Wild Plant Assessment for Heavy Metal Phytoremediation Potential along the Mafic and Ultramafic Terrain in Northern Pakistan

This study investigates the wild plant species for their phytoremediation potential of macro and trace metals (MTM). For this purpose, soil and wild plant species samples were collected along mafic and ultramafic terrain in the Jijal, Dubair, and Alpuri areas of Kohistan region, northern Pakistan. These samples were analyzed for the concentrations of MTM (Na, K, Ca, Mg, Fe, Mn, Pb, Zn, Cd, Cu, Cr, Ni, and Co) using atomic absorption spectrometer (AAS-PEA-700). Soil showed significant (P < .001) contamination level, while plants had greater variability in metal uptake from the contaminated sites. Plant species such as Selaginella jacquemontii, Rumex hastatus, and Plectranthus rugosus showed multifold enrichment factor (EF) of Fe, Mn, Cr, Ni, and Co as compared to background area. Results revealed that these wild plant species have the ability to uptake and accumulate higher metals concentration. Therefore, these plant species may be used for phytoremediation of metals contaminated soil. However, higher MTM concentrations in the wild plant species could cause environmental hazards in the study area, as selected metals (Fe, Mn, Cr, Ni, Co, and Pb) have toxicological concerns.


Introduction
Owing to toxicity, persistent and bioaccumulative nature, the macro and trace metals (MTMs) contamination represent, one of the most burning threats to soil, plants, human health, and environment [1][2][3][4][5]. Among the MTM, Na, K, Ca, Mg, Fe, Co, Cu, and Zn are essential metals for human health and environment. However, these metals may produce toxicity at their higher concentrations. Whereas, others trace metals (TM) including Pb, Cd, Cr, Ni, and As are extremely toxic even at very low concentrations for living organisms and environment [3,6]. These contaminants may have natural (ore deposits or weathering of parent rocks) and anthropogenic (mining, minerals processing, and fly ash) sources [2,[7][8][9]. Although, in most cases, soil enrichment with MTM is due to the hazardous waste pollution, there are many cases where soil derived from mineralized rocks is naturally enriched with these metals [2,7].
Generally, the mafic and ultramafic terrain are enriched with MTM including Cr, Ni, Mg, Cu, Pb, Zn and Cd; and similarly, the weathered soil of ultramafic terrain (serpentine soil) are also enriched with these metals [1,7]. Mining and waste dumping further accelerated the MTM accumulations in soil ecosystem [2,10,11]. pH, electrical conductivity (EC), and soil organic matter (SOM) are main factors that generally affect the chemistry of these metals in soil and their plant uptake, which may cause environmental problems in the area [1,2].
Accumulations of MTM in soil ecosystem, food safety, and potential health risks are of great concern. Food chain contamination is one of the most important pathways for entry of these metals into the human, animals and other living organism [7,12]. Therefore, various techniques have been used to address MTM-contaminated soil. However, phytoremediation has received considerable attention due to being a best and cost-effective technique for metal contaminated soil reclamations [1,2,7,9,13]. Researchers have given more attention to these kinds of soil for better understanding of metal nature, toxicity, sources, and plant accumulations [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, MTM needs more information to identify the soil contaminations and wild hyperaccumulator plant species along the mafic and ultramafic terrain. Therefore, this study was aimed to elucidate the contamination level via metal enrichment factor (EF), pollution load index (PLI), and bioaccumulation factor (BF). Furthermore, selected wild plant species were evaluated for the phytoremediation ability.

Study Area.
The study area is located in the Kohistan region of northern Pakistan between latitude 34 ∘ 50 to 35 ∘ 06 N and longitude 72 ∘ 43 to 73 ∘ 02 E. It covers approximately a total area of 1800 km 2 with <0.91 Million populations. Indus River and its tributaries (Dubair and Khan Khwars) are the main sources of agriculture irrigation. This region is warm in summer (32.5 ∘ C), except in high altitude areas, and very cold in winter (−2.4 ∘ C) receiving an annual precipitation of 650 mm [14].
Geologically, the area is unique as it is composed of the rocks of three different tectonic settings such as the Kohistan island arc (KIA), the Indus suture zone (ISZ), and the Indian plate (IP) (Figure 1). The ISZ is considered as the contact zone between the KIA in the north and the IP in the south. In the study area, the rocks of these different lithologies vary in composition from mafic and ultramafic igneous rocks belonging to KIA and ISZ to metasedimentary rocks of IP [15]. This region has various types of metallic and nonmetallic mineral deposits. According to Miller et al. [16], the huge deposits of chromite are present within the ultramafic rocks (dunite and peridotite) of the ISZ and the KIA at Jijal, Dubair, and Alpuri areas. Presently, the mining of these chromite deposits is carried out on small scale by the local miners.

Plant Sampling and Analyses.
In the study area, wild plant species were randomly collected, properly labeled, and packed in polyethylene bags. Plant species were identified and taxonomically classified with the help of taxonomist in Botany Department, University of Peshawar, Pakistan. For reference, plants grown on the metasedimentary rocks of IP were also collected about 10 km away from the mafic and ultramafic terrain. All plants were washed and cleaned with tape water, oven dried at 70 ∘ C, and ground into powder with electric grinder. Plant samples of 2.0 g were taken in Pyrex beaker and digested with a mixture of acids (HNO 3 + HClO 4 and aqua regia), according to the method adopted from Ryan et al. [17]. Plant extract was diluted to 50 mL with double distilled water (DDW).

Soil Sampling and Analyses.
Surface soil samples of about 1 Kg, collected from the base of each uprooted plant sample, were properly labeled and packed in polyethylene bags. For reference, soil was also collected from the base of uprooted reference plant species collected from IP. Samples were air dried at ambient temperature for 72 h, homogenized, and sieved through 2 mm mesh for further analyses. Soil < 2 mm fraction was used for physical analyses like pH, EC, and SOM. Next, soil was ground into powder in ball mill to a finer than 75 m sieve size for MTM determination. Physical parameters (pH and EC) were measured according to procedure adopted from Das and Maiti [18], while SOM adopted from Konen et al. [19]. Accurately weighed 0.5 g oven dried soil was digested in Teflon beaker with a mixture of acids (HF + HCl) at 130-140 ∘ C for complete digestion. When acids were completely evaporated, diluted HCl was added, and the solution volume was made with DDW [11].

Data Precision and Accuracy.
Digested plant and soil samples were analyzed for MTM using atomic absorption spectrometer (Perkin Elmer, AAS-PEA-700). For data precision and accuracy, blanks and standard reference soil and plant materials were included in digestion and subsequent analyses. Each sample was digested and analyzed in triplicate, and mean values were taken for further interpretation. Reproducibility of the triplicate samples was found within 95% confidence level. To check accuracy of AAS, standards of all metals were prepared by dilution of 1000 mg/L certified standards solutions Fluka Kamica (Buchs, Switzerland) of corresponding metal ions with DDW and analyzed after every 10 samples. All chemicals used in digestion and analyses were of analytical grade, purchased from Merck.

Pollution Quantification.
Pollution quantification was calculated through EF, PLI, and Plant BF.

Enrichment Factor (EF)
. Enrichment factor ratio was obtained from the MTM concentrations in the study area as reported by Shah et al. [1] and Muhammad et al. [2]:

Pollution Load Index (PLI).
For entire sampling site, PLI has been determined as th root of the product of the EF, accordingly to the equation adopted from Usero et al. [20]: PLI provides a simple comparative means of MTM level in the study area.

Plants Bioaccumulation Factor (BF).
Bioaccumulation factor is defined as the ability of a plant to accumulate MTM concentrations. Bioaccumulation factor was obtained from the ratio of MTM concentrations in plant and soil as reported by Rashed [13]: 2.6. Statistical Analyses. Statistical manipulations ranges, mean, and standard deviation were measured using Excel 2007 (Microsoft Office) and one-way ANOVA and correlation analysis using SPSS (17) statistical software.

Physiochemical Parameters.
Soil pH mean values were found highest in Dubair (7.9), while they were lowest in Alpuri (6.9) of the study area. Electrical conductivity means values were found highest in Alpuri (233 S/cm), while they were lowest in Dubair (157 S/cm). Similarly, SOM was found highest in Alpuri (5.3), while it was lowest in Jijal (3.6) ( Table 1). Low pH and high organic matter may have attributed high soluble metallic ions concentration in soil at Alpuri site. As a result, Alpuri site has higher EC values as compared to other sites of the study area and background site. pH, EC, and SOM mean values of the study area were lower than those reported by Muhammad et al. [2] in soil of the Pb-Zn sulfide terrain, northern Pakistan. The concentrations of Na, K, Ca and Mg in soil ranged from 581 to 13870, 2020 to 8035, 1635 to 37305, and 17125 to 77210 mg/Kg, respectively ( Figure 2). The concentrations of Ca and Mg in soil of these areas were found significantly   ( < .001) higher as compared to the background site. Similarly, Fe and Mn concentrations in soil ranged from 1180 to 16840 and 233 to 689 mg/Kg, respectively ( Figure 2). The concentrations of Cr, Ni, Co and Cu ranged from 60 to 2050, 93 to 2631, 25 to 220 and, 10 to 296 mg/Kg and showed significantly ( < .001) higher concentration as compared to the background site ( Figure 2). In the study area, Cr and Ni mean concentrations were found in the order of Jijal > Alpuri > Dubair site, while those of Co and Cu were in the orders of Alpuri > Jijal > Dubair site and Dubair > Jijal > Alpuri site, respectively ( Figure 2). Multifold higher concentrations of these metals in Jijal site can be attributed to the local mafic and ultramafic terrain and chromite mining [7]. Similarly, Pb, Zn, and Cd concentrations ranged from 17 to 240, 88 to 170, and 1 to 3 mg/Kg, respectively ( Figure 2). The concentrations of Pb, Zn, and Cd were almost similar to those of the background site. However, Pb and Zn concentrations were found higher than those reported by Yang et al. [9] in soil of mafic and ultramafic rocks in Mingora and Kabal areas. Figure 3 showed the soil EF of MTM collected along mafic and ultramafic terrain in the Kohistan region. Results showed that EF > 1 for most of MTM such as Mg, Fe, and Mn in soil of the Jijal, Dubair, and Alpuri sites. However, Ca and Mg were enriched in soil of Jijal and Dubair, while those of Alpuri site depleted as compared to background site.

Metal Enrichment Factor (EF) and Pollution Load Index (PLI).
The values of EF for K and Na were found depleted in soil of these sites (Figure 3). Among the TM (Cr and Ni), EF > 3.5, while Co, Cu and Cd showed EF > 1 in soil of the three sites ( Figure 3). The Pb showed enrichment in soil of the Dubair and Alpuri areas and depletion in the Jijal area, while Zn revealed depletion in soil of all these sites. Based on Muller [21], classification in the Dubair site, soil contaminations with Mg, Cr and Ni were classified as moderate to strongly polluted. In the Jijal site, soil contaminations with Ca, Mg, and Co were classified as moderately polluted and Cr and Ni as strongly polluted. Similarly, Alpuri site soil with Co as moderate polluted and Cr and Ni were classified as strongly polluted. In the study area, soil showed that EF values for Mg, Cr, Ni, and Co were higher than those reported by Muhammad et al. [2] for these metals in soil of the Pb-Zn sulfide terrain, northern Pakistan. Values of PLI > 1 in three selected sites were in the order of Jijal > Dubair > Alpuri site ( Figure 3). PLI values showed that Jijal site was highly contaminated which may be attributed due to chromite ore deposits and mining. Pollution load index values were found lower than that reported by Muhammad et al. [2] for soil in the Kohistan region, northern Pakistan and that by Rashed [13] for tailing deposit in Southeast Egypt. Table 2 summarizes the concentrations of MTM in plants along the mafic and ultramafic terrain and background sites. The concentrations of Na, K, Ca and Mg in plants ranged from 221 to 3257, 786 to 16044, 1848 to 40915, and 1293 to 46233 mg/Kg, respectively ( Table 2). Plant species that showed highest concentration of metals were Rumex hastatus (Na), Athyrium schimperi (K), Debregeasia salicifolia (Ca), and Plectranthus rugosus (Mg). Iron concentrations ranged from 114 to 11766 mg/Kg with highest in Olea ferruginea (Table 2). Plant species growing along mafic and ultramafic terrain are enriched with Fe [22]. Iron is one the essential nutrients in plant cell wall, chlorophyll, and protein. Deficiencies symptoms include necrotic lesions and interveinal chlorosis [23]. However, toxic effects include injured or necrotic spots on leaves and reduction of productivity [24,25]. Similarly, Mn concentrations ranged from 125 to 3154 mg/Kg with their highest in Rumex hastatus ( Table 2). Manganese toxicity affects the absorption, translocation, enzyme activity, and utilization of metals (Ca, Fe, Mg, and P), necrotic leaf spots, chlorosis in leaves, and reduction in growth and productivity [26,27].
Lead concentrations ranged from 6 to 17 mg/Kg with a highest concentration in Daphne mucronata and lowest in Olea ferruganea (Table 2). Generally, the Pb concentrations ranging from 2 to 6 mg/Kg are sufficient, while safe agriculture limit is 10 mg/Kg [38]. Lead concentrations in 70% of selected plant species exceeded the limits causing phytotoxicity. However, the Pb concentrations in plant species were found much lower than those reported by Sagiroglu et al. [39] growing (up to 1985 mg/Kg) in the Keban mining district, Turkey. The concentrations of Zn ranged from 6 to 54 mg/Kg with a highest in Fimbristylis dichotoma and lowest in Cirsium vulgarea (Table 2). Toxic limit of Zn in majority of the plant species is 500 mg/Kg [24]. However, plants with Zn < 20 mg/Kg are considered to be Zn deficient [40]. Therefore, majority of plant species were considered to be deficient in Zn contents. Zinc is one of the required metals in a specific amount; however, its high concentration may produce toxic effects in living organism [1,2]. Cadmium concentrations ranged from 0 to 2 mg/Kg with a highest in Berberis lyceum and lowest in Cirsium vulgarea (Table 2). Cadmium concentrations in majority of plant species were equal to that of background site. However, 20% of the plant species showed Cd concentrations below detection limits. Cadmium may cause toxicity in majority of the plant species when its concentration is above 2 mg/Kg [24]. In plants, Cd accumulation causes growth inhibition, browning of root tips, chlorosis, water and nutrient uptake, reduction in photosynthesis, DNA repair inhibition and finally death [36]. Considering the maximum permissible limits of heavy metal in plants, this study revealed that wild flora has accumulated higher concentrations of the Fe, Mn, Cu, and Pb. Therefore, these metals may have adverse effects on flora of the study area [7,26,27,41]. Similarly, Ni and Cr higher concentrations in plant species may lead to phytotoxicity. Enrichment of these metals in wild flora could be a serious threat to community of the area [1,7,9,10]. However, Mewis et al. [42] reported that for detoxification of metal stress and competition the accumulator plant species activate the defense mechanism. Table 3 Table 3. This multifold higher EF of MTM in plants of mafic and ultramafic terrain as compared to that of background site can be attributed due to the serpentine soil which is generally rich in these metals especially Fe, Mg, Cr, and Ni [7,9]. Plant BF values were highest in Dodonaea viscose, Fimbristylis dichotoma (K = 2.0), Sarcococca saligna (Ca = 2.9), Debregeasia salicifolia (Mn = 2.5), and Selaginella jacquemontii (Ni = 1.3, Cu = 2.0)as shown in Table 3.

Statistical Analyses.
One-way ANOVA results revealed that some of metals (Ca, Mg, Cr, Ni, Co, and Cu) have significantly ( < .001) higher concentrations in the Dubair, Jijal, and Alpuri sites of mafic and ultramafic terrain as compared to the background site. These multifold higher concentrations of metals can be attributed to the mafic and ultramafic terrain and chromite mining in the area [1,7,9,10]. Inter-relationship of physiochemical parameters in soil of the study area is summarized in Table 4. Physiochemical parameters showed that some pairs in soil have higher correlations such as pH-Mg ( = 0.692), Fe-Co ( = 0.514), Cr-Ni ( = 0.565), and Cr-Co ( = 0.504) as shown in Table 4. Similarly, in plants, some elemental pairs also showed higher correlation like Na-K ( = 0.541), Mn-Ni ( = 0.533), Cr-Ni ( = 0.516), Cr-Co ( = 0.522), and Ni-Co ( = 0.545)    as shown in Table 5. Interelement relationships are providing interesting information on elements sources and pathways [43]. Correlation metrics showed these relationships were not highly significant in soil and plants, which may be due to the different properties of soil and the physiologies of plant species [7,9]. These weak correlations of physiochemical parameters in soil and plants can be attributed to the variable concentrations of these parameters in soil of the area and variation in plant uptake [2,10].

Conclusions
In the study area, natural processes such as weathering and erosion and anthropogenic processes including mining have caused metals contamination in soil and plants.
Pollution indices (EF and PLI) suggested that the Jijal soil was strongly polluted (EF > 3.5) with Ni and Cr. Generally, some of the selected wild plant species showed higher metal accumulation. Plant species that revealed significant higher