# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 1 Introduction  
Supercritical water oxidation (SCWO), first proposed by American scholar Modell in the 1980s [1], refers to the process, in which organics are completely decomposed into H\({}_{2}\)O and CO\({}_{2}\) as the homogeneous oxidation of organic matters and oxidants (usually excess oxidant) rapidly take place in supercritical water (SCW), water that is in a special state, where its temperature and pressure exceed critical points (374.15 \({}^{\circ}\)C, 22.1 MPa), its density is close to that of liquid, and its viscosity, which is about 1-10% of the corresponding liquid is close to that of gas. In general, ordinary liquid water has good solubility for electrolytes, such as inorganic salts, but has no or low solubility for most organic matters and gases, such as CO\({}_{2}\), O\({}_{2}\), N\({}_{2}\), etc. As a result, many organic substances do not react or incompletely react in ordinary water. However, in the supercritical state, the physical and chemical properties of water, such as the ion product constant, density, dielectric constant, and viscosity, have undergone tremendous changes. Due to its lower dielectric constant, SCW makes a better solvent, which can mix with organics and oxygen into a homogeneous phase in any proportion and remain a terribly low dissociation constant and solubility of inorganic salts. Meanwhile, because of its lower viscosity and higher diffusion coefficient, the reactions in SCW are featured by higher reaction speed and better heat transfer capability. [PERSON] [2] pointed out that, compared with other organic solvents, subcritical/supercritical water is conducive to the realization of green, environment-friendly chemicalprocesses, and has been widely studied in the fields of synthetic fuel production, biomass processing, waste treatment, and material synthesis. Compared with Advanced Oxidation Processes (AOPs) [3], such as Fenton oxidation [4], electrochemical oxidation [5], and wet oxidation [6], the SCWO technology has following advantages: (1) no interphase mass and heat transfer resistance in SCW, extremely fast reaction speed, and short residence time (usually no more than 1 min) [7; 8]; (2) high degradation efficiency. With a 99% removal rate of organic matters and beyond, organic pollutants can eventually be oxidized into CO\({}_{2}\), H\({}_{2}\)O, N\({}_{2}\), and other small harmless molecules, and hetero-atoms, such as sulfur, chlorine, and phosphorus, are converted into corresponding inorganic acids [7; 9]; and (3) low energy consumption. While the mass fraction of organic matters in the wastewater exceeds 3-4%, the heat balance of the reaction can be maintained without any outside heat of fuel but only by the released heat during the exothermic reaction process [10]. According to the six sectors included in the US National Key Technologies List, SCWO is one of the most promising waste disposal technologies of the 21 st century in the \"energy and environment\" sector. China's Ministry of Industry and Information Technology also listed SCWO as a key technical direction in the field of water pollution prevention and control in 2017 [11]. The SCWO technology has become a superior technology for treating stubborn pollutants that are difficult to be effectively treated by traditional methods, and a small number of commercial devices have been put into operation [2]. Due to harsh SCWO reaction conditions, however, strong corrosion, high material requirements and salt deposition in operation have become the biggest obstacles to its commercialization. Some studies have explored the corrosion characteristics of ferritic-martensitic steels [12; 13], TP347H stainless steel [14], Ni-based alloys, [15; 16; 17] and Super304H stainless steel [18] in different SCW circumstances. Furthermore, the developed Point Defect Model SCW_PDM has successfully and directly described the oxidation kinetics of metals in supercritical aqueous systems [19]. Additionally, [PERSON] et al.