## 3 Results and Discussion
### Mechanisms of Phosphate Removal  
The aluminum anode was analyzed before and after fuel cell operation using SEM to clarify the phosphate removal process (Figures 7 and 8). The anode surface formed a corrosion pit after the operation (Figure 7a,b), providing evidence that the anode spontaneously dissolved aluminum ions into the solution. Previous studies have shown that pitting corrosion exists on metal surfaces when they are covered by a compact oxide film [40]. Furthermore, chloride ions in the electrolyte solution are the aggressive ions on the metal surface. Chloride ions can trigger the pitting corrosion reaction, and the passivation layer on the electrode surface will decompose. For the Al anode, a corrosion-protective film consisting of aluminum oxide can easily form on the surface, and the protective film is stable within a pH range of 4-8.5. However, chloride ions can react with the protective film \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Electrode Type** & **Wastewater Type** & \begin{tabular}{c} **Energy** \\ **(kWh/m\({}^{3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Current Density** \\ **(mA/cm\({}^{2}\))** \\ \end{tabular} &
\begin{tabular}{c} **Phosphate** \\ **Removal (\%)** \\ \end{tabular} & **Reference** \\ \hline Al–Al & urban wastewater & 4.5 & 1 & 99 & [39] \\ Al–Fe & synthetic solution & 1.1–6.1 & 1–4 & 97–99 & [3] \\ Al, air cathode & domestic wastewater & 0.4–9.9 & 0.6–1.2 & 97–99 & [14] \\ Al, air cathode, titanium sheet & domestic wastewater & 0.009–0.06 & 1 & 71–98 & [13] \\ Al, air cathode, graphite sheet, & domestic wastewater & 0.05–0.14 & 1 & 85–98 & [16] \\ Al–Al, photovoltaic solar & landscape water & — & 1.3–3.8 & 96–99 & [30] \\ Al–air cathode fuel cell & synthetic solution & — & 0.06–0.2 & 86–98 & This study \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Energy consumption of different electrocoagulation systems and the removal rates of phosphorus reported in different studies with Al electrodes. and expose the anode surface, which will increase the anode dissolution rate [34]. These processes are shown in Equations (7)-(9) [41]:  
\[\text{Al}_{2}\text{O}_{3}+\text{Cl}^{-}+6\text{H}^{+}\to 2\text{AlCl}_{3}+3 \text{H}_{2}\text{O} \tag{7}\]  
\[\text{Al}(\text{OH})_{3}+\text{nCl}^{-}\rightarrow\text{Al}(\text{OH})_{3- \text{n}}\text{Cl}_{\text{n}}+\text{nOH}^{-},\,\text{n}\,=\,1,\,2,\,3 \tag{8}\]  
\[\text{AlCl}_{3}+\text{Cl}^{-}\rightarrow\text{AlCl}_{4}^{+} \tag{9}\]  
The morphology of the precipitates from 10 mmol/L of NaCl and 5 mg/L of phosphate was observed by SEM, and the elemental compositions were analyzed by EDS, as shown in Figure 8a,b. The precipitates had a blocky structure, and the EDS results showed that the precipitates were mainly composed of P, Al, and O elements, which indicated that the phosphate was removed with the aluminum compounds. There are two main mechanisms that exist in the aluminum coagulation phosphate removal process, namely co-precipitation by monomeric aluminum and adsorption by aluminum hydroxide [42]. In the EC process with aluminum electrodes, both direct precipitation and adsorption by aluminum hydroxide were observed, and direct precipitation for phosphate removal was more efficient than hydroxide adsorption removal [43; 44]. The phosphate removal behavior was related to the water conditions and the aluminum species. AlPO4 precipitate can be formed by Al\({}^{3+}\) and phosphate, and the Al\({}_{3}\) species can also induce formation of AlPO4 by complex adsorption [45]. For the coagulation process, pH influences the speciation of aluminum and accordingly affects the phosphorus removal mechanism. When the pH value was below 3.5, the dominant aluminum species was Al\({}^{3+}\). As the pH value increased from 3.5 to 10.0, the dominant species were complex monomeric and polymeric aluminum hydroxides. Al(OH)\({}_{4}\)\({}^{-}\) became the predominant species when the pH value was above 10.0 [46].