Phytoaccumulation of trace elements (As, Cd, Co, Cu, Pb, Zn) by Nicotiana glauca and Euphorbia segetalis growing in a Technosol developed on legacy mine wastes (Domingo Rubio wetland, SW Spain)

Vol.: (0123456789) 1 3 Environ Geochem Health (2023) 45:9541–9557 https://doi.org/10.1007/s10653-023-01523-w ORIGINAL PAPER Phytoaccumulation oftrace elements (As, Cd, Co, Cu, Pb, Zn) byNicotiana glauca andEuphorbia segetalis growing inaTechnosol developed onlegacy mine wastes (Domingo Rubio wetland, SW Spain) C.Barba‑Brioso · P.J.Hidalgo · S.Fernández‑Landero· I.Giráldez · J.C.Fernández‑Caliani Received: 17 June 2022 / Accepted: 27 February 2023 / Published online: 16 March 2023 © The Author(s) 2023 Abstract Sulfidic mine wastes have the potential to generate acid mine drainage (AMD) and release acid leachates containing high levels of iron, sulfate and potentially toxic elements (PTEs). Soils receiving AMD discharges are generally devoid of vegetation. Only a few metal-tolerant plant species can survive under such adverse soil conditions. This work investigates two plant species, Nicotiana glauca and Euphorbia segetalis, that have successfully colonized an AMD-impacted wetland area in south-western Spain. The uptake of PTEs from the soil by roots and their transfer and accumulation in the above-ground biomass were quantified. Results showed that these pioneer plants grew in patches of neutral soil within the wasteland despite the high concentrations of PTEs in the rhizosphere soil (up to: 613mg kg−1 As, 18.7mg kg−1 Cd, 6370mg kg−1 Cu, 2210mg kg−1 Pb and 5250mg kg−1 Zn). The target organs of As, Cu and Pb accumulation were: root > leaf > stem in N. glauca, and root > stem > leaf in E. segetalis. Zinc and Cd showed a significant decrease in roots relative to aerial parts of N. glauca, and Co was preferentially partitioned in stems of N. glauca and leaves of E. segetalis. The soil–plant transfer coefficient values of PTEs in all parts of both plants were well below unity with the only exception of Cd in leaves of N. glauca (1.254), suggesting that roots acted as a barrier limiting the uptake of PTEs by plants. Interestingly, under the same soil conditions, N. glauca absorbed Cd in considerable proportions from soil andaccumulated it in its leaves, while E. segetalis was not effective in transferring PTEs from roots shoots except for Co. In conclusion, soil pH and plant-related factors greatly influence the stabilization of PTE in the rhizospheric soil and produce inconsistencies in PTE phytoavailability. The findings of this study provide criteria to assist in natural remediation in other legacy contaminated sites worldwide. Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s10653- 023- 01523-w. C.Barba-Brioso(*) Department ofCrystallography, Mineralogy andAgricultural Chemistry, University ofSeville, Campus Reina Mercedes, s/n., 41071Seville, Spain e-mail: [email protected] P.J.Hidalgo Department ofIntegrated Sciences, University ofHuelva, Campus El Carmen, s/n., 21071Huelva, Spain e-mail: [email protected] S.Fernández-Landero· J.C.Fernández-Caliani Department ofEarth Sciences, University ofHuelva, Campus El Carmen, s/n., 21071Huelva, Spain e-mail: sandra.fer[email protected] J. C. Fernández-Caliani e-mail: [email protected] I.Giráldez Department ofChemistry, University ofHuelva, Campus El Carmen, s/n., 21071Huelva, Spain e-mail: [email protected] 9542 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol:. (1234567890) Keywords Soil pollution· Heavy metals· Rhizosphere· Phytoextraction· Nicotiana· Euphorbia Introduction Multiple pollution sources are often found around the mouth of rivers and estuaries because population and industry tend to be clustered in distinct geographic areas with favorable natural conditions and resources, such as fertile soils, water availability, and low lands for urban development (Santucci etal., 2017). Many of these conditions converge in coastal wetland areas (Li etal., 2014), where the environmental functions and ecosystem services of the soil may be dramatically disrupted by man-induced reactions involving the formation of acid sulfate soils, oxidation of organic matter and subsequent emission of carbon dioxide to the atmosphere, and potentially toxic trace elements (PTEs) release into the nearby water courses (Fernández etal., 2010). In such degraded wetlands, soils lose their filtering and storing capacity for pollutants that are eventually discharged in high amounts from liquid industrial and agricultural effluents, solid waste deposited on illegal dumping sites, or dust deposition (Conesa etal., 2011; Grantcharova & Fernández-Caliani, 2022). Fortunately, the economic development of countries is commonly accompanied by increased environmental awareness and protection, and so research, legislative and policy efforts are being undertaken to achieve a sustainable land use and management. Thus, for example, the European Commission (2021) has launched a new soil strategy for 2030 to ensure a high level of environmental and health protection. However, for legacy contaminated sites a common approach is lacking in the European Union, which is a relevant legal gap concern. The Domingo Rubio tidal system (Spain) is a prime example of highly polluted wetland where the polluter-pays principle cannot be applied. In addition to diffuse contamination from agriculture and water pollution caused by multiple anthropogenic sources (Barba-Brioso etal., 2010a), local soil contamination occurs by improper disposal of sulfide-rich wastes coming from past mining activities. As a result, the wetland soil adversely affected by acid and heavy metal contamination has become sterile and unproductive. Under these conditions, only some races, ecotypes or edaphoendemisms are able to colonize such altered soils. It is the case of Erica andevalensis, an endemic heather adapted to highly acidic soil conditions along the acidic soils of the Iberian Pyrite Belt (Márquez-García et al., 2006; Rossini-Oliva et al., 2018). Notwithstanding the above, an incipient vegetation usually arises spontaneously over time owing to the tendency for natural processes in soils to attenuate contaminants. Initially, the vegetative cover is composed of annual grasses that are able to colonize technogenic soils developed on mine wastes. These pioneer plants play an interesting role in preventing erosion, promoting soil dynamics, and maintaining other living beings of the trophic change since they are primary producers. Alien plant species may also colonize highly altered soil ecosystems, as they are more resilient and competitive than native species in degraded areas (Dana etal., 2005). Thus, a controversial issue arises when the exotic plants play a significant role in the remediation of contaminated soils, but also pose a threat to biodiversity of native ecosystems (Mendez and Maier, 2008). Interestingly, the species Nicotiana glauca and Euphorbia segetalis are found growing on the sulfidic waste material improperly disposed of in the Domingo Rubio wetland, displaying a patchy distribution, and hence, they have been selected as the focus of this study. It can be hypothesized, therefore, that both species could tolerate high levels of PTEs in surrounding soils. The genus Nicotiana comprises around 67 species distributed in America, South Pacific, Australia and Southwest of Africa. Some of them contain alkaloids derived from nicotinic acid (nicotine). Nicotiana glauca R.C. Graham is a glabrous scrub or small tree (up to 6 m tall), natural from South America (Argentina, Paraguay, and Bolivia) and extensively naturalized in the Mediterranean region. It is a nitrophilous plant commonly found on slopes, embankments, roadsides, river terraces, and well adapted to growing in sandy or stony and disturbed soils (Dana etal., 2005). N. glauca was included in the Spanish Catalogue of Invasive Alien Species (Royal Decree 630/2013) until 2015 when a ruling by the Supreme Court of Spain forced its removal from the list and, since then, its use in natural landscapes is allowed. The reason for the exclusion was that it has a great potential as an energy crop, thus helping to tackle climate change. This species has the ability to survive 9543 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol.: (0123456789) in heavily polluted soils (Barazani etal., 2004), and genes associated with metal tolerance have been encoded (Shingu etal., 2005). Euphorbia segetalis L. is an annual, biennial, or perennial herb (up to 80cm tall), native to Mediterranean Europe, North of Africa, and Macaronesia. It is indifferent to the substrate type and occurs in a wide range of habitats, including ruderal sites, crop fields, stony pastures and rocky areas of coastal places. This species has been phytochemically characterized by its diterpene content (Jakupovic etal., 1998) and antiviral and antimicrobial activities of triterpenes isolated from E. segetalis have been assessed (Madureira etal., 2003). Some species of the genus Euphorbia (Euphorbiaceae) produce a large amount of latex that can cause severe irritation of the skin and eyes. The latex has been extensively investigated from pharmacognostic and phytochemical points of view, and many interesting antitumor molecules have been characterized (Sahai etal., 1981). To our knowledge, this is the first time that E. segetalis is being considered for its potential as phytoaccumulator of PTEs in a contaminated soil. This paper was aimed at determining the environmental conditions, mineral components and degree of contamination of the technogenic soil where N. glauca and E. segetalis are growing, and the concentrations of PTEs in different organ tissues (roots, stems and leaves) in order to quantify the uptake of contaminants from soil by plant roots and their transfer and accumulation in the above-ground biomass of these spontaneous pioneer plants. The final goal is to understand the potential impact of this contamination and how it can affect the environment. Materials andmethods Geoenvironmental setting The Domingo Rubio wetland is a small tidal channel 6 km in length connected to the confluence of the Tinto and Odiel rivers (Pendón et al., 1998), where they form the estuary of Huelva on the southwest coast of Spain (Fig. 1a, b). The tidal wetland is located in a flooded valley, which is incised into Plio-Pleistocene detritic deposits, occupying an area of about 480ha. It has a mild Mediterranean climate moderated by the influence of the Atlantic Ocean (Csa in the Köppen–Geiger climate classification system), withaveragetemperature of9°C inwinterand 27°C in summer, and with most of the rain (mean annual of 525mm) falling inwintermonths. The lower wetland area consists of a tidal marshland with a pronounced marine influence. The marsh soils (Salic Fluvisols according to World Reference Base for Soil Resources) are developed on fluviomarine silty-clayey sediments, and the wetland vegetation is dominated by salt-tolerant plant species, such as Spartina densiflora, Arthrocnemum macrostachyum and Halimione portulacoides (Madejón etal., 2009). The upper wetland is a lacustrine area consisting of a freshwater reservoir artificially created by stream impoundment, which is inhabited by Typha domingensis and Phragmites australis, among others wetland species. This portion of the wetland is fed by Dehesa del Estero stream and many ephemeral creeks from a watershed impacted by intensive irrigated cropping operations (Barba-Brioso etal., 2010b). The tideland area has been catalogued as Natural Area since 1989 and more recently as Special Protection Area of the Natura 2000 Network in October 2002 (Site code: ES6150003, Estero Domingo Rubio, Directive 92/43/EEC) hosting a total of 7 natural Habitats of Community Interest and 156 species (mainly birds) referred to in Article 4 of Directive, 2009/147/EC and listed in Annex II of Directive 92/43/EEC. Despite the legislative framework in place, the wetland receives high loads of pollutants (Online Resource 1) from diffuse agricultural and industrial sources (Barba-Brioso et al., 2009, 2010a) and acidic discharges from a Spolic Technosol developed on legacy sulfidic-rich materials due to improper past waste disposal practices. Plant and soil sampling A stratified random soil sampling was undertaken in three representative sites of the Spolic Technosol developed on the abandoned sulfide-rich mine wastes (sites DR1, DR-2 and DR-3 in Fig.1c). At each site, five topsoil samples (0–20cm depth) were collected with an Edelman auger in crossing directions around the central sampling position (Fig. 1d) and bulked in the field to form one composite sample representing the site. The plants of N. glauca (Fig.1e) and E. segetalis (Fig.1f) and their associated soil samples were collected at ten sites in the wasteland area (five 9544 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol:. (1234567890) sites per plant species) where they spontaneously grew. Plants were harvested and separated into leaves, stems and roots. This sampling scheme allowed us to compare tissue metal concentrations in different groups of organs of the two only shrubs growing on the mine wastes. As a control, plant samples of the same species and soil samples from adjacent roots were collected from seemingly uncontaminated areas elsewhere. The soil from the rhizospheric zone was collected by vigorous hand-shaking and subsequent brushing of the root mass (Luster etal., 2009). Plant and soil samples were placed in zip-lock plastic bags and immediately transported to the laboratory for processing. Analytical methods The soil samples were air dried, gently crushed to break the aggregates, passed through a 2-mm mesh sieve, and thoroughly mixed to ensure optimal homogenization. Soil reaction (pH), redox potential (Eh) and electrical conductivity (EC) were Fig. 1 a Location map of the Domingo Rubio wetland area in the Huelva estuary (SW Spain). b Google Earth® image showing the upper and lower course of the tidal channel, and the main activities developed around the study area. c Aerial view (drone image) of the sampling locations for soils and plants. d Soil sampling method using a hand auger. e Nicotiana glauca on site. f Euphorbia segetalis on site 9545 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol.: (0123456789) potentiometrically measured in a soil/deionized water suspension of 1:2.5 (w/v), after shaking for 15min followed by a 30min equilibration period. Aliquots of sieved material were ground in an agate mortar and pestle until getting a nearly uniform fine powder (< 63μm) for mineralogical and chemical analyses. Mineralogical identification of the major crystalline phases was carried out by powder X-ray diffraction (XRD) on a BRUKER AXS D8 Advance diffractometer, using CuKα radiation at 40kV and 30mA. Scans of randomly oriented bulk powders were run from 3 to 65° 2θ with a step size of 0.02° and a counting time of 0.6s per step. Relative mineral abundance was estimated by empirical intensity factors weighting the integrated peak area values of diagnostic reflections (Kahle et al., 2002). Selected samples were examined by environmental scanning electron microscopy (ESEM) on a JEOL JSM-IT500 instrument coupled with energy-dispersive X-ray spectroscopy (EDS). Back-scattered electron (BSE) images and EDS spectra were acquired at 20kV accelerating voltage to assist in the mineral identification of accessory heavy metal-bearing particles. Soil samples were subjected to multi-acid (HF-HClO4-HNO3-HCl) digestion followed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis using an Agilent 735 ICP-OES instrument to determine the total concentrations of As, Cd, Co, Cr, Cu, Pb, and Zn. The analyses were performed at Activation Laboratories Ltd. (Ancaster, Ontario, Canada), which is accredited to the ISO/IEC 17025:2005 standard. Analytical quality control was monitored by using internationally certified reference materials (OREAS 13b, OREAS 98, OREAS 101b), blank samples and laboratory replicates to check the accuracy and precision of the data. The accuracy expressed as percent recovery ranged from 78.3 to 108.3%, and the precision expressed as percentrelative standard deviation (RSD) was between2.4% and 14.7%. A one-step extraction test with a mild neutral salt solution was used to determine the phytoavailable concentrations of PTEs in the rhizosphere (Menzies etal., 2007). For this purpose, an aliquot (1g) of each rhizosphere soil sample (< 2 mm) was subjected to extraction with 0.01M CaCl2, in a soil/solution ratio of 1:10 (w/v), by end-over-end shaking at 30rpm for 2h (Houba etal., 1990; Rivera etal., 2016). The soil extract solution was then centrifuged at 3000rpm for 10min, and PTEs of concern were measured in the supernatant after filtration through 0.45 μm nylon filters. The analysis of PTEs in the digests was performed by ICP-MS on an Agilent 7700 instrument (Central Research Services, University of Huelva, Spain). The extraction procedure was repeated twice with similar results, and the mean value was taken. Extraction blanks were used to identify possible contamination. In order to extract the PTEs of concern from plant tissues, all plant material (roots, stems and leaves) was washed thoroughly with tap water and rinsed with distilled water repeatedly to remove the surface dust particles and then, sonicated in a bath-type sonicator (Power Sonic 505) for 30min. Afterward, the plant samples were dried in an oven at 40° C for 5days, and ground to 0.5mm in an IKA WERKE MF 10 grinder at 3250rpm. The thicker stems were previously ground in a cutting mill RETSCH(R) model SM 2000 to pass through a 5 mm mesh sieve. An aliquot (0.05g) of the ground sample was digested with 1 ml of concentrated HNO3 (Madejón et al., 2011) using a microwave oven at 350 W for 1.5min. After cooling, 1.5ml of H2O2 (30%) was added to the digest solution and the heating was repeated using the same microwave program. Finally, the resulting solution was diluted to a volume of 5ml with Milli-Q water. The analysis of PTEs in the digests was performed with the Agilent 7700 ICP-MS. The same procedure was applied for control plant samples. The validity of the analytical procedure was assessed by analyzing a certified reference material of plant leaves of Oriental Basma Tobacco Leaves (INCT-OBTL-5) (Samczyński etal., 2012). The average precision was 7.8% RSD, and the average recovery was found to be 92.1%, which is indicative ofgood accuracy. Statistical treatment All results are expressed as mean ± standard deviation. For statistical purposes, the results below the detection limit were assumed as half of the detection limit. Statistical differences between means were evaluated using Student’s t-test and analysis of variance (one-way ANOVA). Prior to ANOVA analysis, the homogeneity of variance was assessed by Levene’s test. The critical level of significance for all statistical tests was set at an α-value of 0.05 (95% confidence interval). Data statistical treatment 9546 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol:. (1234567890) was performed with the Statistica 10.0 and MS Excel 2016 software packages. Results anddiscussion Mineral composition and environmental soil conditions The Spolic Technosol developed on sulfide-rich mine wastes left on the marshland differed drastically in mineral composition and physico-chemical properties from the natural soils in the wetland. It was essentially composed of pyrite, jarosite, gypsum, and quartz in highly variable proportions, with minor feldspars, hematite, and accessory barite and anglesite (Table1). In dry periods, as surface soil dries out it appears covered with yellow efflorescences of readily soluble sulfate minerals, notably ferricopiapite, as reported by Barba-Brioso etal. (2010c). The secondary mineral assemblage was clearly a result of sulfurization processes driven by weathering of the pyriterich wastes, with sulfate minerals being the most abundant oxidation products. Consistently, the Technosol was extremely acid (pH values below 3.0) and strongly oxidant (Eh values ranging between + 659 and + 756mV). These surface conditions are conducive to increased release of acidity, sulfate ions and PTEs, thus limiting the soil capability to support vegetation. Interestingly, the mineral constituents of the soil in which the plants have directly grown differed distinctly from those of the Technosol derived from the sulfidic wastes. The rhizosphere soil samples were composed mainly of quartz and clay minerals, together with minor amounts of feldspars and calcite, which coexisted with accessory gypsum and jarosite in some samples. It was noteworthy the occurrence of carbonates in most samples, indicating a good buffering capacity to neutralize the acid that has been produced, during the oxidative dissolution of pyrite in the abandoned wastes. In fact, soil active reaction was slightly alkaline, with pHH2O values ranging from 7.0 to 8.1 (Table1), which allowed specifically the growth of some species such as N. glauca and E. segetalis. The soil surrounding the plant roots showed Eh values (from + 377 to + 458mV) and electrical conductivity values (up to a maximum of 2.65 mS cm−1) lower than those of the Technosol, and within the typical range reported for natural soils of the wetland (Barba-Brioso et al., 2021). The results suggested, therefore, that these species were able to growin soil patches subjected to contrasting environmental conditions, having less adverse effects in terms of plant tolerance. An alternative hypothesis is the potential transformation of the rhizosphere by these plants using any kind of exudates to detoxify undesirable metal pollutants in soils (Chen etal., 2017). Further research is needed to understand the origin of this extreme modification of the rhizosphere and the reason why only these species are able to survive in this extremely hostile environment. Total and phytoavailable concentrations of trace elements Total PTE concentrations in the technogenic soil greatly varied depending on the element considered (Table 2). The pyrite-rich material (sample DR-1) showed the highest concentrations of Pb (> 5000 mg kg−1), Cu (1710 mg kg−1), Zn (1610 mg kg−1), Co (51 mg kg−1), and Cd (8mg kg−1), whereas the maximum concentration of As (1360mg kg−1) and Cr (89mg kg−1) were measured in the samples DR-2 and DR-3, respectively. In comparison with geochemical baseline concentrations of the local wetland soils (Barba-Brioso, 2011), the Technosol was found contaminated with all PTEs, except Cr. Specifically, the concentrations measured in the Technosol exceeded the local baseline concentrations up to 9.7 times for Pb, 8.9 for As, 8.0 for Cd, 3.4 for Co, 3.2 for Zn, and 2.3 for Cu. The Cr concentration fell within the normal range of the local soils since it is not regarded as a mine-source metal in the Iberian Pyrite Belt (Fernández-Caliani etal., 2009). It was also noticeable that the total concentrations of Pb, As and Cu exceeded the generic reference levels (GRL) or threshold limits established by current regulation (Junta de Andalucía, 2015) for agricultural and other land uses, including natural areas, indicating that exposure to these contaminants may result in an unacceptable risk level for human health or ecosystems. If the GRL are exceeded, the Spanish regulation on contaminated soils establishes that a sitespecific risk assessment must be performed (Tarazona etal., 2005). The rhizosphere soil of both plant species (Table3) contained elevated concentrations of PTEs 9547 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol.: (0123456789) Table 1 Electrochemical parameters and mineral composition of the technogenic soil derived from abandoned mine wastes, of the soil surrounding the plant roots and their control soils Mineral symbols: Ang (anglesite); Brt (barite); Cal (calcite); Dol (dolomite); Fsp (feldspars); Gp (gypsum); Hem (hematite); Jrs (jarosite); Qz (quartz) EC electrical conductivity Sample pH Eh EC Mineral composition (wt %) (in H2O) (mV) (mS cm−1) Pyrite Jarosite Gypsum Quartz Clay minerals Calcite Minor phases (< 10%) Technosol DR-1 1.9 659 11.9 30 − 35 25 − 30 15 − 20 Ang, Brt, Fsp, Gp, Hem DR-2 2.2 756 5.6 15 − 20 70 − 80 Hem, Qz DR-3 2.7 697 3.3 35 − 40 45 − 50 30 − 35 Hem, Brt Rhizosphere N. glauca DR-N-1 7.4 415 2.65 15 − 20 45 − 50 10 − 15 Fsp, Gp, Jrs DR-N-2 7.8 393 0.47 40 − 45 40 − 45 Cal, Gp, Fsp DR-N-3 7.0 458 0.59 35 − 40 50 − 55 Gp, Fsp, Cal DR-N-4 7.7 398 0.13 60 − 65 25 − 30 Cal, Gp, Fsp DR-N-5 7.7 413 0.07 65 − 70 20 − 25 Gp, Fsp, Cal Control DR-N–C 7.5 415 0.68 40 − 45 35 − 40 Cal, Dol, Gp, Fsp Rhizosphere E. segetalis DR-E-1 7.1 403 1.77 10 − 15 45 − 50 25 − 30 Cal, Fsp DR-E-2 7.6 377 0.16 55 − 60 30 − 35 Gp, Fsp, Cal DR-E-3 7.9 384 0.64 35 − 40 45 − 50 Cal, Gp, Fsp DR-E-4 7.0 398 2.42 15 − 20 50 − 55 10 − 15 Fsp, Gp, Jrs DR-E-5 8.1 399 0.13 55 − 60 25 − 30 Fsp, Gp, Cal Control DR-E-C 7.0 453 0.12 60 − 65 25 − 30 Fsp, Gp, Cal 9548 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol:. (1234567890) of concern, particularly Cu (up to 6370 mg kg−1) and Zn (up to 5250 mg kg−1) but also Pb (up to 2210mg kg−1), As (up to 613mg kg−1), and Cd (up to 18.7mg kg−1), compared to local baseline concentrations, yielding contamination factors as high as 8.7 for Cu (sample DR-E-3), 10.3 for Zn (sample DR-E- 4), 4.3 for Pb (sample DR-E-4), 4.0 for As (sample DR-N-1), and 18.7 for Cd (sample DR-N-1). Consistently, the PTE concentrations in rhizosphere were significantly above the control soil levels (p < 0.007) in all sampling sites, except for Cu in sample DR-N- 5. This may be due to the fact that the control soil had an abnormally high concentration (1990 mg kg−1) arising from unexpected contamination in spite of their apparently natural trait. It could be possible due to diffuse contamination originated by re-suspension of waste material. It was also found that the sample DR-N-4 had an anomalously high content of Cr Table 2 Total trace element concentration in the technogenic soil derived from abandoned mine wastes Geochemical baseline concentrations of the local wetland soils and the generic reference levels established for soils of the Andalusia region (South Spain) are given for comparison *Mean value in local wetland soils (Barba-Brioso, 2011) **Junta de Andalucía (2015) Trace element (mg kg−1) As Cd Co Cr Cu Pb Zn Detection Limit (DL) 3 0.3 1 1 1 3 1 Sample DR-1 1250 8.0 51 15 1710 > 5000 1610 Sample DR-2 1360 4.5 24 14 1570 4390 637 Sample DR-3 761 2.2 14 31 1160 2190 525 Baseline concentration* 153 < DL 15 89 732 515 508 Regulatory threshold** 36 25 24 10,000 595 275 10,000 Table 3 Trace element concentrations in the rhizospheric soil of Nicotiana glauca and Euphorbia segetalis Trace element (mg kg−1) As Cd Co Cr Cu Pb Zn Detection Limit (DL) 3 0.3 1 1 1 3 1 Rhizosphere N. glauca DR-N-1 613 10.5 42 32 2640 1070 2150 DR-N-2 163 7.6 59 54 3210 814 4550 DR-N-3 86 1.1 7 30 1340 285 412 DR-N-4 54 3.2 20 223 3360 2210 1420 DR-N-5 69 1.7 6 17 1390 350 585 Mean 197 4.8 26.8 71.2 2388 946 1823 Std. deviation 236 4.1 23 86 972 778 1675 DR-N-control 11 < DL 4 41 65 34 98 Rhizosphere E. segetalis DR-E-1 208 2.5 20 28 2690 303 1220 DR-E-2 160 3.7 15 34 5730 408 1270 DR-E-3 135 6.0 19 37 6370 516 1540 DR-E-4 344 18.7 79 37 3460 1050 5250 DR-E-5 63 2.2 12 45 1110 339 1000 Mean 182 6.6 29 36 3872 523 2056 Std. deviation 105 6.9 28 6 2173 305 1796 DR-E-control 28 0.8 4 24 1990 107 273 9549 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol.: (0123456789) (223mg kg−1) whose source was unclear. The pool of wastes from different sources/time slices made difficult the traceability of the materials. No significant differences were observed among the total PTE concentrations in the rhizosphere soil of the two studied plant species except for Cu, which was significantly higher in soil samples around the roots of E. segetalis than in those of N. glauca (p = 0.018). Surprisingly, the levels of Cu, Zn and Cd in most rhizosphere samples were even higher than those observed in the sulfidic-rich wasteland. The contrasting soil pHvalues between the two soils could be a plausible explanation for the observed difference in PTE accumulation. Soil reaction is a factor of paramount importance concerning metal speciation and mobility in soil systems (McBride, 1994). Thus, a substantial fraction of Cu, Zn and Cd released by sulfide oxidation, which are relatively soluble under the acidic conditions prevailing in the Technosol, was immobilized to a large extent in some patches of neutral soil where N. glauca and E. segetalis grew spontaneously. Chemical fixation might have occurred either by precipitation, adsorption, or any other mechanism involving soil constituents with pH-dependent charge that tend to deprotonate with increasing pH, thus leading to increased metal retention. However, the highest concentrations of As and Pb were found in the Spolic Technosol, in association with relatively insoluble sulfate minerals like jarosite and anglesite. The precipitation of these secondary minerals at low pH was an efficient mechanism for decreasing the mobility of As and Pb at the source of contamination and could be the reason why some races of the studied species were able to survive. It is generally agreed that total concentration of PTEs in soil does not provide predictive insights on mobility and availability of contaminants in relation to plant uptake (i.e., phytoavailability). The PTE pool that may be available in the rhizosphere for plant uptake (Madejón etal., 2011; Menzies etal., 2007) was assessed by single extraction with CaCl2. The results of the analysis (Table4) showed that the extracted element concentrations varied from undetectable levels to values below 8mg kg−1. The maximum concentrations of Zn (7.40 mg kg−1), As (0.07 mg kg−1), Cd (0.06 mg kg−1), and Co (0.06 mg kg−1) were extracted from the soil surrounding the roots of N. glauca, whereas the highest content of extractable Cu (2.53mg kg−1) was noted in the rhizosphere of E. segetalis (sample DR-E-2). The Pb concentration was below the detection limit by ICP methods in all soil extracts. The t-test showed Table 4 Trace element concentrations extracted with CaCl2 from the rhizospheric soil of Nicotiana glauca and Euphorbia segetalis (< DL, below detection limit) Element (µg kg−1) As Cd Co Cu Pb Zn Detection Limit (DL) 3 1 5 47 7 29 Rhizosphere N. glauca DR-N-1 33 ± 2 6 ± 0.4 < DL 75 ± 6 < DL < DL DR-N-2 15 ± 0.3 < DL < DL < DL < DL < DL DR-N-3 7.9 ± 0.2 46 ± 1 63 ± 1 925 ± 6 < DL 4382 ± 14 DR-N-4 66 ± 0.1 3.9 ± 0.03 < DL 805 ± 6 < DL 828 ± 6 DR-N-5 69 ± 0.5 59 ± 0.1 11 ± 0.1 1844 ± 30 < DL 7402 ± 143 Mean 38 ± 28 23 ± 27 15 ± 25 733 ± 690 < DL 2522 ± 3031 DR-N-Control < DL < DL < DL < DL < DL < DL Rhizosphere E. segetalis DR-E-1 33 ± 2 3.0 ± 0.2 < DL 876 ± 38 < DL 51 ± 12 DR-E-2 46 ± 0.003 18 ± 0.1 13 ± 0.01 2570 ± 24 < DL 2340 ± 19 DR-E-3 8.6 ± 0.1 5.1 ± 0.1 < DL 388 ± 3 < DL < DL DR-E-4 22 ± 1 16 ± 1 < DL 65 ± 6 < DL 277 ± 21 DR-E-5 27 ± 1 4.4 ± 0.2 < DL 203 ± 1 < DL < DL Mean 27 ± 13 9 ± 7 4 ± 5 820 ± 949 < DL 534 ± 941 DR-E-Control 36 ± 0.01 104 ± 0.4 72 ± 0.03 14,085 ± 143 12 ± 0.4 98,872 ± 1301 9556 Environ Geochem Health (2023) 45:9541–9557 1 3 Vol:. 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