Accumulation and Translocation of Toxic Elements From Contaminated Soils, Nigeria: Implications for Phytomining and Hazards to Humans

Soil pollution by heavy metals and their health effect on human are pressing issues of the environment caused by human activities. Plant’s accumulation and translocation potentials were investigated to determine their suitability for phytoremedial purposes, their ability to serve as reservoir for recovery of additional economic amount of metals and the potential of the edibles/vegetables to cause harm to humans when consumed. The plant and soil samples were collected, prepared, digested in acid mixture of H 2 O 2 and HNO 3 for plants and Li 2 B 4 O 7 - LiBO 2 for soils and were analysed. The analyses were carried out to determine the concentration of these metals in soil, their accumulation and translocation in plant parts. The data acquired were evaluated using bioconcentration (BCF), translocation factor (TF), bioaccumulation coecient (BAC), metal uptake eciency (ME%) and hierarchical cluster analysis to determine hyperaccumulators, phytoextractors, phytostabilizers, metal source plants and metals that could be toxic to humans through intake of roots, grains/seeds, fruits and leaves as vegetables. ANOVA analysis revealed that the data were signicant at p <0.05. Correlation and cluster analyses were employed to understand the relationships between variables determined. From this study, CA, COA and LA were hyperaccumulators of Co at various points. Arsenic has only phytostabilizers. COA and LA were phytostabilizers of Cd while Sida acuta was the only phytoextractor. Chromium, Co and Cd have prospect of being phytomined from some of the plants. Vegetables/edibles values in shoots and leaves were above permissible levels for Cr, Co and Cd. The metal uptake ecacy (%) were in this order Co (28.99 to 89.08) > Cd (21.74 to 50.96) >Cr (22.90 to 49.06) > and As (9.65 to 39.19). in by correlation. Pairs of variables with strong correlation reected unhindered ability to proportionally translocate soil-leaf from root to shoot. This is irrespective of whether Cd is low in soil Two clusters were revealed. Cluster one is an association between root-shoot, soil-leaf and soil-root. The strongest similarity in the cluster was between root-shoot, followed by soil-leaf and lastly soil-root. In cluster two, BCF-BAC and BCF-EFone were extracted. At a greater distance, TFone and TF were linked to this cluster. From the two clusters, root-shoot, soil-leaf, BCF-BAC and BCF-EFone, the strength of similarities displayed decreases in this order.


Introduction
Trace (some toxic) metals occur naturally in all soils in low concentrations. However, the level of these metals have greatly increased due to human activities such as wastes generation (domestic and municipal), industries such as textile, battery and auto workshops, mining activities, agriculture etc The use of plants is an economically and ecologically friendly alternative to conventional techniques for decontamination/stabilization of toxic metal polluted sites (Suman et al., 2018). Plants have inherent ability to absorb toxic substances, including trace elements along with nutrient materials from the soil. The non-essential, toxic metals are known to cause plant toxicity and occur in various soluble and particulate materials which also affect their bioavailability and mobility. These toxic metals have tendency to accumulate in soft tissues of living organisms (Rangnekar et al, 2013). Remediation of metal polluted soil is still a global challenge to both administrators and researchers due to the non-degradability of these metals in the environment. Phytoremediation, which is a harmless procedure that respect the environment and a biological technology using plants to remove contaminants from soil, has been on the search light due to its cost effectiveness and environmental harmonies ( Hyperaccumulator plants have the capacity to absorb and translocate metals from their roots to the shoots and equally have high tolerance for these metals without phytotoxicity symptoms (Suman et al.,2018). Hyperaccumulators require enormous energy for the mechanisms required to adapt to high metal concentrations in their tissues (Lago-Vila etal, 2015). Plants with potential for phytoextraction are capable of growing in soils with high degree of metals because of their large radicular system, high level of biomass production and are able to accumulate high concentration of metals in shoot (Lago-Vila etal, 2015). Phytoextraction means metal content reduction in soil, translocation to above-ground tissues and this technique is used to reduce damage caused to the soil. In cases where phytoextraction is not feasible, phytostabilization can be adopted. This means immobilizing the metals in the soil, stabilizing/detoxifying contaminated soils and thereby reducing the ow or distribution of contaminants into the environment (Suman et al.,2018). In phytostabilization method, the plants do not accumulate metals in their shoots. This also minimizes the risk in terms of food safety for vegetables and roots, seed/grains, fruits that are edible. Hyperaccumulator plants can take up concentrations of > 10,000.00mg/kg of Zn or Mn; 1,000.00mg/kg and above of As, Co, Cr, Cu, Ni, Pb, Sb, Se and Ti or 100mg/kg of Cd (Verbruggen et al, 2009).According to Lorestani et al., (2011), hyperaccumulator plants must also have TF or EF>1 in addition to above criteria.
This research is hinged on the hypotheses that (i) the native plants are more adapted to the soil and pollutants (ii) these plants can accumulate and translocate metals from the polluted soil to the aboveground tissues (iii) some of these plants have the potential for removal and stabilization of these metal pollutants (iv) because of effective and e cient translocation to shoots and leaves, some plants could serve as reservoir for extraction of these metals (v) some of these plants if consumed by humans and animals could pose health hazards.
The objectives of this study are therefore, to (i) identify and determine metal concentrations in native plants and edibles/vegetables in study soil (ii) compare metal concentrations in soils and plants tissues (iii) determine the metal accumulation and translocation factors from soil to the aerial biomass of the plants (iv) evaluate their potentials for hyperaccumulation, phytoextraction and phytostabilization, (v) highlight plants with both phytomining potential and harmful effects if consumed in excess by humans.
Phytoremediation and phytomining are therefore needed to address environmental and socio-economic concern in dump sites. There is therefore, the need to identify plants which remove metals from soil in large amount, after which valuable metals can be recovered economically and taking note of those with potential to cause harm when consumed. Over time, land is made available for other socio-economic uses once the polluted level has been reduced to minimal and acceptable levels.

Materials And Methods
The area under study is a metropolitan City. The sources of wastes in the area are enormous and include municipal and domestic wastes, hospital wastes, auto body and battery repair workshop wastes, textile and dying activities, agriculture and agricultural produce etc. Waste sites were surveyed and located for sample collection. The study area is between longitude 7 o 10'0''E and 7 o 12'0''E and latitude 7 o 28'0''N and 7 o 31'0''N.

2.1
Plant sample collection The analytical range of elements by EDX3600B metal analyser is between (Mg, Z = 12) and Uranium (U, Z = 92) with high resolution and accuracy of 0.05% and detection limit of 0.01ppm.The pure silver sample was used to calibrate the instrument before use (Aksoy et al. 2014).
Each sample of plants and soil were digested in replicates for consistency of results. Blanks were run in replicates to check the precision of the method with each set of sample (Rangnekar etal, 2013). The standard reference materials for Cr, Cd, As and Co (Merck-E grade, Germany) were used for calibration and quality assurance. Analytical data, the quality of metals were ensured through replicate analysis of standard reference samples. The obtained results were within ±2.05% to 2.85% of certi ed values. The mean recovery of 98% to 99.95% was achieved for different metals.  Hierarchical cluster analysis was used to group similar variables. Evaluations of similarities were based on the average linkage between groups. Cluster analysis was performed on the normalized data sets by means of the Ward's method, using squared Euclidean distances as a measure of similarity  (Table 1b). The average concentration of Cr in soils from study area was higher than the uncontaminated references. The elevated value of Cr observed may be due to pollution from the dumps. In a similar study, 1366 ± 49mg/kg to 2689 ± 82mg/kg of Cr were recorded in soil (Lago-Vila et al., 2015). The accumulation content of Cr in roots varied from 2.00mg/kg in Amaranthus hybridus in site 3 to 120.00mg/kg in Laportea aestuans in site 7. The standard deviation and error were 26.70 and 5.14 respectively (Table  1b and Figure 2a). Compared to the soil concentration of Cr in study area, the level of Cr in roots were higher in three locations. On the average, Cr concentrations in soil were all higher than in roots (Table1a and Figure 2a). In another study, 19.63 ± 1.79mg/kg and 29.49 ± 3.40mg/kg were accumulated in roots of Festuca rubra L. and Juncus sp.L respectively (Lago-Vila et al, 2015).   shoot be > 50mg/kg (ii) that the concentration of Cr in aerial biomass is 10-500 times greater than in the non-metallophyte (0.2-5mg/kg of Cr), (iii) TF or EF>1 and (iv) that Cr concentration in the shoot is greater than in the roots. Based on (i, iii and iv) de nitions, Amaranthus viridis at site 7 (100mg/kg), Abelmoschus esculentus site 7 (100mg/kg); Laportea aestuans (102.00mg/kg of Cr) and Sida acuta (80.00mg/kg of Cr) at site 1 were all hyperaccumulators of Cr. The intake of Cr by the leaves was generally lower compared with the stem and root (   at 0.05 level is strong but at 0.01, it is stronger. It means that between the pair of variables, there exist a good connectivity to the extent that of all accumulations, more than half were translocated to avoe plants parts (Table 1d).The negative correlation (soil-shoot) indicate accumulation but reduced biomass due to soil contamination and low shoot tolerance to Cr.
The dendogram consist of two clusters. Cluster one is a union of TF-TFone, shoot-BAC, leaf-EFone and soil alone. Soil in this cluster, shows the greatest dissimilarity to all the variables. This by implication means that the concentrations of metals in soils were not proportional to that in plants part. In the same cluster, TF-TFone, shoot-BAC and leaf-EFone showed decreasing similarities in that order. The pair of variables with the greatest similarity shows that the metal content in the pair were not different in terms of their concentrations and what was translocated. Cluster two is an association of only root-BCF. The highest degree of dissimilarity is shown in this cluster except the soil. This dissimilarity suggests that the metal content of one variable has no relationship with the content of another variable in the same pair (Figure 2c).
Co concentrations in soil samples range from 1536.67mg/kg in site 1 to 3240.47mg/kg in site 6 (Table 2a and b). Co in uncontaminated soil is 7.00mg/kg (Kabata-Pendias, 2001); 14.90mg/kg (Bradford et al., 1996) and 40.00mg/kg (Papadopoulos et al, 2015). The concentrations cited are way below the study area concentration. This could suggest soil pollution from the wastes dumps in the area (Table 2a). The root accumulated the highest concentration of Co (612.00mg/kg) in Colocasia asculenta and the least (0.00mg/kg) in AH, AV, AE, CM, LA and PA at various points (Table 2a and b).
The shoot on the other hand recorded the highest accumulation of Co (1215.0mg/kg) in Laportea aestuans and the least value of 0.00mg/kg in Amaranthus viridis at location 7. Over all, the shoot accumulated more Co than any other plant tissue (Table 2b).This shows that Co is readily absorbed by roots and transported to the shoot. Sometimes also, Co can be accumulated higher in shoots even though its concentration in soil is low (Ciura et al., 2005   The BCF for Co were all < 1 in the study area. This suggests limited accumulation of Co by the roots from the soil. This is an indication that the roots were excluders of Co (Table 2a and b). Interestingly too, the translocation factor of Co was very high in some locations. Translocation factor ranged from 0.23 in Laportea aestuans to as high as 24.47 in Corchorus aestuans (Tables 2a, 2b and Figure 3b). The high TF could suggest that these two plants and few others at one location each may be suitable source for mining of Co. The BAC for plants were all < 1. The root to leaf translocation was > 1 (in three sampled plants at three locations). Soils to leaf accumulations were all < 1 (Table 2a and Figure 3b). These observations show high translocation but low accumulation in these respective tissues.
The TFs in study area were contrary to TFs of < 1 recorded for Festuca and Juncus. The current result is also in contrast to BCF recorded in roots of Festuca and Juncus which were 1.   (Table 2c).
Two clusters were extracted. The BCF, EFone, TFone, BAC, TF and root showed the same level of similarities and displayed the strongest similarities in cluster one. Also, in cluster one, shoot-leaf showed stronger similarity but not evidenced in the correlation. The more the degree of similarity, the more the likely hood of the variable to retain same concentration of metals in their tissues. Cluster two consists of only the soil. It suggests the soil has no relationship in terms of its concentration and what was accumulated in other parts of the plants (Figure 3c).
The As in soil from study area ranged from 6.27 to 44.00mg/kg. According to Kabata-Pendias & Pendias (1992; 2001), the limit for As in unpolluted soil is 05 to 20mg/kg and 5.00mg/kg (Bowen, 1979). The background concentration of major and trace elements in California soils is 0.80mg/kg (Bradford et al, 1996). The range observed from the study area clearly showed elevated levels of As in sampled soils due to waste dumps (Tables 4a &b). The highest content of As in roots (60.00mg/kg) was recorded in Abelmoschus esculentus in site 4. The lowest concentration of As (1.00mg/kg) in roots was found in Colocasia asculenta (Table 4a,4b and Figure 4a). As accumulated in shoot varied from 1.00mg/kg to 52.00mg/kg. In leaves also, As recorded was between 1.00mg/kg to 9.00mg/kg (Figure 5a). The average of this range in leaves is higher than 0. . Excess of it in edibles/vegetables may not be safe for consumption. On the whole, As accumulation was highest in roots, followed by shoot and lastly leaf (Table 4a and Figure 4a). In another study, the roots of Rumex   (Table 4a and Figure 4a).

The BCF, TF and BAC for roots, shoots and leaves
The soils to roots BCF for Amaranthus hybridus (sites 1, 3 &5), Abelmoschus esculentus, Corchorus aestuans (site 1), Laportea aestuans, Physalis angulata (site 2), Sida acuta (site 1 & 3) and Zea mays (site 3) were all > 1. The corresponding TFs were < 1 except Amaranthus hybridus that is > 1 in site 1 (Table 4a and Figure 4a). Only Amaranthus hybridus (site 1) can serve as both phytoextractor and stabilizer of As. The rest plants were only suitable for phytostabilization of As. The TFs < 1 indicates preference of these plants in storing and accumulation of As in its roots. This property of most of these plants make them not suitable for phytominig and very likely that edibles/vegetables may not accumulate above permissible level of As. Also, BCFs > 1 indicate that more As is accumulated in plants than in soil as seen among few plants (Nonglak et al, 2011;Hosman et al, 2017). The BCF in the study varied from 0.01 to 11.99. The TF from root to shoot ranged from 0.08 to 2.50. The BAC ranged from 0.07 to 5.24. The root to leaf translocation varied from 0.02 to 2.25 (Table 4a and Figure 4b). While the ability of these plants to accumulate As in root and shoot were signi cant in AE, COE, LA, PA,SA and ZM, the study plants also showed good degree of root to leaf translocation in CM and COA. The recorded TFs from the study were lower than 2.4 to 2.8 in A. dubius. The BCF observed was higher (on average) than the 1.0 to 5.7 BCF recorded in A. dubius (Mellem et al, 2012). The likely hood of harm from consumption of any of the edibles/vegetables due to As is remote. The capacity of the plants for phytomining of As was also not attractive. The metal intake e ciency for As varied from 9.65 to 39.19%. This value is the least among study metals (Table 4a).   (Table 4b).
Two clusters were extracted. Cluster one consist of TF-TFone alone. The similarity between the pair is very strong. In cluster two, , the strongest similarity was between shoot-BAC. Lesser similarity was showed between soil-root. Also, between BCF-EFone, and soil-leaf were other similarities in decreasing strength. The greatest dissimilarity was observed between root-BAC ( Figure 4c). These similarities were also revealed in the correlation (Table 4b) Cadmium (Cd).
The concentrations of Cd recorded in soils from this study were 1.53mg/kg to 16.67mg/kg (Tables 5a and b). According to Rudnick and Gao, 2003, Cd limit in uncontaminated soil is 0.10mg/kg. This limit is in contrast and lower than the values obtained from this study (Table 5a). The European commission, Luxembourg Council directive, 1986 limit of Cd in soil is 0.20mg/kg. The obtained range from the study is higher than this value (Table 5b).
Based on other studies such as Papadopoulos et al, (2015); Bradford et al, (1996); the soil contents of Cd were 1.40mg/kg and 0.36mg/kg respectively. The observed range from this study is also higher than these benchmarks.
The accumulated Cd ranged from 1.00mg/kg to 18.00mg/kg in sampled roots. The shoots recorded 1.00mg/kg to 10.00mg/kg of Cd. The leaves on the other hand revealed accumulated range of 1.00mg/kg to 5.00mg/kg. These results showed that more Cd was accumulated in roots than the shoots and leaves (Table 5a and (Table 5a and Figure 5a). This relatively higher than unpolluted soil limit in Cd concentration may not be unconnected with the pollution from the dumps.    (Table 5a). The BAC, that is soil to shoot accumulation ranged from 0.06 to 5.23. This also implies that these plants have potential for translocation and accumulation of Cd above-ground tissues. This indicates that Cd is easily absorbed by roots and transported to the shoots (Nazir et al, 2011). The roots to leaves TF and soil to leaf accumulation were in almost all of the plants less than < 1 except in four plants at various locations ( Figure   5b). This implied ine ciency in translocation and accumulation abilities of the leaves compared to the shoots but even at that, the leaves have reasonable level of Cd. Four plants recorded EF>1, and indication of enrichment of Cd in the sampled plants. The metal uptake e cacy of Cd ranged from 21.74 to 50.96%. This value is slightly higher than that of Cr while Co ME% value is the highest (28.99 to 89.08%). At the signi cant level of 0.01, only soil-BAC (r=-.799) and root-TFone (r=-.832) recorded very strong correlations. This negative correlation indicates active Cd accumulation but reduction in biomass due to soil contamination and low plant tolerance to Cd (Poniedzialek et al.,2010) At P< 0.05 level, soil-leaf (r=.636), and root-shoot(r=.720) displayed strong correlation. Pairs of variables with strong correlation re ected unhindered ability to proportionally accumulate and translocate as much metal from soil-leaf and from root to shoot. This is irrespective of whether Cd is low in soil (Table 5c).
Two clusters were revealed. Cluster one is an association between root-shoot, soil-leaf and soil-root. The strongest similarity in the cluster was between root-shoot, followed by soil-leaf and lastly soil-root. In cluster two, BCF-BAC and BCF-EFone were extracted. At a greater distance, TFone and TF were linked to this cluster. From the two clusters, root-shoot, soil-leaf, BCF-BAC and BCF-EFone, the strength of similarities displayed decreases in this order. Root-shoot showed uninhibited mobility of Cd. Soil-leaf also showed a lesser mobility from soil-leaf. Followed by these two was BCF-BAC displaying good degree of Cd mobility (Figure 5c).
Conclusively, this investigation has shown that Co has three hyperaccumulator plants. As has no hyperaccumulator and phytoextractor but phytostabilizers. Cd has only Sida acuta as phytoextractor at site 3 and COA and LA as phytostabilizers. AH (sites 1 and 4), AE, LA (site 1) were all phytoextractors of Cd. Few other plants were also phytostabilizers of Cd. Cr, Co and Cd can be phytomined from some of the plants while prospect for mining As from the plants is limited. Edible parts/vegetables from some of the plants may have excess of Cr, Co and Cd. It is strongly recommended that these edibles/ vegetables should not be consumed by humans. The metal uptake e cacy (%) were in the order Co (   Average concentration (mg/kg) of As in soil and plant tissues Average As variation in plant tissues Cluster analysis of variables Cluster analysis of the variables