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 Delac?o te
Laboratoire de Chimie Physique et Microbiologie pour l’Environnement,Unit′e Mixte de Recherche UMR 7564,
CNRS—Universit′e H.Poincar′e Nancy I,405rue de Vandoeuvre,F-54600Villers-les-Nancy,France
Received 6October 2004;received in revised form 18November 2004;accepted 18November 2004
Available online 25December 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 1to 4mmol 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 Hg 2+species become dominant.This behavior was found to be signi?cantly 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 bene?cial 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 Hg 2+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 Hg 2+to thiol groups leads to the generation of positively charged complexes in the host material while that of Hg(OH)2involves the formation of neutral moieties.Possible regeneration of sorbents and re-use were also discussed.?2004Elsevier 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 sil-icas were found to be ef?cient for the uptake of mer-cury(II)species and many investigations have been de-voted 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.:+33383685259;fax:+33383275444.E-mail address:walcariu@https://www.doczj.com/doc/d54435435.html,rs-nancy.fr (A.Walcarius).
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 sur-face 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 co-condensation of a mixture of tetraalkoxysilane and mercap-topropyltrialkoxysilane precursors [29].
Thiol-functionalized silicas applied to the removal of Hg(II)from aqueous solutions were ?rst prepared as
4 A.Walcarius,C.Delac?o te/Analytica Chimica Acta547(2005)3–13
amorphous porous adsorbents[4–10].Upon the discovery of the ordered mesoporous silicas(obtained using surfac-tant 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 co-condensation 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 speci?c surface areas(700–1500m2g?1)have been re-ported 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 signi?cantly enhanced accessibility to the binding sites.Loading capacities up to 100%(i.e,one Hg(II)bound to each SH group in the mate-rial)were obtained in ordered structures providing their pore size remained in the mesoporous range(>2nm)while in-complete?lling was always observed with the corresponding amorphous adsorbents[12,15,25].Also,the regular structure of these ordered mesoporous materials resulted in fast ad-sorption 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 brie?y men-tioned in the literature[15,22],is that the ordered thiol-functionalized mesoporous silicas are apparently charac-terized 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 thermody-namic basis suggesting the lack of ability of metal cations other than Hg(II)to coordinate within the con-?ned spaces of the regular pore channels[15],and the unfavorable entropy effect seemed to be better ex-pressed for solids displaying higher level of ordering [22].The effect of pH on this unexpected enhanced se-lectivity for mercury was,however,not considered(ex-periments were usually performed at pH4[15,22])and the in?uence 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 adsorp-tion 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 at-tention was given to point out the in?uence of the materi-ling,or even limiting,their performance for Hg(II)removal (capacity,selectivity).Most measurements have been per-formed from batch experiments involving the suspension of adsorbent particles in aqueous media containing Hg(II)ei-ther 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 adsor-bents: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 SI60(G60)and Kieselgel40(K40), purchased from Merck,or a MCM-41type ordered meso-porous silica sample prepared according to a previously pub-lished procedure[31].These three solids were grafted with n-propyl-SH groups,typically by dispersing250mg of the material into25mL toluene(99%,Merck)to which1mL of mercaptopropyltrimethoxysilane(MPTMS,95%,Lancaster) had been added.This mixture was allowed to re?uxing for 24h under constant stirring.After slow cooling,the solid phase was recovered by?ltration,washed with fresh toluene, and dried overnight under vacuum.The amount of grafted groups was determined by elemental analysis(“Service Cen-tral d’Analyse”of CNRS,Lyon,France).
One-pot synthesis of mesoporous mercaptopropyl-functionalized silica(MPS)samples was performed as pre-viously described[26],by a procedure involving the hy-drolysis and co-condensation of tetraethoxysilane(TEOS, >98%,Merck)and MPTMS in hydroalcoholic medium in the presence of a surfactant template(cetyltrimethylammo-nium bromide,CTAB,98%,Fluka)and ammonia(28% aqueous,Prolabo)as a catalyst.Typically,CTAB(2.4g) was dissolved in50mL deionized water and45mL ethanol (95–96%)to which13mL of28%aqueous ammonia was added.The precursor mixture was prepared by dissolving ap-propriate molar ratios of MPTMS and TEOS(total amount of precursor:16mmol)in5mL ethanol.It was then added to the“surfactant+catalyst”solution and stirred for2h at room temperature.The product was then isolated by vac-uum?ltration on a B¨u chner funnel and washed alterna-tively with water and ethanol.The resulting powder was dried under vacuum(<10?2bar)for24h.The?nal prod-ucts was obtained after surfactant removal by acid/solvent extraction(1g MPS in100mL ethanol+1M HCl for18h under re?ux),?ltration,washing with ethanol and dry-ing.On the basis of the MPTMS ratio in the starting sol, the mercaptopropyl–silica materials have been named af-terwards as MPS-10%,MPS-15%,MPS-20%and MPS-40%.Their thiol group content was measured by elemental
A.Walcarius,C.Delac?o te/Analytica Chimica Acta547(2005)3–135
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)2and Bi(NO3)3)were analytical grade reagents and used without further puri?cation.A certi?ed Hg(II)standard stock solution(1.001±0.002g L?1,Merck) was used for calibration purposes.Mercury and metal ion so-lutions were prepared daily in high purity water(18M cm) obtained from a Millipore Milli-Q water puri?cation system. HNO3and NaOH were used for pH adjustment(this method was preferred over buffers to avoid any unwanted complexa-tion side-reactions,as previously discussed[32]).
Adsorption experiments were carried out in batch condi-tions,by suspending given amounts of adsorbent in aque-ous solutions(typically100or250mL)containing initially Hg(II)(alone or in mixture with other metal ions)at selected concentrations and pH.After equilibration under constant stirring for24h,solid particles were?ltered off and the re-maining metal concentrations in the supernatant were de-termined 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 super-natant was measured with the Metrohm691pHmeter(com-bined 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 main-tained 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,distribu-tion diagrams of mercury species were calculated using the PSEQUAD software[33],and superimposed to the experi-mental data.
Desorption experiments were carried out by suspending 20mg of Hg(II)-loaded adsorbent particles in20mL solution and measuring the desorbed concentrations after24h reac-tion under constant stirring.Several desorption solutions have been tested:12M HCl,3M HCl,5%thiourea in3M HCl,and 5%thiourea in0.1M HCl.Multiple adsorption–desorption experiments have been also performed,for two different pH values(1and4)of the accumulation medium.
2.3.Apparatus
The adsorbents were characterized by various physico-chemical techniques.Speci?c surface areas and pore sizes were evaluated by the BET and BJH methods,respec-tively,from nitrogen adsorption–desorption measurements performed at77K with a Coulter instrument(model SA 3100),in the relative pressure range from about10?5to 0.99(all samples were dried beforehand at50?C for12h under vacuum).X-ray diffraction patterns were obtained using a classical powder diffractometer(X’PERT PRO,with a Cu anode(quartz monochromator,K?1radiation,λ=0.154056nm).Transmission electron microscopy(TEM) was carried out with a Philips CM20microscope oper-ating at200keV.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 pro-cedure[34].Measurements were performed with the aid of a ?-Autolab potentiostat associated to the GPES electrochem-ical analysis system(Eco Chemie),in a conventional three-electrode 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 after30s electrolysis at+0.3V,by scanning potentials in the differential pulse mode up to+1.0V.This led to a linear re-sponse of the technique in the0.1–1?M concentration range. Appropriate dilution of samples was applied prior to analysis in order to?t 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 proper-ties(Table1).The?rst three samples were obtained by graft-ing either amorphous silica gels displaying different average pore diameters(K40and G60)and an ordered mesoporous MCM-41solid.The resulting grafted adsorbents were char-acterized by a thiol content ranging from1to1.5mmol g?1, the ordered MCM41-SH solid displaying a much higher spe-ci?c surface area(about1000m2g?1)in comparison to the amorphous gels which,in turn,were more open(average pore diameters of36and56?A,respectively,for K40-SH and G60-SH)than MCM41-SH(mesopore diameter of23?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 speci?c surface areas,for which the amount of organic ligand can be easily controlled by ad-justing 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%sam-ples.They illustrate clearly that increasing the functional-ization level resulted in(1)a decrease of the materials or-dering(from regular hexagonal packing of cylindrical meso-pores to less ordered mesostructures of wormhole type),and (2)a decrease of porosity(lower speci?c surface area and
6 A.Walcarius,C.Delac?o te /Analytica Chimica Acta 547(2005)3–13
Table 1
Physico-chemical characteristics of the thiol-functionalized silica samples Sample
Nitrogen adsorption Powder XRD d
spacing (?A)Amount of thiol
ligands a (mmol g ?1)Average particle size b (?m)
BET surface area (m 2g ?1)
Total pore volume (cm 3g ?1)Pore diameter c (?A)K40-SH 3550.3236–1.50139G60-SH 3140.4456–1.4589MCM41-SH 10150.6023350.845MPS-10%15980.7621330.956MPS-15%13870.6520311.555MPS-20%10730.50<20292.266MPS-40%
5230.27
<20
274.0312
a Expressed per gram of functionalized material.
b Determined from cumulative particle size distribution analysis.c
Calculated according to the BJH method.
adsorbents that turned from completely mesoporous to the mesoporous–microporous borderline.
These materials represent thus a series of thiol-functionalized adsorbents characterized by different struc-tures (from completely disordered amorphous solids to highly ordered mesostructures),variable density of organic ligands (from 1to 4mmol 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 ?rst series of experiments was directed to a rapid screen-ing on the in?uence 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 1where Hg 2+is dominating,and pH 4at which Hg(II)is mainly hydrolyzed in the form of Hg(OH)2[35].The starting Hg(II)concentration in so-lution was selected to contain an amount of Hg(II)species slightly lower than that expected to be immobilized in the ad-sorbents on the basis of their thiol group content;this
means that in case of unrestricted access to the binding sites,the ad-sorption process would result in total removal Hg(II)species from the solution.Moreover,the sorbent-to-solution ratio was adjusted to 1g L ?1to facilitate the conversion between the solution concentrations and the solid phase contents (i.e.,the total removal from a 1mM Hg(II)solution leads to a Hg(II)content in the solid of 1mmol g ?1).A rapid comparison be-tween 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 ef?ciency at pH 4when 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 restric-tion more pronounced for the small pore K40-SH sample (70%removal)than for the more open G60-SH sample (80%
A.Walcarius,C.Delac?o te/Analytica Chimica Acta547(2005)3–137 Table2
Extent of Hg(II)uptake by various thiol-functionalized silica samples(1g 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 Hg(II)+Cu(II)Hg(II)+Ni(II)Hg(II)+Zn(II)Hg(II)+Cd(II)Hg(II)+Bi(III) pH1pH4pH1pH4pH1pH4pH1pH4pH1pH4pH1pH4
K40-SH 1.230.790.880.670.820.690.820.690.810.700.840.680.78 G60-SH 1.190.930.970.900.930.900.910.920.960.890.900.900.95 MCM41-SH0.860.580.780.500.760.550.750.560.750.550.770.570.77 MPS-10%0.920.720.920.680.880.690.920.700.910.690.910.700.90 MPS-20% 1.45 1.41 1.45 1.39 1.44 1.40 1.44 1.38 1.45 1.40 1.45 1.40 1.45 MPS-40% 2.400.30 2.400.20 2.400.21 2.400.20 2.400.22 2.400.20 2.40 a Expressed with respect to the adsorbent mass as dry powder;estimated error of measurements5%.
removal).These results are consistent with those previously reported for similar porous adsorbents used in similar exper-imental conditions(i.e.,pH4),for which the resort to func-tionalized materials made of mesopore channels of regular dimension ensured an easy access to all the binding sites whereas signi?cant 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 of1),however,the situation was dramatically different as the removal ef?ciency worsened signi?cantly,especially for adsorbents displaying a high level of functionalization and (consequently)low porosity.For example,in conditions as those presented in Table2,less than15%removal of Hg(II) from a2.4mM solution at pH1was 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 Section3.2.
In addition,the presence of potentially interfering species in the medium was found to affect the Hg(II)binding pro-cess and such in?uence 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(Table2).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.2–4for three typical adsorbents:the amorphous small pore grafted K40-SH material(Fig.2),a well-ordered mesostructure of MCM-41type 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)charac-terized by a much higher functionalization level.The starting 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.2–4depict the variation of the amount of Hg(II)species bound to the adsor-bent,as a function of pH,measured from suspensions contain-ing Hg(II)species alone(without any potential interference) at two different solid-to-solution ratios(80and200mg L?1 for K40-SH and MPS-15%,and32and80mg L?1for MPS-40%,as adapted to the respective functionalization levels of the adsorbents).The?rst case(smaller solid-to-solution ra-tios)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 capac-ities 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 re-sults will be useful to evaluate the apparent distribution co-ef?cients associated to the process at various pH.Parts(B) of Figs.2–4represent similar data as in parts(A)but,this time,sorption experiments have been performed from solu-tions 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)(1mM each),which are known to interact with thiol groups immobilized of porous adsorbents[5–7]and,there-fore,are likely to affect the ef?ciency of the Hg(II)binding process(competition effects).The y-axis of all the adsorption plots depicted in parts(A)and(B)of Figs.2–4have been ad-justed to have their upper limit equal to the theoretical capac-ity of the material(i.e.,1.50mmol g?1for K40-SH(Fig.2), 1.55mmol g?1for MPS-15%(Fig.3),and4.03mmol g?1 for MPS-40%(Fig.4)),so that it is easy to check if100% accessibility or less-than-complete?lling was observed;a horizontal dashed line corresponding to the experimentally
8 A.Walcarius,C.Delac?o te /Analytica Chimica Acta 547(2005)3–13
(C)of Figs.2–4are 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 in-terference effects as a function of pH and Hg(II)speciation (distribution diagrams superimposed).
Let us ?rst 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 in?uence on the
ca-
pacity of the materials:while nearly constant at a maximal value for pH values ranging between 4and 7,the exper-imentally observed maximal capacities were found to fall down by decreasing pH,and this trend was somewhat af-fected by the type of adsorbent.The maximal capacities were 1.02mmol g ?1for K40-SH (68%?lling),1.50mmol g ?1for MPS-15%(97%?lling),and 3.38mmol g ?1for MPS-40%(84%?lling);these values are consistent with the fact that restricted accessibility occurred in disordered amorphous sorbents (e.g.,K40-SH)while the regular structure of tem-plated mesoporous structures ensured much easier access to the binding sites providing that the pore aperture remained
in the mesoporous range (≥20?A)
[12,15,20],which is il-lustrated again via the nearly complete ?lling of the well-ordered MPS-15%(97%accessibility)and the high ?lling level of the wormhole-like structured MPS-40%solid (0.68g of Hg(II)per gram of adsorbent).When passing from pH 4to 1,these experimentally observed maximal values were found to decrease by 50%for K40-SH,by 30%for MPS-15%,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 deleteri-ous 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 ef-fect (capacity decrease in strongly acidic media).Low resid-ual Hg(II)concentrations in solutions were achieved when performing sorption experiments in the pH range from 4to 7(i.e.,below the detection limit of the electrochemical tech-nique 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 coef?cients 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 (K40sample),either in the absence (A)or in the presence of other metal ions (B).The experiments were performed in 100mL solution containing initially 0.2mM Hg(II)(alone (A)or in mixture with 1mM Cu(II),Ni(II),Zn(II),Cd(II)and Bi(III)(B))to which either 20mg (( )curve (a))or 8mg (( )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 ?ve-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 ((?)curve (a),200mg L ?1K40-SH)or in excess of (( )curve (b),80mg L ?1K40-SH)the adsorbent capacity.Distribution diagram depicting
A.Walcarius,C.Delac?o te /Analytica Chimica Acta 547(2005)3–139
[36].However,in strongly acidic medium,the distribution
coef?cients 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 20mg MPS-40%in 250mL solution containing 0.05mmol of Hg(II),the evaluated distribution coef?cients were higher than 5×104mL g ?1at pH 5and less than 10mL g ?1at pH
1.
The effect of pH was even more marked when perform-ing sorption experiments from solutions containing mercury in the presence of an excess of several potentially interfer-ing metal ions (i.e.,0.2mM Hg(II)+1mM Cu(II)+1mM Ni(II)+1mM Zn(II)+1mM Cd(II)+1mM Bi(III)).This is illustrated in parts (B)of Figs.2–4.In fact,the maximum ca-pacities of materials,observed in the pH range 4–7,were not affected by the presence of these additional species,as ex-plained by the high strength of the thiol-mercury bond (higher than those reported for the interactions between other diva-lent metal ions and SH groups [37]).Also,the distribution coef?cients remained very high (>5×104mL g ?1)for the or-dered MPS adsorbents and moderately high (ca.500mL g ?1)for grafted silica gels,highlighting again the advantage of the ordered mesoporous solids over their amorphous ana-logues [12,15,22].In strongly acidic medium,however,the deleterious effect observed for Hg(II)alone was signi?cantly 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 decreas-ing pH,but this is also true for the interaction between thiol groups and the other metal ions.The extent of the interfer-ence effect is better evidenced by plotting the relative binding ef?ciency between the situations with and without compet-ing 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,competi-tion did exist in the pH range below 4,where HgOH +and especially Hg 2+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 mer-cury binding ef?ciency as only a small improvement was observed when passing from the amorphous K40-SH mate-rial to the well-ordered MPS-15%solid (comparison of parts (C)in Figs.2and 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 100mL solution containing initially 0.2mM Hg(II)(alone (A)or in mixture with 1mM Cu(II),Ni(II),Zn(II),Cd(II)and Bi(III)(B))to which either 20mg (( )curve (a))or 8mg (( )curve (b))MPS-15%was added.The dashed line represents the experimen-tally 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 adsor-bent in the presence of interference divided by the adsorbed Hg(II)quantity measured in the absence of ?ve-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 ((?)curve (a),200mg L ?1MPS-15%)or in excess of (( )curve (b),80mg L ?1MPS-15%)the adsorbent capacity.Dis-tribution diagram depicting the main chemical forms of Hg(II),obtained by
10 A.Walcarius,C.Delac?o te /Analytica Chimica Acta 547(2005)3–13
MPS-40%adsorbent (part (C)in Fig.4).This competition
trend was further con?rmed by working in Hg(II)solutions containing an interfering cation (Cu 2+)at a concentration 100times over that of Hg(II),for which binding capacities were found to decrease when passing from pH 4to 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 functional-ized 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 par-ticular 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 bind-ing process when changing Hg(II)speciation.Depending on pH,the mercury forms that are likely to interact with thiol groups are Hg 2+,HgOH +,and Hg(OH)2[35].On this basis,the corresponding complexation reactions are described by the following equations:
Si C 3H 6SH +Hg 2++2NO 3?→
Si C 3H 6S Hg +,NO 3?+HNO 3
(1)
Si C 3H 6SH +HgOH ++NO 3?→Si C 3H 6S HgOH +HNO 3(2)
Si C 3H 6SH +Hg(OH)2→Si C 3H 6S HgOH +H 2O
(3)
When Hg(II)is in the form of the dication Hg 2+,its complex-ation 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 com-plexed 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 250mL solution containing initially 0.2mM Hg(II)(alone (A)or in mixture with 1mM Cu(II),Ni(II),Zn(II),Cd(II)and Bi(III)(B))to which either 20mg (( )curve (a))or 8mg (( )curve (b))MPS-40%was added.The dashed line represents the experimen-tally 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 adsor-bent in the presence of interference divided by the adsorbed Hg(II)quantity measured in the absence of ?ve-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 ((?)curve (a),80mg L ?1MPS-40%)or in excess of (( )curve (b),32mg L ?1MPS-40%)the adsorbent capacity.Dis-tribution diagram depicting the main chemical forms of Hg(II),obtained by
A.Walcarius,C.Delac?o te/Analytica Chimica Acta547(2005)3–1311
having recently been reported to form on mercaptopropyl-functionalized 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?lling,the ?rst 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 C3H6SH+M2++2NO3?→
Si C3H6S M+,NO3?+HNO3(4) At the opposite,according to this hypothesis,no electro-static restrictions are expected to occur at pH above4,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 pre-viously 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 con?ned“S HgOH”moieties.However,the hypoth-esis 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 pHs1and4have revealed that free space was still available after mercury uptake(pore volumes of0.28and0.23mL g?1, respectively,at pHs1and4,after reaching maximum?ll-ing values of0.8and1.5mmol g?1,respectively).Indeed, the“Hg+,NO3?”moieties do occupy about twice as much space as the“HgOH”groups,but steric effects are not dom-inant over electrostatic restrictions to explain the results of sorption experiments obtained as a function of pH.
Further evidence to support the hypothesis of capacity re-strictions on the basis of electrostatic considerations is pro-vided in Fig.5,showing the in?uence of pH on Ag+binding to the MPS-40%sorbent.In this case,the complexation re-action 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 C3H6SH+Ag++NO3?→
Si C3H6S Ag+HNO3(5) As shown,even with this highly functionalized MPS-40%
Fig.5.Adsorption isotherms obtained as a function of pH for Ag I on mercaptopropyl-functionalized mesoporous silica(MPS-40%sample),from experiments performed in250mL solution containing initially0.2mM Ag I to which either20mg(( )curve(a))or8mg(( )curve(b))MPS-40%was added.
maximum binding capacity of93±2%of the theoretical one (less-than-complete?lling was explained in this case by steric constraints)being obtained over the whole pH range between 0.5and7.This con?rms thus the key role played by charge effects to restrict the binding capacity and enhance the in?u-ence 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 consid-ered Hg(II)desorption from thiol-functionalized mesoporous silicas[11,14,20],a harsh treatment in12M HCl was found effective to recover more than90%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 struc-ture and/or the density of organo-functional groups on this deleterious behavior,we have performed regeneration and re-use experiments on all the thiol-functionalized mesoporous silicas used in this work,by extending them up to three suc-cessive uptake events carried out at pH1–4.The results are depicted in Fig.6.Several features can be noticed from these data.First,all the adsorbents underwent signi?cant decrease in their sorption capacity upon the successive uptake-leaching steps,by25–50%during the?rst cycle up to50–80%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 re-
12 A.Walcarius,C.Delac?o te /Analytica Chimica Acta 547(2005)
3–13
Fig.6.Variation of the mercury sorption capacity measured at pH 1( )and pH 4(?)for three successive experiments involving desorption in 12M 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 exper-iments.Finally,one have to mention that in spite of this dra-matic capacity loss,the sorbents were still likely to reduce the mercury concentration in solution to insigni?cant 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 con-ditions applied to the desorption step,and the results ob-tained for three typical media are gathered in Table 3.On the one hand,reducing the HCl concentration from 12to 3M 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 signi?cantly the des-orption yields of the 3M HCl medium,in agreement with the fact that acidi?ed thiourea solutions can be used to elute metal ions from mercapto-functionalized organoclay columns [39].thiourea solution containing HCl at lower concentration (i.e.,0.1M)for which the same order of desorption effectiveness (or even better)was achieved (Table 3).In this case,desorp-tion yields of 83–93%were observed for the ordered meso-porous adsorbents.Such “soft”conditions were also found well appropriate for successive uptake-leaching processes be-cause they did not result in signi?cant 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
Extent of desorption in three media (%)3M HCl
3M HCl +5%thiourea 0.1M HCl +5%thiourea K40-SH 256576G60-SH 197391MCM41-SH 368186MPS-10%318793MPS-15%369089MPS-20%318683
A.Walcarius,C.Delac?o te/Analytica Chimica Acta547(2005)3–1313
4.Conclusions
The binding properties of thiol-functionalized meso-porous 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 above4are bet-ter with using ordered mesoporous structures in compar-ison to the amorphous gels functionalized with the same mercaptopropyl groups,in agreement with previous stud-ies[12,15,20].In stronger acidic medium(pH below4), however,both these parameters were found to worsen be-cause 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 ca-pacity 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 pur-poses(i.e.,mercury removal from polluted media),and af-ter testing several desorption solutions,it appears that the use of a5%thiourea medium containing0.1M HCl is the best compromise between suf?ciently high mercury desorp-tion 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.A201(2002)275.
[10]A.Walcarius,M.Etienne,J.Bessi`e re,Chem.Mater.14(2002)2757.[11]X.Feng,G.E.Fryxell,L.-Q.Wang,A.Y.Kim,J.Liu,K.M.Kemmer,
Science276(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,https://www.doczj.com/doc/d54435435.html,mun.(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.E362(1999)33.
[19]A.M.Liu,K.Hidajat,S.Kawi,D.Y.Zhao,https://www.doczj.com/doc/d54435435.html,mun.(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.Delac?o te,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,
Nature359(1992)710.
[31]M.Etienne,B.Lebeau,A.Walcarius,New J.Chem.26(2002)384.
[32]A.Walcarius,M.Etienne,C.Delac?o te,Anal.Chim.Acta508(2004)
87.
[33]L.Z′e kany,I.Nagyral,D.J.Leggett(Eds.),Computational Methods
for the Determination of Formation Constants,Plenum Press,Lon-don,1985(Chapter8).
[34]Y.Bon?l,M.Brand,E.Kirowa-Eisner,Anal.Chim.Acta424(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,Ther-
mochim.Acta328(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.Acta306
(1995)167.