Mercury(II) binding to thiol-functionalized mesoporous silicas: critical effect of pH and sorbent properties on capacity and selectivity

Analytica Chimica Acta 547 (2005) 3–13

Mercury(II) binding to thiol-functionalized mesoporous silicas: critical effect of pH and sorbent properties on capacity and selectivity Alain Walcarius∗ , Cyril Delacˆote Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, Unit´e Mixte de Recherche UMR 7564, CNRS—Universit´e H. Poincar´e Nancy I, 405 rue de Vandoeuvre, F-54600 Villers-les-Nancy, France Received 6 October 2004; received in revised form 18 November 2004; accepted 18 November 2004 Available online 25 December 2004

Abstract The binding properties of mesoporous thiol-functionalized silica sorbents towards mercury(II) species were studied as a function of pH in a wide range (0–8), in the absence or in the presence of competing metal ions, from batch equilibration experiments. To this end, a series of thiol-functionalized adsorbents characterized by different structures (from completely disordered amorphous solids to highly ordered mesostructures), variable density of organic ligands (from 1 to 4 mmol g−1 ), and various degrees of porosity, have been prepared either by post-synthesis grafting or by the co-condensation route. Hg(II) binding to these thiol-functionalized silica samples is strongly dependent on pH, especially in acidic medium (pH < 4) where non-hydrolyzed Hg2+ species become dominant. This behavior was found to be significantly affected by the degree of structural organization of the materials (amorphous or ordered mesoporous solids, short-range versus long-range structural order) and the adsorbent composition (density of functional groups). A beneficial effect of high structural order was observed in both the capacity (access to a high number of binding sites) and selectivity (towards other metal ions) for the ordered mesoporous sorbents in comparison to the amorphous gels, but this was only true for pH values down to 4, where Hg(II) species are mainly in the form of Hg(OH)2 . In more acidic medium, however, the sorption of the non-hydrolyzed Hg2+ species underwent dramatic loss of effectiveness, which resulted in both lower capacities and worse selectivity. These restrictions were more marked when increasing the density of functional groups in the materials and, to lesser extent, when decreasing their level of structural ordering. They were interpreted on the basis of electrostatic considerations as the binding of Hg2+ to thiol groups leads to the generation of positively charged complexes in the host material while that of Hg(OH)2 involves the formation of neutral moieties. Possible regeneration of sorbents and re-use were also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Mercury(II); Thiol-functionalized mesoporous silica; Solid/liquid extraction; Sorption capacity and selectivity

1. Introduction Functionalized porous adsorbents have been widely used as solid-phase extractants for removal of heavy metals from aqueous media [1–3]. Among them, thiol-functionalized silicas were found to be efficient for the uptake of mercury(II) species and many investigations have been devoted to the preparation and characterization of a wide range of polysiloxane-immobilized mercaptopropyl ligands, which were then applied to Hg(II) binding [4–26]. These ∗

Corresponding author. Tel.: +33 3 83 68 52 59; fax: +33 3 83 27 54 44. E-mail address: [email protected] (A. Walcarius).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.11.047

organic–inorganic hybrids belonging to class II (involving a covalent bond between the silica network and the organic component [27]) are indeed attractive because they combine in a single material the physical properties of the inorganic structure with the intrinsic chemical reactivity of the organic moieties. They can be typically obtained by grafting the surface of a silica sample via the reaction of silanol groups with a mercaptopropyltrialkoxysilane [28] or, alternatively, by a direct assembly pathway involving the hydrolysis and cocondensation of a mixture of tetraalkoxysilane and mercaptopropyltrialkoxysilane precursors [29]. Thiol-functionalized silicas applied to the removal of Hg(II) from aqueous solutions were first prepared as

4

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

amorphous porous adsorbents [4–10]. Upon the discovery of the ordered mesoporous silicas (obtained using surfactant micelles structures as templates [30]), however, many efforts were directed to produce mesostructured materials comprising thiol groups [11–26]. This was achieved either via the covalent grafting of mercaptopropylsilyl groups to the framework pore walls of preformed mesoporous silica molecular sieves [11–14,16–19,25], or in one step, by the cocondensation route in the presence of a structure-directing agent [15,20–24,26]. These hybrid materials characterized by an uniform open-framework mesoporosity and exceptionally high specific surface areas (700–1500 m2 g−1 ) have been reported to exhibit improved sorption properties towards Hg(II) species, superior by far to those achieved with silica gels functionalized with the same thiol ligand [12,13,20,25,26]. In particular, they were characterized by significantly enhanced accessibility to the binding sites. Loading capacities up to 100% (i.e, one Hg(II) bound to each SH group in the material) were obtained in ordered structures providing their pore size remained in the mesoporous range (>2 nm) while incomplete filling was always observed with the corresponding amorphous adsorbents [12,15,25]. Also, the regular structure of these ordered mesoporous materials resulted in fast adsorption rates [17,22–26], with mass transfer kinetics usually much higher than in their amorphous analogs [25]. Note that this advantage was even more marked in solids displaying wormhole framework structures with short-range order than in well-ordered materials made of a highly regular hexagonal packing of mesopore channels over an extended length scale [26]. An interesting point, which was only briefly mentioned in the literature [15,22], is that the ordered thiolfunctionalized mesoporous silicas are apparently characterized by a better selectivity for Hg(II) binding (with respect to other metal species) than the corresponding amorphous sorbents. A tentative explanation for such an unexpected behavior was proposed on a thermodynamic basis suggesting the lack of ability of metal cations other than Hg(II) to coordinate within the confined spaces of the regular pore channels [15], and the unfavorable entropy effect seemed to be better expressed for solids displaying higher level of ordering [22]. The effect of pH on this unexpected enhanced selectivity for mercury was, however, not considered (experiments were usually performed at pH 4 [15,22]) and the influence of the materials structure and composition (amorphous, short-range versus long-range order/disorder, density of functional groups, . . .) was not extensively discussed. We have thus examined the effect of pH on the adsorption of Hg(II) species by thiol-functionalized mesoporous silicas, in particular with respect to the accessibility to the active centers and to the selectivity of the binding process in the presence of other metal ions interferences. Special attention was given to point out the influence of the materials characteristics (structure, thiol group content) in control-

ling, or even limiting, their performance for Hg(II) removal (capacity, selectivity). Most measurements have been performed from batch experiments involving the suspension of adsorbent particles in aqueous media containing Hg(II) either in excess of or less than the amount of ligand in the materials. The possibility of adsorbent regeneration was also evaluated. 2. Experimental 2.1. Preparation of thiol-functionalized silica samples Two distinct pathways were used to prepare the adsorbents: grafting of bare silicas and one step synthesis of organic–inorganic hybrids by the sol–gel process. The bare silica samples were either the chromatographic grade silica gels Geduran SI 60 (G60) and Kieselgel 40 (K40), purchased from Merck, or a MCM-41 type ordered mesoporous silica sample prepared according to a previously published procedure [31]. These three solids were grafted with n-propyl-SH groups, typically by dispersing 250 mg of the material into 25 mL toluene (99%, Merck) to which 1 mL of mercaptopropyltrimethoxysilane (MPTMS, 95%, Lancaster) had been added. This mixture was allowed to refluxing for 24 h under constant stirring. After slow cooling, the solid phase was recovered by filtration, washed with fresh toluene, and dried overnight under vacuum. The amount of grafted groups was determined by elemental analysis (“Service Central d’Analyse” of CNRS, Lyon, France). One-pot synthesis of mesoporous mercaptopropylfunctionalized silica (MPS) samples was performed as previously described [26], by a procedure involving the hydrolysis and co-condensation of tetraethoxysilane (TEOS, >98%, Merck) and MPTMS in hydroalcoholic medium in the presence of a surfactant template (cetyltrimethylammonium bromide, CTAB, 98%, Fluka) and ammonia (28% aqueous, Prolabo) as a catalyst. Typically, CTAB (2.4 g) was dissolved in 50 mL deionized water and 45 mL ethanol (95–96%) to which 13 mL of 28% aqueous ammonia was added. The precursor mixture was prepared by dissolving appropriate molar ratios of MPTMS and TEOS (total amount of precursor: 16 mmol) in 5 mL ethanol. It was then added to the “surfactant + catalyst” solution and stirred for 2 h at room temperature. The product was then isolated by vacuum filtration on a B¨uchner funnel and washed alternatively with water and ethanol. The resulting powder was dried under vacuum (<10−2 bar) for 24 h. The final products was obtained after surfactant removal by acid/solvent extraction (1 g MPS in 100 mL ethanol + 1 M HCl for 18 h under reflux), filtration, washing with ethanol and drying. On the basis of the MPTMS ratio in the starting sol, the mercaptopropyl–silica materials have been named afterwards as MPS-10%, MPS-15%, MPS-20% and MPS40%. Their thiol group content was measured by elemental analysis.

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

2.2. Solutions and adsorption procedures All chemicals (HCl, HNO3 , NaOH and thiourea) and metal ion species (Hg(NO3 )2 , AgNO3 , Cu(NO3 )2 , Ni(NO3 )2 , Zn(NO3 )2 , Cd(NO3 )2 and Bi(NO3 )3 ) were analytical grade reagents and used without further purification. A certified Hg(II) standard stock solution (1.001 ± 0.002 g L−1 , Merck) was used for calibration purposes. Mercury and metal ion solutions were prepared daily in high purity water (18 M cm) obtained from a Millipore Milli-Q water purification system. HNO3 and NaOH were used for pH adjustment (this method was preferred over buffers to avoid any unwanted complexation side-reactions, as previously discussed [32]). Adsorption experiments were carried out in batch conditions, by suspending given amounts of adsorbent in aqueous solutions (typically 100 or 250 mL) containing initially Hg(II) (alone or in mixture with other metal ions) at selected concentrations and pH. After equilibration under constant stirring for 24 h, solid particles were filtered off and the remaining metal concentrations in the supernatant were determined quantitatively to allow calculation of the amounts of adsorbed species on the solid phases by difference with respect to the starting concentrations. Final pH of the supernatant was measured with the Metrohm 691 pHmeter (combined glass electrode No. 6.0222.100). A blank experiment (without adsorbent) was performed to check that no Hg(II) consumption occurred other than by adsorption on the solid particles (i.e. not on the vessel walls) and it was also checked in some representative cases that mass balance was maintained by quantitative analysis of Hg(II) in the solid phase. The adsorption isotherms were drawn by plotting the extent of adsorption as a function of pH. When necessary, distribution diagrams of mercury species were calculated using the PSEQUAD software [33], and superimposed to the experimental data. Desorption experiments were carried out by suspending 20 mg of Hg(II)-loaded adsorbent particles in 20 mL solution and measuring the desorbed concentrations after 24 h reaction under constant stirring. Several desorption solutions have been tested: 12 M HCl, 3 M HCl, 5% thiourea in 3 M HCl, and 5% thiourea in 0.1 M HCl. Multiple adsorption–desorption experiments have been also performed, for two different pH values (1 and 4) of the accumulation medium. 2.3. Apparatus The adsorbents were characterized by various physicochemical techniques. Specific surface areas and pore sizes were evaluated by the BET and BJH methods, respectively, from nitrogen adsorption–desorption measurements performed at 77 K with a Coulter instrument (model SA 3100), in the relative pressure range from about 10−5 to 0.99 (all samples were dried beforehand at 50 ◦ C for 12 h under vacuum). X-ray diffraction patterns were obtained using a classical powder diffractometer (X’PERT PRO, Philips) operating at room temperature, which was equipped

5

with a Cu anode (quartz monochromator, K␣1 radiation, λ = 0.154056 nm). Transmission electron microscopy (TEM) was carried out with a Philips CM20 microscope operating at 200 keV. Particle size distribution was measured with the aid of a light scattering analyzer (model LA920, Horiba), and calculated on the basis of the Mie scattering theory. The quantitative analysis of solution-phase Hg(II) was achieved by anodic stripping differential pulse voltammetry on gold electrode, according to a previously published procedure [34]. Measurements were performed with the aid of a ␮-Autolab potentiostat associated to the GPES electrochemical analysis system (Eco Chemie), in a conventional threeelectrode cell: a rotating gold working electrode, an Ag/AgCl reference electrode (Metrohm, No. 6.0733.100), and a Pt wire counter-electrode. Stripping voltammograms were recorded after 30 s electrolysis at +0.3 V, by scanning potentials in the differential pulse mode up to +1.0 V. This led to a linear response of the technique in the 0.1–1 ␮M concentration range. Appropriate dilution of samples was applied prior to analysis in order to fit inside this linear range.

3. Results and discussion 3.1. Materials characteristics and preliminary observations Seven thiol-functionalized materials have been prepared, displaying a rather wide range of characteristics and properties (Table 1). The first three samples were obtained by grafting either amorphous silica gels displaying different average pore diameters (K40 and G60) and an ordered mesoporous MCM-41 solid. The resulting grafted adsorbents were characterized by a thiol content ranging from 1 to 1.5 mmol g−1 , the ordered MCM41-SH solid displaying a much higher specific surface area (about 1000 m2 g−1 ) in comparison to the amorphous gels which, in turn, were more open (average ˚ respectively, for K40-SH and pore diameters of 36 and 56 A, ˚ G60-SH) than MCM41-SH (mesopore diameter of 23 A). The last four samples were obtained by, the co-condensation route in the presence of a surfactant template, by varying the level of functionalization (MPS-10–40%). Consistent with previous works [26], this gave rise to ordered mesoporous solids showing high specific surface areas, for which the amount of organic ligand can be easily controlled by adjusting the MPTMS/TEOS ratio in the synthesis medium. Typical XRD patterns, high-resolution TEM pictures, and nitrogen adsorption–desorption isotherms, are depicted in Fig. 1, respectively, for the MPS-15% and MPS-40% samples. They illustrate clearly that increasing the functionalization level resulted in (1) a decrease of the materials ordering (from regular hexagonal packing of cylindrical mesopores to less ordered mesostructures of wormhole type), and (2) a decrease of porosity (lower specific surface area and pore volume, as well as narrower pore diameter) leading to

6

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

Table 1 Physico-chemical characteristics of the thiol-functionalized silica samples Sample

K40-SH G60-SH MCM41-SH MPS-10% MPS-15% MPS-20% MPS-40% a b c

Nitrogen adsorption BET surface area (m2 g−1 )

Total pore volume (cm3 g−1 )

Pore ˚ (A)

355 314 1015 1598 1387 1073 523

0.32 0.44 0.60 0.76 0.65 0.50 0.27

36 56 23 21 20 <20 <20

diameterc

Powder XRD d ˚ spacing (A)

Amount of thiol ligandsa (mmol g−1 )

Average particle sizeb (␮m)

– – 35 33 31 29 27

1.50 1.45 0.84 0.95 1.55 2.26 4.03

139 89 5 6 5 6 12

Expressed per gram of functionalized material. Determined from cumulative particle size distribution analysis. Calculated according to the BJH method.

adsorbents that turned from completely mesoporous to the mesoporous–microporous borderline. These materials represent thus a series of thiolfunctionalized adsorbents characterized by different structures (from completely disordered amorphous solids to highly ordered mesostructures), variable density of organic ligands (from 1 to 4 mmol g−1 ), and various degrees of porosity. They will be used to point out the main characteristics affecting the mercury binding process (capacity and selectivity). A first series of experiments was directed to a rapid screening on the influence of pH and presence of possibly competing species on the binding of Hg(II) to the thiol-functionalized adsorbents. Two pH values were selected on the basis of the known mercury speciation, pH 1 where Hg2+ is dominating, and pH 4 at which Hg(II) is mainly hydrolyzed in the form of Hg(OH)2 [35]. The starting Hg(II) concentration in solution was selected to contain an amount of Hg(II) species slightly lower than that expected to be immobilized in the adsorbents on the basis of their thiol group content; this means

that in case of unrestricted access to the binding sites, the adsorption process would result in total removal Hg(II) species from the solution. Moreover, the sorbent-to-solution ratio was adjusted to 1 g L−1 to facilitate the conversion between the solution concentrations and the solid phase contents (i.e., the total removal from a 1 mM Hg(II) solution leads to a Hg(II) content in the solid of 1 mmol g−1 ). A rapid comparison between the starting Hg(II) concentration in solution and the amount of Hg(II) bound to the adsorbent after equilibration is thus an easy way to evidence a lack of accessibility to the binding sites. Typical results are gathered in Table 2. Focussing on the Hg(II) removal efficiency at pH 4 when this analyte was present alone in the solution, it appears that 100% removal was achieved when using the ordered mesoporous adsorbents while less than complete removal was observed in case of the amorphous solids K40-SH and G60-SH, with a restriction more pronounced for the small pore K40-SH sample (70% removal) than for the more open G60-SH sample (80%

Fig. 1. Powder X-ray diffraction patterns (A), transmission electron micrographs (B), and nitrogen adsorption (
)–desorption () isotherms (C), obtained for the main two cetyltrimethylammonium-assembled thiol-functionalized silica derivatives used in this work: MPS-15% (a) and MPS-40% (b).

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

7

Table 2 Extent of Hg(II) uptake by various thiol-functionalized silica samples (1 g L−1 ), from solutions containing Hg(II) species at starting concentrations adjusted to be slightly less than the theoretical capacity of the adsorbents (calculated on the basis of their SH group content), for Hg(II) alone and for Hg(II) in the presence of other metal ions at the same starting concentration Adsorbent

Starting metal ion concentration (mM)

Hg(II) bound to the material at equilibrium (mmol g−1 )a Hg(II) alone

K40-SH G60-SH MCM41-SH MPS-10% MPS-20% MPS-40% a

1.23 1.19 0.86 0.92 1.45 2.40

Hg(II) + Cu(II)

Hg(II) + Ni(II)

Hg(II) + Zn(II)

Hg(II) + Cd(II)

Hg(II) + Bi(III)

pH 1

pH 4

pH 1

pH 4

pH 1

pH 4

pH 1

pH 4

pH 1

pH 4

pH 1

pH 4

0.79 0.93 0.58 0.72 1.41 0.30

0.88 0.97 0.78 0.92 1.45 2.40

0.67 0.90 0.50 0.68 1.39 0.20

0.82 0.93 0.76 0.88 1.44 2.40

0.69 0.90 0.55 0.69 1.40 0.21

0.82 0.91 0.75 0.92 1.44 2.40

0.69 0.92 0.56 0.70 1.38 0.20

0.81 0.96 0.75 0.91 1.45 2.40

0.70 0.89 0.55 0.69 1.40 0.22

0.84 0.90 0.77 0.91 1.45 2.40

0.68 0.90 0.57 0.70 1.40 0.20

0.78 0.95 0.77 0.90 1.45 2.40

Expressed with respect to the adsorbent mass as dry powder; estimated error of measurements 5%.

removal). These results are consistent with those previously reported for similar porous adsorbents used in similar experimental conditions (i.e., pH 4), for which the resort to functionalized materials made of mesopore channels of regular dimension ensured an easy access to all the binding sites whereas significant restriction was evidenced when using adsorbents of lower porosity and/or less ordered structures [12,15,20,25]. When operating at lower pH (i.e., at a value of 1), however, the situation was dramatically different as the removal efficiency worsened significantly, especially for adsorbents displaying a high level of functionalization and (consequently) low porosity. For example, in conditions as those presented in Table 2, less than 15% removal of Hg(II) from a 2.4 mM solution at pH 1 was observed when using the MPS-40% adsorbent. This suggests an important role of pH, variable as a function of the materials properties, which will be discussed in Section 3.2. In addition, the presence of potentially interfering species in the medium was found to affect the Hg(II) binding process and such influence was dependent on both pH and the adsorbent characteristics. Similarly to the binding capacity, the interference effect seemed to be more important at low pH values and with materials displaying the highest level of functionalization (Table 2). This will be discussed hereafter. 3.2. Effect of pH and materials properties on capacity and selectivity Hg(II) binding to thiol-functionalized silicas was studied as a function of pH in various conditions (excess Hg(II) with respect to the theoretical capacity of the adsorbent, or not, presence of competing metal ions, or not). Selected results are illustrated in Figs. 2–4 for three typical adsorbents: the amorphous small pore grafted K40-SH material (Fig. 2), a well-ordered mesostructure of MCM-41 type obtained by the co-condensation route, MPS-15% (Fig. 3) containing nearly the same organic groups content as the K40-SH sample, and a less ordered mesoporous solid, MPS-40% (Fig. 4) characterized by a much higher functionalization level. The starting mercury concentration in solution was chosen as 0.2 mM,

a value high enough to provide an amount of Hg(II) large enough to get saturation of the adsorbents (if applicable) and low enough to maintain soluble all the Hg(II) species that are likely to exist in the investigated pH range (0.5–8), i.e., Hg2+ , HgOH+ , Hg(OH)2 [35]. Parts (A) of these Figs. 2–4 depict the variation of the amount of Hg(II) species bound to the adsorbent, as a function of pH, measured from suspensions containing Hg(II) species alone (without any potential interference) at two different solid-to-solution ratios (80 and 200 mg L−1 for K40-SH and MPS-15%, and 32 and 80 mg L−1 for MPS40%, as adapted to the respective functionalization levels of the adsorbents). The first case (smaller solid-to-solution ratios) corresponds to a situation where Hg(II) is in excess of the theoretical capacity of the adsorbent (i.e., more Hg(II) species in solution than SH groups in the solid), and will thus give information on the experimentally observed capacities of the materials as a function of pH (access to each binding sites or restricted accessibility). In the second case (higher solid-to-solution ratios), the adsorbent content in the medium does represent an amount of thiol groups higher than the Hg(II) species in solution (i.e., a situation resembling that applied for remediation purposes), and the corresponding results will be useful to evaluate the apparent distribution coefficients associated to the process at various pH. Parts (B) of Figs. 2–4 represent similar data as in parts (A) but, this time, sorption experiments have been performed from solutions containing Hg(II) species in mixture with other metal species (Cu(II), Ni(II), Zn(II), Cd(II), Bi(III)) in excess of Hg(II) (1 mM each), which are known to interact with thiol groups immobilized of porous adsorbents [5–7] and, therefore, are likely to affect the efficiency of the Hg(II) binding process (competition effects). The y-axis of all the adsorption plots depicted in parts (A) and (B) of Figs. 2–4 have been adjusted to have their upper limit equal to the theoretical capacity of the material (i.e., 1.50 mmol g−1 for K40-SH (Fig. 2), 1.55 mmol g−1 for MPS-15% (Fig. 3), and 4.03 mmol g−1 for MPS-40% (Fig. 4)), so that it is easy to check if 100% accessibility or less-than-complete filling was observed; a horizontal dashed line corresponding to the experimentally observed maximal capacity was added in each cases. In parts

8

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

(C) of Figs. 2–4 are represented the ratios of data obtained in the absence (parts (A)) and in the presence of competing species (parts (B)), expressed in percents, to illustrate the interference effects as a function of pH and Hg(II) speciation (distribution diagrams superimposed). Let us first consider the situation where Hg(II) species were in solutions free of any interference. As shown in parts (A) of Figs. 2–4, pH has a dramatic influence on the ca-

pacity of the materials: while nearly constant at a maximal value for pH values ranging between 4 and 7, the experimentally observed maximal capacities were found to fall down by decreasing pH, and this trend was somewhat affected by the type of adsorbent. The maximal capacities were 1.02 mmol g−1 for K40-SH (68% filling), 1.50 mmol g−1 for MPS-15% (97% filling), and 3.38 mmol g−1 for MPS-40% (84% filling); these values are consistent with the fact that restricted accessibility occurred in disordered amorphous sorbents (e.g., K40-SH) while the regular structure of templated mesoporous structures ensured much easier access to the binding sites providing that the pore aperture remained ˚ [12,15,20], which is ilin the mesoporous range (≥20 A) lustrated again via the nearly complete filling of the wellordered MPS-15% (97% accessibility) and the high filling level of the wormhole-like structured MPS-40% solid (0.68 g of Hg(II) per gram of adsorbent). When passing from pH 4 to 1, these experimentally observed maximal values were found to decrease by 50% for K40-SH, by 30% for MPS15%, and by 40% for MPS-40% (see curves (b) on parts (A) in Figs. 2–4). Once again, a (yet small) advantage of the ordered mesostructures was observed, as this deleterious effect was less with the well-ordered porous solids and much more pronounced with the amorphous gel adsorbents; and this is even much more evident when working with the adsorbent in excess of the amount of Hg(II) in the solution (see curves (a) on parts (A) in Figs. 2–4): nearly complete removal was achieved with MPS-15% independently on pH while the disordered K40-SH and the highly functionalized MPS-40% samples still suffered from the deleterious pH effect (capacity decrease in strongly acidic media). Low residual Hg(II) concentrations in solutions were achieved when performing sorption experiments in the pH range from 4 to 7 (i.e., below the detection limit of the electrochemical technique used in this work, 0.1 ␮M), independently on the fact that the mesoporous materials were prepared with or without surfactant templates, in agreement to what was discussed in a recent paper reporting similarly high distribution coefficients for both ordered and disordered thiol-functionalized sol–gels

Fig. 2. Adsorption isotherms (dotted lines) obtained as a function of pH for Hg(II) on mercaptopropyl-grafted silica gel (K40 sample), either in the absence (A) or in the presence of other metal ions (B). The experiments were performed in 100 mL solution containing initially 0.2 mM Hg(II) (alone (A) or in mixture with 1 mM Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III) (B)) to which either 20 mg ((
) curve (a)) or 8 mg (() curve (b)) K40-SH was added. The dashed line represents the experimentally observed maximum capacity of the adsorbent for Hg(II) species while the upper limit of the y-axis has been adjusted to the theoretical capacity (SH groups content). (C) Variation of the interference effect as a function of pH, expressed as the ratio between the amount of Hg(II) bound to the adsorbent in the presence of interference divided by the adsorbed Hg(II) quantity measured in the absence of five-fold excess of Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III). Data are provided for two solid-to-liquid ratios, corresponding to Hg(II) in solution less than ((䊉) curve (a), 200 mg L−1 K40-SH) or in excess of (() curve (b), 80 mg L−1 K40-SH) the adsorbent capacity. Distribution diagram depicting the main chemical forms of Hg(II), obtained by thermodynamic calculations using the PSEQUAD software [33], has been added.

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

[36]. However, in strongly acidic medium, the distribution coefficients evaluated from sorption experiments carried out at solid-to-solution ratios adjusted to have a small excess of thiol groups with respect to the amount of Hg(II) in solution fell down dramatically: for example, after equilibration of 20 mg MPS-40% in 250 mL solution containing 0.05 mmol of Hg(II), the evaluated distribution coefficients were higher than 5 × 104 mL g−1 at pH 5 and less than 10 mL g−1 at pH 1.

9

The effect of pH was even more marked when performing sorption experiments from solutions containing mercury in the presence of an excess of several potentially interfering metal ions (i.e., 0.2 mM Hg(II) + 1 mM Cu(II) + 1 mM Ni(II) + 1 mM Zn(II) + 1 mM Cd(II) + 1 mM Bi(III)). This is illustrated in parts (B) of Figs. 2–4. In fact, the maximum capacities of materials, observed in the pH range 4–7, were not affected by the presence of these additional species, as explained by the high strength of the thiol-mercury bond (higher than those reported for the interactions between other divalent metal ions and SH groups [37]). Also, the distribution coefficients remained very high (>5 × 104 mL g−1 ) for the ordered MPS adsorbents and moderately high (ca. 500 mL g−1 ) for grafted silica gels, highlighting again the advantage of the ordered mesoporous solids over their amorphous analogues [12,15,22]. In strongly acidic medium, however, the deleterious effect observed for Hg(II) alone was significantly enhanced in the presence of competing ions (comparison of parts (B) and (A) in Figs. 2–4). Of course, the binding strength of the mercury-thiol bond is expected to decrease by decreasing pH, but this is also true for the interaction between thiol groups and the other metal ions. The extent of the interference effect is better evidenced by plotting the relative binding efficiency between the situations with and without competing ions (ratio between the capacity observed for Hg(II) in the presence of added metal species to that attained alone, as seen in parts (C) of Figs. 2–4). A value of 100% would thus reveal the absence of any competition, as aforementioned for the pH range extending above 4. On the contrary, competition did exist in the pH range below 4, where HgOH+ and especially Hg2+ species are expected to dominate. Here, the level of structural order in the porous adsorbent does not seem to be the predominant parameter affecting the mercury binding efficiency as only a small improvement was observed when passing from the amorphous K40-SH material to the well-ordered MPS-15% solid (comparison of parts (C) in Figs. 2 and 3), while a dramatic decrease in capacity (strong interference effect) was pointed out when using the

Fig. 3. Adsorption isotherms (dotted lines) obtained as a function of pH for Hg(II) on mercaptopropyl-functionalized mesoporous silica (MPS-15% sample), either in the absence (A) or in the presence of other metal ions (B). The experiments were performed in 100 mL solution containing initially 0.2 mM Hg(II) (alone (A) or in mixture with 1 mM Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III) (B)) to which either 20 mg ((
) curve (a)) or 8 mg (() curve (b)) MPS-15% was added. The dashed line represents the experimentally observed maximum capacity of the adsorbent for Hg(II) species while the upper limit of the y-axis has been adjusted to the theoretical capacity (SH groups content). (C) Variation of the interference effect as a function of pH, expressed as the ratio between the amount of Hg(II) bound to the adsorbent in the presence of interference divided by the adsorbed Hg(II) quantity measured in the absence of five-fold excess of Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III). Data are provided for two solid-to-liquid ratios, corresponding to Hg(II) in solution less than ((䊉) curve (a), 200 mg L−1 MPS-15%) or in excess of (() curve (b), 80 mg L−1 MPS-15%) the adsorbent capacity. Distribution diagram depicting the main chemical forms of Hg(II), obtained by thermodynamic calculations using the PSEQUAD software [33], has been added.

10

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

MPS-40% adsorbent (part (C) in Fig. 4). This competition trend was further confirmed by working in Hg(II) solutions containing an interfering cation (Cu2+ ) at a concentration 100 times over that of Hg(II), for which binding capacities were found to decrease when passing from pH 4 to 1, by about 10% for K40-SH and MPS-15% materials and by 40% when using the MPS-40% sorbent. One can therefore con-

clude that the interference effect observed in strongly acidic medium is much more pronounced when using adsorbents containing a high density of binding sites; and this restriction cannot be circumvented simply by inducing structural order in the mesoporous solid, as the ordered highly functionalized MPS-40% sample behaved worse than the amorphous K40-SH sorbent containing much less thiol groups. A remaining question relies on how to explain the decrease of the binding capacity at pH values lower than 4. As this particular behavior was observed with all the adsorbent used in this work, even with the highly ordered mesoporous solids for which complete accessibility to the active centers had been demonstrated several times [12,13,20,25,26], such hindered access in strongly acidic medium cannot be simply due to physical diffusion restrictions. A possible explanation could be found in the different mechanisms involved in the binding process when changing Hg(II) speciation. Depending on pH, the mercury forms that are likely to interact with thiol groups are Hg2+ , HgOH+ , and Hg(OH)2 [35]. On this basis, the corresponding complexation reactions are described by the following equations: Si C3 H6 SH + Hg2+ + 2NO3 − →

Si C3 H6 S Hg+ , NO3 − + HNO3

(1)

Si C3 H6 SH + HgOH+ + NO3 − → Si C3 H6 S HgOH + HNO3

(2)

Si C3 H6 SH + Hg(OH)2 → Si C3 H6 S HgOH + H2 O

(3)

When Hg(II) is in the form of the dication Hg2+ , its complexation to thiol groups led to the formation of a positive charge, which requires an anion to be compensated. In the other cases, the reaction of the hydroxylated mercury forms (HgOH+ and Hg(OH)2 ) resulted in the formation of a neutral complexed form ( S HgOH); this monodentate sulfur complex Fig. 4. Adsorption isotherms (dotted lines) obtained as a function of pH for Hg(II) on mercaptopropyl-functionalized mesoporous silica (MPS-40% sample), either in the absence (A) or in the presence of other metal ions (B). The experiments were performed in 250 mL solution containing initially 0.2 mM Hg(II) (alone (A) or in mixture with 1 mM Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III) (B)) to which either 20 mg ((
) curve (a)) or 8 mg (() curve (b)) MPS-40% was added. The dashed line represents the experimentally observed maximum capacity of the adsorbent for Hg(II) species while the upper limit of the y-axis has been adjusted to the theoretical capacity (SH groups content). (C) Variation of the interference effect as a function of pH, expressed as the ratio between the amount of Hg(II) bound to the adsorbent in the presence of interference divided by the adsorbed Hg(II) quantity measured in the absence of five-fold excess of Cu(II), Ni(II), Zn(II), Cd(II) and Bi(III). Data are provided for two solid-to-liquid ratios, corresponding to Hg(II) in solution less than ((䊉) curve (a), 80 mg L−1 MPS-40%) or in excess of (() curve (b), 32 mg L−1 MPS-40%) the adsorbent capacity. Distribution diagram depicting the main chemical forms of Hg(II), obtained by thermodynamic calculations using the PSEQUAD software [33], has been added.

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

11

having recently been reported to form on mercaptopropylfunctionalized mesostructured silicates, as pointed out by X-ray absorption spectroscopy [38]. Therefore, it appears that the formation of positively charged complexes inside the mesoporous adsorbents could prevent its complete filling, the first S Hg+ complexes located on the internal walls of the porous materials acting somewhat as an electrostatic barrier limiting the further ingress of large quantities of positively charged species as Hg2+ . Of course this limitation is even more important in the presence of interfering cations (M2+ that might compete with Hg2+ at the binding sites according to Eq. (4)), and this effect is thought to be more pronounced for sorbents displaying the higher contents of functional groups, in agreement with results of Figs. 2–4 (parts (C)): Si C3 H6 SH + M2+ + 2NO3 − → Si C3 H6 S M+ , NO3 − + HNO3

(4)

At the opposite, according to this hypothesis, no electrostatic restrictions are expected to occur at pH above 4, which is again consistent with the results of sorption experiments (Figs. 2–4). It should be reminded here that such restricted access to reactive centers in mesoporous organic–inorganic hybrids, based on electrostatic considerations, have been previously reported for the protonation of amine-functionalized mesoporous silicas [25]. It is noteworthy that another explanation could have been suggested to interpret the decreased capacities in acidic medium, especially in the presence of competing cations, based on the respective space occupied by the complexes, which is bigger for “ S Hg+ , NO3 − ” in comparison to the more confined “ S HgOH” moieties. However, the hypothesis of such steric hindrance is not valid in the case of, e.g., the MPS-15% sample for which nitrogen adsorption–desorption measurements performed before and after Hg(II) binding at pHs 1 and 4 have revealed that free space was still available after mercury uptake (pore volumes of 0.28 and 0.23 mL g−1 , respectively, at pHs 1 and 4, after reaching maximum filling values of 0.8 and 1.5 mmol g−1 , respectively). Indeed, the “Hg+ , NO3 − ” moieties do occupy about twice as much space as the “HgOH” groups, but steric effects are not dominant over electrostatic restrictions to explain the results of sorption experiments obtained as a function of pH. Further evidence to support the hypothesis of capacity restrictions on the basis of electrostatic considerations is provided in Fig. 5, showing the influence of pH on Ag+ binding to the MPS-40% sorbent. In this case, the complexation reaction occurs according to Eq. (5) indicating the formation of a neutral complex S Ag independently on pH (up to that corresponding to AgOH precipitation, of course): Si C3 H6 SH + Ag+ + NO3 − → Si C3 H6 S Ag + HNO3

(5)

As shown, even with this highly functionalized MPS-40% sorbent, no restriction was observed in acidic medium, a

Fig. 5. Adsorption isotherms obtained as a function of pH for AgI on mercaptopropyl-functionalized mesoporous silica (MPS-40% sample), from experiments performed in 250 mL solution containing initially 0.2 mM AgI to which either 20 mg ((
) curve (a)) or 8 mg (() curve (b)) MPS-40% was added.

maximum binding capacity of 93 ± 2% of the theoretical one (less-than-complete filling was explained in this case by steric constraints) being obtained over the whole pH range between 0.5 and 7. This confirms thus the key role played by charge effects to restrict the binding capacity and enhance the influence of interferences when the sorption process involves the formation of charged complexes inside the mesoporous host. 3.3. Sorbent regeneration and re-use Consistent with previous investigations that have considered Hg(II) desorption from thiol-functionalized mesoporous silicas [11,14,20], a harsh treatment in 12 M HCl was found effective to recover more than 90% of mercury from all the adsorbents considered here. When applying such strong acid-leaching method to the ordered mesoporous adsorbents, however, a partial destruction towards the mesostructures was reported [20]. Moreover, the mercury uptake capacity dropped to ca. 40–60% of their original values [11,14,20]. In an attempt to highlight any eventual effect(s) of the structure and/or the density of organo-functional groups on this deleterious behavior, we have performed regeneration and reuse experiments on all the thiol-functionalized mesoporous silicas used in this work, by extending them up to three successive uptake events carried out at pH 1–4. The results are depicted in Fig. 6. Several features can be noticed from these data. First, all the adsorbents underwent significant decrease in their sorption capacity upon the successive uptake-leaching steps, by 25–50% during the first cycle up to 50–80% in the second one. Secondly, the decrease was a little bit more marked for the ordered mesoporous solids compared to the amorphous gels, suggesting again the partial crashing of the mesostructure in strongly acidic medium. Thirdly, the restricted capacity observed at pH 1 relative to that for pH 4

12

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

Fig. 6. Variation of the mercury sorption capacity measured at pH 1 (
) and pH 4 (䊉) for three successive experiments involving desorption in 12 M HCl between them, using various adsorbents: K40-SH (A), G60-SH (B), MCM41-SH (C), MPS-10% (D), MPS-20% (E), MPS-40% (F).

was maintained lower, especially for the ordered mesoporous sorbents, indicating that the charge-induced restrictions were still observed after successive regeneration and re-use experiments. Finally, one have to mention that in spite of this dramatic capacity loss, the sorbents were still likely to reduce the mercury concentration in solution to insignificant levels (i.e., below the detection limit of the technique used in this work), but this required the addition of much larger amounts of solid particles in the medium to compensate the capacity loss. Attempts were also made to soften the experimental conditions applied to the desorption step, and the results obtained for three typical media are gathered in Table 3. On the one hand, reducing the HCl concentration from 12 to 3 M resulted in unsatisfactory desorption yields (between about 10–35% as a function of the materials type). On the other hand, the addition of a complexing ligand in the medium (i.e., thiourea) was found to increase significantly the desorption yields of the 3 M HCl medium, in agreement with the fact that acidified thiourea solutions can be used to elute metal ions from mercapto-functionalized organoclay columns [39]. Even more interesting were the results obtained when using a

thiourea solution containing HCl at lower concentration (i.e., 0.1 M) for which the same order of desorption effectiveness (or even better) was achieved (Table 3). In this case, desorption yields of 83–93% were observed for the ordered mesoporous adsorbents. Such “soft” conditions were also found well appropriate for successive uptake-leaching processes because they did not result in significant loss of the sorbent capacity, as pointed out in three successive regeneration and re-use experiments. Table 3 Recovery of Hg(II) species bound to the thiol-functionalized silica samples, after desorption in three different media Sample

K40-SH G60-SH MCM41-SH MPS-10% MPS-15% MPS-20% MPS-40%

Extent of desorption in three media (%) 3 M HCl

3 M HCl + 5% thiourea

0.1 M HCl + 5% thiourea

25 19 36 31 36 31 9

65 73 81 87 90 86 84

76 91 86 93 89 83 86

A. Walcarius, C. Delacˆote / Analytica Chimica Acta 547 (2005) 3–13

4. Conclusions The binding properties of thiol-functionalized mesoporous silicas towards Hg(II) species are strongly affected by pH as well as by the physico-chemical properties of the adsorbents (structural order, density of functional groups). The binding capacity and selectivity at pH above 4 are better with using ordered mesoporous structures in comparison to the amorphous gels functionalized with the same mercaptopropyl groups, in agreement with previous studies [12,15,20]. In stronger acidic medium (pH below 4), however, both these parameters were found to worsen because of the formation of positively charged complexes in the mesoporous materials concomitant to the uptake process. This led to electrostatic screening effects, which were more pronounced and restrictive with respect to the binding capacity and selectivity when using adsorbents characterized by a higher density of organo-functional groups. Finally, as far as practical use of these adsorbents for remediation purposes (i.e., mercury removal from polluted media), and after testing several desorption solutions, it appears that the use of a 5% thiourea medium containing 0.1 M HCl is the best compromise between sufficiently high mercury desorption yields and maintenance of high capacity of the sorbent materials. References [1] L.L. Tavlarides, J.S. Lee, Ion Exch. Solv. Extract. 14 (2001) 169. [2] J. Liu, G.E. Fryxell, S. Mattigod, T.S. Zemanian, Y. Shin, L.-Q. Wang, Stud. Surf. Sci. Catal. 129 (2000) 729. [3] L. Mercier, Stud. Surf. Sci. Catal. 129 (2000) 739. [4] U. Koklu, Chim. Acta Turc. 12 (1984) 265. [5] A.R. Cestari, C. Airoldi, J. Braz. Chem. Soc. 6 (1995) 291. [6] E.F.S. Vieira, J. De, A. Simoni, C. Airoldi, J. Mater. Chem. 7 (1997) 2249. [7] A.R. Cestari, C. Airoldi, J. Coll. Interf. Sci. 195 (1997) 338. [8] J.S. Lee, S. Gomez-Salazar, L.L. Tavlarides, React. Funct. Polym. 49 (2001) 159. [9] E.A. dos Santos, R.L. Pagano, J. De, A. Simoni, C. Airoldi, A.R. Cestari, E.F.S. Vieira, Coll. Surf. A 201 (2002) 275. [10] A. Walcarius, M. Etienne, J. Bessi`ere, Chem. Mater. 14 (2002) 2757.

13

[11] X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, J. Liu, K.M. Kemmer, Science 276 (1997) 923. [12] L. Mercier, T.J. Pinnavaia, Adv. Mater. 9 (1997) 500. [13] L. Mercier, T.J. Pinnavaia, Environ. Sci. Technol. 32 (1998) 2749. [14] J. Liu, X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, M. Gong, Adv. Mater. 10 (1998) 161. [15] J. Brown, L. Mercier, T.J. Pinnavaia, Chem. Commun. (1999) 69. [16] X. Chen, X. Feng, J. Liu, G.E. Fryxell, M. Gong, Sep. Sci. Technol. 34 (1999) 1121. [17] S. Mattigod, X. Feng, G.E. Fryxell, J. Liu, M. Gong, Sep. Sci. Technol. 34 (1999) 2329. [18] L. Mercier, T.J. Pinnavaia, NATO Sci. Ser., Ser. E 362 (1999) 33. [19] A.M. Liu, K. Hidajat, S. Kawi, D.Y. Zhao, Chem. Commun. (2000) 1145. [20] J. Brown, R. Richer, L. Mercier, Micropor. Mesopor. Mater. 37 (2000) 41. [21] R.I. Nooney, M. Kalyanaraman, G. Kennedy, E.J. Maginn, Langmuir 17 (2001) 528. [22] B. Lee, Y. Kim, H. Lee, J. Yi, Micropor. Mesopor. Mater. 50 (2001) 77. [23] A. Bibby, L. Mercier, Chem. Mater. 14 (2002) 1591. [24] M. Etienne, S. Sayen, B. Lebeau, A. Walcarius, Stud. Surf. Sci. Catal. 141 (2002) 615. [25] A. Walcarius, M. Etienne, B. Lebeau, Chem. Mater. 15 (2003) 2161. [26] A. Walcarius, C. Delacˆote, Chem. Mater. 15 (2003) 4181. [27] C. Sanchez, F. Ribot, New J. Chem. 18 (1994) 1007. [28] K. Moller, T. Bein, Chem. Mater. 10 (1998) 2950. [29] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403. [30] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [31] M. Etienne, B. Lebeau, A. Walcarius, New J. Chem. 26 (2002) 384. [32] A. Walcarius, M. Etienne, C. Delacˆote, Anal. Chim. Acta 508 (2004) 87. [33] L. Z´ekany, I. Nagyral, D.J. Leggett (Eds.), Computational Methods for the Determination of Formation Constants, Plenum Press, London, 1985 (Chapter 8). [34] Y. Bonfil, M. Brand, E. Kirowa-Eisner, Anal. Chim. Acta 424 (2000) 65. [35] C.F. Baes Jr., R.E. Mesmer, The Hydrolysis of Cations, Wiley, New York, 1976, 301–312. [36] H.-J. Im, C.E. Barnes, S. Dai, Z. Xue, Micropor. Mesopor. Mater. 70 (2004) 57. [37] E.F.S. Vieira, A.R. Cestari, J. De, A. Simoni, C. Airoldi, Thermochim. Acta 328 (1999) 247. [38] C.-C. Chen, E.J. Mckimmy, T.J. Pinnavaia, K.F. Hayes, Environ. Sci. Technol. 38 (2004) 4758. [39] N.L. Dias Filho, Y. Gushikem, W.L. Polito, Anal. Chim. Acta 306 (1995) 167.