Adsorption of cadmium on different granulometric soil fractions: Influence of organic matter and temperature

Geoderma 189–190 (2012) 133–143

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Adsorption of cadmium on different granulometric soil fractions: Influence of organic matter and temperature Estelle Roth a,⁎, Valérie Mancier b, Bernard Fabre c a

Laboratoire Groupe de Spectrométrie Moléculaire Atmosphérique, UMR 6089, Université de Reims Champagne Ardenne, Moulin de la Housse, case 16-17, BP 1039, F-51687 Reims Cedex 2, France LACM-DTI, LRC-CEA 0534/EA 4302, UFR Sciences Exactes et Naturelles, BP 1039, F-51687 Reims Cedex 2, France c Laboratoire Gestion des Risques et Environnement, EA 2334, 3 bis rue Alfred Werner, F-68093 MULHOUSE Cedex, France b

a r t i c l e

i n f o

Article history: Received 7 July 2011 Received in revised form 13 March 2012 Accepted 22 April 2012 Available online 30 July 2012 Keywords: Cadmium Sorption isotherms Sorption kinetics Soil organic matter Soil coarse fraction

a b s t r a c t Kinetic and thermodynamic studies are carried out at low initial cadmium concentration on a soil sample extracted from Aspach le Bas in Eastern France. It can be concluded from kinetic experiments that the implied process is of pseudo‐second order. Rate constants, relaxation times and activation energy are calculated. The energy value determined in this study (40 kJ mol− 1) shows that the process implies both physisorption and chemisorption. The influence of soil particle size (i.e. of the initial soil and its different fractions: sand, coarse silt, fine silt and clay), organic matter and temperature on the cadmium sorption capacity is also investigated. The Langmuir and the Dubinin–Radushkevich models are chosen to analyze these thermodynamic experiments. The isotherms have revealed that the Cd 2+ sorption is enhanced by increasing temperature and organic matter for all the fraction but the sand (in this last case, no temperature or organic matter have changed Cd 2+ sorption). Thermodynamics parameters (ΔG, ΔH and ΔS) are determined from experimental isotherms fitting following three methods used in the literature. It can be deduced from the different results that adsorption (exo or endothermic according to the fraction). Finally, a mean free energy of adsorption is obtained via Dubinin–Radushkevich model and confirms that the two types of sorption (physical and chemical) act in the whole process. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Industrial activities and agricultural soils are an important source of heavy metal pollution due to fertilisers, sewage sludge, pesticides, herbicides and fungicides (Bourrelier and Berthelin, 1995; Juste, 1990). These metals can be either accumulated in plants (Hooda and Alloway, 1995; Sauerbeck and Styperek, 1988) or leached in the groundwater. They induce therefore a risk of food chain contamination (Decloître, 1998; Lacatusu et al., 1996). Studies on metal sorption by soils are considered of great importance to explain the mechanism of soil pollution and to estimate the extent of retention by soils. Among the heavy metals, cadmium, whose concentration in fertilizers ranges from 0.2 to 186.0 μg g − 1 (Grant et al., 2002), is potentially very toxic to mammals, aquatic lives and aerial plant growth (Johri et al., 2010). First studies on cadmium toxicity were published in the 70s (John et al., 1972). Since, the influence of organic matter content, pH (these two parameters having moreover a synergy effect), temperature,

⁎ Corresponding author. Tel.: + 33 326 91 32 31; fax: + 33 326 91 31 47. E-mail address: [email protected] (E. Roth). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.04.010

initial contaminant concentration and ionic strength on the sorption characteristics have been widely investigated (Aguilar-Carrillo et al., 2006; El-Kamash et al., 2005; Huang et al., 2007). Several studies focus on cadmium adsorption on an isolated soil component like clay with or without the presence of humic acid (Hizal and Apak, 2006; Lai et al., 2002) or kaolinite (Angove et al., 1997, 1998). In such cases, when a single soil component is isolated, a surface complexation sorption model is usually derived. Overall soils are tested too and the adsorption capacity is discussed regarding soil properties (Adhikari and Singh, 2003; Appel and Ma, 2002; Hosseinpur and Dandanmozd, 2010; Kim and Fergusson, 1992; Wang et al., 2009). However, the influence of the soil particle size is rarely studied in detail (John et al., 1972; Kim and Fergusson, 1992). In the present work, sorption experiments at 10, 20 and 30°C are conducted on different granulometric soil fractions at very low initial cadmium concentration (0.5 to 10 mg L − 1). This choice allows for the simulation of diffuse trace element contamination. First, kinetic experiments are performed on the whole soil to assess conditions for isotherm studies and adsorption kinetics is discussed. Afterwards, sorption experiments are done on four soil fractions obtained after sieving (the initial soil, sand, coarse silt, fine silt and clay) in the presence of organic matter or after its removal. Isotherms are then

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E. Roth et al. / Geoderma 189–190 (2012) 133–143

analyzed by the Langmuir and the Dubinin–Radushkevich models. These models give information about the soil retention capacity and the strength by which the contaminant is held to the soil. The influence of temperature, organic matter and granulometric size on the Cd sorption has also been researched. Thermodynamic parameters issued from isotherm analysis give an insight on the sorption mechanism from an initial non-equilibrium state. In the literature, several methods have been used to get the adsorption equilibrium constant used for the determination of free energy. A critical discussion about this equilibrium is thus presented. At least, three methods are used to determine sorption constant and derive thermochemical data (ΔG, ΔH and ΔS). 2. Theoretical aspects 2.1. Concerning the kinetic approach

ð1Þ

where qt (mg g − 1) is the amount of sorbed cadmium at t time, qe (mg g − 1) is the amount of sorbed cadmium at equilibrium i.e. at infinite time and k1 is the pseudo-first-order rate constant. The integrated form is the following: logðqe −qt Þ ¼ logðqe Þ−

k1 t 2:303

ð3Þ

the integrated equation is: t 1 t ¼ þ qt k2 :ðqe Þ2 qe

ð4Þ

where k2 is the pseudo-second order rate constant. k2 and qe are t versus t obtained from the slope and the intercept of the plot of qt with Y-axis. 2.2. Concerning the thermodynamic approach 2.2.1. Presentation of fitting models The partitioning coefficient Kd of a metal toward a soil is an important parameter for examining of contaminant migration through the subsurface natural soil to groundwater. If Kd is low, the soil has little or no ability to slow the contaminant movement. For a Kd value of 0 it will travel at the rate of infiltration of water. In contrast, high Kd values signify the substrate does not leach and on site remediation may be possible. Considering the importance of this parameter in risk assessment and remediation management, the United States Environmental Protection Agency dedicated a

qe Ce

ð5Þ

where qe is the substrate concentration in the soil at equilibrium (μg g − 1) and Ce the remaining substrate concentration in the solution at equilibrium (mg L − 1). Kd is given in mL g − 1 by Eq. (5) or L mol − 1 by the following Eq. (6): qe : MCd2þ Ce

ð6Þ

where MCd2+ is molecular the molecular weight of cadmium (i.e. 112.4 g mol − 1). Defining the partitioning coefficient as above implies that the adsorption of the contaminant is a linear isotherm adsorption model. But this is an approximation at low initial contaminant concentration and often contaminant adsorption on soils deviates from linearity. Isotherm models such as Langmuir and Dubinin–Radushkevich models were developed and are largely used in the literature to fit adsorption. The first model (Langmuir) was originally proposed to describe adsorption of gas molecules onto homogeneous solid surfaces (crystalline materials) that exhibit one type of adsorption site (Langmuir, 1918). It has been extended to describe the adsorption of solutes onto solid adsorbents including heterogeneous solids such as soils. The Langmuir model for adsorption isotherm is defined by:

ð2Þ

A plot of log(qe − qt) = f(t) allows for the determination of qe via the intercept of the plot with the Y-axis and k1 via the slope value. The pseudo-second-order model is based on the bulk concentration and is also independent of the sorbed solute concentration (Ho and McKay, 1999b):
dqt 2 ¼ k2 : qe −qt Þ dt

Kd ¼

Kd ¼

Kinetic studies regarding the determination of activation energy constitute a simple way to assess to the mechanisms that control the adsorption process. The two most widely used kinetic models have been used in this study to investigate the sorption mechanism. In the pseudo-firstorder model, based on Lagergren equation, the rate is function of the concentration of non occupied sites (Ho and McKay, 1999b): dqt ¼ k1 :ðqe −qt Þ dt

complete study and discussion about the partitioning coefficient meaning and its determination (USEPA, 1999). The Kd values are usually obtained from laboratory experiments at constant temperature. Three methods to measure Kd can be found: the in-situ batch method (that is used in this work), the laboratory flow-through (or column) method and the field modelling method. In batch experiments Kd is determined as:

qe ¼

q max K L C e ð1 þ K L C e Þ

ð7Þ

where qmax is the maximum adsorption capacity (mg g − 1 or mol g − 1) and KL is the Langmuir constant related to the energy of adsorption (L mg − 1 or L mol − 1), qe being always the substrate concentration in the soil at equilibrium (same unit as qmax). The theory of the Langmuir model implies that adsorption is monolayer and is based on the fact that the bonding energy of all the sites is uniform. But it is often not realistic in complex systems like soils. The second adsorption model is the Dubinin–Radushkevich isotherm (Dubinin, 1975) which does not require homogeneous adsorption sites. It is then well adapted for the adsorption of trace constituents. The equation used here is the following:
 2 qe ¼ qmax ⋅ exp −K DR ⋅ε

ð8Þ

with   1 ε ¼ RT⋅ ln 1 þ Ce

ð9Þ

where qmax is the maximum solute adsorption capacity (mg g − 1) and KDR the Dubinin–Radushkevich constant (mol² J −²). These two parameters are deduced from the intercept and the slope of the plot of ln(qe) versus ε 2. Fitting experiments with the Dubinin–Radushkevich model provides a value of an absolute mean free energy of adsorption |E| in J mol − 1: 1 jEj ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi 2⋅K DR

ð10Þ

E. Roth et al. / Geoderma 189–190 (2012) 133–143

Energies between 1 and 8 kJ mol − 1 signify physical adsorption phenomenon (Van der Walls interactions) and energies between 9 and 16 kJ mol − 1 are characteristic of chemical sorption or ion exchange (ionic or covalent bounding) (Donat et al., 2005). Between 8 and 9 kJ mol − 1, it can be supposed that the both adsorption types are implied in the whole process. The Langmuir model is based on physicochemical processes for which the bonding energy of all the adsorbing sites is the same (Petruzzelli et al., 1985) with a single sorption layer. This model fits well the sorption at low cadmium concentrations where the first sorption layer is still not completed. For high initial cadmium concentration, the Langmuir model is not adapted since it underestimates the adsorbed cadmium, especially for the samples without organic matter. Organic matter provides indeed supplementary sites (for adsorption or ion exchange) and when removed, the first layer of cadmium is completed with a lower initial Cd concentration. The point is particularly clear for the finest granulometric fractions that often provide more sorption sites. The Dubinin–Radushkevich model is suitable at low concentration for trace contaminants and is more general than the Langmuir model as not requiring homogeneous adsorption sites and constant adsorption potential (USEPA, 1999). In this study, the initial concentrations of cadmium [Cd2+]0 are weak (b10 mg L− 1): the Langmuir and the Dubinin–Radushkevich models are then suitable. They provide important data such as maximum adsorption capacities and adsorption affinities by interpreting KL and |E| values.

135

solvent molecular weight, s (cm 2 g − 1) the surface area of the adsorbent i.e. specific surface and N (mol − 1) Avogadro's number. qe, defined before, is expressed here in μg g − 1. This equation, derived from the work of Biggar and Cheung (1973) works that define CS as: CS ¼

ðρ=M i Þ⋅A s N⋅qe

ð15Þ

where A (cm 2), the cross sectional area of the solvent, is obtained via Eq. (16): " A ¼ 1:091  10

−16

Mi  1024 N·ρ

#2=3 ð16Þ

the expression of Cs becomes Eq. (14) if Eqs. (15) and (16) are combined. Cs and Ce being known, K is obtained by plotting ln(CS/Ce) versus CS and extrapolating CS to zero. Papers referring to Biggar and Cheung (1973) usually make the following assumption (Adhikari and Singh, 2003; Mohapatra and Anand, 2007): CS ¼

msoil ·qe V

ð17Þ

2.2.2. Adsorption thermodynamic aspects A great number of papers use isotherms to calculate thermodynamics parameters for adsorption using fundamental thermodynamic relations:

where msoil is the mass of soil (g) and V the volume of solution (L). Thus the adsorption constant becomes the ratio between the mass of metal adsorbed per liter solution in contact with the adsorbent surface, Cs, and the concentration of remaining metal in the solution at equilibrium Ce and Eq. (13) becomes:

ΔG ¼ −RT lnK

K ¼ K adim ¼

ð11Þ

where ΔG is the Gibb's free energy and K is the sorption constant at equilibrium: 2þ

Cd

in solution

kads

⇄ Cd2þ adsorbed on soil fraction

kdes

R is the ideal gas constant (8.31 J K − 1 mol − 1), T is the temperature (K), kads is the adsorption rate constant and kdes is the desorption rate constant. The enthalpy ΔH and the entropy ΔS are then calculated using: ΔG ¼ ΔH−TΔS

ð12Þ

The principal difficulty lies in calculating the real thermodynamic adsorption constant K0 which should be rigorously unitless as a rate of activities (Biggar and Cheung, 1973): K ¼ K adim real ¼

aCd2þ soil fraction γ S C S kads ¼ ¼ aCd2þ solution γ e C e kdes

ð13Þ

where ai is the activity of Cd 2+ in phase i, CS (mol L − 1) and Ce (mol L − 1) are respectively the amount at equilibrium of solute adsorbed on the soil and of solute per volume of solvent in the solution, and γS and γe are the activity coefficients of the adsorbed solute and of the solute in the solution at equilibrium. To calculate K experimental values of Ce and Cs are necessary and if Ce is directly obtained by experiments, Cs has to be calculated by Eq. (14): CS ¼ 1:091:

  qe N·ρ 1=3 s Mi

ð14Þ

where ρ (g.cm − 3) is the solvent volumetric mass, Mi (g mol − 1) the

msoil ·qe V·C e

ð18Þ

the activity coefficient being equal to about 1 since the studied solutions are very dilute in cadmium. In this form Kadim is unitless. Some other authors consider K0 is equal to Kd (Eq. (5)) and perform thermodynamic calculations with this value (Aguilar-Carrillo et al., 2006). In these approach, since Kd (if not in a linear adsorption case) is concentration dependant for the reasons previously detailed, thermodynamic calculations are performed either for each initial contaminant concentration for each temperature or obtained by plotting ln Kd versus Ce and extrapolating to zero. Kd (L mol− 1) is determined using Eq. (6) and then used for the calculation of Gibb's free energy ΔG according to Eq. (11). The plot ΔG versus temperature is then drawn to determine the enthalpy ΔH (equal to ordinate origin) and the entropy ΔS (equal to plot slope). According to a third approach, thermodynamic calculations are performed using coefficients identified in adsorption models namely K = KL (L mg − 1) derived from Langmuir model (Angove et al., 1997; Gupta and Rastogi, 2008; Huang et al., 2007). As a matter of fact the thermochemical adsorption constant is difficult to be properly established and the Gibb’ s free energy will be highly dependent on the magnitude of K. As an example Kd expressed in L mol − 1 (Eq. (6)) and Kadim as defined in Eq. (18) are linked by the equation: K d ¼ K adim ⋅

V ⋅M Cd2þ msoil

ð19Þ

For example, at 20 °C in the present study (msoil = 5 g, V = 0.1 L), the Gibb's free energy calculated with either Kd or Kadim is— 1974 J mol − 1 since:   V −RT lnK d ¼ −RT lnK adim −RT ln ⋅M Cd2þ msoil ¼ −RT lnK adim −1974

ð20Þ

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E. Roth et al. / Geoderma 189–190 (2012) 133–143

3. Experimental

low enough to avoid competition. The pH was adjusted to 6.0 to keep the natural soil pH. The flasks were shaken in a thermostated bath for 2 h at the studied temperature+/−1 °C to reach thermal equilibrium. Shaking period finished, the soil was then exposed to metal solutions for about 6 h and samples were collected at different times. The sorbent was removed by filtration and the filtrate Cd 2+ concentration Ce was determined.

3.1. Soil and granulometric fractions

3.4. Batch sorption experiments

The soil was sampled in Eastern France at Aspach le Bas (Alsace– Haut Rhin). Its properties are listed in Table 1 and indicate that it has very high silt content and low organic matter content. The soil sampling was realised from the surface horizon (0–30 cm). Soil samples were stored at ambient temperature, air dried and first sieved at 250 μm. By further sieving three soil fractions are obtained:

They were performed in the same conditions as before on soil or its granulometric fractions in the presence or absence of OM. The Cd introduction was performed via injection of some micro-litres of a 1 g L − 1 Cd solution in order to obtain initial cadmium concentrations between 0.5 and 10 mg L − 1. These small volumes of Cd additions did not change the final volume of the solution. After 2h, residual cadmium concentration in solution at equilibrium Ce was determined.

These different approaches (K calculated as (α) the partition coefficient Kd in L mol − 1 (Eq. (6)), (β) the dimensionless constant Kadim (Eq. (18)), (γ) the Langmuir constant KL in L mol − 1) have been employed to estimate the thermodynamic adsorption constant. Thermodynamic results derived from these three ways are discussed.

– >50 μm: SanD (SD) – between 20 and 50 μm: Coarse Silt (CS) – b20 μm: Fine Silt and Clay (FSC)

4. Results and discussion

The organic matter (OM) was removed by H2O2 treatment at 70 °C, according to the French standard procedure AFNOR NFX 31‐ 107 (AFNOR, 1983). The removal of organic matter is controlled by the organic carbon level in soils according to the AFNOR French standard procedure NFX 31-109 (AFNOR, 1993). The different fractions obtained are named: Soil+OM (S+OM) for the initial soil without any treatment and Soil−OM (S−OM) for the initial soil after the organic matter removal. The Sand+OM fraction (SD+OM), the Coarse Silt+OM fraction (CS+OM) and the Fine Silt and Clay+OM fraction (FSC+OM) are obtained by successive sieving of the whole soil (Soil+OM). The Sand-OM fraction (SD-OM), the Coarse Silt-OM fraction (CS-OM) and the Fine Silt and Clay-OM fraction (FSC-OM) are isolated from the Soil-OM according to the same sieving procedure. 3.2. Tools 3.2.1. Analytical tools The Cd2+ concentration in solutions was determined using flame atomic absorption spectroscopy VARIAN AA20. The amount of cadmium adsorbed was obtained by calculating the difference between the initial and the residual cadmium concentration in the solution. 3.2.2. Data processing To fit the experimental data, the software Kaleidagraph (version 3.09) was employed.

4.1. Kinetic experiments Fig. 1 shows the kinetic experimental results obtained at 20 °C and 30 °C on the initial soil without removing organic matter. It can be seen that the sorbed amount of cadmium qt is enhanced when temperature increases. The same tendency has already been observed for lead ions by Ho and Ofomaja (2005). This phenomenon can be explained by an endothermic adsorption reaction and will be verified in Sections 4.2 and 4.3. The experimental value of qe is obtained when t tends to infinity in accordance with equilibrium (Table 2). The curves are treated either by pseudo-first-order or pseudosecond-order kinetics. The corresponding plots log (qe − qt) = f(t) and qt ¼ f ðt Þ are shown in Fig. 2a and b. The obtained parameters t are summarized on Table 2. The best fit (higher r²) is clearly found for the pseudo-second-order model and the corresponding values of qe derived from the pseudo-second order are closer to the experimental determination of qe on kinetic curves (Table 2). Pseudo-second-order kinetics often better fit the kinetic of sorption than the Lagergren equation especially at high times. In fact, the latter is not applicable on the whole range of time but only at short time, at the beginning of the adsorption process (Ho and McKay, 1999a, 1999b). Moreover, pseudo-second order process can easily be explained by the retention of metals through several steps: first the metal is fast sorbed by the external soil surface and then enters slowly in the soil pores maybe by diffusion (Brummer et al., 1988; Selim et al., 1992).

3.3. Kinetic experiments 0.09 0.08 0.07

qt abs (mg/g)

Batch kinetic studies were done to determine the rate of cadmium adsorption. The experiments were performed in a High Density PolyEthylene (HDPE) flask to avoid absorption of Cd 2+ on the vessel wall. Before beginning kinetic experiments, these 125 mL receptacles were filled with 5 g of whole soil without any treatment and 100 mL of Ca(NO3)2 10 − 2 mol L − 1 to adjust the ionic strength. This Ca(NO3)2 concentration has been chosen in accordance with Petruzzelli works (Petruzzelli et al., 1985). Indeed, at high ionic strength the soil metal sorption capacity decreases: Ca 2+ ions enter in competition with Cd 2+ ions. The ionic strength chosen for our experiment was

0.06 0.05 0.04 0.03 0.02

20°C

0.01

30°C

0.00

Table 1 Soil characteristic: soil fraction in mass percentage, CEC and pH.

0

100

200

300

400

t (min)

Sand (SD)

Coarse silt (CS)

Fine silt and clay (FSC)

Organic matter (OM)

CEC meq/kg

pH

16.3%

34.4%

47.3%

2.0%

96

5.9

Fig. 1. Sorption of cadmium by initial soil without any treatment at different temperatures. Points are experimental data, lines are fits with the pseudo-secondorder model.

E. Roth et al. / Geoderma 189–190 (2012) 133–143

137

Table 2 Pseudo-first order and pseudo-second order kinetic parameters obtained using the linear method. Experimental

Calculated Pseudo-first order

Pseudo-second order 2

T (°C)

qe (mg g− 1)

qe (mg g− 1)

k1 (min− 1)

r

20 30

0.058 0.078

0.00860 0.00967

0.0163 0.0156

0.710 0.835

El-Kamash et al. (2005), Wang et al. (2009) and Saha and Sanyal (2010) have shown the same kinetic tendency for cadmium on synthetic zeolite, loess soil and clayey soil respectively. It can also be seen in Table 2 that cadmium ions are more quickly adsorbed on soil when temperature increases since the rate constant increases by 25% between 20°C and 30°C. The relaxation time tr is determined according to Roels’ (1983) definition. It corresponds to the time of which half of maximum amount of adsorbate has been adsorbed on the sorbent. Liu (2008) recalls that if the adsorption process is related to a second-order kinetic process, it is dependent on the "availability of adsorption sites on the surface of adsorbent rather than adsorbate concentration in bulk solution". k2 implied in Eq. (4) is related to the true rate constant k2true by Eq. (21): true

k2

¼ k2 :qe

a

ð21Þ

0

k2 (g mg− 1 min− 1)

r2

0.060 0.078

3.73 4.68

0.9997 0.9999

This equation shows that k2 and qe are interrelated and not independent constants as they are determined from the plot of t ¼ f ðt Þ. It comes: qt tr ¼

1 1 ¼ k2 :qe ktrue 2

ð22Þ

The tr values obtained from theory (Eq. (22)) and from experimental data are reported in Table 3. Calculated and experimental tr are close and, as expected from previous results, theoretical and experimental relaxation times decrease when temperature increases. Moreover 95% of the maximum sorption capacity is attained in about 50 min confirming that the procedure employed for batch experiments is consistent with a Ce determination done after 2 h (Section 3.4). Finally, the rise in temperature leads at the same time to an increase in the reaction rate and the sorbed amount of cadmium on sorbent. To qualify the mechanism implied in the sorption (physical or chemical), the activation energy is calculated using Eq. (23): ktrue T :T 2;a Ea ¼ R: a b : ln true Ta −Tb k2;b

-1

log (qe-qt)

qe (mg g− 1)

! ð23Þ

-2 true In the present work (Ta = 20 °C = 293 K; Tb = 30 °C = 303 K; k2, a = true 0.226 g mg− 1 min− 1; k2,b = 0.364 g mg− 1 min− 1), Ea is equal to 35.1 kJ mol− 1. This value of Ea, near to 40 kJ mol− 1, is an intermediate value representing both physisorption and chemisorption (Ho and Ofomaja, 2005; Nollet et al., 2003).

-3 -4 -5

20°C 30°C

4.2. Isotherms

-6 0

50

100

150

200

250

300

350

400

t (min)

b

7000

t/qt (min/mg/g)

6000 5000 4000 3000

4.2.1. Isotherm shape The experimental adsorption isotherms are presented in Figs. 3, 4 and 5. Before modelling the isotherms, special attention is given to their shape. Indeed, for Giles and Smith (1974), the curve shape is largely dependent on the mechanism implied in the adsorption process and is a tool to define it. It can be first noticed that the isotherms do not exhibit any asymptotic values. This point can easily be explained by the fact that the studied cadmium concentrations are low and therefore, saturation of the soil is never reached. In the present work and according to the terms employed by Giles (Giles and Smith, 1974), the shape of most of the experimental curves is the typical L-curves one, corresponding to a classical Langmuir

2000

20°C 30°C

1000 0

0

50

100

150

200

250

300

350

Table 3 Theoretical and experimental relaxation time tr.

400

t (min) Fig. 2. Pseudo-first-order (a) and pseudo-second order (b) plot of Cd(II) ions sorption on initial soil at different temperatures. Points are experimental data, lines are linear regressions.

a b

Temperature (°C)

Theoreticala tr (min)

Experimentalb tr (min)

20 30

4.4 2.8

2.7 2.2

tr is calculated with the Eq. (22). q tr is directly obtained on graphics for qt ¼ 2e .

138

E. Roth et al. / Geoderma 189–190 (2012) 133–143

b) S-MO

0.14

0.14

0.12

0.12

0.10

0.10

qe (mg/g)

qe (mg/g)

a) S+OM

0.08 0.06

10°C

0.04

0

2

4

30°C

0.06 0.04 0.02

30°C

0.00

20°C

0.08

20°C

0.02

10°C

0.00 0

6

2

Ce (mg/L)

0.14

0.12

0.12

0.10

0.10

qe (mg/g)

qe (mg/g)

0.14

0.08 0.06

10°C

0.04

20°C

0.02 0.00 2

4

0.08 0.06

10°C

0.04

20°C

0.02

30°C

30°C

0.00 0

6

2

Ce (mg/L)

0.12

0.12

0.10

0.10

qe (mg/g)

qe (mg/g)

0.14

0.08 0.06

10°C

0.04

20°C

0.02 2

4

4

6

10°C 20°C

0.08

30°C

0.06 0.04 0.02

30°C

0.00

0.00 6

0

2

Ce (mg/L)

Ce (mg/L)

g) FSC+OM

h) FSC-OM

0.14

0.14

0.12

0.12

0.10

0.10

qe (mg/g)

qe (mg/g)

6

f) CS-OM

0.14

0.08 0.06

10°C

0.04

20°C

0.02 4

0.06

10°C

0.04

20°C 30°C

0.00

0.00 2

0.08

0.02

30°C 0

4

Ce (mg/L)

e) CS+OM

0

6

d) SD-OM

c) SD+OM

0

4

Ce (mg/L)

6

Ce (mg/L)

0

2

4

6

Ce (mg/L)

Fig. 3. First type of representation for the isotherms of Cd sorption on the soil (S) and its different fractions (SD, CS and FSC) before removal (+ OM) and after removal (− OM) of organic matter at different temperatures. Points are experimental data (♦ 10 °C, □ 20 °C, △ 30 °C), lines are fits with Langmuir model.

curve. The L shape is observed for soil fractions whatever the temperature except for soil in the presence of organic matter (S+OM). For L-curves the loading of the contaminant on the solid decreases as sites become occupied by solute and leads to the typical curve shape. Giles and Smith (1974) consider that the activation energy for the adsorption/desorption of the studied ion independent on the other components (solvent or other molecules likely to be adsorbed). Thus in each studied soil fraction adsorption site energies are of quite the same range order.

In the case of (S+OM) fraction, the curve shape is S-curve (Giles and Smith, 1974). This type of shape is typical of interactions of solute–solid interactions whose strength increases as the concentration of solute decreases. In this particular case, many different adsorption sites exist resulting in non-unique adsorption energies and/or competition between the studied ion and complexed ligands (Sposito, 1984). Knowing the Langmuir fit for each soil fraction (sand (SD), coarse silt (CS) and fine silt and clay (FSC)), we attempted to sum the curves

E. Roth et al. / Geoderma 189–190 (2012) 133–143

b) 10°C

a) 10°C 0.14

0.14

0.12

0.12

0.10

0.10

qe (mg/g)

qe (mg/g)

139

0.08 0.06 0.04 0.02

0.08 0.06 0.04 0.02

0.00

0.00 0

2

4

0

6

2

Ce (mg/L)

0.14

0.12

0.12

0.10

0.10

qe (mg/L)

qe (mg/g)

0.14

0.08 0.06 0.04

0.08 0.06 0.04

0.02

0.02

0.00

0.00 2

4

6

0

2

4

6

4

6

Ce (mg/L)

Ce (mg/L)

e) 30°C

f) 30°C

0.14

0.14

0.12

0.12

0.10

0.10

qe (mg/L)

qe (mg/g)

6

d) 20°C

c) 20°C

0

4

Ce (mg/L)

0.08 0.06 0.04 0.02

0.08 0.06 0.04 0.02

0.00

0.00 0

4

2

6

Ce (mg/L)

0

2

Ce (mg/L)

Fig. 4. Second type of representation for the isotherms of Cd sorption before removal (+ OM) and after removal (-OM) of organic matter on the soil (♦ S+OM, ◊S−OM) and its different fractions (■ SD+OM, □ SD−OM, ▲ CS+OM, △ CS−OM, ● FSC+OM, ○ FSC−OM) at different temperatures. Points are experimental data, line are fit curves with Langmuir model. Thick lines represent theoretical curves of soil fraction containing organic matter ( S+OM theo) or not ( S−OM theo) following the mass composition of soil.

representative of a granulometric fraction weighted by their respective mass percent in the whole soil. The obtained curve is a theoretical isotherm that models the whole soil and can be compared to the Langmuir fit of the experimental soil isotherm to evaluate the influence of sieving. The theoretical curves of soil with and without OM are given in Fig. 4 (S+OM theo and S−OM theo in thick black and gray lines respectively). The theoretical soil isotherms reveal that the soil cannot be considered as a simple weighted addition of its different granulometric fractions (SD+CS+FSC) (Fig. 4). Competition for cadmium sorption on sites from the different granulometric fractions and/or interaction between the soil fractions due to their structure might occur inside the whole soil and would explain its singular behaviour from this point of view. Noteworthy is the fact that the sand can fit correctly the theoretical behaviour of the soil in the most cases, even if this fraction is not the major one. 4.2.2. Langmuir model parameters Although the Langmuir model is not suitable for the whole soil with organic matter fraction (S+OM) due to its particular shape as discussed previously, all experimental curves have been fitted using the Langmuir Eq. (7) and these fits are represented on the curves

(Figs. 3, 4 and 5) by the lines (dotted or not). Table 4 reports parameters obtained and the correlation coefficient r 2. The Langmuir model gives rather satisfactory results, except for soil+OM fraction (Fig. 3a) for the reasons mentioned before. Except for the whole soil, for which the Langmuir model is not adapted as mentioned before, the values of KL reported in Table 4 are more or less constant for each fraction at a given temperature, with or without removal of organic matter. They range between 0.042 and 0.185 L mg − 1 (Table 4). These KL values are of the same order as those provided by Taqvi et al. (2007) for Cd adsorption on beach sand (0.17 L mg − 1 at 293 K). Some authors report for various soils values between 0.058 and 0.122 L mg − 1 (Adhikari and Singh, 2003) or between 0.04 10 − 3 and 0.045 L mg − 1 (Usman, 2008). A value of 0.087 L mg − 1 was found for clay at 298 K (Baker, 2009). On the contrary, studying adsorption of cadmium on a loess soil, Wang et al. (2009) found ten times stronger KL values. The KL parameter gives the strength of adsorption. When compared to other metal, Cd generally has lower KL, implying a lower adsorption strength and adsorption capacities (Baker, 2009; Usman, 2008). qmax values range between 0.19 and 0.50 mg g − 1 (Table 4). These values are low compared to other studies (Baker, 2009; Wang et al.

140

E. Roth et al. / Geoderma 189–190 (2012) 133–143

d) 10°C

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

Soil+OM

qe (mg/g)

qe (mg/g)

a) 10°C Soil-OM

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

qe (mg/g)

qe (mg/g)

SD+OM SD-OM

e) 20°C

b) 20°C

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

f) 30°C

c) 30°C 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

qe (mg/g)

qe (mg/g)

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

0

4

2

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

6

2

Ce (mg/L)

qe (mg/g)

qe (mg/g)

CS+OM CS-OM

qe (mg/g)

qe (mg/g)

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

i) 30°C

4

6

FSC+OM FSC-OM

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

l) 30°C

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

qe (mg/g)

qe (mg/g)

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

k) 20°C

h) 20°C

0

6

j) 10°C

g) 10°C 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

4

Ce (mg/L)

2

4

Ce (mg/L)

6

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

2

Ce (mg/L)

Fig. 5. Third type of representation for the isotherms of Cd sorption before removal (+ OM) and after removal (− OM) of organic matter on the soil (♦ S+OM, ◊S−OM) and its different fractions (■ SD+OM, □ SD−OM, ▲ CS+OM, △ CS−OM, ● FSC+OM, ○ FSC−OM) at different temperatures. Points are experimental data, line are fit curves with Langmuir model.

E. Roth et al. / Geoderma 189–190 (2012) 133–143

141

Table 4 Langmuir parameters obtained from isotherm of Cd sorption at 10 °C, 20 °C and 30 °C.

10 °C

20 °C

30 °C

qmax (mg g− 1) KL (L mg− 1) r² qmax (mg g− 1) KL (L mg− 1) r² qmax (mg g− 1) KL (L mg− 1) r²

S+MO

S−MO

SD+MO

SD− MO

CS+MO

CS− MO

FSC+ MO

FSC− MO

0.43 0.055 0.97 0.19 0.185 0.99 0.26 0.112 0.99

0.29 0.042 0.99 0.25 0.072 0.98 0.23 0.115 0.99

0.34 0.083 0.98 0.34 0.093 0.99 0.32 0.132 0.99

0.36 0.087 0.99 0.33 0.126 0.98 0.23 0.155 0.99

0.38 0.059 0.98 0.32 0.091 0.99 0.40 0.085 0.99

0.26 0.091 0.99 0.20 0.088 0.99 0.21 0.088 0.99

0.31 0.152 0.99 0.44 0.100 0.99 0.50 0.095 0.99

0.24 0.105 0.99 0.24 0.167 0.99 0.34 0.115 0.98

2009) including that of Taqvi et al. (2007) who report a value of 0.84 mg g − 1 for beach sand. Thus, we can conclude that the adsorption capacity of our soil is quite low. Fig. 4 and Table 4 show that FSC adsorb the most followed by sand. Spark (Spark and Swift, 1994; Spark et al., 1995) and Zachara and Smith (1994) reported that the adsorption capacity is linked to the surface site density and that the presence of clay minerals enhances the adsorption capacity. A general tendency of adsorption capacities can be drawn out. Adsorption capacities decrease as follow: SFC+ OM> CS+OM ~ SD+OM > soil+OM. The work of Kim and Fergusson (1992) concerning Cd adsorption capacities of different granulometric fraction of a New Zealand soil is in agreement with these results. Nevertheless, these maximum adsorption capacities have to be considered carefully: in the experimental curves qe only reaches about 25% to 50% of the maximum adsorption capacities. Fittings are thus done only on the beginning of the isotherm. This could explain the discrepancies with some authors (Baker, 2009; Wang et al. 2009).

then opposite signs from a thermodynamic point of view (exothermic in the first case, endothermic in the second). This point is in accordance with Tables 5, 6 and 7 (Section 4.3). Moreover Fig. 3 allows to study the temperature effect on isotherms, which varies according to the nature of the sorbent, the metal and the solution physical conditions (pH, ionic strength). In our case, generally a temperature increase improves adsorption (only not for CS−MO where a 10 °C isotherm curve is higher than the two others). This observation implies that the sorption process is endothermic and this point will be confirmed by enthalpy calculations (the values must be then positive) in thermodynamics Section 4.3. (Tables 5, 6 and 7). The temperature aids the sorption process through activating the sorption sites (Lee et al., 1998; Raji and Anirhudan, 1977). The FSC fraction is generally the granulometric part of the soil that has the better adsorption, with or without organic matter (Fig. 4). The evolution of qmax with temperature in Table 4 confirms this phenomenon.

4.2.3. Organic matter effect Organic matter widely influences the isotherms excepted for sand (Fig. 5). The concentration of cadmium in soil at equilibrium qe is higher when OM remains (Table 4). For coarse silt samples (Fig. 5g, h, i) and fine silt and clay (Fig. 5j, k, l), this observation is accorded with the fact that these materials have numerous adsorption sites due to the high specific area. On the opposite side, organic matter has relatively little influence on the sand fraction (Fig. 5d, e, f) due to the fact that this material is naturally poor in OM. These points are confirmed in Table 4: except for the SD fraction (for which qmax value is independent of OM). qmax is higher when OM remains in the sample for a specific fraction at a given temperature. Choi (2006) have already shown that cadmium adsorption decreases with organic matter removal. OM favors indeed adsorption and many works are in accordance with this conclusion. So, for Masset et al. (2000), humic substances enhance the sorption of positive ions by making the surface more negative.

4.3. Thermodynamics studies

4.2.4. Temperature effect Temperature influences the role of organic matter or the type of adsorption sites occupied by OM on the material (except for sand) (Fig. 5). Indeed, for the soil (Fig. 5a, b, c) and the FSC (Fig. 5j, k, l) fractions, the gap between the isotherms obtained with and without OM decreases with temperature. For the CS fraction (Fig. 5g, h, i), the inverse phenomenon is observed. The implied mechanisms have

The Gibb's free energies ΔG were obtained from Eq. (11) using three methods to determine K. K is derived either from the Langmuir constant KL (in L mol − 1) (Table 5), from Kd (Table 6) or from the dimensionless constant Kadim as defined by Eq. (18) (Table 7). Although the two latter free energies are linked by Eq. (20) their values are given in Tables 6 and 7 because signs can switch from negative to positive between the two calculations. Free energies derived from the Langmuir constant (Table 5) and partitioning coefficients (Table 6) are all negative indicating a spontaneous sorption process whereas free energies derived from Kadim (Table 7) exhibit positive values for the soil at 10 °C with or without organic matter and for the soil without organic matter at 20 °C. However, it has been already noticed that the soil fraction is not fitted well by Langmuir model. ΔG values calculated from Langmuir are one order of magnitude greater (as they reach around 20 kJ mol − 1 (Table 5)) than the values determined by the two other methods (around 1 kJ mol − 1) (Tables 6 and 7). Nevertheless, all values reveal physical adsorption process in accordance with the kinetic results. The enthalpies and entropies obtained from Eq. (12) have the same sign and are in the same range order whatever the model of K. The entropies are exactly the same when determined from Kd (Table 6) or Kadim (Table 7). The relation between these two calculations (Eq. (20))

Table 5 Thermodynamic calculations using Eq. (11) with K0 = KL (L mol− 1).

10 °C 20 °C 30 °C

−1

ΔG (kJ mol ) ΔG (kJ mol− 1) ΔG (kJ mol− 1) ΔH (kJ mol− 1) ΔS (J mol− 1 K− 1)

S+ MO

S− MO

SD+ MO

SD− MO

CS+ MO

CS− MO

FSC

FSC− MO

− 20.5 − 24.2 − 23.8 24.7 162

− 19.9 − 21.9 − 23.8 35.9 197

− 21.5 − 22.5 − 24.2 16.6 134

− 21.6 − 23.3 − 24.6 20.5 149

− 20.7 − 22.5 − 23.1 12.7 119

− 21.7 − 22.4 − 23.2 − 1.3 73

− 22.9 − 22.7 − 23.3 − 16.6 22

− 22.0 − 23.9 − 23.8 2.9 89

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E. Roth et al. / Geoderma 189–190 (2012) 133–143

Table 6 Thermodynamic calculations using Eq. (11) with K = Kd. (Kd is taken in L mol− 1) and is calculated with Eq. (6) i.e. K d ¼ Cqee :MCd2þ ).

10 °C 20 °C 30 °C

ΔG (kJ mol− 1) ΔG (kJ mol− 1) ΔG (kJ mol− 1) ΔH (kJ mol− 1) ΔS (J mol− 1 K− 1)

S+MO

S−MO

SD+MO

SD−MO

CS+MO

CS−MO

FSC+MO

FSC−MO

− 0.7 − 3.4 − 4.2 47.6 171

− 0.9 − 1.7 − 2.9 26.7 97

− 2.7 − 3.3 − 3.6 10.8 47

− 2.9 − 3.2 − 3.5 5.4 29

− 2.3 − 2.9 − 3.3 10.4 45

− 7.9 − 2.3 − 2.0 − 9.6 − 25

− 3.8 − 4.0 − 4.1 − 2.8 4

− 2.4 − 3.9 − 3.4 11.3 50

Table 7 soil ⋅qe Thermodynamic calculations using Eq. (11) with K ¼ K a dim ¼ mV⋅C (Eq. (18)). e

10 °C 20 °C 30 °C

ΔG (kJ mol− 1) ΔG (kJ mol− 1) ΔG (kJ mol− 1) ΔH (kJ mol− 1) ΔS (J mol− 1 K− 1)

S+MO

SD− MO

SD+ MO

SD− MO

CS+MO

CS− MO

FSC+MO

FSC− MO

1.2 − 1.5 − 2.1 49.6 171

1.0 0.2 − 0.8 28.7 97

− 0.8 − 1.3 − 1.6 12.7 47

− 1.0 − 1.3 − 1.5 7.3 29

− 0.4 − 1.0 − 1.3 12.4 45

− 6.1 − 0.3 − 0.07 − 7.6 − 25

− 1.9 − 2.0 − 2.1 − 0.9 4

− 0.5 − 1.9 − 1.4 13.3 50

Table 8 Sorption energy E in kJ mol− 1 calculated with Dubinin Radushkevich isotherm.

10 °C 20 °C 30 °C

S+ MO

S− MO

SD+MO

SD− MO

CS+ MO

CS− MO

FSC+ MO

FSC− MO

7.5 9.4 9.8

8.8 8.7 8.6

8.3 9.2 9.6

8.7 8.2 9.5

8.3 8.9 9.3

8.6 9.3 8.7

8.7 9.2 9.9

8.8 9.2 9.0

finally concerns only enthalpy values. The positive values of the enthalpies indicate the sorption is endothermic: cadmium ions had probably to displace more than one water molecule so as to can adsorb on soil. Endothermic process is also explained by the need for solvated ions to lose part of their hydration sheath before sorption (Donat et al., 2005). This process is then enhanced with temperature in main cases. Two fractions, namely CS–MO and FSC+MO, have an opposite behaviour where the adsorption is not favored with temperature. Entropies are positive (just not for CS−MO) indicating the adsorption is irreversible. Enthalpies and entropies for the whole soil are of the same order range as those determined on a loess soil by Wang et al. (2009). Sorption energies calculated with the Dubinin–Radushkevich model (Eq. (10)) range between 8.2 and 9.9 kJ mol − 1 (cf Table 8). This order range is between physical and chemical indicating a mix of phenomena and a probable specific adsorption process where surface complexation might occur. This conclusion confirms the interpretation of activation energy value determined by means of the kinetic experiments. For a single soil fraction, generally sorption energies increase slightly with temperature and the highest values (about 10 kJ mol− 1) are found at 30 °C for S+OM, SD with or without OM and FSC+OM indicating a stronger adsorption with temperature. The sorption energy values are roughly constant at a given temperature for all the fractions, maybe because the Dubinin–Radushkevich model is independent of adsorption site homogeneity and consequently the sites’ type in the different fractions.

5. Conclusion The kinetic adsorption of cadmium on the studied soil sample (extracted in Eastern France at Aspach le Bas) is a pseudo‐second order process and the concerned sorption is both chemical and physical under our experiments conditions. Thermodynamic parameters of the adsoprtion process (ΔG, ΔH and ΔS) are determined from experimental isotherm fitting using three methods employed in literature. It turn out

that the Cd adsorption is endothermic and irreversible for most of the soil fractions. Temperature generally favors the sorption process. Adsorption isotherm for the soil show an S-like curve proving adsorption results from complex phenomena including interaction and competition and this, only when OM remains. When sieved, the different fraction isotherms exhibit typical Langmuir curves. OM when present even at a low mass percent (2%) improves the adsorption capacity. Generally speaking it comes that the adsorption capacities decrease as follow: SFC+OM>SC+OM~SD+OM>soil+OM. Cd sorption is moreover generally enhanced by temperature. (déjà exprimé dans la phrase précédente: “Temperature generally favors the sorption process.”, non ? A mon avis, on peut supprimer une des deux phrases du coup). Experiments reveal that the cadmium sorption capacity is low (less to 0.5 mg g− 1) at low initial concentrations (10 mg L− 1 maximum). As a conclusion, this soil provides low retention strength and low adsorption capacity for Cd as indicated by the Langmuir model. Leaching can be expected when diffuse Cd contamination occurs in this soil. Appendix. Nomenclature A cross sectional area of the solvent (cm 2) ai activity of Cd 2+ in phase i Ce amount at equilibrium of solute per volume of solvent in the solution (mg L − 1 or mol L − 1) CS amount at equilibrium of solute adsorbed on the soil per volume of solvent in solution (mg L − 1 or mol L − 1) [Cd 2+]0 introduced initial cadmium concentration (mol L − 1) E free energy of adsorption (J mol − 1) Ea activation energy (J mol − 1) ΔG Gibb's free energy (kJ mol − 1) ΔH enthalpy (kJ mol − 1) k1 pseudo-first-order rate constant of sorption (min − 1) k2 pseudo-second-order rate constant of sorption (g mg − 1 min − 1) k2true true pseudo-second order rate constant (g mg − 1 min − 1)

E. Roth et al. / Geoderma 189–190 (2012) 133–143

kads kdes K

adsorption rate constant desorption rate constant sorption constant (varied unity defined in main text) for the equilibrium 2þ

Cd

in solution

kads

⇄ Cd2þ adsorbed on soil fraction

kdes Kd KDR KL Mi msoil N qt qe

qmax R r2 s ΔS T tr V γS γe ρ

partitioning coefficient (mL g − 1 or L mol − 1) Dubinin–Radushkevich constant (mol² J −²). Langmuir constant related to the energy of adsorption (L mg − 1 or L mol − 1) molecular weight of an element or molecule i (g mol − 1) mass of soil (g) Avogadro's number (mol − 1). amount of solute sorbed at time t (mg g − 1) amount of solute sorbed at infinite time i.e. at equilibrium (generally in mg g − 1, sometimes in μg g − 1 as clarified in main text) maximum adsorption capacity (mg g − 1 or mol g − 1) ideal gas constant (i.e. 8.31 J K − 1 mol − 1) correlation coefficient the surface area of the adsorbent i.e. specific surface (cm2 g− 1) entropy (J mol − 1 K − 1) temperature (°C or K). relaxation time (min) volume of solution (L) activity coefficient of the adsorbed solute at equilibrium activity coefficient of the solute in the solution at equilibrium solvent volumetric mass (g cm − 3)

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