4.2. Metal concentrations in plants
This study showed that heavy metal concentrations were highly variable between plants and also in their roots and aerial parts. Metals uptake by plants from the soil depends on plant species, total metal concentrations and their availability, soil pH, plant physiology, and age of plant (Rascio and Navari-Izzo 2011). The mean concentrations of copper in coal wastes varied between 29.33 mg/kg for Kiasar and 107.5 mg/kg for Karmozd. Similarly, at the coal wastes in Poland (Ciesielczuk et al. 2014), China (Li et al., 2017) and India (Jain et al. 2017) the Cu levels were reported to be 111, 100, and 4 mg/kg, respectively. The normal copper concentrations in plant tissues typically ranges between 3 and 25 mg/kg (Reeves and Baker 2000). In our study the Cu concentration varied between 5.64 and 42.48 mg/kg in aerial parts and 3.03 and 23.91 mg/kg in roots. The levels of Cu in A. scoparia (23.91 mg/kg in root and 41.06 mg/kg in in aerial parts) and C. spinosa (42.48 mg/kg in in aerial parts) exceeded the normal range. Similar to this finding, Li et al. (2017) at coal wastes of China reported that A. scoparia accumulated 37 mg/kg Cu in aerial parts. A copper concentration of 34.01 mg/kg in aerial parts and 15.38 mg/kg in roots was reported for A. scoparia in Azerbaijan (Alirzayeva et al. 2006). According to Shah et al. (2013) the copper concentrations in aerial parts and root of C. spinosa in Pakistan were 21.61 and 4.93 mg/kg, respectively, which were lower compared with our study. Copper hyperaccumulators are described as plants that accumulate more than 1000 mg/kg Cu in dry weight biomass (Baker and Brooks 1989), so no plant species in present study were considered as Cu hyperaccumulator.
The world average level of Arsenic in soil is 5–20 mg/kg (Kabata-Pendias and Pendias 2001). The mean As concentration in coal wastes varied between 4.66 mg/kg in Karmozd and 14.33 mg/kg in Kiasar, which was within the reported ranges (0.5 to 38.3 mg/kg) in different coal mining areas worldwide (Sahoo et al. 2016). The normal concentrations of As in plants varies between 0.01 and 1 mg/kg (Chaney 1989). S. marianum exceeded this limit with 2.01 and 1.48 mg/kg As in root and shoot, respectively. This implies its tolerance to As due to vacuole compartmentalization which cause the prevention of free metal ions in the cytosol and limit them into a spaces where cell division and respiration occur (Singh et al. 2017).
Lead is a non-essential heavy metal for plants and is normally considered at concentrations between 2 and 5 mg/kg in soil (Chaney 1989). In this study, coal wastes contained about 35 mg/kg Pb which was less than some other coal mine areas in Poland (63 mg/kg) and China (91 mg/kg; Ciesielczuk et al. 2014; Li et al. 2017). In the present study, Capparis spinosa was the only plant that accumulated Pb about 7 mg/kg in shoot while it was less than 1 mg/kg for other species. In a lead-zinc mine in semi-arid climate in Iran, C. spinosa had 45–230 µg/gr Pb in shoot (Ghaderian et al. 2007). None of the plants can be considered as hyperaccumulators of Pb. Normally, roots have high potential to concentrate large amounts of Lead, while limited amounts could transfer to aerial parts (Lane and Martin 1977). In this study plants tendency to high lead accumulation in aerial parts represented their ability to Pb translocation from the roots to aboveground parts.
The mean concentration of Zn in coal wastes ranged between 93 mg/kg for Kiasar and 141 mg/kg for Karmozd. Similarly, in the coal wastes of Silesia (Poland), Henan (China) and Meghalaya (India) the Zn levels were 179, 172.38, and 37 mg/kg, respectively (Ciesielczuk et al. 2014; Li et al. 2017; Jain et al. 2017). The Zn ranging between 15 and 150 mg/kg (Chaney 1989) or concentrations less than 100 mg/kg (van der Ent et al. 2012) are normal for the plant growth. The Zn concentrations for studied plants were within the limits considered as normal values (Kabata-Pendias 2011). Similar to van der Ent et al. (2012) C. dactylon had the concentrations of Zn more than 100 mg/kg. Kumar et al. (2015) found that the highest Zn concentrations were in C. dactylon at reclaimed mine land. Zinc concentration in other studied plants was similar to reported levels at a contaminated area (Wei et al. 2005).
In current study, the Cr concentrations were not phytotoxic to plant organs and were found to be in the normal range reported for plant species (0.2 to 5 mg/kg, Reeves and Baker 2000). The accumulation of Cr efficiently occurred in C. dactylon (2.61 mg/kg in aerial parts and 3.4 mg/kg in root) which represents its high ability compared with other plants. In this regard, Abou-Shanab et al. (2007) reported the Cr content in aerial parts of C. dactylon was about 39, 8, 6 and 4 times higher than the shoots of Helianthus annuus, Sorghum bicolor, Zea mays and Conyza discoridies. The concentration of Cr in the aerial parts of most plant species were higher than their roots. Thus, they can effectively translocate Cr from root into the aerial parts.
The Cadmium levels in studied coal wastes were low (ranged 0–0.1 mg/kg) and below the range determined for not contaminated soils (5–30 mg/kg, Kabata-Pendias and Pendias 2001). Cd is a non-essential element for plant growth and its normal concentrations is 0.1–3 mg/kg in soil (Baker and Brooks 1989). Cadmium occurs in plant organs due to the unwanted absorption and translocation into the plant organs (Benavides et al. 2005). The content of Cd in this study ranged from 0.01 to 0.77 mg/kg in aerial parts and roots with greatest concentration in A. scoparia (0.77 mg/kg in aerial parts and 0.63 mg/kg in root). Li et al. (2017) observed the highest mean concentration of Cd in A. scoparia (2.62 mg/kg in aerial parts and 0.46 mg/kg in root) colonized on the coal waste dumps in China. Cadmium accumulated in A. scoparia studied by Alirzayeva et al. (2006) varied between 0.97 and 4.02 mg/kg in the roots and aerial parts, respectively and reported to be significantly higher than the other Artemisia species. Generally, in the present study, cadmium concentrations in plants was within the normal range observed in other studies (Wei et al. 2005; Brunetti et al. 2008).
The Sr concentrations for G. fimbriligerum (145.87 mg/kg in root and 337.11 mg/kg in aboveground parts) and S. marianum (131.17 mg/kg in root and 276.74 mg/kg in aboveground parts) exceeded the phytotoxic level (30 mg/kg; Shacklette et al. 1978), which could be explained by great Sr content of the coal wastes. It seems that these species can be valuable bioaccumulators in their aerial parts for strontium. In accordance with Kabata-Pendias and Pendias (2001), the higher concentrations of Sr were observed in aerial parts of most plant species, despite the strontium translocation from roots into aerial parts of plants is not easily conducted. Generally, Metal uptake by plants occur mainly in two ways: passive transport (apoplastic pathway): water mass flow causing the entrance of metal into roots and active transport (symplastic pathway): ion translocating across the plasma membrane (Shahid et al., 2017). Sasmaz and Sasmaz (2009) indicated that mean Sr values in the aerial parts and roots of plants grown up in mined land were 453 and 243 mg/kg for Euphorbia macroclada; 149 and 106 mg/kg for Verbascum cheiranthifolium; and 278 and 223 mg/kg for Astragalus gummifer, respectively. We also found that Poaceae family accumulated low contents of Strontium in their tissues. Hence, Korobova et al. (1998) have stated that radionuclide absorption by plants decreases in the following order: legumes > herbs > grasses.
The Ni levels in coal waste varied between 40 and 118.18 mg/kg for Kiasar and Karmozd, which were above the range identified for different countries (13–37 mg/kg according to Kabata-Pendias 2011). Jain et al. (2017) has reported the lower content of this metal in Meghalaya (India) coal mine waste (mean value 7 mg/kg). Ni contents of 10 to 100 mg/kg are toxic for different plant species (Brooks 1980). In current study, none of the plants showed Ni concentrations in their tissues that reached the phytotoxic level, except S. marianum (11.44 mg/kg, totally). In line with these findings, Zehra et al. (2009) reported that Ni concentration in S. marianum (12.33 mg/kg) was above the normally reported values. Similarly, lower Ni accumulation reported in the roots of S. marianum than the aerial part (Brunetti et al. 2008).
The average contents of Mn in coal wastes varied between 193 mg/kg and 389.15 mg/kg which was within the normal ranges accepted for worldwide soils (270–525 mg/kg; Kabata-Pendias 2011) and mean concentrations for coal mine areas at different countries (86-3700 mg/kg; Sahoo et al. 2016). All plants had larger concentrations of Mn in their aerial parts, but, none of them reached the toxicity levels considered for plant species (more than 400 mg/kg; Kabata-Pendias 2011). This is because when Mn transfers from roots to shoot by the transpiration flow, it cannot be readily remobilized through phloem to other parts of the plants (Loneragan 1988).
The geochemical characteristics of Vanadium are strongly related to its oxidation states and environmental acidity (Kabata-Pendias 2011; Altaf et al. 2021). The mean concentration of V in coal waste varied between 88.33 and 238.64 mg/kg for Kiasar and Karmozd, respectively, and exceeded the world average soil content (129 mg/kg; Kabata-Pendias 2011). Altaf et al. (2021) found that lower concentration of Vanadium (< 0.002 mg/kg) acts as a growth promoting factor, enhances nitrogen fixation in soil and stimulates chlorophyll production. Phytotoxic concentration of V is reported approximately more than 2 mg/kg which can induce oxidative stress, growth inhibition, leaf chlorosis and necrosis and limits the essential elements absorption (Aihemaiti et al. 2019). Accordingly, V concentrations in S. marianum exceeded the phytotoxic levels and was the only species that accumulated V in its root. Similarly, Franco-Hernández et al. (2010) observed the content of V only in root of Thymophylla setifolia.
The concentration of Cobalt in root and aerial parts (0.028–3.45 mg/kg) were less than the critical or toxic levels (25–100 mg/kg; Kabata-Pendias 2011). The maximum Co was recorded in roots of S. marianum (3.45 mg/kg) and shoots of P. aviculare (3.2 mg/kg). Compared to this results, the highest Co concentration (7.8 mg/kg) was recorded in A. scoparia at a chromite mining site (Nawab et al. 2016). According to current findings, accumulation of Co in Poaceae family plants was lower than the other plants, which is consistent with the results of Kasowska et al. (2017) in copper mine tailings. Generally, less availability of Co decreases its absorption from soil (Li et al. 2009). Only a small amount of Co can be uptaken by plants from the soil and its next distribution in plants is entirely species-dependent mechanisms (Page and Feller 2005).
Molybdenum is an essential micronutrient, but the physiological requirement for this element is relatively low. Sardans et al. (2008) indicated that drought disturbed the Mo uptake by plants and its concentration may increase or decrease depending on the species type. Mean levels of Mo is reported to vary between 0.33 and 1.5 mg/kg in grasses and between 0.73 and 2.3 mg/kg in legumes (Kabata-Pendias 2011). Mo uptake in studied plant species were within the normal range (0.2-5 mg/kg, Kabata-Pendias 2011). G. fimbriligerum effectively accumulated this metal in their root (1.35 mg/kg) and aerial parts (2.64 mg/kg) more than other species.
In the present study, the levels of Fe for all plant species were extremely low. It is well known that interaction of Fe with some other elements can cause low accumulation of this metal in plant organs (Kasowska et al. 2017). In line with present results, Kasowska et al. (2017) observed low concentration of Fe in plants grown up at cooper mining tailing than the reference area.
4.3. Bioconcentration and Transfer Factors
In this study, the values of BCF and TF varied considerably between different plant species. It is accepted that the differences of BCF and TF in plants species represent their different approaches for response to metal concentrations (Chunilall et al. 2005). Most of the plant species had TF > 1 for more than one heavy metal indicating that these plants were capable to translocate metals from roots to the aboveground parts, thereby confirming their efficacy for phytoextraction (Pandey et al. 2016). Transfer factor greater than one (TF > 1) for Al, Fe and Mn in all plants indicates that the studied species can easily translocate these metals from the roots to the aerial parts. Plants with TF and BCF < 1 recognized as excluders and are not suitable for heavy metals extraction from soils (Mganga 2014). Therefore, these plants with extensive root system can be metal resistant and suitable for phytostabilization. In this study, K. prostrate and A. aucheri were classified as Cu, Cr, Ni, Co and Pb excluders and K. prostrate and H. vulgare as Sr excluders. H. vulgare with a highest TF values for Cu, Cr, Ni and Co, C. dactylon for Mn and Mo, G. fimbriligerum for Sr, C. spinosa for Pb and C. album for Cd are recognized as good phytotranslocators of these metals. Three species including C. album, C. spinosa and R. rugosum had a TF > 1 for Copper, Zinc, Lead, Nickel, Manganese, Chromium, Cobalt, and Strontium. This is probably due to efficient metal transport system and immobilization of metals in leaf vacuoles and apoplast (Zhao et al. 2002). C. album showed a high capacity to translocate Manganese, Nickel, Strontium, Zinc, Iron, Copper, Cadmium, and Lead from soil to aerial parts (Tőzsér et al. 2019). Plants with a BCF value more than one have a high capacity to accumulate heavy metals (Branzini et al. 2012). Among the heavy metals measured, only Cd displayed BCF > 1 in most species. Similar to Zhu et al. (2018), the BCF values for Cd in the plants were considerably greater than their associated TF values except for C. dactylon and H. vulgare. This implies that these species had a good bioconcentration potential and efficiently eliminated Cd from the soil. The highest BCF value for Cd was recorded in A. scoparia which indicates its large capability to enrich this metal in root. This is in agreement with the study of Li et al., 2017. This fact suggests that plant species having greater BCF and TF easily absorb metals from soil by roots and transport them to aboveground parts (Zhu et al. 2018). Therefore, C. album and S. marianum can be regarded as the phytoaccumulator and suitable options for remediation of Cd rich soil. In this regard, Bhargava et al. (2007) and Hammami et al. (2018) proposed that C. album and S. marianum could clean up Cd from the soil and may be applied for phytoremediation.