Specific Application Scenarios of Ozone Micro-Nano Bubbles in Soil Environment Improvement (Part Two)

Specific Application Scenarios of Ozone Micro-Nano Bubbles in Soil Environment Improvement (Part Two)

Bubbles are ubiquitous in nature. The common ones visible to the naked eye are large bubbles such as beer foam, carbonated beverages, and soap bubbles, which are prone to burst due to their high internal pressure. This has prompted researchers to explore smaller bubbles. In 1981, Johnson et al. proposed the possibility of bulk nanobubbles, and in 1994, Parker et al. introduced the concept of interfacial nanobubbles. However, the claim that nanobubbles could exist stably for a long time was initially not accepted by the scientific community. It was not until the early 2000s that Lou et al. and Ishida et al. observed nanobubbles on the surfaces of mica and silicon wafers under atomic force microscopy, respectively, that nanobubbles gradually gained recognition from scholars and became a research hotspot.

The existence of nano-bubbles is accompanied by the generation of micro-bubbles and sub-micro-bubbles. People usually refer to this technology as micro-nano bubble technology. Micro-nano bubbles include micro-bubbles, sub-micro-bubbles and nano-bubbles. Due to their large specific surface area and certain buoyancy, they are widely used in flotation and air flotation. After more than 20 years of continuous exploration, a large amount of experimental evidence has shown that nano-bubbles can exist stably for a long time. Subsequently, research on the application of micro-nano bubble technology in environmental improvement has also emerged, and its application has been continuously developing, with particularly prominent applications in water treatment, aquaculture, agricultural planting and other fields.

Soil pollution and the application of micro-nano bubble technology

Soil is an important component of the environment. Once it is polluted, the pollutants will change the physical and chemical properties of the soil, thereby affecting the growth of plants, polluting crops, and endangering human health through the food chain. Therefore, it is of great significance to restore polluted soil and protect soil safety. In recent years, micro-nano bubble technology has attracted the attention of scholars at home and abroad in improving the soil environment. As an emerging soil remediation technology, it has the characteristics of high stability, large specific surface area, high interface potential, the ability to generate a large number of free radicals, and high gas-liquid mass transfer efficiency.

In the actual application process, micro-nano bubble technology is convenient and easy to operate, and has a good treatment effect. When the gas content in the water reaches supersaturation, micro-nano bubbles can still continue to transfer gas and maintain high mass transfer efficiency. This characteristic makes micro-nano bubbles have extremely high bubble density and lateral diffusivity. When the contraction of micro-nano bubbles reaches the limit, the internal gas pressure of the bubbles will tend to infinity. This self-pressurization effect causes the micro-nano bubble-water interface to rupture, thereby achieving efficient gas-liquid mass transfer.

The removal of organic pollutants in soil by micro-nano bubbles

In addition to soil aeration, micro-nano bubble technology also has a good treatment effect on organic pollutants in soil. Minamikawa et al. found that compared with the control water, the total dissolved CH4 emissions from paddy soil treated with oxygen nano-bubble water decreased by 20% - 28%. The oxygen nano-bubble water irrigation technology reduced the CH4 production in flooded paddy soil through oxidation. Besides changing their own physical properties to enhance the utilization efficiency of their own gases, micro-nano bubbles can also adsorb pollutants by taking advantage of their large specific surface area and other characteristics, thereby achieving the purpose of improving the environment.

Jiang et al. investigated the adsorption of perfluoroalkyl sulfonic acid on pristine graphene and functionalized graphene by using density functional theory and molecular dynamics simulation, and found that the removal rate of PFAS increased by 29.2% after aeration. Jenkins et al. combined oxygen micro-nano bubbles with microbial degradation to treat xylene contamination in soil, significantly alleviating the soil clogging problem caused by iron oxidation. Besides oxygen, ozone, due to its oxidation properties, is also an important gas source for treating organic pollutants. The mass transfer coefficient and utilization rate of ozone in water are higher in the micro-nano bubble system, and the total organic carbon removal rate of pollutants in the micro-nano bubble system is also greater. Fan et al. found that the maximum removal rate of 4-chlorophenol in soil by ozone nano-bubbles was 6.9 times higher than that of normal ozone macro-bubbles. Aluthgun-Hewage et al. treated contaminated sediments with ozone nano-bubble technology, achieving a removal rate of up to 91.50% for triphenyl compounds. Ozone micro-nano bubble water also has a good removal effect on trichloroethylene in soil. Micro-nano bubble technology can also be used for the remediation of petroleum-contaminated sites. Choi et al. treated the surface soil of a petroleum-contaminated landfill with a micro-nano bubble washing system, achieving a total petroleum hydrocarbon removal rate of 25.9%.

Micro-nano bubbles can not only effectively remove organic pollutants but also efficiently eliminate inorganic pollutants. Fu Kaibin et al. used a self-made micro-nano bubble flotation device to remediate copper ion-contaminated soil, reducing the copper ion concentration from 10 kg/t to 0.1 kg/t, with a removal rate of 90.2%. Jeong et al.'s research also demonstrated that micro-nano bubbles have a good effect on the removal of copper ions in soil, due to their large surface area and high ζ potential. Batagoda et al. found that under specific conditions, ozone nano-bubbles can achieve a removal rate of up to 97.5% for Cr(Ⅲ) in sediments. Air micro-nano bubbles can also enhance the removal of metal pollutants such as lead ions by adsorbents. Tang et al. discovered that interfacial oxygen micro-nano bubbles can effectively reduce the concentration of As(Ⅲ) in sediments and promote the transformation of As(Ⅲ) to lower-concentration As(V) and methylated As. The mechanism is that the hydroxyl radicals produced by oxygen micro-nano bubbles facilitate the non-biological oxidation of As, and the oxygen micro-nano bubbles improve the anoxic environment of the sediment, thereby enhancing the activity of As-oxidizing bacteria and As-methylating bacteria and promoting the biological oxidation of As. Additionally, oxygen nano-bubbles can effectively mitigate the migration of As in soil. Minamikawa et al.'s research also concluded that oxygen micro-nano bubble water can reduce the solubility of As in paddy soil.

The influence of micro-nano bubbles on soil microorganisms

Improving soil aeration in the root zone not only alleviates soil hypoxia but also enhances soil physical and chemical properties, resolving the water-air contradiction, promoting microbial activity and increasing enzyme activity, further changing the soil environment. The oxygen micro-nano bubble technology can enhance soil enzyme activity, possibly due to the stimulation of microbial growth and the increase in the activity of extracellular enzyme organic complexes. Underground drip irrigation with oxygen micro-nano bubble water can also alleviate the weakened soil permeability caused by long-term underground drip irrigation, increase soil oxygen diffusion rate, and enhance soil enzyme activity.

Pan et al. combined oxygen nanobubble water with clay materials and applied them to the surface of lake sediments, finding that it could effectively improve lake water quality and inhibit the release of phosphorus from sediments to the overlying water, thereby controlling eutrophication of the lake. Ji et al. found that oxygen micro-nanobubbles mainly improve the anoxic state and reduce organic matter concentration by increasing the DO concentration in the overlying water, raising the redox potential and sulfate concentration, which leads to a decrease in the abundance of mercury methylation genes and a reduction in the production capacity of methylmercury, thus achieving the purpose of improving the soil environment. In addition, oxygen nanobubble water can effectively alleviate biological pollution, reduce biodiversity, disrupt the mutualistic relationships among different microbial species, simplify the microbial network structure and reduce its scale, and decrease the sedimentation capacity of minerals in the microbial structure, thereby improving soil quality. Micro-nanobubbles can also enhance the activity of activated sludge. Fu et al. found that the formation of nanobubbles increased the space and pressure between anammox bacteria cells, causing the destruction of cell clusters and changes in their microstructure, which is conducive to the formation of porous microbial particles and avoids the clogging of air channels by cell aggregates, preventing the disintegration of particles.

The Application Prospects and Outlook of Micro-Nano Bubble Technology

Micro-nano bubble technology has demonstrated numerous advantages in soil environmental improvement, achieving remarkable results in removing organic and metal pollutants as well as influencing soil microorganisms. With the continuous deepening of research on micro-nano bubble technology, its application prospects in the field of soil remediation are extremely broad.