# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 2 Fundamental Processes
### Roles of Oxygen and Hydrogen Peroxide  
Oxygen and hydrogen peroxide, two most commonly used oxidants in SCWO, have produced very different results when used by different researchers. [PERSON] et al. [73] compared the effects of hydrogen peroxide and oxygen on oxidation of acetic acid and 2,4-dichlorophenol at 400-500 \({}^{\circ}\)C, and the experiments using H\({}_{2}\)O\({}_{2}\) as oxidant obtained better degradation results under the same conditions. However, [PERSON] et al. [30] claimed that hydrogen peroxide and molecular oxygen had similar oxidation effects. Given this phenomenon, some scholars have studied the oxidation effect and mechanism of oxygen and hydrogen peroxide in the SCWO process. Since hydrogen peroxide can easily decompose and then produce oxygen and water under heating or light conditions, it is likely to decompose in the preheating stage before the reaction. [PERSON] et al. [74] found that when hydrogen peroxide was heated to the supercritical condition, a large amount of oxygen emerged in the product. The total decomposition equation of hydrogen peroxide is shown as follows:  
\[\mathrm{H_{2}O_{2}}\raisebox{-1.72 pt}{\includegraphics[width=14.226378 pt]{.}} \mathrm{H_{2}O}+\
icefrac{{1}}{{2}}\mathrm{O_{2}} \tag{12}\]  
[PERSON] and [PERSON] [75] speculated that the above decomposition reaction was carried out through the following paths: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Initiation** & & \\ \hline
1 & Hydrogen Abstraction & RH+O\({}_{2}\)\(\rightarrow\)R**\(\leftrightarrow\)H**O\({}_{2}\bullet\) \\ \hline
**Propagation** & & \\ \hline
2 & Oxygen Addition & R**\(\leftrightarrow\)O\({}_{2}\)\(\rightarrow\)R**O**\({}_{2}\bullet\) \\
3 & H-abstraction & RO\({}_{2}^{\bullet}\)\(\rightarrow\)RH\(\rightarrow\)ROH \\
4 & Isomerization & RO\({}_{2}\)\(\rightarrow\)HOHO**\(\bullet\) \\
5 & \(\beta\)-Scission & R**\(\rightarrow\)R**\(\leftrightarrow\)C=RH \\ \hline \hline
**Chain Transfer** & & \\ \hline
6 & Disproportionation & 2 RO\({}_{2}\)\(\rightarrow\)O\({}_{2}\) + 2 RO\(\bullet\) \\ \hline \hline
**Branching** & & \\ \hline
7 & Decomposition & ROH\(\rightarrow\)RO\(\bullet\) + HO\(\bullet\) \\ \hline \hline
**Termination** & & \\ \hline
8 & Radical Recombination & RO\({}_{2}\)\(\bullet\)+RO\({}_{2}\)\(\rightarrow\)Products \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elementary steps and reaction families for oxidation in SCW [31]. \[\mathrm{H_{2}O_{2}+(M)}\xrightarrow{\mathrm{k_{1}}}2\mathrm{HO}\bullet+(M) \tag{13}\]  
\[\mathrm{H_{2}O_{2}+HO\bullet}\xrightarrow{\mathrm{k_{2}}}\mathrm{H_{2}O+HO_{2}\bullet} \tag{14}\]  
\[\mathrm{HO_{2}\bullet+HO\bullet}\xrightarrow{\mathrm{k_{3}}}\mathrm{H_{2}O+O_{2}} \tag{15}\]  
\[\mathrm{HO_{2}\bullet+HO_{2}\bullet}\xrightarrow{\mathrm{k_{4}}}\mathrm{H_{2}O_{ 2}+O_{2}} \tag{16}\]  
where M in Equation (13) represents the colliding molecules in the reaction, which can be homogeneous substances (such as water) or heterogeneous substances (such as reactor wall) in the reaction system. As an oxidant with an oxidation capacity only inferior to F\({}_{2}\), OH\(\bullet\) free radical with an oxidation potential of 2.8 eV played a key role in the whole oxidation process of organics [30]. An interesting phenomenon emerged by comparing the reaction mechanism between oxygen and hydrogen peroxide. Systems that initially only contained oxygen produced hydrogen peroxide through a propagation reaction, while systems that initially only contained hydrogen peroxide produced oxygen through a thermal decomposition reaction. In the system containing oxygen and water, [PERSON] et al.