation period, the lettuce biomass and cultivation substrates were collected. Composite samples were prepared from each group for heavy metal analysis (

Table 3. Averaged results of measurements of heavy metals content in biomass after process.

Sample	Element Share [ppm]
	Cu	Cr	Co	Pb	Ni	Zn	Cd	Mn	Fe	Hg
G1—biomass	13	7	2	23	22	54	8	81	714	0.12
G2—biomass	19	10	3	50	37	551	25	118	587	0.11
G3—biomass	14	9	3	58	12	541	27	178	650	0.14

and

Table 4. Averaged results of measurements of heavy metals content in soil after remediation.

Sample	Element Share [ppm]
	Cu	Cr	Co	Pb	Ni	Zn	Cd	Mn	Fe	Hg
G1—soil	19	21	10	113	7	90	6	116	3383	0.24
G2—soil	3	11	3	546	4	355	8	32	5133	0.22
G3—soil	7	16	3	653	4	333	8	52	5092	0.16

). The analysis procedure was identical to that applied before the remediation stage, involving drying, microwave digestion, and quantification using atomic absorption spectroscopy (AAS). The only variation was in the digestion of plant biomass samples: due to the different nature of the material, and following the manufacturer’s guidelines, digestion was performed using 10 cm³ of 65% nitric acid (HNO₃).

The obtained results indicated the need for additional studies focused on the identification of potential sources of heavy metals accumulating in the substrates during the remediation process.

2.3. Detection of the Source of Pollutant Emissions

In investigating the cause of elevated metal concentrations observed during the phytoremediation process, the possibility of emissions from industrial facilities was ruled out, as no such facilities are located within several dozen kilometers of the lettuce cultivation site. No chemical agents, pesticides, or fertilizers were used during cultivation, eliminating them as potential contributors. Additionally, since the experiment was conducted during the summer season, the influence of low-stack emissions was also considered negligible.

The most likely sources of contamination were identified as a local roadway adjacent to the garden and a highway located several hundred meters away. To assess the emission of metals from nearby road traffic, the surrounding environment and various roadside structures were examined. A visible, powdery, gray deposit covered several surfaces, which was suspected to consist of metal particles released during vehicle operation—originating from the abrasion of brake components, tires, or asphalt pavement. To determine its composition, several samples of this observable residue were collected from noise barriers and crash barriers located at the highway entrance (Figure 5). Sampling took place on a sunny day following several dry and windless days, ensuring minimal influence from precipitation or wind-driven particle redistribution. The samples were subsequently transported to the laboratory and tested for metal content—the results are presented in

Table 5. Averaged results of measurements of heavy metals content in the dust from highway.

Sample	Element Share [ppm]
	Cu	Cr	Co	Pb	Ni	Zn	Cd	Mn	Fe	Hg
Dust	136	46	22	606	62	5282	18	1170	25,956	0.11

.

3. Results

3.1. Soil Quality Analyssis

The table below (Table 1) summarizes the average concentrations of heavy metals measured in both the purchased garden soil and the contaminated soils collected for the experiment.

As expected, the soil collected from the vicinity of the zinc and lead smelter exhibited elevated levels of heavy metals. Compared to the dedicated horticultural substrate, the lead content was nearly 100 times higher, zinc was approximately three times higher, and iron levels were about 25% greater. The concentrations of other elements were similar across both soils. Interestingly, manganese was more abundant in the compost-based substrate. This is likely due to the fact that manganese, along with copper, functions as an essential micronutrient for plant development and is therefore intentionally present in trace amounts in commercial horticultural mixes. Cadmium concentration in the soil near the smelter, while low, is still concerning due to its high toxicity, even at minimal concentrations. In contrast, cadmium was not detected at all in the horticultural soil.

The soil sampled from a nearby post-industrial slag heap showed significantly higher levels of contamination than either the compost soil or the smelter-adjacent soil. Only lead was present in similar concentrations to the smelter soil. Zinc levels were over 20 times higher than in the horticultural substrate, and iron was approximately seven times higher. Copper and manganese concentrations were especially elevated—100 times and 4 times greater, respectively—than in the control soil. Such levels are considered toxic for plant growth. Cadmium also reached 15 ppm, a concentration posing a serious toxicological risk.

Although concentrations of other metals (nickel, cobalt, chromium, mercury) remained relatively low, they were still several times higher than in the smelter-adjacent soil. Notably, dark-colored agglomerates found within the slag heap displayed the highest concentrations of all—iron, mercury, zinc, lead, and copper—exceeding tolerable levels even for local metallophyte vegetation.

3.2. Phytoremediation Process Observations

A baseline analysis of the diatomaceous earth (diatomite) used in the third soil group was performed to assess its possible role as a contaminant source (Table 2).

The diatomaceous earth, though naturally containing iron, showed no significantly elevated levels of other heavy metals—except for slight increases in zinc and chromium. This study allowed us to exclude this substrate as a major contamination source.

After just one month of cultivation (June), observable differences in plant development emerged. Lettuce grown in the contaminated soil (G2) exhibited reduced growth, with lower leaves developing brown discoloration and signs of necrosis. In contrast, plants grown in contaminated soil amended with diatomaceous earth (G3) appeared healthier and more vigorous. These formed characteristic compact heads and displayed fewer signs of stress. Predictably, the lettuce grown in the horticultural substrate (G1) showed the most robust growth, producing large, dense heads free from discoloration or spotting. A visual comparison of plants from the different groups is presented in Figure 6.

At the end of the growing season in September, the plants were harvested and compared (Figure 7). Lettuce from the horticultural soil (G1) grew the largest and healthiest, with thick, fleshy leaves. Many of them flowered, suggesting minimal environmental stress. Plants grown in the contaminated soil (G2) exhibited the weakest growth, sparse foliage, and failed to form heads. Only one plant attempted to flower. The third group (G3), with diatomite, showed good overall growth—slightly less than the control group—but many plants flowered, and leaf discoloration was minimal, affecting mainly the lower leaves.

Root system comparisons are shown in Figure 8. Plants from the compost-based substrate (G1) developed healthy root systems, filling the pots uniformly with long, intertwined, pale-colored roots. In contrast, plants from Groups 2 and 3 developed more centralized root masses with fewer lateral roots. Those grown in the unamended contaminated soil developed shorter, weaker roots with a noticeably darker color.

The concentrations of metals in plant biomass and soil after phytoremediation are presented in Table 3 and Table 4, respectively.

From the data above, it is evident that the lettuce plants absorbed a portion of the soil contaminants. For example, those grown in smelter-contaminated soil accumulated approximately 10 times more zinc and cadmium, and around twice as much lead and manganese, compared to the control group. In the case of cadmium, although its share in the substrate was minimal, both the plants and the soil after the process contained its particles. This may indicate a constant emission of this element, and then its deposition in the soil and further extraction by plants. Differences in the uptake of other elements were less pronounced. Iron, despite being abundant in all substrates, was absorbed to similar levels (~600 ppm) across all groups, suggesting that lettuce is not particularly efficient at extracting iron from soil.

More importantly, the post-remediation soil still retained high levels of most metals, often close to their initial concentrations. This supports the hypothesis of a persistent, external contamination source in the cultivation area. This is particularly evident in the horticultural soil, where the initial lead (Pb) concentration was only 6 ppm, but increased to 113 ppm after the process. A similar increase was observed for soil G3 (from 510 ppm to 653 ppm), while for soil G2 it was smaller but still noticeable (36 ppm).

Moreover, the highest concentrations of metals were observed in the soil enriched with diatomite (G3), which could initially suggest a stabilizing effect of the diatomaceous earth. This was also reflected in the condition of the plants: lettuce from Group 3 did not exhibit the same level of stress in response to heavy metal presence in the soil as observed in Group G2. However, the biomass analysis of plants from Group G3 revealed high concentrations of metals, particularly lead (Pb) and cadmium (Cd), very similar to those found in Group G2, which contradicts the assumed stabilizing properties of diatomite. This paradox may be explained by the specific characteristics of diatomite: although it did not effectively reduce metal mobility, its high silicon content could have improved soil structure and water retention, as well as slowed the uptake of toxic elements by plants. Additionally, plants likely absorbed bioavailable silicon from the soil, which is known to strengthen cell walls and enhance plant resistance to various environmental stresses. As a result, despite significant metal accumulation, the physiological condition of the plants remained relatively stable. Furthermore, diatomite’s porous structure may have absorbed a portion of the irrigation and rainwater, thus limiting the downward leaching of metals into the saucers beneath the pots. This means that a greater share of metals may have remained in the soil, available for plant uptake, while the beneficial effects of silicon mitigated the physiological impact of the accumulated contaminants.

3.3. Analysis of the Source of Metal Emissions

The table below (Table 5) presents the results of the analysis of dust collected on the outskirts of a highway.

As expected, the dust contained high levels of heavy metals, particularly iron, zinc, manganese, lead, and cadmium—these same elements were previously detected in earlier post-remediation soil studies. Other elements were also present but in smaller quantities. This analysis confirmed the earlier hypothesis regarding emissions of specific metals from areas with high traffic intensity.

4. Conclusions

The original objective of this study was to analyze the process of phytoremediation and the potential for metal stabilization in soil using diatomaceous earth. However, the results obtained after remediation led to an expanded research scope and deeper investigation into the emission and migration of heavy metals.

The analysis of soils from areas near a zinc and lead smelter, as well as from a nearby waste heap, confirmed the anticipated presence of elevated levels of heavy metals. Near the smelter, especially high concentrations of zinc, lead, and iron were found, directly linked to the facility’s operations. Soils from the heap, located near decommissioned industrial units, showed even higher contamination than those near the smelter. The condition of the surrounding vegetation indicated elevated metal concentrations—most clearly in the dark agglomerates on the heap, which were completely devoid of plant life. A noticeable unpleasant, irritating odor was present on-site, likely caused by other non-metallic contaminants (e.g., sulfur compounds, PAHs, organic compounds), the detection of which would be necessary if the area were to be reclaimed in the future.

Observations of the phytoremediation process in soils from the smelter area revealed a clear and visible impact of heavy metals on the growth and condition of cultivated plants. The stress caused by contaminant uptake manifested in reduced biomass, weak stems, leaf discoloration, and failure to flower—which is a critical limitation for annual crops such as lettuce that rely on rapid growth and seed production.

Through phytoextraction, the plants absorbed some of the contaminants present in the soil. Plants grown in polluted substrates accumulated higher concentrations of heavy metals than those cultivated in horticultural soil. In the case of diatomite amendment, it did not directly affect the total amount of metals absorbed by the plants. Lettuce grown in soil with diatomite accumulated metal concentrations similar to those observed in plants grown without it. Nevertheless, the condition and overall health of the plants were noticeably better in the diatomite group. As previously mentioned, this effect could be attributed to the uptake of silicon from diatomite by the plants, which may have enhanced their resistance to metal-induced stress. Additionally, improvements in soil structure—such as increased water retention and better root zone conditions—may have contributed to the improved plant vitality. Diatomite may also have adsorbed some of the metals onto its surface, thereby reducing the amount leached away during rainfall or irrigation. This phenomenon warrants further investigation in future studies.

However, an ambiguity arose in the post-remediation soil analysis. Elevated levels of some metals—despite their evident accumulation in plant tissues—prompted additional questions and led to further investigation. It is also important to consider that some heavy metals may have been leached from the soil with water from rainfall or irrigation, and subsequently removed during the emptying of the trays under the pots. Therefore, the amount of newly deposited contaminants could have been even higher than measured at the end of the growth. A continuous local source of pollution near the cultivation site was suspected, and subsequent analysis identified a nearby highway. This assumption was confirmed by tests of dust collected along the roadside. This dust, generated through vehicle and road surface wear, can easily migrate beyond the highway area via wind or rainfall, eventually accumulating in nearby soils, water bodies, or entering living organisms via inhalation or ingestion.

It should also be emphasized that phytoremediation, despite its many advantages, is not without limitations. The process is time-consuming and requires regular monitoring of both soil conditions and plant health. Over time, new contaminants may enter the soil, potentially leading to the re-contamination of already remediated areas. The effectiveness of phytoremediation is also highly dependent on environmental conditions such as temperature, precipitation, and soil moisture, which affect plant growth and their uptake capacities. Additionally, plant biomass that has accumulated heavy metals becomes hazardous waste itself and must be properly processed or safely disposed of to avoid secondary contamination.

The series of studies described above highlights the issue of heavy metal pollution. Soils located on former industrial sites, waste heaps, and near factories are degraded and contain toxic elements. The impact of such contamination is often visible even without advanced laboratory analysis—simply through the absence or poor condition of local flora. These soil pollutants can spread further through groundwater, the food chain, or physical disturbance of the soil. Contaminated soils cannot be repurposed or reused without prior remediation and detoxification, which are often costly and energy-intensive. Phytoremediation, as a soil recovery technology characterized by low operational costs, appears to be one of the most effective methods for cleaning such areas due to its flexibility in timing. However, the presented research indicates an additional challenge in soil remediation: the continuous emission of new pollutants. Detoxification through phytoextraction may not yield the desired results if fresh contaminants constantly enter the already remediated soil. The key conclusion drawn from the study is the necessity to first reduce or eliminate local sources of pollution before beginning any remediation or revitalization of contaminated land. In roadside areas, physical barriers such as sound walls or dense hedgerows can be implemented to limit the migration of road dust. Increasingly, the use of so-called “green barriers”, strips of vegetation with high capacity for particle and metal retention, is also being recommended. At the same time, technological improvements in vehicle design, such as low-emission brake systems or more durable tires, should be promoted to reduce non-exhaust emissions. Ultimately, reducing pollutant sources prior to remediation efforts is crucial, as only this integrated approach can ensure long-term detoxification and successful revitalization of contaminated sites.

This work also emphasizes the importance of integrating phytoremediation with environmental monitoring and source control, especially in regions exposed to continuous pollution input. Future research should focus on optimizing plant–substrate combinations tailored to specific contamination profiles, improving biomass handling strategies, and exploring synergistic remediation systems (e.g., combining phytoremediation with microbial or chemical amendments). The findings presented here can serve as a basis for further development of low-cost, sustainable remediation protocols that are scalable and applicable in real-world conditions.

The presented research provides a foundation for further investigations and will be continued in the future. Upcoming studies will focus on a more detailed assessment of the impact of traffic-related emissions depending on the distance of cultivation from high-traffic roads. Planned experiments will include controlled plant cultivation at varying distances from emission sources, allowing for a precise characterization of spatial contamination gradients and their influence on remediation processes.