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Heavy Metals Accumulation of Some Plant Species Grown on Mining Area at Mahad AD’Dahab, Saudi Arabia

A.S. AL-Farraj and M.I. Al-Wabel
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Samples from different plants species, which grown around Mahad AD’Dahab Mine, have been selected to study their ability to accumulate these heavy metals. These plant species were: Pergularia tomentosa, Calotropis procera, Acacia tortilis, Ochradenus baccatus, Salsola sp., Rhiza strica, Convolvalus sp., Euculeptus sp., Family graminaea and Prosopis juliflora. Moreover, some of soil samples under each plant were collected. Plants and soils samples were analyzed for their contents of As, Cd, Cu, Pb and Zn. Two way ANOVA analysis without interaction was performed to examine the effect of plant species and heavy metals concentration in soil on their accumulation by plants. Although significant differences were not found at 0.01 levels among the plant species, it was found that Pergularia tomentosa was the highest to accumulate heavy metals. Considering the mean of accumulating heavy metals, plant species accumulated heavy metals by this order: Pergularia tomentosa, Euculeptus sp. Convolvalus sp. Family graminaea, Rhiza strica, Acacia tortilis, Prosopis juliflora, Salsola sp. Calotropis procera, Ochradenus baccatus. According to the mean of BAF’s, heavy metals concentration of Cd was found to be significantly different than Cu, Pb and Zn. From above, these plants should be described as not-excluder and can be explored further for phytoremediation of metal polluted soils. On other hand, the practice of providing foliage and pods as fodder for live stock should be avoided in Mahad AD’Dahab area.

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A.S. AL-Farraj and M.I. Al-Wabel , 2007. Heavy Metals Accumulation of Some Plant Species Grown on Mining Area at Mahad AD’Dahab, Saudi Arabia. Journal of Applied Sciences, 7: 1170-1175.

DOI: 10.3923/jas.2007.1170.1175



Because of industrialization and mining, soils, plants and water resources are at risk for contamination with heavy metals. Soil highly contaminated with metals may disrupt the physical, chemical and biological balance of the soil. The heavy metals most frequently encountered in include As, Cd, Cu, Pb and Zn, all of which cause risks for human health and the environment (Alloway et al., 1990). Therefore, the reclamation of polluted soils and evaluation of environmental risk necessitate a greater understanding about the remediation of contaminated soils.

Although heavy metals contaminated soils affect the growth of plants, many species of plants have adopted to tolerate high concentrations of heavy metals. Presence of plants in contaminated soils necessarily means that they are tolerant to high levels of heavy metals and able to accumulate or exclude them. Plants that accumulate metals in their above ground tissues are termed metal non-excluders. They could be divided to indicators and hyper accumulators. Indicators species should have levels of metals in their tissues reflected to their levels in soil. Several plant species are well known indicators of heavy metals pollution in soil (Wenzel and Jockwer, 1999). For example, honey mesquite (Prosopis juliflora), was studied as a possible bio-indicator of industrial smelter pollution in Arizona State (Gabriel and Patten, 1994). On other hand, the hyper accumulators can concentrate metals to concentrations far exceeding which in soil (Singh, 2005).

Remediation of contaminated soils using green plants, including grasses, forbs and woody species to remove heavy metals or to render them harmless is currently being studied through the world (Schnoor et al., 1995; Comis, 1996; Wu et al., 1999). This is often called phytoremediation. It is particularly challenging due to the low mobility and generally low bioavailability of heavy metals.

Thangavel et al. (2000) reported that using P. juliflora, for the bio-recovery of aluminium. Moreover, P. juliflora has accumulated Ni, Cr, Cu and Pb (Niverthitha et al., 2002; Senthilkumar et al., 2005). Most kinds of ragweed and Thlapsi rotundifolum can be used to remove lead while Thlaspi caerulescens can be used to remove zinc and cadmium (Comis, 1996). Indian mustard, are well known for removing Pb, S, Ni, Zn, Cu, Cd and Ch. To date, only a few of hyper accumulating plant species have been recognized to uptake more than one metal. Plants belonging to the Brassicaceae family (e.g., Brassica and Thlapi sp.) are the most hopeful ones considered for phytoremediation. Recently, plants of the genus Pelargonium were identified for remediation soils that were contaminated by heavy metals (Singh, 2005).

Because of Mahad AD’Dahab Mine (Saudi Arabia), its soil was approved to be contaminated with As, Cd, Cu, Pb and Zn (Al-Farraj and Al-Wabel, 2006). The huge landfill of Mahad AD’Dahab Mine may lead to rapid accumulation of heavy metals in soils around mining area. Heavy metals contamination can be carried with soil particles swept away from the initial areas of pollution by wind and rain.

In the present research, some plants (wild or agricultural plant) grown in heavy metals contaminated soils around Mahad AD’Dahab Mine have been selected to assess their ability to accumulate heavy metals and their potential use in the reclamation of metal contaminated soils. Therefore, our goals in this research have focused on evaluate concentration of heavy metals in those plant species to identify the plant species best adapted for the uptake of specific metal. Furthermore, the possibility to use some plants as bio-indicator for heavy metals contamination is one of our objectives. Finally, the bioavailability of heavy metals (As, Cd, Cu, Pb, Zn) in soils of Mahad AD’Dahab Mine is one of our goals.


Study area and collection of plant and soil samples: The plant and soil samples used for this research were collected in 2005 from the surrounding area of Mahad AD’Dahab Mine (23°30= N; 40°30= E). Sites of the samples were chosen because of its high contaminated with respect to heavy metals (Cd, Cu, Pb, Zn … etc) (Al-Farraj and Al-Wable, 2006). Plants and Soil samples were collected randomly from 21 different locations, where the depths of soil samples were 0-15, 15-30 and 30-45 cm. Samples for each location were mixed to ensure homogeneity, dried at room temperature and gently ground to pass through a 2 mm sieve. Plant species included Pergularia tomentosa, Calotropis procera, Acacia tortilis, Ochradenus baccatus, Salsola sp., Rhiza strica, Convolvalus sp., Euculeptus sp., Family graminaea and Prosopis juliflora were found at the twenty one selected locations. Deferent branches (leaves and stems) were collected from each plant. These branches were mixed, rinsed well in double distilled water (to incomplete removal of metals on the surface of leaves and stems) and dried at 60°C. The air-dried plant samples were powdered homogenously for further analysis.

Chemical and physical analysis: Soil pH and EC were measured in distilled water extracts after equilibration for 24 h. The ratio of soil and distilled water was 1:5 (Thomas, 1996; Rhoades, 1996). Cations and inions (Ca++, Mg++, Na+, K+, HCO3¯, CO3=, Cl¯, SO4=) were determined in those extracted soil solutions (Richards, 1954; Rainwater and Thatcher, 1979). Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1994). Content of CaCO3 was determined by calcimeter method (Loeppert and Suarez, 1996). Soil organic matter of the soil samples were determined by digested them using concentrated H2SO4 (Nelson and Sommers, 1996).

Heavy metals analysis: Soil samples were digested with HNO3-HClO4-HF (Hossner, 1996). Dried and powdered plant samples were acid digested with HNO3 and HClO4 (Chapman and Pratt, 1966). Digested soil and plant sample were analyzed for heavy metal concentration (As, Cd, Cu, Pb, Zn) using ICP-AES (Perkin elemer, 4300 DV). Due care was taken to avoid metal contamination in the process of sampling, washing, drying and grinding.

Statistical analysis methodology: A two-way analysis of variance (ANOVA) was used to evaluate the effect of the different plant species and concentration of metal in plant over its concentration in soil (which named BAF's) on accumulation of heavy metals by plant. In addition to that, 0.01 significance level will be considered to conclude the differences. ANOVA was followed by LSD test when appropriate to compares all possible pairs of means after the f-test rejects the null hypothesis that groups do not differ. However, in the analysis of this paper and if the p-value of rejecting the effect of any if the independent variables is not too big, LSC test will be applied even if the f-test leads to acceptance of then null hypothesis.


The basic physicochemical properties of studied soil samples are summarized in Table 1. Table 2 shows the concentration of heavy metals of soil samples that surrounding studied plant species compared to the average concentrations and the normal ranges in soils. There is no doubt, all soil samples are contaminated with respect to As, Cd, Cu, Pb and Zn.

Table 1: Basic physicochemical properties of soil samples that surrounding plant species
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia

Table 2: Total heavy metals concentration of soil samples that surrounding plant species studied, the average concentration and common range in soils (mg kg–1)
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia
*Cm: concentration of heavy metals in studied soils; while Ave. m: The average concentration in soil as reported by Lindsay, 1979, 1Lindsay, 1979

Table 3: Concentrations of heavy metals of different species grown surrounding Mahad AD’Dahab Mine (mg kg–1)
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia
*BAF's: Concentration of metal in plant over its concentration in soil

The whole soil samples have concentrations more than the average in soils. Furthermore, most of soil samples have concentration extremely higher than maximum of Cd, Cu, Pb and Zn in soils (Lindsay, 1979).

Table 3 and Fig. 1 illustrate the concentrations of heavy metals in different species of plants. Results showed that Convolvalus sp. accumulated As more than others, while Family graminaea was the lowest. Except As, Pergularia tomentosa accumulated heavy metals extremely higher than other species. It accumulated 720, 333, 89, 76.7 and 11.2 mg kg–1 of Zn, Cu, Pb, Cd and As, respectively (Table 3). It is interesting to note that, the respective of their concentrations in the soil have an approximately similar order (Zn = 1321; Cu = 807; Pb = 636; As = 12.9 and Cd = 6.9) (Table 2).

Generally, the results showed that heavy metals concentrations were much lower in plant shoots compared to their total concentration in the soil (Table 3). There is an exception of As and Cd with some species. Pergularia tomentosa, Convolvalus sp., Family sraminaea and Rhiza strica concentrated Cd in their shoots to levels far exceeding those present in the soil. The calculated BAF's for these species were 11.11, 3.43, 1.88, 1.16, respectively. However, Cd was not detected in the samples of both Calotropis procera and Prosopis juliflora.

Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia
Fig. 1:
Heavy metals concentration (As, Cd, Cu, Pb, Zn) of plant species grown surrounding Mahad AD’Dahab Mine (A.T.: Acacia tortili C.P.: Calotropis procer; C.S.: Convolvalussp.; E.S.: Euculeptus sp., F.G.: Family graminae; O.B.: Ochradenus baccatus; P.T.: Pergularia tomentosa; P.J.:: Prosopis juliflora; R.S. Rhiza strica; S.S.: Salsola sp.)

In contrast, Senthilkumar et al. (2005) reported accumulation of heavy metals including Cd by Prosopis juliflora. That could be explained because of differences of physical and chemical soil properties which can affect the accumulation of metals by plants (Senthilkumar et al., 2005). Moreover, roots of Prosopis juliflora were very deep whereas depth of soil samples < 45 cm. The As concentration of most of species of plant were close to the concentration in their soils (Table 3). The average of BAF's was 0.91 which close to one. Acacia tortilis, Calotropis procera, Euculeptus sp., Rhiza strica and Salsola sp. had concentration of As equal to or higher than its level in the soil. From above, it is clear to recognize that the total heavy metal concentration in soil is the poor indicator of metal availability for plant uptake.

According to the average of BAF's, the order of heavy metals accumulation by plants was Cd> As> Zn> Cu> Pb. The average was 1.84, 0.91, 0.3, 0.28 and 0.11, respectively (Table 3). In the present study, Cd and As were found to be particularly mobile in the soil-plant systems. Both are widespread contaminants in urban environments, however, Cd is a much more zootoxic metal and of more concern to human health and to food chains (Dickinson and Pulford, 2005). On the other hand, Pb, Cu and Zn were low and insufficiently mobile in soil-plant in this research. Other researchers have reported low uptake of As and Pb (French et al., 2006). While, others found that woody biomass may have relevance to clean-up soils contaminated by Zn (Rosselli et al., 2003). The elevated accumulation of Cd agreed with it's high mobility. Whereas, Pb has low mobility, so the lowest accumulated is reasonable.

The results of the present investigation show that P. tomentosa has ability to accumulate all heavy metals (As, Cd, Cu, Pb, Zn) (Fig. 1). That could be explained because of soil of P. tomentosa had relatively higher salinity (3.1 dS m–1), higher soluble SO4= (22.8 meq L–1) and relatively lower pH (7.7). These parameters were observed to increase the solubility of heavy metals (Helal et al., 1996; Alloway et al., 1980; Gabriella and Anton, 2005). Furthermore, P. tomentosa could not be described hyperaccumulator because of less accumulation of heavy metals. Actually, parts of plants which were analyzed for heavy metals didn't include roots. In hyper-accumulator species, heavy metals accumulate both in the shoot and the root (Singh, 2005). Vandecasteele et al. (2005) found that elevated wood, park and foliar concentrations for Cd and Zn, while Cr, Cu, Ni and Pb were less easily accumulated in aboveground biomass compartments. Anyway, P. tomentosa could be used to remediate heavy metal polluted soil at Mahad AD’Dahab Mine and other similar due to its ability to accumulate various heavy metals.

Table 4 shows the result of ANOVA analysis, where it shows a significant differences among the averages of heavy metals (F4, 136 = 2.085, p = 0.086). However, no significance differences have been found among the groups of plant species (F9, 136 = 0.871, p = 0.553).

For the plant species variable, based on the averages of accumulation, the plants can be ordered as follows: Pergularia tomentosa> Euculeptus sp.> Convolvalus sp.> Family graminaea> Rhiza strica> Acacia tortilis> Prosopis juliflora> Salsola sp.> Calotropis procera> Ochradenus baccatus.

Table 4: Two-way ANOVA table of heavy
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia

Table 5: Matrix results (Mean difference and corresponding p-values) of LSD test for the plant species variable
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia
A.T. Acacia tortili C.P. Calotropis procer C.S. Convolvalus sp. E.S. Euculeptus sp. F.G. Family graminae O.B. Ochradenus baccatus P.T. Pergularia tomentosa P.J. Prosopis juliflora R.S. Rhiza strica S.S. Salsola sp.

Table 6: Matrix results (Mean differences and corresponding p-values) of LSD test for the metal variable
Image for - Heavy Metals Accumulation of Some Plant Species Grown  on Mining Area at Mahad AD’Dahab, Saudi Arabia

Although, significance differences were not found among those types of plants, LSD test results in Table 5 shows differences among Acacia tortilis versus Euculeptus sp. and Pergularia tomentosa. That would happened due that the standard error of estimating the difference of the mean differences is too large comparing to its average.

For the heavy metals concentration, the mean of the BAF's for he As, Cd, Cu, Pb and Zn were found 1.073, 1.851, 0.604, 0.0077 and 0.511, respectively where the differences are significant. The LSD test showed that the mean of mean of BAF's at Cd differs significantly than the means of Cu (p = 0.069), Pb (p = 0.008), Zn (p = 0.051) (Table 6). No other significant differences were found between the other pairs of heavy metals concentration. In soils with pH = 5.5, as soils of Mahad AD’Dahab Mine, Pb solubility is controlled by phosphate and carbonate precipitates (Blaylock et al., 1997), so its relatively low bioavailability is reasonable.


The main reservation of our recommendation is that, these plants should be described as not-excluder, so they could be divided to indicators and hyper accumulators. Furthermore, they can be explored further for phytoremediation of metal polluted soils. According to the nature of soil properties, plant and kind of heavy metals, some of these plants species could be successfully used as an efficient and safety method for remediation of Mahad AD'Dahab soil. Moreover, since the area is open to be used for livestock, in view of heavy metals accumulation, a practice should be avoided.


The authors wish to thank Prof. Salm Magrabi, Soil Science department, King Saud Univ., Riyadh, Saudi Arabia, for his assistance and efforts during lap work and his suggestions. Special thanks are due to Dr. Fahad Al-Tekhaifi for his comments and help with the statistical analysis.

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