Using Fluorescein as a Fluorophore to test UV and Light Penetration of Flocculated Particles

S.A.D.A Nilakshi Dissanayake, Michael J Cree, Mark Lay, Lee Streeter, Graeme D.E. Glasgow School of Enginnering, The University of Waikato

UV disinfection is commonly used in water treatment to inactivate pathogens such as cryptosporidium and viruses for preventing 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 a floc 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. While water treatment steps prior to disinfection remove 99% of the particulates, particles can still be present in the 1000’s to 10,000’s per litre after filtration. While typically the chances of a floc particle carrying a virus or oocyst might be 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 if the method of disinfection is appropriate.

This study examines UV and visible light penetration of humic and kaolin floc using fluorescein as a fluorophore. Kaolin and humic samples were prepared in 20, 40, 60, 80 and 100 mg/L concentrations 400 ml of each sample was dosed 200 µl of 1 mg/ml fluorescein and coagulated by adding enough Al2(SO4)3 so the solution had a zero zeta-potential. Flocculation was carried out in a jar tester apparatus by mixing at 100 rpm for 2 minutes, slow mixing at 30 rpm for 5 minutes, and settling for 5 minutes. Floc was also prepared using the same method using 400 ml aliquots of 60 mg/L kaolin, dosing with 50, 100, 200, 300 µl of 1 mg/ml fluorescein. After settling, water samples were measured for turbidity, pH, samples containing the floc for fluorescence intensity using a fluorescence spectrophotometer and particle size under the microscope.

Fluorescence was observed at the 260 nm and 490 nm excitation wavelengths,with fluorescence emissions around the 510 nm wavelength. Fluorescence intensity decreased with increased kaolin and humic concentration, but the intensity could be increased by increasing the dose of fluorescein. At the lower doses of fluorescein and higher concentrations of humic and kaolin, first and second order refraction in the excitation emission matrix was observed which was due to the diffraction grating of the instrument and intensified its excitation light reflecting off the particles rather than being absorbed by the fluorescein. To prevent this from happening, a 400 ml sample containing 20 mg/L kaolin required 333 µl of 1 mg/L fluorescein, a kaolin to fluorescein mass ratio of 60:1. Turbidity increased with increased humic and kaolin concentrations and pH was maintained in the range of 6 to 7. Particle size ranged from 20 to 315 µm, and the particles were a mixture of spherical and non-spherical shapes.