Electrode Materials Comparison for Hydrogen Production from Wastewater Electrolysis of Spiked Secondary Effluent

Abstract

Electrochemical methods show promise for wastewater treatment by removing pollutants, recovering nutrients, and generating hydrogen. To scale this technology, durable and affordable electrode materials are needed. This study evaluates aluminum 6061-T6, titanium grade II, ductile iron, and magnesium to understand their performance in promoting precipitation, gas production, and treating wastewater under several conditions. Electrodes were tested with ammonia-, magnesium-, and phosphate-spiked wastewater samples with induced precipitation at concentrations of 0.033 mol/L and 0.0033 mol/L; the liquid, gas, and precipitation phases were characterized. The results showed up to 35% reduction in ammonia, total phosphate recovery, and up to 70% reduction in magnesium. The cell generates hydrogen with purity levels of 95.6%, 96.1%, 87.9%, and 93.5% when utilizing iron, aluminum, titanium, and magnesium electrodes, respectively. The analyses of precipitants showed formation of vivianite crystals from iron, struvite precipitation from magnesium, and berlinite from aluminum. Overall, these results hold substantial promise for hydrogen generation from wastewater and potential for nutrient recovery and treatment.

1. Introduction

Water scarcity is a global concern, necessitating creative and sustainable wastewater treatment technologies. Electrochemical approaches have arisen as a viable method for environmental sustainability and resource recovery from wastewater [1,2]. Electrocoagulation and electroflotation have been extensively researched for their effectiveness in eliminating contaminants, heavy metals, and suspended solids from contaminated water [3,4], which is a growing problem globally [5,6]. Electrocoagulation is performed through dissolution of sacrificial anodes that release metal cations, which act as coagulants to neutralize charged particles and result in their aggregation and subsequent elimination [1,7]. Electroflotation, on the other hand, utilizes microbubbles produced at the electrodes to transport suspended pollutants to the surface for separation [8].

These electrochemical processes function at ambient temperatures, generate minimal sludge, and can be customized for specific applications by altering electrode materials and configurations [4,9,10]. Despite these benefits, extensive use of these technologies in wastewater resource recovery facilities (WRRFs) is constrained by scalability, optimization of operational conditions, and an insufficient comprehension of fundamental treatment mechanisms [11,12]. Additionally, these techniques are economically limited by factors such as the variability of wastewater quality, cost of electrode material, long-term durability [7,12], and historically low value of the recovered products from treatment (e.g., hydrogen, struvite, vivianite, etc.).

A key advantage of electrochemical treatment is the capacity for resource recovery, which generates additional economic value. This is particularly true for generation of hydrogen (H₂) as a byproduct during electrolysis [13,14,15]. Hydrogen (H₂) produced by electrolysis accounts for 5% of the worldwide production and has seen tremendous growth as a sustainable method for hydrogen generation [16]. Hydrogen production advances clean energy programs and presents an alternative resource recovery pathway, offering economic and environmental benefits [16,17]. Utilizing renewable energy sources such as solar photovoltaic-generated power to facilitate electrochemical processes [17,18] could significantly improve their sustainability [19,20]. Integration of solar energy and electrolysis enables decentralized and off-grid wastewater treatment systems, making these solutions particularly attractive for remote or resource-limited areas [14].

While several laboratory studies have demonstrated the efficacy of electrochemical treatments, additional research is required to validate their performance in real-world conditions, especially for treating complex waste streams such as industrial wastewater or agricultural runoff [21,22]. To overcome these challenges, systematic trials are needed to tune experimental variables for each type of wastewater including current density, electrode geometries, temperatures, and reaction times. It is also important to identify and produce durable and affordable electrode materials to improve the scalability and long-term viability of these technologies [7,23,24].

This study uses an open-source test station designed for electrochemical wastewater treatment research [25]. Evaluations of multiple electrode materials, including aluminum 6061-T6, titanium grade II, ductile iron, and magnesium, have been conducted. These materials were studied to elucidate their performance in promoting precipitation, gas production, and treating wastewater under several conditions. By experimenting on spiked wastewater samples with induced precipitation at two concentrations of ammonia, magnesium, and phosphate (0.033 mol/L and 0.0033 mol/L), this study provides valuable insights into the potential of these materials for sustainable wastewater treatment. Details of the methodology are provided along with results for the water, gas, and precipitants, and the results are analyzed and discussed in the context of the existing literature and future research directions.

2. Materials and Methods

2.1. Electrochemical Cell

This work has been performed using a single-cell electrolyzer constructed of a glass reactor with a working volume of 1 L. To prevent corrosion, a cap was designed and constructed from polyvinyl chloride (PVC) (Figure 1a). Two electrodes were installed through the cap, and a seal was created using silicone O-rings to prevent the escape of gases or liquids during the experiment. The distance between the anode and cathode is 3 cm to avoid possible short circuits due to coagulation. Through a gas outlet derivation on the cell cap (Figure 1b), the gases produced are directed through two impingers in series (Figure 1c); the first is filled with 200 mL of 1mol/L sodium hydroxide (NaOH) solution and 1 mol/L sulfuric acid (H₂SO₄) to capture possible traces of chlorine (Cl) and ammonia (NH₄). Gases are then passed through a silica dehumidifier filter prior to collecting the gases for analysis using 1 L gas bags.

Four different materials were used: ductile iron, aluminum 6061-T6, titanium grade II, and magnesium, described in

Table 1. Electrode characteristics.

Material	Characteristics	Anode Weight [g]	Cathode Weight [g]	Resistance [µΩ]	Refs.
Ductile iron	Commonly used in pipes Environmental friendliness and longer service life	394.2	399.5	229.24	[23,26,27]
Aluminum 6061-T6	Strength and light weight Resistance to corrosion	165.1	165.2	122.26	[23,27,28]
Titanium grade II	Very good corrosion resistance Forms a protective oxide film in the presence of O2	253.1	253	1629.22	[29,30,31]
Magnesium	Commonly used as nutrient recovery Stabilizer for operations	130.1	129.2	136.54	[1,3,23,27]

. The electrodes used were ½-inch 13-unc (unified national coarse) thread rods. The reactor was filled with 1 L of solution, submerging only a portion of the electrodes, resulting in an exposed electrode surface area of 100 cm². Each electrode was weighed before and after the experiments to estimate the mass dissolved from the anode and cathode through the electrochemical process. The materials were selected for their properties and characteristics (Table 1).

2.2. Experimental Procedure

Experiments were conducted in batches in 1 L of wastewater using constant current for 30 min; the reaction was stopped every 10 min to collect 5 mL of solution for analysis. At the end of the experiment, total solids (TS) and precipitants were analyzed. Experiments were conducted with 2 different concentrations of sand-filtered secondary effluent at 2 different amperages. Tests were performed in the same conditions for gas sampling without opening the system to avoid air intrusion.

2.3. Wastewater Characteristics

Sand-filtered secondary effluent from Ilderton, ON, was spiked with ammonium (NH₄) using ammonium chloride (NH₄Cl), phosphate (PO₄) using potassium dihydrogen phosphate (KH₂PO₄), and magnesium (Mg) using magnesium chloride (MgCl₂) at two different concentrations: 0.033 mol/L and 0.0033 mol/L. The water quality was adjusted to match nutrient levels typically observed in different parts of a wastewater treatment plant. The lower concentration corresponded to levels found in raw influent, anaerobic digesters, or sludge treatment processes [32,33], while the higher concentration resembled those in side-stream effluents such as centrate, filtrate, digestate liquid effluent, or streams from nutrient recovery zones within the plant [34]. The results are presented in

Table 2. Wastewater parameters before and after spiking.

Liquid	Total Solids [mg/L]	NH4 [mg/L]	PO4 [mg/L]	Mg [mg/L]
Wastewater initial values	0.1	0.3	1.1	0.05
Wastewater low concentration	0.3	68.5	279	102
Wastewater high concentration	0.6	428.75	2845	772.25

as measured concentrations.

2.4. Analytical Methods

Ammonia, phosphate, and magnesium were analyzed using test kits from Hach company (Ames, IA, USA). Hach method 10031 was used to measure ammonia; total phosphates were measured using Hach method 10127, and Hach method 10292 was used to measure magnesium [35]. Vials were read using a DR6000 benchtop spectrophotometer (Hach, Iowa) [35].

The spiked wastewater was tested using each electrode under standard temperature and pressure (STP) conditions to analyze their electrical behaviour in terms of current draw. For this, an adjustable DC bench power supply (PSC-6932, Circuit Test Electronics, Columbia, British) operating within a range of 0-0.1-32V was utilized. The experiment was run for 30 min at a constant current of 1 A and 2 A at STP. Every 10 min, 5 mL samples were collected to be tested, and the evolution of each parameter was analyzed. Gas samples were collected for analysis using a micro gas chromatograph (MicroGC) Varian CP-4900 (Palo Alto, CA, USA). The energy efficiency of the setup is determined by Equation (1) [16].

$$\mathit{Efficiency} (\%) = {\displaystyle \frac{\mathit{H}\mathit{H}\mathit{V}}{{\mathit{P}}_{\mathit{c}}}}$$

(1)

Efficiency was calculated considering the higher heating value (HHV) of 1 kg of H₂ (39.4 kWhr) divided by the electrical power consumed (Pc) to produce that kilogram of H₂ (kWhr) [16]. At the conclusion of each experiment, the total solids were measured, and electrodes were weighed to establish a mass balance and obtain an understanding of the electrochemical process.

Precipitates from the 4 electrodes from higher-concentration experiments were collected and dried at 50 °C for 8 h prior to analysis. Scanning electron microscope images were taken with a Hitachi SU8230 Regulus Ultra High-Resolution Field Emission (Tokyo, Japan) machine in addition to a sX-Flash detector (10 kV) (Billerica, MA, USA). The phase composition of the samples was analyzed with X-ray diffraction (XRD) using a Rigaku SmartLab (Tokyo, Japan) X-ray diffraction system with Ni-filtered CuKα radiation in Bragg–Brentano geometry (Tokyo, Japan). Diffraction data were acquired over a 2θ range from 10° to 90° with a step width of 0.01° and a scan speed of 2°/min.

3. Results

3.1. Electrical Results

Tests performed at STP on two different spiked wastewater concentrations are shown in Figure 2. The behaviour of each material, according to the conductivity of the wastewater, shows a linear correlation with the m factor of the equation of 5.5 times (

Table 3. Electrical behaviour equations.

Electrode Material	Low Concentration Wastewater	R2	High Concentration Wastewater	R2
Ductile iron	V = 0.07119 I–0.07797	0.997	V = 0.39216 I–0.249	0.993
Aluminum 6061	V = 0.07534 I–0.11096	0.994	V = 0.41667 I–0.633	0.989
Titanium grade II	V = 0.07037 I–0.25185	0.942	V = 0.17857 I–0.61429	0.939
Magnesium	V = 0.07237 I–0.01579	0.999	V = 0.41 I–0.52	0.998

). In higher-conductivity water, it has a lower amount of current with a change in slope of 2.5 times. The change in the slope is due to increased conductivity of the electrolyte, indicating the resistance of the electrolyzer decreases, allowing more current to flow at the same applied voltage [36].

3.2. Water Quality Results

Results of wastewater characterization during experiments (Figure 3) show significant changes in phosphate concentrations for lower-concentration experiments. Figure 3 also shows results from 10 min interval sampling with high and low concentrations of constituents using 1 A and 2 A. The results are presented in tabular form with the columns indicating high and low concentration and the rows indicating the constituents on which this work is focused.

Table 4. Conductivity and pH values before and after experiments.

Material		Initial pH	Final pH	Initial Conductivity [S/m]	Final Conductivity [S/m]
Wastewater		7.05	-	0.07	-
Wastewater low concentration	Fe	6.1	7.9	0.28	0.26
	Al		8		0.25
	Ti		6.6		0.27
	Mg		8.2
Wastewater high concentration	Fe	5	5.6	1.28	1.18
	Al		6.3		1.15
	Ti		5.5		1.21
	Mg		6.7		1.15

shows values of pH and conductivity before and after the experiment.

For phosphate results, aluminum, iron, and magnesium electrodes operating at 2 amperes with a high concentration of constituents showed removal rates of ~62%. Increases in current applied to the cell for these electrodes improved phosphate removal by ~24%, in each case. On the other hand, when titanium electrodes were used, the highest degree of reduction of ~29% was obtained when 2 amperes were applied, and the effect of current increase only resulted in a 5% increase in phosphate reduction. When experiments were conducted in low constituent concentrations, the aluminum, iron, and magnesium electrodes resulted in 100% reduction rates; titanium only showed a 43–46% reduction rate when operated at 1 and 2 amps, respectively [36].

For high concentrations of contaminants, the aluminum, iron, and titanium electrodes had a slight effect on the remaining concentration in the solution, with ~10% reductions in ammonia; increasing the current does not generate significant change. On the other hand, the magnesium electrode high-concentration experiments showed reductions of up to 40% in ammonia, and increasing the current by one ampere resulted in an additional 10% reduction in the contaminant in the solution. At low concentrations of ammonia in the initial solution, the aluminum electrode does not influence ammonia reduction; in contrast, iron, magnesium, and titanium performed better, with titanium showing an ammonia reduction of ~38% during 2 A experiments.

In the case of magnesium, 0.033 mol/L tests show constituent concentration decrease over the test time with a reduction of 20–25%; there is no differential effect due to the increase in the current applied to the cell. However, at low magnesium concentrations, the conversion rate increases significantly, with reductions of 75% for aluminum, 60% for iron, and 20% for titanium electrodes. On the other hand, when the electrode is magnesium, there is a dissolution of the electrode in the solution, which causes the concentration of the solution to increase at up to 40% with respect to the initial concentration, when a current of 2 A is applied. In the case of 0.0033 mol/L, the concentration increases up to 62% above the initial concentration when the same current is applied.

3.3. Gas Production Results

The gas production results from the MicroGC (

Table 5. Gas composition results for high-concentration samples at 2 A.

Electrode Material	Hydrogen [Vol %]	Oxygen [Vol %]	Nitrogen [Vol %]	Total [Vol %]
Ductile iron	95.56	1.01	2.45	99.02
Aluminum 6061	96.13	0.89	1.92	98.94
Titanium grade II	87.93	3.72	6.63	98.28
Magnesium	93.48	1.83	2.44	97.75

) show a composition of almost pure hydrogen with peaks of 96% purity for aluminum electrodes. The MicroGC results do not account for 100% of the gas composition, and this small difference is due to gases that are not detected from the chromatography method used. Even using gas traps, such as NaOH or H₂SO₄, small percentages of chlorine or ammonia are expected. Figure 4 show gas flow measurements at different amperages for higher solution concentration samples; the titanium electrode shows values only up to 4 A because it reached the power supply limits [36].

3.4. Precipitation Results

Reduction and oxidation happen at the cathode and anode, respectively, bringing material into solution to coagulate; a portion of these coagulated materials sink to the bottom, and the rest float to the surface. Figure 5 shows the cell during experiments with different electrode materials, highlighting the role they play in the electrochemical process.

Table 6. Total mass difference for each material.

Electrode Material	Ductile Iron	Aluminum 6061	Titanium Grade II	Magnesium
Phosphate difference [mg]	3220	3215	1866	3685
Ammonia difference [mg]	71	58	195	335
Magnesium difference [mg]	410	379	255	−424
Difference in anode weight [mg]	540	390	150	270
Difference in cathode weight [mg]	11,130	4450	1800	4040
Total solids [mg]	15,020	13,020	3125	5580

shows the mass difference measured through experiments, considering the electrode mass, water quality parameters, and total solids. After four experiments, electrodes lost several grams from reduction at the cathode. Differences are observed among materials; for example, ductile iron lost more than 11 g, while titanium lost only ~2 g, reflecting the solids precipitated and constituents removed from the solution.

The SEM results (

Table 7. SEM high-concentration composition for the four electrode materials tested.

	Electrode Material
Element	Ductile Iron [Wt%]	Aluminum 6061 [Wt%]	Titanium Grade II [Wt%]	Magnesium [Wt%]
Iron	27.4	-	-	-
Aluminum	-	13.8	-	-
Titanium	-	-	26.4	-
Magnesium	2.2	2.8	2.5	16.6
Oxygen	48.	58.7	52.5	59.6
Phosphorus	13.5	18	11.3	19.3
Potassium	2.5	4.2	4.1	1.9
Nitrogen	0.2	-	-	2.5
Chlorine	3.4	1	2.9	0.2
Calcium	2.4	1.4	0.3	-
Silicon	0.1	0.1	-	-

) highlight the oxygen component in all samples, with a peak of 59.6% for the precipitates generated using the magnesium electrode. For each experiment, the element of the electrode material has been measured due to precipitation from the cathode. From the analysis of each electrode, spiking factors like MgCl and KH₂PO₄ have been identified in varying concentrations across individual elements. Phosphorus was most prevalent, with peaks of 19.3% of the precipitate composition. Magnesium found in the precipitate samples was in the range of 2–3% for iron, titanium, and aluminum electrodes, while it reached almost 17% for the magnesium electrode, due to leaking from the electrode itself [36].

The XRD analysis identified the presence of vivianite (Fe₃(PO₄)₂·8(H₂O)) from the iron electrode (Figure 6a), which can also be recognized in Figure 5a from its green colour. Crystals of struvite (NH₄MgPO₄·6H₂O) formed from the magnesium electrode (Figure 6d). The precipitates generated from the aluminum electrode (Figure 6c) had berlinite (AlPO₄) and baricite ((Mg,Fe)₃(PO₄)₂ · 8H₂O). Titanium electrodes produced an amorphous material which could not be identified from XRD analysis (Figure 6c), and similar results have been presented on an amorphous state not associated crystals in XRD analyses [36].

4. Discussion

Testing conducted showed a linear response in the results, suggesting that ohmic losses, influenced by the significant distance between poles (3 cm) in this configuration, are the dominant factor. Coagulation of wastewater during electrochemical treatment may also contribute to hindered anode–cathode charge transport, creating longer paths and requiring higher voltage.

The results from this study are consistent with those by Sahset, who observed 100% removal efficiencies when using aluminum or iron electrodes. Further, they found that removal efficiency is highly sensitive to pH and highly dependent on the initial target constituent concentration [3]. In comparison with Vasudevan et al. (2010), however, who reported cadmium removal with magnesium electrodes, the results here suggest that magnesium electrodes have superior performance for ammonia removal, which could be attributed to the higher pollutant concentrations used in these experiments [3]. In line with studies by Kim et al. (2013), hydrogen production was efficient, especially with aluminum electrodes, reinforcing its potential for industrial applications [22].

Magnesium electrodes have been used satisfactorily for wastewater treatment, yielding a combined effect of flotation and precipitation. Constituents such as phosphates and calcium can be precipitated in the presence of magnesium ions; however, this leads to the eventual destruction of the electrode [37].

Phosphate ions in solutions are generally considered electrochemically inactive, meaning they do not directly participate in reactions or undergo oxidation at the electrodes [38]. Instead, they play a secondary role by interacting with more reactive ions, such as magnesium, iron, or aluminum. Unlike aluminum, iron, and magnesium electrodes, which release ions that promote flocculation, the titanium electrode oxidizes to form titanium dioxide (TiO₂), a chemically stable compound that does not interact with phosphate ions [39]. Consequently, results showed lower phosphate removal rates when titanium electrodes were used. Reactions that occur for each of the electrode materials are grouped into those that occur in the anode, cathode, and reactions related to the formation of solids by the flocculation effect in

Table 8. Chemical reactions description for each electrode.

Electrode	Anode (Oxidation)	Cathode (Reduction)	Flocculation Reactions
Iron (Fe)	Fe → Fe2+ + 2e− (iron oxidation)	2H2O + 2e− → H2 + 2OH− (production of OH− and H2)	Fe2+ + 2OH− → Fe(OH)2 (formation of Fe(OH)2)
	Fe2+ → Fe3+ + e− (in presence of oxygen or oxidizing agents)	Mg2+ + 2OH− → Mg(OH)2 (formation of magnesium hydroxide flocs)	Fe3+ + 3OH− → Fe(OH)3 (formation of Fe(OH)3)
	2H2O → O2 + 4H+ + 4e− (oxygen formation)	NH4+ + OH− → NH3 + H2O (ammonia formation)	3Mg2+ + 2PO43− → Mg3(PO4)2 (magnesium phosphate formation)
	2Cl− → Cl2 + 2e− (chlorine gas formation from Cl−)
Aluminum (Al)	Al → Al3+ + 3e− (aluminum oxidation)	2H2O + 2e− → H2 + 2OH− (production of OH− and H2)	Al3+ + 3OH− → Al(OH)3 (formation of Al(OH)3)
	2Cl− → Cl2 + 2e− (chlorine gas formation)	NH4+ + OH− → NH3 + H2O (ammonia formation)	3Mg2+ + 2PO43− → Mg3(PO4)2 (magnesium phosphate formation)
Magnesium (Mg)	Mg → Mg2+ + 2e− (magnesium oxidation)	2H2O + 2e− → H2 + 2OH− (production of OH− and H2)	Mg2+ + 2OH− → Mg(OH)2 (formation of Mg(OH)2)
	2Cl− → Cl2 + 2e− (chlorine gas formation)	NH4+ + OH− → NH3 + H2O (ammonia formation)	3Mg2+ + 2PO43− → Mg3(PO4)2 (magnesium phosphate formation)
Titanium (Ti)	Ti + 2H2O → TiO2 + 4H+ + 4e− (titanium dioxide formation)	2H2O + 2e− → H2 + 2OH− (production of OH− and H2)	No metal flocs
	2Cl− → Cl2 + 2e− (chlorine gas formation)	NH4+ + OH− → NH3 + H2O (ammonia formation)	3Mg2+ + 2PO43− → Mg3(PO4)2 (magnesium phosphate formation)

.

In the case of iron and aluminum electrodes, the results align with findings in the literature, which explain that ionic decomposition reactions of these metals occur at the anode, and these ions react actively with the OH groups, leading to the formation of hydroxides, and have the capacity to flocculate and precipitate the organic matter present in the wastewater [23,40].

For the iron electrode (Table 8), water oxidation results in the formation of oxygen, which is released into the gaseous phase. Additionally, the formation of chlorine gas at the anode was observed, due to the presence of chorine ions in salts used to increase the concentration of magnesium and ammonia [41]. At the cathode, potential ammonia decomposition reactions were considered, leading to the production of ammonia (NH₃), which can also be released in the gas phase [42].

Further, electrochemical wastewater treatment techniques employed in this study, utilizing the selected electrodes, resulted in hydrogen generation at the cathode due to water reduction. The results obtained are consistent with other literature reports, which indicate that iron, aluminum, magnesium, and titanium electrodes exhibit a high affinity for hydrogen production [15,23]. Specifically, the characteristics of this cell demonstrated a gas generation rate of approximately 25 mL per minute using an aluminum electrode, with hydrogen concentrations exceeding 96% purity. These results appear to outperform those reported by Nasution et al. (2011) and Ali et al. (2012), who used aluminum and iron electrodes in industrial wastewater [15,43]. In these studies, the purity of H2 reached just above 40%, while using similar electrode materials the cell used in this study achieved a purity of 95.56% and 96.13%, respectively. Cho et al. (2014), despite using wastewater with a significantly higher organic load, observed a hydrogen concentration in the generated gases <82% [44]. This difference is reasonable, because in the Cho et al. study, a portion of the applied energy was likely consumed in precipitation of the high-organic-matter content, rather than in hydrogen production [45]. In contrast, this study had a relatively low initial organic load, allowing a larger fraction of the applied potential to be utilized for water reduction and subsequent hydrogen generation.

The energy efficiency calculated for the cell reached more than 40% for the magnesium electrode and 32% for ductile iron. These numbers show a decreasing trend when the current increases, similar to observations by Park et al. (2013) [46]. These results are primarily due to the low conductivity of the solution used, and in this case, the distance between the anode and cathode resulted in a large ohmic resistance. Having achieved such purity in hydrogen gas from a wastewater solution without a membrane has a huge impact on energy requirements, making this treatment approach relevant to industrial applications.

Vivianite detected in the iron electrode experiments is similar to reports by Zhang et al. (2023) [47]. The methods used in this work are also consistent with modelling which indicates that pH and phosphate concentrations are related to the iron being reduced at the cathode, demonstrating the possible removal and recovery of phosphorus via vivianite [47,48]. Berlinite was identified at the aluminum electrode, which could be a recoverable material from the process. Aluminum phosphate enables phosphate recovery and holds valuable applications in electronic devices due to its large mechanical coupling factors, which exceed those of α-quartz. Additionally, its resonant frequency remains nearly temperature-independent for specific orientations [49,50]. The XRD analysis of the magnesium electrode highlights struvite, confirmed by SEM images similar to results reported by Muryanto et al. (2017) [51]. Struvite has been discussed extensively in terms of nutrient recovery and represents a useful application from electrocoagulation of wastewater [52,53,54]. Analysis of the titanium electrode does not yield a precise determination of the precipitates formed. These results are consistent with Zhang et al. (2009), who studied titanium oxide amorous phases from the SEM analysis conducted on the same samples [55].

The experimental reactor supported evaluation of several electrodes and a comparison of the results with the current literature. Promising results were obtained in wastewater treatment, hydrogen production, and mineral recovery, suggesting its value in resource recovery. There are some limitations in these experiments, because they were performed in a batch system. Experiments with continuous operations, in which the constituents’ concentrations vary, as commonly occurs in treatment plants, would allow a better understanding of the real potential for hydrogen production and water treatment [56]. In addition, experiments of longer duration would provide valuable information on the effects of continuous operation on electrode stability and deterioration, as well as on the deposition of oxide layers on the electrodes, which could reduce efficiency, as has been reported by other authors [57]. This study uses real wastewater effluents, spiking them with nutrients so that the experiments could be conducted under controlled conditions. Future work can account for the overall complexity of upstream wastewater, which will involve in-depth analysis as the next steps. In addition, the geometry of the electrodes for all the materials was held constant in this study. Future work can investigate the impact of electrode geometry and dynamic surface roughness on the impacts on the core experiments. The optimal design may not only involve variable electrode materials but also geometries/surface designs for a given type of wastewater. The experimental apparatus used here and fully detailed in [25] is well suited for this necessary future work. Further work is also needed to complete a full lifecycle analysis (LCA) in terms of environmental impact, cost–benefit analysis, and economics to determine the best system and implementation in a large-scale plant. Now that the fundamental promise of this approach has been demonstrated, the further LCA study will quantify the sustainability of the approach. Finally, it should be noted that integrating this hydrogen production system with wastewater treatment for enhanced nutrient recovery could improve energy production through processes such as bio-methanization. The core value of this work is to provide some of the fundamental engineering results to provide wastewater treatment in the future to approach a scalable sustainable state.

5. Conclusions

The experiments conducted with different materials have shown promising and interesting results for electrode characterization, impact on the water quality, gas produced, and solids precipitated. The constituent removal and recovery were most effective for phosphate where the reduction was 100% in lower-concentration solutions and >60% in higher-concentration (0.033 mol/L) batches. Magnesium electrodes showed excellent results in removing phosphate and forming struvite while producing >93% pure hydrogen. From the ductile iron electrode, phosphate recovery was achieved through vivianite precipitation combined with hydrogen production with a purity of 95.56%. Aluminum electrodes had similar performance for recovery of phosphate, magnesium, and ammonia, forming crystals of berlinite with the highest purity of hydrogen (96.1%). All electrodes, except for titanium, had similar electrical efficiencies. The titanium grade II electrodes had several differences compared to other materials; they had the lowest-purity hydrogen, poor nutrient removal, and they had the lowest electrical efficiency, although they had the lowest weight loss, making them more suitable for longer applications.

This work shows there is potential in combining electrolysis and wastewater treatment without the use of membrane separation to achieve very pure hydrogen gas. Wastewater treatment, material recovery, and energy recovery through hydrogen are achievable via electrochemistry. Using these approaches as a foundation provides guidance into sustainable wastewater management in the future.