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Sorption equilibria of lead(II) on some Palestinian soils—the natural ion exchangers
Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
Sorption equilibria of lead(II) on some Palestinian soils—the natural ion exchangers Hasan M. Abdel Aziz ∗ Chemistry Department, Al Azhar University in Gaza, P.O. Box 1277, 001 Gaza-Palestine, Israel Received 28 January 2004; received in revised form 7 September 2004; accepted 27 October 2004 Available online 21 July 2005
Abstract Sorption equilibria of lead(II) on two different types of soils have been studied at 30 ◦ C and 50 ◦ C by a batch process. The sorption data are analyzed in terms of sorption isotherm, the Langmuir equation, the distribution coefficient and various thermodynamic parameters. The adsorption data are in close agreement with the Langmuir equation at both temperatures. Sorption of lead(II) is higher in Biet Hanoun soil than in Biet Lahya soil and lower at higher temperature in both soils. The thermodynamic equilibrium constant Ko and G◦ , H◦ and S◦ values have been calculated for predicting the nature of sorption. © 2004 Elsevier B.V. All rights reserved. Keywords: Lead(II); Natural ion exchanger; Soil; Sorption equilibria; Thermodynamics
1. Introduction Since the environmental pollution is increasing day-byday due to increase in industrialization and urbanization. The land disposal of municipal and industrial sludges has drawn increasing interest in the fixation of heavy metal ions by soil. The content of lead and other heavy metals in sludges can be high [1], and hence, there is a potential hazard of contaminating soils and plants with excess amounts of these meals. There is an abundance of information dealing with the uptake of lead by plants and soils [2,3]. Lead is considered to have a negative effect on the quality of food and fodder [4]. Lead is retained in soils by ion exchange/adsorption and its accumulation in soils may cause clinical problems in animals and human beings. Considerable work has been done on the adsorption behavior of lead(II) on montmorillonite, kaolinite and illite [5]. Harter [6] has studied the adsorption of lead and copper by some soils of Northern United States. In the present paper, sorption equilibrium of lead(II) on two different types of Northern Gaza Strip (Palestine) soils have ∗
Tel.: +972 8 2824020; fax: +972 8 2824020. E-mail address: hasan [email protected].
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.001
been studied. The purpose of this study was to understand the basic chemistry of lead(II) in soils and to evaluate thermodynamics parameters for the interaction of this metal with the soils.
2. Materials and methods The soils used in present investigation were surface samples (depth 0–30 cm) obtained from Biet Hanoun (north east of Gaza Strip) and Biet Lahya (north west of Gaza Strip). Both soils were ground in hammer mill fitted with a sieve to obtain samples of small homogeneous particle size (<2 mm size). The pH, electrical conductance (EC), cation exchange capacity (CEC), percentage of organic matter and surface area of the soils were estimated. The pH was recorded in 1:5 (mass ration) soil/water suspension at 25 ± 1 ◦ C with an Elico pH meter model Li-10 and EC with a Systronic conductivity meter with dip type cell [7]. The CEC, percent of organic matter and surface area of the soil samples were determined by the methods proposed by Ganguli [8] (percolation with ammonium acetate and the bases measured by sodium acetate solution), Walkley and Black [9] (combustion with a
2
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
Table 1 Properties of the two Palestinian soils studied Property
Biet Hanoun soil Biet Lahya soil
pH (1:5) EC ×10−4 (mho/cm) %Organic matter Cation exchange capacity (100 mmol/g) Exchange Ca(II) (100 mmol/g) Exchange Pb(II) (100 mmol/g) Surface area (m2 /g)
9.2 4.86 0.36 5.83 2.81 3.22 562.20
6.1 2.3 0.16 3.89 1.61 2.53 561.31
The statistical average of all the KD values was calculated by a linear regression equation forced through the origin: x Ce (2) KD = m 2 Ce where x/m is the amount of lead(II) sorbed per gram soil (g/g) and Ce , the concentration of lead(II) in equilibrium suspension (g/mL).
3. Results and discussions mixture of potassium dichromate and sulfuric acid at a bout 125 ◦ C) and Dayal and Hendricks [10] (saturation with ethylene glycol monoethyl ether), respectively. The values are summarized in Table 1. For sorption studies, one gram of soil sample was taken in different stoppered conical flasks containing varying amounts of pure Pb(NO3 )2 solution and the volume adjusted to 25 mL with distilled water. The flasks were shaken for 3 h at 30 ◦ C in the first set of experiments and at 50 ◦ C in the second set of experiments in an electric temperature controlled SICO shaker. The suspensions were then centrifuged at 5000 rpm for 10 min and lead(II) was estimated in the supernatant liquid by atomic absorption spectrophotometry (SHIMADZU 6601 F-AAS) at 283.3 nm. The amount of lead(II) sorbed was determined as the difference between the amount of lead(II) added and that left after equilibrium. The distribution coefficient values (KD) were determined by using the formula: KD =
I −F F
×
Total volume of solution (mL) Weight of the soil (gm)
(1)
where I and F are amount of lead(II) added (g) and the present in the solution after equilibrium, respectively.
The two soils under investigation were chosen because of their widely varying properties. It is evident from Table 1 that the pH, EC, organic matter content and cation exchange capacity of Biet Hanoun soil were higher than that of Biet Lahya. Where Biet Hanoun soil has high CEC value (583 mmol/g) with low percentage organic matter (0.36%) because the soil type at the area is vertisoil (Alluvial) of high content of (1:2) montmorillonite clay. The results of the effect of the sorption equilibrium time on the sorption of lead(II) by both the soils are given in Fig. 1. The sorption of lead(II) on both the soils reaches the equilibrium after 2 h for Biet Hanoun soil and 1.5 h for Biet Lahya soil. Therefore, these periods were chosen for sorption studies of lead(II) in the two soils. Sorption of lead(II) was studied by batch process in the concentration range 0–234 g/mL and 0–274 g/mL at 30 ◦ C and 50 ◦ C, respectively, for Biet Hanoun soil and 0–290 g/mL and 0–326 g/mL at 30 ◦ C and 50 ◦ C, respectively, for Biet Lahya soil. Sorption isotherm were plotted between the amount of lead(II) sorbed per gram soil (g/g) and amount of lead(II) in equilibrium suspension (g/mL). It is clear from Fig. 2 that sorption of lead(II) was higher in Biet Hanoun soil than Biet Lahya soil. The higher sorption of lead(II) on Biet Hanoun soil could be due to the
Fig. 1. Time dependence of sorption of lead(II) on Palestinian soils.
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
3
Fig. 2. Sorption isotherm of lead(II) on Palestinian soils.
greater amount of organic matter, higher pH and higher cation exchange capacity. The sorption of lead(II) decreases with the rise in temperature in both the soils, partly due to the weakening of attractive forces between lead(II) and soil and, partly, due to enhancement of thermal energies of the adsorbate; thus making the attractive force between lead(II) and soil weaker for the sorption of lead(II) at the binding sites. The higher statistical average values of all the KD values (Table 2) of Biet Hanoun soil (31.21 mL/g and 28.87 mL/g at 30 ◦ C and 50 ◦ C, respectively) than Biet Lahya soil (22.67 mL/g and 12.46 mL/g at 30 ◦ C and 50 ◦ C, respectively) also conform higher sorption of lead(II) on Biet Hanoun soil at both the temperatures. An examination of Fig. 2 reveals that the sorption isotherm at both temperatures, are of the “H” class [11] for both the soils. This class is caused by very high solute affinity (high-affinity ions exchanging with low-affinity ions). These isotherms result from strong sorption at low concentrations giving an apparent intercept on the ordinate. The shape of the isotherms suggested that the interaction between lead(II) ions was negligible, and therefore, the energy of activation for removal of lead(II) from the surface was independent of coverage [12,13]. The following Langmuir equation [14], Ce /(x/m) = 1/K + Ce /b
(4)
for sorption behaviour of lead (11) on both the soil is applied. The Langmuir constants K and b were obtained from the intercept and slope of curve of Ce /(x/m) versus Ce , respectively (Fig. 3), and the values are given in Table 2 The higher values of the Langmuir constant for Biet Hanoun soil than for Biet Lahya soil at both the temperatures, also confirm higher sorption of lead(II) in the former. Similarly, higher values of the constant at 30 ◦ C also suggest higher sorption of lead(II) on both the soils at lower temperature. These results are in accordance with those of Veith and Sposito [15], who studied the use of Langmuir equation in the interpretation of adsorption phenomena and with those of Harter [6] regarding the adsorption of lead and copper by the soils of northern United States. To study the role of pH in the sorption difference between the two soils, the pH of the soil with a lower pH of 6.0 (the Biet Lahya soil) was raised to 9.3 by adding dilute NaOH solution and was made equal to the pH of the soil with higher pH (the Biet Hanoun soil). It was observed that the sorption of lead(II) at 30 ◦ C on the treated Biet Lahya soil increases with the increase in pH but still remains less than that on Biet Hanoun soil (Fig. 2). The sorption isotherm for the treated Biet Lahya soil remains of the “H” class (Fig. 2) and the sorption behavior remains in close agreement with the linear form of the Langmuir equation. Table 2 shows that the Lang-
Table 2 Statistical average of all the KD Values and the Langmuir constants K and b, at different temperatures Soil source
pH
30 ◦ C KD (mL/g)
Biet Hanoun Biet Lahya Biet Lahya
9.3 6.0 9.3
31.21 22.67 28.45
50 ◦ C K
b (g/g)
KD (mL/g)
K
b (g/g)
0.7709 0.0552 0.2500
6.486 × 103
28.87 12.46
0.3473 0.0376
5.758 × 103 4.211 × 103
5.172 × 103 5.714 × 103
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H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
Fig. 3. Linear form for Langmuir isotherm of lead(II) on Palestinian soils.
muir constants K and b and the statistical average of all the KD values for the treated Biet Lahya soil were found to be higher than those for its untreated counterpart but lower than the Biet Hanoun soil at 30 ◦ C. these results confirm that pH is not the only factor to affect the sorption of lead(II) on the two soils but other factors such as amount of organic matter and cation exchange capacity, seem to play a significant role. The thermodynamic equilibrium constant (Ko ) for the sorption of lead(II) on the soils was calculated by the Bigger and Cheung [16] method as applied by Singh et al. [17]: Ko =
Cs νs × Ce νe
(5)
where Cs (g/g) is the amount of lead(II) adsorbed per gram of the solvent in contact with the soil, Ce (g/mL) the concentration of lead in equilibrium suspension, νs is the activity coefficient of the adsorbed solute and νe is the activity coefficient of the solute in equilibrium suspension. The value of Cs was calculated by using the equation proposed by Fu et al. [18]: Cs =
(P/M)A S/N(x/m)
(6)
where P is the density of the solvent (g/mL), M is the molecular weight of the solvent, A is the cross-sectional area of the solvent molecule (cm2 /molecule), N is Avogadro’s number, S the surface area of adsorbent (m2 /g) and x/m is the specific adsorption (mmole/g). The cross-sectional area of the solvent molecule was calculated by using the equation [19]: 2/3 24 −16 10 M A = 1.091 × 10 (7) NP The ratio of the activity coefficient was assumed to be unity in the dilute range studied [20]. As the concentration of
solute in the solution approached zero, the activity coefficient ν approached unity. Eq. (5) may then be written as: lim Cs →0
Cs = Ko Ce
(8)
the values of Ko were obtained by plotting in Cs /Ce versus Cs and extrapolating to zero Cs . From the values of thermodynamic equilibrium constant, free energy changes (G◦ ) during the sorption were calculated from the relationship: G◦ = −RT lnKo
(9)
where R is the universal gas constant and T the absolute temperature. The standard enthalpy change (H◦ ) was calculated from the integrated form of the Van’t Hoff equation assuming neglectable temperature dependence within the range studied: H ◦ 1 K2 1 =− ln (10) − K1 P T2 T1 and the standard entropy changes (S◦ ) were calculated from H◦ and G◦ values using the equation: S ◦ =
H ◦ − G◦ T
(11)
The values of the thermodynamic equilibrium constant Ko , standard free energy changes G◦ , standard entropy changes S◦ at 30 ◦ C and 50 ◦ C for the sorption of lead(II) on both the soils are summarized in Table 3. These results show higher values of Ko at 30 ◦ C than at 50 ◦ C for both the soils, indicating the higher percentage of lead(II) for the soils at lower temperature. However, these values were higher for Biet Hanoun soil than for Biet Lahya soil, which again confirms that sorption of lead(II) was higher in the former,
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5 Table 3 Various thermodynamic parameter for the sorption of lead(II) on the soils Thermodynamic parameters Ko G◦ (kJ/mol) H◦ (kJ/mol) S◦ (kJ/mol)
Biet Hanoun soil
Biet Lahya soil
30 ◦ C
50 ◦ C
30 ◦ C
50 ◦ C
2.98 × 107 −43.33 −109.95 −0.2195
1.99 × 106 −38.97
3.65 × 104 −26.45 −32.59 −0.0203
1.62 × 104 −26.08
−0.2198
−0.0203
at both temperatures. The results (Table 3) show negative values of G◦ for the sorption of lead(II) on the soils at both the temperatures. It is clear from theses results that the standard enthalpy change G◦ value is also negative, which indicates that sorption of lead(II) on the soils was exothermic and a decrease in temperature favored the reaction. The results of H◦ value together with that of G◦ values confirm that the ion exchange/sorption process had a natural tendency to proceed spontaneously. More negative values of H◦ for Biet Hanoun soil, confirms that lead(II) was more strongly bound to Biet Hanoun soil as compared to Biet Lahya soil. The results of standard entropy change S◦ of the system (Table 3) show a loss in entropy, more during the sorption of lead(II) on Biet Hanoun soil, indicating a greater order produced during the sorption phenomena. The negative values of entropy change suggest the there was a reduction in translational freedom when the solute was sorbed.
Acknowledgments The author is very thankful to Dr. Omar Nasman from the Chemistry Department and the members of the Water
5
Research Center of Al Azhar University for facilitating for the laboratory experiment of this research.
References [1] W.T. Doty, D.E. Baker, R.F. Shipp, J. Environ. Qual. 6 (1977) 421. [2] H.L. Motto, R.H. Daines, D.M. Chilko, C.K. Motto, Environ. Sci. Technol. 4 (1970) 231. [3] G.L. Terttar, R.R. Dedolph, R.H. Holzman, H.F, Lucas. Environ. Res. 2 (1969) 267. [4] J.V. Lagerwerff, Micronutrients, Agric. Proc. Symp. 2 (1972) 593–636. [5] J.E. Bittel, R.J. Miller, J. Environ. Qual. 3 (1974) 250. [6] R.D. Harter, Soil Sci. Soc. Am. Proc. 43 (1973) 679. [7] M.L. Jackson, Soil Chemical Analysis, Prentice Hall, Englewood Cliffs, NJ, 1967, pp. 62–64. [8] A.K. Ganguli, J. Phys. Colloid Chem. 55 (1951) 1417. [9] A. Walkley, I. A. Black, Soil Sci. 63 (1934) 251. [10] R.S. Dyal, S.B. Hendricks, Trans. Int. Conf. Soil Sci 2 (1950) 71. [11] C.H. Giles, T.H. McEwan, S.N. Nakhwa, D. Smith, J. Chem. Soc. 3973 (1960). [12] C.H. Giles, D. Smith, A. Huiston, J. Colloid Interface Sci. 47 (1974) 755. [13] C.H. Giles, A.P. D’silva, I.A. Easton, J. Colloid Interface Sci. 47 (1974) 766. [14] I. Langmuir, J. Am. Chem. Soc. 4 (1918) 1361. [15] J.A. Veith, G. Sposito, Soil Sci. Soc. Am. Proc. 41 (1977) 697. [16] J.W. Biggar, M.W. Cheung, Soil. Sci. Soc. Am. Proc. 37 (1973) 863. [17] R.P. Singh, K.G. Varshney, S. Rani, Ecotoxical Environ. Saf. 10 (1985) 309. [18] Y. Fu, R.S. Hanson, F.E. Bertell, J. Phys. Chem. 52 (1948) 374. [19] K. Kodera, Y. Onishi, Bull. Chem. Soc. Jpn. 32 (1959) 356. [20] R.A. Robinson, R.H. Stokes, Electrolyte Solution, Butterworths, London, 1959.
Sorption equilibria of lead(II) on some Palestinian soils—the natural ion exchangers Hasan M. Abdel Aziz ∗ Chemistry Department, Al Azhar University in Gaza, P.O. Box 1277, 001 Gaza-Palestine, Israel Received 28 January 2004; received in revised form 7 September 2004; accepted 27 October 2004 Available online 21 July 2005
Abstract Sorption equilibria of lead(II) on two different types of soils have been studied at 30 ◦ C and 50 ◦ C by a batch process. The sorption data are analyzed in terms of sorption isotherm, the Langmuir equation, the distribution coefficient and various thermodynamic parameters. The adsorption data are in close agreement with the Langmuir equation at both temperatures. Sorption of lead(II) is higher in Biet Hanoun soil than in Biet Lahya soil and lower at higher temperature in both soils. The thermodynamic equilibrium constant Ko and G◦ , H◦ and S◦ values have been calculated for predicting the nature of sorption. © 2004 Elsevier B.V. All rights reserved. Keywords: Lead(II); Natural ion exchanger; Soil; Sorption equilibria; Thermodynamics
1. Introduction Since the environmental pollution is increasing day-byday due to increase in industrialization and urbanization. The land disposal of municipal and industrial sludges has drawn increasing interest in the fixation of heavy metal ions by soil. The content of lead and other heavy metals in sludges can be high [1], and hence, there is a potential hazard of contaminating soils and plants with excess amounts of these meals. There is an abundance of information dealing with the uptake of lead by plants and soils [2,3]. Lead is considered to have a negative effect on the quality of food and fodder [4]. Lead is retained in soils by ion exchange/adsorption and its accumulation in soils may cause clinical problems in animals and human beings. Considerable work has been done on the adsorption behavior of lead(II) on montmorillonite, kaolinite and illite [5]. Harter [6] has studied the adsorption of lead and copper by some soils of Northern United States. In the present paper, sorption equilibrium of lead(II) on two different types of Northern Gaza Strip (Palestine) soils have ∗
Tel.: +972 8 2824020; fax: +972 8 2824020. E-mail address: hasan [email protected].
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.001
been studied. The purpose of this study was to understand the basic chemistry of lead(II) in soils and to evaluate thermodynamics parameters for the interaction of this metal with the soils.
2. Materials and methods The soils used in present investigation were surface samples (depth 0–30 cm) obtained from Biet Hanoun (north east of Gaza Strip) and Biet Lahya (north west of Gaza Strip). Both soils were ground in hammer mill fitted with a sieve to obtain samples of small homogeneous particle size (<2 mm size). The pH, electrical conductance (EC), cation exchange capacity (CEC), percentage of organic matter and surface area of the soils were estimated. The pH was recorded in 1:5 (mass ration) soil/water suspension at 25 ± 1 ◦ C with an Elico pH meter model Li-10 and EC with a Systronic conductivity meter with dip type cell [7]. The CEC, percent of organic matter and surface area of the soil samples were determined by the methods proposed by Ganguli [8] (percolation with ammonium acetate and the bases measured by sodium acetate solution), Walkley and Black [9] (combustion with a
2
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
Table 1 Properties of the two Palestinian soils studied Property
Biet Hanoun soil Biet Lahya soil
pH (1:5) EC ×10−4 (mho/cm) %Organic matter Cation exchange capacity (100 mmol/g) Exchange Ca(II) (100 mmol/g) Exchange Pb(II) (100 mmol/g) Surface area (m2 /g)
9.2 4.86 0.36 5.83 2.81 3.22 562.20
6.1 2.3 0.16 3.89 1.61 2.53 561.31
The statistical average of all the KD values was calculated by a linear regression equation forced through the origin: x Ce (2) KD = m 2 Ce where x/m is the amount of lead(II) sorbed per gram soil (g/g) and Ce , the concentration of lead(II) in equilibrium suspension (g/mL).
3. Results and discussions mixture of potassium dichromate and sulfuric acid at a bout 125 ◦ C) and Dayal and Hendricks [10] (saturation with ethylene glycol monoethyl ether), respectively. The values are summarized in Table 1. For sorption studies, one gram of soil sample was taken in different stoppered conical flasks containing varying amounts of pure Pb(NO3 )2 solution and the volume adjusted to 25 mL with distilled water. The flasks were shaken for 3 h at 30 ◦ C in the first set of experiments and at 50 ◦ C in the second set of experiments in an electric temperature controlled SICO shaker. The suspensions were then centrifuged at 5000 rpm for 10 min and lead(II) was estimated in the supernatant liquid by atomic absorption spectrophotometry (SHIMADZU 6601 F-AAS) at 283.3 nm. The amount of lead(II) sorbed was determined as the difference between the amount of lead(II) added and that left after equilibrium. The distribution coefficient values (KD) were determined by using the formula: KD =
I −F F
×
Total volume of solution (mL) Weight of the soil (gm)
(1)
where I and F are amount of lead(II) added (g) and the present in the solution after equilibrium, respectively.
The two soils under investigation were chosen because of their widely varying properties. It is evident from Table 1 that the pH, EC, organic matter content and cation exchange capacity of Biet Hanoun soil were higher than that of Biet Lahya. Where Biet Hanoun soil has high CEC value (583 mmol/g) with low percentage organic matter (0.36%) because the soil type at the area is vertisoil (Alluvial) of high content of (1:2) montmorillonite clay. The results of the effect of the sorption equilibrium time on the sorption of lead(II) by both the soils are given in Fig. 1. The sorption of lead(II) on both the soils reaches the equilibrium after 2 h for Biet Hanoun soil and 1.5 h for Biet Lahya soil. Therefore, these periods were chosen for sorption studies of lead(II) in the two soils. Sorption of lead(II) was studied by batch process in the concentration range 0–234 g/mL and 0–274 g/mL at 30 ◦ C and 50 ◦ C, respectively, for Biet Hanoun soil and 0–290 g/mL and 0–326 g/mL at 30 ◦ C and 50 ◦ C, respectively, for Biet Lahya soil. Sorption isotherm were plotted between the amount of lead(II) sorbed per gram soil (g/g) and amount of lead(II) in equilibrium suspension (g/mL). It is clear from Fig. 2 that sorption of lead(II) was higher in Biet Hanoun soil than Biet Lahya soil. The higher sorption of lead(II) on Biet Hanoun soil could be due to the
Fig. 1. Time dependence of sorption of lead(II) on Palestinian soils.
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
3
Fig. 2. Sorption isotherm of lead(II) on Palestinian soils.
greater amount of organic matter, higher pH and higher cation exchange capacity. The sorption of lead(II) decreases with the rise in temperature in both the soils, partly due to the weakening of attractive forces between lead(II) and soil and, partly, due to enhancement of thermal energies of the adsorbate; thus making the attractive force between lead(II) and soil weaker for the sorption of lead(II) at the binding sites. The higher statistical average values of all the KD values (Table 2) of Biet Hanoun soil (31.21 mL/g and 28.87 mL/g at 30 ◦ C and 50 ◦ C, respectively) than Biet Lahya soil (22.67 mL/g and 12.46 mL/g at 30 ◦ C and 50 ◦ C, respectively) also conform higher sorption of lead(II) on Biet Hanoun soil at both the temperatures. An examination of Fig. 2 reveals that the sorption isotherm at both temperatures, are of the “H” class [11] for both the soils. This class is caused by very high solute affinity (high-affinity ions exchanging with low-affinity ions). These isotherms result from strong sorption at low concentrations giving an apparent intercept on the ordinate. The shape of the isotherms suggested that the interaction between lead(II) ions was negligible, and therefore, the energy of activation for removal of lead(II) from the surface was independent of coverage [12,13]. The following Langmuir equation [14], Ce /(x/m) = 1/K + Ce /b
(4)
for sorption behaviour of lead (11) on both the soil is applied. The Langmuir constants K and b were obtained from the intercept and slope of curve of Ce /(x/m) versus Ce , respectively (Fig. 3), and the values are given in Table 2 The higher values of the Langmuir constant for Biet Hanoun soil than for Biet Lahya soil at both the temperatures, also confirm higher sorption of lead(II) in the former. Similarly, higher values of the constant at 30 ◦ C also suggest higher sorption of lead(II) on both the soils at lower temperature. These results are in accordance with those of Veith and Sposito [15], who studied the use of Langmuir equation in the interpretation of adsorption phenomena and with those of Harter [6] regarding the adsorption of lead and copper by the soils of northern United States. To study the role of pH in the sorption difference between the two soils, the pH of the soil with a lower pH of 6.0 (the Biet Lahya soil) was raised to 9.3 by adding dilute NaOH solution and was made equal to the pH of the soil with higher pH (the Biet Hanoun soil). It was observed that the sorption of lead(II) at 30 ◦ C on the treated Biet Lahya soil increases with the increase in pH but still remains less than that on Biet Hanoun soil (Fig. 2). The sorption isotherm for the treated Biet Lahya soil remains of the “H” class (Fig. 2) and the sorption behavior remains in close agreement with the linear form of the Langmuir equation. Table 2 shows that the Lang-
Table 2 Statistical average of all the KD Values and the Langmuir constants K and b, at different temperatures Soil source
pH
30 ◦ C KD (mL/g)
Biet Hanoun Biet Lahya Biet Lahya
9.3 6.0 9.3
31.21 22.67 28.45
50 ◦ C K
b (g/g)
KD (mL/g)
K
b (g/g)
0.7709 0.0552 0.2500
6.486 × 103
28.87 12.46
0.3473 0.0376
5.758 × 103 4.211 × 103
5.172 × 103 5.714 × 103
4
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5
Fig. 3. Linear form for Langmuir isotherm of lead(II) on Palestinian soils.
muir constants K and b and the statistical average of all the KD values for the treated Biet Lahya soil were found to be higher than those for its untreated counterpart but lower than the Biet Hanoun soil at 30 ◦ C. these results confirm that pH is not the only factor to affect the sorption of lead(II) on the two soils but other factors such as amount of organic matter and cation exchange capacity, seem to play a significant role. The thermodynamic equilibrium constant (Ko ) for the sorption of lead(II) on the soils was calculated by the Bigger and Cheung [16] method as applied by Singh et al. [17]: Ko =
Cs νs × Ce νe
(5)
where Cs (g/g) is the amount of lead(II) adsorbed per gram of the solvent in contact with the soil, Ce (g/mL) the concentration of lead in equilibrium suspension, νs is the activity coefficient of the adsorbed solute and νe is the activity coefficient of the solute in equilibrium suspension. The value of Cs was calculated by using the equation proposed by Fu et al. [18]: Cs =
(P/M)A S/N(x/m)
(6)
where P is the density of the solvent (g/mL), M is the molecular weight of the solvent, A is the cross-sectional area of the solvent molecule (cm2 /molecule), N is Avogadro’s number, S the surface area of adsorbent (m2 /g) and x/m is the specific adsorption (mmole/g). The cross-sectional area of the solvent molecule was calculated by using the equation [19]: 2/3 24 −16 10 M A = 1.091 × 10 (7) NP The ratio of the activity coefficient was assumed to be unity in the dilute range studied [20]. As the concentration of
solute in the solution approached zero, the activity coefficient ν approached unity. Eq. (5) may then be written as: lim Cs →0
Cs = Ko Ce
(8)
the values of Ko were obtained by plotting in Cs /Ce versus Cs and extrapolating to zero Cs . From the values of thermodynamic equilibrium constant, free energy changes (G◦ ) during the sorption were calculated from the relationship: G◦ = −RT lnKo
(9)
where R is the universal gas constant and T the absolute temperature. The standard enthalpy change (H◦ ) was calculated from the integrated form of the Van’t Hoff equation assuming neglectable temperature dependence within the range studied: H ◦ 1 K2 1 =− ln (10) − K1 P T2 T1 and the standard entropy changes (S◦ ) were calculated from H◦ and G◦ values using the equation: S ◦ =
H ◦ − G◦ T
(11)
The values of the thermodynamic equilibrium constant Ko , standard free energy changes G◦ , standard entropy changes S◦ at 30 ◦ C and 50 ◦ C for the sorption of lead(II) on both the soils are summarized in Table 3. These results show higher values of Ko at 30 ◦ C than at 50 ◦ C for both the soils, indicating the higher percentage of lead(II) for the soils at lower temperature. However, these values were higher for Biet Hanoun soil than for Biet Lahya soil, which again confirms that sorption of lead(II) was higher in the former,
H.M.A. Aziz / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 1–5 Table 3 Various thermodynamic parameter for the sorption of lead(II) on the soils Thermodynamic parameters Ko G◦ (kJ/mol) H◦ (kJ/mol) S◦ (kJ/mol)
Biet Hanoun soil
Biet Lahya soil
30 ◦ C
50 ◦ C
30 ◦ C
50 ◦ C
2.98 × 107 −43.33 −109.95 −0.2195
1.99 × 106 −38.97
3.65 × 104 −26.45 −32.59 −0.0203
1.62 × 104 −26.08
−0.2198
−0.0203
at both temperatures. The results (Table 3) show negative values of G◦ for the sorption of lead(II) on the soils at both the temperatures. It is clear from theses results that the standard enthalpy change G◦ value is also negative, which indicates that sorption of lead(II) on the soils was exothermic and a decrease in temperature favored the reaction. The results of H◦ value together with that of G◦ values confirm that the ion exchange/sorption process had a natural tendency to proceed spontaneously. More negative values of H◦ for Biet Hanoun soil, confirms that lead(II) was more strongly bound to Biet Hanoun soil as compared to Biet Lahya soil. The results of standard entropy change S◦ of the system (Table 3) show a loss in entropy, more during the sorption of lead(II) on Biet Hanoun soil, indicating a greater order produced during the sorption phenomena. The negative values of entropy change suggest the there was a reduction in translational freedom when the solute was sorbed.
Acknowledgments The author is very thankful to Dr. Omar Nasman from the Chemistry Department and the members of the Water
5
Research Center of Al Azhar University for facilitating for the laboratory experiment of this research.
References [1] W.T. Doty, D.E. Baker, R.F. Shipp, J. Environ. Qual. 6 (1977) 421. [2] H.L. Motto, R.H. Daines, D.M. Chilko, C.K. Motto, Environ. Sci. Technol. 4 (1970) 231. [3] G.L. Terttar, R.R. Dedolph, R.H. Holzman, H.F, Lucas. Environ. Res. 2 (1969) 267. [4] J.V. Lagerwerff, Micronutrients, Agric. Proc. Symp. 2 (1972) 593–636. [5] J.E. Bittel, R.J. Miller, J. Environ. Qual. 3 (1974) 250. [6] R.D. Harter, Soil Sci. Soc. Am. Proc. 43 (1973) 679. [7] M.L. Jackson, Soil Chemical Analysis, Prentice Hall, Englewood Cliffs, NJ, 1967, pp. 62–64. [8] A.K. Ganguli, J. Phys. Colloid Chem. 55 (1951) 1417. [9] A. Walkley, I. A. Black, Soil Sci. 63 (1934) 251. [10] R.S. Dyal, S.B. Hendricks, Trans. Int. Conf. Soil Sci 2 (1950) 71. [11] C.H. Giles, T.H. McEwan, S.N. Nakhwa, D. Smith, J. Chem. Soc. 3973 (1960). [12] C.H. Giles, D. Smith, A. Huiston, J. Colloid Interface Sci. 47 (1974) 755. [13] C.H. Giles, A.P. D’silva, I.A. Easton, J. Colloid Interface Sci. 47 (1974) 766. [14] I. Langmuir, J. Am. Chem. Soc. 4 (1918) 1361. [15] J.A. Veith, G. Sposito, Soil Sci. Soc. Am. Proc. 41 (1977) 697. [16] J.W. Biggar, M.W. Cheung, Soil. Sci. Soc. Am. Proc. 37 (1973) 863. [17] R.P. Singh, K.G. Varshney, S. Rani, Ecotoxical Environ. Saf. 10 (1985) 309. [18] Y. Fu, R.S. Hanson, F.E. Bertell, J. Phys. Chem. 52 (1948) 374. [19] K. Kodera, Y. Onishi, Bull. Chem. Soc. Jpn. 32 (1959) 356. [20] R.A. Robinson, R.H. Stokes, Electrolyte Solution, Butterworths, London, 1959.