Mechanism of heavy metal uptake by a hybrid MCM-41 material: Surface complexation and EPR spectroscopic study

Journal of Colloid and Interface Science 343 (2010) 374–380

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Mechanism of heavy metal uptake by a hybrid MCM-41 material: Surface complexation and EPR spectroscopic study Panagiota Stathi a, Kostas Dimos b, Michael A. Karakassides b, Yiannis Deligiannakis a,* a b

Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece Department of Materials Science & Engineering, University of Ioannina, 45110 Ioannina, Greece

a r t i c l e

i n f o

Article history: Received 29 September 2009 Accepted 10 November 2009 Available online 22 November 2009 Keywords: Hybrid MCM-41 Dithiocarbamate Heavy metals Pb Cd Cu Zn Adsorption SCM FITEQL EPR

a b s t r a c t A novel hybrid MCM-41-based material was synthesized by incorporation of AEDTC [N-(2-aminoethyl)dithiocarbamate] in the MCM-41 pores. The derived MCM-41  AEDTC material possesses high AEDTC loading 35% [w:w], and a well-defined array of regular mesopores with a specific surface area of 632 m2/g. Heavy metal, Cd, Pb, Cu, and Zn, uptake was studied in detail at physiological pH values 6–8, by a combination of analytical and electron paramagnetic resonance (EPR) spectroscopic techniques. The analytical data show a significant improvement, i.e., 200–500%, for Pb, Cu, and Zn uptake by the MCM-41  AEDTC hybrid vs the unmodified MCM-41. In contrast, Cd shows an exceptional behavior: (a) Cd uptake by MCM-41  AEDTC is very low. (b) Competitive metal uptake experiments reveal that Cd ions cause a characteristic inhibition of Cu or Pb uptake by the MCM-41  AEDTC while Cd binding itself always remained low. The present findings are analyzed by a combination of surface complexation modeling and EPR spectroscopy. Accordingly, in the MCM-41  AEDTC the sulfur atoms of AEDTC provide strong binding sites for metal binding, with a stoichiometry [SAEDTC]:[Metal] = 1:1. Cd inhibits accessibility of Cu or Pb ions in the AEDTC sites. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Removal of hazardous heavy metals, such as Cd, Pb, Zn, and Cu, from aqueous solution represents a significant activity of waste treatments. Fruitful research efforts have demonstrated that sorptive materials can provide efficient technologies for removal of metal ions from aqueous solution [1]. Pertinent examples include polymers [2,3], silicates and other metal oxides [4,5] (silica gel, alumina), clays (montmorillonite) [6,7], mud [8], activated carbon [9,10], and zeolites [11,12]. Mesoporous molecular sieves have also been used [13,14]. A second generation of materials is hybrid materials with improved metal uptake properties. The hybrid materials can be prepared by coupling organic ligands (e.g., thiol, amine, or crown ether functionalities) to various matrices, such as organic polymers [15–20] (polystyrene, cellulose, or poly(methyl-methacrylate). More particularly, thiol-modified silica gel [21] showed improved sorption of Ag, Hg, Cu, Zn, and Ni ions from aqueous solution, whereas amine-modified zeolites [22] were shown to have improved sorption properties for Pb and Cd ions

* Corresponding author. Fax: +30 2641074176. E-mail addresses: [email protected] (P. Stathi), [email protected] (K. Dimos), [email protected] (M.A. Karakassides), [email protected] (Y. Deligiannakis). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.11.029

[22]. In another approach, modified clays were prepared and tested as remediation agents. Pertinent examples include modified sepiolite and montmorillonite [23–27] containing various functional groups. Thiol- or carbamate- [28] functionalized layered montomorillonite were shown to have improved metal uptake capacity. In another work, intercalation of 3-mercaptopropyltrimethoxysilane in montmorillonite produced an efficient Pb, Hg, Cd, and Zn sorbing material [25]. Recently, in an extended study of montmorillonite modified with organic ligands containing thiol, amino, or carboxy functionalites [23], theoretical modeling of the observed metal binding for heavy metals, i.e., Cd, Pb, Cu, Zn, demonstrated that the key features which determine the improved performance of the functionalized materials were: (a) high metal loading capacities due to the ligands, and (b) strong binding affinities for the selected metal ions due to the nature of the functional groups. In the past few years, a new class of ordered mesoporous materials has attracted wide attention for industrial and environmental applications. Ordered mesoporous materials, i.e., such as MCM-41, show large BET surface area, high porosity, controllable and narrowly distributed pore sizes, and ordered pore arrangement. The MCM-41 materials—of the M41S family [29,30]—present a high specific surface area that is developed via a wellorganized hexagonal arrangement of cylindrical pores, rendering

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them good candidates for the insertion of organics. Mesoporous materials are also known to be promising candidates for use as catalysts [31,32], adsorbents [33–41], sensors [40], and hosts [41]. In this context, functionalized mesoporous materials have been evaluated as adsorbents of heavy metal ions, organics, dyes, and radionuclide and anionic complexes. Vodified mesoporous silicas have been studied [31–33] as heavy metal adsorbents. Mercier, Pinnavaia, and co-workers [33,34] and Liu et al. [35] presented data which show improved metal-binding capacity by their hybrid MCM materials. Organic functional groups have been grafted or incorporated on the surface of mesoporous channels using ligand-functionalized organosilanes. Experimental data [34,35] suggested that the constricted nature of the microporous channels most likely controls access of Hg ions to the adsorbents binding sites. An important implication of these findings is that, in principle, selectivity for a certain metal can be achieved by modified MCM41 materials, if we take advantage of the geometry in combination with the nature, i.e., metal coordination properties, of an inserted ligand. Equally important, however, is the proper understanding of the contribution of other factor in the overall metal-uptake effect. These should include pH, surface OH groups, and most importantly the binding constants of the metals at each binding site of the modified materials. This highlights the need for a more detailed analysis of the physicochemical factors which determine the metal binding mechanism in the modified mesoporous materials. Electron paramagnetic resonance (EPR) spectroscopy can provide valuable information concerning the local environment of paramagnetic metals, i.e., such as Cu2+ [43,44]. Recently, we have demonstrated that by applying EPR spectroscopy in interfacial phenomena, key-information, e.g., on the type and number of coordinating atoms, can be reliably obtained after proper analysis of the spin-Hamiltonian parameters, i.e., such as g tensor and hyperfine coupling tensor, A, components [44,45]. In the present work, EPR spectroscopy was employed in parallel with analytical metal binding studies. More particularly, in our previous work [46] we have presented the synthesis and characterization of a novel hybrid MCM-41 material [46] prepared by insertion of a short C-chain dithiocarbamate-based complexing agent, i.e., N-(2-aminoethyl)dithiocarbamate (AEDTC). In the present work we present a detailed physicochemical study of Pb, Cd, Zn, and Cu uptake by this MCM-41  AEDTC hybrid material. The pH profile was studied in detail with particular emphasis on competition among Pb, Cd, Zn, and Cu. In the present work specific emphasis is focused on the molecular aspects of the physicochemical mechanism of metal binding, e.g., rather than the mere evaluation of metal loading capacity. Thus the main goals of the present work were: (a) the experimental and theoretical study of Pb, Cd, Zn, and Cu binding by MCM-41  AEDTC; (b) a detailed physicochemical study of the interfacial coordination mode of the metals in the MCM pores, combining in situ information from EPR spectroscopy with metal binding data; (c) to provide a interpretative structural model for the observed inhibitory role of Cd ions.

2. Experimental 2.1. Reagents All solutions were prepared with analytical grade chemicals and ultrapure water (Milli-Q Academic system) with a conductivity of demineralized water of 18.2 lS cm1. All solutions were degassed with 99.999% N2 prior to use. Pb(NO3)2, Cd(NO3)2, Cu(NO3)2, and Zn(NO3)2 (Aldrich > 99.5%) stock solutions were prepared at concentrations of 3 mM and were kept in a polyethylene container at pH value <2. A buffer system of 10 mM MES (N-morpholinoethanesulfonic acid), HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic) acid, and TRIS (tris-hydroxymethylaminomethane) was used at all of the samples. This system presented a significant buffer capacity at range pH 5–9 with an average deviation from the adjusted pH value <5%. 2.2. Materials The preparation and structural characterization of the MCM-41 and MCM-41  AEDTC hybrid materials are described in detail in our previous work [46]. Herein, pertinent physicochemical parameters, taken from Ref. [46], are summarized in Table 1. 2.3. Analytical determination of metal ions The concentrations of metals in the aqueous phase were determined by anodic stripping voltammetry (ASV) by using a Trace Master5-MD150 polarograph by Radiometer Analytica. ASV is well suited for analytical determination on metals, such as Pb, Zn, Cd, and Cu used in this work, at ppb concentrations [23,45]. The measuring cells were borosilicate glass cells from Radiometer. The working electrode was a hanging mercury drop electrode (HMDE), with a 0.4 mm diameter Hg drop generated by a 70 lm capillary. The reference electrode was an Ag/AgCl electrode with a double liquid junction. The counterelectrode was a Pt electrode. Initially, before the stripping step, N2 gas (99.999% purity) was bubbled in the measuring solution to remove O2. During this step the solution was under continuous stirring at 525 rpm. During the stripping step the solution was not stirred. Square-wave (SW) measurements were performed in the anodic direction, i.e., SWASV, to quantify metal ions.. 2.4. Metal sorption experiments Metal ion adsorption was investigated in batch experiments. Four metal ions Pb2+, Zn2+, Cd2+, and Cu2+ were tested. Sorption edge measured the effect of the pH on metal uptake from aqueous solution for an initial metal concentration of 4.5 lM for every metal ion tested. One milligram of material was suspended in polypropylene tubes containing 10 ml of buffered Milli-Q water to yield a concentration of 0.1 g/L. In the following, suitable volumes of metals stock solutions were added to yield a metal concentration of 4.5 lM. The pH of the suspension was adjusted to the

Table 1 Pore and compositional parameters derived from X-ray diffraction patterns, N2 adsorption data, and DTA/TG thermal analyses for samples MCM-41  NY4 and MCM41  AEDTC.a

a

Samples

SBET (m2/g)

Vpore (cm3/g)

d100 (Å)

ao (Å)

dBJH (Å)

p (Å)

AEDTC content (mmol/g)

MCM-41  NY4 MCM-41  AEDTC

948 632

0.80 0.54

41.5 41.6

47.9 48.0

27.1 26.4

21.8 21.6

– 2.57

Data taken from Ref. [46].

P. Stathi et al. / Journal of Colloid and Interface Science 343 (2010) 374–380

2.5. Theoretical analysis: surface complexation of metal binding in MCM-41 materials The H-binding and metal binding properties by the surface groups of the MCM materials used in this work were described by a surface complexation model described previously [46]. Surface complexation models (SCM) [47] can model successfully the adsorption of ions on charged surfaces by assuming that adsorption involves both a coordination reaction at specific surface sites and an interaction between adsorbed ions and the organic ligand on the modified material. This model is based on the concepts originally developed for oxide surfaces and have been extended for the case of clays as well as for organic-clay hybrids [23,24]. 2.6. EPR spectroscopy Electron paramagnetic resonance spectra were recorded with a Brucker ER200D spectrometer at liquid nitrogen temperatures, equipped with an Agilent 5310A frequency counter. Adequate signal to noise was obtained after 5–10 scans. Numerical simulation of the experimental EPR spectra were performed based on second-order perturbation theory, using the software WinEPR Simfonia v. 1.25 by Bruker Analytische. 3. Results and discussion 3.1. Heavy metal (Pb, Zn, Cu, and Cd) adsorption 3.1.1. Metal uptake The metal uptake capacity for the MCM-41 vs the hybrid MCM41  AEDTC was evaluated in pH edge experiments. Table 2 summarises the maximum metal uptake results obtained at pH 7, which is of pertinence for environmental applications of these materials in natural waters. According to Table 2, a significant improvement of the metal uptake is achieved by the MCM41  AEDTC material, i.e., compared with the reference MCM-41 material. Namely, the Pb uptake was almost doubled while the Zn and Cu uptakes were improved, i.e., by 300%. The results in Table 2 show that the hybrid MCM-41  AEDTC can serve as efficient Pb, Zn, and Cu sorbent. However, the case of Cd uptake appears to be different. From Table 2 we see that only a tiny improvement, below 10%, is observed for the Cd uptake by the hybrid MCM-41  AEDTC. Given the improvement observed for the uptake of Pd, Zn, and Cu, the

Table 2 Amount (mmol/100 g) of heavy metals adsorbed on materials at pH 7.

Pb Zn Cu Cd

MCM-41  NHþ 4

MCM-41  AEDTC

Improvement (%)

9.4 8.0 10.0 11.8

18.4 23.8 32.4 12.9

95 198 224 9

low Cd uptake by the MCM-41  AEDTC is rather intriguing. A close inspection of the values in Table 2 reveals that in the unmodified MCM-41 the Cd uptake is higher (12 mmol of Cd per gram of MCM-41) than for Pb, Zn or Cu. In contrast, after incorporation of the AEDTC, the Cd uptake remains practically unaltered while the uptake of Pb, Zn, or Cu shows significant improvement. In an effort to better understand the physiochemical metal binding mechanism, we have carried out detailed pH-dependent, i.e., pH edge, experiments. 3.1.2. pH edge: (a) Pb uptake Fig. 1A (solid squares) shows experimental Pb adsorption data for MCM-41  AEDTC, as a function of the pH. The experimental pH edge data for Pb uptake by the unmodified MCM-41 material are also included for comparison (Fig. 1A, solid circles). By comparing the performance of the two materials with regard to Pb uptake, we observe a global improvement, i.e., at pH > 4 by the MCM41  AEDTC vs the MCM-41. The improvement is maximum at pH 8.2. The experimental data in Fig. 1A show that the Pb uptake increased at alkaline pH values, for both the hybrid MCM41  AEDTC and the MCM-41 material. This pH trend is a well-documented phenomenon for the adsorption of metal cations in metal oxides bearing deprotonable surface groups [23,44,46,47] which serve as metal binding sites. The surface sites of the hybrid MCM-41  AEDTC as well as for MCM-41 material have been studied in detail by H-binding experiments [46]. In the present case, we have incorporated this information in the surface complexation model by assuming the surface reactions (1–3, 4, 9) listed in Table 3. Accordingly, in Fig. 1A the open symbols are theoretical data calculated by FITEQL. The equilibrium constants derived from the fit to the data are also listed in Table 3. According to the speciation scheme in Fig. 1B, the enhanced uptake of Pb by MCM-41  AEDTC can be attributed to the binding of Pb2+ by the AEDTC groups, i.e., with log K[AEDTC– + Pb] = 3.8. The binding of Pb(OH) (pKa = 7.7) was also tested in the fitting procedure; however the contribution was minor relative to the Pb2+ binding. The AEDTC ligands act very efficiently as Pb binding sites; therefore, the Pb uptake observed in Fig. 1A is due to the AEDTC–Pb species. 3.1.3. (b) Zn uptake Fig. 2A shows the pH edge for adsorption of Zn on MCM41  AEDTC (solid squares) and on MCM-41 (solid circles). As in the case of Pb, we observe a global improvement of Zn uptake, i.e., at pH > 4 by the MCM-41  AEDTC vs MCM-41. At pH 8.1, the improvement of Zn uptake is over 400%. The fit to the data, open symbols in Fig. 2A and the ensuing speciation, Fig. 2B, was calculated by assuming the surface reactions

70 60

(A) MCM-41-AEDTC

50 40 30 20 10

MCM-41

0 4

Pb2+ XO Pb

-6

Concentration (M)

desired value, i.e., in the range 4–8 in our experiments, with small volumes of HNO3 or NaOH solutions. Then the samples were allowed to equilibrate at room temperature for 120 min under stirring. Screening experiments showed that equilibrium was attained within 90 min. At the end of the incubation step, measurement of the pH of each suspension showed a pH drift <0.2 during the incubation. In the following each sample was centrifuged and the supernatant solutions were analyzed for metals. Competitive adsorption of heavy metals ions was also investigated by the same protocol, for binary metal solutions at 4.5 lM concentration for each metal.

% Pb-Adsorption

376

5

6

pH

7

8

10

-7

10

-8

10

AEDTC-Pb

-9

10

Pb(OH)

-10

10

-11

10

(B)

-12

10

4

5

6

7

8

pH

Fig. 1. (A) Adsorption pH edge for Pb onto MCM-41  NHþ 4 (d, s) and MCM41  AEDTC (j, h). Solid symbols: experimental data. Open symbols: theoretical fit obtained by assuming the reaction and stability constants listed in Table 3. (B) Theoretical concentrations of the formed species.

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Protonation reactions of XOH M  XO + H+  XOH + H+ M  XOHþ 4 + NHþ 4 M NH3 + H

1 2 3

MCM-41–NHþ 4



[46] [46] [46]

1.6 5.0 9.3

4 5 6 7

Sorption of metals onto MCM-41  NHþ 4  XO + Pb+2 M  [XOPb]+  +2 +   XO + Cd M [XOCd]  XO + Cu+2 M  [XOCu]+  XO + Zn+2 M  [XOZn]+

8

Protonation of AEDTC  AEDTC + H+ M  AEDTC-H

6.9

[46]

Sorption of metals onto MCM-41  AEDTC  AEDTC + Pb+2 M  [AEDTC–Pb]+  AEDTC + Cd+2 M  [AEDTC–Cd]+  AEDTC + Cu+2 M  [AEDTC–Cu]+  AEDTC + Zn+2 M  [AEDTC–Zn]+

3.8 5.2 4.0 3.9

This This This This

9 10 11 12

3.6 3.5 3.1 3.0

This This This This

work work work work

% Zn-Adsorption

100

(A)

80

Zn2+

MCM-41-AEDTC

60

work work work work

40

MCM-41 20 0 4

5

6

pH

7

8

AEDTC-Zn

-6

10

-8

10

XO Zn

-10

10

-12

10

Zn(OH)

-14

10

(B) 4

5

6

7

70

(A) MCM-41-AEDTC

60 50 40

MCM-41

30 20

AEDTC-Cu

-6

10

-7

10

Cu2+

-

XO Cu

-8

10

-9

10

Cu(OH)

-10

10

(B)

-11

10 4

5

6

pH

7

8

4

5

6

7

8

pH

Fig. 3. (A) Adsorption pH edge for Cu onto MCM-41  NHþ 4 (d, s) and MCM41  AEDTC (j, h). Solid symbols: experimental data. Open symbols: theoretical fit obtained by assuming the reaction and stability constants listed in Table 3. (B) Theoretical concentrations of the formed species.

Errors: log K (protons) ± 0.2, log K (metals) ± 0.08.

Concentration (M)

a

Reference

10

Concentration (M)

Log Ka

Reaction

-5

80

% Cu-Adsorption

Table 3 Surface, solution reactions, and stability constants (log K) used to fit the experimental data.

8

pH

Fig. 2. (A) Adsorption pH edge for Zn onto MCM-41  NHþ 4 (d, s) and MCM41  AEDTC (j, h). Solid symbols: experimental data. Open symbols: theoretical fit obtained by assuming the reaction and stability constants listed in Table 3. (B) Theoretical concentrations of the formed species.

(1–3, 5, 10) listed in Table 3. The equilibrium constants derived from the fit to the data are also listed in Table 3. According to the speciation scheme in Fig. 2B the formation of the AEDTC–Zn species is responsible for the enhanced Zn uptake. 3.1.4. (c) Cu uptake The data for uptake of copper(II) ions from aqueous solution are displayed in Fig. 3A. In Fig. 3A a broad upshift occurring for the MCM-41  AEDTC material vs MCM-41 is observed. At pH 8.0 the improvement is near 400%. The fit to the data, open symbols in Fig. 3A, and the ensuing speciation, Fig. 3B, were calculated by assuming the surface reactions (1–3, 6, 11) and equilibrium constants listed in Table 3. According to the speciation scheme in Fig. 3B the formation of the AEDTC–Cu species determines the improved Cu uptake. In summary, the present analytical data demonstrate that the incorporation of the AEDTC moieties in the MCM-41 pores has a major beneficial impact for the Pb, Cu, or Zn uptake. Of particular importance is the significant improvement achieved at near physiological pH values, i.e., 6–8. This is due to two reasons: (a) the pKa value of AEDTC near 7, see Table 3, and (b) the strong binding constant of the –CS 2 group for the Pb, Zn, and Cu ions. From Table 3 we note that the K values for AEDTC–Pd, AEDTC–Zn, and AEDTC–Cu are of the order of 10,000, i.e., K  104.

3.1.5. EPR spectroscopy Fig. 4A (solid lines) shows experimental EPR spectra recorded for Cu2+ ions adsorbed on MCM-41  AEDTC (A, i), or MCM (A, ii) at pH 6.5. This pH value was selected to avoid formation of Cu(OH)+ species. The dotted lines are theoretical simulated EPR spectra. 3.1.5.1. Spectral lineshape analysis. Before we proceed to the detailed analysis of the EPR spectra, we note a considerable spectral resolution in the EPR spectra for MCM–AEDTC–Cu2+ vs Cu2+(H2O), i.e., compare spectra (i) vs (iii) in Fig. 4A. The EPR spectrum for Cu2+(H2O) [trace iii] is broad and weak due to unresolved random dipole–dipole interactions between Cu2+ spins in the frozen solution [43–45]. In Fig. 4A (iii)—for the sake of visibility—the EPR spectrum for Cu2+(H2O) had to be multiplied by a factor of 2. In contrast, incorporation of the Cu2+ ions in MCM—particularly in MCM-41  AEDTC—results in a substantial improvement of spectral resolution. This is indicative that in the MCM matrix the Cu2+ ions are localized in specific sites, where random magnetic interactions are minimized. As we show in the following, specific coordination of the association of the Cu ions with MCM surface groups and S atoms of AEDTC occurs in MCM and MCM-41  AEDTC. Taking into account the BET information on the formation of a uniform array of internal surface sites [46], the EPR data show specific binding of Cu2+ ions at MCM surface groups and/or S atoms inside the MCM and MCM-41  AEDTC tubes, i.e., instead of, for example, polynuclear Cu clustering. This is of immediate relevance for the validity of the surface complexation modeling which assumes specific surface reactions, as well as for the interpretation of the Cu–Cd competition effect, that we discuss in the following. 3.1.5.2. Spin-Hamiltonian parameters. The dotted lines in Fig. 4A are theoretical simulated EPR spectra calculated as detailed previously [43,44] using the spin-Hamiltonian parameters listed in Table 4. Detailed quantitative information on the coordination environment of Cu2+ ions can be obtained by analyzing the Az and gz values. In this context the [Az vs gz] correlation plot, originally suggested by Peisach and Blumberg [42], offers an appropriate method; see Fig. 4B. By comparison with literature data [42,43] the [Az vs gz] data in Fig. 4B indicate that in MCM-41  AEDTC the Cu2+ ions are coordinated by one S atom plus up to three O atoms. The coordinating O atoms most likely can originate from solvent molecules. S coordination is of particular importance since it provides direct evidence for the coordination of the DTC moiety on the Cu2+ ion. This provides direct support for the surface complexation modeling, reaction 11, in Table 3 where we have assumed coordination of one AEDTC per Cu2+ ion. This demonstrates that EPR and surface complexation modeling can

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x2 2+

(v)

([MCM-AEDTC]+Cd )+ Cu 2+

2+

dx"/dH (arb. un.)

([MCM-AEDTC]+Cu )+ Cd

(iv) x2 (iii)

Cu2+(H20)

Az= 108G, gz=2.44

(ii)

200

2+

MCM-AEDTC-Cu

180

Az (Gauss)

2+

160

MCM-Cu

140

(B) 120

2+

Cu (H2O)

2+

Az= 166G, gz=2.41

[MCM]+Cu

100 2.20

2.25

2.30

2.35

2.40

2.45

gz

(i)

Az= 196G, gz=2.22

2+

2+

[MCM-AEDTC]+Cu 2500

3000

(A) 3500

4000

Magnetic Field (Gauss)

Sample

gx

gy

gz

Ax (G)

Ay (G)

Az (G)

Cu(H2O) MCM–Cu MCM–AEDTC–Cu

2.059 2.058 2.057

2.095 2.095 2.063

2.439 2.413 2.218

22 15 18

13 13 28

108 169 196

Errors: g values ± 0.003, A ± 2 G.

be used in a complementary manner for obtaining a consistent picture at the molecular level. In the MCM–Cu2+ case, the [Az vs gz] data in Fig. 4B indicate a coordination sphere for Cu2+ containing only O atoms, which may be attributed to solvent O atoms and MCM surface groups, i.e., as described by reaction 6 in Table 3. Thus both the EPR and the SCM analysis demonstrate that in MCM–AEDTC each Cu2+ is bound by one S atom from one AEDTC and this is the origin of the enhanced Cu2+ binding, i.e., compared with MCM. 3.1.6. (d) Cd uptake Fig. 5A shows the pH edge for adsorption of Cd on MCM41  AEDTC (solid squares) or MCM-41 (solid circles) material, respectively. In contrast to the Pb, Zn, or Cu uptake, discussed in the previous paragraphs, the data in Fig. 5A show comparable Cb uptake by MCM-41  AEDTC and by MCM-41. According to the speciation scheme in Fig. 5B, we note that the contribution to the binding of Cd by AEDTC is marginal at all pH values. A small improvement is observed at pH > 7.5. According to the theoretical analysis, the binding of the Cd ions at the negatively charged surface sites XO accounts for practically all the Cd uptake. The concentration of the AEDTC–Cd species is very low, while the concentration of the XOCd species is of the order of 0.1 lM; see Y axis in Fig. 5B. Compared with the [XOM plus the AEDTC–M] concentrations for the other metals, the total binding Cd is 10–100 times lower. See, for example, the XOPb plus the AEDTC–Pb species in Fig. 1B, the XOZn/AEDTC–Zn species in Fig. 2B, or the XOCu/AEDTC–Cu in Fig. 3B.

45 40 35 30 25 20 15 10 5 0

-4

(A)

MCM-41-AEDTC

MCM-41

4

5

6

pH

7

8

Concentration (M)

Table 4 Cu2+ Spin-Hamiltonian parameters derived from the EPR spectra.

% Cd-Adsorption

Fig. 4. EPR spectra (A) for 200 lM Cu2+ plus 10 lg MCM–AEDTC (A, i), or 10 lg MCM (A, ii), or no material added Cu(H2O) (A, iii). (Solid lines) Experimental spectra; (dotted lines) simulated spectra using the spin-Hamiltonian parameters listed in Table 4. (iv and v) Cu–Cd competition for binding in MCM–AEDTC. In (iv) 10 lg MCM–AEDTC tubes were incubated for 120 min in the presence of 200 lM Cu(NO3)2 followed by incubation (120 min) after the addition of 200 lM Cd(NO3)2. In (v) 10 lg MCM–AEDTC tubes were incubated for 120 min in the presence of 200 lM Cd(NO3)2 followed by incubation (120 min) after addition of 210 lM Cd(NO3)2. Experimental conditions: pH 6.9, T = 77 K, modulation amplitude 10 G, Mod. frequency 100 kHz. (B) [Az vs gz] correlation plot of the spin-Hamiltonian parameters derived from the fit of the experimental spectra.

10 -5 10 -6 10 -7 10 -8 XO Cd 10 -9 10 -10 10 -11 10 Cd(OH) -12 10 -13 10 4

5

Cd2+

AEDTC-Cd

(B) 6

7

8

pH

Fig. 5. (A) Adsorption pH edge for Cd onto MCM-41  NHþ 4 (d, s) and MCM41  AEDTC (j, h). Solid symbols: experimental data. Open symbols: theoretical fit obtained by assuming the reaction and stability constants listed in Table 3. (B) Theoretical concentrations of the formed species.

Finally, we note that the low concentration of the XOCd and AEDTC–Cd species (Fig. 5B) are in striking contrast with the log K constant, i.e., log K[XOCd] = 3.5, log K[AEDTC–Cd] = 5.2; see Table 3. These log K values are high, and this indicates that the formation of the XOCd and AEDTC–Cd species is thermodynamically favorable. Overall this analysis reveals that the low Cd uptake observed for MCM-41 or MCM-41  AEDTC is not due to low affinity of Cd ions for the surface sites. Other possible reasons include accessibility restrictions and/or local geometry effects as noted earlier for Hg uptake by analogous systems [33,34]. Structural irregularities are less likely, based on the information of high pore homogeneity of the present MCM-41  AEDTC material [46]. 3.1.6.1. Competition effects: binary systems. In an effort to better understand accessibility restrictions and/or local geometry effects, we have examined the effect of Cd on the Cu and Pb uptake by the hybrid MCM-41  AEDTC tubes. 3.1.6.2. Pb–Cd. Fig. 6A (squares) shows the pH edge for adsorption of Pb on MCM-41  AEDTC in the absence (solid squares) or in the

70 60

(A)

50

2+

2+

Pb

40

+[Cd ]

30 20 10 0

Pb (Cd) Cd (Pb)

4

5

6

pH

7

8

%Cu, Cd Adsortpion

%Pb, Cd Adsorption

P. Stathi et al. / Journal of Colloid and Interface Science 343 (2010) 374–380

80

2+

Cu

70 60

2+

+[Cd ]

50 40

Cu (Cd)

30 20

Cd

(B)

10

379

the log K values for surface Cd species clearly show that the complexation of the Cd with the surface sites is thermodynamically favorable. (c) Binding of Cd cause a severe inhibition of Pb or Cu ions by the MCM-41 tubes. (d) EPR spectroscopy shows that Cd ions inhibit Cu binding at the surface sites. In the following, based on this information we discuss a physical model which explains the role of Cd based on geometrical/space constriction phenomena.

Cd(Cu) 4

5

6

7

8

pH

Fig. 6. Competitive adsorption of Cd with Pb and Cu ions on MCM-41  AEDTC. (A) Pb adsorption (j, h), Cd adsorption (d, s). (B) Cu adsorption (j, h), Cd adsorption (d, s). Solid symbols: mono-ion adsorption. Open symbols: competitive adsorption.

presence (open squares) of Cd ions, respectively. Strikingly, the presence of Cd ions causes a characteristic inhibition of Pb uptake by the MCM-41  AEDTC; see open circles in Fig. 6A. The inhibitory effect of Cd was global, i.e., for all pH values studied in Fig. 6A. In the same experiment, the Cd binding always remained low (circles). The presence of Pd ions had a, marginal, inhibitory effect on Cd binding. 3.1.6.3. Cu–Cd: (a) metal binding. The data in Fig. 6B show that Cd has a severe inhibitory effect on the Cu uptake by MCM41  AEDTC; i.e., compare solid squares with open squares in Fig. 6B. As in the case of Pb, the presence of Cd ions causes a characteristic inhibition of Cu uptake by the MCM-41  AEDTC, while Cd binding always remained low (circles). The inhibitory effect of Cd over Cu binding was further investigated by EPR spectroscopy. 3.1.6.4. (b) EPR spectroscopy. In Fig. 4A (iv, v) the Cu–Cd competition for binding in MCM-41  AEDTC is investigated. In (iv) the MCM-41  AEDTC tubes were incubated for 120 min in the presence of Cu2+ followed by addition of Cd2+ ions and incubation for 120 min. The obtained EPR spectrum is characteristic of Cu2+ binding at the MCM-41  AEDTC material; i.e., compare spectra (iv) and (i). A quantitative analysis shows that in (iv) the bound Cu2+ ions are 80% relative to those in (v). Based on the analysis of the EPR spectra, this result shows that in (iv) the sequential adsorption of Cu followed by Cd in the MCM-41  AEDTC results in a small inhibition of Cu binding to the MCM-41  AEDTC tubes. Thus after Cu binding, Cd ions have little effect, in accordance with the Cu–Cd binding data in Fig. 6B. In contrast, when the Cd ions were first adsorbed by the MCM41  AEDTC, then subsequent adsorption of Cu ions was largely inhibited; see EPR spectrum in Fig. 4A (v). The obtained EPR spectrum, Fig. 4A (v), is characteristic of Cu2+ ions mainly coordinated by H2O. The vertical lines in Fig. 4A help to visualize the components of two EPR subspectra contributing to spectrum (v). The two subspectra are clearly identifiable as being due to MCM41  AEDTC–Cu (minor fraction) and Cu(H2O) (major fraction). A quantitative comparison shows that in (v) the Cu(H2O) centers account for >80% of the copper ions, while only a minor fraction not exceeding 20% is due to MCM-41  AEDTC–Cu. Thus the EPR spectra demonstrate that when first Cd is adsorbed, Cu binding in the MCM-41  AEDTC frame is inhibited. Interestingly the MCM-41  AEDTC–Cu subspectrum, despite the low abundance, is clearly resolvable in spectrum (v) due to the enhanced resolution of the EPR spectrum of the Cu2+ centers in the MCM-41  AEDTC tubes, as in spectrum (i) in Fig. 4A. Overall the present data reveal that: (a) The fraction of Cd ions bound by the MCM-41  AEDTC tubes is low. (b) The low Cd binding is not due to low binding affinity of cadmium ions. In contrast,

3.1.7. A physical model: space constriction vs binding affinity The constricted nature of the microporous channels controls access of metal ions to the adsorbents binding sites. Previous works provided evidence that the uniform pore structure of thiol- [34] or amino- [35] functionalized mesoporous solid materials facilitates access of the metal ions to the binding sites, resulting in improved metal loading capacities, compared to more irregular substrates. Our recent work [46] provides evidence that MCM-41  AEDTC is characterized by high pore homogeneity. Of immediate relevance to the present work is the observation that in thiol-functionalized mesoporous solids materials very low levels of Cd loading were achieved [34]. Accordingly, Pinnavaja and co-workers have suggested that the immobilization of metal ions is also controlled by the accessibility to the binding sites [34]. Taking together our metal-competition and EPR spectroscopic data, we suggest that restricted accessibility caused by Cd ions occurs in our MCM-41 materials. According to this model, strong binding of Cd ions at surface sites located at the edges of the MCM tubes would block accessibility at the interior of the tube. At this point, geometry and size effects can be of relevance. Selective sorption of other metals over Cd on mesoporous materials is decisively influenced by geometry effects [34,35]. In this context we note that the hydrated Pb cation (hydration energy of Pb2+ 1425 kJ/mol [48]) has on average 6.1 water molecules in the hydration shell with an effective diameter of 5.2 Å [48]. The hydrated Cd cation (hydration energy of Cd2+ 1755 kJ/mol [48]) has 7.6 water molecules in the shell, corresponding to a diameter of 5.5 Å [49]. When bound to oxide surfaces hydrated Cd cations retain most of their H2O molecules. Sherman [48] showed that on binding of hydrated Cd ions on goethite, the Cd ions retain four of their coordinated H2O molecules. In this context, we may consider that the hydrated Cd ions are the bound species at the MCM-41 tubes. Thus binding of the hydrated Cd ions would result in significant space constriction at the entrances of the MCM-41 tubes. This working model could be tested by further spectroscopic or computational data. 4. Conclusions A high level of incorporation of AEDTC in the MCM-41 pores can be achieved due to high homogeneity of the pores of the parent MCM-41 material. Incorporation of AEDTC in the MCM-41 pores results in a significant improvement, 200–500%, for the Pb, Cu, or Zn uptake by the MCM-41  AEDTC hybrid at physiological pH values 6–8. The presence of Cd ions causes a characteristic inhibition of Cu or Pb uptake by the MCM-41  AEDTC while Cd binding always remained low. A physical model for the role of Cd ions in the metal binding by the MCM-41  AEDTC is discussed, which suggests that binding of Cd ions at XO sites located at the vicinity of the edges of the MCM tubes blocks the accessibility of metals at any other binding site located at the interior of the tube. Acknowledgments This research was cofunded by the European Union in the framework of the program ‘‘Pythagoras I” of the ‘‘Operational Program for Education and Initial Vocational Training” of the 3rd Com-

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munity Support Framework of the Hellenic Ministry of Education, funded by 25% from national sources and by 75% from the European Social Fund (ESF).

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