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Competition between monovalent and divalent anions for calcined and uncalcined hydrotalcite: anion exchange and adsorption sites
COLLOIDS AND Colloids and Surfaces A: Physicochemicaland Engineering Aspects 111 (1996) 167-175
ELS EV I ER
A
SURFACES
Competition between monovalent and divalent anions for calcined and uncalcined hydrotalcite: anion exchange and adsorption sites L. Ch~ttelet a, j.y. Bottero a,b, j. Yvon a,., A. Bouchelaghem ¢ a L E M URA 235 CNRS, ENSG, INPL, BP 40, 54501 Vandoeuvre Cedex, France b CEREGE FU 17, LGE URA 132 CNRS, Universitk Aix-Marseille III, P61e EuromkditerranOen de l'Arbois, BP 80, 13767 Les Milles Cedex, France c I N E R T E C , 6 rue de Watford, BP 511, 92005 Nanterre Cedex, France
Received 6 July 1995; accepted 16 January 1996
Abstract
Hydrotalcite compounds are well-known anion exchangers. Their anionic exchange capacity (A.E.C.) increases with thermal treatment together with their specific surface area and porosity. In the presence of mono- and divalent anions such as CI-, SO4 2 - and CrO42- , calcined hydrotalcite behaves not only as an anion exchanger but also as an oxide adsorbent. In the case of divalent anions, the adsorbed quantity is larger than the A.E.C. It corresponds to 1.2 and 3 adsorbed molecules per nm 2 in the case of SO4 2 - and CrO42- respectively. This latter value suggests that CrO42precipitates at high equilibrium concentration. The affinity of each anion in mixed solutions (C1- and SO42-) also depends on their solubility. In this case, the greater affinity of CrO4 2- than SO42- in NaC1 electrolyte for calcined hydrotalcite is governed by its lower solubility. Keywords: Adsorption; Anionic exchange; Hydrotalcite; OH sites; Thermal treatment
1. Introduction Hydrotalcite is a natural layered mineral with a formula of Mg6A12(OH)16CO3"4H/O. The hydrotalcite group of compounds can be represented by the general formula [M2+I_xA13+x(OH)2 ] (A n- )x/nmH20 with M = Mg, A"- = C O 3 2 - , C 1 - , etc and 0 < x < 0 . 3 3 [ 1 - 3 ] . This family of cornpounds can be structurally characterized as containing brucite-like layers in which some divalent metal cations have been substituted by trivalent ions to form positively charged sheets. The cationic charge created in the layers is compensated by the presence of hydrated anions between the stacked
* Corresponding author, 0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(96)03542-X
sheets [4]. Different members of this family have already been synthesized with CO3 2- as an interlayer anion [3,5,6]. Other compensating anions such as SOa 2-, C1- and N O 3 have also been used [4,7,8]. Due to their structural positive charge, hydrotalcites are good anionic exchangers. U p o n thermal activation, they undergo dehydroxylation and decarbonation which increase their exchange capacities [7,9]. Heating also results in an increase in surface area and pore volume [5,10]. This decomposition is reversible if the calcination temperature does not exceed 500-600°C. The calcined product Mgl-xAlxO1 +x/2 can rehydrate and incorporate anions in order to rebuild the initial hydrotalcite structure. The ion exchange equilibrium constant for hydrotalcite-like compounds is greater for divalent anions than for
168
L. Chfitelet et al./Colloids SurJaces A: Physicochem. Eng. A~pects 111 (1996) 167-175
monovalent anions and the exchange affinity for CO32 is the highest [10,11]. This work tries to evaluate the different retention mechanisms of monovalent and divalent anions on unactivated and thermally activated hydrotalcites. The location of adsorption sites and the competition effects between adsorbates are also analyzed,
2. Experimental 2.1. Materials
The Mg A1-CO3 hydrotalcite was prepared according to the conditions described by Reichle et al. [6]. 500 ml of an aqueous solution of 3.50 mol 1-1 N a O H and 0.94 mol 1-1 NazCO3 was added dropwise to 350 ml of an aqueous solution of 1 mol 1-1 Mg(NO3)2"6H20 and 0.5 tool 1-1 Al(NO3)3 "9H20 under vigorous stirring. During synthesis, the temperature was maintained at 35 ° C. The resulting precipitate was heated at 65 _+ 5°C for 18 h with stirring. The slurry was treated by successive cycles of washing and centrifuging, using deionised water, until the resulting product content in sodium was less than 0.1 wt.%. After freezedrying, the final material is a fine white powder, called H T in this work. The thermally activated product was obtained by heating at 450°C for 18 h. The resulting solid is named HTC. 2.2. Experimental techniques
The hydrotalcite-like compounds were char acterized by different methods. X-ray powder diffraction (XRD) spectra were obtained on a Jobin Yvon Sigma spectrometer working by reflection, at a wavelength of 2 = 1.5406 A. Infrared spectra were obtained on a Bruker IFS88 Fourier transform infrared spectrometer equipped with a diffuse reflectance Harrick attachment. Chemical analyses were carried out by emission spectrometry on a Jobin-Yvon 70 quantometer with inductively-coupled plasma. The zeta potential was evaluated using a LaserZee Meter 501 Pen Kern electrophoresis apparatus. The volume fraction of solid employed is 40-50 ppm.
Specific surface areas were deduced from the nitrogen adsorption isotherm at 77 K treated according to the BET method. Prior to the measurement, the samples were outgassed at l l 0 ° C under a residual pressure of 0.1 Pa for 24 h. In the case of HT, an adsorption-desorption isotherm of water vapor at 303 K was also obtained using a quasi-equilibrium gravimetric device built around a Setaram MTB 10-8 symmetrical microbalance. Water vapor was supplied to the sample from a source kept at 45°C at a slow flow rate through a Granvill~Phillips leak valve to ensure quasiequilibrium conditions at all times [12]. Prior to the measurement, the sample was outgassed at 130°C under a residual pressure of 0.1 Pa for 18 h. All anion adsorption isotherms were determined following the same procedure. A volume of 100 ml of 1:1 and/or 1:2 electrolytes was mixed with 100 mg of solid. The tubes were agitated for 24 h at 30°C. This time is sufficient to reach the adsorption equilibrium [ 11 ]. The suspensions were then centrifuged and the equilibrium concentrations in the supernatant were measured. The concentration in CrO4 2 was determined from the intensity of the absorption at 456 nm using a Shimadzu spectrophotometer UV-2100. The amounts of C1 and SO42 were measured using a Waters Quanta TM 4000 capillary electrophoresis equipment. The initial quantity Qi is defined by Qi =(Ci x 100)/m, whereas the adsorbed amount Qads is calculated by Qads = (Ci - Ce) x 100/m, where m (mg) is the solid mass, Ce (mg 1-1 or mmol 1- l) is the equilibrium concentration, Q~ is the initial quantity of anion (mg g- 1 or meq g- ~) and Q~d~is the adsorbed quantity on the solid (mg g - i , meq g 1 or mmol g-l).
3. Results 3.1. Chemical analyses, X R D patterns and specific surface area determination
Chemical compositions of H T and HTC are listed in Table 1. The calculated formulae are: HT: [Mgo.6vAlo.33(OH)z](CO3)o.16.4.8HzO HTC: Mgo.67 Alo.33 O1.16
169
L . Chdtelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175
Table 1 Chemical analysis and properties of HT and HTC Chemical composition
tion is then preserved in H T C [ 11 ] which belongs tO the solid solution M g O - A l z O 3. The BET specific surface areas increase u p o n calcination from 103 to 258 m 2 g - i for H T and H T C respectively (Table 1).
Sample HT (wt.%) HTC (wt.%)
MgO A1203 Na20 Total CO2 Total HzO
33.4 20.9 0.02 9.1 37
Specific area
N 2 adsorption (m2 g-l) Water vapor adsorption (m2 g l)
54.7 33.9 0.07 1.6 10.3
103 805
3.2. Diffuse Reflectance Infra-Red Spectroscopy "DRIFT)
258 n.d.
A.E.C. 330
(meq tool l)
(meqg 1)
2.1
The theoretical anionic exchange capacity (A.E.C.) of H T after calcination and expulsion of H 2 0 is 0.33 eq mol - i or 2.1 meq g - 1 . The X R D patterns of the samples are displayed in Fig. 1. H T presents typical peaks of hydrotalcite. H T C presents a peak characteristic of M g O associated with A1203 (the peak shifts to a higher position and exhibits a little shoulder). The Mg-A1 substitu-
The infrared spectra of H T and H T C are shown m Fig. 2. For HT, the b r o a d band between 3800 and 3 0 0 0 c m -1 is a composite one that results from an overlapping of h y d r o g e n vibrations: stretching vibrations of structural O H - , physically adsorbed water, and vibrations of O H - b o n d e d with carbonate ions in M g and A1 environments. The shoulder at 3 0 5 0 c m - t can be assigned to solvation water molecules highly condensed into the microporosity [ 13]. The low-intensity b a n d at 1591 cm -~ is assigned to bending vibrations of strongly adsorbed water (solvation water of compensating anions). In an asymmetrical environ~
,
,
I.
HTC H.
•
i/
I 20
0
I ,S
I
\, "~.
I s
io
I:LT£,_
] AI203
~\
MgO
'd ~
,\
y
0'~
2~
"
20
I 15
I Io
Fig. 1. XRD patterns of HT and HTC.
6'I000
f i
/
I i
I
//
/
HT
23'00
Wavenumber ¢m "1
Fig. 2. DRIFT spectra of HT and HTC.
600
170
L. Chgttelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167 175
ment, v2 for physisorbed water is shifted toward lower absorption wavenumbers [ 14]. In a symmetric environment, the carbonate anion can be characterized by three bands with frequencies close to 1415cm -1 (v3), 880cm -1 (v2) and 680cm 1 (v4) [15]. In the H T sample, these bands shift to lower frequencies (v3 is observed at 1366 cm 1). This can be explained by a rearrangement of CO32- in the interlayer space in the presence of water molecules. After heating, significant changes can be observed. First, there is a decrease in intensity of almost all bands (around 60%). The band assigned to v3 carbonate vibration splits into two peaks at 1479cm -~ and 1406cm -1. It can be assigned to a change in anion symmetry according to a Mg or A1 environment. This loss of symmetry induces an activation of the v I carbonate vibration around 1050 cm-1. The bands or shoulders at 3700 cm-1 and 3750cm -1 are assigned to O H groups in magnesium oxide environments [ 15]. The v2 vibration of adsorbed water shifts from 1591 cm -1 to
1637 cm-1. Therefore, the IR spectrum of H T C appears to be a combination of the characteristic infrared bands of hydrotalcite and Mg-A1 oxides. Zeta potential measurements Zeta potential was measured as a function of pH in various electrolytes (demineralized water, NaC1, Na/CO3, Na2SO 4, Na2CrO4). Electrophoretic curves are shown in Fig. 3. The IEPs (isoelectric points) of the materials in different electrolytes are listed in Table 2. There is no change in I E P ( p H ~ l l . 1 2 ) with 0.01 M NaC1. With 1:2 electrolytes, the I E P decreases to p H i 9 for SO4/- and p H i 6 . 5 for C032 and CrO42-.
3.4. Retention of anions on H T C 3.4.1. Pure solutions On carbonate-saturated HT, the adsorption of C r O 4 2 - (and also C1- and SO4 z - ) is very weak. The adsorption isotherms vs. Qi of pure solutions of C I - , SO4 2- and C r O 4 2 - o n H T C are shown in Fig. 4. For divalent anions, adsorption is complete. There is no difference between SO4 2- and CrO4 2-
. . . . . . . . . . . .
; w . ~:~2. e Na2S0410-2M . ~o4,~ NlCl
40
~
lC-2 M
~ ~0 ~o 2o "
~ •
1
,
,
i
,
i
7, .
J
,
i
~ ~ . . . . . . . Zeta
.
of HT
potential
. . . . . . . . . . . . .
: .~. . NaC110-2 ~ , ~ 2M~ • ~so,,~2,
= ~ ~ ~° ~ ,0
o
N~C¢O410-2N
° '° . . . . . . . . . . . . . 4
s
pH
a
lo Zeta
12
14
potential of HTC
Fig. 3. Zeta potential of HT and HTC vs. pH in different electrolytes (H20 , NaC1, NazCO 3, Na2SO4 and NazCrO4). Table 2 IEP values for HT and HTC in different electrolytes
Nature of electrolyte
Water 0.01 M NaCI
O.Ol M NazSO4
0.01 M Na2CO 3 0.01 M Na2CrO 4
Sample HT
HTC
(IEP value)
(pH units)
11.1
10.9
> 11.1
> 11 8.9 6.6 6.5
7.6 7.8 6.7
3.4.2. Mixed solutions Chloride and sulfate anions were used for evaluating the competition effects considering that SO42- and CrO42- have the same behavior at low concentrations (Fig. 5). The isotherms were obtained from pure C1- or SO42- solutions and from mixed solutions containing 100 mg 1-1 of SO4 2 and C1- respectively. The sorption of SO4 2- is slightly decreased when C1 is present. The sorption of C1- is strongly
171
L. Chdtelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 120 ,
1000
Hydrolalcit¢HT
t 100
~
80 1
~
"~ E 60 ) -z
•
•
•
o-~---~ 0
o HT CrO4
40
experimental temperature 30°C
700 i
•
6(X)
~.---+ q ? , o, ,
20
800
* HTC Cr04
* * *~
outgassing at 130°C during lgh
D HTC SO4
~
20 q
:
• HTCC1
• *
O' 40 ~
Samplepreparation
900
60
80
100
,
~ 50o
120
Qi (mg/g)
~
Fig. 4. Adsorption, Qaas (rag g t), of Cl , SO42-, CrO4 zonto HTC and CrO4 2 o n t o HT vs. Qi (5-120 mg g 1).
400
~
--
3oo
adsorption
--desorption
200 I IX)
Illlliil I I III
,,!?111.. JJ
40
• Cl
li~
2f)
0 4---
0,1
Ill 1
@ Cl + sO4 t00 mg/l
I0
100
Ce (mg/l)
Fig. 5. Co-adsorption isotherms of C1 and SO42- onto HTC vs. the equilibrium concentration, C~ (mg 1-~). decreased in the presence of S O 4 2 - . The adsorption of divalent anions onto H T C is not strongly affected by the presence of monovalent anions. On the contrary, divalent anions inhibit the adsorption of monovalent anions, 3.5. Adsorption o f water vapor on H T
Fig. 6 shows the adsorption-desorption isotherm of water vapor. For HT, the shape of the isotherm reveals the presence of very thin micropores and large mesopores. The t-plot treatment applied to this isotherm using the values given by Hagymassy et al. [16] is displayed in Fig. 7. Assuming a cross-sectional area of 0.148 nm z for the water molecule, it yields a total specific area of 805 m 2 g ~. Two kinds of micropores are observed, The quantity of micropores observed, 167.5 cm 3 g-~ and 189 cm 3 g-~ for the two classes of pores,
0
PIP•
Fig. 6. Isotherm of water vapor adsorption on HTC at 30° C.
reveals a swelling of H T adsorption.
upon
water vapor
4. Discussion The structures of H T and H T C suggest the presence of two kinds of anion retention sites: sites within the interlayer corresponding to the structural A.E.C. of the material and adsorption onto external surfaces. The nature of these retention sites for different anions can be approached through the study of the variation of zeta potential with pH in the presence of various electrolytes (Fig. 3). Adsorption on the external surfaces should induce changes in zeta potential. In water, the zeta potential decreases slightly for increasing pH up to values near the I E P where a sharp decrease is observed. It must be pointed out that the I E P is close to that of MgO. The zeta evolution can then be assigned to the dissociation of mainly Mg(OH)2 and some AI(OH)2 + sites. When NaC1 is used as an electrolyte, very little difference is observed, suggesting that all the changes (if any) induced by the presence of C1- anions are not occurring on the external surfaces of HT. On the contrary, the presence of
L. Chdtelet et al./Colloids Surfaces A. Physicochem. Eng. Aspects 111 (1996) 167 175
172
i Vadscm3/g
3~.
j
250
200. mica, mi¢I
So
576.3mZ/g 80A.Tm2/g VmicI 167.5cm3/g
150
S out mic
108 - 150
Vmic1I
189cm3/g
Soutmic 70.1-98 100
50 •
,
,
_-
Fig. 7. t-Plot transformation of the isotherm represented in
Fig. 6. CO32-, SO42- or CrO42- strongly affects the zeta potential on H T and HTC. The IEP is reduced to 6.5 (CO32- or CrO4 2-) or 8 (8042-). This suggests that some adsorption of these divalent anions occurs on the external surfaces, Some more insights into the location of anion retention sites can be extracted from the D R I F T experiments displayed in Fig. 8a-c corresponding to HTC before and after adsorption of CI-, SO42and CrO42- respectively. In all cases, the intensity of the band corresponding to interlayer CO32Table 3 Evolution of the relative concentration of CO 2concentrations Anion Initial quantity (ppm) v3 CO 2- (Abs. units) % Removal
Cl5 8.54 0
El50 6.74 21
Cl100 6.18 28
(Y3=1366 cm-1) frO 2 250 3.09 78
CrO~ 500 4.12 70
anions decreases (Table 3) which reveals some exchange of interlayer anions. The adsorption on the external surfaces evidenced from zeta potential experiments does not provoke any changes in the OH bonds in the range 3500-3700cm -1. This suggests the formation of outer sphere complexes. Quantitative information can be obtained from the adsorption isotherms of anions on HTC presented in Fig. 9. In the case of C r O 4 2 - , the adsorption is linear for Qads <2.1 meq g ~, which indicates an exchange mechanism. For Qaas >~2.1 meq g-1, the isotherm is no longer linear; it reaches a plateau for an adsorbed amount of 4.5 meq g-1. The break in the isotherm for Qad~ corresponds exactly to the A.E.C. of HTC. The second part of the isotherm can then be assigned to the adsorption on external sites with partially variable charge. It corresponds to an adsorption amount of 2.6 meq g-X (1.3 mmoles of CrO4) onto sites. If this value is normalized to the specific surface area of 258 m 2 g-1 obtained after heating (Table 1) the surface density o f C r O 4 molecules is ~ 3 nm -2. This value appears to be much too large. However, due to swelling, it seems more appropriate to use the specific surface area measured from water vapor adsorption (805 m 2 g 1). The surface density of C r O 4 molecules would then be ~ 1 nm -2. This number corresponds to the average value of the site density on oxides 1-17]. For SO4 2- , the isotherm is similar but the maximum amount adsorbed is lowered (3.2 meq g-l). The surface density of SO4 molecules is ~0.4 nm 2. For CI-, the maximum adsorbed quantity is 1.7 meq g - l , lower than the A.E.C., which confirms the absence of adsorbed CI- on the external faces shown by zeta measurements.
calculated from D R I F T spectra vs C I - , SO 2 and CrO42 initial
f r O 21000 4.25 69
SO 2100 4.87 43
SO 2 200 3.44 60
SO~250 4.12 52
SO 2500 3.57 58
SO42 750 3.36 60
SO 21000 4.23 50
173
L . Chgttelet et al./ Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 t.-
c~
4O00 (a)
~
S04 750 ppm
~
23~0 w ......
b. . . .
'
600
4000
-1
23; w ......
(b)
'
600
ber cm'l
e4
I
1000 ppm
Cr04 500 ppm
,n "E
,= <
Cr04 250 ppm
Cr04 50 ppm
j
J
J 4(3(/)
(C)
230¢,)
'
600
Wavenumber cm -1
Fig. 8. DRIFT spectra of HTC before and after adsorption of C1- (a), 8 0 4 2 - (b) and C r O 4 2 4.1. A d s o r p t i o n affinity
Hydrotalcite-like compounds exhibit a greater affinity toward divalent anions [10,11]. This
(c)
at Qi in the 5- 1000 mg g-1 range.
trend is confirmed in Fig. 10 which shows the adsorption of CI-, SO42- and CrO4 2- in terms of Qads (mmolg -1) vs. the equilibrium concentration Ce (mmol 1-1). The position of the isotherms
174
L. Chdtelet et al./Colloids Surfaces A." Physicochem. Eng. Aspects 111 (1996) 167 175
Qad~i
(total adsorption)
Q....
•
onto sites on which ] 2"6mext/gads°rbed charge partly variab e
~
4,Tmeq/g
4
'!
" " " •
!
•
@ 1 , 0 ..............I-"-. . . . E E 1)8 "-
V---
CY 0,6
o
AEC 2,1 meq/g
•
1
it
•
"
"
11,4
" 0'0 0 2
, 10
i 20
Qi (meq/g)
D SO4
o C, • CI + SO4 100 mg/l
I
I
•
10 l
10o
C~Cs 30
Fig. 9. Adsorption isotherms of CrO42 , CI- and HTC vs. Qi (meq g-l}.
2,.,
SO42-
onto
2 °°
.'..
Fig. 11. Adsorption isotherms in mixed CI (mmol g-t) vs. log Co/Cs.
/SO42-
solution
co-anion, the more the isotherm is shifted toward the limit of solubility.
.,
"
1,,
no
HI
•
I
0
l
t,2
-".
iI
g ,
....
. ..,.....-s 0
0,00001 0,0001
0,001
0,01
5.
.................... 0,1
I
10
100
Ce(mmom) Fig. 10. Adsorption isotherms of CI-, SO42 and CrO42(mmol g ' vs. Co (retool 1 1).
Conclusion
The retention of monovalent or divalent anions such as C1 , SO42- or CrO42- on hydrotalcite depends on • the A.E.C. value of the solid; it increases with thermal treatment; • the presence of O H sites (namely MgOH); they are located on the external surfaces since the
on the abscissa gives some indication of the affinity, The affinity of CrO42- and SO42 for HTC is very similar at low concentrations (anionic exchange domain), as shown in Fig. 5, and higher than that of monovalents. Once the anionic capacity is completed, more CrO42- ions adsorb cornpared to SO4 z . This difference could be due to some precipitation of chromate anions occurring with Mg 2+ or Mg (OH) + cations. The exchange of monovalents for divalents can be analyzed in terms of mixed solubility of anions. Following the solubility data [18], Fig. 11 shows the adsorption of CI- and SO42 alone and in mixed solutions vs. the solubility of C1 in Na2SO 4 solution and SO42- in NaC1 solution. The affinity of each anion clearly depends on the differential solubility. The higher the concentration of the
adsorption of anions such as SO42- and CrO42shifts the I E P toward the acid domain; • the presence of structural CO32- in uncalcined hydrotalcite which is hardly exchanged. In mixed solutions the affinity of each component depends on its solubility in the mixture. Therefore the affinity decreased in the order C r O 4 2 - > SO42- >monovalents.
References [1] R. Allman and H.P. Jepsen, Neues Jahrb. Mineral., Monatsh, H12 (1969)544. [2] R. Allman, Acta Crystallogr., Sect. B,24 (1968) 972. [3] s. Miyata and T. Kumura, Chem. Lett., (1973) 843. [4] V.R.L Constantino and T.J. Pinnavaia, Inorg. Chem., 34 (1995) 883. [5] H.F.W. Taylor, Miner. Mag., 39 (19731 377.
L. Chfttelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 [6] W.T. Reichle, S.Y. Kang and D.S. Everhardt, J. Catal., 101 (1986) 352. [7] S. Miyata, Clays Clay Miner., 23 (1975)369. [8] S. Miyata and A. Okada, Clays Clay Miner., 125 (1977) 14. [9] S. Miyata, T. Kumura, H. Hattori and K. Tanabe, Nippon Kagaku Zasshi, 92 (1971) 514. [10] S. Miyata, Clays Clay Miner., 28 (1980) 50. [11] T. Sato, T. Wakabayashi and M. Shimada, Ind. Eng. Chem., Prod. Res. Dev., 25 (1986) 89. [12] J. Rouquerol and L. Davy, Thermochim. Acta, 24 (1978) 391. [13] A. Burneau, O. Barr6s, J.P. Gallas and J.C. Lavalley, Langmuir, 6 (1990) 1364.
175
[14] M.J. Hernandez-Moreno, M.A. Ulibarri, J.L. Rendon and C.J. Serna, Phys. Chem. Miner. 12 (1985) 34. [15] M. Del Arco, C. Martin, I. Martin, V. Rives and R. Trujillano, Spectrochim. Acta, Part A, (1993) 1575. [16] J. Hagymassy, S. Brunauer and R.S. Mikhail, J. Colloid Interface Sci., 29(1969)485. [17] W. Stumm and J.J. Morgan, Aquatic Chemistry--An Introduction Emphasizing Chemical Equilibria in Natural Waters, 2nd edn., John Wiley, New York, 1981, Chapter 5. [18] Linke and Seidell, Handbook of Solubilities of Organic and Inorganic Compounds, 4th edn., American Chemical Society, Washington, DC, 1958, p. 931.
ELS EV I ER
A
SURFACES
Competition between monovalent and divalent anions for calcined and uncalcined hydrotalcite: anion exchange and adsorption sites L. Ch~ttelet a, j.y. Bottero a,b, j. Yvon a,., A. Bouchelaghem ¢ a L E M URA 235 CNRS, ENSG, INPL, BP 40, 54501 Vandoeuvre Cedex, France b CEREGE FU 17, LGE URA 132 CNRS, Universitk Aix-Marseille III, P61e EuromkditerranOen de l'Arbois, BP 80, 13767 Les Milles Cedex, France c I N E R T E C , 6 rue de Watford, BP 511, 92005 Nanterre Cedex, France
Received 6 July 1995; accepted 16 January 1996
Abstract
Hydrotalcite compounds are well-known anion exchangers. Their anionic exchange capacity (A.E.C.) increases with thermal treatment together with their specific surface area and porosity. In the presence of mono- and divalent anions such as CI-, SO4 2 - and CrO42- , calcined hydrotalcite behaves not only as an anion exchanger but also as an oxide adsorbent. In the case of divalent anions, the adsorbed quantity is larger than the A.E.C. It corresponds to 1.2 and 3 adsorbed molecules per nm 2 in the case of SO4 2 - and CrO42- respectively. This latter value suggests that CrO42precipitates at high equilibrium concentration. The affinity of each anion in mixed solutions (C1- and SO42-) also depends on their solubility. In this case, the greater affinity of CrO4 2- than SO42- in NaC1 electrolyte for calcined hydrotalcite is governed by its lower solubility. Keywords: Adsorption; Anionic exchange; Hydrotalcite; OH sites; Thermal treatment
1. Introduction Hydrotalcite is a natural layered mineral with a formula of Mg6A12(OH)16CO3"4H/O. The hydrotalcite group of compounds can be represented by the general formula [M2+I_xA13+x(OH)2 ] (A n- )x/nmH20 with M = Mg, A"- = C O 3 2 - , C 1 - , etc and 0 < x < 0 . 3 3 [ 1 - 3 ] . This family of cornpounds can be structurally characterized as containing brucite-like layers in which some divalent metal cations have been substituted by trivalent ions to form positively charged sheets. The cationic charge created in the layers is compensated by the presence of hydrated anions between the stacked
* Corresponding author, 0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(96)03542-X
sheets [4]. Different members of this family have already been synthesized with CO3 2- as an interlayer anion [3,5,6]. Other compensating anions such as SOa 2-, C1- and N O 3 have also been used [4,7,8]. Due to their structural positive charge, hydrotalcites are good anionic exchangers. U p o n thermal activation, they undergo dehydroxylation and decarbonation which increase their exchange capacities [7,9]. Heating also results in an increase in surface area and pore volume [5,10]. This decomposition is reversible if the calcination temperature does not exceed 500-600°C. The calcined product Mgl-xAlxO1 +x/2 can rehydrate and incorporate anions in order to rebuild the initial hydrotalcite structure. The ion exchange equilibrium constant for hydrotalcite-like compounds is greater for divalent anions than for
168
L. Chfitelet et al./Colloids SurJaces A: Physicochem. Eng. A~pects 111 (1996) 167-175
monovalent anions and the exchange affinity for CO32 is the highest [10,11]. This work tries to evaluate the different retention mechanisms of monovalent and divalent anions on unactivated and thermally activated hydrotalcites. The location of adsorption sites and the competition effects between adsorbates are also analyzed,
2. Experimental 2.1. Materials
The Mg A1-CO3 hydrotalcite was prepared according to the conditions described by Reichle et al. [6]. 500 ml of an aqueous solution of 3.50 mol 1-1 N a O H and 0.94 mol 1-1 NazCO3 was added dropwise to 350 ml of an aqueous solution of 1 mol 1-1 Mg(NO3)2"6H20 and 0.5 tool 1-1 Al(NO3)3 "9H20 under vigorous stirring. During synthesis, the temperature was maintained at 35 ° C. The resulting precipitate was heated at 65 _+ 5°C for 18 h with stirring. The slurry was treated by successive cycles of washing and centrifuging, using deionised water, until the resulting product content in sodium was less than 0.1 wt.%. After freezedrying, the final material is a fine white powder, called H T in this work. The thermally activated product was obtained by heating at 450°C for 18 h. The resulting solid is named HTC. 2.2. Experimental techniques
The hydrotalcite-like compounds were char acterized by different methods. X-ray powder diffraction (XRD) spectra were obtained on a Jobin Yvon Sigma spectrometer working by reflection, at a wavelength of 2 = 1.5406 A. Infrared spectra were obtained on a Bruker IFS88 Fourier transform infrared spectrometer equipped with a diffuse reflectance Harrick attachment. Chemical analyses were carried out by emission spectrometry on a Jobin-Yvon 70 quantometer with inductively-coupled plasma. The zeta potential was evaluated using a LaserZee Meter 501 Pen Kern electrophoresis apparatus. The volume fraction of solid employed is 40-50 ppm.
Specific surface areas were deduced from the nitrogen adsorption isotherm at 77 K treated according to the BET method. Prior to the measurement, the samples were outgassed at l l 0 ° C under a residual pressure of 0.1 Pa for 24 h. In the case of HT, an adsorption-desorption isotherm of water vapor at 303 K was also obtained using a quasi-equilibrium gravimetric device built around a Setaram MTB 10-8 symmetrical microbalance. Water vapor was supplied to the sample from a source kept at 45°C at a slow flow rate through a Granvill~Phillips leak valve to ensure quasiequilibrium conditions at all times [12]. Prior to the measurement, the sample was outgassed at 130°C under a residual pressure of 0.1 Pa for 18 h. All anion adsorption isotherms were determined following the same procedure. A volume of 100 ml of 1:1 and/or 1:2 electrolytes was mixed with 100 mg of solid. The tubes were agitated for 24 h at 30°C. This time is sufficient to reach the adsorption equilibrium [ 11 ]. The suspensions were then centrifuged and the equilibrium concentrations in the supernatant were measured. The concentration in CrO4 2 was determined from the intensity of the absorption at 456 nm using a Shimadzu spectrophotometer UV-2100. The amounts of C1 and SO42 were measured using a Waters Quanta TM 4000 capillary electrophoresis equipment. The initial quantity Qi is defined by Qi =(Ci x 100)/m, whereas the adsorbed amount Qads is calculated by Qads = (Ci - Ce) x 100/m, where m (mg) is the solid mass, Ce (mg 1-1 or mmol 1- l) is the equilibrium concentration, Q~ is the initial quantity of anion (mg g- 1 or meq g- ~) and Q~d~is the adsorbed quantity on the solid (mg g - i , meq g 1 or mmol g-l).
3. Results 3.1. Chemical analyses, X R D patterns and specific surface area determination
Chemical compositions of H T and HTC are listed in Table 1. The calculated formulae are: HT: [Mgo.6vAlo.33(OH)z](CO3)o.16.4.8HzO HTC: Mgo.67 Alo.33 O1.16
169
L . Chdtelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175
Table 1 Chemical analysis and properties of HT and HTC Chemical composition
tion is then preserved in H T C [ 11 ] which belongs tO the solid solution M g O - A l z O 3. The BET specific surface areas increase u p o n calcination from 103 to 258 m 2 g - i for H T and H T C respectively (Table 1).
Sample HT (wt.%) HTC (wt.%)
MgO A1203 Na20 Total CO2 Total HzO
33.4 20.9 0.02 9.1 37
Specific area
N 2 adsorption (m2 g-l) Water vapor adsorption (m2 g l)
54.7 33.9 0.07 1.6 10.3
103 805
3.2. Diffuse Reflectance Infra-Red Spectroscopy "DRIFT)
258 n.d.
A.E.C. 330
(meq tool l)
(meqg 1)
2.1
The theoretical anionic exchange capacity (A.E.C.) of H T after calcination and expulsion of H 2 0 is 0.33 eq mol - i or 2.1 meq g - 1 . The X R D patterns of the samples are displayed in Fig. 1. H T presents typical peaks of hydrotalcite. H T C presents a peak characteristic of M g O associated with A1203 (the peak shifts to a higher position and exhibits a little shoulder). The Mg-A1 substitu-
The infrared spectra of H T and H T C are shown m Fig. 2. For HT, the b r o a d band between 3800 and 3 0 0 0 c m -1 is a composite one that results from an overlapping of h y d r o g e n vibrations: stretching vibrations of structural O H - , physically adsorbed water, and vibrations of O H - b o n d e d with carbonate ions in M g and A1 environments. The shoulder at 3 0 5 0 c m - t can be assigned to solvation water molecules highly condensed into the microporosity [ 13]. The low-intensity b a n d at 1591 cm -~ is assigned to bending vibrations of strongly adsorbed water (solvation water of compensating anions). In an asymmetrical environ~
,
,
I.
HTC H.
•
i/
I 20
0
I ,S
I
\, "~.
I s
io
I:LT£,_
] AI203
~\
MgO
'd ~
,\
y
0'~
2~
"
20
I 15
I Io
Fig. 1. XRD patterns of HT and HTC.
6'I000
f i
/
I i
I
//
/
HT
23'00
Wavenumber ¢m "1
Fig. 2. DRIFT spectra of HT and HTC.
600
170
L. Chgttelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167 175
ment, v2 for physisorbed water is shifted toward lower absorption wavenumbers [ 14]. In a symmetric environment, the carbonate anion can be characterized by three bands with frequencies close to 1415cm -1 (v3), 880cm -1 (v2) and 680cm 1 (v4) [15]. In the H T sample, these bands shift to lower frequencies (v3 is observed at 1366 cm 1). This can be explained by a rearrangement of CO32- in the interlayer space in the presence of water molecules. After heating, significant changes can be observed. First, there is a decrease in intensity of almost all bands (around 60%). The band assigned to v3 carbonate vibration splits into two peaks at 1479cm -~ and 1406cm -1. It can be assigned to a change in anion symmetry according to a Mg or A1 environment. This loss of symmetry induces an activation of the v I carbonate vibration around 1050 cm-1. The bands or shoulders at 3700 cm-1 and 3750cm -1 are assigned to O H groups in magnesium oxide environments [ 15]. The v2 vibration of adsorbed water shifts from 1591 cm -1 to
1637 cm-1. Therefore, the IR spectrum of H T C appears to be a combination of the characteristic infrared bands of hydrotalcite and Mg-A1 oxides. Zeta potential measurements Zeta potential was measured as a function of pH in various electrolytes (demineralized water, NaC1, Na/CO3, Na2SO 4, Na2CrO4). Electrophoretic curves are shown in Fig. 3. The IEPs (isoelectric points) of the materials in different electrolytes are listed in Table 2. There is no change in I E P ( p H ~ l l . 1 2 ) with 0.01 M NaC1. With 1:2 electrolytes, the I E P decreases to p H i 9 for SO4/- and p H i 6 . 5 for C032 and CrO42-.
3.4. Retention of anions on H T C 3.4.1. Pure solutions On carbonate-saturated HT, the adsorption of C r O 4 2 - (and also C1- and SO4 z - ) is very weak. The adsorption isotherms vs. Qi of pure solutions of C I - , SO4 2- and C r O 4 2 - o n H T C are shown in Fig. 4. For divalent anions, adsorption is complete. There is no difference between SO4 2- and CrO4 2-
. . . . . . . . . . . .
; w . ~:~2. e Na2S0410-2M . ~o4,~ NlCl
40
~
lC-2 M
~ ~0 ~o 2o "
~ •
1
,
,
i
,
i
7, .
J
,
i
~ ~ . . . . . . . Zeta
.
of HT
potential
. . . . . . . . . . . . .
: .~. . NaC110-2 ~ , ~ 2M~ • ~so,,~2,
= ~ ~ ~° ~ ,0
o
N~C¢O410-2N
° '° . . . . . . . . . . . . . 4
s
pH
a
lo Zeta
12
14
potential of HTC
Fig. 3. Zeta potential of HT and HTC vs. pH in different electrolytes (H20 , NaC1, NazCO 3, Na2SO4 and NazCrO4). Table 2 IEP values for HT and HTC in different electrolytes
Nature of electrolyte
Water 0.01 M NaCI
O.Ol M NazSO4
0.01 M Na2CO 3 0.01 M Na2CrO 4
Sample HT
HTC
(IEP value)
(pH units)
11.1
10.9
> 11.1
> 11 8.9 6.6 6.5
7.6 7.8 6.7
3.4.2. Mixed solutions Chloride and sulfate anions were used for evaluating the competition effects considering that SO42- and CrO42- have the same behavior at low concentrations (Fig. 5). The isotherms were obtained from pure C1- or SO42- solutions and from mixed solutions containing 100 mg 1-1 of SO4 2 and C1- respectively. The sorption of SO4 2- is slightly decreased when C1 is present. The sorption of C1- is strongly
171
L. Chdtelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 120 ,
1000
Hydrolalcit¢HT
t 100
~
80 1
~
"~ E 60 ) -z
•
•
•
o-~---~ 0
o HT CrO4
40
experimental temperature 30°C
700 i
•
6(X)
~.---+ q ? , o, ,
20
800
* HTC Cr04
* * *~
outgassing at 130°C during lgh
D HTC SO4
~
20 q
:
• HTCC1
• *
O' 40 ~
Samplepreparation
900
60
80
100
,
~ 50o
120
Qi (mg/g)
~
Fig. 4. Adsorption, Qaas (rag g t), of Cl , SO42-, CrO4 zonto HTC and CrO4 2 o n t o HT vs. Qi (5-120 mg g 1).
400
~
--
3oo
adsorption
--desorption
200 I IX)
Illlliil I I III
,,!?111.. JJ
40
• Cl
li~
2f)
0 4---
0,1
Ill 1
@ Cl + sO4 t00 mg/l
I0
100
Ce (mg/l)
Fig. 5. Co-adsorption isotherms of C1 and SO42- onto HTC vs. the equilibrium concentration, C~ (mg 1-~). decreased in the presence of S O 4 2 - . The adsorption of divalent anions onto H T C is not strongly affected by the presence of monovalent anions. On the contrary, divalent anions inhibit the adsorption of monovalent anions, 3.5. Adsorption o f water vapor on H T
Fig. 6 shows the adsorption-desorption isotherm of water vapor. For HT, the shape of the isotherm reveals the presence of very thin micropores and large mesopores. The t-plot treatment applied to this isotherm using the values given by Hagymassy et al. [16] is displayed in Fig. 7. Assuming a cross-sectional area of 0.148 nm z for the water molecule, it yields a total specific area of 805 m 2 g ~. Two kinds of micropores are observed, The quantity of micropores observed, 167.5 cm 3 g-~ and 189 cm 3 g-~ for the two classes of pores,
0
PIP•
Fig. 6. Isotherm of water vapor adsorption on HTC at 30° C.
reveals a swelling of H T adsorption.
upon
water vapor
4. Discussion The structures of H T and H T C suggest the presence of two kinds of anion retention sites: sites within the interlayer corresponding to the structural A.E.C. of the material and adsorption onto external surfaces. The nature of these retention sites for different anions can be approached through the study of the variation of zeta potential with pH in the presence of various electrolytes (Fig. 3). Adsorption on the external surfaces should induce changes in zeta potential. In water, the zeta potential decreases slightly for increasing pH up to values near the I E P where a sharp decrease is observed. It must be pointed out that the I E P is close to that of MgO. The zeta evolution can then be assigned to the dissociation of mainly Mg(OH)2 and some AI(OH)2 + sites. When NaC1 is used as an electrolyte, very little difference is observed, suggesting that all the changes (if any) induced by the presence of C1- anions are not occurring on the external surfaces of HT. On the contrary, the presence of
L. Chdtelet et al./Colloids Surfaces A. Physicochem. Eng. Aspects 111 (1996) 167 175
172
i Vadscm3/g
3~.
j
250
200. mica, mi¢I
So
576.3mZ/g 80A.Tm2/g VmicI 167.5cm3/g
150
S out mic
108 - 150
Vmic1I
189cm3/g
Soutmic 70.1-98 100
50 •
,
,
_-
Fig. 7. t-Plot transformation of the isotherm represented in
Fig. 6. CO32-, SO42- or CrO42- strongly affects the zeta potential on H T and HTC. The IEP is reduced to 6.5 (CO32- or CrO4 2-) or 8 (8042-). This suggests that some adsorption of these divalent anions occurs on the external surfaces, Some more insights into the location of anion retention sites can be extracted from the D R I F T experiments displayed in Fig. 8a-c corresponding to HTC before and after adsorption of CI-, SO42and CrO42- respectively. In all cases, the intensity of the band corresponding to interlayer CO32Table 3 Evolution of the relative concentration of CO 2concentrations Anion Initial quantity (ppm) v3 CO 2- (Abs. units) % Removal
Cl5 8.54 0
El50 6.74 21
Cl100 6.18 28
(Y3=1366 cm-1) frO 2 250 3.09 78
CrO~ 500 4.12 70
anions decreases (Table 3) which reveals some exchange of interlayer anions. The adsorption on the external surfaces evidenced from zeta potential experiments does not provoke any changes in the OH bonds in the range 3500-3700cm -1. This suggests the formation of outer sphere complexes. Quantitative information can be obtained from the adsorption isotherms of anions on HTC presented in Fig. 9. In the case of C r O 4 2 - , the adsorption is linear for Qads <2.1 meq g ~, which indicates an exchange mechanism. For Qaas >~2.1 meq g-1, the isotherm is no longer linear; it reaches a plateau for an adsorbed amount of 4.5 meq g-1. The break in the isotherm for Qad~ corresponds exactly to the A.E.C. of HTC. The second part of the isotherm can then be assigned to the adsorption on external sites with partially variable charge. It corresponds to an adsorption amount of 2.6 meq g-X (1.3 mmoles of CrO4) onto sites. If this value is normalized to the specific surface area of 258 m 2 g-1 obtained after heating (Table 1) the surface density o f C r O 4 molecules is ~ 3 nm -2. This value appears to be much too large. However, due to swelling, it seems more appropriate to use the specific surface area measured from water vapor adsorption (805 m 2 g 1). The surface density of C r O 4 molecules would then be ~ 1 nm -2. This number corresponds to the average value of the site density on oxides 1-17]. For SO4 2- , the isotherm is similar but the maximum amount adsorbed is lowered (3.2 meq g-l). The surface density of SO4 molecules is ~0.4 nm 2. For CI-, the maximum adsorbed quantity is 1.7 meq g - l , lower than the A.E.C., which confirms the absence of adsorbed CI- on the external faces shown by zeta measurements.
calculated from D R I F T spectra vs C I - , SO 2 and CrO42 initial
f r O 21000 4.25 69
SO 2100 4.87 43
SO 2 200 3.44 60
SO~250 4.12 52
SO 2500 3.57 58
SO42 750 3.36 60
SO 21000 4.23 50
173
L . Chgttelet et al./ Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 t.-
c~
4O00 (a)
~
S04 750 ppm
~
23~0 w ......
b. . . .
'
600
4000
-1
23; w ......
(b)
'
600
ber cm'l
e4
I
1000 ppm
Cr04 500 ppm
,n "E
,= <
Cr04 250 ppm
Cr04 50 ppm
j
J
J 4(3(/)
(C)
230¢,)
'
600
Wavenumber cm -1
Fig. 8. DRIFT spectra of HTC before and after adsorption of C1- (a), 8 0 4 2 - (b) and C r O 4 2 4.1. A d s o r p t i o n affinity
Hydrotalcite-like compounds exhibit a greater affinity toward divalent anions [10,11]. This
(c)
at Qi in the 5- 1000 mg g-1 range.
trend is confirmed in Fig. 10 which shows the adsorption of CI-, SO42- and CrO4 2- in terms of Qads (mmolg -1) vs. the equilibrium concentration Ce (mmol 1-1). The position of the isotherms
174
L. Chdtelet et al./Colloids Surfaces A." Physicochem. Eng. Aspects 111 (1996) 167 175
Qad~i
(total adsorption)
Q....
•
onto sites on which ] 2"6mext/gads°rbed charge partly variab e
~
4,Tmeq/g
4
'!
" " " •
!
•
@ 1 , 0 ..............I-"-. . . . E E 1)8 "-
V---
CY 0,6
o
AEC 2,1 meq/g
•
1
it
•
"
"
11,4
" 0'0 0 2
, 10
i 20
Qi (meq/g)
D SO4
o C, • CI + SO4 100 mg/l
I
I
•
10 l
10o
C~Cs 30
Fig. 9. Adsorption isotherms of CrO42 , CI- and HTC vs. Qi (meq g-l}.
2,.,
SO42-
onto
2 °°
.'..
Fig. 11. Adsorption isotherms in mixed CI (mmol g-t) vs. log Co/Cs.
/SO42-
solution
co-anion, the more the isotherm is shifted toward the limit of solubility.
.,
"
1,,
no
HI
•
I
0
l
t,2
-".
iI
g ,
....
. ..,.....-s 0
0,00001 0,0001
0,001
0,01
5.
.................... 0,1
I
10
100
Ce(mmom) Fig. 10. Adsorption isotherms of CI-, SO42 and CrO42(mmol g ' vs. Co (retool 1 1).
Conclusion
The retention of monovalent or divalent anions such as C1 , SO42- or CrO42- on hydrotalcite depends on • the A.E.C. value of the solid; it increases with thermal treatment; • the presence of O H sites (namely MgOH); they are located on the external surfaces since the
on the abscissa gives some indication of the affinity, The affinity of CrO42- and SO42 for HTC is very similar at low concentrations (anionic exchange domain), as shown in Fig. 5, and higher than that of monovalents. Once the anionic capacity is completed, more CrO42- ions adsorb cornpared to SO4 z . This difference could be due to some precipitation of chromate anions occurring with Mg 2+ or Mg (OH) + cations. The exchange of monovalents for divalents can be analyzed in terms of mixed solubility of anions. Following the solubility data [18], Fig. 11 shows the adsorption of CI- and SO42 alone and in mixed solutions vs. the solubility of C1 in Na2SO 4 solution and SO42- in NaC1 solution. The affinity of each anion clearly depends on the differential solubility. The higher the concentration of the
adsorption of anions such as SO42- and CrO42shifts the I E P toward the acid domain; • the presence of structural CO32- in uncalcined hydrotalcite which is hardly exchanged. In mixed solutions the affinity of each component depends on its solubility in the mixture. Therefore the affinity decreased in the order C r O 4 2 - > SO42- >monovalents.
References [1] R. Allman and H.P. Jepsen, Neues Jahrb. Mineral., Monatsh, H12 (1969)544. [2] R. Allman, Acta Crystallogr., Sect. B,24 (1968) 972. [3] s. Miyata and T. Kumura, Chem. Lett., (1973) 843. [4] V.R.L Constantino and T.J. Pinnavaia, Inorg. Chem., 34 (1995) 883. [5] H.F.W. Taylor, Miner. Mag., 39 (19731 377.
L. Chfttelet et al./Colloids Surfaces A: Physicochem. Eng. Aspects 111 (1996) 167-175 [6] W.T. Reichle, S.Y. Kang and D.S. Everhardt, J. Catal., 101 (1986) 352. [7] S. Miyata, Clays Clay Miner., 23 (1975)369. [8] S. Miyata and A. Okada, Clays Clay Miner., 125 (1977) 14. [9] S. Miyata, T. Kumura, H. Hattori and K. Tanabe, Nippon Kagaku Zasshi, 92 (1971) 514. [10] S. Miyata, Clays Clay Miner., 28 (1980) 50. [11] T. Sato, T. Wakabayashi and M. Shimada, Ind. Eng. Chem., Prod. Res. Dev., 25 (1986) 89. [12] J. Rouquerol and L. Davy, Thermochim. Acta, 24 (1978) 391. [13] A. Burneau, O. Barr6s, J.P. Gallas and J.C. Lavalley, Langmuir, 6 (1990) 1364.
175
[14] M.J. Hernandez-Moreno, M.A. Ulibarri, J.L. Rendon and C.J. Serna, Phys. Chem. Miner. 12 (1985) 34. [15] M. Del Arco, C. Martin, I. Martin, V. Rives and R. Trujillano, Spectrochim. Acta, Part A, (1993) 1575. [16] J. Hagymassy, S. Brunauer and R.S. Mikhail, J. Colloid Interface Sci., 29(1969)485. [17] W. Stumm and J.J. Morgan, Aquatic Chemistry--An Introduction Emphasizing Chemical Equilibria in Natural Waters, 2nd edn., John Wiley, New York, 1981, Chapter 5. [18] Linke and Seidell, Handbook of Solubilities of Organic and Inorganic Compounds, 4th edn., American Chemical Society, Washington, DC, 1958, p. 931.