Clay Minerals (1994) 29, 17%189
THE COMPETITIVE ADSORPTION
OF METHYLENE
BLUE ON TO MONTMORILLONITE
FROM BINARY
SOLUTION WIH THIOFLAVIN
T, P R O F L A V I N E
AND
ACRIDINE YELLOW. STEADY-STATE
AND DYNAMIC
STUDIES
C.
BREEN
AND
B . ROCK*
Materials' Research Institute, Sheffield Hallam University, Pond Street, Shefffield S1 1WB, UK
(Received 17 February 1993; revised 28 September 1993)
A B S T R A C T : Methylene blue (MB) has been used as a probe molecule to examine how the uptake
dynamics and the equilibria between this dye and the surface of Na +- and H +- montmorillonite were
affected by the presence of a second dye~To prevent spectral interference, the yellow dyes thioflavin T, TFT,
proflavine, PFH, and acridine yellow, ACY, were chosen to compete with MB for the exchange sites. The
MB was initially adsorbed as trimer (MB +)3 and then redistributed via collisions between clay particles until
equilibrium was reached. At equilibrium in the clay/MB systems, the protonated species (MBH2+)
predominated at low loadings (1-5% CEC), whereas at higher loadings the trimer (MB+)3 was the major
species. The presence of the second, competing dye slowed the approach to equilibrium, significantly
reduced the amount of MBH2+ formed and provided evidence for the monomeric MB +, dimeric (MB+)2,
and trimeric (MB+)3 forms of MB. Moreover, the presence of PFH and ACY, which are structurally similar
to MB, resulted in more dimeric character in the aggregated form of MB compared to the aggregate formed
in the presence of the structurally dissimilar TFT.
The adsorption of methylene blue (MB), which is
a cationic dye, on to srnectite surfaces has been
the subject of considerable investigation due to
the utility of the p h e n o m e n o n in determining the
presence of smectites (Taylor, 1985), their cation
exchange capacity, C E C , (Rytwo et al., 1991)
and surface area (Hang & Brindley, 1970).
M o r e o v e r , the aggregation behaviour of this dye,
the spectral properties of which are summarized
in Table 1, on the mineral surface has been
revisited several times since the early study by
Bergmann & O ' K o n s k i (1963), most recently by
Cenens & Schoonheydt (1988), and Yariv et al.
(1990). A large n u m b e r of dye molecules aggregate in aqueous solution to minimize their hydrophobic interaction with water and this impairs
their utility in photochemical applications.
Consequently, if, as some workers suggest (Hang
& Brindley, 1970), it were possible to form
ordered monolayers of dye molecules on the
surface of minerals then this may provide access
* Current Address: Domino Amjet Ltd., Bar Hill,
Cambridge CB3 8TU, UK
to concentrations of m o n o m e r , with its attendant
photochemical properties, which are not normally available in aqueous solution. Pursuit of
this goal has inspired several recent studies into
the adsorption of dye molecules on the surface of
clays in addition to those concentrating on
methylene blue. Monomeric, dimeric and protonated forms of proflavine (PFH) have been
distinguished in clay suspensions (Cenens et al.,
1. Band positions (~max) and extinction coefficients
(e) of methylene blue (Cenens & Schoonheydt, 1988)
TABLE
In aqueous solution
Species
~'max
(nm)
E (din3/
mole. cm)
MB +
664
95,000
MBH2+
(MB+)2
741
605
697
580
76,000
t32,000
22,000
110,000
(MB+)3
9 1994 The Mineralogical Society
Adsorbed on clays
k.....
(nm)
6_53
673
763
596
718
570
~ (dm3/
mole. cm)
100,000
116,000
86,000
80,000
30,000
114,000
180
C. Breen and B. Rock
+
H3C~CH3
HzN/ ~
"-.N--~~
H
methylene blue
"-NH2
acridine yellow
H 2 N ~ N H 2
H
H3C~S\
~L..~L~~ / / ~ - - ~ ~
N(cH3)2
I
CH3
proflavine
thioflavin T
Fl6. 1. Structural formulae of the dye molecules.
1987) a n d in the solid state ( S c h o o n h e y d t et al.,
1986).
In contrast, the competitive a d s o r p t i o n , a n d
resulting p r o p e r t i e s , of mixed-dye clay systems
h a v e received little a t t e n t i o n . A v n i r et al. (1986)
r e p o r t e d t h a t the clay host facilitated long r a n g e
electronic e n e r g y t r a n s f e r f r o m a d o n o r molecule,
r h o d a m i n e 6 G , to various fluorescent cationic
dyes, acting as acceptors. T h e r e was a m a r k e d
e n h a n c e m e n t of energy t r a n s f e r from the d o n o r
to the acceptor in the p r e s e n c e of clay c o m p a r e d
to a q u e o u s solution, b u t the fluorescence of
r h o d a m i n e 6 G was r e d u c e d due to q u e n c h i n g by
t h e acceptor. Margulies et al. (1988) studied the
competitive a d s o r p t i o n of M B a n d thioflavin T
(TFT) f r o m aqueous solution a n d fitted m a t h ematical models to t h e i r a d s o r p t i o n isotherms.
U n f o r t u n a t e l y , they did n o t t a k e a d v a n t a g e of the
excellent p r o p e r t i e s of M B as a p r o b e molecule,
described in detail by C e n e n s & S c h o o n h e y d t
(1988), to confirm the data derived from t h e i r
model. It is interesting to n o t e that the same
workers h a v e s h o w n t h a t clay l o a d e d with dyes
such as T F T a n d proflavine ( P F H ) are able to
p h o t o s t a b i l i z e insecticides in which t h e active
g r o u p is also p h o t o l a b i l e (Margulies et al., 1988).
W e h a v e studied the c o m p e t i t i v e a d s o r p t i o n of
M B from a q u e o u s binary solution with T F T , P F H
a n d acridine yellow ( A C Y ) on to N a +- a n d H +exchanged, low-iron m o n t m o r i l l o n i t e . O u r interp r e t a t i o n s are b a s e d u p o n the effect t h a t the
second dye has o n the very i n f o r m a t i v e spectral
signature of the p r o b e molecule, M B . T h e
c o m p e t i n g dyes, the structural f o r m u l a e for which
are given in Fig. 1, w e r e c h o s e n because: (i) they
a b s o r b in the r a n g e 412-430 n m a n d thus do n o t
interfere with the wide spectral r a n g e of M B a n d
its aggregates (570-770 n m ) ; M B , P F H a n d A C Y
are structurally similar to each o t h e r w h e r e a s T F T
is dissimilar, thus the effect of c o m p e t i n g dye
s t r u c t u r e can b e studied: a n d (iii) the a d s o r p t i o n
of P F H a n d T F T o n to clay has b e e n studied
previously ( C e n e n s et al., 1987; Margulies et al.,
1988) thus e n a b l i n g cross c o m p a r i s o n .
EXPERIMENTAL
T h e Texas b e n t o n i t e , d o n a t e d by English C h i n a
Clays I n t e r n a t i o n a l , h a d a C E C of 100 m E q / 1 0 0 g
clay. T h e n o m i n a l l y < 2 ~tm fraction was separated by s e d i m e n t a t i o n a n d a small p o r t i o n used
w i t h o u t c o n v e r s i o n to t h e N a + - f o r m . X-ray fluorescence analysis of a fused glass b e a d of this
m a t e r i a l indicated t h a t the e x c h a n g e cations were
80% C a 2+ a n d 20% N a +. This sample is subsequently r e f e r r e d to as Ca2+/Na +. T h e r e m a i n d e r
of t h e < 2 ~tm fraction was c o n v e r t e d to the N a +f o r m by washing t h r e e times with 1 M NaCI. T h e
resulting suspension was r e p e a t e d l y w a s h e d with
d e i o n i z e d w a t e r ( p H --5.0) a n d centrifuged at
20,000 r p m until t h e residual conductivity was
< 2 0 ~tS.
T h e centrifugate was r e - s u s p e n d e d in deionized
w a t e r a n d the < 0 . 3 ~tm fraction was collected by
centrifugation. T h e resultant suspension, which
h a d a clay c o n t e n t of 5 g d m -3, was divided into
two portions. O n e p o r t i o n was used as the stock
Competitive adsorption of dyes on clays
N a + - e x c h a n g e d f o r m whilst the o t h e r p o r t i o n was
stirred for 2 h at 20~ after the a d d i t i o n of
sufficient 2 mol d m 3 H 2 S O 4 to give an H +
c o n c e n t r a t i o n of 0.15 mol d m -3 (30 C E C ) . A f t e r
acid t r e a t m e n t , this H + - m o n t m o r i l l o n i t e was
w a s h e d with d e i o n i z e d w a t e r until the p H of the
s u p e r n a t a n t was > 4 . B r e e n (1991) has s h o w n t h a t
a Texas b e n t o n i t e t r e a t e d in a similar m a n n e r
retains its structural integrity.
P r e p a r a t i o n of clay-dye s u s p e n s i o n was identical to the m e t h o d of C e n e n s & S c h o o n h e y d t
(1988) in which t h e c o n c e n t r a t i o n of M B was
m a i n t a i n e d at 2.5 x 10 -6 mol d m -3 to p r e v e n t dye
aggregation in solution prior to a d s o r p t i o n o n to
t h e clay. T h e a m o u n t of dye available to the clay
was increased by r e d u c i n g the a m o u n t of clay
p r e s e n t in suspension. Initial studies i n d i c a t e d
t h a t low loadings of dye o n H + - m o n t m o r i l l o n i t e ,
w h i c h w e r e realized by a h i g h c o n c e n t r a t i o n of
clay in suspension, r e q u i r e d t h e use of a m o r e
c o n c e n t r a t e d buffer solution t h a n t h a t used by
C e n e n s & S c h o o n h e y d t (1988). T h u s all the
d y n a m i c a n d a d s o r p t i o n studies w e r e carried o u t
in solutions which c o n t a i n e d 10 cm 3 of a 0.1
tool d m -3 s o d i u m acetate/acetic acid buffer at p H
4 a n d 10 cm 3 of 10 -5 mol d m -3 dye solution to
which was a d d e d sufficient w a t e r to e n s u r e t h a t
after a d d i t i o n of clay suspension t h e final v o l u m e
was 40 cm 3. In competitive studies, 10 cm 3 of a
10 5 mol d m 3 solution of the s e c o n d dye ( T F T ,
A C Y , P F H ) was a d d e d a n d t h e a m o u n t of w a t e r
r e d u c e d by the s a m e a m o u n t to m a i n t a i n a total
v o l u m e of 40 cm 3. All t h e m i x t u r e s were p r e p a r e d
in n e w p o l y e t h y l e n e vessels to minimize adsorption in cracks. O n c e p r e p a r e d , t h e samples were
placed in a n orbital shaking b a t h , o p e r a t i n g at 250
r p m , at 25~ for a m i n i m u m of 16 h, sufficient for
equilibrium to b e established u n d e r these experim e n t a l conditions. S u b s e q u e n t to recording the
spectra of t h e clay-dye suspensions the a m o u n t of
M B in the s u p e r n a t a n t was d e t e r m i n e d , after
r e m o v i n g t h e clay by c e n t r i f u g a t i o n at 20,000 r p m
for 100 min, using visible spectroscopy. A b s o r p tion spectra of M B a n d mixed-dye clay suspensions were r e c o r d e d in the r a n g e 300-800 n m
against p u r e w a t e r with a Phillips P U 8 7 2 0 spect r o p h o t o m e t e r . T h e spectra of t h e p u r e clay
suspensions were s u b t a c t e d f r o m those of t h e
c o r r e s p o n d i n g clay-dye materials to o b t a i n the
spectra of a d s o r b e d dye only. This s u b t r a c t i o n
e l i m i n a t e d t h e b a c k g r o u n d due to light scattering
by t h e clay particles.
181
RESULTS
Figure 2 illustrates t h e striking c o n t r a s t b e t w e e n
the a b s o r p t i o n spectra of a 2.5 • 10 -6 tool d m -3
solution of M B in t h e a b s e n c e of clay (Fig. 2a),
a n d in the p r e s e n c e of sufficient clay to give M B
loadings of 1% a n d 50% of the C E C (Figs. 2b,2c,
respectively). This d i a g r a m shows quite clearly
t h a t M B was n e i t h e r p r o t o n a t e d n o r did it exhibit
any m e t a c h r o m i c b e h a v i o u r in t h e b u f f e r solution
utilized, a n d this was the case for TFT, A C Y a n d
P F H also. U s i n g t h e a s s i g n m e n t s of C e n e n s &
S c h o o n h e y d t (1988), given in T a b l e 1, it is clear
t h a t w h e n e q u i l i b r i u m is r e a c h e d at a loading of
1% C E C the M B is all in the M B H 2+ form, ~ - m a x
= 770 n m , (Fig. 2b), w h e r e a s at a loading of 50%
C E C the trimeric f o r m of M B , (MB+)3; Xmax =
570 n m , d o m i n a t e s t h e s p e c t r u m (Fig. 2c).
Dynamic studies
Figure 3 indicates h o w t h e a b s o r p t i o n s p e c t r u m
for M B o n H + - m o n t m o r i l l o n i t e at 2.0% of the
C E C varied as a function of time. Two m i n u t e s
after a d d i n g the r e q u i r e d a m o u n t of clay suspension to t h e b u f f e r e d dye solution, m o s t of t h e dye
was p r e s e n t as a d s o r b e d t r i m e r (Xmax = 570 rim).
A f t e r 5 m i n agitation, an a p p r e c i a b l e a m o u n t of
M B H 2+ (X. . . . = 770 n m ) h a d b e e n f o r m e d , a n d
this b a n d c o n t i n u e d to increase at t h e expense of
t h e t r i m e r b a n d at 570 n m . This b e h a v i o u r was
0.30
a
b
i
i
I
o
~ 0.15
!
I
c
u~
az
<
0
400
J
600
i00
),/nm
FIG. 2. Absorption spectra for 2.5 x 10 6 mol dm 3
methylene blue buffered at pH 4 in the absence of clay (a),
and in the presence of sufficient H+-montmorillonite to give
a loading of 1% CEC (b), and 50% CEC (c), respectively.
C. Breen and B. Rock
182
0.3
H+.-H B
a
1.0
=o 0.2
8
0.5
r-.
r'~
r
0
o3
,.0
<
m5%
0.1
b
0
200
'
b
I
~00
I00
400
H+--H B+TFT
1.0
I00
600
3~0
Timelmin
klnm
Fl6. 3. Time dependence of the visible spectrum of
methylene blue on H+-montmorillonite at a loading of
2.0% of CEC (a) 2 min, (b) 5 min, (c) 35 min, (d) 130 min
and (e) 410 rain.
o b s e r v e d with all the samples which e x h i b i t e d a
b a n d for M B H 2+ at e q u i l i b r i u m including those
using N a + - m o n t m o r i l l o n i t e a n d mixed M B / d y e
sytems. A d j u s t i n g the solution c o m p o s i t i o n s
accordingly it was d e t e r m i n e d that, within the
accuracy of t h e e x p e r i m e n t , the rate of M B H 2+
f o r m a t i o n increased with increasing clay c o n t e n t
b u t was i n d e p e n d e n t of t h e a q u e o u s dye
concentration.
Figure 4 shows h o w the a b s o r b a n c e of the
M B H 2+ b a n d at 770 n m , n o r m a l i z e d to the
a b s o r b a n c e value at equilibrium, varied with time
in single a n d mixed dye systems. T h e rate at which
M B H 2+ was f o r m e d d e c r e a s e d with increased dye
loading, which p r o b a b l y reflects the c o n c o m i t a n t
decrease in clay c o n c e n t r a t i o n necessary to
achieve these higher loadings. F u r t h e r m o r e , the
rate of f o r m a t i o n of M B H 2+ was slower on the
less acidic N a + - m o n t m o r i l l o n i t e a n d was markedly r e d u c e d in the p r e s e n c e of a second,
c o m p e t i n g dye (Fig. 4b). A t t e m p t s to fit the rate
data to simple first- a n d s e c o n d - o r d e r kinetic
f o r m u l a t i o n s which considered b o t h t h e p r e s e n c e
and a b s e n c e of an equilibrium, w h e r e appropriate, were unsuccessful.
Equilibrium studies--single dye adsorption
Figure 5 shows t h a t as the loading of M B
increased from 5% to 50% of the C E C , the
~
0.5
m5+5%
o
2'6o
360
Time/min
FIG. 4. Time dependence of the absorbance of the MBH 2+
band (770 nm) for the systems (a) H+-montmorillonite/
methylene blue and (b) H+-montmorillonite/methylene
blue/thioflavin T.
0.30
0
.e 0.15
o,?,
%CEC
so o..
/.X ~
/7'x
/ \/I
12. s
600
600
BOO
k/nm
FIG. 5. Variation of the visible absorption spectrum of
methylene blue on H+-montmorillonite as a function of the
equilibrium dye toadings indicated.
183
Competitive adsorption of dyes on clays
intensity of t h e characteristic M B H 2+ b a n d at 770
n m d e c r e a s e d a n d t h a t of t h e trimer, (MB+)3, at
570 n m increased. Careful inspection of the
relative p e a k intensities a n d t h e a s y m m e t r y of
p e a k s indicated t h a t as t h e loadings i n c r e a s e d
m o n o m e r i c (kma• = 660 n m ) a n d dimeric species
(X~nax = 596 n m ) occurred. F o r e x a m p l e , C e n e n s
& S c h o o n h e y d t (1988) r e p o r t e d t h a t the relative
a b s o r b a n c e s of the b a n d s at 7 7 0 , 6 8 0 a n d 620 n m ,
which are v i b r o n i c c o m p o n e n t s of a d s o r b e d
M B H 2+, are 1.00 : 0.52 : 0.17, respectively. T h u s ,
w h e n the ratio of the b a n d s at 770 n m a n d 680 n m
was n o t --2 : 1, t h e n t h e additional intensity n e a r
680 n m was t a k e n to indicate t h e p r e s e n c e of
M B + n e a r 660 nm. M o r e o v e r , an assymetry o n
t h e high w a v e l e n g t h side of the 570 n m , (MB+)3,
b a n d was a t t r i b u t e d to a small a m o u n t of d i m e r ,
(MB+)2, b e c a u s e its a b s o r b a n c e was t o o high to
b e a t t r i b u t e d to t h e M B H 2+ vibronic c o m p o n e n t
at 620 nm.
T h e close proximity of t h e a b s o r p t i o n b a n d s of
the m o n o m e r , d i m e r a n d p r o t o n a t e d species of
T F T , A C Y a n d P F H p r e c l u d e d any definitive
resolution of the a b s o r p t i o n b a n d s in the 400-460
nrn range. T h u s the changes in ~-max for the
principal a b s o r p t i o n b a n d s a n d t h e i r respective
a b s o r b a n c e values for the clay/TFT a n d clay/MB/
T F T systems are illustrated in Fig. 6. H o w e v e r , as
t h e p r i m e objective was to study the effect of t h e
second dye o n the a b s o r p t i o n s p e c t r u m of M B , n o
a t t e m p t was m a d e to ascribe changes in p r o t o n a tion a n d / o r aggregation to t h e s e variations
a l t h o u g h this has b e e n c o n s i d e r e d by o t h e r s
( C e n e n s et al., 1987; C o h e n & Yariv, 1984;
G r a u e r et al., 1987).
t 70
I:=
r
x
E
e<
"
I
0
g 0.0
o
50
[TFT]o/%CEC
100
Vv
0.0
Equilibrium studies--mixed dye adsorption
T h e c o m p e t i t i v e a d s o r p t i o n studies were
carried o u t using e q u i m o l a r c o n c e n t r a t i o n s of M B
plus the s e c o n d dye (TFT, A C Y , P F H ) in which
t h e final a q u e o u s c o n c e n t r a t i o n of each dye was
2.5 X 10 _6 mol d m 3. T h e dye loadings in these
b i n a r y systems will b e p r e s e n t e d s u b s e q u e n t l y as
(1 + 1)% which d e n o t e s 1% loading of M B a n d
1% loading of t h e second dye. Figure 7 c o m p a r e s
t h e a b s o r p t i o n s p e c t r u m of M B a n d T F T , b o t h
p r e s e n t at 12.5% C E C , a d s o r b e d o n H + - m o n t morillonite with M B a l o n e o n the same a d s o r b a t e
at loadings of 12.5% a n d 25% C E C . Clearly, the
a m o u n t of M B H 2+ f o r m e d in the p r e s e n c e of
50
[TFT] o / ~ CEC
100
FIG. 6. Variation of (a) k..... and (b) absorbance as a
function of loading of thioflavin T (TFF) alone and in binary
mixture with methylene blue (MB) on Na +- and H §
montmorillonite. A H+/TFT; (3 Na+/TFT; 9 H+/TFT/
MB; (1 Na+FFFT/MB; [] dual point. In the binary systems,
the abscissa represents the equimolar amount of each dye
present in the initial mixture.
e q u i m o l a r a m o u n t s of M B a n d T F T was significantly less t h a n t h a t f o r m e d at e i t h e r 12.5% or
25% loading in the p r e s e n c e of M B alone.
M o r e o v e r , the a b s o r p t i o n b a n d n e a r 600 n m in
184
C. Breen and B. R o c k
the M B / T F T system was red shifted c o m p a r e d to
the single dye system and the relative intensities
of the bands at 690 and 770 nm (which should be
in the ratio of 2 : 1 in the absence of m o n o m e r ,
MB +) indicated that, in contrast to when MB was
adsorbed alone, there was m o n o m e r present. The
spectrum for the second dye in the region 400--460
nm did change slightly with loading (Fig. 6) but
the change was not sufficient to warrant replotting
the spectrum at each loading.
After careful consideration of many single and
mixed-dye systems in the presence of clay it was
determined that the extinction coefficients
reported by Cenens & Schoonheydt (1988) were
applicable and thus permitted the calculation of
the amount of M B H 2§ formed. Figure 8 shows, as
anticipated, that the acid-treated clay contained
m o r e sites capable of protonating MB + to
M B H 2+ than both the sedimented Ca~+/Na + and
the Na+-form. These observations are in good
agreement with data reported by other workers
for the synthetic mica-montmorillonite, Barasym
(Cenens & Schoonheydt, 1988) and Cu 2§
exchanged montmorillonite (Yariv & Lurie,
1971). Clearly, the presence of the second dye
severly reduced the amount of M B H 2+ formed as
illustrated in Fig. 7. T h e r e was a small, but
significant, difference between the a m o u n t of
M B H e+ formed in the presence of T F T and A C Y ,
whilst P F H suppressed the protonation of MB to
1% of the C E C for the H+-montmorillonite.
2
~ 10
m
~: 5
50
100
[H B+]o/%CEC
FIG. 8. Variation of [MBH2+] with methylene blue loading
for a selection of single and binary dye systems in the
presence of Na +- and H+-montmorillonite. Inset:
Expanded portion detailing the results for Na+-montmoril lonite. A H+/MB; 9 (Ca2+/Na+)/MB; C) Na+/MB; 9 H+/
MB/TFT; 9 H+/MB/ACY; 9 Na+/MB/ACY. In the binary systems the abscissa represents the equimolar amount of
each dye present in the initial mixture.
Further information concerning the adsorption
behaviour of MB in the binary dye systems can be
obtained from a study of the kroax value for the
band near 580 nm. Cenens & Schoonheydt (1988)
were able to attribute absorption bands near 600
nm and 570 nm to dimer, (MB+)e, and external
trimer, (MB+)3, respectively. Consequently, a
plot of km,x, in the 570-620 nm region, vs. % C E C
should provide evidence concerning the state of
aggregation of MB. Figure 9 uses the H +0.30
m o n t m o r i l l o n i t e / M B / T F T system to illustrate
both the spectral quality in the binary dye systems
and how, as the loading increased, the intensity of
the band at 580 nm increased whilst that at 770 nm
decreased ( c f Fig. 5). The inset in Fig. 9 shows
that at low loadings of MB and TFT, the
rr
dominant MB aggregate was, following Cenens &
Schoonheydt's assignments (1988), the dimer
0
(k~n~x = 595-605 nm). H o w e v e r , as the MB and
t,"n
T F T loadings both approached 12.5% of the
<~
CEC, ~'maxm o v e d to a value diagnostic of the
HB
trimer before increasing again as the loading
m o v e d to higher values. A t loadings above (50 +
50)% C E C , both MB and T F T were determined
in the supernatant; thus, the values of ~ m a x n e a r
/+00
600
800 620 nm were probably influenced by unabsorbed,
aqueous phase MB + m o n o m e r , which exhibits
Xlnm
two
bands in the absorbance ratio 1 : 2 at 618 and
FIG. 7. Comparison of the visible absorption spectra for
methylene blue alone and in binary mixture with thioflavin 664 nm, respectively.
T on H+-montmorillonite at the loadings indicated.
Figure 10a builds on Fig. 9 and compares how
0~25.0%
0.1s
12.5%HB+12.S%TFT //~/I'
185
Competitive adsorption of dyes on clays
630
030
62~F
'
e,~.
/ r
o
W
0.15
0
rt~
E
600
x
.<
i::
400
570i
BOO
~/nm
FIG. 9. Variation of the visible absorption spectrum of
600
0
methylene blue from binary solution with thioflavin T as a
function of dye loading on H+-montmorillonite. Inset: The
variation of ~.maxfor the aggregated form of methylene blue
as a function of dye loading.
the value of ~-rnax, for the a g g r e g a t e d forms of M B ,
in the region 560~530 n m varied with dye loading
for b o t h the single a n d b i n a r y dye systems in the
p r e s e n c e of H + - m o n t m o r i l l o n i t e . H o w e v e r , these
values should n o t b e c o n s i d e r e d i n d e p e n d e n t l y of
the c o r r e s p o n d i n g a b s o r b a n c e values which are
plotted in Fig. 10b. In the a b s e n c e of a s e c o n d
dye, n o a d s o r b e d (MB+)3 was o b s e r v e d until the
loading o n H + - m o n t m o r i l l o n i t e e x c e e d e d 5 %
C E C (Fig. 5), a n d this b e h a v i o u r is reflected by
t h e o p e n triangles in Fig. 10a,b. T h u s , up to a M B
loading of 25% C E C , any aggregate p r e s e n t was
the e x t e r n a l t r i m e r (Xmax = 570 n m ) . A b o v e this
loading the Xma• value b e g a n to increase a l t h o u g h
the a b s o r b a n c e value stabilized. In the p r e s e n c e
of a s e c o n d dye t h e initial values for b o t h ~ m a x a n d
a b s o r b a n c e were higher, reflecting the p r e s e n c e
of aggregated M B . Figure 8 s h o w e d t h a t the
a m o u n t of M B H 2+ f o r m e d in the p r e s e n c e of a
second dye varied as T F T > A C Y > P F I - I . This
same s e q u e n c e was e v i d e n t h e r e insofar as t h e
m i n i m u m in the kmax vs. % C E C curve follows t h e
sequence TFT<ACY<PFH.
Incidentally, t h e
m a x i m a n e a r 600 n m in t h e spectra for H +montmorillonite/ACY/MB and H+-montmoril l o n i t e / P F H / M B were m u c h b r o a d e r t h a n t h o s e
for H + - m o n t m o r i l l o n i t e / T F T / M B . This m a y indicate t h a t at loadings below (25 + 25)% t h e r e was
significantly m o r e (MB+)2 p r e s e n t w h e n t h e dye
~
~ '
50
100
[H B+]o/%GEe
m
,--~
0.1
(~
t.--O
~
--
I
~0.05
A:l
<
0
I
I
50
I
~00
[H B+10/%CEC
FIG. lO. Variation of (a) ~-max and (b) absorbance for the
aggregated form of methylene blue (MB) as a function of
loading of MB alone (ZS) and in binary mixture with
thioftavin T (V), acridine yellow (&) and proflavine ([3) on
H+-montmorillonite. In the binary systems the abscissa
represents the equimolar amount of each dye present in the
initial mixture.
in c o m p e t i t i o n with M B was A C Y or P F H t h a n
w h e n T F T was used. A t loadings in which b o t h
M B a n d the second dye were in excess of 50% of
the C E C , t h e value of ~-maxwas influenced by t h e
p r e s e n c e of the a q u e o u s M B b a n d at 660 n m .
V e r y similar o b s e r v a t i o n s were m a d e w h e n the
186
C. Breen and B. R o c k
a b s o r p t i o n spectra of b i n a r y dye mixtures were
recorded
after
equilibration
with
Na + - m o n t m o r i l l o n i t e .
T h e equilibrium loadings of t h e dye molecules
r e c o r d e d in b o t h the single dye/clay systems a n d
the mixed-dye/clay systems are p r e s e n t e d in
T a b l e 2 a n d it is i m m e d i a t e l y obvious t h a t the
loadings always e x c e e d e d the C E C of t h e clay
(100 m E q / 1 0 0 g clay). In general Na+-clay
a d s o r b e d 5 - 1 0 % m o r e dye t h a n H+-clay a n d t h e
a m o u n t of single dye a d s o r b e d increased as
ACY>MB>TFT>PFH.
In the c o m p e t i t i v e
e x p e r i m e n t s M B was always a d s o r b e d in slight
p r e f e r e n c e to T F T and P F H , b u t t h e r e was little
distinction b e t w e e n the a m o u n t of M B a n d A C Y
adsorbed.
DISCUSSION
B o t h the d y n a m i c a n d e q u i l i b r i u m studies cond u c t e d here confirm t h a t M B t e n d e d to aggregate
on the clay surface e v e n t h o u g h the a q u e o u s
c o n c e n t r a t i o n (2.5 • 10 - 6 mol d m 3) was m u c h
lower t h a n t h a t n o r m a l l y r e q u i r e d for m e t a c h r o matic b e h a v i o u r . In water, M B readily forms
dimers at c o n c e n t r a t i o n s as low as 10 -5
mol d m -3. T h u s the clay, w h e t h e r it was in the
Na +- or H + - f o r m , exerted a c o n c e n t r a t i n g effect
u p o n M B as suggested by previous investigators
( B e r g m a n n & O ' K o n s k i , 1963; C e n e n s &
S c h o o n h e y d t , 1988). M o r e o v e r , the clay in this
study p r o d u c e d little e v i d e n c e of a d s o r b e d m o n o meric M B +, in the single clay dye systems, which
TABLE2. Maximum adsorption capacities (mEq/100 g clay)
for single and binary dye mixtures adsorbed on clay.
Exchange
cation
MB
158(95)
-
Na +
TFT
ACY
TOTAL
73(90)
-
185
96
158
145
185
171
196
-
-
140
131
178
170
184
145(110)
-
-
98(20)
100
140
-
H+
1
3
1
-
93
91
-
-
77
-
178
93
Values in parentheses from Margulies et al. (1988)
contrasts with b o t h t h e earlier cited work a n d
o t h e r studies in these l a b o r a t o r i e s o n different
clays. G r e a t e r difference was initially e x p e c t e d
b e t w e e n the a g g r e g a t i o n b e h a v i o u r of M B o n the
N a +- a n d H + - e x c h a n g e d forms since a difference
in tactoid size, a n d / o r floc s t r u c t u r e h a d b e e n
anticipated. H o w e v e r , B a n i n & S h a k e d (1969)
r e p o r t e d t h a t freshly p r e p a r e d H + - m o n t m o r i l l o nite is well dispersed a n d each tactoid c o n t a i n e d
only o n e or two plates, which m a y explain w h y t h e
H + - f o r m used in this study s h o w e d little t e n d e n c y
to form visible flocs. M o r e o v e r , it was necessary
to use a higher ionic s t r e n g t h buffer (0.025 mol
d m -3) in these studies, c o m p a r e d to t h a t utilized
by C e n e n s & S c h o o n h e y d t (1988) b e c a u s e of the
acidity of the H + - f o r m . It is c o n c e i v a b l e t h a t this
c o m p r e s s e d the thickness of the d o u b l e layer to
the same e x t e n t in b o t h cationic forms a n d thus
controlled the initial surface available for adsorption of M B . Finally, it is p e r t i n e n t to n o t e t h a t the
dye itself m a y control the available surface. X i a n g
et al. (1992), using scanning e l e c t r o n microscopy,
h a v e o b s e r v e d g l o b u l a r particles (ca. 5 ~tm in
d i a m e t e r ) in air-dried samples of Ca2+-exchanged
m o n t m o r i l l o n i t e in which 50% of the exchange
cations have b e e n replaced by m e t h y l viologen.
S c h o o n h e y d t & H e u g h e b a e r t (1992) h a v e
recently s h o w n t h a t , at low loadings o n N a +L a p o n i t e , M B molecules are a d s o r b e d o n the first
sites t h a t they e n c o u n t e r d u e to t h e i r e x t r e m e l y
high affinity for the clay surface. If aggregates of
clay platelets occur, it was suggested that adsorption would occur p r e d o m i n a n t l y o n the e x t e r n a l
surface of these aggregates, followed by: (i) slow
m i g r a t i o n of the M B molecules o v e r the surface to
assume an e q u i l i b r i u m distribution, (ii) rearr a n g e m e n t of t h e clay aggregates; or (iii) simult a n e o u s occurrence of b o t h p h e n o m e n a . Schoonh e y d t & H e u g h e b a e r t (1992) p r e f e r r e d process
(ii) b e c a u s e V i a e n e etal. (1987, 1988) s h o w e d t h a t
the r e o r g a n i z a t i o n of the fluorescent p r o b e [3-(1pyrenyl)propyl]trimethylammonium,
a much
b u l k i e r cation, o v e r t h e surface o c c u r r e d o n a
time scale of 500-1000 s. T h e rate of f o r m a t i o n of
M B H 2+ on b o t h N a + - m o n t m o r i l l o n i t e a n d H +m o n t m o r i l l o n i t e in this study was linearly d e p e n dent o n clay c o n c e n t r a t i o n , b u t i n d e p e n d e n t of
dye c o n c e n t r a t i o n . T h e r e f o r e , given (i) the high
ionic s t r e n g t h used h e r e a n d (ii) t h a t p r e l i m i n a r y
studies indicated t h a t the rate of M B H 2+ formation increased w h e n the suspensions were s h a k e n ,
it is clear t h a t the aggregated dye molecules m u s t
Competitive adsorption of dyes on clays
c o m e into contact with p r o t o n sites a n d b e
transfel?red. I n d e e d the o b s e r v a t i o n s m a d e in this
study are c o m m e n s u r a t e with a m e c h a n i s m in
which the a g g r e g a t e d dye molecules o n t h e
external surface c o m e into contact with p r o t o n s
o n basal surfaces of s e p a r a t e particles w h e n t h e
aggregates collide with o t h e r aggregates or
particles.
If r e d i s t r i b u t i o n of (MB+)3 o c c u r r e d by migration within, or r e a r r a n g e m e n t of, a single
floccule t h e n the rate of M B H 2+ f o r m a t i o n would
b e i n d e p e n d e n t of clay c o n c e n t r a t i o n . M o r e o v e r ,
since n o free M B + was o b s e r v e d at the low
loadings used for t h e d y n a m i c studies a n d the rate
of M B H e+ f o r m a t i o n was i n d e p e n d e n t of the
initial M B c o n c e n t r a t i o n , t h e n clearly a d s o r b e d
t r i m e r was i n v o l v e d in the distribution process.
Clearly, as Fig. 2 shows, n o M B is p r o t o n a t e d in
the a b s e n c e of clay at p H 4 a n d thus the clay m u s t
b e the source of p r o t o n s . F u r t h e r m o r e , N a +m o n t m o r i l l o n i t e has fewer p r o t o n sites t h a n H +m o n t m o r i l l o n i t e a n d the rate of f o r m a t i o n of
M B H 2+ was slower o n N a + - r n o n t m o r i l l o n i t e at
t h e same M B loading. T h u s the n u m b e r of
available p r o t o n s was a crucial factor as would b e
a n t i c i p a t e d if t h e floc structure is a s s u m e d to b e
identical.
T h e c o n c e p t of dye t r a n s f e r b e t w e e n dyel o a d e d and dye-free particles was first suggested
by Yamigishi & S o m a (1981) to explain t h e i r
results for n-alkylated acridine orange. T h e s e
w o r k e r s were able to describe this t r a n s f e r
process by simple s e c o n d - o r d e r kinetics in which
the initial c o n c e n t r a t i o n s of dye a n d clay were n o t
identical. In this study, with the e x c e p t i o n of very
low dye loadings, a n e q u i l i b r i u m was established
usually involving the trimer, d i m e r a n d M B H 2+ ,
a l t h o u g h at i n t e r m e d i a t e loadings, some e v i d e n c e
for the the m o n o m e r was also o b t a i n e d (vide
infra). C o n s e q u e n t l y , n o single kinetic f o r m u l a tion would b e e x p e c t e d to fit the data at all
loadings. H o w e v e r , as stated previously, n o
success was a c h i e v e d in fitting data at low loadings
to any realistic formulation.
Figures 7-10 p r e s e n t and s u m m a r i z e the way in
which the M B molecules were p r o t o n a t e d a n d
aggregated w h e n a d s o r b e d on the clay from
b i n a r y mixtures with TFT, A C Y a n d P F H . It is
impractical to p r e s e n t all the spectra resulting
from the c o m p e t i t i v e a d s o r p t i o n systems, b u t
Fig. 10 serves to s u m m a r i z e t h e aggregation
b e h a v i o u r of M B in these systems. H o w e v e r , n o
187
single diagram can s u m m a r i z e t h e f o r m s in which
M B was o b s e r v e d , a n d t h e r e f o r e T a b l e 3 is
included to m e e t this r e q u i r e m e n t . N o t e t h a t t h e
a m o u n t of m o n o m e r o b s e r v e d was always small
a n d identified only b e c a u s e the 680 n m b a n d
a t t r i b u t e d to M B H 2+ was m o r e i n t e n s e t h e n
a p p r o p r i a t e a n d t h e d i m e r b a n d s n o t e d for b i n a r y
solutions c o n t a i n i n g A C Y a n d P F H were b r o a d
a n d p r o b a b l y included a c o n t r i b u t i o n f r o m trimer.
Clearly, t h e p r e s e n c e of the second dye
decreases t h e a m o u n t of M B H 2+ o b s e r v e d a n d
the obvious i n t e r p r e t a t i o n is t h a t the s e c o n d dye
c o m p e t e s effectively for the available p r o t o n s .
U n f o r t u n a t e l y , it is n o t possible to confirm this b y
scrutinizing the spectral shifts of the second dye,
as C e n e n s et al. (1987) h a v e shown. T h e y only
o b s e r v e d t h e p r o t o n a t e d f o r m of proflavine,
PFH22+, ()v.... = 458 n m ) at 0.3% loading
b e c a u s e it is n o r m a l l y s w a m p e d by the d i m e r b a n d
at 430 n m and the m o n o m e r b a n d at 453 nm.
A n o t h e r possibility is t h a t the dye molecules
aggregate the clay particles m a k i n g the p r o t o n
sites less accessible to t h e i n c o m i n g dye molecules. I n d e e d , p r e l i m i n a r y studies of the t r a n s f e r
kinetics of a d s o r b e d (MB+)3, which has b e e n
allowed to equilibrate o n the clay surface, to
freshly a d d e d dye-free clay indicated t h a t the
process was m u c h slower t h a n those r e c o r d e d in
Fig. 4. This suggests t h a t the e q u i l i b r a t e d t r i m e r
was relatively inaccessible, unlike the initial
t r i m e r which was easily r e d i s t r i b u t e d (Fig. 3).
This would suggest t h a t e q u i l i b r a t e d (MB+)3
TABLE 3. Summary of the differm~t forms of methylene
blue (MB) observed during the competitive adsorption
onto H+-clay from binary solution with thioflavin T (TFT),
proflavine (PFH) and acridine yellow (ACY). Adsorbed
monomer M, dimer D, trimer T, and protonated
monomer P.
% LOADING
MB/TFT
MB/ACY
MB/ACY
0.5 + 0.5
1.0 + 1.0
2.5 + 2.5
5.0 + 5.0
8.3 + 8.3
12.5 + 12.5
25.0 + 25.0
37.5 + 37.5
50.0 + 50.0
P
D,T,P
M,D,T,P
M,D,T,P
M,D,T,P
M,D,T,P
Ms,D,T,P
Ms,D
Ms,D
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D,P
M,D
M,D
M,D
MS,D
Ms = MB monomer
centrifugation.
M,D
detected in solution after
188
C. Breen and B. Rock
~max value,
resides o n the internal surfaces of a c a r d - h o u s e
structure consisting of individual particles or
tactoids, similar to the structures suggested by
Yariv et al. (1991) except t h a t the p l a n e of the dye
molecules lie parallel to the p l a n e of t h e basal
surface.
Figure 11 illustrates h o w ~-max for the aggregated form of M B varied with time for single a n d
mixed-dye systems and thus gives an insight into
the m e c h a n i s m for dye distribution o v e r the
available surface. O n H + - m o n t m o r i l l o n i t e , at 1%
a n d 2% loadings of M B , t h e r e was a gradual red
shift in k .... with time which was a t t r i b u t e d to the
d i s a p p e a r a n c e of the 570 n m b a n d of t h e t r i m e r
a n d the increase of the M B H 2+ b a n d at 618 n m .
T h e kma• value was c o n s t a n t at 5% loading
because, unlike the case at 1% a n d 2 % C E C , the
t r i m e r was p r e s e n t at equilbrium. T h e v a r i a t i o n of
~-max with time in the M B f l T ' F binary m i x t u r e
exhibited the most m a r k e d time d e p e n d e n c e (Fig.
10b). A t a loading of (1 + 1 ) % , the b a n d shifts
from a value diagnostic of the t r i m e r to t h a t for
the dimer. A t a loading of (2 + 2 ) % , the shift was
in the same direction b u t not so m a r k e d , w h e r e a s
w h e n the loading was increased to (5 + 5 ) % , the
610
a
H+-MB
600
a l t h o u g h red-shifted slightly,
r e m a i n e d characteristic of t h e trimer. T h e variation in w a v e l e n g t h was m i n i m a l in t h e M B / A C Y
system a n d t h e final value of ~-max was r e a c h e d
very rapidly (Fig. 10c). H o w e v e r , this rapid
a t t a i n m e n t of t h e e q u i l i b r i u m value for ~maxm a y
mask subtle c h a n g e s in the relative a m o u n t s of
d i m e r a n d t r i m e r b e c a u s e this b a n d was quite
b r o a d . N o n t h e l e s s , it is satisfying to note t h a t the
final values of ~'maxo b t a i n e d in t h e kinetic studies
were identical to t h o s e f o u n d in the equilibrium
e x p e r i m e n t s e v e n t h o u g h they were p e r f o r m e d
using different samples.
T h e results f r o m the kinetic a n d e q u i l i b r i u m
studies c o n v e r g e to p r o v i d e the following picture
of the a d s o r p t i o n a n d r e d i s t r i b u t i o n m e c h a n i s m .
T h e majority of M B molecules are initially
a d s o r b e d as t r i m e r o n the e x t e r n a l surfaces of the
clay a n d t h e r e f o r e the a q u e o u s c o n c e n t r a t i o n of
M B has little influence o n the r e d i s t r i b u t i o n
dynamics at low loadings. In contrast, the q u a n tity of clay p r e s e n t at low loadings of M B has a
significant role to play in providing a large
n u m b e r of p r o t o n sites. T h e t r i m e r is redistrib u t e d by t r a n s f e r f r o m o n e particle or floc to
b
H+--MBITFT
I
C
H+-M B/A C Y
F
0
[] []
E 590
.<E 580
560 F
,
,
100
200
I
0
300
0
I
100
200
Time/min
300
0
I
I
100
200
300
FIG. 11. Variation of ~nlax for the aggregated form of methylene blue as a function of time for methylene blue alone (a),
methylene blue in binary solution with thioflavin T (b), and acridine yellow (c) on H+-montmorillonite. O = 1% or
(l + l)%, 9 = 2% or (2 + 2)%, [] = 5% or (5 + 5)%.
Competitive adsorption of dyes on clays
a n o t h e r particle o r floc. T h e p r e s e n c e of t r i m e r ,
dimer, m o n o m e r a n d M B H 2+, in varying
amounts,
suggests the following t e n t a t i v e
mechanism:
(MB+)3 + H + ___>(MB+)2 + M B H 2+
(MB+)2 + H + ~ M B + + M B H 2+
M B + + H + ~ M B H 2+
T h e M B aggregate o b s e r v e d at i n t e r m e d i a t e
loadings of a b i n a r y mixture of M B a n d T F T o n
H + - m o n t m o r i l l o n i t e has c o n s i d e r a b l e trimeric
c h a r a c t e r , w h e r e a s in the p r e s e n c e of A C Y a n d
P F H this aggregate is a mixture of b o t h d i m e r a n d
t r i m e r resulting in a b r o a d a b s o r p t i o n b a n d
c e n t r e d n e a r 600 n m .
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