The Suitability of Selected Naturally Growing Plant Species for the Phytostabilization of Heavy Metals at Different Locations on the Slopes of a Zinc Smelting Waste Landfill: The Second Case Study

Abstract

This case study is the second of three which we have been conducting on different industrial waste landfills. We are planning a fourth study comparing the three landfills. Phytostabilization, including assisted phytostabilization, is a measure of reducing the negative impact of industrial waste landfills on the environment. It is particularly important in the case of old unprotected and often abandoned landfills. Most studies investigate how phytostability depends on the plant species but do not consider its dependence on the specific location at the landfill where the plants are growing. We assumed that the habitat conditions within the landfill had been modified unequally over the years. The most heterogeneous habitat conditions were found on the slopes of the landfill. The aims of the study were to assess the impact of the location on the landfill, i.e., the site of growth; the impact of the plant species or organ; and the combined and simultaneous impact of the location and species/organ on the phytostabilization of cadmium, lead, zinc, and copper. All bioaccumulation factor (BCF) values calculated for each metal and each location (base, middle, and top) differed statistically significantly from one another. In the case of lead, zinc, and copper the highest BCFs, irrespective of species, were obtained for plants growing at the top of the landfill, whereas the highest value for cadmium was recorded at the base. Additionally, all interactions analyzed between location and species/organ were statistically significant. Variations in the BCF values, including the variation influenced by the interaction between location and species/organ, followed four distinct patterns along the slope of the landfill from the base, to the middle, and to the top.

1. Introduction

The technological and economic development of civilization is usually accompanied by anthropogenic impacts of varying intensity on the natural environment. This applies in particular to areas associated with past and current extraction and processing of coal and metal ores and with the storage of industrial waste [1,2,3,4,5,6]. Secondary emissions of pollutants from post-industrial areas can result in contamination of the environment with heavy metals. These elements can persist at high concentrations for decades or even millennia, and their gradual migration to agricultural soil or sediments of watercourses, often even in trace amounts, can pose a real threat to all living organisms, including people [7,8,9,10,11,12]. Today, sustainability is a legal, moral, social, engineering, and economic obligation. In the third decade of the 21st century it is impossible or even unthinkable to build an industrial waste landfill with no environmental safeguards, which is an achievement of humankind. However, there are undesirable remains from industrial times, such as landfills, where waste deposition was terminated several decades ago. The technologies used thus far do not fully protect the surrounding areas from the negative impacts of industrial waste landfills. Both external factors (topography, microclimatic conditions, vegetation, or the type of protective barrier) and internal factors (the deposition technology, the physical and chemical properties of the waste, or the means of protecting the substrate) influence the type and degree of nuisance, as well as the quantity of pollutants originating at a given site [13,14]. Negative environmental effects are particularly perceptible in the region of old landfills, in which waste was usually deposited with no form of protection and without reclamation procedures [15,16,17,18,19]. One of the most important factors limiting the negative environmental impact of these sites is vegetation. Plants stabilize the surface of landfills, reduce wind and water erosion, eliminate negative effects on the landscape, and limit the migration of toxins into the environment by accumulating them in their organs. Vegetation also takes part in soil formation processes, in part through the accumulation of biomass in the soil of the landfill. Finally, it enables and supports microbiological processes of the colonization of the landfill soil. Irrespective of the type of succession (by reclamation or natural), developing plants generally face unsuitable humidity and air temperature conditions, extreme (low or high) pH, a lack of basic nutrients, extreme water content (usually drought), and phytotoxic substances present in the Technosols of landfills [19,20,21]. These negative traits of the substrate exert a strong selective pressure on colonization by plants, and consequently landfills are often located on large areas entirely devoid of vegetation or have a specific, unique composition of plant species growing there spontaneously [6,22,23]. This is why it is so important to identify plant species with high adaptability to the habitat conditions prevailing at landfills as well as the ability to accumulate heavy metals, which is of practical significance for the reclamation of these sites. Apart from a qualitative assessment, a quantitative assessment is important as well, i.e., the determination of how much of a given metal can be accumulated and temporarily excluded from the circulation of matter. According to Bolan et al., ‘phytostabilization aims to contain contaminants within the vadose zone through accumulation by roots or precipitation within the rhizosphere’ [24]. Many studies on phytoremediation have assessed the suitability of various species in this process. Pot and plot experiments have been conducted with different planting or sowing systems, fertilizers and fertilizer application rates, and levels of soil pollution with heavy metals. The pollution levels may be due to previous contamination or experimentally induced to obtain specific conditions for the purpose of the research [25,26,27,28]. Many studies have also been conducted in post-industrial areas including industrial waste landfills. These studies evaluate the remediation potential of selected plants, which may be sown or planted during reclamation or may appear naturally during succession [22,29,30,31]. Unlike pot and plot experiments, in this case the soils are very often techno soils which do not create soil ecosystems. Some of the authors cited above indicate variability in heavy metal bioaccumulation within the same species of plants growing under the same contamination pressure. However, it is difficult to find studies in which this variability is explained by the location where the plants grow on a landfill [5,6]. Bolan et al. tested the effect of location on the stabilization of heavy metals on an industrial waste landfill, i.e., at its base, in the middle, and at the top. In that study, the metals were stabilized by microbes, and the location on the landfill was confirmed to influence biostabilization of metals. This was the only study we found in our analysis of the literature that was devoted to the effect of location on stabilization of metals. In contrast to our study; however, the variation in habitat conditions between the three areas also affected the vegetation, as different plants were growing at the bottom of the slope, in the middle, and at the top. At the landfill in Krze, the same plants were present on all parts of the landfill [6]. Our previous research on a zinc and lead post-flotation landfill showed phytostabilization variability depending on the location of growth on the landfill. This was a site with a relatively short history which underwent reclamation treatments [19]. In our subsequent research, we decided to test this hypothesis on landfills covered by natural plant succession. Our studies, conducted at a coal ash waste landfill, confirmed this approach [5]. The research presented in the present study was conducted at a zinc smelting waste landfill site dating back about 200 years and covered exclusively with vegetation originating from natural plant succession.

In the case described here, a technologically unvarying metallurgical process contributed to the homogenization of the waste produced. Nevertheless, a landfill is the site of complex processes—geological, ecological, hydrological, and microclimatic—which can affect the physicochemical and microbiological properties of the substrate. The habitat conditions in which plants develop on different parts of the landfill may vary. Szwalec et al. and Sun et al., 2022, demonstrated that the location at which plants grow on a landfill affects their capacity for the bioaccumulation of metals [5,6].

The following research questions were posed: Which species accumulate heavy metals, and in what amounts? Does the location on the landfill influence the content of metals? And, under the conditions of this case study, did the above-mentioned factors interact? And, finally, is sustainability a tool which may help to solve the problem of abandoned landfills?

The aim of the study was to assess the suitability of the selected natural and permanent plant species growing at various locations on a zinc smelting waste dump for the phytostabilization of cadmium, lead, zinc, and copper.

2. Research Methods

2.1. Research Site

The landfill is located in the Krze housing estate in the city of Trzebinia. The city lies in Chrzanów County, on the border of Lesser Poland and Upper Silesia in southern Poland, Eastern Central Europe (Figure 1). The county is located within the Krakow–Upper Silesian ore-bearing area, where the mining and smelting of lead, silver, and later also zinc date back to the beginnings of the Polish state, about a thousand years ago.

The landfill was the site of the deposition of waste from the former Artur zinc smelter. The waste arose during zinc production carried out from about 1820 to about 1910 by the Silesian (fire–muffle) process, as well as from the preparation of ceramic consumables. The waste consisted of slag and silt constituting a mixture of oxides and silicates of zinc, lead, iron, and other heavy metals. Used muffles in the form of ceramic retorts also ended up on the waste dump. No reclamation was carried out on the landfill, and the plants growing on it developed in a secondary succession. It should be noted that this area (Krakow and Upper Silesia) was responsible for about 11% of the world’s zinc production in the 19th century [33,34,35,36]. Today (autumn of 2024), the landscape of this area fits in with that of the physiogeographic unit in which it is situated, i.e., Jaworzno Hills.

2.2. General Description of Research Methods

The site of this case study is the second of three landfills which we have already sampled. The first study was published as Szwalec et al. [5] in Sustainability. Plant species may appear naturally (secondary succession) or be planted in a reclamation process. There were two conditions for the selection of species for the study. First, the plant species should be a permanent element of the phytocoenosis of the landfill (i.e., present for several decades). The second criterion was that the species must grow on the entire surface of the landfill slope, so that they could represent its base, middle, and top. The landfill is located at 50°10′48″ N latitude and 19°27′25″ E longitude. The climate is moderate with both maritime and continental elements. The growing season lasts 220 days, and the annual rainfall in the town of Trzebinia is 700 mm. The heavy metal contents in the soil and other parameters are presented in Table S1 of the Supplementary Materials.

The plant and soil (substrate) material was collected from the slope (Figure 2), because the diversification of biotope conditions on this part of the landfill takes place fastest and with the greatest intensity. The slope was divided into three characteristic locations—the base, the middle, and the top. For each of these locations, four research plots were established. Each plot had an area of about 112.5 m² and was further divided into four sampling points as replicates. From each sampling point (22.5 m²), one averaged sample was taken, consisting of five homogenized primary samples of moss thalli, herbaceous plant shoots, and the shoots and leaves of trees—silver birch (Betula pendula Roth), rowan (Sorbus aucuparia L.), and English oak (Quercus robur L.), as well as soil samples. Following homogenization, each averaged sample weighed about 600 g DW in the case of the plants (for each species/organ) and about 500 g DW in the case of the soil. The aerial parts of the plants were collected using stainless steel secateurs, and soil samples were collected using a soil sampler from a 0–0.1 m layer. Moss thalli were dried and then thoroughly cleaned of soil and leaf residue using a small brush. In the case of higher plants, shoot and leaf samples were analyzed separately. The plant samples were dry-digested, and the soil substrate samples were wet-digested in mixtures of concentrated nitric and perchloric acids. Metals were extracted from the ash using hydrochloric acid. A detailed description of the research methods, experimental design, means of sample collection, and handling in the laboratory is given by Szwalec et al. [5], available at https://www.mdpi.com/2071-1050/14/12/7083 access on 1 February 2025.

3. Results and Discussion

Analysis of the phytoremediation properties of the plants and the factors determining phytostabilization was based on the bioaccumulation factor (BCF), which is calculated as the ratio of the metal content in the plant to its content in the soil. Depending on the author, the content in the plant can be understood as the content in the root or in the shoot or leaves. In this study, it was the content of the metal in the shoot/leaf. The roots of the plants were deliberately omitted from the analysis, because it would not have been possible to collect a representative number of root samples without destabilizing the slope and substantially damaging the vegetation. The bioaccumulation factor used in the study is modified by the translocation factor of a given metal between the root and the shoot. The exception was moss, a plant without tissues, and its bioaccumulation factors. BCF makes it possible to present, assess and discuss variation in the content of metals in the substrate and in plant organs. According to Szöcs and Schäfer [37], indices such as BCF are more suitable for use in statistical tests than the metal content itself.

3.1. Results and Overall Assessment

There is no information at all concerning reclamation of the landfill which was the subject of this study. It is highly likely that the top layer covering the waste deposited here was formed as a consequence of natural processes, including soil formation processes resulting from secondary succession, which have taken place at this site for nearly 200 years. This Technosol, with a relatively low and irregular thickness of 0.05–0.1 m, has been typologically classified [38] as argillaceous clay. The pH of the samples ranged from slightly acidic (pH 6.5) to basic (pH 7.3), and they had a high content of organic matter (15.8–33%). On the basis of these physicochemical properties and the content of the metals tested, the soil samples could be classified according to the Kabata-Pendias et al. method [39] in a range from natural content to severe contamination (Table S2). In the vegetation, the cadmium bioaccumulation factor was highest in the mosses growing in the middle part of the landfill (BCF = 0.558). These plants, irrespective of the part of the landfill, had much higher cadmium BCF values than the other species, with an average level of accumulation. A similar pattern was observed for bioaccumulation of the other metals. Mosses are pioneer plants which, together with lichens, are among the first to colonize post-industrial areas. They are often used as bioindicators to assess the state of the environment [40,41,42,43,44,45,46]. The cadmium concentrations in the mosses at the site were lower than those reported by Pajak and Jasik [44] in mosses in the nearby Brynica region in Upper Silesia, also a typical industrial area. Ite et al. [45] reported much lower concentrations in mosses growing in a petroleum exploitation region in Nigeria. Comparable cadmium content in mosses collected during the rainy season at two sampling points in the metropolitan area of the Toluca Valley were reported by Macedo-Miranda et al. [46]. The next species in the bioaccumulation chain of the plants analyzed on the landfill was silver birch. This is a common tree all over Europe and can be found fairly frequently in degraded areas as well. The species is considered a hyperaccumulator, especially of zinc [5,19,47,48]. These bioaccumulation abilities of silver birch were also confirmed in the present study. Like mosses, it accumulates zinc and copper to a moderate degree, and the other metals poorly. It should be stressed that this was bioaccumulation in the shoots and leaves, and not the most typical (highest) in the roots. In this case, the translocation of these elements in the plant is necessary. Equally high contents of zinc in the leaves of birch growing in industrialized areas were reported by Dmuchowski et al. [48]. On the other hand, concentrations of zinc in the leaves of this species growing on the site of a former lead and zinc mine in northern Spain were several times higher, as reported by Marguí et al. [49]. The next tree in the accumulation chain of the metals analyzed was rowan. Both the shoots and leaves of this tree, irrespective of the part of the landfill, accumulated more lead than the next species, English oak. The lead content reported by Reimann et al. [50] in the leaves of rowan trees in Oslo, Norway, was several times lower. On the other hand, Szwalec et al. [19] recorded higher concentrations of this metal in the leaves and shoots of this species growing in the middle part of a zinc and lead tailings dump, located near the landfill investigated in the present study. A higher average copper content in the leaves of English oaks growing in northwestern Spain was noted by Aboal et al. [51]. These results are comparable to the concentrations in the leaves of this species reported by Stojnić et al. [52]. In herbaceous plants growing at the study site, the concentrations of all elements were lowest in the samples collected at the base of the landfill. Higher concentrations of cadmium, lead, and zinc in this group of plants were reported by Szwalec et al. [19], while Jankowski et al. [53] noted lower contents of cadmium and lead in grasses growing near an expressway.

3.2. Detailed Assessment

The research questions were transformed into the following research hypotheses, separately for each metal:

• Location, i.e., the site of growth on the slope of the landfill, affects the bioaccumulation of the metal.
• The species of plant affects the bioaccumulation of the metal.
• The species and location affect the bioaccumulation of the metal jointly and simultaneously.

The first statistical test conducted was the Kolmogorov–Smirnov test, which confirmed a normal distribution for the values of the bioaccumulation factors of each of the metals tested. Two-way analysis of variance made it possible to reject the null hypothesis and accept the alternative hypotheses, stating that the plant species (or part); the location of growth on the slope; and the interaction of these two factors determined the uptake, translocation—and finally the bioaccumulation of cadmium, lead, copper, and zinc—in the shoots and leaves of the plants. The probability values are presented in

Table 1. Probability values for pairs of null hypotheses and alternative hypotheses I, II, and III for the bioaccumulation factors of cadmium, copper, lead, and zinc; alpha = 0.05.

	Cd BCF	Cu BCF	Pb BCF	Zn BCF
	Probability Value
Location	4.28 × 10−6	7.2 × 10−166	7.9 × 10−107	6.9 × 10−241
Plant species/part	1.7 × 10−264	8 × 10−289	2.7 × 10−307	9.5 × 10−300
Interaction	5.98 × 10−6	4.2 × 10−176	1.1 × 10−201	4.3 × 10−240

.

The statistical tests used in this study, including ANOVA and Tukey’s post hoc test, depend on the number of factors, combinations, and repetitions. The more there are, the more precisely the errors can be calculated. According to Gauss, errors are always additive, so they should be calculated as precisely as possible in the statistical analysis. In our study, due to the relatively small number of species used, birch, rowan, and oak were divided into two parts—shoots and leaves. This division was employed at the level of analysis of the samples in the laboratory. By increasing the number of factors from five to eight, we increased the accuracy of the error calculation.

3.2.1. Effect of Location

Current studies on the phytostabilization of pollutants from industrial waste dumps omit location as a factor independently influencing the bioaccumulation abilities of plants. The concept of location is itself unclear and therefore not used, and so it must be defined. It is a set of many factors, treated in phytoremediation studies as tertiary or quaternary, whose effect on bioaccumulation is not considered in practice. This effect may be additive, antagonistic, or synergistic. However, the combined effect may generate a heterogeneity of growth and development conditions for plants of the same species at a single site, thereby determining bioaccumulation. The greatest diversity of habitat conditions potentially takes place on the slopes of landfills. This area, however, is not identified as independent in studies of metal phytostabilization on industrial waste landfills.

There is also individual variation for phytostabilization properties within a species. In this case, however, it can be excluded, because traits (genes) are inherited randomly in the sexual propagation process. Thus, individual plants of the same species growing at selected locations will have random phytostabilization abilities. This would most likely not be the case if the landfill had been planted with maple trees, i.e., plants with the same genotype with the capacity for hyperaccumulation, in a reclamation process. In this case, however, no reclamation procedures had been carried out.

All of the BCF values calculated for each metal and for each location differed statistically significantly from one another (

Table 2. BCF values with determination of statistical significance of differences between locations (without taking into account species/organ) (vertically); statistical significance of differences (s.s.d), numbers with different letters are statistically significantly different, n = 384.

HSD BCF Cd	0.002	s.s.d	HSD BCFPb	0.001141	s.s.d
Base	0.066	a	Top	0.074119	a
Middle	0.055	b	Base	0.031613	b
Top	0.053	c	Middle	0.023417	c
HSD BCFCu	0.0019	s.s.d	HSD BCFZn	0.0016	s.s.d
Top	0.2509	a	Top	0.2570	a
Base	0.1374	b	Base	0.1138	b
Middle	0.1224	c	Middle	0.0641	c

). The highest bioaccumulation factors, without taking species into account, were noted for the plants growing on the top of the landfill in the case of lead, copper, and zinc, whereas for cadmium, the highest value was noted at the base of the slope (0.066), followed by the middle (0.055), while the value was lowest on the top (0.053). In the substrate, the values for cadmium were 1.031 mg·kg⁻¹ DW (top), 0.749 mg·kg⁻¹ DW (middle), and 0.651 mg·kg⁻¹ DW (base). The lowest bioaccumulation was recorded in the middle of the slope for lead, copper, and zinc, and at the top in the case of cadmium. The content of cadmium, zinc, copper, and lead was always lowest at the base of the landfill.

One of the key soil properties influencing the bioavailability of elements (macro- and micro-) and xenobiotics for plants is the microbial activity of the rhizosphere. In the case of metallurgical waste, it is zero; moreover, very high levels of zinc and cadmium significantly impede microbial colonization of the soil. Szwalec et al. [5] hypothesize that a potentially varied biodiversity of microorganisms may be an important reason for the varied phytostabilization of metals on the slope of a hard coal combustion waste landfill. In zinc smelting waste, there is none at all. However, it is observed in the soil on which the landfill was built. Another factor directly linked to microbial activity is organic matter content. This is a factor which significantly determines the microbial activity of soil, thus indirectly influencing phytostabilization. Organic matter reduces the mobility of heavy metals in the soil, thus strongly influencing the uptake of metals by plants. The middle of the slope contained much more organic matter than the top—26.6% and 17.1%, respectively. Prior to the enactment of legal regulations requiring environmentally friendly waste management, landfills were created and exploited according to the principle of the lowest possible cost. For this reason, the base of our landfill was formed from the local soil, potentially containing organic matter. This is also the oldest part of the landfill; the top may have been formed as many as 70 years later.

A third factor may be the edge effect. Plants growing on the base of the landfill can exploit the resources of the soil surrounding them, and not only the Technosol. Similarly, the flora and fauna in the vicinity of the landfill may attempt to colonize the base of the landfill or use its resources, thereby enriching it ecologically. Another factor with potential importance for BCF in the context of location is the hydrogeological processes taking place in the waste dump. These may include the gravity-induced flow of water and the minerals dissolved in it and rainwash of organic matter, clay minerals, and insoluble elements.

Analysis of the average contents of metals in the plants (without taking into account the species/organ), shown in Table 2, reveals that the lowest concentrations in the biomass were always found in the base of the landfill. In the case of bioaccumulation factors (Table S3 in the Supplementary Materials), the BCF values were lowest on the middle of the slope, except for cadmium, for which they were lowest in the plants growing on the top. At the same time, this part of the landfill had the highest BCF values for the other three metals. All of these values were statistically significantly different (Table 2 and Table S3).

The top, i.e., the youngest part of the landfill, was most recently colonized by microflora, fauna, and flora, and therefore had the most difficult conditions for plant growth and development in the secondary succession processes. The landfill is one of the oldest in this part of Europe. With the development of metallurgical technology, waste from all landfills older than 200 years has been reused as raw material. Despite the lush vegetation on the waste dump discussed here and its natural fit into the surrounding hilly landscape, a study of ground beetles (Carabidae) has shown that it is still in the initial stage of succession [4].

Szwalec et al. [5], in a study on a slag and ash landfill, showed that the capacity of the plants growing on it for bioaccumulation of heavy metals was modified by the site of their development, i.e., the part of the landfill they grew on. That study identified a weak but statistically significant effect of location on the content of cadmium, lead, zinc, and copper in the organs of black locust (Robinia pseudoacacia L.), silver birch (Betula pendula Roth.), European aspen (Populus tremula L.), common oak (Quercus robur L.), and the herbaceous species wood small-reed (Calamagrostis epigejos L.), European goldenrod (Solidago virgaurea L.), and common reed (Phragmites australis (Cav.) Trin. ex Steud). That landfill was only a few decades old, whereas the present study was carried out on a much older landfill, where the deposition of waste began about 200 years ago [35,36].

In addition, to reduce error size and thus reveal even weak associations, and in post hoc tests even small differences between mean contents as statistically significant, the number of replicates was increased from 12 to 16 [5]. The data for two-way analysis of variance were arranged so that location was the first factor, and thus the interaction measured was a modification of the effect of location on the species of plant (specifically its organ/part).

3.2.2. Plant Species or Organ Without Location

The BCF values for individual metals are presented in

Table 3. BCF values and content of metals for plant species/organs with determination of statistical significance of differences (vertically), without taking into account the sampling location. Statistical significance of differences (s.s.d), numbers with different letters are significantly different, n = 384.

BCFCd HSD	0.004	s.s.d	BCFPb HSD	0.00242	s.s.d
moss	0.364	a	moss	0.29467	a
herbaceous.plants	0.023	b	birch.shoot	0.01281	b
birch.shoot	0.020	b,c	birch.leaves	0.00875	c
birch.leaves	0.018	c	herbaceous.plants	0.00822	c
rowan.leaves	0.010	d	rowan.shoot	0.00744	c,d
oak.shoot	0.010	d	rowan.leaves	0.00556	d,e
oak.leaves	0.009	d	oak.leaves	0.00428	e,f
rowan.shoot	0.008	d	oak.shoot	0.00267	f
BCFCu HSD	0.0039	s.s.d	BCFZn HSD	0.0033	s.s.d
moss	0.5311	a	moss	0.3593	a
rowan.leaves	0.1366	b	birch.leaves	0.3428	b
herbaceous.plants	0.1312	c	birch.shoot	0.1645	c
rowan.shots	0.1280	c	oak.leaves	0.0839	d
birch.shoot	0.1220	d	rowan.shoot	0.0677	e
birch.leaves	0.1122	e	rowan.leaves	0.0632	f
oak.leaves	0.1119	e	herbaceous.plants	0.0497	g
oak.shoot	0.0885	f	oak.shoot	0.0284	h

, and the contents of metals in individual plants (species) and their organs are given in Table S4 (in the Supplementary Materials).

In contrast to the location, the patterns of values for BCF and the content of corresponding metals are nearly identical (Table 3 and Table S3). The higher plants have developed a number of mechanisms for controlling the transfer of heavy metals within them [54,55,56]. Mosses, as lower, thallus plants without such abilities, had the highest contents of the metals and the highest BCFs. Unfortunately, the bioremediation properties of mosses are negligible, due to their very small mass. Nevertheless, it should be noted that they are very important organisms for the succession and stabilization of the higher plants succeeding them, and thus they indirectly influence phytostabilization. The values for the factors (BCF moss/BCF higher plants in descending order) and metal concentrations (metal moss/metal higher plants in descending order) were very similar. For cadmium they ranged from 14- to 20-fold between the first three plants (arranged in descending order in Table 3 and Table S4), from 22- to 35-fold for lead, and from 3.5- to 4-fold for copper. For zinc, these differences did not exist in practice, and the greatest difference was 1.9-fold. Only in the oak shoots did this element have characteristics of physiological content (57.90 mg·kg⁻¹ DW), while in the other plants its concentrations exceeded 100 mg·kg⁻¹ DW, considered by Kabata-Pendias [39,57] to be a toxic level. The lowest bioaccumulation factors for lead and copper and a low bioaccumulation of cadmium were also found in the shoots and leaves of this species. Concentrations of copper, the second microelement after zinc, were low (except in mosses), at the threshold of deficiency in plants [57]. Li et al. (2022) studied the phytoremediation of nine species of the oak genus (Quercus sp.) and found that the most suitable were Quercus texana and Quercus fabri, while the English oak (Quercus robur), which was also included in our study, appeared to be one of the less suitable species for the phytostabilization of zinc and cadmium [58]. A very interesting observation was reported by Domínguez et al. [59] who were carrying out the reclamation of an area flooded and polluted with liquid waste. The most suitable plant species for this purpose was the olive tree (Olea europea) as it was able to survive the summer stress. The research was carried out in the vicinity of Seville (southern of Spain). The best in terms of phytoremediation capacity was Quercus ilex var. Ballota; however, this plant was much more sensitive to drought and direct sun radiation [59]. Birch (Betula pendula) has been reported to be good phytoaccumulator of zinc and lead. Birch grew on a zinc and lead ore flotation waste landfill in northern Spain (Pyrenees, Val d’Aran) [49]. In the present study, this tree was the second-best phytoaccumulator (after moss) for cadmium, lead, and zinc. In the case of copper, the BCF for birch was below the median. Wójcik et al. reported extremely high accumulations of cadmium, lead, and zinc in plants growing naturally on landfills of zinc and lead smelting and mining waste. For Anthyllis vulneraria L., Echium vulgare L., Hieracium piloselloides Vill., and Reseda lutea L. the value of that study was enhanced by the fact that the plants were naturally growing and by the fact the landfill is in the same post-industrial region, i.e., Krakow–Upper Silesia, as the landfill analyzed in the present study [22]. These are herbaceous plants, with considerable biomass (except for Hieracium piloselloides). They may be interesting potential additives in the case of the reclamation of the ‘Artur’ landfill in Krze. Dong et al. studied abandoned farmland in the vicinity of a non-ferrous mine in Gansu Province in China. As in the present study, naturally growing plants were tested in that study as well—Kalidium foliatum, Kalidium gracile, Neotrinia splendens and Reaumuria songarica. Unfortunately, these plants do not grow in southern of Poland [60].

The following plant species/organs proved to be the least suitable for the phytostabilization of pollutants on the Artur landfill in Krze (Table 3):

-

For the BCF of copper—oak shoots, followed by oak leaves; these values differed statistically significantly.

-

For the BCF of zinc—oak shoots, followed by herbaceous plants; here too the differences were statistically significant.

-

For the BCF of cadmium—rowan shoots, followed by oak leaves; in this case, however, the differences were not statistically significant.

-

For the BCF of lead—oak shoots and oak leaves; these values did not differ statistically significantly.

3.2.3. Interaction of Location and Plant Species/Organ

This is the most interesting and important part of the study. In our search of the literature, we did not find any study in which location was the first factor determining phytostabilization and the interaction of location and species was analyzed, rather than the reverse, i.e., species and location (

Table 4. The combined and simultaneous effect of location and species/organ on BCF with a determination of statistical differences (vertically); significance level 0.05; a, b, c… statistically significantly different values (s.s.d.); n = 384.

BCFCd HSD	0.005	s.s.d	BCFPb HSD	0.0032	s.s.d
base-moss	0.389	A	top-moss	0.5071	a
middle-moss	0.369	B	base-moss	0.2117	b
top-moss	0.335	C	middle-moss	0.1652	c
base-birch.shoot	0.034	D	top-birch.shoot	0.0199	d
top-herbaceous.plants	0.034	D	top-birch.leaves	0.0178	d
base-birch.leaves	0.028	E	top-herbaceous.plants	0.0168	d
middle-herbceous.plants	0.019	F	top-rowan.shoot	0.0134	e
top-birch.leaves	0.018	f,g	base-birch.shoot	0.0133	e
base-rowan.leaves	0.017	f,g	top-rowan.leaves	0.0083	f
base-herbaceous.plants	0.017	f,g	top-oak.leaves	0.0065	f
base-rowan.shoot	0.016	f,g,h	base-rowan.shoot	0.0061	f,g
base-oak.shoot	0.015	f,g,h	base-birch.leaves	0.0061	f,g
middle-birch.shoot	0.014	h,i	middle-birch.shoot	0.0053	f,g
top-birch.shoot	0.013	h,i,j	middle-herbaceous.plants	0.0050	g,h
base-oak.leaves	0.011	i,j,k	base-rowan.leaves	0.0048	g,h,i
middle-birch.leaves	0.009	j,k	base-oak.leaves	0.0046	g,h,i
middle-oak.shoot	0.009	j,k,l	middle-rowan.leaves	0.0036	g,h,i
top-oak.leaves	0.008	j,k,l	base-oak.shoot	0.0033	g,h,i
middle-oak.leaves	0.008	j,k,l	top-oak.shoot	0.0032	g,h,i
top-oak.shoot	0.007	k,l,m	base-herbaceous.plants	0.0029	h,i
top-rowan.leaves	0.007	k,l,m	middle-rowan.shoot	0.0028	h,i
middle-rowan.leaves	0.007	k,l,m	middle-birch.leaves	0.0023	h,i
top-rowan.shoot	0.006	l,m	middle-oak.leaves	0.0017	i
middle-rowan.shoot	0.003	m	middle-oak.shoot	0.0016	i
BCFCu HSD	0.005	s.s.d	BCFZn HSD	0.004	s.s.d
top-moss	0.829	a	top-birch.leaves	0.707	a
base-moss	0.457	b	top-moss	0.617	b
middle-moss	0.307	c	base-moss	0.319	c
top-herbaceous.plants	0.249	d	top-birch.shoot	0.214	d
top-oak.leaves	0.175	e	base-birch.leaves	0.209	e
top-birch.leaves	0.160	f	base-birch.shoot	0.172	f
top-birch.shoot	0.155	f	top-oak.leaves	0.144	g
top-rowan.shoot	0.154	g	middle-moss	0.142	g
top-rowan.leaves	0.154	g	top-rowan.shoot	0.131	h
base-rowan.shoot	0.148	h	top-rowan.leaves	0.119	i
middle-rowan.leaves	0.144	h	middle-birch.leaves	0.113	j
top-oak.shoot	0.130	i	middle-birch.shoot	0.107	k
base-birch.shoot	0.113	j	top-herbaceous.plants	0.081	l
base-rowan.leaves	0.112	j	base-oak.leaves	0.080	l
middle-herbaceous.plants	0.098	k	top-oak.shoot	0.044	m
middle-birch.shoot	0.097	k	middle-rowan.leaves	0.041	m
middle-oak.leaves	0.096	k	base-rowan.shoot	0.040	m
middle-birch.leaves	0.094	k	base-herbaceous.plants	0.035	o
base-birch.leaves	0.082	l	middle-herbaceous.plants	0.033	o,p
middle-rowan.shoot	0.081	l	middle-rowan.shoot	0.033	o,p
base-oak.shoot	0.075	m	base-rowan.lives	0.030	p,r
base-oak.leaves	0.065	n	middle-oak.lives	0.028	r,s
middle-oak.shoot	0.061	n	base-oak.shoot	0.025	s
base-herbaceous.plants	0.047	o	middle-oak.shoot	0.016	t

and Table S5).

All analyzed interactions of location and species/organ were statistically significant. This is in contrast to the findings of Szwalec et al. [5] regarding the stabilization of metals by the vegetation on an energy waste landfill, where wood small-reed and common reed did not react to the interaction of species and location. At the same time, the bioaccumulation factors were low. However, wood small-reed and common reed were not present on the ‘Artur’ landfill in Krze.

The values of the bioaccumulation factor changed along the slope of the landfill in the same way for mosses (asymmetrical letter V for Cu, Pb, and Zn and inverted V for Cd) and for higher plants, while the values of the factors themselves were several times greater, except for the BCF of Zn, especially for birch leaves (b.l.) (Figure 3 and Figure 4).

Changes in the BCF values of higher plants for individual metals at the locations tested (base—middle—top) were arranged in the following patterns:

The first (most common) was an asymmetrical letter V, where the values for the base and top were the maxima and the value for the middle was the minimum, but these changes (the arms of the V) are asymmetrical. The asymmetry can be leftward or rightward. Thus, for the bioaccumulation factors for cadmium, the asymmetry is leftward, i.e., the values for the base are highest. The only exception was for herbaceous plants (h.p.), where the asymmetry of the letter V is rightward (Figure 3A,B).

Rightward asymmetry in the shape of the letter V, i.e., the maximum for the top, was much more common in the case of the other metals: Cu—birch shoots (b.s.) and oak shoots (o.s.) (Figure 3C); Pb—rowan shoots (r.s.), rowan leaves (r.l.), oak leaves (o.l.), and oak shoots (o.s.) (Figure 3E); herbaceous plants (h.p.) and birch leaves (b.l.) (Figure 3F); Zn—oak leaves (o.l.) (Figure 3G) and birch leaves (b.l.) (Figure 3H). Full symmetry occurred in only one case: Cu—rowan shoots (r.s.) (Figure 3D).

The second pattern of changes was an increase in BCF from the base to the top: Cd—herbaceous plants (h.p.) (Figure 3B); Cu—oak leaves (o.l.) and birch leaves (b.l.) (Figure 3C); Pb—herbaceous plants (h.p.) (Figure 3F); Zn—herbaceous plants (h.p.), rowan shoots (r.s.) and rowan leaves (r.l.) (Figure 3G).

The most common V formation is not described by Szwalec et al. [5]. Only isolated examples of this formation can be found in the figures included by the authors in their article. The differences in the variation in BCF values on the slope of the landfill in the study cited and in the present study may be due to the following:

• Differences in the species composition of the plants growing on the landfills;
• Differences in area cover—a parameter in phytosociological research which presents the total area of rectangular projections of vegetation onto the landfill surface;
• Different types of industrial waste;
• Different ages of the landfills—200 vs. 50 years;
• Different technology used to build the landfills;
• Different surroundings;
• Differently calculated interaction—location–species in Krze vs. species–location in Skawina.

We would like to take a closer look at factors 4 and 5. Colonization has been taking place on the older landfill for a longer time. Every lifeform has an inherent need to spread and colonize new habitats. Biocoenoses near old landfills have had more time to attempt to colonize them. In the case of the older landfill, colonization has been part of the process of secondary succession. We do not know what was present at the site where the landfill was created more than 200 years ago. No doubt it was land that was not needed at that time, most likely a depleted area of low utility value. In terms of the type of waste, it may have been depleted fields that had been the site of zinc mining or the extraction of clay, which was also needed in the metallurgical industry. In the 1820s and 1830s, Poland did not yet have railways, including the narrow-gauge railways later used in zinc mining and metallurgy. Human and animal labour was used at that time, supported by simple machines. There were also no excavators, bulldozers, conveyor belts, or pumps for the hydrotransport of waste. The smelter was built next to the zinc deposit, and waste was deposited next to the smelter. At the same time, there was less demand for land and far more land resources. This meant the formation of a flatter landfill with longer slopes and a lower height, as well as a significantly greater presence of organisms (horses and people) during its formation, which enriched the barren industrial waste with organic matter (human and animal feces and urine). Due to the longer border of the lower and larger landfill, the edge effect was stronger and wildlife colonization was easier. Finally, the site where the waste heap was formed was significantly less degraded, because the land was not damaged by heavy machinery; only minimal earthworks were carried out, and only when necessary. During the existence of the landfill, urban settlement arose around it and around the Artur smelter. The pressure exerted by the presence of humans on the landfill increased over time. During its existence, the city of Trzebinia (where it is located) has belonged to the Austro-Hungarian Empire, Poland, Germany, the area was occupied and plundered by the Red Army (Soviet Union), the Polish People’s Republic, and after 1989, free Poland again. Each of these hostile changes was accompanied by a change in the owners, i.e., administrators, of the area, and it was significantly modified—how it was developed, the value assigned to its components, and how its resources were used. At the same time, all documentation was destroyed, which today makes it difficult to discuss results describing the ecotoxicological condition of the landfill and the waste accumulated in it.

4. Conclusions

Based on this case study, phytostabilization studies and detoxification measures using phytoremediation on industrial waste landfills should take into account the effect of location on the magnitude and efficiency of this process, because waste which has been homogenized by temperature as a result of zinc production becomes diversified again over time by biogeochemical processes. In this case study, both the location itself and, more importantly, from a scientific and engineering perspective, the interaction of location and plant species/organ was statistically significant, and the averages obtained differed statistically significantly in the comparison using Tukey’s test.

To analyze the interaction of the study factors (location and species/organ), mosses had to be separated from higher plants and cadmium from other metals, i.e., zinc, copper, and lead.

For cadmium, lead, and copper bioaccumulation, the stabilization indicators of the mosses were many times greater than those of the higher plants. This is due to the anatomical structure of these plants. These are lower plants, i.e., thallophytes, which means that they do not form organs and have no roots, so no translocation of metals takes place. Unfortunately, due to their low biomass, they are of limited use for phytostabilization.