CHARACTERIZATION OF THE NATURAL ORGANIC MATTER IN THE COOLING WATER CIRCUITS AT

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CHARACTERIZATION OF THE NATURAL ORGANIC MATTER IN THE COOLING WATER CIRCUITS AT

LETHABO POWER STATION

by

MPHONYANA THANJEKWAYO

Supervisor: Dr M. Shumane Co-supervisor: Prof C.A Buckley

Submitted in fulfillment of the requirements for the degree of

Master of Technology in the Faculty of Science, Department of Advanced Chemical Technology, University of Johannesburg

December 2005

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Abstract

Scaling is a major problem in cooling water circuits as it reduces water flow and therefore affects the efficiency of the circuit. The natural organic matter has been suggested in earlier studies to limit the formation of calcium carbonate scaling by complexing the calcium ion. It was therefore the aim of this study to characterize the natural organic matter in the cooling water circuits at Lethabo power station (Vereeniging) and to investigate its potential to complex with calcium.

The cooling water and raw water samples were comprehensively analyzed for major metal ions, anions and dissolved organic carbon using AAS, ICP-OES, IC and TOC analyzer and the results entered into MINTEQA2 speciation program to determine the precipitation potential of aragonite and calcite in the water samples. The natural organic matter from the cooling water and raw water were isolated initially through the cross- flow ultrafiltration using a polysulfone membrane with a molecular weight cut-off of 45 kDa. The collected isolates were characterized by ultraviolet-visible spectrophotometer, Fourier transform-infrared spectroscopy and (carbon, hydrogen and nitrogen) elemental analysis. The natural organic matter was also fractionated on ultrafiltration stirred cells using membranes with molecular weight cut-off of 1 kDa, 10 kDa and 100 kDa and then characterized using high performance size-exclusion chromatography. The isolated fractions were also titrated with sodium hydroxide and with calcium chloride to determine the amounts of carboxylic and phenolic groups available for complexation and the extent of the complexation of the fractions with calcium respectively.

The speciation results from MINTEQA2 indicated that the raw water had a potential to be corrosive and the cooling water had a potential to scale. Higher concentrations of the

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This low molecular weight fraction was further confirmed by the results from the high performance size-exclusion chromatography analysis of the fractions obtained from the fractionation process using ultrafiltration stirred cells. The specific ultraviolet absorbance and ratios of 465 to 656 nm absorbances results indicated that the organic compounds were mostly aliphatic in character. Titration of the concentrated organic isolates with sodium hydroxide solution revealed that the fractions had a high content of titratable acidic groups and titrating with calcium chloride showed that there is considerable complexation with calcium to affect calcium carbonate precipitation.

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Declaration by the candidate

I, MPHONYANA THANJEKWAYO, declare that unless indicated, this dissertation is my own work and to the best of my knowledge it has not been submitted for a degree at another University or Institution.

………

MPHONYANA THANJEKWAYO December 2005

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Acknowledgements

The ideas that have found expression in this thesis have their roots in discussion with many exceptionally bright individuals. I may not fully understand or trace the paths by which their ideas were constructed and shaped, but I am most grateful to all of you from whom I have been privileged to learn.

I am particularly grateful to:

• Dr Manelisi Shumane, former Head of Advanced Chemical Technology department and Professor Chris Buckley, Head of the Pollution Research Group at the University of KwaZulu-Natal, for their supervision, input and advice.

• Associate Professor Lingam Pillay of Durban Institute of Technology for the induction course on membranes especially on ultrafiltration membranes.

• Mr. Gerhard Gericke, Chief Consultant at Eskom for organizing the water samples, advice and subsequent funding.

• Research Committee at the University of Johannesburg and National Research Foundation (Technikon Research Development Programme) for funding of the project.

• Water Research Commission for funding my visits to the University of Kwazulu- Natal and Durban Institute of Technology.

• Department of Labor in conjunction with the National Research Foundation for awarding the scholarship for my studies.

• Finally, to all my fellow postgraduate students in the M. Tech room at the university, especially to Mokae Bambo, the shared times of laughter were often a welcome distraction from the reality of my work. I am most humbled, very thankful and eternally grateful to you all.

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3.4.1 Elemental and ash content analyses 47 3.4.2 Ultraviolet-visible analysis 48 3.4.3 Specific ultraviolet absorbance analysis 48 3.4.4 Ultraviolet-visible absorbance ratio 48 3.4.5 Fourier transform infrared analysis 49 3.4.6 Total organic carbon analysis 49 3.4.7 High performance size exclusion chromatography analysis 49 3.4.8 Alkalimetric titration of dissolved organic matter 50 3.4.9 Total dissolved solids 50 3.4.10 Calcium-Natural Organic Matter complexation 51

3.5 MINTEQA2 simulation 51

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Chapter 4: Results and Discussions 52

4.1 Chemical composition of the raw water and cooling water samples

and the scaling potential 53

4.1.1 General parameters 54

4.1.2 Metal ion analysis 55

4.1.3 Anion analysis 57

4.1.4 Cation-anion balance of the raw and cooling water 58 4.1.5 Saturation indices of calcite and aragonite 59 4.1.6 Natural organic matter interactions with metal ions 62

4.1.7 MINTEQA2 modeling 64

4.1.8 Conclusions 66

4.2 Isolation and characterization of the Natural Organic Matter 68

4.2.1 Isolation 68

4.2.1.1 Isolation of commercial humic acid 69 4.2.1.1.1 Ultraviolet-visible spectroscopy 69 4.2.1.1.2 Alkalimetric titration of the commercial humic acid sample 72 4.2.1.1.3 Isolation of Natural Organic Matter from raw water and

cooling water samples 73

4.2.2 Characterization of the Natural Organic Matter 75

4.2.2.1 Ultraviolet-visible characterization 75 4.2.2.2 Fourier transform infrared spectrophotometer 78 4.2.2.3 Elemental analysis 80

4.2.3 Fractionation of ultrafiltration stirred cells 81 4.2.4 High performance size-exclusion chromatography analysis 86

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4.3 Calcium-Natural Organic Matter interactions 93 4.3.1 Proton titration curves for the raw water and cooling water samples 94

4.3.2 Calcium binding 97

Chapter 5: Conclusions and recommendations 100

5.1 Conclusions 100

5.2 Recommendations 102

References

Appendix A Atomic absorption spectrophotometry Appendix B Ion chromatography

Appendix C Statistical analysis of random errors Appendix D Calcium-ion selective electrode

Appendix E MINTEQA2 calibration and validation Appendix F MINTEQA2 OUTPUT

Appendix G Polystyrene sulfonate standards Appendix H Isolation of Natural Organic Matter Appendix I Anion-Cation balance calculations

Appendix J Calculations of the carboxyl and phenolic acid concentrations from proton titrations

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List of Figures

Figure 1.1 Hypothetical structure of humic acid 2 Figure 2.1 Classification of the composition of Natural Organic Matter

(after separating natural organic matter into different components) 12 Figure 2.2 Model structure of humic acid 13 Figure 2.3 Model structure of fulvic acid 14 Figure 2.4 Typical porous membrane for ultrafiltration and microfiltration 19 Figure 2.5 Typical porous membrane for reverse osmosis and nanofiltration 19 Figure 3.1 Schematic representation of Triton X-100: Hydrophobic head

towards membrane and hydrophilic tails towards aqueous phase 43 Figure 3.2 Schematic diagram of cross-flow ultrafiltration membrane system 45 Figure 3.3 Schematic procedure for ultrafiltration fractionation 47 Figure 4.1 Efficiency graph for the isolation of commercial humic acid

through a cross-flow ultrafiltration membrane module 70 Figure 4.2 Comparison between the UV-Vis absorbance of NBW and

CHA at pH 8 76

Figure 4.3 UV-Vis spectra of concentrated samples of the raw water 76 Figure 4.4 UV-Vis spectra of concentrated samples of the cooling water 77 Figure 4.5 FT-IR difference spectra of extracted humic acid samples 78 Figure 4.6 Fourier transform infrared spectrum of cooling and raw water

samples 79

Figure 4.7 High performance size-exclusion chromatogram of polystyrene sulfonate standards 87 Figure 4.8a Chromatograms of natural organic matter fractions of the

raw water 88

Figure 4.8b Chromatograms of Natural Organic Matter fractions of the

cooling water 89

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Figure 4.9 Titration curves of cooling water and raw water solutions 95

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List of Tables

Table 2.1 Comparison of four membrane processes 18 Table 2.2 Dissolved organic carbon fraction distributions 23 Table 2.3 Dissolved organic carbon values of natural samples

predicted using fluorescence spectroscopy model 28

Table 3.1 Chemicals used 39

Table 4.1 Raw water and cooling water important parameters 54 Table 4.2 Atomic absorbance spectroscopy and ICP-OES analyses of the

raw water and cooling water samples 56 Table 4.3 Ion chromatography of the raw water and cooling water samples 58 Table 4.4 Percentage differences of the cation-anion balance 59 Table 4.5a Raw water saturation indices of aragonite and calcite

at 24 ^oC and 40 ^oC 60

Table 4.5b Cooling water saturation indices of aragonite and calcite

at 23.70 ^oC and 40 ^oC 61

Table 4.6 Comparison of calcium concentrations determined

by EDTA titration, Ca-ion selective electrode, AAS and ICP-OES 62 Table 4.7 Percent distribution of calcium determined with MINTEQA2

at 25 ^oC 66

Table 4.8 Total organic carbon results of the initial feed, permeate and

concentrated commercial humic acid samples 71 Table 4.9 Titration of commercial humic acid solution with 0.01 N NaOH 72 Table 4.10 Concentration of dissolved organic matter in raw water and

cooling water using UV₂₅₄ 74 Table 4.11 Concentration of dissolved organic matter in raw water and

cooling water using total organic carbon 75 Table 4.12a: Literature elemental analyses of the commercial humic acid 80

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Table 4.12b Elemental analyses of the commercial humic acid,

cooling water and raw water powders 81 Table 4.13a Distribution of dissolved organic carbon fractions obtained

from raw water 82

Table 4.13b Distribution of dissolved organic carbon fractions obtained

from cooling water 83

Table 4.14 Guidelines for the nature of Natural Organic Matter using

SUVA 84

Table 4.15 UV254 and specific ultraviolet absorbance 84 Table 4.16 E4/E6 ratio for the raw water and cooling water samples 85 Table 4.17a Approximated molecular weights of Natural Organic Matter in

raw water samples 91

Table 4.17b Approximated molecular weights of Natural Organic Matter in

cooling water samples 92 Table 4.18 Estimated carboxyl and phenolic contents of the raw water and

cooling water from proton titration curves 96 Table 4.19 Tabulation of log β_ovfor raw and cooling water 98

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Abbreviations

Abbreviation Explanation

AAS Atomic absorption spectroscopy CHA Commercial humic acid

CHN Carbon, Hydrogen and Nitrogen C-NMR Carbon nuclear magnetic resonance

CW Cooling water

DOC Dissolved organic carbon (<0.45 µm filtrate) DOM Dissolved organic matter

DBP Disinfection by-product

EDTA Ethylene diamine tetraacetic acid

FA Fulvic acid

FAF Fulvic acid fractions FT-IR Fourier transform infrared

HA Humic acid

HAA Haloacetic acid HAF Humic acid fractions HFi-A Hydrophilic acid HFi-B Hydrophilic base HFi-N Hydrophilic neutral HFo-A Hydrophobic acid HFo-B Hydrophobic base HFo-N Hydrophobic neutral HS Humic substances

HPLC High performance liquid chromatography

HPSEC Higher performance size exclusion chromatography HMWCO High molecular weight cut-off

IC Ion chromatography

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ICP-OES Inductive coupled plasma-optical emission spectroscopy

ISE Ion selective electrode

LMWC Lower molecular weight compounds LRFA Laurentian river fulvic acid

LRHA Laurentian river humic acid

MF Microfiltration MIEX Magnetic ion exchange resin

MWCO Molecular weight cut-off NBW Natural brown water

NF Nanofiltration NOM Natural organic matter

PES Poly(ether)sulfone

PWF Pure water flux

RW Raw water

SUVA Specific ultra-violet absorbance TDS Total dissolved solids

THM Trihalomethane THM-FP Trihalomethane formation potential

TOC Total organic carbon

UF Ultrafiltration UV Ultraviolet UV-Vis Ultra-violet-visible XAD Synthetic polymer resin (adsorbent)

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Units

cm centimetre

cm^-1per centimetre

g gram

kDa kilo Daltons

kPa kilo Pascal

L litre

mg milligram

mg L^-1 milligram per litre

mg C L^-1 milligram of Carbon per litre

ml millilitre

mm millimetre

N normality

nm nanometre

oC degree Celsius

Pa Pascal

ppm parts per million

µm micrometre

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CHARACTERIZATION OF THE NATURAL ORGANIC MATTER IN THE COOLING WATER CIRCUITS AT