Sorption of cadmium on palygorskite, sepiolite and calcite: Equilibria and organic ligand affected kinetics

Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

Sorption of cadmium on palygorskite, sepiolite and calcite: Equilibria and organic ligand affected kinetics Mehran Shirvani a,∗ , Hosein Shariatmadari a , Mahmoud Kalbasi a , Farshid Nourbakhsh a , Bijan Najafi b a

Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran b Department of Chemistry, Isfahan University of Technology, Isfahan, Iran Received 30 July 2005; received in revised form 16 March 2006; accepted 29 March 2006 Available online 7 April 2006

Abstract Quantity, affinity and rate of heavy metals retention by soil constituents can greatly influence the mobility and bioavailability of these metals in soil environments. Equilibrium and kinetic studies of cadmium (Cd) sorption on palygorskite, sepiolite and calcite minerals were carried out. Effects of low molecular weight organic acid anions, acetate and citrate, and siderophore desferrioxamine B (DFOB) on the Cd sorption rates were also investigated. Langmuir and Freundlich isotherms adequately fitted the equilibrium sorption data with r2 values >0.89. Among the minerals studied, sepiolite showed the highest sorption capacity and chemical affinity for Cd. Uptake of Cd by the minerals was initially rapid and then slowly continued until approached a pseudo equilibrium. The time-dependent Cd sorption data were well-described by pseudo second-order (0.98 < r2 < 1), Elovich (0.84 < r2 < 0.99) and power function (0.80 < r2 < 0.98) kinetic models. Sepiolite and calcite possessed the highest and the lowest Cd sorption rate values, respectively. Acetate and citrate ligands generally induced decreases in the Cd retention rates; however, the inhibitory effect of citrate was more pronounced. The DFOB siderophore, on the other hand, enhanced the rate of Cd uptake by the sorbents. © 2006 Elsevier B.V. All rights reserved. Keywords: Cd; Kinetic; Equilibrium; Desferrioxamin B; Citrate; Acetate

1. Introduction Sorption of trace metals onto the natural particles largely determines the mobility and bioavailability of these metals in soils and sediments. Sorption is a general term which may include adsorption, surface precipitation, coprecipitation and diffusion into the crystal [1]. Various equilibrium-based models have been used to describe sorption reactions on soil constituents. These models can provide useful information about the retention capacity of and the metals affinity to the sorbents. Equilibrium studies show the final state of reactions; however, cannot explain the reaction rates. To understand the fate of heavy metals in soils, one needs to understand the rate of sorption reactions and their mechanisms as well. The rate of metal sorption could be determined through kinetic studies. Kinetics of metals sorption in soils has generally been characterized by an initial rapid rate

∗

Corresponding author. Fax: +98 311 3912254. E-mail address: [email protected] (M. Shirvani).

0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.03.052

followed by a much slower approach to equilibrium. The fast reactions are usually related to surface adsorption and the slow reactions are ascribed to diffusion phenomena, different sites of reactivity, and surface precipitation and coprecipitation [2,3]. Palygorskite, sepiolite as well as carbonate minerals are naturally abundant in arid and semiarid pedospheres worldwide and can thus affect the quality of linked hydrospheres, biospheres and atmospheres. Palygorskite [Si8 Mg5 O20 (OH)2 (OH2 )4 ·4H2 O] and sepiolite [Si12 Mg8 O30 (OH)4 (OH2 )4 ·8H2 O] belong to the phyllosilicate minerals with a ribbon-like structure formed from two inversed silica tetrahedral sheets and a magnesium octahedral sheet between them making alternate hollow channels allowing penetration of solutes into the structure. In addition, some isomorphic substitutions in the tetrahedral layer, such as Al3+ for Si4+ , develop negatively charged adsorption sites able to electrostatically adsorb cations. Palygorskite and sepiolite are used in many industrial and environmental applications because of their high sorption capacity for organic and inorganic materials [4,5]. Surface complexation and precipitation reactions are the main mechanisms for heavy metal uptake by

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

these fibrous clay minerals [6–8]. Metals may also be adsorbed or precipitated on calcite as one of the most common carbonate minerals associated with palygorskite and sepiolite in calcareous soils and deposits. Sorption of metal ions on calcite has been characterized as a fast initial chemisorption followed by a slow ion uptake as a result of defect-enhanced diffusion into the lattice and (Cd,Ca)CO3 solid-solution formation through a dissolution–recrystalization mechanism [9]. Low molecular weight organic acids (LMWOAs) can be excreted from plant roots and microorganisms or be produced during plant residue decomposition in soils. Formation of organically bound metal ions can greatly affect the metal ions behavior in the soils as well as their uptake by and accumulation in plants [10,11]. Anions of LMWOAs may depress metal sorption on minerals via formation of stable aqueous nonsorbing metal–anion complexes or competition between metals and anions for available surface sites. Conversely, ligands may increase the metal sorption through electrostatic interactions, formation of ternary metal-ligand-surface complexes and/or surface precipitation [12]. Siderophores and phytosiderophores are chelating agents secreted by microorganisms and plant roots, respectively, facilitating the uptake of iron and other micronutrients like Zn [13]. Forming very strong bounds with metal ions, siderophores can considerably influence metal behavior in the environment. The mobility of heavy metal ions in soils may be enhanced or depressed through complex formation with siderophores depending on the nature of the complexes and mineral surfaces. For example, Kraemer et al. [14] revealed that sorption of Pb on goethite is depressed in the presence of desferrioxamine B (DFOB) sideophore because of competition between siderophore ligands and goethite surface sites for lead. They showed that H2 PbDFOB+ was the dominant soluble complex of Pb and DFOB at pH 7–8. Neubauer et al. [15], on the other hand, reported the enhancement of Cd sorption on montmorillonite in the presence of DFOB as a result of attraction of cationic Cd–DFOB complexes by high negative charge of the mineral surface. Kinetics of heavy metals sorption on iron and manganese oxides, aluminosilicates and organic substances are extensively studied [16–18]. However, information on the sorption kinetics of these elements onto palygorskite, sepiolite and calcite is hardly found. The role of LMWOAs and siderophores on the sorption rates of heavy metals on these minerals are also rarely understood. It is expected that Cd sorption on the minerals is affected by the presence of these organic ligands. Therefore, the objective of this research was to investigate the influence of acetate and citrate as LMWOAs’ anions, and the DFOB as a hydroxamate siderophore on the kinetic parameters of Cd sorption on palygorskite, sepiolite and calcite minerals.

183

Table 1 Specific surface area (BET SSA) and cation exchange capacity (CEC) of the minerals used in this research

BET SSA (m2 g−1 ) CEC (cmolc kg−1 )

Palygorskite

Sepiolite

Calcite

136 ± 4.0 19.5 ± 0.5

240 ± 4.0 11 ± 0.5

0.8 ± 0.1 –

tory (Purdue University, IN, USA) with structural formula of (Mg0.33 Ca0.62 Na0.04 K0.13 )[Al1.50 Fe(III)0.52 Fe(II)0.01 Mn0.01 Mg1.91 Ti0.06 ][Si7.88 Al0.22 ]O20 (OH)4 as given by Van Olphen and Fripiat [19], sepiolite of Vicalvaro, kindly supplied by TOLSA (Madrid, Spain) and calcite (99.5% pure) purchased from BDH Chemicals (Poole, UK). The fibrous clay minerals were treated to remove possible carbonates, organic matter and iron/manganese oxides present according to Kittrick and Hope [20]. The <2 ␮m clay fractions were then separated by centrifugation and converted to the homoionic Ca form with 0.5 M CaCl2 (Merck, Germany). Excess salts were rinsed with deionized water until the electrical conductivity of eluents reached about 30 ␮S m−1 . The Ca-saturated clay samples were then freezedried. X-ray diffraction patterns obtained for orientated samples with a XD-610 Shimadzu X-Ray Diffractometer (Cu k␣) indicated the samples were mainly contained palygorskite, sepiolite or calcite. Subsequently, the minerals were analyzed for their specific surface area (SSA) using BET N2 adsorption method and cation exchange capacity (CEC) by the ammonium acetate method of Rhoads [21] (Table 1). Stock suspensions of the minerals in 0.01 M CaCl2 solution were kept at room temperature for 24–48 h and their pH were measured periodically until reached constant values. The pH values were 7.5, 8.1 and 7.7 for palygorskite, sepiolite and calcite suspensions, respectively. 2.2. Reagents

2. Materials and methods

The reagents used in this study were extra pure analytical grades: CdCl2 ·2.5H2 O (from BDH Chemicals, UK), CaCl2 ·2H2 O, CH3 COONa·3H2 O, and C6 H5 Na3 O7 ·2H2 O (all from Merck, Germany) and C25 H48 N6 O8 ·CH4 O3 S (desferrioxamine mesylate) (from Sigma-Aldrich, USA). Desferrioxamine B (DFOB) is a hydroxamate siderophore exudated by microorganisms in iron-limiting environments. This siderophore possesses three hydroxamic acid functional groups (–CONHOH) and a terminal amine group which is positively charged at pH <8 and is not involved in the complex formation (Fig. 1). Acetate, citrate and DFOB were chosen as representatives of monocarboxilic and tricarboxilic LMWOAs’ anions and hydroxamate ligands presented in soils, respectively. These ligands, being significant organic substances in the rhizospheric soils available in chemical marketings, have been used in several metal sorptions/desorption studies, make it feasible to compare our results with those of other workers.

2.1. Sorbents

2.3. Sorption equilibrium experiments

The sorbents used in this study were palygorskite of Florida (PFl-1) obtained from the Source Clay Minerals Reposi-

Sorption isotherms for Cd were determined using a batch experiment. Ten milliliter samples from each of the stock mineral

184

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

activity. Duplicate samples (each one 5 mL) were pippeted out at designated time intervals (from 10 to 1440 min), centrifuged and analyzed for the residual Cd concentration in solutions. The supernatants were subsequently analyzed for total Cd with the AAS. The amount of Cd sorbed on each solid phase was calculated from the difference between the initially added and finally measured Cd concentrations. The time-dependent Cd sorption data were then tested to be described by various kinetic models. 3. Results and discussion 3.1. Equilibrium modeling Fig. 1. Schematic molecular structure of desferrioxamine B.

suspensions were pipetted into 50 mL centrifuge tubes and solutions containing nine different Cd concentrations were added to the samples. The initial Cd concentrations varied from 8.9 to 313 ␮M with pH values of 6.7 and the sorbent/solution ratio were 1/100 for clays (0.3 g sorbent) and 5/100 (1.5 g sorbent) for calcite. A control sample for each Cd concentration level (with no sorbent) was also included and a few drops of toluene were added to inhibit microbial interference. The samples were then shaken on an orbital shaker (160 rpm) for 24 h at room temperature (23 ± 2 ◦ C). At the end of this period, the final pH values were measured and the equilibrium solutions were separated by centrifugation at 5000 g for 20 min. The contents of Cd and Mg in the supernatants were then determined by an AAnalyst 200 Perkin-Elmer Flame Atomic Absorption Spectrophotometer (FAAS). The detection limits of FAAS instrument were 10 and 3 ppb for Cd and Mg, respectively. Cadmium species in discarded equilibrium supernatants were also estimated using the Visual MINTEQ (ver. 2.32) chemical equilibrium model [22]. The Visual MINTEQ is a Windows version of MINTEQA2 ver. 4.0, which was released by the USEPA in 1999. The amount of sorbed Cd in each sample was calculated from the difference between initial and final Cd concentrations. The sorbed Cd values were then plotted against the corresponding equilibrium Cd concentrations and various adsorption isotherms were tested to describe the sorption data. 2.4. Sorption kinetic experiments The rate of Cd uptake by each mineral was evaluated using a batch experiment. Sorption was started by simultaneously adding 50 mL of 400 ␮M cadmium chloride and 50 mL of 4 mM acetate, 4 mM citrate or 1 mM DFOB solution to 100 mL of mineral suspensions, prepared in Section 2.1, agitated vigorously by a magnetic stirrer in a 250 mL glass reaction vessel in an ambient atmosphere at room temperature (23 ± 2 ◦ C). The total dissolved concentrations were 100 ␮M for Cd, 1 mM for acetate or citrate and 250 ␮M for DFOB at the beginning of the experiment. The mineral/solution ratios were 1, 0.2, and 4% for palygorskite, sepiolite, and calcite, respectively, selected based on the capacity of each mineral to sorbe 40 ␮mol of Cd introduced to the systems estimated from sorption equilibrium experiments. A few drops of toluene were also added to inhibit microbial

Two common models, Langmuir and Freundlich, were fitted to the equilibrium sorption data well. Determination coefficients (r2 ), standard errors of estimate (S.E.E.) and sorption constants for the fitted models are presented in Table 2. The S.E.E. values were calculated from:
 1/2 (qe − qe )2 S.E.E. = (1) n−2 where, qe and qe are measured and model estimated amounts of Cd sorbed at equilibrium, respectively, and n is the number of measurements. The Langmuir equation provided better correlations for Cd sorption on palygorskite and calcite, while Cd sorption on sepiolite was better described by the Freundlich equation. The Langmuir equation can be written as: qe =

qm aL Ce 1 + a L Ce

(2)

where, qe is the amount of Cd sorbed at equilibrium (␮mol g−1 ), Ce is the equilibrium Cd concentration (␮mol L−1 ), qm is the maximum sorption capacity of the sorbent (␮mol g−1 ), and the aL is an affinity parameter (L g−1 ). The linear form of Langmuir equation is: 1 Ce Ce = + qe qm aL qm

(3)

Table 2 Constants, determination coefficients (r2 ) and standard errors of estimate (S.E.E.) for the Langmuir and Freundlich equations fitted to the Cd sorption dataa Model

Palygorskite

Sepiolite

Calcite

Langmuir qm (␮mol g−1 ) aL (L g−1 ) r2 S.E.E. (␮mol g−1 )

9.746 0.049 0.988 0.541

46.083 0.101 0.895 1.186

2.341 0.049 0.995 0.104

Freundlich KF (L g−1 ) 1/n (mg(1 − 1/n) g−1 L1/n ) r2 S.E.E. (␮mol g−1 )

1.044 0.428 0.964 0.813

3.961 0.801 0.978 1.502

0.211 0.473 0.969 0.240

a All of the determination coefficients are significant at probability level <0.001.

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190 Table 3 Percentage of the two dominant Cd species (Cd2+ and CdCl+ ) and saturation index (SI) with respect to CdCO3 (otavite) in supernatants equilibrated with palygorskite, sepiolite or calcite, calculated using Visual MINTEQ speciation program Ce a

Cd2+

CdCl+

SIb

Palygorskite 1.22 2.07 6.67 13.88 30.89 65.40 128.7 159.2 234.3

48.87 48.90 48.91 48.92 48.94 49.00 49.06 49.09 49.16

48.07 48.08 48.09 48.08 48.08 48.04 48.00 47.97 47.92

−1.12 −0.99 −0.55 −0.23 0.06 0.31 0.58 0.63 0.76

Sepiolite 0.268 0.375 0.625 1.187 2.406 4.272 6.500 9.808 11.91

47.84 48.56 48.63 48.68 48.81 48.81 48.84 48.87 48.89

47.10 47.84 47.92 47.97 48.11 48.11 48.14 48.17 48.20

−0.63 −0.99 −0.87 −0.69 −0.79 −0.54 −0.51 −0.60 −0.91

Calcite 1.12 2.22 6.33 9.91 26.16 59.73 106.38 138.66 221.43

48.77 48.76 48.81 48.82 48.88 48.91 48.98 49.01 49.09

48.07 48.06 48.11 48.11 48.15 48.13 48.12 48.10 48.03

−0.96 −0.62 −0.37 −0.19 −0.09 0.25 0.16 0.23 0.42

185

0.76 L g−1 for their sepiolites, respectively. These dissimilarities are expected because the source of sepiolite we used and its preparation process was different from those they used. Besides, the experimental conditions applied in our research, such as Cd concentration range, background solution composition and pH values, were greatly varied from those applied in their research. Experimental data were also fitted to the Freundlich adsorption model: qe = KF Ce (1/n)

(4)

where, qe and Ce are the same as the Langmuir equation, and KF (mg(1 − 1/n) g−1 L1/n ) and n (dimensionless) are constants. The linear form of Eq. (4) is:   1 ln qe = ln KF + (5) ln Ce n In accordance with the sequence observed for Langmuir sorption maxima, the highest and the lowest KF values belong to the sepiolite and calcite, respectively, indicating that the Freundlich KF value may be used to compare sorption capacities of the sorbents. 3.2. Sorption mechanisms

a

Ce = total measured soluble Cd concentration in equilibrium supernatants (␮M). b SI = log IAP − log K , where IAP is the ion activity product [(Cd2+ )(CO 2− )] s 3 and Ks is the solubility constant of otavite (CdCO3(s) ). The SI values greater than zero indicate oversaturation with respect to otavite.

therefore, the qm and aL values may be obtained from the slope and intercept of a plot of Ce /qe versus Ce . Inserting the Ce values into the chemical speciation model (Visual MINTEQ) showed that Cd2+ and CdCl+ were the main Cd species, comprising approximately 49 and 48% of the total soluble Cd contents in the equilibrium supernatant, respectively (Table 3). The sequence of sorption maxima estimated by the Langmuir equation (qm ) were as 46.08 ␮mol g−1 (sepiolite) >9.75 ␮mol g−1 (palygorskite) >2.34 ␮mol g−1 (calcite). The affinity parameter aL for the sorbents also were as follows: 0.101 L g−1 (sepiolite) >0.049 L g−1 (palygorskite) = 0.049 L g−1 (calcite). These results clearly demonstrate the much higher Cd sorption potential of sepiolite compared to the other two sorbents. The sorption capacity and chemical affinity of Vicalvaro sepiolite calculated in this study are much lower than those obtained by Garcia-Sanchez et al. [23] and Alvarez-Ayuso and Garcia-Sanchez [6] for Orera sepiolite. They calculated sorption capacities of about 74 and 153 ␮mol g−1 and the affinity constants of 1.67 and

3.2.1. Clay minerals The amounts of Mg and proton released in the equilibrium solutions of clay suspensions increased with the amount of Cd sorbed on the minerals (Figs. 2 and 3). These results corroborate two possible mechanisms proposed by Vico [7] for adsorption of Cu2+ and Zn2+ on sepiolite, i.e. replacement of Mg2+ in the edges of octahedral layer by Cd2+ and inner-sphere complexation of Cd2+ on the functional groups on the broken edges of the mineral and subsequent release of H+ . Alvarez-Ayuso and Garcia-Sanchez [6] also suggested that replacement of structural Mg at the edges of octahedral sheets is the main mechanism involved in Cd and Zn sorption on sepiolite considering similar hydrated ionic radii of Mg, Cd and Zn. Presence of 0.01 M Ca2+ in the background solutions weakened the possibility of non-specific outer-sphere adsorption of Cd on the negatively charged sites on the mineral surfaces. That is probably the reason why the sorption capacities of the minerals follow their SSA rather than their CEC. Alvarez-Ayuso and Garcia-Sanchez [24] proposed that reaction of heavy metals with numerous silanol groups is the major mechanism responsible for metal ions sorpion on palygorskite. Precipitation possibly is another mechanism involved in Cd sorption in our systems. Speciation analysis using Visual MINTEQ program showed that solutions equilibrated with some of the high Cd loaded palygorskite systems were supersaturated with respect to CdCO3(s) (otavite) (Table 3). In this analysis, a priority order of thermodynamic stabilities of each solid is established by comparing the appropriate ion activity products (IAP) with the corresponding formation constant (k) after the aqueous phase has been equilibrated. The logarithmic ratio of these terms (saturation index, SI) is calculated and used to establish the stability order for precipitation or dissolution of the soilids.

186

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

Fig. 3. Effect of Cd sorption on the Mg released from palygorskite () and sepiolite (
) (***, significant at probability level <0.001).

barrier for nucleation, particularly in case of crystallographical similarity between the surface and precipitating phase [25].

Fig. 2. Effect of Cd sorption on the equilibrium pH of the mineral suspensions (***, significant at probability level <0.001).

If the SI (log IAP − log k) for a particular mineral is positive, the system is supersaturated with respect to that mineral. Formation of amorphous copper hydroxychloride (CuCl2 ·3Cu(OH)2 ) was also demonstrated by Alvarez-Puebla et al. [8] as the sorption mechanism of Cu on palygorskite. All of the solutions in equilibration with the sepiolite, however, were undersaturated with respect to otavite as estimated by Visual MINTEQ (Table 3). This may be due to the high adsorption capacity of sepiolite capable of reducing the Cd concentration in solution to a level, low enough to inhibit the formation of otavite. Even so, surface nucleation of CdCO3(s) might occur on the clays in unsaturated aqueous systems. This is because surface can catalyze the nucleation step of crystallization by reducing or removing the energy

3.2.2. Calcite In agreement with the results obtained by McBride [26], the pH of the calcite suspensions decreased as the amount of Cd sorbed increased (Fig. 2) suggesting that Cd precipitation on the calcite surface has probably occurred in our systems. McBride [26] proposed that during Cd precipitation (according to the reaction: Cd2+ + H2 CO3 = CdCO3(s) + 2H+ ), the precipitate coats the CaCO3 surface and calcite no longer can react with the acidity, produced through CdCO3 precipitation, resulting in a lower pH value. Martin-Garin et al. [9] illustrated that adsorption of Cd on calcite inhibited the dissolution of calcite crystals up to 75% at the maximum Cd adsorption coverage. Speciation calculations on the solutions in equilibrium with calcite using Visual MINTEQ also showed that at higher Cd concentrations some systems were supersaturated with respect to otavite (Table 3). Undersaturation state of the other low Cd loaded systems, however, does not eliminate the possibility of Cd precipitation or coprecipitation on the calcite surfaces. Davis et al. [27] revealed that mechanism of Cd sorption on calcite is a continuum between an initial rapid exchange of Cd2+ for Ca2+ on the lattice surface sites followed by a slow diffusion of the metal into the hydrated CaCO3 and formation of CdCO3 –CaCO3 solid solution during dissolution–recrystalization of calcite. 3.3. Kinetic modeling Cadmium sorption on the minerals was initially rapid and then continued slowly till a pseudo equilibrium condition was reached. The amounts of Cd sorbed versus time were plotted with and without addition of the organic ligands (Fig. 4). Presence of organic ligands did not alter the apparent Cd sorption pattern for sorbents. However, analyzing the time-dependent data with several kinetic models showed that the presence of organic ligands could remarkably affect the Cd sorption rates in the sys-

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

187

Table 4 Values of rate constants, determination coefficients (r2 ) and standard errors of estimate (S.E.E.) for kinetic equations fitted to the Cd sorption data on palygorskite in the absence and presence of organic ligandsa Kinetic model Second-order qe (␮mol g−1 ) k2 × 103 (g ␮mol−1 min−1 ) h (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 ) Elovich α (␮mol g−1 min−1 ) β (g ␮mol−1 ) r2 S.E.E. (␮mol g−1 ) Power function a b ab (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 )

No ligand

Acetate

Citrate

DFOB

11.59 3.27 0.439 0.999 1.769

11.61 2.69 0.363 0.998 1.823

11.38 2.76 0.358 0.998 1.860

11.67 4.11 0.560 1 1.341

263.50 1.141 0.974 0.250

67.02 1.017 0.958 0.357

96.71 1.082 0.948 0.414

181.64 1.068 0.990 0.161

5.80 0.094 0.545 0.978 0.193

5.19 0.108 0.563 0.970 0.266

5.25 0.103 0.542 0.959 0.290

5.76 0.010 0.575 0.984 0.224

a

All of the determination coefficients are significant at probability level <0.001.

Eq. (6) for the boundary conditions t = 0–t and q = 0–qe , results in: 1 1 = + k2 t qe − q t qe

(7)

Eq. (7) can be rearranged to obtain a linear form: t 1 t = + 2 qt k2 q e qe

(8)

Table 5 Values of rate constants, determination coefficients (r2 ) and standard errors of estimate (S.E.E.) for kinetic equations fitted to the sorption data of cadmium on sepiolite in the absence and presence of organic ligandsa

Fig. 4. Sorption of Cd on the minerals as a function of time in the absence of organic ligands () and in the presence of acetate (
), citrate ( ) or DFOB ().

tems. Pseudo second-order, Elovich and power function models were used to describe the time-dependent Cd sorption data. The r2 and S.E.E. values used for comparison of fitted models are presented in Tables 4–6 for different sorbents. The pseudo second-order model was well-fitted to the Cd sorption data on the minerals (Tables 4–6). The rate equation for the second-order model can be expressed as: dqt = k2 (qe − qt )2 dt

(6)

where, qt and qe (␮mol g−1 ) are the amount of Cd sorbed on the mineral at time t and at equilibrium, respectively and k2 (g ␮mol−1 min−1 ) is the second-order rate constant. Integrating

Kinetic model

No ligand

Acetate

Citrate

DFOB

Second-order qe (␮mol g−1 ) k2 × 104 (g ␮mol−1 min−1 ) h (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 )

47.17 2.319 0.516 0.994 3.799

49.50 2.097 0.514 0.994 3.958

50.00 0.907 0.227 0.985 2.531

49.50 3.146 0.771 0.998 3.776

Elovich α (␮mol g−1 min−1 ) β (g ␮mol−1 ) r2 S.E.E. (␮mol g−1 )

1.602 0.125 0.934 3.759

1.624 0.120 0.940 3.622

0.655 0.106 0.927 4.525

2.884 0.128 0.950 3.079

Power function a b ab (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 )

4.700 0.325 1.5257 0.964 3.946

4.809 0.327 1.571 0.968 3.814

1.0202 0.547 0.558 0.949 6.691

7.277 0.274 1.993 0.959 3.967

a

All of the determination coefficients are significant at probability level <0.001.

188

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

Table 6 Values of rate constants, determination coefficients (r2 ) and standard errors of estimate (S.E.E.) for kinetic equations fitted to the sorption data of cadmium on calcite in the absence and presence of organic ligandsa

Table 7 The instantaneous sorption rate at 50 (SR50 ) and 90 (SR90 ) percent of Cd sorption capacity and the half life (t1/2 ) of Cd sorption in the presence and absence of organic ligands

Kinetic model

Parameter

Second-order qe (␮mol g−1 ) k2 × 103 (g ␮mol−1 min−1 ) h (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 )

No ligand

Acetate

Citrate

DFOB

2.499 11.359 0.0705 0.999 0.223

2.547 6.512 0.0422 0.997 0.280

2.596 4.438 0.0299 0.995 0.219

2.489 11.465 0.0710 0.999 0.225

Elovich α (␮mol g−1 min−1 ) β (g ␮mol−1 ) r2 S.E.E. (␮mol g−1 )

0.191 2.33 0.836 0.329

0.110 2.191 0.904 0.258

0.078 2.104 0.918 0.246

0.326 2.728 0.924 0.182

Power function a b ab (␮mol g−1 min−1 ) r2 S.E.E. (␮mol g−1 )

0.347 0.306 0.1062 0.805 0.502

0.273 0.331 0.0904 0.910 0.377

0.211 0.363 0.0764 0.935 0.326

0.542 0.226 0.1225 0.919 0.247

a

All of the determination coefficients are significant at probability level <0.001.

the qe and k2 can be calculated from the slope and intercept of the plot t/qt versus t. Sorption of lead on natural sepiolite has also been shown to follow the pseudo-second order kinetics [28]. The h = k2 qe2 is the initial Cd sorption rate (␮mol g−1 min−1 ) since as qt → 0, dqt /dt → k2 qe2 in Eq. (6). The initial sorption rate “h” has been widely used for evaluation of the sorption/desorption rates [17,18,28]. In the present study the values of “h” decreased as follows: 0.516 ␮mol g−1 min−1 (sepiolite) > 0.439 ␮mol g−1 min−1 (palygorskite) > 0.071 ␮mol g−1 min−1 (calcite) indicating that sepiolite can sorb Cd more rapidly than palygorskite and calcite from solution. According to the second-order kinetic model, the rate of a sorption reaction nonlinearly decreased with time. For example, the instantaneous rates at 50 and 90% of Cd sorption (SR50 and SR90 , respectively) can be calculated from Eq. (6): SR50

k2 qe2 h = k2 [qe − (0.5qe )]2 = = 4 4

SR90 = k2 [qe − (0.9qe )]2 =

h k2 qe2 = 100 100

(9) (10)

therefore, SR50 and SR90 values are one-fourth and onehundredth of initial sorption rate “h”, respectively, (Table 7) and comparisons reported here based on “h” values can be extended to the entire experiment duration. Based on the second-order kinetic law the half-life of Cd sorption (the time at which half of the sorption process is completed) directly depends on the sorption capacity of the mineral and inversely relates to the initial sorption rate: t1/2 =

1 qe = (k2 qe ) h

Sepiolite

Calcite

109.82 90.75 89.50 140.00

129.00 128.42 56.72 192.72

17.73 10.55 7.47 17.75

SR90 × 103 (␮mol g−1 min−1 ) No ligand 4.39 Acetate 3.63 Citrate 3.58 DFOB 5.60

5.16 5.14 2.27 7.71

0.70 0.42 0.30 0.71

× 103

(11)

the t1/2 values for the minerals followed the order: sepiolite > calcite > palygorskite (Table 7). Mineral sorption capac-

Palygorskite (␮mol g−1

SR50 No ligand Acetate Citrate DFOB

min−1 )

t1/2 (min) No ligand Acetate Citrate DFOB

26.4 32.0 31.8 20.8

91.4 96.3 220.4 64.2

35.2 60.3 86.8 35.0

ities determined from the fitted pseudo second-order kinetic model (qe values in Tables 4–6) were quite comparable to the maximum sorption capacities calculated from the Langmuir isotherms (qm ) presented in Table 2. Sorption data of Cd also can be satisfactorily described by the Elovich model with r2 values ranging from 0.836 to 0.990 (Tables 4–6). The Elovich equation can be written as: dqt = α exp (−βqt ) dt

(12)

where, qt is the amount of Cd sorbed at time t and α and β are constants. Since dqt /dt → α as qt → 0, α can be regarded as the initial rate constant. After integrating with the boundary condition q = qt at t = t and q = 0 at t = 0, Eq. (12) changes to:   1 ln (1 + αβt) (13) qt = β rearrangement of Eq. (13) and assuming that αβt  1 results in the linear form of the Elovich equation:     1 1 ln (αβ) + ln t (14) qt = β β The power function model also adequately described the time-dependent Cd sorption on the minerals. The correlation coefficients and the constants of the fitted power function model are presented in Tables 4–6. This model can be expressed as: qt = a t b

(15)

where, qt (␮mol g−1 ) and t (min) are defined elsewhere and a and b are constants. The product of power function model constants “ab” is defined as the “specific sorption rate at unit time” since: dqt = ab t (b−1) dt and at t = 1 min: dqt/dt = ab.

(16)

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

3.4. Effects of ligands on Cd sorption rates By comparison of kinetic parameters “h”, “α” and “ab” presented in Tables 4–6, it can be concluded that sorption rate of Cd on the minerals generally decreased in the presence of citrate and acetate. For instance, initial sorption rate of Cd on palygorskite “h” decreased from 0.439 ␮mol g−1 min−1 in the absence of organic ligands to 0.363 and 0.358 ␮mol g−1 min−1 in the presence of acetate and citrate, respectively. Cadmium sorption rate on the sepiolite was negatively affected; however, only by citrate as evidenced from rate constants of Table 5. The negative effect of citrate and acetate on the Cd sorption rates on calcite is also evident from the rate parameters values given in Table 6. For example, the “α” value decreased from 0.191 ␮mol g−1 min−1 in the absence of ligands to 0.110 and 0.078 ␮mol g−1 min−1 in the presence of acetate and citrate, respectively. Acetate clearly had a weaker effect on the Cd sorption rate than citrate. In several studies, the observed decrease in the amount of metal sorption on the minerals in the presence of LMWOAs’ anions has been proposed to be due to the formation of soluble metal-ligand complexes that have lower affinities to the surfaces compared to free metal ions or competition between metals and ligands for complexation with functional groups on the clay surfaces [12]. Brooks and Herman [16], for example, suggested that formation of stable anionic aqueous complex [Co(citrate)]− which comprised more than 90% of the total soluble Co, was the reason of decreased rate and extent of Co adsorption on silica at pH 7. The LMWOAs induced sorption reduction of aluminum on kaolinite [29] and rare earth elements on Chinese soils [30] were also attributed to the competition of aqueous organic ligands with adsorption sites for sorbates. Speciation analysis using Visual MINTEQ program on model solutions with similar compositions to the real solutions used in our kinetic experiment at t = 0 (100 ␮M Cd and 1 mM citrate or acetate in 10 mM CaCl2 background solution, pH 7–8), however, showed that large percentages of soluble Cd were un-bounded to LMWOAs’ ligands, and [Cd(citrate)]− and [Cd(acetate)]+ , as dominant complexes of Cd with citrate and acetate, comprised only about 6 and 2% of the total dissolved Cd, respectively (data not shown). Since the total Ca concentration in the solutions used in our experiments was 10 times as much as that of the Cd, most of the citrate anions were bounded to Ca ions. More than 90% of the acetate anions were also unbounded in solution because of low affinity of these anions to Cd and Ca ions. This may decrease the possibility of formation of stable soluble complexes of Cd with acetate and citrate as the main mechanism for Cd sorption rate decrease on the sorbents in the systems investigated here. As a result of reduction of Cd sorption rates, the t1/2 values increased in the presence of LMWOAs’ anions (Table 7). This may implies that Cd reside for a longer time in soil liquid phase in the presence of such organic ligands which may, in turn, influence the movement and plant absorption of this metal in soil environments. In contrast to the LMWOAs’ anions, DFOB siderophore induced more rapid uptake of Cd on the minerals as evidenced from the values of the rate constants presented in Tables 4–6. For example, in case of sepiolite, addition of DFOB increased the initial Cd sorption rate “h” from 0.516 to 0.771 ␮mol g−1 min−1 ,

189

Elovich “α” value from 1.602 to 2.884 ␮mol g−1 min−1 and “ab” from 1.526 to 1.993 compared to the control treatment. Kraemer et al. [31] denoted that metal ions sorption enhancement in the presence of DFOB may be caused by ternary surface complexation, electrostatic attraction, or surface precipitation of trace metal-sideophore complex. Neubauer et al. [15] found that heavy metal sorption on montmorillonite is promoted in the presence of DFOB. They attributed their observation to the high amount of negative charges on the montmorillonite surface leading to the strong affinity of positively charged metal-DFOB chelates to this mineral surface. Another mechanism for the increased metal sorption by DFOB may be simultaneous binding of some functional groups of DFOB molecule with the clay surface and some others with metal ions forming a ternary clayDFOB-metal complex. Rosenberg and Maurice [32] noted that even though DFOB is a tri-hydroxamate (six-dentate) ligand, Fe binds only to the hydroxamate group farthest from the amine group (and therefore farthest from the positive charge) forming a 1:1 complex. Farkas et al. [33] also stated that taking into account the large ionic radius of Ca and the short connecting chain of DFOB, only one of the hydroxamate groups of DFOB may coordinate to the Ca ion. Ternary clay-ligand-metal complexation has been proposed by Ali and Dzombak [34] and Naidu and Harter [35] for sorption enhancements of Cu on goethite and Cd on soils in the presence of some organic acids. Sorption rate of Cd on calcite also increased in the presence of DFOB (Table 6). The reasons for metal sorption rate changes by acetate, citrate and DFOB additions are not clear and more elaborate experimental set up is required to study the phenomena. 4. Conclusions Equilibrium data of Cd sorption on palygorskite, sepiolite and calcite was adequately described by the Langmuir and Freundlich equations. Capacities of the minerals for Cd uptake followed the order: sepiolite > palygorskite > calcite. Sepiolite also showed the highest affinity for Cd among the sorbents. The increase observed in the content of Mg and proton released to equilibrium solutions with increasing the amount of Cd sorbed supported the suggestion of Mg substitution on the edges of octahedral sheet by Cd as well as complexation of Cd on surface functional groups of the clay minerals as the mechanisms involve in Cd sorption. The rate of Cd sorption on the minerals decreased in the presence of acetate and citrate. This may be due to the competition of ligands with surfaces for Cd or competition of ligands with Cd for mineral surfaces. Presence of DFOB siderophore, on the other hand, increased the sorption rate of Cd on the minerals. The observed effect may be explained by Cd-ligand-surface ternary complex formation and electrostatic attraction of positively charched Cd-DFOB complexes to the clay surfaces. References [1] D.L. Sparks, Elucidating the fundamental chemistry of soils: past and recent achievements and future frontiers, Geoderma 100 (2001) 303–319.

190

M. Shirvani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 182–190

[2] M.M. Benjamin, J.O. Leckie, Multi-site adsorption of Cd, Zn and Pb on amorphous iron oxyhydroxide, J. Colloid Interface Sci. 79 (1981) 209–221. [3] A.M. Scheidegger, M. Fendrof, D.L. Sparks, Mechanisms of nickel sorption on pyrophyllite: macroscopic and microscopic approaches, Soil Sci. Soc. Am. J. 60 (1996) 1763–1772. [4] E. Galan, Properties and applications of palygorskite-sepiolite clays, Clay Miner. 31 (1996) 443–453. [5] H.H. Murray, Traditional and new applications for kaolin, smectite, and palygorskite: a general overview, Appl. Clay Sci. 17 (2000) 207–221. [6] E. Alvarez-Ayuso, A. Garcia-Sanchez, Sepiolite as a feasible soil additive for the immobilization of cadmium and zinc, Sci. Total Environ. 305 (2003) 1–12. [7] L.I. Vico, Acid-base behavior and Cu2+ and Zn2+ complexation properties of the sepiolite/water interface, Chem. Geol. 198 (2003) 213–222. [8] R.A. Alvarez-Puebla, C. Aisa, J. Blasco, J.C. Echeverria, B. Masquera, J.J. Garrido, Copper heterogeneous nucleation on a palygorskitic clay: an XRD, EXAFS and molecular modeling study, Appl. Clay Sci. 25 (2004) 103–110. [9] A. Martin-Garin, P. Van Cappellen, L. Charlet, Aqueous cadmium uptake by calcite: a stirred flow-through reactor study, Geochim. Cosmochim. Acta 67 (2003) 2763–2774. [10] G.S.R. Krishnamurti, G. Cieslinski, P.M. Huang, C.J. Van Rees, Kinetics of cadmium release from soils as influenced by organic acids: implication in cadmium availability, J. Environ. Qual. 26 (1997) 271–277. [11] R. Nigam, S. Srivastava, S. Prakash, M.M. Siravastava, Effect of organic acids on the availability of cadmium in wheat, Chem. Speci. Bioavail. 12 (2000) 125–132. [12] C.R. Collins, K.V. Ragnarsdottir, D.M. Sherman, Effect of inorganic and organic ligands on the mechanism of cadmium sorption to goethite, Geochim. Cosmochim. Acta 63 (1999) 2989–3002. [13] V. Romheld, The role of phytosiderophores in acquisition of iron and other micronutrients in graminaceous species: an ecological approach, Plant Soil 130 (1991) 127–134. [14] S.M. Kraemer, S.F. Cheah, R. Zapf, J. Xu, K.N. Reymond, G. Sposito, Effect of hydroxamine siderophores on Fe release and Pb(II) adsorption by goethite, Geochim. Cosmochim. Acta 63 (1999) 3003–3008. [15] U. Neubauer, B. Nowack, G. Furrer, R. Schulin, Heavy metal sorption on clay minerals affected by the siderophore desferrioxiamine B, Environ. Sci. Technol. 34 (2000) 2749–2755. [16] S.C. Brooks, J.S. Herman, Rate and extent of cobalt sorption to representative aquifer minerals in the presence of a moderately strong ligand, Appl. Geochem. 13 (1998) 77–88. [17] Y.S. Ho, G. McKay, Application of kinetic models to the sorption of copper(II) on to peat, Adsorp. Sci. Technol. 20 (2002) 797–815. [18] A. Benhammou, A. Yaacoubi, L. Nibou, B. Tanouti, Study of the removal of mercury(II) and chromium (VI) from aqueous solutions by Moroccan stevensite, J. Hazard. Mater. 117B (2005) 243–249.

[19] H. Van Olphen, J.J. Fripiat, Data Handbook for Clay Minerals and Other Non-metallic Materials, Pergamon Press, Oxford, 1979, p. 346. [20] J.A. Kittrick, E.W. Hope, A procedure for the particle size separation of soils for X-ray diffraction analysis, Soil Sci. 96 (1963) 312– 325. [21] J.W. Rhoads, Cation exchange capacity, in: C.A. Page (Ed.), Methods of Soil Analysis, ASA, Madison, WI, 1986, pp. 149–158. [22] J.P. Gustafsson, http://www.lwr.kth.se/english/ OurSoftware/Vminteq (2005). [23] A. Garcia-Sanchez, A. Alastuey, X. Querol, Heavy metal adsorption by different minerals: application to the remediation of polluted soils, Sci. Total Environ. 242 (1999) 179–188. [24] E. Alvarez-Ayuso, A. Garcia-Sanchez, Palygorskite as a feasible amendment to stabilize heavy metal polluted soils, Environ. Pollut. 125 (2003) 337–344. [25] M.B. McBride, Chemisorption and precipitation reactions, in: M.E. Sumner (Ed.), Handbook of Soil Science, CRC Press, Boca Raton, FL, 2000, pp. B265–B302. [26] M.B. McBride, Chemisorption of Cd2+ on calcite surfaces, Soil Sci. Soc. Am. J. 44 (1980) 26–28. [27] J.A. Davis, C.C. Fuller, A.D. Cook, A model for trace metal sorption processes at the calcite surfaces: Adsorption of Cd2+ and subsequent solid solution formation, Geochim. Cosmochim. Acta 51 (1987) 1477–1490. [28] N. Bektas, B.A. Agim, S. Kara, Kinetic and equilibrium studies in removing lead ions from aqueous solutions by natural sepiolite, J. Hazard. Mater. 112B (2004) 115–122. [29] R. Xu, G. Ji, Effect of anions of low molecular weight organic acids on adsorption and desorption of aluminum by and from a kaolinite at different pH, Soil Sci. 168 (2003) 39–44. [30] X. Shan, J. Lian, B. Wen, Effect of organic acids on adsorption and desorption of rare earth elements, Chemosphere 47 (2002) 701–710. [31] S.M. Kraemer, J. Xu, K.N. Raymond, G. Sposito, Adsorption of Pb(II) and Eu(III) by oxide minerals in the presence of natural and synthetic hydroxamate siderophores, Environ. Sci. Technol. 36 (2002) 1287– 1291. [32] D.R. Rosenberg, P.A. Maurice, Siderophore adsorption to and dissolution of kaolinite at pH 3 to 7 and 22◦ C, Geochim. Cosmochim. Acta 67 (2003) 223–229. [33] E. Farkas, E.A. Enyedy, H. Csoka, A comparison between the chelating properties of some dihydroxamic acids, desferrioxamine B and acetohydroxamic acid, Polyhedron 18 (1999) 2391–2398. [34] M.A. Ali, D.A. Dzombak, Interactions of copper, organic acids, and sulfate in goethite suspensions, Geochim. Cosmochim. Acta 60 (1996) 5045–5053. [35] R. Naidu, R.D. Harter, Effect of different organic ligands on cadmium sorption by and extractability from soils, Soil Sci. Soc. Am. J. 62 (1998) 644–650.