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Sorption of anions by the cation exchange surface of muscovite
Sorption of Anions by the Cation Exchange Surface of Muscovite K. W. PERROTT Ruakura Soil Research Station, Hamilton, New Zealand AND
A. G. LANGDON AND A. T. WILSON School of Science, University of Waikato, ttamilton, New Zealand Received October 19, 1972; accepted August 15, 1973 The retention of phosphate and sulfate by the negatively charged 001 face of muscovite was investigated using techniques previously developed for the study of single planar surfaces. Muscovite is a layer silicate with a readily obtainable smooth surface of known structure particularly suitable for such studies. The results obtained indicate that this negatively charged surface retains anions by the following mechanisms: a. Adsorption of positively charged colloidal particles of insoluble stoichiometric compounds such as Pba(P04)2 and BaSO~. Once sorbed these particles were not desorbed under conditions which cause charge reversal in suspension. b. Adsorption of positively charged colloidal particles of hydrous oxides (or polymeric hydroxo-complexes) containing absorbed phosphate or sulfate. c. Sorption of the anion by positively charged colloidal particles of hydrous oxides (or polymeric hydroxo-complexes) previously adsorbed by the negatively charged surface. As the interaction of the colloidal particles with the surface is electrostatic these mechanisms would also operate with other negatively charged surfaces which are common in nature. INTRODUCTION
many of the problems found with finely dispersed materials can be overcome. As only a relatively small area (approx 1 cm 2) is studied by these techniques, problems associated with the analysis of very small amounts of material and with contamination have had to be solved (1, 2). Early results (2) showed that enhanced anion retention by muscovite surfaces pretreated with FeC13 solutions is due to the presence of colloidal particles of hydrous iron oxide on the surface. It has been suggested that this effect would also be observed when other similar anion sorbing hydrous oxides were present on the surface. Sorption of anions by precipitation of insoluble salts at the surface
The sorption of ionic species from solution by solids has usually been studied with samples of high specific surface area. It has often been impossible to determine the nature of the sorption process from the results of such studies, mainly because the nature of the surface of such small sized particles is not usually known and impurities or decomposition products are often present. Recently; techniques have been developed which have enabled sorption by the 001 face of large mica crystals to be studied (1, 2). Because the structure of this surface is well known and can be obtained clean and smooth down to molecular level by crystal cleavage i0 Journal of Colloid and Interface Science, Vol. 48, No. 1, July 1974
Copyright ~ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
SORPTION OF ANIONS BY MUSCOVITE is another possible mechanism. I t was the aim of the present work to test these suggested mechanisms of anion sorption by cation exchange surfaces. This paper reports the results of an investigation into the effect of pretreatment of the surface with a range of cations, and hydrolysis products, on the subsequent sorption of phosphate and sulfate anions. The sorption of insoluble phosphates and sulfates was also investigated. The results enabled conclusions to be drawn about the mechanism of the sorption of anions by cation exchange surfaces. EXPERIMENTAL
METHODS
Materials Clear, ruby muscovite was obtained as large sheets from Mica and Micanite Supplies Limited, London. Strips of mica with freshly cleaved faces and trimmed edges were used in the experiments. Gloves were worn and freshly cleaned scissors and tweezers were used in all manipulations to prevent contamination of the surfaces. Carrier-free 35S sulfate and high specific activity 3~p orthophosphate were obtained from the Radiochemical Centre, Amersham, U.K. KH~PO4 and K2SO4 solutions used were prepared from the analytical grade salts and all other reagents were of the highest purity available.
Methods The preparation of mica samples and the precautions taken to maintain clean conditions and prevent surface contamination were described by Langdon, Perrott, and Wilson (2). It was found that if A1CI~ solution entered the frayed edges of the mica strip during pretreatment, anion sorption by the resulting surface was nonuniform [-see Sec. (a) in Results and Discussion below~. All pretreatments were therefore performed by a method which limited the treatment solution to a spot on the mica face. After rinsing thoroughly anion sorption was carried out by partial immersion of the
ll
strip in the appropriate solution, ensuring that the pretreated area was covered. Sulfate was sorbed from carrier-free ~5S solutions obtained by dilution of the stock isotope solution. The concentration of the diluted solutions was calculated to be about 10-s M SO~-. Phosphate was sorbed from 10-6 MKH2P04 labeled with 32p. Mter sorption of the phosphate anion the mica samples were rinsed for about 30 sec with distilled water, dried and autoradiographed. It was found that a similar rinse to remove excess reagent from sulfate treated samples also removed most of the sorbed sulfate. These samples were therefore simply drained for 10 sec and then dried. Sorption of sulfate was indicated when the amount of sulfate on the surface was greater than that present in the liquid film after draining. The blank value for sulfate in the liquid film was determined by counting the activity remaining on a surface treated with ~5S labeled 10-2 M K2SO4, where negligible ~5S sorption could be assumed due to the very low specific activity. The sorption ratio, X -~ (moles/cm 2 surface)/ (moles/cm 3 solution), was determined for each surface by counting the solution and surface activities as described previously (2). Although the measurement errors involved in sampling and counting are such that an overall reproducibility of better than ± 12% is possible, in most cases larger variations (-t-20% or more) were found. Edroth (3) studied the sorption of thorium on various surfaces by a similar method and also found variations larger than expected from the measurement errors. This probably arises from the lack of reproducibility inherent in processes occurring in some of the sample treatments. For example, the reproducibility of the sorption of colloidal particles or of hydrolysis processes occurring during rinsing could be expected to limit the overall reproducibility. The above methods were described in more detail by Langdon, Perrott, and Wilson (2). a. Effect of edges during pretreatment. Strips of mica were pretreated for 10 rain with Journal of Colloid and interface Science, Vol. 48, No. 1, J u l y 1974
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PERROTT, LANGDON AND WILSON
1 M FeCla or A1C18 by the method described above where the edges are protected from the treatment solution. Duplicate strips were pretreated by partial immersion in the solutions. After rinsing for 30 min with distilled water, the samples were treated with 82p phosphate (no carrier added) for 30 min, rinsed for 30 sec and then dried. Autoradiography of the samples enabled comparison of the amount and uniformity of phosphate sorption. b. Pretreatment with i M salts. Samples were pretreated for 30 mill with a range of 1 M solutions of salts of alkali metal, alkaline earth, transition metal and hydrolyzable cations [-a 0.1 M solution was used in the case of Th(NO3)4~. Although 1 M solutions could be prepared from most salts used, in the case of Bi(NO3)~ it was necessary to add HNO3 to dissolve the precipitated hydrolysis products. Precipitation of hydrolysis products also occurred in preparation of Ce(SO~)~ solution. In this case the clear supernatant solution was used for the surface treatment. The low pH values of some of the pretreatment solutions (e.g., approx 1 for 1 M FeCI~) introduces the possibility of decomposition of the mica surface. However, published information on the kinetics of acid induced clay mineral decomposition (4) indicate that this would not be significant at the low temperature and relatively short treatment times used. After pretreatment the surfaces were rinsed for 30 rain and treated with the phosphate or sulfate solutions for 30 rain. The pH values of both the phosphate and sulfate solutions were found to be about 5.5. As anions such as acetate would compete for sorption sites no attempt was made to control the pH of these solutions by the use of buffers. Mica surfaces which had been treated with 1 M Zr(NO~)4, 1 M Bi(NO3)~ and 0.1 M Th(NO~)4 and washed were examined in the electron microscope using a replication technique. After washing, separate samples of each surface were freeze dried or dried under an infrared lamp. Replicas were prepared by shadowing the surface at an angle of approx Journal of Colloid and Interface Science. Vol. 48, No, 1, J u l y 1974
10° with a mixture of platinum and carbon and then depositing a thin film of carbon at normal incidence. The preshadowed replicas were floated off on water and picked up from below on electron microscope grids. As the replica on the freezedried Th(NO3)4 treated surface could not be floated off on water a dilute HC1 solution was used and the replica was washed by transferring to a water surface before picking it up on a grid. c. Pretreatment with hydrolysis products. Pretreatment solutions were prepared by mixing known volumes of standardized NaOH solution and of the 0.1 M salt solutions with distilled water to give solutions approx l0 -3 M with respect to the cation and a degree of neutralization corresponding to midpoints of buffer regions in previously determined titration curves. These solutions were prepared on the day of use to reduce aging effects. Other experimental details are the same as for Sect. (b).
d. Sorption from solutions containing other salts. A range of 10-~ M sait solutions containing carrier-free 35S sulfate was used to treat mica samples for 30 min. Phosphate sorption from 10-6 M KH~PO4 containing cations of interest at a range of concentrations was carried out for 100 min. RESULTS AND DISCUSSION
a. E/ffect of edges during pretreatment. The autoradiograms of these samples show that phosphate is sorbed uniformly by surfaces pretreated with FeCI~ by either method. Surfaces pretreated with AICI~ on the face only do not sorb significant amounts of phosphate, whereas surfaces pretreated by partial immersion of the mica strip in the A1C13 solution sorb phosphate in streaks. Although pretreatment has been performed in this manner with many cations only AI(III) caused this streaking effect. The streaks are generally observed to start from the mica edges or from a "step" on the face. By changing the orientation of strips during the various steps in the experiment, it was found that the streaks always
SORPTION OF ANIONS BY MUSCOVITE occur along the same direction as that taken by water flowing over the strip during rinsing after A1C13 treatment. These results indicate that A1CI3 pretreatment of the face does not enhance phosphate sorption and suggest that phenomena occurring during leakage of A1CI~ solution from the edges during rinsing is somehow responsible. Leakage of AIC13 through a fine capillary tube into the bottom of the rinsing device used (water flows upwards over the strip in this device) was also found to cause phosphate sorption by freshly cleaved mica exposed to such rinsing thus confirming the above hypothesis. These results are compatible with a mechanism involving adsorption of hydrolysis products formed by dilution of the A1C13 solution leaking from the edge spaces during washing. Hydrolysis on dilution is indicated by the findings of Frink and Peech (5) that AICIa solutions become supersaturated with respect to A1(OH) a at concentrations below 5 X l0 -~ M, and precipitation of Al(OI-I)a occurs on dilution to below l0 -5 M. Such a mechanism would also be expected for other hydrolyzable cations and has been confirmed for FeCla(2). As found for the FeCla case the species on the surface after A1Cla treatment by the strip method are not exchangeable with Na + and Ca2+ and are acid soluble. Because AI(III) is less extensively hydrolyzed than the other cations causing anion retention, no hydrolysis products are adsorbed during rinsing of surfaces pretreated on the face only. When A1Cla solution is leaking out of the edges and passing over the face during rinsing, conditions are more favorable for adsorption of hydrolysis products. b. Pretreatment with 1 M salts. (Table I). Under the conditions of the experiment, blank samples (i.e., no pretreatment) were found to give sorption ratios of from 0 to 8 X 10-5 cm (corresponding to about 500 ions/~m 2) for phosphate sorption and about 2 X 10-4 cm for sulfate sorption. Preatreatment with chlorides of a range of alkali and alkaline earth metals (including Ba) or with MnC12, CoSO~, CdCI~, ZnSO4, Cn(NO~)2, UO2(N03)2,
13
TABLE
I
RETENTION OF ANIONS BY MUSCOVITE 001 FACE PRETREATED WITH 1 M SALTS Pretreatment solution (I M)
Blank A1Cla FeCI~ CrCI3 Th(NOa)4 (0.1 M) Zr(NO3)4 Bi(NO3)3 Ce(SO02 b
Sorption ratio (cm)a Phosphate (from 10-6 M KH~PO*)
Sulfate (from carrier-free solution)
0-8 2 4.5 5.6 1.7 1.7 9.7 1.2
2 2 6 1.0 1.6 3.2 8.7 7.5
X X X X X X X X
10-5 10-s 10-2 10-3 10-1 10-1 10.2 10-1
X X X X X X X X
10-4 10-4 10-4 10-a 10-~ 10-3 10-2 10.2
See text. b Clear supernatant solution after precipitation of hydrolysis products. Concentration less than 1 M.
Pb(NO3)2 or La(NO3)a did not enhance retention of either anion. Values of the sorption ratio for the other pretreatment solutions investigated are given in Table I. Of all the pretreatment solutions investigated only FeCI~, CrCla, Th(NQ)4, Zr(SO4)~, Bi(NO3)3, and Ce (SO4)2 caused enhanced phosphate retention and sulfate retention. The hydrolysis reactions occurring on dilution of 1 M salts of hydrolyzable cations are probably similar to those for FeC13 which has been studied in some detail (6, 7). FeCIa solutions with concentrations less than about 0.1 M are supersaturated with respect to hydrous ferric oxide. It is thought that after reducing the concentration below 0.1M, primary colloidal particles form and subsequently hydrolysis proceeds by growth of these nuclei. This initial stage of nuclei formation occurs more rapidly at lower concentrations. Such rapid formation of colloidal hydrous ferric oxide would be expected to occur durnig rinsing of mica surfaces treated with 1 M FeC13 and as these colloidal particles are likely to be positively charged they would be adsorbed by the negatively charged surface. A similar mechanism could be expected to operate for the other hydrolyzable cations. Journal of Colloid and Interface Science, Vol. 48, No. 1, July 1974
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PERROTT, LANGDON AND WILSON
FIG. 1. Electron micrographs of repficas of mica surfaces: (a) Untreated surface; (b) 1 M Zr(NQ)4 treated surface, air dried; (c) 0.1 M Th(NQ)4 treated surface, freeze dried; (d) as (c) but air dried; (e) 1 M Bi(NO3)2 treated surface, freeze dried; (f) as (e) but air dried.
The electron micrographs presented in Fig. 1 confirm this mechanism for Zr (NO3) 4, Th(NO~)4, and Bi(NO3)3. The grain size of the replicating material is about 4 nm which means that particles smaller than about 5 nm would not be seen by this method. No particles were detected on the freeze-dried Journal of Colloid and Interface Science,
Vol. 48, No. 1, July 1974
Zr(NO3)~ treated surface (not shown in Fig. 1) but the fact that particles about 5 nm in diameter were seen on the air-dried surface suggests that very small particles are present which aggregate on drying to form larger visible particles. The particles on the Th (NO3)4 treated surface range in size up to about 50 nm
15
SORPTION OF ANIONS BY MUSCOVITE TABLE II RETENTION OF ANIONS BY MUSCOVITE 001 FACE PRETREATED WITI-IPARTIALLY )$EUTIRALIZED SALT SOLUTIONS Pretreatment solution~
Blank A1CI~ FeC13 CrC13 A CrC13 B Th(NO~)4 Co(NO3)2 MnC12A MnC12B Zr(NO3)4A Zr(NOa)4B
Molar ratio Hydroxide/cation (approx) -1.1 1.2 0.3 1.5 1.6 0.8 0.1 0.9 1.3 2.9
Sorption ratio (cm) . Phosphate Sulfate (from 10-6M KH2PO*) (from carrier-free solution) 0-8 2.0 5.6 4 1.1 1.7 6.1 2.7 6.6 1.2 3.3
X X )< X X X X X X X X
10-5 10-~ 10-2 10-~ 10-~ 10-1 10-4 I0 -4 10-4 10-3 10-2
2 X 10-~ 3.0 X 10-2 2.4 X 10-3 9 X 10-4 1.6 X 10-2 1.0 >< 10-2 2 X 10-4 1 X 10-~ 1 X 10-~ 3.3 X 10-3 5 X 10-4
See text. Cation concentration, 10-5 M.
and do not appear to be affected by air drying. Particles with a diameter of about 200 nm are seen on the freeze-dried Bi(NO~)3 treated surface. These appear to be aggregates and the smaller particles on the air-dried surface may have resulted from shrinkage and breakup of the aggregates on drying. Similar electron micrographs were obtained with FeC13 treated surfaces (2) but particles were not detected on surfaces treated with A1Cla by leaking the solution through a capillary into the bottom of a rinsing device as described above. The nonexchangeability of the phosphate sorbing species (see above) indicate that they are unlikely to be simple monomeric cations (8) and in this case colloidal particles of hydrous aluminum oxide, or polymeric hydroxo-complexes, with a diameter smaller than 4 nm are probably present on the surface. The size of the particles formed during rinsing would be determined by the extent of hydrolysis, i.e., by the nature of the cation, the concentration and the pH of the solution and the rinsing conditions. For instance, the larger size of particles on the Th (NOa)4 treated surface compared with the Zr(NO3)4 case probably arises from the higher pH of the 0 . 1 M Th(NO3)4 solution. In the case of
Bi(N03)8 the particles on the surface will be insoluble bismuthyl nitrates rather than the hydroxide or hydrous oxide. The adsorption of insoluble basic salts by the mica face may also occur in some other cases [-e.g., treatment with Zr (SO02]. c. Yretreatment with hydrolysis products (Table II). Anion retention was not enhanced by pretreatment with partially neutralized solutions of ZnSO4, Pb(NO3)2, Cu(NO~)2, UO2(NO3)~, or La(NOa)3. Values of the sorption ratio for the other partially neutralized pretreatment solutions are given in Table II. Of the partially neutralized solutions studied A1C13, FeC13, CrCt3, Th(NOa)~, and Zr(SO4)2 enhanced phosphate and sulfate retention. The solutions containing Co (NO3)2 and MnC12 also enhanced phosphate retention. The partially neutralized solutions used would contain colloidal particles of hydrous oxides or hydroxides. However, not all such species would have anion sorbing properties. The results obtained in parts (b) and (c) should therefore be explained by: i. The adsorption of positively charged colloidal particles of hydrous oxides (or poIyJournal of Colloid and Interface Science, Vol. 48, No. 1, July i974
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PERROTT, LANGDON AND WILSON TABLE
III
I~ETENTION O1~SULFATE BY MUSCOVITE 001 FACE ~'RO~f SOLUTIONS CONTAINING OTHER SALTS Salt present in sulfate solutionb
pH
Sorption ratio (cm). (sulfate carrier free)
Blank BaCi~ A1C13 FeC13 CrCI~ Bi(N0~)3
5.5 5.4 3.4 2.1 3.1 1.6
2 9.5 4 2.6 1.2 6.7
X X X X X X
10-* 10-* 10-4 10-3 I0 -s 10-2
a See text. b Cation concentration, 10-2 M.
meric hydroxo-complexes) formed either by dilution on rinsing or by addition of NaOH. ii. Whether or not these adsorbed species react with anions. Some idea of the extent of specific sorption of.'sulfate and phosphate by the species present on the surface can be obtained from published reports of~Complex formation by the catiOns concerned (9, 10). With three exceptions, the results listed in Tables I and II are those that would be expected from literature evidence for the existence or nonexistence of sulfate and phosphate complexes of the cations concerned (10). The exceptions are those surfaces pretreated with partially neutralized solutions of CrC13(A), Co(NO3)~, and MnCI~. When CrC13 was being titrated with NaOH to obtain the titration curve it was noted that the color of the solution changed from violet to green at the beginning of the curve indicating formation of chloro-comp!exes (11). This would reduce the hydrolysis of Cr(III) in the least basic (A) solution and also reduce the charge on any polymeric species formed. Although pretreatment with 1 M CrCI~ in part (b) gives enhanced anion retention it is of interest to note that pretreatment with freshly prepared CrCI3, which is green and contains the chloro-complex, does not enhance retention. Very little hydrous chromic oxide would therefore be adsorbed from the less basic solution. Co (II) and Mn (II) Journal of Colloid and Interface Science, Vol. 48, No. 1, J u l y 1974
are known to be oxidized by air in basic solutions (11, 12) and phosphate retention would be due to the presence of hydrous Co(In) or Mn(III) oxide on the surface ill these cases.
d. Sorption from solutions containing other salts (Tables III and IV). Apart from solutions containing Ba 2+ and SO42- no enhancement of anion retention occurred with salts of the alkali and alkaline earth metals or with ZnSO4. Sulfate retention was also not affected by La(NO~)~. Sorption ratios for the other salts studied are given in Table III for sulfate and Table IV for phosphate. The amounts of phosphate present in a solid phase in these solutions were determined by paper chromatography with distilled water. The paper was washed with 0.50-/0 oxalic acid, rinsed and dried before use. This removed contaminants in the paper which prevented orthophosphate ions from moving with the solvent front. The fraction of 32p remaining at the origin, as determined by scanning with a collimated G.M. tube, was assumed to be present in a solid and the values obtained are listed in Table IV. In some cases at high cation concentrations where the pH was low, all the phosphate was present in a streak starting from the origin. The results indicate that anions can be retained as insoluble compounds [-i.e., PbsCPO4)2 and BaSO4~. It would be expected that such compounds are adsorbed as positively charged colloidal particles. The effect of particle charge was determined using ~S labeled suspensions of BaSO4. The face of a mica strip was placed on the surface of a K2SO4 solution in a continuous monitoring vessel of the type described by Langdon, Perrott, and Wilson (2). Additions of 3~S and BaCI~ were made through the side arm and sorption of sulfate was monitored with a Geiger tube placed on the other face of the mica strip. As the fl particles from 35S have a maximum range of only 28 mg/cm 2 (13), the solution activity detected by the Geiger tube was limited to a thin film at the surface. Sorption was indicated by an increase in count rate above this background solution activity.
17
SORPTION OF ANIONS BY MUSCOVITE TABLE IV DATA ~'OR SOLUTIONSO:F 10-6 M KH~PO~ CONTAININGOTHER SALTS AT A RANGE OF CONCENTRATIONS Original cation conen. (moles/liter)
Salt present 10-~
10-~
Pb (NO3) 2 pH °'/osolid phosphate ~ Sorption ratio (cm) b
5.5 0 8 X 10-5
5.5 2 2 X 10-5
MC13 pH % solid phosphate Sorption ratio (cm)
5.3 0 i X 10-~
4.9 0 4 X 10-5
FeCI~ pH % solid phosphate Sorption ratio (cm)
5.0 2 1.8 X 10-~
4.7 44 3.3 X 10-2
Th(NO3) 4 pH % solid phosphate Sorption ratio (cm)
5.4 50 1.8 X 10-2
4.6 100 5.8 X 10-2
5.2 19 2.2 X 10.4
4.7 93 7 X 10.5
1 0 -4
10 -2
10 -I
5.3 27 1.7 X 10-4
4.9 100 5.5 X 10-4
4.3 100 2.0 X 10-~
4.4 2 2.2 X 10-~
4.1 17 5.0 )< 10-a
3.4 2.9 Streaking ~ Streaking 1.5 X 10-~ 1.6 X 10-4
3.6 100 1.8 X 10-2
2.7 100 3.8 X 10-e
5.5 25 3 X 10-5
10-~
4.0 3.4 100 100 7.2 X 10-~ 4.2 X 10.2
2.1 100 6.8 X 10-a
1.6 Streaking 2.3 X 10-~
3.0 2.4 100 Streaking 2.5 X 10-3 8 X 10-a
Zr(S04)~ pH % solid phosphate Sorption ratio (cm)
3.9 100 8.0 ;K 10.4
3.1 2.2 100 100 4.5 X 10-~ 5.7 ?K 10-4
1.3 Streaking 5 X 10-5
a Determined by paper chromatography. b See text. Average of at least three samples. See text.
I t was f o u n d t h a t sulfate is r e t a i n e d f r o m 10- ~ M K2SO4 solutions w h e n the BaC12 conc e n t r a t i o n is 10- 3 M b u t n o t w h e n it is 10 .4 M or less. A l t h o u g h the solubility p r o d u c t of BaSO4 is 10 -1° mole2/liter 2 (10), solid p h a s e B a S Q is n o t p r o d u c e d until t h e critical supers a t u r a t i o n ratio is exceeded. M e a s u r e m e n t s m a d e b y L i e s e r (14) i n d i c a t e t h a t this wouId n o t occur until the ion p r o d u c t [-Ba~+-][-SO~ 2--] exceeded 5.5 X 10 -9 mole2/liter 2. T h i s explains the l a c k of sulfate r e t e n t i o n w h e n t h e BaC12 c o n c e n t r a t i o n was 10-4 M . A l t h o u g h t h e sulfate on t h e surface is r a p i d l y r e m o v e d b y rinsing w i t h distilled w a t e r ( p r e s u m a b l y b y a dissolution m e c h a n i s m ) , it is n o t r e m o v e d b y t r e a t m e n t w i t h 0.5 M K2SO4.
N o sulfate was r e t a i n e d f r o m 10 -3 M K2SO4 w h e n the BaCI~ c o n c e n t r a t i o n was 10 .5 M . H o w e v e r , the lower specific a c t i v i t y of t h e solution in this case increased the possibility of s o r p t i o n being m a s k e d b y t h e b a c k g r o u n d due to t h e solution a c t i v i t y . T o check this result, m i c a was t r e a t e d w i t h BaSO4 suspensions a n d t h e n rinsed w i t h 0.5 M K~SO4 i n s t e a d of distilled water. T h i s rinse r e m o v e d the thin film of a c t i v e solution b u t did n o t r e m o v e a d s o r b e d sulfate. I t was f o u n d t h a t the surface r e t a i n e d 1.0 X 10- 9 moles SO~2-/cm 2 (6 ions S O ~ - / 1 n m 2) f r o m the 10- 3 M B a ~+, 10 -~ M SO42- solution, b u t o n l y 2.8 X 10 -11 m o l e S O ~ - / c m 2 (0.17 ions SO~2-/1 n m ") was r e t a i n e d f r o m the 10- 5 M B a 2+, 10- ~ M SO~2- solution. Journal of Colloid and fnterface Science, Vol. 48, No. 1, J u l y 1974
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PERROTT, LANGDON AND WILSON
The last value is the same as that obtained for a 10-'~ M K2SO4 solution containing no BaCI2, confirming that no BaSO4 is retained from the solution containing excess sulfate. As Ba2+ and SO42- are the potential determining ions for BaSO4 these resultsconfirm that only positively charged particlesare adsorbed by the surface. The fact that the BaSO4 on the surface is not removed by treatment with 0.5 M K2SO4 solution indicates that the positive charges on the BaSO4 crystals responsible for their retention at the surface do not undergo charge reversal as would be expected in suspension. It seems very unlikely that any specific chemical bond could be formed between mica and BaSO4. Langdon, Perrott, and Wilson (2) also found that colloidal particles of hydrous ferric oxide cannot be removed from mica surfaces by charge reversal. Exclusion of anions by the negatively charged mica surface and steric effects are probably responsible for this effect. As sulfate was sorbed from solutions containing A1C13, FeCI~, CrCla, and Bi(NO~)3, even though the corresponding sulfates are soluble, a simple precipitation mechanism cannot be operating in these cases. Similarly, although chromatography indicates the presence of phosphate in an insoluble phase where sorption of this anion occurs from solutions containing A1CI,, FeC13, Th(NO3)4, and Zr(SO4)2 (Table IV), comparison of ion products with the solubility products available for stoichiometric phosphates does not indicate sorption of such insoluble compounds. However, at the pH values and cation concentrations where sorption of sulfate and phosphate is observed all these cations will exist as hydrolyzed species, i.e., hydrous oxides or polymeric hydroxo-complexes. The observed results in these cases can therefore be explained by adsorption of positively charged colloidal hydrous oxides or polymeric hydroxo-complexes containing adsorbed sulfate or phosphate anions. The importance of hydrolysis rather than precipitation of stoichiometric phosphates was confirmed in similar experiments where the pH, phosphate concentration and aluJournal of Colloid and Interface Science, Vol. 48, No. i, J u l y 1974
minum or iron concentration were all varied. It was found that there was a minimum cation concentration below which retention was negligible and that although this minimum concentration was independent of phosphate concentration it decreased with an increase in pH value. It has often been found that ion products for solutions containing phosphate and iron or aluminum do not agree with the thermodynamic solubility products of stoichiometric compounds like variscite and strengite. Bache (15) found such agreement only for pH values below 3.1 for variscite and 1.4 for strengite and suggested hydrolytic decomposition of the surface layers at higher pH values. Such hydrolytic decomposition or incongruent solubility has also been reported by Taylor and Gurney (16) and Huffman et al. (17) for a range of aluminum and iron phosphates. Similar hydrolysis was reported for thorium phosphate by Chukhlantsev and Stepanov (18) and it most likely occurs for all phosphates of such hydrolyzable cations. The precipitates formed in neutral or slightly acid solutions containing phosphate and such cations must therefore be basic Phosphates of variable composition dependent on pH value as well as cation and phosphate concentrations (19, 20). The species adsorbed by the surface from such solutions could therefore also be considered as positively charged colloidal particles of basic phosphates, although this term is usually used to describe compounds containing greater amounts of phosphate than are probably present in this case. CONCLUSIONS Observations on the retention of phosphate and sulfate by the 001 surface of muscovite lead to the conclusion that the cation exchange surface of muscovite can sorb these anions in the following ways: a. Adsorption of positively charged colloidal particles of insoluble compounds such as Pb3(PO4)2 and BaSO4, the positive charge arising from the relative concentrations of the charge determining ions in solution.
SORPTION OF ANIONS BY MUSCOVITE b. Adsorption of positively charged particles of hydrous oxides, or polymeric hydroxocomplexes, containing adsorbed phosphate or sulfate. c. Sorption of sulfate or phosphate b y positively charged colloidal particles of hydrous oxides or polymeric hydroxo-complexes, which have been previously adsorbed by the surface. Results obtained with BaSO4 suspensions indicate that particles adsorbed on the muscovite surface are not removed when the surface is treated with a solution containing an excess of the negative charge determining ion. Reversal of the positive charges on the particles responsible for retention does not occur as it wouId in suspension. As the interaction between the cation exchange surface of muscovite and the positively charged species is a nonspecific electrostatic one anion retention would be expected to occur at other negatively charged surfaces in a similar manner. One would also expect the reverse process of cation retention on a positively charged surface to occur by a similar mechanism. However, the former process would dominate in nature as the surfaces of most minerals and living organisms have a negative charge. ACKNOWLEDGEMENTS We thank the New Zealand University Grants Committee for the award of postgraduate scholarships (K.W.P. and A.G.L.), the New Zealand Golden Kiwi Lottery Grants Committee for a research grant and J. Chalcroft and G. Leet of the New Zealand Meat
19
Industry Research Institute for assistance in preparation of the electron micrographs. REFERENCES 1. HoA~, R., Ph.D. thesis, Victoria Univ. of Wellington, 1967. 2. LANGDON, A. G., PERROTT, K. W., ASm WILSON, A. T., J. Colloid Interface Sci. 44, 486 (1973). 3. EDROTH,B., Acta Chem. Scan& 23, 2636 (1969). 4. COLEgaN,N. T., Econ. Geol. 57, 1207 (1962). 5. FRINK, C. R., AND PEECtI, M., Inorg. Chem. 2, 473 (1963). 6. LAMB,A. B., ANn JACQUES,A. G., J. Amer. Chem. Soc. 60, 967, 1215 (1938). 7. BULATOV,N. K., AND MOKRUSI~IN,S. G., Kolloid. Zh. 27, 158 (1965). 8. JACKSON,M. L., Soil Sci. Soc. Amer., Proc. 27, 1
(1963). 9. K~AUS, K. A., PHILLIPS, H. O., CARLSON, T. A., AND JOHNSON, J. S., PI"OC. U.]~. Int. Conf. Peaceful Uses At. Energy, 2nd 28, 3 (1958).
10. SILLEN, L. G., AND MAR~ELL, A. E., "Stability Constants of Metal-Ion Complexes." Special Publ. No. 17, The Chem. Soc., London (1964). 11. CO~TON, F. A., AND WILKINSON,G., "Advanced Inorganic Chemistry--A Comprehensive Text." Wiley (Interscience) New York, 1962. 12. KRANZ,M., Chem. Abstr. 65, 3318b (1966). 13. FRIEDLANDER, O., KENNEDY, J. W., AND MILLER~ J. M., "Nuclear and Radiochemistry" 2nd ed., p. 107. Wiley, New York, 1966. 14. LIESER, K. H., Angew. Chem. Int. Ed. Engl. 8, 188 (1969). 15. ]]ACHE, B. W., J. Soil Sci. 14, 113 (1963). 16. TAYLOR,A. W., AND GURNEY,E. L., J. Soil Sci. 13, 187 (1962). 17. HVFF~AN, E. O., CATE, W. E., DENNING,M. E., AND ELMORE,K. L. Trans. Int. Congr. Soil Sci. 7th 2, 404 (1960). 18. CIZUKi=ILANTSEV,V. G., AND STEPANOV, S. I., Zh. Neorg. Khim. 1, 478 (1956). 19. MATTSON,S., Soil Scl. 30, 459 (1930). 20. PucH, A. J., Soil Sci. 38, 315 (1934).
Journal of Colloid and Interface 5"eience, VoL 48, No. I, July 1974
A. G. LANGDON AND A. T. WILSON School of Science, University of Waikato, ttamilton, New Zealand Received October 19, 1972; accepted August 15, 1973 The retention of phosphate and sulfate by the negatively charged 001 face of muscovite was investigated using techniques previously developed for the study of single planar surfaces. Muscovite is a layer silicate with a readily obtainable smooth surface of known structure particularly suitable for such studies. The results obtained indicate that this negatively charged surface retains anions by the following mechanisms: a. Adsorption of positively charged colloidal particles of insoluble stoichiometric compounds such as Pba(P04)2 and BaSO~. Once sorbed these particles were not desorbed under conditions which cause charge reversal in suspension. b. Adsorption of positively charged colloidal particles of hydrous oxides (or polymeric hydroxo-complexes) containing absorbed phosphate or sulfate. c. Sorption of the anion by positively charged colloidal particles of hydrous oxides (or polymeric hydroxo-complexes) previously adsorbed by the negatively charged surface. As the interaction of the colloidal particles with the surface is electrostatic these mechanisms would also operate with other negatively charged surfaces which are common in nature. INTRODUCTION
many of the problems found with finely dispersed materials can be overcome. As only a relatively small area (approx 1 cm 2) is studied by these techniques, problems associated with the analysis of very small amounts of material and with contamination have had to be solved (1, 2). Early results (2) showed that enhanced anion retention by muscovite surfaces pretreated with FeC13 solutions is due to the presence of colloidal particles of hydrous iron oxide on the surface. It has been suggested that this effect would also be observed when other similar anion sorbing hydrous oxides were present on the surface. Sorption of anions by precipitation of insoluble salts at the surface
The sorption of ionic species from solution by solids has usually been studied with samples of high specific surface area. It has often been impossible to determine the nature of the sorption process from the results of such studies, mainly because the nature of the surface of such small sized particles is not usually known and impurities or decomposition products are often present. Recently; techniques have been developed which have enabled sorption by the 001 face of large mica crystals to be studied (1, 2). Because the structure of this surface is well known and can be obtained clean and smooth down to molecular level by crystal cleavage i0 Journal of Colloid and Interface Science, Vol. 48, No. 1, July 1974
Copyright ~ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
SORPTION OF ANIONS BY MUSCOVITE is another possible mechanism. I t was the aim of the present work to test these suggested mechanisms of anion sorption by cation exchange surfaces. This paper reports the results of an investigation into the effect of pretreatment of the surface with a range of cations, and hydrolysis products, on the subsequent sorption of phosphate and sulfate anions. The sorption of insoluble phosphates and sulfates was also investigated. The results enabled conclusions to be drawn about the mechanism of the sorption of anions by cation exchange surfaces. EXPERIMENTAL
METHODS
Materials Clear, ruby muscovite was obtained as large sheets from Mica and Micanite Supplies Limited, London. Strips of mica with freshly cleaved faces and trimmed edges were used in the experiments. Gloves were worn and freshly cleaned scissors and tweezers were used in all manipulations to prevent contamination of the surfaces. Carrier-free 35S sulfate and high specific activity 3~p orthophosphate were obtained from the Radiochemical Centre, Amersham, U.K. KH~PO4 and K2SO4 solutions used were prepared from the analytical grade salts and all other reagents were of the highest purity available.
Methods The preparation of mica samples and the precautions taken to maintain clean conditions and prevent surface contamination were described by Langdon, Perrott, and Wilson (2). It was found that if A1CI~ solution entered the frayed edges of the mica strip during pretreatment, anion sorption by the resulting surface was nonuniform [-see Sec. (a) in Results and Discussion below~. All pretreatments were therefore performed by a method which limited the treatment solution to a spot on the mica face. After rinsing thoroughly anion sorption was carried out by partial immersion of the
ll
strip in the appropriate solution, ensuring that the pretreated area was covered. Sulfate was sorbed from carrier-free ~5S solutions obtained by dilution of the stock isotope solution. The concentration of the diluted solutions was calculated to be about 10-s M SO~-. Phosphate was sorbed from 10-6 MKH2P04 labeled with 32p. Mter sorption of the phosphate anion the mica samples were rinsed for about 30 sec with distilled water, dried and autoradiographed. It was found that a similar rinse to remove excess reagent from sulfate treated samples also removed most of the sorbed sulfate. These samples were therefore simply drained for 10 sec and then dried. Sorption of sulfate was indicated when the amount of sulfate on the surface was greater than that present in the liquid film after draining. The blank value for sulfate in the liquid film was determined by counting the activity remaining on a surface treated with ~5S labeled 10-2 M K2SO4, where negligible ~5S sorption could be assumed due to the very low specific activity. The sorption ratio, X -~ (moles/cm 2 surface)/ (moles/cm 3 solution), was determined for each surface by counting the solution and surface activities as described previously (2). Although the measurement errors involved in sampling and counting are such that an overall reproducibility of better than ± 12% is possible, in most cases larger variations (-t-20% or more) were found. Edroth (3) studied the sorption of thorium on various surfaces by a similar method and also found variations larger than expected from the measurement errors. This probably arises from the lack of reproducibility inherent in processes occurring in some of the sample treatments. For example, the reproducibility of the sorption of colloidal particles or of hydrolysis processes occurring during rinsing could be expected to limit the overall reproducibility. The above methods were described in more detail by Langdon, Perrott, and Wilson (2). a. Effect of edges during pretreatment. Strips of mica were pretreated for 10 rain with Journal of Colloid and interface Science, Vol. 48, No. 1, J u l y 1974
12
PERROTT, LANGDON AND WILSON
1 M FeCla or A1C18 by the method described above where the edges are protected from the treatment solution. Duplicate strips were pretreated by partial immersion in the solutions. After rinsing for 30 min with distilled water, the samples were treated with 82p phosphate (no carrier added) for 30 min, rinsed for 30 sec and then dried. Autoradiography of the samples enabled comparison of the amount and uniformity of phosphate sorption. b. Pretreatment with i M salts. Samples were pretreated for 30 mill with a range of 1 M solutions of salts of alkali metal, alkaline earth, transition metal and hydrolyzable cations [-a 0.1 M solution was used in the case of Th(NO3)4~. Although 1 M solutions could be prepared from most salts used, in the case of Bi(NO3)~ it was necessary to add HNO3 to dissolve the precipitated hydrolysis products. Precipitation of hydrolysis products also occurred in preparation of Ce(SO~)~ solution. In this case the clear supernatant solution was used for the surface treatment. The low pH values of some of the pretreatment solutions (e.g., approx 1 for 1 M FeCI~) introduces the possibility of decomposition of the mica surface. However, published information on the kinetics of acid induced clay mineral decomposition (4) indicate that this would not be significant at the low temperature and relatively short treatment times used. After pretreatment the surfaces were rinsed for 30 rain and treated with the phosphate or sulfate solutions for 30 rain. The pH values of both the phosphate and sulfate solutions were found to be about 5.5. As anions such as acetate would compete for sorption sites no attempt was made to control the pH of these solutions by the use of buffers. Mica surfaces which had been treated with 1 M Zr(NO~)4, 1 M Bi(NO3)~ and 0.1 M Th(NO~)4 and washed were examined in the electron microscope using a replication technique. After washing, separate samples of each surface were freeze dried or dried under an infrared lamp. Replicas were prepared by shadowing the surface at an angle of approx Journal of Colloid and Interface Science. Vol. 48, No, 1, J u l y 1974
10° with a mixture of platinum and carbon and then depositing a thin film of carbon at normal incidence. The preshadowed replicas were floated off on water and picked up from below on electron microscope grids. As the replica on the freezedried Th(NO3)4 treated surface could not be floated off on water a dilute HC1 solution was used and the replica was washed by transferring to a water surface before picking it up on a grid. c. Pretreatment with hydrolysis products. Pretreatment solutions were prepared by mixing known volumes of standardized NaOH solution and of the 0.1 M salt solutions with distilled water to give solutions approx l0 -3 M with respect to the cation and a degree of neutralization corresponding to midpoints of buffer regions in previously determined titration curves. These solutions were prepared on the day of use to reduce aging effects. Other experimental details are the same as for Sect. (b).
d. Sorption from solutions containing other salts. A range of 10-~ M sait solutions containing carrier-free 35S sulfate was used to treat mica samples for 30 min. Phosphate sorption from 10-6 M KH~PO4 containing cations of interest at a range of concentrations was carried out for 100 min. RESULTS AND DISCUSSION
a. E/ffect of edges during pretreatment. The autoradiograms of these samples show that phosphate is sorbed uniformly by surfaces pretreated with FeCI~ by either method. Surfaces pretreated with AICI~ on the face only do not sorb significant amounts of phosphate, whereas surfaces pretreated by partial immersion of the mica strip in the A1C13 solution sorb phosphate in streaks. Although pretreatment has been performed in this manner with many cations only AI(III) caused this streaking effect. The streaks are generally observed to start from the mica edges or from a "step" on the face. By changing the orientation of strips during the various steps in the experiment, it was found that the streaks always
SORPTION OF ANIONS BY MUSCOVITE occur along the same direction as that taken by water flowing over the strip during rinsing after A1C13 treatment. These results indicate that A1CI3 pretreatment of the face does not enhance phosphate sorption and suggest that phenomena occurring during leakage of A1CI~ solution from the edges during rinsing is somehow responsible. Leakage of AIC13 through a fine capillary tube into the bottom of the rinsing device used (water flows upwards over the strip in this device) was also found to cause phosphate sorption by freshly cleaved mica exposed to such rinsing thus confirming the above hypothesis. These results are compatible with a mechanism involving adsorption of hydrolysis products formed by dilution of the A1C13 solution leaking from the edge spaces during washing. Hydrolysis on dilution is indicated by the findings of Frink and Peech (5) that AICIa solutions become supersaturated with respect to A1(OH) a at concentrations below 5 X l0 -~ M, and precipitation of Al(OI-I)a occurs on dilution to below l0 -5 M. Such a mechanism would also be expected for other hydrolyzable cations and has been confirmed for FeCla(2). As found for the FeCla case the species on the surface after A1Cla treatment by the strip method are not exchangeable with Na + and Ca2+ and are acid soluble. Because AI(III) is less extensively hydrolyzed than the other cations causing anion retention, no hydrolysis products are adsorbed during rinsing of surfaces pretreated on the face only. When A1Cla solution is leaking out of the edges and passing over the face during rinsing, conditions are more favorable for adsorption of hydrolysis products. b. Pretreatment with 1 M salts. (Table I). Under the conditions of the experiment, blank samples (i.e., no pretreatment) were found to give sorption ratios of from 0 to 8 X 10-5 cm (corresponding to about 500 ions/~m 2) for phosphate sorption and about 2 X 10-4 cm for sulfate sorption. Preatreatment with chlorides of a range of alkali and alkaline earth metals (including Ba) or with MnC12, CoSO~, CdCI~, ZnSO4, Cn(NO~)2, UO2(N03)2,
13
TABLE
I
RETENTION OF ANIONS BY MUSCOVITE 001 FACE PRETREATED WITH 1 M SALTS Pretreatment solution (I M)
Blank A1Cla FeCI~ CrCI3 Th(NOa)4 (0.1 M) Zr(NO3)4 Bi(NO3)3 Ce(SO02 b
Sorption ratio (cm)a Phosphate (from 10-6 M KH~PO*)
Sulfate (from carrier-free solution)
0-8 2 4.5 5.6 1.7 1.7 9.7 1.2
2 2 6 1.0 1.6 3.2 8.7 7.5
X X X X X X X X
10-5 10-s 10-2 10-3 10-1 10-1 10.2 10-1
X X X X X X X X
10-4 10-4 10-4 10-a 10-~ 10-3 10-2 10.2
See text. b Clear supernatant solution after precipitation of hydrolysis products. Concentration less than 1 M.
Pb(NO3)2 or La(NO3)a did not enhance retention of either anion. Values of the sorption ratio for the other pretreatment solutions investigated are given in Table I. Of all the pretreatment solutions investigated only FeCI~, CrCla, Th(NQ)4, Zr(SO4)~, Bi(NO3)3, and Ce (SO4)2 caused enhanced phosphate retention and sulfate retention. The hydrolysis reactions occurring on dilution of 1 M salts of hydrolyzable cations are probably similar to those for FeC13 which has been studied in some detail (6, 7). FeCIa solutions with concentrations less than about 0.1 M are supersaturated with respect to hydrous ferric oxide. It is thought that after reducing the concentration below 0.1M, primary colloidal particles form and subsequently hydrolysis proceeds by growth of these nuclei. This initial stage of nuclei formation occurs more rapidly at lower concentrations. Such rapid formation of colloidal hydrous ferric oxide would be expected to occur durnig rinsing of mica surfaces treated with 1 M FeC13 and as these colloidal particles are likely to be positively charged they would be adsorbed by the negatively charged surface. A similar mechanism could be expected to operate for the other hydrolyzable cations. Journal of Colloid and Interface Science, Vol. 48, No. 1, July 1974
14
PERROTT, LANGDON AND WILSON
FIG. 1. Electron micrographs of repficas of mica surfaces: (a) Untreated surface; (b) 1 M Zr(NQ)4 treated surface, air dried; (c) 0.1 M Th(NQ)4 treated surface, freeze dried; (d) as (c) but air dried; (e) 1 M Bi(NO3)2 treated surface, freeze dried; (f) as (e) but air dried.
The electron micrographs presented in Fig. 1 confirm this mechanism for Zr (NO3) 4, Th(NO~)4, and Bi(NO3)3. The grain size of the replicating material is about 4 nm which means that particles smaller than about 5 nm would not be seen by this method. No particles were detected on the freeze-dried Journal of Colloid and Interface Science,
Vol. 48, No. 1, July 1974
Zr(NO3)~ treated surface (not shown in Fig. 1) but the fact that particles about 5 nm in diameter were seen on the air-dried surface suggests that very small particles are present which aggregate on drying to form larger visible particles. The particles on the Th (NO3)4 treated surface range in size up to about 50 nm
15
SORPTION OF ANIONS BY MUSCOVITE TABLE II RETENTION OF ANIONS BY MUSCOVITE 001 FACE PRETREATED WITI-IPARTIALLY )$EUTIRALIZED SALT SOLUTIONS Pretreatment solution~
Blank A1CI~ FeC13 CrC13 A CrC13 B Th(NO~)4 Co(NO3)2 MnC12A MnC12B Zr(NO3)4A Zr(NOa)4B
Molar ratio Hydroxide/cation (approx) -1.1 1.2 0.3 1.5 1.6 0.8 0.1 0.9 1.3 2.9
Sorption ratio (cm) . Phosphate Sulfate (from 10-6M KH2PO*) (from carrier-free solution) 0-8 2.0 5.6 4 1.1 1.7 6.1 2.7 6.6 1.2 3.3
X X )< X X X X X X X X
10-5 10-~ 10-2 10-~ 10-~ 10-1 10-4 I0 -4 10-4 10-3 10-2
2 X 10-~ 3.0 X 10-2 2.4 X 10-3 9 X 10-4 1.6 X 10-2 1.0 >< 10-2 2 X 10-4 1 X 10-~ 1 X 10-~ 3.3 X 10-3 5 X 10-4
See text. Cation concentration, 10-5 M.
and do not appear to be affected by air drying. Particles with a diameter of about 200 nm are seen on the freeze-dried Bi(NO~)3 treated surface. These appear to be aggregates and the smaller particles on the air-dried surface may have resulted from shrinkage and breakup of the aggregates on drying. Similar electron micrographs were obtained with FeC13 treated surfaces (2) but particles were not detected on surfaces treated with A1Cla by leaking the solution through a capillary into the bottom of a rinsing device as described above. The nonexchangeability of the phosphate sorbing species (see above) indicate that they are unlikely to be simple monomeric cations (8) and in this case colloidal particles of hydrous aluminum oxide, or polymeric hydroxo-complexes, with a diameter smaller than 4 nm are probably present on the surface. The size of the particles formed during rinsing would be determined by the extent of hydrolysis, i.e., by the nature of the cation, the concentration and the pH of the solution and the rinsing conditions. For instance, the larger size of particles on the Th (NOa)4 treated surface compared with the Zr(NO3)4 case probably arises from the higher pH of the 0 . 1 M Th(NO3)4 solution. In the case of
Bi(N03)8 the particles on the surface will be insoluble bismuthyl nitrates rather than the hydroxide or hydrous oxide. The adsorption of insoluble basic salts by the mica face may also occur in some other cases [-e.g., treatment with Zr (SO02]. c. Yretreatment with hydrolysis products (Table II). Anion retention was not enhanced by pretreatment with partially neutralized solutions of ZnSO4, Pb(NO3)2, Cu(NO~)2, UO2(NO3)~, or La(NOa)3. Values of the sorption ratio for the other partially neutralized pretreatment solutions are given in Table II. Of the partially neutralized solutions studied A1C13, FeC13, CrCt3, Th(NOa)~, and Zr(SO4)2 enhanced phosphate and sulfate retention. The solutions containing Co (NO3)2 and MnC12 also enhanced phosphate retention. The partially neutralized solutions used would contain colloidal particles of hydrous oxides or hydroxides. However, not all such species would have anion sorbing properties. The results obtained in parts (b) and (c) should therefore be explained by: i. The adsorption of positively charged colloidal particles of hydrous oxides (or poIyJournal of Colloid and Interface Science, Vol. 48, No. 1, July i974
16
PERROTT, LANGDON AND WILSON TABLE
III
I~ETENTION O1~SULFATE BY MUSCOVITE 001 FACE ~'RO~f SOLUTIONS CONTAINING OTHER SALTS Salt present in sulfate solutionb
pH
Sorption ratio (cm). (sulfate carrier free)
Blank BaCi~ A1C13 FeC13 CrCI~ Bi(N0~)3
5.5 5.4 3.4 2.1 3.1 1.6
2 9.5 4 2.6 1.2 6.7
X X X X X X
10-* 10-* 10-4 10-3 I0 -s 10-2
a See text. b Cation concentration, 10-2 M.
meric hydroxo-complexes) formed either by dilution on rinsing or by addition of NaOH. ii. Whether or not these adsorbed species react with anions. Some idea of the extent of specific sorption of.'sulfate and phosphate by the species present on the surface can be obtained from published reports of~Complex formation by the catiOns concerned (9, 10). With three exceptions, the results listed in Tables I and II are those that would be expected from literature evidence for the existence or nonexistence of sulfate and phosphate complexes of the cations concerned (10). The exceptions are those surfaces pretreated with partially neutralized solutions of CrC13(A), Co(NO3)~, and MnCI~. When CrC13 was being titrated with NaOH to obtain the titration curve it was noted that the color of the solution changed from violet to green at the beginning of the curve indicating formation of chloro-comp!exes (11). This would reduce the hydrolysis of Cr(III) in the least basic (A) solution and also reduce the charge on any polymeric species formed. Although pretreatment with 1 M CrCI~ in part (b) gives enhanced anion retention it is of interest to note that pretreatment with freshly prepared CrCI3, which is green and contains the chloro-complex, does not enhance retention. Very little hydrous chromic oxide would therefore be adsorbed from the less basic solution. Co (II) and Mn (II) Journal of Colloid and Interface Science, Vol. 48, No. 1, J u l y 1974
are known to be oxidized by air in basic solutions (11, 12) and phosphate retention would be due to the presence of hydrous Co(In) or Mn(III) oxide on the surface ill these cases.
d. Sorption from solutions containing other salts (Tables III and IV). Apart from solutions containing Ba 2+ and SO42- no enhancement of anion retention occurred with salts of the alkali and alkaline earth metals or with ZnSO4. Sulfate retention was also not affected by La(NO~)~. Sorption ratios for the other salts studied are given in Table III for sulfate and Table IV for phosphate. The amounts of phosphate present in a solid phase in these solutions were determined by paper chromatography with distilled water. The paper was washed with 0.50-/0 oxalic acid, rinsed and dried before use. This removed contaminants in the paper which prevented orthophosphate ions from moving with the solvent front. The fraction of 32p remaining at the origin, as determined by scanning with a collimated G.M. tube, was assumed to be present in a solid and the values obtained are listed in Table IV. In some cases at high cation concentrations where the pH was low, all the phosphate was present in a streak starting from the origin. The results indicate that anions can be retained as insoluble compounds [-i.e., PbsCPO4)2 and BaSO4~. It would be expected that such compounds are adsorbed as positively charged colloidal particles. The effect of particle charge was determined using ~S labeled suspensions of BaSO4. The face of a mica strip was placed on the surface of a K2SO4 solution in a continuous monitoring vessel of the type described by Langdon, Perrott, and Wilson (2). Additions of 3~S and BaCI~ were made through the side arm and sorption of sulfate was monitored with a Geiger tube placed on the other face of the mica strip. As the fl particles from 35S have a maximum range of only 28 mg/cm 2 (13), the solution activity detected by the Geiger tube was limited to a thin film at the surface. Sorption was indicated by an increase in count rate above this background solution activity.
17
SORPTION OF ANIONS BY MUSCOVITE TABLE IV DATA ~'OR SOLUTIONSO:F 10-6 M KH~PO~ CONTAININGOTHER SALTS AT A RANGE OF CONCENTRATIONS Original cation conen. (moles/liter)
Salt present 10-~
10-~
Pb (NO3) 2 pH °'/osolid phosphate ~ Sorption ratio (cm) b
5.5 0 8 X 10-5
5.5 2 2 X 10-5
MC13 pH % solid phosphate Sorption ratio (cm)
5.3 0 i X 10-~
4.9 0 4 X 10-5
FeCI~ pH % solid phosphate Sorption ratio (cm)
5.0 2 1.8 X 10-~
4.7 44 3.3 X 10-2
Th(NO3) 4 pH % solid phosphate Sorption ratio (cm)
5.4 50 1.8 X 10-2
4.6 100 5.8 X 10-2
5.2 19 2.2 X 10.4
4.7 93 7 X 10.5
1 0 -4
10 -2
10 -I
5.3 27 1.7 X 10-4
4.9 100 5.5 X 10-4
4.3 100 2.0 X 10-~
4.4 2 2.2 X 10-~
4.1 17 5.0 )< 10-a
3.4 2.9 Streaking ~ Streaking 1.5 X 10-~ 1.6 X 10-4
3.6 100 1.8 X 10-2
2.7 100 3.8 X 10-e
5.5 25 3 X 10-5
10-~
4.0 3.4 100 100 7.2 X 10-~ 4.2 X 10.2
2.1 100 6.8 X 10-a
1.6 Streaking 2.3 X 10-~
3.0 2.4 100 Streaking 2.5 X 10-3 8 X 10-a
Zr(S04)~ pH % solid phosphate Sorption ratio (cm)
3.9 100 8.0 ;K 10.4
3.1 2.2 100 100 4.5 X 10-~ 5.7 ?K 10-4
1.3 Streaking 5 X 10-5
a Determined by paper chromatography. b See text. Average of at least three samples. See text.
I t was f o u n d t h a t sulfate is r e t a i n e d f r o m 10- ~ M K2SO4 solutions w h e n the BaC12 conc e n t r a t i o n is 10- 3 M b u t n o t w h e n it is 10 .4 M or less. A l t h o u g h the solubility p r o d u c t of BaSO4 is 10 -1° mole2/liter 2 (10), solid p h a s e B a S Q is n o t p r o d u c e d until t h e critical supers a t u r a t i o n ratio is exceeded. M e a s u r e m e n t s m a d e b y L i e s e r (14) i n d i c a t e t h a t this wouId n o t occur until the ion p r o d u c t [-Ba~+-][-SO~ 2--] exceeded 5.5 X 10 -9 mole2/liter 2. T h i s explains the l a c k of sulfate r e t e n t i o n w h e n t h e BaC12 c o n c e n t r a t i o n was 10-4 M . A l t h o u g h t h e sulfate on t h e surface is r a p i d l y r e m o v e d b y rinsing w i t h distilled w a t e r ( p r e s u m a b l y b y a dissolution m e c h a n i s m ) , it is n o t r e m o v e d b y t r e a t m e n t w i t h 0.5 M K2SO4.
N o sulfate was r e t a i n e d f r o m 10 -3 M K2SO4 w h e n the BaCI~ c o n c e n t r a t i o n was 10 .5 M . H o w e v e r , the lower specific a c t i v i t y of t h e solution in this case increased the possibility of s o r p t i o n being m a s k e d b y t h e b a c k g r o u n d due to t h e solution a c t i v i t y . T o check this result, m i c a was t r e a t e d w i t h BaSO4 suspensions a n d t h e n rinsed w i t h 0.5 M K~SO4 i n s t e a d of distilled water. T h i s rinse r e m o v e d the thin film of a c t i v e solution b u t did n o t r e m o v e a d s o r b e d sulfate. I t was f o u n d t h a t the surface r e t a i n e d 1.0 X 10- 9 moles SO~2-/cm 2 (6 ions S O ~ - / 1 n m 2) f r o m the 10- 3 M B a ~+, 10 -~ M SO42- solution, b u t o n l y 2.8 X 10 -11 m o l e S O ~ - / c m 2 (0.17 ions SO~2-/1 n m ") was r e t a i n e d f r o m the 10- 5 M B a 2+, 10- ~ M SO~2- solution. Journal of Colloid and fnterface Science, Vol. 48, No. 1, J u l y 1974
18
PERROTT, LANGDON AND WILSON
The last value is the same as that obtained for a 10-'~ M K2SO4 solution containing no BaCI2, confirming that no BaSO4 is retained from the solution containing excess sulfate. As Ba2+ and SO42- are the potential determining ions for BaSO4 these resultsconfirm that only positively charged particlesare adsorbed by the surface. The fact that the BaSO4 on the surface is not removed by treatment with 0.5 M K2SO4 solution indicates that the positive charges on the BaSO4 crystals responsible for their retention at the surface do not undergo charge reversal as would be expected in suspension. It seems very unlikely that any specific chemical bond could be formed between mica and BaSO4. Langdon, Perrott, and Wilson (2) also found that colloidal particles of hydrous ferric oxide cannot be removed from mica surfaces by charge reversal. Exclusion of anions by the negatively charged mica surface and steric effects are probably responsible for this effect. As sulfate was sorbed from solutions containing A1C13, FeCI~, CrCla, and Bi(NO~)3, even though the corresponding sulfates are soluble, a simple precipitation mechanism cannot be operating in these cases. Similarly, although chromatography indicates the presence of phosphate in an insoluble phase where sorption of this anion occurs from solutions containing A1CI,, FeC13, Th(NO3)4, and Zr(SO4)2 (Table IV), comparison of ion products with the solubility products available for stoichiometric phosphates does not indicate sorption of such insoluble compounds. However, at the pH values and cation concentrations where sorption of sulfate and phosphate is observed all these cations will exist as hydrolyzed species, i.e., hydrous oxides or polymeric hydroxo-complexes. The observed results in these cases can therefore be explained by adsorption of positively charged colloidal hydrous oxides or polymeric hydroxo-complexes containing adsorbed sulfate or phosphate anions. The importance of hydrolysis rather than precipitation of stoichiometric phosphates was confirmed in similar experiments where the pH, phosphate concentration and aluJournal of Colloid and Interface Science, Vol. 48, No. i, J u l y 1974
minum or iron concentration were all varied. It was found that there was a minimum cation concentration below which retention was negligible and that although this minimum concentration was independent of phosphate concentration it decreased with an increase in pH value. It has often been found that ion products for solutions containing phosphate and iron or aluminum do not agree with the thermodynamic solubility products of stoichiometric compounds like variscite and strengite. Bache (15) found such agreement only for pH values below 3.1 for variscite and 1.4 for strengite and suggested hydrolytic decomposition of the surface layers at higher pH values. Such hydrolytic decomposition or incongruent solubility has also been reported by Taylor and Gurney (16) and Huffman et al. (17) for a range of aluminum and iron phosphates. Similar hydrolysis was reported for thorium phosphate by Chukhlantsev and Stepanov (18) and it most likely occurs for all phosphates of such hydrolyzable cations. The precipitates formed in neutral or slightly acid solutions containing phosphate and such cations must therefore be basic Phosphates of variable composition dependent on pH value as well as cation and phosphate concentrations (19, 20). The species adsorbed by the surface from such solutions could therefore also be considered as positively charged colloidal particles of basic phosphates, although this term is usually used to describe compounds containing greater amounts of phosphate than are probably present in this case. CONCLUSIONS Observations on the retention of phosphate and sulfate by the 001 surface of muscovite lead to the conclusion that the cation exchange surface of muscovite can sorb these anions in the following ways: a. Adsorption of positively charged colloidal particles of insoluble compounds such as Pb3(PO4)2 and BaSO4, the positive charge arising from the relative concentrations of the charge determining ions in solution.
SORPTION OF ANIONS BY MUSCOVITE b. Adsorption of positively charged particles of hydrous oxides, or polymeric hydroxocomplexes, containing adsorbed phosphate or sulfate. c. Sorption of sulfate or phosphate b y positively charged colloidal particles of hydrous oxides or polymeric hydroxo-complexes, which have been previously adsorbed by the surface. Results obtained with BaSO4 suspensions indicate that particles adsorbed on the muscovite surface are not removed when the surface is treated with a solution containing an excess of the negative charge determining ion. Reversal of the positive charges on the particles responsible for retention does not occur as it wouId in suspension. As the interaction between the cation exchange surface of muscovite and the positively charged species is a nonspecific electrostatic one anion retention would be expected to occur at other negatively charged surfaces in a similar manner. One would also expect the reverse process of cation retention on a positively charged surface to occur by a similar mechanism. However, the former process would dominate in nature as the surfaces of most minerals and living organisms have a negative charge. ACKNOWLEDGEMENTS We thank the New Zealand University Grants Committee for the award of postgraduate scholarships (K.W.P. and A.G.L.), the New Zealand Golden Kiwi Lottery Grants Committee for a research grant and J. Chalcroft and G. Leet of the New Zealand Meat
19
Industry Research Institute for assistance in preparation of the electron micrographs. REFERENCES 1. HoA~, R., Ph.D. thesis, Victoria Univ. of Wellington, 1967. 2. LANGDON, A. G., PERROTT, K. W., ASm WILSON, A. T., J. Colloid Interface Sci. 44, 486 (1973). 3. EDROTH,B., Acta Chem. Scan& 23, 2636 (1969). 4. COLEgaN,N. T., Econ. Geol. 57, 1207 (1962). 5. FRINK, C. R., AND PEECtI, M., Inorg. Chem. 2, 473 (1963). 6. LAMB,A. B., ANn JACQUES,A. G., J. Amer. Chem. Soc. 60, 967, 1215 (1938). 7. BULATOV,N. K., AND MOKRUSI~IN,S. G., Kolloid. Zh. 27, 158 (1965). 8. JACKSON,M. L., Soil Sci. Soc. Amer., Proc. 27, 1
(1963). 9. K~AUS, K. A., PHILLIPS, H. O., CARLSON, T. A., AND JOHNSON, J. S., PI"OC. U.]~. Int. Conf. Peaceful Uses At. Energy, 2nd 28, 3 (1958).
10. SILLEN, L. G., AND MAR~ELL, A. E., "Stability Constants of Metal-Ion Complexes." Special Publ. No. 17, The Chem. Soc., London (1964). 11. CO~TON, F. A., AND WILKINSON,G., "Advanced Inorganic Chemistry--A Comprehensive Text." Wiley (Interscience) New York, 1962. 12. KRANZ,M., Chem. Abstr. 65, 3318b (1966). 13. FRIEDLANDER, O., KENNEDY, J. W., AND MILLER~ J. M., "Nuclear and Radiochemistry" 2nd ed., p. 107. Wiley, New York, 1966. 14. LIESER, K. H., Angew. Chem. Int. Ed. Engl. 8, 188 (1969). 15. ]]ACHE, B. W., J. Soil Sci. 14, 113 (1963). 16. TAYLOR,A. W., AND GURNEY,E. L., J. Soil Sci. 13, 187 (1962). 17. HVFF~AN, E. O., CATE, W. E., DENNING,M. E., AND ELMORE,K. L. Trans. Int. Congr. Soil Sci. 7th 2, 404 (1960). 18. CIZUKi=ILANTSEV,V. G., AND STEPANOV, S. I., Zh. Neorg. Khim. 1, 478 (1956). 19. MATTSON,S., Soil Scl. 30, 459 (1930). 20. PucH, A. J., Soil Sci. 38, 315 (1934).
Journal of Colloid and Interface 5"eience, VoL 48, No. I, July 1974