# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 2 Fundamental Processes
### Ionic Reactions and Free Radical Reactions  
The ion product of water, which is related to both density and temperature, increases with temperature rise [49], and is also significantly affected by the density. Relative to the lower ion product of water under the standard state (10\({}^{-14}\) mol\({}^{2}\)/L\({}^{2}\)), the ionic product of water reaches the maximum value (\(>\)10\({}^{-11}\) mol\({}^{2}\)/L\({}^{2}\)) when the pressure is 30 MPa and the temperature is slightly less than 350 \({}^{\circ}\)C. This means that the concentration of H\({}^{+}\) and OH\({}^{-}\) are more than 10 times larger than that under the standard state and that the acid-base catalyzed reactions such as hydrolysis are promoted. The ion product of water plunges with the decrease of density and the increase of temperature. For example, it is merely \(\sim\)10\({}^{-21.6}\) mol\({}^{2}\)/L\({}^{2}\) when the water density is \(\sim\)18 kg/m\({}^{3}\) at 450 \({}^{\circ}\)C and 25 MPa. Under this condition, the concentration of hydrogen ion and hydroxide ion is so low, and the free radical reaction is dominant. The special properties of water near the critical point have attracted wide attention. Some scholars have studied the solvation effect of SCW employing quantum chemistry and molecular dynamics simulation. [PERSON] et al. investigated the effect of solvation on chloride ion and methyl chloride, and found that relative to ambient liquid water, the solvation of ions in SCW increased the reaction rate by 9-12 orders of magnitude [50; 51]. [PERSON] and [PERSON] et al. [52] calculated the dissociation of H\({}_{2}\)O\({}_{2}\) in SCW by combining density functional theory (DFT) and molecular simulation (MD). The dissociation activation energy barrier of H\({}_{2}\)O\({}_{2}\) in SCW decreased by 2.1 kJ/mol than that in gas-phase due to the solvation. Moreover, as the water approaches to the critical point, the dissociation constant (K\({}_{\rm w}\)) is about three orders of magnitude higher than that in ambient liquid water. The concentration of H\({}^{+}\) produced by the dissociation of water near the critical point is high enough so that some acid-catalyzed organic reactions occur without the addition of any acid. Based on this property, because of hydrolysis as the most typical ionic reaction, many scholars studied the hydrolysis of organics such as indole, phenol and guaiacol under subcritical conditions [53; 54; 55]. Biomass resources (such as lignin and microalgae) and wastes (such as sludge and chemical wastewater) were also gasified or partially oxidized under subcritical conditions to produce bio-oil or hydrogen and thus realize clean utilization of biomass and resource utilization of pollutants [56; 57; 58; 59; 60; 61]. K\({}_{\rm w}\), however, drops sharply above the critical point. At 600 \({}^{\circ}\)C and 250 atm, it is about nine orders of magnitude lower than that in the liquid state [2]. Therefore, the ion reactions are not as significant under supercritical conditions as under subcritical conditions. [PERSON] et al. [62] found that only 2.1% methanol was converted by hydrolysis reaction at 544 \({}^{\circ}\)C and 6.6 s residence time, but in the SCWO process above 530 \({}^{\circ}\)C, complete conversion (\(>\)98%) could be basically achieved. In the presence of alkali metal ions and hydroxide ions, ion dissociation could enhance the ion reaction in SCW and promote the degradation of organic matters. [PERSON] et al. [63]  
Figure 3: The competitive PCB reaction pathways in SCW [48]. discovered that the addition of NaOH effectively reduced the formation of dimers and promoted the dechlorination of 2-chlorophenol (2-CP) in the SCWO process of 2-CP and phenol. In the SCWO process, the oxidants as requisites mainly cause the degradation of organic matters by free radical reactions, which thus tend to play a dominant role rather than ionic reactions.