water interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects 166 (2000) 153 – 159 www.elsevier.nl/locate/colsurfa

Adsorption of lead and cadmium ions from aqueous solution to the montmorillonite/water interface F. Barbier, G. Duc, M. Petit-Ramel * Laboratoire d’Instrumentation et de Chimie Analytique en Solution-LICAS, Uni6ersite´ Claude Bernard Lyon I, 43 boule6ard du 11 No6embre 1918, 69622 Villeurbanne cedex, France Received 19 May 1999; accepted 26 October 1999

Abstract In order to prevent contamination of subsoil and groundwater by leachates containing heavy metals, montmorillonite linings are used on landfill bottoms. It is therefore important to understand ion uptake by this clay. The sorption of Pb(II) and Cd(II) from aqueous solution to the montmorillonite/water interface has been studied as a function of pH, for three concentrations. Two montmorillonites have been used: an industrial bentonite and a reference clay. Clay acidity constants have been determined, indicating that both clays seem to have characteristics similiar to each other. Adsorption increases with pH. Whatever the clay or pH, lead is retained more than cadmium. A model that assumes two kinds of binding sites can describe metallic ion uptake: XNa groups, responsible of ion exchange, and ampholitic surface hydroxyls SOH, responsible of surface complexation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Heavy metals; Montmorillonite; Ion exchange; Surface complexation.

1. Introduction Lead and cadmium, as a result of their numerous uses can pollute water and soils, and it is well known that heavy metal pollution is a serious threat to the environment. Clay linings are used as barriers in landfills to prevent contamination of groundwater by leachates [1] containing metals, but a possible acidification can lead to an increase in mobility of metals bond to the soil [2 –5]. * Corresponding author. Tel.: +33-4-72448495; fax: + 334-72446202. E-mail address: [email protected] (M. Petit-Ramel)

In order to predict the fate of these contaminants in soil, it is necessary to improve the knowledge of their behaviour in such a clay system. The most important processes regulating the free concentration of heavy metals in natural media, such as the adsorption by solid surfaces and the release from sediments, highly depend on interfacial chemical reactions. Consequently, many research studies have focused on the interactions of dissolved metals with surfaces of naturally occurring oxides [6–10]. Adsorption of heavy metals at oxide/water interface can be described as surface complexation mechanism [11]. The uptake of heavy metals by

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 5 0 1 - 4

154

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159

clay minerals is obviously more complicated. The chemical nature of the metal clay interaction changes with increasing pH: at low pH values cation exchange is the dominating process [10,12], whereas at high pH values, the uptake of heavy metal ions is accompanied by a release of hydrogen ions, and seems to be more specific than the uptake at low pH values. Thus, the classical ion exchange model does not cover the whole range of adsorption phenomena and a part of heavy metal adsorption occurs at sites created by displacement of protons from surface hydroxyls (i.e. surface complexation) [13]. In this paper, we will determine acido-basic characteristics of two montmorillonites and we will study the uptake of Pb(II) and Cd(II) as a function of pH.

2. Materials and methods

2.1. Electromoti6e force measurements The pH is measured with a Metrohm glass electrode at 25°C. Metal ions concentrations are obtained with the aid of ion-sensitive electrodes (Tacussel for Pb2 + and Orion for Cd2 + ). The reference electrode is a Metrohm Ag, AgCl/3M KCl separated from the bulk by a salt bridge filled with NaNO3. Buffer standardisation is realised with 0.05 mol l − 1 of potassium hydrogenophtalate and 0.05 mol l − 1 of sodium tetraborate, as described in NF T01-012 and NF T01-013 [14]. The pH values used in this report are defined by: pH = −log[H+]yH Table 1 Chemical composition of BSAB Components

% By weight

SiO2 Al2O3 Fe2O3 MgO Na2O CaO

58.83 20.92 3.26 4.39 3.40 1.43

where yH is the activity coefficient of the hydrogen ions obtained from Davies equation: log yi = − 0.5z 2i (I 1/2/(I+ I 1/2)− 0,3I)

2.2. Materials Two bentonites are studied. The first one, provided by the Clay Mineral Society, is the SWy-2 montmorillonite, used as a reference. The second one, called BSAB, is a technical commercial bentonite, containing 80% of Na-montmorillonite and commonly used as landfill liner. Its chemical composition was provided by supplier (Table 1). In order to respect BSAB characteristics and to evaluate the retention phenomenon in landfill liners, these two clays have not been treated. Specific surface areas were measured by N2, BET method: SSWy − 2 = 29.4 m2 g − 1 and SBSAB = 87 m2 g − 1. These values are lower than those expected for montmorillonites [15], but it may be due to the lack of treatment. In fact, Bourg [16] and Goldberg et al. [17] have measured, for a montmorillonite without pretreatment, specific surface areas of 23.9 and 18.6 m2 g − 1: it is possible that N2 molecules can not penetrate easily the interlayer regions between the layer sheets, involving an underestimation of specific surface areas [18–21].

2.3. Titration All the reagents are of analytical grade. All the experiments are realised with a solid concentration of 1 g l − 1. The titration cell was thermostated at 25°C. Fifty milligrams of clay and 50 ml of NaNO3 0.05 mol l − 1 solution are introduced in the cell and stirred for an hour in order to permit the particles ageing. Carbon dioxide contamination is avoided by passing dry and oxygen-free nitrogen through the investigated system. After this step, HNO3 is added until a pH value of about 4 is reached, and the solution is stirred for 15 min. Aliquots of NaOH 0.02 mol l − 1 are added every 3 min. In these conditions, total titration time is lower than 1 h, and consequently the solid dissolution is reduced; moreover, a kinetics study shows that

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159

155

step, leading to metallic concentrations of 1, 2 and 4× 10 − 4 mol l − 1. This suspension is stirred for 15 min, titration is realised as described above and metal concentration is measured. In order to realise a blank titration, a solid suspension is prepared as described previously. After the ageing step, the solid phase is separated from the aqueous phase by centrifugation and filtration at 0.45 mm. The aqueous phase is the blank and is titrated in the same conditions as suspension. Fig. 1. BSAB titration. 2Experimental curve; —: calculated curve (DLM)

3. Results and discussion

3.1. Clay titration The titration curves of montmorillonites (Fig. 1 and Fig. 2) display two buffer regions, indicating the presence of two kinds of acidic groups. We can postulate the following equilibria [22]: XNa+ H+ ? XH+ Na+ KXH = {XH}[Na+]yNa/{XNa}[H+]yH + SOH+ 2 ? SOH + H

Fig. 2. SWy-2 titration. 2Experimental curve; —: calculated curve (DLM)

/ {SOH+ 2 } Ka2 = {SO−}[H+]yH exp(− FC/RT)/{SOH}

Table 2 Equilibrium constants I= 0

TXNa (mmol g−1) TSOH (mmol g−1) log KXH log K sa1 log K sa2 VY

Ka1 = {SOH} [H+] yH exp(− FC/RT)

BSAB

SWy-2

480 2479 4 8.239 0.03 −3.709 0.14 −4.349 0.06 6.03

260 36.8 9 3 8.67 9 0.07 −4.3890.45 −5.2690.27 1.26

equilibrium state is reached after 2 min. The total proton concentration, T(H) net, is the sum of solution proton concentration and surface proton concentration. When titration is performed in presence of metal ions, an aliquot of concentrated metallic solution is added in the system after acidification

where, SOH: surface hydroxyl groups; XNa: ionic exchange sites; c: acting surface potential; exp(9 Fc/RT): coulombic correction. The equilibrium constants and the SOH sites density are evaluated with the aid of FITEQL [23]. The model chosen is the diffuse layer model (DLM) because it does not need any fixed value, like, for instance, capacitance in the constant capacitance model. Furthermore, according to Westall and Hohl [24], all surface complexation models are equivalent to describe surface properties from potentiometric titrations, even if acidity constants calculated are a little bit different. The constants are fitted for I= 0.05 and calculated for I= 0 using the Davies equation. In a first time, all the parameters, i.e. XNa sites density, SOH sites density, KXH, Ka1 and Ka2, are fitted but there is

156

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159

no convergence of the adjustment process. Consequently, in a second time, XNa concentration is fixed and the fit is successful. The obtained values are presented in Table 2. The symbol VY represents the fitting quality, and a value comprised between 0.1 and 20 indicates an acceptable fitting quality [23]. Fig. 1 and Fig. 2 show that the DLM

Fig. 3. BSAB: surface sites speciation. 2SOH+ 2 ; SOH;  SO−; — XNa; + XH.

with the optimised values gives an acceptable fit of the experimental data. However, systematic errors may originate from non-ideal behaviour of the adsorbed species and may explain the nonperfect fit. The cationic exchange sites density is much more important than the surface hydroxyls groups density. It is well known that montmorillonites have high CEC values and bibliographic references indicate values between 800 and 1300 mmol kg − 1 [25,26]. These values are significantly higher than ours and the difference may be explained by the fact that clays have not been pretreated. Moreover, usual methods used to determine CEC (saturation by Ba2 + , NH4 + or Na+) measure in fact the total site density, i.e. CEC and SOH sites density [27]. Surface sites speciation, presented in Fig. 3 and Fig. 4, shows that XNa groups, which are able to be exchanged with protons and metal ions, are mainly on the XH form, which will compete with metal sorption. As the surface hydroxyl groups SOH are ampholitic, they are able to form inner sphere complexes as SOM+ and (SO)2M. The SOH groups can be silanol groups (SiOH) or aluminol groups (AlOH): they compete for available adsorbent, but otherwise do not interact with each other [28]. We can compare our results with the stability of the surface complexes with deprotonated SiOH, AlOH, Al(OH)(OH2) and FeOH [11] presented in Table 3. The Ka2 values of this study are closed to SiOH constants. However, the Ka1 values are closer to Al(OH)(OH2) values, since they are lower, than other group’s values. Therefore, we can do the assumption that our clays contain SiOH and Al(OH)(OH2) groups, and that reactions are: Ka1

Fig. 4. SWy-2: surface sites speciation. SO−; — XNa; + XH.

2SOH+ 2 ;

SOH; 

Al(OH2)(OH2)+ = Al(OH)(OH2) + H+ Ka2

Al(OH)(OH2) = Al(OH)(OH)− + H+ Ka2

Table 3 Acidity constants of surface hydroxyl groups I= 0 Group

log K sa1

log K sa2

AlOH Al(OH)(OH2) SiOH FeOH

−7.31¯−7.51 −5.38 – −6.51¯−6.71

−9.39¯−9.89 −7.94 −5.57¯−6.69 −8.99¯−9.14

SiOH = SiO− + H+ This is in accordance with the results obtained by Stadler and Schindler [29], even if acidity constants are different. In fact, in bibliography, acidity constant values for montmorillonites are very different, varying between − 2.81 and − 7.38 for Ka1, and between −1.26 and − 9.09 for Ka2 [17].

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159

157

In conclusion, BSAB and SWy-2 have characteristics close to each other, BSAB being a little more acidic. Their affinities for metallic ions should then be comparable.

3.2. Adsorption of metal ions

Fig. 5. BSAB: lead sorption versus pH. 2: 1.10 − 4 mol l − 1;  2.10 − 4 mol l − 1; : 4.10 − 4 mol l − 1

Fig. 6. SWy-2: lead sorption versus pH. 2: 1.10 − 4 mol l − 1;  2.10 − 4 mol l − 1; : 4.10 − 4 mol l − 1

Fig. 7. BSAB: cadmium retention versus pH. 2: 1.10 − 4 mol l − 1;  2.10 − 4 mol l − 1; : 4.10 − 4 mol l − 1

Adsorption of Pb(II) and Cd(II) versus pH is presented in Figs. 5–8. Whatever the clay and the metallic ion, sorbed fraction remains constant in acidic conditions, and retention highly increases in a short pH range: 5.5BpHB 6.5 for Pb2 + on BSAB, 6B pHB7 for Pb2 + on SWy-2, and 8B pH B 9 for Cd2 + on the two clays. The low sorption in acidic conditions may be explained by the competition between protons and metallic cations. The differences between lead sorption and cadmium sorption concern the pH value at which sorption begins to increase and the pH value of maximum adsorption. For instance, lead sorption increases at lower pH values than cadmium sorption, and whatever the pH, Pb(II) is more retained than Cd(II). Nevertheless, at high pH values, i.e. pH 7 for Pb2 + and pH 9 for Cd2 + , determination of metal sorbed is overvalued because of metal precipitation. Consequently, in order to neglect precipitation phenomena, only experimental values under these pH are taken into account to fit the constants. The sorption reactions are:

Fig. 8. SWy-2: cadmium retention versus pH. 2: 1.10 − 4 mol l − 1;  2.10 − 4 mol l − 1; : 4.10 − 4 mol l − 1

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159

158

Table 4 Equilibrium constants I= 0

Pb(II) Cd(II)

BSAB SWy-2 BSAB SWy-2

log KX2M

log KSOM

VY

10.83 90.20 9.85 90.11 5.25 90.16 5.61 90.08

−3.079 0.36 1.56 9 0.10 −4.699 0.42 2.44 9 0.10

15.2 36 21 1.7

2 XNa + M2 + ? X2M + 2 Na+ KX2M = {X2M} [Na+]2 y 2Na / {XNa}2 [M2 + ]

The uptake of heavy metal ions by montmorillonite from solution, from acidic to basic conditions can be describe by a model assuming two kinds of binding sites: “ ion exchange sites, XNa; “ specific adsorption sites, surface hydroxyl groups Al(OH)(OH2), which are ampholitic. Ion exchange seems to be the main sorption phenomena, but at low pH values, competition with protons is very important. Whatever the clay, lead uptake is more important than cadmium uptake.

× yM SOH + M2 + ? SOM+ + H+

References

KSOM = {SOM+}[H+]2y 2H exp(FC/RT) / {SOH}

[1] D. Pywell, Wastes Manag. C93/85 (1985) 15. [2] G.D. Campbell, H.F. Galicia, P.W. Schindler, Aust. J. Soil Res. 25 (1987) 391. [3] M.L. Luncan-Bouche´, F. Habets, S. Biagianti-Risbourg, G. Vernet, Fresenius Environ. Bull. 6 (1997) 719. [4] G.D. Redden, J. Li, J. Leckie, in: E.A. Jenne (Ed.), Adsorption of Metals by Geomedia, Academic Press, New York, 1998, p. 13. [5] B. Batchelor, Environ. Sci. Technol. 32 (1998) 1721. [6] C. Tiffreau, J. Lu¨tzenkirchen, P. Behra, J. Coll. Interface Sci. 172 (1994) 82. [7] P.W. Schindler, P. Liechti, J.C. Westall, Neth. J. Agri. Sci. 35 (1987) 219. [8] W.R. Roy, I.G. Krapac, J.D. Steele, J. Environ. Qual. 22 (3) (1993) 537. [9] Y.C. Sharma, G. Prasad, D.C. Rupainwar, Int. J. Environ. Anal. Chem. 45 (1) (1991) 11. [10] L. Alberga, T. Holm, G. Tiravanti, D. Petruzzelli, Environ. Technol. 15 (1994) 245. [11] P.W. Schindler, W. Stumm, in: W. Stumm (Ed.), Aquatic Surface Chemistry, Wiley, New York, 1987. [12] K.G. Tiller, J. Gerth, G. Bru¨mmer, Geoderma 34 (1984) 1. [13] M. Stadler, P.W. Schindler, Clays Clay Miner. 41 (6) (1993) 680. [14] Recueil des Normes Franc¸aises: Analyse, Normes Fondamentales, AFNOR, 1988, Tome1. [15] D. Hillel, Environmental Soil Physics, Academic Press, New York, 1998. [16] Bourg A., Mode´lisation du comportement des me´taux traces a` l’interface solide-liquide dans les syste`mes aquatiques. The`se de doctorat, Bordeaux I, France, 1983. [17] S. Goldberg, C. Su, H.S. Forster, in: E.A. Jenne (Ed.), Adsorption of Metals by Geomedia, Academic Press, New York, 1998. [18] D. Sparks, Environmental Soil Chemistry, Academic Press, New York, 1995.

× [M2 + ] yM Results are presented in Table 4. The DLM, associated with cationic exchange and surface complexation reaction, gives an acceptable fit of the experimental data. KX2M constants are significantly higher than KSOM, indicating that lead and cadmium are principally sorbed by cation exchange. Nevertheless, even if the constants are high, competition phenomenon with H+ is very important, decreasing metal sorption. KX2M values for Pb2 + are significantly higher than those for Cd2 + , which confirms that lead is more retained by clays than cadmium. Moreover, the values for KX2M constants obtained are significantly higher than the corresponding values reported for kaolinite [7]. These results are in agreement with the fact that cationic exchange is an important property of Na-montmorillonite [26]. Furthermore, in the case of cadmium sorption on SWy-2, the KSOM value is somewhat higher than those of kaolinite are [30], despite a lower surface hydroxyl groups density. We can assume that this phenomenon is due to a higher reactivity of these sites. Aluminol sites are known to be more reactive than silanol groups, confirming the hypothesis of a majority of Al(OH)(OH2) groups in our clay. The differences with bibliographic values can be due to impurities and to the lack of pretreatment.

F. Barbier et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 166 (2000) 153–159 [19] I. De´ka´ny, F. Sza´nto´, A. Weiss, G. Lagaly, Berichte Der Bunsen-Gesellschaft fu¨r Physikalische Chemie 89 (1985) 62. [20] I. De´ka´ny, F. Sza´nto´, L.G. Nagy, J. Coll. Interface Sci. 109 (1986) 376. [21] I. De´ka´ny, L.G. Nagy, J. Coll. Interface Sci. 147 (1991) 119. [22] W. Stumm, Chemistry of the Solid–Water Interface, John Wiley, New York, 1992. [23] Herbelin A.L. Westall J.C., FITEQL, version 3.2, A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data, Report 9601, Department of Chemistry, OR State University, Corvallis, Oregon, USA, 1996. [24] J.C. Westall, H. Hohl, Advan. Coll. Interface Sci. 12 (1980) 265. [25] Winiarski T., Analyse Syste´mique du Fonctionnement de

.

[26]

[27] [28]

[29] [30]

159

l’Interface Syste`me Naturel et Syste`me Anthropise´: Exemple de l’Interface De´charge/Aquife`re. The`se de Doctorat, Lyon I, France, 1994. S. Caille`re, S. He´nin, M. Rautureau, Mine´ralogie des argiles, II. Classification et Nomenclature, Masson, Paris, 1982, p. 3. G. Sposito, The Chemistry of Soils, Oxford University Press, New York, 1989. R.T. Pabalan, D.R. Turner, F.P. Bertetti, J.D. Prikryl, in: E.A. Jenne (Ed.), Adsorption of Metals by Geomedia, Academic Press, New York, 1998. M. Stadler, P.W. Schindler, Clays Clay Miner. 41 (3) (1993) 288. Benyahya L., Complexation de Ligands Organiques et de Me´taux-Trace a` la Surface de la Silice, de l’Alumine et de la Kaolinite: Expe´rimentation et Mode´lisation. The`se de Doctorat, Paris VI, France, 1996.