Clay Minerals (1970) 8, 445.

T H E I N T E R A C T I O N O F K A O L I N I T E W I T H P O L Y -

P H O S P H A T E A N D P O L Y A C R Y L A T E I N A Q U E O U S

S O L U T I O N S - - S O M E P R E L I M I N A R Y R E S U L T S

J. I. B I D W E L L , W. B. J E P S O N AND G. L. TOMS

Central Laboratories, English Clays Levering Pochin & Co. Ltd., St Austell, Cornwall

(Read at the meeting of the Clay Minerals Group of the Mineralogical Society in London in November 1969)

ABSTRACT: The choice of kaolinite sample for surface chemical studies is examined. An adsorption apparatus, particularly suited for those studies where the adsorbate acts as a deflocculant, is described. Some preliminary results are given for the adsorption of total phosphate from tetrasodium pyrophosphate solutions and of polyacrylate from sodium polyacrylate solutions. The release of aluminium and silicon from the kaolinite was measured; with tetrasodium pyrophosphate, aluminium-phosphate com- plexes are formed.

Competitive adsorption measurements suggest that the polyacrylate is adsorbed on the positively charged edges of the kaolinite.

I N T R O D U C T I O N

Both the rheology and the change of rheology with time (stability) of deflocculated clay slips are important in ceramic and in paper coating applications. Here we are concerned with the deflocculation behaviour of the so-called paper-coating clays, that is, refined china clays of small particle size (80-90% less than 2 t~ m) and high brightness.

When a coating clay such as 'Dinkie A '<R> is made down to a 70 wt. % slurry in the presence of about 0"3 wt. % tetrasodium pyrophosphate and sufficient alkali to give a final pH of around 7"0, the viscosity is between 3 and 5 P depending upon the work input during mixing. If the slurry is left with gentle stirring for some days, the viscosity may rise by a factor of between two and three. Such thickening is un- desirable in that it can create problems in the pumping of the slurry itself and in the rheology of the final mix of clay slurry with adhesive (synthetic latices and starch). If however the clay is deflocculated instead, either with a polyacrylate (of suitable molecular weight) alone or with a mixture of tetrasodium pyrophosphate and the same polyacrylate (5:1 weight ratio), the thickening is absent and the viscosity then changes little with time. This practical solution to the problem of thickening is relatively well known (Dennison & Toms, 1967).

(R) Registered Trade Mark of English Clays Levering Pochin & Co., Ltd.

446 J. L Bidwell, W. B. Jepson and G. L. Toms

The present work arises out of the above and represents a continuation of earlier work which should eventually lead to a better molecular understanding of the chemistry of the interaction between clay minerals and the deflocculants, tetrasodium pyrophosphate and the polyacrylates. This paper gives some preliminary results on adsorption 'isotherms', mixed adsorption, clay-deflocculant reaction as evidenced by the appearance of the elements aluminium and silicon in solution, and chroma- tographic analysis of the aqueous phase to determine the species present there. The actual choice of kaolinite sample for an investigation of this kind is discussed.

C H O I C E OF K A O L I N I T E FOR STUDY

All workers in clay science are confronted at intervals with the problem of proper choice of material and this is particularly true in those areas where the surface properties are of dominant interest.

Commercially available grades of kaolinite are generally unsuitable for funda- mental surface studies. Any high grade kaolinite intended for use in paper coating and from whatever commercial source, is the end product of a sophisticated refining and treatment process whose aim is to optimize such user-desirable properties as rheology, brightness and particle size. The surface chemistry of such material will certainly differ from that of the same kaolinite found in its natural state.

Most research workers are aware of these facts and adopt the proper course of using matrix as source material, the kaolinite being extracted by simple elutriation with water.

The kaolinite prepared for study should, of course, contain minimal amounts of ancillary minerals. In the present work, which relied upon the differing particle size distributions of the kaolinite, mica and quartz for separation, it was possible by careful selection of matrix to obtain a product containing 98"2 wt. % kaolinite with the balance present as mica.

All kaolinites show some degree of 'iron staining' arising from the presence of particulate iron oxides (goethite, lepidocrocite and hematite) as evidenced by electron microscope observations (Follet, 1965; Greenland, Oades & Sherwin, 1968). Iron staining cannot be avoided but can certainly be minimized by suitable choice of matrix.

The association of organic matter with kaolinite is important in surface studies. Loosely termed humic acid (Steelink, 1963), it may be present in association with polyvalent cations, notably Fe 8+ and A1 s+. Organic matter is usually concentrated in the finer fraction of a clay because of the greater specific surface presented there. In the context of surface studies, it may provide adsorption sites (De Haan, 1965) and is certainly desorbe0~ to some extent in those adsorption systems where the adsorbate acts as a deflocculant. The amount of organic matter associated with kaolinite is variable from deposit to deposit and indeed within a deposit. Mild oxidative treatment, as for example with hydrogen peroxide, reduces the amount but complete elimination is not possible except by drastic treatment.

Kao l in i t e - p o l y p h o s p h a t e - p o l y a c r y l a t e in terac t ion 447

E X T R A C T I O N A N D P R O P E R T I E S OF T H E K A O L I N I T E

Some 3 Mg of matrix were obtained from a selected 'well-kaolinized' stope in a pit in the St Austell area. To release the clay, the matrix was slurried with water and then agitated with an air stream. Passage through a 200-mesh B.S.S. screen separated the unwanted coarse material including quartz-sand from the finer d a y particles. The screened slurry was then passed through a multi-stage hydrocyclone* operating under such conditions as to yield a product nominally containing no particles greater than 5 ~ m. Finally the product slurry was filtered and the clay dried at 50 ~ C to 0.1 wt. % water. The yield was 6%.

Physical and chemical properties

The particle size distribution of the clay was measured by the sedimentation technique: 83 wt. % of the particles were less than 2 ~m and 1 wt. % were greater than 5 ~m. The specific surface area of the clay, measured by nitrogen adsorption at --183 ~ C and calculated by the BET method (Gregg, 1961), was 8"8 m~/g assuming that an adsorbed nitrogen molecule occupies an area of 16.2 A s.

The chemical analysis of the clay is shown in Table 1. X-ray analysis showed kaolinite and mica as the only detectable minerals present. Calculation, assuming the mica to be present as muscovite KAl=(A1Si~OI0)(OH)~, gives a rational analysis of 98.2 wt. % kaolinite plus l-8 wt. % mica which is in good agreement with the X-ray values of 98% and 2% respectively.

The exchangeable cations present on the clay were Li + (0"7), Na + (18.3), K§ (10.2), Ca 2+ (40"7), Mg 2+ (29"0), AP + (0.3) and Fe 3+ (0-8) with the relative amounts

TABLE 1. Chemical analysis of the clay before conversion to the homoionic form

wt. %

SiO2 46.20 A1203 39-20 Fe203 0.23 TiO2 0.09 CaO 0.06 MgO 0.07 K20 0.21 Na20 0-O9 C 0.O3 Loss on ignition 13.80

Sum 99.98

* In the hydrocyclone (Bradley, 1965), the clay is deflocculated by mechanical action (shearing forces) rather than chemical action and the particle size separation achieved through centrifugal action.

448 J. L Bidwell, W. B. Jepson and G. L. Toms

L.

~ - -

20cm

'v-

@ . . . . .

FIG. I. Continuously stirred filtration apparatus used for the adsorption studies.

in mole % indicated. The base exchange capacity, expressed as NH, +, was 36 t~ mol/g (3.6 meq/100g).

The reflectance of a compact of the clay was compared against magnesium oxide at two wavelengths (Gate & Windle, 1968). The per cent reflectances were 96"5 at

Kao l in i t e - p o l y p h o s p h a t e - p o l y a c r y l a t e in terac t ion 449

574 nm and 95.2 at 458 nm. The closeness of these values to 100% suggests the virtual absence of iron staining. This was confirmed by treating a sample of the clay with excess sodium dithionite (Mitchell & Mackenzie, 1954) when the iron release was 0.015 wt. % as FezO3, or 7% of the total present (Table 1). Indeed, whilst a proportion of the iron will be present in the mica fraction, it is likely that most is present as iron (III) atoms occupying octahedral sites in the kaolin lattice itself (Malden & Meads, 1967).

Finally, a determination of the organic carbon content of the clay by heating at 900 ~ C gave 0.027 wt. % carbon which is equivalent to about 0"06 wt. % organic matter. Experience in these laboratories based upon data from a large number of clays indicated that this value of 0"027 wt. % is particularly small, and is certainly acceptable. Indeed an earlier batch of clay refined in a similar manner was rejected because its organic carbon content, 0.084 wt. % expressed as carbon, was considered too large.

E X P E R I M E N T A L T E C H N I Q U E (With J. F. Williams)

When measuring the adsorption of an adsorbate from a liquid phase onto a solid surface, it is usually necessary to separate a quantity of solution free from solid for analysis. The amount adsorbed is then calculated by difference. In experiments with clays, one common technique is to withdraw a sample of the suspension and bring about a separation by centrifugal action. This practice is not free from objection in those systems where the adsorbate acts as a deflocculant. Here, the technique becomes tedious, particularly when working with dilute suspensions of fine particles, and the temperature of the suspension may rise by several o C during the separation. Furthermore, previous work in these laboratories on the tetrasodium pyrophosphate- kaolinite system has indicated an increasing adsorption with increasing solids content. The possibility therefore exists that the centrifuging action may disturb the equilibrium or pseudo-equilibrium which has been so carefully established.

Apparatus An apparatus based upon a filtration technique was developed for the present

adsorption studies. The clay-adsorbate solution is equilibrated by stirring in a flask for the required interval of time and then transferred by applying a smaU positive pressure of air into the filtration apparatus (Fig. 1) through the side arm A. A mem- brane filter (Sartorius, Type MF 12) is sandwiched between the flange joints J and compressed by the metal discs B to give a leak-free seal A strip of flexible Polythene S is attached to the stainless steel rod R which is rotated by a small electrically driven motor at M. The rotating strip sweeps away any clay deposit which builds up on the membrane filter and which would otherwise severely reduce the rate of filtration. The filtrate is collected in the container C. Both the flask and the filtration apparatus are immersed in a water thermostat; the filtration apparatus is contained in a polythene bag to prevent water seepage through the unlubricated joint K.

Analyses of the filtrate were made by radiotracer techniques and by atomic

450 J. L Bidwell, 14I. B. Jepson and G. L. Toms

absorption analysis (Ferris, Jepson & Shapland, 1969) and a total quantity of 4cm 3 proved adequate. The small filtration rate (4cm~/hr) was acceptable and the solids content of the slip only increased by 2% (50 wt. % solids; 70 cm s of slip). With the particular membrane filter used (average pore size of 0.05/~m by mercury intrusion), no clay could be detected in the filtrate. The clay slip was discarded after each individual adsorption measurement. Blank experiments, made in the absence of clay, established that there was no detectable loss either of phosphate or of polyacry- late through adsorption on the glass walls of the apparatus or on the membrane itself.

Materials The clay described above was converted to the sodium form by stirring for 24 hr

with an excess of Amberlite 120 resin which had been converted to the sodium form.* The clay suspension was separated from the resin by passage through a 300-mesh screen and the pH then adjusted to the value required for the subsequent adsorption measurements by addition of either NaOH or HC1. The suspension was allowed to stand for 16 hr and the pH adjusted again as necessary. Finally the suspension was filtered and the clay dried at 50 ~ C to 0"1 wt. % water.

Tetrasodium pyrophosphate was recrystallized (Quimby, 1954) three times from a water-ethanol mixture to remove any orthophosphate which might be present; stock solutions, 0"4 mol/dm s, were made up weekly from the recrystallized material. The pyrophosphate was labelled with TSPP-P32 (Radiochemical Centre, Amersham); specific activities of 0.1 Ci/mol and 7 Ci/mol were used for the adsorption and chromatographic studies respectively.

The sodium polyacrylate was supplied by Allied Colloids Ltd. as S-35 labelled material; it had been prepared by copolymerizing thioglycollic acid-S25 (Radio- chemical Centre, Amersham) with acrylic acid (1:20 ratio on a molecular basis). It was almost certainly atactic. Measurements with inactive material prepared by the same route indicated a number-average molecular weight of about 1500. In subsequent calculations, the degree of polymerization (D.P.) was assumed to be 16 exactly. The polymerization mechanism is such that one of the terminal molecules in the polymer chain is sometimes --S.CH:.COOH rather than ---CHz.CH2.COOH. The polymer had a specific activity of about 2 mCi/g when prepared.

RESULTS

Kaolinite-TSPP System The rate of adsorption of tetrasodium pyrophosphate (TSPP) by the sodium-kaolin

was measured at pH 7"8 and 25 ~ C in separate experiments using 50 wt. % slips. Adsorption appeared complete after 10 hr (Fig. 2) and an equilibration time of 24 hr was adopted for convenience.

* Carbon analyses indicated a small but d~teztable contamination of the clay with this technique, possibly through contamination with fragments of resin in the sam0 size range as the clay itself. In future work, the ion exchange rosin route will be avoided.

Kaol ini te - po lyphosphate - polyacrylate interaction 451

E ::L

o

E

fl--

7/r " ' f

A/A• ~ ~ , I 0 4 8

T S P P

I I I 12 16 20

T i m e / hr

�9 Si - - - -~

t 24

4

-6 E

3 :t x

> ~

2

E aJ

tad

FxG, 2. Interaction of TSPP with sodium-kaolin at pH 8.0. The amount of TSPP adsorbed and the moIalities of aluminium, silicon and iron in solution are plotted

against time.

An 'adsorption isotherm' corresponding to 50 wt. % solids and 25 ~ C is shown in Fig. 3 for an initial pH of 7.8. The experimental values, calculated from the counting data, are expressed as tt tool of pyrophosphate. There was a small increase of pH, generally about 0"4, over the 24 hr equilibration periods. Most of this change occurred over the first hr of each adsorption experiment.

Evidence for reaction between the clay and the pyrophosphate was provided by analysis of the aqueous phase after each equilibration experiment. Molalities of

TABLE 2. Aluminium, silicon and iron release from sodium-kaolinite after contact with TSPP for 24 hr at pH 7,8

Equilibrium molality of TSPP

rnolalityh~rnol g-1

Molality of stated element molalityh,mol g-1

A1 Si Fe

3-08 0"41 0-63 0" 14 5-80 1"0 0"60 0"18 8"52 1"7 0"87 0"25

11-3 1"6 0"78 0-41 14"4 1"9 0"75 0"29 20"0 2"3 0"87 0-30 43"6 3"7 1 "2 0"48

452 J. !. Bidwell, IV. B. Jepson and G. L. Toms

I I I I 5 I0 15 20

TSPP in solution (molality/Fmol g-i)

FIG. 3. Adsorption of TSPP on sodium-kaolin at pH 7.8. The mount adsorbed is expressed as t~mol of pyrophosphate per g of clay.

aluminium, silicon and iron were measured. The results (Table 2) indicate significant amounts of each element in solution; the extent of reaction increases with increasing TSPP molality. In a further experiment, corresponding to point A on the isotherm, the rate of reaction was measured over 24 hr. The results (Fig. 2) show that aluminium continues to be released from the clay after TSPP adsorption is complete. Blank experiments were made at pH 8 in the absence of deflocculant at 20 wt. % solids. After 24 hr the silicate molality was equivalent to 0.2-0.3/~ mol per g of solution with only traces of aluminium and iron.

Thin layer chromatography using TSPP-32 and automatic scanning was used to determine the phosphate species present in solution after contact with the clay. The solution used is represented by point B on the isotherm of Fig. 3 (the equilibration time was, however, 6 hr). The chromatograph, obtained with a solvent system based on isopropanol (Ebel, 1953), is shown in Fig. 4; peaks corresponding to orthophos- phate (A), pyrophosphate (B) and higher molecular weight phosphate species (C) are present. There is a suggestion of a further peak B', seen as a shoulder on the pyrophosphate peak. On a phosphorus atom basis, the relative peak areas are 8 (A), 83 (B) and 9 (C). Essentially similar results were obtained using a solvent system based on dioxan (RtJssel, 1963).

Since the solution which has been in contact with the clay had been passed through a membrane filter, the peak C can hardly be attributed to particulate material. Furthermore, if a mixture of aluminium sulphate solution and TSPP solution (molar ratio similar to that in filtrate used above) is analysed by thin-layer chromatography, a trace similar to that shown in Fig. 4 is obtained.

That adsorption of pyrophosphate is accompanied by desorption of organic matter was demonstrated as follows. A 50 wt. % slip was equilibrated with 15.0/~ mol of TSPP per g of clay at pH 8 for 24hr. The slip was centrifuged, the supernatant

Kaolinite - p o l y p h o s p h a t e - polyacrylate interaction

A Ii

453

S I

d

/ x A

Direction of scan

FIG. 4. Chromatograph indicating the phosphate species present in the aqueous phase after contact with the sodium-kaolin. The broken line is for TSPP alone.

I-0 "T

E

c

E

.~ 0.5(

-g

I 1 I O O~5 I'O 1"5

PAA in solution (mololity/l~ tool g-i )

�9 pH 7 ' 3

FIG. 5. Adsorption ofpolyacrylate on sodium-kaolin. The amount adsorbed is expressed as pmol of polyaerylate per g of clay assuming a degree of polymerization of 16.

454 J. L Bidewll, W. B. Jepson and G. L. Toms

poured off and the clay dried. The carbon content of the clay was redetermined: the value indicated a loss of 0.008 wt. %.

Clay--SPA systems Rates of 'adsorption' of sodium polyacrylate (SPA) by the sodium-kaolin

were measured at pH 7"0 and at 25 ~ C using 50 wt. % slips. About 40 hr were required for equilibration and a time of 72 hr was used in all other experiments.

'Adsorption isotherms' corresponding to 50 wt. % solids and 25 ~ C are shown in Fig. 5 for initial pH values of 4"7 and 7.3. The amount of polyacrylate (PAA) adsorbed is seen to decrease with increasing pH. In the experiments at pH 7"3, there was no detectable change in pH ( • 0"05) during adsorption. In the experiments at pH 4-7, there was an increase in pH of about 0-2 over each 72-hr period.

Experiments were made at pH 7"3 to determine if the adsorption was reversible. A quantity of unlabelled SPA was added to the 50 wt. % slip in an amount to correspond to point A on the isotherm in Fig. 5. After 72 hr, a small quantity of SPA-S32 (3 wt. % of that present) was added and the slip equilibrated for a further 72 hr. Exchange occurred indicating that at least 90% of the PAA was reversibly adsorbed. A similar conclusion was reached after a second experiment, correspond- ing to point B on the isotherm.

TABLE 3. Aluminium, silicon and iron release from sodium-kaolinite after contact with SPA for 72 hr at pH 7-3

Equilibrium molality of SPA

molality/~mol g-1

Molality of stated element molalityhzmol g-1

A1 Si Fe

0.055 0.03 0.20 0.02 0.101 0.10 0-28 0.01 o. 163 o. 11 0-30 0.03 0.161 0.16 0-43 0.04 0-551 0.56 0-80 0.04 0.880 0.44 0.65 0.03 1.30 0.37 0-52 0.04 1-65 0-44 0-67 0.05

As with TSPP, adsorption of SPA is accompanied by the appearance of aluminium, silicon and iron in solution. The molalities, whilst significantly less than those obtained in the corresponding experiments with TSPP (compare Tables 2 and 3) again show the feature of increasing with increase of amount of SPA adsorbed. Rates of reaction were rtot measured.

Indirect evidence that adsorption of SPA is accompanied by desorption of organic matter was provided by treating a 70 wt. % clay slip with SPA (0"3 wt. %) and then filtering through a membrane filter. The filtrate was yellow in colour. A blank solution containing similar concentrations of SPA and of iron was eolourless.

K a o l i n i t e - p o l y p h o s p h a t e - p o l y a c r y l a t e i n t e r a c t i o n 455

Clay--TSPP + SPA systems

A limited number of experiments was made to study mixed adsorption of TSPP and of SPA onto the sodium-kaolinite. The experimental conditions were 25 ~ C, a starting pH of 7"7 and 50 wt. % solids. Complementary measurements were made with TSPP-P32 plus unlabelled SPA and with SPA-S35 plus unlabelled TSPP. In other experiments TSPP was first adsorbed on the clay and the SPA added after 24 hr. Similarly, SPA was first adsorbed on the clay and the TSPP added after 72 hr.

The results in Table 4 show that SPA depresses the adsorption of TSPP and that the converse equally applies; evidontly, there is competitive adsorption.

Table 4. Mixed adsorption of TSPP and SPA on sodium-kaolinite at pH 7-7

Adsorbate Equilibration TSPP SPA

Time (hr) a/t~mol g -1 mh~mol g-1 a/~mol g-1 m/t~mol g -1

TSPP alone 24 3-59 4.93 -- - - SPA alone 72 - - - - 0.407 0.114 TSPP + SPA together 72 1.68 6.69 0.355 0.166 TSPP followed by SPA 24 + 72 1.99 6.39 0.360 0.165 SPA followed by TSPP 72 + 24 1.23 7.58 0.372 0.148

a is the amount adsorbed; m is the molality

D I S C U S S I O N

It is generally accepted that the kaolinite particle carries a negative charge on its faces and a positive charge on its edges. The overall charge is negative as shown by electrophoretic mobility measurements. The well-known edge-to-face flocculation of kaolinite has been interpreted on the basis of these charges.

The negative charge on the faces is thought to arise from isomorphous substitu- tion in the layer lattice. It is compensated by adsorption of cations on the exterior faces (Van Olphen, 1968). The increase of cation exchange capacity with pH has led to the suggestion that ionization of hydroxyl groups attached to weakly acidic aluminium may also contribute to the negative charge. Direct experimental demon- stration of the negative charges on the face is provided by the adsorption there of positively charged particles of 'ferric hydroxide' and of silver iodide (Follet, 1965; Weiss & Russow, 1963).

The classical work of Thiessen (1942) showing adsorption of negatively charged gold particles on the edges of kaolinite platelets provides good evidence for the presence there of positively charged sites. Such sites are consistent with work on the adsorption of chloride ions (Schofield & Samson, 1954) and have been attributed (Schofield, 1949) to adsorption of protons on exposed edge aluminium atoms :

456 J. L Bidwell, IF. B. Jepson and G. L. Toms

AIOH+ H++OH - ~ Al§ ~ H~O (1) " - O H -

with the O H - ions held in the electrically double layer. The degree to which these positive sites persist with increasing pH was in some doubt (Follett, 1965; Chakravarti & Talibudeen, 1961; Miclmels & Morelos, 1955) for a number of years. The question was subsequently resolved by Quirk (1960), who demonstrated the positive adsorp- tion of chloride ions above pH 7. Such adsorption was concentration independent up to pH 11, the limit of the experiments.

Adsorption of TSPP The work to date has been confined to adsorption of TSPP at a single pH, 7"8.

Qualitative experiments indicate that the adsorption is at least partly reversible. The chromatographic results demonstrate the presence of some orthophosphate so that the isotherm is in principle a mixed one involving orthophosphate, pyrophos- phate and possibly aluminium-phosphate complexes. A similar conclusion was reached by Michaels (1958).

Extensive work on the adsorption of orthophosphate on kaolinite (Schofield, 1949; Hsu & Rennie, 1962; Muljadi, Posner & Quirk, 1968a, b, c) has led to the view that adsorption occurs at the edge sites. Similar conclusions have been reached from studies of polyphosphate adsorption (Michaels, 1958; Lyons, 1964). The adsorption sites are thought to be charged edge aluminium atoms [cf.-equation (1)] and this is consistent with the observation (Quirk, 1960) that the majority of chloride ion adsorption sites are eliminated after phosphate adsorption.

Considering TSPP adsorption, the predominant ionic species at pH 8 are HP207 s- (72%) and P2Or 4- (26%) so that adsorption on the positive edge sites would certainly reverse the edge charge thus accounting for its excellent deflocculat- ing power (Van Olphen, 1968). On geometrical grounds, there is certainly sufficient edge area to accommodate the experimentally observed adsorption. Taking the measured specific surface area of the clay and assuming an aspect ratio (diameter: thickness) of 10: 1, the edge area is 1.65 m2/g. The plateau on the isotherm of Fig. 3 corresponds to a surface coverage of 1.8 pyrophosphate ions per I00 A 2 at pH 8. Additional adsorption sites may be provided by unknown amounts of aluminium hydroxyl species (Kafkafi, 1968) as a consequence of possible silica removal during the washing stages in the preparation of the homoionic clay.

The precise orientation and position of the pyrophosphate ion in respect of the kaolinite edge is not yet resolved. Some chemisorption or interaction with the edge surface itself as distinct from adsorption in the double layer must occur in view of the significant quantities of aluminium and silicon which appear in solution. Indeed, the mole ratio AI:Si found in solution always exceeds unity implying a preferential dissolution of aluminium from the kaolinite. The data of Fig. 2 where the rate of phosphate adsorption is compared with the rate of aluminium release, shows the latter to be a consequence of phosphate adsorption. The suggestion (Michaels, 1958)

Kaolini te - po lyphospha te - po lyacry la te interaction 457

that polyphosphate adsorption follows disruption of the kaolinite lattice (as at single sheet exfoliations) is unlikely to hold because aluminium release proceeds after the pyrophosphate adsorption is complete. The increased rate of hydrolysis of TSPP in the presence of clay is consistent with earlier work with tripolyphosphates (Quevedo, 1967).

The chromatographic experiments suggest that part of the aluminium in solution is present as a complex with phosphate. Fortunately the molar ratio of pyrophos- phate toaluminium is such that no aluminium phosphate complexes should be precipitated (Lyons, 1964; Hsu, 1967). The stoichiometry of the complexes formed in the present work will be investigated when separation techniques (Ohashi, Yoza & Ueno, 1966; Felter, Dirheimer & Ebel, 1968) using Sephadex columns have been perfected.

Adsorption of SPA

The observation that the adsorption of PAA on kaolinite decreases with increas- ing pH is in agreement with earlier work (Michaels & Morelos, 1955; Mortenson, 1962). The failure of Michaels & Morelos (1955) to detect any adsorption of PAA at pH values greater than 7"7 may be ascribed to their analytical method.

Mortenson concluded that the adsorption was irreversible whilst here some 90 % of the adsorbed PAA could be exchanged. This discrepancy may be due in part to the higher molecular weight of material used in his studies, 60,000 compared with 1,500 in the present work.

The adsorption sites on the kaolinite and the type of bonding have not been resolved. Michaels & Morelos (1955) considered a number of bonding types and advocated hydrogen bonding as the predominant mechanism. Mortenson (1962) proposed that the ionized polyacrylate ion is bonded to positively charged sites on the edges of the kaolinite particles.

Either hydrogen bonding of the PAA to the aluminium face hydroxyls or ion adsorption at the positive edges is consistent with the observed change of adsorption with pH. However the degrees of ionization at the hydrogen ion concentrations of interest here would appear to rule out hydrogen bonding. Thus, estimates (Nestler, 1968) indicate that the polymer is 25% ionized at pH 4"7 and that this increases to 65 % at pH 7"3. Furthermore, as discussed below, the mixed adsorption measurements indicate that PAA is adsorbed on the same site as the pyrophosphate ion. Hence the Mortenson (1962) mechanism of adsorption at the positive edges is advocated.

Using the calculated edge area of 1.65 m2/g, the plateaux on the isotherms of Fig. 5 correspond to 6"7 and 4"5 molecules of monomer adsorbed per 100 A 2 at pH values of 4"7 and 7.3 respectively. The area of a monomer segment in a PAA molecule has been measured (Nestler, 1968); the value is 20 A 2. Accepting the approxi- mate nature of the calculations, there is sutlieient area for the PAA to be accommo- dated on the edges, particularly since the polymer will be attached by only a few of its segments, with the remainder present as loops stretching out into the aqueous phase.

The observed decrease in adsorption with increasing pH may reflect the increas- G

458 J. L Bidwell, W. B. Jepson and G. L. Toms

ing size of the polyacrylate ion and the decreasing number of positively charged edge sites.

We have made a few measurements of the adsorption of acetic acid onto kaolinite. Here the adsorption again decreases with increasing pH (.oH range 3-10) but perhaps the most striking feature is the small amount of acetate adsorbed. At pH 7, the plateau is found at 1 tL mol/g of clay at a solution molality of 0.25 t~ mol/g. Since acetic acid is about 95 % ionized at pH 7, the bonding to the kaolinite is presum- ably Coulombic. Furthermore the amount adsorbed corresponds to 0"36 molecules per 100 A. ~ of edge area implying a small surface concentration of positively charged sites. Such a concentration would be adequate to account for the adsorption of PAA at pH 4.5 if it is assumed on average that only one carboxylate ion per molecule is attached to the clay surface.

The number of sites calculated from the acetate adsorption results is however significantly less than that indicated from the TSPP adsorption results so that the apparent discrepancy is unresolved.

Adsorption of TSPP + SPA The mixed adsorption experiments (Table 4) show that when the sodium-kaolin is

exposed to TSPP and SPA in the same solution, both species are adsorbed. Further- more SPA will displace part of the TSPP already adsorbed on the clay surface; the converse is equally true.

These results provide clear evidence that TSPP and SPA are competing for a common adsorption site. The possibility of more than one adsorption site is not excluded. However taking a simple view, since there is good evidence that the pyrophosphate ion is adsorbed at the positive edges, it follows that PAA must be likewise adsorbed.

It is important to note that the kinetics of adsorption and desorption in the mixed systems have yet to be determined. The equilibration times in Table 4 were merely the same as those chosen for the single adsorption experiments and have not been confirmed. Until such times have been determined and more data obtained it is not possible to deduce whether the adsorption is on a simple mole per mole basis (taking into due account the number of PAA segments attached) or if it is necessary to invoke the suggestion of Hingston et al. (1967) developed by Nagarajah, Posner & Quirk (1968) that phosphate is displaced because the PAA increases the net negative charge on the oxide surface.

Role el organic material The amount of organic material associated with the clay used in the present work

was small based on comparative data for a number of clays. The value (0"065 wt. %) is however by no means negligible compared with the amount of SPA used in the adsorption measurements'; the final point on the isotherm at pH 7"3 in Fig. 5 corresponded to only 0.37 wt. % SPA. Desorption of organic material consequent upon absorption of either TSPP or SPA has been demonstrated and this desorbed material may well subsequently act as a competitive adsorbate.

K a o l i n i t e - p o l y p h o s p h a t e - p o l y a c r y l a t e i n t e r a c t i o n 459

Working with another clay (0"08 wt. % carbon), mild oxidative treatment with H202 was found to increase the amount of acetate adsorption by 50%. Similar experiments, with TSPP and with SPA, have not yet been made.

A C K N O W L E D G M E N T

We thank the Directors of English Clays Lovering Pochin and Co. Ltd. for permission to publish this work.

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