Upload
loweschevy12
View
217
Download
0
Embed Size (px)
Citation preview
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
1/119
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
2/119
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
3/119
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
4/119
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
5/119
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
6/119
vi
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
7/119
vii
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
8/119
viii
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
9/119
ix
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
10/119
x
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
11/119
xi
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
12/119
xii
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
13/119
xiii
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
14/119
xiv
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
15/119
xv
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
16/119
1
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
17/119
2
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
18/119
3
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
19/119
4
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
20/119
5
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
21/119
6
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
22/119
7
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
23/119
8
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
24/119
9
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
25/119
10
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
26/119
11
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
27/119
12
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
28/119
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
29/119
14
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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
30/119
15
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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
31/119
16
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
32/119
17
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
33/119
18
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
34/119
19
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
35/119
20
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
36/119
21
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+.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
37/119
22
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
38/119
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
39/119
24
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
40/119
25
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
41/119
26
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
42/119
27
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):
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
43/119
28
++ 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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
44/119
29
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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
45/119
30
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
46/119
31
)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,
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
47/119
32
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
48/119
33
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
49/119
34
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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
50/119
35
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
51/119
36
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)
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
52/119
37
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
53/119
38
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).
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
54/119
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
55/119
40
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
56/119
41
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.
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
57/119
42
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
8/11/2019 Removal of Phosphate Ions From Aqueous Solutions Using Bauxite and Kaolinite Obtained From Malawi
58/119
43
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