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Article

Sustainable Ecological Restoration of Sterile Dumps Using Robinia pseudoacacia

by
Adriana Mihaela Chirilă Băbău
1,
Valer Micle
1,*,
Gianina Elena Damian
2 and
Ioana Monica Sur
1,*
1
Department of Environmental Engineering and Sustainable Development Entrepreneurship, Faculty of Materials and Environmental Engineering, Technical University of Cluj Napoca, 400641 Cluj Napoca, Romania
2
Department of Cadaster, Civil and Environmental Engineering, “1 Decembrie 1918” University of Alba Iulia, 510009 Alba Iulia, Romania
*
Authors to whom correspondence should be addressed.
Sustainability 2021, 13(24), 14021; https://doi.org/10.3390/su132414021
Submission received: 27 October 2021 / Revised: 8 December 2021 / Accepted: 17 December 2021 / Published: 19 December 2021
(This article belongs to the Section Environmental Sustainability and Applications)
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Abstract

:
The feasibility of using Robinia pseudoacacia in phytoremediation of sterile dumps was determined. The potential of Robinia pseudoacacia seeds to grow in a medium contaminated with high concentrations of Pb, Cd, and Cu was firstly evaluated by applying germination tests on acacia seeds in the presence of various extractants prepared by mixtures of sterile material (SM) collected from the “Radeș” dump (Romania), calcium carbonate (CaCO3) and dehydrated sludge (DS) from Someș Water Treatment Plant (Cluj Napoca, Romania), fertilizer (N.P.K.), and potassium monobasic phosphate (KH2PO4-99.5%). The results indicated that Robinia pseudoacacia seeds grow much better in an acidic than in a neutral medium and in the absence of carbonates. The capacity of metal uptake from SM by Robinia pseudoacacia and the development of the plants were then investigated at the laboratory scale. During the phytoremediation process, 92.31% of Cu was removed from SM, and the development of the Robinia pseudoacacia plants was favorable. However, although the results of the present study indicated that Robinia pseudoacacia can be successfully used in the phytoremediation of sterile dumps, making a sustainable decision for the current situation of sterile dumps located in mining areas may be difficult because an optimal point between people, profit, planet, and diverse ethical views must be found.

1. Introduction

Everyone is completely dependent on Earth’s ecosystems and the services they provide (e.g., water, food, climate regulation, aesthetic enjoyment, spiritual fulfillment, etc.) [1]. Even though natural systems are the biological life support, society practices do not acknowledge the dependence on these ecosystems [2]. Consequently, humans have changed natural systems, largely to meet their own needs [1].
In the last few decades, environmental pollution and the threat it poses have become challenges that concern the entire world, especially specialists in environmental engineering sciences [3,4]. One of the major environmental problems is the toxicity of heavy metals and the possibility for them to enter the food chain [5]. Soil pollution with heavy metals represents one of the most pressing threats to water and soil resources, as well as human health [6,7,8,9]. Some heavy metals, such as lead and cadmium, which are not essential nutrients in the plant, animal, or human organism, can cause increased toxicity following exposure, even at low concentrations [10,11]. In many parts of the world, mining activities performed in the past have left significant amounts of sterile material improperly deposited. Sterile material contains high levels of heavy metals that contaminate soil substrates, destroying its texture and causing groundwater pollution and a decline in biological diversity [12]. The concentration of metals in sterile material is typically approximately 1000 ppm (0.1%), so it is toxic to plants [13]. Thus, sterile dumps are impractical for plant growth and are exposed to erosion. According to a national inventory made in 2017 on potentially contaminated sites, 1628 potentially polluted sites were reported in Romania, of which 151 sites were affected by mining and metallurgy [14].
Given that few ecosystems on earth have been unaffected by humans, restoring them holds great promise for stemming the biodiversity crisis [15]. Both science and ethics are involved in making sustainable decisions on environmental restoration. The most persuasive argument supporting sustainable ecological restoration is the ethical responsibility for the wellbeing of other life forms [2]. Saving nature because humankind has an ethical responsibility for the fate of more than 30 million species with which it shares the planet is not the same as saving ecosystems for present and future enlightened use by humankind (sustainable use of the planet) [13]. To help environmental decision makers and experts, and thus to better protect the environment, the ethical dimensions need to be harmonized and adopted to the scientific, economic, technological, social, and legal aspects of ecological restoration. The ethical dimensions characterize the totality of human activities described by moral values without which society could not function and would self-destruct.
Forty years ago, ecological restoration was conceptualized through a natural science lens. Today, ecological restoration has evolved into a social and scientific concept [16]. Restoration is not only aimed at improving the state of a damaged area but also to restore it to pre-settlement conditions [17]. Thus, ecological restoration should involve all major levels of organization, from component species to the entire systems. From an ecological viewpoint, ecological restoration represents the intentional activity that initiates or accelerates ecosystem recovery in terms of its integrity: species composition and structure and the functions and capacity of the physical environment to support the biota. From a socio-economic point of view, it should recover the flows of natural goods and services of economic impact that the ecosystem provides to society [18]. On the other side, sustainability science deals with the interactions between natural and social systems, and with how those interactions affect the challenge of sustainability: “meeting the needs of present and future generations and conserving the planet’s life support systems” [19].
Technology and science are steadily increasing and support ecological restoration, so the stimulus required for the implementation of sustainable ecological restoration is an ethical responsibility in this respect. Ethics means a value-based system of constraint and individual behavior [20]. The knowledge provided by science and technology alone does not ensure motivation to act; a value or belief system is the key in determining any action, and there is a need to harmonies environmental knowledge and sustainability with ethical behavior and thus achieve behavioral change and the internalization of environmentally ethical values [21]. Therefore, from an ethical and cultural viewpoint, sustainable ecological restoration represents the moral duty to restore ecosystems by making the right choices, without dominance, and in a manner that renews the human relationship with nature. Additionally, increased responsibility, respect, and consciousness towards nature are required for the well-being of both present and future species living on Earth [22].
But what is needed to make the right choices regarding the sustainable ecological restoration of polluted mining areas? Albert Einstein has said, “We can’t solve problems by using the same kind of thinking we used when we created them” [23]. The lack of a system of values or beliefs intended to determine an action in harmonizing environmental knowledge and sustainability with ethical behavior is one of the major problems involved today in making the right environmental decisions. Therefore, environmental decisions must be viewed primarily as ethical rather than as technically dictated conclusions. Given this, the sustainable decision-making process may be manageable only by morally developed experts and decision-makers [24].
One issue that repeatedly divides ethical responses to the sustainable ecological restoration of mining sites is related to the comparison of classical and unconventional restoration methods of these sites. Often, remediation is achieved through long-lasting procedures or costly or very difficult procedures [25]. Chemical methods are very costly and can lead to another form of pollution, depending on the substances used. These are temporary solutions that do not ensure the restoration of the land from a biological or landscape point of view [26]. Physical methods, such as coating with materials, are effective but expensive, [27]. Among the available restoration methods for sterile material dumps, coverage with vegetation is the most economical and environmentally friendly method. It allows the stabilization, bioremediation, and rehabilitation of sterile dumps, but to achieve a sustainable ecological restoration, ecological, socio-economic, political, and ethical aspects must be integrated into the restoration process.
The human aspiration to live sustainably on the planet must recognize that the integration, elimination, or replacement of plant species and habitats during the ecological restoration of the sterile dumps is not a sustainable practice that considers the ethical dimension. For instance, by using exotic plant species (e.g., Indian mustard “Brassica juncea”) for ecological restoration of a mining site, the indigenous species that had initially colonized mining sites could be threatened or even eliminated. In these conditions, total ecological destruction exceeds total ecological repair, and a balance between destruction and restoration rates must be achieved [2]. Furthermore, ecological restoration efforts might eliminate those species that had initially colonized mining sites and were able to tolerate anthropogenic stress. Maybe the species that are removed might be quite desirable someday.
Several researchers have demonstrated that plants have the ability to withstand relatively high concentrations of heavy metals without apparent visual toxic effects [28,29,30,31,32]. Zerkout et al. (2018) investigated the germination percentage, seedling vigor index, tolerance index, germination index, mean germination time, and relative injury rate of Acacia auriculiformis seeds exposed to Pb and concluded that Acacia auriculiformis seeds are able to germinate in high concentrations of Pb (up to 1.5 g/L), indicating the resistance of those species to Pb [33]. Moreover, plants can be used to accumulate metals/metalloids in their harvestable biomass (phytoextraction), but also to convert and release certain metals/metalloids in a volatile form (phytovolatilization) [34,35,36,37]. Penkova and Petkova (2015) investigated the Pb accumulation characteristics of Robinia pseudoacacia leaves from polluted soil (30.7 mg kg−1). The results showed that the leaves of Robinia pseudoacacia accumulated Pb in parallel with their increase in the contaminated soil [38]. Plants can also be used to limit the mobility and bioavailability of heavy metals in soil and to prevent their migration by the erosion of wind and water. Budău and Timofte (2016) and Budău et al. (2014) have shown that the species Robinia pseudoacacia can be successfully used in stabilizing slopes, against landslides and stabilizing sand dunes or even abandoned sterile dumps [39,40].
Although the literature revealed that some plants (metalloids) can tolerate high levels of heavy metals, their growth will depend on the existence of humidity and sufficient nutrients (P, N, and K). A quantity of 25–30 kg/year of N, P, and K for a period of 6 years is sufficient to maintain the growth of the plants after the initial installation period [41]. Additionally, the uptake of heavy metals from soil varies from species to species within a genus and from plant to plant [42].
In this context, the adequate selection of plant species plays an important role in the development of remediation methods (decontamination or stabilization), especially on low- or medium-polluted sites [43]. To select suitable plants, it is necessary to understand their characteristics before planting as regards their capability to accumulate heavy metals [44]. According to their accumulation capability, there are various distinct groups of plant species [45]. Among these, hyperaccumulators, a particular sub-group within the accumulators, can tolerate, grow, and finish their life cycle without symptoms of metal phytotoxicity [46]. Robinia pseudoacacia species have been recognized as tolerant to heavy metals, have a deep root system, and have the ability to fix heavy metals around their roots [47,48,49]. However, much less attention has been paid to the use of Robinia pseudoacacia as a solution for the phytostabilization of sterile dumps, and no attention at all has been paid to discussing the related ethical aspects of the ecological restoration of sterile dumps in order to achieve a complete and sustainable ecological restoration. Robinia pseudoacacia (black locust) is part of the Leguminosae family. This species is resistant to frost but sensitive to late frosts, is drought-resistant, and prefers lean light soil, and is fertile, so it is suitable for cultivation on sandy soil [50].
Consequently, this paper aims to investigate the ability of the Robinia pseudoacacia species to tolerate and grow in sterile material (SM) collected from the “Radeş” sterile dump from Almașu Mare commune, Alba County, Romania and to discuss some ethical aspects in ecological restoration of sterile dumps using plant species in order to achieve a sustainable ecological restoration of sterile dumps.

2. Materials and Methods

The potential of Robinia pseudoacacia seeds to grow in a medium contaminated with high concentrations of Pb, Cu, and Cd was evaluated by applying germination tests on acacia seeds in the presence of various liquid extractants prepared by mixtures of sterile material (SM), calcium carbonate, dehydrated sludge, fertilizer, and potassium monobasic phosphate in various weight ratios. The capacity of metal uptake from SM by Robinia pseudoacacia plants and the development of the plants in SM was investigated at the laboratory scale in order to determine the feasibility of using Robinia pseudoacacia in phytoremediation of heavy metal polluted industrial sites. The present study provides insights for seeking sustainable strategies to remediate heavy metal polluted sterile dumps.

2.1. The Contaminated Site

The “Radeș” exploitation (GPS coordinates: 46°07′14.7″ N, 23°07′27.2″ E), the researched subject in this paper, falls within a mining field located in the south-east of the Zlatna-Stănija volcanic alignment, located in the subunit of Metaliferi Mountains of the South Apuseni Mountains (Romania). Since 1900, mining activities have been performed in this area. Today, the mining activity has been completely stopped at “Radeș” exploitation. However, the industrial area of the “Radeș” mine is highly polluted with heavy metals (soil and waters) due to the improper storage of the gangue material extracted from the “Radeş” coastal gallery, which allowed an unimpeded flow of mine wastewaters (acid mine drainage). The material is characterized by its strongly acidic nature and significant levels of iron, sulphate, and other heavy metals ions [51], which have accumulated in the soil and receiving rivers, creating large imbalances in the soil ecosystem and in the entire local ecosystem. Additionally, human homes and their households are very close to this source of pollution.

2.2. Robinia Pseudoacacia Seeds Germination Test

The germination test was performed using commercially available Robinia pseudoacacia seeds. The potential of Robinia pseudoacacia seeds to grow in a medium contaminated with high concentrations of Pb, Cu, and Cd was evaluated by applying germination tests on acacia seeds in the presence of various liquid extractants prepared by mixing 200 mL of tap water and 100 g of mixtures prepared from sterile material (SM) collected from the “Radeș” dump (Romania), calcium carbonate (CaCO3) and dehydrated sludge (DS) from Someș Water Treatment Plant (Cluj Napoca, Romania), fertilizer (N.P.K.), and potassium monobasic phosphate (KH2PO4-99.5%), in the weight ratios indicated in Table 1. One liquid extract was prepared from commercially uncontaminated soil (US) and used as a reference. Six liquid extracts, called Liq.US, Liq.SM, Liq.SM-CaCO3, Liq.SM-CaCO3-DS, Liq.SM-CaCO3-DS-N.P.K, and Liq.SM-CaCO3-DS-N.P.K-KH2PO4, with the compositions indicated in Table 1, were investigated. CaCO3 was added, and for the nutritional improvement of the soil and for the increase of the germination efficiency of the plants, was added dehydrated sludge, fertilizer (N.P.K.), and KH2PO4 (99.5%), a substance that contributes to the absorption of heavy metals from the soil solution in the plant [52].
Experiments were performed in duplicate, and the average values were reported. The SM was air-dried and used in experiments in its natural state. All extraction experiments were conducted in an orbital rotation-oscillation and thermostat cupola stirrer at 125 oscillations/min and an oscillation amplitude of 32 mm for 1 h. After homogenization, samples were collected and filtered through a 0.45-μm pore size filter for 8 h.
The initial heavy metal concentrations of the sterile material (SM), uncontaminated soil (US), dehydrated sludge (DS), and prepared mixtures (called SM-CaCO3, SM-CaCO3-DS, SM-CaCO3-DS-N.P.K, and SM-CaCO3-DS-N.P.K-KH2PO4), were determined through Atomic Absorption Spectrometry (AAS) using a SHIMADZU AA-6800 spectrometer. Prior to the AAS analysis, the US, DS, SM, and the prepared mixtures were dried, crumbled, and milled to dimensions of less than 250 µm. Then, 1 mL distilled water, 21 mL of concentrated HCl (Hydrochloric Acid), and 7 mL of concentrated HNO3 (Nitric Acid) were added over 3 g of the prepared material in a 100 mL glass flask. The glass flask was covered and left for mineralization (heated on a sand bath) for 2 h. After cooling, the mineralized sample was filtered (0.45 µm pore size filter) into a 100 mL volumetric glass flask, filled to the mark with distilled water, and analyzed for heavy metal concentration. The results regarding the initial content of heavy metals in the prepared mixtures are reported in Table 2. Additionally, the properties of the sterile material and the soil on the investigated site can be found in our previous published paper [53].
The Robinia pseudoacacia seed germination process was conducted at the laboratory scale in a 250 mL flask, under temperature-controlled conditions (20–22 °C) and under natural light so that seed germination occurred naturally. Nine Robinia pseudoacacia seeds were placed at a distance of 2 cm on absorbent papers, which were then carefully rolled up. The placement of every seed on the absorbent papers was maintained using adhesive tape. The rolled absorbent papers containing the Robinia pseudoacacia seeds were then placed in 250 mL flasks containing the liquid extractants obtained in the extraction experiments. After 10 days, the rolled papers were opened, and germination rate and development capacity of the seedling was evaluated.
The Germination Rate (GR) was calculated using the following equation [54]:
GR (%) = 1 + n/N · 100
where n is the number of germinated seeds and N is the number of tested seeds.
Briefly, the pH of the liquid extractants prepared for the Robinia pseudoacacia seed germination test was determined using a Hanna HI 3512 pH-meter.

2.3. Phytoremediation of Sterile Material

In order to determine the feasibility of using Robinia pseudoacacia (black locust) in the phytoremediation of heavy metal polluted industrial sites, the capacity of metal uptake from SM by Robinia pseudoacacia plants and the development of the plants in SM was investigated at the laboratory scale.
Prior to the phytoremediation experiments, the Robinia pseudoacacia seeds were soaked in tap water with a temperature of 40° for 24 h with the purpose of soaking the shell that wraps the seeds, thus allowing the seedlings to easily open the shell and come to the surface. The sterile material (1000 g) was placed in polypropylene pots of 20 × 15 cm with the weight of 130 g and with a diameter of 20 cm. Then, a number of 10 Robinia pseudoacacia seeds were seated in each polypropylene pot at a depth of 2–3 cm in the sterile material. In order to assure the required humidity for Robinia pseudoacacia seed germination, the polypropylene pots containing sterile material and the seeds were submerged in a water bucket for a few seconds. The germination rates of Robinia pseudoacacia seeds in sterile material were determined after 4 days. The phytoremediation experiment lasted 12 weeks, was performed in triplicate, and the average values were reported.
The capacity of Robinia pseudoacacia plants to extract heavy metals form sterile material was determined by analyzing the heavy metal content from the roots and aerial parts of the plants and from the SM after the phytoremediation process. The concentration of heavy metals from sterile material concentrated around the Robinia pseudoacacia roots was also determined in order to identify if Robinia pseudoacacia can be used for the phytostabilization of sterile dumps.
The vegetal parts of the Robinia pseudoacacia plants used in the phytoremediation experiments were analyzed for heavy metal content through atomic absorption spectrometry (AAS). Prior to the AAS analysis, vegetal parts were dried at 105 °C for about 2–3 h and milled using a laboratory mill. Then, 0.5 g of vegetal material was placed in a 25-mL glass flask with 2 mL of 30 % hydrogen peroxide (H2O2). The glass flask was covered with a glass plate and left to react. After 3 h, 6 mL of concentrated HNO3 (nitric acid) was added, and samples were left for mineralization for about 2 h (samples were heated on a sand bath). After cooling, every mineralized sample was filtered (0.45-μm pore size filter) into a 100-mL glass flask, filled to the mark with distilled water, and analyzed for heavy metal concentration.
To determine the concentration of Pb, Cd, and Cu in SM through AAS analysis, sterile material was subjected to the same treatment as the one described in Section 2.2.
The removal efficiency of metal ions from SM by phytoremediation is defined by the following equation [55]:
Removal efficiency (%) = me/mi × 100
where me is the metal concentration (mg kg−1) extracted from the SM and mi is the initial metal concentration (mg kg−1) in the SM.
All chemicals used were of analytical grade or ultrapure. The water was distilled with a Technosklo water distiller, and all glassware used was acid-washed.
The data presented (tabular or graphic) is the average of the replicated experiments.

3. Results and Discussion

3.1. Germination of Robinia pseudoacacia Seeds in Liquid Extracts

The germination rates (GR) of the Robinia pseudoacacia seeds in the liquid extractants tested in the present study are indicated in Table 3. After 10 days, five seeds had germinated in the liquid extract obtained from uncontaminated soil, corresponding to a 55% germination rate, while all seeds had germinated in the liquid extract obtained from the sterile material, corresponding to a 100% germination rate. This indicates that Robinia pseudoacacia seeds are able to germinate much better in sterile material than in uncontaminated soil.
Contrary to expectations, in investigated experimental conditions, the addition of CaCO3, dehydrated sludge, fertilizer (N.P.K.), or KH2PO4 in the sterile material does not improve the germination process of Robinia pseudoacacia seeds, the germination rate being 0%.
Besides the germination rate, to ensure the maintenance of the vegetation on the sterile dumps, it is mandatory to analyze the development of the plants. The Robinia pseudoacacia seedlings for every investigated liquid extract are presented in Figure 1. Basically, the development capacity of the seedlings was evaluated by measuring each root, stem, and leaf of each seedling.
The biometric measurements of the seedlings grown in the liquid extract obtained from US indicated a normal development of root, stem, and leaf, while in the liquid extract obtained from SM, a poor development of the seedlings with growth abnormalities were identified. Although the germination rate was higher in the sterile material than in the liquid extract obtained from uncontaminated soil, the roots and stems were shorter and disfigured, as can be observed in Figure 1.
The best development of the seedling was observed in the case of liquid extracted from uncontaminated soil. The roots of the seedlings were longer, as were the stems (Table 3). Regarding the development of the seedling’s leaves, numerous and longer leaves were identified in the case of liquid extracted from sterile material compared with the control.
The pH values of the tested liquid extracts are indicated in Figure 2. According to the literature [56,57,58], the optimal pH for plant growth is between 6.0 and 8.0 pH units. Calcium carbonate is one of the most used amendments on acidic substrates in the remediation of soils polluted with heavy metals. CaCO3 and/or Ca(OH)2 increases the pH of the substrate, correcting acidity levels that would be toxic to plants and limit the mobility of heavy metals and, at the same time, bioavailability and export by water [59].
Regarding the optimum pH for Robinia pseudoacacia seeds germination, the results indicated that the process of germination occurs very well, with a germination rate of 100%, at pH values of 4.3 and in the absence of CaCO3. In the presence of CaCO3 and at pH values of 5.3 and 5.6, the germination of Robinia pseudoacacia seeds does not occur. At higher pH values, such as the one corresponding to the pH of the liquid extract obtained from uncontaminated soil (6.8), the germination of Robinia pseudoacacia seeds is 55%.

3.2. Germination and Development of Robinia pseudoacacia Seeds in Sterile Material

The values of germination rates of Robinia pseudoacacia seeds in the three pots (1A, 1B, and 1C) containing sterile material after 4 days are indicated in Figure 3. The data presented is the average of the three replicated experiments. The mean value of the Robinia pseudoacacia germination rate in sterile material was greater than 63%, indicating that Robinia pseudoacacia seeds germinate quite well and relatively fast in sterile material as compared with the germination rate of the seeds obtained in liquid extracted from SM. The results obtained within the germination test of Robinia pseudoacacia seeds in the SM liquid extract indicated a 55% germination rate. The differences between the obtained values of germination rates could be due to the differences in the pH value of the substrates. The liquid extract obtained from the sterile material has a pH value higher, with 0.9, than the pH value of the sterile material (3.4 pH units), indicating that germination of Robinia pseudoacacia seeds occurs much better and faster at relatively low pH values (3.4 pH units).
After 4 days, by analyzing the development of the seedling in the sterile material, a better development was observed from the leaves of the seedling compared to when Robinia pseudoacacia seeds germinated in the liquid extracts.
During the phytoremediation process, the development of the Robinia pseudoacacia plants was favorable, as can be seen in Figure 4. After 12 weeks, the plants were harvested, and biometric measurements were performed. The biometric measurements indicated a more accentuated development of the roots than of the plant stems. The root length of the Robinia pseudoacacia plants ranged from 4 to 18 cm, while the stem lengths ranged from 6 to 16 cm. These results concern all three of the triplicates. The dry weights of roots, stem, and leaves of all Robinia pseudoacacia plants harvested were 2.8, 1.9, and 4.1 g, respectively.

3.3. Phytoremediation of Sterile Material Using Robinia pseudoacacia

The concentration of Pb and Cu in the sterile material and in vegetal parts of the Robinia pseudoacacia plants after the phytoremediation process is indicated in Figure 5 and Figure 6. The data presented is the average of the three replicated experiments. After 12 weeks, the Pb concentration from SM decreased by more than 2 times against its initial concentration in the sterile material, while the Cu concentration decreased by 13 times compared with the initial concentration. As regards the Cd concentration, it decreased insignificantly during the phytoremediation process using Robinia pseudoacacia.
The heavy metal concentration (Pb, Cu, Cd) in the sterile material is significant and was compared with the threshold limits established by the Romanian legislation (Order No. 756 of 3 November 1997) [60]. Thus, the Pb concentration (3090 mg kg−1) in sterile material collected from “Radeș” exploitation exceeds more than three times the intervention threshold limit established by the Romanian legislation (1000 mg kg−1), as can be seen in Figure 5. Additionally, the Cu concentration (424.1 mg kg−1) exceeds more than 1.6 times the intervention threshold limit (250 mg kg−1) established by the same Romanian regulatory standard (Figure 6).
The Cd concentration in sterile material collected from “Radeș” exploitation exceeds more than 3.7 times the warning threshold limit established by the same regulatory standard.
After the phytoremediation process of sterile material using Robinia pseudoacacia, the concentration of Cu in the sterile material was below the warning threshold limit established by Romanian legislation, with more than 217.4 mg kg−1 (Figure 6). Additionally, the Pb concentration in the sterile material decreased at a value of 1328 mg kg−1, but the intervention threshold limit was not reached (Figure 5). Regarding Cd concentration, the obtained values were almost the same as the initial concentration and could not be attributed to the phytoremediation process.
Regarding the concentration of Pb and Cu from SM concentrated around Robinia pseudoacacia roots, from Figure 5 and Figure 6, it is obvious that both Pb and Cu were concentrated around the roots, indicating that Robinia pseudoacacia can be used for the phytostabilization of sterile dumps, especially for lead contaminated sterile material. Cd was not concentrated around Robinia pseudoacacia roots.
Regarding the Cu and Pb concentration in vegetal parts of the Robinia pseudoacacia, the results indicated that Robinia pseudoacacia is able to uptake large amounts of Cu and Pb from sterile material. Thus, high concentrations of Cu and Pb were identified in the roots and the aerial parts of the investigated plant. An amount of 425 mg kg−1 of Pb and 136 mg kg−1 of Cu were accumulated in the roots of the Robinia pseudoacacia, while 140.96 mg kg−1 of Pb and 189 mg kg−1 of Cu were accumulated in the aerial parts of the Robinia pseudoacacia. More than 13.75% of Pb and more than 32% of Cu were accumulated in the roots of the Robinia pseudoacacia from the initial amount identified in the sterile material. In the aerial parts of Robinia pseudoacacia that comprise stems and leaves, 44.5% of the initial amount of Cu was accumulated. As seen in Figure 5 and Figure 6, Cu has been accumulated more in the aerial parts than in the roots, while Pb has been accumulated more in the roots of the Robinia pseudoacacia.
The results of the present study indicate that Robinia pseudoacacia can be successfully used in the phytoremediation of sterile material from sterile dumps. In investigated experimental conditions, the highest removal efficiency was observed in the case of Cu because 92.31% of Cu was removed from the sterile material by Robinia pseudoacacia, whereas the lowest removal efficiency was observed in the case of Cd. The removal efficiency of Pb from SM by phytoremediation using Robinia pseudoacacia was 46.64%. Additionally, the results indicated that this particular plant did not remove a significant amount of Cd, at least in the trials as conducted, because the concentration of Cd in the aerial parts of Robinia pseudoacacia were below the instrument detection limit.

3.4. Sustainable Ecological Restoration of Sterile Dumps

Taking into account that sterile material collected from “Radeș” exploitation contains significant concentrations of Cu, Pb, and Cd, and that it is improperly deposited, posing a great risk to the entire local ecosystem, it is mandatory to identify a sustainable remediation strategy for this particular problem. Currently, biotechnologies and the restoration of sterile dumps in an ecological manner are of interest. One ecological manner to restore sterile dumps is phytoremediation. Phytoremediation is an eco-friendly, a plant-based approach that involves the use of plants to extract and remove elemental pollutants or lower their bioavailability in soil [61]. The results obtained in the present study indicated that Robinia pseudoacacia can be successfully used in the phytoremediation of sterile dumps because this plant has the capacity to uptake important amounts of Cu and Pb. The importance of studying and applying the ecological restoration of polluted areas has emerged as a feed-back reaction to the impact of various human activities, which, over time, has created large imbalances in the quality of life on Earth.
Ecological restoration as defined by the Society for Ecological Restoration (SER) is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed [62]. As stated by Higgs (2003), recovery refers to all biogeochemical processes that enable an ecosystem to return to conditions before the disturbance [63]. But in most cases, disturbed ecosystems may not be restored to the naturalistic assemblage of plants and animals that existed before disturbance. This realization is very unpleasant if it means the loss of restored habitat essential for a bioregion or landscape.
At present, most ecological restoration is carried out to repair disturbance caused by human mismanagement of industrial areas, but many restoration projects are carried out through the destruction of another habitat relatively undamaged. For example, the cover of the lands contaminated with heavy metals or sterile dumps is a treatment that is limited to the upper part of the soil; practically, the contaminated land is covered with unpolluted soil, destroying the vegetal carpet, the microflora, and the fauna of the soil through stripping soil. Normally, this secondarily damaged land should be subjected to another restoration action. Sacrificing a relatively undamaged habitat may cause ecological harm [64] and does not assure sustainability. Practically, the natural systems and their products are treated as commodities with a marketplace value. In terms of sustainability, progress should move to the acknowledgment of dependence upon the planet’s support.
Additionally, at the current state of knowledge, the projects of ecological restoration of sterile dumps are likely to have unforeseen outcomes. For example, if exotic plant species (e.g., Indian mustard “Brassica juncea”) is planted for the remediation of sterile material polluted with heavy metals near the “Radeș” mine, the indigenous species that had initially colonized the area and were able to tolerate anthropogenic stress (e.g., Robinia Pseudoacacia, as indicated by the experimental results if the present study) could be eliminated by the invasive exotic species. In these conditions, the ecological destruction exceeds the ecological repair. It is unpleasant to realize that the species eliminated from “Radeș” may be quite desirable someday. Therefore, due to the uncertainty of the outcome involved, choices in sustainable ecological restoration of sterile dumps are value judgments and matters of morality, [13] as stated by Cairns (2003). All these issues must be discussed and evaluated before the ecological restoration is ever started in order to maximize the probability of success and avoid doing further damage. Sustainable decision-making process regarding ecological restoration of sterile dumps using phytoremediation processes must find an optimal point between humans, profit, planet, and ethical aspects. One condition of an ecosystem is the measure to judge the rightness of our thinking, actions, and behavior, and we must be sure that we understand completely the full context of these factors.

4. Conclusions

The results obtained in the present study indicated that Robinia pseudoacacia can be successfully used in an ecological manner to remediate sterile dumps (phytoremediation) because it is able to extract and remove important quantities of heavy metals from sterile material. In investigated experimental conditions, 92.31% of Cu and 46.64% of Pb were removed from sterile material by Robinia pseudoacacia. Additionally, the experiment performed indicated that Robinia pseudoacacia is not suitable for the removal of Cd from sterile material.
Robinia pseudoacacia seeds germinate in more than 60% of cases, and relatively quickly, in sterile material. Therefore, the results obtained recommend Robinia pseudoacacia as a short-term crop. However, the complete phytoremediation of sterile material could be achieved in a couple of years. Additionally, the results indicated that this plant could accumulate more than 44% of the initial amount of Cu in the aerial parts. Thus, to overcome, in the long term, the reintroduction of heavy metals accumulated in Robinia pseudoacacia leaves in sterile material, a textile geomembrane could be settled under the surface of the sterile dump after the initial installation of the plants. Periodically, the leaves could be collected, embedded in resin, and used as decoration and filling material for furniture and handicraft object manufacture. The contribution of the present study to future research is related to the implementation of phytoremediation technology on a real scale and the identification of whether the heavy metals removed by the investigated plant are not redeposited during Robinia pseudoacacia trees defoliation. Another aspect to be considered by future research is the analysis of the uncertainties involved in making correct decisions for sustainable ecological restoration of sterile dumps using Robinia pseudoacacia species, considering all the aspects involved in this process.

Author Contributions

Conceptualization, A.M.C.B. and V.M.; methodology, A.M.C.B. and V.M.; investigation, A.M.C.B., G.E.D. and I.M.S.; resources, V.M. and I.M.S.; writing—original draft preparation, A.M.C.B. and G.E.D.; writing—review and editing, G.E.D.; funding acquisition, V.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Romanian Ministry of Education and Research, PN-II-PT-PCCA-2013-4-1717.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The research was financed by the Romanian Ministry of Education and Research, PN II PT-PCCA 2013-4-1717 Program, (Project BIORESOL No. 91/2014).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Robinia pseudoacacia seed germinated in tested liquid extracts. Liq.US: liquid extract obtained from uncontaminated soil. Liq.SM: liquid extract obtained from sterile material. Liq.SM-CaCO3: liquid extract obtained from sterile material and CaCO3. Liq.SM-CaCO3-DS: liquid extract obtained from sterile material, CaCO3, and dehydrated sludge. Liq.SM-CaCO3-DS-N.P.K: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer. Liq. Liq.SM-CaCO3-DS-N.P.K-KH2PO4: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate.
Figure 1. Robinia pseudoacacia seed germinated in tested liquid extracts. Liq.US: liquid extract obtained from uncontaminated soil. Liq.SM: liquid extract obtained from sterile material. Liq.SM-CaCO3: liquid extract obtained from sterile material and CaCO3. Liq.SM-CaCO3-DS: liquid extract obtained from sterile material, CaCO3, and dehydrated sludge. Liq.SM-CaCO3-DS-N.P.K: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer. Liq. Liq.SM-CaCO3-DS-N.P.K-KH2PO4: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate.
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Figure 2. The pH values of the tested liquid extracts. Liq.US: liquid extract obtained from uncontaminated soil. Liq.SM: liquid extract obtained from sterile material. Liq.SM-CaCO3: liquid extract obtained from sterile material and CaCO3. Liq.SM-CaCO3-DS: liquid extract obtained from sterile material, CaCO3, and dehydrated sludge. Liq.SM-CaCO3-DS-N.P.K: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer. Liq.SM-CaCO3-DS-N.P.K-KH2PO4: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate.
Figure 2. The pH values of the tested liquid extracts. Liq.US: liquid extract obtained from uncontaminated soil. Liq.SM: liquid extract obtained from sterile material. Liq.SM-CaCO3: liquid extract obtained from sterile material and CaCO3. Liq.SM-CaCO3-DS: liquid extract obtained from sterile material, CaCO3, and dehydrated sludge. Liq.SM-CaCO3-DS-N.P.K: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer. Liq.SM-CaCO3-DS-N.P.K-KH2PO4: liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate.
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Figure 3. The germination rates of Robinia pseudoacacia seeds in sterile material.
Figure 3. The germination rates of Robinia pseudoacacia seeds in sterile material.
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Figure 4. The development of the Robinia pseudoacacia plants after (a) 7 weeks and (b) 12 weeks.
Figure 4. The development of the Robinia pseudoacacia plants after (a) 7 weeks and (b) 12 weeks.
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Figure 5. Lead concentration in sterile material and in vegetal parts of the Robinia pseudoacacia plants.
Figure 5. Lead concentration in sterile material and in vegetal parts of the Robinia pseudoacacia plants.
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Figure 6. Copper concentration in sterile material and in vegetal parts of the Robinia pseudoacacia plants.
Figure 6. Copper concentration in sterile material and in vegetal parts of the Robinia pseudoacacia plants.
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Table
Table 1. Materials used to prepare liquid extractants for Robinia pseudoacacia seeds germination test.
Table 1. Materials used to prepare liquid extractants for Robinia pseudoacacia seeds germination test.
Liquid ExtractComposition of the MixturesWeight Ratios (g)
Liq.US aCommercially uncontaminated soil1000
Liq.SM bSterile material1000
Liq.SM-CaCO3 cSterile material + CaCO3950 + 50
Liq.SM-CaCO3-DS dSterile material + CaCO3 + dehydrated sludge550 + 50 + 400
Liq.SM-CaCO3-DS-N.P.K eSterile material + CaCO3 + dehydrated sludge + N.P.K. fertilizer525 + 50 + 400 + 25
Liq.SM-CaCO3-DS-N.P.K-KH2PO4 fSterile material + CaCO3 + dehydrated sludge + N.P.K fertilizer + KH2PO4524.95 + 50 + 400 + 25 + 0.05
a liquid extract obtained from uncontaminated soil. b liquid extract obtained from sterile material. c liquid extract obtained from sterile material and CaCO3. d liquid extract obtained from sterile material, CaCO3, and dehydrated sludge. e liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer. f liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate.
Table
Table 2. Heavy metal content of the prepared mixtures.
Table 2. Heavy metal content of the prepared mixtures.
MixtureUnitValue of Pb ConcentrationValue of Cd ConcentrationValue of Cu Concentration
US amg kg−1<IDL b<IDL b<IDL b
SM cmg kg−13090.018.7424.1
SM-CaCO3mg kg−11540.021.0405.0
SM-CaCO3-DS dmg kg−11550.017.6225.0
SM-CaCO3-DS-N.P.K emg kg−12736.315.0208.0
SM-CaCO3-DS-N.P.K-KH2PO4 fmg kg−11487.018.0196.3
DS dmg kg−1<IDL b<IDL b<IDL b
a US: uncontaminated soil. b IDL: Instrument detection limit. c SM: sterile material. d DS: dehydrated sludge. e N.P.K: N.P.K fertilizer. f potassium monobasic phosphate, all in the weight ratios indicated in Table 1.
Table
Table 3. Germination rates of Robinia pseudoacacia seeds and parameters indicating the development of Robinia pseudoacacia seedlings (the results are the average of two replications that were not significantly different from each other).
Table 3. Germination rates of Robinia pseudoacacia seeds and parameters indicating the development of Robinia pseudoacacia seedlings (the results are the average of two replications that were not significantly different from each other).
Liquid ExtractThe Number of Seeds that GerminatedDevelopment ParametersGermination Rate (%)
Plant Height (cm)Root Length (cm)Stem Length (cm)Leaf Length (cm)Number of Leaf (cm)
Liq.US a56.53.52.01.0155
1.50.51.000
3.52.01.500
9.25.03.01.22
8.55.52.01.02
Liq.SM b95.72.52.01.22100
3.81.01.81.02
2.00.50.51.02
6.31.53.51.32
7.75.01.51.21
4.51.720.82
5.01.82.21.02
5,52.02.51.01
2.90.71.50.71
Liq.SM-CaCO3 c0-----0
Liq.SM-CaCO3-DS d0-----0
Liq.SM-CaCO3-DS-N.P.K e0-----0
Liq.SM-CaCO3-DS-N.P.K-KH2PO4 f0-----0
a liquid extract obtained from uncontaminated soil. b liquid extract obtained from sterile material. c liquid extract obtained from sterile material and CaCO3, in the weight ratios indicated in Table 1. d liquid extract obtained from sterile material, CaCO3, and dehydrated sludge in the weight ratios indicated in Table 1. e liquid extract obtained from sterile material, CaCO3, dehydrated sludge, and N.P.K fertilizer in the weight ratios indicated in Table 1. f liquid extract obtained from sterile material, CaCO3, dehydrated sludge, N.P.K fertilizer, and potassium monobasic phosphate in the weight ratios indicated in Table 1.
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Băbău, A.M.C.; Micle, V.; Damian, G.E.; Sur, I.M. Sustainable Ecological Restoration of Sterile Dumps Using Robinia pseudoacacia. Sustainability 2021, 13, 14021. https://doi.org/10.3390/su132414021

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Băbău AMC, Micle V, Damian GE, Sur IM. Sustainable Ecological Restoration of Sterile Dumps Using Robinia pseudoacacia. Sustainability. 2021; 13(24):14021. https://doi.org/10.3390/su132414021

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Băbău, Adriana Mihaela Chirilă, Valer Micle, Gianina Elena Damian, and Ioana Monica Sur. 2021. "Sustainable Ecological Restoration of Sterile Dumps Using Robinia pseudoacacia" Sustainability 13, no. 24: 14021. https://doi.org/10.3390/su132414021

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