Role of polymer adsorption in the behaviour of the electrical double layer at the calcium carbonate—electrolyte interface

Power

Technology.

0 Elsevier Sequoia

27 (1980) 227 - 231 S.A., Lamanne -Printed

227

in the Netherlands

Role of Polymer Adsorption in the Behaviour of the Electrical at the Calcium Carbonate-Electrolyte Interface ALICJA

Double

Layer

MONIES

of Radiochemistry University. Lublin

Department SkIodowska

(Received

March 6.1980;

and Application (Poland)

of

Instirute

of

Chemistry,

Maria

Curie-

in revised form July 23,198O)

SUMMARY

Isotherms of polyacrylamide adsorption on calcium carbonate are presented_ It was found that the presence of KCIOl and KNOB electrolyte slightly increases adsorption_ Potentiometric measurements showed that PAM adsorption causes a small reduction of the surface charge in the range of the positive charge values comparable to the growth of absolute value of charge, Ial, in the range of its negative values. A further effect of the polymer adsorption is the shift of pH,,, towards lower pH values.

INTRODUCTION

Studies of flocculation have proved that the process proceeds if requirements such as incomplete particle surface coverage by polymer, satisfactory length of the polymer chain and considerable strength of polymer bondage with the surface are met. These ensure formation for polymer bridges between solid particles of an aqueous suspension resulting in destabilization of the dispersed system [l] . The sum of forces operating in a flocculated system can be expressed as: v, = v,

Radioisotopes,

electrical double layer. On the other hand, the steric effects must be taken into account even at a small polymer coverage of the surface. Thus, in general an equation expressing interparticle interactions effects of polymer adsorption on V,, V, and V, must be taken into account. In the case of mineral suspensions which, as a rule, are stabilized electrostatically, the effect of adsorbed polymer on the component V, is of considerable interest, as the macromolecules - or rather macro-ions - can change the structure of the electrical double layer. The nature and extent of these changes have not been explained adequately so far. There have been very few publications dealing with the effect of adsorbed polymer on the electrical double layer structure [2 9]_ Most of these - mainly by Lyklema, Fleer and co-workers - relate to the best known colloid system, i.e. AgI [2 - 6] _ In this paper an attempt is made to describe the electrical double layer in the system: precipitated calcium carbonate-polyacrylamide, PAM solution. This system was chosen because of its practical importance. Calcium carbonate is the main component of many flotation wastes and its properties are known from the literature 110 - 121. On the other hand, polyacrylamide is the main component of an important group of flocculants.

+ v, + Vb

where V, = attractive forces according to DLVO theory, V, = repulsive forces, eiectrostatic by nature, according to DLVO theory, and V, = bridging interactions. In reality, systems in which only one factor changes the stability are not met with. It must be kept in mind that polymer adsorption on particles of a suspension at zero point of charge, zpc, change the parameters of the

MATERIALS

AND

METHODS

Calcium carbonate was precipitated from concentrated solutions of NasCOs and CaCls. The precipitate was rinsed with distilled water until the chloride ions were washed out, then filtered off and dried at 100 “C. The crystal size as determined under the microscope was in the range 2 - 4 nm. The specific surface

228

area of t’ne precipitate was measured by a radioisotopic method [13] using Ca-45 in SOIUtion with CaCOs. The specific surface area of three successive batches of the precipitate was: 0.920 m2 g-l, 1.240 mz g-l, 1.153 m* g-’ respectively. Polyacrylamide, PAM, was obtained by radical polymerization of acrylamide using the solvent method. A redox system composed of amonium persulfate and sodium sulfide was used as an initiator, redistilled and deoxidized, boiling water being the solvent. The mean molecular weight of PAM as determined by the viscosimetric method was 827,000. Solutions of KCIO,, KN03, HCI and KOH were prepared from analytical grade reagents and deoxidized, redistilled water, of specific conductance 1 ps. Adsorption

measurements

The amount of polymer adsorbed on the CaCOs surface was calculated from the difference of polymer concentration in the solution before and after the adsorption process. The PAM concentration was determined by the nephelometric method according to Cruramet and H ummel [14] _ Measurements were carried out on a mono-beam spectral photometer (“Specol”, manufactured by Carl Zeiss, Jena), equipped with a countershaft for turbidity control at the wavelength 500 nm. All adsorption measurements were carried out using a 5% suspension of CaCO,. Electrochemical

measurements

The surface charge was determined by the method of potentiometric titration. The measurement procedure did not differ basically from that described in the litmature [15,16] _ A suspension of CaCOs (2 g of solid in 50 cm3 of water) was titrated with solutions of HCl and KOH. The hydrogen ion concentration was determined by means of a glass electrode as an indicator, calomel electrode as a reference electrode and pH meter N-512 (manufactured by ELPO-Poland, Lit. Polymetron). Measurements were performed at constant temperature (25 “C). All solutions used in the titration were saturated with nitrogen previously purified by conventional methods [17] _ The following titration procedure was used: Electrolyte solution was bubbled with nitrogen until a constant pH value was achieved - about 15 minutes. Then the

solution was titrated with KOH up to pH 11 and then titrated with HCl down to pH 7.5. The titration curves were used as the reference ones. Titration of the suspensions was made following the above procedure except that it was started after a constant pH of the suspension had been established, which took place some 2 ho-urs after addition of the solid phase to the electrolyte solution. Subsequent portions of titrant were added every 15 minutes. The procedure was also followed in the titration of electrolyte solution + PAM and suspension of CaC03 + electrolyte + PAM systems, except that the time needed to reach the equilibrium pH values was about 4 hours with the polymer and about 3 hours with suspension of CaC03 in the polymer solution. Acid-base titration of the system containing PAM and CaC03 was found to be reversible in pH range from 8.5 to 10.5. The surface charge was estimated by.relating the titration curves to the reference titration curves obtained for the electrolyte or electrolyte and PAM. RESULTS

AND

DISCUSSION

The kinetics of PAM adsorption on CaCO, are presented in Fig. 1. It is seen that the adsorption equilibrium is reached in less than 1 min, irrespective of pHo of the adsorbate solution. Similar results were obtained at all initial concentrations of PAM and ionic strengths of the solution-used. In Fig. 2 isotherms of PAM adsorption within the range of initial concentrations 5 X lo+ - 1 X 10m4 g cm3 are presented. The run of the curves suggests that even at highest concentrations of polymer in the solution, adsorption saturation 2 E

P

I

:: I

5

15

Fig_ 1. Kinetic curves of PAM adsorption on CaCO3 from the solutions of following initial PI&: 0, 6.84; 0.11.98. Initial a, 7.95; Q,9.10;@. 9.88; 0.10.55; concentration of PAM 5 X 10m6 g cmm3-

229

-5

-3

-3

-2

-i

agc,ig cm’1

Fig_ 2. Adsorption of PAM on CaC03, r, from mixed solutions of PAM and KC104 or KNO3 of various initial pHo us. equilibrium concentrations PAM, c,. 0, pHO ?;a, pHo S;@, pHO lo;*, pH, ll;n, pHO 12.

of the surface has not been reached. This is in agreement with the data related to some other solids [l] _ Sitilarly, the dependence of adsorption, r, on pHo is clear in view of CaC03 properties [lo - 121 and polyacrylamide features [18, 191. The decrease of adsorption observed at pH values higher than pHpac CaC03 is probably due to the growth of potential barrier between negatively charged surface and the polyions of the polymer_ The significant adsorption of the polymer within the range of pH studied proves that the adsorption was by interactions other than the electrostatic ones as the CaC03 surface charge sign changed at pH about 9.9. It seems probable that the mechanism of polymer adsorption on CaC03 involves the formation of hydrogen bonding [l, 203. The isotherms of PAM adsorption from solutions containing KClO_+ and KN03 quoted in Fig. 3 show some increased polymer adsorption resulting from the presence of simple electrolytes. It is known from the literature 1121 that C104- ions as well as NOB- ions do not adsorb on the CaCOs surface, thus their influence on the polymer adsorption can be attributed more to changes o? the macromolecule conformation in the solution and at the interface_ A reduction of the coil’s effective size which follows the increase of ionic strength of the solution is well known from the literature [l, 191. It must be kept in mind that even a non-ionic polymer gains a certain charge in the solution and the magnitude of the charge depends on the ionic strength of the solution. An increase in ionic strength of the solution is accompanied by a fall in degree

-I

-7

bgC,Igcti?

Fig. 3. Adsorption of PAM on CaCOa, r, from mixed solutions of PAM and KC104 or KNO3 U.S. equilibrium concentrations PAM ce. 0, solution of PAM only; c) with 0.1 mol dm-’ KC104; 0, with 0.1 mol dm-’ KNO3.

of dissociation of the polymer function groups and the impairment of interrelations between particular groups which is followed by a coiling macromolecule. This effect may cause an increase in the polymer adsorption on the surface due to reduction of both the energetic and steric barriers. In the next series of experiments the potentiometric titrations of the system CaC03electrolyte solution were carried out. Results obtained using KN03 as the indifferent electrolyte are presented in Fig. 4. The run of the curves (Tuersus pH as well as the sequence of the curves in relation to ionic strength of the solution indicate that KNOB is a true indifferent electrolyte. pH, determined by this technique is equal to 9.95. Snzilar results were obtained using KC104 where pHpz determined was equal to 9.87. Having in mind that the accuracy of measurement, dependent

Fig. 4. Surface charge of CaC03 particles, U, us_ equilibrium pH, of the solution_ Indifferent electrolyte KNO3 1 mol dmw3 (O), 0.1 mol dmm3 (O), 0.01 mol dmd3 (a)_

on the precision of the pH meter, is of the order of 0.01 pH unit, it can be assumed that KC104 was an indifferent electrolyte also- The effect of PAM adsorption at the interface (Figs. 5 - 7) is to shift the zero point of charge towards lower pH. Precise determination of is difficult because of considerable *PH, experimental error. Nevertheless, at constant concentration of the indifferent electrolyte, this value seems to depend on adsorption density. The shift of pzc in the system discussed is smaller than that in the system AgI/ PVA 163 _ When we compare the-slopes of the curves (Tuersus pH, obtained for various concentrations of the same electrolyte at constant surface coverage, a certain increase of * ~Hpac accompanying a decrease of indifferent electrolyte concentration can be noticed. It must be kept in mind, however, that results of potentiometric measurements obtained in dilute electrolytes are associated with considerable error- Changes of the surface charge caused by the presence of polymer, i.e. the reduction of absolute charge, 101, on the positive potential side, can be attributed to replacement of simple ions in Stern’s layer by polyions of a small negative charge. On the other hand, the increase of Ial on the negative potential side can be explained by progressive ionization of carboxyl groups following the increase in pH and consequently by the growth of the negative charge carried out into the Stern’s layer by adsorbed polyions_ As is known from numerous studies on high molecular organic compounds adsorption, even at the highest adsorption a part of the

l-10-% KNO,

Fig. 5. Surface

charge

particles,(T,us_equilibrium

pHe of the solution. Indifferent electrolyte: O-01 M KNOB. PAM concentrations used: 0 0 g cmd3; 0, 5 x 10-6 g cm-3; a, 1 x 10-5 g cms; l, 5 x 10-5

g cm+.

[PC

“-.j cm

I-10-‘nKCI0,

Fig. 6. Surface charge of CaC03 particles, (T, us. equilibrium pH, of solution. Indifferent electrolyte: 0.1 Af IU.304. Meaning of the symbols as in Fig. 5.

G

4

WC

-20 0

I

-20 -LO -60 1 -E3 i -750, -120 I 7

10-2n KClOr

7_ Surface

charge of CaCO3 particles, a, vs. equilibrium pHa of solution. Indifferent electrolyte: O-01 M KClO4. Meaning of the symbols as in Fig. 5. Fig.

solid surface remains uncovered [l, 2 - 6,20, 21-J _ Polymer adsorbed in the interface exists in two energetic states: some segments lie flat in the surface (trains) whereas others form loops and tails protruding from the solution. Distribution of the density of the segments depends on such factors as magnitude of the adsorption, nature of the solvent, presence and concentration of electrolyte etc. Potentiometric titration gives information merely of the Stern’s layer, thus it reflects the influence of trains segments. Following Lyklema [6], the shift of pHIBc observed in the system CaCOa-PAM solution can be considered as a result of the replacement of specifically adsorbed ions and water dipoles by the train segment. This is in disagreement with data by Kavanagh et al. for the gibbsite-PVA system

CSI-

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