Metal ion removal by ultrafiltration of colloidal suspensions of organically modified silica

Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

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Metal ion removal by ultrafiltration of colloidal suspensions of organically modified silica Marc Hébrant a,b,∗ , Maureen Rose-Hélène a,b , Alain Walcarius a,b a b

Université de Lorraine, LCPME, UMR 7565, 405 rue de Vandoeuvre, 54600 Villers-lès-Nancy, France CNRS, LCPME, UMR 7565, 405 rue de Vandoeuvre, 54600 Villers-lès-Nancy, France

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Low metal ion concentrations extraction using colloidal organically modified fumed silica.  High extraction yields and concentration factors are attained.  The loss of extracting phase through the membrane is very low and non toxic.

a r t i c l e

i n f o

Article history: Received 18 July 2012 Received in revised form 25 October 2012 Accepted 27 October 2012 Available online 6 November 2012 Keywords: Metal ion extraction Ultrafiltration Colloidal silica particles Organo-modified silica

a b s t r a c t Porous fumed silicas (FS) covalently grafted with 5-phenyl-azo-8-hydroxyquinoline (5Ph8HQ) are considered as extractants for copper(II) and nickel(II) at low concentration levels (10–50 ␮M). The separation of the extracting colloidal functionalized silicas particles from the bulk aqueous phase is achieved by means of the frontal ultrafiltration technique. The stability of the colloidal dispersion of the organically modified silica and of the covalent grafting itself is studied via the loss of silica or of 5Ph8HQ through the ultrafiltration membrane during the separation process. The effects of the anion nature, specific surface area, surface ionization state on the metal ions extraction are assessed. For the sake of comparison, the extraction of copper(II) by 5Ph8HQ solubilized in neutral Triton X-100 (TX-100) micelles has been studied in the most similar experimental conditions, as well as copper(II) extraction by the same ligand in a biphasic water–chloroform system. High copper extraction yields are attained, and taking into account the low volume of extracting phase of the dispersed colloidal silica, high separation factors are obtained. The stoichiometry of the formed complex is the same (1:1) in the two microheterogeneous systems considered here whereas a 2:1 complex is formed in the biphasic liquid–liquid system. The apparent stability constant is greater by a factor 10 in the micelles than for the silica bounded 5Ph8HQ systems but the true affinity of the ligand for copper is 5 times lower in the micelle than on the functionalized systems. Comparing to the literature, it is shown that the type of silica considered (colloidal fumed silica in this work versus silica gel in the literature) greatly influences the affinity of the ligand toward the copper. Finally, the apparent extraction constant in 5Ph8HQ modified silica is ten times higher for copper(II) than for nickel(II). © 2012 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author at: Université de Lorraine, LCPME, UMR 7565, 405 rue de Vandoeuvre, 54600 Villers-lès-Nancy, France. Tel.: +33 03 83 68 52 54; fax: +33 03 83 27 54 44. E-mail address: [email protected] (M. Hébrant). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.10.046

Many research works are devoted to the development of new solutions for the removal of heavy metal ions from industrial wastewater. Among the solvent free processes, various ways are still explored: micellar extraction coupled with ultrafiltration [1],

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M. Hébrant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

HO O O

silica

Si

N N

OH

Once the metal was extracted onto the particles, its separation from the bulk was achieved by the means of the ultrafiltration technique. For the sake of comparison the extraction of copper by the same molecular ligand was studied in a non-ionic micellar system (Triton-X-100), and in a chloroform–water biphasic system. The stoichiometry and the stability constants are compared and discussed in both systems taking into account the zeta potential of the silica particles and the counterion present in the system.

N

Scheme 1. 5-Phenyl-azo-8-hydroxyquinoline (5Ph8HQ) grafted on a silica surface.

adsorption on NiO [2], polystyrene supported complexing agent [3], silica supported biosorbents [4], adsorption onto carbon black [5], extraction by magnetic hydrogel [6], adsorption on particle of clays, iron oxide or silica gel coupled with nanofiltration [7]. We are studying metal ion extraction from model solutions by silica colloidal particles bearing a covalently bound highly selective complexing agent for two reasons: - From the fundamental point of view, the understanding of reactivity variation change for a given extractant induced by its grafting onto a solid surface is far from being complete. This point was discussed in a preceding paper [8]. - From the applied point of view, the development of a process analogous to micellar extraction but without surfactant can be valuable. The main drawback of micellar ultrafiltration is a small but unavoidable loss of surfactant through the membrane. Would the micelles be replaced by silica particles in a process that an eventual loss of material would be environmentally friendly since silica is naturally present in water. Such a simple way of thinking does obviously not completely hold for industrial processes since the cost has to be taken into account, micellar extraction coupled to ultrafiltration has proven to be cost feasible at the industrial scale [9] and the particles studied here could not meet the same criteria. The aim here is to compare at the lab scale the results obtained with a ligand either dispersed in micellar systems or grafted onto silica particles when using the same complexing agent in the same ultrafiltration conditions. We selected the 5-phenylazo-8-hydroxyquinoline (5Ph8HQ) as the extractant (Scheme 1) because its synthesis and its complexing ability were described some time ago [10]. Among the existing synthetic routes to graft this molecule on a solid, we used the diazo coupling of HQ to p-amino phenyl silica [11]. One already knows from previous literature the protonation constants (pKa1 = 2.7 and pKa2 = 8.6) of the 5Ph8HQ grafted on silica gel [12] or of its sulfo-derivative soluble in water (pKa1 = 3.78 and pKa2 = 7.94) [13]. Some other information, such as the stability constant of the complex between the Ph8HQ ligand grafted onto silica gel and Cu2+ was also reported [12]. Moreover the influence of the surface potential on the keto–enol isomeric ratio and its consequences on the copper complexation was also by grafting 5Ph8HQ onto a silver electrode [14]. These data are important since they constitute the starting point for the present study dealing with the reactivity of our functionalized colloidal silica particles. Among the commercially available systems, the fumed silicas present important interest for such a study, as they are constituted of well defined monodispersed nanoparticles (14 nm for FS200 and 7 nm for FS390) covalently linked in bundles which are able to form stable colloidal dispersions [15], at least in their pristine form. We focused our efforts in this work on studying the extraction of copper and nickel by colloidal dispersions of variable amount FS200 and FS390 silicas organically modified with grafted 5Ph8HQ.

2. Materials and methods 2.1. Materials FS200 and FS390 are fumed silicas purchased from Sigma exhibiting specific surfaces of 200 and 390 m2 g−1 , respectively, according to the provider. Before use, they were calcined at 600 ◦ C for 5 h. After calcination the samples were stored in a 70% relative humidity atmosphere. Specific surface areas of 184 m2 g−1 and 343 m2 g−1 were experimentally determined by N2 gas adsorption volumetry (BET isotherm) for FS200 and FS390, respectively. The radii of the nanoparticles given by the manufacturer are 14 and 7 nm for FS200 and FS390, respectively. All other reagents were of analytical grade and used as received. 2.2. Preparation of 5-phenylazo-8-hydroxyquinoline grafted particles The grafting of 5-phenylazo-8-hydroxyquinoline onto pyrogenic silicas has been described in a preceding paper [8]. The amount of 5Ph8HQ grafted was typically 0.12 ± 0.02 mmol g−1 for FS200-5Ph8HQ and 0.15 ± 0.03 mmol g−1 for FS390-5Ph8HQ. 2.3. Preparation of colloidal suspension of modified particles Typically 20 mg of dry modified or unmodified silica were poured in a beaker, 10 mL of solution were added and sonication (Branson® 200w tip sonifier, 10 mm tip) was applied for 2 min. Less than 25% of the max power was delivered by the tip. To avoid overheating of the solution during sonication the beaker was maintained in a cool water bath. A slightly turbid suspension was obtained. 2.4. Characterization of the colloidal suspensions The hydrodynamic radius of the particles and zeta potentials were determined for FS200, FS390, FS200-5Ph8HQ and FS3905Ph8HQ using a Zetasizer 3000 apparatus (Malvern Instruments). For the dynamic light scattering experiments, no refraction index of the particles was taken into account, the mean average radii are thus based on the light intensity. 2.5. Metal ion content analysis The concentrations of metal ions in solution were quantitatively determined an Inductively Coupled Plasma-spectrometer (AES-ICP Jobin Yvon-Horiba, Ultima 2). 2.6. Ultrafiltration 2.6.1. Frontal ultrafiltration device Ultrafiltration experiments were conducted in 10 mL ultrafiltration cells (type Amicon 8010, PLGC10 ultrafiltration membranes). Typically 50% of 10 mL of silica suspension was filtered under stirring (300 rpm) and N2 pressure (4 bar).

M. Hébrant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

40

(a)

12

67

(b)

10

30 20

Y(%)

Y(%)

8

10

6 4 2

0

0 -2

-10 4,0

4,4

4,8 pH

5,2

3,4

5,6

3,6

Fig. 1. Adsorption yields of Cu2+ (a) and Ni2+ (b) on pristine silica particles as a function of pH. FS200: ( C0∗ = 50 ␮M (counter ion: acetate).

The amount of adsorbed metal ions is calculated as (it is assumed here that only mononuclear species are formed): [adsorbed] = [M2+ ]0 − [M2+ ]p

(1)

where [M2+ ]0 and [M2+ ]p are the initial and permeate analytical metal concentrations, respectively. In this equation, and throughout the following text, all concentrations indicated as [i] are calculated as a number of mole reported to the overall volume. Considering entities exclusively located at the surface of particles themselves representing less than 1% of the overall volume and calculating physico-chemical constants on the overall volume is the most practical way to proceed nevertheless it has to be kept in mind for any discussion of the obtained thermodynamic constants. Blank experiments were conducted to verify that no retention of metallic species was obtained with the membrane alone. Moreover, since silanol groups have a non negligible affinity for metal ion, we have determined the extraction yields observed for the pristine silica as a function of pH, in a range close to that investigated here for the functionalized silicas. The results are given in Fig. 1. At pH = 4 ± 0.1 no significant adsorption on pristine silica occurs with copper (compared to the experimental incertitude). In the case of Nickel ion, a small yield of 5% was observed but it will not be taken into account in the interpretation of the results with the modified silica for 2 reasons: first the experimental incertitude is at its maximum for low extraction yields, second in the ultrafiltration experiments, the Donnan effect can impact the extraction [16] depending on the ionization state of the silica surface which obviously is not be the same when considering the modified silica. This view is reinforced by the fact that the calculated yields do not depend neither on the amount of dispersed silica nor on its specific surface area. The negative yield values observed for copper and low pH in Fig. 1 may only be understood taking into account at the same time the yield definition (Section 2.6.2) and the fact that the Donnan effect may induce concentrations in the permeate higher than the initial one. 2.6.2. Extraction yield All along this paper, the extraction yield in ultrafiltration of microheterogeneous systems experiments is defined as: Y (%) = 100

[M2+ ]0 − [M2+ ]p [M2+ ]0

(2)

when liquid–liquid systems are considered, experiments being conducted with 1 volume of chloroform and 1 volume of water, the

3,8

) 0.5 g L−1 , (

4,0 pH

4,2

4,4

) 0.05 g L−1 ; FS390: (

4,6

) 0.5 g L−1 , (

) 0.05 g L−1 ;

calculation of the yield is the same except that [M2+ ]p is replaced by [M2+ ]aq in Eqs. (1) and (2). 2.6.3. Analyzing the results The curves “yields versus [LH]0 /[M2+ ]0 values” are fitted using a simple non linear least square procedure. Briefly, a complex stoichiometry is postulated, an initial value of Kext is considered for each experimental point. A theoretical yield is calculated taking into account the mass balance equation in both phases, the mass of the square balance in the permeate and the retentate. The sum errors between experimental and theoretical yields ( 2 ) is calculated. An iterative procedure (routine Solver from Excel software)  2 allows to minimize  by adjusting the Kext value. When comparing (as in Table 1) two sets of data for which  2 selected numbers  will be divided by of experiments have been performed the the number N of experiments. 3. Results 3.1. Characterization of colloidal suspensions 3.1.1. Hydrodynamic radius The stability of the colloidal suspension was described in a preceding paper [8]. It was proven by studying the hydrodynamic radius that the colloidal FS-5Ph8HQ suspensions are stable for over 24 h. At the time scale of the ultrafiltration experiment a simple UV–visible absorbance measurement allows to control that no settling occurs. The hydrodynamic radius ranging between 175 and 200 nm indicates that no dislocation of the bundle of particles of the pristine fumed silica occurs during the sonication or the various steps of synthesis. In the following, we will speak in term of “aggregate” to refer to these bundles of nanoparticles in suspension. As it can be seen from the top curve in Fig. 2 no significant effect of the extracted amount on the aggregate size can be seen. To obtain this result a sample was prepared and split in two parts: one was used to determine the hydrodynamic radius and the particle zeta potential whereas the second one was ultrafiltrated as indicated in the experimental part to determine the extraction yield. 3.1.2. Zeta potential Careful examination of the surface charge of the adsorbent is important to select the experimental conditions. Indeed, if the silica surface is negatively charged at the working pH, non selective binding of the positively charged metal ions could occur via favorable

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Table 1 Experimental extraction constants K  ext and sums of the square errors (divided by the number of experiments) of the best fits of the extraction data. Metal ion

Stoichiometry

Anion

FS200-5Ph8HQ K  ext

Cu2+

1:1 1:1

Ni2+ 2:1

FS390-5Ph8HQ ˙2 /N

TX100-Ph8HQ

 Kext

˙2 /N

 Kext

˙2 /N

AcO− Cl−

10.7 ± 2.0 9.5 ± 1.8

32.3 23.4

6.5 ± 0.9 3.8 ± 0.5

32.1 14.7

100.5

26.1

AcO− Cl− AcO− Cl−

0.8 ± 0.1 1.2 ± 0.2 1.83 4.34

32.8 42.6 53.8 123

0.8 ± 0.1 0.5 ± 0.1 2.05 1.2

30.2 13.9 24.2 34.3

4.4

78.5

electrostatic interaction and one could not distinguish between this process and complex formation at the ligand sites. The zeta potential was found to be close to zero at pH 4 for the pristine silicas used in this work. This pH value avoiding non-specific binding, as confirmed by the blank experiments, was thus selected for this study. Nevertheless, since the extractant itself is ionizable and the complex can be also charged, the electrokinetic behavior of the 5Ph8HQ-modified particles has been investigated in the course of the complexation process. It is clearly seen from Fig. 2 that the zeta potential of the modified particles is positive and that it does not change with the amount of extracted copper. This implies that no extraction is expected by a simple electrostatic effect and only Cu2+ (or Ni2+ ) complexation by the ligand is responsible for the extraction. In fact a small exclusion Donnan effect can be expected, potentially inducing metal ion concentration higher in the permeate than in the retentate at pH at which the extractant is not effective. This possibly leads to some negative calculated yields. This positive surface potential can be attributed to the partial protonation (≈10%) of the ligand as expected on the basis of  = 2.85 ± 0.1 and pK  = its apparent protonation constants (pKa1 a2 9.8 ± 0.3, see Fig. SM1). Supplementary material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2012.10.046. The fact that it remains constant is particularly important because it allows to calculate a conditional constant which is much more simple than to take into account the electrostatic potential variations that might also induce variation of local concentrations. The most straightforward and plausible explanation of this constancy is that the formed complex bears also one positive charge and that complex formation is more or less compensated by a proportional decrease of the number of protonated ligands.

3.2. Extraction of metal ion 3.2.1. Chemical stability of the particles After the colloidal stability, we checked the chemical stability of the modified chemical particles by checking both the amounts of extractant and those of silicon released in the permeate during the ultrafiltration. The absorbance ratio Apermeate /Aretentate at 450 nm (i.e. close to the maximum of absorbance of the ligand) in all the control experiments we performed with both FS200-5Ph8HQ and FS390-5Ph8HQ in the presence of whether 50 ␮M of copper or nickel acetate remained lower than 1.0%. Unexpectedly this ratio decrease with the amount of modified silica in the permeate. In other words, a maximum absorbance of 0.01 absorbance unit at 450 nm is reached in the permeate, as if a saturation level was observed. The leaking of surfactant through the membrane during a micellar ultrafiltration, frontal or tangential has been shown to be limited to the critical micelle concentration [17] nothing is known with silica particles. We decided to investigate the silicon release for two reasons: first the amount of matter released in the permeate is interesting by itself in view of potential environmental applications, second to understand if released silicon is proportional to the amount of released extractant or if it occurs in the same extent without chemical modification. The amount of silicon in the permeate is very limited. The solubility of amorphous silica (as that of silicic acid) is about 2 × 10−3 mol/L at pH < 6 and 298 K [18]. It is noticeable from Fig. 3 that there is no need to evoke colloidal silica species to explain the amount of silicon measured in the permeate, indicating that the nanoparticles are totally retained by the ultrafilter. It can also be seen that the amount of silicon in the permeate is slightly higher in the permeate with the modified silicas than with the pristine ones. This can be explained by the ionization state of the ligand, which generates a positive zeta potential. In such conditions it is known that the local pH (i.e. close to the interface) is higher, so the hydrolysis is expected to be enhanced.

100 200 80 160

ξ(mV)

120 40

80

20

0

Rh(nm)

60

40

0

10

20

30

40

Y(%)

50

60

0 70

Fig. 2. Hydrodynamic radius (Rh , right scale) and zeta potential (, left scale) of FS200-5Ph8HQ ( , ) and FS390-5Ph8HQ ( ) versus the copper extraction yield Y; pH = 3.92 ± 0.03; [Cu2+ ]0 = 50 ␮M; counter anion acetate.

3.2.2. Effect of the L/M ratio The experimental results for copper extraction by FS-5Ph8HQ are given together with their best fits in Fig. 4 while Fig. 5 is related to nickel extraction. High extraction yields are reached, up to 97% with copper at [LH]0 /[Cu2+ ]0 ≥ 3 for an initial metal ion concentration of 5 × 10−5 M. Taking into account a ratio [LH]0 /[Cu2+ ]0 ≈ 3 and the amount of 5Ph8HQ grafted on FS200-5Ph8HQ allows to calculated that the minimum modified silica concentration necessary to reach yields ≥97% is about 1.25 g L−1 . The silica having a density of ∼2.2, one can calculate a concentration factor (the ratio of the aqueous upon extracting phase volumes) of about 1700. This is of particular importance in view of an eventual applied cleanup process. Moreover the amount of 5Ph8HQ attached to the adsorbent is rather limited (0.15 mm/g at the maximum) in our experiments, thus this result could be even improved by using a more efficient way of grafting the extractant.

M. Hébrant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

0,10

(a)

0,16

0,08

0,12

0,06

[Si] filt(mM)

[Si] p(mM)

0,20

0,08 0,04 0,00

69

(b)

0,04 0,02

0

2

4

6

8

0,00

10

0

2

4

6

8

10

[Si]0(mM)

[Si]0(mM)

Fig. 3. Silicon released during the ultrafiltration as a function of the initial amount of silicon in the colloidal suspension: (a) Cu2+ , (b) Ni2+ () FS200-5Ph8HQ; ( ) is what is expected on the basis of the results pristine silicas FS200 and FS390. 5Ph8HQ; [Cu2+ ]0 = 50 ␮M or [Ni2+ ]0 = 50 ␮M; pH = 3.91 ± 0.11. The line (

100

100

(a)

80

60

40

R(%)

R(%)

(b)

80

60

40

20

20

0

0

-20 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

-20 0,0

4,0

0,5

1,0

Fig. 4. Yields of extraction of Cu2+ by (a) FS200-5Ph8HQ and (b) FS390-5Ph8HQ versus: ( ) counter ion acetate; ( that might be attained with a 2:1 stoichiometry complex. [Cu2+ ]0 = 50 ␮M; pH = 3.83 ± 0.07.

The anion present in the system may interact with the complex by entering in the complexation sphere of the metal ion but it may also play a role with the charged interface and thus modify the electrostatic potential. As may be seen from Fig. 4, there is no significant effect of exchanging the counter anions chloride and acetate for the extraction of copper with FS200-5Ph8HQ. Small differences may be seen between the other curves in Figs. 4 and 5. One taking into

2,5

3,0

3,5

4,0

100

(a)

80

80

60

60

40 20

) counter-ion chloride; (. . .) maximum extraction yields

account the experimental incertitude on both the metal ion concentration determinations and on the amount of extractant available in the suspension may hardly conclude on the significance of these differences. Moreover considering Fig. 5, the effect of exchanging the chloride against the acetate seems to be reversed between FS200 and FS 300. We finally assume that there is no significant effect of the counter ion in our experiments. This point is important since

R(%)

R(%)

2,0

[LH]0/[Cu ]0

[LH]0/[Cu ]0

0 0,0

1,5

2+

2+

100

) FS390-

(b)

40 20

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0 0,0

0,5

2+

[LH]0/[Ni ]0 Fig. 5. Yields of extraction of Ni2+ by (a) FS200-5Ph8HQ and (b) FS390-5Ph8HQ versus [LH]0 /[Ni2+ ]0 : ( pH = 3.85 ± 0.05.

1,0

1,5

2,0

2,5

3,0

3,5

4,0

2+

[LH]0/[Ni ]0 ) counter ion acetate; (

) counter-ion chloride; [Ni2+ ]0 = 50 ␮M;

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M. Hébrant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

100

Y(%)

80 60 40 20 0

0

1

2 3 2+ [LH]0/[Cu ]0

4

5

Fig. 6. Extraction yields of Cu2+ by 5Ph8HQ solubilized Triton X-100 micelles ( ) or in chloroform ( ) versus [LH]0 /[M2+ ]0 . The dashed line represents the maximum extraction yields that might be attained with complexes of 2:1 stoichiometries, respectively. [Cu2+ ]0 = 10 ␮M; [TX-100] = 2 × 10−2 M; pH = 3.99 ± 0.04; counter ion acetate.

stoichiometry were observed during the extraction of europium by pyrazolone ligands [20]. We have no clear explanation for this behavior; we can only notice that the complex 1:1 seems to be more stable than the 2:1 in TX-100 micelles. To explain the high efficiency of the 5Ph8HQ extractant solubilized in micelles, we determined its pKa ’s in this medium: the found values are pKa1 = 5.5 ± 0.1 and pKa2 = 9.1 ± 0.2. These values are very different from obtained for the same ligand grafted on FS200 (pKa1 = 2.85 ± 0.1 and pKa2 = 9.8 ± 0.3). This can be attributed to a solvation difference, the grafted molecule being in direct contact to the water surrounding the particles, with an influence of the positive electrostatic potential of the surface on the local proton concentration. No electrostatic effect is expected in TX-100 neutral micelles, moreover the molecules are, at least partly, solubilized in the low dissociating medium constituted by the oily core of the micelles. To further investigate this point the complex formation was studied in a CHCl3 –water biphasic system and typical results are reported in Fig. 6. In this case the best data fit obtained is by far by considering the formation of a 2:1 complex, with an extraction constant of Kext = 584. 4. Discussion

the species formed at the interface we consider in the following do not imply the counterion present in the system. The dotted line in the graphic is the representation of the function Y(%) = 1/2(100[LH]0 /[Cu2+ ]0 ), i.e. the maximum extraction that can be observed keeping the mass balance in mind. On the basis of this simple line, we were able to exclude the formation of a 2:1 complex at low [LH]0 /[Cu2+ ]0 ratio. This is not so surprising since most of the reported complexes with an extractant covalently bound on a silica are of stoichiometry 1:1 [19]. Moreover, one could remark that the number of complexing sites available is rather low in this work (see experimental part) and assuming a homogeneous distribution, one can estimate a maximum average value of 0.5 extractant molecule per nm2 . This makes unlikely the formation of a 2:1 complex for geometrical consideration. Figs. 4 and 5 do not exhibit an obvious difference on the yields when considering FS390 instead of FS200. This point will be discussed on the basis of Table 1 with the overall extraction data obtained in this work. Nickel extraction is less favorable than copper, the observed maximum extraction yield being 65%. This is at least partly expected when considering with the Irving-Williams series. Nevertheless the selectivity toward copper is particularly pronounced in our result suggesting that an eventual effect of the interfacial location of the extractant on the symmetry or the hydration of the formed complex would be more important with nickel than for copper. The low stability observed for the complex formed with nickel is the main distinction with the results relative to copper. The other difference is that the formation of a 2:1 complex cannot 2 /N be excluded as simply as for copper. In this case, even the does not allow to distinguish between 1:1 and 2:1 stoichiometry. Nevertheless as it would be hardly understandable to observe a 2:1 complex on silica when only the 1:1 complex is seen with copper, we consider that the 1:1 is by far the most likely. Two experimental facts are evidenced from the results depicted Fig. 6: (i) for a given [LH]0 /[M2+ ]0 ratio, the sorption yields are higher in Triton X micelles than with the modified silicas at the same pH; (ii) the stoichiometry of the copper complex formed is also the same in both systems. That the complex formed was a 1:1 with copper and 5Ph8HQ in TX-100 micelles was really surprising as in very similar conditions a 2:1 complex was observed in C11 -HQ (i.e. another hydrophobic derivative of 8-hydroxyquinoline) [16], and even higher

4.1. Extraction conditional constants and intrinsic affinity of the ligand to the metal ion The very first point we aim at discussing is the eventual effect of the medium composition on the affinity of the extractant toward the metal ion. The starting point of this discussion is based the experimental constants listed in Table 1. Considering that the anion is not implied in the complex formation, the equilibrium of formation of the 1:1 complex is: LHint + M2+ = LM+ + H+ int

(3)

Contrarily to what happens in biphasic liquid–liquid system in which the apolar organic phase imposes the complex formed to be neutral, microheterogeneous media (micelles, nanoparticles, etc.) allow to extract metal ion with ionic complexes [21]. In other words, the complete separation of the dispersed solid extracting phase from the bulk aqueous phase being not envisaged in ultrafiltration processes there is no need for electroneutrality in the colloidal solid phase. And the corresponding constant:  Kext =

[LM+ ][H+ ] int [LMint ][M2+ ]

(4)

 Kext should be understood as a conditional constant since a zeta potential of about 30 mV takes places at the silica particle surface. Applying a correction to the concentrations of the mobile species allows to calculate the thermodynamic constant:  Kext = Kext exp

 F  RT

(5)

This expression is only valid if the zeta potential can be used instead of the true electrostatic potential of the surface. Since the complexation site, owing to the geometry of the grafted molecule and its rigidity, at some distance of the surface this seems to be a fair approximation. Moreover this approximation was shown to fairly  values are in our apply with this ligand grafted on silica gel [12]. Kext view the most directly related to the experimental results, without any calculated correction, and thus the more reliable. Nevertheless, to compare our results with the literature or between different

M. Hébrant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 65–72

71

Table 2 K0 values in various systems. System 0

K a

Colloidal FS200-5Ph8HQ

Colloidal FS390-5Ph8HQ

5Ph8HQ in TX-100 micellesa

Sulfo 5Ph8HQ in water [13]

5Ph8HQ grafted on silica gel [12]

7.0 × 10

6.2 × 10

1.27 × 1011

1.4 × 1010

4.8 × 108

11

11

K0 in the TX-100 micelles was calculated using Eq. (6) and considering 0 as zeta potential.

systems, it was necessary to calculate K0 , the formation constant of the complex, from the L− form of the ligand at zero potential. K K 0 = ext exp Ka1

 2F  RT

(6)

One must bear in mind that K0 has been calculated taking into account concentrations relative to the overall volume instead of local concentration relative to the volume of each phase. The 1:1 stoichiometry is the only case in which this K0 value can be regarded as reflecting the intrinsic affinity of the ligand toward the metal ion. Would there be a 2:1 complex formed that a volume correction would be required. The K0 values in FS modified silica are slightly higher (a factor of ∼5) than the one obtained in TX-100; nevertheless, the extraction yields obtained in the micellar systems are higher than those with the FS-5Ph8HQ. This illustrates the high impact of the ionization constant on the extraction yields. The main feature that can be extracted from Table 2 is that grafting 5PH8HQ on colloidal silica can contribute to enhance its affinity for copper when comparing to the same ligand in water. The opposite was reported in the literature when 5Ph8HQ was grafted on silica gel having 60 A˚ pore size [12]. The only experimental difference is that in the present work we are separating the solid and liquid phases (at least partly) whereas the value reported in the literature was established in situ. In other words there is a factor of ∼1000 between K0 in colloidal FS and in silica gel system. To understand this unexpected result, neither the different zeta potential (12 mV against 30 in this work), nor the very different pKa2 (1.2 log unit) may be evoked, since they have already been accounted for. Nevertheless it is noticeable that considering the crude experimen , the value that can be calculated from Ref. [11] is tal result Kext differing by “only” a factor 20. Assuming the corrections to obtain K0 are valid we are led to think that two silicas can really generate very different local conditions. Such a difference is of the order of what might be observed by changing the solvent nature, it may thus indicate that the local polarity of the water is changing (and thus the solvation of the metal ion reaching the interface and of the extractant) when comparing the silica gel and the colloidal particles. This is the very first time to our knowledge that such an observation is done, this encourage us to explore other systems to go further in the understanding.  No sensitive influence of the particle size on Kext may be seen from Table 1 with both copper and nickel ions. Comparing results obtained with modified FS and those in TX-100 micelles, with both metals, the difference is in favor of the micelles. Considering the 2 /N with nickel in the micellar system one may conimportant   sider that the ratio of Kext in the modified FS systems and Kext in the micellar system is the same for copper and nickel.  The Kext value for Ni2+ calculated from an extraction curve reported for 5Ph8HQ grafted on silica gel in the literature [22] is 5.9. Considering that the experimental conditions (pH buffer, initial concentration of the metal ion) are not the same, we assume that it is rather comparable to our determination.  About a factor 10 distinguishes between the Kext for copper and nickel. This is of the order of what is expected from the literature data [22]. A complete selectivity study is in progress and will be the subject of a next paper.

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