Reactions of Aqueous 

Aluminum Species 

at Mineral Surfaces

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1827-F
Reactions of Aqueous 

Aluminum Species 

at Mineral Surfaces

By D. W. BROWN and J. D. HEM

CHEMISTRY OF ALUMINUM IN NATURAL WATER

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1827-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Brown, David Wayne, 1949-

Reactions of aqueous aluminum species at mineral surfaces. (Chemistry of aluminum 

in natural water)

(Geological Survey Water-supply Paper 1827-F

Bibliography: p. 48.

1. Aluminum salts. 2. Surface chemistry. 3. Water chemistry. I. Hem, John David,

1916- joint author. II. Title. III. Series. IV. Series: United States.

Geological Survey. Water-supply Paper 1827-F. 

TC801.U2 [QD181.A4] 553'.7'0973s [551.4'9] 75-619064

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402

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CONTENTS

Page

Abstract__________________________________________ Fl

Purpose and scope ____________________________________________________ 1

Acknowledgments __________________________________________ 2

Experimental plan and methods-_____________________________ 3

Characterization of surfaces___________________________________ 4

Long-term aging effects.___________________________________ 6

Interpretation of results ____________________________________ 7

Possibility of aluminosilicate formation __________________________ 10

Gibbsite equilibrium solubility ______________________________ 17

Changes in pH during aging __________________________________ 19

Changes in Ala and Alb concentration during aging.____________ 20

Concentration of Alb at time zero ________________________________ 21

Significance of cation exchange capacity __________________________ 23

Short-term adsorption experiments __________________________________ 25

Adsorption of Al a ________________________________________________ 29

Adsorption of Alb ___________________________________ 29

Equilibrium models for adsorption of Alb -------_______________ 41

Behavior of Alc -------_______________________________________________ 43

Effects of surfaces on approach to equilibrium ______________________ 45

Conclusions regarding sorption and polymerization effects _________________ 47

References cited ______________________________________________________ 48

ILLUSTRATIONS

Page

FIGURE 1. Graph showing relation between specific surface area and cation

exchange capacity for materials studied _______ ___ F7

2. Electron micrograph of Wyoming bentonite __________ 8

3. Electron micrograph of volcanic ash ______________ 9

4. Electron micrograph of kaolinite __________________ 10

5. Diagram comparing compositions of experimental solutions with

clay-mineral solubilities _______________________ 16

6. Diagram comparing compositions of experimental solutions with

gibbsite solubilities ___________________________ 18

7. Graph showing first-order disappearance of Alb and pH changes for 

solution F 0.02VA __________________________ 22

8. Graph showing first-order disappearance of Al^ and pH changes for

solution H O.IK ____________________________ 22 

9-12. Graphs showing initial loss of Al^ as a function of surface area in:

9. Solutions D ____________________________ 25

10. Solutions F ____________________________ 26

III
IV CONTENTS

Page

FIGURES 9-12. Graphs showing initial loss of Alb as a function of surface area 

in Continued

11. Solutions G _______________.____________ 27

12. Solutions H _____________________________ 28

13-18. Graphs showing:

13. Adsorption of Ala at 1 and 48 hours in solutions DS as a

function of surface area ____________________________ 32

14. Gibbsite ion activity product in solutions DS as a function of

cation exchange capacity ____________________________ 36

15. Adsorption of Alb at 1 and 48 hours in solutions DS as a

function of surface area ____________________________ 38

16. Adsorption of Al^ at 1 and 48 hours in solutions DS as a

function of cation exchange capacity ______________ 39

17. Adsorption of Al^, as a function of time of solution-surface

contact ____________________________________________ 40

18. Test of Alb adsorption at 48 hours for correspondence to

Langmuir Isotherm ______________________ _ 43

19-22. Electron micrographs of:

19. Adsorbed microcrystalline gibbsite on Wyoming bentonite 44

20. Adsorbed microcrystalline gibbsite on volcanic ash______ 45

21. Adsorbed microcrystalline gibbsite on kaolinite _______ 46

22. Microcrystalline gibbsite adhering to kaolinite after 48

hours aging ________________________________________ 47

TABLES

Page

TABLES 1-4. Concentrations of two types of dissolved aluminum and pH in 

relation to age and amount and type of mineral surface in:

1. Solutions D _________________________________________ Fll

2. Solutions F ___________________________ 11

3. Solutions G ____________________________ 12

4. Solutions H_____________________________ 13

5. Aluminosilicate and gibbsite ion activity products and related data

for solutions D, F, G, and H after 2-3 months aging 15

6. Short-term losses of Alb in solutions D, F, G, and H____________ 24

7. Adsorption experiment data________________-__________----_-_ 30

8. Calculated sorbed species and aqueous Alc concentrations ____ 30

9. Calculated aqueous Ala species distribution at 48 hours _______ 31

10. Dissolved and sorbed species and selectivity constants for three ion

exchange mechanisms ____________________________________ 34

11. Data used for development of Langmuir isotherm for absorption of

Alb ____________________________________ 42
CHEMISTRY OF ALUMINUM IN NATURAL WATER

REACTIONS OF AQUEOUS ALUMINUM 

SPECIES AT MINERAL SURFACES

By D. W. BROWN and J. D. HEM

ABSTRACT

Aqueous aluminum solutions containing 4.5x10 molar aluminum in 0.01 molar 

NaClC>4 were partly neutralized with NaOH to give OH: Al mole ratios from 1.40 to 2.76. 

Measured amounts of montmorillonite, kaolinite, volcanic ash, or feldspathic sand were 

added to provide an area of inert surface. Reactions that occurred during 100 days of 

aging were compared with those in similar solutions without added surfaces, studied in 

earlier work.

Adsorption of monomeric species Al(H2 O)6+3,AlOH(H2O)5+2, and A1(OH)2 (H2O)4 + on 

the added surfaces follows a cation exchange mass law equilibrium model, and adsorption is essentially complete in 1 hour. Only minor changes in monomeric aluminum 

species occurred after that.

Rapid adsorption of polynuclear aluminum hydroxide species also occurs ard follows 

the pattern of the Langmuir adsorption isotherm. In the absence of surfaces, the 

polynuclear ions slowly increase in size and become microcrystalline gibbsite during 

aging. Electron micrographs showed microcrystalline gibbsite was present or surfaces 

after aging only 2 days. However, the analytical data suggest this material must have 

been adsorbed after it had already attained a near-crystalline state. Adsorbed polynuclear aluminum hydroxide species were not extensively converted to microcrystalline 

gibbsite during 100 days of aging.

PURPOSE AND SCOPE

In an earlier paper in this series, Hem and Roberson (1967) dealt 

with the form and stability of aqueous aluminum hydroxide complexes 

at various ranges of pH. They reported that below neutral pH the 

dissolved aluminum species consist of three forms, (1) the monomeric 

octahedral A1(H2 O)6 +3 , A1OH(H2 O) 5 + 2 , A1(OH) 2 (H2 O) 4 +, and 

A1(OH)4(H2 O)2 ~ ions, (2) a positively charged polymeric species consisting of octahedral units held together by double hydroxide bridges 

and containing as much as several hundred aluminum atoms, and (3) 

hexagonal microcrystalline gibbsite particles with maximum diameters near 0.1/um (micrometre). In a later report, Smith and Heir (1972) 

described the effect of aging on these species designated by thorn Ala , 

Alb, and Alc , respectively. No clearly denned boundary exists between
F2 CHEMISTRY OF ALUMINUM IN NATURAL WATER

Alb and Alc with regard to molecular size; the boundary used for 

convenience is an analytical one. When Ala, Alb, and Alc are placed in 

contact with the complexing agent ferron (7-iodo-8-hydroxyquinoline-5-sulfonic acid) under mildly acidic conditions, the Ala 

reacts completely in less than 1 minute, and the Alb reacts in a 

relatively slow first-order reaction having a half-time of 5-20 minutes. 

The Alc form does not readily react with the ferron and is stable for 

several days in this environment.

Solutions were prepared by addition of differing amounts of hydroxide to solutions of A1+3 , so that they were supersaturated in Ala with 

respect to gibbsite. The solutions approached equilibrium over several 

years aging as the polymeric Alb species grew in size. This process 

lowered the pH, because the units lost hydrogen ions from some of the 

attached water molecules when hydroxide bridges were formed. The 

Ala concentration remained relatively constant during aging; eventually the pH decreased to an equilibrium value. Between age times of 

10-20 and 100-120 days, the disappearance of Alb (to form Alc) appeared to obey a first-order rate law.

The experimental aging-study solutions used by Smith and Hem 

(1972) constituted a relatively simple system in comparison to the 

aluminum hydroxide species occurring in ground or surface water 

systems. The aluminum hydroxide species were studied separately 

and apart from other complicating factors such as the effects on dissolved aluminum of dissolved silica, fluoride, sulfate, and organic 

ligands, and of various types of mineral surfaces. These factors are 

being considered in other papers of the series "Chemistry of Aluminum 

in Natural Water."

The purpose of this paper is to describe the effects of representative 

mineral surfaces on the behavior of dissolved and suspended 

aluminum hydroxide species, especially with regard to adsorption of 

dissolved aluminum species by the surfaces and their effect on the rate 

of polymerization of Alb to Alc .

The general approach used in this investigation was to duplicate 

some of the experimental solutions prepared by Smith and Hem 

(1972), but with various known quantities of relatively insoluble minerals added in order to provide solid surfaces. Comparison of the 

results with those obtained in the absence of such surfaces permits a 

quantitative evaluation of their effects on the chemical reactions of 

aluminum that occur in systems moderately supersaturated with respect to gibbsite.

ACKNOWLEDGMENTS

Reviews of this paper by Professors R. W. Smith, Ur iversity of 

Nevada (Reno) and G. A. Parks, Stanford University, were helpful in
ALUMINUM SPECIES AT MINERAL SURFACES F3

developing the presentation and are acknowledged with thanks. We 

also wish to thank C. J. Lind for furnishing all the, electron micrographs used in the report.

EXPERIMENTAL PLAN AND METHODS

The work was begun by preparing a series of solutions containing 

the same total concentration of aluminum (4.53 xlO~4 molar) but 

different amounts of added base. These aging-study solutions were 

prepared by a technique identical to that described by Smith and Hem 

(1972). The OH:A1 ratio (rn) in the solutions as made up varied from 

1.40 to 2.76. The four solutions prepared were designated D, F, G, and 

H and were closely comparable with similarly designated solutions 

studied by Smith and Hem.

Smith and Hem (1972) found that the rate of addition of base in 

preparation of the aging-study solutions had a marked effect on the 

amount of Alb and Alc initially formed. Alb concentration was found to 

decrease and Alc concentration increase when the rate of base addition 

increased. Al a concentration was found to be independent of this rate. 

The slower rates of base addition thus yielded solutions with a higher 

pH and farther from equilibrium than those obtained by fast addition 

of base. In this set of experiments, base was added dropwis? over a 

period of 90-120 minutes in order to start the solutions with as much 

Alb and as little Alc and as far from equilibrium as practicable.

Each 3,000 ml (millilitres) solution (D, F, G, and H) was divided into 

12 aliquots of 250 ml each and placed in polyethylene bottles in which 

measured amounts of the various surfaces had been placed. Tl us from 

solution D were formed the following 12 solutions:

Amount and type Abbreviation Added surface Surface per volume 

of surface (m2 ) (m 2/l)

No added surface (a) ___-_____ D NASA 0.0 0.0

No added surface (b) ________________ D NASB .0 .0

0.010 g Wyoming bentonite ______ D0.01WB 5.5 22

0.10 g Wyoming bentonite _______ D 0.1WB 55 220

0.20 g Wyoming bentonite ______ D 0.2WB 110 440

0.020 g volcanic ash ________________ D 0.02VA 2.2 8.8

0.10 g volcanic ash ___________ D 0.1VA 11 44

0.50 g volcanic ash ___________ D 0.5VA 55 220

0.10 gkaolinite #17 ______________ D O.IK 1.5 6.0

0.50 gkaolinite #17 ________________ D 0.5K 7.5 30

2.0 gkaolinite #17__ _ _ D 2.0K 30 120

1.0 g Monterey sand __________ D l.OMS .5 2.0

Solutions F, G, and H were similarly apportioned.

Each of these 48 solutions was analyzed for aqueous Ala and Alb in 

the manner described by Smith and Hem (1972). The aliquots used for
F4 CHEMISTRY OF ALUMINUM IN NATURAL WATER

analysis were centrifuged at 2,000 G's for 1 minute in order to remove 

particulate suspended material that might interfere with the spectrophotometric measurements. A modification of the ferron-orthophenanthroline analytical method for aluminum (Brown and others, 

1970) was used in which the pH of sample aliquots was never allowed 

to go below 5.0. Under these conditions, Ala reacts immediately with 

the ferron, while Alb reacts by first-order kinetics with ferron as these 

polymer species are attacked and dismantled by the acid. The halftime of the reaction ranges from 5 to 20 minutes, probably depending 

on the characteristic sizes and shapes of the Alt, polymers encountered. 

In any event, the reaction is at least 95 percent completed after 90 

minutes of analysis time. Alc does not react at a significant rate at pH 

5. At a given analysis time, the amount of ferron-aluminum complex 

formation is determined spectrophotometrically at 370 nanometres. 

The concentration of Alt, present at zero analysis time is determined by 

extrapolation of the first-order reaction rate to zero time us ing a plot of 

analysis time versus the negative logarithm of aluminum concentration recovered. The intercept is the negative logarithm of the analyzed 

Alb concentration (pAlb). Al a concentration is found by subtracting Alb 

concentration from the total concentration recovered after 90 minutes. 

The remainder of aluminum not recovered is aqueous and £ dsorbed Alc 

and also adsorbed Ala and Alt,. At the same time as the aliquots for 

aluminum determinations were withdrawn, pH measurements were 

made using a Corning model 12 pH meter, 1 accurate to ±0.01 pH unit. 

These Ala , Alb, and pH measurements were repeated at intervals of 

1-3 days during the first 20 days of aging and at 5-15 day intervals 

after that. After the final analysis near the 100th day, dissolved silica 

concentrations were determined for each solution in order to evaluate 

the extent to which the added mineral surfaces might have undergone 

chemical attack and released solutes that influenced the processes of 

aluminum hydroxide polymerization.

CHARACTERIZATION OF SURFACES

The mineral materials used were selected to represent commonly 

occurring types similar to those that might be associated with natural 

water but more specifically definable than most natural soil or sediment. Two clays, kaolinite and montmorillonite, were used, and 

coarser grained material included volcanic ash and a feldspathic sand. 

The solids and their pretreatment are described subsequently.

The Wyoming bentonite (WB) was from a sample collected in 

January 1965 at Belle Fourche, S. Dak. It was dried at 90°C for 10

'Use of trade names or commercial products in this report is for identification only and does not constitute 

endorsement by the U.S. Geological Survey.
ALUMINUM SPECIES AT MINERAL SURFACES F5

minutes, then roller crushed to particle sizes of <200 mesh. Dry sieve 

analysis was 90 percent <20/xm, while wet analysis showed 99 percent 

<20/xm and 92 percent <0.5/Am.

Ward's standard volcanic ash (New Mexico) fragments (VA) were 

ground in a mortar and pestle until all particles could pass through a 

32-mesh sieve, were thrice washed in distilled water, and then d^ied at 

90°C. Dry sieve analysis showed 100 percent <500/xm, 67 percent 

<250/xm, 48 percent <150/Jim, 42 percent <125/xm, 35 percent 

<105jum, 19 percent <74/xm, and 12 percent <44/xm. Microscopic 

examination showed most of the particles to have irregular jagged 

surfaces to which no simple geometric model for evaluating surface 

areas could be applied.

Ward's standard kaolinite #17 (Lewistown, Montana) (K) was 

treated by the same method as that used on volcanic ash. No sieve 

analysis was performed.

Monterey sand, a beach sand from Monterey, Calif. (MS), was obtained from the Braun-Knecht-Heimann Company, San Francisco, 

Calif. It was twice washed with 35 percent Na2 SO3 solution at pH 3 in 

order to remove MnO2 formed on the sand surface in previous experimental studies by Hem (1964), who reported 55 percent of the grains 

were feldspar and 45 percent quartz. The grains were moderately 

rounded, and 85 percent of them were between 0.25 and 0.8 mm 

(millimetre) in diameter. None were larger than 0.8 or smaller than 

0.1 mm.

The specific areas of the surfaces used were determined by the 

orthophenanthroline and ethylene glycol monolayer adsorption 

methods by Lawrie (1961) and Bower and Goertzen (1959), respectively. The orthophenanthroline adsorbed by a known amount of solid 

was determined by subtracting the concentration remaining in a solution shaken with the solid from the concentration of the saturated 

orthophenanthroline solution used. Analyses were made colorimetrically using the iron (II) orthophenanthroline complex. Specific surface 

areas obtained by this method are as follows:

Wyoming bentonite ______ ___ 550±40 rr^/gfmetres squared per gram)

Volcanic ash ____________ 110±10

Kaolinite #17____________ 15±2

Monterey sand ______ ________ 0.5±0.05 vcP/g

Ethylene glycol adsorbed by a known amount of solid was determined 

gravimetrically. Values obtained using this method are as follows:

Wyoming bentonite ________ 550±110 m2 /g

Volcanic ash ____________ 118±10

Kaolinite #17____________ 17±2
F6 CHEMISTRY OF ALUMINUM IN NATURAL WATEF

Cation exchange capacities of the surfaces used were determined 

using the method of Chapman (1965). By this procedure the clay or soil 

is saturated with sodium acetate, then ammonium acetate is used to 

replace the sodium which is then analyzed by atomic absorption spectrophotometry. Values obtained are as follows:

Wyoming bentonite __________ 922 /aeq/g (microequivalents per gram)

Volcanic ash _________________ 264 /aeq/g

Kaolinite #17 _____________ 69 /ueq/g

Monterey sand ________________ 6.6 /aeq/g

Determinations of the point of zero charge were made by measuring 

adsorption of hydrogen or hydroxide using the titration method described by T. W. Healy (oral commun., 1970), and the rerults obtained 

are as follows:

Wyoming bentonite _________ pHS.l

Volcanic ash ____________ 8.4

Kaolinite #17____________ 4.3

Cation exchange capacity for the four materials studied is closely 

related to surface area, as shown by the plot of these data (fig. 1). The 

degree to which this relationship may be more generally applicable is 

uncertain, however. For one thing, the two properties, exchange capacity and surface area, were measured by chemically similar techniques. 

The results thus have an internal bias that might lead to a relationship 

of this type. However, the wet adsorption procedures for surface area 

gave results that were in approximate agreement with a few measurements on the same materials made by the Brunauer-EmmettTeller (BET) procedure. The surfaces of all four materials are similar 

in the sense that all are dominated by oxygen ions bonded to silicon, 

and some similarity in behavior should not be surprising. Surfaces 

with radically different structure would perhaps behave differently 

from the ones studied.

Electron micrographs of the Wyoming bentonite, volcanic ash, and 

kaolinite materials used in this study are given as fig ires 2-4. The 

thin plates of montmorillonite are well displayed in the bentonite. The 

volcanic ash is composed of irregular glassy shards without a 

well-defined crystal structure. The kaolinite consists me inly of rather 

large layered plates with the angular outline typical of this mineral.

LONG-TERM AGING EFFECTS

The solutions studied by Smith and Hem (1972) were analyzed at 

intervals over about 9 months, and their paper also included some data 

on equivalent solutions aged as much as 3 years. Most cf the changes
ALUMINUM SPECIES AT MINERAL SURFACES F7

1000

200 400 600 

SPECIFIC SURFACE AREA, IN METRES SQUARED PER GRAM

FIGURE 1. Relation between specific surface area and cation exchange capacity for

materials studied.

occurred in the first 4 months of study, however, and it therefore 

appeared possible to evaluate adequately the influence of the surfaces 

on the attainment of equilibrium solubility of aluminum with a maximum of 4 months of aging of the test solutions. The results of these 

experiments and factors considered in their interpretation follow. Observed pH and concentrations of aluminum species for each solution 

are given in tables 1-4.

INTERPRETATION OF RESULTS

The concentrations of dissolved species that can be measured, Ala,
F8 CHEMISTRY OF ALUMINUM IN NATURAL WATER

FIGURE 2. Electron micrograph of Wyoming bentonite.

Alb, and H+ , constitute the information that is available about the 

processes that occurred. Where correlated with what is known about 

the mineral surfaces, some insight into the nature of the processes 

ought to be possible. Several alternative hypotheses can be suggested 

as a starting point for the interpretation of the experimental data. The 

following are possible conditions and ways for evaluating them. 

1. Chemical reactions may have occurred in which the surfaces were 

actually attacked and partly brought into solution. Such reactions might have produced a new solid aluminosilicate whose 

behavior would affect aluminum solubility in a way different 

from simpler aluminum hydroxide species. The extent of attack 

on the surfaces can be evaluated by determining dissolved silica 

in the aged solutions. A comparison of the halloysite ion activity 

products calculated from Si(OH)4°, A1+3 , and H+ activities in 

these solutions with those observed by Hem, Roberson, Lind, and 

Polzer (1973) for such material should indicate the importance of
ALUMINUM SPECIES AT MINERAL SURFACES F9

0.1 fJ,rn

FIGURE 3. Electron micrograph of volcanic ash.

reactions involving surface dissolution. If some of the solutions 

appear to be producing an aluminosilicate solid they should not 

be included in an evaluation of equilibria involving gibbsite.

2. Mineral surfaces catalyze or facilitate the processes of polymerization of aluminum hydroxide previously observed. Essentially 

this involves the conversion of Alb to Alc (gibbsite). It should be 

feasible to evaluate this possibility by comparing rates of 

dissappearance of Alb and testing for adherence to gibbsite 

solubility equilibria or by observing the rate and extent of pH 

shift in the presence or absence of surfaces.

3. Mineral surfaces alter concentrations of cations in solution by 

chemisorption. This can be studied by applying cation exchange 

equilibria.

4. Physical adsorption of polymerized species of low surface charge 

density occurs. The behavior of Alb and Alc might be so affected 

and can be explored by surface adsorption models.
F10 CHEMISTRY OF ALUMINUM IN NATURAL WATER

0.1 fj.m

FIGURE 4. Electron micrograph of kaolinite.

A combination of certain of these processes may well occur. From a 

systematic examination of the experimental data, however, it should 

be possible to decide which processes predominate. In addition to the 

chemical data, electron micrographs of solids before and after exposure 

to aluminum solutions are available. These provide qualitative 

evidence of crystallization processes at the surfaces.

POSSIBILITY OF ALUMINOSILICATE FORMATION

As pointed out earlier, if the mineral surface in a particular solution 

were attacked and partly dissolved, this could be detected by the 

presence of aqueous silica. Studies by Hem, Roberson, Lind, and Polzer 

(1973, p. 16) indicate that solutions containing aluminum in 

concentrations similar to the ones studied here, with 2.0 mg/1 

(milligrams per litre) SiO2 or less, precipitated only crystalline 

A1(OH)3. Where more silica was present, there was a mixed product;
ALUMINUM SPECIES AT MINERAL SURFACES Fll

TABLE 1. Concentrations of two types of dissolved aluminum and pH in relation to age 

and amount and type of mineral surface in solutions D 

[Asterisks indicate solutions which attacked surfaces]

Age 

(days) NASA NASB 0.01WB 0.1WB 0.2WB 0.02VA 0.1VA 0.5VA* O.IK 0.5K* 2.0K* l.OMS

Moles Ala/litre x 104

4_ _

8 ___

18 ___

34_ ___

50 _ 

69 _

103 

1.90

2.02

2.08

2.14

2.02 

1.92

1.99

2.09

2.21

2.15

1.95

2.03 

1.89

1.93

1.73

1.96

1.90

1 94

1.93 

1.86

1.88

1.08

1.22

1.13

1.07

1.05 

1.07

1.04

1.96

.22

.21

.09

 -

1 89

2.03

1.97

2.10

2.06 

1.92

1.91

1.96

1.92

1.92

1.96

1.98 

1.82

1.95

1.03

.90

.68

.82

.94

1.06

2.01

1.99

2.09

1.98 

1 89

1.80

1.96

1.93

1.76

1.82

1.74

1.46

1.44

2.12

1.91

1 79

1.67

Moles Alb/litre x 104

4 _

8_ _ 

18_ ---

34 

50_.

69 

103 

2.04

2.00

1.95

2.00 

1.78

1.91 

1.91

2.19

2.04

2.00

1.95

1.82

1.66 

1.41

1.12

1.12

.95

.85 

.72

.79 

.71

0.12

.07

.08 

.10

.09 

.13

1.78

.00

.00

.00

1.91

1.74

1.58 

1.48

1.55 

1.51

1.20

.62

.48 

.32

.41

.48

1.29

.06 

.04

.05 

.02

0.19

1.66

1.51

1.55 

1.38

1.38 

1.38

0.07

.56 

.62

.6f 

.6?

0.28

.32

.28

.22

.41

.34 

.42

2.09

1.95 

1.82

1.91 

2.00

pH

50_ 

69 _ 

103_

4.34 4.54 4.49 4.43 4.72 4.54 4.55 4.39 4.54 4.32 4.29 4.58

4.30 4.52 4.38 4.46 4.87 4.53 4.47 4.70 4.52 4.31 4.35 4.54

4.41 4.49 4.41 4.44 4.86 4.47 4.42 4.40 4.49 4.4C 4.35 4.51

4.43 4.54 4.44 4.46 4.90 4.51 4.45 4.42 4.50 4.41 4.35 4.54

4.42 4.45

4.44 4.43

4.27 4.36

4.42

4.42

4.38

4.45

4.46

4.44

4.50 4.45

4.51 4.45

4.45 4.41

4.38

4.43

4.38

4.44

4.47

4.40

4.42 

4.4E 

4.4C

4.32 4.51

4.33 4.53

4.30 4.47

TABLE 2. Concentrations of two types of dissolved aluminum andpH in relation to age 

and amount and type of mineral surface in solutions F

[Asterisks indicate solutions which attacked surfaces]

Age 

(days) NASA NASB 0.01WB 0.1WB* 0.2WB* 0.02VA 0.1 VA

Moles Ala/litre

2 __

3 -

6_ -___

12 

19 _

32 

45 _ _

69 

82 

98 

1.25

.85

.95

.97

1.02

.88

.95

1.18

1.00

1.21

0.95

.86

.92

.97

.97

1.16

1.06

1.20

1.49

.92

.99

.90

.88

1.06

.95

1.16

1.09

1.14

0.00

.22

.27

.16

.19

.18

.20

.20

0.09 0.97

.07 .82

.00 1.04

.35 .98

.02 .20

.25 .98

.04 .96

.24 1.27

1.00

1.25

Moles Alb/litre

2__ __

3__ ____

6__ _

12 __

19 _

32 

45 

69 

82 __

98 __ ___

3.02

2.88

2.75

2.57

2.19

1.66

1.17

.59

.32

2.88

2.95

2.81

2.57

2.14

1.58

1.05

.49

.22

2.14

2.29

2.09

1.82

1.74

1.29

1.05

.51

.35

0.43

.04

.00

.02

.05

.02

.00

.00

0.06 2.51

.03 2.69

.02 2.57

.10 2.34

.07 1.45

.02 1.29

.00 .85

.00 .25

.16

 _ .10

xlO4

0.84

.69

.97

.98

.85

.95

.90

1.18

.88

1.06

xlO4

1.66

1.70

1.48

1.35

1.20

.85

.56

.09

.08

.10

0.5VA

0.50

.25

.00

.16

.12

.11

.07

.19

1.38

.04

1.15

.05

.04

.02

.01

.02

O.IK

1.07

1.13

1.07

.98

.83

.91

1.01

1.17

1.08

1.26

2.45

2.63

2.45

2.19

2.14

1.51

1.17

.60

.37

.26

0.5K*

1.61

1.5C

l.OC

.9?

.9?

.9?

.94

1.11

.8f

1.1?

1.5?

1.7C

1.45

1.2f

1.2?

l.OC

.9?

.71

.62

2.0K*

1.44

1.29

.66

.76

.82

.79

.80

.86

.63

.80

0.21

.15

.21

.10

.10

.06

.03

.04

.07

.04

l.OMS

0.93

.66

.91

.92

.85

.93

.93

1.25

.97

1.19

2.75

2.88

2.69

2.51

2.23

1.58

1.10

.58

.38

.24

pH

0 ___

2 -

3 ___

6__- 

12__ _

19_ _

32 __

45_ ____

69 _

98 

4.75

4.65

4.64

4.56

4.57

4.48

4.03

4.38

4.04

4.29

4.75

4.65

4.48

4.59

4.57

4.48

4.29

4.38

4.25

4.40

4.74

4.65

4.64

4.62

4.58

4.48

4.47

4.45

4.38

4.69

4.73

4.70

4.67

4.78

4.66

4.72

4.75

4.72

5.33 4.75

5.36 4.69

5.39 4.65

5.38 4.60

5.53 4.59

5.27 4.28

5.41 4.43

5.36 4.42

5.35 4.33

4.31

4.30

4.75

4.68

4.64

4.61

4.58

4.52

4.49

4.46

4.41

4.31

4.68

4.74

4.74

4.80

4.79

4.76

4.76

4.68

4.85

4.76

4.67

4.66

4.63

4.61

4.60

4.48

4.44

4.41

4.38

4.70

4.65

4.63

4.62

4.60

4.63

4.52

4.53

4.54

4.54

4.53

4.55

4.54

4.55

4.51

4.63

4.47

4.45

4.45

4.48

4.76

4.72

4.67

4.67

4.60

4.48

4.43

4.40

4.36

4.35
F12 CHEMISTRY OF ALUMINUM IN NATURAL WATER

TABLE 3 Concentrations of two types of dissolved aluminum and pH in relation to age 

and amount and type of mineral surface in solutions G

[Asterisks indicate solutions which attacked surfaces]

Age 

(days) NAS A NAS B 0.01WB 0.1WB* 0.2WB* 0.02VA 0.1 VA

Moles Ala/litre

3_____

14 _ ___ 

25_ -

51 _____ 

64______

85______ 

95 _____

0.24

.13

.24 

.31

.54

.55

.59

.70

19

.32 

.34

.60

.58 

.98

.22 

.31

49

.51

.57

.08 

.04

09

0.00 0.18

.05 .20

.08 .24 

.08 .34

.57

____ .78 

.61

Moles Alb/litre

!_-___.

3_ -_

14_ ___

25 _ ___ 

51______

64 _ ___

85_- _- 

95______

2.88

2.81

2.69

2.29 

1.74

.56 

.32 

.20

2.88

2.75

2.63

240 

1 74

56 

.30 

.22

2.19

2.14

2 09

1.86 

1.51

.50 

.23

.17

0.01

.01

.01 

.01

----

0.04 2.63

.01 2.63

.00 2.40

.03 2.19 

.00 1.51 

.02 .62

.38

____ .14 

____ .13

xlO4

0.19

.15

.21

.32 

.33

.46

.55

xlO4

1.91

1.70

1.35

1.15 

.85 

.21

.01

.00

0.5VA

0.00

.00

.00

.00 

.00

.07

.00

.00

0.15

.02

.00

.00 

.00 

.01

.01

.00

O.IK

0.21

.18

.32 

.34

.53

.53

.59 

.66

2.63

2.51

2.40

2.23

1.74 

.89

.63

.37 

.23

0.5F--

0.29

.29

.29

.33 

.31

.40

.£2

1.95

1.78

1.66

1.48 

1.29 

.93

.79

.74 

.62

2.0K1 l.OMS

0.31

.33

.23

.32 

.25

.27

 -

0.41

.19

.06

.02 

.00 

.01

 -

0.30

.24

.21

.25 

.30

.45

.49 

60 

.43

2.75

2.69 

2.51

2.34 

1.74 

.85

.59 

.34 

.30

pH

0 ___

3_-___-

7___-_-

25_-____

64______

85______ 

95 -___-

4.92

4.74

4.81

4.66

4.53

4.40

4.41 

4.46

4.95

4.91

4.79

4.49

4.42 

4.43

4.93

4.84

4.65

4.59

4.58 

4.53

6 29

6.25

6.3

6 49

6.35

----

6.40 4.96

6.69 4.97

6.76 4.86

6.73 4.72

6.92 4.60

6.35 4.61

6.65 4.51

4.50

4.44 

____ 4.42

4.96

5.00

4.90

4.86

4.76

4.68

4.59

4.60 

4.55

5.08

5.38

5.41

5.48

5.55

5.40

5.59

5.75 

5.64

4.95

4.91

4.86

4.70

4.58

4.57 

4.52 

4.48

4.89

4.65

4.83

4.73

4.70

4.74 

4.72 

4.67

4.82

4.74

4.77

4.69

4.96

4.91

4.87

4.86

4.69

4.68

4.51

4.53 

4.48 

4.46

however, silica concentrations as much as 9.0 mg/1 SiC>2 still gave solids 

that contained gibbsite identifiable by X-ray diffraction, and solubility 

data indicated a gibbsite equilibrium was reached. These observations 

suggest that if concentrations of SiC>2 remained below 9.0 mg/1, 

gibbsite would be precipitated in solutions similar to the ones prepared 

here. Nonetheless, it is expedient to examine aluminosilicate 

equilibria in all the solutions where silica was detected even if the 

amount is less than 9.0 mg/1, because higher levels might have been 

present in some of these systems at some time during the aging 

process.

To say whether a solution may be at equilibrium with respect to a 

silicate, we must first determine from the H+ , Na+ , Al +3 , and Si(OH)4 

activities whether various solutions are saturated or supersaturated 

with respect to possible aluminosilicates. It can be shown that within 

the ranges of these activities in our solutions the most likely minerals 

for which saturation or supersaturation would occur are halloysite and 

kaolinite, for which the equilibrium can be written

H2 O

and
ALUMINUM SPECIES AT MINERAL SURFACES F13

TABLE 4. Concentrations of two types of dissolved aluminum andpH in relation to age 

and amount and type of mineral surface in solutions H

[Asterisks indicate solutions which attacked surfaces]

Age 

( days ) NAS A NASB 0.01WB 0.1WB* 0.2WB* 0.02VA 0.1VA

Moles Ala/litre

l _____ 

3______ 

6 _____ 

12 ___ 

20-_ 

45______ 

72 

81 

97____._

0.11 

.02 

.00 

.09 

.22 

19

.25 

.46 

.27 

.48

0.12 

.12 

.00 

.08 

.23

01

.33

.54

.37 

.42

0.15 

.21 

.00 

.08 

.02 

.16

.17 

.27

.21 

.34

0.00 

.31 

.09 

.04 

.00

.10

0.03 0.00 

.12 .09 

.31 .01 

.00 .08 

.00 .17

.18 .32 

.23 .49

.44

.46

Moles Alb/litre

l 

3______

6 ____

12 

20 _ 

32 

45- -

72 ____

97______

2.57 

2.57 

2.57 

2.09 

1.51 

1.05 

.71

.31

.12

2.57 

2.57 

2.51 

2.14 

1.66 

1.15 

.78

.38

no

.14

1.95 

2.04 

2.04 

1.41 

1.23 

.89 

.55

.19

.02

0.20 

.19 

.01 

.05 

.02 

.08

.01

0.05 2.34 

.19 2.23 

.01 2.14 

.03 1.82 

.03 1.26 

.02 .85

.14 .19

.03

xlO4

0.08 

.06 

.00 

.12 

.13 

.22

.08 

.27

xlO4

1.58 

1.29 

1.02 

.78 

.39 

.12

.10

0.5VA*

0.02 

.10 

.00 

.00 

.00 

.00

.00 

.09

0.23 

.17 

.05 

.07 

.10 

.02 

.03

.02

O.IK

0.08 

.52 

.02 

.12 

.19

.20 

.32

.26

2.40 

2.34 

2.29 

1.86 

1.45 

1.50

.32

.19

0.5K

0.48 

.14 

.47 

.14 

.06 

.15

.19

.27

.22 

.30

1.74 

1.41 

1.74 

1.23 

1.05 

.87 

.71

.50

.33

2.0K*

1.21 

.00 

.00 

.04 

.02 

.02

.11

.05

034 

.08 

.58 

.05 

.02 

.03 

.01 

.02

_-__

l.OMS

0.09 

.05 

.00 

.13 

.26 

.21

.27 

.44

.34 

.40

2.51 

2.45 

2.40 

1.91 

1.41 

.98 

.66 

.30 

.19 

.13

pH

o _ __

!______ 

3__ - 

6______

20______ 

32__ _

45______ 

72_ __ 

81______

97 ___

5.10 

5.19 

5.01 

5.06

4.40 

4 59

4.54 

4.47 

4.51

4.46

5.12 

5.31 

5.17 

5.08

4.50

4.63 

4.55

4.55

5.13 

5.39 

5.30 

5.10

4.83

4.68 

4.66 

4.59

4.64

6.33 

6.59 

6.59 

6.45

6.63

6.45 

6.40

6.89 5.18 

6.53 5.33 

7.01 5.22 

6.45 5.18

7.03 4.75

6.85 4.64 

6.9 4.63 

4.53

4.52

5.18 

5.45 

5.34 

5.22

4.93

4.75

4.70 

4.66

5.50 

6.09 

5.92 

5.90

5.87

5.75

5.93 

5.95

5.14 

5.40 

5.22 

5.18

4.87

4.70 

4.66 

4.61

5.07 

5.25 

5.32 

5.12 

5.07

4.99 

4.93

4.87 

4.87 

4.85

5 12 

519 

520 

509 

503 

501 

4.97 

4.97 

515

5.16 

5.36 

5.21 

5.17 

4.88 

4.82 

4.67 

4.66 

4.58 

4.55 

4.61

[Si(OH)4]

[H

=K.

From the following thermodynamic data, it is possible to evaluate this 

equilibrium constant for three forms of clay, ranging from relatively 

unstable synthetic halloysite to well-crystallized kaolinite.

Species AG°f (kcal/mole )

A1+3 ______________________________ -115.0

Si(OH)4 ___________________ -312.7

H2O ______________________ -56.69

Al2Si2 O5 (OH)4 :halloysite__________ -898.4

Synthetic halloysite ___________ -897±1.0

Al2 Si2 O5(OH)4 :kaolinite ________ -902.87

Reference

Latimer (1952).

Polzer and Hem (1965).

Latimer (1952).

Robie and Waldbaum (1968).

Hem, Roberson, Lind, and

Polzer (1973). 

Robie and Waldbaum (1968).

Mineral

Halloysite _______________ -6.84

Synthetic halloysite _____________ 7.54

Kaolinite _________________ -4.61

K at 25°C for

[tf+P 

105.01

105.53

103.38
F14 CHEMISTRY OF ALUMINUM IN NATURAL WATER

The following relation can be used to calculate the thermodynamic 

activity of Al +3 in solutions of 0.01 ionic strength when pH and total 

Ala are known:

.. «. (Ala)

 1.47xlO~5 1.91xlO~10 7.6xlO~23 2.34+ + + - 

[H+ ] [H +] 2 [H + ] 4

The quantity (Ala) is the analytically determined value in moles per 

litre. Calculated values for [Al +3] after approximately 100 days of 

aging are given in table 5. The activities of silicic acid (Si(OH)4 ) in 

table 5 were calculated from the determined silica, assuming that this 

uncharged species has an activity coefficient of unity. The activity of 

OH~ was calculated from the measured pH, using io~1400 as the ion 

activity product for water.

The data in table 5 all represent the analysis made at the maximum 

aging time that gave useful information for calculating equilibrium 

solubility. Generally this was near 100 days. For some of the solutions, 

however, especially those having the larger amounts of silica, the 

concentration of Ala dropped below the detection limit before that 

time. For these solutions a somewhat shorter aging period was used, 

for which a specific value for Ala was available. The cation exchange 

capacity values in table 5 represent concentrations in equivalents per 

litre, calculated from the weights of solids added and their measured 

cation exchange capacity per unit weight.

From inspection of the plot of pAl + pSi(OH)4 versus pH (fig. 5), it is 

apparent that several solutions from each group are supersaturated or 

near saturation with respect to synthetic halloysite. Generally those 

solutions containing 0.1 and 0.2 g Wyoming bentonite and 0.5 and 2.0 

g kaolinite are those whose aluminum solubility may b^ controlled by 

silicate equilibria. The aluminum concentrations in these solutions 

may therefore be controlled by precipitation of aluminosilicate, although it is not possible to verify the existence of this kind of solid 

material in the presence of the added mineral material. A few of the 

solutions contained more than the 9 mg/1 SiO2 specified by Hem, 

Roberson, Lind, and Polzer (1973) as the upper limit for dissolved 

silica in solutions in which microcrystalline gibbsite vas ultimately 

identified. It appears reasonable and proper to leave data from these 

solutions out of consideration in evaluating the behavic1" of aluminum 

in systems where dissolved silica is absent. It has been noted that 

Wyoming bentonite and volcanic ash, both having beer found to have 

high points of zero charge (8.1 and 8.4, respectively), could have 

increased the pH by direct sorption of H+ . Kaolinite, having a low
ALUMINUM SPECIES AT MINERAL SURFACES F15

TABLE 5. Aluminosilicate and gibbsite ion activity products and related data for solutions D, F, G, and H after 2-3 months aging

Solution

1 D NASA

3 D 0.01WB

4 D 0.1WB _

5D0.2WB _

6 D 0.2VA _

7 D 0.1VA _

8D0.5VA _

9 D O.IK ___

10 D0.5K ___

11 D2.0K ___

12 D l.OMS _

13 F NASA

14FNASB___

15 F 0.01WB

17 F 0.2WB _ 

18 F 0.02VA _ 

19F0.1VA___

20F0.5VA___

21 F0.1K ___ 

22 F 0.5K

24 F l.OMS. _

25 G NASA _

26 G NASB _

27 G0.01WB_

28G0.1WB _ 

29G0.2WB _

30 G 0.02VA _

31 G 0.1VA

33 G O.IK ___

34G0.5K ___

35G2.0K 

36 G l.OMS _

37 H NASA _

38 H NASB _

39H0.01WB.

40H0.1WB _

41 H 0.2WB _

42 H 0.02VA _

43 H 0.1 VA _

44 H 0.5VA _

45 H O.IK _

46H0.5K ___

47 H 2.0K __

48 H l.OMS _

0.

_ ________4.27

___________4.38

___ __ __4.44

___________4.90

___ ______4.45

___ _____ 4.41

___________4.38

_ ___ _____4.40

______ __ 4.40

430

_______ ___4.47

4 29

_ _ _____4.38

___________4.72

___________5.35 

___________4.30 

_____ __ _4.31

___________4.85

___________4.38 

___________4.54

___________4.35

___ _ 4.43

___________5.97 

___________6.65

___________4.42

__ _ ___4.55

___________4.48

___________4.67

___________4.70

_________ _4.46

____ _ 4.46

______ 4.64

_____ _ -6.40

___________6.90

___________4.52

___________4.66

___________5.95

_____ _ ___4.87

__ ______5.15

___________4.61

(moles/lxlO (A 4)a

1.99

1.93

1.88

1.04

.09

1.91

1.95

.94

1.80

1.76

1.44

1.67

1.21

1.20

1.14

.20

.24 

1.25 

1.06

.19

1.26 

1.13

.80

1.19

.70

.98

.57

.09 

.18

.61

.55

.00

.66

.52

.27

.43

.48

.42

.34

.11

.23

.46

.27

.09

.31

.30

.05

.40

~6b

d1 w

0.0

.9

1.6

1.4

.7

.8

5.3

1.2

3.4

13.6

.6

.0

.0

.5

2.5 

.4 

1.4

1.0

1.4 

3.7

13.4

.3

.0

.0

.0

.9 

.9

.6

1.2

1.4

2.4

7.7

.0

.0

.4

.7

1.2

1.2

4.2

1.7

3.3

9.7

.3

TO 

+

3 

^.

4.13

4.17

4.44

5.72

4.18

4.16

4.47

4.19

4.21

4.27

4.25

4.37

4.39

5.26

5.81 

4.34 

4.41

5.36

4.35 

4.44

4.57

4.37

4.62

4.47

4.73

7.43 

9.28

4.67

4.75

4.65

4.82

5.12

4.83

4.79

4.99

8.59

10.14

4.82

5.10

7.38

5.03

5.17

6.22

4.91

O

S3 

o.

4.82

4.57

5.18

4.93

4.88

4.05

4.70

4.25

3.65

5.00

5.08

4.38 

5.18 

4.63

4.78

4.63 

4.21

3.65

5.30

4.82 

4.82

5.00

4.70

4.19

4.63

4.40

3.89

5.18

4.93

4.70

5.18

4.70

4.55

4.26

3.79

5.30

1

9.73

9.64

9.62

9.56

9.10

9.55

9.59

9.62

9.60

9.60

9.70

9.53

9.71

9.60

9.62

9.28

8.65 

9.70 

9.69

9.15

9.62 

9.46

9.52

9.65

9.54

9.57

9.47

8.03 

7.35

9.58

9.45

8.36

9.52

9.33

9.30

9.54

9.54

9 45

9.36

7.60

7.10

9.48

9.34

8.05

9.36

9.13

8.85

9.39

a* 

o

2L

< 

a

8.99

9.01

10.90

9.11

9.04

8.52

8.89

8.46

7.92

9.25

10.34

10.19 

9.52 

9.04

10.14

8.98 

8.65

8.22

9.67

12.25 

14.10

9.67

9.45

9.28

9.22

9.01

10.17

13.52

14.84

10.00

9.80

11.54

9.58

9.43

10.01

10.21

[H]-3 [Si(OH ] p[Al]4

4.15

4.31

3.80

4.24

4.19

4.62

4.31

4.74

4.98

4.16

3.82

5.86 

3.38 

3.89

4.41

4.16 

4.97

5.22

3.38

5.66 

5.85

3.59

4.20

3.92

4.79

5.09

3.75

5.68

5.86

3.56

4.18

6.31

5.18

3.62

[OHP p[Al] 

33.32

33.08

33.03

33.12

33.02

32.82

32.93

33.33

32.99

33.01

33.37

32.84

33.48

33.47

33.25

33.10

31.76 

33.44 

33.48

32.81

33.21 

32.82

33.13

33.32

33.24

33.18

33.14

31.52 

31.33

33.41

33.10

33.21

32.81

33.02

33.45

33.41

33.07

31.39

31.44

33.26

33.12

31.53

33.11

32.56

32.77

33.08

exchange cation 

capacity 

(equivalents/ IxlO 4)

0.369

3.69

7.38

.211

1.06

5.28

.276

1.38

5.52

.264

.369

3.69

7.38 

.211 

1.06

5.28

.276 

1.38

5.52

.264

0.369

3.69 

7.38

.211

1.06

5.28

.276

1.38

5.52

.264

.369

3.69

7.38

.211

1.06

5.28

.276

1.38

5.52

.264

isoelectric point of 4.3, should have had little effect on the pH's of the 

solutions containing it.

As shown in the section on Alb measurements, those solutions that 

are here shown to be supersaturated with respect to aluminum silicate 

showed very rapid decreases in Alb concentrations. Within 2-5 days 

Alb was below the approximately 10~6 molar detection limit, with only 

two or three points to trace the rate of polymerization. As to whether 

this sudden decrease is really due to surface catalyzed polymerization 

to form gibbsite or to reaction of the aluminum hydroxide polymer
F16 CHEMISTRY OF ALUMINUM IN NATURAL WATER

15

PH

FIGURE 5. Clay-mineral solubilities compared with compositions of experimental 

solutions. Numbers beside points correspond to solutions likewise numbered in 

table 5.
ALUMINUM SPECIES AT MINERAL SURFACES F17

with silica released from the surfaces, we can say with some ceHainty 

that in the cases mentioned, the clay mineral surfaces have caused 

chemical interferences and have introduced factors other than those of 

an inert surface catalyzing aluminum hydroxide polymerizat; on. Because of this effect, those solutions calculated to be supersaturated or 

near saturation with respect to aluminosilicates will not be further 

studied for comparison of polymerization rates and rate constants. 

However, since the effects of aluminosilicate solubility most likely 

become dominant only after several weeks aging and any adsorption of 

Ala or Alb by the surface took place after only a few hours, as later 

shown, the adsorption of Ala and Alb m these solutions can be 

considered.

GIBBSITE EQUILIBRIUM SOLUBILITY

The solubility constant for gibbsite given by Smith and Hem (1972) 

for the expression #=[A1+3 ] [H+ ] ~3 is 108- 22. This represents the 

behavior of well-crystallized material produced by long agir g (more 

than 2 years in some instances). If written as a conventional solubility 

product,

KSO =[A1 +3][OH-] 3,

this becomes 10~33- 78. Hem and Roberson (1967) reported a value for 

this solubility product of 10~32- 65 , representing micro crystalline 

gibbsite. Their solid was aged for a shorter time.

Values are given in table 5 for KSQ for solutions D, F, G, and H after 

2-3 months of aging. The activity products for these system s lie between the KSQ values for microcrystalline gibbsite and the more stable 

final form. This is illustrated in another way by plotting log[Al +3] 

versus pH in figure 6.

A few points would have plotted substantially to the right (supersaturation) of even the microcrystalline gibbsite line. These are for 

solutions that gave evidence of attack on the surfaces and cannot be 

considered significant when treated in this simplified equilibrium 

model.

In general the positions of the points where effects of silica are 

absent are about what would be predicted for the length of aging, using 

the criteria of Smith and Hem (1972). As the solutions age, it would be 

expected the points would move toward the left hand gibbsite solubility limit.

Although the data in tables 1-5 show a considerable loss in Ala 

occurred when the surfaces were present, the behavior of Ala generally 

seems to fit that of a gibbsite equilibrium system. A more detailed 

examination of the data is obviously required before the behavior of 

Al a can be fully explained.
F18 CHEMISTRY OF ALUMINUM IN NATURAL WATER

FIGURE 6. Gibbsite solubilities compared with compositions of experimental solutions. 

Numbers beside points correspond to solutions likewise numbered in table 5.
ALUMINUM SPECIES AT MINERAL SURFACES Fl9 

CHANGES IN pH DURING AGING

Smith and Hem (1972) observed a substantial decrease in pH during 

aging of their solutions when the ratio of bound hydroxide to 

aluminum was 0.94 or greater. This decrease was attributed to the 

process of polymerization of Alb, and they were able to demorstrate a 

stoichiometric relationship. In solutions where polymerization did not 

occur, the pH remained essentially constant. The approach of the 

solutions to equilibrium with respect to gibbsite occurred mainly 

through the decreasing pH. It is of interest to compare pH changes in 

the solutions having mineral surfaces with corresponding ones that 

had no mineral surfaces.

Although the solutions D, F, G, and H prepared for this work were 

made by mixing amounts of stock solutions that were identical to those 

used by Smith and Hem (1972), the composition of the final mixtures 

differed somewhat from those of the earlier study. The stock solutions 

were mixed slowly with the aim of maximizing the Alb concentration. 

As a result the Alb content was substantially higher, Ala somewhat 

lower, and pH a little lower than in corresponding solutiors of the 

earlier study. As noted by Smith and Hem (1972), slow addition of the 

basic solution during mixing tends to produce a solution farther from 

equilibrium than rapid addition does, in that slow addition promotes 

the formation of Alb, a nonequilibrium species.

The solutions that did not contain added mineral surfaces did generally follow the previously observed patterns after mixing. The pH 

usually decreased in these solutions by an amount similar to that 

observed by Smith and Hem in the first 100 days that their solutions 

were aged.

The values of pH observed in the final analysis of our aging solutions, made after about 100 days of aging, are given in table 5. It is 

evident that the solutions to which the larger amounts of solid minerals were added generally had a higher pH after aging than the other 

solutions. Furthermore, the observations of pH made durirg aging 

(tables 1-4) show that many of these solutions had only small changes 

in pH after the initial observation, made just after time zero vrhen the 

solutions were first prepared and mineral surfaces were adc'ed.

It seems obvious that the mineral surfaces influenced th*> aging 

process in some way; however, the systems involved are more complex 

than those discussed in Smith and Hem (1972), and several possible 

processes may have been involved. As previously stated, actual attack 

and partial dissolution of the surfaces probably occurred in some of the 

solutions, resulting in losses of H+ by reaction with surficial material.
F20 CHEMISTRY OF ALUMINUM IN NATURAL WATER

Also, reactions involving aluminosilicate precipitation could occur. 

The final equilibrium condition might thus be shifted away from one 

involving only gibbsite. Another possible effect is the adsorption of 

cations by the mineral surface. Direct sorption of H+ would, of course, 

increase the pH, and preferential sorption of any of the various forms of 

aqueous aluminum also would have indirect effects on pH. It is 

noteworthy that the solutions with highest pH generally are the ones 

in each group with large exchange capacity associated with bentonite 

and volcanic ash; the solutions containing the largest amount of 

kaolinite show a less prominent pH increase. In any event, these are 

only three of possibly several causes for differences observed in these 

solutions.

The effects of adsorption and cation exchange on aluminum species 

concentrations will be considered later in this paper.

CHANGES IN Ald AND Alb CONCENTRATION DURING AGING

As mentioned earlier, measurements of aqueous Ala ?nd Alb were 

made periodically on each group of solutions. In the analytical procedure where no surfaces have been added to the solution, the amount of 

Alc is calculated by difference, since it is assumed that 

2Al=Ala+Alb +Alc .

It should be noted that the Ala species concentration is a directly 

determined value. Differences between the Ala content of the blank 

solution and the solutions containing surfaces, therefore, are valid 

measurements of the Ala losses (or AAla). The results may be used to 

evaluate the nature and rates of processes at the mineral surfaces that 

are responsible for the loss of aqueous Al a.

Similarly, aqueous Alb can be measured in a somewhat less direct 

way, and AAlb can be computed by comparing values for the blank 

solution with the comparable values for solutions containing surfaces. 

If there are significant differences in behavior of Ala and Alb, the 

experimental results should permit at least a generalized evaluation.

The third form of aqueous aluminum evaluated by Smith and Hem 

(1972), Alc , presents more difficult problems. In the absence of surfaces, it can perhaps be assumed, as was done previously, that the 

difference between the total initial aluminum concentration and the 

measured values of Ala -(-Alb at time t represents the concentration of 

Alc . This material is in the colloidal size range, or near it, and cannot 

be directly measured, even where solid mineral surfaces are not present, although qualitative evidence of its presence is readily obtainable. When surfaces are present, there is still anothe^ species of 

aluminum to be considered, the adsorbed aluminum, which cannot be 

readily determined separately from the Alc . The total of both can be 

calculated as the difference between Ala +Alb and the total initial
ALUMINUM SPECIES AT MINERAL SURFACES F21

aluminum for any of the solutions at a specific time after the surface 

was added.

Changes in the concentration of Ala between 1 and 100 days were 

observed in many of these solutions, although the changes generally 

were small. Smith and Hem (1972) reported no change of significance 

occurred in Ala in their solutions during aging.

It does not seem possible to attribute much significance to the 

changes in Ala that can be seen in tables 1-4. Some solutions show 

small gains in Ala, and others show losses. There is no clear-cut 

relationship to amount or kind of solid present. Some depolymerization of aqueous Alb or desorption of aluminum species might have 

occurred as the pH changed during aging and polymerization.

The Al^ contents show a much more consistent trend and, as in the 

solutions containing no added surface, the trend was downward 

throughout the aging period.

Smith and Hem (1972) observed a first-order disappearance of Alb 

over a considerable part of the aging period, and the half-time of this 

process appears from their data to have been about 20 days. However, 

the Alt, content of most of their solutions changed much mere slowly 

during the first 20 days than during the next 100 days, vhere the 

first-order rate mechanism was well defined.

Values of log Alb concentration and time in days were is ken from 

tables 1-4 and plotted. The results for solutions in the F, G, and H 

series generally correspond to those obtained by Smith and Hem, in 

that the data generally show a first-order rate mechanism and the 

half-time of the process is about 20 days. Most of the solutions in the D 

series lost very little Alb during 103 days of aging. The initial pH of 

these D solutions was in a range where Smith and Hem's result shows 

little polymerization took place during an equivalent period of time.

Typical graphs of log Alb concentration versus time in days are 

given as figures 7 and 8. The polymerization rate is evidently not 

significantly influenced by the presence of mineral surfaces of the 

types used here. However, the surfaces do have a readily discernible 

effect in the early part of the aging process, because the amount of Alb 

present at the time of the first measurement can be seen in tables 1-4 

to differ from one solution to another and the lower values represent 

solutions to which mineral surfaces had been added. A closer study of 

the effects produced immediately upon addition of the solid materials 

seems, therefore, to be required.

CONCENTRATION OF Al b AT TIME ZERO

From examining first-order rate curves drawn across the plots of 

-log (Alb) versus age time, it can be observed that the value for -log 

(Alb) at zero age time, that is immediately after addition of mineral
F22 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

5.5

5.0

o 

o

1 4.5

DC 

O

Q.

4.0

3.5

o o

20

O

o o o

40 60 

DAYS AGED

80 100 120

FIGURE 7. First-order disappearance of Alb, pH (circles) and -log (.Alb) (dots) as a 

function of aging time for representative solution F 0.02VA.

O 

O

DC 

O

X 

Q.

60 

DAYS AGED

1CO 120

FIGURE 8. First-order disappearance of Alb, pH (circles) and -log (Alb) (dots) as a 

function of aging time for representative solution H O.IK.
ALUMINUM SPECIES AT MINERAL SURFACES F23

surfaces, varies widely within a given set of solutions. Thie initial 

value appears to be dependent on the amount and type of surface 

added.

Within each set of solutions, those with no added surface contained 

greater Alb concentrations than any of the ones to which mineral 

surfaces were added. This indicates that on addition of surface, a 

substantial amount of Alb was lost. Because the first-order polymerization rates are too slow to account for such rapid polymerization 

during the 1 or 2 days before the first analysis, this loss of Alb is 

probably attributable to adsorption by the surface. If the difference in 

initial values of log (Alb) is attributable to adsorption by the surface, 

the amount adsorbed, in terms of concentration, may be calculated by 

subtracting the Al^ concentration in question from the Al^ corcentration of the solution to which no surface was added. This quantity is 

shown in the last column of table 6.

From data in table 6, it appears that the greatest losses of Alb 

occurred in solutions containing the greatest area of solid surface. The 

relationship between Al^ loss and surface area is demonstrated in 

figures 9-12. Although some of the points deviate from the regression 

lines, the results suggest that differences in the properties of the 

mineral surfaces used in this study are of relatively small significance 

and loss of Al^ is primarily related to area of surface available.

Smith and Hem (1972, p. 32) attributed the decline in pH that they 

observed in their solutions during aging to the production of Alc by 

polymerization of Al^. A somewhat similar decline can be seen in the 

pH of solutions without the added surface listed in tables 1-4 of this 

paper. However, the solutions to which mineral surfaces wer^ added 

do not show pH changes that correlate with Alb loss. From data in 

tables 1-4, it is evident that most solutions where Alb loss occurred 

immediately after the surfaces were added did not decrease in pH to 

the extent one might expect had the Alb polymerization gone to completion at the surface. It would seem, therefore, that the los-^ of Alb 

represents an adsorption rather than catalysis of the polymerization 

process.

It is possible the surfaces also adsorbed some H+ ard thus 

influenced the pH. This effect cannot be completely evaluated, but it is 

believed to be minor. The solutions with surfaces in which Alb loss 

continued during aging decreased in pH, suggesting that the effect of 

the polymerization reaction can be observed, when it occurs, even if 

the surfaces are present.

SIGNIFICANCE OF CATION EXCHANGE CAPACITY 

The CEC (cation exchange capacity) of added surfaces is sbown in
F24 CHEMISTRY OF ALUMINUM IN NATURAL WATER

TABLE 6. Short-term losses of Al^, in solutions D, F, G, and H

[Dash leaders indicate Aljj loss so rapid that Al(j at time zero could not be confidently extrapolated]

Solution '

DNASA _____

D NASB _ _.

D0.01WB_____

D0.1WB _____

D 0.2WB _____

D 0.02VA _ _.

D 0.1VA _____

D 0.5VA _____

D0.1K _______

D0.5K _______

D 2.0K ___ __

D l.OMS _____

FNASA _____

F NASB ____

F0.01WB_____

F0.1WB _____

F0.2WB _____

F0.02VA _____

F0.1VA _____

F0.5VA _____

F0.1K _______

F0.5K _______

F2.0K _______

F l.OMS _____

G NASA _____

GNASB _____

G0.01WB_____

G0.1WB _____

G0.2WB _____

G0.02VA_____

G0.1VA ____.

G0.5VA _ _.

G0.1K _______

G0.5K _______

G2.0K _______

G l.OMS

HNASA _____

HNASB _____

H0.01WB_____

H0.1WB _____

H 0.2WB _ __

H0.02VA____.

H0.1VA _____

H0.5VA _____

H0.1K _______

H0.5K _______

H2.0K _______

H l.OMS _____

Added surface area

(m2 /!)

_______ 0.0

________ .0

________ 22

________ 220

._ __ 440

________ 8.8

________ 44

________ 220

______ 6.0

________ 30

________ 120

________ 2.0

________ .0

________ .0

________ 22

________ 220

.__ ___ 440

________ 8.8

________ 44

________ 220

________ 6.0

________ 30

________ 120

________ 2.0

.0

________ .0

. ____ 22

. _ _ _ 220

________ 440

________ 8.8

.____ _ 44

220

________ 6.0

________ 30

________ 120

________ 2.0

________ .0

________ .0

_______ 22

__ ___ 220

.___ _ _ 440

8.8

________ 44

________ 220

..______ 6.0

________ 30

.____.__ 120

_______ 2.0

Extrapolated 

-log Alb

3.70\ mean,

3.671 3.68

3.94

4.90

3.72

4.07

3.76

3.89

4.49

3.66

3.50\ mean,

3.501 3.50

3.75

3.65

3.78

3.65

3.83

4.60

3.57

3.53) mean,

3.55J 3.54

3.67

6.08

3.58

3.72

3.72

3.58

3.70

4.3

3.56

3.58J 3.58

3.69

4.65

3.65

3.78

4.63

3.63

3.85

5.0

3.60

Corcentration differences 

(moles/litre xlO4 )

0.00

.94

1.96

.18

1.24

.35

.80

1.77

.00

.00

1.38

.92

1.50

.92

1.68

2.91

.47

.00

.75

2.88

.25

.98

.98

.25

.89

2.38

.13

.00

.59

2.41

.39

.97

2.40

.29

1.22

2.53

.12

table 5 for each solution. The exchange positions on the surfaces 

present would, of course, be occupied at all times by cations taken up 

from the associated solution. Moreover, the nature of th? ions so held 

and the proportions of species in the exchange positions would be 

functions of the solution composition as well as of properties of the 

exchange sites.
ALUMINUM SPECIES AT MINERAL SURFACES F25

X

o 

cc

2.0

1.5

0.5 -

50 100 150 200 

SURFACE AREA, IN METRES SQUARED PER LITRE

250

FIGURE 9. Initial loss of Alb due to apparent short-term adsorption from solutions D as a 

function of effective area of surface added.

Cation exchange reactions are generally rapid and can bo treated 

quantitatively by equilibrium chemical models. A completely quantitative treatment, however, becomes difficult in a system containing 

several different cationic species. In the absence of aluminum, the 

cation exchange positions would be occupied by sodium and hydrogen 

ions in the proportion dictated by pH and the specific behavior of the 

exchange sites. In a s^nse, the PZC (point of zero charge) measurement 

characterizes the behavior of a surface toward H+ ; surfaces with high 

PZC, such as bentonite and volcanic ash, should retain H+ more 

strongly in the neutral or mildly acid pH range than would be expected 

for the kaolinite surface which has a much lower PZC.

The CEC values for the aging-study solutions range from 

0.211 x 10" 4 to 7.38x 10~4 equivalents per litre. The CEC is considered 

negligible for the solutions to which no solid was added. By 

comparing CEC values with the concentrations of aluminum available in the test solutions, some indication of the potential importance of 

ion exchange processes can be gained. In the solutions containing 0.50 

g of volcanic ash, the CEC is sufficient to take up almost all the 

aluminum as A1+3 , and substantial fractions of total aluminum could 

be adsorbed in several other solutions. Also, the capacity for H+ 

adsorption is significant.

SHORT-TERM ADSORPTION EXPERIMENTS

To study more closely the rate and magnitude of the apparent initial
F26 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

3.0

50 100 

SURFACE AREA, IN METRES SQUARED PER LITRE

150

FIGURE 10. Initial loss of Al^ due to apparent short-term adsorption from solutions F as 

a function of effective area of surface added.

adsorption of Ala and Al^ 3,000 ml of solution DS (rn = 1.40) were made 

and divided into 12 250-ml aliquots. A solution having this low an rn
ALUMINUM SPECIES AT MINERAL SURFACES F27

50 100 150 200 

SURFACE AREA, IN METRES SQUARED PER LITRE

250

FIGURE 11. Initial loss of Alb due to apparent short-term adsorption from solutions G as 

a function of effective area of surface added.

value was used here because the rate of Alb polymerization is slow 

under these conditions. Therefore, any sudden disappearar^.e of Alb 

would most likely be attributable to adsorption by the surface. The 

amounts of solids added were 0.010, 0.030, 0.10, and 0.20 gram Wyoming bentonite, 0.020, 0.050, 0.10, and 0.50 g volcanic ash, and 0.10, 

0.50, and 2.0 g kaolinite. One solution was left untreated to act as a 

blank. These 12 solutions were then placed on a wrist-action shaker 

and shaken vigorously for 48 hours. Aliquots were withdrawn and 

analyzed for Ala and Alb at 1, 4, 32, and 48 hours age time and also 

were analyzed for total aqueous aluminum at 48 hours. These aliquots 

were centrifuged for 5 minutes at 2,000 G's prior to analysis in order to 

separate the suspended clays. (Comparison of Ala, Alb, anc^ Alc 

analyses for a centrifuged and a noncentrifuged aliquot of the blank 

showed that no important quantity of aqueous aluminum was lost by 

this centrifuging.) They were then analyzed for Ala and Alb using the 

analytical procedure previously described, and aqueous Alc vas calculated by difference from the total aqueous aluminum determined after
F28 CHEMISTRY OF ALUMINUM IN NATURAL WATER

50 100 150 200 

SURFACE AREA, IN METRES SQUARED PER LITRE

250

FIGURE 12. Initial loss of Al), due to apparent short-term adsorption from solutions H as 

a function of effective area of surface added.

48 hours. Also at 48 hours silica content and pH were measured in 

order to determine the extent of dissolution of solids and possible 

interference at that time and also to be able to determine the distribution of the four Ala monomeric species. The pH values ranged from 

4.32 for DS 2.0K to 4.67 for DS 0.2WB. The blank solution had a pH of 

4.45. Silica concentrations at 48 hours were low, the highest being 

0.18 mg/1 SiO2 for DS 2.0K.

From these data it was concluded the dissolution of the added minerals was negligible and aluminosilicate reactions did not need to be 

considered. For each solution the amounts of Ala, Alb, and Alc adsorbed were calculated by subtracting their concentrations from those 

for the blank (DS NAS), defined to have zero surface prea and no 

aluminum loss by adsorption. Although we know that loss of Alb 

within a 48-hour span is unlikely to be caused by increased polymerization of Alb to form more Alc as Alc is adsorbed, we may not be able to 

assume the same for Ala with respect to Alb- Since we have not 

thoroughly examined the kinetics of formation of Alb from Ala, we
ALUMINUM SPECIES AT MINERAL SURFACES F29

have no direct experimental evidence to show whether loss of Ala is 

due to adsorption or to polymerization to form more Alb to compensate 

for the Alb adsorbed. If the latter were the case, actual adsorption of 

Ala would have been somewhat less and Alb adsorption somewhat 

more than expressed here. The methods used here to calculate adsorbed Ala and Alb may be slightly suspect in that they are based on an 

adsorption assumption where there is no such interaction between Ala 

and Alb species. The experimental and calculated aluminum values 

are given in tables 7 and 8.

ADSORPTION OF Al,

Ala, unlike species Alb and Alc , can be completely defined in terms 

of species concentrations and effective ionic charge if the pH is 

known. The concentrations of A1(H2O)6+3 , A1OH(H2 O)5+2 , and 

A1(OH)2(H2O)4+ are calculated from Ala concentration and pH using 

the equilibrium relationships and activity coefficients quoted earlier 

in this paper; A1(OH)4(H2O)2~ is negligible below pH 5.0. The calculated values are given in table 9. At 48 hours the pH values observed in 

the solutions ranged from 4.35 to 4.67, and most of the Ala in all the 

solutions was in the form Al(H2O)g+3 at this low pH.

From the data on Ala and pH at 48 hours, it is possible to gain some 

further insight into the nature of the process by which this form of 

aluminum is taken up by the mineral surfaces. In a general way it is 

obvious that solutions with the larger amounts of mineral surface area 

show the larger losses of Ala. The relationship between surface area 

and loss of Ala is rather well defined, as shown by the plots in figure 13, 

in which the points lie close to a straight line at 48 hours and have 

almost the same relationship at 1 hour.

The ion exchange sorption mechanism can be explored by a mass 

law equilibrium model. For the systems to be examined, the cationic 

species present are Na+ , H + , and the three Ala monomers, and as 

noted, the trivalent aluminum ion is predominant. At the pH range of 

these solutions, the sodium activity is not sufficiently great to retain a 

significant amount of sodium in exchange sites on the solid surfaces. 

Consequently, the system can be considered as involving H+ and Al+3 

in solution and adsorbed on the surfaces. Whether the Al+3 ion itself is 

reversibly adsorbed at exchange sites or the reaction is more complex, 

involving the conversion of Al +3 to hydroxide complexes, can also be 

studied, but without directly measuring all the species.

The surfaces in some of the solutions have a CEC sufficient to adsorb 

much more H+ than the active free concentration represented by the 

pH, and as noted earlier, the pH in some of the solutions probably 

shows this influence. The kaolinite surfaces may have carried enough 

sorbed H+ when they were introduced to explain the lower pH's ob-
F30 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

TABLE 7. Adsorption experiment data

A. Molar Ala concentration xlO4 

Solution_______________________1 hour________4 hours________32 hours________48 hours

DSNAS ________________ 27l4272227Tl2.08

DS0.01WB -____________ 2.08 2.18 2.00 2.06

DS0.03WB ______________ 1.77 2.07 1.96 1.74

DS 0.1WB ______________ 1.34 1.30 1.28 1.30

DS0.2WB _-_________-_--- .39 .26 .31 .45

DS0.02VA__-__-__-_____ 2.06 2.16 2.03 2.06

DS0.05VA_-_______----- 2.01 2.15 1.97 2.05

DS0.1VA _____________ 1.83 1.85 1.87 1.91

DS0.5VA _____________ 1.02 1.10 1.12 1.12

DS0.1K _____________ 1.95 2.11 1.97 1.96

DS0.5K ________________ 1.91 2.02 1.91 1.92

DS 2.OK _____________ 1.70 1.81 1.86 1.86

B. Molar Alj concentration x704 

Solution_______________________1 hour________4 hours________32 hours________48 hours

DSNAS _____________ L87 L84 L90 1.94

DS0.01WB ___________ 1.51 1.45 1.54 1.54

DS0.03WB _____________ .96 .69 .75 .70

DS0.1WB __________________ .11 .14 .17 .14

DS0.2WB _______-----__ .06 .03 .02 .01

DS0.02VA____________ 1.63 1.61 1.65 1.63

DS0.05VA_-___---_____ 1.53 1.38 1.38 1.38

DS0.1VA __________________ 1.22 .99 .98 .92

DS0.5VA __________________ .18 .15 .13 .13

DS0.1K ________________ 1.79 1.60 1.77 1.77

DS0.5K ____________________ 1.51 1.40 1.42 1.27

DS2.0K _ ______ .42________.34________.36________.29

C. Other analyses at 48 hours

Total aqueous Al, SlC>2 

Solution____________________molar xlO4____________________pH__________(mg/1)

DSNAS_________________ 4.71 4.46 0.00

DS0.01WB __---________ 4.20 4.45 .03

DS0.03WB __________ 2.82 4.47 .01

DS0.1WB ____________. 1.51 4.46 .05

DS0.2WB _---__________ .35 4.67 .03

DS0.02VA _ _ _ 4.30 4.48 .02

DS0.05VA______________ 3.78 4.45 .02

DS0.1VA __________________ 3.17 4.44 .04

DS0.5VA ______________ 1.37 4.41 .07

DS O.IK ---------______ 4.34 4.48 .02

DS0.5K _________-_-____ 3.80 4.40 .06

DS2.0K ----__-_____--- 2.25________________4.35________.18

TABLE 8. Calculated sorbed species and aqueous A\. concentrations

[Concentrations expressed in moles/litre x 10"; sorbed species are expressed in terms of the concentration change 

effected by the addition of the surface. For solution DS NAS, sorbed concentration is denned as zero for all species]

Solution Sorbed Added Cation Aqueous Alc

 surface area exchange capacity

Ala Alj, AJc (m2/!) (equivalents/1 x 10")

__________________________1 hour__________________________ DS NAS -_-___ o.oo 67oo777767o67o7777

DS0.01WB____ .06 .36 ____ 22 .37 ____

DS0.03WB__- .37 .91 ____ 66 1.11 ____

DS0.1WB ____ .80 1.76 ____ 220 3.69 ____

DS0.2WB ____ 1.75 1.81 ____ 440 7.38 ____

DS 0.02VA ____ .08 .24 ____ 8.8 .21 ____

DS0.05VA-- .13 .34 ____ 22 .53 ____

DS 0.1VA ____ .31 .65 ____ 44 1.06 ____

DS0.5VA ____ 1.12 1.69 ____ 220 5.28 __
ALUMINUM SPECIES AT MINERAL SURFACES F31

Solution Sorbed

Ala Alb Alc

Added

surface area

(m2 /!)

Cation

exchange capacity 

(equivalents/lx 104 )

Aqueous Alc

1 hour Continued

DS0.1K _____

DS0.5K ____

DS 2.0K ____

__ .19

__ .23

.44

.08 _____

.36 ___.

1.45 _____

6.0

30

190

0.28 ____

1.38 ____

5.52 ____

4 hours

DS NAS ___ 0.00

DS0.01WB____

DS0.03WB___. 

DS 0.1WB ___. 

DS0.2WB ___. 

DS 0.02VA __.. 

DS 0.05VA ____

DS0.1VA ____

DS0.5VA ____

DS0.1K ______

DS0.5K ______

DS2.0K ______

.04

.15

.92

1.96

.06

.07

.37

1.12

.11

.20

.41

0.00

.39

1.15

1.70

1.81

.23

.46

.85

1.69

.24

.44

1.50

32 hours

DS NAS ____ 0.00 0.00

DS0.01WB____

DS 0.03 WB ____

DS0.1WB ____

DS0.2WB _____

DS 0.02VA __

DS 0.05VA __

DS0.1VA ____

DS 0.5VA .___

DS0.1K ______

DS0.5K ______

DS2.0K ______

.11

.15

.83

1.80

.08

.14

.24

.99

.14

.20

.25

.36 ____ ____ -___ ____

1.15

1.73 _ ____

1.88 ____ ____ ____ ____

.25 __ __ __ ____

.52 __ __ __ ____

.92

.77 ____ ____ -___ __-_

.13

.48 ____ ____ ____ ____

1.54

48 hours

DSNAS ______

DS 0.01WB____.

DS0.03WB____

DS0.1WB _____

DS0.2WB _____

DS 0.02VA _____

DS 0.05VA .___

DS0.1VA _____

DS0.5VA _____

DS0.1K _______

DS0.5K _______

DS2.0K _______

0.00

.02

.34

.78

1.63

.02

.03

.17

.96

.12

.16

.22

0.00

.40

1.24

1.80

1.93

.30

.56

1.02

1.81

.17

.67

1.65

0.00

.09 ____ -___

.31

.62 ____ ____

.69

.07 ____

.34 ____

.35 ____ ____

.57 ____ ____

.08 ____

.08

.59 ---_ ____

0.69

.60

.38

.07

.00

.61

.35

.34

.12

.61

.61

.10

TABLE 9. Calculated aqueous Alq species distribution at 48 hours

Molar concentration x 104

Solution PH (Ala ) (Al+3) (A1OH + 2) ,A1(OH) 2 +")

DS NAS _______

DS0.01WB ___

DS 0.03WB ___

DS0.1WB _____

DS 0.2WB _____

DS0.02VA_____

DS 0.05VA ____

DS0.1VA

DS0.5VA _____

DS0.1K _______

DS0.5K _____ __

DS2.0K _____

___ 4.46

___ 4.45

_ 4.47

 _ 4.46

___ 4.67

_ _ 4.48

_ 4.45

_ _ 4.44

___ 4.41

___ 4.48

__ 4.40

____ 4.35

2.08

2.06

1.74

1.30

.45

2.06

2.05

1.91

1.12

1.96

1.92

1.86

1.67

1.65

1.38

1.04

.31

1.65

1.64

1.55

.92

1.55

1.59

1.58

0.30

.30

.26

.19

.09

.30

.30

.27

.15

.30

.25

.22

0.11

.11

.10

.07

.05

.11

.11

.09

.05

.11

.08

.06
100 200 300 400 500 0 

SURFACE AREA, IN METRES SQUARE

FIGURE 13. Adsorption of Ala at 1 hour and 48 hours in solutions DS as a function of effect

correlation coefficient.
ALUMINUM SPECIES AT MINERAL SURFACES F33

served in solutions DS 2. OK and DS 0.5K, and some of the other 

surfaces may have had other ions in exchange sites that were replaced 

by H+ when the solutions were prepared. The higher pH of solution DS 

0.2WB might, for example, have been related to exchange of adsorbed 

Na+ for H+ . In any event, the equilibrium model, if valid, should be 

capable of treating H+-A1+3 relationships regardless of the cutside 

influences on pH.

In its simplest form the cation exchange equilibrium might be 

written

and

[H+] 3(A1,3 ) _

 /V£)g.

[A1 +3](HX )3

The x terms represent adsorbed material, expressed as concentrations 

in moles per litre. For solutions containing the amounts of the solids 

shown, the CEC in equivalents per litre is known and can be expressed 

as

CEC=3(A1X 3)+(HX ).

The amounts of adsorbed Ala shown in the 11 solutions in table 9 were 

considered to represent Alx ; therefore, the amount of Hx can be calculated for each solution. The value of [ Al+3] was computed by multiplying the determined concentration of A1+3 by the appropriate activity 

coefficient for these solutions. If ionic strength is near 0.01, 

^Al+3 =0.45. Results of the calculation forKps (table 10) show a wide 

range (10~3- 62 to 10+1 - 45 ) for what is supposed to be a constant. It may 

be concluded, therefore, that the form of the exchange equilibrium 

used was not applicable.

The alteration of Al +3 to a hydroxide complex at the surface at the 

time of adsorption could be represented by the equation

or

[H+] 3(A10H^2 )

 ^--=Krt9 on this system,

[Al+^ffi,)2 D2 

where

CEC=2(AlOHx2 )+(Hx )

and a recalculation of (Hx ) is needed. Using this new value for (Hx ) and 

the mass law equation for KD2 , a set of K values is obtained with a
F34 CHEMISTRY OF ALUMINUM IN NATURAL WATER

TABLE 10. Dissolved and sorbed species and selectivity constants for three ion exchange 

mechanisms (solutions aged 48 hours)

Solution [H+] [Al + 3]

DSNAS _______

DS 0.01WB _____

DS 0.03WB _____

DSO.l WB _____

DS 0.2WB _____

DS 0.02VA _____

DS 0.05VA _____

DS0.1VA_______

DS0.5VA_______

DS0.1K _______

DS0.5K _______

DS 2.0K _______

10-4.46 iO-4-i3

1Q-4.46 1Q-4.13

1Q-4.47 lQ-4.21

1Q -4.46 1Q-4.33

10 -4.67 1Q-4.86

lQ-4-48 1Q-4.13

1Q-4.45 1Q-4.14

1Q-4.44 10-4-16

1Q-4.41 1Q-4.39

1Q-4.48 1Q-4.16

1Q-4.40 1Q-4.15

1Q-4.35 1Q-4.16

1Q-5.70

lQ-4.47

10 -3.79

10-5.70

10 -5.52

10 -4.77

10 -4.02

10 -4.92

1Q-4.80

10-4.66

10-4.51

10 -4.48 

10-4-46 

1Q-5.04

10-4.37 

10 -3.87

10 -3.67 

10 -3.54 

1Q-3.60

10 -3.38 

1Q-3.24 

1Q-4.83

10-4.77 

10 -4.72 

1Q-4.36

1Q-4.33 

1Q-4.30 

10 -4.26

1Q-4.14 

10-4.05 

1Q -3.62

10 -3.47 

10-3.36 

<0

1Q-5.44 

10 -4.85 

10 -4.05

10 -3.97 

10 -3.91 

10 -3.31

10 -3.29 

10 -3.27

10-1.39

1Q-5.96 

1Q-10.46 

10 + 1.45

1Q-4.93 

1Q-9.64 

10 -1.55

10 -5.82 

10 -9.62 

10-2.14

10 -6.18 

1Q-9.70 10- - 52

1Q-5.47 

1Q- 10.29 

1Q-1.65

10-6.07 

10-10.43

10 -5.65 

10-9.88 

10 -2.00

1Q-5.95 

1Q-9.50

10 -3.32 

10-9.71 

10 -1.70

10-5.91 

10 -9.94 

10-3.62

10-6.17 

10-10.28

range of 10~6- 18 to 10~3-32 . This spread is not greatly decreased from 

the one obtained for KD%.

A further alteration of the equilibrium equation car be made, however, which perhaps is more in accord with what might normally be 

expected, in that a single Al +3 ion is unlikely to neutralize two or three 

unit surface charges which could be spread some distance apart over 

the surface. Attachment of the Al +3 ion at a single charge site can be 

represented as

Al +3 +2H2O+Hx ^ Al(OH)2x +3H+ ,

[H+] 3 Al(OH)2x)

and

=Kr

CEC=(Al(OH)2x )+(Hx ).
ALUMINUM SPECIES AT MINERAL SURFACES F3 5

Recalculation of (H.x ) using this model and solving for Kj)\ gave values 

ranging from lO" 10- 46 to lO" 9- 50. Although the "constant" still has a 

fairly wide range, the improvement obtained by postulating a 1:1 

exchange of H+ for A1+3 is obvious and suggests that the process takes 

place in some such fashion. Numerous references in the literature 

relate the apparent preference for hydroxide complexes over uncomplexed species where polyvalent metals are being adsorbed at inorganic surfaces.

In addition to the cation adsorption process, the activities of Al +3 

and H+ also are influenced in these solutions by the precipitation of 

gibbsite. When the exchange capacity is small compared with the total 

aluminum present in the system, the adsorbed Ala species will be 

withheld at the expense of Alc , the precipitated form, or Al^, the 

metastable potentially precipitable form. Thus at equilibrium both 

the adsorbed and solid forms could be present. Some of the solutions in 

this study contained an exchange capacity a little greater than the 

total molar concentration of aluminum, but the amount of Ala sorbed 

at the pH of the solutions was never large enough to preclude gibbsite 

precipitation. The adsorption of H + by the solids tends to obscure the 

stoichiometry of Al +3 and OH~ species involved in complexing or 

polymerization, and the pH changes in solutions with solid surfaces 

are not comparable with those where surfaces are absent.

Effects of mineral surfaces on Ala concentrations can be summarized as follows,

1. Ala species are adsorbed by the mineral surfaces, and the process 

appears to follow a cation exchange mass law equilibrium model.

2. The adsorption of Ala is essentially complete in 1 hour of reaction 

time.

3. Adsorbed Ala is produced within the system at the expense of 

precipitated forms of Al when the solution is initially supersaturated with respect to gibbsite. Gibbsite solubility will control Al a 

concentration ultimately unless the exchange capacity is large 

enough to remove almost all Ala. Where a large excess of exchange capacity is present, the Ala concentration may be maintained at a level governed by ion exchange equilibria.

4. Adsorbed Ala evidently can be desorbed and returned to solution 

more readily than precipitated Alc can be redissolved or than 

polymerized Al^ can be degraded to Ala.

5. In the process of being adsorbed, Ala species appear to become 

essentially monovalent, perhaps through formation of hydroxide 

complexes at one or more sites in the hydration shell of the A1+3 

form. In any event, the processes of adsorption and hydrolysis or 

the first stages of polymerization of Ala are closely intermir gled.
F36 CHEMISTRY OF ALUMINUM IN NATURAL WATER

ADSORPTION OF Al b

Data in table 7 show ihat in all the 11 solutions that contained 

mineral surfaces there was significantly less Al^ than in the blank. It 

is also evident that the loss of Alt, had already mostly taken place 

before the first set of determinations were made 1 hour after the 

solutions were prepared and that the changes in aqueous Alt, 

concentrations are greater on addition of the minerals for the solutions 

having the greater surface areas. These concentration changes could 

have resulted from adsorption of Al^ species by the surfaces, or the 

surfaces might have promoted or catalyzed the polymerization of Al^, to 

form crystalline gibbsite.

If the latter hypothesis were correct, one would expect a lower pH 

and a closer approach to the equilibrium solubility of gibbsite in 

solutions having the larger surface areas and CEC's. In figure 14 the 

log of the calculated solubility product for A1(OH)3 for each solution is 

plotted against the log of the CEC in that solution. Although these 

points are rather scattered, they suggest a trend toward a more stable 

A1(OH)3 solid is produced by increasing the CEC. For the blank, the 

value for Ks0 is 10~32 -75 , essentially that of microcrystalline gibbsite.

Although the number of points is rather small, figure 14 does indicate that the three materials have somewhat different effects. Both 

the kaolinite and volcanic ash surfaces show, as CEC increases, a more

33.2

33.1 -

Y 33.0

i 

o

 T 32.9

O 

O

32.8

32.7

32.6

*

A EXPLANATION

 Volcanic ash

A Wyoming buntonite

 Kaolinite

4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 

NEGATIVE LOGARITHM OF CATION EXCHANGE CAPACITY

3.0

FIGURE 14. Gibbsite ion activity product in solutions DS as a function of negative 

logarithm of equivalents of cation exchange capacity added per litre of solution.
ALUMINUM SPECIES AT MINERAL SURFACES F3 7

strongly decreasing trend in KSQ than does the bentonite surface. 

However, the significance of the apparent correlation is limited and 

uncertain, because the KSQ values for the longer term aging-study 

solutions do not display a discernible pattern. It can only be concluded 

that the surfaces may have influenced the early stages of the reactions 

that precipitated A1(OH)3 .

Plots of the changes in concentrations of Al^ as functions of surface 

area and of CEC (figs. 15, 16) indicate that the adsorption of Alt, is 

somewhat more closely related to surface area than to CEC. This 

seems reasonable, because Alt, units are large polymers with a considerable range in net positive charge per included aluminum atom. They 

cannot be expected to exhibit the same behavior as Ala species wherein 

each individual aluminum atom is bound to a charge site. We should, 

therefore, not expect the adsorbed Alb to be strictly dependent on the 

number of surface charge sites, or CEC. The relationship between Alb 

adsorption and surface area is linear if Alb remains in solution in 

substantial excess. Complete adsorption of Alb is approached asymptotically if an excess of surface area is present. This behavior is characteristic of adsorption equilibria.

Unlike the adsorption of Ala, some of the adsorption of Alb occurs 

between 1 and 48 hours. The slopes of the linear portions of the 

adsorption-surface area plots at 1 and 48 hours are.l.3xlO~6 and 

2.2xlO~6 moles Al^/m2 , respectively. For those solutions in which 

there is a large excess of Alb, about 40 percent of the adsorption occurs 

between 1 and 48 hours. A possible explanation for the longer time of 

completion of adsorption of Alt, is that the Al^ polymers, being larger 

and having less charge per aluminum atom than Ala, are less mobile. 

Figure 17, the plot of adsorption of Al^ against the time of contact 

between the solutk ns and the surfaces, shows how quickly the adsorption does occur. Seemingly, it would completely stabilize after several 

days, thereby allowing us to account for Alb polymerization as the sole 

process causing disappearance of Alb after those several days. At 48 

hours, the slope of Al^ adsorption, 2.2x 10~ 6 moles/m2 , is considerably 

larger than the slope of Ala adsorption at that time, 3.0xlO~7 

moles/m2 . This is in general agreement with many observations in the 

literature which report strong adsorption effects for polynuclear ions; 

hovv ever, the polynuclear species are generally large units, each containing many metal ions. Thus, each Al^ unit that is adsorbed has a 

much larger effect on analytically determined aluminum than does 

the adsorption of a single aluminum monomer.

The Alb units represent a range of compositions, with a net charge 

per aluminum atom that probably is commonly less than one, and so 

an ion exchange equilibrium model probably cannot be expected to be 

appropriate for this kind of material. Also, the techniques used by
100 200 

300 400 500 0 1

SURFACE AREA, IN METRES SQUARED 

FIGURE 15. Adsorption of Alb at 1 hour and 48 hours in solutions DS as a 
ALUMINUM SPECIES AT MINERAL SURFACES F39

or

UJQ.

CO 

UJ

<

5 OLJU

o

Ovx

LU 

O

o

Nl '3ONVHO
F40 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

2.0

0 10 20 30 40 50 

HOURS OF SOLUTION-SURFACE CONTACT, IN SOLUTIONS DS

FIGURE 17. Adsorption of Alb as a function of contact time for different amounts and 

types of mineral surfaces. Effective surface area in metres squared per litre is noted for 

each solution.
ALUMINUM SPECIES AT MINERAL SURFACES F41

Smith and Hem (1972) for estimating the size and charge of such 

species cannot be readily applied where the pH may not reflect correctly the increasing proportion of bridging hydroxide ions.

The apparent adsorption of Alb can, however, be studied by other 

procedures. Nearly all adsorption of Ala and most of that of / l^ took 

place during the first hour of contact between the surfaces and solutions. Furthermore, for many of the solutions, aqueous concentrations 

of Alb also decreased between 1 and 48 hours. This effect is more 

pronounced for those solutions having the higher surface areas and is 

due to further adsorption by the surfaces or to an increased rate of 

polymerization of Alb catalyzed by the surfaces, or to a combination of 

both. In order to study this effect more closely, first-order rate constants were computed where possible, for the disappearance of Alb 

from the solutions during the period between 1 and 48 hours after 

preparation. Although there was a general tendency for a faster rate 

in solutions containing the greater amounts of surface, the relationship was not very well defined. Rates of disappearance of Alb from 

comparable aging-study solutions during longer aging periods generally are slower than rates for the 1-48-hour aged solutions observed 

here, but this comparison could only be made in a small number of 

solutions. Where a substantial amount of Alb was adsorbed before the 

first measurement, at 1 hour, the continuing loss of Alb mav have been 

attributable to continued adsorption rather than to polymerization. 

We cannot definitely conclude from comparison of these rates with 

surface areas that the effect of surface area is to increase the p^ymerization rate; however, 'f such an effect exists, it evidently is not great 

enough to be of major importance.

EQUILIBRIUM MODELS EOR ADSORPTION OF Al b

The plots of adsorption of Alb as a function of surface ares clearly 

show that the adsorption per unit area of adsorbent approaches a 

limiting value as the surface area approaches zero. Kipling (1965) 

presented a form of the Langmuir adsorption isotherm that describes 

this type of behavior:

a be

a^ l+bc

where a is the adsorption of solute per mass (or area) of adsorbent at 

aqueous solute concentration c. The limiting value of a at saturation is 

a^, and b is a constant. This equation may be rearranged as

. 

c

By plotting the quantity ale as a function of a, a a> and b can be
F42 CHEMISTRY OF ALUMINUM IN NATURAL WATER

determined. The relationship should define a straight line of intercept 

a OQ& and slope b. The quantity a represents the observed decrease in 

Al species concentration (AC) times sample volume (V) in litres divided by area of surface (A). The quantity c is the observed aluminum 

species concentration.

Data required for calculation of a are given in table 8 for the 48 hour 

observations for Al^. The results are given in table 11. Figure 18 is a 

plot of the results.

TABLE 11. Data used for development of Langmuir isotherm for adsorption o

Solution

DSNAS ______

DS0.01WB __._.

DS 0.03WB ____

DS0.1WB ___.

DS 0.2WB ___.

DS 0.02 VA __..

DS 0.05VA ___.

DS0.1VA_____.

DS0.5VA__ __.

DS0.1K ______

DS0.5K ____..

DS2.0K ____..

A/V 

(m2/! 1

0.0

.__ 22

. 66

.__ 220

440

8.8

.__ 22

.__ 44

.__ 220

6.0

 30

.__ 120

CxlO4 

(moles/litre)

1.94

1.54

.70

.14

.01

1.63

1.38

.92

.13

1.77

1.27

.29

ACxlO1 

(moles/litre)

0.00

.40

1.24

1.80

1.93

.30

.56

1.02

1.81

.17

.67

1.65

ACxlO" 

AIV 

(moles/m2 )

0.0182

.0188

.0082

.0044

.0341

.0254

.0232

.0082

.0280

.0224

.0137

AC

CAIV 

(litres/m2 )

0.0118

.0268

.0584

1 .438

.0209

.0184

.0252

.0633

.0160

.0176

.0473

lrThe measurement of aqueous Al^ for solution DS 0.2WB gave a value at or below the detection limit (10 molar) 

and, therefore, is a very highly uncertain AC value. Consequently, this point is not considered in determining slope 

and intercept.

From inspection of figure 18, it is seen that the adsorption of Alb 

could be explained by the Langmuir isotherm. The results are used to 

determine a^ and b, which are found to be 3.7xlO~6 moles/m2 and 

1.8 x 104 litres/mole, respectively. This says that at saturation, with an 

excess of Alb remaining in solution, the surface would adsorb 

3.7x 10~6 moles Alt, Per square metre of surface area, or one atom in 

Alj, Per 45 square angstroms. Therefore, the maximum adsorption a oo 

corresponds to less than a monolayer for our Alb polymeric ions. The 

derived adsorption coefficient, 3.7xlO~6 moles Al^/m2 , compares 

reasonably well with the observed adsorption of 2.2xlO~6 moles 

Alb/m2 in the test solutions where concentrations were as much as 

2xlO-4 moles/litre.

From the foregoing discussion the following general conclusions are 

drawn:

1. Alb is rapidly removed from solution by adsorption when mineral 

surfaces are introduced into a system containing aqueous 

polynuclear aluminum hydroxide ions.

2. The adsorption process is related to surface area of the mineral 

available, and the relationship follows the pattern of the Langmuir isotherm.
ALUMINUM SPECIES AT MINERAL SURFACES F43

0.08

Linear regression correlation coefficient

= -0.816 

Slope=-b=-18,200 litres/mole 

lntercept= 3,^=0.067 litre/m2

=3.7x10~6 moles/m2 , or 0.022 Al

atom in Alb per A2 , or one Al atom in

Alb per 45 A2

0.01 0.02 0.03

a OR A£><1P_ | N MOLES PER METRE SQUARED

AIV

FIGURE 18. Fit of Alb adsorption at 48 hours to Langmuir isotherm parameters.

3. Polymerization of Alb occurs in the solution at about the same rate 

whether a surface is present or not.

4. The initial take up of Alb by the surfaces does not appear to involve 

extensive polymerization because the differences in pH between 

blank solutions and those having various amounts of surfaces are 

small compared with the difference in Alb content. Polymerization to the extent required to account for Alb conversion to Alc 

should produce enough H+ to materially affect the pH.

BEHAVIOR OF Alc

As noted previously, aqueous Alc cannot be directly determined from 

the available information. It can be calculated for solutions to which no 

mineral surface was added, and we know some Al c was
F44 CHEMISTRY OF ALUMINUM IN NATURAL WATER

present at 48 hours in DS NAS (table 8). It seems reasonable that this 

material or its precursor also occurred in the other solutions before the 

mineral surfaces were added to them, but the development of recognizable crystallinity in this material is probably more rapid at the 

mineral surfaces. Because the change in solution pH is not large 

enough, it seems unlikely that very much of the Al^ that disappeared 

was fully converted to Ale; however, the values for aqueous Alc in table 

8 were computed from the difference between determined Ala+Al^ 

and the determined total aqueous aluminum and thus should be 

reasonably valid. The quantity of adsorbed Alc given for the DS solutions in table 8 represents the difference between aqueous Alc in 

solution DS NAS and in solutions with mineral surfaces. In any event, 

the assumptions and calculations involved probably give the numbers 

for Alc sorbed a rather high degree of uncertainty; efforts to correlate 

adsorbed Alc with surface area indicated only a rather tenuous relationship, and no quantitative treatment is presented here.

It is possible, however, by means of the electron microscope to detect 

the presence of Alc at the mineral surfaces as gibbsite crystals, which 

can be discerned in the electron micrographs of solids recovered from 

these solutions after approximately 100 days (figs. 19-21). The hexFIGURE 19. Gibbsite crystallization centers on Wyoming bentonite after 100

days aging.
ALUMINUM SPECIES AT MINERAL SURFACES F45

O.ljum

FIGURE 20. Gibbsite crystals on surface of volcanic ash fragments after 100

days aging.

agonal plates adhere to the other mineral surfaces. Some of these 

crystals may have grown through polymerization of adsorbed Al^, but 

direct sorption of the small crystals that had developed in the solution 

also has probably occurred. In solutions without surfaces that have 

been aged for a comparable length of time, one would be able to find 

similar gibbsite crystals that had developed independently.

Figure 22 shows a few gibbsite platelets on the surfaces of kaolinite 

removed from solution DS after 48 hours of aging. No crystals of 

gibbsite could be found in solutions without surfaces at this pH range 

in any of our previous work until after a longer aging period. Thus, the 

surfaces may facilitate the development of crystalline gibbsite, but as 

indicated by other evidence previously cited, the principal effect of the 

surfaces is that of adsorption.

EFFECTS OF SURFACES ON APPROACH TO EQUILIBRIUM

In previous work the solutions were assumed to have reached a state 

of equilibrium when the Al^ had all disappeared, by conversion to Al c.
F46 CHEMISTRY OF ALUMINUM IN NATURAL WATER

0.1 /Jim

FIGURE 21. Gibbsite crystals on surface of kaolinite after 100 days aging.

After that time, the pH remained constant. When the mineral surfaces 

were added, a substantial loss of dissolved aluminum commonly ensued, although it appeared generally not to be accompanied by a 

change in pH that was as large as would be expected were the process a 

polymerization of Al^.

The approach to equilibrium in the absence of surfaces was traceable by the decreasing pH. Actual changes in Ala, although generally 

small, were observed in these solutions, probably because they had 

initially larger amounts of Al^ than previously studied solutions 

where Al a remained nearly constant. In any event, the surfaces did 

help speed the attainment of equilibrium by adsorbing nonequilibrium species, where this was all that was needed to attain equilibrium. 

Where more extensive changes in solution composition were needed to 

attain equilibrium, the surfaces may not always promote such 

changes and could even inhibit them. However, the latter effect is 

probably rather rare.
ALUMINUM SPECIES AT MINERAL SURFACES F47

0.1 /urn 

FIGURE 22. Gibbsite platelets on surface of kaolinite after 48 hours aging.

CONCLUSIONS REGARDING SORPTION 

AND POLYMERIZATION EFFECTS

When a solution that is supersaturated with respect to gibbsite 

comes in contact with mineral surfaces having CEC, the experiments 

performed here suggest that a rapid decrease in dissolved aluminum 

species concentration occurs. The changes in concentration are most 

extensive during the first hour of reaction time and are closely correlated with effective surface area or CEC per unit volume of solution.

The weight of experimental evidence suggests that the aluminum 

species are removed by adsorption and that the adsorbed material does 

not undergo rapid polymerization to crystalline gibbsite. There may 

be some hydrolysis of monomeric dissolved aluminum species at the 

surface to increase the amount of hydroxide bound to aluminum, and 

microcrystals of gibbsite may be sorbed by the surfaces.

Surfaces used in these studies had some relatively subtle effects on 

distributions of dissolved species and rates of equilibration. However, 

after the initial adsorptive phase of the reactions was completed, the
F48 CHEMISTRY OF ALUMINUM IN NATURAL WATER

continued polymerization of the remaining Al^ proceeded at the same 

rate and probably by the same mechanism as observed previously in 

solutions that had no added surface.

It should be noted that because surfaces were added after the initial 

mixing of A1+3 and OH~, the experiments do not permit evaluating 

the effect surfaces might have had on the initial formation of Alb. Also, 

the pH range of the solutions was relatively narrow and was too low to 

include systems in which A1(OH)4~ would have been a significant 

constituent. However, the results do indicate that the adsorption process can have significant effects on the behavior of aluminum in 

natural systems.

REFERENCES CITED

Bower, C. A., and Goertzen, V. O., 1959, Surface areas by an equilibrium ethylene gycol

method: Soil Sci., v. 87, p. 289-292. 

Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, Methods for collection and

analysis of water samples for dissolved minerals and gases: U.S. Geol. Survey

Techniques Water Resources Inv., book 5, chap. Al, 160 p. 

Chapman, H. D., 1965, Cation exchange capacity, in Black, C. A., ed., Methods of soil

analysis, part 2: Am. Soc. Agronomy ser. no. 9, p. 891-900. 

Hem, J. D., 1964, Deposition and solution of manganese oxides: U.S. Geol. Survey

Water-Supply Paper 1667-B, 42 p. 

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complexes in dilute solution: U.S. Geol. Survey Water-Supply Paper 1827-A, 55 p. 

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aluminum with aqueous silica at 25°C: U.S. Geol. Survey Water-Supply Paper

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related substances at 298.15°K(25.0°C) and 1 atmosphere (1.013 bars) pressure and

at higher temperatures: U.S. Geol. Survey Bull. 1259, 256 p. 

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dilute aqueous solution: U.S. Geol. Survey Water-Supply Paper 1827-D, 51 p.

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