-M) *4 3^

UNIVERSITY OF UMEÅ

F r o m t h e D e p a r t m e n t o f I n o r g a n i c C h e m i s t r y

U n i v e r s i t y o f U m e à ^ - 9 0 1 8 7 U m e å , S w e d e n

EQUILIBRIUM STUDIES OF TERNARY ALUMINIUM (III) HYDROXO COM PLEXES

WITH LIGANDS RELATED TO CONDITIONS IN NATURAL WATERS

By

LARS-OLOF ÖHMAN

U M E Å 1 9 8 3

To Elsmarie

Johan y Daniel & Jenny

EQUILIBRIUM S TUDIES O F T ERNARY ALUMINIUM(III) HYDROXO C OMPLEXES

WITH L IGANDS RE LATED T O C ONDITIONS IN NATURAL W ATERS

by

LARS-OLOF ÖHMAN

AKADEMISK A VHANDLING

som med tillstånd av re ktorsämbetet vid Umeå U niversitet för erhållande av filosofie doktorsexamen framlägges till offentlig granskning vid Kemiska institutionen, sal B, LuO, fredagen den 20 ma j kl 10.00

Umeå 198 3

Title: Equilibrium Studies of Ternary Aluminium(III) Hydroxo Co mplexes with Ligands Related to Conditions in Natural Waters

Author: Lars-Olof öhman

Address: Department o f Inorganic Chemistry, University of Umeå, S-901 87 Umeå, Sweden

Abstract: The thesis is a summary and discussi on of eight papers. During the last decades, p recipitation has become i ncreasingly acidic due to the extensive use of fossil fuels. In areas o f poorly buffered bedrocks, e.g. Scandinavia, northeastern United States, this phenomenon ha s re­sulted in elevated amounts of Al(III) being leached in to streams and lakes. Recent findings reveal that these e levated Al-concentrations could cause f ish death and decreasing forest production.

In the present thesis, the importance of taking naturally occurring substances into consideration when dis cussing Al(III) in natural wa­ters, is emphasized. On th e basis of a chemical characterization of relevant ligand classes in a n atural water, the complex formation be­tween A l^+, hydroxide ions and the inorganic ligand carbonic acid, the low-molecular weight organic ligand citric acid and the high-molecular weight model substances gallic acid, 1,2-dihydroxynaphtha-lene-4-sulfonate, 1,2-naphthoquinone-4-sulfonate, pyrocatechol and salicylic acid were i nvestigated. The investigations were performed as series of Potentiometrie titrations and data were processed by means of the least-squares computer progra m LE TAGROPVRID using a technique called pqr-analysis, permitting an unbiase d search f or com­plex model (and corresponding e quilibrium constants) to be m ade. In most systems studied, the complexation a t high ligand excesses can be described by a series of mononuclear complexes AIL -AIL^. Tentatively, the whole series consists of octahedrally coordinated (water and l i­gand oxyg ens) AI(III). At lower ligand excesses, the significance and in some case s even predo minance of ternary mono- and po lynuclear hydroxo complexes i s demonstrated. In two o f the systems, b inary aluminium hydroxo species are evaluated.

The p otential importance of the substances with respect to Al-com-plexation in natural waters are indicated in a num ber of model calcu­lations. The s olubility of the clay mineral kaolinite is calculated as a function of -lg[H+] and l igand concentration. It is shown that citric acid, gallic acid, 1,2-dihydroxynaphthalene-4-sulfonate, pyro­catechol and s alicylic acid contribute quite significant to the total solubilities, even a t very low c oncentrations.

As a complement and backgro und to the equilibrium studies, the cor­rosion rate for one o f the naturally occurring Al-bearing minerals, corundum, is reported. In this investigation, performed w ith a leach-ant solution of ground-water composition, an experimental technique was em ployed which ma de i t possible to divide the corrosion into che­mical and mech anical losses.

Key word s: aqueous solution, binary and ternary hydroxo complexes o f Al^+, car­bonic acid, citric acid, phenolic ligands, emf-titrations, equilib­rium analysis, model calculations towards natural waters, corrosion rates of (X-AI2O3.

ISBN 91-7174-124-0 74 pages + 8 appendices (191 pages)

i

EQUILIBRIUM STUDIES OF T ERNARY ALU MINIUM(III) HYDROXO C OMPLEXES

WITH L IGANDS R ELATED T O C ONDITIONS IN NATURAL W ATERS

LARS-OLOF ÜHMAN

Department o f Inorganic Chemistry, University of Umeå,

S-901 87 Umeå, Sweden

This thesis presents a review of the results presented i n Papers

I-VIII. In the text they will be re ferred to by their roman numerals.

I. Equilibrium and S tructural Studies of Silicon(IV) and Alu mi-

nium(III) in Aqueous S olution. 1. The Formation o f Ternary

Mononuclear and Polynuclear Complexes i n the System A l^+-

Gallic Acid-0H~. A Potentiometrie Study i n 0.6 M Na(Cl).

Uhman, L.-O. and Sjöberg, S. Acta Ch em. Scand. A 35 (1981) 201.

II. Equilibrium and Stru ctural Studies of Silicon(IV) and Alum i-

nium(III) in Aqueous Solution. 3. A Potentiometrie Study o f

Aluminium(III) Hydrolysis and A luminium(III) Hydroxo Carb onates

in 0.6 M Na(Cl).

Uhman, L.-O. and Fo rsling, W. Acta Ch em. Scand. A 35 (1981) 795.

III. Equilibrium and Stru ctural Studies of Silicon(IV) and Alumi-

nium(III) in Aqueous S olution. 4. A Potentiometrie Study o f

Polynuclear Aluminium(III) Hydroxo Complexes with Gallic Acid

in Hydrolyzed Al uminium(III) Solutions.

öhman, L.-O. and Sjöberg, S. Acta Ch em. Scand. A 36 (1982) 47.

i i

IV. Equilibrium and S tructural Studies of Sili con(IV) and A lumi-

nium(III) in Aqueous S olution. 7. Redox, Hydrolysis and

Complexation Equilibria in the System Al^ +-1,2-Naphthoquinone-

4-sulfonate/l,2-Dihydroxynaphthalene-4-sulfonate-0H". A Poten­

tiometrie Study i n 0.6 M Na(Cl).

öhman, L.-O., Sjöberg, S. and In gri, N. Acta Ch em. Scand, A 37.

Accepted for publication.

V. Equilibrium and Stru ctural Studies of Silicon(IV) and Alum1-

nium(III) in Aqueous S olution. 8. A Potentiometrie Study o f

Aluminium(III) Salicylates and A luminium(III) Hydroxo Sa licy­

lates in 0.6 M Na(Cl).

öhman, L.-O. and Sjöberg, S. Acta Ch em. Scand. Submitted f or

publication.

VI. Equilibrium and Stru ctural Studies of Silicon(IV) and Alumi­

nium^ II) in Aqueous S olution. 9. A Potentiometrie Study o f

Mono- and Polynuclear Aluminium(III)-Citrates.

ühman, L.-O. and Sjöberg, S. J. Chem. Soc., DaHon Tran s.

Submitted f or publication.

VII. Equilibrium and S tructural Studies of Silicon(IV) and Alum1-

nium(III) in Aqueous S olution. 10. A Potentiometrie Study o f

Aluminium(III) Pyrocatecholates and Aluminium(III) Hydroxo

Pyrocatecholates in 0.6 M Na(Cl).

öhman, L.-0. and Sjöberg, S. Polyhedron. Submitted f or publi­

cation.

i i i

VIII. Corrosion of Dense Polycrystalline a-AlgOg i n NaHCQg-Buffered

Water S olut ion o f pH 8 .5 a t 40°-100°C,

öhman, L.-O., Ingri, N. and T egman, R . Am, Ceram. So c. Bull»

61 (1982) 567-571, 581.

i v

TABLE O F C ONTENTS

INTRODUCTION 1

ALUMINIUM(III) IN NATURAL W ATERS, A B ACKGROUND 3 Regulating sol id phases 3 Experimentally found sp eciation 4 Substances o f relevance to compiexation 5

A13+-H,0 6 3+ Al -inorganic substances 7

Al^+-low molecular weight organic substances 8 Al^+-high molecular weight organic substances 9

EXPERIMENTAL M ETHODS 10 Equilibrium studies 10

The choice of ionic medium 10 The em f mea surements 12 Complementary me thods 13

Leaching study 14

MATHEMATICAL TR EATMENT O F D ATA 15 Equilibrium studies 15

Mass balances 15 Evaluation of equilibrium model 16 Concluding remarks 20

Leaching study 20

RESULTS A ND D ISCUSSION 21 The bi nary aluminium hy drolysis 21 The system H +-Al3+-C02(g) 24 The system H+-Al3+-Citric acid 25 The system H+-Al3+-Gallic acid 27 The system H +-A13+-1,2-Dihydroxynaphthalene-4-sulfonate/1,2-Naphthoquinone-4-sulfonate 28 The system H +-Al3+-Pyrocatechol 29 The system H +-Al3+-Salicyli c acid 30 The system H +-Al3+-Ftalic acid 30 Concluding remarks, Papers I-VII 31 The c orrosion rate of a-A^O^ 32

v

SOLUBILITY MODELLING O F AI-COMPLEXES IN A SI MPLIFIED NATURAL W ATER SYSTEM 33

FUTURE P LANS 36

ACKNOWLEDGEMENTS 37

REFERENCES 39

1

INTRODUCTION

In the middle of 1960: s, the research activities at the Department

of Inorganic Chemistry in Umeå commenced under the guidance of

Professor Nils Ingri, a former me mber of the late Lars-Gunnar Sillen

group at KTH in Stockholm. The major part of the research done a t

this department has been devote d to investigations concerning the

formation of 3- and 4-c omponent e quilibrium species in aqueous solu­

tion.

The basi c experimental technique has been em f titrations. During the

early 1970:s automatic systems for precise titrations were co nstruc­

ted and b uilt at this department (Ginstrup^). The e valuation of data

has been perform ed w ith the aid of the least squares computer pro-2 gram L ETAGROPVRID which has bee n developed fu rther (Lyhamn ). Fur­

thermore, a computer mode lling program S OLGASWATER, by m eans o f

which t heoretical titration curves, distribution and predo minance

area diagrams etc. could be ca lculated, has been develo ped a t the 3 department (Eriksson ).

As emf-investigâtions only give gross compositions and equ ilibrium

constants for the different species formed, several other experimen­

tal techniques such as U V-vis., IR, Raman and E SR spectro metry, ca-

lorimetry (Danielsson), stopped-flow kinetic experiments and r e­

cently multi nuclear NMR-spectrometry, have bee n a pplied.

In close relation to the different systems studied, extensive work

has also been devote d to the preparation and s tructure determination

(X-ray, neutron techniques) of single crystals (Hedman, Ivarsson,

2

Nenner, Strandberg) and, in order to correlate the single crystal

results with the findings made i n aqueous solution, large angle X-

ray scattering methods have bee n empl oyed (Lyxell).

During the first ten years of research, the investigations followed

two main lin es with one group studying the formation of heteropoly-4 anions with Mo(VI) as "central" ion (Pettersson ) and the other

studying mainly transition metal complexes w ith imidazole, OH" and 5 Cl (Granberg, Sjöberg ). One o f the aims of this research, as p oin­

ted out by Sjöberg in his dissertation, was to develope a "know-how"

at the department concerning suitable experimental and c alculation

techniques in the study of complicated equilibria.

During the second p art of 1970:s, three new pr ojects were started

with the aim of applying this "know-how" into fields of geochemical

and b iological interest.

The first of these p rojects attempts to apply an eq uilibrium ap­

proach to certain natural waters (e.g. groundwaters in contact with

mineral ores, stagnant deep-water in lakes and dee p sea basins) by

applying field measurements and chemi cal m odelling (Hedlund,

Liden6).

The second pro ject deals with primary production and met al toxicity

in algae and i s a cooperative project with the Department o f Plant

Physiology in Umeå (Hofslagare).

Finally, the third project, of which t his thesis forms a p art, is a

"traditional" laboratory project, dealing with the ability of Si(IV)

3

and Al(III) to form complexes w ith ligand classes occurring in natu­

ral waters. A spec ial interest is thereby paid to the occurrence o f

ternary mixed hydroxo species.

This third project was or iginally initiated by r eports claiming that

the presence of octahedrally coordinated Al(III) complexes i n nearly

neutral and mildly alkaline solutions was o f vital importance for

the formation of clay minerals (e.g. kaolinite, AT2(OH)i2O5) at 7 8 low temperatures 9 . However, during the last several years, the sub­

ject of Al(III) speciation in natural waters has received a mu ch

greater interest due to reports on environmental hazards such as

fish death^'^, failure in bird reproduction^ and decreasing forest 12 production from the elevated aluminium co ncentrations in acidified

natural waters.

The main aim o f the present thesis is to report and com pare th e com-

plexation features of Al(III) with representatives of some o f the

relevant ligand classes occurring in a n atural water. As a complement

and bac kground t o these investigations, the dissolution kinetics of

one of the naturally existing aluminium s ources, i.e. the mineral

corundum (a -A^Og), is reported.

ALUMINIUM(I11) IN NATURAL W ATERS, A B ACKGROUND

Regulating solid phases

Although aluminium i s the most co mmon metal ion in the outer crust

of the earth, very low concentrations of this element is normally

+ o < a o i X O < a

3+

AI(OH) J

pH

Fig. 1: Experimentally found values of Al (III) (from ref. 21).

4

registered in natural waters. Indeed, until recent years the analy­

tical methods have not been s ensitive enough to permit its reliable 13 determination . The po ssible reasons for these low n atural levels

are: i) the slow d issolution kinetics of the primary aluminium b ear-14 ing minerals (e.g. feldspars, amphiboles, corundum ( c.f. Paper

VIII)) and i i) the low solubility of secondary formed Al-phases such 15 as aluminiumhydroxi de (Al (OHJ^gibbsi te), different aluminosi 1ica-

tes^6 (e.g. Al2(0H)4Si205, kaolinite) and di fferent basic aluminium

sulfates^7 (e.g. Al(OH)SO^-, jurbanite). It is generally accepted

that, with the exception of fresh rain water passing over solid rocks,

the second explana tion is valid in nature^7. This has been explaine d

either as a result of the large particle areas o f normal mineral 18 soils or, as suggested by Johns on et al. , that the soil contains

some re active aluminium mineral. Of the secondary phases mention ed

above, the basic aluminium s ulfates are suggested to regulate the

aluminium co ncentration in solutions of high acidity (-lg[H+] < 4) 17 -4 and high sulfate concentrations (> 10 M). The que stion of

whether i t is aluminium hydroxide or aluminosilicates that regulate

the aluminium con centrations at lower acidity has been discussed in 18-20 a large number of papers ~ . Some expe rimentally found values of

Al (III) are illustrated in Fig. 1, from Ref. 21, where it can be seen

that neither kaolinite nor gibbsite regulate the aluminium concen­

trations at pH (= -lg[H+]) lower than » 4.

Experimental1y found s peciation

In most natural waters, the solid phases discussed above are in con­

tact with an aqu eous pha se which c ontains a large number of inorga­

nic and organ ic molecules and ions. It is important to realize that

Schematic Representation of Aluminum Fractionation

aluminum measurement

aluminum fraction

-total aluminum , acid digested

Jtotal monomeric aluminum . no acid digestion

cation desalted, monomenc aluminum non-labile monomeric aluminum

labile monomeric aluminum

acid soluble

aluminum

monomeric fraction a lumino-organic composition complexes

free aluminum; monomeric

aluminum sulfate, fluoride,and

hydroxide complexes

colloidal, polymeric, aluminum;

strong alumino-organic complexes

Fig. 2: A method to fractionate Al (III) in a natural water (from ref. 2 2 ) .

O)

Al-Org -o

Al-OH Al-F aJ

time (months)

Fig. 3: Experimentally found Al(III)-speaiation (from ref. 22).

5

if any of these substan ces forms a soluble complex w ith the alumi­

nium io n, the speciation in the aqueous phase woul d chan ge and

therefore the solubility of the solid. In fact, aluminium co ncentra­

tions 10-100 times higher than the values predicted by the solid

phase eq uilibria have bee n re ported to occur in near-neutral solu-o

tions of high organic matter contents .

22 Recently, Driscoll developed a meth od t o fractionate the total

aluminium co ncentration in natural waters into monomeric, anionic

and a cid soluble aluminium fractions (Fig. 2). When t his method wa s

applied to some stream and lak e waters, it was found (Fig. 3) that

the Al^+-ion was a minor constituent and that almost 50 % of the to­

tal aluminium co ncentration was pre sent as n egatively charged (orga­

nic) complexes. Thus, the importance of taking natural complex for­

ming substa nces i nto consideration when dis cussing aluminium in na­

tural waters is emphasized.

Substances o f relevance to compiexation

In an ac id aqueous solution, the ion Al^+ is octahedrally surrounded 3+ 23 with six water mole cules, forming a hyd rated Al^O)^ ion . When

-lgh (h = [H+]) of the solution increases to values above « 3-4, 23 this ion begins to hydrolyze into different hydrolysis products .

As the -lgh in most na tural waters lies above th is limit, the reac­

tions between A l^+ and water itself are of great importance i n the

speciation of natural waters.

With knowledge of these bas ic reactions of the aluminium io n in an

aqueous solution, it follows that the only ligands that could com-

6

plexate Al^+ in natural waters, are those which cou ld compete wi th

the coordinated water molecules and t he hydroxide ions. When th e 24-26 complexation s trength of different ligand classes is compared

with that of the hydrated water mole cules, it is found that only the

fluoride ion and the oxygen coo rdinating ligands are able to do so.

The present work has, due to the fact that several reports on the 3+ 27 complexation betw een Al and the fluoride ion already exist , been

focused on the complexation a bility of the oxygen coo rdinating li­

gands. In nature, a great variety of presumptive oxygen coo rdinating

ligands occurs. These comprise inorganic as w ell as an almost inde­

finite number of organic molecules and i ons.

This enormous number of potentially oxygen coordinating chemical sub­

stances occurring in a humified natural water inevitably requires a

selection procedure. The s trategy has thereby been t o divide the

oxygen coo rdinating ligand class into relevant subgroups, characte­

rize each subgroup chemically according to present knowledge and,

finally, pick out representati ve compounds from each group f or in­

vestigation. It has been found convenient to divide the class into

four subgroups as follows:

i) A13+-Hg0. This is the basic system under lying all other complex

forming systems in an aqu eous s olution. The system ha s been the sub-23 28 ject of a large number of investigations but, quoting Aveston ,

"although the aluminium io n perhaps i s the best known exam ple of

metal-ion hydrolysis, there is little agreement i n the literature

on th e formulae of the species that are formed". This divergence

seems to have mainly two causes : a) the extremly slow a ttainment of

7

equilibrium in near-neutral solutions and b) the low solubility of

A1(0H)3(S) in neutral solutions leading to precipitations at the

moment of base a dditions. In order to increase the reaction rates it

has been co mmon t o perform the measurements at elevated temperatu­

res. The mos t comprehensive work i n this direction has been pe rfor-29 med by Mesmer and Ba es who have also critically discussed the sub­

ject of Al^+-hydrolysis^.

It is, however, important to realize that the difficulties with the

slow and comp lex hy drolysis reactions can be ove rcome by performing

the different Al^-I^O-ligand measurements a t such high excesses of

ligand that the hydrolysis reactions are negligible. By performin g

the measurements at successively lower and lower ligand excess i t is

then possible to reach b riefly the equilibrium conditions of the

Al^-H^O reactions. This method thus offers an a lternative way to

study the hydrolysis reactions. The draw back w ith the method is, of

course, that the two-component e quilibria have t o be determined i n

the presence o f three-component complexes. The met hod has, however,

turned out to be very fruitful in Papers I I and IV of this work.

ii) A1^+-inorganic substances. Inorganic oxygen coo rdinating sub­

stances which frequently occur in natural waters are sulfate ions,

hydrogen phos phate and dihydroge n phosphate ions, silicic acid and

hydrogencarbonate ions^. Of these, the sulfate^ (A1(S0^) + ,

A1(S04)2") and the phosphate32 (Al(H2P04)2+, A1(HP04)+) ions have

been sho wn to form r elatively weak soluble complexes with Al^+. It

has also been sh own that solid phases^7,32 (e.g. A1(0H)S0^, Al(PO^))

exist in these systems. Concerning the interaction with silicic

acid, the occurrence of low-solubility phases has already been m en­

8

tioned^ whereas, in this system, no in dications of soluble species

seem to have bee n r eported. Finally, the lack of data concerning

interactions with the hydrogencarbonate ion, i.e. the major inorga­

nic anion in many n atural waters, made t his ligand an obvious choice

for investigation. The r esult, which sho wed on th e existence of rela­

tively weak soluble complexes, is presented i n Paper I I.

i i i) Al^+-low molecular weight organic substances. The biodégrada­

tion of dead organic material releases a great number of water sol-33 uble low molecular weight organic substances . After pre-concen-

tration with different extraction solvents, the identity of these 33 substances could be determined using for example gas chromatography .

34 In all, more than 500 different substances have bee n i dentified .

Common f or all these are, however, from an Al-complexation view,

that they could be ch aracterized as al i fati c polyhydroxycarboxyli c

acids. When the compiexation strength of this kind of substances /M 9 ft

(in relevant concentrations) are compared 5 with the hydrolysis

reactions of Al^+, it is found that the complexes between A l^+ and

the monodentate ligands (e.g. acetic acid) are too weak to contri­

bute appreciably to the total aqueous concentration of aluminium in

near-neutral solutions. The same holds for the a-hydroxycarboxylic

acids (e.g. lactic acid) and the whole group of polyhydroxy sub­

stances (sugars). On th e contrary, the complexation strength of the

polycarboxylic acids (e.g. oxalic acid, citric acid) seems to be o f

such magnitu de that elevated aluminium co ncentrations could be ex­

pected. As mod el substance under this heading on e o f the most fre­

quently occurring compounds, c itric acid, has bee n chosen. The r e­

sult of this investigation is presented i n Paper VI.

Degradation of lignin (coniferous lignin)

Nitrogenous compounds from microorganisms

(and plants)

Phenols microbial synthesized

(e.g. Epicoccum nigrum)

Aliphatic carbon source

CH3 } COOH Rl H R. Z°H

y Ri=COOH

Composition: C6H7.12°2 (H20) 0.40 Proteins

Peptide« (OCH 3) 0.92 acids

Ammonia

J 65.08%

5.88% 29.04% 12.87%

,o Lignin degradation products

0H, OC H 3

COOH (phenol

(heterocycl

COOH COOH

COOH

Other phenols of p lants and

microorganisms

4a: The formation of himic substances

(from ref. 36).

V f C è -

C OH ,L

^ T oh 0 9 OH C OH °H ?H 9 Y« f VCH^fV j'° Q0H f

o » HO—C<

HO—C

HO—C

«s.

^0H OH

C=0--

Ç=0 OH OH

OH OH o

0"bóc° S^OH 0

0#c\ r°

?h < °*c^c'"oh

V°">

JQUÛL

Fig. 4b:

The formation of humic substances from

lignines (from ref. 39).

Fig. 4c:

Model of a fulvi c acid (from ref. 41).

COOH

C OOH COOH

Fig. 4d: Model of a humus molecule (from ref. 40).

9

iv) Al^-high molecular weight organic substances. In most stream

and lake waters in the tempered pa rts of the world, a faint yellow

colour of the waters can be observed. This colour of the waters is

normally caused by h igh molecular weight organic substances and 3 5 36 these are usually referred to as humi c and fu lvic acids ' . It is

generally agreed t hat these substances ma inly originate from l ignin

degradation and that they thus should c ontain large amounts o f aro-13 matic subunits. This has also been confirmed by us ing the C-NMR

37 technique on pre-co ncentrated solutions .

The c haracterisation of these substances has bee n the subject of an 36 38 enormous number of investigations. It has bee n shown t hat ' :

a) the main eleme nts are carbon, oxygen an d hydrogen, b) the mole­

cular weights continuously range from a few hundr ed up to over two

hundred thousand, c) main par ts of the substances have an aromatic

structure and d) the substances have a large number of hydrolyzable

protons, with the dissociation constants continuously spread over

the -lgh range 3-11. By using different degradation techniques, it

has also been pos sible to identify a large number of chemically

well-defined subunits in these substances.

On th e basis of this characterization, several models for the che-36 39-41 mica! shape o f these compounds hav e been constr ucted 9 . Some

of these mode ls are shown i n Fig. 4 and i t is interesting to note,

from an Al-complexation point of view, that all these mod els con­

tain several probable oxygen-coordinating subgroups.

As no method e xists to obtain a chemically pure and un iform natu­

ral humic substance, the strategy of the present work has been t o

Summary of ligands

q CHj-COOH

I HO—C—COOH HO^ ^OH

CH^COOH Carbonic acid Citric acid

(paper II) (paper VI)

COOH

è HO/y ^OH OH

Gallic acid

(papers I and III)

ls 2-Dihydroxynaphthalene-

4-sulfonate (paper IV)

132-Naphthoquinone-

4-sulfonate (paper IV)

OH

OH

Pyrocatechol

(paper VII)

COOH

OH

Salicylic acid

(paper V)

.COOH

-COOH

F tali c acid

10

investigate the complex formation between the aluminium ion and se­

veral of these oxyg en co ordinating subgroups. Those subgroups which

have bee n investigated are a) two ortho-coordinated phenolic groups

(gallic acid: Paper I and I II; pyrocatechol: Paper VII and 1,2-

dihydroxynaphthalene-4-sulfonate: Paper I V), b) two keto-groups in

ortho-position, i.e. the oxidized state of c)-diphenols (1,2-

naphthoquinone-4-sulfonate: Paper IV ) and c) one phenolic and one

carboxylic group in ortho-position (salicylic acid: Paper V). It

may be men tioned that, at present, investigations concerning d) two

carboxylic groups in ortho-posi tion (ftalic acid) and e) one keto

and one ph enolic group i n o-position (kojic acid) are in progress

at this department.

EXPERIMENTAL M ETHODS

Equilibrium studies

The me thod employed throughout the whole series of complexation

studies has been the emf titration technique. This is a technique

which, combined w ith the use o f solutions containing a constant

ionic medium, has proved to be one of the most suitable ones i n the

study of complicated equilibria. By the development of automated

measuring and data collecting systems at the department^, it has

become possible to collect the large amount o f high precision data

needed f or these types of investigations in a reasonably short time

period.

The choi ce of ionic medium. I t is well known that in the mathemat-

i i

ical treatment of equilibrium reactions, the activity of the parti-23 cipating species should be ap plied . Concerning so luble species,

these a ctivities are expressed as the product of an a ctivity factor

and the concentration of the species. It has, however, been found

that in a d ilute solution, the activity factors are concentration 23 dependent . In an ea rly contribution from the Sillén group i n

42 Stockholm in the beginning of 1950 : s , the method o f constant

ionic medium w as demonstrated to give a solution to this problem.

(For an h istorical outline of the "ionic medium" method the reader 42 is referred to the article by Biedermann and Sillén ). In this pa­

per it was shown t hat if, in addition to the system under i nvesti­

gation, a high and c onstant salt medium w as added to the solution,

the activity factors of the participating species became p ractically

constant. This has been found to hold as long as the equivalent

concentrations of the investigated compounds do n ot exceed ™ 10 % 78 of the salt medium conc entration . Thus, w ithin this area, the ac­

tivities of the participating species could be replace d by their

concentrations.

The ionic medium o riginally chosen by the Sillén school was 3 M

NaClO^ and, depending on whether mainly cationic or anionic species

were formed, the C10^" resp. Na+ concentration was kept constant. A

description of advantages and disadvantag es with this choice of me-43 dium ha s been given by F orsling . In the present study, aiming at

interpreting the compiexation under naturally occurring conditions,

it has been lo gical to perform the measurements in a simplified sea

water medium and, accordingly, a me dium of 0.6 M NaCl has been cho­

sen. (This medium gives the solutions a salinity of 35 °/oo). Since

the main pa rt of the species formed i n the different systems has

12

been neg atively charged, i t has also been lo gical to keep the sodium

ion concentration constant.

The em f measurements. As will be sho wn under the heading "MATHEMAT­

ICAL T REATMENT O F D ATA", it is possible to interpret the complex

formation in a system i f, besides the analytical concentrations of

the participating components, the free concentration of one o f the

components could be me asured. In the present case, i.e. different

H+-Al3+-ligand systems, t he only measurable free component is the

free H+ concentration, denoted by h. This quantity has bee n me asured

employing a cell

-Ag, AgCl/0.6 M NaCl //eq^J^^JJm /ME+,

where M E den otes a glass or hydrogen elec trode.

The em f of this electrode (expressed i n millivolts) may be w ritten

E = EQ + 59 .157 * lgh + E. (1)

where

E. = - 77 • h + 4 2 • 1 .875 • IO" 1 4 • h"1 (2)

is the liquid junction potential at the junction 0.6 M NaCl//equilib-

rium solution. Eq is a constant, determined w ithin each titration in

solutions of known h .

In Paper IV, where the (hypothetical) activity of electrons {e~} was

measured, the same c ell with ME denoting a p latinum foil, was em ­

ployed. In this case, the Eq p t had to be determined i ndirectly, by

determining the absolute potential of the -Ag,AgCl/0.6 M NaCl refe-

13

rence h alf cell, i.e. the potential versus the normal hydrogen ele c­

trode (NHE).

In Paper I I, i.e. the system H+-Al^+-C02(g), carbon dioxide of a

known p artial pressure was bubble d through the solution. Thus, in

this system, the free concentration of carbonic acid, iftCC^aq)] +

[H^COg]), was kn own together with h, while the total concentration

of carbonic acid, i.e. l([C02(aq)] + [h^COg] + [HC0g~] + [C0^~])

varied as a function of -Igh.

In the titrations, the quotient between th e total concentrations of

Al^+, B, and l igand, C, has, except for the system H +-Al^+-C02(g)»

been kept constant. The a nalytical concentration of H+, H, has been

varied by m eans of additions of H+ or OH"". When po ssible, the "0H~-

additions" have bee n performed co ulometrically (i.e. reduction of H+

or H2O), thus leading to titrations at constant B and C. In Paper I V,

where ele ctrons were add ed s electively into the solution, an i nte­

grating potentiostat, working a t + 240 mV versus NH E, was employed.

Since several of the ligands investigated have been oxyg en s ensitive,

special precautions have bee n taken to protect these solutions from

oxidation during titration.

Complementary method s. Even i f emf titrations is one o f the most ad­

vantageous methods in the study of complicated e quilibria, it is ad­

visable that a complementary met hod should be use d to confirm the

results. One co mmon met hod, which has been use d w ith great success at 4 44 45 the department (cf. Pettersson -Hedman , Strandberg ), is to com­

bine the investigations with LAXS-measurements ("X-ray on concentra ­

ted solutions") and single-crystal studies of crystals precipitated

14

from s olutions of known comp ositions. In the present project the

preparation of single crystals has, however, been pr actically im­

possible due to several factors, for instance, the oxygen s ensiti­

vity of the ligands.

Thus, when a m ultinuclear NMR-instrument (Bruker WM 250) in 1980

became a vailable to the institution, large efforts were m ade to 27 apply an Al-NMR technique to the systems under st udy. However,

due to the quadrupolar p roperties^ of ^A1 this method also turned

out to have a rather limited application in practice. For example,

in the H+-Al^+-gal1ic acid system, the only recognizable species

were the most acidic one A l^O)^* (6 = 0 ppm; = 2.7 Hz) and

the most a lkaline one A IL^" (6 = 33 ppm; = 1100 Hz). In the

whole -lgh range i n between, no signal intensity what so ever could

be regi stred. This is a consequence of the quadrupolar moment (I = 27 5/2) of Al, which re sults in enormous half-line widths of all the

unsymmetrical species formed. This example i s general to all the

systems studied and i t might perhaps be stated that in an aqu eous 27 solution, the best application of Al-NMR t o solution chemistry,

3+ is to use the instrument as an Al(H^O)^ probe, i.e. to quantita­

tively measure the concentration of free aluminium io ns in solu­

tions of known - lgh and total composition. In the present project,

no attempts in this direction have bee n ma de due t o the limited

existence area of Al(H20)g3+ (-lgh < 3-4) (cf. Figs. 13, 15-18).

Leaching study

In the investigation of the corrosion rate of a-A^O^ a technique

which m ade i t possible to divide the corrosion into particle and

BASEPLATE ROD

tio er /•••••\ (•••••) ii \t3 mm » "i •* ]••••/

3D C 28 mm 50 mm 3

From the top From the front

Fig. S

Liquid l e v e 1

Hg. 6

Fig. 7

15

chemical losses, was employ ed. The test body wa s prepared by hot

i sostati c pressing of a h igh purity a-A^O^ powder and had th e

appearance given in Figs. 5 and 6. As the dissolution rate of this

compound i s known to be very low, the objective was to prepare a

test body w ith a well-defined and ma ximal surface area without

thereby causing stagnant leaching solution between th e rods, due to

capillary effects. The t est body wa s exposed to a continuous flow

of a hydrogencarbonate/carbonate-buffered(-lgh = 8 .5) leaching so­

lution. This was achieved wi th a leaching system setup shown i n

Fig. 7. The co rrosion of the test body wa s followed by a) regular

measurements of the weight of the body and b) Al-analysis of the

reacted leaching solution (i.e. from the collection vessel).

MATHEMATICAL T REATMENT O F D ATA

Equilibrium studies

Mass balances. The e quilibrium reactions in a H +-A1^+-1igand (L)

system can be describ ed by the three general reactions

pH+ + qA13+ « HpAlP+3q ; ßpjq (3a)

pH+ + rL « Hpl_P ; (3b)

pHV+qAl3+ + rL«HpAlqLP+3c> ; (3c)

where (3a) denotes aluminium h ydrolysis, (3b) denotes the acid-base

properties of the 1igand and (3c) denotes the formation of alumi-

nium-ligand complexes. The law of mass action and the conditions

for the concentrations applied to these r eactions give

16

H = h + Iipßpjqhpbq + ixpßpjrhpcr + Iiipßpjq5rhpbqcr (4)

B = b + IIqßp)qhPbq + HIqßpjq>rhPbqcr (5)

C = c + Urs hpcr + inre . „hpbqcr (6) P>' P 9 M 9 r

where h, b and c are the free concentrations of H+, Al^+ and L .

Generally in the titrations, H, B and C have bee n known from a naly­

sis, while h has been m easured by me ans of the emf cell described

above. In each experimental point, b anc c have bee n c alculated

using eqns (5) and (6), with the assumption of certain values of

^p q5 ^p r and ^p q r" These calculated values have then been used,

together with the measured value of h, to obtain a c alculated value

of H, H . . Through a stepwise variation of one o r several of the ca i c 9

3-constants, the model has be en adjust ed until U = I(^ca]c"H) 9

where the summation i s taken over all experimental points, approved

its minimum value.

In the system H+-Al(g), where h and c were known, eqn (5) was

used to calculate b, whereafter Hca^c was cal culated from eqn (4).

Thus, in this system, eqn (6) was not used a t all.

All these calculations have been perfo rmed w ith the least squares 47 4ft 4Q computer program L ETAGROPVRID (version ETITR ' ).

Evaluation of equilibrium model. The e valuation of the equilibrium

model has proceeded through a num ber of steps. First of all, the

model describing the acid-base properties of the ligand (eqn 3b)

has been evaluated in separate experiments. Secondly, titrations

0.10 0.08 0.06 0.04 0.02

-0.02

(Zr) -(Zc) exp ^ calc

B/mM C/mM • 101 9.77 A 101 5.03 ' O 1.01 2.47 • 402 9.03 V too 1.99 • V <

m •

O

*7 O l

- T . ? —?<Z

* a 7 a a * 4Vtf-lgh ®7

0.02

•Q02

"^c'exp^Ccalc v v ^7

l 3 ^ i r V £ ̂ V ° 6 * * 7 j o 9 " l g h

Fig. 8: Residual plots in the system E -Al -1,2-dihydroxynaphtalene-4-sulfonate. The upper figure with species obtained at high C/B-quotients3 the lower with final model.

17

with relatively high excess o f ligand over the aluminium concen­

tration have bee n performed. In these titrations, the influence of

Al-hydrolysis (eqn 3a) has normally been ne gligible and a ll effects

above th e ligand level could thus be t reated as cause d by 3-compo-

nent complexes. The mathematica l treatise of this data has normally

been s tarted with a calculation of the average nu mber o f ligands 50 bound per aluminium i on, n. As first introduced by N iels Bjerrum ,

the plot n(lg[L]) is often referred to as the "Bjerrum-plot" or the 43 "formation curve". It could be sho wn (cf. Forsling ) that if these

curves coincide within defined ranges o f B, C and C/B, the complex

formation could be described by a series of complexes Al Ln within

these ranges. Thus, w ith this function, it has been pos sible to

quickly obtain important information concerning the types of com­

plexes formed. The data fulfilling the demand on coincidence has

been use d to calculate the formation constants for these complexes

by me ans of LETAGROP ca lculations.

The third step in the complex formation studies has been t o collect

data at lower C/B. This data has then been com pared w ith the model

evaluated at high C/B by m eans of residual plots, i.e. the devia­

tions between experimental data and complex mo del (cf. Fig. 8) have

been ca lculated. In many cases, additional systematic "effects"

have then been re gistered, indicating either shortcomings in the

hydrolysis model (eqn 3a) or the occurrence o f additional three-

component complexes. The search f or explanation to these d eviations

has been perform ed by a procedure called pqr-analysis. In this pro­

cedure i t is assumed th at within a certain part of the data space,

only one ne w com plex H^Al^Lr occurs, and d ifferent combinations o f

integers p, q and r were systematically tested. That combination

12 16 955

20 _ 431 24 _ 220

-P 11 _ 612

-P

15 -205

-P

19 -99

-P

23 -440 74

10 -1774 328

14 -559 245

18 21! 232

22 -92 235

9 -4314 172 1838

13 -1543 28 1133

17 -682 28 857

21 -341 52 716

8 -1495 396

12 -472 355

16 -196 341

20 -107 335

7 -509 I t - 264

15 -208

19 -198

6 -1336

i

q «2

i

10 -927

1

q >3 14 -728

i

q=4

1

18 -619

1

q

1

= 5

1 1 2 3 r 2 3 4 r 3 4 5 r 4 5 6 i

+ 3+ Fig. 9: Result of a pqr-analysis in the system H -Al -Citric acid.

The figures give error squares sums for "best possible fit"

of one additional complex H^Al^(H^Cit)

2 giving the lowest error squares sum U = ^(Hcaic-H) or U =

l(Z^ _al -ZJ2» where l n = (h-H)/C, is considered to be th e "best" C ^ Oct ICC C one. I f systematic deviations still remained w ith this "best" com­

plex, pairs of pqr-triplets with a m ean composition approximating

the pqr-composition of the "best" single complex, were tested (cf.

Paper I I). The r esult of these pqr-analysis have, in several cases,

indicated the need f or complementary me asurements at certain B, C

and C/B-ranges.

In the system H+-Al^+-citric acid (Paper VI), the pqr-analysis,

performed on data with C/B > 2, clearly indicated that a dominating

complex with a C /B-quotient equal to 1 was forme d (cf. Fig. 9). It

was, however, impossible to judge whether the species formed contai­

ned 3 or 4 aluminium i ons. As a consequence, complementary titra­

tions were performed a t C/B = 1 . On th e basis of this new data, it

was then possible to rule out the tetra-nuclear species.

In the system H+-Al3+-salicy!ic acid (Paper V), it was found that

the difference between two mod els mainly occurred as a concentra­

tion dependence d ifference at -Igh « 7-8. Therefore, complementary

dilution titrations (titrations with pure ionic solutions) were

performed in this -lgh range.

3 In the second case, the computer program S OLGASWATER was most

valuable for modelling. With this program, the equilibrium concen­

trations of all species, assumed in a c ertain model of a chemical

system, could be ca lculated when total or free concentrations of

the components and s tability constants are given as input data.

This program, equipped wi th plotting procedures, has also been used

B/mM

32.50 o 0.50

0.5

-lg h

Fig, 10: A figure Z^(-lgh) in the system it-AV**-Gallic acid. The

fulldrawn curves have been calculated using the final

complex model.

19

for other purposes as de scribed below:

i) Theoretical Z(-lgh) and n( lg[L]) curves as w ell as diagram s

showing the distribution of different complexes w ithin certain con­

centration ranges were comp uted.

ii) When the approximate solubility product of the precipitate

formed i n the system H+-Al^+-gal1 i c acid (Paper III) was determin­

ed, this program w as use d to calculate the concentration of the

soluble species of corresponding composition in the last non-

precipitate point of each titration. With this program, it was als o

possible to estimate the formation constant within the precipita­

tion area. In this case, the formation constant of the solid phase

was man ually varied until the calculated and me asured -Igh agreed.

i i i) Conditions occurring in natural waters were s imulated. In

these calculations relevant amounts of ligands have been e quilib­

rated with the clay mineral kaolinite and the total amount o f sol­

uble aluminium as well as the concentration of each Al-containing

species has been fol lowed as a function of -Igh.

The final stage in the evaluation of the complex mo del has been t o

expose the whole data set to the model. Eventual shortcomings in

the model would then be see n as remaining systematic deviations in

some o f the data range. This test has been perform ed in the form o f

residual plots (cf. the lower part of Fig. 8) and the degree o f fit

has normally been il lustrated in figures giving experimental and

theoretical Z(-lgh) and/or n(lg[L]) curves (cf. Fig. 10). The ave-51 52 rage d eviation has also been obtained ' as a ( Z ) and/or a(H) in

20

the LETAGROP ca lculations.

Concluding remarks. In conclusion about the evaluation of a complex

model in a three-component system i t may be st ated that:

i) access to a h igh speed computer and a suitable least-squares

program (i.e. LETAGROPVRID) is very helpful and i n most cases even

necessary, especially in the evaluation of the different ternary

complexes,

ii) it is most valuable to use a mo delling program ( i.e. SOLGASWATER)

along with the LETAGROP ca lculations, as this provides information

on s uitable conditions for complementary meas urements, and

iii) the use of residual plots and the pqr-analysis technique have

proved t o be v ery powerful tools in the evaluation of complex mo ­

dels in complicated systems.

Leaching study

In the corrosion study, the two mea surable q uantities were a ), the

weight loss of the test body and b) the aluminium c oncentration in

the leachant solution. The co rrosion was fol lowed as the accumula­

ted Al-loss as a function of time. In the calculation, the two

measurable q uantities were r ecalculated to an a-Al2 0g-thickness,

employing the density (6) and geometric surface area (A) of the

test body, i.e. c = Am/ô-A, where c is the corrosion and A m the to­

tal weight loss at a given time. The c orrosion rates were o btained

as the slopes of these curves. From the temperature dep endence o f

21

these rates, the activation energy, according to the Arrhenius 53 -E /kT equation k = A-e a' , was calculated.

RESULTS A ND D ISCUSSION

The b inary aluminium h ydrolysis

The r esults in this field have been obtained as "byproducts" of in­

vestigations of the three-component systems H +-Al^+-C02(g) (Paper II)

and H +-A1^+-1,2-dihydroxynaphthalene-4-sulfonate (Paper IV), but

the basic importance of these reactions justifies a separate ac­

count.

In the H+-Al3+-C02(g) system, it was found that stable emf-readings

could be obtained within reasonable time for -lgh <4-4.2, even a t

relatively low carbon diox ide partial pressures. A data analysis,

performed on data with p^ = 0.10 atm, showed th at, at this p^Q ,

the binary aluminium hy drolysis was dominating over ternary complex

formation. In the search f or a complex mo del explaining these e f­

fects the assumption of one single hydrolytic species was, however, 2+ insufficient, but with the introduction of two species (Al(OH)

5+ and A l^OH)^ ) a good expla nation of data was obtained. The i ntro­

duction of ternary complexes, evaluated from data with higher p^ , 3+ did not change t his picture. The Al -hydrolysis model suggested by

23 Baes and Mesmer comprises, in this -lgh range, the complexes

A1(0H)2+; A12(0H)24+; A13(0H)45+ and Al1304(0H)247+. In the final

refinement of data, an attempt was ma de t o include the two " missing"

species A12(0H)24+ and Al-j30^(OH) into e m°del. The r esult of

Table 1. Evaluated e quilibrium constants in the system H +-A1

The e quilibria are defined according to the reaction

pH+ + qAl3+ <=> HpAl^+3q; The e rrors given are 3a(lg 3p ^

p q Proposed formula lg 3p q

-1 1 A1(0H)2+ -5.52 + 0.04

-4 3 A13(0H)45+ -13.57 + 0.02

-32 13 A11304(0H)247+ -109.2 + 0.12

-4 1 Al (OH)/ -23.46 + 0.11

22

this was, that while the complex Al-|30^(0H)24^+ was accepted w ith a

moderate standard deviation in its stability constant, the complex

Al2(OH)9ave no significant contribution to the model. As final

model for the hydrolysis at -lgh < 4, the complexes A1(0H)^ +,

5+ 7+ A13(0H)4 and Al-|^0^(OH)24 with equilibrium constants given in

Table 1 were suggested. It is important to note that although the

constant given for the species AI-jßO^OH^^"1" has to be regarded as

approximate due t o the impossibility of obtaining stable emf-

readings when l arger amounts of this species were formed (extremely

slow formation kinetics), the value suggested by Baes and Mesmer

(lg3 = -104.5) could definitely not be adapt ed to the data. It may

be men tioned th at the incorrectness of this proposed constan t value

also has been in dicated in several of the other three-component

systems studied.

54 In the autumn 1982, Biedermann et al. published a comprehensive

investigation concerning the hydrolysis of In^+ in acid solution.

In this work, based on mea surements o f h as w ell as the free con­

centration of In^+, a h ydrolysis model including the species InOH^+,

In(0H)2+, ^(OH^* and I n^OH)^"1" was est ablished. As the measure­

ment of both free concentrations obviously improves the reliability

in the evaluation of a binary complex model, it was most interest­

ing to compare these two cl osely related systems. It was noted that 3+ 4+ while, in the In -system two low-polynuclear complexes

6+ and M^(0 H)g exist, the author has suggested only one species with 5+ the composition M^( OH)^ . Obviously there is a r isk that the lat­

ter is a m ean composition of two c o-existing species. To t est this 5+ assumption, the species Alo(OH)^ was replaced by the two species

Al2(OH)and A1^(0H)66+ in a LETAGROP ca lculation. This led to an

23

even be tter fit to the experimental data (U^ - 0.151;

U« n /nu\ i a i /nu\ = 0 .033). The formation constants with corre-"'2* '2 4* '6

sponding standard d eviations obtained were 1 g(3_2 2 0 - ^ =

+ 0.028 and l g(3 _g 4 q + 3a) = - 20.01 + 0.029 respectively. Thus,

this calculation points out that the assumption m ade abov e seems to

be r ight. However, when the theoretical Z(-lgh) curves, calculated

for the two mo dels by me ans of S0LGASWATER, were comp ared, it was »3

found t hat, even at the highest Al-concentration, 20*10 M, the

differences hardly exceeded the experimental uncertainties. In or­

der to obtain experimentally ascertained differences the total alu-_3 minium c oncentration must exceed « 50-10 M but then, in a 0.6 M me­

dium, the rule of a 10 % replacement o f the background m edium i s se­

verely violated. As a final conclusion, it may be st ated that the

possibility of two co -existing low-polynuclear species remains and

that measurements with the same technique in a h igher salt medium,

3.0 M Na(Cl), probably would giv e a conclusive answer. It must be

pointed out that as the differences between the two model s are very

small for B < 20-10"3 M, the eventual replacement o f A13(0H)45+

with A12(0H)24+ and Al ^(0H)g6+ does n ot seriously influence the

evaluation of ternary complex mode ls (Paper I I and I II).

At -Igh > 7 the dominating h ydrolysis species is A1(0H)4". This has

been sho wn i n numerous investigations concerning the solubility of 15 23 Al(OH)3(s) in alkaline solution 9 . The s tability constant for

the species has ma inly been evaluated from these solubility curves.

In the system H +-A1^+-1,2-dihydroxynaphthalene-4-sulfonate (Paper

IV), it was found that stable emf-readings could be obtained at

such low C/B-quotients that the ternary complexes p artially were

transformed i nto this complex. Thus, in this data range, the stabi-

*s1

10.7 12.4

1 2 r

Is2

23.3 2.0 2.9

<1=3 2 r

2.2

2.5 5.4 20.4 0.3 2.8 1.5 1.6

2.3 5.7 -5 2Û4 1.1 4.5 -6 0.8 2.7

-4 - 4 4 7.9 -7 - 1.7

+ 3+ Fig. pqr-analysis in the system H -Al -CO (g).

24

1 i ty constant for the complex c ould be evalua ted. The value obtai­

ned, lg(3_4 -j q) = -2 3.46 + 0.11 , is somewhat higher than what is 23 suggested by Ba es and Mesmer , but in view o f the experimental un­

certainties with the solubility determination method, the revised

value obtained by the author is fully reliable. It could also be

mentioned t hat a verification of this value was obtained in the

system H +-Al^+-salicyli c acid (Paper V) but that, in this system,

the resulting standard deviation was relatively large.

Finally, in the -Igh range 4-7, no st able emf-readings in solutions

containing hydrolysis products have bee n o btained. This is, in the

more a cidic part of the range, due to the extremely slow formation

kinetics of the complex Al -j3O4(OH)anc*» i n more neutral solu­

tion, due to the low solubility of Al(0H)3(s). In this range, the

model suggested by Ba es and Mesmer^, i.e. Al (OH ) ; lgß_2 1 q =

-10.3 and Al (OH)^(aq) ; lg3_3 -j q = -16.1, has bee n adopted.

The system H +-Al^+-C02(g)

This system (Paper I I) was investigated through titrations where

gas mi xtures of different C02(g)-Ar(g) compositions were bubbled

through solutions in which H was var ied by m eans o f additions of a

carbonate s olution. Binary hydroxy species of Al^+ (given and d is­

cussed abo ve) and hydr ogencarbonate ions as w ell as ternary alumi­

nium hydro xocarbonate comp lexes were formed i n this system. The va­

lue found f or the formation of HCOg", according to the reaction

C02(g) + H^O HCO3- + was in agreement w ith earlier in-55 56 vestigations 5 . The composition of the main te rnary complex wa s

evaluated by m eans of a pqr-analysis (cf. Fig. 11) and the composi­

tion was found t o be H_ ^A12(CO2)- In addition, a trinuclear species,

H_5A13(CO2)» was needed t o explain data at the highest aluminium

Table 2. Equilibrium constants in the system H+-Al^+-C02(g).

The e quilibria are defined according to the reaction

pH+ + qAl3+ + rC02(g) « HpAlq(C02)P+3q; ßp jC)>r- The e rrors given

are 3o(lg 3p#qjr).

p q r Proposed formu la lg ßp r

-1 0 1 HC03" -12.539 + 0.001

-4 2 1 A12(0H)2C032+ -20.41 + 0.02

-5 3 1 A13(0H)4HC034+ -22.74 + 0.06

-3.75 '8[hco3 -3.5 -3.25 -4.25 0.3

0.2

AW0Hl/ t

3.6 3.8 4.2 3.4 -Igh

-5.0 'g^ccQ -4.75 0.3

_Fj

0.2

4.0 3.8 3.2 -Igh

Fig. 12: Distribution of complexes in the system H -Al -CO^(g).

25

concentrations. The formation constants are given in Table 2. (Note

that these a re given with 1 Pa as the standard state for p^Q , i.e. -20 41 ^ -1 ^

3_4 2 i = 10 " M *Pa ). Based on structures found i n solid 57 phases and probable resemblance with hydrolysis products the two

2+ species were t entatively assigned the formula A^OH^COg and 4+ A13(OH)^HCO^ respectively. The r esults of this investigation are

new and could be regarded as of basic interest from both a solution

and st ructural standpoint. Thus, in applications where aluminium and

carbonates are used i n combination, e.g. antacids, sewage-treatment

and in dustrial processes like the Pedersen process, this study

should be of value. From str uctural point of view, it is known t hat 58 59 at least two mine rals, dawsonite [NaAl ( C O 3 )(OH)^] and dundasite

[PbA^CO^^OH^-H^O], consist of chains of AlOg-octahedra bound

together with double hydroxo brid ges and carbonate io ns. It is

tempting to assume t hat these chains have been develo ped i n an 2+ aqueous solution containing A^OH^COg -ions.

The system H +-A1 ̂ "-Citric acid (H^Cit)

In this system (Pape r VI), mainly investigated through coulometric

titrations, the well established tri-basic behaviour of citric acid

in acidic solutions^ was confirmed. It was, however, found that if

C exceeded approxim ately 0.01 M, concentration dependent titration

curves were ob tained. This effect, which w as mo st pronou nced near

the citrate(3-) composition, could be explain ed as activity factor

variations, high diffusion potentials for the anions^0, self-associa-

tion or complex formation with thé sodium ions in the background me ­

dium. To avoid the problem, the data range wa s mainly restricted to

C < 0.008 M. The general features in the three-component t itrations

Table 3. Equilibrium constants in the system H+-Al3+-Citric acid

(H 2 C i t). The e quilibria are defined according to the reaction

pH+ + qAl3+ + rHgCit « HpAlq(H^Cit); 3p ^ p. The e rrors given are

3a(lg 3 ). v y Pp,q

p q r Proposed formula lg ß

-1 0 1 H2Cit" -2.769 + 0.003

-2 0 1 HCit2" -6.850 + 0.003

-3 0 1 Cit3" -12.067 + 0.004

-2 1 1 AI HC it+ -2.68 + 0.02

-3 1 1 AlCit -4.925 + 0.008

-6 1 2 Al(Cit)23~ -12.53 + 0.12

-13 3 3 Al3(0H)4(Cit)34" -21.77 + 0.02

B= 0.008 M C = 0.008 M AL(OH), L.

1.0

0.8

0.6

AKOHf

-igh

ALIOHIL. B = 0.00025 M C=0 008 M 1.0

AIL 0.8 AIL

0.6

AIHL' 0.2

Fig. 13: Distribution of complexes in the system H+-Al^+-Citric acid.

26

were, that while equilibria were q uickly established for -lgh <

2.75-3, the system be haved very sluggishly (waiting times up to 6

hours) at higher -lgh. The data analysis was divided into two

parts, according to fast and slow attainment of equilibrium. The

"fast equilibrium" part was found t o be expla ined by the two com ­

plexes (H)_£A1 (HgCit)+ and (H J^Al (H^Cit), tentatively assigned as

Al(HCit)+ and Al(Cit) respectively. The a nalysis of data in the

"slow equilibrium" range, showed th at a major polynuclear complex

with C/B = 1 formed and, on t he basis of measurements at this quo-4-tient, the composition (H^-jgA^H^CitJg was esta blished. In ad­

dition, a complex (H)_^A1(H^Cit)^ was found t o be formed i n minor

amounts at higher C/B-quotients. The formulas proposed f or these 4- 3-species were A l^OH^Cit)^ and Al ( Ci t) 2 • A com pilation of

complexes and corresponding equilibrium constants are given in

Table 3.

This system ha d been the subject of two e arlier investigations with

quite contradictionary results. Thus, w hile Pattnaik and Pa ni^ had

interpreted the complexation as a series of mononuclear species

AlCit-Al(0H)2Cit^~, Wiese and V eith^ reported the formation of se­

veral polynuclear species (Al3(0H)Cit3~, Al3(0H)^Cit2~ and

Al10(°H)i5cit63-).

Accordingly, the present investigation can be regarded as a valu­

able contribution to the understanding o f this probably both bio-

inorganically and geochemically important system. It is interesting

to note both the insignificance of any monodentate complex 2+ Alb^Cit and the extreme s tability of the trinuclear species

4-Al^OHJ^Citg . All attempts to crystallize this trinuclear spe-

32130 B=0.02M >-430 C=0.001 M

' \ \ X5 31

-35 lg h

B=Q02M C=Q02M

B=a005M C= 0.005 M

-15 lg h -3

B=Q02M C=Q01 M

Fig. 15: Distribution of complexes in the system it-Al^+-Gallio aoid.

27

cies, whose s tructure naturally would be most interesting to know,

from an aqueous solution, have f ailed. Thus, in a solution prepared

from AlCl^-öh^O; Na^Cit and N aOH with Al : Ci t: OH = 1 :1:1.33, sodium

chloride turned out to be th e most insoluble compound and even w hen

the solution was evaporated to syrup-consistency only NaCl was pre -27 ci pi tating. The AI-NMR spectra of a one molar solution of this

composition was g iving one broad peak (v-j^ = 1600 Hz) at 6 = 12

ppm ind icating the aluminium ions to be oc tahedrally coordinated. A

tentative structure, based on maxi mal coordination and conditions fi? found i n the solid phase magnesiumcitratedecahydrate , is presen­

ted in Fig. 14.

The system H +-Al3+-Gal1ic acid

This ligand (3,4,5-trihydroxybenzoic acid, H^L) was the first humic

substance m odel in this series (Papers I and I II). Due to its oxy­

gen s ensitivity in neutral and a lkaline solutions, the measurements

were performed under an inert (Ar(g)) or reducing ^(g)) atmosphere.

The a cidity constants were found t o be con centration independent

for C < 0.035 M. The c ollection of three-component data was some­

what complicated by the fact that a solid ternary phase formed i n

the -Igh range « 4-5.5 at C/B < 4 . This, together with the oxygen

sensitivity in neutral solutions, made a special titration proce­

dure necessary. In the first part (Paper I ), where data with C/B >

3 and -Igh < 9 was reported, the data analysis showed th at, besides

four mononuclear species (H)_2A1(H^L); H__3A1(H^L); H_gAl(H3L)2 and

H_gAl (H^L)3, a dinuclear proton series H^gA^H^L^-H^ -|Al^(H^L)3

also formed. Comparisons with conditions in other Al—organic acid 64 58 59 systems and c rystal structure considerations 5 made i t cred-

Table 4. Equilibrium constants in the system H+-Al^+-Gal1ic acid

(H^L). The e quilibria are defined according to the reaction

pH+ + qAl3+ + rH^L HpA1q(H3L)p+3q» ßp q r- The errors given are

3ö(lg ßp,q,r)-

p q r Proposed fo rmula lg 3p q r

-1 0 1 H2L" -4.152 + 0.002

-2 0 1 HL2" -12.590 + 0.005

-3 0 1 L3" -23.674 + 0.007

-2 1 1 A1HL+ -4.933 + 0.009

-3 1 1 AIL or Al(OH)HL -9.43 + 0.02

-6 1 2 A1L23~ -21.98 + 0.03

-9 1 3 A1L36~ -37.69 + 0.02

-8 2 3 A12(0H)2(HL)32" -22.65 + 0.04

-9 2 3 A12(0H)2(HL)2L3" -27.81 + 0.07

-10 2 3 A12(0H)2(HL)(L)24" -32.87 + 0.03

-11 2 3 A12(0H)2L35" -39.56 + 0.05

-9 4 3 a14L33+ -20.25 + 0.05

-5 3 1 A13(0H)4(H2L)4+ -12.52 + 0.01

-3 1 1 AlL-4H20(s) -6.2 + 0.5

28

i bl e to assume t hat the ligand was coordinated via two ^-coor dinated

phenolic groups. Thus, the formulas A1HL+, AIL or A1(0H)HL, All^3-,

AIL36 ' and A 12(0H)2(HL)32"-A12(0H)2L35" were proposed. In Paper

III, the complex formation at C/B < 3 and - lgh < 4 was reported to­

gether with a determination of composition and appro ximate s olubi­

lity product of the ternary phase. Two a dditional complexes,

H-5AI3(H3L)and 4(H3*~)» tentatively considered as

A13(OH)^(H2L)and Al^L^3*, were found. The s olid phase wa s found

to have the composition H_^A1(H^L) '41^0, probably having the same

structure as the soluble species of the same com position. A compi­

lation of equilibrium constants for the system is given in Table 4. 27 Note, also, the results of the AI-NMR measurements reported under

"Complementary methods".

The system H +-A13+-1,2-Dihydroxynaphthalene-4-sulfonate (H2L~)/1,2-

Naphthoquinone-4-sulfonate (Q~)

The main reason for investigating this system (Paper I V) was t o de­

termine whether complexes were formed betw een Al^ + and the oxidized

state of c>-diphenols, i.e. jD-quinones. A li terature survey^'^'^

showed th at it was necessary, in order to obtain a stable quinonic

state, to perform this investigation on a stabilized system, i.e. a

naphthalenic molecule. In the measurements, performed under an i n­

ert atmosphere of Ar(g), electrons were s electively added to the

naphthoquinone so lutions by me ans o f an in tegrating potentiostat.

After the complex formation between Al^+ and the dihydroxynaphtha-

lene ligand (showing on the species H ^Al^L), H^Al^L)",

H^Al (H^L)^3", H_5A1(H2L)2^" and H_gAl(H^L)^ 6") had bee n e valuated,

two types of Potentiometrie measurements wer e carried out to deter-

Table 5. Equilibrium constants in the system H +-A13+-1,2-dihydroxy-

naphthalene-4-sulfonate (H2L~)/1,2-naphthoquinone-4-sulfonate (Q").

The e rrors given are 3a(lg ß).

System Equilibrium reaction formula^ ®

e~-H+-Q~ Q"+2H++2e"«H2L"; -lg h < 4.5 21.27+0.09

Q" decomposes for -lg h > 4.5

H+-A13+-H2L" pH++qAl 3++rH2L%*HpAl q (HgL) f+3q"r

p,q5r: -1 0 1 HL2- -7.798+0.002

-2 1 1 A1L -5.343+0.006

-4 1 2 ail23" -13.115+0.009

-6 1 3

I U

D

CO

_

l < -24.47+0.02

-3 1 1 Al(OH)L' -11.24+0.08

-5 1 2 A1(0H)L24" -21.15+0.04

A13+-Q- qAl3++sQ~«A1qQ3q~s

No s table complexes are formed.

B=0.001 M C =0.020 M

-412 -613 010. -211

0.8

0.6

0.2 -311

-10

B= 0.001 M C = 0.002 M -412

010

-211 0.8

0.6 •613

-512

0.2 -311 -110

-10 lg h

Fig. 16: Distribution of complexes in the system H*-AlZ+-l,2-dihydroxy-

naphthalene-4-sul fonate.

29

mine whether A l-quinonic complexes form or not. Neither of these

experiments indicated any complex formation and i t was concluded

that Al^+ does no t form any stable complexes w ith o-quinones. Du­

ring the course o f this investigation, the equilibrium constant for

the reduction of 1,2-naphthoquinone-4-sulfonate to 1,2-dihydroxy-

naphthalene-4-sulfonate as w ell as a formation constant for the

aluminate ion, Al(OH)^", were a lso evaluated. A com pilation of

complexes and corresponding e quilibrium constants is given in Table 5.

The system H +-Al^+-Pyrocatechol (HgL)

This investigation (Paper VII) was carried out to obtain a b asis

for the comparison o f complex formation between A l^+ and two pheno ­

lic, one phe nolic and one ca rboxylic (salicylic acid) resp. two

carboxylic (ftalic acid) groups in ortho-position. The system has

been the subject of several earlier investigations^""^ but the

stability constants reported differ by several orders of magnitude.

The data analysis showed th at, besides the main se ries of complexes

(H_2A1(H2L)+, H_4A1(H2L)2" and H_ 6A1(H2L)33~, assigned as A1L+, 3-A1L-2 and Alresp.) two hydrolyzed species, H^gAlg^LOg and

2-H^Al^L^ s form. Of these, the former species is believed to be

some m ean composition of (Al(0H)L)n, while the latter species prob-2-ably has the formula Al(0H)l_2 . These assump tions are supported by

the findings made i n the H+-A1^+-1,2-dihydroxynaphthalene-4-sulfo-

nate system. Compositions of the species formed i n this system and

their stability constants are given in Table 6.

Table 6. Equilibrium constants in the system H +-Al^+-Pyrocatechol

on (H^L). The e quilibria are defined according to the reacti

pH+ + qAl3+ + r(H2L) HpA1q(H2L^r+3q; q r' The errors 91ven are

30(19 ßp,q,r)-

p q r Proposed formu la lq 3 r 3 P>q>r

-1 0 1 HL" -9.198 + 0.001

-2 1 1 A1L+ -6.337 + 0.005

-4 1 2 A1L2" -15.44 + 0.02

-6 1 3 a1L33~ -28.62 + 0.02

-9 3 3 A13(0H)3L3 -29.91 + 0.07

-5 1 2 A1(0H)L?2" -23.45 + 0.08

B = 0.001 M C = 0.030 M

\AyoH)3L3

-AIIOH1L. A1IOHIL AïoH vAy°Hw

* •/" ^ f Fzg. 17: Dis t r ibution of complexes in the system H - Al -Pyrooatechol.

The system H +-A13+-Sa1icyli c acid (HgL)

As mentioned, this investigation (Paper V) was undertaken to compare

the ^-phenolic-carboxylic binding site with the o-diphenolic site in

pyrocatechol. During the measurements, in order to avoid precipita­

tions at neutral -lgh, it was found necessar y to perform the titra­

tions at considerably higher C/B-quotients than in the £-diphenolic

systems. A probable cause f or this was found d uring the data analy­

sis, where AIL ^, which i s a major complex i n all o>-diphenolic systems,

was r ejected. Instead, the complex formation in neutral and slightly

alkaline solutions was explained as a hydrolysis of the complex

A1L2"(H_4A1(H2L)2"); i.e. Al(0H)L22~(H_5A1(H2L)22~) and

AT(0H)2L2^"(H_gA1(H2L)2^~)• It was also found t hat considerable

amounts of the aluminate ion, Al(OH)^", form together with these

species. An attempt to evaluate the stability constant for this spe­

cies showed consistency with the value found i n the H+-A1^+-1,2-

dihydroxynaphthalene-4-sulfonate system. The standard deviation in

the constant obtained was, however, rather large. The stability con­

stant for the species H_ 2A1(H2L)+(A1L+), formed i n the -lgh range

1.5-3, was evaluated through titrations with H+-solution. The com ­

plex reactions and corresponding e quilibrium constants are given in

Table 7.

The system H +-Al3+-Ftalic acid (H^L)

This system i s still under investigation and t he full results will

not be given in the present thesis. It could, however, be pr elimina­

rily stated that the complexes which se em to be formed are

H_2A1(H2L)+(A1L+), H_4A1(H2L)2"(A1L2"), H.3A1(H2L)(A1(0H)L) and

Table 7. Equilibrium constants in the system H+-Al^+-Salicylic acid

(H2L). The e quilibria are defined according to the reaction

pH+ + qAl3+ + rH2L ** HpAlq(H2L)P+3q; q r* T^e errors 9 lven are

3a ( lq 3 ) . v y p,q,ry

p q r Proposed for mula lg ßp

-1 0 1 HL* -2.724 + 0.001

-2 1 1 A1L+ -3.052 + 0.005

-4 1 2 AIL,," -8.391 + 0.011

-5 1 2 A1(0H)L22" -15.99 + 0.02

-6 1 2 A1(0H)2L23' -25.31 + 0.11

B = 0.001 M C=0.015 M A l l ~

B=0.001 M C = 0.0075 M

AI(OH)4" AL3' AIL'

AI(OH)L Al(OH)L22", '

AKOHÌ •

/'ÂKOHU \

AKOHk(aq)

Al(OH)3(aq)

AlOH),L

+ 3+ Fig. 18: Distribution of complexes in the system H - Al -Salicylic acid.

31

H_4A1(H2L)"(Al(0H)2L~). Thus, i n a comparison w ith the salicylic

acid system, it seems as though the carboxylate group(s) inhibits 3-the formation of Al• In this system eve n the formation of A1L2

is somewhat hindered (i.e. a low formation constant) and this leads

to an increased significance of "early" hydroxo complexes.

Concluding remarks, Papers I-VII

The i nformation from the different equilibrium studies may be syn­

thesized with the following concluding remarks. Firstly, it can be

noted that in most systems studied, the complexation a t high ligand

excesses can be described by a series of mononuclear complexes

AlL-AlLn, in which th e end species AlLn has six ligand oxygens co­

ordinated to the aluminium i on. It is most credible that these oxyge n

atoms are giving the Al-ion an octahedral surrounding. Thus, i t is

tempting to assume that the whole ser ies of complexes can be d escri­

bed as a successive exch ange o f water oxygens for ligand oxygens and

that, accordingly, the Al-ion has an octahedral coordination through­

out the series.

Secondly, a comparison betw een the three systems with o-diphenolic

coordinating ligands shows the clear connection between complexation

strength and pka-value of the first hydrolyzable phenolic group.

Thirdly, the absence of AlL^-species in the salicylic and ftalic

acid systems illustrates the importance of steric hindrance ("un­

wieldy" carboxylate groups) in solution chemistry.

Fourthly, the significance and i n some case s even predo minance of

32

the mixed hydroxo species i s striking. This concept has bee n mo re o r

less overlooked i n earlier investigations. It is interesting to note

that the nucleari ti es of these species range from one to three, in

the same range as the early Al-hydrolytic species and i t is tempting

to assume that some s tructural and/or equilibrium connection could

exist. Concerning the mononuclear ternary species several comments

can be ma de: i) the pka-values for Al(H^Ö)2^2 in the two o- diphenolic

coordinating systems pyrocatechol and 1,2-dihydroxynaphthalene-4-

sulfonate are quite equal (8.01 and 8.04 resp.) while the correspon­

ding value in the salicylic acid system is considerably lower (7.60).

ii) The abse nce o f Alin the salicylic acid system re sults in a

further hydrolysis of Al (F^O) (0H)l_2 to A1(0H)2L2 (pka = 9.3). i i i )

The pk a for Al^OJ^L is much lower in the ftalic acid system (4.3)

than in the 1,2-dihydroxynaphthalene-4-sulfonate system (5.90). A

tendency f or this hydrolyzed species, i.e. Al (h^O^OHJL, to poly­

merize has been found in the naphthalenic as w ell as in the pyro­

catechol and the gallic acid systems. The p ossibility of creating a

dimeric species with a double hydroxo bridge, i.e. A^^O^OH^L »

probably favours this polymerization. The hig her acidity of the sa­

licylate and ftal ate species than of the ^-diphenol i c species, as i n­

dicated under i) and i ii), is probably caused by the higher electron

demand from the carboxylate groups than from the phenolic oxygens.

The c orrosion rate of a-AlgO^

The r esults of this investigation (Paper VIII) could be sum marized

by F ig. 19, where th e corrosion rates found a t different temperatu­

res are plotted versus the inverse of the absolute temperature. As

seen i n the figure, the corrosion rates determined by the weight

o

2.7 3.0

-1 -

Fig. 19: Weight loss measurements - open symbols; Al-oono. in

leachant solution - filled symbols.

IgC

Al (OH)"

C/M igS/

n-5.5

8 -Igh 6

Fig. 20: The influence of Al-citvates on the solubility of kaolinite.

33

loss measurements, ln(K/nnryr~^) = 28.2-(71•10^/RT), differ signifi­

cantly from the value obtained by the measurements o f aluminium con­

tent in the leachant solution, ln(K/nnryr~^ ) = 24.5-(64-10^/RT). (In

these e quations, T denotes the absolute temperature and R = 8.3143

J-mol^K"1). A probable explanation for this difference would be

that the weight loss measurements include both a mechan ical (col­

loidal Al particles) and a chemical corrosion while the latter

method only registers the chemical part of the corrosion.

A small extrapolation of the investigation to 20 °C, i.e. a r elati­

vely high natural water temperature, yields the weathering rates of

0.39 and 0.17 nm*yr~^ respectively. Thus, an o riginally 10 mm sized

corundum cr ystal could be ca lculated to resist rain and ri ver water

for at least 10-15 million years. A comp arison w ith leaching rates 13 for different feldspars under similar conditions give at hand t hat

these are approximately 20 times higher. It is therefore not surpris­

ing that corundum cry stals or sapphires can be found as weathering

residues in lateritic soils where mo st of the minerals in the origi­

nal corundum-bearing rocks have d isintegrated.

SOLUBILITY MO DELLING O F A l-COMPLEXES IN A SI MPLIFIED N ATURAL W ATER

SYSTEM

In an attempt to indicate the potential importance o f complexation Q

of Al + in a n atural water, a number of model calculations using the

computer program S0LGASWATER has been performed. In these c alcula­

tions, a h ypothetically one molar solution of kaolinite composition 3 (n^-|3+ = = 2 moles; n^+ = - 6 moles per dm ) has been allowed

AKOH), Al(OH)3(aq)

Fig. 21: The influence of Al-gallates on the solubility of kaolinite.

IgC

AIL AIL AIL

Al(OH)L

-5

AKOH),"

6 -Igh 8 u

u C/M

5

6

7

8

6 7 8 9 U 5

Fig, 22: The influence of Al-dihydroxynaphthalenes on the solubility of

kaolinite.

34

to equilibrate with water (i.e. hydrolysis) and organic ligands,

permitting kaolinite (Al2(OH)^Si2O5)» gibbsite (Al(OH)g) and qu artz

(SÌO2) (equilibrium constants from Helgeson^) to be formed.

In a first series of calculations, the investigated ligands have

been "added", one at a time, to the solution in relevantly low con­

centrations. The t otal solubility of aluminium, , as w ell as the

dominating aqueous species were c alculated as a function of -1gh and

the (logarithm of) total concentration of ligand. The r esults of

these calculations are presented i n Figures 20-24. The c alculation

performed on the carbonate system ha s not been il lustrated, as i t

was found that the compiexation a bility of this system gav e quite

insignificant contributions to the Al-sol ubi 1 i ty. Even i n a solution

equilibrated with a p ^ of 1 atmosphere (an improbable situation in

an ope n n atural water system) the total solubility was only raised

by a t maximum 0.4 % at -Igh = 5. On the other hand, most of the or­

ganic substances studied show quit e significant contributions to the

solubilities, even at very low concentrations. Noteworthy is the

predominancy o f mixed hydrox o species in some areas and the extreme

stability of the trinuclear aluminium-hydroxocitrate species. It is

also interesting to note that the -lgh range a t which t he carboxylate-

coordinating ligands act (citric and salicylic acid) differs signi­

ficantly from that of the ^)-di phenolic-coordinating ligands (pyro-

catechol, gallic acid, 1,2-dihydroxynaphthalene-4-sulfonate). Thus,

while the former substances act exclusively in acidified solutions

(-lgh < 6-7) the latter substances are showing their most s ignifi­

cant influence in neutral and a lkaline solutions. This difference is

illustrated in Fig. 25, where equal amounts (30-10 ^ M) of salicylic

and g allic acid were e quilibrated with the solid phases and the

AKOHUaq) AKOHf

Fig. 23: The influence of Al-pyrocatecholates on the solubility of

kaolinite.

Al(OH)3(aq) AKOHf

Fig. 24: The influence of Al-salicylates on the solubility of kaolinite.

35

fraction of Al^+, Al-salicylates, Al-gallates and Al- hydroxo species

followed as a function of -1gh.

Another interesting notation which can be ma de i s that the compiexa-

tion ability of the o-diphenolic group i s highly dependent o f substi­

tution effects on the benzene ring (cf. pyrocatechol-gal1ic acid).

Thus, in an analo gous calculation as abov e w ith gallic acid exchanged

for pyrocatechol, the significance of Al-salicylates becomes som ewhat

greater but, as can be seen in Fig. 26, the main re sult is that alu­

minium hy drolysis becomes hi ghly predominating over compiexation in

neutral and a lkaline solutions.

Concerning the o-diphenolic substances it is also important to bear

in mind t hat, in a n atural water, these substances could be oxi dized

to the corresponding ci-diquin ones, and that these quinones show n o

complex formation ability towards Al^+ (cf. Paper I V). To il lustrate

this, a calculation was performed wher e the solid phases were e qui­

librated with a total of 30*10"^ M 1,2-dihydroxynaphthalene-4-

sulfonate plus 1,2-dinaphthoquinone-4-sulfonate. The total Al-

solubility and predominating aqueous species were c alculated as a

function of -1gh and pe, i.e. the minus-logarithm of the (hypotheti­

cal) electron activity (Fig. 27). It is, however, important to rea­

lize that the redox l evel (pe°, E°) at which the o-diphenolic group

is oxidized is dependent on substitution effects on t he benzene 65 ring and th at, therefore, this functioning group probably could

exist at considerably higher pe-values in an actu al humic substance.

1.0

rF; Al-OH I Fi 0.8 0.8 Al-G

0.6 0.6

Al-OH

Al-P Al-S Al-S .0.2 0.2

-Igh -Igh

Fig. 25 Fig. 26

Al(OH) (aq) AKOH),

Fig. 27

36

FUTURE P LANS

In view o f the growing i nterest of Al^+-speciation in natural waters

and the shortcomings of existing thermodynamic data, the research

concerning complex formation between A1^ + and na turally occurring

oxygen-coordinating ligand classes could by no me ans be considered

as completed. The studies should be regarded as a type of fundamen­

tal mapping and, at present, the organic ligands ftalic, kojic and

oxalic acid, are the subjects for investigation at the department.

Concerning the inorganic substance of major complexation im portance,

i.e. the fluoride ion, several investigations have sho wn the occur-2+ 3-rence o f a series of complexes A1 F -AlFg and formation constants

27 31 for these complexes have bee n evalua ted ' . However, as all these

measurements have bee n performed i n solutions of relatively high

acidity, the probable occurrence o f mixed h ydroxo-fluoride complexes

in solutions of lower acidity has never been detec ted. From a na tu­

ral water point of view, an in vestigation in this direction would be

most valuable.

As mention ed i n the introduction, the present project was o riginally

initiated due to reports claiming that the existence of octahedrally

coordinated Al(III) was o f vital importance for the formation of 7 8 clay minerals under na tural conditions ' . Thus, on the basis of

well defined conditions in systems H +-A1^-organic ligands, an ex­

tension towards the field of aqueous ternary and qu arternary com­

plexes in systems H +-Al^+-Si(OH)^-organic ligands would be a logical

evolution of the project. In such systems, the composition and e xist­

ence area of solid phases would also be o f vital importance.

Finally, concerning aluminium complexation, it has been sho wn i n nu­

merous re cent articles that Al(III) could cause hu man b rain diseases 72 73 74 75 (Alzheimers disease ' , dialysis encephalopathy 9 ). Therefore,

it would be very interesting to extend t he choice of ligands into

the area of biological interest. In this area comparative studies

with Ga^+, another group 3B metal which has proved to have cancer 76 77 inhibitory effects ' , would also be mo st interesting.

In the field of corrosion (weathering) rates for the naturally oc­

curring Al-bearing minerals, it would be very interesting to study

if, and to what e xtent, these rates are influenced by the presence

of Al-complex forming substances in the leaching solution. Work in

this direction has recently been st arted as a cooperation project

with the Department o f Forest Soils, Swedish Un iversity of Agricul­

tural Sciences. These c orrosion studies will be re lated to the study

performed i n Paper VIII.

ACKNOWLEDGEMENTS

I wish to express my sincere gratitude to Professor Nils Ingri for

introducing me in to the field of complex chemistry, for his sugges­

tion and initiation of this project, for his continuous interest and

encouragement and for all the facilities placed at my d isposal. I am

also greatly indebted to Docent St affan Sjöberg for his excellent

supervision and n ever-failing patience throughout the course o f this

work.

I would also like to thank Docent Willis Forsling for the stimula-

33

ting educational and scientific cooperation and Docent U lf Edlund

for introducing me t o the field of nuclear magnetic resonance. My

thanks are also due to Mrs Yvonne Hägglund for the careful and va­

luable help with some of the experimental work and t o Mr Sture

Pettersson for valuable technical assistance.

For the good cooperatio n and w illing assistance from a ll the members

of the solution chemistry research group which, at present, in addi­

tion to Professor Nils Ingri, Docent Staffan Sjöberg and Mrs Yvonne

Hägglund comprises Mrs s Ingegärd Andersson and Agneta Nordin and Drs

Bo Daniels son, Inger Granberg, Tomas Hedlund, O lof Hofslagare, Jan

Liden and Lag e Pet tersson I would li ke to express my h eartfelt gra­

titude.

I would also like to express my gra titude to Miss C hristina Broman

for skilful typing of the manuscripts, to Mr Lage Bo den for excellent

drawings and to Docent Michael Sharp and Professor Surendra Sax ena

for linguistic revision.

To a ll the colleagues and fri ends who have helped m e i n some wa y I

also express my app reciation.

Financial support by the Swedish Natura l Science Research Council is

hereby gratefully acknowledged.

39

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