# Remediation of Copper Contaminated Soils Using Water Containing Hydrogen Nanobubbles
## 1 Introduction  
The use of chemical substances is increasing, and the resulting outflows from factories, gas stations, and industrial complexes are also increasing. Soil contamination, an obstacle to development, needs to be addressed, and contaminated land needs to be reclaimed. While the buffering capacity of land is comparatively large compared to the volume of contaminants, much depends on the soil characteristics and environmental factors, and so the buffering capacity of the land varies [1]. In particular, soilcontamination sourced from chemical facility effluents has characteristics that can be difficult to effectively treat as compared to water or air pollution. Soil remediation methods can be categorized into in situ or ex situ, depending on where treatment takes place. Offsite processing, the ex situ method of restoration, is more effective and faster, but it is also more expensive. By contrast, onsite processing, the in situ method, has a lower cost and uses technologies pertinent to the characteristics of the site [2]. While some in situ remediation methods employ ecofriendly enhancers, chemical solvents are also employed, due to the inherent difficulty in desorbing heavy metals from fine-grained soil. Toxic heavy metals exist in water media, and effective removal represents a continuing challenge for the scientific community as a whole. To remedy this, numerous studies have been published discussing the use of adsorbent materials for aqueous media decontamination from heavy metal ions [3; 4; 5; 6]. However, the improper extraction of enhancers employed for remediation may result in enhancers remaining in the soil at the conclusion of treatment, causing secondary contamination [7]. The diameter of the nanobubbles ranges from 1 to 999 nm [8]. Bubbles with a diameter greater than 50 \(\upmu\)m are classified as microbubbles, while bubbles with a diameter size between 10 and 50 \(\upmu\)m are classified as microbubbles, and bubbles with a diameter under 200 nm are nanobubbles [9]. The microbubbles have a fast rate of rise due to their large buoyant force. It does not last long in the water and rapidly rises to the surface, and it bursts when it reaches the surface. On the other hand, the microbubbles become smaller over time as they stay in water due to their slow rate of increase. If the diameter of the microbubble is 10 \(\upmu\)m, the internal pressure of the microbubble is about 0.3 atm. As the diameter of the microbubble decreases to 1 \(\upmu\)m and 100 nm, the internal pressure of the microbubbles rises up to 3 and 30 atm, respectively. During this process, the internal gas spreads according to Henry's Law, so all gases in the microbubble spread into the surrounding water until the microbubble disappears [10]. On the other hand, the nanobubbles exist in water for a long time once they are created. This is known to be due to the strong hydrogen bonds found in ice and gas that hydrate the surface of the nanobubbles. These bonds prevent the spread of internal gases and maintain an adequate mechanical balance against high internal pressures, as observed via Attenuated Total Reflectance Infrared (ATR-IR) [11]. These various characteristics of the nanobubbles have been observed by many researchers in different areas, including water purification, drag reduction, purification of surface contamination, removal of tumors using shock waves caused by the collapse of bubbles, growth promotion of animals and plants, and contrast medium. The characteristics of the nanobubbles are that they have a wide specific area, they generate radicals on their boundary surfaces, and they permit high internal pressure in the liquids. Thus, studies looking into how to exploit these beneficial characteristics of the nanobubbles are being actively pursued in various fields, including surface cleaning, remediation of poor water quality, and as a supersonic contrast agent [9].