Evaluation of heavy metal mobility in contaminated soils between Abu Qurqas and Dyer Mawas Area, El Minya Governorate, Upper Egypt

Abstract

Background

Heavy metals have dangerous effect on the biota. So, this work aimed to determine the mobility and bioavailability of these metals through the application of pollution indices and geochemical fractionation technique. Thirty-one topsoil samples (20 cm) were collected from the area between Abu Qurqas and Dyer Mawas, El Minya Governorate, and subjected to chemical analysis.

Results

The results revealed that Cd and As are mainly associated with carbonate fraction while Cr, Pb, Ni, and Cu are more present in the residual fraction. The relative amounts of easily dissolved phase of heavy metals in the soils are in the order of As > Cd > Cr > Pb > Ni > Cu.

Conclusion

The results revealed that As, Cd, and Cr have high mobility and risks more than Pb, Ni, and Cu metals in the studied soils. As, Cd, and Cr were the heavy metals that caused pollutants in the soils of the studied area.

Introduction

Metal contamination of soil is an important concern to human and environmental health worldwide. The potential toxicity of heavy metals in soil is a function of their mobility and bioavailability. Metal mobility is depending on the phase in which the metal occurs as well as physical and chemical processes that control transformations between phases. Some heavy metals cause damage to the nervous system and internal organs as well as carcinogenic effects (Lee et al. 2007; Maas et al. 2010). One of the most critical properties of these metals that distinguish them from other contaminants is their persistence and non-biodegradability in the environment (Ghaderi et al. 2012; Yang et al. 2012).

Heavy metals in soils are derived from natural sources such as weathering of rock, volcanoes, and forest fires as well as anthropogenic activities such as the industrial effluent, fertilizers, atmospheric deposition, traffic, agricultural drainage, and phosphate industry (Ghaderi et al. 2012; Wali et al. 2013).

Several studies have estimated the total metal contents in polluted and unpolluted soils (Vega et al. 2004; Covelo et al. 2007; Kierczak et al. 2008; Maas et al. 2010; Nemati et al. 2011; Wali et al. 2013, Mohamed et al. 2015, Elnazer et al. 2015; Salman et al. 2016; Salman et al. 2018; Said et al. 2019). However, the total contents of trace metals cannot give enough information about their various forms, mobility, bioavailability, or potential risks to the environment (Davutluoglu et al. 2011; Nemati et al. 2011). In contrast, the speciation analysis for metals in soil can give a good indicator of the ecosystem quality. Metals may be present in soil in several physicochemical phases, including soluble or exchangeable, bound to amorphous material (Fe/Mn oxides), bound to organic matter, sulfides, and bound to mineral fractions (residual) (Rauret et al. 1999; Zemberyova et al. 2006; Perez-Lopez et al. 2010 and Nemati et al. 2011). The mobility and bioavailability of trace metals are highly dependent on their specific chemical forms or behavior of binding to each soil phase. Sequential extraction procedures have recently been developed to detect the speciation of heavy metals in soil matrix; these procedures provide more information on the origin, physicochemical availabilities, mobilization, and transport of trace metals in natural environments (Kierczak et al. 2008; Rao et al. 2010; Nemati et al. 2011; Aiju et al. 2012).

In case of plants, free metal ions in the soil solution may be more toxic than metals in the complex states (Gupta and Sinha 2006). Accordingly, only the labile metal species (soluble, exchangeable, and chelated) are available to plants. Thus, the detection of total concentrations must be complemented by evaluating available fractions of metals (Buccolieri et al. 2010). The specific objective of this work was the determination of the mobility and bioavailability of heavy metals (As, Cd, Cr, Pb, Ni, and Cu) through the application of pollution indices and geochemical fractionation technique.

Location

The study area occupied the middle part of the Nile Valley between longitudes 30° 29′ and 30° 54′ E and latitudes 27° 37′ and 27° 56′ N (Fig. 1) between Abu Qurqas and Dyer Mawas Area, El Minya Governorate. The stratigraphic succession in El Minya Area is represented by Tertiary and Quaternary sedimentary rocks. The distribution of the different rock units was indicated in Said (1981). The stratigraphic sequence is built up from base to top as follows: Middle Eocene limestone intercalated with shale (Samalut Formation); Pliocene undifferentiated sands, clays, and conglomerates; Plio-Pleistocene sand and gravel with clay and shale lenses; Pleistocene sand and gravel with clay lenses; and Holocene silt and clay (Fig. 2); the main economy of the study area is based on agricultural and agro-industrial activities. The cultivated areas are represented by two plains, the older alluvial plain (reclaimed land) and the younger alluvial plain (agriculture soil) which originated from the Ethiopian Plateau (Abd Elsanad 2010).

Fig. 1

Location map of the study area and sampling sites

Fig. 2

Geologic map of the study area

Material and methods

In November 2014, 31 surface soil samples (20 cm) have been collected from the study area (Fig. 1); the samples were air-dried, crushed, passed through a 2-mm sieve, and stored at ambient temperature. Afterwards, the coning and quartering method was used to obtain representative sub-samples; each quarter was thoroughly homogenized, and about 50 g was milled to a diameter of < 0.01 mm and another 50 g to a diameter of < 63 μm for further analysis. Afterwards, 1 g of soil was digested with aqua regia (3:1 HCl:HNO₃) and analyzed for heavy metals. Enrichment factor (EF) was used to assess the accumulation or leaching processes of heavy metals in soils for the detection of their behaviors. It may be determined by comparing concentrations of certain heavy metal with a reference element (Kabata-Pendias and Pendias 2001). This factor has been calculated from the following equation according to Buat-Menerd and Chesselt (1979). According to the equation, EF = (E_s/E_background)/(R_s/R_background), where E_s is the content of the element in the soil, E_background is the content of the same element in soils worldwide, R_s is the content of the reference metal in the soil, and R_background is the content of the same reference element in soils worldwide.

Geoaccumulation (I_geo) index was applied to assess the pollution of metals in the soil. The geoaccumulation index (I_geo) is defined by the equation of Muller (1979)): I_geo = Log₂ (C_n/1.5B_n), where C_n is the measured concentration in soil sample and B_n is the background concentration of the metals in soils worldwide (Kabata-Pendias and Pendias 2001). The classes of soils according to EF and I_geo are illustrated in Table 1.

Table 1 Classification of enrichment factor (EF) and geoaccumulation indices (I_geo)

The contaminated soils were subjected to sequential extraction method which was described previously by Tessier et al. (1979) and modified after by Phuong (2008); this method is based on the partitioning of particular metal traces of 1 g of soil which was attacked by 25 mL of 1 M CH₃COONH₄ at a pH of 7 in 50 mL centrifugation tubes in order to liberate exchangeable fractions; metals associated and bound to carbonate phases have been solubilized using 25 mL of 1 M CH₃COONa adjusted to 5 pH with CH₃COOH; the previous residue has been added to 25 mL of 30% H₂O₂ adjusted to 2 pH with HNO₃ which was added to liberate the metals associated with the bound to organic matter phases; the previous residue has been extracted with 25 mL of 0.2 M NH₄-oxalate acidified 3.25 pH with oxalic acid solution in order to release the metals associated with the bound to Fe-Mn oxide phases. Finally, the contaminants have been released by a mixture of strong acid 15 mL of HCl and 5 mL of concentrated HNO₃. The extracts were analyzed by using atomic absorption spectrometer instrument (model: Perkin Elmer 400) in the National Research Centre laboratories in order to assess the metal (Cd, As, Cr, Pb, Ni, and Cu) concentrations.

The risk assessment code (RAC) was applied for evaluating the availability of metals in soils. According to Perin et al. (1985), RAC was calculated from the equation RAC = (100 × F₁ + F₂)/(F₁ + F₂ + F₃ + F₄ + F₅), where F₁ is the water-soluble and exchangeable fraction (Ex), F₂ is the carbonate-bound fraction (Ct), F₃ is the organic matter-bound fraction (Ob), F₄ is the Fe-Mn oxide-bound fraction (FM), and F₅ is the residual fraction (Rd).

Results

The concentration range of Cd, As, Cr, Pb, Ni, and Cu was 0.35–2.6, bdl–140, 89–260, bdl–300, 18–170, and 35–140 ppm, respectively (Table 2). The obtained values of EF and I_geo were presented in Table 3. Cadmium fractionation data is recorded in Table 4 and demonstrated in Fig. 3. Carbonate fractions of As vary from 205 to 943 ppm with an average 608.41 ppm in the selected sample soils (Table 5 and Fig. 4). Chromium, lead, nickel, and copper fractionation data is tabulated in Tables 6, 7, 8, and 9 and explained in Figs. 5, 6, 7, and 8. According to Perin et al. (1985), values of risk assessment code of heavy metals in the selected soils are listed in Table 10. The risk assessment code had been classified into five categories according to Perin et al. (1985) as summarized in Table 11.

Table 2 Heavy metal content in the studied soil samples (ppm)

Table 3 Enrichment factor (EF) and geoaccumulation index (I_geo) of heavy metals in soil samples

Table 4 Cadmium fractionation results (ppm)

Fig. 3

Distribution of Cd among the various soil fractions in the study area

Table 5 Arsenic fractionation results (ppm)

Fig. 4

Relative distribution of As among the various soil fractions in the study area

Table 6 Chromium fractionation results (ppm)

Table 7 Lead fractionation results (ppm)

Table 8 Nickel fractionation results (ppm)

Table 9 Copper fractionation results (ppm)

Fig. 5

Relative distribution of Cr among the various soil fractions in the study area

Fig. 6

Relative distribution of Pb among the various soil fractions in the study area

Fig. 7

Relative distribution of Ni among the various soil fractions in the study area

Fig. 8

Relative distribution of Cu among the various soil fractions in the study area

Table 10 Heavy metals risk assessment code (RAC%) in selected soils

Table 11 Classification of risk assessment code (RAC) in the studied soil

Discussion

The concentrations of As, Cr, and Ni exceed the maximum allowable concentration (MAC) set by Kabata-Pendias and Mukherjee (2007), while Cd, Pb, and Cu were within MAC. Based on the EF values, it is noticed that the samples which have a value exceeding 5 are considered rich in heavy metals according to the EF classification (Table 1). Also, the I_geo values of the studied samples which have a value over 2 represented polluted according to the EF classification (Table 1).

Sequential extraction

The most contaminated samples with As, Cr, Pb, Ni, and Cu are chosen based on the high enrichment factor and geoaccumulation index of these metals (Tables 1 and 3). The high contaminated samples are S₁, S₇, S₈, S₉, S₂₅, and S₃₁. So, these samples were subjected to sequential extraction methods. The metal forms have been obtained from the sequential extraction schemes which will be discussed as the following:

Cadmium

Cadmium has no essential biological function, but it tends to accumulate in plants and aquatic biota with consequent problems of toxicity. It is toxic to humans through the inhalation of dust causing lung damage and may cause cancer from long-range exposure (WHO 1996). According to mean levels, the comparative mobility and bioavailability of Cd in the studied soils were decreased in the following order: Ct > Rd > Ob > FM > Ex (Table 4 and Fig. 3). It is well known that Cd associated with the exchangeable fraction is really available, whereas that associated with both of carbonate, oxides, and organic matter is less available to biota. The residual fraction of Cd is actually unavailable to every plants or microorganism (Abollino et al. 2011; Wuana et al. 2012).

Arsenic

Anthropogenic activity has resulted in the widespread atmospheric deposition of arsenic from the burning of coal and the smelting of non-ferrous metals and phosphate fertilizers; the hazards resulting from geogenic load are generally regarded as lower than from anthropogenic contamination (Skala et al. 2011), and they clear that As is only associated with the non-residual carbonate fraction (Table 5 and Fig. 4), referring that As may be adsorbed, precipitated, or co-precipitated with carbonates (Smith and Naidu 2009). Then, the non-residual carbonate fraction of As is potentially available to plants or microorganism.

Chromium

The human body needed a small amount of Cr for insulin action and the metabolism of proteins and carbohydrates (Frausto da Silva and Williams 1991). Chromium has a varying toxicity depending on speciation in the environment. It is highly toxic, causing liver and kidney damage and acting as a carcinogen (WHO 1996). According to average values, the comparative mobility and bioavailability of Cr in the studied soil samples were decreased in the following order: Rd > Ct (Table 6 and Fig. 5). This point to the authentic Cr was primarily concentrated in the clay minerals or incorporated in the lattice structure in the studied soils (Rao et al. 2007; Hseu 2006). Therefore, the non-residual carbonate fraction of Cr is likely available to plants or microorganism. The residual fraction of Cr is actually unavailable to biota.

Lead

Lead has unknown biological role in plants or animals and is highly toxic to mammals and aquatic life; it can cause mental impairment in young children, causes neuropathy and hypertension in adults, and may be lethal at high levels, e.g., over 25 μg kg^− 1 of body weight (WHO 1996). According to mean contents, the comparative mobility and bioavailability of Pb in the examined soil samples cleared a decrease in the following order: Rd > FM > Ct > Ob >Ex (Table 7 and Fig. 6). This indicates that the native Pb was mainly fixed in the lattice structure in the studied soils (Rao et al. 2007; Hseu 2006; Sarkar et al. 2014). The residual fraction of Pb is indeed unavailable for biota.

Nickel

Nickel has been shown to be fundamental for microorganisms and having an essential role in human metabolism (McGrath 1995); most Ni compounds are comparatively non-toxic, but some compounds are highly toxic, and extreme excesses of Ni are both toxic, causing dermatitis and gastric irritation and carcinogenic illnesses (WHO 1996). According to average concentrations, the comparative mobility and bioavailability of Ni in the investigated soil samples were reduced in the following order: Rd > Ct > Ob > FM >Ex (Table 8 and Fig. 7); this refers that the primary Ni was mostly concentrated in the clay fraction or may be incorporated in the lattice structure in the studied soils (Rao et al. 2007; Hseu 2006; Ayodele and Mohammed 2011). The total bulk of Ni in soil is really unavailable for biota.

Copper

Copper is an essential trace element for all organisms and humans which can bear levels up to 12 mg per day, although the element can be toxic at extremely high levels (WHO 1996). Reimann and de Caritat (2005) report examples of kidney failure in small children resulting from drinking water polluted with copper in low pH environments containing high concentrations of Cu up to about 1 ppm. According to mean values, the comparative mobility and bioavailability of Cu in the studied soil samples were decrease in the following order: Rd > FM > Ob > Ct >Ex (Table 9 and Fig. 8). This refers that the authentic Cu was largely concentrated in the clay fraction of the studied soils or may be integrated in the lattice structures in soils (Rao et al. 2007; Hseu 2006; Rutkowska et al. 2013). The bulk of total Cu in soil is rightly unavailable to every plants and microorganism.

Risk assessment code

The risk assessment code (RAC) mainly compares the sum of the exchangeable and carbonate fractions with the total extracted for evaluating the availability of metals in soils. These fractions are considered to be weakly bonded metals that may equilibrate with the aqueous phase and thus become more rapidly bioavailable (Singh et al. 2005; Li et al. 2011).

Tables 10 and 11 show that heavy metals are considered to be easily dissolved into the water by acidity (Perin et al. 1985; Jain 2004). The relative amounts of easily dissolved phase of heavy metals in the soils are in the order of As > Cd > Cr > Pb > Ni >Cu. According to RAC values, the risks of both of As, Cd, and Cr were very high. So, both As, Cd, and Cr should be recognized as priority pollutants in the soils of the study area.

Conclusion

The results indicated that the pollution with As, Cd, and Cr metals of some samples were the most of polluted samples according to the EF and I_geo and were subjected to sequential extraction, which indicated that Cd and As are mainly associated with carbonate fraction. On the other hand, Pb, Cr, Ni, and Cu are mainly incorporated in the residual fraction, and calculated RAC cleared the potential availabilities of As, Cd, and Cr in the study area.