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Clays and Clay Minerals, VoL 47, No. 5, 5 6 7 ~ 7 2 , 1999.
R E M O V A L OF N I C K E L A N D COBALT F R O M A Q U E O U S SOLUTIONS B Y
Na-ACTIVATED B E N T O N I T E
STELLA TRIANTAFYLLOU,EIRINI CHRISTODOULOU,AND PARASKEVI NEou-SYNGOUNA
National Technical University of Athens, Department of Mining and Metallurgical Engineering, Laboratory of Metallurgy,
Heroon Polytechniou 9, 15780, Zografou, Greece
Abstract--The ability of Na-activated bentonite to remove Ni2+ and Co2+ from aqueous solutions at room
temperature (22 _+ I~ was studied under various experimental conditions. The parameters studied were
solid-to-liquid ratios and initial cation concentrations. Experiments involved the behavior of bentonite vs.
Ni and Co separately and where Ni and Co were present in solution at different concentrations and ratios.
Bentonite retained substantial amounts of both metals readily, but it showed a higher affinity for Ni.
Over-exchange appears when initial metal concentration exceeds the concentration corresponding to the
cation exchange capacity (CEC) of bentonite. The presence of both metals in solution may be either
synergistic or antagonistic sorption, depending on the initial ion concentrations.
Key Words--Adsorption, Bentonite, Cation Exchange, Clays, Cobalt Removal, Heavy Metals, Nickel
Removal, Wastewater.
INTRODUCTION
Heavy metals such as Ni and Co are common constituents of runoff from mining operations, urban
stormwater runoff, and industrial effluents. Although
in small concentrations they are essential to life, excessive levels may be detrimental. Therefore, heavy
metals in wastewater should be minimized to prevent
accumulation in the biosphere. Adsorption on smectite-rich clays, i.e., bentonites, is a reliable method of
metal removal owing to its simplicity, effectiveness,
and low cost, and bentonites are abundant. C o m m o n
environmental uses of bentonites involve the protection of aquifers from contamination by the sealing of
ponds and landfill sites; bentonite holds pollutants and
prevents downward movement of wastewater. Hence,
it is important to know the retention capacity of bentonite and the factors that govern cation retention.
Bentonite is commercially available and consists
mainly of the clay mineral montmorillonite, a member
of the smectite family. Smectites display nearly threefold higher sorption of Cd, Cu, Zn, and Ni than illite,
with Ni and Zn occupying interlayer sites rather than
external surfaces of the clay particles. Ni and Zn have
a lower release during desorption. (Rybicka et al.,
1995; Brigatti e t al., 1994).
Zn z+ and Pb 2+ follow Langmuir-type exchange, with
Zn preferred slightly to Pb (Brigatti et al., 1995). Adsorption of Zn 2+ and Cd 2+ by smectites might be improved by heat treatment (Pradas e t al., 1994). Smectites also absorb Hg at acidic pH through an endothermic process (Viraraghavan and Kapoor, 1994). Reactive sorption sites in soils might be specific or
non-specific (Tiller et al., 1984a). Short contact time
gives an approximation of total adsorption that underestimates the potential of any soil or clay system. Verburg and Baveye (1994) proposed a two-stage kinetics
Copyright 9 1999, The Clay Minerals Society
model for ion exchange. The model emphasizes the
key role of activation-energy barriers.
The high affinity of smectites for Cd and Ni was
attributed to the hard-soft acid-base rule (Puls and
Bohn, 1988; Pradas et al., 1994). Montmorillonite exhibits a harder base character than kaolinite; thus in
CI-, f i t 4 - , and SO42- solutions, montmorillonite is
preferred by Ni, which is a relatively hard Lewis acid
(Puls and Bohn, 1988). However, both minerals do not
readily sorb Ni, especially in the presence of SO42-,
reflecting their relative softness in an absolute sense.
In contrast, kaolinite shows greater relative affinity for
Ni than montmorillonite and soil clay fractions rich in
2:1 phyllosilicates display greater preference for Zn
than for Cd and Ni, and higher affinities for each metal
at pH < 5 than clays rich in Fe (Tiller e t al,, 1984b).
The objectives of the study are: (1) to investigate
the effectiveness of bentonite in removing Ni and Co
from aqueous solutions; (2) to examine the effects of
contact time and contaminant concentration on the adsorption capacity of bentonite and; (3) to evaluate
sorption in the presence of both Ni and Co in aqueous
solutions.
MATERIALS AND METHODS
Bentonite characterization
The bentonite used in this study comes from M i l t s
island (Greece) and was supplied by Silver and Baryte
Co. in the Na-activated form. It is light tan and has a
surface area of 65.6 m 2 g-1 measured by N2 adsorption
following A S T M D4567 (Flowsorb Micromeritics Instrument Corp). The sample was ground to < 1 0 0
mesh.
A sample of the original calcium bentonite (pre-exchanged form) was analyzed for particle size distribution, by wet sieving for the sand fraction and by
567
568
Triantafyllou, Christodoulou, and Neou-Syngouna
100
I
I---
g
"~ 95
"3
90
100
10000
Particle size (~tm)
Figure 1. Particle size analysis of Milos-Island bentonite.
Point A separates sand from silt fraction whereas point B
separates silt from clay fraction.
sedimentation (Andreassen pipette method) for the silt
and clay fractions. This sample was a more reliable
indicator of the clay content since it was not ground.
Cation exchange capacity (CEC) of the Na-activated
bentonite was determined for a sample dried at 100~
for 24 h, by saturation with ammonium acetate at pH
7, subsequent distillation with a Kjeldahl microstream
apparatus, and titration against H2SO 4 (0.05 N). All
reagents used were pro analysis quality. Chemical
analysis for SiO2 content was performed by fusion at
1000~ in a Pt crucible with a mixture of NaCO 3,
KCO3, and borax as flux at a 15:1 flux:sample ratio.
Major elements were determined by acid digestion using a mixture of concentrated HE HCIO4, and H N O 3.
Loss on ignition (LOI) was determined by weight difference after calcination of an air-dried bentonite sample at 1000~
Mineralogy of the Na-activated bentonite was determined by X-ray diffraction analysis (XRD) using a
Siemens D5000 X-ray diffractometer (40 kV, 30 mA,
Ni-filtered C u K a radiation, and graphite monochromator).
Experimental procedure
Three series of experiments were performed to
study the sorption behavior of bentonite for Ni 2+ and
Co > ions. The first series were performed using Ni
solutions of 5 to 500 ppm initial concentration and 1:
50, 1:100, 1:500, and 1:100 solid-to-liquid ratios. An
analogous set of experiments was performed for Co.
In the third series, the solid-to-liquid ratio remained
constant (1:100) and the solutions contained Ni and
Co. Each solution was characterized by one primary
and one secondary cation depending on which cation
was of interest. The concentration ratios used for the
primary-to-secondary cation were: 1:10, 1:2, 2:1, and
10:1, and the concentration values were 0.5, 2.5, 5, 10,
50, and 100 ppm. Also, control experiments with Ni
and Co in the absence of bentonite were conducted to
Clays and Clay Minerals
determine if the glass of the containers absorbed any
Ni or Co.
All experiments were conducted in 500-mL mechanically-stirred spherical-glass reactors equipped
with a Hg contact thermometer and a pH electrode to
measure variations in temperature and pH. The aqueous solutions of Ni and Co were obtained from dilutions o f stock solutions prepared by dissolving
NiSOn-6H20 and COSO4-7H20 (Merck standards) in
deionized water. The experimental procedure was the
following: a known amount by weight of bentonite,
dried at 100~ for 4 h, was mixed with 400 mL of
reacting solution with a specific metal concentration,
agitated at a speed of 500 rpm at room temperature
(22 _+ I~
Preliminary tests showed that the pH of a 5% bentonite suspension was 10.5, and this high value can
lead to precipitation of metal hydroxides formed in the
over-alkaline en v i r o n m en t (Barrer and Townsend,
1976). The chemistry and valences of the hydrolysis
products may influence the results in the following
way: (1) adsorption of the dissolved Ni and Co on the
surface of the bentonite, (2) coagulation of metal species to form colloidal particles, and (3) oxidation or
reduction of the metal. Therefore, the pH value of the
solutions was adjusted to 3,5 immediately after bentonite dispersion, using 12 N H2SO 4 to avoid any precipitation of metal hydroxides pH values < 3 were
avoided because of dissolution of bentonite (Thomas
et al., 1950).
In each experiment, samples of 2 mL of the suspension were withdrawn over time. Initial tests indicated that a contact time of 120 min suffices for all
experiments. The samples were subsequently filtered
through a Whatman 42 filter and analyzed for Ni or
Co by atomic absorption spectrophotometry (AAS).
The quantity o f metal ion sorbed by the bentonite was
determined by the difference between the initial metallic concentration and the remaining concentration in
the filtrate, since the control tests showed negligible
loss from the container. The pH value of the suspensions at equilibrium was found to be < 5 in all experiments.
E X P E R I M E N T A L RESULTS
Wet-sieve and Andreassen-particle size analyses are
presented in Figure 1 as cumulative percentage of material whose particle size is smaller than the size noted
on the x axis. The chemical analysis, based on wt. %
oxide of the dry sample, is: SIO2, 50.54; A1203, 19.23;
Fe203, 5.77; TiO2, 1.97; MgO, 4.74; CaO, 5.62; 15220,
0.74; Na20, 3.29; LOI, 8.10. X-ray data (Figure 2)
show the presence of smectite and calcite. Surface area
of the bentonite is 65.6 m 2 g 1 and the CEC value
based on NHg exchange is 0.96 meq g 1,
The adsorption results for the binary systems of
Vol. 47, No. 5, 1999
Na-activated bentonite
Cr
S
$
,
i
'
'
'
I
S
'
,
. . . .
I
0
'
'
,
,
I
. . . .
10
I
,
,
,
,
I
. . . .
I
20
. . . .
I
'
30
2-thet~ scale
Figure 2. X-ray diffraction pattern of Milts-Island Na-activated bentonite. S = Smectite, Cc = Calcite.
bentonite and either N i or Co are presented in Figure
3 and equilibrium data are s h o w n in Figure 4. Figure
5 s h o w s the effect o f Co upon the r e m o v a l o f N i b y
bentonite and v i c e versa.
569
T h e h i g h smectite content e x p l a i n s the h i g h C E C
value, w h i c h c o m b i n e d w i t h the c h e m i c a l analysis,
w a s u s e d to d e t e r m i n e the structural formula (van O1p h e n , 1 9 7 7 ) o f the s m e c t i t e present: (Si3.6A10.4) (mll.4Fe3+0.2Mg0.6)O10(OH)2(K0.2, Nat.6)0. 8. T h e C E C value is a measure o f the degree o f substitution, since the
e x c h a n g e a b l e c a t i o n s c o m p e n s a t e the u n b a l a n c e d
charge from the 2:1 layers o f the smectite o w i n g to
i s o m o r p h o u s substitutions.
T h e bentonite is an industrial product and contains
calcium, e v e n after Na-activation. T h e Ca 2+ e x c h a n g e able cations react to form f a C t 3 as indicated in Figure 2.
H e a v y m e t a l e x c h a n g e . Figure 3 s h o w s that the re-
DISCUSSION
Bentonite characterization
In Figure 1, all material < 2 p~m in size essentially
corresponds to smectite and calcite, as indicated b y the
X R D pattern (Figure 2) o f the c l a y fraction. T h e possibility exists that smectite is present in the coarse
fractions also, b e c a u s e smectite m a y form q u a s i c r y s -
tals several microns thick and it can exist in the fine
and m e d i u m - s i l t fractions ( 2 - 6 and 6 - 2 0 Ixm, respectively) as well. Point B (Figure 1) indicates that the
Na-activated bentonite sample contains > 9 3 % s m e c tite.
m o v a l o f metal cations reaches a steady state quickly.
After 3 0 rain, a h i g h amount (Qs(i)/Q0(i~ w h e r e Qs(i~ is
the quantity o f the I cation sorbed b y bentonite, Q0 is
the initial quantity, based on m e q g-a) o f each metal
w a s sorbed b y the N a - a c t i v a t e d bentonite. This time is
similar to that d e t e r m i n e d b y R y b i c k a e t al. ( 1 9 9 5 ) ,
Viraraghavan and K a p o o r ( 1 9 9 4 ) , Brigatti e t al.
( 1 9 9 4 ) , and Pradas e t al. ( 1 9 9 4 ) . Tiller e t al. ( 1 9 8 4 a )
reported a t w o - s t a g e adsorption and e m p h a s i z e d that
a short contact time o f several hours or e v e n d a y s
gives only an approximation of total adsorption. Thus,
the results are an underestimation o f the sorptive po1
b
--II
0,8
0,8
~(~ 0,6
0,6
1
-"
~/ 0,4
"
"_kNi
~Ni,
0,2
0
0
1:50
1:500
(~ 0,4
~
Ni,-1:50
...~._ Ni, 1:500
_.11_ Co, 1:50
Co, 1:500
0,2
--II-- Co, 1:50
9"-N---Co, 1:500
0
i
i
i
i
a
i
20
40
60
80
100
120
140
i
i
i
i
I
I
20
40
60
80
100
120
t (miR)
t
Co,
1:50
Ni,
0,8
0,8
0,6
,6
Co,
Ni,
~
1:1000
1:1000
1:50
i
,
1:50
~0,4
0,4
:
o,1:10oo
'
Co,
0,2
0
N
140
(m)
1:1000
0,2
i
i
i
i
i
i
20
40
60
80
100
120
t (~'~)
0
140
i
i
i
i
i
i
20
40
60
80
100
120
140
t (r~n)
Figure 3. (a) Plot of Qs(i)/Q0(i) vs. time for Ni 2+ and Co 2+ absorbed by the Na-activated bentonite. Initial concentration of
i cation: 5 ppm for solid-to-liquid ratios of 1:50 and 1:500. Q~(i) = the quantity of i cation sorbed by bentonite (meq g ] of
bentonite), Q0(i) = the initial quantity of i cation (meq g 1 of bentonite). (b) Plot of Qs(i)/Q0(i) vs. time for Ni 2+ and Co 2+
absorbed by Na-activated bentonite. Initial concentration of i cation: 20 ppm for solid-to-liquid ratios of 1:50 and 1:500. (c)
Plot of Qs(i)/Q0(i ) vs. time for Ni 2+ and Co 2+ absorbed by the Na-activated bentonite. Initial concentration of i cation: 200
ppm for solid-to-liquid ratios of 1:50 and 1:1000. (d) Plot of Qs(i)/Q0(i) vs. time for Ni 2+ and Co 2+ absorbed by the Naactivated bentonite. Initial concentration of i cation: 500 ppm for solid-to-liquid ratios of 1:50 and 1:1000.
570
Clays and Clay Minerals
Triantafyllou, Christodoulou, and Neou-Syngouna
1,2
0,8
." ideal
*~176
~ .~
. ~176176176
CEC
CEC
Ni
/
r
0,8
m 0,6
ideal
. .- "" ~
~176
Co
0,4
c~
0,4
0,2
0
0,2
0,4
0,6
0,8
0,5
1
Qo (meqg I BE)
1,5
Q0 (meq g i BE')
ideal
... -" ""
ideal
~176176176
6
oa
.-*"
Ni
Ni
.-'~
o
Co
o
2
~~176176
-~
2
1
A
CEC
4
6
8
10
Q0 (meq g'l BE)
0
2
4
6
Qo ( ~ q g l BE~
Figure 4. (a) Removal of Ni 2+ and Co 2+ by Na-activated bentonite at equilibrium. Solid-to-liquid ratio: 1:50. (b) Removal
of Ni 2+ and Co 2+ by Na-activated bentonite at equilibrium. Solid-to-liquid ratio: 1:100. (c) Removal of Ni 2+ and Co 2+ by Naactivated bentonite at equilibrium. Solid-to-liquid ratio: 1:500. (d) Removal of Ni 2+ and Co 2+ by the Na-activated bentonite
at equilibrium. Solid-to-liquid ratio: 1: 1000.
tential o f the clay. In fact, the time required to approach a steady state of adsorption differs with metal
ion, sample, and pH. A l t h o u g h this occurs for static
systems, the criterion for an industrial application
(e.g., wastewater treatment) o f bentonite is less stringent and a short contact time is sufficient.
Solid-to-liquid ratio o f 1:50. The solid-to-liquid ratio
of 1:50 resulted in an increased N i or Co r e m o v a l (Figure 3) and a longer time was needed to reach a steady
state. Cation r e m o v a l increased o w i n g to the availability o f m o r e sorption sites. The greater time required to reach a steady state was extended until bentonite was satisfactorily dispersed. The sorbent at this
density f o r m e d aggregates which probably result in a
reduced surface area o f the smectite.
Ni at a solid-to-liquid ratio o f 1:50 (Figure 3d) appeared to be an exception, since the solution showed
high Ni depletion. However, the increase in the p H
value o f the suspension from 3.5 at the beginning of
the experiment to 5.0 at the end, m a y h a v e led to precipitation rather than adsorption of the e x a m i n e d cation (Baes and Mesmer, 1976).
Solid-to-liquid ratio o f 1:100. These systems showed
that s o m e over-exchange for Ni was o b s e r v e d after the
initial metallic concentration e x c e e d e d CcEc (concentration corresponding to the cation e x c h a n g e capacity
o f bentonite) (Figure 4b). This result m a y be attributed
to broken bonds at the edges of the tetrahedral sheets,
which require adsorbed cations for charge balance.
T h e n u m b e r o f such bonds is high for smaller particle
sizes. In addition, there is a high possibility o f formation of c o m p l e x e s in the metal solutions b e t w e e n
the e x c h a n g e cations and the co-anions, so that hydrogen also exchanges onto the bentonite, which w o u l d
result in an apparent over-exchange. (Loizidou and
Townsend, 1987).
Solid-to-liquid ratio of 1:500. The b e h a v i o r of the sorbent at the solid-to-liquid ratio o f 1:500 (Figure 3a and
3b), is characterized by a small uptake at initial concentrations lower than CcEc; but w h e n the concentration of the cation in the reacting solution b e c o m e s
larger than the C C E C , the adsorption o f additional cations is promoted, o w i n g to sorbate-sorbate forces
(Gregg and Sing, 1982). T h e s e attractive forces exist
o w i n g to van der Waals forces, w h i c h p r o m o t e contact.
A t these high concentrations, the sorption of N i by
bentonite reaches 80% o f its C E C and for Co it g o e s
twice b e y o n d this capacity. During the experiments,
the solid dispersed w e l l and there was no increase in
the p H value f r o m the initial value o f 3.5.
Solid-to-liquid ratio of 1:1000. The b e h a v i o r of the
sorbent at the solid-to-liquid ratio of 1:1000 is similar
to that for 1:500 (Figure 3c and 3d). The sorption o f
both cations exceeds the bentonite capacity at a con-
Vol. 47, No. 5, 1999
Na-activated bentonite
,r'~ ~ "~
Co
9
0,9
0,8
o
0,7
0,6
I
t
I
J
t
0,2
0,4
0,6
0,8
1
1,2
Q0(Co)/Q0(Ni+Co)
0,9
o
o
f
.
nu..~
--~176
~,,.,..,.
0,8
\ co
0,7
0,6
0,2
0,4
0,6
0,8
1
1,2
Q0(Co)~0fNi+Co)
0,9
9
9 ..............
Co
"m
O 0,8
0,7
0,6
0,2
0,4
0,6
0,8
1
1,2
Qo(Co),Q0(Ni+Co)
Figure 5. (a) T h e effect of C o 2+ present in the Ni2§
nated solutions and vice versa. Initial concentration of i cation
is 5 ppm and solid-to-liquid ratio is 1:100. (b) The effect of
Co 2+ present in the Ni2+-dominated solutions and vice versa.
Initial concentration of i cation is 10 ppm and solid-to-liquid
ratio is 1:100. (c) The effect of Co 2+ present in the Ni 2+dominated solutions and vice versa. Initial concentration of i
cation is 100 ppm and solid-to-liquid ratio is 1:100.
centration higher than the C C E C ; for Ni it is five times
the C E C , whereas for Co it is a value twice the C E C .
Generally, at the same experimental conditions, the
sorption results indicated an affinity of bentonite for
Ni, which was confirmed by the steady state experiments (Figure 4). The relative abilities o f the soluteion species to c o m p e t e for surface sites o f bentonite
is g o v e r n e d by intrinsic factors such as valence, ionic
radius, pH, and the solution activities. Because the p H
o f the solution was kept constant and both Ni and Co
are divalent cations, the selectivity depends entirely on
the hydrated radii o f the ions. With a radius o f 4.04
,~, Ni is smaller than Co (radius = 4.23 ,~) (Nightin-
571
gale, 1959). Ni is more effective in reacting with the
clay particles of bentonite because of its smaller size.
We assume that the solutions are d o m i n a t e d by the
cation under study. Thus, the addition of Co to the Ni
d o m i n a t e d solutions (Figure 5) acts to reduce the rem o v a l of the latter by bentonite. Nevertheless, at initial
Ni concentrations of 5 p p m and C o b e t w e e n 5 0 - 7 0 % ,
the p h e n o m e n o n b e c o m e s " s y n e r g i s t i c " (Figure 5a)
and there is a substantial r e m o v a l of Ni. Similar results
occur at an initial Ni concentration of 10 p p m and Co
< 5 0 % (Figure 5b).
In contrast, the presence o f Ni apparently enhances
Co removal. The results are generally synergistic, but
there is an exception at low initial Co concentrations;
Co retention is reduced ( " a n t a g o n i s t i c " ) w h e n Ni is
b e t w e e n 1 0 - 5 5 % (Figure 5c).
The above results clearly indicate that N i and Co
r e m o v a l by bentonite is influenced significantly by the
ion species present in the reacting solution with results
either synergistic or antagonistic. N o t e that there are a
n u m b e r o f possible c o m p o u n d s of Ni and Co at a given valence. T h e s e c o m p o u n d s result f r o m surface
c o m p l e x formation o f the metal cations with a single
coordinated O H group on the edges of the surfaces o f
smectites. Thus, a surface sorption m e c h a n i s m m a y
d o m i n a t e in the simple i o n - e x c h a n g e mechanism. Further discussion of the possible c o m p l e x e s is b e y o n d
the scope of this paper. However, the nature o f these
c o m p o u n d s rather than the quantity is of prime importance for the adsorption process ( A d e l e y e e t al.,
1995).
CONCLUSIONS
Adsorption studies indicate that Na-activated bentonite readily retains Ni and Co. Contact time m a y be
short (several minutes) for m o s t adsorption to occur.
L o w solid-to-liquid ratios result in better sorption, but
l o w e r values of r e m o v a l per unit mass o f bentonite.
O v e r - e x c h a n g e appears w h e n initial metal concentration exceeds the concentration corresponding to C E C .
C o m p a r e d to Co, Ni r e m o v a l is favored probably due
to its smaller ionic radius. The presence of both metals
in the solution m a y favor greater sorption or reduce
sorption effects depending on the initial ion concentrations.
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(Received 25 February 1997; accepted 25 October 1998; Ms.
97-020)
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