Abstract
Eskom, the chief power generating and supply company in South Africa, has power stations that are
mainly coal-fired. The surface water used in the condenser tubes for cooling purposes at these stations
is rich in dissolved organic and inorganic compounds. Due to the relatively high concentration of alkaline
earth metals (notably Ca and Mg) in the water, super-saturation of mineral phases that are responsible
for scale formation occurs in a range of physico-chemical parameters (such as pH, ionic strength and
temperature). Natural organic matter (NOM) in aquatic environments influences the speciation and
mobility of metals, and is available in high concentrations in different fractions. The humic substances
fraction, for instance, has concentrations ranging 10-200 mg/L. Studies in this thesis typify the
characteristics and role of NOM with respect to complexation to metal cations. This complexation is
understood to minimize scaling potential at the Lethabo and Kriel power stations, as archetypical power
generating stations in South Africa. Furthermore, PHREEQC based simulations using the modified
Tipping and Hurley database with Windermere’s Humic Acid Model (T_H-WHAM) were used to speciate
and quantify complexation outputs.
Conventional NOM characterization techniques such as ultra-violet (UV) analysis, total and dissolved
organic carbon (TOC/DOC) analysis, specific ultra violet absorbance (SUVA) measurements have been
found lacking in terms of analysis time and robustness. As an alternative, detailed characterization of
NOM compounds in raw and cooling water (extracted by solid phase extraction) was achieved by the use
of liquid chromatography-organic carbon detection-organic nitrogen (LC-OCD-OND), fluorescence
excitation emission matrices (FEEM) and 2 dimensional gas x gas chromatography with a time-of-flight
mass spectrometer (GC x GC-TOFMS). Organic compounds with different molecular weights were
characterized i.e. those with >600 g by LC-OCD-OND and <600 g by GC x GC-TOFMS).
Fluorescing/chromophoric NOM compounds determined by FEEM were mainly humic substances. The
fractions obtained by GC x GC-TOFMS (such as aliphatic and aromatic), highly depended on pre...
Ph.D. (Chemistry)