Colloids and Surfaces, 32 (1988) 127-138
Elsevier Science Publishers B. V., Amsterdam -
Printed in The Netherlands
127
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Adsorption Properties of Ca2+on N a-Kaolinite and
I ts Effect on Flocculation Using Polyacry lamides
G. ATESOK*, P. SOMASUNDARAN and LoIJ.MORGAN
Henry Krumb Schoolof Mines. Columbia University, New York, NY 10027(U.S.A.)
(Received20 April 1987;acceptedin final Conn26 October 1987)
ABSTRACT
Adsorption behavior of calcium ions on Na-kaolinite and the adsorption and flocculation behavior of Na-kaolinite by polyacrylamidein the presenceof calcium speciesin solution havebeen
investigated.Parametersexaminedin calcium adsorption experimentsinclude pH, conditioning
time of the suspensionwith calcium, and calciumconcentration.The effect of calcium on adsorption of polyacrylamideand hydrolyzedpolyacrylamideby the kaolinite and on its flocculation is
determined.Also the effect of the polymer on calcium adsorption is examined.
Adsorption of calcium occurs only under pH conditions where it hydrolyzesto CaOH+ and
Ca(OH)2. Calcium adsorption (abstraction) is sensitiveto the presenceof polymer as well as to
stiuing conditions. Interestingly,abovepH 8, stirring and addition of polymer (HPAM 33) were
found to causecalciumdesorptionsuggestingdetachmentof precipitate from the kaolinite surface.
INTRODUCTION
Mineral fines can be flocculated by reducing the electrostatic repulsion between particles, e.g.by using inorganic salts, and by using polymers which are
capableof bridging the particles with one another. Polyacrylamides are widely
used as a flocculating agent for fine particle suspensionsand their adsorption
hasbeengenerally characterized [ 1-4 ] . Most of thesesystemscan alsocontain
ionic specieswhich may alter the surface physicochemical properties of the
particles significantly [5,6]. For example, ions such as Ca2+ and Mg2+ have
been reported to changethe zeta potenti~oi-9c-u_artz(7,8] as_~~ll~t9~ffect
its-flocculatiuncnaractefiStlr:s:-Such-effects can iri turnmastically affeCt-m:dustrial selective flocculation processes[6,9].
In the last decade,a number of studies of cation adsorption have been conducted [10,11]. Many of these have shown the important role of pH in controlling cation adsorption on hydrous metal oxides [12]. In some cases,
.Present address:Department of Mineral Dressing,Mining Faculty of Istanbul Technical University, Tesvikiya, Istanbul, Turkey.
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128
increaseduptake of the metal ion occurs in a narrow pH range. This is particularly true for strongly hydrolyzable cations [13,14]. For these systems,adsorption has been correlated primarily with the hydrolysis characteristics of
the adsorbing ion rather than with the charge properties of the mineral. On
the other hand, a recent work detailing the effect of conditions in the double
layer hasshown that in a number of cases,phenomenaattributed to adsorption
can in fact be explained by surface precipitation [15].
Somework has been done in the past on the adsorption of inorganic ions on
kaolinite. For example, the selectivity sequenceon kaolinite is found to be
Li+ <Na+ <K+ <Cs+ [16,17]. However, there is little information on divalent cation selectivity on kaolinite [18,19]. There has also been somework on
the adsorption of and flocculation by polymers on kaolinite [1-4,20]. Both of
these processeshave been shown to depend significantly on the charge characteristics of the polymer and the mineral as determined by the pH and ionic
strength. Indeed, the behavior of these systems is extremely complex when
both the inorganic and polymer speciesare present and almost no information
is available for such caseseven though both are invariably present in most
flocculating systems.
In this study, the adsorption. responseof calcium on Na-kaolinite and the
adsorption and flocculation ofNa-kaolinite by polyacrylamides in the presence
of calcium in solution have been investigated. Results obtained for adsorption
and fl6Cculation are discussedwith the help of electrokinetic data.
EXPERIMENTAL
Materials
.
Mineral Na-kaolinite
A well-crystallized sample of Georgia kaolinite of 9.94 m2 g-l surface area
was obtained from the clay repository at the University of Missouri. A homoionic sampleof sodium kaolinite waspreparedfrom this using an ion-exchange
treatment discussedelsewhere [20,21].
;
---Potymers
c-
:c-::--=---
",cccc=-=,,;
The 14C-Iabelednonionic polyacrylamide (PAM) of 5.2XIO6 molecular
weight and 33% hydrolyzed polyacrylamide (HP AM 33) of 6.6 X lOSmolecular
weight used here were synthesizedand characterizedby American Cyanamid
Company.
*The word adsorptionis usedsomewhatlooselyhere as it is clear there may alsobe precipitation
in the system.It might be more correct to use"calcium abstraction" to include both the calcium
which adsorbsand that which precipitates.However,for simplicity, "adsorption" is usedand the
likelihood of precipitation is kept in mind.
:1"
29
.,
1
Jnorganicreagents
;;,
Fisher certified HCI and NaOH wereusedto adjust the pH of the suspension.\
Amend Drug and Chemical Company reagent grade NaCI (99.96% pure) was.
"
usedto preparethe 3 X 10-2 kmol m -3 salt solution usedin all the experiments~:
to control ionic strength, J.
.~;i
:J~
,~
I
Methods
. L:A
' ~f'~
.~
~:~.
Flocculation procedure
For flocculation tests, 5 g samples of Na-kaolinite were pre-conditioned in
150 cm3 salt solution in 250 cm3 Teflon bottles for 2 h using a wrist-action
shaker and for an additional 30 min after adding desired amounts of calcium
salt. Polymer which is in 40 ml salt solution was then added and the pulp
stirred for 10 min using a 1 inch diameter propeller at 1200 rpm. These conditions were identified as sufficient to achieve equilibrium adsorption (i.e., no
additional change measured). Sedimentation tests were carried out in 250 cm3
beakers at 2.5% (by weight) pulp density. The slurry was allowed to settle for
30 s and then 100 ml of the supernatant was removed using a suction device.
The pH of the pulp was measured at the end of each test.
~'
~
r
Polymer determinations
Supernatant sampleswere centrifuged for 10 min at 10,000rpm, using an
IEC centrifuge and analyzed for residual polymer using a Beckman Liquid
Scintillation Counter (LSC).
~
..
Cakium determination
i Supernatants were analyzed for total calcium, CaT, also using the LSC to
~ount 45Ca.Ca2+concentration in someof the sampleswas also determinedby
measuringthe calcium potential using a Ca2+selectiveelectrode.The effectof
the presenceof the hydrolyzed (anionic) polymer on the calcium measurement was also monitored and taken into account in the calibrating system
(Fig. 1). There is significant_interference but only above 100 ppm polymer.
Evidently the-anIonic polymer quenchesthe calcium to a certain extent or
there is someprecipitation of calcium with the negatively chargedpolymer. .
Electrokinetic studies
Electrokinetic mobility of Na-kaolinite was measuredusing a Zeta-Meterin
mineral supernatants at 0.1% solids content.
.:~.~.
1.10
> 1501
E
.140
.J
«
~
i
I
I: 3110-2kmol/m3 NaCI
T: 23; 2.C
CondoSpeed: 1200rpm
Cond.Time : 10mln
Conc. of Co..: 100 ppm (2.5 11O-3M)
Polymer:
33% HydrolyzedPolyacrylamide
pH: 6.60-6.44
~--
1301
~
1201
0
~.f!:T!..Q~ !~.fi.t~~
o--.o
:--:
~
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~ 110
J
1
\01
100'
I
-
0
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=J
I
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I
400
600
800
POLYMER CONCrNTRATION, p pm
200
INITIAL
Fig. 1. Effect of polymer on measuring calcium potential
"
,@
~
.
1000
by using a Caz+ selective electrode.
.0
0.9
e 0.8
..:
..
0
u07
0
~0.6
in
z 0.5
\II
Q
z 0.4
2t: 0.3
«
gO.2
Q
--
.
pH: 11.13-11.26
~ 9.53-9.70
0
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., "--- '-'--
~ ; ~; 1.53- 1.60
c( 0.1
0
-
0
I
I
I
I
I
I
I
I
I
100 200 300 400 ~
600 700 800 900 1000
INITIAL CALCtUMCONCENTRATION
01 Co, ppm
Fig. 2. Effect of initjaJ calcium concentration on adsorption.
RESULTSAND DISCUSSION
" dsorption
Results obtained for adsorption of calcium by Na-kaolinite are given in Figs
land 3 as a function of calcium concentration and pH, respectively. Calcium
Idsorption density increaseswith concentration and is higher at elevated pH
( Fig. 2) . The more interesting feature, however,is the sharp rise in adsorption
131
N
e
I : 3 x 10-2 kmol/m3 NoCI
T: 22'f2.C
CondoTime: 30minute,
0.30
,
~t
c.
,r~'
e
-~O25
100
fa
(.)
~
u
9O_E
00.
80 Z 0.
0.20
>0.15
Z
0
i=
IL
70
60
. CoT
\&J
0
50
-z
~o
~§
z
IAJ .J
uO
zU)
40 °
0.10
30
~
~
0.05
0
ct
~.,
,
-
0
,,,
2
3
"
4
-
0
0 CO'.
DoCoT
t:
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,',',
6
cl°
~u
.
./
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So
u+
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I
1 \.
'.~O«
2000
-z
10 ~ cI
7
8
9
10 11 12 13 14
pH
Fig. 3. Adsorption of calcium from 100 ppm aqueous solutions by Na-kaolinite.
above pH 10 (Fig. 3). Similar behavior has been observedin the past for calcium adsorption on silica [22]. The sudden increase in adsorption may be
attributed to formation of calcium-hydroxide complexesin solution and at the
~
1.12
+20
+10
~
>
E
-10
.J
~
.Z
1&1
.0
a.
-20
-30
I: 3Klo-2kmol/m3 NoCI
T: 23+28C
CondoTime: 30 minutes
~
1&1
N
-40
-50
-60
I
0
I
I
I
I
200
400
600
800
CONCENTRATION
of CALCIUM
I
I
1000 1200
os Co++,
ppm
Fig. 5. Zeta potential valuesof Na-kaolinite after adsorption of calcium as a function of concentration of-calciumas Ca2+.
CII
0.30
E
0-
E 0.25
..:
.
0
u
0.20
0
>I-
~ 0.15
1&1
0
Z
0
0.10
~
Q.
Q:
0.05
0
0
.
.
1
2
Fig. 6. Effect of conditioning
.
.
.
.
3
4
5
6
CONDITIONING TIME, hours
.
.
.
7
8
9
10
time of the suspension with calcium on adsorption.
~
133
.
mIneral
surface
15,22,23]
.
s;
~"~
The presenceof calcium alsocausessignificant changein zetapotential above','
pH 9 (Figs 4 and 5). While increasing calcium concentration causessome
reduction in negativezeta potential at pH 7.5,the effect is dramatic at pH 11.3
~~~)_.~~h_~~~_otential
chan~~!~1!!c2P!!~edto~dueto
adsorption
of tile hy<Iioxycomplex, CaOH~ by hydrogen bonding or water forming reactions betweenthe complex and protons on the solid surface [ 13] . It hasbeen
shown, however,that under theseconditions there can be surfaceprecipitation
of Ca (OH) 2 [15]. The negativepotential in the interfacial region givesriseto
an excessof Ca2+ and the solubility product is exceededthen at the interface
before the bulk solution attains the critical concentration. The precipitated
Ca (OH) 2,which has a point of zero chargeabove 12 [23], will impart positive
potential to the surface which accounts for the observedincrease in zetapotential abovepH 9.
There is another possibleexplanation for the observedbehavior. Carbonate
in solution open to atmosphereicCO2 (g) increasesdramatically at high pH
and can lead to calcium carbonate precipitation [24]. Formation of CaCO3is
thermodynamically favored over Ca(OH)2 and it too has a higher PZC than
kaolinite [24]. Presenceof either of these specieson the kaolin surfacewill
reducemeasuredzeta potential.
The effect of conditioning time of the suspensionin calcium salt solution on
Ca adsorption by kaolinite at pH 7.4 and 11.4is shown in Fig. 6. While condie
tioning time had no effect on Ca adsorption at pH 7A, a sudden marked risein
adsorption resulted at pH llA at 2 h followed by a steady decreasethereafter.
The reasonsfor the sharp rise in calcium adsorption are not evident at present.
It is suggestedthat it may be due to onset of precipitation of calcium species
at the surface followed by somedetachment of crystallites.
Flocculation
Calcium alone has only a small effect on kaolinite aggregation. When the
percent solids settled is comparedin the absenceand presenceof calcium (Fig.
7), there is a small decrease(dispersion) below pH 11 and a measurableincreaseabovepH 11.This latter is evidently dueto the decreasein zeta potential
caused by the calcium in alkaline solutions. At pH 11, the zeta potential is
reducedfrom - 55 mV to - 20 mV in the presenceof 100ppm calcium.
The results in Fig. 7 show the flocculation of the clay by nonionic polyacrylamide in the presenceof calcium and as a function of pH. Adsorption of the
polymer in these systems was total at the tested level (50 ppm) both in the
absenceand presenceof calcium and was independent of pH. As expected,the
polyacrylamide causedconsiderableflocculation of the kaolinite and, interestingly enough, addition of calcium a further increasein flocculation.
Flocculation of kaolinite by anionic polymer, HPAM (33% hydrolyzed), in
13.
80
-
70
~~~~::::::=.
~
6a
- -- -
~-~
.:;;
..~
050
-
,;.-'-'
:~=-~~~.
1&1
-J
.-
Somple: No-Koolinlte
1;j 40
II)
0
~.2 x 106MW
CI : 50mg/kg
Conc. of Co'.:
100ppm
Polymer: PAM,
"
0
D
4
~,
.-~--:::=
Without Cdclum and Polymer
With Polymer without Calcium
With Calcium and Polymer
With Calcium without Polymer
I
0.3
~30
II)
~
i~
a:
U)w
20
L.
0
0
S/L:
10
T
I
0
QI
0
:;.-=
0
0
z~
r
0.2 0w)- ..J
8
0.025
: 23i28C
: 3x10-2kmol/m3 NoCI
~
~
i=-0.1 ~
g
Cond.tlme Colcium- 30minute.
Polymer -10mlnute.
0< 0
0
2
3
4
5
6
7
8
9
10 11 12 13 14
pH
Fig. 7. Effect of PAM on flocculation of Na-kaolinite in the presence of calcium (CaT, total
calcium (~ithout distinguishing among species); Ci, initial concentration; SIL, solidf1iquid ratio;
I, ionic.strength).
80r-:
I
Sample: No-KaolinIte S/L: 0.025
Polymer: HPAM,33%
T
: 23+ 2.C
(6.6 x ioe MWI I
: 3xlO-2 kmal/m3
CI
: 50mg/kg
NaCI
Cone.of Co++: 1O0ppm CondoTime:
-
.
70
60
CalcIum -30mlnutes
Polymer -lOminutes
050
.
IIJ
..J
~40
IIJ
II)
Without Colcium or Polymer
Ne
0 With Polymer, Without Calcium
0 With Calcium and Polymer
e
~
~
-.
~
0.3 t=
:J 30
U)..
0
II)
at
~20
.
0.1 O~
%°
.1°.20-0%
:;.
'4
~
~
i i
%e
UJ»-
...
-
%
0
[ 50
UJ
U
0
u
10
'
0-«0
100~III
8
9 10 11 12 13 14
pH
Fig. 8. Effect of HPAM on noccuJation of Na.kaolinite in the presence of calcium.
0
2
3
4
5
6
7
'
I .
135
I: 3xlO-2kmd/m'
T: 23i2.C
E
a.
a.
HoCI
(33% 6.6.tO5MW)
CondoSpeed: 1200rpm
( HPAM)
Polymer -1 Omkl
. Canc. of Co..: 100 ppm
.: 100
.
100 ...
.G
~
8
G
Z 80
80 ~
.2
...
0 Before Polymer oddition
~ Effect of stirring on desorption
of calcium before adding
60
.
Polymer (with blank lolutlon)
Z
\II
D Af ter Polymer addition
U
% De,orptlon of Colcium os totol
40 .
c(
a:
...
~
:»
e
20.
.-,,'
.
a: 02
I
I
I
I
I
3
4
5
6
7
I
I
60
'~T
. Percentage amount of the adsorbed
Calcium de,orbed by HPAM
In
\II
~£
.
8
I
~
-~:»
c(
u
0
1
40
z
~
20
~
g
\II
.
0
I
I
I
~,
;~';
, ,(
c.
~~
"~
Conc.of HPAM: 50ppm
Condotime: Calcium- 30mln
1
0
~
8
9
10 11 12 13 14
pH
Fig. 9. Adsorption of cakium by Na-kaolinite before and afteredding polymer.
1~6
the absenceand presenceof calcium was investigated as a function of pH.
Polymer enhancesflocculation only at quite low pH and calcium has no additional effect. Solids settled is the sameas with calcium addition (no polymer)
up to pH 10.Results obtained for polymer adsorption and flocculation are also
givenin Fig. 8. It can beseenfrom this figure that the adsorption of the polymer
was increasedby the addition of calcium abovepH 9. However, there was insignificant changein the flocculation as me-asuredby the solids settled.
= In=om el'-to--study-thiSC'
pc}}en0me:n:o
rffu~t
he-effeCf:Ofpo
lymer-ijf1~
sorption by kaolinite was also examined. Adsorption of calcium before and
after polymer addition was determined. Results obtained are presentedin Fig.
9. As seen from this figure, addition of the hydrolyzed polymer did causedesorption of calcium. To determine the nature of desorption or detachment,
control experiments were carried out in the absenceof polymer. It can be seen
from Fig. 9 that the stirring process itself (without polymer) does produce
measureabledesorption of calcium from the clay.
These results suggestthat calcium depletion from solution might be due to
surfaceprecipitation of calcium specieswhich can be detachedby stirring. It
appearsthat HPAM enhancedthe detachment possibly due to increasedparticle-particle abrasion in the flocculated system. It is also likely that the calcium speciesform a complex with HPAM and then detach from the surface.
In this.:regard,it is interesting that the flocculation of kaolinite by the anionic
polYn1erwas not affected by the presenceof calcium.
The zeta potential of kaolinite in the presenceof both calcium and HPAM
as a function of pH is given in Fig. 10. These results show that polymer alone
reducesthe magnitude of the zeta potential only slightly. The addition of polymer.with calcium causescomplex zeta potential response.However, abovepH
10,the negativepotential is restored at least partially, suggestingthat someof
what adsorbedor precipitated on the surfacewas removedby the polymer.
SUMMARY
Clay flocculation by calcium, by nonionic and anionic (33% hydrolyzed)
polyacrylamide and by mixtures of calcium and polymer hasbeenstudied. Calcium adsorption and polymer adsorption have beenexaminedas well. Electrokinetic data support the results.
Calcium has its maximum effect at high pH where kaolinite aggregationis
slightly enhanced,the magnitude of zeta potential is reducedand calcium adsorption showsa sharp increase.These effects are attributed to the formation
and adsorption ofCaOH+. At high pH, surface precipitation of the hydroxide
or carbonatecan occur but additional stirring is able to causesomesubsequent
desorption.
Nonionic polyacrylamide flocculates the clay and this is enhancedby the
addition of calcium even though calcium has no effect on polymer adsorption.
Calcium may be increasing bridging among particles.
The anionic polyacrylamidehasno effect on flocculation exceptat very acidic
pH. The addition of calcium does not affect these results significantly even
though it seemsto leadto higher polymeradsorptionat high pH. Adsorbed~?1
calcium speciesmay p~ovideadditional adsorption site~ for _~~eanionic ~1x-~,J
mer:ccoo
t.c-due-to=repulslv~ffeGtsfdonor tea~t1occ-u latWn. ze~a'l>OfeIn
tJa~t
results at high pH support the hypothesis that the anionic polymer interactst,tl
with calcium species,removing them from the surface.
'~~~,!I
ACKNOWLEDGEMENTS
The authors gratefully acknowledgethe support of the Multiphase and Interfacial Phenomenaprogram of the National ScienceFoundation.
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1:18
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i
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,