Thesis Title: Iron-activated persulfate for the inactivation of bacteria in the water

Friday 12 July 2019, at: 11:00, Venue: Hall Κ2.A11

Examination Committee

• Associate Professor Danae Venieri (advisor)
• Assistant Professor Paraskevi Panagiotopoulou
• Assistant Professor Nikolaos Xekoukoulotakis

Abstract

Protection of the environment and public health requires integrated and efficient water treatment, as it is already required by European legislation. In this context, the ever-increasing demand for high standards of hygiene has led to the exploration and development of effective disinfection techniques.

Waterborne diseases documented worldwide and their uncharted rates of transmission through the consumption of contaminated water demonstrate the importance of effective inactivation of pathogenic microorganisms, including bacteria, viruses and protozoa. Typically, water disinfection is achieved by chlorination and its compounds, which are considered powerful oxidizing agents and are capable of inactivating a wide variety of microorganisms.

Despite the efficacy of these chemicals, they are associated with significant health risks. New methods of disinfection have been discovered such as advanced oxidation process (AOPs), which has been described as an emerging group of techniques with high oxidation potential and biocide effect on various microorganisms in the water.

AOPs based on hydroxyl (HO•, E0= 1.8–2.7 V) and sulfate radicals (SO4•−, E0= 2.5–3.1 V) have been widely used as novel oxidants to degrade many pollutants. The formation of highly reactive sulfate radicals can be achieved by activating persulfate salt by heat, ultrasound, transition metals, and ultraviolet (UV) light or other means. Activated persulfate reactions have a wide application for environmental recovery as the formed roots react with organic chemicals that cause partial or complete mineralization.

The purpose of the present study was to activate persulfate salt with iron ions, ultrasound, but also to combine these two methods as well as to find the most effective concentrations for the inactivation of Escherichia coli and Enterococcus faecalis in water. The concentrations used were 100, 200 and 300 mg/L of Na2O8S2 and was activated with 10, 20 and 30 mg/L Fe2SO4 while concentrations of 100, 200 and 300 mg / L Na2O8S2 were used for ultrasonic activation. Finally, a combined experiment with concentrations of 200 mg/L Na2O8S2 activated with 30 mg/L Fe2SO4 and ultrasound was also tested.

The initial bacterial concentration in the suspension in each experiment was 106 CFU /mL while bacterial counts were performed by applying the serial dilution plate procedure and using the Nutrient Agar (LAB M) as culture medium.

According to the experimental results, the method of activating persulfate proved to be more effective at higher concentrations of both persulfate and iron ions. Faster inactivation was performed by iron activation, and particular for concentrations of 200 or 300 mg/L Na2O8S2 activated with 30 mg /L Fe2SO4. A 6 Log reduction of bacterial concentration occurred in about 60 min for both bacteria. However, in the majority of the study these two bacteria showed variations. Escherichia coli,  Gram (-) bacteria was deactivated faster than Enterococcus faecalis, Gram (+) bacteria, which also consists a thick cell wall, and this fact tends to be due to this difference. Inactivation of both bacterial species retarded when activation of persulfate carried out by ultrasound, although after about 105 min a 6 Log reduction was performed to 300 mg/L Na2O8S2 concentration. Ultimately, inactivation was very effective for both bacteria with a 6 Log reduction of approximately 40 minutes with 200 mg/L Na2O8S2 persulfate concentration activated with 30 mg/L Fe2SO4 and ultrasound.

Based on the results, the application of activated salts seems to be innovative, and effective for disinfecting aqueous samples and for inactivating fecal bacterial markers. Investigation of the effects of process variables is required to successfully reduce bacteria to acceptable limits for the protection of public health.