Assessing the heavy metal transfer and translocation by Sida acuta and Pennisetum purpureum for phytoremediation purposes

Abstract

Field sampling of two prevalent weeds (Sida acuta and Pennisetum purpureum) growing on cement-polluted soil was carried out seasonally to assess the root and leaf contents of Pb, Cu, Cr, Cd and Zn. Concentrations of heavy metal in corresponding soils (total and bioavailable) were also determined and data generated were used to calculate the respective transfer factors (TF) and translocation indices (Tis) of metals in the weed species. The results of the TFs and Tis were used to evaluate the potential ability of these weed species to accumulate heavy metals in their tissues and suitability or phytoremediation. Findings indicated that, S. acuta and P. purpureum are suitable for phytostabilization of Cr, Cd and Zn in cement-polluted soil while Cr and Cd can be phytoextracted by these two weed species from cement-polluted soil.

Headnote

Abstract

Field sampling of two prevalent weeds (Sida acuta and Pennisetum purpureum) growing on cement-polluted soil was carried out seasonally to assess the root and leaf contents of Pb, Cu, Cr, Cd and Zn. Concentrations of heavy metal in corresponding soils (total and bioavailable) were also determined and data generated were used to calculate the respective transfer factors (TFs) and translocation indices (Tis) of metals in the weed species. The results of the TFs and Tis were used to evaluate the potential ability of these weed species to accumulate heavy metals in their tissues and suitability or phytoremediation. Findings indicated that S. acuta and P. purpureum are suitable for phytostabilization of Cr, Cd and Zn in cement-polluted soil while Cr and Cd can be phytoextracted by these two weed species from cement-polluted soil.

Keywords: Accumulator plant, phytoremediation, weeds species, phytoextractor, transfer factor, translocation index.

(ProQuest: ... denotes formulae omitted.)

1. Introduction

Plants have developed the process for the acquisition of relatively less abundant micronutrients such as Cu, Zn, Ni and Mn from the soil. These essential micronutrients are also highly reactive and potentially toxic to plants [27]; hence the uptake, transport and accumulation of these micronutrients are thereby highly coordinated and regulated by necessity in plants. Many of these micronutrients of plants are also heavy metals which makes the contamination of the soil environment a problem through either the accumulation of essential micronutrients (Zn, Mn, Cu and Ni) in high amount or metals that can acts as analogues or replacements (such as Cd, Pb or Hg) [27]. Therefore, plants have evolved potential mechanisms like detoxification of metals in the roots and true tolerance to heavy metal stress which may involve primarily the avoidance of build-up of toxic concentrations at sensitive sites [26, 27]. Also, escaping from heavy metal toxicity by reducing or excluding the uptake of heavy metals may be another strategy for plants to resist metal toxicity [8].

Some plants are also tolerant to specific metals and are able to accumulate such metals in substantial quantities in their biomass due to their effective uptakes and translocations. Such plants with effective transfer and translocation concentrate metals in above-ground parts from low to great soil concentrations and are regarded as accumulators [8]. According to Nan et al. [31], the uptake and transfer of heavy metals from soil to plants is a process of significant importance and determining of transfer factor (TF) and translocation index (Ti) have been considered as a key parameter to assess the availability of elements in soil and hyper-accumulation capacity of the plants. So a detailed study on the metal transfer from soils to plants and translocation to aerial parts could possibly shed more light on the metal accumulation potentials of plants species and their phytoremediation potentials. D'Souza et al. [13] stated that all plants with phytoremedial potentials are not alike; some may be metal-specific while others may perform well for metals in combination. But the phytoremedial potential of plants is reported to be influenced by the mobility and availability of heavy metals in soil and plants, and thus the transfer factor (TF) (ratio of bioavailable metal content in root to that in soil) and translocation index (Ti) (ratio of metal content in leaves to that in root) can be used to assess the accumulation potential and hence the phytoremedial potential of plants [8, 17, 43]. And a critical limitation to the study of phytoremediation is that most studies are carried out ex situ in pots and hydroponic [14, 45,] without assessing the potentials on-field and even inbetween seasons. Native or wild plants should be preferred for phytoremediation according to Antonsiewicz et al. [6] and Yoon et al. [44] because they are often better in terms of survival, growth and reproduction under environmental stress and require no agronomic inputs [14]. Therefore, this study aimed at assessing the transfer and translocation of heavy metals in native S. acuta and P. purpureum growing on soil in the environment of a mega cement factory. The seasonal assessment of TFs and Tis in the two weed species were used to evaluate possible phytoremedial potentials of heavy metals.

2. Material and Methods

The study was carried out around the factory plant of the Lafarge-Cement WAPCO in Sagamu, southwestern Nigeria (6°50' and 7°00' N; 3°45' and 4°00' E) (Fig. 1). This cement facility has been in full operation for the past 3 decades and Ogunkunle and Fatoba [33] have reported significant pollution of this soil with heavy metals. The area stands on a low-lying gently undulating terrain with altitude ranging between 30 and 61 m above sea level. The area is characterized by high annual temperature, high rainfall, high evapo-transpiration and high relative humidity which make it to be classified as humid tropical region [4].

The soil type of Sagamu is Oxic Haplustults according to USD A Soil Taxonomy [42] and Ferric Acrisols according to FAO System of Classification [15]. The climate is classified as humid tropical climatic zone [1] and controlled by the Tropical Maritime and Tropical Continental air masses [7]. The geology of Sagamu comprises of sedimentary rocks which consists of Abeokuta formation [2]. It is reported to be highly fossiliferous and consists of deposits of limestone [18, 22], sand with sandstone, siltstone, clay, mudstone and shale interbed. According to Agbaje [3], the basal conglomerate of Abeokuta formation has poorly rounded quartz pebbles with a silicified and ferruginous sandstone matrix.

2.1 Sampling method

Topsoil (0-15 cm) at different locations within a radius of 3km of factory was randomly collected between August and October and between November to December of 2011 and 2012 respectively for rainy and dry seasons. Thirty-eight (38) and twenty-six (26) representative soil samples were collected randomly around the factory in wet seasons and dry seasons respectively, each sample representing a composite sample of 3 subsamples. The soil samples were airdried in the laboratory to constant weight; after which the samples were grounded and passed through 2mm sieve to get fine fractions for chemical analyses. Two native species (Pennisetum purpureum and Sida acuta) were used for the study due to their relative abundance around the factory. At every point where soil sample was collected, matured plants (P. purpureum and S. acuta) were harvested, cleaned with tap water, sorted into roots and leaves while the reproductive parts and the stems were discarded. The plant samples were properly air-dried, powdered and well-tagged for chemical analysis.

2.2 Chemical and statistical analyses

The soil pH was measured in 1:2.5 (soil: water) suspension [38] with glass electrode pH meter (PHS3C model) while the soil organic matter content was determined by Loss on Ignition method of Reddy et al., [37]. In order to assess the mobility and potential phyto-availability of the metals in the soil, the exchangeable/bioavailable fractions of the metals was estimated. The methods of Tessier et al. [41] and Chojnacka et al. [12] were adapted to determine the exchangeable/bioavailable fraction by extraction of bioavailable metal from soil with 8ml of 0.5M Magnesium chloride (Scharlau, Spain) at pH 7.0 for 20mins in a test tube with continuous agitation using a motorized shaker JK VXR S17 at 1000 rpm. Digestion of plant samples (roots and leaves) was performed using concentrated HN03 (70%, SigmaAldrich Corp, Germany) [21]. The solutions from the extraction and digestion processes were filtered using Whatman No 42 filter paper, diluted with deionized water to make up 25 ml and analyzed for Pb, Cu, Cr, Cd and Zn by Atomic Absorption Spectrophotometry method using AAS Peckin Elmer A Analyst 200. Quality assurance and control were assessed using reference materials (CRM 193 BG 310-Meadow soil and IAEA 359-cabbage), duplicates and blanks method [9]. Recovery percentage of the reference materials was within the range of 96% to 127-9% and 80% tol05% respectively for the soil and plants. The AAS was calibrated for each element determination using series of working standard solutions and the obtained calibration graphs of absorbance against standard concentrations were excellent with correlation coefficients r2>0-980.

Analysis of Variance (ANOVA) was used to identify the significance of differences of the concentration of each element in the soil and plant samples. A multiple separation of means was determined by the Duncan Multiple Range Test (DMRT) and significance level of P<0.05 was used throughout the study. Statistical analysis was carried out using the SPSS 16 version at P<0.05 while Origin 7 software was employed to present the figures.

Transfer factor (TF) for the heavy metals was calculated using the method of Chojnacka et al. [12] and Prabu [35].

...Eq. 1

Cp is the root/leaves metal content and Cts is the exchangeable metal content in the soil. According to Mendez and Maier [29] and Garba et al. [14], TF>1 refers to plant as phytostabilizer.

Translocation index (Ti) for the heavy metals was determined according to Ghosh and Singh [19] and Kidd et al. [25].

... Eq. 2

Cpl is the leaves metal content and Cpr is the root metal content and according to Garba et al. [14] and Baker [8], Ti>l denotes accumulator plant while Ti<l denotes excluder plant.

3. Results and Discussion

3.1 Distribution of total and bioavailable metal contents of the soil around the cement factory.

The distributions of total metal contents of the soil for the wet (mean of two wet seasons) and dry (mean of two dry seasons) seasons are presented in Table 1. In the wet season, heavy metal contents of the soil were significantly high; Pb has mean value of 469-2 mg/kg, Cu=404-4 mg/kg, Cr=185-2 mg/kg, Cd=298-9 mg.kg and Zn had a mean value of 168.1 mg/kg (Table 1). There was great variability in the metal contents of the soil in the wet season especially for Cu and Cd, thereby indicating the presence of outliner values. The high values in the wet season indicated that the soil was highly polluted as reported earlier by Ogunkunle and Fatoba [32] and Ogunkunle and Fatoba [33]. This pollution level was attributed to the activities of cement production in the study area by these authors. Significant reductions at P<0-05 were observed in the metal contents of the soil in the dry season especially in soil contents Pb, Cu and Cd. The main reason for the reduction was the adoption of filter bay dust collection as air pollution control in 2011 which reduced the dust emission level to less than 10mg/m3 [28]. Another adducible reason could be the leaching of metals into the sub-soil layers by rainfall as reduction in metal concentrations in soils over time may occur due to breakdown and leaching [23]. And this is explained the significant reduction in total soil Cd in this study due to its low potential to adsorb to soil and organic matter [30], thereby readily leached into sub-soil.

Bioavailable metal contents extracted with MgCl2 from the soil were presented Table 2. The soil around the cement factory was slightly acidic (pH=6-5) in the wet season whereas in the dry season, the soil tended towards alkalinity with a mean pH value of 7-6. The soil was rich in organic matter content with mean value of organic matter content of 5-6% in the wet season and significant increase was recorded in the dry season to 11-7% at PO-05. Organic matter forms complexes with metals in the soil, so that it can either reduce mobility or increase bioavailability of metals when the complexes are soluble in soil solution [20]. Extractable Pb in the wet season was 4-% of the total metal content of the soil, 1-5% of Cu was extracted, 57% of Cr, 0-8% of Cd while 4-5% of Zn was extracted with MgCl2. Significant increase (P<0-05) was observed in the amount of all the extractable bioavailable metals of the soil in relation to total metal concentrations in the dry season except for Zn that dropped significantly (P<0.05) (Table 2). Extractable Pb content was 26-0% of the soil total metal, Cu recorded 8-4%, Cr recorded 6-3%, and Cd recorded 82% whereas extractable Zn content dropped to 0-6% of the soil total metal content. Generally, the recovered bioavailable metal contents of the soil in the two seasons were low compared to the total metals and this could be linked to the pH of the soil and maybe the mineralogy of the soil [10]. Several reports have indicated that higher pH of soil reduces metal mobility [11, 40] but the considerable increase in the dry season could be linked to the large amount of organic matter content of the soil. Halim et al. [20] stated that bioavailability of metals could be increased when these metals form soluble complexes with organic matter content of the soil.

3.2 Uptake and metal partitioning in plants' parts.

Large amounts of extractable Cu, Cd and Zn (3.15 mg/kg, 3.15 mg/kg and 9.90 mg/kg respectively) were taken up by the roots of S. acuta from the phytoavailable metals in the soil in the wet season (Table 3). And this is not a peculiar case for these plant species because Nan et al. [31] and Kalavrouziotis et al. [24] had reported that most plants ordinarily accumulate heavy metals mainly in plant roots system and to a more limited extent in leaves and/or in edible parts. Little variation was observed in the tissue partitioning of S. acuta in the wet season. There was no significant difference in the pattern of partitioning of Pb, Cu, Cr and Cd in roots and leaves of S. acuta whereas the amount of Zn in the roots was significantly greater than the accumulated Zn in the leaves at P<0.05 (Table 3). This lower amount of Zn in the leaves could be adduced to utilization of Zn for growth since wet season is a growing period for most plants and reports has it that Zn is mostly accumulated in green tissues like leaves [36]. A different pattem of partitioning was observed in S. acuta in the dry season. Leaf contents of Pb (2.39 mg/kg) and Cu (2.87 mg/kg) in the leaves were significantly higher (P<0.05) than roots contents (0.97 mg/kg and 0.84 mg/kg respectively) of S. acuta. This change of pattem could be due to the intense évapotranspiration experience in the dry season and also increase foliar uptake of metals by plants since report by Oluyemi et al. [34] stated that there is increase foliar deposition of metals the dry season. Oluyemi et al. [34]) also reported that dehydration of plant leaves by transpiration increases metal concentrations in the leaves. Cr and Cd partitioning between the roots and leaves in S. acuta in the two seasons did not differ statistically (P<0.05) which indicates that S. acuta has affinity for bioaccumulation of these metals.

A different pattem of tissue partitioning of heavy metals was observed in P. purpureum (Table 3). This supports the earlier assertion of Alloway [5] that plant species types dictate the patterns of accumulation of heavy metals. Accumulations of Pb (16.79 mg/kg), Cu (3.79 mg/kg) and Zn (7.25 mg/kg) in the roots were significantly greater than the leaves contents of 14.18 mg/kg, 3.05 mg/kg and 5.18 mg/kg respectively (Table 3).

These partitioning pattern could be peculiar to grasses because Yoon et al, [44] also reported higher Pb, Cu and Zn contents in the roots of some grasses (bahia, bermuda and wire grasses) than the leaves contents. This occurrence of larger amount of heavy metals (Pb, Cu and Zn) in the roots of P. purpureum in the wet season could be due to the complexation and sequestration of metals in cellular structures like the vacuoles in the plant root and render them unavailable for translocation to the shoot [39]. There was no difference in the tissue partitioning of all the metals in the dry season (Table 3) and this could probably be due to the low concentration of bioavailable metals in the soil in the dry season since the pH was slightly alkaline thereby rendering the metals non-mobile. It is also important to note that root and leaf contents of P. purpureum showed no significant difference in the tissue partitioning of Cr and Cd in the two seasons and this also indicates the high bioaccumulation potential of this plant species for Cr and Cd.

3.3 Heavy metal transfer and translocation in S. acuta and P. purpureum.

The TF and Ti values (Figures 2 &3) help to identify the suitability of plants for phytoextraction and phytostabilization by depicting the characteristics of accumulation and the patterns of translocation of metals in plants. Figure 1 presents the seasonal variations in the transfer factor (TF) of S. acuta and P. purpureum growing around the cement factory. TFs of Cr, Cd and Zn in S. acuta were all greater than 1.0 in both the wet and dry season while TFs of Pb and Cu for both season were below 1.0 but still higher than 0.5 (Fig. 2a).

This is an indication that S. acuta has the potential of taking up Cr, Cd and Zn from phytoavailable pool into the roots without much restriction and this could be a possible indication of S. acuta being a phytostabilizer of Cr, Cd and Zn. Garba et al. [17] have indicated that plants with TF>1.0 for a particular metal are good phytostabilizers. From Figure 2b, TFs of Cr, Cd and Zn in P. purpureum were also above 1.0 in both seasons while Pb had TF that was above 1.0 in the wet season and Cu with TF above 1.0 in the dry season. This is also an indication the suitability of P. purpureum as phytostabilizer of Cr, Cd and Zn in cement-polluted soil without restriction by seasons. Phytostabilization uses the ability of plants' roots to limit mobility of contaminants and bioavailability in the soil through the process of sorption, precipitation, complexation or metal valence reduction [19]. Therefore, S. acuta and P. purpureum possess the potentials to phytostabilizer Cr, Cd and Zn contaminated soil. The inconsistency in the TF of Pb and Cu for both weed species is an indication these plants species had limited capabilities of mobilizing these metals in the roots zone cross seasons and hence possess limited phytostabilization potential.

Translocation indices of metal for S. acuta and P. purpureum for the two seasons are presented in Figure 3. Pb had Tis >1.0 in S. acuta for both seasons, likewise Cr and Cd and according to Baker [8] and Garba et al. [17], a Ti that is above 1.0 denotes plant to be an accumulator. Tis of Cu and Zn were below 1.0 in the wet season and greater than 1.0 in the dry season (Fig. 3a). S. acuta cannot be regarded as accumulator of Cu and Zn because of its inconsistency in the Tis for Cu and Zn in the two seasons. Figure 3b showed that P. purpureum could also be regarded as accumulator of Cr and Cd because the Tis were also above 1.0. Due to the inconsistency in the Ti of P. purpureum in the two seasons (Tis of Pb, Cu and Zn <1.0 in the wet season and Tis>1.0 in the dry season), this plant species cannot be regarded as accumulator. Though, a study on similar grass {Eleusine indica) revealed the potential of Eleusine indica to exclude Pb, Cu and Zn and this supported the earlier assertion of Alloway [5] that plant species type determines metal absorption and accumulation patterns. The Ti showed the movement of metals from soil to root/shoot and indicate the efficiency of uptake by plants which is an important factor in determining accumulator or excluder plants.

4. Conclusions

The results from this study showed that S. acuta and P. purpureum displayed different metal partitioning in their tissue biomass. In spite of the different patterns of metal partitioning, these two weed species qualify as potential phytostabilizers of Cr, Cd and Zn in cement-polluted soil. This study has also been able to identify S. acuta and P. purpureum as accumulators (phytoextractors) of Cr and Cd which could be a sustainable option in remediating these metals from cement-polluted soil. This is indicated by the fact that these two weed species allocated greater proportion of Cr and Cd absorbed from the soil to aboveground biomass (leaves). And this is a convenient way of phytoremediation because these leaves are going to be senesced later thereby helping to discarding these toxic metals.

References

5. References

1. Adamson KY: Towards an Environmental Action Plan for Ogun State. World Bank

Assisted Project Report, Abuja, Nigeria; 1996: 139p

2. Adegoke OS: Eocene stratigraphy of Southern Nigeria. Bureau Recherches Géologiques Minieres Mem. 1996; 3: 23-48.

3. Agbaje NG: Geology and Mineral Resources of Nigeria. Springer, 2009,22 lp

4. Akanni CO: Climate In: Ogun State in Maps. Onakomaya, S. O., Oyesiku, K. and Jegede, J. (Eds): Ibadan, Rex Charles Publisher; 1992, 134-187.

5. Alloway BJ: Heavy Metals in Soil: New York, John Wiley and Sons 1990, 1-168.

6. Antonsiewicz DM, Escude-Duran C, Eierzbowska E, Sklodowska A : Indigenous plant species with potential for the phytoremediation of arsenic and metal contaminated soil. Water, Air and Soil Pollution 2008, 19:197-210

7. Aweto AO: Fallowing and Soil Fertility Restoration in South-west Nigeria. Malaysian Journal of Tropical Geography 1981: 3:1-6.

8. Baker AJM: Accumulators and Excluders-Strategies in the response of Plants to Heavy Metals. Journal of Plant Nutrition 1981, 3(1-4): 643-654.

9. Bai J, Cui B, Chen B, Zhang K, Deng W, Gao H, Xiao R: Spatial distribution and ecological risk assessment of heavy metals in surface sediments from a typical plateau lake wetland, China. Ecological Modeling 2011,222: 301-306

10. Bani A, Echevarria G, Sulce S, Mullai A, Morel J L: In-situ phytorextraction of Ni by native populations of A. murale on an ultramafîc site (Albania). Plant Soil 2007, 293: 79-89

11. Basta NT, Pantone DJ, Tabatabai MA: Path Analysis of Heavy metal Adsorption by soWAgron. J1993, 85: 1054-1057

12. Chojnacka K, Chojnacki A, Gorecka H, Górecki H: Bioavailability of Heavy Metals from Polluted Soils to Plants. Sci.Total Environ.: 2005, 337: 175-182.

13. D'Souza R, Varun M, Masih J, Paul MS: Spatial distribution of heavy metals in soil and flora associated with the glass industry in north-central India : implications for phytoremediation. Soil and Sediment Contamination: An International Journal 2013, 22: 1-20.

14. D'Souza R, Varan M, Masih J, Paul MS: Identification of Calotropis procera L. as a potential phytoaccumulator of heavy metals from contaminated soils in Urban North Central India. Journal of Hazardous Material 2010, 184: 457^164.

15. F AO: FAO-UNESCO soil map of the world, Revised Legend. World Soil Resources Report 60, FAO, Rome, 1988. Reprinted as Technical Paper 20, 1989, ISRIC, Wageningen

16. Garba ST, Nkafaminya II, Barminas JT: Phytoremediation: Influence of different level of EDTA on the phytoextraction ability of Pennisetum pedicellatum for the metals; cadmium and zinc. International Journal of Engineering and Management Sciences 2013, 4(2): 92-97

17. Garba ST, Santuraki AH, Barminas JT, Kamai N: Ethylenediaminetetraacetic acid (EDTA) Assisted Phytoextraction of the Metals; Cu, Cd, Cr, Ni and Zn by Crowfoot Grass {Dactyloctinium aegyptium). International Journal of Environmental Sciences 2012, 1: 1-10

18. Gbadebo AM, Bankole OD: Analysis of Potentially Toxic Metals in Airborne Cement Dust around Sagamu, Southwestern Nigeria. Applied Science 2007, 7: 35-40

19. Ghosh M, Singh SP: A review on phytoremediation of heavy metals and utilization of its byproducts. Applied Ecology and Environmental Research 2005, 3(1): 1-19

20. Halim M, Conte P, Piccolo A: Potential availability of heavy metals in phytoextraction from contaminated soils induced by exogenous humic substances. Chemosphere 2003, 52: 265-275

21. Hseu ZY: Evaluating heavy metal content in nine compost using four digestion methods. Bioresource Technology 2004, 95: 53-59

22. Ilalokhoin PO, Otara AJ, Odigure JO, Abdulkareem AS, Okafor JO: Environmental impact assessment of a proposed cement plant in southwestern Nigeria. IO SR Journal of Environmental Science, Toxicology and Food Technology 2013, 3(5): 83-98.

23. Imray P, Langley A: Health-based soil investigation levels: Paragon Printers, fyshwick, Australia; 2001: 3-25

24. Kalavrouziotis IK, Robólas P, Koukoulakis PH, Papadopoulos AH: Effects of municipal wastewater on the macroand micro - elements status of soil and of Brassica olerácea var. Itálica, and B. olerácea var Gemmifera. Agr Water Manage.2008, 95: 419-426.

25. Kidd PS, Dominguez-Rodriguez DMJ, Monterroso C: Bio availability and plant accumulation of heavy metals and phosphorus in agricultural soils amended by long-term application of sewage sludge. Chemosphere 2007, 66:1458-1467.

26. Kochian L: Adaptive strategies for plant response to toxic metals in the soil. The proceeding of the International Plant Nutrition Coloquium XVI, UC Davis, 2009: http://escholarship.org/uc/item/2w73r71d

27. Kochian L, Pence NS, Letham DLD, Pineros MA, Magalhaes JV, Hoekenga OA, Garvin DF: Mechanisms of metal resistance in plants: aluminium and heavy metals. Plant and Soil, 2002, 247:109-119.

28. Lafarge-WAPCO: Lafarge WAPCO: Supporting Nigeria's Development. Annual Report of Lafarge-Cement WAPCO Nigeria Pic, Nigeria: 2011,1-32

29. Mendez MO, Maier RM: Phytostabilization of mine tailings in arid and semiarid environments-an emerging remediation technology. Environ Heal Perspect. 2008, 116: 278-283.

30. McLaughlin MJ, Hamon RE, McLaren RG, Speir TW, Rogers SL: Review: a bioavailability based rationale for controlling metal and metalloid contamination of agricultural in Australia and New Zealand. Aust. J. Soil Res. 2009, 38: 1037-1086

31. Nan Z, Li J, Zhang G, Cheng G: Cadmium and zinc interaction and their transfer in soil crop system under actual field conditions. Sei. Total Environ. 2002, 285: 187-195.

32. Ogunkunle CO, Fatoba PO: Contamination and spatial distribution of heavy metals in topsoil surrounding a mega cement factory. Atmospheric pollution Research 2014, 5. DOI- 10.5094/APR.2014.033

33. Ogunkunle CO, Fatoba PO: Pollution loads and the ecological risk assessment of soil heavy metals around a mega cement factory in southwest Nigeria. Polish Journal of Environmental Studies 2013, 22(2): 487-493

34. Oluyemi EA, Feuyit G, Oyekunle JA, Ogunfowokan AO: Seasonal Variations in Heavy Metal Concentrations in Soil and Some Selected Crops at a Landfill in Nigeria. African Journal of Environmental Science and Technology 2008, 2(5): 89-96

35. Prabu PC: Impact of heavy metal contamination of Akaki River of Ethiopia on soil and metal toxicity on cultivated vegetable crops. EJEAFChe 2009, 8(9): 818-827.

36. Probst A, Liu H, Fanju M, Liao B, Hollande E: Response of Vicia faba L. to metal toxicity on mine tailing substrate: geochemical and morphological changes in leaf and root. Environmental and Experimental Botany 2009, 66: 297-308

37. Reddy KR, Hettiarachci H, Gangathulasi J, Bogner JE, Lagier T: Geotechnical properties of synthetic municipal solid waste. International Journal of Geotechnical Engineering 2009, 3(2): 429-438

38. Shukla PK: Nutrient dynamics of teak plantations and their impact on soil productivityA Case Study from India. XIII World Forestry Congress, Buenos Aires, Argentina, 2009: 1-11

39. Singh S, Sinha S: Accumulation of metals and its effects in Brassica juncea grown on various Amendments of Tannery Waste. Ecotox. Environ. Safety 2005, 62: 118-127

40. Smejkalova M, Mikanova O, Boruka L: Effects of heavy metal concentrations on biological activity of soil micro-organisms. Plant Soil Environ. 2003, 49: 321-326

41. Tessier A, Campbell PGC, Basson M: Sequential extraction procedure for spéciation of particulate trace metals. Annal. Chem. 1979, 5: 844-851.

42. USD A: Soil taxonomy, a basic system for soil classification for making and interpreting soil surveys. Agricultural Handbook No. 436, Washington, 1975.

43. Va run M, D 'Souza R, Pratas J, Paul MS: Metal contamination of soils and plants associated with the glass industry in north central India: Prospect of phytoremediation. Environ Sei Pollut Res. 2012, 19: 269-281

44. Yoon J, Cao X, Zhou Q, Ma LQ: Accumulation of Pb, Cu and Zn in native plants growing on a contaminated Florida site. Science of the Total Environment 2006, 368: 456-464

45. Zhuang PQ, Yang W, Wang HB, Shu WS: Phytoextraction of heavy metals by eight plant species in the field. Water, Air, and Soil Pollution 2007, 184: 235-24.