3.1. Physiochemical Characteristics of Soil
Soil contamination with heavy metals (HMs) and metalloids was ubiquitous and considerably increased over the past few decades with rapid industrialization thereby conferring serious risks to the environment and human health [
38]. HM concentrations in soil mainly depends upon soil pH, moisture, organic matter, and texture [
39].
Table 1 shows the detailed information about the physicochemical information of soil collected from the solid waste dumpsite and control site. In the dump site’s soil, pH, EC, TDS, OM, porosity, bulk density, and texture were 6.9 ± 2.6, 700 ± 340 μS cm
−1, 170 ± 65 mg L
−1, 18.42 ± 5.54 mg kg
−1, 80 ± 19%, 3.54 ± 1.43 g cm
−3, and sandy loam, while in the control site, the values were as 7.1 ± 0.87, 180 ± 98 μS cm
−1, 44 ± 11 mg L
−1, 0.65 ± 0.14 mg kg
−1, 21 ± 7%, 1.02 ± 0.33 g cm
−3, and silty clay loam, respectively.
Likewise, HMs concentrations in the dumpsite were in the range of 784–1234, 543–894, 1322–1643, 879–1368, and 445–879 mg/kg with mean values of 980 ± 230, 742 ± 180, 1465 ± 163, 1168 ± 256, 660 ± 217 mg kg
−1 for Cr, Ni, Fe, Pb, and Zn, respectively (
Table 2). While in the control site, Cr, Ni, Fe, Pb, and Zn ranged at 21–56, 23–78, 104–327, 19–76, and 19–287 mg kg
−1, respectively. A significant difference (
p < 0.05) was observed between the HM concentration in plants and soil samples collected from the dumpsite to that of the control site. In the dumpsite, all analyzed metals were above the permissible ranges given by US-EPA [
40]. The elevated metal concentration could be attributed to the industrial waste disposal in the studied dumpsite. For instance, during Pb-acid battery recycling, different processes such as crushing, fusion, refining, reduction, etc., release different species of Pb in the form of anglesite (PbSO
4), cerussite (PbCO
3), metallic lead (Pb), and Pb oxide (PbO), ultimately landing in the dumpsite [
41]. Similarly, higher Zn contents were observed in the dumpsite’s soil, which might be because of the disposal of bottle caps, blades, and different pharmaceutical leftovers.
Ni pollution is widely distributed around the world due to its abundance in soil. Elevated Ni and Fe concentrations could be attributed to vehicular exhaust, agriculture fertilizer, incinerated hospitals, municipal waste, and other industrial waste disposals at the dump site. A likely explanation for the higher bioavailability of the HMs is the soil chemistry of the dump soil. Sorption–desorption reactions strongly influence HMs’ mobility and bioavailability in soil [
42]. Soil physiochemical properties have a stronger influence on HMs bioavailability. For instance, soil OM can significantly influence metal behavior by binding with toxic metals, thereby alleviating metal toxicity in soil [
43]. Earlier, Chandra and Kumar [
44] mentioned that soil with higher OM contents showed higher Pb contents than Cd in the control environment, which demonstrates that Pb has higher affinity and stability towards OM than Cd. The difference in relative binding affinities among the metals is mainly because of soil chemistry. In the same manner, soil pH was slightly acidic to basic in the dumpsite’s soil. Soil pH holds a significant influence on the bioavailability of HMs in different media and their subsequent toxic effects on biota. In low pH soil, the mobility and bioavailability of HMs are greater compared to soil with high pH [
45]. Similarly, similarly to the mobility and bioavailability of metal, pH plays a significant role in metal speciation in soil. For instance, Cr-OM complexes can affect the bioavailability of metals. Similarly to other factors, soil texture is one of the deciding factors that induce metals bioavailability in soil. It is reported that crops grown on sandy soil are more metal deficient than those grown on soil with a loamy texture, which is likely to have low metal retention capacity. A very close association was found between soil texture and HM concentrations [
46].
3.2. Heavy Metals in Plants
The soil-plant transfer of metals and nutrients is natural and a part of the nutrient cycle [
47]. Metals are taken up in different concentrations by plants, most often through soil solutions, and higher metal accumulation indicates higher metal contents in soil. Regarding the potential contamination and toxicity to biota, some metals such as Zn, Cu, Mn, Mo, and Ni are essential at low concentrations. However, higher accumulation than the threshold limits can cause serious damage [
48]. In the current study, five HMs, i.e., Cr, Pb, Ni, Zn, and Fe, were analyzed in the selected plant roots and shoots (
Table 2). Cr concentrations in the analyzed plant roots and shoots ranged from 56 to 567 and 13 to 165 mg kg
−1, and the highest Cr concentration was observed in
B. lycium Royle, while the lowest concentration was in
A. creticus Lam. Cr concentrations in the dumpsite’s plants was significantly (
p < 0.05) higher than in the control site’s plants. Cr is potentially toxic at a higher concentration for the normal growth and development of plants. The concentration of heavy metals in plant samples was higher in comparison to the other studies [
18]. Toxicity occurs due to the mutagenic and inhibitory impact of heavy metals on enzymatic activities, which ultimately results in reduced root growth and low yields [
9,
13].
Elevated Cr contents can remarkably decrease water potential in leaf air spaces, adversely affecting the transportation rate of different nutrients in plants and reducing Fe accumulation, total protein, chlorophyll contents, and CAT activity in plants [
49].
Ni concentrations in plant roots and shoots ranged from 20 to 453 and 13 to 214 mg kg
−1.
D. stramonium L. accumulated the highest concentration, while
A. creticus Lam accumulated the lowest concentration of Ni (
Table 2). Ni is an essential trace element for plants when available within threshold limits; however, its excess can render toxicity symptoms such as necrosis and chlorosis in different plant species. It is known to cause toxicity and affects protease and ribonucleic enzyme activities, which can lead to retarded seed germination and crop growth [
50]. It has also been reported that Ni concentrations equally influence the mobilization and digestion of carbohydrates and proteins in germinating seeds, consequently decreasing root length, plant height, pigment contents, and fresh and dry weight and increasing malondialdehyde contents and electrolyte leakage. The elevated concentration of Ni affects photosynthetic pigments and can decrease water potential and anti-oxidative enzymes, H
2O
2 contents, lipid peroxidation, and proline levels [
51].
Fe showed varied concentrations in plant roots and shoots and ranged from 98 to 4689 and 45 to 356 mg kg
−1, respectively (
Table 2). The highest and lowest Fe concentration was observed in
C. intybus L. and
D. stramonium L., respectively. Fe plays an important role in plant photosynthetic activities [
45]. Fe phytotoxicity normally exists in the form of bronzing and stippling of plant leaves. It is required for key biological functions such as photosynthesis, nitrogen fixation, sulfur assimilation, hormones, DNA synthesis, and mitochondrial respiration. However, Fe is a very abundant element in Earth’s crust but is very poorly available to the plants under oxidative and alkaline conditions. Fe concentration >500 mg kg
−1 can disrupt the cell redox balance towards a pro-oxidant state, leading to the changes in different metabolic activities and the morphological and physiological traits of plant species [
52].
Pb concentrations in plant roots and shoots ranged from 123 to 678 and 29 to 287 mg kg
−1, respectively. Among the plant species,
B. lycium Royle and
P. hysterophorus L. accumulated the highest and lowest concentration of Pb, respectively (
Table 2). Pb does not play any known metabolic and biological roles in plants, and plants are even equipped with an active defense mechanism against Pb stress that keeps its interaction in sensitive biological tissues. However, a mobile fraction of Pb present in soil can accumulate in plants and, thereby, enter the food chain. Total Pb concentration > 30 mg kg
−1 in plant tissues is considered toxic for plants species [
53]. For most plant species, a higher level of Pb accumulation can reduce seed germination, the inhibition of chlorophyll synthesis, reduction in plant biomass, and negative impacts on enzymatic reactions and nutritional imbalance. One of the major impacts of Pb toxicity on plants is the quick inhibition of the root cells’ growth, which may be because of the blockage of cell division in root tips [
54].
Zn concentrations varied in the plant roots and shoots at 28–456 and 12–321 mg kg
−1, respectively.
C. intybus L. and
A. creticus Lam. showed the highest and lowest Zn concentrations, respectively (
Table 2). Although Zn is an essential micronutrient for plants, it becomes toxic at higher concentrations. Under normal conditions, Zn concentration weigh up to 60 mg kg
−1 dry weight; however, if this concentration increases and reaches 500 mg kg
−1 dry weight, it inhibits root elongation [
55]. It has a long biological lifetime and is an important micronutrient that significantly affects metabolic activities of plants. The phytotoxicity of Zn reduces both plant root and shoot growth, leading to chlorosis in fresh younger leaves, and it can extend to mature leaves after prolonged exposure to higher Zn concentrations. At a higher level, Zn toxicity induces oxidative stress, which is usually very unstable and short-lived but chemically very reactive. The reactive oxygen species (ROS) produced inside plants as a result of Zn toxicity induces oxidative stress, which causes lipid peroxidation, membrane damage, and enzyme inactivation in the cell [
56].
HM bioaccumulation in plants is controlled by multiple factors, such as plant species diversity, preferential uptake, and the binding of some metal species, growing stage, and elemental characteristics of soil [
38]. Total phosphorous concentrations in the soil can also affects HMs uptake by plants. Moreover, the manner in which HMs interact with each other in soil defines the amount of particular metal that is taken up by the plant species. The enrichment of toxic HMs in indigenous plant species can lead to serious problems for the local community of the area, as it ultimately can end up in the food chain [
57]. Hyperaccumulator plants must be protected, and the produced waste in the form of biomass should be treated in a separate chamber to restrict the HM’s reach. It is high time to search for hypertolerance strategies to minimize and restrict HMs in the environment by retaining and detoxifying the underground parts of the plants.
3.3. Phytoremediation Potential of the Studied Plants
Plants possessing the ability to survive in metal-rich soils are known as metallophytes. Prolonged exposure of metallophytes to an excessive amount of HMs enables evolutions in their tolerances via a defensive mechanism and the development of a unique capacity to withstand, survive, and reproduce in a metal-rich environment [
58]. Plants can be classified into three basic categories, excluder, accumulator, and hyperaccumulator, based on their responses when exposed to HMs.
Plants with BAF > 10, BAC > 1 and TF > 1 can be called hyperaccumulators [
59]. The highest BAF values were observed for Pb (0.81), Zn (0.74), Fe (0.61), Cr (0.59), and Ni (0.54) in
A. maurorum Medic.,
A. creticus Lam.,
C. intybus L.,
P. hysterophorus L., and
B. lycium Royle, respectively (
Table 3). Similarly, BAC values were the highest for Cr (11.19), followed by Zn (0.40), Ni (0.38), Pb (0.37), and Fe (0.23) in
A. creticus Lam.,
P. hysterophorus L.,
A. maurorum Medic., and
P. hysterophorus L., respectively. The maximum value of TF was for Ni (1.64) followed by Zn (0.62) in
P. hysterophorus L. and
C. intybus L., respectively, while the minimum value was for Cr (0.11) in
D. stramonium L. (
Table 3). TF and BAC values > 1 can be regarded as hyperaccumulators for the respective metals [
44,
60,
61,
62]. The results revealed that based on TF and BAC values,
A. maurorum Medic.,
A. creticus Lam.,
C. intybus L.,
B. lycium Royle, and
D. stramonium L. were hyperaccumulators for Cr while
P. hysterophorus L. was promising species for both Ni and Cr. Our results demonstrate that
D. stramonium L. was the most efficient species for Cr phytoextraction followed by
C. intybus L. and
A. maurorum Medic., respectively. From these results, it is very clear that the hyperaccumulation of different metals differs with different plant species tested, and these results are in line with other researchers [
60,
61,
62].
3.4. Pollution Indices
Pollution indices such as the geoaccumulation index (Igeo), contamination factor (CF), and enrichment factor (EF) were used to gauge HMs pollution levels in the dumpsite soil of Peshawar (
Table 4). The highest CF value was calculated in the case of Fe (41.86), followed by Ni (18.99), Pb (17.18), and Cr (13.16), while the lowest values were observed for Zn (4.22), designating very high contamination (CF > 6) for Fe, Ni, Pb, and Cr and considerable contamination (3 < CF < 6) from Zn. As shown in
Figure 2, the degree of contamination values ranged from 35.08 to 78.76, showing a very high degree of contamination (DC > 24). Likewise, the highest geoaccumulation was observed for Fe (27.90), followed by Ni (12.66), Pb (11.45), and Cr (8.77), while the lowest value was observed for Zn (0.24), indicating that the soil was strong to extremely polluted (class 6, >5) for Fe, Ni, and Pb and low to moderately polluted (class 1, 0–1) for Zn (
Table 4). Ni showed the highest EF value of 3.52, followed by Pb (3.38) and Cr (3.37), and the lowest value was demonstrated by Zn (2.04), depicting moderate enrichment (2 < EF ≤ 5) for all analyzed metals. The Eri values for the analyzed metals were Ni, (92.95), Pb (83.82), Fe (27.90), Cr (8.77), and Zn (2.86), showing moderate risks (40 < Eri ≤ 80) for Ni and Pb and low risks (Eri > 40) for Fe, Cr, and Zn (
Table 4). RERI values ranged from 67.31 to 195.84, employing considerable risks from the dumpsite’s soil (
Figure 2). From these results, it has become clear that the dumpsite was contaminated with different heavy metals that have the potential to cause certain risks to human health through various mechanisms [
63].
Such higher values for HMs consolidate our hypothesis that the dumpsite is illegally in use for both hazardous and non-hazardous waste, which is not technically and legally allowed otherwise. Wastes such as biosolids and manures, e.g., livestock manures, compost, and municipal sewage sludge, when disposed of in open dumpsites, can cause HMs accumulation such as As, Cd, Cr, Pb, Hg, Se, Zn, Ti, Sb, and so forth in the soil. Although most of organic waste contains a lower amount of HMs, continuous dumping can lead to HM accumulation in soil. In the current study, the higher pollution indices values for Ni, Cr, Pb, and Fe can be regarded as industrial waste, incinerated waste from hospitals, barbershop wastes, and other mixed types of waste streams coming from the local community.