Application of ozone nanobubbles in wastewater treatment

Abstract

Ozone nanobubbles, in recent times, seems to have wide range of applications in wastewater treatment. Several unique characteristics of nanobubbles such as high stability, increased mass transfer rates etc. make them eminent in improving the efficiencies of the conventional advanced oxidation processes (AOPs). These properties of ozone nanobubbles have been studied in detail. Many practical applications of ozone nanobubbles have been presented. In addition, directions for future research of ozone nanobubble technology and their application in real life problems have been identified. Ozone is known to be one of the most powerful oxidant used in wastewater treatment processes. One of the primary disadvantages associated with ozone is its restricted solubility and instability when dissolved in an aqueous solution. These characteristics impose limitations on its potential applications and need the use of specialized systems to facilitate gas-liquid interaction. In this work, the novel advanced oxidation process a so-called ozone nanobubble technology for degradation of the pollutants at high salinity conditions has been investigated. Enhancing the ozonation process through the utilization of ozone nanobubbles was studied in this work. The findings of the experiment and subsequent analysis indicate that the presence of nanobubbles enhances the process of ozonation through three key mechanisms: (i) an increased mass transfer coefficient, (ii) a higher rate of reactive oxygen species (ROS) generation attributed to the charged interface, and (iii) the nanobubble interface serving as an active surface for chemical reactions. The study showcased the degradation of methylene blue dye exhibited a much higher rate of dye degradation compared to ozone microbubbles. The confirmation of the radical degradation mechanism was achieved by the utilization of electron spin resonance (ESR) measurements. The developed process has high potential for application in industrial scale textile wastewater treatment. In pharmaceuticals, especially antibiotics, in industrial and domestic effluents causes serious damage to the environment. Classical wastewater treatment processes, in particular conventional biological treatment methods, are not sufficient to rapidly eliminate antibiotics. Typically, Advanced Oxidation Processes (AOPs) based on activation of hydrogen peroxide, ozone or persulfate for production of particular type of radical species or singlet oxygen are used. One of the cutting edge technologies to increase effectiveness of AOPs based on ozone are nanobubbles based processes. Thus, this thesis focuses on utilization of ozone in the form of nanobubbles for degradation of tetracycline (TC). This studies revealed, that the presence of ozone nanobubbles had a substantial positive impact on the degradation of TC. This improvement may be attributed to the enhanced mass transfer and the production of reactive radicals that occur during the collapse of the nanobubbles. Identification of radical species revealed a significant contribution of hydroxyl radicals (•OH) and superoxide O2 •- in the degradation of the antibiotic. Based on identified by LC-MS intermediates a degradation mechanism has been described. Degradation of TC and intermediates transformations include demethylation, hydroxylation, ring-opening steps as well as cleavage of C-N bonds. Organic pollutants, especially dyes, present in substantial amounts with high molecular weight and complex structure are highly toxic and can contaminate natural waterways if improperly treated. AOPs have proven to be an effective solution for chemical wastewater treatment. Ozonation is recognized as one of the most prevalent AOPs. Nevertheless, some cases show a lowered efficiency of O3 utilization is attributed to its inadequate distribution in the treated water causing low mass transfer coefficient as well as shorter half-life. This study demonstrates the application of ozone nanobubbles to enhance the degradation of organics under high loading conditions. We propose an integrated method that utilizes bulk nanobubbles to enhance the reactivity of ozone for the degradation of organics. The degradation of the organic pollutant by ozone nanobubbles demonstrated a threefold increase in reaction rate constants compared to microbubbles. The degradation reaction exhibited second-order kinetics by ozone nanobubbles while ozonation alone offers first-order kinetics. The efficiency of removing the organic pollutant was higher under acidic pH conditions, as well as with lower concentrations of salts and surfactants. The presence of reactive oxygen species and hydroxyl radicals was verified using scavenging tests. Arsenic, a highly toxic element, is present in various water resources as As(III) and/or As(V). The removal of As(V) using adsorption is considered to be easier than the removal of As(III). In this work, we report the oxidation of As(III) to As(V) using ozone nanobubbles. Ozone has been widely used in the advanced oxidation process (AOP) to remove organics and inorganics in wastewater. The major advantage of ozone nanobubbles is their enhanced half-life, which leads to higher ozone solubility in water. In addition, the rate of mass transfer by using nanobubbles can be enhanced drastically. The present results revealed that ozone nanobubbles positively impacted the oxidation of As(III) to As(V). AOP process based on O3 NBs (nanobubbles) was most efficient with 99% oxidation rates of 1 ppm of As(III) within 20 minutes at 6.5 ppm ozone concentrations. An acidic pH of 3-7 promoted quick oxidation due to the high mass transfer coefficient of ozone nanobubbles. About ∼ 70% degradation of As(III) to As(V) was achieved at acidic (pH = 3) compared to ∼ 39% degradation at pH =11. The presence of salt (∼ 50 mM) in the solution not only hindered the ozone mass transfer coefficient but also reduced the stability of nanobubbles. The detection of reactive oxygen species, particularly hydroxyl radical was unravelled by utilizing 2-propanol as a scavenger.