Adsorption Properties of Ca2+ on N a-Kaolinite and I ts ...ps24/PDFs/Adsorption Properties... · INITIAL CALCtUM CONCENTRATION 01 Co, ppm Fig. 2. Effect of initjaJ calcium concentration

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127Colloids and Surfaces, 32 (1988) 127-138Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands

Adsorption Properties of Ca2+ on N a-Kaolinite andI ts Effect on Flocculation Using Polyacry lamides ~

G. ATESOK*, P. SOMASUNDARAN and LoIJ. MORGAN

Henry Krumb School of Mines. Columbia University, New York, NY 10027 (U.S.A.)

(Received 20 April 1987; accepted in final Conn 26 October 1987)

ABSTRACT

Adsorption behavior of calcium ions on Na-kaolinite and the adsorption and flocculation be-havior of Na-kaolinite by polyacrylamide in the presence of calcium species in solution have beeninvestigated. Parameters examined in calcium adsorption experiments include pH, conditioningtime of the suspension with calcium, and calcium concentration. The effect of calcium on adsorp-tion of polyacrylamide and hydrolyzed polyacrylamide by the kaolinite and on its flocculation isdetermined. Also the effect of the polymer on calcium adsorption is examined.

Adsorption of calcium occurs only under pH conditions where it hydrolyzes to CaOH+ andCa(OH)2. Calcium adsorption (abstraction) is sensitive to the presence of polymer as well as tostiuing conditions. Interestingly, above pH 8, stirring and addition of polymer (HPAM 33) werefound to cause calcium desorption suggesting detachment of precipitate from the kaolinite surface.

INTRODUCTION

Mineral fines can be flocculated by reducing the electrostatic repulsion be-tween particles, e.g. by using inorganic salts, and by using polymers which arecapable of bridging the particles with one another. Polyacrylamides are widelyused as a flocculating agent for fine particle suspensions and their adsorptionhas been generally characterized [ 1-4 ] . Most of these systems can also containionic species which may alter the surface physicochemical properties of theparticles significantly [5,6]. For example, ions such as Ca2+ and Mg2+ havebeen reported to change the zeta potenti~oi-9c-u_artz (7,8] as_~~ll~t9~ffectits-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 con-ducted [10,11]. Many of these have shown the important role of pH in con-trolling cation adsorption on hydrous metal oxides [12]. In some cases,

.Present address: Department of Mineral Dressing, Mining Faculty of Istanbul Technical Uni-versity, Tesvikiya, Istanbul, Turkey.

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128

increased uptake of the metal ion occurs in a narrow pH range. This is partic-ularly true for strongly hydrolyzable cations [13,14]. For these systems, ad-sorption has been correlated primarily with the hydrolysis characteristics ofthe adsorbing ion rather than with the charge properties of the mineral. Onthe other hand, a recent work detailing the effect of conditions in the doublelayer has shown that in a number of cases, phenomena attributed to adsorptioncan in fact be explained by surface precipitation [15].

Some work has been done in the past on the adsorption of inorganic ions onkaolinite. For example, the selectivity sequence on kaolinite is found to beLi+ <Na+ <K+ <Cs+ [16,17]. However, there is little information on diva-lent cation selectivity on kaolinite [18,19]. There has also been some work onthe adsorption of and flocculation by polymers on kaolinite [1-4,20]. Both ofthese processes have been shown to depend significantly on the charge char-acteristics of the polymer and the mineral as determined by the pH and ionicstrength. Indeed, the behavior of these systems is extremely complex whenboth the inorganic and polymer species are present and almost no informationis available for such cases even though both are invariably present in most

flocculating systems.In this study, the adsorption. response of calcium on Na-kaolinite and the

adsorption and flocculation ofNa-kaolinite by polyacrylamides in the presenceof calcium in solution have been investigated. Results obtained for adsorptionand fl6Cculation are discussed with 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 homo-ionic sample of sodium kaolinite was prepared from this using an ion-exchangetreatment discussed elsewhere [20,21]. ;

---Potymers c- :c-::--=--- ",cccc=-=,,;

The 14C-Iabeled nonionic polyacrylamide (PAM) of 5.2XIO6 molecularweight and 33 % hydrolyzed polyacrylamide (HP AM 33) of 6.6 X lOS molecularweight used here were synthesized and characterized by American CyanamidCompany.

*The word adsorption is used somewhat loosely here as it is clear there may also be precipitationin the system. It might be more correct to use "calcium abstraction" to include both the calciumwhich adsorbs and that which precipitates. However, for simplicity, "adsorption" is used and thelikelihood of precipitation is kept in mind.

:129"1 .,

J norganic reagents ;;,

Fisher certified HCI and NaOH were used to adjust the pH of the suspension. \Amend Drug and Chemical Company reagent grade NaCI (99.96% pure) was .

"used to prepare the 3 X 10-2 kmol m -3 salt solution used in all the experiments~:

to control ionic strength, J. .~;i:J,~ ~

I . L:AMethods ' ~f'~

.~~:~.

Flocculation procedureFor 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-actionshaker and for an additional 30 min after adding desired amounts of calciumsalt. Polymer which is in 40 ml salt solution was then added and the pulpstirred for 10 min using a 1 inch diameter propeller at 1200 rpm. These con-ditions were identified as sufficient to achieve equilibrium adsorption (i.e., noadditional change measured). Sedimentation tests were carried out in 250 cm3beakers at 2.5% (by weight) pulp density. The slurry was allowed to settle for30 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 determinationsSupernatant samples were centrifuged for 10 min at 10,000 rpm, using an

IEC centrifuge and analyzed for residual polymer using a Beckman LiquidScintillation Counter (LSC). ~..

Cakium determinationi Supernatants were analyzed for total calcium, CaT, also using the LSC to~ount 45Ca. Ca2+ concentration in some of the samples was also determined bymeasuring the calcium potential using a Ca2+ selective electrode. The effect ofthe presence of the hydrolyzed (anionic) polymer on the calcium measure-ment 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 quenches the calcium to a certain extent orthere is some precipitation of calcium with the negatively charged polymer. .

Electrokinetic studiesElectrokinetic mobility of Na-kaolinite was measured using a Zeta-Meter in

mineral supernatants at 0.1 % solids content. .:~.~.

1.10

> 1501E.140

.J« I

~

i 1301

~ 12010~ \~ 110J 1\01

I: 3110-2kmol/m3 NaCIT: 23; 2.CCondo Speed: 1200rpmCond.Time : 10mlnConc. of Co..: 100 ppm (2.5 11O-3M)Polymer: 33% Hydrolyzed PolyacrylamidepH: 6.60-6.44 ~--

~.f!:T!..Q~ !~.fi.t~~ ~~ o--.o :--: = J

100' I - I I I I .0 200 400 600 800 1000

INITIAL POLYMER CONCrNTRATION, p pm

Fig. 1. Effect of polymer on measuring calcium potential by using a Caz+ selective electrode.

.0",@ 0.9~e..: 0.80

u07. .0

~0.6inz 0.5\IIQz 0.42-t: 0.3«gO.2Qc( 0.1

. --" 0 pH: 11.13-11.26

"--- ~ 9.53-9.70., '-'-- ~ ; ~; 1. 53 - 1. 6 0

0 - I I I I I I I I I

0 100 200 300 400 ~ 600 700 800 900 1000INITIAL CALCtUM CONCENTRATION

01 Co, ppm

Fig. 2. Effect of initjaJ calcium concentration on adsorption.

RESULTS AND DISCUSSION

" dsorption

Results obtained for adsorption of calcium by Na-kaolinite are given in Figsland 3 as a function of calcium concentration and pH, respectively. CalciumIdsorption density increases with concentration and is higher at elevated pH( Fig. 2) . The more interesting feature, however, is the sharp rise in adsorption

131

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~ 0.20>-t:~ 0.15\&J0

Z0 0.10i=IL~

~ 0.050ct

I : 3 x 10-2 kmol/m3 NoCIT: 22'f2.CCondo Time: 30minute,

0 CO'.

Do CoT

. CoT

100 ~u

9O_E00.80 Z 0.

0 --z70 ~o60 ~§

z50 IAJ .J

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30 .J +cl°~u. 2000./ -z

- ~ ., , 1 \. 1 0 ~ cI0 ,,, " ,',', I I '.~O«

2 3 4 5 6 7 8 9 10 11 12 13 14pH

Fig. 3. Adsorption of calcium from 100 ppm aqueous solutions by Na-kaolinite.

above pH 10 (Fig. 3). Similar behavior has been observed in the past for cal-cium adsorption on silica [22]. The sudden increase in adsorption may beattributed to formation of calcium-hydroxide complexes in solution and at the

1.12

+20

+10

~

~

>E

-10

-20

-30

-40

.J~.-Z1&1.-0a.

~1&1N

I: 3Klo-2kmol/m3 NoCIT: 23+28CCondo Time: 30 minutes

-50

-60 I I I I I I I

0 200 400 600 800 1000 1200CONCENTRATION of CALCIUM os Co++, ppm

Fig. 5. Zeta potential values of Na-kaolinite after adsorption of calcium as a function of concen-tration of -calcium as Ca2+.

CII 0.30E0-E 0.25

..:0u. 0.200

>-I-~ 0.151&10Z0 0.10~Q.Q:

0.05

0 . . . . . . . . .

0 1 2 3 4 5 6 7 8 9 10CONDITIONING TIME, hours

Fig. 6. Effect of conditioning time of the suspension with calcium on adsorption.

~133

. s;mIneral surface 15,22,23] . ~"~

The presence of calcium also causes significant change in zeta potential above ','pH 9 (Figs 4 and 5). While increasing calcium concentration causes somereduction in negative zeta potential at pH 7.5, the effect is dramatic at pH 11.3

~~~)_.~~h_~~~_otential chan~~!~1!!c2P!!~edto~dueto adsorptionof tile hy<Iioxycomplex, CaOH~ by hydrogen bonding or water forming re-actions between the complex and protons on the solid surface [ 13] . It has beenshown, however, that under these conditions there can be surface precipitationof Ca (OH) 2 [15]. The negative potential in the interfacial region gives rise toan excess of Ca2+ and the solubility product is exceeded then at the interfacebefore the bulk solution attains the critical concentration. The precipitatedCa (OH) 2, which has a point of zero charge above 12 [23], will impart positivepotential to the surface which accounts for the observed increase in zeta po-tential above pH 9.

There is another possible explanation for the observed behavior. Carbonatein solution open to atmosphereic CO2 (g) increases dramatically at high pHand can lead to calcium carbonate precipitation [24]. Formation of CaCO3 isthermodynamically favored over Ca(OH)2 and it too has a higher PZC thankaolinite [24]. Presence of either of these species on the kaolin surface willreduce measured zeta potential.

The effect of conditioning time of the suspension in calcium salt solution onCa adsorption by kaolinite at pH 7.4 and 11.4 is shown in Fig. 6. While condietioning time had no effect on Ca adsorption at pH 7 A, a sudden marked rise inadsorption resulted at pH llA at 2 h followed by a steady decrease thereafter.The reasons for the sharp rise in calcium adsorption are not evident at present.It is suggested that it may be due to onset of precipitation of calcium speciesat the surface followed by some detachment of crystallites.

Flocculation

Calcium alone has only a small effect on kaolinite aggregation. When thepercent solids settled is compared in the absence and presence of calcium (Fig.7), there is a small decrease (dispersion) below pH 11 and a measurable in-crease above pH 11. This latter is evidently due to the decrease in zeta potentialcaused by the calcium in alkaline solutions. At pH 11, the zeta potential isreduced from - 55 m V to - 20 m V in the presence of 100 ppm calcium.

The results in Fig. 7 show the flocculation of the clay by non ionic polyacryl-amide in the presence of calcium and as a function of pH. Adsorption of thepolymer in these systems was total at the tested level (50 ppm) both in theabsence and presence of calcium and was independent of pH. As expected, thepolyacrylamide caused considerable flocculation of the kaolinite and, interest-ingly enough, addition of calcium a further increase in flocculation.

Flocculation of kaolinite by anionic polymer, HPAM (33% hydrolyzed), in

13.

80

-70

6a

0501&1-J.-

1;j 40II)0

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~ 20

~~~~~::::::=. - - - - ~,

~-~ .:;; - .-~--:::=

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Somple: No-KoolinltePolymer: PAM, " Without Cdclum and Polymer

~.2 x 106MW 0 With Polymer without CalciumCI : 50mg/kg D With Calcium and PolymerConc. of Co'.: 100ppm 4 With Calcium without Polymer

I i0.3 ~ a:

U)wz~w)-0 0 0 QI 0 :;.-= 0 0 8 0.2 0 ..J

S/L: 0.025 r ~ ~T : 23i28C i=--I : 3x10-2kmol/m3 NoCI 0.1 ~Cond.tlme Colc ium - 30 minute. g

Polymer -10mlnute. 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, totalcalcium (~ithout distinguishing among species); Ci, initial concentration; SIL, solidf1iquid ratio;I, ionic.strength).

L.

10

0

80 r-:70

60

050IIJ..J

~40IIJII)

30

~20

I Sample: No-KaolinIte S/L: 0.025- Polymer: HPAM, 33% T : 23+ 2.C

(6.6 x ioe MW I I : 3xlO-2 kmal/m3CI : 50mg/kg NaCICone. of Co ++: 1O0ppm Condo Time:. CalcIum -30mlnutes

Polymer -lOminutes-

. Without Colcium or Polymer0 With Polymer, Without Calcium0 With Calcium and Polymer

:;.10

Ne ~%

~ 0e -~ '40.3 t= - ~

~U)...%e UJUJ»- U

. ... .1°.20-0% %° 0 O~ u 0.1 i [ 50 i- 0 III0 ' 0-« 100~

2 3 4 5 6 7 8 9 10 11 12 13 14pH

Fig. 8. Effect of HPAM on noccuJation of Na.kaolinite in the presence of calcium.

:J0II)at

'I .

1~,

;~';, ,(c.

135

~~"~Conc. of HPAM: 50ppm(33% 6.6.tO5MW)

Condo Speed: 1200rpm( HPAM)

Ea.a.

I: 3xlO-2kmd/m' HoCIT: 23i2.CCondo time: Calcium - 30mln

Polymer -1 Omkl.: 100 . Canc. of Co..: 100 ppm 100 ...

~ 8. .G GZ 80 80 ~.2 :»... -c( ~a: 60 . 60 c(... uZ -\II 0U z8 40 . 40 ~~ ~:» 20.

1 20 g e \II

In . 0\II .a: 0- I I I I I I I I I I I 0 ~

2 3 4 5 6 7 8 9 10 11 12 13 14pH

Fig. 9. Adsorption of cakium by Na-kaolinite before and afteredding polymer.

~£'~T

0 Before Polymer oddition~ Effect of stirring on desorption

of calcium before addingPolymer (with blank lolutlon)

D Af ter Polymer addition. % De,orptlon of Colcium os totol. Percentage amount of the adsorbed

Calcium de,orbed by HPAM ~.-,,'

1~6

the absence and presence of calcium was investigated as a function of pH.Polymer enhances flocculation only at quite low pH and calcium has no addi-tional effect. Solids settled is the same as with calcium addition (no polymer)up to pH 10. Results obtained for polymer adsorption and flocculation are alsogiven in Fig. 8. It can be seen from this figure that the adsorption of the polymerwas increased by the addition of calcium above pH 9. However, there was in-significant change in the flocculation as me-asured by the solids settled.

= In= om e l'-to--study-thiSC' pc }}en 0 me:n:o rffu~t he -effeCf:Ofpo lymer-ijf1~sorption by kaolinite was also examined. Adsorption of calcium before andafter polymer addition was determined. Results obtained are presented in Fig.9. As seen from this figure, addition of the hydrolyzed polymer did cause de-sorption of calcium. To determine the nature of desorption or detachment,control experiments were carried out in the absence of polymer. It can be seenfrom Fig. 9 that the stirring process itself (without polymer) does producemeasureable desorption of calcium from the clay.

These results suggest that calcium depletion from solution might be due tosurface precipitation of calcium species which can be detached by stirring. Itappears that HP AM enhanced the detachment possibly due to increased par-ticle-particle abrasion in the flocculated system. It is also likely that the cal-cium species form a complex with HPAM and then detach from the surface.In this.:regard, it is interesting that the flocculation of kaolinite by the anionicpolYn1er was not affected by the presence of calcium.

The zeta potential of kaolinite in the presence of both calcium and HPAMas a function of pH is given in Fig. 10. These results show that polymer alonereduces the magnitude of the zeta potential only slightly. The addition of poly-mer. with calcium causes complex zeta potential response. However, above pH10, the negative potential is restored at least partially, suggesting that some ofwhat adsorbed or precipitated on the surface was removed by the polymer.

SUMMARY

Clay flocculation by calcium, by nonionic and anionic (33% hydrolyzed)polyacrylamide and by mixtures of calcium and polymer has been studied. Cal-cium adsorption and polymer adsorption have been examined as well. Electro-kinetic data support the results.

Calcium has its maximum effect at high pH where kaolinite aggregation isslightly enhanced, the magnitude of zeta potential is reduced and calcium ad-sorption shows a sharp increase. These effects are attributed to the formationand adsorption ofCaOH+. At high pH, surface precipitation of the hydroxideor carbonate can occur but additional stirring is able to cause some subsequentdesorption.

Nonionic polyacrylamide flocculates the clay and this is enhanced by the

addition of calcium even though calcium has no effect on polymer adsorption.Calcium may be increasing bridging among particles.

The anionic polyacrylamide has no effect on flocculation except at very acidicpH. The addition of calcium does not affect these results significantly eventhough it seems to lead to higher polymer adsorption at high pH. Adsorbed ~?1

calcium species may p~ovide additional adsorption site~ for _~~e anionic ~1x-~,Jmer:ccoo t.c-due-to=repulslv~ffeGtsfdonor tea~t1occ-u latWn. ze~a'l>OfeIn tJa~tresults at high pH support the hypothesis that the anionic polymer interactst,tlwith calcium species, removing them from the surface. '~~~,!I

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of the Multiphase and In-terfacial Phenomena program of the National Science Foundation.

REFERENCES

1 P. Somasundaran, Principles of Flocculation, Dispersion, and Selective Flocculation. in P.Somasundaran (Ed.), Fine Particle Processing, AIME, New York, Vol. 2,1980, p.947.

2 A.F. Hollander, P. Sornasundaran and C.C. Gryte, Adsorption Characteristics of Polyacryl-amide and Sulfonate-Containing Polyacrylamide Copolymers on Na- Kaolinite, J. Appl. Polym.Sci., 26 (1981) 2123.

3 P. Somasundaran, Y.H. Chia and R. Gorelik, Adsorption of Polyacrylamides on Kaoliniteand Its Flocculation and Stabilization, ACS Symp. Ser., 240 (1984) 393-410.

4 P.J. Dodson and P. Somasundaran, Desorption of Polyacrylamide and Hydrolyzed Polyacryl-amide from Kaolinite Surface, J. Colloid Interface Sci., 97 (1984) 481-487.

5 K.P. Ananthapadmanabhan and P. Sornasundaran, Role of Dissolved Mineral Species in

Calcite-Apatite Flotation, Miner. Metall. Process., 1 (1984) 36-42.6 R.H. Heerema and I. Iwasaki. Chemical Precipitation of Alkaline Earth Cations and Its

Effect on Flocculation and Flotation of Quartz, Trans. Am. Inst. Min. Metall. Eng.. 268

(1980) 1510-1516.7 J.K. Critchley and S.H. Jewitt, The Effect of CU2+ Ions on Zeta Potential of Quartz, lost.

Min. Metall. Trans. Sec. C, 88 (1979) 57-59.8 S. V. Krishnan, Influence of Surface Precipitation on Separation Process, Presented in the

15th Annual Metting of the Fine Particle Society, Orlando, FL, August 1984.-9 R.H. Heerema, R.J. Lipp and I. Iwasaki, Complexation of Calcium Ion in Selective Floccu-

lation of Iron Ore, Presented at the Annual AIME Meeting, New Orleans, 1979.10 G.M. Zhabrova and E. V. Egorov, Sorption and Ion Exchange on Amphoteric Oxides and

Hydroxides, Russ. Chem. Rev., 30 (1961) 338-346.11 V. Vesely and V. Pekarek, Synthetic Inorganic Ion-Exchanger. I. Hydrous Oxides and Acidic

Salts of Multivalent Metals, Talanta, 19 (1972) 219-262.12 A. Breeuwsma and J. Lyklema, Physical and Chemical Adsorption of Ions in the Electrical

Double Layer on Hematite [a-Fe203] , J. Colloid Interface Sci., 43 (1973) 437-448.13 R.O. James and T.W. Healy, Adsorption of Hydrolyzable Metal Ions at the Oxide-Water

Interface, J. Colloid Interface Sci., 40 (1972) 65-81.14 V.V. Pushkarev, Adsorption of Radioactive Isotopes on Ferric Hydroxide, Russ. J. Inorg.

Chem., 1 (1956) 176-185.

1:18

l.r; K.P. Ananthapadmanabhan and P. Somasundaran, Surface Precipitation of Inorganic andSurfuct.'1nts and Its Effect on Flotation, Colloids Surfaces, 1:l (1985) 151-167.

16 A.P. Ferris and W.B. Jepson, The I-:xchange Capacities of Kaolinite and the Preparation ofHomoionic Clays, J. Colloid Interface Sci., 51 (1975) 245-259.

17 M.D.A. Bolland, A.M. Posner and J.P. Quirk, Surface Charge on Kaolinite, Aust. .J. Soil Res.,14 (1976) 197-216.

18 E.J. Udo, Thermodynamics of Potassium-Calcium and Magnesium-Calcium Exchange Re-actions on a Kaolinite Soil Clay, Soil Sci. Soc. ~m. J., 42 (1978) 556-560.

-:l9=~~'; E. B i tt.e I and-.R;.}.M ille~L-I!:ad;: £admiumandCalciumSelcctMtyceuefficien t,s{}~on to;-morillonite, Illite and Kaolinite, J. Environ. Qual., 3 (1974) 250-253.

20 A.F. Hollander, P. Somasun.daran and C.C. Gryte, in P.R. Tewari (Ed.), Adsorption fromAqueous Solutions, Plenum, New York, 1981, pp. 393-410.

2:1 H. Van Olphen, An Introduction to Clay Colloid Chemistry, Wiley, New York, 1977.22 S.W. Clark and S.R.B. Cooke, Trans AIME, 241 (1968) 334.23 M.C. Fuerstenau and B.R. Palmer, Anionic Flotation of Oxides and Silicates, in M.C. Fuer-

stenau (Ed.) , Flotation, A.M. Gaudin Memorial, Vol. I, AIME, NewYork,1976,pp.148-196.24 P. Somasundaran, J. Ofori Amankonah and K.P. Ananthapadmanabhan, Mineral-Solution

Equilibria in Sparingly Soluble Mineral Systems, Colloids Surfaces, 15 (1985) 309-333.

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