removal of phosphate ions from aqueous solutions using bauxite and kaolinite obtained from malawi

8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi

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REMOVAL OF PHOSPHATE IONS FROM AQUEOUS SOLUTIONS USING

BAUXITE AND KAOLINITE OBTAINED FROM MALAWI

M.Sc. (Applied Chemistry) Thesis

By

MOSES WITNESS KAMIYANGO

B.Ed (Science)-University of Malawi

Submitted to the Department of Chemistry, Faculty of Science, in fulfilment of the

requirements for the degree of Master of Science (Applied Chemistry)

University of Malawi

Chancellor College

September, 2009


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DECLARATION

I the undersigned hereby declare that this thesis is my own original work which has

not been submitted to any other institution for similar purposes. Where other peoples

work has been used acknowledgements have been made.

___________________________________________________________________________________________________

__Full Legal Name

____________________________________

Signature

____________________________________

Date


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CERTIFICATE OF APPROVAL

The undersigned certify that this thesis represents the students own work and effort

and has been submitted with our approval.

Signature: _______________________ Date: ____________________________

Masamba W.R.L., PhD (Professor)

Main Supervisor

Signature: _______________________ Date: ____________________________

Sajidu S.M.I., PhD (Associate Professor)

Member, Supervisory Committee

Signature: _______________________ Date: ____________________________

Fabiano E., PhD (Senior Lecturer)

Member, Supervisory Committee

Signature: _______________________ Date: _____________________________

Mwatseteza J.F., PhD (Senior Lecturer)

Head, Chemistry Department


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DEDICATION

I dedicate this thesis to my sister Funny and brother William who supported me

throughout my formal education. I also dedicate this work to my late grandmother

Motsetse and late sister Miriam who through their demise I have found courage and

strength to complete my work.


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ACKNOWLEDGEMENTS

I wish to thank all members of my supervisory team: Professor W.R.L. Masamba,

Associate Professor S.M.I. Sajidu and Dr. E. Fabiano for their support. I

acknowledge invaluable support offered by the late Professor E.M.T. Henry during

the early stages of my studies. Acknowledgements should also go to the laboratory

staff in the Chemistry Department of Chancellor College for their assistance.

I also thank the International Science Programme (ISP) through the International

Programme in the Chemical Sciences (IPICS) at Uppsala University for the

fellowship. Finally, I thank God for the good health during my study, and for allowing

me to complete this work.


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ABSTRACT

Surface waters in some parts of Malawi are known to contain high concentrations of

phosphate ions due to, among other reasons, the discharge of incompletely treated

municipal and industrial wastewaters into streams. High levels of phosphates in

surface waters pose a threat to aquatic life as such there is a need for materials that

can bind phosphate ions in wastewater during treatment. This thesis therefore

concerns characterisation of locally sourced kaolinite to determine its point of zero net

proton charge and bench studies on removal of phosphate ions from aqueous solutions

by locally sourced kaolinite and bauxite.

Raw bauxite, raw kaolinite, and treated kaolinite (at a dosage of 10 g/L) reduced

concentration of phosphate ions in solutions by 93.2 0.152, 19.3 0.344, and 50.6

0.436 % respectively. Raw bauxite was more effective than kaolinite because of the

presence of minerals that have high affinities for phosphate ions, namely, goethite and

gibbsite.The phosphate removal capacity of kaolinite increased after acid treatment

arguably due to the release of more calcium ions from calcium carbonates that were

present in the kaolinite samples as impurities.


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The percent removal of phosphate ions increased with decreasing pH for bauxite, with

high removal achieved below pH 5. This trend was attributed to a ligand exchange

reaction mechanism involving phosphate ions and reactive hydroxyl groups on the

goethite and gibbsite surfaces. Phosphate removal by both raw and treated kaolinite

was achieved through an ion exchange mechanism between pH 2 and 5, whilst

precipitation of hydroxyapatite dominated above pH 7.

Carbonate ions inhibited precipitation of hydroxyapatite and competed with

phosphate for adsorption sites resulting in reduced capacities for kaolinite and bauxite

respectively. Sulphate ions reduced the phosphate removal capacity of bauxite by

competing for active sites with phosphate ions. Both calcium and magnesium ions

enhanced phosphate precipitation by kaolinite and phosphate adsorption on bauxite.

Calcium and magnesium ions enhanced adsorption of phosphate ions through

electrostatic interactions whereas precipitation was enhanced through an increase in

calcium ions, and reduction of carbonate ions in solution as a result of the formation

of magnesium-carbonate ion pairs. Kaolinite recorded a very low phosphate removal

capacity as such cannot be used as a phosphate removing agent during wastewater

treatment. This is in contrast to bauxite which recorded a higher phosphate removal

capacity.


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TABLE OF CONTENTS

ABSTRACT .................................................................................................................. vi

LIST OF FIGURES ...................................................................................................... xii

LIST OF TABLES .......................................................................................................xiv

LIST OF ACRONYMS AND ABBREVIATIONS .................................................. ...... xv

CHAPTER ONE: INTRODUCTION ...........................................................................1

1.1 Background ................................................. ......................................... ........1

1.2 Problem statement ................................................................................ ........2

1.3 Aim and objectives of the study and thesis outline ........................................4

1.3.1 Aim of the study .....................................................................................4

1.3.2 Specific objectives ..................................................................................4

1.3.3 Thesis outline .........................................................................................4

CHAPTER TWO: LITERATURE REVIEW ..............................................................5

2.1 Chemical forms of phosphorus in water .............................. ..........................5

2.2 Sources of phosphates in wastewater ........... ......................................... ........8

2.3 Effects of excess phosphorus on aquatic ecosystems: Eutrophication .......... 10

2.4 Levels of phosphate pollution in Malawi .................................................... 11

2.5 Methods for phosphate removal during wastewater treatment ..................... 12

2.5.1 Chemical precipitation ................... ......................................... ...... 12

2.5.2 Biological phosphorus removal ..................................................... 14

2.5.3 Crystallisation technology ............................................................. 15


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2.5.4 Ion exchange ........... ...................................................................... 16

2.5.5 Magnetic attraction ........................................................................ 17

2.5.6 Low cost materials for phosphate removal from wastewater .......... 17

2.5.6.1 Phosphate precipitating low cost materials ............................... 17

2.5.6.2 Low cost phosphate adsorbents ............................................... 18

2.6 Chemical composition and acid-base properties of Linthipe kaolinite and

Mulanje bauxite. ................... ...................................................................... 19

2.6.1 Gibbsite ................... ...................................................................... 20

2.6.2 Goethite ........................................................................................ 22

2.6.3 Kaolinite ........... ............................................................................ 23

2.7 Complexation and adsorption ..................................................................... 26

2.7.1 Development of reactive functional groups at the metal oxide-

solution interface ........................................................................... 27

2.7.2 Adsorption of ions at the metal (hydr)oxide-solution interface :

The electrostatic double layer model ............................................. 28

2.8 Protonation of surface functional groups and charge balance ...................... 30

2.9 Patch-wise surface charge heterogeneity on kaolinite and the point of

zero charge ................................................................................................. 34

2.10 Modeling phosphate adsorption on kaolinite and bauxite ............................ 34

2.11 Precipitation of calcium phosphates ............................................................ 35

CHAPTER THREE: MATERIALS AND METHODS ............................................. 38

3.1 Materials .................................................................................................... 38

3.1.1 Adsorbents .................................................................................... 38

3.1.2 Chemicals, reagents and instruments ..................... ........................ 38

3.2 Methods ..................................................................................................... 39


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3.2.1 Preparation of kaolinite and bauxite samples ................................. 39

3.2.2 Preparation of solutions ................................................................. 39

3.2.2.1 Reagents .................................................................................... 39

3.2.2.1.1 Sodium Carbonate (0.01 mol/L and 1000 mg/L) .................. 39

3.2.2.1.2 Nitric acid (0.02359 mol/L and 1+1) ........... ........................ 40

3.2.2.1.3 Lanthanum solution ............................................................. 40

3.2.2.1.4 Sodium hydroxide (0.020 mol/L) ........................................ 40

3.2.2.1.5 Hydrochloric acid (0.3 mol/L) ..................... ........................ 40

3.2.2.1.6 Sodium nitrate (1.0 mol/L) .................................................. 41

3.2.2.1.7 Sulphate solution (1000 mg/L) .................... ........................ 41

3.2.2.1.8 Magnesium solution (1000 mg/L) ....................................... 41

3.2.2.1.9 Calcium solution (1000 mg/L) ..................... ........................ 41

3.2.2.2 Standard solutions ...................................................................... 42

3.2.2.2.1 Standard phosphate solution ................................................ 42

3.2.2.2.2 Standard calcium solution (for AAS determination of

calcium). ............................................................................ 42

3.2.3 Determination of phosphate ions in solution using ion chromatography ...... 42

3.2.4 Determination of Calcium ions in solution............. ........................ 43

3.2.5 Potentiometric titration of raw kaolinite samples ........................... 44

3.2.6 Effect of suspension pH on the amount of phosphate ions

removed by bauxite and kaolinite .................................................. 45

3.2.7 Determination of calcium ions released into solution from

kaolinite ........................................................................................ 45

3.2.8 Effect of bauxite and kaolinite dosage on amount of phosphate

ions removed ........... ...................................................................... 46

3.2.9 Effect of contact time on the amount of phosphate ions removed

by bauxite and kaolinite................................................................. 47


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3.2.10 Effect of initial phosphate concentration on the phosphate

removal capacity of bauxite and kaolinite ...................................... 48

3.2.11 Effect of magnesium, calcium, sulphate and

carbonate/bicarbonate ions on amount of phosphate ions

removed by bauxite and kaolinite .................................................. 49

3.2.12 Multi-interactive effect of magnesium, calcium, sulphate, and

bicarbonate ions on the phosphate removal efficiency of bauxite. .. 50

CHAPTER FOUR: RESULTS AND DISCUSSION ......... .......... ......... .......... .......... .. 51

4.1 Point of zero net proton charge for kaolinite ............................................... 51

4.2 Effect of suspension pH on the amount of phosphate ions removed by

kaolinite and bauxite .................................................................................. 53

4.2.1 Description of reactions resulting in removal of phosphate ionsby kaolinite and bauxite................................................................. 56

4.3 Effect of kaolinite and bauxite dosage on the amount of phosphate ions

removed ..................................................................................................... 61

4.4 Effect of contact time on the amount of phosphate ions removed by

bauxite and kaolinite .................................................................................. 64

4.5 Effect of initial phosphate concentration on the phosphate removal

capacity of bauxite and kaolinite ................................................................ 71

4.6 Effect of calcium, magnesium, sulphate and carbonate ions on phosphate

uptake by raw and treated clay............................................ ........................ 75

4.7 Effect of magnesium, calcium, sulphate and carbonate/bicarbonate ions

on phosphate uptake by bauxite ................... ............................................... 79

4.8 Multi-interactive effect of magnesium, calcium, sulphate, and bicarbonate

ions on the phosphate removal efficiency of bauxite. .......... ........................ 83

CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS ................. ...... 85

5.1 Conclusions ................................................. ............................................... 85

5.2 Recommendations ...................................................................................... 88

REFERENCES ............................................................................................................ 90


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LIST OF FIGURES

Figure 1: Structures of cyclotriphosphate and cyclotetraphosphate ...................................6

Figure 2: A schematic representation of a polyphosphate .................................................7

Figure 3: A schematic representation of a branched inorganic phosphate .........................7

Figure 4: Chemical phosphate precipitation. ................... ............................................... 12

Figure 5: Biological phosphorus removal ....................................................................... 14

Figure 6: The DHV Crystalactor .................................................................................... 15

Figure 7: Sphere model of gibbsite structure.. ........................................ ........................ 21

Figure 8: Ring structure model for gibbsite .................................................................... 21

Figure 9: Ball and stick model for goethite. .................... ............................................... 22

Figure 10: Silica tetrahedron and layer. .................................................. ........................ 24

Figure 11: Clay octahedron and octahedral sheet. ........... ............................................... 24

Figure 12: Kaolinite structure ........................................................................................ 25

Figure 13: The electrostatic double layer model ............................................................. 29

Figure 14: Experimental net proton surface density curve for Na-kaolinite .................... 52

Figure 15: Plot of % phosphate removal against pH for raw and treated kaolinite .......... 53

Figure 16: Plot of % phosphate bound against pH for bauxite ................ ........................ 55

Figure 17: Fractional composition of phosphate species in solution at different pH ........ 57

Figure 18: Plot of % phosphate removal against dosage for kaolinite ............................. 61

Figure 19: Plot of % phosphate removal against dosage for bauxite ............................... 62

Figure 20: Plot of phosphate uptake against time for kaolinite ....................................... 65

Figure 21: Plot of phosphate uptake against time for phosphate adsorption on bauxite ... 66

Figure 22: Second order fits for phosphate precipitation by kaolinite ............................. 68

Figure 23: First order fits for phosphate precipitation by kaolinite ......... ........................ 68

Figure 24: Second order fits for phosphate adsorption on bauxite .......... ........................ 69


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Figure 25: First order fits for phosphate adsorption on bauxite ....................................... 69

Figure 26: Plot of phosphate uptake against initial phosphate concentration for

phosphate removal by kaolinite ..................................................................... 71

Figure 27: Phosphate uptake by bauxite fitted to the Freundlich equation using non-

linear regression ............................................................................................ 73

Figure 28: Vant Hoff plot for phosphate adsorption on bauxite ..................................... 75

Figure 29: Effect of competing ions on phosphate removal by raw kaolinite .................. 76

Figure 30: Effect of competing ions on phosphate uptake by treated kaolinite ................ 76

Figure 31: Effect of competing ions on adsorption of phosphate ions on bauxite ........... . 80

Figure 32: Multi-competitive effect of calcium, magnesium, carbonate, and sulphate

ions on phosphate adsorption on bauxite ........................................................ 83


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LIST OF TABLES

Table 1: Chemical composition of Linthipe kaolinite .................................................... 20

Table 2: Solution combinations for the effect of initial phosphate concentration ........... 48

Table 3: Solution combinations for the effect of competing ions ........... ........................ 49

Table 4: Ca2+

concentration in solution and calculated saturation index values .............. 60

Table 5: Equilibrium constants and Gibb's free energy change values ........................... 74


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LIST OF ACRONYMS AND ABBREVIATIONS

ACP Amorphous Calcium Phosphate

APHA American Public Health Association

CSC Correctional Service of Canada

DDL Diffuse Double Layer

GSoM Geological Survey department of Malawi

HAP Hydroxyapatite

PZC Point of Zero Charge

PZNPC Point of Zero Net Proton Charge

TCP Tricalcium Phosphate

UNEP United Nations Environmental Programme


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CHAPTER ONE: INTRODUCTION

1.1 Background

Freshwater availability and use, as well as the conservation of aquatic resources, are

key to human well-being, but water quality degradation from human activities

continues to harm human and ecosystem health (UNEP, 2007). According to the

fourth global environmental outlook assessment (UNEP, 2007), the most ubiquitous

freshwater quality problem is high concentrations of nutrients (mainly phosphorus and

nitrogen) resulting in eutrophication, and significantly affecting human water use.

Nutrient pollution from municipal wastewater treatment plants and from agricultural

and urban non-point source run-off remains a major global problem, with many health

implications.

The Malawi State of the Environment Report indicates that the country faces

contamination of its water resources arising mainly from poor sanitation and improper

disposal of wastes, agro-chemicals and effluent from industries, hospitals and other

institutions (Malawi Government, 2002). According to Ferguson and Mulwafu

(2004), the release of untreated sewage directly into rivers and streams is one of the

major causes of water pollution in Malawi. It is partly through the discharge of

untreated or inadequately treated sewage, that some rivers and streams are becoming

loaded with phosphate ions, resulting in degradation of water quality.


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The Malawi Government, however, seeks to promote effective water pollution

monitoring and prevention programmes based on enforceable water quality guidelines

and standards (Malawi Government, 2002).

1.2 Problem statement

Presence of excess phosphorus (mainly in the orthophosphate form) in stagnant and

flowing water bodies pose a threat to aquatic life. This is due to stimulated growth of

aquatic plants that result in depleted oxygen levels when they decompose, as well as a

bloom of blue-green algae some of which produce cyanotoxins (such as saxitoxins

and anatoxin-a) that are more toxic than cobra venom (Skulberg et al., 1984;

Carpenter et al., 1998).

Sources of phosphate ions in flowing and stagnant water bodies include domestic and

industrial effluents (mainly as a result of use or manufacturing of products containing

phosphate formulations) and excessive fertilizer application to soils (Carpenter et al.,

1998; Smith et al., 1999). A study by Chipofya and Matapa (2003) revealed that the

Mudi reservoir, which is a raw water source for Blantyre water board, is infested with

blue green algae as a result of nutrient inflow from various catchments. Removing

phosphate ions during wastewater treatment is thus essential to minimize phosphorus

loading of receiving rivers and dams (Hammer and Hammer Jr., 2001).

Studies in Blantyre and Zomba, Malawi, have revealed presence of excessive

phosphate ions in effluent from wastewater treatment plants (Kwanjana, 2003; Sajidu

et al., 2007). This indicates inefficiency of the conventional biological filter plants

that are in use, in removing phosphate ions from wastewater. The established methods


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for wastewater phosphate removal, i.e. biological uptake and chemical precipitation,

are either expensive to run in developing countries or have poor operation stability

(Morse et al., 1998; Akhurst et al., 2006; Li et al., 2006; Karageorgiou et al., 2007;

Huang et al., 2008). As a result of high operational costs and poor operation stability

associated with the established methods, there is a growing interest in search for

cheap materials that can remove phosphate ions either through adsorption or

precipitation of phosphate salts (Pradhan et al., 1998; Johansson and Gustafsson,

2000; Zeng et al., 2004; Kostura et al., 2005; Akhurst et al., 2006; Ozacar, 2006;

Chen et al., 2007; Huang et al., 2008).

Locally sourced kaolinite and bauxite were chosen to be tested for their phosphate

removal capacities following reports that indicated adsorption of phosphate on

kaolinite, gibbsite and goethite minerals obtained from other parts of the world (Chen

et al., 1973; Persson et al., 1996; Kubicki et al., 2007; Stachowicz et al., 2008). Huge

reserves of kaolinite (800 million metric tonnes) and bauxite (26 million metric

tonnes) are available in Malawi (Yager, 2006). The natural abundance of kaolinite and

bauxite provides the possibility of long term or sustainable use if the materials can be

recycled. Waste materials from digestion of bauxite for alumina (red mud) can be an

alternative for application in wastewater treatment systems if the available bauxite

resources are mined and digested for alumina (Pradhan et al., 1998; Huang et al.,

2008).

Use of locally sourced kaolinite and bauxite in wastewater treatment systems would

require well designed preliminary bench studies to obtain information on the

interaction of the adsorbents with phosphate ions in terms of kinetics, reaction

mechanisms, and effects of solution physical and chemical composition. This study


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was therefore carried out to obtain information on the chemical interactions between

the adsorbents and phosphate ions.

1.3 Aim and objectives of the study and thesis outline

1.3.1 Aim of the study

The study was undertaken to investigate the use of locally sourced kaolinite and

bauxite as adsorbents for phosphate ions in aqueous solutions.

1.3.2 Specific objectives

(i) To determine the point of zero net proton charge for the locally sourced kaolinite

(ii) To determine the effects of conditions such as dosage, initial suspension pH,

contact time, initial phosphate concentration, presence of competing ions

(calcium, magnesium, carbonate and sulphate), and temperature on phosphate

removal using kaolinite and bauxite.

1.3.3 Thesis outline

The outline of the thesis is as follows: Chapter 2 provides the literature review on

chemical forms of phosphorus in water, phosphate sources and effects, phosphate

removal technologies, and chemical properties of kaolinite and bauxite ; chapter 3

presents the materials and methods whereas results and discussions are presented in

chapter 4. Chapter 5 presents conclusions and recommendations for further study.


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CHAPTER TWO: LITERATURE REVIEW

This chapter starts with a description of chemical forms of phosphorus found in water

and also the various sources of the abundant phosphorus chemical form (phosphate) in

wastewater. Building up on the sources of phosphates, this chapter then presents the

effects of excess phosphates on aquatic ecosystems, and later provides a review of

phosphate levels in Malawi and conventional methods being used to remove

phosphate from wastewater worldwide. This is followed by a review of low cost

materials that have been studied for their phosphate removal capacity in other

countries and later a presentation of the chemical composition of materials that will be

tested for their phosphate removal capacity (locally sourced kaolinite and bauxite)

along with a comprehensive review of their chemical behaviour in aqueous solutions.

2.1 Chemical forms of phosphorus in water

Phosphorus exists in water in either a particulate phase or a dissolved phase.

Particulate matter includes living and dead plankton, precipitates of phosphorus,

phosphorus adsorbed to particulates, and amorphous phosphorus. Dissolved

phosphorus occurs in natural waters and in wastewaters almost solely as phosphates

(APHA, 1989). Phosphates occur in various forms in water and are classified as

monophosphates, condensed phosphates (pyro-, meta-, and polyphosphates), and

organically bound phosphates (APHA, 1989).


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Monophosphates (orthophosphates) are compounds whose anionic entity, [PO4]3-

, is

composed by an almost regular tetrahedral arrangement of four oxygen atoms centred

by a phosphorus atom. Among the various categories of phosphates, monophosphates

are the most abundant mainly because they are the most stable. (Averbuch-Pouchot

and Durif, 1996). Additionally, all polyphosphates gradually hydrolyze in water to the

stable ortho form (Hammer and Hammer Jr., 2001).

The term condensed phosphates is applied to salts containing polymerized phosphoric

anions. Condensed phosphates are further classified as cyclophosphates,

polyphosphates and branched inorganic phosphates (or ultraphosphates).

Cyclophosphates (metaphosphates) are built up from cyclic anions and have the

composition MPO3where M is hydrogen or a monovalent metal. Representatives of

this group are cyclotriphosphate, M3P3O9, and cyclotetraphosphate, M4P4O12, shown

in Figure 1.

Figure 1 (a) Cyclotriphosphate (b) Cyclotetraphosphate

Highly polymerized cyclic phosphates containing as many as 10 to 15

orthophosphoric acid residues have been observed in some samples of condensed

phosphates (Kulaev et al., 2004).


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Polyphosphates in contrast to cyclophosphates have anions that are composed of

chains in which each phosphorus atom is linked to its neighbours through two oxygen

atoms, thus forming a linear, unbranched structure that may be represented

schematically by Figure 2.

M O P O P O P O ... P O M

O O O O

O M OM O M OM

Figure 2 A schematic representation of a polyphosphate

Branched inorganic phosphates are high molecular weight condensed phosphates,

which unlike the linear polyphosphates contain branching points, i.e. phosphorus

atoms that are linked to three rather than two neighbouring phosphorus atoms (Figure

3).

Figure 3 A schematic representation of a branched inorganic phosphate

P O P O P O .. ... .

O O O

O M O O M

P

O

O O


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Branched inorganic phosphates undergo unusually rapid hydrolysis in aqueous

solutions irrespective of pH as such they have not been found in living organisms

(Kulaev et al., 2004).

Organic phosphatesare phosphates that are bound to plant or animal tissue formed

primarily through biological processes. They include nucleic acids, phospholipids,

inositol phosphates, phosphoamides, phosphoproteins, sugar phosphates, phosphoric

acids, organophosphate pesticides, humic associated organic phosphorus compounds

and organic condensed phosphates in dissolved, colloidal and particle-associated

forms (McKelvie, 2005).

2.2 Sources of phosphates in wastewater

The principle sources of phosphates are point sources such as domestic and industrial

wastewater treatment plant effluents and natural runoff (non-point) from surrounding

uses such as land application of fertilizers and farming operations. Orthophosphates

and certain polyphosphates are major constituents of many commercial cleaning

agents. For example, many synthetic detergents contain 25-45% sodium

tripolyphosphate (Na5P3O10) which acts mainly as a water softener, by chelating and

sequestering Mg2+

and Ca2+

in hard water (Greenwood and Earnshaw, 1984).

Trisodium phosphate (Na3PO4) has scouring, bleaching, and bacteria killing

properties as such it is available in formulations of automatic dish washing powders

(Greenwood and Earnshaw, 1984). Domestic use of phosphate containing synthetic

detergents contributes towards high levels of ortho and polyphosphates in domestic

wastewaters.


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Various monophosphates are widely used at the industrial level. Sodium hydrogen

phosphate (Na2HPO4) is widely used in the food industry as an emulsifier in the

manufacture of pasteurized processed cheese and as a starch modifier. Sodium

dihydrogen phosphate (Na2H2PO4) is a solid, water-soluble acid which finds its use

(with NaHCO3) in effervescent laxative tablets and in the pH adjustment of boiler

waters. It is also used as a mild phosphatising agent for steel surfaces and as a

constituent in the undercoat for metal paints. Potassium hydrogen phosphate has

buffering properties as such it is added to car-radiator coolants as a corrosion inhibitor

(Greenwood and Earnshaw, 1984). Calcium phosphates also have a broad range of

applications both in the food industry and as bulk fertilizers. The wide application of

phosphate compounds in various industrial processes result in higher phosphate levels

in industrial wastewaters that are in some cases discharged onto surface water bodies

without proper treatment.

Organically bound phosphates are contributed to sewage through body waste and food

residues, and may also be formed from orthophosphates in biological treatment

processes or by receiving water biota. Organic phosphates may occur as a result of the

breakdown of organic pesticides which contain phosphates and they may exist in

solution, as loose fragments, or in the bodies of aquatic organisms (Smith et al.,

1999).

Phosphorus loading of surface water bodies through non-point sources is greatly

linked to excessive application of phosphate containing fertilizers and manure to farm

lands. In many areas, phosphate inputs from fertilizers and manures greatly exceed

phosphorus outputs in farm produce resulting in yearly phosphorus accumulation in


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the soil. This trend has important implications for phosphate levels in surface waters

because the total amount of phosphates transported in runoff from the landscape to

surface waters increases linearly with the soil phosphorus content (Smith et al., 1999).

2.3 Effects of excess phosphorus on aquatic ecosystems: Eutrophication

Of the many mineral resources required for plant growth, inorganic nitrogen and

phosphorus are the two principle nutrients that limit the growth of terrestrial plants as

well as algae and vascular plants in freshwater and marine ecosystems (Smith et al.,

1999). Waters having relatively large supplies of nutrients are termed eutrophic (well

nourished), and those having poor nutrient supplies are termed oligotrophic (poorly

nourished). Waters having intermediate nutrient supplies are termed mesotrophic and

those receiving greatly excessive nutrient inputs are termed hypertrophic.

Eutrophication is defined as the process by which water bodies become well

nourished through an increase in their nutrient supply (Van den Brandt and Smit,

1998).

The most common effects of increased nitrogen and phosphorus supplies on aquatic

ecosystems are perceived as increases in the abundance of algae and aquatic plants.

The environmental consequences of excessive nutrient enrichment are more serious

and far-reaching than nuisance increases in plant growth alone. Decomposition of

dead nuisance plants can result in oxygen depletion in the water causing death of

aquatic animals such as fish (Carpenter et al., 1998). Microorganisms use much

oxygen during decomposition of dead plants, thus resulting in lowered oxygen levels

in the water. Eutrophication also brings about a shift in phytoplankton species towards


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dominance of the phytoplankton by blue-green algae (cyanobacteria), some of which

produce cyanotoxins that are more toxic than cobra venom (Skulberg et al., 1984).

Other effects of eutrophication include, reduced water clarity, bad odour and taste of

the water, and a general decrease in perceived aesthetic value of the water body

(Smith et al., 1999). Eutrophication is not limited to lakes and reservoirs, it can also

occur in rivers and streams (Smith et al., 1999). Both phosphorus and nitrogen limit

plant growth, but phosphorus is the primary limiting nutrient in most lakes and

reservoirs consequently most eutrophication management frameworks focus primarily

on phosphorus loading (Hecky and Kilham, 1988).

2.4 Levels of phosphate pollution in Malawi

Reported studies on effluent quality from wastewater treatment plants in Zomba and

Blantyre indicate phosphate levels above the minimum limit of 1.0 mg/L (CSC,

2000). The Soche wastewater treatment plant in Blantyre is a conventional biological

filter plant that receives wastewater from industries such as cloth making and food

processing as well as latrine and septic tank emptyings. A study by Sajidu et al.,

(2007) reported a phosphate influent concentration of 5.39 0.66 mg/L, and an

effluent concentration of 3.86 0.76 mg/L for the plant. In a separate study, a

phosphate effluent concentration of 5.18 0.00 mg/L was reported for the Zomba

wastewater treatment plant (Kwanjana, 2002). Lower phosphate concentrations were

reported for the Limbe wastewater treatment plant with an influent and effluent

concentration of 0.79 0.93 mg/L and 0.63 0.23 mg/L respectively (Sajidu et al.,

2007). Fluctuations in composition of the wastewater influent may result in lower or

higher effluent phosphate concentrations than those reported. Besides the wastewater


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treatment plants, high phosphate concentrations were reported for Nasolo river (3.20

0.69 mg/L), Limbe stream (3.42 0.00 mg/L), and Mudi river (5.50 3.20 mg/L)

in Blantyre (Sajidu et al., 2007).

2.5 Methods for phosphate removal during wastewater treatment

2.5.1 Chemical precipitation

Chemical precipitation is a physico-chemical process, comprising the addition of a

divalent or trivalent metal salt to wastewater, causing precipitation of an insoluble

metal phosphate that is settled out by sedimentation. Iron and aluminium are the most

suitable metals and are added as chloride or sulphate salts. Lime may also be used to

precipitate calcium phosphate. Chemical precipitation is a flexible technology

allowing for application of the metal salts at several stages during wastewater

treatment (Figure 4).

Influent

Metal salt

Rapidmix

Flocculant Aid

Primaryclarifier

AerationBasin

Seco-ndaryclarifier

Alternative metal saltaddition points

Sludge to processing

Figure 4: Chemical phosphate precipitation (Sourced from Morse et al., 1998).


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13

Removal of phosphates from wastewater using metal salts can be described by simple

chemical reactions involving direct combination of phosphate and the metal ions to

form precipitates, for example a reaction between aluminium sulphate and

orthophosphate can be given by Equation 1 (Hammer and Hammer Jr., 2001).

OH3.14SO3AlPO22POO.14.3H)(SOAl 2-2

44

-3

42342 +++ (1)

Recent evidence on phosphate removal from aqueous solutions using aluminium

sulphate and aluminium hydroxide (Georgantas and Grigoropoulou, 2007) has shown

that orthophosphate and metaphosphate ions are also removed through a ligand

exchange mechanism in which surface hydroxyl groups on the surface of the

precipitated aluminium hydroxide are exchanged for phosphate ions.

Chemical precipitation typically produces phosphorus bound as a metal salt within the

wasted sludge. The wasted sludge has the potential value of being used as fertilizer

although research on bioavailability of the bound phosphorus is inconclusive (Morse

et al., 1998). Chemical precipitation is an established technology that is easy to

install and operate and can achieve high phosphate removal, however, it requires high

doses of chemicals, there is an increase in sludge production and phosphorus

recyclability is variable (Morse et al., 1998).


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2.5.2 Biological phosphorus removal

The development of biological phosphorus removal was based on evidence that under

certain conditions, some heterotrophic bacteria in activated sludge could take up

phosphorus in considerable excess to that required for normal biomass growth (luxury

uptake) (Sidat et al., 1999). Biological phosphorus removal is achieved in the

activated sludge process by introducing an anaerobic zone ahead of an aerobic stage

(Figure 5).

In the anaerobic zone, sufficient readily degradable chemical oxygen demand (COD)

must be available, typically as volatile fatty acids provided by pre-fermenting the

sludge using storage or thickeners, or from the addition of acetic acid or sodium

acetate (Sidat et al., 1999). In the absence of oxygen and nitrates, bacteria, such as

Acinetobacter take up the acids and release phosphorus into solution, but in the

aerobic stage luxury uptake occurs, increasing overall phosphorus removal rates to as

much as 80-90%. However, phosphorus removal is variable and, in practice, the

achievement of a low and consistent effluent standard may require complementary

InfluentAerobic Clarifier

Return Sludge

Effluent

Anaerobic zone

Figure 5 Biological phosphorus removal (Adapted from Morse et al.,1998)


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chemical simultaneous precipitation (Morse et al., 1998). This technology does not

require use of high doses of chemicals, removal of phosphorus and nitrate can be

achieved simultaneously, and the phosphorus is more recyclable (Sidat et al., 1999).

However, biological phosphorus removal has poor operational stability and handling

of huge volumes of sludge maybe more difficult (Morse et al., 1998).

2.5.3 Crystallisation technology

Phosphorus removal is achieved through crystallisation of a phosphate mineral on a

seeding grain in a reactor. The DHV crystalactorTM process is an example of

phosphate crystallisation technologies. The DHV CrystalactorTM

process is based on

the crystallisation of calcium phosphate on a seeding grain, typically sand, within a

fluidized reactor, as shown in Figure 6 (Scholler, undated).

Chemicals

Influent

Periodic injection of seeding grains (0.2 0.6 mm)

Periodic removal of pellets (1 2 mm)

Fluidised bed Grains:

0.2 2 mm

Effluent

Injection

nozzles

Height:

6m

Diameter: 0.5 4 m

Figure 6 The DHV Crystalactor for phosphorus crystallization (Sourced fromScholler, undated)


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Process conditions are adjusted to promote calcium phosphate crystallization by

adding either sodium hydroxide or calcium hydroxide. Pellets formed during

crystallization are periodically removed and replaced by small diameter seed grains

and the removed pellets can be recycled by the phosphate industry. A major

advantage of the CrystalactorTM

technology is that phosphorus removal produces no

additional sludge, only a quantity of water-free pellets consisting almost exclusively

of calcium phosphate (40 - 50%) and seed material (30 - 40%), with other materials

present in small amounts (Scholler, undated). Disadvantages of this technology

include use of chemicals and requirement for operation skills (Morse et al., 1998).

2.5.4 Ion exchange

The RIM-NUT ion exchange-precipitation process is widely used to remove

phosphates and ammonia from wastewater through formation of struvite [magnesium

ammonium phosphate hexahydrate, OH.MgPO)NH(

)]. The process uses a

cationic resin to remove the ammonium ions and a basic resin to remove phosphate

ions. Regeneration of the ion exchange resins releases the ammonium and phosphate

ions that are then precipitated as struvite. Struvite is a good slow release fertilizer, as

such the technology has a high phosphorus recycling potential for agriculture. Despite

having a high potential for phosphorus recycling, the technology is complex, it

requires use of chemicals and disposal of waste eluate is a problem (Morse et al.,

1998).


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2.5.5 Magnetic attraction

Magnetic attraction systems such as the Smit-Nymegen process uses calcium oxide to

precipitate calcium phosphate attached to magnetite that is later separated using an

induced magnetic field. After isolation, the magnetite is uncoupled from the

phosphate in a separator unit by shear forces and a drum separator. The separated

suspension of calcium phosphate or carbonate in water is further processed depending

on the use of the final product. Magnetic attraction systems can achieve high

phosphate removal; however they are unnecessarily complex and require use of

chemicals (Morse et al., 1998).

2.5.6 Low cost materials for phosphate removal from wastewater

The potential low cost materials for removing phosphate ions during wastewater

treatment can be categorized into two groups, those that involve phosphate

precipitation, and those that adsorb phosphate ions.

2.5.6.1 Phosphate precipitating low cost materials

Materials that have been extensively studied in this group include fly ash, calcite, and

blast furnace slag. Fly ash is a waste product from coal-fired power plants composed

of various metal oxides. Presence of calcium allows for precipitation of calcium

phosphates at high pH levels, whereas iron oxides bind phosphate at low pH levels

through ligand exchange reaction mechanisms (Li et al., 2006; Chen et al., 2007).


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Generally, favourable conditions for phosphate precipitation are adequate calcium

ions, and high equilibrium pH (>9) (Can and Yildiz, 2006; Chen et al., 2007; Moon et

al., 2007). Phosphate removal by calcite involves a combination of precipitation of

hydroxyapatite and adsorption of phosphate ions on the calcite surface (Karageorgiou

et al., 2007).

Blast furnace slag is an industrial by-product derived from the separation of iron from

iron ore. It is a complex CaO-MgO-Al2O3-SiO2 system that also incorporates a

number of minor components that can concentrate on the slag surface during

crystallization or transition to a glassy state (Kostura et al., 2005). Phosphate is

precipitated as hydroxyapatite under strongly alkaline conditions (pH > 9) and large

amounts of soluble calcium ions (Johansson and Gustafsson, 2000).

2.5.6.2 Low cost phosphate adsorbents

Low cost phosphate adsorbents that have been studied include iron oxide tailings, red

mud, and alunite. Iron oxide tailing is an industrial waste derived from iron ore

processing that contain significant amounts of iron oxides. This adsorbent has a high

affinity for phosphate ions and adsorption is high under low pH conditions (Zeng et

al., 2004).

Red mud is a waste material formed during the production of alumina when the

bauxite ore is subjected to caustic leaching (Pradhan et al., 1998). It is a brick

coloured highly alkaline (pH 10 12) sludge containing mostly oxides of iron,

aluminium, titanium, and silica. Red mud has a high phosphate adsorption capacity

because of the presence of iron oxides, but acid treatment is required to lower the pH.


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For example, red mud can be neutralized by seawater to pH 9.0 0.5 to produce a

commercial adsorbent known as BauxsolTM

(Akhurst et al., 2006). BauxsolTM

can be

activated by an acid to improve its phosphate adsorption capacity. Initial

neutralization by seawater entails requirement for small amounts of acid during acid

treatment and lower preparation costs as compared to acid activation of raw red mud

(Akhurst et al., 2006). Huang et al., (2008) confirmed low phosphate removal

efficiency for raw red mud as compared to acid activated red mud.

Alunite, KAl3(SO4)2(OH)6, is one of the minerals of the jarosite group and is not

soluble in water in its original form (Ozacar, 2006). Alunite gives thermal

decomposition reaction products such as Al2O3, Al2(SO4)3 and K2SO4 when it is

calcined at 973 1023 K (Ozacar and Sengil, 2003). Phosphate ions are adsorbed

onto the resultant metal oxide surfaces via ligand exchange mechanisms. Despite

having a high phosphate removal capacity, requirement for high temperature

calcinations can be a drawback for use of alunite in countries where it is found.

2.6 Chemical composition and acid-base properties of Linthipe kaolinite andMulanje bauxite.

Bauxite from Mulanje mountain and kaolinite from Linthipe, Dedza, were

investigated for their potential use as phosphate adsorbents. According to the

Geological Survey Department of Malawi (GSoM), the bauxite is mainly a trihydrate

gibbsite which lies over kaolinite and has free quartz and geothite as the main

contaminants. The kaolinite from Linthipe contains iron oxides and calcium

carbonates (reported as calcium oxide) as the main contaminants (Table 1).


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Table 1 Chemical composition of kaolinite obtained from Linthipe, asdetermined by the GSoM

Parameter Composition (wt.%)

SiO2 46.7

Al2O3 33.8

Fe2O3 2.0

CaO 1.1

MgO 0.26

K2O + Na2O 0.28

Gibbsite, kaolinite and iron oxides are the major minerals present in the locally

sourced bauxite as such a detailed understanding of their structural composition and

acid-base properties in aqueous environment is necessary for elucidation of phosphate

binding mechanisms.

2.6.1 Gibbsite

The gibbsite ( ( )3OHAl ) structure consists of double layers (AB) of close packed

OH groups with Al atoms occupying two thirds of the octahedral interstices within the

layers. Each Al atom is octahedrally bonded to three O atoms of layer A and three

atoms of layer B. The AB layers are stacked in the sequence AB-BA-AB-BA-

(Figure 7).


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The structure of gibbsite can also be viewed from the perspective of the Al3+ ions that

are arranged in a pattern of coalesced hexagonal rings. A single ring of six Al3+

ions,

joined above and below by six pairs of bridging OH-ions, is the smallest recognizable

unit of the gibbsite structure (Goldberg et al., 1996). The6)OH(Al unit can be

represented schematically as an octahedron with each apex being at the centre of a

hydroxyl ion (Figure 8). Two octahedra are joined along one edge by sharing a pair of

OH- ions, and six octahedra each sharing two edges yields a +6126 )OH(Al ring.

Figure 8 Ring structure model for gibbsite

Layer A

Layer B

Figure 7 Sphere model of gibbsite structure. Large spheresrepresent OH- ions; smaller spheres represent Al3+

ions in octahedral coordination sites. Part of theupper OH layer (layer A) is removed to showarrangement of Al3+.


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Surface functional groups on gibbsite are located at the basal planes and on the edges.

At the basal planes of gibbsite, all OH- groups are coordinated to two Al

3+ ions i.e.

OHAl2 . Both singly coordinated hydroxyl groups ( AlOH ) and doubly coordinated

hydroxyl groups are found in equal amounts on the gibbsite edges.

The singly coordinated hydroxyl groups are considered to be the most reactive and

they occur in pairs on the edge of an 6AlO octahedron (Goldberg et al., 1996).

2.6.2 Goethite

The structure of goethite is based on the hexagonal close packing of oxygen atoms

with 6-fold coordinated Fe atoms occupying octahedral position (Frost et al., 2003).

Each oxygen atom or hydroxyl group coordinates with three Fe3+

ions. The Fe atoms

are arranged in a double row to form what can be described as double chains of

octahedra, which run the length of the c- axis (Figure 9).

b c

OFe

Ha

Figure 9 Ball and stick model for goethite (Sourced from Frost etal., 2003).


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23

Within the double chains in theb-c plane, all bonds are covalent with each octahedron

sharing four of its edges with neighbouring octahedra. In contrast, bonding between

double chains consists of relatively weak hydrogen bonding directed through apical

oxygen ions directed along the a- axis. In this case, stacking of double chains along

the a- axis can be easily disrupted and this consequently induces structural defects,

such as non-stoichiometric hydroxyl units incorporated into the goethite structure

during crystal growth (Frost et al., 2003).

In the bulk of the goethite mineral two types of triply coordinated oxygen groups are

found, one protonated ( OHFe3 ) and the other non protonated ( OFe3 ). Different

types of surface groups are present at the crystal faces that are singly-( (OH)FeOH ),

doubly- ( OHFe2 ), and triply-coordinated ( O(H)Fe3 ) hydroxyl groups

(Stachowicz et al., 2008). The primary charging behaviour and adsorption reactions

between goethite and ions in solution is attributed to the singly and triply coordinated

surface oxygens located on the dominant 110 crystal face (Hiemstra and van

Riemsdijk, 1996).

2.6.3 Kaolinite

Clays are finely divided aluminosilicates. The principal building elements of the clay

minerals are the two-dimensional arrays of silicon-oxygen tetrahedral (tetrahedral

silica sheet) and that of aluminium- or magnesium-oxygen-hydroxyl octahedral

(octahedral, alumina or magnesia sheet) (Grim, 1995). The tetrahedral layer is

constituted by the coordination of several silica tetrahedrons. In a silica tetrahedron a

silicon atom is at the centre of a structure where the corners are filled with oxygen


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atoms (Figure 10). Each tetrahedron is formed by one atom of Si4+

and 4 atoms of O2-

;

so that its chemical formula is

SiO : the arrangement in a sheet leads to a general

averaged formula of the type (Si4O10)4-

n

Figure 10 Silica tetrahedron and layer (Grim, 1995).

The coordination of aluminium, magnesium, or iron atoms with oxydrils or hydroxyls

gives rise to structural units with an octahedral shape. Octahedrons also have the

capacity to join in sheets (Figure 11).

Figure 11 Octahedron and octahedral sheet (Grim, 1995).

The central cations in the octahedrons have to balance the negative electrical charge

of the octahedral arrangement of oxydrils and hydroxyls, equal to two electrons per

unit. Therefore while the magnesium ion needs to be present in every unit, the


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aluminium ion will be required just in two cells over three. Sharing of oxygen atoms

between silica and alumina sheets results in two- or three-layer minerals such as 1:1

kaolinite built up from one silica and one alumina sheet (TO), or 2:1 type

montmorillonite, in which an octahedral sheet shares oxygen atoms with two silica

sheets (TOT) (Van Olphen, 1963; Schulze, 2002).

Kaolinite, [ ] 81044 )OH(O)Al(Si , is a dioctahedral 1:1 layer aluminosilicate. A kaolinite

unit cell consist of a layer of silica tetrahedral bound to an octahedral alumina layer

whose structure is very similar to that of gibbsite except that some hydroxyls are

replaced by oxygens (White, 2007). Kaolinite particles are formed by the repetition of

the basic layer by overposition, where bonding between subsequent layers is provided

by hydrogen bonds and van der Waals forces (Grim, 1995).

The kaolinite surface consists of three morphologically different planes with different

chemical compositions: a gibbsite type basal plane, a silica type basal plane and edge

G

G

G

7.2

Silica tetrahedral

sheet

Alumina octahedralsheet

Figure 12 Kaolinite structure


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planes represented by a complex oxide of the two constituents 3)OH(Al and 2SiO

(Rosenqvist, 2002).

Surface functional groups on kaolinite are located on the octahedral and tetrahedral

basal planes as well as along the edge of the sheets. These functional groups include

doubly coordinated hydroxyl groups ( OHAl2 ) on the octahedral basal plane, the

siloxane group ( OSi2 ) on the tetrahedral basal plane as well as the

aluminol, OHAl , silanol ( OHSi ) and the 2AlOSi groups located along

the edge of the sheets (Rosenqvist, 2002). The siloxane group , OSi 2 , which is the

only group present at the basal surface of a 2:1 clay or at one basal surface of

kaolinite is unreactive (Avena et al., 2003). For the 2AlOSi sites, the charge

on the oxygen is fully neutralized and the group is therefore probably also not reactive

(Rosenqvist, 2002). The silanol and aluminol functional groups are therefore

considered reactive and hence contribute towards acid-base behaviour of kaolinte and

complexation reactions with solution speciation.

2.7 Complexation and adsorption

A complex is a unit in which an ion, atom, or molecule binds to other ions, atoms, or

molecules. The binding species is termed a central group and a bound species is

termed a ligand (Goldberg et al., 1996). Adsorption is described in terms of a set of

complex formation reactions between dissolved solutes and surface functional groups.

Ligands can be associated with the surface in different ways. In the formation of

inner-sphere complexes, a chemical (largely covalent) bond between the central atom


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and the ligand is formed (White, 2007). In outer-sphere complexes on the other hand,

one or more water molecules remain between the ligand and the central atom and no

direct bond is formed. Outer-sphere complexes are held together mainly by

electrostatic forces. Inner-sphere complexes can be classified by the ligands mode of

binding to the surface. If the ligand is attached to only one surface functional group,

the complex is termed monodentate, whereas a ligand connected to two surface

functional groups forms a bidentate complex. Bidentate complexes can in turn be

further classified by considering the number of central atoms (in the solid material)

included in the complex. If a bidentate complex involves two central atoms, the

complex is generally referred to as a bridging complex, whereas a bidentate complex

involving only one central atom is referred to as a mononuclear chelate (Goldberg et

al., 1996).

2.7.1 Development of reactive functional groups at the metal oxide-solutioninterface

Oxygen and metal atoms at an oxide surface are incompletely coordinated; i.e., they

are not surrounded by oppositely charged ions as they would be in the interior of a

crystal (White, 2007). Consequently, mineral surfaces immersed in water attract and

bind water molecules that can dissociate leaving a hydroxyl group bound to the

surface metal ion as indicated by Equation 2:

++ ++ HMOHOHM 2 (2)

where M denotes a surface metal ion. In a similar fashion, incompletely coordinated

oxygens at the surface can also bind water molecules, which can then dissociate,

again creating a surface hydroxyl group (Equation 3):


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++ OHOHOHO 2 (3)

Thus the surface on an oxide immersed in water very quickly becomes covered with

hydroxyl groups, which are considered to constitute part of the surface rather than the

solution. Different surface hydroxyl groups can be identified based on the number of

metal atoms to which they are coordinated. OH groups coordinated to only one metal

atom are called singly coordinated or terminal hydroxyls, whereas OH groups

coordinated to more than one metal are called bridging hydroxyls (Rosenqvist, 2002).

Bridging hydroxyls might be coordinated to two, three, or four metal atoms and are

therefore called doubly, triply and quadrupely coordinated, respectively.

2.7.2 Adsorption of ions at the metal (hydr)oxide-solution interface : Theelectrostatic double layer model

As noted previously, oxygen atoms on the surface of a metal (hydr)oxide are

neutralized by both metal ions belonging to the solid and a variable number of

adsorbed protons. Depending on the solution pH, an excess or deficiency of protons

can occur at the interface resulting in a positively or negatively charged surface

respectively (Hiemstra and Riemsdijk, 1999). This surface charge, 0 , is

compensated by electrolyte ions in a double layer, normally assumed to be a diffuse

double layer (DDL) (Hiemstra and Riemsdijk, 1996). The ions present in the DDL are

hydrated and have a finite size, thereby preventing charge neutralization starting

directly from the close-packed surface. This result in counter and co-ions having a

minimum distance of approach to the surface and hence formulation of a charge free

layer called the Stern layer. This double layer picture has been described as the basic

Stern (BS) model (Hiemstra and Riemsdijk, 1996). Hiemstra and Riemsdijk (1996)


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argued that outer sphere complexes have a minimum distance of approach to the

surface as counter and co-ions. This implies that outer sphere complexes are adsorbed

in an electrostatic plane positioned on the solution side of the Stern layer near the

head end of the diffuse double layer (Figure 13). The innersphere complexes on the

other hand, are closer to the surface, penetrating the stern layer.

In the presence of specific adsorption, a hypothetical electrostatic plane (1-plane)

emerges in the Stern layer in which solution oriented ligands of the specifically

adsorbed complexes are located. This double layer picture, consisting of three

electrostatic planes (0-, 1-, and 2-or d-plane) is called the three plane (TP) model

(Hiemstra and Riemsdijk, 1996). In the absence of specifically adsorbing ions, the TP

model simplifies to the BS model.

Figure 13 The electrostatic double layer model (Sourcedfrom Hiemstra and Riemsdijk, 2006)


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2.8 Protonation of surface functional groups and charge balance

Adsorption reactions on metal (hydr)oxide surfaces are pH dependent (Manning and

Goldberg, 1996) as such an understanding of acid-base reactions on the metal

(hydr)oxide surfaces is essential for the description of the effect of pH on the amount

of ions adsorbed. It is assumed that protonating oxygen atoms on an oxide surface can

bind two protons in a pH range. In such approach, surface groups are considered to be

diprotic reacting according to the following scheme presented by Equations 4 and 5:

SOHHSO + + (4)

++ +

SOHHSOH (5)

where SO , SOH and + 2SOH are deprotonated, monoprotonated and diprotonated

surface groups respectively (Kraepiel et al., 1998; Avena et al., 2003). To describe

these two consecutive protonation/deprotonation steps, two pKa values are required

and this conceptual model is therefore known as the two pKa model. Recent

theoretical and experimental evidences however, indicate that two protonation steps

as indicated by Equations 4 and 5 seldom occur at oxygen atoms in aqueous media

either at the surface of solids or in true solutions (Borkovec et al., 2001). In most Al

and Fe containing hydroxides in nature, the metal (M

3+

) ions are most often

octahedrally coordinated to six oxygen atoms. This means that each oxygen atom will

neutralize one sixth of the charge on the metal resulting in 0.5 charge units. If the

oxygen atoms are coordinated to only one metal ion, the half unit charge from the

metal means that the OH group cannot be neutral; it will have either +0.5


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)MOH(. +

or 0.5 (

5.0MOH ) charge. Any other protonation steps are unlikely to

occur within the normal pH range, and therefore the protonation of the surface can be

described using a single protonation step and one pKa value. For an oxygen atom that

is singly coordinated to the metal ion (Me), the protonation step can be written as

++ + 1/223-1/2

OHMeHMeOH whereas for a triply coordinated surface oxygen one

finds ++ + 1/23-1/2

3 OHMeHOMe (Hiemstra and Riemsdijk, 1999). Generally, a

protonation reaction in the one-pKa approach is given by (Equation 6):

1xx AHHA ++ =+ (6)

where xA denotes a functional surface group carrying a charge x (fractional or

integer, negative or positive) and 1xAH + is the protonated group.

The balance of surface charge on an aluminium oxide mineral in aqueous solution is

given by Equation 7,

0dosisH =+++ (7)

where H is the net proton charge, defined by H = )(F OHH , where is a

surface excess concentration; is is the inner-sphere complex charge resulting from

the formation of inner-sphere complexes between adsorbing ions (other than H+

and

OH-) and surface aluminol groups; os is the outer-sphere complex charge resulting

from the formation of outer-sphere complexes between adsorbing ions and surface

aluminol groups or ions in inner-sphere complexes; d is the dissociated charge,


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equal to minus the surface charge neutralized by electrolyte ions in solution that have

not formed adsorbed complexes with surface aluminol groups (Goldberg et al., 1996).

Consideration of the surface charge balance as a function of pH leads to the definition

of the point of zero charge. The point of zero charge, p.z.c, is the solution pH value

where total net particle charge is zero: 0dosisH ==++ . The adsorbent surface

develops a net positive charge below the p.z.c., and a net negative charge above it.

Adsorption of anions is generally higher below the p.z.c. whilst that of cations is

higher above the p.z.c. Determination of the p.z.c. is therefore necessary in

discussions of ion adsorption on metal hydroxide surfaces. The .c.z.p can be

measured directly using electrokinetic measurements or colloidal stability

experiments (Goldberg et al., 1996). When the p.z.c. is measured using electrokinetics

it is often called the isoelectric point (i.e.p.). The point of zero net proton charge,

p.z.n.p.c, is the solution pH value where the net proton charge is zero. The p.z.n.p.c.

can be measured using a potentiometric titration if only selective aluminol groups are

titrated (Goldberg et al., 1996). The point of zero salt effect (p.z.s.e.) is the solution

pH value where the net proton charge is independent of solution ionic strength,

0I

H =

. The p.z.s.e. can also be measured by potentiometric titration using either

batch or continuous titrations.

The p.z.n.p.c. is determined as the point where a plot of the apparent proton surface

charge density, titr,H , against suspension pH crosses the x-axis, i.e. pH value where

titr,H = 0. Negative and positive proton surface charge density indicates net coverage

of the surface by hydroxyl groups and protons, respectively. Schroth and Sposito

(1997) described equations used to calculate the apparent proton surface charge


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density for kaolinite that corrects for proton consumption by aluminium species in

solution, introduced through dissolution of aluminium at low pH values (generally

below pH 4).

The apparent proton surface charge density, titr,H , is given by Equation 8:

=

++++

S][H

K

][H

K)]H[]H([M w

b

w

SbSolntitrH, (8)

where Msoln is the mass of electrolyte solution (per unit dry mass) equilibrated with

kaolinite, [H+] is the solution proton concentration, Kw is the dissociation product of

water, and the subscripts s and b refer to sample and blank solutions, respectively. For

highly acidic samples in which there is significant proton release caused by

dissolution of kaolinite, the apparent surface charge density is corrected for Al in

solution by Equation 9:

[ ] [ ] ( )+++ ++= 223

lnsotitr,HAl,titr,H OHAlAlOH2Al3M (9)

where the concentrations of the 3 Al species are calculated using the total Al

concentration in solution. This correction accounts for protons that would be

consumed in the release of Al and those that would be generated by Al hydrolysis at

pH 6. (Schroth and Sposito, 1997).


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2.9 Patch-wise surface charge heterogeneity on kaolinite and the point of zerocharge

Clay mineral particles hold both permanent negative charges on the faces and pH

dependent either negative or positive charges developing mainly on OHAl active

sites at the broken edges and exposed hydroxyl-terminated planes (Tombcz and

Szekeres, 2006). The permanent negative charge sites on the basal planes (faces) are a

result of substitution of the Si- and Al-ions in the crystal lattice for lower positive

valence ions. Since these two types of sites are situated on the given parts of the

particle surface, different charge patches exist on the basal planes and edges of clay

particles (Koopal, 1996). Development of the different surface charge patches on clay

particles affect reactivity of the functional groups on the basal planes and edges

towards adsorption of various ions as well as colloidal behaviour of the particles

(Tombcz and Szekeres, 2006). The size of the surface charge patches and the lateral

interactions of the surface sites are affected simultaneously by suspension pH and

ionic strength of the background electrolyte. Determination of the p.z.c. for clay

particles is with respect to pH-dependent functional groups located on the edges of the

particles as such the point of zero charge is referred to as p.z.c. edge.

2.10 Modeling phosphate adsorption on kaolinite and bauxite

Phosphate adsorption on kaolinite and bauxite will be modelled using the Freundlich

isotherm equation (Equation 10). The Freundlich equation is a semi-empirical model

used to describe heterogeneous systems (Milonjic, 2007):

n

1

efe cKq = (10)


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Where Kfis the Freundlich constant (dm3g

-1), qeis phosphate uptake (mg/g), Ceis the

equilibrium phosphate concentration (mg/L) and 1/n is the heterogeneity factor. The

Freundlich equation is consistent with an exponential distribution of electrical

potentials or of binding constants on metal hydroxide surfaces (Barrow et al., 2005).

The empirical form of the Freundlich isotherm equation is applicable to both

monolayer adsorption (chemisorption) and multilayer adsorption (van der Waals

adsorption) (Yang, 1998). It is always inappropriate to use the Langmuir equation as a

simple equation to describe sorption by soil because soils do not comprise one

uniform surface; adsorption always induces a change in the properties of the surface

and this is inconsistent with this equation; and the equation usually describes sorption

poorly (Mead, 1981; Barrow, 2000; Barrow et al., 2005; Barrow, 2008). The kaolinite

and bauxite samples used in this study are heterogeneous materials just like soil hence

the use of only the Freundlich equation is justified.

2.11 Precipitation of calcium phosphates

Kaolinite samples used in this study had a significant amount of calcium impurities

(1.1% CaO), giving the possibility of precipitation of calcium phosphates during

treatment if high dosages are used. Under proper physical and chemical environment,

different kinds of calcium phosphates, such as OHCaHPO 24 2. (dicalcium

phosphate dihydrate, DCPD), OHPOHCa 2344 5.2.)( (octacalcium phosphate, OCP),

243 )(POCa (tricalcium phosphate) and OHPOCa 345 )( (hydroxyapatite, HAP) may

precipitate from saturated solutions, among which HAP is thermodynamically the

most stable one (Koutsoukos et al., 1980; Van Kemenade and De Bruyn, 1987). The


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thermodynamic driving force to a chemical reaction is the Gibbs free energy G, and

it is the criterion to judge whether a reaction is spontaneous, in equilibrium, or

impossible, corresponding to G < 0, = 0, or > 0, respectively. Considering a calcium

phosphate precipitation reaction, the Gibbs free energy is given by Equation 11:

spK

IAPln

n

RT-G=

(11)

where R is the ideal gas constant (8.314 JK-1mol-1), T is the absolute temperature, IAP

and Ksp are respectively the free ionic activities product and the thermodynamic

solubility product of the precipitate phase, and n is the number of ions in the

precipitated compound (Song et al., 2002a). Supersaturation is a measure of the

deviation of a dissolved salt from its equilibrium value, for a solution departing from

equilibrium is bound to return to this state by the precipitation of excess solute. The

saturation index, SI, of a solution with respect to a precipitate phase provides a good

measurement of supersaturation of a system and is defined by Equation 12:

Ksp

IAPlogSI= (12)

Gibbs free energy is therefore related to SI by Equation 13:

SIn

2.303RT-G = (13)


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When SI = 0, hence G = 0, the solution is in equilibrium; when SI < 0, G > 0, the

solution is undersaturated and precipitation is impossible; when SI > 0, G < 0, the

solution is supersaturated and precipitation is spontaneous. SI is a good indicator to

show the deviation of a salt from its equilibrium state, i.e. the thermodynamic driving

force for the precipitation of a calcium phosphate phase. But considering precipitation

kinetics, supersaturation does not certainly mean the quick occurrence of a

spontaneous precipitation. Between the undersaturated zone and spontaneous

precipitation zone there is still a metastable zone, where the solution is already

supersaturated but no precipitation occurs over a relatively long period (White, 2007).

The boundary between metastable zone and spontaneous precipitation zone is called

the critical supersaturation (Joko, 1984).


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CHAPTER THREE: MATERIALS AND METHODS

3.1 Materials

3.1.1 Adsorbents

Kaolinite and bauxite samples used in this study were collected from kaolin deposits

located at Linthipe in Dedza district, and from Mulanje Mountain, Malawi,

respectively. Both the kaolinite and bauxite samples were identified by the Geological

Survey Department of Malawi (GSoM).

3.1.2 Chemicals, reagents and instruments

The following analytical grade chemicals and instruments were used: anhydrous

potassium dihydrogen phosphate, KH2PO4, (Glassworld, SA); Lanthanum oxide,

La2O3, (SAARCHEM, SA); Sodium carbonate, Na2CO3, (Glassworld, SA); Sodium

hydrogen carbonate, NaHCO3, (BDH); Sodium Fluoride, NaF, and (SAARCHEM,

SA); anhydrous Sodium sulphate, Na2SO4, (ACE, SA); Calcium nitrate, Ca(NO3)2,

(SAARCHEM, SA); and Magnesium nitrate, Mg(NO3)2, (BDH); Sodium nitrate,

NaNO3, (BDH); Sodium hydroxide, NaOH, and nitric acid, HNO3, (Associated

Chemical Enterprises (Pty) Ltd, RSA); Hydrochloric acid, HCl, (Glassworld, SA), pH

7.00 0.02 and pH 4.01 0.02 buffer solutions (Mettler-Toledo Inc., Switzerland);

Hettich Rotixa/AP centrifuge (Hettich lab technology, Germany); Gallenkamp

Thermostirrer 85 water bath (Weiss Gallenkamp, UK); Stuart mini orbital shaker

SSM1(Bibby Scientific Ltd, UK).


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3.2.2.1.2 Nitric acid (0.02359 mol/L and 1+1)

This solution was prepared by diluting 1.7 mL of 55% nitric acid (of density 1.34

g/mL) with distilled water to 1000 mL in a volumetric flask. The dilute nitric acid was

standardized with 0.01 mol/L sodium carbonate (Na2CO3) through an acid-base

titration, using methyl orange indicator to determine the end point.

The 1+1 nitric acid was prepared by mixing equal amounts of concentrated nitric acid

(55 %) and distilled water.

3.2.2.1.3 Lanthanum solution

The lanthanum solution was prepared by dissolving 58.65g of lanthanum oxide,

La2O3, in 250 mL concentrated HCl followed by dilution to 1000 mL with distilled

water.

3.2.2.1.4 Sodium hydroxide (0.020 mol/L)

This solution was prepared by dissolving 0.8000g of sodium hydroxide pellets in

deionised water. The solution was poured into a 1 L volumetric flask and diluted to 1

L with distilled water.

3.2.2.1.5 Hydrochloric acid (0.3 mol/L)

The dilute hydrochloric acid solution was prepared by diluting 29.5 mL concentrated

HCl (of density 1.16g/mL) to 1000 mL with distilled water in a volumetric flask.


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3.2.2.1.6 Sodium nitrate (1.0 mol/L)

This electrolyte solution was prepared by dissolving 42.4985g of NaNO3(dried in an

oven at 105 C for 12 hours) and making to the mark in a 500 mL volumetric flask

with distilled water. Working electrolyte concentrations were prepared by diluting

calculated volumes of the 1.0 mol/L sodium nitrate solution.

3.2.2.1.7 Sulphate solution (1000 mg/L)

This solution was prepared by dissolving 1.4790g of Na2SO4(dried in an oven at 105

C for 6 hours) and making to the mark in a 1000 mL volumetric flask with distilled

water.

3.2.2.1.8 Magnesium solution (1000 mg/L)

This solution was prepared by dissolving 6.1024g of Mg(NO3)2(dried in an oven at

106 C for 6 hours) and making to the mark in a 1000 mL volumetric flask with

distilled water.

3.2.2.1.9 Calcium solution (1000 mg/L)

The calcium solution was prepared by dissolving 4.0943g of Ca(NO3)2 (dried in an

oven at 106 C for 6 hours) and making to the mark in a 1000 mL volumetric flask

with distilled water.


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3.2.2.2 Standard solutions

3.2.2.2.1 Standard phosphate solution

A standard phosphate stock solution (1000 mg/L) was prepared by dissolving 1.4330g

of analytical grade anhydrous KH2PO4 (dried for 1 hour in an oven at 105C) in

distilled water and making to the mark in a 1000- mL volumetric flask. Intermediate

standard solutions (100 mg/L) were prepared by diluting 25 mL of the stock solution

in a 250 mL volumetric flask. The intermediate standard solutions were used to

obtain working phosphate concentrations.

3.2.2.2.2 Standard calcium solution (for AAS determination of calcium).

A standard calcium solution (100 mg/L) was prepared by suspending 0.2497g of

CaCO3 (dried at 180C for 1 hour) in water followed by addition of 20 mL of 1+1

HNO3to dissolve. 10 mL of concentrated HNO3was added to the solution which was

later diluted to 1000 mL with distilled water.

3.2.3 Determination of phosphate ions in solution using ion chromatography

Phosphate ions in solution were determined using an ion chromatography technique.

In principle, a small volume of aqueous sample is injected into an ion chromatograph

to flush and fill a constant-volume sample loop. The sample is then injected into a

flowing stream of carbonate-bicarbonate mobile phase. The sample is pumped

through two different ion exchange columns, then a conductivity suppressor device,

and into a conductivity detector. The two ion exchange columns, a precolumn or

guard column and a separator column, are packed with an anion exchange resin. Ions

are separated into discrete bands based on their affinity for the exchange sites of the


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resin inside the guard and analytical columns. The guard column extends the life of

the analytical column by trapping organic compounds and other species that could

destroy the analytical column. The guard column also adds about 20 % to the total

separation capacity of the analytical system. The conductivity suppressor is an ion

exchange-based device that reduces the background conductivity of the mobile phase

to a low or negligible level and simultaneously converts the anions in the sample to

their more conductive acid forms. The separated anions in their acid forms are

measured using an electrical conductivity cell. Anion identification is based on the

removal of phosphate ions from aqueous solutions using bauxite and kaolinite obtained from malawi

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