## 2 Materials and Methods
### The Removal Performance of COD in Secondary Effluent  
The secondary effluent from a wastewater treatment plant in Wuxi, China, was selected as a real complex water matrix to evaluate the catalytic activity of the Fenton iron-peat catalyst. The initial COD of the wastewater is 70 mg/L in the study. The initial pH of the reaction system was adjusted using a borate buffer solution. In a 200 mL beaker, a specific amount of Cat\(-\)2 was mixed thoroughly with the wastewater by ultrasound. After reaching adsorption equilibrium, a predetermined amount of H\({}_{2}\)O\({}_{2}\) and K\({}_{2}\)FeO\({}_{4}\) was added, and stirring was continued. The concentrations of H\({}_{2}\)O\({}_{2}\), potassium ferrite, and catalyst were 0.8 mM, 0.1 mM, and 0.5 g/L, respectively. In the experiment, to study the effect of H\({}_{2}\)O\({}_{2}\) concentration on COD degradation, the concentrations of H\({}_{2}\)O\({}_{2}\) were 0.8 mM, 1.0 mM and 1.5 mM, respectively. At specific time intervals, 2 mL of supernatant was sampled, filtered through a 0.22 \(\upmu\)m aqueous filter, and mixed with Hach reagents. The low-range COD ampoules (HACH Chemical) containing the water samples were then placed in a preheated digestion instrument at 150 \({}^{\circ}\)C for 2 h. After cooling the digestion instrument to 120 \({}^{\circ}\)C, the COD ampoules were removed and shaken several times. The ampoules were then cooled to room temperature for measurement. Furthermore, the effects of key operational parameters on system performance were investigated, including catalyst dosage, pH, oxidant concentration, and the oxidant concentration ratio. All experiments in this study were repeated three times. In addition, the potential application of the Cat\(-\)2/H\({}_{2}\)O\({}_{2}\)/Fe(VI) system was evaluated in a 4 L continuous-flow reactor for COD removal from wastewater. In a small-scale continuous flow experiment, the reflux ratio is 1:4, the pH is 8.1 (\(\pm\)0.1), the concentration of hydrogen peroxide is 0.8 mM, the concentration of potassium ferr is 0.1 mM, and 120 g of catalyst is placed in each layer.  
The reactor was shown in Figure 1. The reactor operated in a bottom-in and top-out mode with partial reflux. Initially, 3 L of sewage and 306.44 \(\upmu\)L of H\({}_{2}\)O\({}_{2}\) were added to the mixer. The water inlet pump flow rate was then set to maximum, and raw water was added to the reactor at the fastest possible speed. Once all the water samples from the mixer had flowed into the reactor, another 3 L of sewage, 306.44 \(\upmu\)L of H\({}_{2}\)O\({}_{2}\), and 200 \(\upmu\)L of H\({}_{2}\)SO\({}_{4}\) were added to the mixer. The raw water was again pumped into the reactor through the water inlet pump. When the water level in the reactor reached the outlet, the water inlet pump was turned off. Reflux was then initiated, and 0.09902 g of potassium ferrite was added to the upper port of the reactor. The potassium ferrite was quickly mixed evenly through reflux. After the reactor had reacted for 2.5 h, the water inlet pump was turned back on, with the flow rate set to 26.01 mL/min. Simultaneously, the potassium ferrite drug inlet pump was activated, with a flow rate of 1 mL/min, and the potassium ferrite was fed into the reactor from the upper reflux port. This process continues for 72 h.  
Figure 1: Reactor operation diagram.