## 3 Results and Discussion
### Effects of Initial pH  
The pH value can be a significant factor in the electrocoagulation process. In this work, the initial solution pH ranged from 5.0 to 9.0, the initial phosphate concentration was 5 mg/L, and the NaCl concentration was 10 mmol/L. As shown in Figure 4a, different initial pH values influenced the phosphate removal rate in the aluminum-air fuel cell system. When the solution pH was alkaline, the phosphate removal rate was faster than the rate under acidic conditions in the early stage. Within 50 min, all phosphate removal rates reached values higher than 90% with different initial pH values. At 60 min, as the initial pH 5.0 changed to pH 9.0, the phosphate removal reached 96.3%, 98.5%, 96.3%, 97.0%, and 95.6%. In contrast to the removal rate trend, the highest level of final phosphate removal was obtained at pH 6.0, which was likely due to the higher aluminum dissolution with a higher current output at pH 6.0 [31]. The average current density of the aluminum-air fuel cell ranged between 0.18 and 0.20 mA/cm\({}^{2}\), which changed slightly during electricity production. At pH values of 5.0 to 9.0, the calculated aluminum ion concentrations were 30.7 mg/L, 33.9 mg/L, 33.1 mg/L, 31.9 mg/L, and 32.2 mg/L. These results also indicated that the self-driven electrocoagulation could adapt to different pH conditions, as shown in previous research [20]. Figure 3: (**a**) Power density and polarization curves and (**b**) electrode potential curves for the aluminum–air fuel cell with different electrolyte concentrations. Figure 2: Effects of different electrolyte concentrations on (**a**) phosphate removal and (**b**) current generation in the aluminum–air fuel cell. The power density of the aluminum-air fuel cell varied slightly under different pH conditions (Figure 5a,b). The maximum power densities of the aluminum-air fuel cell were higher at pH 6.0 and pH 7.0, being 338.1 mW/m\({}^{2}\) and 332.8 mW/m\({}^{2}\), respectively. Under alkaline conditions, the cell maximum power densities decreased; at pH 9.0, the maximum power density was 292.8 mW/m\({}^{2}\). The difference in the fuel cell electrode potential under different pH conditions was not obvious. The electrocoagulation performance was influenced by the aluminum dissolution and the final pH. In this experiment, pH variation was also investigated with different initial pH values, as shown in Table 1. After 60 min of operation with the initial pH value ranging from 5.0 to 9.0, the final pH changed in different trends. For the acidic and neutral conditions, the pH value increased, while with initial pH values of 8.0 and 9.0, the final pH value decreased. The variation trends were similar to previously reported trends [20]. The dissolution rate of aluminum was also related to the current density and the pH value. The behaviors of the electrode and aluminum ions in solution also affected the solution pH. For the aluminum-air fuel cell, the electrode and overall reactions were as shown in Equations (2)-(4). At the aluminum anode, electrons were released from the aluminum, and then the aluminum ions reacted with water, as shown in Equation (5) [32,33]:  
\[\text{Anode}:\text{Al}\rightarrow\text{Al}^{3+}+3\text{e}^{-}\text{E}^{0}=1.66 \text{ V} \tag{2}\]  
\[\text{Cathode}:\text{O}_{2}+4\text{H}^{+}+4\text{e}^{-}\to 2\text{H}_{2} \text{O}\text{E}^{0}\text{= 1.23 \text{ V}} \tag{3}\]  
\[\text{Overall}:\text{Al}+\text{O}_{2}+\text{H}^{+}\rightarrow\text{Al}^{3+}+ \text{H}_{2}\text{O}\text{E}^{0}=\text{ 2.89 \text{ V}} \tag{4}\]  
\[\text{Al}^{3+}+3\text{H}_{2}\text{O}\rightarrow\text{Al}\left(\text{OH} \right)_{3}+3\text{H}^{+} \tag{5}\]  
Figure 4: Effects of different initial pH values on (**a**) phosphate removal and (**b**) current generation in the aluminum\(-\)air fuel cell. Figure 5: (**a**) Power density and polarization curves and (**b**) electrode potential curves for the aluminum\(-\)air fuel cell with different initial pH.