Effect of Nanoscale Pore Space Confinement on Cadmium Adsorption

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Effect of Nanoscale Pore Space Confinement on Cadmium Adsorption from Aqueous Solution onto Ordered Mesoporous Silica: A Combined Adsorption and Flow Calorimetry Study Benedicte Prelot,*,† Sebastien Lantenois,†,‡ Claude Chorro,† Marie-Christine Charbonnel,‡ Jerzy Zajac,† and Jean Marc Douillard† †

Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Agregats, Interfaces et Materiaux pour l'Energie, C. C. 1502, Place Eugene Bataillon, 34095 Montpellier cedex 5, France ‡ CEA Marcoule, Nuclear Energy Division, RadioChemistry & Processes Department, Service of Separation Processes, Laboratory of Interactions Ligand-Actinide, F-30207 Bagnols-sur-Ceze, France

bS Supporting Information ABSTRACT: The competitive adsorption of Cd(II) ions has been studied from aqueous solutions of cadmium nitrate in the presence of 0.1 M sodium nitrate at 298 K onto nonporous Spherosil XO75LS and ordered porous silica of the SBA-15 type with the purpose of tracking the effect of confinement in pores. Four SBA-15 mesoporous samples, prepared using pluronic P103 and P123 as templates at two temperatures of 303 and 373 K and differing in both the pore size (from 3.8 to 6.4 nm) and the areal density of acid sites reactive toward gaseous ammonia, were used. The equilibrium adsorption isotherms and the enthalpy curves for the related displacement process were determined by means of solution depletion method and liquid flow microcalorimetry, respectively. The experimental adsorption curves were transformed into normalized plots showing the ratio between the amount of cadmium retained at the solidliquid interface and the number of surface silanol groups. Estimates of the standard molar Gibbs free energy and entropy of adsorption were made in the pseudoplateau adsorption region. It was postulated that the adsorbing Cd2+ cations ion exchanged with the initially adsorbed Na+ ions, the overall displacement process being endothermic and entropy-driven in the whole concentration range studied, mostly due to small modifications of the hydration shells of adsorbing and desorbing species. The areal density of surface silanols, influenced by the surface curvature within pore space, appeared to be the major factor that induced significant changes in the molar enthalpy of displacement.

1. INTRODUCTION In environmental science and technology where the control of depollution processes is of vital importance, the knowledge of the behavior of heavy metal and radionuclide ions in solution and their interactions at various solidliquid interfaces still remains a great challenge. A large number of adsorption studies involving these ions and mineral oxides or clays have been undertaken,1 including determination of the adsorption edge2 or the adsorption capacity,3 as well as modeling attempts with the use of various surface complexation models (1-pK, 2pK, Music, ... and CCM, BSM, TLM)49 or model equations for adsorption isotherms.1012 The enthalpies of adsorption are usually evaluated from isotherm measurements performed at different temperatures, based on the general GibbsHelmholtz relation or the Van’t Hoff equation.13 In contrast with such studies reported on the temperature effect of ion adsorption,1416 direct calorimetric measurements of the enthalpy effects accompanying the phenomenon are not easy to carry out. The results of ion adsorption onto carbons published by Groszek et al.17,18 or those r 2011 American Chemical Society

obtained with functionalized siliceous materials by the research group of Airoldi1923 can be cited as examples of the pioneering work in the field, in addition to other papers on phosphates,24,25 soils,26 layered double hydroxide,27,28 and more recently clays or oxides.29,30 However, it is crucial to note that all of these studies deal with solids exhibiting high adsorption capacity and then producing relatively high, therefore easily measurable, adsorption energies. Measurements of the enthalpy effects in systems characterized by both low amounts and low energies of ion adsorption at the solidliquid interface are very rare because of the detection limits of the most frequently used calorimeters and the related difficulties in treating the signal.31 Nevertheless, there is an increasing need for such experimental studies since calorimetry offers an opportunity for monitoring the various interactions Received: February 17, 2011 Revised: September 6, 2011 Published: September 06, 2011 19686

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The Journal of Physical Chemistry C

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Table 1. Textural and Surface Properties of the Four Ordered Porous Silicas of the SBA-15 Type and Nonporous Reference Spherosil XO75LS38a sample

SBET (m2 g1)

CBET

Sm (m2 g1)

Vp (cm3 g1)

Vmes (cm3 g1)

Vmic (cm3 g1)

dp (nm)

Nacid (site nm2)

SBA-15P103303K

794

341

637

0.62

0.56

0.07

3.8

0.055

SBA-15P123303K SBA-15P103373K

905 876

215 113

704 885

0.93 1.11

0.85 1.11

0.08 0

5.6 5.6

0.091 0.053

SBA-15P123373K

1037

112

1022

1.32

1.32

0

6.4

0.051

XO75LS

69.4

178

69.4

0.348

a

The BET specific surface area, SBET, and energetic constant, CBET, the specific area including the external surface and the mesopore walls, Sm (determined from the αS-plot analysis of the N2 adsorption isotherms at 77 K), the overall pore volume, Vp, volumes of mesopores, Vmes, and micropores, Vmic, the most probable pore diameter, dp (inferred from the BJH pore size distribution), as well as the number of surface acid sites per sq. nm, Nacid (evaluated based on the two-cycle adsorption of gaseous ammonia).

involved in the adsorption phenomena. Furthermore, the experience gained from the previous research on the adsorption of ionic surfactants interacting specifically with mineral oxides3234 indicates that the nature of the solidliquid interface involved and, in particular, various partial processes accompanying ion adsorption from aqueous solution show up more clearly on the enthalpy curves than on the adsorption isotherms. Calorimetry is thus another independent experimental method which assists in detection of changes in the interfacial properties with surface coverage. From this standpoint, calorimetric data may be very useful for theoretical consideration and modeling. Nowadays, the knowledge of the influence of pore size on such surface-involving phenomena as adsorption, dissolution, or precipitation occurring at a boundary between oxides and aqueous solutions is yet far from being complete. Commonly, the term ‘confinement’ is used in the sense of modification of physical and chemical properties (e.g., phase behavior, molecular mobility, etc) of molecular species confined in the pore space and it may include such effects as alteration of the solidification behavior of gases35 or some drastic changes in the dielectric constant of water in a pore space.36 The intention of the present work is to extend the use of this term so as to describe the effects of adsorbent porosity on the adsorption of ions from solutions on oxide surfaces. Since the mechanism of such an adsorption phenomenon seems to be dictated by the properties of vicinal water, it should be to a great extent sensitive to the porosity. Among many questions still waiting to be answered, this paper addresses the problem related with the nature and accessibility to adsorbing species of surface reactive sites located on the pore walls, the effect of curvature on the structure of the electrical double layer (EDL) forming inside the pore space, as well as the possible consequences on the charge behavior of the solid surface which would impose the surface complexation environment and adsorption mechanism different from those observed on a flat solid surface. The main question arises as to whether the confinement effect has an impact on the thermodynamics of heavy metal adsorption. To avoid any uncertainty due to the heterogeneity of pore size distribution, pore arrangement, or heteroatom incorporation,37 purely siliceous materials of the SBA-15 type possessing pores in the lower end of the mesopore size range were utilized as ordered porous adsorbents. For this purpose, four SBA-15 silica samples were prepared with the use of block copolymer templates of different sizes at two temperatures of the hydrolysis polycondensation stage in order to tune the pore structure of the resulting materials. In a previous work,38 these silica-based

adsorbents served to demonstrate the confinement effect on surface reactivity of amorphous silica at the solidgas and solidliquid interface. The areal densities of surface sites reactive toward either gaseous ammonia or hydrated protons/hydroxide anions in mesoporous samples were compared with those present on the surface of Spherosil XO75LS used as nonporous reference. The increased surface heterogeneity, as evidenced by the presence of micropores in the material, smaller pore ordering, and greater values of the BET energetic constant, was found to enhance surface acidity of amorphous silica in the gas phase. The surface reactivity of porous materials was significantly enhanced when passing from the solidgas to solidwater interface, contrary to the behavior of Spherosil XO75LS. In the present work, similar research strategy of comparison between the four mesoporous SBA-15 silicas and the nonporous reference was applied in the thermodynamic study of the competitive adsorption of cadmium cations from nitrate aqueous solutions containing sodium nitrate as extra salt under constant pH 7 at room temperature. The amount of cadmium cations adsorbed on the adsorbent surface and the integral enthalpy of displacement accompanying the adsorption phenomenon were the main observables determined separately by the solution depletion technique and liquid flow microcalorimetry as a function of the solute concentration in the equilibrium bulk solution. The differences in the adsorption isotherms and enthalpy curves among the five silica samples were rationalized in regard with the adsorbent porosity.

2. EXPERIMENTAL SECTION 2.1. Materials. Ordered mesoporous silicas of the SBA-15 type were prepared following the method described by Zhao et al.39,40 with some modifications concerning the structuredirecting agent used and the temperature of hydrolysis polycondensation. The synthesis route and successive treatments were detailed in the previous article.38 Four samples were achieved with the use of Pluronic triblock copolymers P123 or P103 and by aging the reaction mixture during 48 h at 303 or 373 K. These materials are further referred to as SBA15P103303K, SBA-15P103373K, SBA-15P123303K, and SBA15P123373K, as the template type and the aging temperature have been included in the material name. Table 1 sums up the following surface characteristics of the calcined samples, as determined previously38 from the BET, αS-plot, and two-cycle adsorption analysis41 applied to the isotherms of nitrogen adsorption at 77 K and gaseous ammonia adsorption at 373 K: the BET specific surface area, SBET, and energetic constant, CBET, 19687

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The Journal of Physical Chemistry C the specific area including the external surface and the mesopore walls, Sm, the overall pore volume, Vp, volumes of mesopores, Vmes, and micropores, Vmic, the most probable pore diameter, dp, as well as the number of surface acid sites per sq. nm, Nacid. The detailed interpretation of all data obtained in studying the porosity and surface properties (described in ref 38) points toward mesoporous materials with a 2D hexagonal pore arrangement, presenting some subtle differences in the texture due to the combined effect of the type of the template used and the synthesis temperature. SBA-15P123303K, SBA-15P123373K, and SBA-15P103373K possess uniformly sized tubular-like mesopores (H1-type hysteresis loop, narrow pore size distributions), whereas the pore structure of SBA-15P103303K appears less organized (H2-type hysteresis loop, the second and third order peaks in the XRD pattern are less intense, broader pore size distribution). Furthermore, the porosity of the silica materials synthesized at 303 K includes some microporous contribution, which accounts for the enhanced interaction between the adsorbed nitrogen molecules and the hydroxylated silica surface (much greater CBET constants). In the case of nonmicroporous SBA-15P103373K and SBA-15P123373K, the CBET values are smaller than those usually obtained for other nonmicroporous silicas, pointing out the low surface energy linked to the increased amorphous character of the surface. The principal textural parameters (i.e., SBET, Sm, Vp, Vmes, and dp) increase when the synthesis temperature is raised or the larger template is used. The order of increasing mean pore diameter is as follows: SBA-15P123373K > SBA-15P103373K ≈ SBA15P123303K > SBA-15P103303K. SBA-15P123303K is characterized by the highest density of surface sites reactive toward gaseous ammonia, whereas the three other samples exhibit similar surface acidity. It is thus important to note that two samples, SBA-15P123303K and SBA-15P103373K, with the same mean pore size may be still differentiated based on the areal density of acid sites. Hydrophilic precipitated silica, Spherosil XO75LS, received from Procatalyse (France), was used as the nonporous silica reference. Some of its surface properties are recalled in Table 1. This sample contains about 6 times more acid sites per unit area of its surface than the mesoporous materials. 2.2. Isotherms of Cd(II) Adsorption from Aqueous Solution. All Cd-containing solutions were prepared in a 0.1 M NaNO3 aqueous solution utilized as mixed solvent in order to maintain the ionic strength constant. The pH of this mixed solvent was adjusted to 7.0 making use of NaOH or HNO3 standard solutions. Then the stock solution of the heavy metal at a fixed concentration of 0.01 M was prepared from solid Cd(II) nitrate dissolved in the mixed solvent. The pH of the resulting aqueous phase was measured and adjusted again to 7.0, if necessary. Solutions of various concentrations ranging between 1 104 and 0.01 M for different experimental points on the adsorption isotherm were achieved by diluting the 0.01 M Cd(NO3)2 stock solution with the mixed solvent. Adsorption quantification was carried out in 30 mL Nalgene reactors agitated using a rotary shaker. In each reactor, a solid sample of Spherosil (0.5 g) or mesoporous silica (0.05 g) was added to 20 mL of the appropriate cadmium solution and equilibrated for 2 h at 298 K. Afterward the rotation was stopped, the pH of the solidliquid suspension measured and adjusted to 7.0. Equilibration was continued overnight at 298 K. Then the pH was measured again to monitor the pH evolution and the solid phase was separated from the

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supernatant by centrifugation at 10 000 rpm for 15 min. The concentration of the equilibrium bulk solution (the supernatant) was determined with the aid of ionic chromatography analyzer (Shimadzu HPLC) equipped with a CDD-6A conductivity detector operating at 313 K. A Shim-Pack IC-C1 column (with an IC-GC1 precolumn) was used, the mobile phase being 4 mM tartaric acid/1.5 mM ethylenediamine at a flow rate of 1.5 mL per min. The amount of Cd(II) adsorbed at the solidliquid interface per unit area of this interface was calculated as follows: Γads ¼

V i ðCi Ceq Þ mS AS

ð1Þ

where Ci and Ceq are respectively the initial and final concentration of Cd(II) expressed in mol L1, Vi denotes the initial volume (in L) of Cd(II) solution in a given Nalgene reactor, and mS and AS are the mass (in g) and the specific surface area (in m2 g1) of the adsorbent available to the liquid phase, respectively. 2.3. Liquid Flow Microcalorimetry Measurements of Cd(II) Adsorption. The direct measurements of the integral enthalpy of Cd(II) adsorption were performed at 298 K by means of a Microscal flow microcalorimeter.42 Depending on the effective density of the solid bed and the solid particle size, a sample of 1560 mg of porous or nonporous silica was placed in the calorimetric cell to fill the constant cell volume (about 50 μL). The sample was pumped in situ for 1 h at 298 K using online vacuum facilities. Then the solid bed was wetted and subsequently equilibrated during 3 h with a 0.1 M aqueous solution of NaNO3 at pH 7 flowing through the calorimetric cell at a constant flow rate of 3.3 mL h1 fed by a Shimadzu LC-20AD pump with parallel double pistons. The pump was operating in a low-pressure configuration in order to enhance the thermal resolution.31 After attaining thermal equilibrium in the system, the flow of the mixed solvent was replaced by that of a solution containing Cd(II) ions at a fixed concentration in the mixed solvent (i.e., 0.1 M NaNO3 aqueous solution at pH 7). The attainment of the adsorption equilibrium was detected through the return of the thermal signal to the steady baseline. The thermodynamic reversibility of the adsorption phenomenon was then tested. For this purpose, a desorption stage was carried out under exactly the same experimental conditions, just by returning to the flow of the mixed solvent through the adsorbent bed. Altogether, each calorimetry run was composed of three subsequent steps: thermal equilibration step with the mixed solvent (0.1 M aqueous solution of NaNO3 at pH 7), adsorption of Cd(II) ions at the solidliquid interface from appropriate solution, and desorption of Cd(II) ions from the interface by the flow of the mixed solvent. Thermal peaks related to adsorption and desorption were recorded independently for several concentrations of the heavy metal ion. Calibration of areas under these thermal peaks was performed by dissipating a known amount of energy in the adsorbed bed with the aid of a calibration Joule effect probe incorporated into the outlet tube of the flow calorimeter.42

3. RESULTS 3.1. Isotherms of Cd(II) Adsorption onto Nonporous and Mesoporous Silicas. In the case of ions adsorption at electrified

solidliquid interfaces, the adsorption equilibrium is a function of the pressure, temperature, and solute concentration in the equilibrium bulk phase, but it may also depend on the charging 19688



Figure 1. Adsorption isotherms for Cd(II) ions onto silica supports at 298 K from aqueous solution of Cd(NO3)2 containing 0.1 M NaNO3 at fixed pH 7: Spherosil XO75LS (f), SBA-15P103303K (4), SBA15P103373K (2), SBA-15P123303K (O), and SBA-15P123373K (b). The dashed lines are drawn to guide the eye, and they have no theoretical significance.

behavior of the solid surface in a given environment of the liquid phase. It is usually recommended to carry out adsorption experiments in the presence of indifferent electrolyte at constant pH, though some researchers prefer to work with adsorption systems at “free” pH randomly imposed by the experimental conditions used and without any electrolyte.4346 With hydrolyzable metal cations, the adsorption measurements should be absolutely performed at a controlled pH in order to avoid ions hydrolysis and their concomitant precipitation at the solid liquid interface. The speciation diagram of cadmium nitrate under the experimental conditions of adsorption applied in the present study (cation concentration below 0.01 M, background NaNO3 electrolyte at a fixed concentration of 0.1 M) was reported previously.31 Since the hydrolysis of cadmium was found to occur beyond pH 8, the pH for the present adsorption experiments was fixed at 7. According to this speciation diagram, the predominant species at pH 7 was Cd2+. The cadmium concentration of the equilibrium solution was varied between 1 104 and 9 103 M. Since one of the objectives was to compare adsorption capacities of the various samples and quantify the effect of porosity, the amount of cadmium adsorbed was normalized when dividing the adsorption value by the adsorbent surface AS available to the adsorbate. As a first approximation, the value of AS was taken as being equal to SBET from Table 1. The experimental adsorption curves for Cd2+ onto Spherosil and the four mesoporous silicas are shown in Figure 1. The absolute uncertainties in determining Γads were evaluated by taking into account contributions from solution preparation, measuring procedures and further data processing. As far as the nonporous reference sample is concerned, the quantity of adsorption per unit surface area of the adsorbent increases steadily for equilibrium concentrations ranging from 1 104 to 4 103 M. The slope of the adsorption curve begins to decrease beyond this interval, indicating that the phenomenon is about to level off around 0.3 μmol m2. In the

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case of ordered mesoporous silicas of the SBA-15 type, somewhat different shapes can be distinguished. The amount of Cd(II) adsorbed onto four samples still increases with increasing concentration of the equilibrium bulk solution. However, two adsorption regimes can be clearly seen at low and high equilibrium concentrations which results in a distinct ‘knee’ or even a first pseudoplateau on the isotherm. The tendency to level off at high equilibrium concentrations is observed again. For the two mesoporous silicas prepared making use of P123 as template, the high-concentration pseudoplateau value is evaluated at 0.35 μmol m2 for SBA-15P123303K and 0.19 μmol m2 for SBA-15P123373K. In the case of SBA15P103303K and SBA-15P103373K, the cadmium uptake per sq. m is quite small at low concentrations and then it increases slowly to about 0.12 μmol m2. Surprisingly, the maximum adsorption capacity of the SBA15 silicas in the concentration range studied here is generally smaller than that of nonporous Spherosil XO75LS. Only SBA15P123303K adsorbs somewhat more Cd(II) ions per unit area of its surface. The question arises as to whether the enthalpy effects accompanying the adsorption phenomenon show the same trends depending on the adsorbent porosity. 3.2. Displacement Enthalpy Effects Accompanying Cd(II) Adsorption onto Nonporous and Mesoporous Silicas. Although titration microcalorimetry offers an opportunity of measuring the pseudodifferential molar enthalpy effects accompanying ion adsorption, thereby giving a valuable insight into the adsorption mechanism, it is technically impossible to adjust the pH to the desired value.46 The use of liquid flow microcalorimetry should be sometimes preferred since the continuous percolation of the liquid phase at a constant composition through the solid bed located inside the calorimetric cell allows such important physical parameters as pH, ionic strength, and the solute concentration to be maintained constant.31 Here changes in the enthalpy correspond not only to the formation of a solidliquid interface being in thermal and material contact with the percolating stock solution, but also include wetting/ dewetting and displacement effects. Altogether, the thermal balance in the flow calorimetry experiment represents the integral enthalpy of displacement ΔdplH. For the purpose of the present study, the values of ΔdplH were measured by means of a liquid flow microcalorimeter for several stock solutions with Cd(II) concentration ranging between 1 103 and 1 102 M. It should be emphasized that, irrespective of the type of solid, the integral enthalpy effect of displacement due to Cd(II) adsorption from appropriate aqueous phase was positive in the whole concentration range, clearly pointing out the endothermic character of the overall process. Repeated adsorption and desorption steps with the aqueous phase in contact with the solid surface for a time required to reach equilibrium were used to assess reversibility of the interfacial phenomena involved. The desorption stage was systematically exothermic, giving an integral enthalpy value equal to that of the preceding adsorption cycle but opposite in sign. Based on the observed reversibility of the adsorption and desorption phenomena, the integral molar enthalpy of displacement Δdplh was determined from the experimental ΔdplH value and the corresponding quantity of Cd2+ adsorption taken from the adsorption isotherm obtained separately under the same experimental conditions (Figure 1). 19689



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follows: 1.9 kJ mol1 for SBA-15P103303K, 2.3 kJ mol1 for SBA15P103373K, and 2.8 kJ mol1 for SBA-15P123373K. The integral molar enthalpies of displacement onto Spherosil obtained in the present work are consistent with the literature data for ion adsorption onto macroporous silica.47,48 For example, Sahai derived the adsorption enthalpies for monovalent cations from theoretical terms describing their electrostatic interaction with silica and reported enthalpy values of about 10 kJ mol1.47 According to Kosmulski,48 the enthalpy of adsorption for cobalt cation onto silica should be about 20 kJ mol1, as determined from the van’t Hoff procedure applied to the measurement of the temperature dependence of adsorption isotherms.

Figure 2. Variations of the integral molar enthalpy of displacement Δdplh accompanying adsorption of Cd(II) ions from an aqueous solution of cadmium nitrate in the presence of 0.1 M sodium nitrate at 298 K onto silica supports: Spherosil XO75LS (f), SBA-15P103303K (4), SBA-15P103373K (2), SBA-15P123303K (O), and SBA15P123373K (b). In the inset the Δdplh vs Ceq plots for the porous solids are presented on a reduced scale to better show the differences between them. The repeatability of the enthalpy measurement, estimated from two separate runs, was within 3%. The dashed lines are drawn to guide the eye, and they have no theoretical significance.

The enthalpy curves showing the variations of the integral molar enthalpy of displacement induced by the progressive adsorption of Cd(II) and the concomitant desorption of Na+ are presented in Figure 2. At this level, to avoid any oversimplifying assumption as regards the structure of the adsorbed layer, the experimental enthalpy values have been plotted as a function of the Cd(II) concentration in the percolating stock solution. Furthermore, the Δdplh values below a bulk Cd2+ concentration of 4 103 M have not been reported for all porous samples. The main reason is that small amounts adsorbed were accompanied by very small integral enthalpies, thereby leading to great uncertainty in the Δdplh determination. Given the complexity of measuring procedures and further data processing, the absolute error calculation was replaced by the estimation of the repeatability of the enthalpy measurement based on two separate runs. Generally speaking, the differences in Δdplh among the five silica samples fall in line with the various slopes of the corresponding adsorption isotherms shown in Figure 1, especially at low equilibrium concentrations. At the boundary between nonporous silica (Spherosil XO75LS) and aqueous solutions of various compositions, the displacement is strongly endothermic and the value of Δdplh decreases continuously with increasing bulk concentration from about 15 to about 7 kJ mol1. For all mesoporous silicas, the adsorption and related phenomena still produce the overall endothermic effect, but the Δdplh values do not exceed 3 kJ mol1 within the concentration range studied. The initial value is near to 1 kJ mol1 at a concentration of 4 103 M. For SBA15P123303K, the enthalpy of displacement curve shows a little variation between 0.7 and 0.8 kJ mol1 as the adsorption of Cd(II) progresses. With SBA-15P123373K, SBA-15P103303K, and SBA15P103373K, a small increase in Δdplh is observed with increasing equilibrium concentration. The maximum enthalpy value is as

4. DISCUSSION The uptake of Cd(II) ions by nonporous and mesoporous silicas from aqueous solution containing background electrolyte ions is an endothermic phenomenon in the whole adsorption range studied in the present work, with the intensity of the effect being dependent on the adsorbent porosity. This points out the complex, competitive character of the overall adsorption process. It is important to mention here that the marked endothermicity is not a feature of heavy metal adsorption at all solid-solution interfaces. There are some examples of systems where the adsorption of cations other than cadmium (e.g., lead) onto nonporous silica is an athermal process or where the phenomenon is highly exothermic in certain intervals of surface coverage by the adsorbing cation.4951 It is commonly admitted that preferential ion adsorption on the oppositely charged surfaces involves primarily (i) nondirectional electrical attractions between ions and the surface charge, leading irrevocably to the formation of an electric double layer, and (ii) specific (i.e., nonelectrical) interactions, when the approaching distance between the adsorbed species and the reactive surface site is small enough. To rationalize the variations of the enthalpy of displacement in the present adsorption systems, several contributions to the overall thermal effect should be considered (see the related discussion in section A of the Supporting Information). They are mainly related to the direct silicateCd(II) cation interaction and the dehydration of the adsorbing cation when approaching the surface (at least on the side next to the charged plane). Generally speaking, the first enthalpy term including transfer and immobilization of the ion is negative (i.e., exothermic contribution to Δdplh). The second term associated with the hydration shell effects balance can be positive (i.e., endothermic contribution to Δdplh) since the hydration enthalpies are usually negative.52 Nevertheless, the adsorbing ions may exchange with electrolyte or some other exchangeable ions present at the solidliquid interface, because of the limited extent of the adsorption space and also to ensure the electric neutrality of the interface as a whole. Taking into account the fact that the negative surface charge is initially neutralized by sodium cations in the presence of background electrolyte, these enthalpy terms should be compared with those related to the desorbing Na+ ions which are subsequently rehydrated in the bulk solution. When bringing together similar contributions, the total change in the molar enthalpy may be simply represented as follows: Δdpl h ¼ ΔhNa f Cd ¼ ðΔads hCd rΔads hNa Þ þ ðΔhyd H Na Δhyd H Cd Þ 19690

ð2Þ



Figure 3. Normalized isotherms for Cd(II) adsorption onto silica supports showing the ratio between the number of Cd2+ ions retained at the solidliquid interface (see Figure 1) and the number of surface silanol groups reactive toward gaseous ammonia (see Table 1) as a function of the Cd(II) concentration in the equilibrium bulk: Spherosil XO75LS (f), SBA-15P103303K (4), SBA-15P103373K (2), SBA15P123303K (O), and SBA-15P123373K (b). The dashed lines are drawn to guide the eye, and they have no theoretical significance.

where ΔadshCat is the molar enthalpy of adsorption for Cd2+ or Na+ cation, r is the molar ratio of displacement (the number of Na+ cations released by one Cd2+), and ΔhydHCat represents the total enthalpy change in the hydration of cation Cat between the interfacial region and the bulk solution (corresponding to one adsorbing Cd2+ ion). The second term on the right-hand side of eq 2 depends on the hydration parameters for both the exchanging cations (i.e., the successive enthalpies of cation hydration, cation radius, and number of water dipoles in the hydration shell), as well as the hydration balance upon exchange process depending on the modes of cation fixation to the silica surface and its specific location within the electric double layer. It should be remembered that the values of Δdplh analyzed on the basis the above equation strictly correspond to the fixed pH and ionic strength, at which the measurements have been carried out, and the same consideration cannot be applied for other experimental conditions. For the purpose of further rationalization of the displacement effect, some simplifying assumptions are necessary. Based on the assumptions that cadmium is adsorbed on the silica surface from aqueous solution as Cd2+ ions and the surface silanol groups are the major adsorption sites,53 the experimental adsorption isotherms from Figure 1 were normalized by dividing each Γads value by the number of acid surface sites reactive toward gaseous ammonia Nacid, as determined by two-cycle ammonia adsorption (cf., Table 1). In the absence of Lewis acid groups on the silica surface, the value of Nacid can be regarded as a first approximation of ionizable surface silanols that interact with cadmium cations in aqueous solution. It should be noted that Nacid does not necessarily include surface OH groups interacting with NH3 molecules only through hydrogen bonding. The normalized adsorption curves are plotted in Figure 3. In the case of Spherosil, the maximum ratio value of 0.5 indicates that one Cd2+ ion is statistically adsorbed on two

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silanol groups. The specific interactions of sodium with the charged surface at the silicawater interface, as well as its role in heavy metals adsorption onto silica, have been thoroughly considered.1,5458 Since the pH and ionic strength are maintained constant in the adsorption and calorimetry experiments, it may be assumed here that one adsorbing Cd(II) cation displaces two compensating Na+ ions to the bulk solution. Even though the locations of both types of cations within the ionic double layer are different, this “indirect” ion exchange is regulated through the concomitant changes in the electric charge and potential.56 The mechanistic models of the adsorption phenomenon (e.g., CD MUSIC), which involve multiple surface reactions and several model parameters, are not suitable for handling the present calorimetry results. Therefore, further analysis of the displacement effect has been based on a simplified interaction scheme which does not pretend to describe the actual displacement mechanism. Cd2+ and Na+ are considered to be bound to oxygen atoms of the ionized surface silanols and each such bond leads to the loss of one water dipole from the hydration layer of the adsorbing cation. In view of the small enthalpy values observed here, only the weakly bonded water molecules composing the two hydration shells are concerned by the hydration balance. The seventh water molecule in the hydration layer of Na+ is bound with an energy of 20.7 kJ mol1, whereas the loss of the seventh and eighth molecule from the vicinity of Cd2+ requires 183 kJ mol1.52 When two Na+ ions are released from the surface by one adsorbing Cd2+, the enthalpy change corresponding to the hydration balance per mole of Cd2+ ions (ΔhydHNa ΔhydHCd) is equal to +141.6 kJ mol1. As far as the energies of Na+O and Cd2+O bonds are concerned, several different values have been reported for the two parameters in the literature;59 the maximum values being 75 and 138 kJ mol1, respectively. It should be remembered that such data are related to bond dissociation enthalpies which have been derived from the heats of formation of the species involved in the appropriate “bulk” reactions. With all these data in mind, the enthalpy Δdplh corresponding to an ideal 1:2 exchange between Cd2+ and Na+ modifying only the external part of the hydration shell can be evaluated at +15.6 kJ mol1. This estimate falls within the same order of magnitude as the experimental Δdplh obtained for the adsorption of Cd2+ on the flat surface of nonporous Spherosil at low concentrations of the equilibrium aqueous solution. Even though the adsorption energies appearing in the above approximation have been obviously overestimated with respect to the real interaction distances between the cations and the charged surface, this result clearly indicates the competitive character of the phenomenon studied. When the equilibrium concentration Ceq increases, and so does the surface coverage with the adsorbed Cd2+ ions, the displacement process becomes less and less endothermic and the energy balance behavior differs from the above ideal one. Even though the ion-exchange ratio remains unchanged (i.e., r = 2), it is easy to admit that the energies of ion binding to the surface strongly depend on the local topology of heterogeneous silica surface and also on their lateral interactions with other ions adsorbed in the vicinity. For example, Cd2+ ion is considered as a soft species, when compared with Na + , and its specific (nonelectrostatic) binding to the silicate surface may produce more energy than that indicated in the literature for the “bulk” Cd2+O ion pairing. The ionic or covalent contributions to such bonds may also undergo alterations depending on the 19691


The Journal of Physical Chemistry C orientation and location (distances) of the adsorbed ions with respect to the neighboring surface silanols. Changes in the hydration energies of the adsorbed ions rendering the overall displacement effect to be endothermic should be also taken into consideration. It is obvious that the problem cannot be solved without resorting to appropriate modeling of the system, which is out of the scope of the present work. The mechanism of Cd2+ adsorption onto porous SBA-15 samples seems more complicated since not only the ion exchangehydration balance is far from the “ideal” value but also the Cd(II)/site ratio is generally greater than 0.5. Within the experimental error, the curves in Figure 3 corresponding to the same template employed during the synthesis are quite similar, irrespective of the synthesis temperature. Since silica samples prepared at 303 K possess micropores in addition of mesopores (see Table 1), the normalized adsorption curves show no clear influence of the microporosity content on the stoichiometry of Cd2+ adsorption for the two block copolymer templates. This conclusion is not necessarily consistent with the results reported by De Keizer et al.60 and Szekeres et al.61 since these authors claim that the micropores contribute strongly to the overall particle charge of amorphous silica. The unavailability of the micropore space cannot be really postulated in the present case as the ionic radius of Cd2+, estimated at 0.426 nm on the basis of the hydrodynamic radius reported by Nightingale,62 is smaller than the micropore sizes. Furthermore, there is a regular increase in the Cd(II)/site ratio until a maximum value of about 2.3 (P123 template) or 1.3 (P103 template). Note that the Cd(II)/site value near to 1 can be interpreted as the consequence of adsorption of one Cd(II) cation on one silanol site, the nearest silanols being too far from each other to interact with the same bicharged metal cation. From the point of view of electrical interactions, ratio values greater than 1 have no physical significance. One of the plausible explanations of such discrepancies is that Nacid is not always a correct estimate of the number of reactive sites in aqueous media for some mesoporous solids, which indicates the confinement effect on surface reactivity of amorphous silica depending on whether this reactivity is studied at the solidgas or solidliquid interface.38 Among the other parameters, it is commonly admitted that the extent and energetics of adsorption should be chiefly dependent on the number of reactive sites and the charging behavior of the solid surface (i.e., the location of the point of zero charge PZC, the density and variation of the pH-dependent surface charge).63 The short review of the literature in section B of the Supporting Information shows that there is no consensus on this matter. Nevertheless, it is hardly conceivable that the apparent enhancement in the cation adsorption on a per site basis for the mesoporous silica samples in the present work may be ascribed to marked changes in the surface charge due to the overlapping of the portions of the double layer surrounding the opposite pore walls. The curvature effect on the surface charge is valid only when the thickness of such a double layer is comparable with the pore diameter of the adsorbent. For the present adsorption systems, the inverse of the so-called DebyeH€uckel length k calculated from the well-known dependency on the ionic strength63 is about 1 nm in a 0.1 M solution of 1:1 electrolyte at 298 K. From this viewpoint, the Cd(II)/site ratio should be independent of the pore diameter contrary to the image presented in Figure 3.

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The working hypothesis in the present paper is that the observed change in the extent and energetics of Cd2+ adsorption when passing from the nonporous reference to mesoporous silica samples may rather be ascribed to the varying mode of interactions between the adsorbing cations and the surface silanols as a function of the relative conformations of the interacting species. Some indications can be obtained from the analysis of the porosity effect on the entropy changes accompanying the adsorption process, even though one knows that the evaluation of entropy terms always needs simplifying assumptions. The adsorption equilibrium constant Keq should be calculated from the following expression: K eq ¼

Φs Γads ¼ s Ceq δ Ceq

ð3Þ

where Ceq and Φs refer to the concentration of Cd2+ in the equilibrium bulk solution and the related concentration of the adsorbed cation in a surface monolayer, respectively, Γads is the Gibbs adsorption of Cd(II) cation (amount adsorbed per unit surface area), and δs the thickness of the surface layer. Then the classical equations which relate the equilibrium constant with the standard molar changes in Gibbs free energy Δg0 and entropy Δs0 may be used to calculate the fundamental thermodynamic functions of the adsorption phenomenon at a given Ceq when the corresponding enthalpy changes are known ! Γads 0 Δads g ¼ RT ln K eq ¼ RT ln s and δ Ceq TΔads s0 ¼ Δdpl h Δads g 0

ð4Þ

For the purpose of further comparison, estimates of such thermodynamic parameters were made in the region where the Cd(II)/siteNH3 ratio leveled off at high Cd(II) concentrations in the equilibrium bulk solution. As mentioned before, the values usually considered for modeling the thickness of the electric double layer in a 0.1 M solution are narrowly spread around 1 nm, being somewhat dependent on the model used. It is worth mentioning that the highest value of 1.3 nm was obtained by Panagiotou et al.64 who modeled the thickness of the compact layer together with the stagnant-diffuse and the mobile-diffuse parts of the electrified interface. Therefore, a 1 nm-thick layer covering the whole surface area (i.e., SBET) was taken here as a realistic model of the surface layer, especially when searching for trends rather than absolute numbers. The resulting values are given in Table 2. The positive TΔadss0 values in Table 2 clearly show that the adsorption of Cd(II) cations in the pseudoplateau adsorption region is an entropy-driven phenomenon. Furthermore, the entropy term per mole of Cd2+ adsorbed increases from SBA15P103303K, possessing the less organized structure of small mesopores, to large-pore samples with a better organized mesoporous structure, and finally to the planar silica surface, thereby indicating a decrease in the degree of order in the surface system. The differences among all samples can be explained by taking into account the hypothesis of varying distances between the neighboring surface silanols, being a function of both the areal density of active sites and the pore volume. No simple trends can be observed in the molar Gibbs free energy and enthalpy of displacement. For SBA-15P123303K and SBA-15P103373K characterized by the same pore diameter, the influence of the density of active sites 19692



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Table 2. Molar Changes in the Gibbs Free Energy, Enthalpy, and Entropy, Ratio between the Number of Surface Silanol Groups Reactive Towards Gaseous Ammonia and the Number of Cd2+ Ions Retained at the SolidLiquid Interface (i.e., the Inverse of Cd(II)/SiteNH3 from Figure 3), Related to the Adsorption of Cd(II) Cations from an Aqueous Solution of Cadmium Nitrate in the Presence of 0.1 M Sodium Nitrate at 298 K onto Silica Supports, Taken in the Region of High Ceq Concentrations (Pseudo-Plateau Adsorption Range) Δadsg0 sample

Δdplh

1

TΔadss0

siteNH3/

1

dp (nm) (kJ mol ) (kJ mol ) (kJ mol1) Cd(II)

SBA-15P103303K

3.8

6.3

1.8

8.1

0.77

SBA-15P103-373K SBA-15P123303K

5.6 5.6

6.3 9.1

2.3 0.7

8.6 9.8

0.77 0.43

SBA-15P123373K

6.4

7.5

2.8

10.3

0.43

9.1

7.5

16.6

2.0

XO75LS

should be taken into account. The enthalpy of displacement is about 3 times greater (more positive) for cadmium adsorption onto SBA-15P103373K, whereas this sample possesses a smaller number of reactive sites per sq.m (Table 1). Differences in the density of reactive sites are not the only reason for samples to exhibit unlike thermodynamic behavior. Although SBA15P123373K, SBA-15P103303K, and SBA-15P103373K are characterized by very similar Nacid values, the corresponding enthalpies of displacement differ from one another. A plausible explanation may be rather searched in the changing distance between oxygen atoms belong to the neighboring OH groups. According to the areal site density, the mean distance between two silanols is as follows: 1.7 nm, XO75LS; 3.2 nm, SBA15P123303K; 4.5 nm, three other samples. It is obvious that appropriate distances are necessary for the adsorption of Cd2+ to fall in line with the idealized ion exchange behavior described previously. Evidently, such conditions are satisfied only in the case of nonporous XO75LS where the siteNH3-to-Cd(II) ratio points toward bonding of one Cd2+ by two surface oxygens. The Δdplh effect at this surface coverage with Cd(II) cations is different from the idealized exchange-hydration balance (+15.6 kJ mol1) calculated previously based on the idealized interaction modes in the bulk phases more related to the dilute case. Therefore, the enthalpy value of +7.5 kJ mol1 may be considered for this coverage as a “surface reference” for Δdplh involving the 1:2 exchange stoichiometry between Cd2+ and Na+, lateral interactions between the neighboring adsorbate species, as well as the actual dehydrationrehydration balance. Increasing distance between two neighboring surface silanols lowers the possibility of establishing such an interaction mode, which likely accounts for the enthalpy differences between the nonporous reference and the porous SBA-15 samples. The values of Δdplh in Table 2 also indicate that the displacement phenomenon becomes much less endothermic when the Cd2+ fixation mode differs from the ideal one. The effect of surface curvature should be also mentioned in this context since the pore sizes are of the same order of magnitude as the distances between silanols. It may happen that a distance between two silanols located on the opposite pore walls is smaller than the mean value resulting from the overall density of reactive sites. Within small pores, there may be a possibility for the adsorbing Cd2+ ions to interact with such ‘neighboring’ silanols. For mesoporous samples with the same

areal density of reactive sites, the enthalpy of displacement slightly decreases with decreasing pore diameter and attains its minimum (+1.8 kJ mol1) for SBA-15P103303K where the probability of Cd(II) interactions with silanol groups from the opposite walls is maximal. This means that the energetics of cation adsorption within the small pores has to be different (less positive enthalpy) from that observed within larger pores or on the flat silica surface, even though the siteNH3-to-Cd(II) ratio were the same.

5. CONCLUSION It is a common premise that a condensed phase reduced to nanometer dimensions should have physical and chemical properties different from that of its bulk counterpart and that interfacial phenomena in small pores follow mechanisms that may be somewhat different from those occurring on flat surfaces. It is often attributed to modifications of the solvent structure. In the present paper, the adsorption of Cd(II) ions from aqueous solutions and the accompanying phenomena have been studied in view of tracking the effect of pore confinement. Contrary to irregular pore size-dependent behavior of adsorption isotherms per unit area of adsorbent surface, the enthalpy of displacement per mole of Cd2+ adsorbed, Δdplh, undergoes significant quantitative and qualitative changes when passing from nonporous Spherosil XO75LS to mesoporous silicas of the SBA-15 type. There appears that the areal density of reactive sites (i.e., surface silanols), in combination with the surface curvature within pore space, is the major factor that induces significant changes in the molar enthalpy of displacement, the overall process being endothermic and entropy-driven in the whole concentration range studied. It is postulated that the adsorption of one Cd(II) ion is accompanied by (i) the release to the bulk solution of appropriate amount of Na+ co-ions (preadsorbed at the solidliquid interface) and (ii) the modifications of the hydration shells surrounding the adsorbing and desorbing ions. The positive Δdplh values observed in all systems studied indicate that adsorption is to a great extent dependent on the ion dehydration/rehydration effects. The idealized adsorption mode involves bonding of one Cd2+ by two neighboring surface oxygens, desorption of two sodium cations, and hydration changes concerning only a small part of the cation hydration shells. The main porosity effect is to modulate the effective density of surface silanol groups in the 3D space and thus the mean distance between two neighboring sites. The latter certainly decide the number of silanols to which is bound one adsorbing Cd2+ cation. When the adsorption mode is different from the ideal one (i.e., the site-to-Cd2+ ratio is smaller than 2), the endothermic character of the displacement process greatly decreases corresponding to a decreasing influence of the direct interaction Cd2+O. It should be also emphasized that in small pores, unlike the corresponding flat surface, two remote silanols located on the opposite pore walls may become sufficiently close to each other for the adsorbing species to interact with more than one surface site, the overall interaction effect being less endothermic again. ’ ASSOCIATED CONTENT

bS

Supporting Information. A short review of the literature on the endothermicity of competitive adsorption and effect of adsorbent porosity on the charging behavior of mineral oxides.

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The Journal of Physical Chemistry C This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +33 (0)4 67 14 33 05. Fax: +33 (0)4 67 14 33 04.

’ ACKNOWLEDGMENT The authors greatly acknowledge the financial support of this work by the ANR project CILSAMES (ANR-07-JCJC-0044) and the GNR PARIS 1115 (Physico-chimie des Actinides et des radioelements aux interfaces et en solution). This work has been carried out as a part of the ICSM (Institut de Chimie Separative de Marcoule) research topics, as a CNRS-CEA Valrho, University Montpellier II joint project. ’ REFERENCES (1) L€utzenkirchen, J., Surface Complexation Modelling; Academic Press: New York, 2006; Vol. 11, p 652. (2) Bradbury, M. H.; Baeyens, B. Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides. Geochim. Cosmochim. Acta 2005, 69 (4), 875–892. (3) Pagana, A. E.; Sklari, S. D.; Kikkinides, E. S.; Zaspalis, V. T. Microporous ceramic membrane technology for the removal of arsenic and chromium ions from contaminated water. Microporous Mesoporous Mater. 2008, 110 (1), 150–156. (4) Bonten, L. T. C.; Groenenberg, J. E.; Weng, L.; van Riemsdijk, W. H. Use of speciation and complexation models to estimate heavy metal sorption in soils. Geoderma 2008, 146 (12), 303–310. (5) Gustafsson, J. P.; Dassman, E.; Backstrom, M. Towards a consistent geochemical model for prediction of uranium(VI) removal from groundwater by ferrihydrite. Appl. Geochem. 2009, 24 (3), 454–462. (6) Hanna, K.; Lassabatere, L.; Bechet, B. Zinc and lead transfer in a contaminated roadside soil: Experimental study and modeling. J. Hazard. Mater. 2009, 161 (23), 1499–1505. (7) Hiemstra, T.; Van Riemsdijk, W. H.; Rossberg, A.; Ulrich, K. U. A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate. Geochim. Cosmochim. Acta 2009, 73 (15), 4437–4451. (8) Jordan, N.; Marmier, N.; Lomenech, C.; Giffaut, E.; Ehrhardt, J. J. Competition between selenium (IV) and silicic acid on the hematite surface. Chemosphere 2009, 75 (1), 129–134. (9) Villalobos, M.; Cheney, M. A.; Alcaraz-Cienfuegos, J. Goethite surface reactivity: II. A microscopic site-density model that describes its surface area-normalized variability. J. Colloid Interface Sci. 2009, 336 (2), 412–422. (10) Altin, O.; Ozbelge, H. O.; Dogu, T. Use of general purpose adsorption isotherms for heavy metal clay mineral interactions. J. Colloid Interface Sci. 1998, 198 (1), 130–140. (11) Mana, M.; Ouali, M. S.; Lindheimer, M.; de Menorval, L. C. Removal of lead from aqueous solutions with a treated spent bleaching earth. J. Hazard. Mater. 2008, 159 (23), 358–364. (12) Peric, J.; Trgo, M.; Vukojevic Medvidovic, N. Removal of zinc, copper and lead by natural zeolite--a comparison of adsorption isotherms. Water Res. 2004, 38 (7), 1893–1899. (13) Fokkink, L. G. J.; De Keizer, A.; Lyklema, J. Temperature dependence of Cadmium Adsorption on Oxides. J. Colloid Interface Sci. 1990, 135, 118–131. (14) Almazan-Torres, M. G.; Drot, R.; Mercier-Bion, F.; Catalette, H.; Den Auwer, C.; Simoni, E. Surface complexation modeling of uranium(VI) sorbed onto zirconium oxophosphate versus temperature:

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