###### Abstract  
A self-powered electrocoagulation system with a single-chamber aluminum-air fuel cell was employed for phosphate removal in this study. Electricity production and aluminum hydroxides in solution were also investigated. When the NaCl concentration increased from 2 mmol/L to 10 mmol/L, the phosphate removal increased from 86.9% to 97.8% in 60 min. An electrolyte composed of 10 mmol/L of NaCl was shown to obtain a maximum power density generation of 265.7 mW/m\({}^{2}\). When the initial solution pH ranged from 5.0 to 9.0, 98.5% phosphate removal and a maximum power density of 338.1 mW/m\({}^{2}\) were obtained at pH 6.0. Phosphate was mainly removed by aluminum hydroxide adsorption. These results demonstrate that the aluminum-air fuel cell can be applied as electricity-producing electrocoagulation equipment. Aluminum-air fuel cells provide an alternative method to meet the goal of carbon neutrality in wastewater treatment compared with traditional energy-consuming electrocoagulation systems. aluminum-air fuel cell; electrocoagulation; phosphate removal; electricity production +
Footnote †: journal:  
Electrocoagulation (EC) is considered a promising wastewater treatment technology that comprises three aspects: coagulation, flotation, and electrochemistry [8]. In the EC process, a sacrificial metal anode, such as Al or Fe, dissolves and releases metal ions into the solution with an impressed current [8]. Various metal hydroxides are formed by the hydrolysis process. Hydrogen bubbles and OH\({}^{-}\) ions are discharged at the cathode. Pollutants and impurities are mainly removed by multiple physicochemical reactions with metal hydroxides and generate precipitates simultaneously [9; 10]. EC is an effective method for phosphate removal, and the advantages of the EC system compared with chemical coagulation are its easy operation, no additional chemical requirements, and less sludge production [10; 11; 12]. However, the electrical energy demands need to be reduced to make the EC process economically viable and eco-friendly. Recently, a new type of EC process using an air cathode has been investigated to reduce EC energy demands [13; 14; 15]. At the air cathode, an oxygen reduction reaction proceeds with a related catalyst, and oxygen gas is obtained through air diffusion without aeration. With a minor added operation voltage of 0.5 V, 98% of phosphate was removed in a shortened time of 15 min [16]. The reason for this decrease in the electrical energy requirement of the EC with the air cathode is that the metal anode and the oxygen reduction air cathode exist in a potential gradient. Metal-air fuel cells, which consist of a metal anode and an air cathode, have been investigated for synchronous contaminant removal and energy generation [17]. Over the last few decades, metal-air fuel cells have been implemented to remove arsenate, chromium, p-arsanilic acid, and humic acid, and for algal recovery [18; 19; 20; 21; 22; 23; 24]. Some researchers have also applied metal-air fuel cells for phosphate removal and recovery. When wastewater contained nitrogen and phosphorus together, Mg-air fuel cells were employed to recycle the nitrogen and phosphorus by forming struvite precipitation [25; 26]. Iron-air fuel cells are a new option for recovering phosphate from wastewater. Phosphate was recovered by the formation of vivianite precipitation. Within 3 months of continuous operation, the iron-air fuel cell output voltage achieved approximately 400 mV, and 97% of phosphate was removed from synthetic industrial wastewater [27]. Different configurations of iron-air fuel cells were constructed to treat wastewater containing anaerobically digested sludge. The phosphate removal rate of a two-chamber fuel cell obtained a rate of 11.60 mg-P/L/h [28]. Aluminum is also commonly used as an electrode in electrocoagulation for phosphate removal, and aluminum-air fuel cells have been utilized to remove arsenate.