Sorption and lability of cadmium and lead in different soils from Egypt and Greece

Geoderma 153 (2009) 61–68

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a

Sorption and lability of cadmium and lead in different soils from Egypt and Greece Sabry Mohamed Shaheen ⁎ Department of Soil Sciences, Faculty of Agriculture, University of Kafrelsheikh, 33 516-Kafr El-Sheikh, Egypt

a r t i c l e

i n f o

Article history: Received 19 November 2008 Received in revised form 6 July 2009 Accepted 27 July 2009 Available online 9 September 2009 Keywords: Sorption Lability Cadmium Lead Soil types

a b s t r a c t Reactions of heavy metals with soils are important in determining their availability and fate in the environment. Mono-metal sorption and lability of sorbed cadmium (Cd) and lead (Pb) in different representative soils from Egypt and Greece as influenced by their properties were investigated in this study. For this purpose eleven surface soil samples varying widely in their origin and properties were selected. Four of them were from Egypt representing the main soil orders i.e., Entisols and Aridisols and the rest, seven, from different sites of Greece belonging to the orders Entisols, Alfisols, Vertisols, Mollisols, and Histosols. In these samples sorption isotherms were developed from which sorption parameters, and distribution coefficient (Kd) of Cd and Pb were determined. In addition lability of these metals was estimated by DTPA extraction following their sorption. The results showed that Freundlich model described satisfactorily sorption of both metals. In all the soils studied Kd values of Pb were higher than that of Cd indicating that this was retained by the soils stronger than Cd. Sorption parameters (kf, n) and Kd values of Egyptian Entisol developed on lacustrine deposits showed higher affinity for Pb, Greek Histosol for Cd while acidic Alfisols showed the lowest affinity for both metals studied. Permanent charge clayey soils with relatively low Fe, Al and Mn oxides content sorbed more Cd and Pb than the variable charge red soils with higher content of these oxides. In variable charge red soils with similar sesquioxides content, Pb and Cd sorption was pH dependent. However, in variable charge soils with similar soil pH, no significant differences were recorded for Pb sorption, while Cd sorption capacities differed significantly depending on the active ratios of Fe and Al oxides. Sorption parameters of Cd were correlated to clay content, cation exchange capacity, organic matter content, total free and amorphous aluminum oxides, amorphous iron oxides and CaCO3 content while Pb sorption parameters were correlated with clay content, total free and amorphous silica oxides, and amorphous aluminum oxides content as well as cation exchange capacity (CEC). Lability of the adsorbed Cd was higher than Pb in all the studied soils and may pose more threats to the ground water and plants. The Greek acidic Alfisols (Rhodoxeralf) showed the lowest lability of sorbed Cd, while the alkaline one exhibited the highest Cd lability. The lowest lability of sorbed Pb was in Greek Histosols and the highest in acidic Alfisol (Typic Haploxeralf). Labile Cd was negatively correlated only with CaCO3 content in all the soils studied while labile Pb was negatively correlated with CEC, clay, organic matter, total free aluminum oxides and the amorphous iron and aluminum oxides content. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The environmental hazards derived from the recent increasing disposal of heavy metals into soils particularly cadmium and lead, which are highly toxic to humans even at low concentrations have arisen growing concern in recent years (USEPA, 1992). The interest in heavy metals existence and behaviour in soil arises from the fact that soil is the main source for human food and from the possibility of soil pollution due to industrial wastes. Like other heavy metals, Cd and Pb in soils come from both soil parent materials and anthropogenic activities (Alloway, 1995). Lead and Cd are used in many industrial, urban, and

⁎ Tel.: +20 473247324, +20 104525256(mobile); fax: +20 473232032. E-mail address: [email protected]. 0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2009.07.017

agricultural applications (Kabata-Pendias and Pendias, 1992; Adriano, 2001) and are often found together at sites contaminated with heavy metals. Therefore, it is important to understand the chemistry of Pb and Cd in soils in order to assess their bioavailability. Heavy metals in soil are to a large extent sorbed to soil particles. Sorbed heavy metals can be desorbed into soil solution, and thus uptaken by plants or move down to lower soil horizons and groundwater. The fate of Cd and Pb that reaches soil from anthropogenic sources depends essentially on its sorption and lability in the host medium, which in turn depends on soil properties such as pH, organic matter and clay content. Heterogeneous soil systems consist of both organic and inorganic constituents with different affinities for heavy metals. In addition, heavy metals themselves exhibit varying affinities for soil surfaces. Variability in heavy metals affinity for soil sorption sites has been attributed to a given metal's hydrolysis constant (pKH),

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S.M. Shaheen / Geoderma 153 (2009) 61–68

electronegativity, Lewis acidity, charge density and solubility (KSP) of precipitates, including hydroxide and carbonate (McBride, 1994; Pardo, 2000; Sparks, 2003). Hence, sorption and lability of Cd and Pb in different soil types is of potential interest for determining their loading capacity and distribution in the soil profile. These data can be used to predict the environmental impact of Cd and Pb from anthropogenic sources, as well as to establish government policies on the usage of metals containing materials in agricultural soils. Sorption of heavy metals onto soil components can be described by a linear (McLaren et al., 1990), Langmuir, or Freundlich sorption model (Sposito, 1989). The Freundlich equation is often useful for modeling sorption of metals onto solids with heterogeneous surfaces and has frequently proved superior than the Langmuir equation for cations or anions sorption by soils (Sposito, 1980; Arias et al., 2005). Although, there is disagreement regarding the effectiveness of Langmuir and Freundlich models to interpret sorption of metal cations in soils (Sparks, 1995), some parameters of these models, such as maximum sorption quantity and the distribution coefficient are widely acceptable in characterizing Pb and Cd sorption capacity of soils (Basta and Tabatabai, 1992a; Gomes et al., 2001; Holm et al., 2003; Usman, 2008). Distribution coefficient is a useful parameter for comparing the sorptive capacity of different soils or materials for any particular ion, when they are measured under the same experimental conditions (Alloway, 1995). The mobility of metals in the soil environment are directly related to their partitioning between soil and solution (Sparks, 1995) and, therefore, are directly related to their distribution coefficients, which indicate the capability of a soil to retain a solute and the extent of its movement to the liquid phase (Reddy and Dunn, 1986). The agricultural soils of Egypt and Greece are one of the oldest agricultural areas in the world, having been under continuous cultivation for at least 5000 years. The soils in these two countries differ widely in their origin and properties. Most of the Egyptian soils are classified as Entisols and Aridisols. The Egyptian Entisols were developed on different deposits i.e., (fluvial, lacustrine and sandy marine deposits) differ widely in their physico-chemical properties, while the Aridisols were developed on calcareous deposits. Most of the Greek soils belong to the orders Entisols, Inceptisols, Alfisols, and Vertisols, and in a less extent to the orders Mollisols and Histosols. These soils differ in pH, sesquioxides content and the other physical and chemical properties, which affect heavy metals sorption and mobility. Sorption of Cd and Pb by soils has been studied by several researchers (Gomes et al., 2001; Appel and Ma, 2002; Appel et al., 2008) but usually in soils developed under different than the Mediterranean conditions. Few relevant studies have been conducted in Egyptian and Greek soils but mainly on one soil group such as fluvial deposits (Usman, 2008), calcareous soils (Elkhatib et al., 1991) and Rhodoxeralf (Ponizovsky and Tsadilas, 2003). In addition, either in Egypt or Greece Cd and Pb sorption and lability characteristics as influenced by soil properties in different soil types are limited. So, the main objective of this study was to investigate the influence of soil properties in different soil types from Egypt and Greece on (i) sorption characteristics and distribution coefficient of Cd and Pb, (ii) lability of sorbed Cd and Pb in the studied soils. 2. Materials and methods

deposits). The rest seven soils were from Greece belonging to the orders Entisols, Alfisols, Vertisols, Mollisols, and Histosols (Table 1). Soil samples were air-dried, ground to pass through a 2-mm sieve and analyzed for their basic physical and chemical properties according to Sparks (1996). Soil pH was measured in deionized water with a soil solution ratio of 1:1. Organic matter content was determined by the wet oxidation procedure. Cation exchange capacity (CEC) was measured by the 1 M ammonium acetate (pH 7.0) method. Total calcium carbonates equivalent (TCCE) were determined by using Collins calcimeter. Particle size analysis was performed according to Gee and Bauder (1986). Total free iron (Fed), aluminum (Ald), silicon (Sid) and manganese (Mnd) oxides content were extracted with 3 M sodium citrate + 1 M sodium bicarbonate + 1 g sodium dithionite (CBD) in a water bath heated at 85 °C (Mehra and Jackson, 1960). Amorphous iron (Feo), aluminum (Alo), silicon (Sio) and manganese (Mno) oxides content were extracted with 0.175 M ammonium oxalate + 0.1 M oxalic acid adjusted to pH 3.0. Alkaline soils were pretreated with 1 M ammonium acetate (pH 5.5) to remove carbonates according to Loeppert and Inskeep (1996). 2.2. Sorption experiment A batch equilibrium experiment was conducted using Cd and Pb solutions as follows: 2 g of soil samples were equilibrated with 20 mL of 0.01 M NaNO3 solutions containing 0.25, 0.5, 0.75, 1.0, 1.25, 1.5 and 2.0 mM Cd as cadmium nitrate or 1.0, 1.5, 2.0, 2.5, 3.0 and 4.0 mM Pb as lead nitrate in 50-mL centrifuge tubes (pre-weighed) for 24 h on a reciprocating shaker at room temperature. Some drops of toluene were added to suppress microbial activity. A relatively high initial Cd and Pb concentration range was used in this study, because after a preliminary experiment it was shown that the large initial Cd and Pb concentrations range were necessary for the alkaline soils with high Cd and Pb sorption capacity. After equilibration the samples were centrifuged and the supernatant was filtered through a Whitman No. 42 filter paper. Two replicates were used for collecting each data point. Cadmium and Pb concentrations in the supernatant were measured by atomic absorption spectrometry (Varian, SpectrAA-400 Plus, Australia). The amount of Cd and Pb sorbed was calculated as the difference between the initial and final concentration. Cadmium and Pb sorption data were fitted to Freundlich equation using the formula: q = kf c

n

where q is the sorbed Cd and Pb amount in mg kg− 1; c is the equilibrium solution concentration in mg L− 1. From this equation, the following sorption parameters were determined: kf, which represents the Cd and Pb sorbed at c = 1 mg L−, n which is an empirical parameter expressing the Cd and Pb sorption intensity. To fit the data, the model was linearized by using the logarithmic transformation resulting in the predictive equation log (q) = log (Kf) + n log(c). The linearized model fitted to each soil using analysis of covariance to estimate log (Kf) and n and test if the log (Kf) and n coefficients differed across the tested treatments. Estimates of Kf were obtained using exp (log (Kf)). The distribution coefficient (Kd) values were calculated according to Alloway (1995), Anderson and Christensen (1988), and Gomes et al. (2001) by using the formula: n

Distribution coefficientðKd Þ = q = c = kf c = c = kf c

n−1

2.1. Soil selection and characterization 2.3. Lability of sorbed Cd and Pb Eleven surface soil samples varying widely in their origin and properties were selected for this study corresponding to the dominant soil orders from Egypt and Greece. Four of them were from Egypt representing belonging to the orders Entisols (developed on fluvial, lacustrine and sandy marine deposits) and Aridisols (calcareous

Lability of sorbed Cd and Pb was evaluated at the end of sorption experiment by DTPA extraction (Cottenie et al., 1982). A 10-mL DTPA solution was added to each centrifuge tube containing Cd or Pb sorbed samples from the sorption experiment after washing with isobutyl

S.M. Shaheen / Geoderma 153 (2009) 61–68

63

Table 1 Classification and selected properties of the studied soils. Soil order

Classification

pH

CEC

OM

TCE

Sand

Clay

Fed

Mnd

Ald

Sid

Feo

Mno

Alo

Sio

g kg− 1 Egyptian soils Entisol1 fluvial Entisol2 lacustrine Entisol3 marine Aridisol Greek soils Entisol fluvial Vertisol Mollisol Histosol Alfisol1 Alfisol2 Alfisol3

Typic Typic Typic Typic

Ustifluvent Xerofluvent Xeropsament Calcorthids

8.1b⁎ 8.1b 8.7a 8.6a

56.9b 64.2b 5.4f 13.1e

22.2b 26.8b 5.7d 16.1c

31.5e 86.0d 11.2f 333.0b

55.4e 28.0e 917.6a 331.9d

452.4b 528.0a 47.3e 273.3c

5.7c 6.6c 0.7f 4.1d

0.59b 0.64b 0.08e 0.26d

0.64c 0.95b 0.27e 0.53d

1.33a 1.59a 0.27c 0.78b

2.12c 3.87b 0.25e 1.06c

0.38b 0.46b 0.05d 0.14c

1.95b 2.54b 0.35e 1.09c

0.94b 1.33a 0.23d 0.45c

Typic Typic Typic Typic Typic Typic Typic

Xerofluvent Chromoxerert Calcixeroll Haplofibrists Rhodoxeralf Rhodoxeralf Haploxeralf

7.8b 7.9b 7.7b 7.4b 7.8b 5.1c 5.2c

13.6e 37.5c 31.0c 87.6a 28.1c 13.7e 23.4d

13.1c 24.1b 23.1b 361.5a 17.2c 12.7c 16.1c

104.5c 132.0c 173.8c 423.5a 17.3f nd nd

750.0b 390.0d 520.0c – 471.0c 550.0c 530.0c

90.0e 360.0b 270.0c – 260.0c 220.0c 170.0d

2.9e 3.5d 3.8d 7.8b 13.6a 12.5a 7.9b

0.30d 0.44c 0.38c 0.41c 0.58b 1.08a 0.44c

0.30e 0.46d 0.41d 1.9a 0.97b 1.01b 0.70c

0.84b 1.15a 0.78b 0.45b 0.91b 0.17c 0.15c

1.12d 0.99d 0.46e 10.2a 1.47c 1.49c 4.19b

0.19c 0.36b 0.13c 0.20c 0.38b 0.94a 0.37b

0.81d 1.23c 1.39c 3.44a 1.30c 0.91c 1.05c

0.56b 0.55b 0.44c 0.36c 0.59b 0.19d 0.25d

pH (1:1 H2O); OM: organic matter; CEC: Cation Exchange Capacity (cmol(+)/kg); TCE: total CaCO3 equivalent; Fed, Ald, Mnd & Sid: citrate-bicarbonate–dithionate extractable-Fe, Al, Mn, Si; Feo, Alo, Mno & Sio: ammonium oxalate–oxalic acid extractable-Fe, Al, Mn & Si; nd: not detected; (–): not measured. ⁎Values accompanied by different letters are significantly different within columns at the probability level (P < 0.05).

alcohol. Then the tubes were shaken for 120 min on a reciprocating shaker to extract Cd and Pb, centrifuged, and the supernatant solutions were filtered and analyzed for Cd or Pb content by atomic absorption spectrometry (Varian, SpectrAA-400 Plus, Australia). The amounts of Cd and Pb extracted by DTPA at the end of Cd and Pb sorption experiment were designated as the portion of sorbed Cd and Pb retained in the labile pool, whereas Cd and Pb un-extractable by DTPA was attributed to soil Cd and Pb in the non-labile pool. The Proc GLM procedure in the SAS software package was used for computation (SAS Institute, 2003). 3. Results and discussion 3.1. Classification and characterization of the studied soils The classification of studied soils (Soil Survey Staff, 1998) and their basic properties are presented in Table 1. The Egyptian soils were classified as Entisols (developed on fluvial, lacustrine and sandy marine deposits) and Aridisols (calcareous deposits). The Greek soils belonged to the soil orders Entisols, Alfisols, Inceptisols, Vertisols, Mollisols, and Histosols. The soils studied differed widely in their physical and chemical properties. The diverse geological nature of these deposits is reflected in the wide variation of soil pH, clay, carbonates, and total free and amorphous forms of Fe, Al, Mn and Si oxides content. All the Egyptian soils were alkaline with pH values ranging from 8.1 to 8.7, while the Greek soils differed widely in their pH values which ranged from 5.1 in Alfisol2 (Rhodoxeralf) and Alfisol3 (Haploxeralf) to 7.9 in Vertisol. Clay content ranged from an average of 47.3 g kg− 1 in marine soils to 528.0 g kg− 1 in lacustrine soils. Cation exchange capacity ranged from 5.4 in marine Egyptian Entisols to 87.6 cmol kg− 1 in Greek Histosols, organic matter content from 0.57 g kg− 1 in marine Egyptian Entisols to 361.5 g kg− 1 in Histosol, total calcium carbonates equivalent from non detected values in the acid Alfisols to 423.5 g kg− 1 in Histosol (Table 1). Total Fe, Al and Mn concentrations differed greatly between soils. The Feo, Mno and So values were low compared to Fed, Mnd, and Sio for all soils except for Fe in Histosol and Si in the acidic two Alfisols suggesting that the majority of Fe, Mn and Si existed in crystalline forms in the clayey inorganic soils. However, iron exists in the amorphous form in the organic soils and the same was true for Si in the acidic Alfisols. On the other hand, Alo values were higher than those of Ald in all the soils especially in the clayey ones suggesting that Al oxides mainly existed in amorphous form. The values of all the aforementioned properties differed significantly between soil groups (Table 1). Consequently, the great differences in these properties were expected to affect cadmium and lead sorption characteristics in the studied soils.

3.2. Effect of soil types and properties on Cd and Pb sorption and distribution coefficient (Kd) Data in Table 2 and Fig. 1 showed that large variations in Cd and Pb sorbed were recorded between soils developed on different parent materials as influenced by their physico-chemical properties as suggested by the high variations in Kf values. Freundlich model described very well Cd and Pb sorption since R2 values were found to be higher than 0.93. The effectiveness of Freundlich equation in describing Cd and Pb sorption was reported also by others (Ponizovsky and Tsadilas, 2003; Shaheen et al., 2007; Usman, 2008). The distribution coefficient (Kd) is a useful index for comparing the sorptive capacities of different soils or materials for a particular ion under the same experimental conditions (Alloway, 1995). It is defined as the ratio of the metal concentration in the solid phase to that in the equilibrium solution after a specified reaction time (Anderson and Christensen, 1988; Alloway, 1995). Such coefficient represents the net result of all the various processes by which metal ions can be transferred between soil and solution, and are satisfactory for comparing the behaviour of different soils with respect to a given cation under fixed conditions. It is especially useful when the irregularity of empirical sorption and/or desorption isotherms hampers or prevents the fitting of simple empirical curves or theoretical models such as the Freundlich and Langmuir isotherms, as is often the case when the presence of more than one metal results in competition for sorption sites. A high Kd value indicates high metal retention by the solid phase through chemical reactions, leading to low metal bioavailability. Similarly, a low Kd value indicates that a Table 2 Calculated Freundlich parameters of the sorption isotherm of Pb and Cd by the studied soils. Soils

Egyptian soils Fluvial Lacustrine Marine Aridisol Greek soils Entisol Vertisol Mollisol Histosol Alfisol1 Alfisol2 Alfisol3

Pb

Cd

Kf, L kg− 1

n

R2

Kf, L kg− 1

n

R2

7128.5 7998.3 2398.8 13,031.7

0.29 0.28 0.14 0.67

0.98 0.98 0.98 0.98

639.1 1074.0 245.5 628.1

0.53 0.64 0.36 0.54

0.99 0.99 0.99 0.98

3715.3 6918.3 6309.6 13,152.2 3630.8 1737.8 1548.8

0.23 0.36 0.34 0.63 0.14 0.12 0.17

0.94 0.96 0.99 0.93 0.93 0.95 0.97

255.7 575.4 467.7 1737.8 354.8 20.4 75.9

0.44 0.58 0.54 0.95 0.42 0.74 0.60

0.99 0.99 0.99 0.93 0.99 0.98 0.99

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S.M. Shaheen / Geoderma 153 (2009) 61–68

Fig. 1. Sorption isotherm of cadmium and lead in the studied soils. On the y-axis q represents the metal concentration sorbed onto solid phases (mg kg− 1), and on the x-axis C represents metal equilibrium concentration in solution (mg L− 1).

high metal amount remains in the solution (Anderson and Christensen, 1988; Gomes et al., 2001; Covelo et al., 2004a,b,c,d; Covelo et al., 2007a,b,c,d; Shaheen et al., 2009a). Therefore, a further analysis of the obtained data based on the distribution coefficients was done. The distribution coefficient (Kd) was calculated over the whole range of the added concentrations of Pb and Cd. Also, the sorption selectivity sequence of Pb and Cd by the studied soils has been established at Kd medium values to obtain one comparable value for each metal and each

soil (Vega et al., 2006; Shaheen et al., 2007; Usman, 2008; Shaheen et al., 2009b; Tsadilas et al., 2009) as shown in Table 3. The data of Table 3 show that in all the soils studied the percentage of total sorbed Cd and Pb decreased with increasing the initial Cd and Pb concentration, as suggested by the decrease of Kd values with the increase Cd and Pb addition. This indicates that changes occur in the nature of the sites involved in the sorption process, depending upon the metal concentration as it was suggested by Sastre et al. (2006).

Table 3 The distribution coefficient, Kd (L kg− 1) calculated for each added metal concentration and Kd Initial conc., mM Pb 0.5 1.0 1.5 2.0 3.0 4.0 Kd medium Cd 0.25 0.50 0.75 1.00 1.25 1.50 2.00 Kd medium

medium

in the studied soils.

Egyptian soils

Greek soils

Fluvial

Lacustrine

Marine

Aridisol

Entisol

Vertisol

Mollisol

Histosol

Alfisol1

Alfisol2

Alfisol3

112,445.9 58,932.7 27,160.5 18,429.0 9873.8 4372.6 38,535.8ab

220,291.5 81,192.2 60,635.8 27,491.7 12,633.4 7278.2 68,253.8a

5640.8 490.0 210.0 51.8 34.1 18.4 1074.2bc

31,339.7 25,549.1 22,541.8 21,477.2 19,605.9 15,423.7 22,656.2b

13,346.4 9901.3 3053.9 1863.6 575.5 261.5 4833.7bc

54,285.1 32,306.1 16,800.3 9025.9 7619.4 6508.9 21,091.0b

52,801.0 28,840.4 13,384.5 8300.5 6437.7 4046.8 18,968.5b

39,845.8 30,832.0 23,857.3 21,350.0 20,049.2 19,604.4 25,923.1b

47,740.6 7984.2 4261.8 827.7 136.0 50.4 10,166.8b

404.6 78.6 28.1 17.5 11.5 7.6 91.3c

354.5 101.8 45.5 25.3 17.2 10.4 92.4c

2202.6 1545.3 1278.7 1060.8 999.2 819.2 681.3 1226.7b

199.0 77.5 39.0 29.0 22.1 17.5 13.0 56.7de

1128.1 717.0 523.2 420.9 356.8 267.8 196.8 515.8c

231.0 106.7 69.2 48.6 40.1 31.5 24.4 78.8de

906.5 604.4 470.7 381.4 339.0 267.5 212.7 454.6c

1270.0 716.1 525.7 405.3 362.5 272.2 204.8 536.7c

Means accompanied by different letters are significantly different within rows at the level (P < 0.05).

685.2 412.9 306.7 232.6 202.2 159.7 123.8 303.3cd

1641.1 1652.0 1681.2 1706.8 1712.9 1741.9 1761.2 1699.6a

463.0 203.8 124.6 86.2 67.7 50.9 36.1 147.5de

10.3 8.4 7.4 6.8 6.4 6.0 5.6 7.3e

37.8 24.7 19.6 17.2 15.4 13.9 11.9 20.1de

S.M. Shaheen / Geoderma 153 (2009) 61–68

The higher Kd value obtained in the experiment with lower metal concentrations is associated with the sorption sites of high selectivity, which have relatively strong bonding energies. Otherwise, heavy metal sorption becomes unspecific at higher metal concentrations, when the specific bonding sites become increasingly occupied, resulting in lower Kd values (Basta and Tabatabai, 1992b; Yu et al., 2002; Sastre et al., 2006). Increasing rates of Cd and Pb addition to the soils may result in saturation of Cd and Pb sorption sites, thereby decreasing the sorption capacity (Gomes et al., 2001). In this respect, Saha et al. (2002) explained that at low metal concentrations metals are mainly adsorbed onto specific sorption sites, while at higher metal concentrations soils lose some of their ability to bind heavy metals as sorption overlap, becoming thus less specific for a particular metal. This in turn induces a reduction in metal sorption. Lead and Cd sorption differed significantly between the studied soils as it resulted from the high variation of Kd values (Table 3, Figs. 2 and 3). The Egyptian lacustrine Entisol showed the highest affinity for Pb followed by Egyptian fluvial Entisol, Histosol, Aridisol, Vertisol, Mollisol, Alfisol1 (Alkaline Rhodoxeralf), Greek fluvial Entisol, Egyptian sandy marine Entisol, Alfisol3 (Acidic Haploxeralf) and Alfisol2 (Acidic Rhodoxeralf). On the other hand, Histosol showed the highest affinity for Cd followed by Egyptian lacustrine Entisol, Egyptian fluvial Entisol, Aridisol, Vertisol, Mollisol, Alfisol1 (Alkaline Rhodoxeralf), Greek fluvial Entisol, Egyptian sandy marine Entisol, Alfisol3 (Acidic Haploxeralf) and Alfisol2 (Acidic Rhodoxeralf). In general, the total amounts of Pb and Cd sorbed within the concentrations range used in the sorption experiments were as expected larger in alkaline than in acidic soils as suggested by Kd medium values, which were significantly (P < 0.05) higher than those of acidic soils (Table 3). Soil pH plays a major role in the sorption of heavy metals as it directly controls the solubilities of metal hydroxides, as well as metal carbonates and phosphates (Appel and Ma, 2002; Silveira et al., 2003). Soil pH also affects metal hydrolysis, ion-pair formation, organic matter solubility, as well as surface charge of iron and aluminum oxides, organic matter, and clay edges (Sauve et al., 1998; McBride, 1994). Soil pH increase, increases cationic heavy metal retention to soil surfaces via sorption, inner-sphere surface complexation, and/or precipitation and multinuclear type reactions (McBride, 1994; Sparks, 1995). This phenomenon has been demonstrated by many researchers in a variety of temperate region soils and soil mineral analogs in both batch and column studies (Basta et al., 1993; Rose and Bianchi-Mosquera, 1993; Yong and Phadungchewit, 1993; Altin et al., 1999). Soil sorption of Cd and Pb in our experiment followed the expected trend of increased metal sorption with increased soil pH, where, total amount of Cd and Pb

Fig. 2. Effect of soil types on Cd distribution coefficient.

65

Fig. 3. Effect of soil types on Pb distribution coefficient.

sorbed within the experimental concentrations range was extremely greater in alkaline than in acidic soils. In order to determine the influence of soil properties on the metal sorption capacity, coefficients of determination (R2) between the Kd medium values and the values of soil chemical properties were calculated (Table 4). These data show that the sorption affinity of Cd represented is influenced mainly by amorphous aluminum oxides (Alo) (R2 = 0.94, P < 0.001), CEC (R2 = 0.90, P < 0.001), clay content (R2 = 0.86, P < 0.001), organic matter content (R2 = 0.78, P < 0.001), amorphous iron oxides (Feo) (R2 = 0.79, P < 0.001), total free aluminum oxides (Ald) (R2 = 0.72, P < 0.001) and CaCO3 (R2 = 0.64, P < 0.05). On the other hand, Pb sorption is related to clay content (R2 = 0.90, P < 0.001), amorphous silica (R2 = 0.90, P < 0.001), total free silica (Sid) (R2 = 0.82, P < 0.001), amorphous aluminum (Alo) (R2 = 0.69, P < 0.001), and CEC (R2 = 0.70, P < 0.001). Egyptian lacustrine Entisol had the greatest distribution coefficient of Pb compared to all other tested soils. This may be attributed to its higher clay content, total free silica (Sid) and amorphous silica (Sio) as well as to its high cation exchange capacity (CEC) compared to the rest soils (Table 1). The statistical analysis showed that, Kd was Table 4 Coefficients of determination (R2) between selected soil properties and distribution coefficient (Kd) and lability of sorbed Cd and Pb. Soil properties

Kd, L Kg− 1 Cd

Pb

Cd

Pb

Clay Sand CEC OM pH TCCE Fed Ald Mnd Sid Feo Alo Mno Sio Cd _Kd Pb_Kd Labile Cd Labile Pb

0.86⁎⁎⁎ − 0.81⁎⁎⁎ 0.90⁎⁎⁎ 0.78⁎⁎⁎ ns 0.64⁎ ns 0.71⁎⁎ ns 0.98⁎⁎⁎ 0.79⁎⁎⁎ 0.94⁎⁎⁎ ns ns 1 0.72⁎⁎ ns −0.88⁎⁎⁎

0.90⁎⁎⁎ − 0.85⁎⁎⁎ 0.70⁎⁎⁎ ns ns ns ns ns ns 0.82⁎⁎⁎ ns 0.69⁎⁎ ns 0.90⁎⁎⁎ 0.72⁎⁎ 1 ns −0.68⁎⁎

ns ns ns ns ns − 0.67⁎⁎ ns ns ns ns ns ns ns ns ns ns 1 0.60⁎⁎

− 0.81⁎⁎⁎ 0.74⁎⁎⁎ − 0.88⁎⁎⁎ − 0.71⁎⁎ ns ns ns 0.65⁎⁎ ns ns − 0.64⁎⁎ − 0.88⁎⁎⁎ ns ns − 0.88⁎⁎⁎ − 0.68⁎⁎ 0.60⁎⁎ 1

Lability, %

OM: organic matter; CEC: Cation Exchange Capacity (cmol(+) kg− 1); TCCE: total CaCO3 equivalent; Fed, Ald, Mnd, & Sid: citrate-bicarbonate–dithionate extractable-Fe, Al, Mn, Si; Feo, Alo, Mno, & Sio: ammonium oxalate extractable-Fe, Al, Mn & Si. ⁎⁎⁎Significant at P ≤ 0.001; ⁎⁎significant at P ≤ 0.01; ⁎significant at P ≤ 0.05. ns = not significant.

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significantly correlated with clay, Sio and Sid content, and CEC as mentioned above and as shown in Table 3. These interpretations are in agreement with those reported by Sipos et al. (2005) and Usman (2008). In this respect, Serrano et al. (2005) reported that soils with higher pH and clay content had the greatest sorption capacity of Pb and Cd. On the other hand, the higher values of Cd _Kd in Histosol compared to the other tested soils are due to its higher organic matter and CaCO3, amorphous iron (Feo) and total free (Ald), and amorphous (Alo) aluminum oxides content and high CEC, (Table 1). Cadmium Kd was significantly correlated with Alo, CEC, organic matter, Feo, Ald and CaCO3 content (Table 4). Several previous studies identified that organic matter it as one of the main components controlling the distribution of Cd in soils, (Christensen,1989; Boekhold and Van der Zee, 1992; Sauve et al., 2000; Holm et al., 2003). The significant correlation of the heavy metals sorption with CEC was expectable, since these metals occur as cations adsorbed onto the soil exchange complex (Harter and Naidu, 2001). The higher the CEC value, the more exchange sites on soil minerals available for metal retention (Silveira et al., 2003). Egyptian fluvial Entisol contain more clay and has higher CEC values, organic matter and total free and amorphous Fe, Al, Mn and Si content (Table 1), and thus sorbed more Cd and Pb than the Greek fluvial Entisol (Tables 2 and 3). In this respect, Adriano (2001) reported that, the soil sorption capacity for most heavy metals increases with increasing pH and is controlled by soil components such as clay and sesquioxides. The high proportion of the clay fraction favors the creation of a high soil micro-porosity, which also influences the potential physical retention of Pb (Ma and Uren, 1998). On the other hand, Cd and Pb sorption by the Egyptian Aridisol may be attributed to the presence of high amounts of total calcium carbonate in this soil compared to all the other tested soils except for the Histosol (Table 1). This is in agreement with those reported by Elkhatib et al. (1991) who studied the sorption of Pb in Egyptian calcareous soils. Also, McBride (1980) reported that calcite had a high affinity for Cd sorption. From the two Greek red soils included in this study i.e. one alkaline (pH 7.81, Alfisol1) and the other acidic (pH 5.14, Alfisol2), although they contained similar amounts of clay, Fe and Al oxides (Table 1), the alkaline one showed significantly higher Cd and Pb_ Kd values (Table 3; Figs. 2 and 3). This is in agreement with those reported that Pb and Cd sorption by variable charge soils like ours, is pH dependent (Appel and Ma, 2002; Ponizovsky and Tsadilas, 2003; Moreno et al., 2006). Increased pH values may increase variable charges and hence metal sorption by clay minerals and also by iron oxides (Wu et al., 2003). Many researchers have shown increased Cd and/or Pb sorption in variable charges soils with increasing pH (Naidu et al., 1994; Naidu et al., 1997; Naidu et al., 1998) due mainly to increased negative surface charge. From the two Greek acidic Alfisols included in this study i.e. one Rhodoxeralf (pH 5.14, Alfisol2) and the other Haploxeralf (pH 5.25, Alfisol3), although they contained similar acidic pH, no significantly differences recorded for Pb sorption, while the Haploxeralf one showed significantly higher Cd _Kd values (Table 3,, Figs. 2 and 3). The higher Cd distribution coefficient of Haploxeralf compared to Rhodoxeralf may be attributed to its higher CEC and amorphous oxides content as well as and the higher active ratios of Fe & Al oxides (Feo/Fed and Alo/Ald respectively) (Table 1). This means that in variable charge soils with similar pH, Cd sorption capacities depends on the active ratios of Fe and Al oxides, while under similar sesquioxides content Cd sorption is pH dependent. On the other hand, the permanent charge clayey soils with relatively low Fe, Al and Mn oxides content adsorbed more Cd and Pb than the variable charge red soil with higher content of these oxides. Interestingly, although alkaline soils exhibited greater Cd and Pb sorption than acidic soils, the relationship between soil pH values and sorption of Cd and Pb was not significant (Table 4). This may be due to

the lower Cd and Pb distribution coefficient in some high alkaline soils such as sandy marine soil and the Greek Entisol as a result of its low clay, organic matter and sesquioxides content compared to the other tested soils (Table 1 and 3). These data showed that, with similar amounts of clay, organic matter and sesquioxides, the alkaline soils adsorb more Cd and Pb than acid soils. 3.3. Distribution coefficient values and selectivity sequences of Cd and Pb in the studied soils Lead presented the highest Kd values compared to Cd in all the studied soils, showing that it was retained stronger than Cd (Table 3, Figs. 2 and 3). These data demonstrated the preference of all the studied soils for Pb compared to Cd (Fig. 1). This is usually attributed to differences in metal characteristics and resultant affinity for sorption sites (McBride, 1994; Appel and Ma, 2002; Appel et al., 2008). For example, the hydrated radius of Pb2+ is smaller than that of Cd2+ (Pb2+ = 0.401 nm; Cd2 + = 0.426 nm; Nightingale, 1959), favoring Coulombic interactions of Pb with exchange sites. Furthermore, Pb has a greater affinity for most functional groups in organic matter including carboxylic and phenolic groups, which are hard Lewis bases. This is mainly attributed to the differences in chemical properties between the two metals. Lead as a harder Lewis acid (Pb2+ is a borderline Lewis acid while Cd2+ is a soft Lewis acid), has a higher electronegativity (2.33 and 1.69 for Pb and Cd, respectively) and lower pKH (negative log of hydrolysis constant; 7.71 and 10.1 for Pb and Cd, respectively) than Cd. These factors favor Pb for inner-sphere surface sorption/complexation reactions compared to Cd (McBride, 1994; Wulfsberg, 2000; Gomes et al., 2001). Lead (Pb2+ ) also has 2 valence electrons in its 6s atomic orbital (and empty p orbitals of only slightly higher energy), which can form, depending on the Pb–O symmetry, molecular orbitals with O 2p atomic orbitals originating from an oxide surface. This orbital overlap stabilizes the Pb–O complex. On the other hand, Cd2 + has a filled 4d valence atomic orbital, which participates minimally in electron sharing with O 2p atomic orbitals from oxide surfaces. The previously mentioned support that the sorption preference exhibited by these soils for Pb over the Cd may be attributed to: (i) the greater hydrolysis constant (ii) the higher atomic weight, (iii) the higher ionic radius, and subsequently smaller hydrated radius, and (iv) its larger Misono softness value, making it a better candidate than other metals for electrostatic and inner-sphere surface complexation reactions. These differences in soil affinity for Pb and Cd have been observed by others for soils and/or pure oxidic mineral systems (Basta and Tabatabai, 1992a; Pardo, 2000; Gomes et al., 2001; Appel and Ma, 2002; Saha et al., 2002; Lu et al., 2005; Vega et al., 2006; Appel et al., 2008; Usman, 2008). 3.4. Lability of the adsorbed Cd and Pb The sorbed Cd and Pb were partitioned into labile and non-labile pools distinguished by extracting with DTPA at the end of sorption experiment. The amount of labile Cd and Pb as a mean values differed among the tested soils. Generally, on average, about 58 to 70% of the total sorbed Cd was labile, while for Pb it was about 45 to 65% of the total sorbed Pb (Table 5). These data indicate that the lability of Cd was higher than Pb in all the studied soils. Therefore, Cd may pose more threats to the ground water contamination and plants than Pb. In this respect, Appel and Ma (2002) reported that, Pb demonstrated a higher affinity for soil sorption sites relative to Cd. The former metal also confirmed its ability to take part in inner-sphere surface reactions, rendering it much less bio-available and mobile in the soil environment, compared with Cd. Also, Appel et al. (2008) indicated that lead appears to be more readily undergo inner-sphere surface complexation with soil surface functional groups than Cd. Thus, it should be less labile than Cd. Cadmium tends to be more mobile in

S.M. Shaheen / Geoderma 153 (2009) 61–68 Table 5 Average percentage of labile forms⁎ of total sorbed Cd and Pb in the studied soils. Soils Egyptian soils Fluvial Lacustrine Marine Aridisols Greek soils Entisols Vertisols Mollisols Histosols Alfisols1 Alfisols2 Alfisols3

Cd

Pb

63.8 60.3 68.1 62.3

50.6 50.6 60.7 57.8

66.3 65.2 66.9 58.9 57.5 69.6 68.2

57.3 55.4 58.5 44.6 57.6 58.3 65.3

⁎Lead and cadmium extracted by DTPA at the end of sorption experiment. Average percentage of labile Pb and Cd was calculated from all concentration levels of sorption.

soils and therefore more available to plants than many other heavy metals including Pb (Alloway, 1995). Lability of sorbed Cd and Pb differed among the tested soils. Alfisols2 (Acidic Rhodoxeralf) showed the lowest lability of Cd, while Alfisols1 (Alkaline Rhodoxeralf) exhibited the highest values of labile Cd. On the other side the lowest labile Pb was in Histosols and the highest was in Alfisol3 (Typic Haploxeralf). In order to evaluate the influence of soil properties on the lability of sorbed Cd and Pb, the coefficients of determination (R2) between the mean values of DTPA-extractable Cd and Pb after sorption experiment and the basic soil properties were calculated (Table 4). These values showed that the lability of sorbed Cd was negatively correlated only with the amount of CaCO3 in the soils studied (R2 = −0.67, P < 0.01). This means that the presence of CaCO3 increase Cd sorption and reducing its lability. This interpretation was in agreement with Alloway (1995) who reported that soils containing free CaCO3 can sorb Cd and reduce its bioavailability. In contrast, lability of sorbed Pb correlated negatively with CEC (R2 = −0.88, P < 0.001), amorphous aluminum oxides (Alo) content (R2 = −0.88, P < 0.001), clay content (R2 = − 0.81, P < 0.001), organic matter content (R2 = − 0.71, P < 0.01), total free aluminum oxides (Ald) (R2 = −0.65, P < 0.01) and amorphous iron oxides content (Feo) (R2 = −0.64, P < 0.01). Lability of sorbed Cd and Pb in the studied soils depends on the mechanisms of metal sorption in soils, where it is important as these reactions dictate the strength of the metal–soil surface interaction. The stronger the interaction of Cd and/or Pb with the soil surface, the less the likelihood of environmental contamination (plant and ground water). On a relative basis, exchange reactions (i.e., reversible electrostatic or outer-sphere reactions) render the metals most labile, whereas inner-sphere complex formation and co-precipitation with soil surfaces (i.e., bond formation between contaminant metal and soil surface) cause the Cd and Pb to be retained strongly and in many cases nearly irreversibly (McBride, 1994). So, more studies are needed to understand the mechanisms of Cd and Pb sorption in the studied soils to explain the influence of these mechanisms in the lability of the sorbed metals to the close environment. 4. Conclusion In this study sorption characteristics and lability of the sorbed cadmium and lead in different soils from Egypt and Greece developed on different parent materials were assessed at varying metal concentrations. Freundlich model described well Cd and Pb sorption. Apparently due to Pb's chemical characteristics (relatively high electronegativity, lower pKH, small hydrated radius and electronic structure), this metal was sorbed stronger than Cd showing thus lower lability in all the soils studied and posing thus less threat to

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ground water systems and growing plants. Clayey inorganic soils showed higher affinity for Pb, while the organic Histosol showed the highest affinity for Cd. Acidic Alfisols showed the lowest affinity for both metals studied. However, acidic Alfisols (Rhodoxeralf) showed the lowest lability of sorbed Cd, while the alkaline one exhibited the highest Cd lability. The lowest lability of sorbed Pb was in Histosols and the highest in acidic Alfisol (Typic Haploxeralf). Acknowledgments This paper is a part of the work accomplished in Institute of Soil Mapping and Classification (ISMC) of the National Agricultural Research Foundation (NAGREF) under the scholarship of the State Foundation for Scholarship (IKY) of Greece. Thanks are expressed to IKY for financial support and Dr. Christos Tsadilas for supervising and reviewing the paper. Technical support of the laboratory staff of the Institute is greatly acknowledged. 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