## 3 Results and Discussion
### Mechanisms of B/C Improvement
#### 3.3.2 Biodegradability Improvement Analysis  
The relative abundance of organic pollutants in the petrochemical water after the Fenton oxidation treatment for 60 min is shown in Figure 4b, and the quality of the petrochemical water was significantly improved. Some of the large molecule organic pollutants were completely removed, such as Myristic-1-13C Acid, 2-Acetyl-2-thiazoline, Guanidine, N,N'-dimethyl-. Small organic molecules, such as acetic acid and ammonium acetate were produced, and the relative abundance values of acetic acid and ammonium  
acetate were 30.5% and 28.14%, respectively. It can be inferred that the organic species were reduced by the Fenton oxidation treatment, and the large molecules were oxidized to small molecules by the \({}^{\bullet}\)OH generated from the Fenton reaction [28; 29].  
The result of ozone oxidation treatment is shown in Figure 4c. The effluent organics were mainly aromatic hydrocarbons and small molecule organics, among which the relative abundance of Di-tert-butylphenol, 2-Amino-4,5,6-methylbenzoic acid were still high, at 19.53% and 22.53%, respectively. However, it can be found that there are still many small molecules of organic matter generated in the water samples oxidized by ozone, such as the relative abundance of acetic acid at 15.34%, from which it can be inferred that ozone molecules directly attack the chemical bonds of organic matter to break them, breaking the unsaturated bonds of organic matter and changing the benzene ring into an oxygen-containing heterocyclic ring. Other methods to form small molecules of organic matter also take place.  
Macromolecular organic matters are difficult to biodegradable because they are too large and, thus, cannot be injected into microorganisms, preventing their effective biodegradation. Oxidation reactions break down these large molecules into short-chain intermediates, such as small organic acids, which can enter the cell and become readily biodegradable [30].  
In summary, both Fenton and ozone oxidation could effectively improve the biodegradability of pretreated petrochemical wastewater, and Fenton oxidation had a more obvious improvement effect. The above difference was mainly caused by the different process mechanisms of Fenton and ozone oxidation. \({}^{\bullet}\)OH, which generates from the Fenton reaction, mainly degrades organic pollutants through attacking double bonds in order to form carbocation, or by combining with carbocation intermediate to form additional product, capturing H\({}^{+}\) by the C-H bond fracture of organic pollutants (Equation (4)) and degrading organic pollutants by electron transfer [31]. By contrast, O\({}_{3}\) degrades organic pollutants through reacting with organic compounds containing unsaturated bonds or receiving electrons to replace some functional groups of organic pollutants [32]. Compared with ozone oxidation, a much higher amount of small molecular organic acids was generated in the Fenton oxidation process according to the GC-MS results, which could be attributed to the fact that O\({}_{3}\) was a highly selective oxidant, which made it difficult to degrade organic  
Figure 4: Types and relative abundance of pollutants in different water samples: (**a**) Wastewater; (**b**) Fenton oxidation; (**c**) Ozone oxidation.  
pollutants without electron-rich fraction [33; 34]. On the other hand, the generated \({}^{\bullet}\)OH can attack macromolecular organic pollutants non-selectively. Therefore, Fenton oxidation is more suitable for the pretreatment of petrochemical wastewater.  
\[\text{R-H}+{{}^{\bullet}}\text{OH}\rightarrow\text{H}_{2}\text{O}+\text{R}^{\bullet} \tag{4}\]