Sorption and desorption behavior of lead in four different soils of India

doi:10.4236/as.2011.21007

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Vol.2, No.1, 41-48 (2011) Agricultural Sciences

doi:10.4236/as.2011.2 1007

Sorption and desorption behavior of lead in four

different soils of India

Sudarshan K. Dutta1,2*, Dhanwinder Singh1

1Department of Soils, Punjab Agricultural University, Ludhiana, Punjab, India;

2Department of Plant and Soil, University of Delaware, Newark, USA; *Corresponding Author: sudarshandutta@gmail.com

Received 20 October 2010; revised 11 November 2010; accepted 20 November 2010

ABSTRACT

Sorption and desorption mechanisms of lead

(Pb) were determined in four different soils col-

lected from different agro-climatic regions of

India. The soils were classified as: Fine loamy

mixed Typic Dystrudepts, fine sandy loam Typic

Ustochrepts, fine loamy Typic Ustochrept, and

fine sandy loam Udic Haplustalfs. Seven differ-

ent Pb solutions [Pb(NO3)2 dissolved in 0.01M

Ca(NO3)2] in a range of 400 to 2000µgL-1 were

applied to study the sorption amounts at 25-

(±2)oC and 45(±2)oC temperatures. With the in-

crease in application rate and temperature,

sorption amounts of Pb increased; however,

percentages of sorption of applied Pb were de-

creased. Sorptions were positively and signifi-

cantly (p ≤ 0.01) correlated with Langmuir ad-

sorption isotherm. Thermodynamic parameters

of sorption (i.e. Ko, Go, Ho, and So) were also

determined at two temperatures, 25(±2)oC and

45(±2)oC. Increase in Ko with the increase in

temperature indicated positive effect of tem-

perature on Pb sorption. High absolute values

of Go, and positive values of Ho, and So

suggested that the sorption reaction was spon-

taneous and endothermic. Sorbed Pb were de-

sorbed in Pb free 0.01 M Ca(NO3)2 solutions at

25(±2)oC and 45(±2)oC. Desorption amounts in-

creased with increase in the Pb application rate,

but not always with the increase in temperature.

Keywords: Desorption; Isotherm; Lead; Sorption;

Thermodynamics

1. INTRODUCTION

Lead (Pb) is a widespread non–biodegradable chem-

ical contaminant found in the soil receiving disposal of

city wastes, sewage, and industrial effluents [1]. The

bioavailability of Pb in soil is highly dependent on the

sorption (adsorption + precipitation) and desorption be-

havior of soil [2-4]. Sorption increases the immobility of

Pb while desorption increases the Pb concentration in

soil solution from the sink. Therefore, an understanding

of adsorption and desorption processes and their mecha-

nism is crucial for the assessment of the Pb contamina-

tion and the reclamation of such polluted soils [1]. Con-

siderable research has been done on the adsorption be-

havior of Pb on different soils. Adhikari and Singh [1]

studied the adsorption of Pb by some Indian soils. Aziz

[5] reported the sorption equilibrium of Pb in two dif-

ferent types of Palestine soils. However, Pb sorption

depends on the chemical and mineralogical characteris-

tics of the soils, and therefore varies among soil types

[1]. Therefore, more information related to Pb sorption

in different types of soils with different soil physico-

chemical properties is always helpful to strengthen our

present understanding of sorption.

The sorption of Pb is controlled by the thermody-

namic parameters related to the soil-metal interactions

[1]. Therefore, determination of the thermodynamic pa-

rameters can assist in the prediction of the final state of

metal in the soil system from an initial non-equilib-

rium state [6]. These thermodynamic parameters include

equilibrium constant, Ko; standard free energy, Go;

standard enthalpy, Ho; and standard entropy, So. For

example, Adhikari and Singh [1] reported that the sorp-

tion process can be better expressed by the evaluation of

the free energy change (Go) corresponding to the trans-

fer of element from bulk solution into soil surface. They

also reported that an understanding of the change in en-

thalpy (Ho) and entropy (So) helps in determining the

free energy change and disorders occurred during sorp-

tion process. In the present study, we focused on the

sorption of Pb (II) in four different soils of India, and

evaluated the thermodynamic parameters (Ko; Go; Ho;

and So) related to the interaction of the metal with soils

during sorption.

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

Soil can sorb excessive amount of trace metal ions,

like Pb, from soil solution, and therefore has drawn at-

tention of a lot of researchers [7]. However, only a few

studies are focused on the desorption kinetics of the

sorbed trace metals [8]. The thermodynamics related to

the sorption and desorption assumed to be different as

the sorption reactions involving trace metals are ex-

tremely rapid, and desorption reactions can be slower by

orders of magnitude [7,9,10]. Moreover, adsorption-

desorption reactions are often not completely reversible,

which is known as non-singularity, or hysteresis [7]. On

the contrary, number of studies related to Pb desorption

is very limited, especially for Indian soils. Therefore, in

the present study we also focused on the desorption

characteristics of the sorbed Pb.

Specific questions that were addressed in this study

include: 1) How do the sorption amounts of Pb by the

soil vary with the variation in application rate and soil

temperature, and how well do they fit with Langmuir

adsorption isotherms? 2) How do the thermodynamic

parameters (Ko, Go, Ho, and So) of Pb sorption vary

at two different temperatures? 3) How do the desorptions

of added Pb vary with the initial application rate and soil

temperature, and how well do the desorption data fit

with the Langmuir desorption isotherm?

2. MATERIALS AND METHODS

2.1. Soil Sampling and Analysis

Four surface soil samples (0-15 cm depth) expected to

differ in physicochemical properties, were collected

from four different agro-climatic zones (ACZs) of India

(based on the classification of Gajbhiye and Mandal,

[11]). The samples were collected from Palampur (ACZ

1, Himachal Pradesh; soil type: fine loamy mixed Typic

Dystrudepts), Jalandhar (ACZ 6, Punjab, soil type: fine

sandy loam Typic Ustochrepts ), Purulia (ACZ 7, West

Bengal, soil type: fine loamy Typic Ustochrept), and

Pudukkottai (ACZ 11, Tamil Nadu, soil type: fine sandy

loam Udic Haplustalfs) during summer (April-June) of

2004. The USDA system of soil classification was used

for the determination of soil textural classes. The soil

samples were collected mainly from the agricultural

fields with no history of receiving Pb applications. Sam-

ples were then air dried, crushed in wooden mortar and

pestle, and then passed through a 2 mm sieve. Then the

samples were stored in HDPE (high-density polyethyl-

ene) containers in laboratory until analyzed. The details

of the measurements of the soil physico-chemical proc-

esses were explained elsewhere [12]. Total Pb values

were measured by following the method of Tessier et al.

[13]. The DTPA extractable Pb contents were determined

by DTPA method [14].

2.2. Sorption/Desorption Studies

Batch equilibrium technique was adopted for the Pb

sorption study. Sorption of Pb at soil was determined by

equilibrating the soil samples (2 g of soil samples in

duplicate) with 20ml of 0.01M calcium nitrate [Ca-

(NO3)2] solutions containing seven different concentra-

tions of Pb. The concentrations of Pb used in our study

include 40, 80, 100, 120, 140, 160, and 200 µg ml-1 of

Pb (equivalent to 400, 800, 1000, 1200, 1400, 1600, and

2000 µg Pb g-1 soil, respectively). We used lead nitrate

[Pb(NO3)2] as a source of Pb. Calcium nitrate [Ca(NO3)2]

was used as the supporting electrolyte for reducing the

non-specific adsorption of Pb [15]. Previous studies also

reported a high (> 80%) reduction of non-specific sorp-

tion of heavy metals by using Ca(NO3)2 solution [16].

We preferred nitrate (NO3

-) as the electrolyte anion over

others (like chloride, Cl-) as NO3

- is less likely to form

complexes with metal ions [16,17].

To measure the thermodynamic parameters, sorption

and desorption experiments were carried out at two dif-

ferent temperatures, 25  2

˚C and 45  2˚C. Samples

were allowed to equilibrate at two temperatures (in trip-

licate concentrations as well as temperatures) for 24 hrs

followed by a 2 hrs shaking to attain the sorption equi-

librium; therefore, the total number of sample was 168

(i.e. 4 soils  3 replications  7 concentrations  2 tem-

peratures). The average values of these 3 replications are

presented in this paper. We also performed an initial

study prior to actual experiment to find out the time re-

quired to attain sorption equilibrium at both tempera-

tures (25  2˚C and 45  2˚C) and observed that sorption

did not increase after 2 hrs of shaking. Concentrations of

Pb were measured in these decanted solutions using

Avanta PM Flame Atomic Absorption Spectrophotome-

ter (AAS) (GBC Scientific Equipment, Dandenong,

Australia). The detection limit of the instrument at lower

side was 10 µg L-1 (0.01 ppm). Difference in the mass of

Pb in the solutions before and after equilibrium was con-

sidered as the amount of Pb sorbed per 2 g of soil.

Desorption was determined by measuring the Pb con-

centrations in the extracted solutions. Four to six extrac-

tions based on a pre-study were performed with pure (Pb

free) 0.01 M Ca(NO3)2 solution for the same soil sam-

ples used for sorption studies. Ca(NO3)2 solution was

used for the extraction of the heavy metals (like Pb) as

this could be a good indicator of the bioavailability of Pb

[1]. The number of extraction required for maximum

desorption was decided based on initial experiments

performed at both 25 and 45˚C (described in [12]).

2.3. Analysis of Sorption and Desorption

Data Using Langmuir Adsorption

Isotherms

To further our understanding of sorption and desorp-

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

tion processes, we fitted our Pb sorption and desorption

data to Langmuir adsorption and desorption isotherms.

The details of data analyses steps were described in

Adhikari and Singh [1]. In short, Langmuir adsorption

isotherm was used to calculate the adsorption parameters,

such as: Adsorption maxima ‘b’, and bonding energy

coefficient, ‘K’. The conventional form of the Langmuir

adsorption isotherm of Pb can be written as: Ce/(x/m) =

1/(K.b) + Ce/b (Eq. 1).

Where, Ce is the equilibrium Pb concentration (mg

L-1), x/m is the amount of Pb adsorbed by soil (mg kg-1),

‘b’ and ‘K’ are Langmuir constants. The constant ‘K’ is

related to the bonding energy of the adsorbent for the Pb

(L mg-1).

The Langmuir isotherm (Eq. 1) is a straight line

where Ce/(x/m) were plotted against Ce. The best fit val-

ues of the coefficients ‘b’ and ‘K’ were derived using the

isotherm equations, where b = 1/slope, and K = slope/

intercept.

Cumulative Pb desorption data were fitted to a modi-

fied Langmuir desorption isotherm, which can be written

as: De/R = 1/Kd Dm + De/Dm (Eq. 2).

Here, De is the concentration of Pb desorbed (mg L-1),

and Dm is the desorption maxima (mg kg-1 soil). R is the

amount of Pb desorbed per gram of soil (µgg-1 soil), Kd

is the constant related to the mobility of lead. Dm and Kd

were calculated from the linear plots of De versus De/R.

Previous studies [18] also applied this equation in their

Pb desorption studies in different soils of Punjab, India.

2.4. Analysis of Thermodynamic

Parameters

The Pb sorption data obtained at two different tem-

peratures (25  2˚C and 45  2˚C) were used to measure

the thermodynamic properties of the reactions. Among

the different thermodynamic parameters, the variation of

thermodynamic equilibrium constant (Ko) were com-

puted following the procedure described by Adhhikari

and Singh [1].

The value of Ko for the adsorption reaction can be ex-

pressed as: Ko = as/ae = s Cs/e Ce (Eq. 3).

Where, as denotes activity of adsorbed metal, ae is the

activity of metal in equilibrium solution. Cs is the

amount (milligrams) of metal adsorbed per unit volume

(liter) of solution in contact with the adsorbent surfaces,

Ce is the amount (milligrams) of solute per unit volume

(liter) of solution in contact with the adsorbent surfaces

at equilibrium, s is the activity coefficient of the sorbed

metals, and e denotes the activity coefficient of metal at

equilibrium. In physical chemistry it is assumed that

with the lowering of concentration (Cs and Ce), the ac-

tivity coefficients (s and e) approach unity. Therefore,

the equation becomes: Ko = Cs/ Ce (Eq. 4).

The Ko values were determined by plotting ln (Cs/ Ce)

versus Cs and extrapolating to Cs = 0. The standard free

energy (ΔGo) was determined by using the following

equation: (ΔGo) = –R T ln Ko (Eq. 5).

The standard enthalpy (∆Ho) was obtained by using

the integrated Vant Hoff equation:

ln K2

o/K1

o = ∆Ho/R [1/T1 – 1/T2] (Eq. 6).

The standard entropy (So) was measured as:

∆So = (∆Ho – ΔGo)/T (Eq. 7).

Statistical operations were performed using the statis-

tical software JMP (Version 8, SAS Institute Inc., Cary,

NC, USA)

3. RESULTS AND DISCUSSION

3.1. Physicochemical Properties of the

Experimental Soils

The physicochemical properties of the four experi-

mental soils have been described in Ta bl e 1 . The pH of

the soils ranged from 5.6 to 6.2; EC values were from

0.08 to 0.15 dSm-1; CEC varied from 11.5 to 23.4 cmol

(p+) per kg soil; clay contents were from 15 to 22%, and

% organic C ranged from 0.29 to 0.91%. The experi-

mental soils were non-calcareous and therefore did not

have any CaCO3. Total native Pb content of the soils

varied from 16.8 to 34.8 µg g-1 soil; whereas, the DTPA

Pb content ranged from 0.21 to 1.48 µgg-1 (Table 1). The

percentage of labile Pb, i.e. amount of Pb over total na-

tive Pb varied from 1.3 to 7.4%. Overall, the soils varied

in their physicochemical characteristics; and expected to

show variation in sorption and desorption amounts of

Pb.

3.2. Variation of Pb Sorption with Pb

Application Rate and Temperature

All the soil samples exhibited high capacities to sorb

Pb at both 25(±2)˚C and 45(±2)˚C. The amounts of Pb

sorption with varying application rates (400, 800, 1000,

1200, 1400, 1600, and 2000 gg-1 soil) have been re-

ported in Table 3. We observed that, with increase in Pb

concentrations from 400 to 2000 gg-1 soil, the amount

of Pb sorption also increased in all the four soils (Table

2).

This was expected because more Pb was available for

sorption on soil with the increase in application. However,

the percentage of Pb sorbed on soils decreased. This was

also expected because the availability of the binding sites

decreased with the increase in concentration. Previous

studies also reported an increase in Pb sorption with the

increase in application rate. Shaheen et al. [19] observed

Pb sorption in 11 different types of soils. They found

increase in Pb sorption with the increase in Pb applica-

tion rate from 1 to 4 m ML-1; however, they reported de-

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

Table 1. Physicochemical properties and total (Pb) contents of the collected soil samples.

Sampling Loca-

tion

/Soil type

pH EC

(dS m-1)

Organic

Carbon (%)

CaCO3

(%)

CEC

(cmol(p+)/kg

)

Sand

(%)

Silt

(%)

Clay

(%)

Total Pb

(µg g-1 )

DTPA

(µg g-1 )

Labile

Pb (%)

Palampur

(Fine loamy

mixed Typic

Dystrudepts)

6.2 0.08 0.91 ND*22.9 64.5 12 22 34.8 1.31 3.8

Jalandhar

(Fine sandy loam

Typic Usto-

chrepts)

5.6 0.08 0.60 ND 23.4 80.9 5 15 20.1 1.48 7.4

Purulia

(Fine loamy Typic

Ustochrept)

6.1 0.15 0.48 ND 17.9 47.8 33 19 24.9 0.94 3.8

Pudukkottai

(Fine sandy loam

Udic Haplustalfs)

6.1 0.09 0.29 ND 11.5 73.0 6 20 16.8 0.21 1.3

*ND = Non detectable

Tab le 2. Amount and percentage of Pb sorbed (g g-1) at four different soils receiving seven different initial Pb treatments at two

different temperatures (25 ± 2˚C and 45 ± 2˚C).

Amount of Pb applied per gram soil (g g-1) at 25 (±2)˚C

Sampling

locations 400 800 1000 1200 1400 1600 2000

Palampur 381.6 (96.5) 757.6 (94.7) 927.1 (92.7)1100.1 91.7) 1268.9 (90.0)1427.8 (89.1) 1763.4 (88.2)

Jalandhar 368.2 (92.05) 732.0 (91.5) 889.6 (89.1)1057.8 (88.1) 1221.1 (87.2)1349.5 (84.3) 1670.0 (83.5)

Purulia 386.6 (96.7) 758.1 (94.8) 921.8 (92.2)1066.3 (88.9) 1179.1 (84.2)1334.1 (83.4) 1629.8 (81.5)

Pudukkottai 355.4 (88.9) 678.9 (84.9) 803.8 (80.4)934.3 (77.9) 991.6 (70.83)1054.6 (65.3) 1216.3 (60.8)

Amount of Pb applied per gram soil (g g-1) at 45 (±2)˚C

Palampur 395.6 (98.9) 786.6 (98.5) 965.0 (96.5)1154.4 (96.3) 1320.9 (94.4)1480.6 (92.5) 1845.1 (92.3)

Jalandhar 397.7 (94.9) 755.2 (94.4) 930.2 (93.0)1115.1 (92.9) 1283.1 (91.7)1455.6 (91.0) 1802.8 (90.1)

Purulia 389.1 (97.3) 766.3 (95.8) 929.5 (93.0)1111.8 (92.6) 1271.2 (90.8)1435.6 (89.7) 1668.8 (83.4)

Pudukkottai 367.5 (91.8) 689.6 (86.2) 830.8 (83.1)993.5 (82.8) 1055.0 (77.1)1155.0 (72.2) 1269.4 (63.2)

(Figures within parentheses denote percent of Pb sorption)

Table 3. Langmuir adsorption isotherms, correlation coefficients, and Langmuir parameters for four experimental soils at two

different temperatures.

Sampling locations

At 25(±2)˚C

Linear Langmuir equations Correlations of

equations (R)

Adsorption maxima

(b) (mg kg-1)

Bonding energy coeffi-

cient (k) (L/mg)

Palampur y = 0.442x + 5.165 0.97* 2.26 0.086

Jalandhar y = 0.379x + 8.731 0.93* 2.39 0.043

Purulia y = 0.697x + 2.682 0.99* 1.44 0.259

Pudukkottai y = 0.701x + 11.207 0.99* 1.43 0.063

At 45(±2)˚C

Palampur y = 0.508x + 1.397 0.98* 1.97 0.360

Jalandhar y = 0.335x + 4.797 0.98* 2.99 0.070

Purulia y = 0.501x + 3.059 0.99* 1.99 0.164

Pudukkottai y = 0.692x + 7.797 0.99* 1.54 0.088

* Statistically significant at p ≤ 0.01.

crease in Pb sorption percentage due to less availability

of binding space.

Although we observed increase in Pb sorption with

increase in application rates for all the four experimental

soils, there were variations in the sorption amounts. The

sorption trend was Typic Dystrudepts > Typic Usto-

chrepts (loamy sand) > Typic Ustochrept (loamy) >

Typic Haplustalfs (i.e. Palampur soil > Jalandhar soil >

Purulia soil > Pudukkottai soil) at all concentrations.

This variation might be a consequence of the variation in

the % organic C, and CEC values of the soils. The %

Organic C and CEC values were also higher for Palam-

pur and Jalandhar soils as compared to that of Purulia

and Pudukkottai soils (Table 1). Previous studies also

reported that % organic C, and CEC played important

roles in Pb sorption [2,4,20,21]. Singh and Sekhon [18]

observed a significant correlation of sorption maxima

with CEC, clay contents, and % organic C for some of

the Punjab soils. Adhikari and Singh [1] reported that

variations in sorption maxima were correlated with the

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

pH, CEC, and organic carbon contents of the soils.

The effect of temperature on the amount of Pb sorp-

tion can also be observed in Ta b l e 2. We found that Pb

sorption and percentage of applied Pb sorbed increased

with increase in temperature from 25˚C to 45˚C at all the

levels of Pb application (Table 2). Adhikari and Singh [1]

also reported an increase in Pb sorption with increase in

temperature from 25˚C to 45˚C. They concluded that the

Pb sorption reaction was endothermic which resulted in

an increase in Pb sorption with the increase in the tem-

perature.

We also compared the sorption isotherms at the two

different temperatures with the different solute sorption

isotherms based on the classification of Giles et al. [22].

According to Giles et al. [22], there are four different

solute sorption isotherms: namely ‘S’, ‘L’, ‘H’, and ‘C’

curves; and each of them corresponds to a different sol-

ute/sorbent interaction. They mentioned that the shape of

these isotherms also provide information related to the

strength by which the sorbate is held/attached to the soil.

The Pb sorption isotherms of our four experimental soils

were presented in Figures 1(a)-(d). By comparing our

present graphs with Giles et al. [22], we observed ‘L’

type of curve for all the four soils at 25˚C temperature

(Figure 1(a)-(d)). However, in 45˚C, the curve types

varied among the soils. In Palampur and Jalandhar soils,

the sorption isotherms were approaching to be ‘H’ type

(Figure 1(a), (b)); whereas, for Purulia and Pudukottai

soils, the sorption isotherms were ‘L’ type (Figure 1(c),

(d)). According to Giles et al. [22], ‘L’ type of curves

corresponds to a strong affinity between metallic cations

and the sorbent surface, which favors specific sorption.

On the other hand, ‘H’ type of curve indicates strong

sorbate-substrate attraction force and may lead to pre-

cipitation. Martı́nez-Villegas et al. [17] also observed the

same Pb sorption isotherm pattern with the increase in

Pb concentrations from 10 to 400 mg L-1 in different

Mexican soils.

3.3. Pb Sorption and Langmuir Adsorption

Isotherm

The Pb sorption data were fitted to the Langmuir and

Freundlich adsorption isotherms. In the case of Lang-

muir adsorption isotherm, we observed that the sorption

data fitted significantly (p ≤ 0.01) in the isotherms for all

the experimental soils and at both 25˚C and 45˚C tem-

peratures (Table 3). The observed correlations were also

high (R = 0.93 to 0.99) (Ta b le 3). Previous studies also

reported significant correlations of their sorption data

and the Langmuir adsorption isotherms [18,23]. For

example, Singh and Sekhon, [18] described that their Pb

sorption in alkaline soils fitted significantly (p ≤ 0.05)

with Langmuir adsorption isotherm. In all of our ex-

(a) (b)

Figure 1. (a) Lead sorption isotherms for Palampur soil. At 25˚C the sorption isotherm is ‘L’ type, and

at 45˚C the isotherm trends towards ‘H’ type. (b) Lead sorption isotherms for Jalandhar soil. At 25˚C

the sorption isotherm is ‘L’ type, and at 45˚C the isotherm is ‘H’ type. (c) Lead sorption isotherms for

Purulia soil. At both 25˚C and 45˚C the sorption isotherm is ‘L’ type. (d) Lead sorption isotherms for

Pudukkottai soil. At both 25˚C and 45˚C the sorption isotherm is ‘L’ type.

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

perimental soils, we observed that the Langmuir adsorp-

tion maxima (‘b’) increased with the increase in tem-

perature from 25˚C to 45˚C (Table 3). The ‘b’ values

were in the order of Jalandhar soil > Palampur soil >

Purulia soil > Pudukkottai soil at both the temperatures.

The physicochemical properties of the Jalandhar soil and

the Palampur soil were almost similar and might have

resulted in similar ‘b’ values (2.26 and 2.39µgml-1, re-

spectively). The higher 'b’ values for Palampur and

Jalandhar soils compared to Purulia and Pudukkottai soil

can be attributed to the higher % organic C, and CEC

values of these soils. Soil CEC and % organic C play

important role in Pb sorption [4,12]. Adhikari and Singh

[1] also reported highest Pb sorption maxima for the soil

which had the highest CEC, and % organic C contents

among their experimental soils. The bonding energy

coefficient (k) of experimental soils varied from 0.043 to

0.259 L mg-1 at 25˚C; and with the increase in tempera-

ture to 45˚C, the ‘k’ values also increased (Table 3).

Adhikari and Singh [1] also studied sorption of Pb in

four different soils of India at 25 and 45˚C and reported

the range of bonding energy coefficient (k) from 0.094

to 0.135 L mg-1.

3.4. Thermodynamic Parameters of

Pb Sorption in Soils

The thermodynamic parameters which include ther-

modynamic equilibrium constant (Ko), standard free

energy (Go), standard enthalpy (Ho), and standard

entropy (So), provide an insight into the Pb sorption

mechanism in the soils [1]. We calculated all these men-

tioned parameters at both 25˚C to 45˚C (Table 4). We

observed that, ‘Ko’ increased with a rise in the tempera-

ture from 25˚C to 45˚C for all the four soils. The calcu-

lated free energy (Go) values were negative. Since the

negative ‘Go’ denotes the amount of energy diminished

or required before reaching equilibrium, therefore, more

negative ‘Go’ value indicates more Pb sorption [1]. We

also observed the highest amount of Pb sorption in our

Palampur soil which had the highest absolute value of

‘Go’. Moreover, in all the experimental soils, the ‘Go’

values were more negative at a higher temperature,

which suggested more spontaneity of the sorption reac-

tion at a higher temperature [6]. The values of isosteric

heat or enthalpy (Ho) of Pb sorption were positive

ranging from 27.5 to 92.9 KJ mol-1. This suggests that

the Pb sorption reactions are endothermic in nature. Pre-

vious studies [1] also found good correlation between

the thermodynamic parameters (Go, and Ho), and the

soil physico-chemical properties (pH, CEC, and % or-

ganic C) of the soils. The standard entropy (So) values

for all the four soils at both the temperatures were posi-

tive ranging from 112.61 to 348.12 J mole-1 K-1 at 25˚C

and 107.42 to 334.68 J mole-1 K

-1 at 45˚C (Table 4).

Adhikari and Singh, [1] also reported a decrease in So

with increase in temperature and suggested that, the dis-

order in the sorption process was lower at higher tem-

perature.

3.5. How does Pb Desorption Vary with

Initial Application Rate and

Temperature

The cumulative desorption of sorbed Pb increased with

the increase in the rate of application in all the experi-

mental soils (Table 5). The percentage of the Pb de-

sorbed from the amount sorbed also increased with Pb

application rate (Table 5). Padmanabham [24] suggested

that, with the increase in Pb concentrations, the available

sites for Pb binding become lower and the required

bonding energies for the Pb sorption become higher;

therefore, a larger amount of applied Pb become avail-

able for desorption. The Pb desorption data followed the

pattern of Pudukkottai soil > Purulia soil > Jalandhar soil

> Palampur soil. Therefore, we observed an overall op-

posite trend between the sorption and desorption data for

our four experimental soils; i.e. the soils having higher

sorption of Pb, desorbed less and vice-versa. It empha-

sizes that, sorption and desorption does not follow the

same patterns [7,8]. However, we did not observe any

certain pattern in Pb desorption with the increase in

temperature.

We hypothesize that, at high application rates some of

the Pb might get physically precipitated instead of

chemically bonded and therefore, a portion of sorbed Pb

did not show any certain desorption pattern.

3.6. Pb Desorption Isotherm

We fitted Pb desorption data with Langmuir desorption

equations for the experimental soils at both 25˚C and

45˚C (Table 6). We found the desorption data fitted well

with the Langmuir desorption isotherms. We found high

and significant correlation (p ≤ 0.01) among the Pb de-

Table 4. Thermodynamic parameters for Pb sorption in four experimental soils at two different temperatures (25 ± 2˚C and 45 ± 2˚C).

Ko Go (kJ mol-1) So (J mol-1 K-1)

Sampling locations 25(±2)oC 45(±2)oC 25(±2)oC 45(±2)oCHo (kJ mol-1) 25(±2)oC 45(±2)oC

Palampur 152.24 161.04 –10.75 –13.44 92.9 348.12 334.68

Jalandhar 72.99 73.63 –10.03 –11.37 38.5 163.76 156.82

Purulia 23.42 25.52 –7.81 –8.56 33.8 139.63 133.21

Pudukkottai 25.97 26.47 –6.07 –6.66 27.5 112.61 107.42

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

Table 5. Amount of Pb desorbed (g g-1) and percentage of desorbed Pb over sorption at four different soils receiving seven

different initial Pb treatments at two different temperatures (25 ± 2˚C and 45 ± 2˚C).

Amount of Pb applied per gram soil (g g-1) at 25 (±2)˚C

Sampling

locations

400 800 1000 1200 1400 1600 2000

Palampur 4.1 (1.1) 75.8 (10) 103.9 (11.2)136.5 (12.4)184.2 (14.5)289.9 (20.3) 360. 8 (20.5)

Jalandhar 7.8 (2.1) 88.3 (12.1) 122.8 (13.8)163.9 (15.5)217.7 (17.8)262.8 (19.5) 376.1 (22.5)

Purulia 7.9 (2) 92.2 (12.2) 164.4 (17.8)257.1 (24.1)320.2 (27.2)424.5 (31.8) 500.4 (30.7)

Pudukkottai 11.9 (3.3) 123.2 (18.1) 234.2 (29. 2)304.9 (32.6)339.3 (34.2)379.1 (35.9) 460.4 (37.9)

At 45 (±2)˚C

Palampur 4.8 (1.2) 42.8 (5.4) 82.8 (8.6) 106.5 (9.2) 161.4 (12.2)202.1 (13.6) 311.7 (16.9)

Jalandhar 22.9 (5.8) 72.2 (9.6) 106.8 (11.5)166.8 (14.9)240.2 (18.7)308.4 (21.2) 484.6 (26.9)

Purulia 13.2 (3.4) 60.1 (7.8) 100.9 (10.9)147.5 (13.3)223.1 (17.6)306.3 (21.3) 411.6 (24.7)

Pudukkottai 23.9 (6.5) 126.1 (18.3) 205.5 (24.7)287.4 (28.9)339.9 (32.2)394.9 (34.2) 486.2 (38.3)

(Figures within parentheses denote percent of Pb desorbed from the Pb sorbed during sorption study)

Table 6. Langmuir desorption equations, correlation coefficients and Langmuir parameters for various soils at 25 (±2)˚C and

45 (±2)˚C.

Sampling locations Linear Langmuir equationCorrelation of equations

(R)

Desorption maxima

Dm (mg kg-1 )

Desorption coefficient

Kd (L/mg)

At 25 (±2)˚C

Palampur y = 0.005x + 0.045 0.95* 182.62 0.116

Jalandhar y = 0.005x + 0.058 0.96* 192.31 0.10

Purulia y = 0.006x + 0.059 0.95* 166.67 0.101

Pudukkottai y = 0.007x + 0.087 0.95* 136.99 0.084

At 45 (±2)˚C

Palampur y = 0.005x + 0.032 0.96* 204.08 0.152

Jalandhar y = 0.005x + 0.066 0.99* 222.22 0.068

Purulia y = 0.005x + 0.047 0.98* 192.31 0.111

Pudukkottai y = 0.007x + 0.085 0.97* 149.25 0.080

* Statistically significant at p ≤ 0.01

sorption values and the Langmuir desorption equation (R

= 0.95-0.99) at both the temperatures. Desorption pa-

rameters i.e. desorption maxima (‘Dm’), and desorption

coefficient (‘Kd’) were also reported (Table 6). Previous

studies [23] also reported a significant (p ≤ 0.05) corre-

lation between Pb desorption and Langmuir desorption

isotherm.

Interestingly, although we observed higher amount of

‘Dm’ values for the Palampur as well as Jalandhar soils

compared to that of Purulia and Pudukkottai soils, the

desorption was higher for the last two soils. Less Go

values for Purulia and Pudukkottai soil signifies that the

sorption process was less spontaneous for them as com-

pared to the other two soils.

4. CONCLUSION

In our Pb sorption and desorption study, performed in

four different soils of India, we observed an increase in

Pb sorption at all of these soils with an increase in Pb

application. Thermodynamic parameters revealed that Pb

sorption was an endothermic reaction. We observed an

increase in the sorption amount with the increase in

temperature. On the basis of isotherm classification de-

scribed by Giles et al. [22], we observed that our Pb

sorption data followed the ‘L’ type of curve, which sig-

nifies a strong affinity between metallic cations and the

sorbent surface. The sorption data fitted well with Lang-

muir adsorption isotherms. At both 25˚C and 45˚C

(R2 = 0.96-0.99). In all the soils, Langmuir sorption

maxima ‘b’ and bonding energy coefficient ‘k’ increased

with the increase in temperature. Desorption of sorbed

Pb showed that sorption and desorption did not follow

the same pattern. Desorption data also fitted well to

Langmuir desorption isotherms (R2 = 0.91-0.98) for all

the experimental soils.

The sorption and desorption studies performed to date

were mostly in laboratory scale with sampled soil and

therefore have disturbed soil profile. On the contrary,

field scale study with undisturbed soil column is missing

although in-situ study is extremely important, especially

for remediation aspects. This is especially true as the

factors controlling the sorption and desorption behavior

of Pb in the soil are more varied, less human controlled,

and act simultaneously. However, the in-vitro studies

S. K. Dutta et al. / Agricultural Sciences 2 (20 11) 41-48

determining the fate of heavy metals like Pb could be an

initial step to understand the environmental processes.

Therefore, with the knowledge and concept of laboratory

analyses, in-situ field study related to heavy metal sorp-

tion is required under various environmental conditions.

5. ACKNOWLEDGEMENTS

Grateful acknowledgement is extended to Punjab Agricultural Uni-

versity, India, for providing laboratory facility. Indian Council of Ag-

riculture Research provided fellowship to the graduate student (Sudar-

shan Dutta) to complete the research. Authors show special gratitude to

H. S. Hundal, PhD, Department of Soils, and S. S. Bhardwaj (Phd),

Department of Chemistry, Punjab Agricultural University; Pritha Dey,

Department of Bioinformatics, Sikkim Manipal University of Health

Science & Technology, and Obaidur Rahaman, Chemistry Department,

University of Delaware for their overall support.

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