Tsinghua University | Research on In-situ Remediation Technology of Groundwater Based on Ozone Micro-nano Bubbles

Tsinghua University | Research on In-situ Remediation Technology of Groundwater Based on Ozone Micro-nano Bubbles

Groundwater, an important source of water supply, is facing increasingly severe organic pollution problems. The development of green and sustainable in-situ remediation technologies is extremely urgent, and the ozone micro-nano bubble technology has shown great application potential in the field of in-situ remediation of polluted groundwater.

The sources of groundwater organic pollution are numerous and widespread, mainly including industrial pollution sources, agricultural pollution sources and domestic pollution sources. Industrial activities such as the discharge of industrial wastewater by enterprises, production activities in the petrochemical industry, and leakage from underground oil tanks at gas stations, agricultural practices such as extensive use of pesticides and fertilizers on farmland and irrigation with sewage, as well as domestic activities such as leakage of urban domestic sewage and leachate from landfill sites, have all caused severe pollution to groundwater.

Conventional Remediation Technology Analysis
Polluted groundwater remediation technologies are divided into ex-situ and in-situ remediation. Ex-situ remediation involves extracting contaminated groundwater and excavating contaminated soil for treatment, while in-situ remediation treats the contamination at the original location with minimal environmental disturbance. Compared to ex-situ remediation, in-situ remediation is characterized by lower costs and less impact on the surrounding environment, thus receiving extensive attention. Currently, widely applied in-situ remediation technologies for organic contaminated groundwater include monitored natural attenuation, enhanced bioremediation, groundwater aeration combined with soil vapor extraction, and in-situ chemical oxidation.

The monitored natural attenuation method relies on physical, biological and chemical processes in nature to reduce the content and toxicity of organic pollutants, mainly including convection dispersion, adsorption, biodegradation and chemical transformation. This method mainly depends on the degradation of organic matter by microorganisms in the remediation process of organic contaminated sites, and has a good removal effect on volatile and semi-volatile petroleum hydrocarbons and halogenated organic compounds. It is often applied to the remediation of sites with a relatively low degree of pollution. However, the degradation effect of the natural attenuation method on pollutants is dependent on environmental conditions and is often affected by factors such as dissolved oxygen concentration, temperature and pH within the site. The remediation time required is relatively long, and the remediation effect is not good for sites with severe pollution, so its application range is limited. Enhanced bioremediation is a further development of the natural attenuation method, which achieves the remediation of contaminated groundwater by improving the site environment or artificially cultivating and introducing specific organisms. However, there is currently a lack of technology that can efficiently and continuously provide dissolved oxygen to the site, and the treatment effect is also limited for sites with high pollutant concentrations or high salinity.

Ozone and micro-nano bubble technology
Ozone has strong oxidizing properties, with an oxidation-reduction potential of +2.1V, showing stronger oxidizing properties than commonly used hydrogen peroxide and potassium permanganate. Ozone can be generated through chemical reactions, ultraviolet radiation, and corona high-voltage discharge, among which the corona high-voltage discharge method has a large ozone output and low energy consumption, making it more suitable for industrial applications. The ozone oxidation method has received increasing attention in the fields of drinking water disinfection and pollution removal due to its significant treatment effect and low secondary pollution. The oxidation pathways of ozone on pollutants mainly include direct oxidation and indirect oxidation. It can directly oxidize and degrade some pollutants and also oxidize and degrade most organic pollutants by decomposing to generate hydroxyl radicals. However, the low dissolution efficiency of ozone and the rapid self-decomposition rate of dissolved ozone limit its practical application in the remediation of contaminated groundwater.

Micro-nano bubbles refer to bubbles with a particle size ranging from 10 nm to 100 μm, including both microbubbles and nanobubbles. Due to their size being much smaller than that of millimeter-sized bubbles, micro-nano bubbles have a larger specific surface area and internal pressure, significantly enhancing the mass transfer efficiency of gases. Moreover, their upward movement speed is relatively slow, allowing them to remain in water for a longer period. In recent years, the technology for generating micro-nano bubbles based on the shear mixing of gas-liquid two-phase fluids has developed rapidly, enabling the production of even smaller micro-nano bubbles and further improving the mass transfer efficiency of gases. The longer existence time of micro-nano bubbles in water is mainly attributed to their slower rising speed, the presence of surface interface charges, and their existence in clusters. The surface interface charges can prevent the coalescence of bubbles, extending their lifespan in water. When existing in clusters, the dissolution rate of bubbles slows down, allowing them to persist for a long time.

Advantages of Ozone Micro-nano Bubble Technology

Ozone micro-nano bubble technology combines the strong oxidation of ozone with the excellent characteristics of micro-nano bubbles, providing a new approach for the application of ozone in the remediation of organic contaminated sites and groundwater. Compared with ozone millimeter-sized bubbles, ozone micro-nano bubbles have a faster ozone dissolution rate, a longer ozone retention time, and a higher ozone utilization efficiency. They can also further promote the generation of hydroxyl radicals, significantly enhancing the treatment effect of ozone on pollutants. In the field of wastewater treatment, ozone micro-nano bubbles can greatly improve the mass transfer efficiency of ozone, increasing the removal efficiency of organic pollutants by nearly double. In the remediation of contaminated sites, ozone micro-nano bubbles also show good application effects, capable of large-scale migration within the site along with groundwater movement, effectively reducing pollutant concentrations.

The results of physical model tests show that ozone micro-nano bubbles have high mass transfer efficiency, wide migration range and good remediation effect. Micro-nano bubbles increase the dissolution efficiency of ozone by three times, extend the half-life of dissolved ozone by 14 times, and effectively provide dissolved oxygen to the water body. Ozone micro-nano bubbles have strong degradation ability and high removal efficiency for pollutants. When combined with hydrogen peroxide, the removal efficiency of pollution can be significantly improved. Micro-nano bubbles can migrate over a wide range in the soil with groundwater, showing obvious hydrodynamic dispersion and adsorption characteristics, and have a high remediation efficiency for contaminated groundwater. Field test results also prove that the in-situ remediation technology of groundwater based on ozone micro-nano bubbles has significant advantages of being environmentally friendly, fast and efficient. After six days of remediation, the removal rate of trichloroethylene in the groundwater within the site exceeded 99%.