Towards identification of floc compounds in water using multi-frequency fluorescence lifetime analysis

Abstract

UV disinfection is commonly used in water treatment to inactivate pathogens such as Cryptosporidium and viruses to prevent diseases such as cryptosporidiosis and norovirus in communities. Disinfection typically follows water treatment steps, such as coagulation, flocculation, clarification, and filtration. However, particles in water, for example, flocs 0.1 to 100 μm in diameter, made from humic and inorganic substances present in the water, surrounding a Cryptosporidium oocyst or virus, can protect the pathogens from UV exposure. Although water treatment steps prior to disinfection remove 99% of the particulates, particles can still be present in the 1000s to 10,000’s per litre after filtration. While the chances of a floc particle carrying a virus or oocyst are typically low, in some regions, particularly during calving in the dairy industry, oocyst concentrations in the water might be high due to cryptosporidiosis in calves. Therefore, it is useful to test the properties of the floc compound for UV penetration to determine whether the disinfection method is appropriate.

In this thesis, a technique that uses multi-frequency analysis to measure the fluorescence lifetime of a fluorophore to provide information on particle composition is presented. Frequency-domain fluorescence fluorometry was used to determine the fluorescence lifetime. This was achieved using an experimental setup that used a laser diode operating at 100 mW and modulated at 10–60 MHz to excite the fluorophores, optical elements to focus and filter the light, and detectors to collect the fluorescence emission signal via a storage oscilloscope. The signals were then processed using a MATLAB program to determine the fluorescence lifetime.

Fluorescence lifetime measurements were challenged by the chemical and physical behaviour of the fluorophore and the adsorption of the fluorophore to the floc particles. Therefore, standard measurements such as turbidity, pH, particle size, and fluorescence were used to understand the absorption/adsorption of fluorescein to flocs. Fluorescence was observed at the 260–490 nm excitation wavelengths, with fluorescence emissions at approximately 510 nm. The particle size and turbidity measurements showed that fluorescein acted as a flocculant, with the particle size increasing with increasing fluorescein concentration. Fluorescence intensity measurements from a standard fluorescence spectrophotometer were used to calculate fluorescein adsorption on humic acid and kaolin to generate adsorption isotherms. Fluorescein was bonded to kaolin 10 times more than to humic acid. Adequate flocculation required a pH of 6.5 to produce reasonably flocculated particles in the sample. Surface charge analysis showed that the use of buffer to control pH required more alum to neutralise the surface charge of humic acid and kaolin.

Multi frequency measurements and subsequent analysis showed that the fluorescence lifetime and contamination ratio were 4.2 ± 0.3 ns and 0.09 ± 0.05 for fluorescein. The fluorescence lifetime of fluorescein was compatible with the results of previous studies using different techniques. The samples with floc particles had a larger excitation light contamination ratio than those without particles; therefore, the contamination ratio could be used as a measure of particle contamination in the samples. The fluorescence lifetime of fluorescein did not change when fluorescein was attached to humic acid particles, but increased by 0.6 ns for kaolin floc particles.