# Remediation of Copper Contaminated Soils Using Water Containing Hydrogen Nanobubbles
## 2 Experimental Methods and Condition
### Nanobubbles  
The hydrogen nanobubbles were created by using the equipment shown in Figure 1. Nanobubbles were created with hydrogen gas that was pressurized and filtered through a ceramic filter, so as to create tiny bubbles. These tiny bubbles were transferred to the surface of the water and crushed, thereby creating other nanobubble nuclei [14]. The nanobubbles were created for a period of 24 h. Upon completion, their zeta-potential was measured so as to appraise their stability. The parameters for the zeta-potential measurements and the measured value are presented in Table 2. The prepared nanobubbles were measured by using zeta-potential analysis equipment (ZetaPALS, Brookhaven Corp., USA), to measure the zeta potential of the nanobubbles (Figure 2c). The size and quantity of the generated nanobubbles were measured, using a laser within the Nanoparticles Tracking Analysis (NATA) equipment, as shown in Figure 1. Nanobubbles with an average diameter of 171 \(\pm\) 6.11 nm, a mode of 130 \(\pm\) 9.85 nm, and a concentration of 1.5 \(\pm\) 0.03 \(\times\) 10\({}^{8}\) particles/mL were thus obtained. The pH measured after the manufacture of the nanobubbles was confirmed to be neutral 7.  
The stability of the nanobubbles was confirmed over two months. For the experiment, stable 14-day-old nanobubbles were used [15]. Figure 3 shows how long nanobubbles are maintained. Figure 3a confirms the persistence by making nanobubbles in gasoline, and the results confirm that the nanobubbles are maintained for 120 h. Figure 3b shows a comparison of the changes in the concentration of the nanobubbles over 14 days, and it is the same as the nanobubbles used in this  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Soil Content** & **Sand** & **Silty Clay (\textless{}0.075 nm)** \\ \hline Soil Classification & SW & CL \\ Liquid Limit (\%) & 19.81 & 44.20 \\ Plastic Limit (\%) & NP & 23.82 \\ Specific Gravity (G\({}_{\text{s}}\)) & 2.59 & 2.46 \\ pH & 7.6 & 9.4 \\ Cation Exchange Capacity & - & 18.23 \\ (Cmol/kg) & - & 16.0 \\ BET Surface Area (m\({}^{2}\)/g) & - & 16.0 \\ Organic Matter (\%) & 0.22 & 0.85 \\ Water content (\%) & 17.3 & 40 \\ \hline \hline \end{tabular}  
* SW: well-graded sand, CL: low plastic clay, NP: non plastic.  
\end{table}
Table 1: Material properties of soils.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Measurement** & **Values** \\ \hline \multirow{4}{*}{Parameters} & Refractive Index & 1.331 \\  & Dielectric Constant & 78.54 \\  & Electric Field (V/cm) & 27.79 \\  & Viscosity (cP) & 0.890 \\ \hline \hline \multicolumn{2}{c}{Zeta-potential (mV)} & \(-\)20.13 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters for \(\zeta\)-potential measurement and value of zeta-potential of nanobubble.  
experiment. After day 14, approximately 15% of the nanobubbles were lost. Thus, the long-term existence of the nanobubbles was confirmed.  
Figure 1: Schematic view of a nanobubble Generator [16]. (**a**) Pressure gauge (air pressure in water tank), (**b**) pressure gauge (air pressure in air tank), (**c**) water tank, (**d**) air, (**e**) gas tank, (**f**) water, (**g**) pressure gauge (outflow), (**h**) bubble, (**i**) filter, and (**j**) pressure gauge (inflow).  
Figure 2: Nanobubble (**a**) NTA Instrument (Malvern Nanosight LM10, Malvern Panalytical Ltd, Unites Kingdom), (**b**) image of nanobubbles [17], and (**c**) Zeta Potential Analyzer (ZetaPALS).