Chlorite: Geochemical properties,

Dissolution kinetics and Ni(II) sorption

sa Zazzi

Doctoral Thesis in Chemistry KTH Chemical Science and Engineering

Stockholm, Sweden, 2009 ________________________________________________________________

AKADEMISK AVHANDLING Som med tillstnd av Kungliga Tekniska Hgskolan i Stockholm framlgges till offentlig granskning fr avlggande av Filosofie Doktorsexamen i Kemi fredagen den 24 april 2009, kl. 10.00 i D2, Lindstedsvgen 5, Entreplan, Stockholm. Fakultetsopponent r Ph. D. Peter Vilks, AECL, Whiteshell Laboratories, Canada. Avhandlingen frsvaras p engelska.

Chlorite: Geochemical properties, Dissolution kinetics and Ni(II) sorption sa Zazzi Doctoral Thesis KTH Chemical Science and Engineering Royal Institute of Technology Stockholm, Sweden, 2009

ISBN 978-91-7415-247-0 ISSN 1654-1081 TRITA-CHE Report 2009:9 sa Zazzi, Mars 2009 Printed by E-PRINT AB, Stockholm 2009.

Nr man r en bjrn med en mycket liten hjrna och tnker ut saker, upptcker man att en id som verkade vara riktigt idaktig inne i hjrnan, r annorlunda nr den kommer ut i det fria och andra mnniskor ser p. Nalle Puh When you are a Bear of Very Little Brain, and Think of Things, you find sometimes that a Thing which seemed very Thingish inside you, is quite different when it gets out into the open and has other people looking at it. Winnie the Pooh

i

Abstract

In Sweden, among other countries, a deep multi-barrier geological repository, KBS-3, is

planned for the burial of nuclear waste. One of the barriers is identified as the grantic

bedrock itself and in this environment chlorite is present at surfaces in fracture zones.

This thesis is focused on characterisation of chlorite samples and studies of their dissolution

and sorption behaviour, in order to verify chlorites capacity to retard possible radionuclide

migration in the case of leaking canisters.

Chlorite dissolution of has been studied in the pH interval 2-12, and as expected the

dissolution is highest at acidic pH and at most alkaline pH, whereas dissolution is lowest at

near neutral pH values. Chemical and physical properties of chlorites clearly influence the

dissolution rates, and at steady-state dissolution rates in the interval 10-12 - 10-13 mol g-1 s-1

was observed.

Sorption studies were performed since Ni(II) is one of the important activation products in

spent nuclear fuel and sorption data on minerals like chlorite are lacking. Ni(II) sorption onto

chlorite was studied using batch technique as a function of; pH, concentration of Ni(II),

ionic strength and solid concentrations. As expected, the sorption of Ni(II) onto chlorite was

pH dependent, but not ionic strength dependent, with a sorption maximum at pH ~ 8, and

with a Kd of ~ 103 cm3/g. This confirms that the Ni(II) sorption onto chlorite is primarily

acting through surface complexation. The acid-base properties were determined by titrations

and described by a non-electrostatical surface complexation model in FITEQL. Further, the

sorption results were fit with a 2-pK NEM model and three surface complexes,

Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit using FITEQL.

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iii

Sammanfattning

Sverige r ett av de lnder som planerar ett geologiskt slutfrvar kallad KBS-3, bestende av

ett antal barrirer, fr placering utav det anvnda krnbrnslet. En av dessa barrirer r

identifierad som sjlva berggrunden dr det tilltnkta frvaret kommer att byggas och i denna

milj frekommer klorit p granitytor i sprickzoner.

Denna doktorsavhandling karakteriserar kloriter och studerar deras upplsnings- och

sorptionsbeetende, fr att kunna bestmma huruvida kloriter r utav betydelse som naturlig

barrir fr eventuell radionuklidtransport frn det anvnda krnbrnslet.

Upplsning av klorit har underskts i pH intervallet 2-12 och graden av upplsningen r som

frvntat hgst vid sura respektive mest basiska pH och lgst dr pH r neutralt. Denna

studie bekrftar att den kemiska sammansttning och de fysikaliska egenskaper hos kloriterna

pverkar upplsningshastigheterna och vid steady-state har upplsningshastighet bestmts till

10-12 - 10-13 mol g-1 s-1.

Sorptionsstudier genomfrdes d Ni(II) r en viktig aktiveringsprodukt och data rrande

Ni(II) sorption till klorit saknas. Ni(II) sorption till klorit har studerats i; varierande pH, olika

initiala Ni(II) koncentrationen, olika jonstyrka och olika fastfas frhllanden dr individuella

satser i serie har nyttjats. Som frvntat r sorptionen av Ni(II) till klorit pH beroende men

inte jonstyrkeberoende och ett sorpions maximum observerades vid pH ~ 8, med ett

Kd-vrde p ~ 103 cm3/g. Frn detta dras slutsatsen att sorptionen av Ni(II) till klorit sker

mestadels genom ytkomplexering. Syra-bas egenskaperna hos kloriterna bestmdes genom

titreringar och bekrevs med en icke-elektrostatisk modell i FITEQL. Vidare har passning av

sorptionsresultaten utfrts med en 2-pK NEM-modell och tre ytkomplex, Chl_OHNi2+,

Chl_OHNi(OH)+ och Chl_OHNi(OH)2, vilket gav den bsta passningen av data med

FITEQL.

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v

List of publications This thesis is based on the following papers:

I. The effect of pH on chlorite dissolution rates at 25 C sa B. Gustafsson1 and Ignasi Puigdomenech In: Scientific Basis for Nuclear Waste Management, XXVI (R. J. Finch, D. B. Bullen, eds.), Material Research Society, Boston, MA, USA, 2002, vol. 757, p. 649-655.

II. Study of Ni(II) Sorption on Chlorite-A Fracture Filling Mineral In Granites . Gustafsson1, M. Molera, and I. Puigdomenech In: Scientific Basis for Nuclear Waste Management XXVIII (J.M. Hanchar, S. Stroes-Gascoyne, L. Browning, eds.), Material Reseach Society, San Fransisco, CA, USA, 2004, vol. 824 p. 373-379.

III. Structural Investigations of natural and synthetic chlorite minerals by X-ray

diffraction, Mssbauer spectroscopy and Solid-state Nuclear Magnetic Resonance sa Zazzi, Tomas K. Hirsch, Ekaterina Leonova, Andrei Kaikkonen, Jekabs Grins, Hans Annersten, and Mattias Edn In: Clays and Clay Minerals; April 2006; v. 54; no. 2; p. 252-265

IV. Ni(II) sorption on natural Chlorite sa Zazzi, Anna-Maria Jakobsson and Susanna Wold Submitted to: Applied Geochemistry

V. Ni(II) sorption on the fracture filling mineral Chlorite

sa Zazzi and Susanna Wold Accepted for publication in: Scientific Basis for Nuclear Waste Management XXXII (R.B. Bebak, N.C. Hyatt and D.A. Pickett, eds). Material Research Society, Boston, MA, USA, 2008, vol 1124.

VI. Dissolution rates and stoichiometry of two different chlorites as

a function of pH sa Zazzi, Maria E. Malmstrm and Susanna Wold Manuscript

1 Maiden name, changed to Zazzi by way of matrimony.

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Comment on my contribution to the publications

Paper I: I performed the experimental work, participated in evaluation of the data and wrote

most part of the manuscript.

Paper II: I participated in the design of experiments and performed most of the

experimental work. I performed parts of the simulations, participated in evaluation of the

data and wrote the manuscript.

Paper III: I provided the chlorite samples, prepared the synthetic chlorite and wrote parts of

the Introduction section of the manuscript.

Paper IV: I designed most of the experiments and performed most of the experimental

work. I participated in discussions about the simulations as well as in evaluation of data and

wrote most of the manuscript.

Paper V: I designed and performed the experiments, evaluated the data and wrote the

manuscript.

Paper VI: I designed and performed the experiments, participated in data evaluation and

prepared parts of the manuscript.

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ix

List of abbreviations

Abbreviations used in text. AFM Atomic Force Microscopy

BET Brunauer-Emmett-Teller, a method for measuring the surface area of powders

CCM Constant Capacitance Model

CEC Cation Exchange Capacity

DLM Diffuse layer model

ICP-MS Inductively Couple Plasma equipped with Mass Spectrometry

ICP-OES Inductively Couple Plasma equipped with Atomic Emission Spectroscopy

KBS-3 Krnbrnsleskerhet-3, the Swedish concept for spent nuclear fuel, the

abbreviation is always used.

LSC Liquid Scintillation Counting

MES 2-(N-morpholino)ethanesulfonic acid

MUSIC Multi Site Complexation Model

NEM Non Electrostatic Model

SCM Surface Complexation Model

SEM Scanning Electron Microscopy

SEM-EDS Scanning Electron Microscopy-Energy Dispersive Spectroscopy

SKB Swedish Nuclear Fuel and Waste Management Company

TLM Triple Layer Model

TOT Tetrahedral-Octahedral-Tetrahedral coordination

TRIS 2-Amino-2-hydroxymethyl-1,3-propanediol

UV-VIS Ultraviolet-Visible Spectroscopy

XRD X-ray Diffraction

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Table of contents Abstract ................................................................................................................................................. i Sammanfattning ............................................................................................................................ iii List of publications ........................................................................................................................ v Comment on my contribution to the publications ................................................. vii List of abbreviations.................................................................................................................... ix 1. Introduction................................................................................................................................. 1

1.1 Background .......................................................................................................................... 1 1.2 The mineral chlorite ....................................................................................................... 3 1.3 Theory of weathering processes............................................................................ 5 1.4 Sorption processes, surface complexation and surface complexation models.............................................................................................................. 7 1.5 Literature survey of the area .................................................................................12 1.6 Objectives of this work ..............................................................................................14

2. Experimental.............................................................................................................................15 2.1 Chlorites used...................................................................................................................15 2.2 Reagents ..............................................................................................................................16 2.3 Experimental methods ...............................................................................................16 2.4 Treatment of data..........................................................................................................20

3. Results and discussion .......................................................................................................23 3.1 Characterisation of the used chlorites used ................................................23 3.2 Dissolution results .........................................................................................................29 3.3 Sorption results ...............................................................................................................41 3.4 Titration results ...............................................................................................................53 3.5 Fitting the sorption and titration data .............................................................55

4. Conclusions ................................................................................................................................59 5. Future work ...............................................................................................................................61 6. Acknowledgements...............................................................................................................63 7. References..................................................................................................................................67 Appendix ............................................................................................................................................73

Appendix A: Experimental details for sorption isotherm experiments..............................................................................................................................................................73 Appendix B: Summary of dissolution results.......................................................74 Appendix C: Summary of sorption results. ...........................................................78 Appendix D: Table of titration data ............................................................................79 Appendix E: Literature data.............................................................................................81

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1

1. Introduction

1.1 Background Swedish nuclear reactors will produce a total amount of 12 000 tons of spent nuclear fuel, if

todays nuclear power plants will be active for a period up to 60 years [1]. According to

todays Swedish regulations, spent nuclear fuel has to be taking care of within Swedish

borders, and the company SKB was created for this purpose. In 1983 they proposed a

design capable of storing spent nuclear fuel named KBS-3 [2]. The KBS-3 repositories are

planned to be situated approximately 500 m below the surface in the Swedish granitic

bedrock. The Swedish repository design is a multi-barrier concept developed for storage up

to 100 000 years where the first 1000 years are the most critical for nuclear waste disposal,

since a dramatic decrease in radioactivity occurs during that time [3]. KBS-3 consists of four

different barriers, Figure 1, where each barrier will act independently of the other. The first

barrier is the fuel itself, which has a low solubility in reducing groundwater. The second

barrier is a corrosion- resistant canister made of copper and iron to contain the spent fuel.

The canisters will be placed in deposition holes and surrounded by the third barrier,

bentonite clay [4, 5]. The tunnels will be refilled with a mixture of bentonite clay and the

original bedrock. The fourth and final barrier consists of the surrounding bedrock itself.

In a worst case scenario any leakage will reach the bentonite clay and radionuclides, fission

products or activation products will be transported through the bentonite clay and reach the

surrounding geosphere. The fourth barrier should retard the migration of these products

through sorption reactions in the near field and will concentrate them to the fracture

systems and the wall rock adjacent to these, since the surrounding rock volume has a large

sorption capacity coupled with the specific surface area [6]. The activation product 63Ni is

estimated to be present in the surrounding area of the repository upon leakage of fuel 300-

1000 years after closure. During this time the maximum calculated activity released into the

near field will be approximately 1000 Bq yr-1, peaking at 600 years after closure [7]. Nickel is

a small surface-complexing divalent cation and is representative for cations such as Co2+,

Mn2+, Cd2+ and Zn2+.

2

Figure 1.1. A schematic model of the KBS-3 structure designed for storing spent nuclear

fuel.

Most of the Swedish bedrock is of crystalline granitic type formed during different

intrusions of magma [8, 9] and is stable where mechanical and chemical changes occur

extremely slowly [10]. The granitic bedrock consists of a number of closely related

rocktypes2 e.g. different granitoids consist of different silicates, among those chlorite [11,

12]. Mapping of these fracture zones, with their minerals and adjacent rock walls, is

important for example in choosing a building area with low frequency of fractures for the

repository. Different areas of Swedish granitic bedrock have been investigated, the main

sites of the geological investigations being sp, within the underground Hard Rock

Laboratory (HRL) [13, 14] and the two areas of interest for future repositories, namely

Forsmark and Laxemar/Simpevarp (divided into two sub-areas) [15, 16]. Mapping of

drillcores shows that only 0.2% of Forsmark granite consists of chlorite, whereas vr

granite from Laxemar contains 4.4%, but chlorite is still one of the dominant minerals in

fractures, comprising 30-70% of the fracture surface [15]. In the tunnel situated within the 2 A rocktype is defined by a number of different minerals. A mineral is a naturally occurring, homogenous solid with a defined chemical composition and highly ordered atomic arrangement (The new penguin dictionary of Geology, second edition). In a more general way minerals are the building blocks of the Earth formed by the history and show us the diversity of formation.

3

sp HRL, the fresh granite consists of 10% biotite and 1% magnetite, whereas altered

granite consists of by 5-10% chlorite, 0.1% pyrite and 1-2% hematite [17]. The fracture

coatings is composed of 35% chlorite, 13% epidote, 0.2% pyrite, 30% of calcite and 18%

other clay minerals [17].

1.2 The mineral chlorite

Chlorite is a phyllosilicate arranged in a 2:1 structure type with an interlayer, Figure 1.2. The

ideal structure of chlorite can be described as alternating talc-like layers (TOT) together with

brucite-like layers (O), which give it a unit structure of 14 in the stacking direction [18].

Figure 1.2. The unit structure of chlorite.

Within the structure the TOT-layer has a negative permanent charge, whereas the brucite

layer has a positive permanent charge [19]. Major contributors to surface charge are defects

in the lattice and isomorphous substitution [19]. Isomorphous substitution occurs in the

crystal lattice of the mineral, for example when Si4+ is replaced by Al3+ in the tetrahedral

layer of clays. In addition, in the octahedral layer Al3+ may be replaced by divalent cations,

such as Mg2+ [20]. In isomorphous substitution the replacement occurs between atoms of

4

similar sizes but with different charges, as described above. Meanwhile, the configuration of

oxygen and hydroxide groups stays essentially unchanged.

Chlorite may be a product resulting from hydrothermal alteration of pyroxenes, amphiboles

and biotite. The overall alteration reaction of biotite to chlorite can be described as [21]:

Biotite + anorthite + H2O + O2 + H+

chlorite + sphene + epidote + muscovite + quartz + magnetite + K+

where the K+ from the biotite diffuses through the fluids, responsible for the alteration, and

water within the bedrock provides H+, which diffuses into the interlayer of biotite and

substitute for K+. When the K+ diffuses out from the structure the attached H+ weakens the

Si-O or Al-O bonds and a brucite-like layer replaces the K interlayer while the talc-like layer

is inherited directly from biotite [22, 23]. The process requires two biotites to become one

chlorite with a volume loss of approximately 35% [21].

The colour of the chlorite varies from white to almost black or brown with a tint of green

where these optical properties of chlorites are coupled to the chemical composition of

chlorite. An increase in Fe/(Fe+Mg) ratio is followed by an increase in absorption, together

with the Fe2O3 content, degree of oxidation, total iron in octahedral sites and Si/Al ratio in

tetrahedral sites, different schemes describing the optical properties [24-26]. Using these

schemes, Mg-Al rich varieties of chlorites are more or less colourless whereas Fe-rich

members are of different green colours and Mn and Cr segments add orange-brown, pink

or lavender colours to the chlorites.

The family of chlorite minerals has the generic formula [19, 27, 28]:

(R2+6-y-zR3+y z)2(Si4-kR3+k)2O20(OH)16 (1.1)

where the parameters y and k denote the degree of substitution of trivalent cations in the

octahedral and tetrahedral sheets, respectively, and z accounts for vacancies [29]. In

general, the cation distribution of divalent (R2+) and trivalent (R3+) ions is not a priori

known either within each layer or between them. Nevertheless, the net formula, Equation

1.1, may schematically be decomposed into contributions from the two types of alternating

layers. The composition of the brucite-like layer conforms to the formula:

+

2(R

116 zy+3

R 1y 1z)(OH)12 (1.2)

whereas the generic composition of the 2:1 layer is

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( +

2R 226 zy

+3

2R y 2z)(Si4-kR3+k)2O20(OH)4 , (1.3)

and the coefficients combine as y1+y2=y and z1+z2=z. R3+ in the tetrahedral sheets is usually

Al3+. However, other cations are also often present.

Expressing the general formula as Equation 1.1 implies that the chlorite is trioctahedral in

both the 2:1 layer and the brucite-like layer, assuming eight tetrahedral cations and twelve

octahedral cations per unit cell. A chlorite may be dioctahedral in the two sub-structures, as

well as a combination of di-and trioctahedral [30]. The most common natural form of

chlorite include clinochlore, chamosite and penninite [19], which differ in the nature of the

dominant divalent octahedral cation; Mg2+ in clinochlore, Fe2+ in chamosite and Mn2+ in

penninite.

1.3 Theory of weathering processes

The phenomenon of weathering can be divided into chemical weathering and physical

weathering, where physical weathering is the result of wind, water and root abrasion, where

part of the rock is worn down without any change in its elemental composition. Chemical

weathering is a dissolution process through chemical reactions, where a release of ions gives

a change in chemical composition. For most silicate minerals the dissolution and dissolution

rates are strongly pH-dependent [31]. A schematic relationship between the dissolution rate

for silicates and pH is shown in Figure 1.3. In the acid and basic pH regions, the dissolution

rates increases while in the near neutral pH region the rate is independent of pH.

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0 2 4 1086 12

pH

Log

Dis

solu

tion

Rat

e

0 2 4 1086 12

pH

0 2 4 1086 12

pH

Log

Dis

solu

tion

Rat

e

Figure 1.3. Schematic relationship between dissolution rate and pH for silicates.

Dissolution is a process undertaken in a number of different steps [32, 33]:

1. Migration of the reactants (H+, OH- or ligands) to the surface.

2. Surface adsorption of the reactants.

3. Formation of surface species.

4. Detachment of the surface species from the surface.

5. Transport of reactants from surface to solution.

The rate-determining step is attachment of a reactant to the surface [32]. Rearrangement

may occur where the coordination of the metal ions changes and bonds are weakened or

broken, which facilitates the detachment of the surface metal species into the bulk solution.

These steps indicate surface-controlled dissolution, Equation 1.4 and 1.5 [34].

Hydrolysed metal surface sites + reactants (H+, OH- or ligands) surface species (1.4)

Surface species Me(aq) (1.5)

This surface-controlled model was developed by Stumm and co-workers and when the

system is not in equilibrium, the back reactions can be excluded [35]. Silicates have a

non-stoichiometric dissolution i.e. the different cations detach from the mineral surface at

different rates [32].

Chemical weathering can be described as congruent or non-congruent dissolution. In

congruent dissolution no new solid phases are formed after dissolution and the dissolution

7

is complete where the products formed are all soluble. In contrast, incongruent dissolution

is an exchange process and gives rise to new solid phases.

An example of congruent dissolution is the dissolution of quartz:

SiO2 (s) + 2H2O (l) H4SiO4 (aq) (1.6)

whereas an example of incongruent dissolution is the weathering of olivine, where quartz is

formed as a precipitate [20]:

Mg2SiO4 (s) + 4H+ (aq) 2Mg2+ (aq) + SiO2 (s) +2 H2O (l) (1.7)

A steady state is achieved when there is no concentration change over time. However, in

experimental studies true steady-state may not be reached within a reasonable experimental

period of time [36]. Therefore, steady state under experimental conditions is denoted as the

time when the cation concentration within the outflow solution is nearly constant [37] [36].

1.4 Sorption processes, surface complexation and surface

complexation models

There are a number of different processes taking place in the surface water interface region,

such as adsorption, surface precipitation and ion exchange.

In terms of surface complexation, adsorption reactions are described as chemical reactions

between surface functional sites and dissolved chemical species, where the protonation of

surface oxygen and deprotonation of hydroxyl groups at the surface take place. For ion

exchange, and more specifically cation exchange, the reactions can be described as an

interchange reaction between an ion in solution and another ion with electrostatic

attraction.

Cation exchange and surface complexation reactions may take place simultaneously, where

sorption through cation exchange is generally pH independent and sorption through surface

complexation is pH dependent [38].

Sorption may be used as a more general term instead of adsorption. The term sorption can

be used when investigations are performed on the macroscopic level and not combined

with microscopic studies [39].

8

A general cation exchange reaction can be written as:

XH + Me+ XMe + H+ (1.8)

where XH is a cation-exchange site [40].

SOH or XOH are common symbols to denote a general neutral surface site.

A cation exchange reaction at the surface can be written as:

SOH + Me+ SOMe + H+ (1.9)

and a general surface complexation reaction can be written as:

SOHrn + Mez+ + yH2O SOHcMe(OH)yr+z-y +(n-c+y) H+ (1.10)

where SOHrn is a surface species with charge r, Mez+ is a sorbing cation, y is the number

of protons released in the reaction and SOHcMe(OH)yr+z-y is the surface complex formed.

For the surface precipitation the general reaction can be written using S(OH)3(s) as the

hydrous oxide:

SOH + Me2+ + 2H2O S(OH)3(s) + =MeOH2 + H+ (1.11)

=MeOH2 + Me2+ + 2H2O MeOH2(s) + =MeOH2 + 2H+ (1.12)

The uptake and release of protons of a hydrolysed surface complex describes the acid-base

reactions which take place at the surface. These reactions can be described either by two

acidity constants called the 2-pK approach, which is more commonly used [41], or by one

acidity constant, the 1-pK approach.

The reactions of the 2-pK model can be written as:

SOH + H+ SOH2+ (1.13)

SOH SO- + H+ (1.14)

The reaction with only one step can be written as:

SOH-0.5 + H+ SOH2+0.5 (1.15)

The acid-base properties of the surface hydroxyl groups are affected by the ionic strength

used and the permanent charge of the surface itself.

9

There are mathematical models such as Freundlich or Langmuir which applied to the

sorption system describe the metal sorption as a function of the equilibrium concentration

in solution [20, 42]. The Langmuir model assumes that the sorption takes place in a

monolayer on a homogeneous surface, that the binding sites are evenly distributed over the

surface, that all of them have the same adsorption affinity and that surface complexation is

the sorption mechanism. The Freundlich model is an empirical model that can be applied to

heterogeneous surfaces where all sites are not considered to be equal and no monolayer of

adsorption is assumed. Instead accumulation of the adsorbate and no sorption maximum

are achived with this model. Freundlich does not assume any specific sorption mechanism

and is found to be most suitable in trace metal concentrations because surface precipitation

possibly takes place [43].

The cation may attach to the surface in different ways, as an inner sphere, as an outer

sphere or as a diffuse swarm of ions within the double layer [44, 45]. When an inner sphere

is formed there is a bond between the cation and the electron-donating ions at the surface.

When an outer sphere complex is formed, the ion pairs between the cation and the surface

are separated by one or more water molecules. Furthermore, the outer sphere complexes

are less stable than the inner sphere complexes since they involve electrostatic bonding

whereas inner sphere complexes involve covalent bonds and some ionic bonds.

To identify whether inner sphere or outer spheres complexes are formed different

spectroscopic techniques are used, for example EXAFS, since this technique enables the

atomic distances between adsorbed species and neighbouring cations to be seen [46]. A

simpler way is to study the sorption of the cation as a function of ionic strength, since a

dependence on the ionic strength is assumed to be the result of outer sphere complex

formation [47].

To predict the experimental outcome involved and to understand the different reactions

involved in surface complexation, several models have been developed. The Surface

Complexation Model (SCM) describes the reactions in relation to the charging properties of

mineral surfaces and adsorption mechanisms. Within the SCM it is assumed that the

adsorbing ion forms surface complexes at the specific sorbing sites on the surface and

different SCM use different descriptions for the distribution of the ions near and around the

charged surface. Electrostatic effects and surface charge are incorporated and introduced in

different ways in the different models. The surface charge can be determined with different

10

methods; titration with acid or base or applying an electrical field to the surface. If titration

data are used for the determination, the surface charge can be calculated according to:

= F(H+ - OH-) (1.16)

where F is Faraday constant (96490 C mol-1) and H+ and OH- are the adsorption densitiy

of H+ and OH- (mol m-2).

The surface complexation models have been thoroughly discussed [46, 48-50] and here only

a short summary is given with the major differences between the different models. The

abbreviations for the different models are used in the text. The models summarised are; the

Constant Capacitance model (CCM), the Diffuse Layer Model (DLM), the Triple Layer

Model (TLM) and a Charge-Distribution Multi Site Complexation Model (CD_MUSIC).

The differences are mainly coupled to how the charges and potential relationship are

included and corrected for in the mass action equations for surface equilibrium see Figure

1.4.

In DLM the protonation and deprotonation as well as ion adsorption, occur in only one

plane in the interface region (the o-plane), and only these specifically sorbed ions contribute

to the total surface charge density. CCM and DLM both assume only one layer of interface

region but in CCM the charged surface is assumed to be separated from the bulk solution

by a layer of constant capacitance. In TLM the different reactions take place in three

different planes. The deprotonation and protonation naturally take place in the inner plane

and specific sorption in the middle plane whereas the outer plane is described as the diffuse

layer.

CD-MUSIC is based on crystallographic knowledge and within this model the different

crystal planes (the surface basal planes and the surface edges) are included. NEM does not

include any charge or potential relationships instead the adsorption is treated purely as

chemical reactions. The generalised two-layer model was introduced by Dzombak and

Morel [49] and this model is based on the DLM model but includes two site types for cation

binding. These different surface sites are classified as strong or weak sites, depending on the

difference in the affinity to bind a sorbing ion or molecule. The weak sites, with a low

binding affinity, correspond to the total number of reactive sites available for sorption of

cations and are defined by their observed sorption maximum. The strong sites, with a high

binding affinity, are defined by their sorption isotherms, meaning the point where the

sorption density has a slope lower than 1.0 in a log-log isotherm plot [49]. The

11

concentration of strong and weak sites together corresponds approximately to the

concentration of proton binding sites [49]

Figure 1.4. Summary of surface complexation models desctibed in the text.

A difficulty when applying SCM is the distinction between surface complexation and

surface precipitation, reactions that may occur in parallel [42, 51]. The main differences are

that surface complexation takes place in just one single layer, while surface precipitation can

form multiple layers, and that ions complexed to the surface behave differently to

precipitated ionss and thus surface precipitation requires a different model than surface

complexation (SC). It is very difficult to determine the breakpoint where the SC monolayer

formation changes to surface precipitation, in multi-layers [52]. However, Dzombak and

Morel [49] introduced a rule of thumb for when surface precipitation should be considered,

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namely if the dissolved sorbate concentration exceeds (i) one-tenth of its solubility or (ii)

one-half of the total surface site concentration.

To compute the different models, different types of software such as FITEQL [53] and

PHREEQC [54] are frequently used.

The pH where the net surface charge is zero is called zero point of charge (ZPC) or point

of zero charge (PZC). If the change in surface charge is only dependent on the adsorption

of H+ or OH-, the point of zero net proton charge (PZNPC) or the isoelctric point (IEP) is

defined [48, 55]. Stumm and Morgan [44] reported a pHPZNPC for Mg-silicates in the pH

range 9-12.

1.5 Literature survey of the area

Chemical weathering has been thoroughly studied in terms of dissolution rates and

dissolution kinetics and the dissolution kinetics of sheet silicates have been reviewed [56].

The silicates studied include kaolinite [57], biotite [58] and smectite [59] and some studies

even involve chlorite. The early publications regarding chlorite weathering, published in the

mid-1950s, include studies of alteration of chlorite to vermiculite, using techniques such as

XRD. It was found [60, 61] that one parameter has an impact on this alteration, the Fe(II)

content, with a chlorite with high Fe(II) altering faster and directly to vermiculite.

Recent studies of chlorite dissolution are listed in Table 1.1.

13

Table 1.1. Recent publications on dissolutions rates of chlorites.

Reference Reference number pH Dissolution rates(mol m-2s-1)

Comment

Brandt et al. [62] 2-5 10-12 and 10-13 Also AFM Hamer et al. [63] 3.5 8.2810-12 (nitric

acid) 25.5310-12 (citric acid)

Organic and inorganic acid solutions

Ross [61, 64, 65] < 1 10-9.29 Based on graphical data for Mg

May et al. [66] 5 10-12.52 Malmstrm et al.

[67] 8.2 10-11.59 Flow through reactor technique

Rochelle et al.

[68] 9.9 10.2

25C 10-12 70C 1.510-11

25C and 70C

Lowson et al. [69] 4 10-11.66 Using buffers and flow through cell

Like dissolution, sorption has been extensively studied using both macroscopic and

microscopic techniques. Experimental studies of cation and anion sorption to a number of

oxides and minerals have been reviewed [41, 49, 50]. Several studies regarding cation

sorption on chlorite have been reported, but Ni(II) sorption on chlorite is scarcely studied,

Table 1.2.

Table 1.2. Sorption studies onto chlorite. Reference Reference

number Cation studied

Comment

Shawan and Erten [70] Co2+ and Cs+ Different temperatures,30 ,40 , 50 and 60C . Data represented by Freundlich isotherm.

Bond et al. [71] U(VI) and Pu Sorption maximum at pH ~ 8. TLM used.

Zorn [72] U(VI) Sorption maximum at pH ~ 6.5. DLM used

Krawczyk-Brsch et al.

[73] U(VI) Secondary Fe-oxyhydroxide phases formed during dissolution of chlorite

Li et al. [74] (article in Chinese)

Cs+and Yb3 Concentrations used are in the 10-6 and 10-3 M rangr. Data represented by Freundlich isotherm.

Eylym et al. [75, 76] Ba2+ Using synthetic groundwater Koppelman et al. [77] Ni(II) Using XPS technique

14

Ni(II) sorption onto other surfaces has also been studied, as listed in Table 1.3.

Table 1.3. Ni-sorption onto different mineral surfaces. Reference Reference

number Surface studied Comment

Bradbury and Baeyens

[78, 79] Na-montmorillonite Sorption edge and isotherms studies

Scheidegger et al. [80] Pyrophyllite, X-ray absorption fine structure (XAFS)

Dhn et al. [81] Montmorillonite P-EXAFS technique

Titration using suspensions of oxides or minerals is the traditional way to determine their

acid-base properties and a large number of minerals, such as goethite, boehmite and

hydrous ferric oxide, have been thoroughly investigated using this technique [41, 49, 50].

Chlorite, on the other hand, is scarcely studied and this is mainly due to the structural

features of chlorite, primarily due to the high number of existing hydroxyl groups in the

structure [72].

1.6 Objectives of this work

The aims of this work were to characterise chlorite samples and study their dissolution and

sorption behaviour in order to verify the capacity of chlorites to retard radionuclide

migration from the repositories for spent nuclear fuel. These aims were achived through the

following specific objectives:

Characterisation of chlorite - by its composition.

Determination of the dissolution rate of different chlorites in a wide pH range.

Determination of the sorption capacity of chlorite for Ni(II), a divalent metal cation,

under various experimental conditions.

Determination of the acid-base behaviour of chlorite by titrations.

15

2. Experimental

2.1 Chlorites used

Table 2.1 shows the origin and source of the natural chlorites used. The chlorites are named

after their place of origin with the exception of KOV:01, which is the name of the drill core.

Table 2.1. Origin and source of the natural chlorites used. Sample Origin Source Catalog number Taberg Taberg, Vrmland,

Sweden Swedish Museum of Natural History, Stockholm, Sweden

89530

Karlsborg Karlsborg, Vstergtland, Sweden

Swedish Museum of Natural History, Stockholm, Sweden

630491

FlagStaff Hill FlagStaff Hill, USA Source Clay Repository, University of Missouri, Colombia, USA

CCa-2

KOV:01 Oskarshamn, Smland, Sweden

SKB, Sweden Drillcore KOV:01

The chlorite pieces from the collections weighed approximately 30 grams and since

experiments were performed with powder the chlorites were first mechanically crushed to

pieces of ~1 cm2 and a couple of millimetres thick, then either treated with liquid N2 in

cooling-heating cycles or further treated by mechanical crushing using a mill. The powder

was then dry-sieved into different size fractions. During the mechanical treatment and the

sieving, the finest fraction was repeatedly removed so that only the larger mineral particles

were treated. In order to remove ultra-fine particles, the chlorite powder was ultrasonically

washed in ethanol.

The synthetic chlorite was prepared from SiO2, -Al2O3 and MgO in different ratios placed

inside an autoclave at 1.2 kb and 650C for approximately 7 weeks [82].

The chlorite from the KOV:01 core pieces was scraped off the surfaces using a carbide

blade since a iron free tool was preferable in order not to contaminate the samples with Fe.

16

2.2 Reagents

Milli Q water was used in all solutions and either NaClO4H2O (Merck p.a.) or NaCl (Merck

p.a.) was used in different concentrations as background electrolyte throughout the

experiments. pH adjustments were made with small additions of HClO4, HCl, HNO3 or

NaOH (analytical grade).

For the sorption isotherms, the buffers TRIS and MES were used.

The tracer 63Ni was used in the sorption experiments and was prepared from a stock

solution of 63Ni in Ni(II)Cl2 with an activity of 740 MBq mL-1 (PerkinElmer LifeScience,

Inc.). Tracer solutions were prepared by adding small amounts of the 63Ni solution to

inactive solutions of Ni(NO3)2 at a concentration of 10-4, 10-5 or 10-6M, resulting in a total

Ni concentration in the experimental tube of 10-6 M, 10-7 M or 10-8 M.

2.3 Experimental methods

2.3.1 Analytical techniques

The specific surface area was determined by the BET method [83] using a Micrometrics

Flow Sorb II with N2 as adsorbing gas. The relative error from the BET measurement has

been evaluated as 10-15% [59].

The amounts of elements released i.e. Mg, Fe(II,III), Al and Si, were determined using

ICP-AES with a iCAp 6500 Themo Fischer instrument (Nuclear Chemistry, Chalmers

Technical University, Gothenburg) or with a Applied Research Laboratory (ARL) model

3520 B.

In the sorption experiments, the Si concentration was instead determined colorimetrically

by the molybdate method [84] using UV-VIS spectrophotometry with a WPA lightwave

S2000 and plastic cuvette.

In the CEC experiments, the Cu(II)-complex concentration [85] was determined by

UV-VIS using a Varian Cary 300 at 620 nm in a 10 mm cuvette using Milli-Q water as

blank.

17

The Fe2+/Fe3+ ratio was determined by 57Fe Mssbauer spectroscopy at room temperature

at the Department of Earth Science, Solid Earth Geology, Uppsala University, Sweden. The

data collection took 1-2 days depending on the iron content of the chlorites. The Fe2+/Fe3+

ratio of the chlorites investigated was calculated from the fitted areas under the absorption

doublets assuming similar recoil-free fractions of the iron sites.

-activity of 63Ni in aqueous phase was measured using Beta LSC on a Beckman LS1801

instrument or a PACKARD Tri-Carb Liquid Scintillation Analyser Model 1500.

The SEM analysis was performed either with XL 30 ESEM-FEG with a back-scattered

electron detector situated at the Department of Geology and Geochemistry, Stockholm

University (Stockholm, Sweden) or with Zeiss DSM 940 with EDS (Oxford Instruments

link) or Hitachi S-3400N SEM equipped with an INCADryCool Energy Dispersive X-Ray

Spectrometer (EDS) both at the Department of Earth Sciences Centre, University of

Gothenburg, Sweden) or with a JEOL JSM 6490LV with SEI and EDS detector at

Inorganic Chemistry, KTH, Stockholm, Sweden.

XRD was used for identification of chlorite as well as for identification of impurities and

other phases and other minerals in the different samples prior to the experiments. A Rigaku

powder diffractometer and a DANalytical Xpert pro with software packages were used.

The AFM images were taken using a Veeco DIMENSION 3100 system with a Nanoscope

IV controller at INE, Karlsruhe, Germany, and images were recorded using contact mode.

Grains of chlorite were attached to a sample holder with a thin layer of Tempfix

(Neubauer Chemicals, Germany), using a method described by Bickmore et al. [86]. The

AFM images obtained were later processed using the WSxM software (Nanotech

Electronica, Spain).

Solid state NMR was performed by one of the co-authors of Paper III. The instruments

used were Varian/Chemagnetics Infinity-200 and 400 spectrometers operating at 4.7 T and

9.4 T respectively, and the work was carried out at the Department of Physical Chemistry,

18

Arrhenius Laboratory, Stockholm University, Stockholm, Sweden. The experimental details

are given in Paper III and by Edn et al. [87] and Levitt [88]

pH was measured using a Mettler-Toledo InLAB423 combined electrode, saturated with

NaCl or with a Metrohm 713 Ph-meter and a Sentron Steam-Line IntelliProbe combined

glass/reference electrode filled with 3 M LiCl or with an Hamilton slimtrode.

Further details concerning the analytical techniques are given in Papers I-VI.

2.3.2 Experimental set-up

The weathering experiments were performed with a thin-film continuous flow-through

technique [58, 89] in ambient atmosphere, Figure 2.1. The flow rate was adjusted with a

peristaltic pump (ISAMETC IPC High Precision Multichannel Dispenser) to 5 or

2.7 mL h-1. The flow rate was determined for every specific sample using the sampling time

and weight of the sample. Samples were regularly taken during a period up to 30 days. The

pH of the inlet and outlet solutions was regularly measured during the experimental period.

Figure 2.1. The experimental set-up used for the weathering experiments.

The sorption experiments were performed as individual samples using a batch sorption

technique at room temperature, Figure 2.2. The experiments were performed inside a

glovebox in an Ar atmosphere. The sealed tubes were centrifugated in the ambient

atmosphere and then passed into the glove-box for sampling. The pH measurements were

performed after sampling, in ambient atmosphere.

19

Figure 2.2. The experimental set-up used for the sorption experiments.

The Ni(II) sorption was studied as a function of pH, concentration of sorbent,

concentration of sorbate or concentration of background electrolyte, with a reaction time of

7 or 14 days.

The acid-base properties of the chlorite surface were studied using two types of titrations,

conventional continuous titration using a suspension of the chlorite powder and batch

titration on the supernatant with varying periods of contact with the chlorite surface.

All titrations were performed in a glove-box in an N2 atmosphere using a polypropylene

titrations vessel, Figure 2.3. The time between the additions was controlled by an automatic

titrator, and volumes of the additions were controlled by computer software.

More experimental details concerning the different techniques are given in Papers I-VI and

in Appendix A.

20

Figure 2.3. The experimental set-up used for the titration experiments, inside a glove-box.

2.4 Treatment of data

The distribution of the sorbate between the surface and the aqueous phase can be calculated

in different ways. One way is to use the distribution coefficient (Kd) which can be calculated

according to the following formula:

( )

=

gcm3

mCVCC

Kf

fid (2.1)

where Ci and Cf are initial and final activity in solution, V is the volume of solution (cm3)

and m is the mass of dry chlorite (g). The values of Kd obtained from this formula are

expressed in cm3 g-1. Since Kd is system-specific [46], it is important to take the distribution

between the solid phase and the solution into consideration. Percentage sorption can be

calculated using the equation:

( )

i

fi

CCC

sorbed

=100

(%) (2.2)

The percentage of sorbed metal ion does not relate to the actual concentration of sorbed

ion, since the percentage of sorbed ions is a comparison between the initial concentration

and the concentration after reaction. The degree of reversibility of sorption is the relationship between sorption and desorption,

where the fraction really sorbed can be described by:

21

s

d

AA

sorbedcationoffractionthe100

(%)

= (2.3)

where Ad is the amount of desorbed cation and As is the amount of sorbed cation before

the desorption experiments.

The Freundlich isotherm equation is : n

eF CKQ = (2.4)

where Q is the equilibrium concentration of the adsorbed ion (mg g-1) and Ce is the

equilibrium concentrations in the liquid phase (mg L-1). KF is the Freundlich adsorption

coefficient and n is the Freundlich exponent.

The Freundlich isotherm equation can be written in the linear form as :

eF CnKQ logloglog += (2.5)

where KF and n can be calculated from the slope and intercept of the linear plot where KF is

related to the amount sorbed and n the sorption intensity [90]. If n = 1 KF = Kp, where Kp is

the partition coefficient [42].

The dissolution rate for separate elements within the chlorite as well for the chlorite in

general can be expressed in a number of equations. The release rate of an individual

element, rj is calculated based on Equation 2.6 and as dissolution rate of the chlorite

mineral, Rj, see Equations 2.7.

The surface area normalised release rate of an element in the chlorite can be calculated

according to:

= 2

,,reactor,, m s

cation of molesAmcF

dtdC

mAV

AmcF

r jiijijiiji (2.6)

where Fi is the flow rate (L s-1); ci,j is the concentration (mol L-1) of element j in the sample

i; m is the mass of the dry mineral powder (g) and A is the specific surface area (m2 g-1).

The dissolution rate of chlorite based on element j can be calculated according to:

= 2

,, m s

chlorite of moles

j

jiji P

rR (2.7)

22

where Pj is the stoichiometric coefficient for element j (moles of element j per moles of

chlorite).

The accumulated dissolution of chlorite based on element j up to the time ti from the start

of the experiments can be described by:

( )

= 21,

0 mchlorite of moles)()(

iiiji

t

jij ttRdttRti

(2.8)

where Rj is the dissolution rate for each element and ti is the time elapsed from the start of

the experiments.

The dependence of the dissolution rate as a function of pH, taking silica into consideration,

can be described by the empirical rate law:

[ ] [ ]nmj OHkkHkR + + ++= OH0H (2.9)

where ki are the rate constants and m and n are empirically determined constants [31, 32,

91].

Depending on the model for fitting sorption data, a number of input parameters are

needed; surface site density, surface acidity constants, hydrolysis constants and the

concentration of sorbate. For the electrostatic models, values for capacitance and dielectric

constant are also needed [42, 48]. Minor changes in some parameters, such as surface site

density, have a large impact on the results in the fitting procedure, and therefore these

parameters should be changed with caution.

23

3. Results and discussion

3.1 Characterisation of the chlorites used

The characterisation results are general and are not included in one specific publication.

However, Papers III and V includes some of the characterisation results.

The sample from FlagStaff Hill was light green, Taberg sample darker green with black

intrusions, KOV:01 was also dark green with black parts and the Karlsborg sample was

predominantly black. The iron content affects the colour and for the chlorites studied the

higher the Fe content the darker the colour [26].

The specific surface areas for Taberg and Karlsborg chlorites are listed in Tables 3.1 and

3.2.The limited amounts of KOV:01 chlorite made BET determinations impossible to

perform for that sample. Table 3.1 lists the surface areas determined for different fractions

obtained by liquid nitrogen crushing, whereas Table 3.2 shows the surface areas of

mechanically crushed chlorite. These results show that the fractions used for the

experiments were nearly identical, regardless of crushing method, which is particularly

interesting since it has been implied that the mechanical crushing procedure influences the

particles shape and increases reactivity, whereas liquid nitrogen will break along natural

grain boundaries [92].

Table 3.1. Specific surface area of Taberg and Karlsborg, liquid N2 crushing. Fraction m Taberg m2 g-1 Karlsborg m2 g-1 63-118 7.95 0.46 0.59 0.05 118-180 6.73 0.65 0.51 0.06 180-250 6.61 0.79 0.51 0.05 250-335 6.06 0.62 0.48 0.05 335-425 5.22 0.50 0.37 0.01 Table 3.2. Specific surface area of Taberg and Karlsborg, mechanical crushing. Fraction m Taberg m2 g-1 Karlsborg m2 g-1 118-180 6.68 0.73 0.50 0.07 180-250 6.55 0.59 0.49 0.05

24

The range of relative error from BET measurements for the fractions investigated was

estimated to be 4-13% which is in agreement with earlier evaluations on silicates [59].

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

50 150 250 350

size (m)

surf

ace

area

(m2 /g

)

KarlsborgTabergMalmstrm et alBrandt et alLowson et alHamer et alRochelle et alSverdrup

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

50 150 250 350

size (m)

surf

ace

area

(m2 /g

)

KarlsborgTabergMalmstrm et alBrandt et alLowson et alHamer et alRochelle et alSverdrup

Figure 3.1. Surface area as a function of particle size, for the actual chlorite samples together with literature values, references listed in Appendix E.

When the values for Taberg and Karlsborg were compared with other reported surface

areas of chlorite, Taberg was the chlorite with the largest surface area. Malmstrm et al. [67]

reported values half those of Taberg, but they used an another sorbing gas. The specific

surface are was lower than the area reported by Brandt et al. [62] of 1.1 m2 g-1 but quite

similar to that of the chlorite used by Rochelle et al. [68] of 0.89 m2 g-1.

A strong relationship between surface area and size was found for Taberg, while Karlsborg

showed a weaker dependence. Malmstrm et al. [67] reported surface areas for two different

fractions for their chlorite, butr since they used a different sorbing gas for the BET

determinations, comparison with their results is not possible. The specific gas molecules

have unique cross-sections, which affects BET surface area determination [93, 94].

Furthermore, nitrogen has a permanent quadropolar moment whereas argon and krypton

25

are apolar monoatomic gases [95]. The surface area determination is affected by the surface

area and the surface roughness [96].

Chlorites from Taberg and Karlsborg were dissimilar concerning their dependency on

surface area, Figure 3.1. It would be interesting to analyse the surface area in relation to

fraction size for the various chlorites described in the literture [31, 61, 62, 67-69], in order to

compare them with the behaviour observed in our study.

After crushing our samples were treated in the same way. SEM images were obtained in

order to look at the particles and determine wheter the methods gave rise to any differences

which did not affect the surface area, Figure 3.2.

Taberg untreated particle mill used Karlsborg untreated particle mill used

Taberg untreated particle liquid nitrogen used

Karlsborg untreated particle liquid nitrogen used

Figure 3.2. SEM images of untreated Taberg and Karlsborg samples, frozen or mechanically crushed. Note different scales.

The images in Figure 3.2 show that the shape of particles was in agreement and that

ultra-fine particles were formed in both methods used. However, these small particles were

much more frequent for the Taberg samples.

26

AFM was used to characterise topographical structures, on a nanometer scale, of the

chlorite (001) surface. The images clearly reveal that the basal surface of the chlorite used in

our experiments was not atomically flat but contained molecular scale steps. Such

nano-topographic heterogeneity provided additional sorption sites, Figure 3.3.

100 nm100 nm

Figure 3.3. Contact mode AFM image of a chlorite (001) basal surface (scan area: 700 x 700 nm). Molecular scale steps with a height down to one unit cell (14 ) are clearly visible

Even though the chlorite underwent different degrees of isomorphous substitution,

resulting in a negative charge on the surface, the CEC observed at pH 7.5 for both Taberg

and Karlsborg was low. This is in agreement with earlier observations [20, 97]. Taberg has a

CEC of 4.0 meq/100 g (=cmol/kg) and Karlsborg a CEC of 1.4 meq/100 g. Both these

results are in the same range as published previously for chlorites [20, 97] and both results

indicate a very low CEC.

Chemical composition of the chlorites Taberg, Karlsborg and FlagStaff Hill were

determined by ICP-AES at Analytica AB, Lule, Sweden, Table 3.3. SEM-EDS analysis

confirmed the content of the major oxides (SiO2, Al2O3, Fe2O3, MgO).

Values of the ratio between Fe(II) and Fe(III) obtained from Mssbauer spectroscopy,

presented as percentage Fe(III) of the total amount Fe(II + III) for the chlorites used, are

listed in Table 3.5. For Karlsborg, it was found that the Fe(III) was tetrahedrally

coordinated whereas the other samples had octahedral coordination of both Fe(II) and

700 nm700 nm

27

Fe(III). Octahedrally coordinated Fe(II) and Fe(II) is in accordance with other

characterisations of chlorites [62, 66, 67, 69].

The FlagStaff Hill chlorite had too low iron content for Mssbauer determination.

Table 3.3. Chemical composition of chlorite samples, determined by Analytica AB, Lule, Sweden. % TS Taberg Karlsborg FlagStaff Hill SiO2 33.6 30.0 30.7 Al2O3 13.7 19.3 22.9 MgO 32.9 19.8 31.1 Fe2O3 5.95 16.1 1.42 K2O 0.68 0.84 0.063 CaO

28

KOV:01: (Mg6.34FeII1.12FeIII0.78Al2.47 1.29)(Si6.91Al1.09)O20(OH)16

Table 3.4. Major oxides of KOV:01 sample, achivied from SEM-EDS determination. % oxide SiO2 Al2O3 Fe2O3 MgO CaO K2O MnO2 Na2O

KOV:01 34.4 15.4 12.9 21.6 0.55 0.12 0.51 n.d.

The vacancies ( ) are not always presented within the chemical formulas since they may be

a result of random isomorphous substitution, which will leads to less crystallised structures.

For a tri-octahedral chlorite, all the tetrahedral and octahedral positions should be occupied,

but when using the percentage of oxides and filling the eight tetrahedral positions there is a

divergence within the octahedral position, which may be due to LOI (loss of ignition) or, as

mentioned, isomorphous substitution.

Due to the limited amount of synthetic chlorite its chemical composition was not

determined. The FlagStaff Hill sample was not used for sorption or weathering experiments

due to its very low iron content, since iron was among the elements of major interest.

Table 3.5 Fe(III)/(Fe(II)+Fe(III)) data for the different chlorite samples.

Chlorite Fraction of

Fe(III)/(Fe(II)+Fe(III)) %

Taberg 29

Karlsborg 16

FlagStaff Hill n.d.

KOV:01 41

Taberg and Karlsborg chlorites were chosen based on differences in their chemical

composition.

29

3.2 Dissolution results

This chapter summarises and discusses the results obtained in Paper I and Paper VI.

The release of the elements Al, Fe, Mg and Si was monitored over time throughout the

flow-through experiments. Figure 3.4 shows the release rates of the different elements for

the Taberg and Karlsborg samples. A high initial release rate declining to a stable value is

the general dissolution behaviour for silicates. This behaviour was observed for the Taberg

sample at pH 2, where the first 50 hours showed a drastic decrease in cation release, Figure

3.4. For the Karlsborg sample at pH 2, this behaviour was monitored for Mg, but when the

other three elements were studied, a continuous release at low level was detected. Both

chlorites displayed a minimum released concentration at neutral pH, Figure 3.5. The

behaviour of the Taberg sample at pH 2 was in agreement with other mixed-flow reactor

investigations, and it acted in the same manner as chlorite, kaolinite and biotite [57, 58, 62].

Experimental steady-state conditions were reached after approximately 15 days, but

sampling continued for several days after this in order to ensure that no temporal plateau

had been reached. In recent studies of chlorite dissolution, Lowson et al. [69] reported that

the time needed to reach experimental steady state conditions varied between 10 to 50 days

and Brandt et al. [62] reported that in their system the steady state was reached after 48

hours. The buffer solutions used by Lowson et al. [69] seem not to affect the time elapsed to

steady state. However, comparison in the initial dissolution phase however is difficult, since

few samples were collected during that period [69].

The high release rate in the beginning of the experiments depended on 1) fine particles at

the surface not been removed during the washing processes 2) fresh surface being used for

each experiments and 3) dissolution of reactive sites at the chlorite surface introduced

during the crushing procedure.

SEM images confirmed that there were some really small particles, ~ 2-5 m, left after the

washing procedure, independent of crushing method.

30

Taberg pH 2Lo

g r j

(mol

es/m

2 s)

time (days)

-12

-11

-10

-09

-08

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Taberg pH 2Lo

g r j

(mol

es/m

2 s)

time (days)

-12

-11

-10

-09

-08

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Taberg pH 4

Log

r j (m

oles

/m2 s

)

time (days)

-14

-13

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Taberg pH 4

Log

r j (m

oles

/m2 s

)

time (days)

-14

-13

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Log

r j (m

oles

/m2 s

)

Taberg pH 12

time (days)

-13

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Log

r j (m

oles

/m2 s

)

Taberg pH 12

time (days)

-13

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

31

time (days)

Log

r j (m

oles

/m2 s

)Karlsborg pH 2

-11

-10

-09

-08

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

time (days)

Log

r j (m

oles

/m2 s

)Karlsborg pH 2

-11

-10

-09

-08

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Log

r j (m

oles

/m2 s

)

time (days)

Karlsborg pH 4

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

Log

r j (m

oles

/m2 s

)

time (days)

Karlsborg pH 4

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

MgFeAlSi

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

time (days)

Log

r j (m

oles

/m2 s

)

MgFeAlSi

Karlsborg pH 12

-12

-11

-10

0 2 4 6 8 10 12 14 16 18 20 22

time (days)

Log

r j (m

oles

/m2 s

)

MgFeAlSi

Karlsborg pH 12

Figure 3.4 Logarithm release rates of the different elements over a time-scale up to 22 days for Taberg and Karlsborg in mol m-2 s-1. Note that there are different scales in the plots.

32

For the dissolution rates at steady state conditions, the last measured values were used after

approximately 22 days, and are tabulated in Table 3.6.

In Paper I, Figure 3, the steady-state release rates of Si as a function of pH are published.

Lowson et al. [69] have misinterpreted our release rates, since they automatically assumed

that the data were normalised, which was not the case.

Table 3.6 Dissolution rates, normalised with respect to the stoichiometry and the final values after 22 days. Sample RAl mol m-2 s-1 RFe mol m-2 s-1 RMg mol m-2 s-1 RSi mol m-2 s-1 Taberg pH 2 1.00 10-12 1.24 10-12 6.86 10-13 1.29 10-12 Taberg pH 4 3.94 10-13 2.77 10-14 2.24 10-12 2.6610-13 Taberg pH 10 2.64 10-13 4.91 10-14 1.10 10-11 4.74 10-14 Taberg pH 12 6.20 10-13 2.02 10-13 - 1.79 10-13 Karlsborg pH 2 6.75 10-11 5.07 10-11 3.91 10-13 4.20 10-12 Karlsborg pH 4 1.79 10-12 3.90 10-12 5.36 10-12 2.33 10-12 Karlsborg pH 10 2.82 10-12 8.5010-14 2.74 10-13 8.99 10-13 Karlsborg pH 12 5.9710-11 7.8310-13 1.6610-12 9.57 10-13

Since Table 3.1 and Table 3.2 together with Figure 3.1 and Appendix E show that the BET

surfaces were quite different between the chlorites normalisation to mol g-1s-1 was used

instead of the mol m-2 s-1 which are listed in Table 3.6.

Table 3.7 Dissolution rates, normalised with respect to the stoichiometry and the final values after 22 days. Sample RAl mol g1 s1 RFe mol g1 s1 RMg mol g1 s1 RSi mol g1 s1 Taberg pH 2 6.70 10-12 8.31 10-12 4.60 10-13 8.64 10-12 Taberg pH 4 2.64 10-11 1.86 10-13 1.50 10-11 1.7810-11 Taberg pH 10 1.77 10-12 3.29 10-13 6.70 10-11 3.18 10-13 Taberg pH 12 4.15 10-12 1.35 10-12 - 1.20 10-12 Karlsborg pH 2 3.44 10-11 2.59 10-11 1.99 10-13 2.14 10-12 Karlsborg pH 4 9.10 10-13 1.99 10-12 2.73 10-12 1.19 10-12 Karlsborg pH 10 1.44 10-12 4.34 10-14 1.40 10-13 4.58 10-13 Karlsborg pH 12 3.05 10-11 3.99 10-13 8.48 10-13 4.88 10-13

33

-14.00

-13.50

-13.00

-12.50

-12.00

-11.50

-11.00

-10.50

-10.000 2 4 6 8 10 12 14

pHLo

g R

(Si)

mol

/gs

TabergKarlsborgBrandt et alLowson et alRochelle et alMalmstrm et alHamer et al

-14.00

-13.50

-13.00

-12.50

-12.00

-11.50

-11.00

-10.50

-10.000 2 4 6 8 10 12 14

pHLo

g R

(Si)

mol

/gs

TabergKarlsborgBrandt et alLowson et alRochelle et alMalmstrm et alHamer et al

Figure 3.5 Logarithm steady state chlorite dissolution rates (based on Si data) as a function of pH.

The dissolution rates of chlorite for both Taberg and Karlsborg showed that the assumed

pH dependency and the behaviour of chlorite followed the schematic picture for silicates,

even though the specific dissolution rates diverged. For example, for pH 2 the dissolution

rates calculated from Si data differed by almost one order of magnitude. The same

difference was observed for rates calculated for Al, while Fe and Mg differed by a factor

~ 2.

At pH 2 the dissolution rate for Karlsborg was similar to that obtained by Brandt et al. [62]

and at pH 7.4 the dissolution rate for Taberg was in agreement with that reported by

Rochelle et al. [68] and Lowson et al. [69]. Larger differences in dissolution rate were

observed in the acidic pH range in comparison to basic pH l, which could be caused by the

difference in composition between the chlorites studied and the higher degree of dissolution

at that pH.

The dissolution rate for chlorite reported by May et al. [66] increased with increasing acidity,

which was also observed for Taberg and Karlsborg chlorites. Rochelle et al. [68] analysed Si,

Mg, Al and Fe at different pH, in the neutral to basic range, varying the pH and

34

temperature. They observed that the dissolution rate increases with increasing pH, which is

in line with our observations.

When the empirical rate law, Equation 2.9, was applied to the Si data in Table 3.7, the

constants and reaction orders were calculated using a weighted non-linear least-square

regression with kH = 1.9010-11 mol g-1 s-1, kn 1.010-13 mol g-1 s-1 and kOH = 1.0410-12

mol g-1 s-1 and the reaction orders in this study were m = 0.32 and n = 0.14. The vaules

obtained in athis study were than the m= 0.5 obtained by May et al. [66], m = 0.49 and n =

0.43 determined by Lowson et al. [69] and n = 0.30 obtained by Rochelle et al. [68], although

our value was in agreement with m= 0.29 0.05 obtained by Brandt et al. [62]. The reaction

order of the overall rate law shows the release of H4SiO4 into solution, where Si from the

surface is released due to proton-promoted dissolution, which is apparent as the pH

dependency of the dissolution [98]. A higher number of reaction order m indicates that the

dissolution requires a higher degree of protons present.

-14

-13

-12

-11

-10

1 3 5 7 9 11 13pH

Log

RSi

[mol

g-1

s-1

]

TabergKarlsborg

Figure 3.6. Logarithm steady-state dissolution rates calculated from Si data. The curve shows the calculated rate according to the empirical rate law in Equation 2.9.

The accumulated dissolution for the Taberg and the Karlsborg chlorites based on the

elements studied as a function of time for some pH values investigated is for the Taberg

presented in Figure 3.6. As in Figure 3.1, the fast reaction of chlorite in the beginning of the

experiments indicates by a rapid increase in the accumulated dissolution in Figure 3.6.

35

Since no studies of eventual formation of secondary phases were performed in these

dissolution studies, the present data were not interpreted in terms of congruent or non-

congruent dissolution. Instead, the terms stoichiometric and non-stoichiometric dissolution

were used.

A release ratio lower than start ratio indicates that Si is preferentially released, while if the

ratio is higher than the start ratio Mg, Fe or Al is preferentially released compared with Si.

Kalinowski and Schweda [99] discuss the stoichiometry as ratios as in Table 3.8.

Holdren and Speyer [100] and Stillings and Brantley [101] discuss stoichiometry as relative

release ratios, RRRx. When RRRx = 1 the observed dissolution is stochiometric and when

the ratios is < 1 Si is preferentially released compared with the other cation, while if the

ratio is > 1 the cation x is preferentially released compared with Si.

RRRx = (x/Si)solution/(x/Si)starting material (3.2)

where x = Al, Mg or Fe.

For Taberg the following observations were made: At pH 2 in the beginning of the

experiments there was preferential release of Mg, Fe and Al, while at steady-state Si was

preferentially released. Both initial and final releases were non-stoichiometric, but Si and Al

were close to stoichiometry at the end of the experiments. For pH 4 Si was preferentially

released compared with Fe and Al throughout the whole experiments, while when Mg and

Si were compared a different behaviour was observed, for the first 15 days Si was clearly

preferentially released, while at the end of the experiments the reverse occured. At pH 12,

Al and Si were close to stochiometric release in the beginning of the experiment but at

steady state Al was preferentially released. When comparing Fe and Si, in the beginning of

the experiments Si was preferentially released and at steady state the release was close to

stoichiometry.

For Karlsborg the following observations were made: At pH 2 in the beginning of the

experiments Al, Mg and Fe were preferentially released compared with Si and at steady state

Fe and Al were still preferentialyl released compared witho Si, while for Mg and Si the

reversed occured. When pH was increased to 4, Mg and Al were still preferentially released

in the beginning of the experiment. When comparing Fe and Si, Si was the element that was

preferentially released. At steady state, on the other hand, Si was preferentially released

compared with Al, Fe and Mg. At the most basic pH studied, pH 12, the Fe/Si ratio was

more or less constant throughout the whole experiment, which could be explained by low

release of Fe and possible formation of secondary phases during the dissolution experiment,

36

which PHREEQC calculations indicated. This has also been observed by AFM

investigations [73]. The same possible formation of secondary phases during the dissolution

was observed for Mg. However, Mg and Al were preferentially released in the beginning of

the experiments and for Al and Si the same releationsship was observed at steady-state,

while the reverse occured for Mg and Si.

To summarise, Taberg and Karlsborg cation release was non-stochiometric for the elements

studied, which is in agreement with the observations by Brandt et al. [62] for pH 2-4. Hamer

et al. [63] reported non-stoichiometric dissolution with a preferential release of Si relative to

Al and Fe and in some cases even to Mg. Ross [65] also reported that Mg, Fe and Al

dissolved at the same rate in Si-saturated 2N HCl at 60C and suggested a dissolution

process where the hydroxide sheet and the 2:1 sheet within the chlorite structure were

equally dissolved. Our observations contradict the rates reported by Lowson et al. [69] Ross

[61, 64] since they stated that congruent dissolution takes place.

Table 3.8. Release rates ratios compared with ratio in starting material for Taberg and Karlsborg samples. Mg/Si Fetot/Si Al/Si TABERG Starting material 1.46 0.13 0.48 pH 2 1 day 2.62 0.22 0.59 22 days 0.88 0.08 0.43 pH 4 1 day 0.001 0.004 0.13 22 days 7.05 0.01 0.25 pH 12 1 day 0 0.05 0.51 22 days 0 0.14 2.69 KARLSBORG Starting material 0.97 0.40 0.75 pH 2 1 day 22.05 0.94 1.83 22 days 0.07 6.31 15.4 pH 4 1 day 1.34 0.06 0.96 22 days 0.68 0.24 0.40 pH 12 1 day 40.17 0.14 4.16 22 days 0.60 0.14 2.69

37

We let the accumulated release of Mg represent the octahedrally coordinated cations and Si

represent the tetrahedrally coordinated cations as suggested by Lin and Clemency [102].

Using the accumulated releases at steady state, Figure 3.8, we observed that Mg amount was

twice as high as the Si amount, which indicates a preferential release of the octahedrally

coordinated cations at pH 2. At pH 4 and 12 the opposite behaviour was observed for

Taberg. For Karlsborg, for the three pH presented in Figure 3.8, Mg was preferentially

released when steady state was achived. At pH 4 for Karlsborg, the accumulated amount of

Mg was six times higher than that of Si while at pH 12 Mg was approximately ten times

higher than Si, which indicates that the octahedrally coordinated cations were preferentially

released for this sample.

Brandt el al. [62] observed preferential release of octahedrally coordinated cations up to pH

4, while Hwang [103] observed that Al, Mg and Fe are preferentially released compared

with Si under acidic conditions.

The Karlsborg sample and the chlorite used by Brandt et al. [62] behaved in the same way in

the pH range 2-4 while Taberg exhibited different behaviour at pH.

SEM images were collected for the weathered samples after the flow-through experiments,

Figure 3.7. At the surface of the weathered particles from pH 2 and 12 a pattern of channels

was observed, which is in agreement with the observations at acidic pH for other chlorites

following dissolution experiments [61, 62]. These channels have been denoted dissolution

channels in previous work [61].

Figure 3.7. SEM imgages of weathered Taberg chlorite particles at pH 2.

38

0

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 2

Time (days)

Mol

es c

hlor

ite/g

0

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 2

Time (days)

Mol

es c

hlor

ite/g

0

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 2

Mol

es c

hlor

ite/g

Time (days)

0

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 2

0

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 2

Mol

es c

hlor

ite/g

Time (days)

0

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

9.00E-06

1.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 4

Time (days)

Mol

es c

hlor

ite/g

0

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

9.00E-06

1.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 4

0

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

9.00E-06

1.00E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 4

Time (days)

Mol

es c

hlor

ite/g

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

0 2 4 6 8 10 12 14 16 18 20 22

SiFeMgAl

Karlsborg pH 4

Mol

es c

hlor

ite/g

Time (days)

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

0 2 4 6 8 10 12 14 16 18 20 22

SiFeMgAl

Karlsborg pH 4

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

0 2 4 6 8 10 12 14 16 18 20 22

SiFeMgAl

Karlsborg pH 4

Mol

es c

hlor

ite/g

Time (days)

39

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 12M

oles

chl

orite

/g

Time (days)

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 12

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

Al

Fe

Si

Mg

Taberg pH 12M

oles

chl

orite

/g

Time (days)

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 12

Mol

es c

hlor

ite/g

Time (days)

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 12

0

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

0 2 4 6 8 10 12 14 16 18 20 22

AlSiFeMg

Karlsborg pH 12

Mol

es c

hlor

ite/g

Time (days) Figure 3.8. Accumulated dissolution of chlorites, in moles chlorite g-1. Note: Difference in the vertical scale at pH 4 for Taberg and Karlsborg.

40

41

3.3 Sorption results

This chapter presents and discusses the sorption of Ni(II) on chlorites. The experiments

were performed under a number of varying experimental conditions, see further in Papers II,

IV and V for details and Appendix C for tabulated sorption results.

In Figure 3.9 the sorption is presented as Kd as a function of pH for three different

background electrolyte concentrations and one of the initial Ni(II) concentrations, 10-6 M for

Karlsborg chlorite at a concentration of 5 g/L. The Kd values increased by three orders of

magnitude in the pH range 4-9.5, with a Kd maximum of 1700 cm3/g at pH 8.4. Figure 3.10

shows the three different initial nickel concentrations together with the three different

background electrolyte concentrations as a function of pH for Karlsborg chlorite with a solid

to solution (s:s) ratio of 5g/L. The typical sorption edge appearance of the sorption curve

was observed where the sorption increased over a limited pH range which is typical for

pH- dependent sorption.

[Ni]initial = 106 M

0.00.51.0

1.52.02.53.0

3.54.0

4 5 6 7 8 9 10 11pH

log

Kd

(cm

3 /g)

0.01 M [NaClO4]0.1 M [NaClO4]0.5 M [NaClO4]

[Ni]initial = 106 M

0.00.51.0

1.52.02.53.0

3.54.0

4 5 6 7 8 9 10 11pH

log

Kd

(cm

3 /g)

0.01 M [NaClO4]0.1 M [NaClO4]0.5 M [NaClO4]

Figure 3.9. Sorption as a function of pH and ionic strength for Ni concentration 10-6 M, Karlsborg chlorite.

42

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4 5 6 7 8 9 10

pH

log

Kd

(cm

3 /g)

0.01 M, 10-6 M0.1 M, 10-6 M0.5 M, 10-6 M0.01 M,10-8 M0.1 M,10-8 M0.5 M, 10-8 M0.1 M,10-7 M0.01 M 10-7 M0.5 M, 10-7 M

[NaClO4] [Ni(II)]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4 5 6 7 8 9 10

pH

log

Kd

(cm

3 /g)

0.01 M, 10-6 M0.1 M, 10-6 M0.5 M, 10-6 M0.01 M,10-8 M0.1 M,10-8 M0.5 M, 10-8 M0.1 M,10-7 M0.01 M 10-7 M0.5 M, 10-7 M

[NaClO4] [Ni(II)]

Figure 3.10. Sorption as a function of pH and ionic strength for the three different Ni(II) concentrations onto Karlsborg chlorite.

Figure 3.11 shows the sorption data for Taberg and Karlsborg as percentage sorbed Ni(II) as

a function of pH. One initial Ni(II)l concentrations and three different ionic strengths were

analysed, using an s:s ratio of 5 g/L for both chlorites.. Both chlorites behaved in a similar

way, showing a strong pH dependency and no dependence of background electrolyte

concentration, which was the expected behaviour for Ni(II) sorption onto chlorite.

Bradbury and Baeyens [78, 79] studied sorption as a function of pH and found that for Ni

there were two main mechanisms that controlled sorption, one pH independent and one pH

dependent. The pH independent mechanism was identified as ion exchange and the pH

dependent mechanism as surface complexation. However, those authors studied the sorption

onto montmorillonite which has a high CEC value. They observed that for the region where

the sorption is more or less constant, the plateau-like area appears at different pH values for

the different background electrolyte concentrations, but no such observation was found in

the Ni(II) chlorite system.

43

[Ni(II)] [NaClO4]

Karlsborg 10-6 M, 0.01 M

0

10

20

30

40

50

60

70

80

90

100

4 5 6 7 8 9 10 11 12

pH

% s

orbe

dN

i(II)

Taberg 10-6 M, 0.01 M

Taberg 10-6 M, 0.1 M

Karlsborg 10-6 M, 0.1 M

Taberg 10-6 M, 0.5 M

Karlsborg 10-6 M, 0.5 M

[Ni(II)] [NaClO4]

Karlsborg 10-6 M, 0.01 M

0

10

20

30

40

50

60

70

80

90

100

4 5 6 7 8 9 10 11 12

pH

% s

orbe

dN

i(II)

Taberg 10-6 M, 0.01 M

Taberg 10-6 M, 0.1 M

Karlsborg 10-6 M, 0.1 M

Taberg 10-6 M, 0.5 M

Karlsborg 10-6 M, 0.5 M

Figure 3.11. Percentage of Ni(II) sorbed onto Karlsborg and Taberg chlorites.

Lack of dependence on ionic strength confirms that Ni(II) sorbs onto chlorite by surface

complexation and that the sorption occurs through inner-sphere complex rather than outer-

sphere [47], still Koppelman et al. [77] in their work found that Ni(II) adsorbed to chlorite as

a hexa-aquo ion, Ni(H2O)62+, due to the binding energies observed in the XPS spectrum. The

observed binding energies are not similar to the binding energies of Ni(OH)2 or NiO.

According to Ltzenkirchen [47], Ni(II) could not been sorbed as an aquo ion since that

wo